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Interpreting carbon-isotope excursions: Stranglove oceans
Abstract
Large negative excursions in marine carbonate δ¹³C are commonly associated with period boundaries and mass extinctions. Explanations for these events must be consistent with limitations imposed by carbon-isotope mass balance. At steady state (i.e., for excursions lasting more than 10ⵠyr), the surface ocean δ¹³C is set by the organic fraction of the total carbon burial rate and the magnitude of the photosynthetic isotope effect. The δ¹³C of the deep ocean and the surface-to-deep isotope gradient are set by both the organic fraction of the ocean's remineralized particulate flux and the magnitude of the isotope effect. Thus it is the carbon-isotope composition of the deep ocean that is most reflective of internal oceanic processes; the surface ocean records changes in the longer term throughput of carbon in the system. The cessation of organic export from the surface ocean, such as is presumed to have caused the Strangelove ocean condition of the Cretaceous/Tertiary (K/T) boundary, leads to an isotopically homogeneous ocean in decades to centuries. If this condition persists, the ocean's isotopic composition approaches that of the riverine weathering input (in 10ⵠyr). Failure to approach this value during the K/T event suggests continued production and burial of organic carbon, dominantly in either terrestrial or shallow-marine environments.
... If δ 13 C values of carbonate reflect a mixture of marine and porewater dissolved inorganic carbon (low value due to remineralization of organic matter in pore fluid), δ 13 C of contemporaneous seawater must be more positive than − 3‰. This value is significantly higher than the assumed mantle-like seawater δ 13 C carb value of − 5‰, which assumes marine organic matter production was suppressed by the global glaciation (Javoy et al., 1986;Kump, 1991;Hoffman et al., 1998). But, in all carbonates from the Sturtian glacial deposits except Namibia, including North America, Scotland, Svalbard, Australia, and South China, the δ 13 C carb value is higher than the assumed value (Fig. 11A, Fairchild et al., 1989;Zhou et al., 2007;Fairchild et al., 2018;Mackey et al., 2020;Hoffman et al., 2021;Hood et al., 2021). ...
... These results and sedimentological observations are inconsistent with the snowball Earth model. A snowball Earth event would shut down the marine primary productivity and generate a mantle-like seawater carbon isotope value (− 5‰) (Javoy et al., 1986;Kump, 1991;Hoffman et al., 1998). Furthermore, in a snowball Earth event, the MSR would mainly occur near the ice grounding line. ...
The Cryogenian Sturtian glaciation (717–660 Ma) may have completely frozen global oceans, which would have resulted in marine anoxia due to the oceans' prolonged separation from the atmosphere. Sedimentological evidence reveals that the ice sheets are dynamic throughout this glacial. The ice sheets' retreat increased the availability of nutrients and led to active marine biogeochemical cycles. However, direct geochemical constraints on the synglacial marine chemistry, such as data from carbonate rocks, are rare. We conducted detailed sedimentological and geochemical analyses on the Cryogenian Fulu formation in the Yangtze Block's southeastern margin. The Fulu Formation mainly consists of sandstones with rare carbonate rocks. Facies analysis shows that these sandstones were deposited in deep water environment during the waning Sturtian glaciation. Three facies were recognized, including the sand lobe, lobe fringe and basin plain facies. According to the geochemical results, the Fulu Formation has a large variation in organic carbon isotope (δ13Corg) and pyrite sulfur isotope (δ34Spy) (δ13Corg: −29.5‰ ~ −22.0‰, mean = −25.7‰, n = 143; δ34Spy: −19.4‰ ~ −29.0‰, mean = 1.7‰, n = 88), but shows low values of TOC (<0.1 wt%) and pyrite content (<0.4 wt%). Dolostone layers are mainly deposited in the basin environment and are characterized by: conspicuous Fe and Mn content (Fecarb: 15746–64,428 ppm, mean = 43,457 ppm; Mncarb: 4436–8576 ppm, mean = 6078 ppm; n = 13), stable inorganic carbon isotope (δ13Ccarb: −3.7‰ ~ −2.5‰, mean = −3.1‰, n = 23), and absence of Ce anomaly (0.93–1.07, mean = 0.99, n = 13). These stable δ13Ccarb, low TOC and pyrite content suggest that sustaining primary productivity and low seawater sulfate levels. Turbidity deposition, rather than marine chemistry controls the oscillations of δ13Corg and δ34Spy records under these conditions. The remineralization of Mn-oxide causes the coupling variation trend of REE and Mncarb content result from the Mn-oxide releases adsorbed REE. Mn-oxides in turbidity deposits from shallow water indicate that the shallow ocean is oxic/suboxic. The absence of the Ce anomaly, on the other hand, suggests that synglacial deep ocean remained anoxic. Thus, despite low primary productivity and seawater sulfate concentration, sustaining turbidity deposition maintained the ocean redox stratification during the waning Sturtian glaciation.
