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Expression of the Early Toarcien negative carbon-isotope excursion in separated carbonate microfractions (Jurassic, Paris Basin)
Abstract
The causes of the pronounced negative excursion in carbon-isotope values that was recorded during the Early Toarcian Oceanic Anoxic Event (T-OAE) are still under debate, particularly with regard to the local versus global pattern of the excursion, and the extent to which recorded signals are under a diagenetic control. In this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a Toarcian section recovered from the Sancerre–Couy borehole, southern Paris Basin. Beyond the recognition of a − 6‰ δ13C excursion in the bulk carbonate content, our data also demonstrate that biogenic particles (such as coccoliths) and inorganic grains precipitated as early diagenetic phases (including dolomite) both record the excursion with the same magnitude. Although several black shales occur through the Paris Basin Toarcian section, it is only that associated with the onset of the OAE that coincides with a large negative carbon-isotope excursion. Taken together these observations indicate that during this event, the entire water column was characterized by homogeneous carbon-isotope values; such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re-mineralized organic carbon (cf. “the Küspert model”), since this would have required a strong vertical gradient in the water column. Additionally, the Paris Basin data show that the decrease in carbonate δ13C values during the OAE occurred in several discrete steps (each of some − 2‰), as has previously been found for organic carbon substrates in other European sections. The stepped nature of the isotopic profile, which is part of a stratigraphic signature previously ascribed to Milankovitch forcing, is compatible with regular pulsed input of light carbon into the whole atmosphere–ocean system from a climatically sensitive source such as gas hydrate, or from thermal methanogenesis of organic-rich sediments in the Karoo–Ferrar large igneous province. Contrasts in the amplitude of the negative carbon-isotope excursion on a regional scale remain an important unexplained aspect of the Toarcian record.
... The most widespread deoxygenated conditions were experienced in north European epicontinental basins and sub-basins (e.g., Schouten et al., 2000;van Breugel et al., 2006;McArthur et al., 2008;Hermoso et al., 2009aHermoso et al., , 2013Trabucho-Alexandre et al., 2012;Lézin et al., 2013;Dickson et al., 2017;Houben et al., 2021) and, albeit to a lesser extent, in Fig. 1. A. Palaeogeographic position of the study area during the Toarcian (~180 Ma) (modified after Ruhl et al., 2016). B. Paleogeographic map of the Northwestern Tethys during the Toarcian (~180 Ma) (modified after Ruebsam et al., 2018). ...
... The black shale interval is, instead, completely devoid of benthic fauna. Furthermore, high concentrations of redox-sensitive elements (e.g., Fe/Al, Mn/Al, V/Al, and Mo), depletion in Mn and the presence of framboidal pyrite (on average about 5 μm in diameter) suggest that the sedimentation occurred under euxinic bottom waters that persisted from the middle J1 to the earliest J2 (Hermoso et al., 2009a(Hermoso et al., , 2009b(Hermoso et al., , 2013. Following the interpretation proposed by Hermoso et al. (2009b), redox conditions then shifted to suboxic across the J2 interval before returning to fully oxic conditions shortly after the end of the Jenkyns Event. ...
... Réka Valley: δ 13 C org (Müller et al., 2017), black shales stratigraphic position and redox conditions (Ruebsam et al., 2018). Sancerre: δ 13 C carb (Hermoso et al., 2009a(Hermoso et al., , 2012 and δ 13 C org (Hermoso et al., 2012), black shales stratigraphic position (Hermoso et al., 2009a), detrital input (Hermoso et al., 2013), redox conditions (Hermoso et al., 2009b(Hermoso et al., , 2013, bioturbation (Hermoso et al., 2009a(Hermoso et al., , 2013 no available data on PZE. Rietheim: δ 13 C carb and δ 13 C org (Montero-Serrano et al., 2015), black shales stratigraphic position (Montero-Serrano et al., 2015;Fantasia et al., 2018), detrital input, productivity and redox conditions (Montero-Serrano et al., 2015), bioturbation (Montero-Serrano et al., 2015;Fantasia et al., 2018) no available data on PZE. ...
The Sogno Core represents one of the deepest pelagic records of the Toarcian Oceanic Anoxic Event (T-OAE) in the Alpine-Mediterranean Tethys. New sedimentological, elemental, Rock-Eval, and biomarker data are presented here, with the aim of reconstructing the depositional conditions that characterized the sedimentation of this succession during the broad positive carbon-isotope excursion of the T-OAE, and its negative isotopic anomaly (Jenkyns Event). Higher Mn xs concentrations, inversely correlated with the δ 18 O curve, were observed starting slightly below the onset level of the Jenkyns Event up to the lowermost part of the negative carbon-isotope anomaly, reflecting a combination of a gradual warming and a progressive increase in fresh waters delivered by runoff. Increased weathering intensity was accompanied by enhanced detrital input with higher concentrations in lithogenic elements, and by a relative increase in palaeoproductivity, consistently with higher Si xs concentrations and changes in calcareous nannoplankton assemblages. The enhanced freshwater input favored water-column stratification, less efficient deep-water circulation and oxygen depletion. Except for two discrete black shales characterized by anoxic pore waters, redox conditions at Sogno turned at maximum suboxic, as indicated by rare to absent bioturbation, limited enrichments in redox-sensitive elements, and molecular biomarkers. The Sogno Core record shows that dominant oxic-suboxic conditions were present also at bathyal depths in the Alpine-Mediterranean Tethys during the Jenkyns Event, thereby contrasting with the anoxic-euxinic environments present both in the shallower water basins and sub-basins of epicontinental northern Europe and, locally, in the relatively deep-water areas of the Alpine Tethys.
... However, the causes of such fluctuations and associated regionally varying oxic conditions in bottom waters remain unclear (Silva et al., 2011;Silva and Duarte, 2015;Schöllhorn et al., 2020aSchöllhorn et al., , 2020b. The early Jurassic of the Sancerre-Couy borehole (NE France) has been studied for numerous stratigraphic and environmental proxies (Gély and Lorenz, 1991;Hermoso et al., 2009aHermoso et al., , 2009bHermoso and Pellenard, 2014;Bougeault et al., 2017;Peti et al., 2021). The current study analyzed the same samples as studied in , and Peti et al. (2021). ...
... In the early Jurassic, Europe was located at the NW margin of the Tethys Ocean and characterized by an extensional tectonic regime ( Fig. 1). Numerous shallow basins delimited by shoals and smaller landmasses were part of a wide North-European epicontinental shelf (Bassoullet and Baudin, 1994;Hermoso et al., 2009a). One such basin was the Paris Basin located at approximately 30 • N paleolatitude and characterized by wet and warm paleoclimatic conditions (Bassoullet et al., 1993;Baudin et al., 1990). ...
... One such basin was the Paris Basin located at approximately 30 • N paleolatitude and characterized by wet and warm paleoclimatic conditions (Bassoullet et al., 1993;Baudin et al., 1990). Hemipelagic sedimentation of marllimestone alternations, silty shales and relatively deep-water carbonate on subsided continental margins was established due to sea-level rise and widespread drowning of carbonate platforms (Baudin et al., 1990;Gély and Lorenz, 1991;Mattioli et al., 2008;Hermoso et al., 2009a). The Sancerre-Couy borehole was drilled in 1986-1987 in the southernmost Paris Basin (central France, near Bourges city; GPF-Sancerre; 40 • 1 ′ 10.8 ′′ N, 2 • 47 ′ 6 ′′ E) and encompasses Carboniferous to Middle Jurassic strata (Gély and Lorenz, 1991). ...
The early Jurassic saw large fluctuations in global temperature, sea-level and bottom-water oxic conditions prior to the Toarcian-Oceanic Anoxic Event (T-OAE), in several oceanic basins of NW Europe. Links between such changes, ocean nutrient concentrations and progressively increasing abundance and diversity of calcareous nannoplankton remain unclear. We have investigated the early Jurassic nannofossil assemblages of the Paris Basin across the mid-Sinemurian to lowermost Toarcian. Comparisons to geochemical proxy data let us decipher evolutionary trends from paleoenvironmental responses, test paleoecological affinities of coccolith species and improve our understanding of paleoenvironmental changes. Despite a strong evolutionary trajectory of the coccolith assemblages, multivariate analyses allowed to identify taxa indicating cold (Parhabdolithus, Crucirhabdus, Crepidolithus pliensbachensis), open ocean (Mitrolithus elegans, Mitrolithus lenticularis, Crepidolithus crassus sensu lato, Crepidolithus granulatus (both variants), Crepidolithus crucifer) and high fertility conditions (placoliths: Bussonius, Lotharingius, Similiscutum novum, Similiscutum finchii). Our data support that: (1) stable cool conditions persisted through the Upper Sinemurian, (2) progressive warming and high sea-levels onsetting across the Sinemurian/Pliensbachian transition, persisting to the lower margaritatus ammonite zone (AZ) may have favored the evolution of placoliths, (3) a first significant increase in abundance of placoliths was triggered in the stokesi ammonite subzone (ASz) by elevated fertility levels which preceded immediately the late Pliensbachian organic matter preservation interval (OMPI), (4) the OMPI and remaining subnodosus ASz were characterized by warm temperatures but low fertility surface waters, (5) another large increase in abundance of placoliths suggests again higher fertility levels across the uppermost margaritatus AZ, spinatum cold event and lowermost Toarcian.
... The Early Toarcian "Oceanic Anoxic Event" (T-OAE) is a very significant paleoenvironmental perturbation of the Mesozoic that coincided with marked disruptions to both the climate system and marine ecosystems (Jenkyns, 1988;Jenkyns and Clayton, 1997;Bailey et al., 2003;Hesselbo et al., 2007;Hermoso et al., 2009;Hermoso and Pellenard, 2014;Kemp and Izumi, 2014;Han et al., 2018). This event is associated with widespread burial of marine carbon (Jenkyns et al., 2002) and a large negative carbon-isotope excursion (CIE) in carbonates, organic matter, and fossil wood (Kemp et al., 2005;Hesselbo et al., 2007;Hermoso et al., 2009;Hermoso and Pellenard, 2014;Kemp and Izumi, 2014;Montero-Serrano et al., 2015;Fu et al., 2016a;Izumi et al., 2018;Xu et al., 2018;Remírez and Algeo, 2020). ...
... The Early Toarcian "Oceanic Anoxic Event" (T-OAE) is a very significant paleoenvironmental perturbation of the Mesozoic that coincided with marked disruptions to both the climate system and marine ecosystems (Jenkyns, 1988;Jenkyns and Clayton, 1997;Bailey et al., 2003;Hesselbo et al., 2007;Hermoso et al., 2009;Hermoso and Pellenard, 2014;Kemp and Izumi, 2014;Han et al., 2018). This event is associated with widespread burial of marine carbon (Jenkyns et al., 2002) and a large negative carbon-isotope excursion (CIE) in carbonates, organic matter, and fossil wood (Kemp et al., 2005;Hesselbo et al., 2007;Hermoso et al., 2009;Hermoso and Pellenard, 2014;Kemp and Izumi, 2014;Montero-Serrano et al., 2015;Fu et al., 2016a;Izumi et al., 2018;Xu et al., 2018;Remírez and Algeo, 2020). These features have been attributed to a large-scale injection of isotopically light carbon into the ocean-atmosphere system, possibly from the release of volcanogenic CO 2 from the emplacement of the Karoo-Ferrar Large Igneous Province (LIP) in southern Gondwana (e.g., Pálfy and Smith, 2000) and/or from dissociation of methane hydrates (Kemp et al., 2005). ...
... The T-OAE has been well documented in the western Tethyan and Boreal realms (e.g., Kemp et al., 2005;Hesselbo et al., 2007;Hermoso et al., 2009;Hermoso and Pellenard, 2014;Han et al., 2018;Xu et al., 2018). Only a few studies on the T-OAE have been reported from the northwest, central, and northeast Panthalassic Ocean (e.g., Al-Suwaidi et al., 2010Caruthers et al., 2011;Kemp and Izumi, 2014;Izumi et al., 2018). ...
The Early Toarcian “Oceanic Anoxic Event” (T-OAE) is recorded by marked disruption to both the climate system and marine ecosystems. Here, we present intergraded high-resolution carbon-isotope data (δ13C), bulk geochemistry, mineral characterization from an open-ocean setting in the eastern Tethys. With these data, we (1) construct the high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys; (2) show that the T-OAE in the Sewa succession was marked by coarser-grained deposits associated with high-energy conditions within the otherwise low-energy claystone deposits that likely linked to a globally increased supply of clastic sediments into marginal and deeper marine basin; (3) propose that the low Corg:Ptotal ratios, in combination with bioturbated structure and depletion or slight enrichment in redox-sensitive trace elements of V, Mo, and U suggest a long-term oxygenation event throughout the T-OAE interval at the Sewa succession, and hence, anoxia may not play a fundamental role during the Toarcian negative CIE in this setting; (4) exhibit that a warming and more humid climate began at the start of the T-OAE, and many episodic changes in sediment provenance throughout the T-OAE interval occurred at this location; and (5) suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic changes, but a regional environmental perturbation also might influence the preservation of organic matter.
... environmental perturbations (e.g. Küspert, 1982;Jenkyns and Clayton, 1986;Jenkyns, 1988Jenkyns, , 2010Hesselbo et al., 2000aHesselbo et al., , 2007Röhl et al., 2001;Jenkyns et al., 2002;Cohen et al., 2004;Röhl and Schmid-Röhl, 2005;Hermoso et al., 2012Hermoso et al., , 2013Hermoso et al., 2009;Al-Suwaidi et al., 2010Bodin et al., 2010;Littler et al., 2010;Bodin et al., 2016;Suan et al., 2010Suan et al., , 2011Suan et al., , 2015Lézin et al., 2013;Sabatino et al., 2013;Kafousia et al., 2014;Kemp and Izumi, 2014;Reolid et al., 2014;Caruthers et al., 2014;Pieńkowski et al., 2016;Martinez et al., 2017;Silva et al., 2017;Them et al., 2017;Xu et al., 2017bXu et al., , 2018; Ruvalcaba Baroni et al., 2018;Fantasia et al., 2018Fantasia et al., , 2019bRuebsam et al., 2019;Danise et al., 2019;Remírez and Algeo, 2020;Schöllhorn et al., 2020a;Storm et al., 2020;Rodrigues et al., 2021). However, there is increasing evidence that organic carbon burial and preservation, even when associated with widespread carbon sequestration events such as the T-OAE, were modulated by local geography, bathymetry, climate, ocean chemistry, nutrient supply, biological communities and, consequently, the local depositional environment (e.g. ...
... In particular cases, such as the T-OAE, the δ 13 C record may respond in a more complex way if deposition of OM is accompanied by large-scale carbon release from internal or external sources with different isotopic composition (e.g. Küspert, 1982;Jenkyns and Clayton, 1986;Jenkyns, 1988Jenkyns, , 2010Hesselbo et al., 2000aHesselbo et al., , 2007Pálfy and Smith, 2000;Röhl et al., 2001;Jenkyns et al., 2002;Cohen et al., 2004;Röhl and Schmid-Röhl, 2005;Hermoso et al., 2012Hermoso et al., , 2013Hermoso et al., 2009;Al-Suwaidi et al., 2010Bodin et al., 2010;Littler et al., 2010;Bodin et al., 2016;Suan et al., 2010Suan et al., , 2011Suan et al., , 2015Lézin et al., 2013;Sabatino et al., 2013;Kafousia et al., 2014;Kemp and Izumi, 2014;Reolid et al., 2014;Caruthers et al., 2014;Pieńkowski et al., 2016;Martinez et al., 2017;Silva et al., 2017Silva et al., , 2020aThem et al., 2017;Xu et al., 2017bXu et al., , 2018Fantasia et al., 2018Fantasia et al., , 2019bRodrigues et al., 2019Rodrigues et al., , 2020bRuebsam et al., 2019;Danise et al., 2019;Storm et al., 2020;Ullmann et al., 2020). Two primary debates exist today regarding the processes and mechanisms leading to the occurrence and preservation of OM in sediments (Forsman and Hunt, 1958;Huc and Durand, 1974;Pedersen and Calvert, 1990;Largeau and Derenne, 1993;Tyson, 1995;van Buchem et al., 1995;Vandenbroucke and Largeau, 2007;Zonneveld et al., 2010;Jenkyns, 2010;Suárez-Ruiz et al., 2012;Silva et al., 2012Silva et al., , 2013Silva et al., , 2020aHemingway et al., 2019;Hülse et al., 2019). ...
