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Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism
Abstract
The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the “least-altered” portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny.
... The analysis of stable isotopes, such as δ 13 C and δ 18 O, is valuable for recon the fluid properties during dolomite precipitation, especially the δ 13 C value, wh fluenced by the diagenesis after deposition [78][79][80][81]. The δ 18 O value alongside th ated crossplot of δ 13 C and δ 18 O, as the oxygen isotope exchanges between seawate carbonate and meteoric/burial water, has tended to occur easily relative to oth isotopes during diagenetic alteration [79,80,[82][83][84][85][86]. ...
... The analysis of stable isotopes, such as δ 13 C and δ 18 O, is valuable for recon the fluid properties during dolomite precipitation, especially the δ 13 C value, wh fluenced by the diagenesis after deposition [78][79][80][81]. The δ 18 O value alongside th ated crossplot of δ 13 C and δ 18 O, as the oxygen isotope exchanges between seawate carbonate and meteoric/burial water, has tended to occur easily relative to oth isotopes during diagenetic alteration [79,80,[82][83][84][85][86]. By comparing the carbon isoto acteristics of the stylolites and matrix, it is found that the carbon isotope characte the stylolites are lighter than that of the matrix ( Figure 8A). ...
... The analysis of stable isotopes, such as δ 13 C and δ 18 O, is valuable for reconstructing the fluid properties during dolomite precipitation, especially the δ 13 C value, which is influenced by the diagenesis after deposition [78][79][80][81]. The δ 18 O value alongside the associated crossplot of δ 13 C and δ 18 O, as the oxygen isotope exchanges between seawater/marine carbonate and meteoric/burial water, has tended to occur easily relative to other stable isotopes during diagenetic alteration [79,80,[82][83][84][85][86]. ...
Carbonate rocks exhibit significant heterogeneity as both a source rock and reservoir. Stylolite formation plays a crucial role in the enrichment of organic matter and the migration of geofluids within carbonate rocks. In order to study the enrichment mechanism of organic matter and the geofluid migration mode in the stylolites developed in carbonate rocks, stylolite-bearing core samples from the Dachigan structural belt in the eastern Sichuan Basin were collected. The stylolites and matrix were subjected to the total organic carbon (TOC) test and Rock-Eval pyrolysis, thin-section observation under fluorescent light, whole-rock X-ray diffraction, carbon and oxygen isotope analysis, and scanning electron microscopy. The organic matter occurring in the stylolites is mainly in the form of three types: soluble organic matter, pyrobitumen, and bitumen. This suggests that the organic matter within the stylolites mainly consists of secondary migrated organic matter. The stylolites also exhibit well-developed secondary dolomite and pyrite resulting from late-diagenetic recrystallization. These minerals contribute to the preservation of intercrystalline pore spaces and fractures, providing favorable conditions for oil and gas accumulation and migration. The strong cementation observed at the contact between the stylolites and matrix, along with the presence of secondary minerals nearby, may be attributed to the fractionation of light and heavy oil components during the migration of hydrocarbon fluids from the matrix to the stylolites. The thicknesses of the stylolites vary within the bulk, indicating severe diagenesis in thinner areas. Consequently, this leads to significant fractionation effects. The fractionation of crude oil components by stylolites poses challenges for the study of definitive oil–source correlations. To overcome these challenges, future research could investigate biomarker compounds to attempt oil–source correlations. Additionally, future efforts should take into consideration the spatial variation in the crude oil properties. Understanding the role of stylolites in organic matter enrichment and geofluid migration is crucial for optimizing exploration strategies in the Sichuan Basin, a region of growing importance in the energy industry. Moreover, our findings shed light on the complex interactions within stylolite-bearing rocks, which are not limited to this specific basin. These insights offer valuable contributions to the broader field of geology and reservoir characterization, enhancing our ability to predict and interpret similar geological formations globally.
... However, compared to Mesozoic and Cenozoic rocks, Paleozoic carbonate rocks are more commonly affected by diagenesis, which can alter the isotopic characteristics of carbonate samples and hence affect the retrieval of information about the original seawater (Kaufman and Knoll, 1995 . The d 13 C signature is more conservative than the d 18 O signature in its response to later diagenesis, so d 18 O values may be an effective proxy for identifying diagenesis (Kaufman et al., 1991;Veizer et al., 1999). It is generally believed that d 18 O values less than around -10‰ indicate that the samples have undergone strong diagenetic alteration, and that the oxygen and carbon isotope values are no longer reliable . ...
... The different responses of the d 13 C and d 18 O signatures to diagenesis enable their relationship to be used to discriminate carbonate samples that have undergone diagenetic alterations (Burdett et al., 1990;Frank et al., 1997;Irwin et al., 1977;Kaufman et al., 1991). If there is no positive correlation between the d 13 C and d 18 O values in marine carbonate samples then it can be inferred that they have preserved the original carbon and oxygen isotopic signatures Kaufman et al., 1991;Veizer et al., 1999). ...
... The different responses of the d 13 C and d 18 O signatures to diagenesis enable their relationship to be used to discriminate carbonate samples that have undergone diagenetic alterations (Burdett et al., 1990;Frank et al., 1997;Irwin et al., 1977;Kaufman et al., 1991). If there is no positive correlation between the d 13 C and d 18 O values in marine carbonate samples then it can be inferred that they have preserved the original carbon and oxygen isotopic signatures Kaufman et al., 1991;Veizer et al., 1999). Reference to Fig. 5a shows that there is no significant covariation of the d 13 C and d 18 O values, which excludes the possibility of post-depositional fluid alteration (Fig. 5a). ...
The biosphere experienced episodic perturbations during the Neoproterozoic–early Cambrian, accompanied by major fluctuations of the carbon cycle. However, compared to the Ediacaran–early Cambrian, geochemical records of the Cambrian Toyonian Stage (Longwangmiao), which contains the significant Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), are scarce. To address this data gap, we conducted measurements of carbon, oxygen and strontium isotopes and element contents of a continuous series of carbonate samples from the Longwangmiao Formation of Well HS2, located in the central Sichuan Basin of South China. Our specific aims were to reconstruct the paleoenvironment and to determine the origin of the ROECE. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. The ⁸⁷Sr/⁸⁶Sr values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative δ¹³C excursion in the late Toyonian is globally associated with the ROECE, which is widely thought to have resulted from extinctions caused by a transgression. However, a transgression is not shown in the ⁸⁷Sr/⁸⁶Sr profile of Well HS2, which we attribute to local karst formation and erosion caused by frequent regional regressions. The interrelationships among several paleoenvironmental proxies show that continental weathering was climatically controlled, which affected the supply of terrigenous detritus, and that the carbonate platform was semi–restricted. Furthermore, profiles of paleoenvironmental proxies indicate that the environment was warm and humid, favoring high biological productivity and diversity during Interval Ⅰ (early Toyonian), and that it gradually became colder and drier during Interval Ⅱ (middle Toyonian). We suggest that by the time of Interval III (late Toyonian), the adverse effects of a cold and arid environment were superimposed on the transgression, triggering the ROECE. Our results provide a reference and possible interpretation for other carbon isotope excursion events.