... Furthermore, if respiration rates are high in the surface ocean, high primary productivity will not have a major effect on o 13 C 0 1 c The size of the organic carbon ,reservoir (biomass and sedimentary organic carbon) controls the o 13 C 0 1 c values of the global ocean. A net increase in the burial of organic carbon will enlarge the organic carbon reservoi;· and increase the S 13 c of seawater and consequently of marine carbonates (Garrels and Lerman, 1984;Berger and Vincent, 1986;Kump, 1991). Alternatively, increased weathering rates-as might accompany increased tectonism-may result in a net decrease in the organic carbon reservoir and lower the global ocean o 13 C value. ...
Stable isotopes can be used to monitor global and paleoceanographic changes associated with supercontinent formation and breakup. Substantial amounts of isotopic data have been produced for the time interval represented by the evolution of Pangea, but data free of diagenetic effects are uncommon. Thick nonluminescent brachiopod shells and marine cements of original miner ology a nd chemistry arc the materials most likely to retain their original isotopic composition. Altered marine cements can be used to infer an original marine signature. Whole rock samples are the least reliable for isotopic study, providing only an approximation of marine δ13C values and diagenetically altered δ180 values. These different techniques have different shortcomings but tend to complement each other. The isotopic record for Pangea is too sparse for detailed interpretation, but several robust features are evident Carbon isotopic values of brachiopod shells and former marine cements from Eurasia (Paleotethyan) and Arctic Canada increase by about 27 °C during the mid-Carboniferous. This increase is not seen in samples from the central and south-central United States (Panthalassan) and in part reflects changes in ocean circulation with the closing of the equatorial seaway as Gondwana collided with Laurussia. The Paleotethyan-Panthalassan δ13C difference appears to extend into the Permian. Permian δ13C values arc high but decrease by roughly δ13C at or near the Permian-Triassic boundary. This decline in δ13C is attributed to a decrease in the inventory of buried organic carbon. δ13C values remain low in the Triassic (<4‰) compared to Permian values (generally >4‰). Oxygen isotopic values for the Carboniferous are generally between-3‰ and
... The abundance of hiatuses with significant and variable duration in vertical sampling transects creates difficulties for the temporal interpretation of recorded data if environmental conditions and hence geochemical proxies vary temporally (secularly), laterally (spatially) or both. Systematic and highly variable carbonate 13 C values can occur in different, but timeequivalent, facies, which in turn are used to infer regional or global scale stratification of 13 C with water depth (e.g., Kump, 1991). We recognise that some of the proximal-to-distal differences seen in 13 C records may be diagenetic in origin, or caused by differing fractions of transported (e.g. ...
It is well known that the sedimentary rock record is both incomplete and biased. Correlation between geographically distinct records is problematic in the absence of absolute age constraints, which can profoundly affect interpretations of geological and geochemical data. We use a computational process model to create canonical examples of carbonate-siliciclastic stratigraphic cross-sections from oscillatory overall-rising and -falling sea level, in which we can quantify how spatial and temporal geochemical records are biased by the inherent processes of marine sedimentation and preservation. Sedimentary hiatuses are shown to span ~50 to 95% of geological time, and even in these simple cases, infinitely dense sedimentology and geochemical data collected along spatial transects crossing the geological strata are shown to confound expert interpretations of paleo-environmental signals. Observations of secular variation of inferred paleo-environmental changes are shown to be irreversibly disguised as lower frequency signals by an aliasing effect, caused by cyclical and multiscale relative sea-level changes. Nyquist’s theorem correctly predicts their biased signatures, proving that aliasing is caused by natural geological processes. Both effects are compounded by finite spatial sampling intervals adopted in practice, illustrated using real data case studies. These combined effects imply (a) that deeper marine records are significantly more likely to provide unaliased environmental signatures, and (b) that careful sedimentological field observations still allow such records to be correctly correlated in age given aliased shallow marine intervals.