... Hesselbo et al., 2007), up to 5‰ in bulk carbonate (e.g. Hermoso et al., 2009;Kafousia et al., 2011), and up to 4‰ in macrofossil carbonate (e.g. brachiopods (Suan et al., 2010;Ullmann et al., 2020) superimposed on a broader Early Toarcian pCIE, which encompasses the tenuicostatum-bifrons time interval (Fig. 4) (Küspert, 1982;Jenkyns and Clayton, 1986;Jenkyns, 1988Jenkyns, , 2010Hesselbo et al., 2000aHesselbo et al., , 2007Röhl et al., 2001;Jenkyns et al., 2002;Cohen et al., 2004;Röhl and Schmid-Röhl, 2005;Woodfine et al., 2008;Hermoso et al., 2009Hermoso et al., , 2012Hermoso et al., , 2013Al-Suwaidi et al., 2010Bodin et al., 2010;Littler et al., 2010;Bodin et al., 2016;Suan et al., 2010Suan et al., , 2011Suan et al., , 2015Lézin et al., 2013;Sabatino et al., 2013;Kafousia et al., 2014;Kemp and Izumi, 2014;Pittet et al., 2014;Reolid et al., 2014;Caruthers et al., 2014;Pieńkowski et al., 2016;Martinez et al., 2017;Silva et al., 2017;Them et al., 2017;Xu et al., 2017bXu et al., , 2018Fantasia et al., 2018Fantasia et al., , 2019bRuebsam et al., 2019Ruebsam et al., , 2020Boulila et al., 2019;Danise et al., 2019;Jin et al., 2020;Ruebsam and Al-Husseini, 2020). ...
Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ¹³C record. This survey combines an in-depth analysis of literature on the distribution of OM in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco.
Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ¹³C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressed in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ¹³C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ¹³C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ¹³C (recovery) trend following the δ¹³C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of OM (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval.
The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).
... However, some other authors point to a high primary production occurring before the event and to a phytoplankton blackout concomitant with it (Bucefalo Palliani et al. 2002;Mattioli et al. 2009). The Jenkyns Event is coincident with a characteristic negative carbon isotope excursion (CIE) with a magnitude of c. 5‰ (Hesselbo et al. 2000;Hermoso et al. 2009;Suan et al. 2015). This negative CIE is present in different substrates, e.g. ...
... This negative CIE is present in different substrates, e.g. marine biogenic carbonate and micrite, and in marine and terrestrial organic matter, implying a major perturbation of the carbon cycle affecting the exogenic carbon reservoirs (Suan et al. 2008a;Hermoso et al. 2009;Bodin et al. 2010;Suan et al. 2010;Hesselbo and Pienḱowski 2011;Müller et al. 2020b). Furthermore, this carbon isotope anomaly can be also traced at multiple localities around the world, suggesting a global extent of this event (Caruthers et al. 2011;Gröcke et al. 2011;Suan et al. 2011;Izumi et al. 2012;Al-Suwaidi et al. 2016). ...
... Additionally, the most negative δ 13 C values Jenkyns Event (T-OAE) in a Tethyan pelagic basin in Hungary (c. −5-6‰) are lower than in published early Toarcian bulk carbonate records (Hesselbo et al. 2007;Hermoso et al. 2009;Bodin et al. 2016). Such low values are commonly related to organic matter remineralization and precipitation of 13 C-depleted carbonate in organic-rich mudstones, forming during early diagenesis under anaerobic conditions when sulfate reduction takes place (Bodin et al. 2016;Wohlwend et al. 2016;Arabas et al. 2017). ...
The Jenkyns Event or Toarcian Oceanic Anoxic Event (T-OAE) was an episode of severe environmental perturbations reflected in carbon isotope and other geochemical anomalies. Although well studied in the epicontinental basins in NW Europe, its effects are less understood in open marine environments. Here we present new geochemical (carbon isotope, CaCO 3 , [Mn]) and nannofossil biostratigraphic data from the Tölgyhát and Kisgerecse sections in the Gerecse Hills (Hungary). These sections record pelagic carbonate sedimentation near the margin of the Tethys Ocean. A negative carbon isotope excursion of ∼6‰ is observed in the Tölgyhát section, in a condensed clay and black shale layer where the CaCO3 content drops in association with the Jenkyns Event. At Kisgerecse, bio- and chemostratigraphic data suggest a gap in the lower Toarcian. The presence of an uppermost Pliensbachian hardground, absence of the lowermost Toarcian Tenuicostatum ammonite zone, and the condensed record of the Jenkyns Event at Tölgyhát, together with a condensed Tenuicostatum Zone and the missing negative carbon isotope anomaly at Kisgerecse implies arrested carbonate sedimentation. A calcification crisis and sea-level rise together led to a decrease in carbonate production and terrigenous input, suggesting that volcanogenic CO2-driven global warming may have been their common cause.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5355342
... Hesselbo et al., 2007;Ruebsam et al., 2020a). This global change led to a biotic crisis constituting a secondorder mass extinction for benthic organisms (Little and Benton, 1995;Aberhan and Fürsich, 2000;Cecca and Macchioni, M. Reolid: Microfossil assemblages and geochemistry for interpreting the Jenkyns Event probably driven by oxygen depletion affecting many platforms and deep oceanic environments (Röhl et al., 2001;Bucefalo Palliani et al., 2002;Wignall et al., 2005;Hermoso et al., 2009;Caruthers et al., 2014;Them et al., 2018;Reolid et al., 2012aReolid et al., , 2019a. Oxygen-depleted conditions in marine ecosystems in some areas reached generalized anoxia and euxinia (e.g. ...
... In the Cueva del Agua section, the abrupt decrease in the CaCO 3 content and the negative CIE when thin-laminated dark marls are recorded (Fig. 4) are well-correlated with the T-OAE and the perturbation of the carbon cycle, as well as the carbonate production located in the lower part of the Serpentinum Zone (Erba, 2004;Mattioli et al., 2004Mattioli et al., , 2008Tremolada et al., 2005;Wignall et al., 2005;Hermoso et al., 2009;Suan et al., 2010;Ruebsam et al., 2020a). In the Cueva del Agua section, values of CaCO 3 and δ 13 C equivalent to those of the Pliensbachian are not recorded until the thinbedded limestones. ...
... Thus, for the Serpentinum Zone (Levisoni Zone for Mediterranean biozonation and Falciferum Zone for sub-boreal biozonation), the TOC values range from 5 % to 15 % in the outcrops of northern Europe (Saelen et al., 2000;Röhl et al., 2001;Bucefalo-Palliani et al., 2002;Mailliot et al., 2006;McArthur et al., 2008;Baroni et al., 2018;Thibault et al., 2018;Fantasia et al., 2019;among others), whereas in the Tethys region (southern Europe and North Africa) the values range from 0.5 % to 3 % (Jenkyns, 1985(Jenkyns, , 1988Jenkyns et al., 2002;Hesselbo et al., 2007;Bodin et al., 2010;Reolid et al., 2012b;Rodrigues et al., 2019;Ruebsam et al., 2020a, b;among others). This stands as evidence of the geographic variations of the T-OAE signal (see also Hermoso et al., 2009;Ruebsam et al., 2020b). The thin-laminated dark marls registered during the organic-matter-rich intervals of the T-OAE, similar to the ones recorded in the Cueva del Agua section, are likewise reported for the T-OAE elsewhere (Parisi et al., 1996;Röhl et al., 2001;Hermoso et al., 2009;Izumi et al., 2012, among others). ...
By studying the facies, geochemistry, and microfossil assemblages of the
uppermost Pliensbachian and lower Toarcian of the Cueva del Agua section, I
was able to appraise the impact of the Jenkyns Event in the eastern part of
the South Iberian Palaeomargin (Western Tethys). Depleted oxygen conditions
are envisaged for the Polymorphum–Serpentinum Zone boundary (lower
Toarcian), represented by dark marls, precisely in a laminated interval. The
decrease in the α diversity of foraminifera and ostracods, along
with greater proportions of opportunists such as Lenticulina, Eoguttulina, and Cytherella just before the
negative carbon isotopic excursion (CIE), would indicate a disturbance of the
environmental conditions during the initial phase of the biotic crisis. The
peak of the biotic crisis is characterized by an absence of trace fossils,
increased organic matter content, an increased Mo∕Al ratio, and negative CIE and
δ18O, as well as fewer specialist forms and more opportunists.
This biotic crisis peak is related to oxygen-depleted conditions in the
bottom waters and in the sediment pore water, while warming negatively
affected microfauna – to the point of leaving a barren benthic horizon in
the record. Recovery is evidenced by the occurrence of carbonate layers with
hummocky cross-stratification and a decrease in organic matter content, the
Mo∕Al ratio, and the trace fossil record. In terms of microfauna, the
first phase of recovery shows highly abundant foraminifera, ostracods, and
microgastropods, mainly opportunist forms. After the proliferation of
opportunist forms, a second phase of recovery is marked by a progressive
increase in α diversity.
... Ammonite zonation schemes for the German and Paris basins are after Page (2003). The thick black line marks intervals of global black shale deposition, the stippled line indicates regional black shale deposition 2022a), the Mediterranean realm (e.g., Jenkyns 1988;Pittet 2002, 2004;Reolid et al. 2018;Ruebsam et al. 2020a;Visentin and Erba 2021), the Paris Basin (e.g., van Breugel et al. 2006;Hermoso et al. 2009Hermoso et al. , 2013Ruebsam et al. 2022b), the South-German Basin (Prauss et al. 1991;Röhl et al. 2001;Frimmel et al. 2004;Schwark and Frimmel 2004;Montero-Serrano et al. 2015;Ajuaba et al. 2022), and the British Islands (e.g., Clayton 1986, 1997;McArthur et al. 2008;Xu et al. 2018). Detailed and stratigraphically well-constrained data from the central area of the NGB are, however, relatively rare. ...
... This biocalcification crisis might be expressed by moderate carbonate abundances in interval III (Fig. 7). Carbonate abundances increase in interval IV and maximize in interval V (Lower Elegans Bed), indicative of a recovery of calcareous nannoplankton subsequent to the Jenynks Event (Mattioli et al. 2008(Mattioli et al. , 2009Hermoso et al. 2009;Ruebsam et al. 2022c). Marked fluctuations occur in carbonate contents of interval III. ...
The Pliensbachian/Toarcian boundary interval represents a transition from a coldhouse into a hothouse climate state, involving the demise of a land-based cryosphere, initiating a third-order global sea-level rise. Within the intensely studied Northwest Tethyan shelf region, the South-German Basin has been investigated in more detail than the North-German Basin (NGB). We here provide a palaeoenvironmental reconstruction of the Pliensbachian/Toarcian transition from the Hondelage fossil excavation site located in the NGB employing organic, isotope, and major/trace element proxies. Here, the late Pliensbachian was characterized by cold climate, low sea level, and a slow hydrological cycle, causing minor terrigenous sediment and nutrient fluxes to the basin, instigating low marine productivity. Shallow, well-mixed shelf waters of normal salinity favored aerobic degradation of planktonic biomass, preventing sedimentary accumulation of organic matter. These conditions changed in the earliest Toarcian, where increased temperatures led to sea-level rise via meltdown of land-based ice and accelerated the hydrological cycle, causing salinity stratification. Enhanced riverine sediment and nutrient supply from nearby landmasses promoted marine primary productivity, which caused anoxic conditions in bottom and pore waters favoring enhanced preservation and accumulation of organic matter. A short-lived sea-level fall at the Lower Elegans Bed coincided with lowered productivity and enhanced carbonate precipitation, due to reduced runoff and recovery of the carbonate factory. Increased redox-sensitive trace element concentrations above the Lower Elegans Bed suggest a renewed inflow of low-salinity arctic water masses via the Viking Corridor and potentially increased freshwater input, promoting water column stratification, enhanced planktonic productivity, and re-establishment of bottom water anoxia/euxinia.
... This was an Early Jurassic period of (geologically) abrupt and extreme global warming, where seawater temperatures increased by up to 10°C (Bailey et al. 2003;Ruebsam et al. 2020b) over ,500 000 years. Early Toarcian warming was linked to elevated continental weathering (Cohen et al. 2004;Them et al. 2018), sea-level rise, a large negative C-isotope excursion (up to −6‰ δ 13 C) indicating substantial perturbations to the oceanic (Röhl et al. 2001;Hermoso et al. 2009;Al-Suwaidi et al. 2010;Caruthers et al. 2011;Gröcke et al. 2011;Kemp et al. 2011) and atmospheric carbon cycles (Hesselbo et al. 2000(Hesselbo et al. , 2007Hesselbo and Pienkowski 2011), widespread organic carbon burial (Jenkyns 1988) and global increases in the extent of ocean deoxygenation (Pearce et al. 2008;Them et al. 2019). These changes were associated with a mass extinction of benthic and pelagic taxa across the Boreal, Tethyan and Panthalassa oceans (e.g. ...
... Reolid et al. 2012;Nikitenko et al. 2013;Rita et al. 2016), phytoplankton and zooplankton assemblages changed across the NW Tethys, and so did the size and morphology of selected taxa groups (e.g. Palliani and Riding 1999;Palliani et al. 2002;Schwark and Frimmel 2004;van Breugel et al. 2006;Hermoso et al. 2009; Van de Schootbrugge et al. 2013;Reolid et al. 2014Reolid et al. , 2019Clemence et al. 2015;Correia et al. 2017;Galasso et al. 2021;Reolid and Ainsworth 2022), showing that both primary and secondary production changed over this period. Changes in the biogeographical distribution of marine nekton including ammonites (Dera et al. 2011) and belemnites, and changes in the body-size of the latter are also apparent in NW Tethys (Caswell and Coe 2014;Rita et al. 2018Rita et al. , 2019. ...
Climate change is undermining the health and integrity of seafloor ecosystems, with declines in bioturbation expected to impact future ecosystem functioning. We explored changes in the nature and degree of bioturbation during Early Jurassic global warming and ocean deoxygenation. Understanding how these communities responded can help anticipate how bioturbation and ecosystem functioning might change over large spatial and temporal scales. Trace and body fossils from outcrop and core in the Cleveland Basin, UK show how healthy seafloor communities deteriorated through the Pliensbachian spinatum Zone, and macroinfaunal behaviour fluctuated across the Pliensbachian-Toarcian boundary coincident with mass extinction. Deoxygenation began above the stage boundary, and conditions deteriorated until bioturbation ceased completely (upper tenuicostatum Zone) for 0.6-2.5 Ma, longer than anywhere else in NW Tethys. The macroinfaunal record revealed new details on the progression and timing of deoxygenation, benthic recovery and fluctuations in the palaeoredox boundary. After the OAE infauna were fewer, smaller and did not mix sediments to depth, whilst the depth and diversity of bioturbation increased by the fibulatum Subzone ( bifrons Zone) the benthos had not recovered to late Pliensbachian pre-OAE state. Bioturbation collapse over large parts of the northern hemisphere, likely contributed to regional-scale changes in ecosystem functioning.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6370710
... A maximum depositional time of 184.4 ± 0.61 Ma for the base of the oil shale layer (Fig. 1) suggests the Early Toarcian age (Fu et al. 2016). Interestingly, 4-step negative CIEs are also clearly identified in the Qiangtang Basin (Fig. 2), which is the typical feature of the T-OAE carbon-isotope records (Hermoso et al. 2009). Recently, Xia et al. (2021) had reported a solid pollen data to constrain an early Toarcian age for the lower part of the Biluoco (Bilong co in in Xia et al. 2021) section based on the first appearance of marker dinocysts taxa of Mancodinium semitabulatum, Nannoceratopsis gracilis, Scriniocassis priscus, and Scriniocassis weberi, and along with index pollen grains of Callialasporites turbatus and C. microvelatus. ...
... The Biluoco section is not the only location to study T-OAE. The Toarcian carbon-isotope profiles from the Qiangtang Basin, Eastern Tethys are compatible with those in contemporaneous strata in the western Tethyan and Boreal realms (e. g., Hesselbo et al. 2007, Hermoso et al. 2009, Hermoso and Pellenard 2014, Xu et al. 2018, supporting the global expression of the T-OAE. ...
... Using a combination of enhanced enrichment in organic carbon in globally distributed outcrops, and the accompanying carbonate and organic-matter carbon-isotope signature from a number of localities, suggests a duration from the mid-tenuicostatum zone (over which interval carbon-isotope values are generally rising) to the mid-serpentinum zone where values are typically at their highest, thereby taking in the intervening pronounced stepped negative carbon-isotope excursion that characterizes the T-OAE and interrupts the overarching positive trend (e.g. Jenkyns & Clayton, 1997;Hesselbo et al. 2000;Hermoso et al. 2009;Jenkyns, 2010;Kemp et al. 2011;Xu et al. 2018;Storm et al. 2020 for UK sequences). In northern and southern European outcrops, the most organic-rich level, typically developed as millimetre-laminated black shale, lies primarily in the lower part of the exaratum subzone of the serpentinum zone or its stratigraphic equivalents (Jenkyns et al. 2002). ...