... Compiling a radiometric chronology of diagenetic events in ancient carbonate archives is a prerequisite for complex carbonate archives. Platform-or basin-wide events, contact metamorphism (Kaufman et al., 1991;Melezhik et al., 2003;Holness and Fallick, 2004;Laskar et al., 2016), or orogenic and other far-field tectonic phases may be recorded in the cross-cutting relationships of cemented veins and their geochemical data (Burley et al., 1989;Campbell et al., 2002;Guo et al., 2016;Dong et al., 2017). A given regional or even local tectonic framework may respond to tectonic processes > 1000 km away, such as the opening of the Proto-Atlantic from the Early Jurassic to the Early Cretaceous Mueller et al., 2020;Burisch et al., 2022), and these events may, in turn, be recorded in proxy data. ...
... Neoproterozoic n/a X X X n/a n/a X n/a X X X Kamber and Webb, 2001 Neoarchean n/a n/a n/ a n/a n/a n/a n/a n/a n/a X X Kennedy, 1996 Neoproterozoic n/a X n/ a n/a n/a n/a n/a n/a X X n/a Kaufman et al., 1991 Neoproterozoic n/a X X n/a n/a n/a n/a n/a X n/a n/a Burley et al., 1989 Jurassic n/a X X X n/a n/a X n/a n/a n/a n/a ...
Carbonate archives record a brief snapshot of the ambient Earth’s surface conditions at their deposition. However, the geologically reasonable extraction and interpretation of geochemical proxy data from ancient, diagenetically altered rock archives is fraught with problems. Three issues stand out: the dichotomy between petrographic and geochemical alteration; the lack of quantitative age constraints for specific diagenetic phases resulting in a poorly constrained admixture of local, basin-wide and over-regional (far-field) features; and an often insufficient understanding of the temperatures and compositions of diagenetic fluids. Here, the archive of Devonian marine limestones exposed to multiple far-field diagenetic events is used as an example to explore the above-listed issues. Methods applied include petrography, U-Pb dating, micro XRF, fluid inclusion data, clumped isotopes, δ13C and δ18O isotopes, 87Sr/86Sr ratios and quartz trace element data. Devonian limestones studied here were overprinted by two cross-cutting regional fault zones (T ≈ 230 °C) by multiple events between the Variscan Orogeny and the late Paleogene. The following processes are recorded: (i) protolith deposition and partial dolomitisation during rapid burial in the Middle/Late Devonian (T ≈ 180 °C); (ii) deep burial to ca 6.5 km and tectonic/hydrothermal overprint during the Variscan Orogeny in the Carboniferous (T ≈ 90–230 °C); (iii) rapid uplift to 1–2 km burial depth at the end of the Variscan Orogeny and hypogene karstification (T ≈ 50 to 100 °C) initiated by regional geology in the Permian/Triassic; (iv) tectonic/hydrothermal overprint during the opening of the Proto-Atlantic Ocean between the Early Jurassic and the Early Cretaceous (T ≈ 50 to 130 °C); (v) tectonic/hydrothermal overprint including renewed hypogene karstification and hydrothermal calcite cement precipitation (T ≈ 50 to 180 °C) during Alpine Orogeny between the Late Cretaceous and late Paleogene. Despite this complex series of diagenetic events, the protolith limestones largely preserved their respective Middle/Late Devonian dissolved inorganic carbon (DIC) and 87Sr/86Sr signatures. This study documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over-regional context, significantly increases the ability to extract quantitative information from ancient carbonate rock archives. Research shown here has wider relevance for carbonate archive research in general.
... The Nama Group has been the subject of multiple chemostratigraphic studies, which have included carbonate carbon and oxygen isotopes (δ 13 C and δ 18 O; Kaufman et al., 1991;Nelson et al., 2022;Ries et al., 2009;Saylor et al., 1998;Wood et al., 2015), sulfur isotopes (δ 34 S; Ries et al., 2009;Tostevin et al., 2017), calcium isotopes (δ 44 Ca; Tostevin et al., 2019a), uranium isotopes (δ 238 U; Tostevin et al., 2019b), iron speciation (Bowyer et al., 2020;Darroch et al., 2015;Sperling et al., 2015;Wood et al., 2015), phosphorus speciation (Bowyer et al., 2020), total organic carbon (TOC; Bowyer et al., 2020;Darroch et al., 2015;Sperling et al., 2015;Wood et al., 2015), and cerium anomalies (Tostevin et al., 2016). However, the resolution is generally too coarse to permit direct comparison of chemostratigraphic variations with smalland medium-scale sequences or to perform cyclostratigraphic analysis. ...
... The δ 13 C of the Nama Group displays globally-recognized trends for the late Ediacaran, suggesting that the Nama Basin was connected to the open ocean (Kaufman et al., 1991;Saylor et al., 1998). The lower Kuibis Subgroup records a recovery from strongly negative δ 13 C values, termed the basal Nama excursion (BANE; Bowyer et al., 2022), while values in the Urusis Formation remain relatively constant between ~0-3‰ (Ries et al., 2009;Saylor et al., 1998). ...
... Then, the mixed water was conveyed to continental shelves and intracratonic basins due to sealevel rise. Therefore, the cap carbonates were formed extensively within the oceanic basins (Grotzinger & Knoll, 1995;Kaufman et al., 1991;Knoll et al., 1996;Rothman et al., 2003). (c) The gas hydrate destabilization model considered that warming or submarine permafrost instability triggered local methane destabilization and release during deglaciation. ...
... However, there are still some disputes over the major contributing factors to negative δ 13 Ccarb excursion in the Ediacaran cap carbonate, and multiple hypotheses have been proposed (Fabre & Berger, 2012;Font et al., 2010;Grotzinger & Knoll, 1995;Hoffman et al., 1998Hoffman et al., , 2017Higgins & Schrag, 2003;Jiang, Kennedy, & Christie-Blick, 2003;Kaufman et al., 1991;Knoll et al., 1996;Kennedy et al., 2001Kennedy et al., , 2008Myrow et al., 2018;Nédélec et al., 2007;Retallack, 2011;Rothman et al., 2003;Shields, 2005;Shields et al., 2007;Spence et al., 2016;Wallace et al., 2019;Wang et al., 2008;, and sedimentary environment during cap carbonate precipitation (Hoffman et al., 2007Hoffman & Macdonald, 2010;Jiang, Kennedy, et al., 2006;Kuang et al., 2019Kuang et al., , 2022. Nevertheless, one of the most important questions why was the deposition of dolostone rather than limestone after post-Marinoan glaciation was rarely mentioned in previous researches. ...
Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post‐glacial warm environment. Some consensuses on the distinctive negative excursion of δ ¹³ Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong‐Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong‐Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post‐Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygen‐rich condition with syn‐depositional or pene‐contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow‐ to deep‐water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep‐water basinal condition, rather than within the entire oceanic basin.