... The transition from a decoupling to a coupling relationship at the onset of the NCIEs interval suggests that terrestrial runoff may play a role in increasing light carbon towards more negative values. Carbonate carbon isotope values (δ 13 C carb ) are primarily influenced by the carbon isotope of riverine inputs, ratio of organic carbon to carbonate burial, and fractionation of carbon isotopes between the atmosphere and organic matter during photosynthesis (Dong et al., 2021;Hayes et al., 1999;Kump, 1991;Kump and Arthur, 1999;Wendler, 2013). Kump and Arthur (1999) simulated the changes in marine isotopic composition resulting from increased riverine carbon flux. ...
Global climate change has profound implications for human survival and prosperity. The Carnian (early Late Triassic, ~233 Ma) was a time of global environmental perturbations, climatic change, and biotic turnover, commonly known as the Carnian Pluvial Episode (CPE). However, the understanding of possible triggering mechanisms of the CPE is still partially untraveled due to the lack of a high-resolution chronostratigraphic framework. Here we present an astrochronology of episodic negative carbon isotope excursions (NCIEs) observed in the shallow marine sediments of the Bagong Formation in the Qiangtang Basin (Tibetan Plateau, China) to explore the role of orbital forcing during the CPE. This study marks the first identification of five episodic NCIEs in the Carnian strata of the Bagong Formation. These NCIEs correspond with five distinct periods of increased detrital influx. The coupling relationship between each phase of NCIE and related terrigenous input indicates that episodic NCIEs may be influenced by pulses of active continental weathering. Mercury isotope is contempora-neous with the active volcanism that triggered the onset of the NCIEs; however, the impact of volcanism in the Qiangtang Basin was weak and almost non-existent at this time. An anchored floating astronomical timescale (ATS) for the episodic NCIEs is established, revealing evidence for a 405 kyr eccentricity in high-resolution gamma ray data series using time series analysis. A ~13.16 Myr-long ATS of the Bagong Formation is developed by astronomical tuning of gamma ray logs to the stable 405-kyr long-eccentricity cycles. This floating ATS takes the U-Pb zircon as its anchor point and establishes an anchored floating ATS of the Bagong Formation from 233.16 ± 1.37 Ma to 220.4 ± 1.1 Ma. The innovative astrochronology approach, employing a sedimentary noise model, has successfully reconstructed sea-level changes during the Late Triassic period. These findings align well with the documented global sea-level fluctuations of the same era. The antiphase relationship of the filtered ~1.2 Myr cycles between the sedimentary noise model sea-level curve and the obliquity modulation cycles demonstrates that the ~1.2 Myr modulation cycles may be the main driver of sea level changes during the Late Triassic. The ~1.2 Myr obliquity modulation maxima correlate well with the high sea level, episodic NCIEs, global warming, and marine life crisis, suggesting that obliquity forcing could have played a prominent role during the CPE. Our results reveal that the orbital forcing enhanced the hydrological cycle during the CPE, which provides a broader perspective of the CPE-related to the astronomical forcing.
In the previous chapters we have mentioned several times the oceans. The main example is the treatment of the carbonate chemistry related to the \(\textrm{CO}_2\) problem. Now we cannot pretend to exhaust the field of oceanography so we concentrate on some neglected topics and will be inspired mainly by the quite old book by Wally Broecker Chemical Oceanography published almost 50 years ago.
Carbon isotopic studies have documented the effects of mass extinctions and biological recoveries on the global carbon cycle. Stable carbon isotopes can also be used to document the disappearance and re-appearance of specific ecological strategies (such as life span, seasonality of growth, relative depth of habitat, and photosymbiont reliance) during mass extinctions and evolutionary recoveries. Nitrogen isotopes have never been used to study ecological collapse and recovery from mass extinctions. However, they have strong potential for testing the effects of mass extinctions and biological recoveries on the biologically mediated global nitrogen cycle. They may also be very helpful for documenting the effect of mass extinctions and taxonomic radiations on the trophic structure of terrestrial and marine ecosystems.
Mass extinctions shape the history of life and can be used to inform understanding of the current biodiversity crisis. In this paper, a general introduction is provided to the methods used to investigate the ecosystem effects of mass extinctions (Part I) and to explore major patterns and outstanding research questions in the field (Part II). The five largest mass extinctions of the Phanerozoic had profoundly different effects on the structure and function of ecosystems, although the causes of these differences are currently unclear. Outstanding questions and knowledge gaps are identified that need to be addressed if the fossil record is to be used as a means of informing the dynamics of future biodiversity loss and ecosystem change.
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