... Given that a pronounced negative carbon-isotope excursion, typically interrupting an overarching positive trend, is a feature of the T-OAE worldwide and recorded in pelagic and shallowwater carbonate, marine and terrestrial organic matter (e.g. Jenkyns & Clayton, 1997;Kemp et al. 2005;Hesselbo et al. 2007;Woodfine et al. 2008;Hermoso et al. 2009;Al-Suwaidi et al. 2010;Caruthers et al. 2011;Sabatino et al. 2013;Kemp & Izumi, 2014;Bodin et al. 2016;da Rocha et al. 2016;Them et al. 2017;Xu et al. 2017Xu et al. , 2018Han et al. 2018;Ikeda et al. 2018;Kemp et al. 2020), it is possible that this signal is recorded in the Junction Bed. Where well dated, the core of the negative excursion lies in the basal exaratum subzone or equivalent (basal serpentinum zone), which is the interval that appears to be unrecorded in the Junction Bed and is likely lost to a hiatus (Howarth, 1992). ...
Two fallen blocks of the Marlstone and stratigraphically overlying Junction Bed sampled on the beach below Doghouse Cliff in Dorset, UK (Wessex Basin) have been examined for carbon and oxygen isotopes of bulk carbonate as well as for strontium, carbon and oxygen isotopes and Mg:Ca ratios in the contained belemnites. The sequence, which contains most of the Toarcian zones and subzones within a metre or less of grey to yellow to pink, red and brown fossil-rich nodular limestone, is extremely condensed and lithologically similar to pelagic red limestones of the Tethyan Jurassic that are locally mineralized with Fe-Mn oxyhydroxides (e.g., Rosso Ammonitico). Strontium-isotope ratios of the contained belemnites are compatible with existing reference curves and both blocks show a rise to more radiogenic values post-dating the Pliensbachian–Toarcian boundary. The high degree of correlation between the relatively negative carbon and oxygen isotopes of the bulk carbonate is compatible with significant diagenetic overprint, and contrasts with higher carbon-isotope values in coeval condensed coccolith-rich limestones elsewhere. Evidence for the characteristic signature of the Toarcian Oceanic Anoxic Event, as represented by organic-rich sediment, is absent, possibly owing to a stratigraphic gap. Both blocks exhibit abrupt carbon-isotope shifts to lower values, one of which could represent the limbs of an incompletely recorded negative excursion associated with the Toarcian Oceanic Anoxic Event. That the Toarcian Oceanic Anoxic Event was also a significant hyperthermal is illustrated in both blocks by a drop in oxygen-isotope values and rise in Mg:Ca ratios of belemnites close to the base of the Junction Bed in the lowest part of the serpentinum zone.
... This event is characterized by the presence of δ 13 C excursions, and especially a prominent negative carbon isotope excursion (CIE) documented in different materials, such as bulk carbonates, carbonate microfractions, marine organic matter, brachiopod calcite and continental fossil wood worldwide (i.e. Jenkyns and Clayton 1986;Jenkyns 1988;Hesselbo et al. 2000Hesselbo et al. , 2007aHesselbo et al. , 2013Röhl et al. 2001;Jenkyns et al. 2002;Mattioli et al. 2004a;Gómez et al. 2008;Suan et al. 2008Suan et al. , 2010Hermoso et al. 2009Hermoso et al. , 2012Mailliot et al. 2009;Jenkyns 2010;Littler et al. 2010, Caruthers et al. 2011Bodin et al. 2014Bodin et al. , 2016Al-Suwaidi et al. 2016;Ait-Itto et al. 2017, 2018Bougeault et al. 2017;Xu et al. 2017;Fantasia et al. 2018Fantasia et al. , 2019Ruebsam et al. , 2019Krencker et al. 2019;Menini et al. 2019;Storm et al. 2020). Furthermore, changes in the oceanic productivity owing to enhanced continental weathering, which could have led to shelf sea eutrophication, have been recorded during this event (i.e. ...
... Some authors support the hypothesis that the biotic crisis related to the Jenkyns Event was probably driven by oxygen depletion affecting both platforms and oceanic deep environments (i.e. Jenkyns and Clayton 1986;Jenkyns 1988;Baudin et al. 1990;Röhl et al. 2001;Bucefalo-Palliani et al. 2002;Mattioli et al. 2004a, b;Röhl and Schmid-Röhl 2005;Wignall et al. 2005;Hermoso et al. 2009;Gill et al. 2011;Reolid et al. 2012Reolid et al. , 2019aCaruthers et al. 2014;Them et al. 2018), although the occurrence of an oceanic anoxic event in the early Toarcian (eT-OAE), based on the deposition of black-shale facies in basins all around the World has recently been discussed in the literature (Gómez et al. 2008;Gómez and Arias 2010;Gómez and Goy 2011;García Joral et al. 2011, 2018Fraguas et al. 2012;Ruebsam et al. 2020). ...
Quantitative analysis performed on latest Pliensbachian-early Toarcian calcareous nannofossil assemblages from the Camino section (Basque Cantabrian Basin), allowed to decipher their response to the environmental changes recorded during this time interval, characterized by an extinction event. The results were introduced within a principal component analysis and compared to the stable isotope and total organic carbon curves. During the latest Pliensbachian, the Mirabile and the lowermost part of the Semicelatum Ammonite Subzones, Schizosphaerella , Bussonius prinsii , Biscutum finchii , Calcivascularis jansae and Similiscutum avitum , taxa that probably thrived in rather cold waters, dominated the calcareous nannofossil assemblages. Coinciding with the warmer and probably wetter conditions, which probably led to an increase in surface water fertility, recorded slightly below the extinction boundary, the mesotrophic taxa B. novum , L. hauffii and Calyculus spp. were dominant. Nevertheless, T. patulus and C. jansae , which became extinct just below the extinction boundary, show preferences for oligotrophic conditions. Salinities similar to those of modern oceans have been inferred around the extinction boundary, considering the coupling between the abundances of Calyculus spp. and the species richness, and the absence of black-shales. After the extinction boundary, nannofossil assemblages were dominated by the deep-dweller C. crassus and the shallow-dweller Lotharingius species, interpreted as opportunistic taxa. This work confirms that calcareous nannofossils are a useful tool for palaeoceanographic and palaeoenvironmental reconstructions, especially in terms of climatic changes.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5481527
... During the Early Jurassic, the supercontinent Pangaea broke up, leading to the formation of an epicontinental seaway in NW Europe as part of the Western Tethys Ocean (Bassoullet & Baudin 1994;Hermoso et al. 2009). During that time the investigated site, the Mochras Farm (Llanbedr) borehole, was located between palaeolatitudes of 30-45° N (Coward et al. 2003;Korte et al. 2015). ...
The Mochras core, UK, an historic reference site for the study of Early Jurassic calcareous nannofossils, has been reinvestigated here across the Hettangian to Lower Pliensbachian. Both relative and absolute abundances of 24 calcareous nannofossil species reveal patterns and occurrences that define not less than 37 defined biohorizons. However, due to the low abundance of many of those species towards the base and top of their stratigraphical range, we observe large inconsistencies in first and last occurrences of these species. Instead, we consider records of first consistent and last consistent occurrences (FCOs and LCOs), as well as base and top of acme events, as more reliable biohorizons. Accordingly, we assess the reliability of our biohorizons and show that, in addition to previously established zonal and sub-zonal biomarkers, nine non-zonal biohorizons prove to be robust. Based on that, an updated biozonations scheme with additional subzones for NJB has been proposed and tested against other localities.
... The Toarcian Oceanic Anoxic Event (T-OAE,~183 Ma) was the first in a series of Mesozoic Oceanic Anoxic Events (Jenkyns, 2010). The deposition of black shales in a geographically widespread area, a prominent negative shift in carbon isotope composition in carbonates and organic matter, as well as a reduction in biodiversity coincide with the environmental changes during this time (Little and Benton, 1995;Gahr, 2005;Kemp et al., 2005;Hesselbo et al., 2007;Hermoso et al., 2009;Danise et al., 2013;Hermoso and Pellenard, 2014;Kemp and Izumi, 2014;Montero-Serrano et al., 2015;Izumi et al., 2018;Xu et al., 2018;Caswell and Dawn, 2019;Remírez and Algeo, 2020). Given the absence of detailed information on land plant and marine phytoplankton responses to environmental disturbances at the T-OAE, Galasso et al. (2022) published a comprehensive dataset on the marine and terrestrial palynology of Dormettingen, southwest Germany. ...
The environmental perturbations of the Toarcian Oceanic Anoxic Event (T-OAE, ~ 183 Ma) were associated with the widespread deposition of black shales, such as the Posidonia Shale in South Germany. We recently published a detailed palynological analysis of the Posidonia Shale deposit at Dormettingen, investigating both pollen, spores, and marine plankton. In addition to the comprehensive palynological analysis, we point out the occurrence of possible teratological features (i.e. unseparated spore tetrads and dark-coloured sporomorphs) before the T-OAE. These features have been tentatively linked to environmental changes, such as increased UV-B irradiation or heavy metal pollution associated with the large-scale volcanism of the Karoo Ferrar province (Galasso et al., 2022). Our interpretation has drawn criticism (Riding et al., 2022). Here we present a more substantiated discussion of why spore tetrads and darkened spores might not be sedimentological artefacts but indicators of aggravating environmental conditions prior to the T-OAE. The combined occurrence of several teratomorphic features (i.e. spore tetrads, dark sporomorph walls, and even polyploidy) during the earliest Toarcian (Tenuicostatum Zone) is interpreted as a signal of environmental stress in land plants, although for each individual morphological variation a much easier explanation exists.
... Moreover, a significant decline in schizosphaerellid abundance outlines the "Schizosphaerella crisis" starting just prior to the T-OAE black shale interval and representing the temporary breakdown of this rock-forming taxon (Claps et al., 1995;Erba, 2004;Tremolada et al., 2005;Casellato and Erba, 2015). Similar and coeval decreases in abundance of Schizosphaerella punctulataand in general in nannofossil total abundancesare reported from various sections from western Tethys and the Boreal realm (Bucefalo Palliani et al., 2002;Mattioli et al., 2008;Mattioli et al., 2009;Fraguas et al., 2012Fraguas et al., , 2021Hermoso et al., 2012;Clémence et al., 2015;Erba et al., Erba et al., 2019bErba et al., , 2022 (c) Paleogeographical map of western Tethys in the Toarcian (modified after Ruebsam et al., 2018) and paleo-location of the Sancerre-Couy borehole (Hermoso et al., 2009), Peniche (Hesselbo et al., 2007) and Sogno (this study). 2019a,b; Menini et al., 2021; suggesting a major change in Schizosphaerella biocalcification at supra-regional scale. ...
Abundance and size variations of nannofossil Schizosphaerella punctulata were quantified in the uppermost Pliensbachian–Lower Toarcian succession recovered with the Sogno Core (Lombardy Basin, Northern Italy). High-resolution nannofossil biostratigraphy and C-isotopic chemostratigraphy identified the Jenkyns Event within the Toarcian oceanic anoxic event (T-OAE) interval. Absolute abundances and morphometric changes of “small S. punctulata” (< 7 μm), S. punctulata (7–10 μm; 10–14 μm; > 14 μm) and “encrusted S. punctulata” (specimens with a fringing crust) show large fluctuations across the negative δ¹³C Jenkyns Event. The Schizosphaerella crisis is further characterized by a decrease in average valve size in the early–middle Jenkyns Event. The abundance fall was caused by the failure of S. punctulata specimens >7 μm and “encrusted S. punctulata” that along with the increased relative abundance of small specimens, produced the reduction of average dimensions also documented in the Lusitanian and Paris Basins, although with a diachronous inception. The average valve size from the Lombardy Basin is ~2 μm smaller than in these other basins. Hyperthermal conditions associated with excess CO2 and ocean acidification possibly forced the drastic reduction of S. punctulata abundance/size. In the pelagic succession of the Sogno Core there is a strong positive correlation between the S. punctulata (> 7 μm) absolute abundance/size and the CaCO3 content, with a negligible contribution by small specimens (< 7 μm). Encrusted specimens testify selective neomorphic processes: the diagenetic crust seems diagnostic to separate S. punctulata from Stephanodiscus astraea.
... For instance, we observe the coldest D 47 temperatures in the Late Pliensbachian where a cold climate has also been inferred from oxygen isotope measurements of carbonate fossils (e.g., Dera et al., 2011;Korte et al., 2015;Ruebsam et al., 2019b), and from the occurrence of glendonites and glacial deposits (Price, 1999;Rogov and Zakharov, 2010;Suan et al., 2011). Similarly, warming across the Early Toarcian is well supported by oxygen isotope measurements (e.g., Bailey et al., 2003;Dera et al., 2011;Ullmann et al., 2014;Korte et al., 2015), large perturbations of the global carbon cycle (e.g., Hesselbo et al., 2000Hesselbo et al., , 2007Hesselbo et al., , 2009Them et al., 2017;Fantasia et al., 2018a), higher abundances of clay minerals typical of warm and humid conditions (e.g., Dera et al., 2009;Fantasia et al., 2018c), and the latitudinal migrations of marine faunas (e.g., van de Schootbrugge et al., 2005;Zakharov et al., 2006). These cooling and warming patterns are also reflected in the reordered belemnites, which show cooling towards the end of the Pliensbachian and warming in response to both the Pl-To boundary event and the T-OAE (Figs. 3 and 7). ...
The magnitude of temperature changes in the Early Jurassic are not well known. Clumped isotope measurements can potentially be used to provide better constrains, but unfortunately many of the well-studied sedimentary successions that preserve Lower Jurassic fossils experienced burial temperatures above the limits of preservation of Δ47, which for geological timescales is thought to be between 80–120 °C. Samples from these basins are expected to be partially reordered and yield apparent clumped isotope temperatures that are warmer than original values. Here, we explore whether useful paleoclimate information can be recovered from these samples. We test the hypothesis that relative temperature differences are preserved in partially reordered samples when they experience a common burial history. This was done with the use of reordering models and Δ47 measurements of early Jurassic belemnites from the Aubach section of the SW German Basin, a basin that has a relatively well constrained burial history with maximum burial temperatures above 90 °C. We find that even though partial reordering progressively erases the Δ47 difference between samples, the majority (>50%) of the signal is preserved when samples are buried at temperatures as high of 150 °C for up to 200 Ma. Moreover, the models demonstrate that – regardless of burial conditions – partially reordered samples always preserve minimum records of temperature change across climate events. These inferences are supported by the belemnite Δ47 data that show partially reordered compositions and warming/cooling patterns across the Early Jurassic that closely mimic observations from independent proxies. Model observations are used to interpret a 13 ± 4 °C (95% ci) temperature increase that is observed in the belemnite data across the Early Toarcian. The large magnitude of the temperature excursion is explained as a combination of warming and a change in belemnite habitat before and after the Toarcian Ocean Anoxic Event. Our results demonstrate the usefulness of partially reordered samples and further open the use of this proxy in deep time settings.
... Given that chronostratigraphic correlation is challenging, the pCO 2 falling-rising cycle might correspond to the quickly shifting cycle of stable isotopes during the T-OAE (Fig. 8a, c-d). In detail, the rapid falling-rising of pCO 2 is consistent with (1) the quick negative-positive carbon isotope excursion of marine carbonates from Italy (Jenkyns and Clayton, 1986;Sabatino et al., 2009), England and Wales (Jenkyns and Clayton, 1997), northern Spain (van de Schootbrugge et al., 2005), the Lusitanian Basin of Portugal (Hesselbo et al., 2007), the Paris Basin (Hermoso et al., 2009), and Morocco (Bodin et al., 2016); (2) the invertebrate calcareous shells from the Cleveland Basin in the UK (Korte and Hesselbo, 2011) and northwestern Algeria (Baghli et al., 2020); (3) the marine organic matter from Morocco (Bodin et al., 2016), Yorkshire (Cohen et al., 2004;Kemp et al., 2005) and the Cardigan Bay Basin in the UK (Xu et al., 2018), northern Germany (van de Schootbrugge et al., 2013), Alberta and British Columbia in Canada (Them II et al., 2017), northern Tibet (Fu et al., 2016), and Japan (Izumi et al., 2018); (4) the terrestrial organic matter from the Sichuan Basin in China (Xu et al., 2017); and (5) the quick oxygen isotope negative-positive shifting (seawater warming) of bra-chiopods (Suan et al., 2008) and fossilized wood (Hesselbo et al., 2007) from the Lusitanian Basin in Portugal. ...
Climatic oscillations had been developed through the (Early) Jurassic from marine sedimentary archives, but remain unclear from terrestrial records. This work presents investigation of climate-sensitive sediments and carbon and oxygen isotope analyses of lacustrine and pedogenic carbonates for the Early Jurassic Ziliujing Formation from the grand Sichuan paleobasin (GSB), Southwest China. Sedimentary and stable isotope proxies manifest that an overall secular (semi-) arid climate dominated the GSB during the Early Jurassic except for the Hettangian. This climate pattern is similar to the arid climate in the Colorado Plateau region, western North America, but distinct from the relatively warm-humid climate in North China and high latitude in Southern Hemisphere. The estimated atmospheric CO2 concentration (pCO2) from carbon isotopes of pedogenic carbonates shows a range of 980-2610 ppmV (~ 3.5-10 times the pre-industrial value) with a mean of 1660 ppmV. Three phases of pCO2 (the Sinemurian 1500-2000 ppmV, the Pliensbachian 1000-1500 ppmV, and the early Toarcian 1094-2610 ppmV) and two events of rapid falling pCO2 by ~1000-1300 ppmV are observed, illustrating the pCO2 perturbation in the Early Jurassic. The perturbation of pCO2 is compatible with seawater temperature and carbon cycle from the coeval marine sediments, suggesting a positive feedback of climate to pCO2 through the Early Jurassic.