... However, as 87 Sr/ 86 Sr values of carbonates are susceptible to the diagenesis, altered samples should be excluded (Marshall, 1992;Derry et al., 1994). Elements and their ratios not only can reflect the paleoenvironmental fluctuations (Riquier et al., 2006;Tribovillard et al., 2006;Algeo and Rowe, 2012;Algeo and Liu, 2020) but also can be used for determining the diagenesis (Kaufman et al., 1991;Derry, 2010). Therefore, records of 87 Sr/ 86 Sr and elements are suitable for further restricting the sedimentary environment during the latest Ediacaran (~551-542 Ma). ...
... Diagenetic processes can change the primary geochemical signatures of marine carbonates, further affecting the analysis of the coeval seawater (Kaufman et al., 1991;Derry et al., 1994). Elemental proxies have been widely used to evaluate the influence Frontiers in Earth Science frontiersin.org ...
The Ediacaran–Cambrian period witnessed episodic extinctions, oxygenation of seawaters, Cambrian explosions, and tectonic events. However, compared with the various high-resolution geochemical records of the early–middle Ediacaran and Cambrian, the available geochemical record of the latest Ediacaran (551–542 Ma) is scarce (especially the strontium isotope and elements), which leads to the ambiguous interpretation of the paleoenvironment of the latest Ediacaran. Therefore, we conducted measurements of strontium isotopes and elemental content of a continuous series of carbonate samples from the Dengying Formation of Well PT1, located in the Sichuan Basin, southeastern Tibetan Plateau, in order to constrain the paleoenvironment of the latest Ediacaran. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. Our analyses show that the environment and geochemical records of the seawater were controlled by tectonic activities, especially the Gondwana assembly. The global strontium isotope correlation indicates that the Sichuan Basin was a restricted basin (high ⁸⁷Sr/⁸⁶Sr values, ∼0.7090), which can be attributed to the existence of a submarine high. Under the background of oxic environment, there were two episodes of anoxic expansion. During the initial stage, the stable terrigenous detrital input and oxic environment provided the prerequisite for the emergence of aerobic organisms in the restricted platform. Then, the decreasing sea level and intense tectonic activities improved the terrigenous detrital input with higher ⁸⁷Sr/⁸⁶Sr values (∼0.7095), which stimulated the emergence of aerobic organisms, further resulting in the first episode of anoxic environment. Lastly, a global transgressive resulted in a high sea level, and thus, the Sichuan Basin changed to an open platform. The exchange with extensive oceans led to the increased paleoproductivity, which consumed oxygen and nutrients, further resulting in the second episode of anoxic environment. Thus, the restriction degree, eustatic variations, and the terrigenous detrital input affected the biological evolution and redox conditions.
... Since their early insights on basin developments and conditions in the middle of the last century, these isotopes ratios are extensively used as geochemical proxies and basin evolution markers with severe consequences to climate, environment and Earth's dynamic reconstructions (e.g. Knoll et al., 1986;Kaufman and Knoll, 1991;Hoffman et al., 1998;Walter et al., 2000;Santos et al., 2000;Bartley et al., 2001;Azmy et al., 2008;Knauth and Kennedy, 2009;Frimmel, 2009Frimmel, , 2010Halverson et al., 2005;Teixeira et al., 2017;Caxito et al., 2012;McKenzie et al., 2014;Madhavaraju et al., 2016;Uhlein et al., 2019;Alvarenga et al., 2019). ...
... However, high Mn concentrations are also attributed to diagenetic changes (Kaufman and Knoll, 1995;Derry, 2010). In unaltered samples, Mn values are usually low (Kaufman et al., 1991). ...
Rocks of the Vazante Group in the Morro Agudo region (SE-Brazil) integrate a thick clay-carbonate sequence composed of twelve sedimentary facies and four facies associations (FAs), deposited in a mixed marine passive margin environment. This work carried out systematic isotope analyses of δ¹³C and δ¹⁸O on the two upper stratigraphic units of the Vazante Group (Morro do Calcário and Serra da Lapa formations) and on the Serra do Landim Formation. Despite the pervasive secondary dolomitization and hydrothermal overprint, most samples can be classified as least-altered dolostone, with some exceptions. Samples of Serra do Landim Formation are the ones with the highest degree of alteration, together with a small group of dolomites collected in a sphalerite-mineralized zone, at the top of the Morro do Calcário Formation. The isotopic profiles and the facies analysis showed the Morro do Calcário Formation presents homogeneous values of δ¹³C, and the Serra da Lapa Formation negative values of δ¹³C at the base, upwards inflected towards positive values. These coupled data suggest a significant change of environment in the depositional setting and conditions between these two formations, passing from calmer and stable (Morro do Calcáreo) to energetic and disturbed (Serra da Lapa). The strong negative values of δ¹³C, namely Lapa anomaly, are associated with diamictite facies below thick layer of carbonates, suggesting the influence of the Sturtian glacial event, in the lower Serra da Lapa Formation. The Lapa anomaly is the best chemostratigraphic marker throughout the sequence and guides the regional correlation of the Vazante Group. The basal diamictites of the Serra da Lapa Formation include dolomitic fragments of Morro do Calcário facies, indicating an intra-basinal source. Based on the isotopic profile correlation, it is inferred that the upper units (Morro do Calcário and Serra da Lapa formations) of the north and south portions of the Vazante Group were deposited in the same basin, or in connected basins, possibly separated, in part, by the Rio Escuro basement High.
... F4 that caps several of the measured sections throughout Farm Hansburg represents the Mooifontein Member of the Kuibis Subgroup (Grotzinger and Miller, 2008;Vickers-Rich et al., 2013; see section 4.3.1 below) based on the distinctly positive δ 13 C values (Figs. 1, 5, 7, and Supplementary information;Kaufman et al., 1991;Grotzinger et al., 2000). We interpret the depositional environment as a slightly lower energy than the rest of F4 based on the limited mud content above the base of the bed. ...
... The Mara has distinctly negative δ 13 C values, while the Mooifontein values are significantly more posi Maloney et al. Palaeogeography, Palaeoclimatology, Palaeoecology xxx (xxxx) xxx-xxx tive (Figs. 1, 5, 7;Kaufman et al., 1991;Grotzinger et al., 2000). Fig. 7). ...
Ediacaran fossils from the Nama Group (Kuibis Subgroup) of southern Namibia have a long history of scientific scrutiny, however many of the fossil localities still require investigation from a sedimentary facies and sequence stratigraphic standpoint. Detailed sedimentary analyses utilizing chemostratigraphy and facies-based approaches resulted in five proposed facies and two paleoenvironmental settings that allow for interpretation of the fossiliferous sections as a nearshore to a protected shallow marine paleoenvironment. The classic Ediacaran taxon Ernietta was restricted to the Kliphoek Member of the Nama Group, which limits the stratigraphic range of these organisms to younger depositional system of the Kuibis Subgroup. The paleoenvironment has been interpreted as a mixed carbonate-siliciclastic, protected shallow marine environment at Farm Hansburg, suggesting that these organisms thrived in environments with medium to high flow velocities and periodic clastic sediment supply. Studies that utilize detailed paleoenvironmental reconstructions may aid in constraining phylogenetic affinities of the Ediacara biota by placing reasonable bounds on the local habitat.