... The increased anoxia during the T-OAE has been interpreted to contribute to the demise of many belemnite taxa, while Acrocoelites might have survived and radiated in its immediate aftermath [49]. The T-OAE coincides with an abrupt negative carbon isotopic excursion (CIE) which disrupts an overarching positive excursion [36,[50][51][52], reflecting the enhanced burial of organic carbon and its preservation at the sea bottom. Although deoxygenation has been interpreted to increase since the Pli-Toa boundary, the negative CIE and widespread black shale deposition are still interpreted to reflect the peak of anoxia during the T-OAE [53]. ...
Body-size reduction is considered an important response to current climate warming and has been observed during past biotic crises, including the Pliensbachian-Toarcian crisis, a second-order mass extinction. However, in fossil cephalopod studies, the mechanisms and their potential link with climate are rarely investigated and palaeobiological scales of organization are not usually differentiated. Here, we hypothesize that belemnites reduce their adult size across the Pliensbachian-Toarcian boundary warming event. Belemnite body-size dynamics across the Pliensbachian-Toarcian boundary in the Peniche section (Lusitanian Basin, Portugal) were analysed based on the newly collected field data. We disentangle the mechanisms and the environmental drivers of the size fluctuations observed from the individual to the assemblage scale. Despite the lack of a major taxonomic turnover, a 40% decrease in rostrum volume is observed across the Pliensbachian-Toarcian boundary, before the Toarcian Oceanic Anoxic Event where belemnites go locally extinct. The pattern is mainly driven by a reduction in adult size of the two dominant species, Pseudohastites longiformis and Passaloteuthis bisulcata. Belemnite-size distribution is best correlated with fluctuations in a palaeotemperature proxy (stable oxygen isotopes); however, potential indirect effects of volcanism and carbon cycle perturbations may also play a role. This highlights the complex interplay between environmental stressors (warming, deoxygenation, nutrient input) and biotic variables (productivity, competition, migration) associated with these hyperthermal events in driving belemnite body-size.
Extreme global warming and environmental changes associated with the Toarcian (Lower Jurassic) Oceanic Anoxic Event (T-OAE, ~ 183 Mya) profoundly impacted marine organisms and terrestrial plants. Despite the exceptionally elevated abundances of fossil insects from strata of this age, only assemblages from Germany and Luxembourg have been studied in detail. Here, we focus on the insect assemblage found in strata recording the T-OAE at Alderton Hill, Gloucestershire, UK, where <15% of specimens have previously been described. We located all known fossil insects (n = 370) from Alderton Hill, and used these to create the first comprehensive taxonomic and taphonomic analysis of the entire assemblage. We show that a diverse palaeoentomofaunal assemblage is preserved, comprising 12 orders, 21 families, 23 genera and 21 species. Fossil disarticulation is consistent with insect decay studies. The number of orders is comparable with present-day assemblages from similar latitudes (30°–40°N), including the Azores, and suggests that the palaeoentomofauna reflects a life assemblage. At Alderton, Hemiptera, Coleoptera and Orthoptera are the commonest (56.1%) orders. The high abundance of Hemiptera (22.1%) and Orthoptera (13.4%) indicates well-vegetated islands, while floral changes related to the T-OAE may be responsible for hemipteran diversification. Predatory insects are relatively abundant ( ~ 10% of the total assemblage) and we hypothesise that the co-occurrence of fish and insects within the T-OAE represents a jubilee-like event. The marginally higher proportion of sclerotised taxa compared to present-day insect assemblages possibly indicates adaptation to environmental conditions or taphonomic bias. The coeval palaeoentomofauna from Strawberry Bank, Somerset is less diverse (9 orders, 12 families, 6 genera, 3 species) and is taphonomically biased. The Alderton Hill palaeoentomofauna is interpreted to be the best-preserved and most representative insect assemblage from Toarcian strata in the UK. This study provides an essential first step towards understanding the likely influence of the T-OAE on insects.
Globally, considerable coverage has been devoted to marine environment affected by the Jenkyns Event, but little attention has been given to lacustrine system. Here, siliciclastic‑carbonate samples of the Da'anzhai Member (J1dn) were examined for petrology and geochemistry in order to evaluate lithofacies characteristics, depositional processes and environmental changes of the palaeo-Sichuan lacustrine basin contemporaneous with the Jenkyns Event. Unique sedimentological and petrologic features were used to identify five principal facies associations (FA) and depositional environments, including the distal delta front, accretionary bioclastic shoals, proximal bioclastic shoal ramp, distal bioclastic shoal ramp and (semi-)deep lacustrine environment. The vertical transitions of FA reveal that J1dn forms a complete lacustrine transgressive-regressive sedimentary system during the Toarcian, which can be further divided into 3 units in combination with standard gamma logging. The lower unit is characterised by frequent intermixing of multiple FA interpreted as frequent lake-level fluctuations. The middle unit is characterised by the deposition of clay-rich mudstone and the demise of bioclastic shoals interpreted as intensive lacustrine transgression that were at times affected by storm-induced waves or oscillatory currents coinciding with the initiation of the Jenkyns Event. The upper unit is characterised by the thicker and higher proportion of biogenic carbonate-rich lithofacies interpreted as the lake regression, contemporaneous with the end of the Jenkyns Event. Overall, clay-rich lithofacies from suspension settling, turbidity current or slumping triggered by storm events were deposited during a high relative lacustrine-level, accompanied by organic carbon-rich deposition and preservation. In contrast, carbonate-rich lithofacies were mainly controlled by biogenic carbonate sedimentation, associated with a low relative lacustrine-level unconducive to organic matter accumulation. Finally, the proposed depositional evolution model at the lake-level highstand stage and lowstand stage of the palaeo-Sichuan lacustrine basin provides new insights and understanding on the changing palaeolimnological history of the Toarcian lacustrine system.
The Toarcian Oceanic Anoxic Event (T-OAE) is characterized by significant environmental perturbation and climatic instabilities due to a substantial release of greenhouse gases. Several investigations of the T-OAE were conducted based on reliable and well-developed integration of multi-proxy approaches, revealing the global distribution of this event in the Panthalassic, Boreal, and Western Tethyan regions. However, a few marine shelf T-OAE records are reported from the Eastern Tethys. This study presents high-resolution organic and inorganic and isotope geochemistry, mineralogical, and sedimentological analyses from the marine Suobucha section of the Qiangtang Basin to assess the role of continental weathering and paleoceanographic conditions, including paleoredox and paleoproductivity conditions, and factors that govern organic carbon accumulation during the T-OAE in the Eastern Tethys. The carbon isotope profile of Suobucha displays a long-term negative excursion characterizing the Jenkyns Event, which is recognized in geographically widespread sedimentary archives indicating its global nature. Continental weathering was intensified at the onset of the Jenkyns Event. Increased values of fluvial detrital proxies (i.e., Si/Al and Ti/Al) and the occurrence of coarser-grained sediments (i.e., silty mudstones) at Suobucha indicated enhanced terrigenous input at the onset of the Jenkyns Event, which was driven by accelerated continental weathering at this time. The lower Toarcian interval in the marine shelf environment of the Qiangtang Basin is characterized by fully oxidizing conditions intermittent with minor phases of dysoxic setting, especially during the Jenkyns Event interval. Given that a stratified water column with anoxic bottom water occurred in the proximal and more restricted lagoonal area (i.e., the Bilong Co area), redox conditions were spatially variable and controlled by basin hydrography and water depth during the Jenkyns Event in the Qiangtang Basin. Oxygen-enriched conditions and low marine bioproductivity were responsible for the low organic carbon burial within the Suobucha section.
We present high‐resolution (every 2 cm) magnetic susceptibility (MS) data from the Sancerre‐Couy drill‐core (Paris Basin), spanning the latest Sinemurian to the earliest Aalenian (early Jurassic). This record allows to build a 20‐million‐year cyclostratigraphic interval using the stable 405 kyr (g2–g5) orbital eccentricity cycle and to focus on long‐period cyclicities and their potential implications for the chaotic diffusion in the inner Solar System and sea level changes. Time series analysis indicates evidence of two long‐period cyclicities of 1.6 and 3.4 Myr. These Early Jurassic cyclicities likely correspond to the Cenozoic orbital cyclicities of 2.4 Myr (g4–g3) and 4.7 Myr eccentricity terms. Shortening of eccentricity terms during the Early Jurassic is potentially related to the chaotic orbital motion of the inner planets expressed in the resonant argument θ = 2(g4 – g3) − (s4 – s3). The 1.6 Myr (g4–g3) cycle matches the third‐order eustatic sequences, while the 3.4 Myr cycle has no equivalent in the reference eustatic chart. To these cycles of several million years are superimposed a cyclicity of ~7.5 Myr, which may correspond to the eccentricity term of 9.5 Myr, previously detected in the Cenozoic. Such cyclicity matches the global, “shorter” second order sea level sequences, and is strongly documented in the sedimentological and mineralogical proxy data, hence supporting the potential key role of orbitally paced climate and sea‐level changes at this timescale.
The Pliensbachian/Toarcian boundary (Pl/To) event precedes by ca. 1 Myr the onset of the Toarcian Oceanic Anoxic Event. It corresponds to a second order mass extinction associated with an outstanding collapse of shallow marine ecosystems at global scale. Yet, our knowledge about its exact driver(s) and unfolding is relatively ambiguous due to the numerous hiatuses present in the sedimentary record during this critical time interval. In this study, an integrated carbon isotope chemostratigraphy and sequence stratigraphy approach is applied to two case studies (the upper Pliensbachian in South-East France and the Pliensbachian–Toarcian transition in Morocco) to demonstrate how the major changes in sea-level and sedimentation supply accompanying the Pl/To event led to the formation of ubiquitous, often cryptic hiatal surfaces in the sedimentary record. Hence, as a consequence of strongly progradational stacking pattern during the latest Pliensbachian related to a global sea-level lowstand associated with cold greenhouse climate, proximal settings were characterized by bypass and/or erosion, inducing an incomplete record of the Spinatum chronozone in localities situated in the outer part of sedimentary basins. In the earliest Toarcian, the collapse of the neritic carbonate factory led to a halt of carbonate mud export into the basin, resulting in sediment starvation in most basins characterized by a carbonate-dominated sedimentation regime before the environmental perturbation. Only localities where vigorous siliciclastic sediment supply took over are likely to have a more complete sedimentary record of the immediate aftermath of the carbonate production collapse. This combination of causes explains the ubiquitous incompleteness of the record of the Pliensbachian/Toarcian transition in numerous European localities where the bulk of our current understanding about the Pl/To event derives from. A comparison between the two known most expanded and complete records of the Pliensbachian–Toarcian transition of the Llanbedr (Mochras Farm) core in Wales and Bou Oumardoul n'Imazighn section in Morocco shows that the onset of the environmental perturbations is associated with a positive carbon isotope excursion spanning the Pliensbachian/Toarcian boundary. This is followed by a negative carbon isotope excursion during the earliest Toarcian that coincides with the global collapse of neritic carbonate factory and an ample sea-level fall.
The early Toarcian, as registered in a variety of sedimentary archives, was characterized by an abrupt negative carbon-isotope excursion (CIE) typically superimposed on a long-term positive trend, and was accompanied by significant climatic and environmental changes. However, the changes in continental weathering influx and oceanic deoxygenation in shallow waters and their possible role in causing carbonate-platform crises in low latitudes remains poorly constrained. Here, we present carbonate content and carbonate-hosted elements for the Pliensbachian–Toarcian transitional interval from the Kioto Carbonate Platform (KCP) in the Tibetan Himalaya. The most water-insoluble elements (e.g. Ti, Sc, Th and total rare earth elements) show an obvious increase starting at the Pliensbachian–Toarcian boundary, followed by a weak increase or relatively high-level values during the negative phase of the T-OAE CIE, suggesting that the enhanced terrigenous input can be linked to rapid global warming during this time interval. The Mn, Ce and Ce anomaly start to increase immediately following the rise in abundance of the water-insoluble elements, followed in turn by enhanced values over the interval of the negative CIE. These observations indicate that the deoxygenation process and development of manganous (suboxic) conditions occurred in shallow water during this time interval and were likely linked to enhanced continental weathering and nutrient input, favoring both primary productivity and oxygen consumption. Stratigraphically higher, the water-insoluble elements show a gradual decreasing trend parallel with elevated values of redox proxies during the recovery phase of the CIE, suggesting decline in continental weathering intensity and a corresponding second deoxygenation in shallow waters. In this instance, deoxygenation might have been caused by a slackening of ocean circulation and/or enhanced recycling of bioessential nutrients. The coupled relationship between biotic changes, carbonate content and geochemical data suggest that: (1) the onset of enhanced terrigenous influx and deoxygenation in shallow waters likely led to slight deterioration to the KCP around Pliensbachian–Toarcian boundary time, and (2) the drastically enhanced terrigenous flux and deoxygenation likely played a pivotal role in the more severe crisis for benthic carbonate producers during the negative phase of the CIE.
The Triassic–Jurassic transition, which is here broadly defined as extending from the Late Triassic through the Early Jurassic (~237 Ma to 174 Ma), was an important interval in Earth history. The end-Triassic mass extinction (ETME), at ~201 Ma, ranks among the ‘Big Five’ Phanerozoic mass extinctions. It largely completed the shift from the ‘Paleozoic Evolutionary Fauna’ to the ‘Modern Evolutionary Fauna’ that had been initiated by the end-Permian mass extinction, and may have contributed to the ‘Mesozoic Marine Revolution’ and rise of dinosaurs to dominance in terrestrial environments. In addition, the Triassic–Jurassic transition encompasses a second-order mass extinction during the early Toarcian oceanic anoxic event (T-OAE), at ~181 Ma. The ETME was triggered by Central Atlantic Magmatic Province (CAMP) magmatism, and the T-OAE by Karoo-Ferrar Large Igneous Province (KFLIP) magmatism, both associated with the stepwise disintegration of the Pangean supercontinent. These events led to major changes in continental and marine habitats, including climatic warming, ocean acidification, and widespread watermass anoxia, that produced a cascade of lethal environmental stresses. This article undertakes a review of the ETME and T-OAE mass extinctions, the large igneous province eruptions that triggered those biotic events, and the web of environmental changes that linked them together.
The early Toarcian oceanic anoxic event (T-OAE, ~183 Ma), also known as the Jenkyns Event, was one of the most important hyperthermal events of the Phanerozoic, caused by the large-scale release of carbon from massive volcanism and/or surficial carbon reservoirs. Organic-rich facies record the event in basins worldwide, but the precise controls on organic carbon burial, and the relative importance of this burial as a mechanism of sequestering excess carbon from the Toarcian atmosphere, are uncertain. In this study, we have compiled total organic carbon (TOC) data from 67 lower Toarcian sections to reconstruct the pattern of organic enrichment through the T-OAE. Shallow marine sites dominate the compilation, and rates of organic carbon burial during the T-OAE were low relative to shallow water margins at the present day. Redox was a major control on organic enrichment and burial rates, but the very high TOC (>5%) observed in hydrographically restricted anoxic-euxinic basins in northern Europe and elsewhere were largely a consequence of low sedimentation rates. Globally, organic enrichment and redox conditions were highly variable, but a majority of sites show an increase in TOC at the T-OAE relative to pre-event values. TOC increases were generally highest where deoxygenation was most severe. In anoxic-euxinic marine basins in Europe organic carbon burial rates may have increased ~500% on average during the T-OAE, potentially sequestering an extra ~791 Gt of carbon relative to the same time period immediately prior to the event. Sites outside of Europe and those that remained oxic-suboxic or were dominated by terrestrial organic matter show variable, and often negligible, changes in organic enrichment across the T-OAE. Globally, an extra ~9000 Gt of carbon may have been buried in shallow seas during the T-OAE relative to before the event. Nevertheless, a paucity of data and significant uncertainties mean that the precise amount of excess carbon sequestered during the T-OAE remains uncertain.
Predictions of how marine calcifying organisms will respond to climate change rely heavily on the fossil record of nannoplankton. Declines in calcium carbonate (CaCO3) and nannofossil abundance through several past global warming events have been interpreted as biocalcification crises caused by ocean acidification and related factors. We present a global record of imprint-or "ghost"-nannofossils that contradicts this view, revealing exquisitely preserved nannoplankton throughout an inferred Jurassic biocalcification crisis. Imprints from two further Cretaceous warming events confirm that the fossil records of these intervals have been strongly distorted by CaCO3 dissolution. Although the rapidity of present-day climate change exceeds the temporal resolution of most fossil records, complicating direct comparison with past warming events, our findings demonstrate that nannoplankton were more resilient to past events than traditional fossil evidence suggests.