... The oceanic overturn model was the first attempt to account for cap carbonates in the context of the SEH (Kirschvink, 1992). In this model, global glaciation led to a stagnant anoxic deep ocean under the sea ice, and cap carbonates formed during deglaciations in response to overturn of stratified oceans, mixing high-alkalinity anoxic deep waters into an oxic ocean-surface layer, and to eustatic rise, introducing such waters to continental shelves and intracratonic basins (Kaufman et al., 1991(Kaufman et al., , 1997Grotzinger and Knoll, 1995;Knoll et al., 1996). Alkalinity (i.e., HCO 3 − and CO 3 2− ) was generated through organic matter oxidation either by MSR in the deep watermass during glaciations (Grotzinger and Knoll, 1995;Knoll et al., 1996) or in the oxic surface layer during deglacial overturn (Rothman et al., 2003). ...
... The oceanic overturn model accounted for some key features of cap carbonates, such as their negative δ 13 C excursions, through oxidation of 13 C-depleted organic matter (Kaufman et al., 1991;Hoffman et al., 1998). The model also explained concurrent deposition of banded iron formations (BIFs) in shallow-water environments through upwelling of Fe 2+ ions from the ferruginous deep ocean (Grotzinger and Knoll, 1995). ...
... Most, if not all, carbonate sediments/rock have undergone varying degrees of diagenetic alterations. However, the extent to which diagenesis has affected these primary geochemical signals is widely debated Banner and Hanson 1990;Veizer 1980, 1981;Denison et al. 1994;Derry et al. 1992 Jacobsen and Kaufman 1999;Kaufman et al. 1991;Kaufman and Knoll 1995;Knauth and Kennedy 2009;Marshall 1992;Nabelek 1987;Swart and Eberli 2005;Swart and Kennedy 2012). A number of factors can dictate the ultimate chemical compositions of diagenetic carbonates, e.g., carbonate mineralogy, fluid compositions, fluid/rock ratio, trace element distribution coefficients, isotopic fractionation factors. ...
... A number of factors can dictate the ultimate chemical compositions of diagenetic carbonates, e.g., carbonate mineralogy, fluid compositions, fluid/rock ratio, trace element distribution coefficients, isotopic fractionation factors. However, it generally agrees, or at least in many cases, that (1) increasing levels of diagenesis will lead to the gain of Mn and loss of Sr, and (2) the δ 13 C is less susceptible to diagenetic alterations than the δ 18 O and 87 Sr/ 86 Sr, especially when diagenesis is related to fluid-rock interaction in a closed system Banner and Hanson 1990;Veizer 1980, 1981;Derry et al. 1992;Husson et al. 2015;Jacobsen and Kaufman 1999;Kaufman et al. 1991;Kaufman and Knoll 1995;Marshall 1992;Siahi et al. 2018). Our previous investigation on bulk carbonate from section 13ZS (Zhang et al. 2017) showed that (1) LBF fragments and micritic parts has very weak luminescence; (2) Bulk carbonate has relatively high Sr concentrations (mostly between 800 and 1400 ppm) and generally low Mn concentrations (~40-200 ppm, mostly < 70 ppm); (3) Mn/Sr ratios (< 0.2) in bulk carbonate are much lower than the empirical guideline (Mn/Sr < 1-2) indicative of good preservation of primary δ 13 C signals in ancient carbonate rock; (4) No evident correlation between Mn/Sr ratios and δ 13 C values can be observed in bulk carbonate; (5) 87 Sr/ 86 Sr ratios from bulk carbonate still preserve the primary signal of contemporary sea water in the open ocean. ...
The Paleocene-Eocene thermal maximum (PETM) was an extraordinary pulse of global warming that left an indelible mark on the Earth approximately 56 Ma ago. This warming event is associated with an addition of large amounts of 13 C-depleted carbon into the atmosphere-ocean system, but the magnitude of the negative carbon isotope excursion (CIE) signaling the PETM onset and often used to estimate mass of the released carbon is still debated. Here we gauge the CIE magnitude through the use of secondary ion mass spectrometry (SIMS) to perform in situ δ 13 C measurements within individual larger benthic foraminifera preserved in a tropical shallow-marine limestone section at Tingri, south Tibet. This SIMS-based δ 13 C record yields a CIE (Δ~7‰) comparable in magnitude to that registered by some terrestrial PETM records but larger than the ~4‰ CIE returned by surface-dwelling planktonic foraminifera in deep-sea records. We posit that the CIE magnitude in the surface ocean and atmosphere was ~7‰, and that previous ~4‰ estimates are attenuated by incomplete preservation and/or diagenetic overprinting. Mass balance calculations indicate that the released carbon mass during the CIE would not exceed 28,000 petagrams, given that the carbon was sourced from organic matter, permafrost, thermogenic methane, methane hydrate, or any of their combinations. Our study demonstrates that δ 13 C records from some shallow-marine carbonate sections can avoid strong diagenetic alteration, preserving primary signals of deep-time carbon perturbations.
... However, studies employing isotopic proxies suggest that meteoric diagenesis or fluid-mediated alteration was not a prevalent phenomenon in Otavi platform carbonates. Kaufman et al. (1991) arrived at this conclusion by comparing δ 13 C values in microspar (a less altered phase) with those of the whole rock, which showed a nearly 1:1 ratio. Similarly, Halverson et al. (2002) reached the same conclusion in the Ombaatjie Formation (pre-Ghaub) after examining δ 18 O and δ 13 C values along with the Mn/Sr ratio. ...
In this study, we investigate the paleomagnetism of the basal Maieberg Formation (Namibia) cap carbonate sequence to elucidate its magnetic properties and paleolatitude of deposition, establish global correlations, and contribute to the understanding of Snowball Earth postglacial dynamics. Two distinct magnetization components, C1 and C2, were identified. C1 is interpreted as a depositional or post-depositional remanent magnetization carried by detrital pseudo-single domain (PSD) magnetite, while the C2 component is a thermochemical remanent magnetization carried by fine authigenic single domain (SD)−PSD magnetite. The deposition paleolatitude provided by C1 is 33.3° ± 3.2°, which gives an initial quantitative approximation of the paleolatitude for the underlying Marinoan Ghaub diamictites. The thickness of the Keilberg Member cap dolostone is anomalously high for the paleolatitude calculated with C1, which suggests that other factors besides the influence of the paleolatitude on carbonate oversaturation may have influenced the sedimentary production of cap dolostones and the overall thickness of the flooding cap carbonate sequence. Possible explanations could include the influence of alkalinity input combined with local tectonic subsidence during a long glacial period with unusually low sedimentation rates, which appear to be in a favorable configuration for the substantial thickness of the Keilberg Member. Paleomagnetic field reversals at the Keilberg cap dolostone and analogous units globally suggest a longer duration of marine transgression than energy-balance deglaciation models and sedimentological-geochemical observations have constrained. Factors such as ocean warming, thermal expansion, and local glacio-isostatic adjustments imply extended marine transgressions beyond the deglaciation period. Still, magnetostratigraphic estimates for postglacial transgressive sequences require longer time scales by a factor of five or more. Thus, the conflict arising between estimates derived from paleomagnetic data and the constraints imposed by climate physics underscores uncertainties regarding an unconventional field state or a remanence acquisition mechanism within these cap carbonates that is not fully understood. Importantly, if such a phenomenon proves to be primary and global, the widespread occurrence of these stratigraphically compressed reversals would support the precise temporal correlation between Marinoan cap dolostones. The C2 pole correlates with Cambrian remagnetization poles observed in carbonates from West Gondwana, which now extend to the Congo craton. The remanence acquisition of C2 likely stems from diagenesis-related low-temperature authigenic magnetite formation after the conversion of iron-rich smectite to iron-poor illite. Cooling associated with the Kaoko orogen’s exhumation and tectonic uplift possibly locked the magnetic system at ca. 520 Ma, supported by the C2 pole position on the West Gondwana apparent polar wander path, although other explanations remain valid.
... The Nama Group of southern Namibia and northwest South Africa is one of the most important successions for the calibration of terminal Ediacaran to lower Cambrian chronostratigraphy (Figs. 2, S2, S3;Kaufman et al., 1991;Grotzinger et al., 1995;Saylor et al., 1995Saylor et al., , 1998Bowring et al., 2007;Wood et al., 2015;Linnemann et al., 2019;Nelson et al., 2022). Here, mixed carbonate and siliciclastic deposits of the lower Kuibis Subgroup stratigraphically underlie an ash bed dated at 547.36 ± 0.23 Ma (Bowring et al., 2007 et al., 1998;Boggiani et al., 2010;Wood et al., 2015;Parry et al., 2017;Yang et al., 2021;Bowyer et al., 2022). ...
We present new stratigraphic and fossil occurrence information from a previously undescribed section of the Nama Group that outcrops in the Namib Desert of southwest Namibia. This section records the oldest confirmed regional occurrence of tubular skeletal metazoan fossils of the Nama assemblage. We also present an updated and revised global age model that shows the current understanding of global biostratigraphic information for the mid-late Ediacaran and lower Cambrian.
... Global temporal equivalents for the Maieberg δ 13 C excursion show depletion in 13 C relative to typical seawater values of 0.0 ‰ (Halverson et al. 2010). The evidence of the excursion is also found in C isotopic data reaching an average nadir between -5 to -7 ‰, which agree with the ranges reported in previous studies of the Marinoan (Halverson et al. 2002;Hoffman et al. 1998;Hoffman and Maloof 1999;Kasemann et al. 2005;Kaufman et al. 1991). ...
The "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth’s surface was covered in ice. Paleoenvironmental reconstructions of these events utilize isotopic systems, such as Δ¹⁷O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ⁶⁶Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates’ Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production.
... near the Precambrian/Cambrian boundary). Both "glacial" events correspond with low δ 13 C isotope ratios (Kaufman et al., 1991). Some volcanic tuffs occur in the Kuibis and Schwarzrand Subgroups (Germs, 1972a(Germs, , 1974. ...
Provenance in Neoproterozoic to Early Paleozoic successions in SW Gondwana
... This is consistent with previous work that reports δ 13 C org values in levée deposits elsewhere at Castle Creek ranging from −18.0 to −28.5‰ (Davis, 2011; see also Table 2), and confirms that all OM is marine in origin. It is important to note that thermal alteration may have altered the isotopic signature of primary organic carbon (McKirdy & Powell, 1974;Hayes, 1983;Kaufman et al., 1991;Des Marais, 2001), but in this case (burial diagenesis and low-grade metamorphism) would have increased the δ 13 C org signatures of these strata by at most a few per mil (Arthur et al., 1983;Hayes, 1983Hayes, , 1993Des Marais, 2001;Razvozzhaeva et al., 2007;Osterhout et al., 2019), and therefore insufficient to alter the results and interpretations in this study. ...
... This is consistent with previous work that reports δ 13 C org values in levée deposits elsewhere at Castle Creek ranging from −18.0 to −28.5‰ (Davis, 2011; see also Table 2), and confirms that all OM is marine in origin. It is important to note that thermal alteration may have altered the isotopic signature of primary organic carbon (McKirdy & Powell, 1974;Hayes, 1983;Kaufman et al., 1991;Des Marais, 2001), but in this case (burial diagenesis and low-grade metamorphism) would have increased the δ 13 C org signatures of these strata by at most a few per mil (Arthur et al., 1983;Hayes, 1983Hayes, , 1993Des Marais, 2001;Razvozzhaeva et al., 2007;Osterhout et al., 2019), and therefore insufficient to alter the results and interpretations in this study. ...
Deep‐marine levées are areally extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial and preservation. Although modern deep‐marine levées have been shown to sequester a large proportion of the world's total buried organic carbon, few studies have attempted to assess carbon deposition and preservation in ancient deep‐marine levée deposits. Observations of well‐exposed levée deposits of the Neoproterozoic Windermere Supergroup in British Columbia, Canada, have shown that depositional processes in levées can result in the concentration and enrichment of sedimentary marine organic matter. In contrast to many previous studies where organic‐rich strata occur as black shales formed in anoxic conditions, organic matter in this study occurs mostly in banded, mud‐rich sandstones deposited under oxic conditions. Scanning electron microscopy shows that this organic carbon occurs primarily as nano‐scale coatings on clay particles, but also as uncommon sand‐sized organomineralic aggregates and discrete sand‐sized amorphous grains. As flows overspilled the channel margins the rapid collapse of the turbulent suspension resulted in elevated rates of sediment fallout that promoted aggregation of organic matter and clay particles, increasing cohesive forces in the flow, and ultimately en masse deposition. Rapid burial plus association with clay mineral surfaces prevented organic matter degradation, thereby effectively sequestering significant amounts of carbon in the sediment and therefore a net sink for atmospheric CO2. However, the mechanisms and efficiency by which organic matter is buried and preserved on geological timescales is influenced by many factors, including glacial, sea level and tectonic cycles. This work elucidates the fundamental physical and chemical processes that control organic matter accumulation and preservation in deep‐marine levées, and how these processes have evolved throughout geological time.