Large-scale release of isotopically light carbon is responsible for the carbon isotope excursion (CIE) of the Toarcian Oceanic Anoxic Event during the Lower Jurassic. Proposed sources include methane hydrate dissociation, volcanogenic outgassing of carbon dioxide and/or thermogenic methane release from the Karoo-Ferrar magmatic province (southern Africa). Distinct small-scale shifts superimposed on the long-term CIE have been interpreted as rapid methane pulses linked to astronomically forced climate changes. In the Peniche reference section (Portugal), these small-scale shifts correspond to distinct brownish marly layers featuring markedly high mercury (Hg) and magnetic mineral concentration. Total organic carbon and Hg increase are uncorrelated, which suggests input of Hg into the atmosphere, possibly released after the intrusion of the Karoo-Ferrar sills into organic-rich sediments. Enhanced magnetic properties are associated with the presence of martite, washed-in oxidized magnetite, inferred to be due to increased aridity on the continental hinterland. This study provides strong evidence for a direct link between the Karoo-Ferrar magmatism, the carbon-isotope shifts and the resulting environmental changes.
Calcium carbonate (CaCO3) biomineralizing organisms have played major roles in the history of life and the global carbon cycle during the past 541 Ma. Both marine diversification and mass extinctions reflect physiological responses to environmental changes through time. An integrated understanding of carbonate biomineralization is necessary to illuminate this evolutionary record and to understand how modern organisms will respond to 21st century global change. Biomineralization evolved independently but convergently across phyla, suggesting a unity of mechanism that transcends biological differences. In this review, we combine CaCO3 skeleton formation mechanisms with constraints from evolutionary history, omics, and a meta-analysis of isotopic data to develop a plausible model for CaCO3 biomineralization applicable to all phyla. The model provides a framework for understanding the environmental sensitivity of marine calcifiers, past mass extinctions, and resilience in 21st century acidifying oceans. Thus, it frames questions about the past, present, and future of CaCO3 biomineralizing organisms.
Oceanic anoxic events (OAEs) represent changes in global carbon cycle as well as biogeochemical cycles and are robust recorders of major changes brought in the ocean-atmosphere system of the Earth. In the present study, a comprehensive compilation of well-documented OAEs in the Earth’s geological history indicates that compared to the plethora of OAE studies in different parts of the world, the Indian part lacks sufficient such geological studies. Also, it has been observed that despite the variety of causes referred by researchers for the occurrence of OAEs based on various geological proxies, their development tends to cluster in particular periods having unique geological settings under specific climate conditions of the Earth. OAEs usually coincided with Earth’s greenhouse condition and in marine to shallow marine depositional settings, which have been associated with rapid alternating phases of transgression and regression. The Paleocene-Eocene thermal maximum (PETM) at the Paleocene-Eocene boundary comprises the last OAE of the Phanerozoic Eon and the only identified OAE in the Cenozoic. The deposition of Himalayan foreland basin sediments during Paleocene-Eocene time coincides with the India-Eurasia collision and PETM. Characteristic litho sections of Paleocene-Eocene HFB shallow marine sediments represented by the Subathu Formation occur in and around the Jammu region (of the Ramngar sub-basin) and Simla region (of the Subathu sub-basin) of NW Himalaya (India) and have been explored for OAE records. Besides the fact that deposition of these sediments coincides with PETM, there are many other reasons which suggest possible representation of these as records of OAE. For example, these sediments show OAE specific sedimentary and biostratigraphic facies associations which are characteristic of alternating transgressive-regressive successions. The base of these early HFB sediments consists of sideritic ironstone, phosphorite, and black shale association indicating commencement of basinal sedimentation under euxinic, shallow marine conditions. Though these Paleocene-Eocene HFB sediments comprising dominantly black to gray-green shales are regionally extensive, however, these occur in discontinuous patches all along the HFB and also greatly vary in thickness due to the tectonic complexity of the Himalayan region.
The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO 2 ( p CO 2 ) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a ¹² C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric p CO 2 and δ ¹³ C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian p CO 2 and δ ¹³ C proxy data, and that thermogenic carbon (δ ¹³ C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.
The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; ∼183Ma) and the associated negative C-isotope excursion is the massive release of ¹² C favouring greenhouse and continental weathering. The nutrient delivery to shallow-basins supported productivity and, because of O 2 -consumption by organic-matter respiration, anoxia development. However, several works showed that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr borehole (Mochras Farm; Wales, UK) were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep-dwellers ( Crepidolithus crassus ) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE likely because of prolonged thermohaline sea-water stratification.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5541440
The organic-rich upper Lower Jurassic Da'anzhai Member (Ziliujing Formation) of the Sichuan Basin, China is the first stratigraphically well-constrained lacustrine succession associated with the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma). The formation and/or expansion of the Sichuan mega-lake, likely one of the most extensive fresh-water systems to have existed on the planet, is marked by large-scale lacustrine organic productivity and carbon burial during the T-OAE, possibly due to intensified hydrological cycling and nutrient supply. New molecular biomarker and organic petrographical analyses, combined with bulk organic and inorganic geochemical and palynological data, are presented here, providing insight into aquatic productivity, land-plant biodiversity, and terrestrial ecosystem evolution in continental interiors during the T-OAE. We show that lacustrine algal growth during the T-OAE accounted for a significant organic-matter flux to the lakebed in the palaeo-Sichuan mega-lake. Lacustrine water-column stratification during the T-OAE facilitated the formation of dysoxic-anoxic conditions at the lake bottom, favouring organic-matter preservation and carbon sequestration into organic-rich black shales in the Sichuan Basin. We attribute the palaeo-Sichuan mega-lake expansion to enhanced hydrological cycling in a more vigorous monsoonal climate in the hinterland during the T-OAE greenhouse.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5433544
The chemostratigraphic record of the Toarcian Oceanic Anoxic Event (T-OAE) is generally well documented in epicontinental basins; however, the duration of anoxia and recovery remains poorly constrained in the Tethys Ocean owing to limited stratigraphic resolution of oceanic sections with hiatuses. In this paper, we describe a relatively continuous hemipelagic record in the Central Western Carpathians of Slovakia (Skladaná Skala section), which contains the Pliensbachian/Toarcian boundary, the T-OAE, and the subsequent recovery phase. In our section, the Pliensbachian Spinatum Zone is represented by a ~ 7 m thick alternation of spotted, spiculitic marly limestones and limestones of the Allgäu Formation that contain moderately diverse trace fossils including abundant Zoophycos, Lamellaeichnus and Teichichnus. In the uppermost part of the Spinatum Zone, a ~ 0.8‰ decline in δ13Ccarb values marks the carbon isotope excursion (CIE) at the Pliensbachian/Toarcian boundary. This isotope anomaly is overlain by a ~ 3 m-thick interval of spotted marly limestones and marls, which contains a positive CIE that represents the base of the T-OAE. The T-OAE interval comprises a 60 cm-thick pyritic black shale interval that lacks bioturbation and shows a negative CIE (δ13Corg values drop by 4–5‰), coincident with an increase in TOC values (from 0.3% to 2–3%), and a drop in CaCO3 values (from 50 to 80% to 3–7%). The black shale is overlain by ~8 m thick marls with rare intercalations of marly limestones, which show ammonites of the Exaratum Subzone in the lowermost part (~1 m) succeeded by Serpentinum Zone beds. This unit exhibits a second positive CIE (δ13Corg increases by 2.5‰ and δ13Ccarb increases by 1.5‰), significantly more negative δ18O values relative to the pre-T-OAE interval, and a low diversity of trace fossils dominated by Chondrites and extremely thin Zoophycos. Sponge spicules re-appear in the uppermost 6.5 m-thick interval of the section formed by the alternation of spotted marls and limestones, with ammonites of the Falciferum Subzone in the lower part and of the Bifrons Zone in the uppermost parts; however, diversity of trace fossils remains low. Based on these findings, we argue that the 60 cm-thick pyritic black shale that represents the T-OAE is a strongly condensed succession because it includes the onset of the negative CIE and the subsequent rebound, and thereby encompasses the full duration of the anoxic event. Anoxia in basins open to the oceanic Tethys thus coincided not only with reduced carbonate production but also with reduced siliciclastic supply, generating very thin black shale deposits. In contrast to current-swept swells where black shales associated with hardgrounds are thin or missing owing to winnowing, carbonate starvation was induced by a calcification crisis coincident with the T-OAE and siliciclastic starvation was probably induced by rapid sea-level rise. Although bottom waters were not anoxic in the aftermath of T-OAE, bioturbation remained restricted during the Falciferum Subzone in basinal settings, indicating that environmental stress induced by limited ventilation and/or high bottom-water temperatures were persistent. The reduced diversity of trace fossils and the re-appearance of sponge spicules close to the lower/middle Toarcian boundary indicate that the ecological recovery in basins open to the oceanic Tethys was delayed, a pattern similar to the epicontinental seas that experienced anoxia.
The Pliensbachian-Toarcian sedimentary rocks of the Median Subbetic (Betic Cordillera, SE Spain) were deposited in the South Iberian Palaeomargin under hemipelagic conditions. In the Arroyo Mingarrón section (SE Spain) these deposits are composed by a marl and marl and limestone alternation with abundant radiolarians. The application of magnetic susceptibility and gamma ray spectrometry to these deposits allows to interpret environmental changes related to the fluctuations in the continental influx of terrigenous and phytodetritus to a hemipelagic setting in the South Iberian Palaeomargin. The increase of terrigenous input was coincident with the negative carbon isotopic excursion (CIE) at the base of the NJT6 nannofossil Zone that correlates with the Jenkyns Event. The increase of Th and K content is related to enhanced detrital input, especially oxides and silicates, including clay minerals. The gamma ray signal derived from U is comparatively low with respect to K and Th. Nevertheless , the U content shows an increase in the base of NJT6 nannofossil Zone that indicates a relative increase of organic matter content coincident with the Jenkyns Event. Changes in the calcareous nannofossil assemblages at the beginning of the NJT6a nannofossil Subzone and, in particular, the sharp decrease in abundance of the species Mitrolitus jansae during the negative CIE of the Jenkyns Event are interpreted as related to seawater stratification and a probable poor oxygenation making inhospitable the deep photic zone during this event.
Cyclostratigraphical analysis of the foraminiferal assemblages from the Early Toarcian at the Mochras Farm Borehole (Wales) was conducted in order to evaluate the incidence of cyclic palaeoenvironmental changes on the foraminiferal community. Different variables such as type of morphogroup, evolutionary strategy, habitat, particular taxa, diversity and abundance were studied using the Lomb–Scargle periodogram implemented in the computer program SLOMBS. A well‐developed cyclostratigraphical pattern is recognized, with the presence of several cycles (in metres) at 3.4–4/7.2–7.5/10.1–10.6/32.1–33.3/104.2–111.2/128.2/166.7, belonging to the high‐, middle‐, middle‐/low‐, and low‐frequency bands. The incidence and relevance of the cycles is found to be related to particular variables marking the global and local character of the involved processes. Cyclic changes in the organic matter input are found to be the most relevant palaeoenvironmental factor, oxygenation being secondary. A correspondence with specific Milankovitch cycles is, at present, difficult to determine.
The Early Jurassic Toarcian oceanic anoxic event (T-OAE) is characterized by an extinction event, a major sea-level rise, enhanced marine primary productivity, elevated seawater temperatures, widespread deposition of organic-rich sediments, and a negative carbon-isotope excursion (CIE). However, the T-OAE exhibits significant interregional variation in its expression, with strong spatial variation in lithology, total organic carbon (TOC) content, and the magnitude of the negative CIE in both carbonate and organic carbon profiles. It is better developed as a distinct geological event on the Northwest European Shelf (NWES), exhibiting both a large organic CIE (to –5 to –7 ‰) and high TOC content (to ~10 %), than in other regions globally. The reason for the regionally variable expression of the T-OAE has been a matter of debate, with models based on both regional and global factors proposed. We review these models in the context of the global sedimentary record of the T-OAE. The T-OAE records a global carbon-cycle perturbation that has been linked to Karoo-Ferrar Large Igneous Province magmatism, suggesting that volcanic emissions of greenhouse gases were the main cause of contemporaneous paleoenvironmental changes. Increases in seawater temperature have been documented only on the NWES to date, although lithologic and geochemical evidence of enhanced chemical weathering intensities is transregional. Mercury (Hg) enrichments are found mainly in shallow-marine settings, and their provenance in volcanic emissions remains uncertain. The exceptional expression of the T-OAE on the NWES points to regional oceanographic factors in the development of watermass stratification, deepwater anoxia, and enhanced organic matter accumulation. At a global scale, shifts toward more reducing oceanic redox conditions were spatially variable, and the T-OAE may therefore be more widely recognizable by its negative CIE than by paleo-environmental redox changes.
The Early Jurassic Toarcian oceanic anoxic event (T-OAE) was marked by deposition of organic-rich sediments
and a pronounced negative carbon-isotope excursion across the Northwest European Sea (NWES), representing a
signal that was stronger regionally than globally. Various models for the T-OAE have been proposed, some
focusing on general atmospheric-oceanic controls to reflect its global character and others emphasizing local or
regional factors to account for its strong expression in the NWES. In this study, we evaluate watermass dynamics
of the Early Jurassic Cleveland Basin (UK) during the T-OAE using a combination of paleosalinity proxies (B/Ga,
Sr/Ba, and S/TOC). These proxies suggest that brackish conditions existed prior to onset of the T-OAE, with a
shift to nearly freshwater conditions during the T-OAE, reflecting a large increase in terrestrial runoff due to a
stronger hydrological cycle, possibly combined with shallowing of the marginal sill of the Cleveland Basin
through sea-level fall. Following the T-OAE, basinal salinity conditions first returned to brackish and then
gradually shifted to fully marine. This hydrographic history accounts for low Mo concentrations in T-OAE sediments
of the Cleveland Basin, demonstrating control by watermass restriction and local (rather than global)
aqueous Mo drawdown. These findings demonstrate the importance of local hydrographic controls on the sedimentary
expression of the T-OAE. The more general significance of this study is that it highlights the need for
application of paired salinity and redox proxies in order to correctly interpret paleoenvironmental variations in
marginal-marine basins.
During the Toarcian oceanic anoxic event (OAE) considerable environmental changes occurred that were associated with global warming, perturbations to the C-cycle and ocean deoxygenation which resulted in a mass extinction of marine fauna. Recovery of the biota after the event was protracted and has to date undergone limited study. However, understanding the patterns and processes of recovery are critical to anticipating ecosystem responses to the environmental changes predicted for the near future. Results showed that increases in benthic diversity, and the re-establishment of the Toarcian infauna was gradual and followed the changing redox conditions. Pioneering infauna, such as Dacryomya ovum that dominated the seafloor after the event in the Cleveland Basin, Yorkshire, UK, can modify the physico-chemical environment and thus facilitate ecological succession after disturbance. The length of D. ovum increased >8 mm throughout the bifrons Zone and these body-size changes were linked with total organic carbon (TOC) content suggesting a link to primary productivity, although only at intermediate levels of deoxygenation. Major changes in the phytoplankton, and so food supply, seem to have driven changes in bivalve body size, across trophic guilds, both during and after the event in Yorkshire, and on the mid to lower shelf in Spain and France, respectively. Primary productivity collapse seems then to have been a major driver of biotic change throughout the Toarcian event, as it was during the Permian–Triassic, Triassic/Jurassic and Cretaceous/Tertiary mass extinctions. Further investigation of both the palaeontological and geochemical changes that occurred within early successional Toarcian infaunal communities are required to more fully understand the pattern of recovery after the OAE.
The Toarcian oceanic anoxic event (T-OAE, ~183 Ma) marks a geologically brief and severe global warming, associated with a profound perturbation in the global carbon cycle. The carbon cycle perturbation has been documented worldwide in marine and continental sedimentary records with a pronounced negative carbon isotope excursion (CIE) in the long-term δ ¹³ C profile. However, the cyclostratigraphically inferred duration of the CIE, which was mainly derived from the Paris (France) and Lusitanian (Portugal) basins, remains controversial, resulting in two notably different estimates of 300–500 and 900 kyr. Here, we present an early Toarcian cyclostratigraphic record from the High Atlas in Morocco (Talghemt section), based on high-resolution δ ¹³ C and %CaCO 3 data, which capture the Pliensbachian-Toarcian (Pl-To) transition event and the T-OAE, and strongly correlate to previous δ ¹³ C key records. Orbital tuning based on the short and long, stable 405 kyr (g2–g5) eccentricity cycles, provides a duration of ~400 to ~500 kyr for the T-OAE. This duration is very close to that previously inferred from the Sancerre Core in the Paris Basin (300 to 500 kyr), and similar to that recently revised from the Peniche section (Lusitanian Basin, Portugal) (~472 kyr). In addition, the 405 kyr%CaCO 3 timescale at Talghemt calibrates high-frequency δ ¹³ C variations at the Pl-To transition and the initiation part of the T-OAE to the obliquity cycle band, thus concuring with previous studies for obliquity forcing during these time intervals. The 405 kyr calibrated O1 obliquity period (~30 kyr) is shorter than the astronomically predicted one (~35 kyr), hence supporting the hypothesis of shortened obliquity periods during the Early Jurassic, and providing constraints on Earth's tidal dissipation factor during this geologic epoch. Finally, a remarkable phase change between %CaCO 3 and δ ¹³ C orbitally paced cycles is observed for the first time at the T-OAE, suggesting a change in the carbon reservoir in relation with volcanically released greenhouse gases and major carbonate crisis. However, this phase shift is not observed at the Pl-To event implying different causal mechanisms on the carbon cycle perturbation between the Pl-To and T-OAE events.