... 54 Fe) in the reduced Fe phase (Craddock and Dauphas, 2011a;Johnson et al., 2008a;McCoy et al., 2017), and the distinction of kinetic effects associated to mineral precipitation (Jiang et al., 2022;Jiang and Tosca, 2019). Carbon isotope composition in carbonates is easily measured in iron formations through gas extraction via reaction of whole-rock powders with orthophosphoric acid (Becker and Clayton, 1972;Busigny et al., 2013;Fischer et al., 2009;Garcia et al., 2021;Kaufman et al., 1991;Lebeau et al., 2014). In contrast, the analysis of Fe isotope composition of distinct Fe-mineral phases such as carbonates, as well as oxides (e.g. ...
The iron isotope composition of iron-bearing carbonates is commonly used to obtain insights into ancient environmental conditions. However, it is often challenging to target only Fe‑carbonates (e.g. siderite and ankerite) from samples containing a variety of other Fe-bearing minerals, such as observed in Precambrian iron formations. Chemical extraction (i.e. leaching) methods of Fe‑carbonates could be an alternative to in-situ measurements and/or micro-drilling techniques applied to isotopic studies. Yet, only a few studies have looked at the effects of leaching carbonates (e.g. partial and/or total dissolution) on their Fe isotope composition. Here, we tested several leaching protocols, using 5 to 20% acetic acid (HAc) and 0.4 M HCl, on a siderite standard and three natural samples, including an iron formation, Fe-rich and Fe-poor carbonates. We showed that carbonate mineralogy has a strong control on how much of each mineral phase was being dissolved, and that variations in HAc concentration from 5% to 20% are less likely to change how much siderite dissolves (e.g. ~30% dissolution) under a 12 h period at room temperature. Importantly, the Fe isotope composition of partially dissolved siderite had indistinguishable values within error from the whole-rock composition (i.e. complete dissolution) as shown with HAc and HCl attacks. Carbonates from the three natural samples were almost completely dissolved under the same protocol with 5 to 20% HAc, while 0.4 M HCl attacks dissolved additional mineralogical phases, which might contribute to the Fe leachate. Moreover, the iron isotope composition of carbonate leachates was preserved without generating anomalous results. Hence, weak chemical leaches represent a reliable tool to study Fe isotopic composition of carbonate to understand how the Fe cycle was operating throughout Earth's history.
... Based on the above equations, one can infer the fractional burial rate of organic carbon (f org ) for specific δ 13 C carb excursions, assuming that δ 13 C in and ε p remain largely invariant. Within this framework, positive δ 13 C carb excursions have typically been interpreted as resulting from a higher rate of organic carbon burial, therefore representing an atmospheric oxygenation event (Broecker, 1970;Scholle and Arthur, 1980;Veizer et al., 1980;Knoll et al., 1986;Baker and Fallick, 1989;Kaufman et al., 1991;Des Marais et al., 1992;Karhu, 1993;Karhu and Holland, 1996;Kaufman et al., 1997;Shields, 1997;Shields et al., 1997;Cui et al., 2018); conversely, negative δ 13 C carb excursions have often been interpreted as resulting from a lower rate of organic carbon burial and/or enhanced oxidative weathering event Kump et al., 2011). ...
The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ¹³Ccarb) values from +6‰ down to ca. –10‰, representing the largest δ¹³Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ¹³Ccarb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ¹³Ccarb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (μXRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of δ¹³Ccarb in authigenic calcite cements, including extremely negative values down to –37.5‰ (VPDB). We interpret these calcite cements as methane-derived authigenic calcite (MDAC) resulting from microbial sulfate reduction (MSR) and anaerobic oxidation of methane (AOM) during early diagenesis. Based on the new results, we propose that the heterogeneous SE in South China — manifest on micrometer, centimeter, and basinal scales — was modulated by methane oxidation under variable local redox and water depth conditions. The SE, therefore, was coupled with different degrees of methane oxidation in individual basins, and globally triggered by enhanced seawater sulfate during an atmospheric oxygenation event. In light of this study, the potential role of redox variability in methane oxidation during the SE may have been underestimated. Our study demonstrates that integrated SIMS-SEM analysis can distinguish different generations of isotopically distinct carbonates otherwise undetected by conventional analysis, and is thus an effective approach to assess the origin and diagenetic history of δ¹³Ccarb anomalies in the sedimentary record.
... These are the Sturtian or Middle Cryogenian glaciation (750-710MY), Marionoan or Late Cryogenian glaciation (630MY) and Gaskiers or Moelv/Biskop or Late Ediacaran glaciation (570MY) (Knoll, 2000;Knoll et al., 13 2004). The two major -ve ä C anomalies within the Rohtas are correlated to some significant geological events worldwide, the lower one may represent the third and last Neo-Proterozoic glaciation and the lowermost negative excursions of isotope values (down to -5 to 7‰) is considered to be associated with cap carbonates and may represent the PC-C (Fig. 22, Jacobsen and Kaufman, 1999 (Brasier et al., 1990;Burdett et al., 1990;Kaufman et al., 1991;Veizer et al.,1992;Hall and Veizer,1996). Alternately, the uppermost -ve excursion may also represent an unconformity surface at the Rohtas top that may lead to profuse meteoric diagenesis as a result the -ve excursion of isotope values are observed. ...
The present paper describes an integrated and
comprehensive evaluation of Rohtas Limestone, by
subdividing it into different units/zones by
multidisciplinary approach (i.e. sedimentology,
geochemistry and electrologging). Sedimentological
analysis revealed that Rohtas Limestone is
broadly divided into three lithounits with an
argillaceous-rich middle lithounit formed within a
transgressive phase in between the carbonate-rich
lower and upper units developed in response to a
punctuated progradation of the relative sea-level
that are well documented and correlated in 10 wells
covering the entire Damoh-Nohta-Jabera PEL
area. An attempt has been made to further subdivide
these three lithounits based on electrolog and
chemo-stratigraphic studies.
The carbonates of upper and lower units are
micritic in nature showing varied degree of
aggrading neomorphism of a continuous spectrum
of the end members micrite and microsparite and
are also selectively dolomitized with degree of
dolomitization more in the lower unit (max. 23%) as
compared with upper (8%) depicting effects of
burial diagenesis. The reservoir in general is tight
with minor micritic porosity and occasional
presence of secondary porosity due to
dolomitization and in form of partially open
tensional cracks/ fractures. The degree of
compaction of micritic matrix has also resulted
occurrences of low and high amplitude stylolamination.
The 3 broad lithounits have been
further subdivided into 10 electrofacies on the basis
of log signatures, viz. 5 within the upper, 2 within the
middle and 3 within the lower unit. Fracture
analysis revealed two fracture-sets with the
dominant NE-SW trend indicating presence of two
mutually perpendicular stress directions.
Isotopic trends of ä13C and ä18O along with major
and trace elements were analyzed to subdivide this
formation on the basis of chemo-stratigraphy that
have subdivided into different chemo-stratigraphic
zones which correlates with the electrofacies and
facies units. The two prominent negative excursions
of isotope values observed within the Rohtas
Limestone are correlated with some major
worldwide geological events. The lower negative
excursion of isotope values (down to -5 to -7‰) is
considered to be associated with cap carbonates
immediately overlying the third and last Neo-
Proterozoic glaciation events (Biskhop glaciation)
whereas the upper negative shift represents close to
the unconformity surface and is comparable with ä 13C
global curve present close to the Precambrian-
Cambrian boundary. Thus both the independent
studies suggest Ediacaran to basal Early Cambrian
age for the Rohtas Limestone.