The Toarcian Oceanic Anoxic Event (T-OAE, ∼183 Myr) was a long-lasting episode of ocean deoxygenation during the Early Jurassic. The event is related to a period of global warming and characterized by major perturbations to the hydrological and carbon cycles with high rates of organic matter burial in shelf seas. Ocean circulation during the Toarcian and its influence on marine biogeochemical cycles are still not fully understood. Here we assess the spatial extent of anoxia in the NW Tethys Ocean during the T-OAE, the relationship with ocean circulation and the impact on organic carbon burial, using new and existing sedimentary records from the European Epicontinental Shelf in combination with general circulation model results. We demonstrate that bottom waters on the southwestern part of the shelf were mainly oxic during the T-OAE, while those in the northeastern basins were mostly anoxic or even sulfidic. Results for two ocean-atmosphere models (Fast Ocean-Atmosphere Model and Massachusetts Institute of Technology general circulation model) suggest the presence of a strong clockwise gyre over the European Epicontinental Shelf, which brought oxygenated equatorial waters from the Tethys Ocean to the southern shelf. The northward limb of the gyre was significantly weakened due to the rough bathymetry of the northern shelf, making this relative small region highly sensitive to local ocean stratification. These sluggish ocean dynamics promoted bottom water anoxia and enhanced burial of organic carbon in the northeastern basins, which accounted for 3–5% of the total carbon extracted from the ocean-atmosphere system as recorded by the positive carbon isotope shift.
A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock‐Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north–south transect, from the Jura through the Alpine Tethys (Sub‐Briançonnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon‐isotope excursion in carbonate (ca 2 to 4‰) and organic matter (ca 4 to 5‰) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (≤11 wt.%) in the Jura, to thin‐bedded marl (≤5 wt.% total organic carbon) in the Sub‐Briançonnais Basin and to hemipelagic reddish marly limestone (total organic carbon <0.05 wt.%) in equivalent levels from the Lombardian Basin. The carbon‐isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub‐Briançonnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely‐bedded laminae and homogeneous mud layers containing rip‐up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian–Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea‐level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings. This article is protected by copyright. All rights reserved.
Significance
Declining oxygen contents in today’s oceans highlight the need to better understand ancient, natural marine deoxygenation and associated extinctions. In the Early Jurassic, the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) is associated with significant perturbations to the Earth system, historically defined by carbon isotopes. We reconstructed global oceanic (de)oxygenation using thallium isotopes from two ocean basins that suggest a stepwise decline of oxygen that initiated before and extended well after the classically defined T-OAE interval. This initial deoxygenation occurs with the start of massive volcanism and marine extinctions, while a later shift corresponds to the traditional T-OAE. This emphasizes the need for more nuanced records of ancient environmental and biogeochemical feedbacks that lead to and maintain widespread marine anoxia.
The Lower Jurassic Toarcian Stage ( c. 183–174 Ma) is marked by one of the largest global exogenic carbon-cycle perturbations of the Phanerozoic, which is associated with the early Toarcian Oceanic Anoxic Event (T-OAE; c. 183 Ma). Climatic and environmental change at the T-OAE is reasonably well constrained in the marine realm, with marine anoxic or euxinic conditions developing locally across both hemispheres, at the same time as the T-OAE negative carbon-isotope excursion. However, high-resolution stratigraphic comparison between different palaeo-ocean basins and with the continental realm can be complicated. Palaeomagnetic reversals can provide a precise and accurate stratigraphic correlation tool between marine and continental sedimentary archives, and even between sedimentary and igneous successions. Here, we present a high-resolution magnetostratigraphic record for the Toarcian Stage in the biostratigraphically complete and expanded Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, Wales. This study provides the first geomagnetic polarity reversal scale that is integrated with high-resolution biostratigraphy and carbon-isotope stratigraphy for the entire Toarcian Stage. This stratigraphic framework also provides a new, precise correlation with the basalt lava sequence of the Karoo–Ferrar Large Igneous Province, linking the Pliensbachian–Toarcian boundary and T-OAE climatic and environmental perturbations directly to this episode of major volcanic activity.
Supplementary material: Details of the palaeomagnetic data and dip direction are available at https://doi.org/10.6084/m9.figshare.c.4052720
Comparison of rates of accumulation or organic carbon in surface marine sediments from
the central North Pacific, the continental margins of Northwest Africa, northwest and southwest America, the Argentine Basin, and the western Baltic Sea with primary production rates suggests that the fraction of primary produced organic carbon preserved in the sediments is universally related to the bulk sedimentation rate. Accordingly, less than 0.0l% of the primary production becomes fossilized in slowly accumulating pelagic sediments [[2 to 6 mm (1000 y) -1] of the Central Pacific, 0.1 to 2%, in moderately rapidly accumulating [(2 to 13 cm (1000 y) -1] hemi-pelagic sediments off northwest Africa, northwest America (Oregon) and southeast America (Argentina) and 11 to 18 % in rapidly accumulating [66 to 140 cm (1000 y) -1] hemi-pelagic sediments off southwest America (Peru) and in the Baltic Sea.
The empirical expression:
0.0030 · R · S 0.30 / ps(1- Θ) = % org-C
implies that the sedimentary organic carbon content (% org-C) doubles with each 10-fold increase in sedimentation rate (S), assuming that other factors remain constant; i.e. primary production (R), porosity (Θ) and sediment density (ps). This expression also predicts the sedimentary organic carbon content from the primary production rate, sedimentation rate, dry density of solids, and their porosity; it may be used to estimate paleo-productivity as well. Applying this relationship to a sediment core from the continental rise off northwest Africa (Spanish Sahara) suggests that productivity there during interglacial oxygen isotope stages 1 and 5 was about the same as today but was higher by a factor of 2 to 3 during glacial stages 2, 3 and 6.
APTICORE at the Cismon Valley (Belluno Basin, Southern Alps) penetrated 131.8 m of pelagic limestones, marlstones and black shales. The cored interval extends from the upper Albian to the lower upper Hauterivian. Freshly cored material and logs from the Cismon drill site provide a very informative record. Detailed, multidisciplinary studies show high and low frequency changes in lithologic, paleontologic, paleomagnetic, and geochemical records. Moderate to good preservation of calcareous and siliceous plankton and palynomorphs allowed the identification of several calcareous nannofossil, planktonic foraminiferal, radiolarian and dinoflagellate events. Magnetostratigraphy is available for the entire interval and Chrons CM0 through CM9 were detected. Stable C and O, and Sr isotopes were analysed and chemostratigraphy of the Cismon core comprises both δ13Ccarb curve and the 87Sr/86Sr record. Integrated stratigraphy of the Cismon APTICORE resulted in the first coherent scheme derived from direct calibration of bio-, magneto-, and chemo-stratigraphy for the Upper Hauterivian-Aptian interval. In the upper portion of core, an unconformity removed part of the upper Aptian, the lower Albian and most of the middle Albian. The Upper Hauterivian is characterized by folding and some slumps resulting in high sedimentation rates. The uppermost Hauterivian consists of red pseudonodular limestone with low sedimentation rate. We propose the Cismon core as a "reference section" for the Barremian-Aptian at low latitudes, containing both a complete carbonate and organic carbon record through the critical black shale intervals.
Analogies concern not only types and gross vertical evolution of facies, but also composition and textures of the fine sediment and the pattern of diagenetic alteration. The occurrence of the nanno-organism Schizosphaerella Deflandre and Dangeard (sometimes as a conspicuous portion of the fine-grained carbonate fraction) is of particular interest. Specifically, Schizosphaerella was often the only component of the initial fine-grained fraction of a sediment that was able to resist diagenetic obliteration. Intraskeletal cementation is usually followd by the growth of a radially structured crust of bladed to fibrous calcite around the valves. Suggests that the crusts formed and (inferentially) mineralogic stabilization occurred at a relatively early time in the diagenetic history in the shallow burial realm. An enhanced rate of lithification at relatively shallow burial depths and thus the change for neomorphism to significantly influence the textural evolution of the buried sediment may be related to a lower Mg/Ca concentration ratio in the oceanic system and, hence, in marine pore waters in pre-Late Jurassic times.-from Authors
Detailed paleogeographic studies in the Southern Alps suggest that an oxygen-minimum model is appropriate for interpreting conditions on Tethyan continental margins. Advection of manganese in this low-oxygen layer may explain the local occurrence of carbonate ores of this element. Carbon-sulfur and iron-sulfur ratios, although showing considerable scatter, suggest that bottom waters at this time were locally euxinic, containing free hydrogen sulfide. This Oceanic Anoxic Event was preceded by significant faunal turnover of ammonites in Tethys and accompanied by widespread extinction of benthos in northern Europe in response to the lateral spread of anoxic bottom waters during transgression. Similar changes may be recognized in other parts of the world. Other anoxic events may have taken place during the Jurassic, but documentation is as yet meagre. Furthermore, models for such phenomena remain largely speculative, although upwelling and increased planktonic productivity, commencing in pre-Toarcian time, are favored for the falciferum-Zone event documented here.-from Author
Comparison of oxygen and carbon isotope and manganese evolution curves in bulk carbonate from the historical Bedoulian stratotype (Cassis-La Bedoule area, Provence, France) reveals an important geochemical event (negative δ¹³C and high Mn content) located within the D. deshayesi ammonite Zone and at the base of the R. hambrowi ammonite Subzone. This worldwide event, which can be observed in environments ranging from the fluvial to the pelagic realm (Selli/Goguel level), seems to be related to methane hydrate destabilization. Scenarios for manganese, carbon and oxygen evolutions are proposed for early Bedoulian oxic conditions and for dysoxic/anoxic conditions related to methane hydrate destabilization at the early/late Bedoulian transition. The impacts of this global event on the biosphere (nannoconid crisis) and its stratigraphic implications are considered. Comparison of geochemical and biostratigraphical data from the Cassis-La Bedoule stratotype with that of the Cismon-Apticore reference borehole shows that the La Bedoule sequence records geochemical evolution during the Goguel/Selli Event in more detail than that of any other previously published section.
Belemnite rostra from the Grey Shale and Jet Rock Members of the Lower Toarcian Whitby Mudstone Formation (England) record fluctuations in oxygen and carbon stableisotope compositions (δ 18O and δ 13C, respectively) of the ambient waters. Salinity modelling based on the rostral calcite δ 13C values from the most organic-rich intervals of the Jet Rock (Beds 34 and 35) indicates that surface waters were up to ∼15‰ less saline compared to contemporaneous seawater. In addition to sea-surface salinity fluctuations, δ 13C of the rostra probably records (a) various rates of primary production and burial of organic carbon, as well as (b) mixing of iso-topically light DIC into the lower salinity and higher temperature surface water, or (c) migration of belemnites into sub-pycnoclinal waters with isotopically light DIC. A positive correlation between modelled Δ t values (i.e., the density contrast between bottom and surface waters based on the belemnite temperature and salinity proxy data) and hydrogen index values of the sedimentary organic matter, indicates that water-column stratification was a significant factor in preserving the latter. Long periods of water-column stratification probably caused stagnation and concomitant dysoxia and anoxia in the bottom waters. These conditions were favourable to the preservation of marine organic matter and, combined with relatively low clastic input, resulted in high total organic carbon contents and hydrogen index values. The primary production was probably generally low, but may intermittently have been boosted by irregular up-welling of nutrient-rich intermediate and bottom-waters.
A new composite delta18O record, generated from calcareous fine-fraction and bulk sediments from the Exmouth Plateau, details long-term Cretaceous climatic change at mid-latitudes in the Southern Hemisphere. Assessment of new and previously published delta18O data indicates that a mid-Cretaceous global climatic optimum was achieved sometime between the time of the Cenomanian-Turonian boundary and the middle Turonian, when surface-ocean paleotemperatures were the highest of the past 115 m.y. Periods of cooling and warming that reversed the general patterns were superimposed on long-term Aptian-Turonian warming and Turonian-Maastrichtian cooling trends, respectively. Extrapolation of Southern Hemisphere paleotemperature trends to Maastrichtian paleotemperature data from a low-latitude Pacific guyot implies that maximum mid-Cretaceous low-latitude paleotemperatures could have been in excess of 33 °C.
A detailed multidisciplinary investigation (sedimentology, palaeoecology, geochemistry) of the Lower Toarcian Posidonia Shale revealed that the depositional environment was mainly controlled by sea level changes and palaeoclimate. Carbon isotope values of both, carbonates and organic matter are closely related to environmental conditions. Highly unfavourable living conditions for benthic fauna prevailed during a relative sea level low stand resulting in an enclosed stagnant basin environment. Long-term benthic colonization could not occur until water circulation improved during sea level high stand. The carbon isotope record of the Posidonia Shale is primarily controlled by redox conditions. Due to the dependency of the isotopic composition on regional palaeoecological, sedimentological and geochemical conditions, on oxygen availability and sea level changes, there is no need to infer global atmospheric changes of pCO2 or oceanwide anoxic events.
The geochemical events (negative shifts in delta C-13 and delta O-18, and Mn peak) associated with the "Schistes carton" of Quercy (SW France) do not appear to be due only to lithological variations and a differential diagenesis. They correspond to the Lower Toarcian global event and seem to be connected to a destabilization phase of gas hydrates. During its oxidation by seawater, the release of methane gas with a very low carbon isotopic ratio (-60%) led to the production of CO2 and carbonates with a negative delta C-13. The consumption of oxygen in the seawater resulted in conditions that were first dysoxic and then anoxic. The reduction of burrowing and bioturbation associated with this oxidation resulted in the laminated appearance of the "Schistes carton". The decrease in the oxygen content led to a decrease in MnO2 microparticles present in the water column and sediments. This created a complementary source of Mn2+ that was incorporated into the carbonates produced during this event. Furthermore, an increase in the average seawater temperature (delta O-18) appears associated with this event. Finally, the apparent diachronism of geochemical events in the Boreal and the Tethyan realms is discussed.
A composite Tethyan Late Jurassic-Early Cretaceous carbon and oxygen isotope curve is presented. C-isotope data provide information on the evolution and perturbation of the global carbon cycle. O-isotope data are used as a palaeotemperature proxy in combination with palaeontological information. The resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification. Positive C-isotope anomalies in the Valanginian and Aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide. A major warming pulse accompanies the Aptian but not the Valanginian C-isotope event. The observed change in Early Aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion. Both C-isotope anomalies are accompanied by biocalcification crises that may have been triggered by pCO2-induced changes in climate and in surface water chemistry. Elevated nutrient levels in river-influenced coastal waters and in upwelling regions further weakened marine calcification. These conditions contrast with 'normal' trophic conditions prevailing in the latest Jurassic and favouring biocalcification. The C-and O-isotope curves record a stable mode of carbon cycling and stable temperatures. We conclude that biocalcification is mostly triggered (and inhibited) by CO2 conditions in the atmosphere-ocean system.
The causes and duration of the early Toarcian anoxic event are controversial. Integration of data from calcareous and organic phytoplankton provides a biochronological framework that allows precise correlations across the western Tethys. In particular, the Carinolithus superbus nannofossil Zone can be used to correlate the levels enriched in organic matter and the related δ13 negative excursion. Although a variable duration is given in the literature for this negative excursion, it is likely that it lasted between 520 and 650 ka. Increased atmospheric pCO2, related to excess volcanic emissions (magmatic activity in the Karoo and Ferrar Provinces), had an impact on climate and ocean chemistry, and marked the inception of a biotic crisis affecting many organisms. The beginning of the crisis within shallow carbonate platforms, documented at southern latitudes, predates the levels enriched in organic matter. Dinoflagellate cysts experienced a decrease in abundance in the C. soperbus Zone, until they temporarily disappeared. The nannoplankton crisis was twofold: a decrease in size and low calcified specimens are observed in addition to a drastic decrease in absolute abundance. The increased atmospheric pCO2, as a result of the magmatic activity and temporarily amplified by transient methane release, could have been the trigger for the biocalcification crisis, which first affected the probably more reactive neritic system, and eventually the nannoplankton community.
Calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early Toarcian oceanic anoxic event (T-OAE). We quantify the abundance of nannofossils across the T-OAE at three locations in Western Europe, where the event is marked by a prominent negative carbon isotope excursion (CIE). Data were treated by statistical analysis, and the Shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution. In the basal Toarcian, before the T-OAE, high proportions of taxa with an affinity for low temperature (e.g., P. liasicus, T. patulus, Bussonius) occurred. This observation is consistent with interpretations of published oxygen isotope records. During the T-OAE, the lowest abundance of nannofossils is observed, but there is a peak of a coccolith (Calyculus) from an organism that probably thrived in low-saline surface waters depleted of nitrate. At the end of the perturbation, the lowest diversities of nannofossils occurred, and assemblages are dominated by Crepidolithus crassus, a deep dweller. This interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton. The highest abundance of nannofossils is recorded above the CIE and testifies to the recovery of the entire nannoplankton community. The T-OAE was widespread (perhaps global) and probably linked to major changes in the atmosphere and hydrosphere. However, the record of this event varies in the basins surrounding the western Tethys, suggesting regional imprints on the global signal. The regional variability may attest to establishment of effective connections between the Arctic and Tethys oceans, which allowed cool, low-saline water formed at high latitudes to stream toward the western Tethys.
Thirty years passed since the Cretaceous Oceanic Anoxic Events were proposed by Schlanger and Jenkyns (1976). Three events, OAE1 to 3 were recognized at the beginning of the research. Nowadays, however, OAE1 is subdivided into four subevents, OAE1a, b, c, and d. The OAE1b subevent is further subdivided into three components, Jacob, Paquier, and Leenhardt. The existence of Mid-Cenomanian Event (MCE) between OAE1d and OAE2 is drawing attentions of world scientists. A concise review of the researches on these events and subevents is followed by more detailed descriptions of the synonymy (local names), geographic distribution, age, stable carbon isotope fluctuations, extinction and/or radiation of fossils, major synchronous events, characteristics including the duration and the types of kerogen, and their causal factors. It is recognized that all OAEs do not have the same causal factor. The western part of the Tethys and the narrower early Atlantic were rather closed basins, where anoxic to dysoxic conditions easily occurred through the stratification of water column by run off, like the Sapropel event 1 in the Holocene Mediterranean Sea. The Cretaceous global OAEs may have occurred either by the stagnation of the deep water associated with the global warming or by propagation of marine organisms. Due to a large amount of input of terrestrial siliciclasitics, the regional influence of OAEs in the Japanese Cretaceous strata may differ from that of the Tethyan/Atlantic region, even if the occurrence of OAEs is ocean wide or globally synchronous.
A positive δ13C excursion reveals an oceanic anoxic event in the Subbetic, corresponding to the westernmost end of the Tethys during the Early Jurassic. This positive δ13C excursion exists in five stratigraphic sections consisting of Lower and Middle Toarcian pelagic sediments. The positive δ13C excursion in the Subbetic is located in the upper part of the Early Toarcian, serpentinus (≈ falciferum) biozone, while in other Tethyan regions (Alps, Apennines, and Dinarides) it has been found in the lower part of the same biozone. The most significant positive δ13C excursion detected in the Subbetic is therefore about half a million years younger, although other relative peaks had been detected. This isotopic anomaly can be correlated with a maximum transgression that occurred earlier in the regions closer to the open sea of the Tethys ocean. Other features of the anoxic event (faunal crises and black-shale deposits), appear at different ages, related to the activity of the transgressive phenomenon that occurred through the Early Toarcian.
A well-known second-order mass extinction took place during the Pliensbachian and Toarcian Stages of the Early Jurassic. First recognized as a minor Pliensbachian peak in the global extinction rate, it has alternatively been interpreted as a regional response to the early Toarcian oceanic anoxic event. Detailed studies established it as a global long-term event spanning five successive ammonoid zones. Here we present a revised time scale based on high-precision U-Pb ages resolved to the zone level, which suggests that elevated extinction rates were sustained for about 4 m.y. and peak extinction occurred at 183 Ma. Recent isotopic dating of flood basalts from the southern Gondwanan Karoo and Ferrar provinces documents a culmination in volcanic activity ca. 183 Ma. The onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater 87Sr/86Sr curve. The synchrony of voluminous flood basalt eruptions and biotic crises, as already noted for three of the major mass extinctions, permits a causal relationship, which in this case may be mediated by widespread oceanic anoxia.
Small but significant differences exist among stable carbon and oxygen isotopic excursions measured in coccolith-dominated bulk carbonate and planktic foraminifera during the Paleocene-Eocene thermal maximum ( PETM). One hypothesis suggests that the bulk carbonate isotopic record is compromised by changing nannofossil assemblages, since modern nannofossils show a large ( 5 permil) range of interspecific vital effects. New techniques are employed here to separate different size fractions of coccoliths from PETM sediments at ODP Site 690 for isotopic analysis, removing a major portion of the variation in nannofossil assemblages. Isotopic compositions of coarse and fine coccolith fractions dominated by coccoliths of genus Chiasmolithus and Toweius, respectively, differ by less than 0.5 permil for both oxygen and carbon. The near-monogeneric Toweius record closely parallels the main trends in the bulk carbonate isotope records, including multiple steps in the negative carbon isotopic excursion, suggesting that the trends in the bulk carbonate record are not artifacts of changing species assemblages. Because both coccolithophorids and symbiont-bearing foraminifera like Acarinina must inhabit the photic zone, it is unlikely that the 10(3) year lags in isotope event onset between coccoliths and Acarinina reflect true time-transgressive invasion of isotopically depleted CO2 into the water column. The small range of vital effects among Paleocene coccoliths is unlikely to result from diagenetic homogenization, and instead may reflect more similar carbon acquisition strategies of Paleocene coccolithophorid algae due to larger and/or more similar cell sizes and higher atmospheric carbon dioxide. The small range of vital effects suggests that bulk carbonate records are likely reliable for other early and pre-Cenozoic sediments where foraminifera are often scarce.
Isotopic records across the "Latest Paleocene Thermal Maximum' (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (~55.6 Ma). There also was a coeval -2 to -3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. A rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present-day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately -60‰. -from Authors
Carbonate rocks (limestones and dolomites) constitute a major part of the geological column and contain not only 60% of the world's known hydrocarbons but also host extensive mineral deposits. This book represents the first major review of carbonate sedimentology since the mid 1970's. It is aimed at the advanced undergraduate - postgraduate level and will also be of major interest to geologists working in the oil industry. Carbonate Sedimentology is designed to take the reader from the basic aspects of limestone recognition and classification through to an appreciation of the most recent developments such as large scale facies modelling and isotope geochemistry. Novel aspects of the book include a detailed review of carbonate mineralogy, non-marine carbonate depositional environments and an in-depth look at carbonate deposition and diagenesis through geologic time. In addition, the reviews of individual depositional systems stress a process-based approach rather than one centered on simple comparative sedimentology. The unique quality of this book is that it contains integrated reviews of carbonate sedimentology and diagenesis, within one volume.
In sections through Lower Toarcian oil shales, parallel variations in the 13-C/12-C ratios of carbonate and organic matter point to pronounced changes in the oxygen level of the water column. Fauna and flora, fossil preservation, paleogeography, and certain sedimentological features support this interpretation and justify the distinction of three lithologicisotopic facies types.Carbon isotopes might be generally useful in tracing the extent of “stagnation” during deposition of bituminous sediments.
Although much lower in δ 13 C than comparable Recent marine plankton, the bulk of the organic matter (as well as the oil shale carbonate) in the Posidonia Shales is ultimately derived from phytoplankton which inhabited the oxygenated surface layer of the ocean. Diagenetic processes such as oxidation, cementation, dolomitisation, isotope exchange, impregnation and migration have led to partial redistribution of carbon and oxygen isotopes within the sediment-pore water system.
The Couy-Sancerre borehole (Deep Geology Program of France) had the target of reaching the Paris Basin magnetic anomaly. It was located in the eastern part of the Biturige subblock, more precisely between the Armorican Block and the Burgondy Block, on the edge of the Loire Trough, on the site of the Mesozoic Marly Trough. It was located on a small intermediate horst (Fig. 1). The sedimentary series has been entirely cored, and this has been completed by a complete set of modern well logs (microresistivity, sonic, gamma ray, formation density, compensated neutron, photoelectric effect, dipmetering). This series is often comprehensive, but sometimes shows condensations. Petrographic and mineralogic studies (Delavenna et al., 1989; Mélières, 1991) and the determining of numerous ammonites (Lorenz et al., 1987) make this borehole a first-rate stratigraphic reference concerning the Lower and Middle Jurassic series in the southern part of the Paris Basin. The lithostratigraphic analysis has already been the subject of a publication (Lorenz et al., 1992), and a description of the sequences has been sketched out (Gely and Lorenz, 1991). At the same time, the implementation of lithostratigraphic data and the description of the well logs provide greater accuracy in interpretation in terms of desposit sequences, while the stratigraphic calibration of the well-log signatures gives a reference on the scale of the Paris Basin. The sequences defined in the Couy borehole are compared to the ones already published elsewhere on a global scale (Haq et al., 1988; Vail et al., 1987) and on a regional scale (Rioult et al., 1991; Gonnin et al., 1992, 1993; Bessereau and Guillocheau, 1994, (Figs. 2 and 3). Discontinuities corresponding to a sedimentation gap are usually represented by traces of bioturbation or by perforations in the top surface of a bed or else by a surface of gullying. In other cases, the sequence boundaries do not seem to be so clearly expressed but correspond to highly bioturbated bands, a single limestone bed in the midst of marls or perhaps an abrupt lithological change. For a limestone bed situated at the upper boundary of a sequence, we can see that this latter is often perforated or bioturbated. This enables them to be distinguished from other limestone levels situated at the transgressive maximum, and which contain pelagic fossils or authigenic minerals. On well logs, these two types of beds often have a comparable signature. The sequence boundaries are clearly shown by the well logs, which show clearcut curve breaks and are often capable of orienting or confirming the choice of identification criteria of the boundaries in the core samples. However, it can be seen that there is no direct relationships between the visible size of the discontinuity and the existence of an appreciable biostratigraphic gap. The Domerian-Toarcian boundary does not show any biozone gap, but it is particulary well expressed by gullying. Whereas the very large gap in the Lower Bajocian is not marked by any appreciable discontinuity, but only by a perforated surface, the gap nonetheless corresponds to four biozones. From the absence of erosion traces of a subaerial nature, we can deduce that the drop in sealevel at the end of the sequences was less than subsidence (Type 2 discontinuity) in the area around the Couy borehole. Located between the Biturige subblock and the Loire Trough, the Couy borehole, as the result of the evolution of local tectonic subsidence, reveals either carbonate facies of the Berry platform (sequence of dominantly carbonate deposits) or thick clayey facies of the Marly Trough (sequence with dominantly clayey deposits). In these sequences, maximum transgression corresponds to the minimum values of the gamma-ray log in the basin series and to maximum values on the platform domains (Lorenz and Gely, 1994). Hence, in the platform domain, the transgressive maximum may be shown by clayey limestones with rare ammonites, indicating a maximum value of the gamma-ray log. However, in other places this must not be confused with a comparable well-log signature provided on the platform by lagoonal marls situated at the top of the sequences. In the basin domain, the transgressive maximum is linked to the maximum distancing of continental areas from a fixed point, thus having a good chance of involving a slowing down of sandy and clayey terrigenous influxes. For the other components of the sediment, the slowing down of clayey sedimentation causes a decrease in dilution. Enrichments in phosphate appear (Bifrons and Bomfordi subzones, Opalinum, Niortense and Garantiana zones), in glauconite (Hodsoni zone), in ferruginous ooliths (Niortense and Garantiana zones, and above in the region, the Macrocephalus, Lamberti and Cordatum zones). The influence of regional subsidence is superimposed on the successions of deposits, linked to sealevel changes. These two factors control the accomodation and the hydrodynamic conditions of the basin. Tectonic activity can obliterate the role of eustatism to the extent of preventing any deposit, and entire sedimentary sequences may then not be expressed locally, as is the case for the Humphrie-sianum zone in the Poitou and Berry regions. On the other hand, a speeding up of the subsidence of the Marly Trough occurred during the highstand interval in the Lotharingian (Sequence 3), causing the deposition of back basin clays, thus situating the maximum paleodepth exceptionally at the top of the sequence underneath a nonerosive discontinuity. A contrast can also be seen between the marly series of the Upper Pliensbachian, such as the Bathonian, and the condensed limestone series of the Lower and Middle Bajocian. The installation of the carbonate platform in the Aalenian-Lower Bajocian (platform with an accumulation of bioclast sands of the "keep-up carbonate platform" type, Sarg, 1989), with already a first glimpse of the upermost Toarcian (micritic limestone platform with gryphites, of the "catch-up carbonate platform" type, according to Sarg, 1989), was probably caused by a great slowing down of subsidence. The same pattern can be found in the regional Callovian with periods of erosion or nondeposition. We are led to highlight three periods of tectonic activity in the Couy area (Lotharingian, Aalenian-Bajocian, Callovian), which do not always correspond to the tendencies described in other paleogeographic domains, for example such as Champagne in the Middle Jurassic (Guillocheau, 1991). A sequential analysis of the Couy borehole has revealed 18 sequences in the Lower Jurassic and part of the Middle Jurassic. In the Lower Jurassic, 11 characterized sequences (the last one transiting into the Middle Jurassic) are similar to the ones proposed by Haq et al. (1988), which for Western Europe must stem from the relative uniformity of Lower Jurassic deposits, often with clearcut boundaries, with generally abundant ammonites, and with a biostratigraphic scale that has been widely accepted by researchers. For the Middle Juassic, the 8 recognized sequences are rather difficult to correlate with the ones proposed by Haq et al. (1988) as well as by Rioult et al. (1991) in Normandy, or by Gonnin et al. (1993) in Poitou. In the Middle Jurassic, the shallow carbonate platform facies, with numerous more or less prolonged sedimentation gaps, generally have fewer ammonite fauna than the Lower Jurassic deposits, and their lithological units are less clearcut and vary laterally very quickly. To complicate the analysis, at least in France, this was a period of slight tectonic instability linked to both the opening up of the Atlantic and the rifting period of the future Alpine domain. In distensional conditions this resulted in tiltings of blocks causing local variations in facies and thickness, and even emersions (Lorenz and Pomerol, 1985; Gabilly et al., 1985). These relatively slight movements "disturb" the recording of eustatic variations. The biostratigraphic dating of these phenomena is not the same in the Middle Jurassic in the Tethysian province and in the Boreal province. The Paris Basin is situated at the fluctuating boundary between these two provinces. Likewise, these two scales have not been definitively accepted by researchers, which sometimes makes the correlations of their results more delicate. In the Middle Jurassic, only a few major breaks reflecting the eustatism can be considered. It is to be feared that the others, even though biostratigraphically well dated, risk, for some time to come, representing local phenomena rather than general event. Because of its lithostratigraphic and biostratigraphic data, as well as its well-calibrated well-log recordings, the Couy borehole is a reference borehole for the stratigraphy of the Paris Basin. It has been possible to recognise third-other deposit sequences described on a global scale and to characterize them by a specific well-logging signature (curve break, ramps and major peaks) used for correlations on a basin scale.
Dolomite sampled during Leg 201 occurs as hard lithified layers surrounded by small amounts of friable dolomite, and its presence is 'related to organic carbon-rich diatomaceous sediments. Petrographic relationships indicate a shallow depth of dolomite precipitation on the Peru margin. Dolomitic breccia layers found on the lower slope, however, were precipitated contemporaneous with brecciation, which probably took place at greater depth under the influence of tectonic deformation processes in the accretionary prism. In most cases,the formation of coexisting framboidal pyrite postdates the growth of dolomite rhombs. No crystal morphologies indicating a direct influence of bacteria in the precipitation process were observed. Occurrence and morphology of the carbonate layers, however, suggest focused precipitation within the sedimentary sequence. We hypothesize that microbial hot spots discovered during Leg 201 at geochemical boundaries such as the methane/sulfate boundary may be sites of strongly enhanced microbial mediation of dolomite precipitation.
Calcareous nannofossils play an important role in biostratigraphy and paleoecology, though their study is often complicated by their reduced size. The methodology presented herein allows to separate the nannofossil assemblage into granulometrically and taxonomically homogeneous fractions. This method permits :a) the concentration of rare taxa in order to simplify their morphometric study; b) the geochemical analysis (stable isotopes and trace elements) of these fractions. The latter is helpful in many cases, especially when the analysis of major or minor taxa constituent of the assemblage is greatly complicated by the reduced particles dimensions. In this way, it will be possible to bring light to the ecological parameters of these taxa, as calcification temperature and other ecological requirements (salinity, nutrient concentrations).