... At present, the age of 80 the Ediacaran-Cambrian boundary is 541.0 ± 1.0 Ma (ICC 2021), however the radiometric age of 81 a tuff deposit in the Nama Group, Namibia, on the Kalahari Craton, provides a current best estimate 82 of 538.8 Ma for the maximum age of the first appearance of T. pedum (Linnemann et al., 2019; 83 Xiao and Narbonne, 2020). 84 The carbon isotopic composition of marine carbonates (δ 13 Ccarb) is most commonly considered 85 to reflect secular changes in the ratio of 13 C to 12 C in seawater that are associated with changes in 86 the relative export/burial rates of inorganic versus organic carbon (Kaufman et al., 1991;Keith 87 and Weber, 1964; Veizer et al., 1980;Veizer and Hoefs, 1976). As a result, secular δ 13 Ccarb profiles 88 have been used for regional and global correlation (Halverson et (Cui et al., 2020b). ...
The Ediacaran-Cambrian transition, which incorporates the radiation of animals, lacks a robust global temporal and spatial framework, resulting in major uncertainty in the evolutionary dynamics of this critical radiation and its relationship to changes in palaeoenvironmental geochemistry. We first present a new δ13Ccarb composite reference curve for the Ediacaran Nama Group of southern Namibia, and we then outline four new possible global age models (A to D) for the interval 551-517 million years ago (Ma). These models comprise composite carbonate-carbon isotope (δ13Ccarb) curves, which are anchored to radiometric ages and consistent with strontium isotope chemostratigraphy, and are used to calibrate metazoan distribution in space and time. These models differ most prominently in the temporal position of the basal Cambrian negative δ13Ccarb excursion (BACE). Regions that host the most complete records show that the BACE nadir always predates the Ediacaran-Cambrian boundary as defined by the first appearance datum (FAD) of the ichnospecies Treptichnus pedum. Whilst treptichnid traces are present in the late Ediacaran fossil record, the FAD of the ichnospecies T. pedum appears to post-date the LAD of in situ Cloudina and Namacalathus in all environments with high-resolution δ13Ccarb data. Two age models (A and B) place the BACE within the Ediacaran, and yield an age of ~538.8 Ma for the Ediacaran-Cambrian boundary; however models C and D appear to be the most parsimonious and may support a recalibration of the boundary age by up to 3 Myr younger. All age models reveal a previously underappreciated degree of variability in the terminal Ediacaran, incorporating notable positive and negative excursions that precede the BACE. Nothwithstanding remaining uncertainties in chemostratigraphic correlation, all models support a pre-BACE first appearance of Cambrian-type shelly fossils in Siberia and possibly South China, and show that the Ediacaran-Cambrian transition was a protracted interval represented by a series of successive radiations.
... The carbon isotopic composition of marine carbonates (δ 13 C carb ) is most commonly considered to reflect secular changes in the ratio of 13 C to 12 C in seawater that are associated with changes in the relative export/burial rates of inorganic versus organic carbon (Kaufman et al., 1991;Keith and Weber, 1964;Veizer et al., 1980;Veizer and Hoefs, 1976). As a result, secular δ 13 C carb profiles have been used for regional and global correlation (Halverson et al., 2010;Macdonald et al., 2013;Maloof et al., 2010;Yang et al., 2021;Zhu et al., 2007). ...
The Ediacaran-Cambrian transition, which incorporates the radiation of animals, lacks a robust global temporal and spatial framework, resulting in major uncertainty in the evolutionary dynamics of this critical radiation and its relationship to changes in palaeoenvironmental geochemistry. We first present a new δ¹³Ccarb composite reference curve for the Ediacaran Nama Group of southern Namibia, and we then outline four new possible global age models (A to D) for the interval 551–517 million years ago (Ma). These models comprise composite carbonate‑carbon isotope (δ¹³Ccarb) curves, which are anchored to radiometric ages and consistent with strontium isotope chemostratigraphy, and are used to calibrate metazoan distribution in space and time. These models differ most prominently in the temporal position of the basal Cambrian negative δ¹³Ccarb excursion (BACE). Regions that host the most complete records show that the BACE nadir always predates the Ediacaran-Cambrian boundary as defined by the first appearance datum (FAD) of the ichnospecies Treptichnus pedum. Whilst treptichnid traces are present in the late Ediacaran fossil record, the FAD of the ichnospecies T. pedum appears to post-date the LAD of in situ Cloudina and Namacalathus in all environments with high-resolution δ¹³Ccarb data. Two age models (A and B) place the BACE within the Ediacaran, and yield an age of ~538.8 Ma for the Ediacaran-Cambrian boundary; however models C and D appear to be the most parsimonious and may support a recalibration of the boundary age by up to 3 Myr younger. All age models reveal a previously underappreciated degree of variability in the terminal Ediacaran, incorporating notable positive and negative excursions that precede the BACE. Nothwithstanding remaining uncertainties in chemostratigraphic correlation, all models support a pre-BACE first appearance of Cambrian-type shelly fossils in Siberia and possibly South China, and show that the Ediacaran-Cambrian transition was a protracted interval represented by a series of successive radiations.
The Ediacaran-Cambrian radiation occurred over a protracted interval without global mass extinctions and with generally diachronous metazoan appearances.
... Previous studies of Paleozoic and Proterozoic carbonates have shown that δ 18 O is a very sensitive indicator of diagenetic processes; and trace element (Fe, Mn, and Sr) contents are also a powerful tool for assessing the degree of diagenesis (Banner and Hanson, 1990;Kaufman et al., 1991;Banner and Kaufman, 1994;Bartley et al., 2007). In this study, in addition to the carbon and oxygen isotopes measured in the six studied cap dolomite sections, the whole rock major and trace elements were measured in three sections (Gaoqiaohe, Guanmenshan, and Yazikou) to investigate the diagenetic processes in the Nantuo and Doushantuo formations. ...
Ediacaran cap dolomites are strong evidence for the glaciation during the Neoproterozoic. Stratigraphic-sedimentological studies combined with δ¹³C and δ¹⁸O isotope analyses are used for defining the processes of post-glacier environmental changes and sea-level rise caused by glacier melting and the reconstruction of the depositional environments for the Doushantuo cap dolomites in Shennongjia, northern Yangtze Craton (YC). Like many other cap dolomites in the YC and worldwide, those in Shennongjia exhibit strongly negative δ¹³Ccarb excursions, ranging from −7.3 to −2.5 ‰ (averagely from −5.4 to −4.1 ‰). A laminated clast-free calcareous siltstone commonly occurs between the cap dolomite and the underlying diamictites, indicating a low-energy, shallow-water depositional environment, wherein alkaline conditions developed before the deposition of the cap dolomite. The most prominent features of the cap dolomites in Shennongjia are the very thin thickness and monotonous sedimentary structures. The maximum thickness of the cap dolomites from the six sections in Shennongjia is 1.8 m, and the thinnest cap dolomite is 0.4 m with simple laminations. Sedimentary structures such as low-angle cross-bedding, giant wave ripples, sheet-crack cement, and tepee breccias are rare in Shennongjia except in the area near the Three Gorges. These evidences, therefore, point to the deposition of the Shennongjia cap dolomite in restricted shallow-water tidal environments.