In sections through Lower Toarcian oil shales, parallel variations in the 13-C/12-C ratios of carbonate and organic matter point to pronounced changes in the oxygen level of the water column. Fauna and flora, fossil preservation, and sedimentological features support this interpretation. Although much lower in delta 13 C than comparable Recent marine plankton, the bulk of the organic matter in the Posidonia Shales is ultimately derived from phytoplankton which inhabited the oxygenated surface layer of the ocean. Diagenetic processes have led to partial redistribution of carbon and oxygen isotopes within the sediment-pore water system. -from Author
The Early Toarcian transgression is marked by the occurrence of organic carbon-rich shales in large parts of western Europe and in other parts of the world as well. Based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum Zone in several of the Tethyan sections, the widespread occurrence of the Early Toarcian shales was explained by an Oceanic Anoxic Event (OAE) (Jenkyns, 1988; Jenkyns and Clayton, 1997). The rapid burial of large amounts of organic carbon, which is rich in 12 C, would have led to a relative enrichment in 13 C of the global carbon reservoir and hence to an increase in 13 C of the limestones. In Southwest Germany (and a number of other locations) both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum Zone and the lower part of the falciferum Zone synchronous to black shale deposition and predating the positive isotopic excursion (Kuspert, 1982, 1983). It is presently unclear what the exact cause for this widespread negative isotopic excursion is (Jenkyns and Clayton, 1997). The objective of our study was to examine if the negative carbon isotope excursion of organic matter (OM) in the Southwest German Toarcian shales can be attributed to compositional changes of the OM, or if the excursion is related to variations in 13 C(CO2)aq in the photic zone. To this end, we analyzed the molecular and stable carbon isotope composition of OM in ten samples spanning the Early Toarcian (TOC 2.5-10.5 percent; HI 350-700). Biomarker distributions and kero- gen pyrolysates differ only slightly among the samples, and 13 C values of primary production markers follow closely those of associated carbonate and kerogen ( 13 CCARB/TOC). This strongly suggests that differences in 13 CTOC are related to differences in 13 Co f (CO 2)aq in the photic zone supporting Kuspert's model of advection of isotopically light, organic matter-derived CO2 from anoxic bottom waters. Evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids, such as isorenieratane, from anoxygenic phototrophic sulfur bacteria. Their concentrations are ex- tremely low in black shales from the tenuicostatum Zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum Subzone suggesting that photic zone euxinia (PZE) was a common feature of the Early Toarcian sea in Southwest Germany. Our data, therefore, support the Kuspert model of advec- tion of isotopically light, organic matter-derived CO2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike. The enrichment in 13 C in the middle of the falciferum Zone, which followed the negative spike, may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition.
Pore waters and Quaternary sediments at ODP Leg 172 sites on the Carolina Slope (CS; Site 1054), Blake Outer Ridge (BOR; Sites 1057 and 1060), Bahama Outer Ridge (BAOR; 1062) and Bermuda Rise (BR, Site 1063) were studied. The sediments are mainly clayey and silty mud with intercalations of nannofossil-rich and lutite-rich beds towards the top. Sedimentation rate ranges from 4.2 cm/ky at Site 1054 to 23 cm/ky at Site 1060. Average total organic carbon (TOC) contents of the upper sediment units range from 0.40% to 0.58% at Sites 1057, 1060, 1062 and 1063, and from 0.86% to 1.25% at Site 1054. The organic matter at all the sites is mostly degraded, except at Site 1054. Sediments at Sites 1062 and 1063 have lower total sulfur (TS) (0.00–0.62%; average: 0.06%) than those at the other sites. Site 1054 has the highest TS (0.8–1.4%) and the lowest reactive iron (0.003–0.074%) contents among all the sites.
Main steps of the evolution of selected marine organisms (Benthic Invertebrates, Plancton, Algae) between LatePliensbachian and Late Toarcian are analyzed in Western European terrigeneous platform environments and Tethyan basins and carbonate platforms. In W-European platforms and Tethyan basins, characterized by dominantly argillaceous sediments, well marked episods of crisis were recognized on a large scale. It principally affects the benthic organisms at the Pliensbachian-Toarcian boundary and inside Early Toarcian (top of Tenuicostatum zone). Evidence of another crisis period, which seems contemporary, is shown in Tethyan carbonate platforms. These crisis proceed from multiple factors in relation with paleogeographic modifications and sea-level changes during this time.
The Early Jurassic (early Toarcian, ca. 183 Ma) carbon cycle perturbation is characterized by aabout -5 parts per thousand {delta} {sup 13}C excursion in the exogenic carbon reservoirs, a 1000 ppm rise in atmospheric CO{sub 2}, and a 6-7 degrees warming. Two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light CH4 from (1) Gondwanan coals by heating during the intrusive eruption of the Karoo-Ferrar large igneous province (LIP) or (2) the thermal dissociation of gas hydrates. Carbon cycle modeling indicates that the release of CH4 from Gondwanan coals synchronous with the eruption of the Karoo-Ferrar LIP fails to reproduce the magnitude or timing of the CO{sub 2} and {delta} {sup 13}C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated {delta}{sup 13}C record indicate that both features of geologic record can be explained with the huge input of about 15,340-24,750 Gt C over about 220 k.y., a result possibly pointing to the involvement of hydrothermal vent complexes in the Karoo Basin. The simulated release of > 6000 Gt C from gas hydrates also reproduces aspects of the early Toarcian rock record, but the large mass involved raises fundamental questions about its formation, storage, and release.
The study of calcareous nodules from four organic-rich black shale series allows us to suggest a prominent part for benthic organisms in creating heterogeneities in the sediment, thus favouring microbially mediated early diagenesis based on organic matter consumption. To cite this article: J.-G. Bréhéret et al., C. R. Geoscience 336 (2004).
Calcareous nannofossils play an important role in biostratigraphy and paleoecology, though their study is often complicated by their reduced size. The methodology presented herein allows to separate the nannofossil assemblage into granulometrically and taxonomically homogeneous fractions. This method permits :a) the concentration of rare taxa in order to simplify their morphometric study; b) the geochemical analysis (stable isotopes and trace elements) of these fractions. The latter is helpful in many cases, especially when the analysis of major or minor taxa constituent of the assemblage is greatly complicated by the reduced particles dimensions. In this way, it will be possible to bring light to the ecological parameters of these taxa, as calcification temperature and other ecological requirements (salinity, nutrient concentrations).
Shows that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m.y. Three major features of the simulated Early Jurassic climate include the following. 1) A global warming, compared to the present, of 5°C to 10°C, with temperature increases at high latitudes five times this global average. Average summer temperature increases at high latitudes five times this global average. Average summer temperatures exceed 35°C in low-latitude regions of western Pangaea where eolian sandstones testify to the presence of vast deserts. 2) Simulated precipitation and evaporation patterns agree closely with the moisture distribution interpreted from evaporites, and coal deposits. High rainfall rates are associated primarily with monsoons that originate over the warm Tethys Ocean. These systems are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography. 3) Decreases in planetary albedo, occurring because of reductions in sea ice, snow cover, and low clouds, and increases in atmospheric water vapor are the positive climate feedbacks that amplify the global warming. Large seasonal temperature fluctuations occurred over mid- and high-latitude continental interiors. -from Authors
In Western Tethyan areas, the Toarcian stage begins with two important evolutionary events in ammonite faunas: (1) the disruption of Tethyan–Boreal provinciality; (2) a biological crisis linked with the oceanic anoxic event OAE. The analysis of these events has been addressed by constructing curves of ammonoid diversity (species richness, origination and extinction rates) in the Late Pliensbachian (= Domerian)–Early Toarcian interval in selected localities. Two diversity drops are recognized. The first one is recorded at the end of the Dactylioceras mirabile subzone and reflects the disruption of Tethyan–Boreal provinciality, through the progressive extinction of the Boreal endemic family Amaltheidae that occupied the north-western European seas during the whole Pliensbachian on the one hand, and the extinction of Late Domerian Ammonitina endemic to the Mediterranean areas on the other hand. The Early Toarcian homogeneization of Mediterranean and north-western European ammonoid faunas was reached via elimination of both Boreal and Mediterranean endemics with differential rates of extinction in the two palaeogeographic domains and the subsequent geographical expansion of Tethyan-derived ammonoids. The second, dramatic drop in ammonite diversity in the upper part of the Dactylioceras semicelatum subzone coincided with the onset of OAE. It also affected epioceanic ammonoid clades like Phyllocerataceae and Lytocerataceae. These two drops are interpreted as two distinct extinctions and not as episodes of a single, stepwise event. Complex relations between ammonoid diversity and sea-level changes are suggested by trends in endemism, which may be reversed during either a single transgression or a single regression.
The Lower Toarcian Posidonia Shale is famous for its excellently preserved fossils and its high amount of organic matter (up to 16%). Both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions. High-resolution geochemical, sedimentological and palaeoecological investigations of various sections of the Posidonia Shale in SW-Germany, however, indicate that oxygen availability was variable and ranged from short oxygenated periods to longer-term anoxia. The benthic macrofauna consists of nine fossil communities and was used, in combination with geochemical data, to reconstruct a time-averaged oxygen curve. Anoxic conditions prevailed during the deposition of the Toarcian black shales; they were, however, punctuated by various short periods (weeks to years) with oxygenated bottom water conditions. Sedimentological (e.g. distinctiveness of microlamination, siliciclastic content) and geochemical parameters (e.g. organic matter content, isotopic signatures: δ18O and δ13C, molecular redox parameters: pristane/phytane ratio, arylisoprenoids) exhibit a remarkable covariation and seem to be controlled by sea level fluctuations. Maximum oxygen depletion and an extreme negative shift of δ13Corg values (−34‰) occurred during the early falciferum-zone. This is explained by the recycling of 12C-enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia. The subsequent transgression permitted enhanced water exchange with the Tethyan Ocean and caused improvement of living conditions at the end of the falciferum-zone.
There were three negative seawater strontium-isotope excursions (shifts to lower 87Sr/86Sr values) during the Jurassic and Cretaceous that were of relatively short duration (5-13 my) and showed a relatively quick recovery to pre-excursion 87Sr/86Sr ratios. These excursions occurred in the Pliensbachian-Toarcian (Early Jurassic), Aptian-Albian, and Cenomanian-Santonian (Early and Late Cretaceous respectively). Each excursion coincided closely in time with an Oceanic Anoxic Event (OAE) marked by sediments unusually rich in organic carbon. The Jurassic OAE occurred at the end of the strontium-isotope excursion, whereas the two Cretaceous OAEs occurred at the onset of the accompanying strontium-isotope excursions. The possible causes of these excursions were evaluated by successively examining the changes in the riverine strontium fluxes, riverine 87Sr/86Sr ratios, or hydrothermal strontium fluxes required to produce each excursion. A range of seawater strontium budgets was used to encompass the uncertainties in modern and ancient cycles. To produce the excursions, we calculate that the riverine strontium fluxes would have had to decrease by 6 to 15 percent or the fluvial 87Sr/86Sr ratios by 0.00019 to 0.00046. The uncertainties largely stem from the assumed magnitude of the hydrothermal strontium flux at the onset of each excursion. Alternatively, increases in sea-floor hydrothermal activity of 7 to 104 percent could also have produced the strontium-isotope excursions. This large range is due mostly to uncertainties in the relative flux of strontium from axial high-temperature hydrothermal systems and low-temperature off-axis systems. Only a small portion of this range stems from uncertainties in the riverine strontium terms. The possible causes of the excursions were further evaluated by examining several geologic factors that could have affected riverine strontium, including climate change, sealevel, and the eruption of flood basalts. We conclude that neither variations in riverine strontium fluxes nor in 87Sr/86Sr ratios is the likely cause of the strontium-isotope excursions. The most probable explanation is increased rates of hydrothermal activity related to increased ocean-crust production at the mid-ocean ridges. The close correlation in time between the strontium-isotope excursions and the major Oceanic Anoxic Events (OAEs) is compatible with a causal linkage. We propose that increased ocean-crust production led to enhanced CO2 outgassing and global warming, which in turn led to several processes that acted to make surface ocean waters more productive. However, because OAEs did not occur throughout the proposed periods of enhanced hydrothermal activity, it appears that these processes only preconditioned the oceans for the OAEs: sealevel rise may have been the final trigger. This model explains why all three OAEs did not occur at the same time relative to the onset of excess hydrothermal activity and why OAEs are not associated with every sealevel rise documented in the stratigraphic record.
A new analysis of Lower and Middle Jurassic bivalve faunas of western North America and the southern Andes in comparison to Europe, supported by evidence from other molluscs, throws further light on the establishment of a “central Atlantic” epicontinental seaway prior to the formation of a true oceanic strait between eastern North America and Africa-South America. Some restriction to free communication persisted until the early Callovian but there was an episode of relatively free intermigration in the Toarcian to early Bajocian time interval, which is associated with major rise of sea level. One of the most significant consequences of this event was a spread into northwest Europe of taxa which were previously confined to the Pacific margins, to occupy niches vacated as a result of an early Toarcian extinction event.
Emiliania huxleyi is numerically the most important coccolithophorid in the modem ocean and has been intensely studied in the contexts of biogeochemistry (especially relating to the global carbon cycle), plankton ecology, biomineralization, and cellular carbon transport. This paper reviews older as well as more recently acquired information on reproduction, morphology, ecophysiology, and cell physiology of E. huxleyi, emphasizing aspects that are relevant to coccolith formation and calcification-photosynthesis interactions. The existence of a number of ecotypes, which probably accounts for the wide distribution of this species in nature, complicates comparisons between laboratory studies in which different clones have been used. Coccolith formation is a strongly regulated process; use of mutants may be helpful in elucidating the control mechanisms involved. Conceptual models illustrating the role of calcification in photosynthetic carbon supply are supported by extensive experimental evidence, but the exact mechanisms of calcium and bicarbonate ion transport and of CO2 entry into the cell remain to be established.
A detailed analysis of species diversity increase, zone by zone up the Liassic section in northwest Europe, has been undertaken for the six fossil groups for which adequate data are available: bivalves, ammonites, rhynchonellid brachiopods, crinoids, foraminifera and ostracods. The general pattern is of a rapid increase through the Hettangian from a very low level after the end-Triassic mass extinction, followed by a slower rate of increase until the late Pliensbachian. Thereafter there was a drastic fall in the early Toarcian as a consequence of the mass extinction of that time, followed by a further rise continuing into the Middle Jurassic. A more general study takes into account data from southern Europe and extra-European localities, the best of which for the Liassic being in Argentina and Chile. The best data come from brachiopods and bivalves. -from Author
The timing and causal relationships between the pronounced negative C isotope excursion and paleoenvironmental perturbations associated with the Toarcian oceanic anoxic event (Early Jurassic) remain unclear, particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main C isotope anomaly. Here we present a new quantification of Late Pliensbachian-Early Toarcian calcareous nannofossils abundance and size from the Peniche reference section (Portugal) together with O and C isotope records of well-preserved brachiopod shells from the same section. The brachiopod shell delta 13C curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions, close to the Pliensbachian-Toarcian boundary (~-20/00) and during the Toarcian oceanic anoxic event (~-3.50/00). Our results indicate that both C isotope negative excursions were characteristic of benthic and shallow-water environments, suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs. Coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of Mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures, probably as a result of increased CO2 levels and enhanced greenhouse conditions. CO2-induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by synchronous drops of platform-derived carbonate accumulation and drastic reductions in size (>3 mum) of the main pelagic carbonate producer Schizosphaerella. We suggest that the Early Toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the Karoo-Ferrar province.
Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1000 closely spaced samples were analyzed during this study. At least seven major delta/sup 13/C excursions can be correlated from section to section. The most important heavy events occur near the Aptian-Albian and Cemonanian-Turonian boundaries, whereas light events are near the Jurassic-Cretaceous, Albian-Cemonanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of events with stage boundaries and the consistent correlation of events between stratigraphic sections provides a significant new tool for time-rock correlation independent of standard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic anoxic events demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins. 12 figures, 1 table.
The carbon isotope record in four pelagic carbonate sections from the
Southern Alps (northern Italy) across the Jurassic-Cretaceous boundary
has been correlated to biostratigraphy and magnetostratigraphy. The
carbon isotope curve from bulk carbonates shows a decrease from
Kimmeridgian to Early Tithonian (CM24-CM22) values of
δ13C=+2.07 (± 0.14)‰ to Late Tithonian
and Berriasian (CM18-CM14) values of δ13C=+1.26
(± 0.16)‰. The change in the carbon isotope record
coincides with changes in Tethyan calcite and silica accumulation rates,
with a drop in the calcite compensation depth in the Atlantic and Tethys
oceans and with changes in organic carbon burial along the Eurasian
margin of the Tethys. Reduced surface water productivity due to
diminished transfer rates of biolimiting elements into the Atlantic and
Tethys oceans can explain these observations. The decreased transfer
rates of elements such as silica or phosphorus from continents into the
oceans resulted from drier climatic conditions and decreased water
runoff on continents bordering the Tethys and Atlantic oceans. The
proposed changes in Tithonian - Berriasian ocean chemistry and
paleoclimate suggest that variations in the global carbon cycle were
coupled with changes in the global hydrological cycle and in associated
material cycles.