The space–time distribution of sedimentary facies in Shennongjia and the southeastern areas to Shennongjia indicate that the Ediacaran cap dolomite was formed on a ramp. Accordingly, we propose a sedimentary facies model for the formation of the cap dolomite, covering the inner and middle ramp facies. This model explains the thickness variations and distributions of the cap dolomites at both the local and regional scale, including their sedimentary structures, compositions, and barite occurrences. The model successfully illustrates why the cap dolomites in the Shennongjia area share a unique set of sedimentary features. Finally, the cap dolomite is interpreted as a diachronous deposit (bottom to top), tracking glacioeustatic flooding and recording ocean-wide changes over the time following deglaciation, and a correlation and cause of negative δ¹³Ccarb excursions in cap dolomites worldwide, particularly in those in the Yangtze Carton, are further discussed.
... However, these depleted δ 13 C values in the carbonates of the Dabis Formation, including the Mara Member, have also been interpreted as heralding the end of the Shuram excursion Wood et al. 2015). The excursion recorded in the Mara Member elsewhere (Kaufman et al. 1991;Wood et al. 2015) is not fully expressed in the sampled section, possibly because the Pockenbank area was on a palaeo-high compared with that of other localities that record the Shuram anomaly in full (see Vickers-Rich et al. 2016). Additional micropalaeontological investigation is needed to evaluate the extent of the association's range through the lower Nama Group and in relation to its chemostratigraphy. ...
New occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxon Granomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range of Granomarginata extends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence of Granomarginata in the Indreelva Member (< 563 Ma) of the Stáhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. Although Granomarginata is rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution of Granomarginata through the middle–late Ediacaran interval. A new flask-shaped microfossil Lagoenaforma collaris gen. et sp. nov. is also reported in horizons containing Granomarginata from the Stáhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. The Granomarginata – Lagoenaforma association, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences of Granomarginata add to a growing record of body fossil taxa spanning the Ediacaran–Cambrian boundary.
... Consequently, the organic carbon isotopic analysis of the sediments is generally used to reconstruct the palaeo-environmental settings. In marine OM, the discrepancies in the composition of carbon isotopes have also been used to interpret the environs (Arthur, Dean, & Pratt, 1988;Dean, Arthur, & Claypool, 1986) as well as for chemostratigraphic correlations (Kaufman, Hayes, Knoll, & Germs, 1991;Knoll, Hayes, Kaufman, Swett, & Lambert, 1986). In sedimentary organic carbon, the measured δ 13 C value shows a distinct pattern of 13 C enrichment (Figure 17). ...
In the Yangtze block, the Early Cambrian Niutitang Formation is mainly composed of mudstone, shale, and carbonates are important for investigating the depositional environment and evolution of the Early Cambrian rocks. The Niutitang Formation in the study area has a greater geologic interest due to its polymetallic beds, depositional age, variation in environmental conditions, Cambrian explosion, organic matter enrichment, algal boom, etc. This research represents the sedimentary geochemistry of the Early Cambrian Niutitang Formation to reconstruct the paleo-depositional environment and evaluate the organic matter (OM) enrichment mechanism by means of total organic carbon (TOC), biomarkers, carbon isotopes, mineralogy, scanning electron microscope, etc. Based on the variation in TOC content, the Niutitang Formation is divided into three parts (upper, middle, lower). The majority of the samples from the middle part of the Niutitang Formation exhibit an excellent source of hydrocarbon (TOC> 4.0 wt.%) relative to the upper and the lower part. The lighter carbon isotopic composition (<-30.7%) in these sediments reveals the presence of the I-amorphous kerogen group. Moreover, these lighter δ13Corg values suggest the presence of type-I oil-prone kerogen. Saturated hydrocarbon in these rocks showed the dominance of short-chain n-alkanes maximizing at C18. The predominance of these n-alkanes represents the OM is chiefly derived from algal/bacterial input. Similarly, the dissymmetric V shape of C27-C28-C29 steranes with a predominance of C27 and the higher values of Σn-C21-/Σn-C22+ in all three parts reflects the OM in these rocks are chiefly originated from the lower aquatic marine organisms. Based on the Pr/Ph, it is predicted the middle part of the Niutitang Formation was deposited in extreme anoxic conditions (Pr/Ph < 0.5), whereas the upper and the lower parts were deposited in relatively less anoxic conditions. Some biomarkers have more stable stereochemistry, which can't be affected by diagenetic processes. These stable configurations are utilized to measure the maturity of OM i.e., Ts/(Ts+Tm), C29ββ/(ββ + αα), C29αα20S/(20S + 20R) steranes and homohopane C31 22S/(22S + 22R). These geochemical indices reveal the Early Cambrian Niutitang Formation in the studied area is mature to the post-mature in the gas generation phase. Moreover, the hydrothermal fluids rich in metallic elements (e.g., Mo, Zn, V, U) from the deeper part of the earth owing to elongational forces amongst Yangtze and Cathaysian plate over Early Cambrian time, entered the oceanic basin via remnant features (fissure and cracks) and through upwelling phenomena interacted with shelf sediments. At the ocean's surface, these nutrient-rich fluids enhanced the breeding and evolution of marine life (bio-productivity), which created hypoxic water conditions suitable for the preservation of OM in these rocks.
... Secular variations in carbon isotopic composition of unaltered carbonates are sometimes effectively used to estimate apparent depositional ages (e.g. Melezhik et al., 2008;Halverson et al., 2010;Johnston et al., 2012;Halverson et al., 2005), High amplitude carbon isotope excursions have also been reported in Proterozoic meta-carbonates, especially a Paleoproterozoic positive carbon isotope excursion related to the Lomangudi-Jatuli event (2.3 to 2.1 Ga) and excursions related to Neoproterozoic global glaciation events (0.7-0.63 Ga) are notable (Des Marais et al., 1992;Frimmel, 2010;Gilleaudeau et al., 2018;Halverson et al., 2010;Kah et al., 1999;Kaufman et al., 1991;Knoll et al., 1986;Shields and Veizer, 2002). On the other hand, d 13 C V-PDB values of carbonates deposited in the Mesoproterozoic have a restricted range. ...
In this contribution we review the possibility of establishing the depositional age and tectonic settings of metamorphosed carbonate rocks from continental collision zones in the East African-Antarctic Orogen. The geochemical characteristics of regionally distributed meta-carbonate rocks from the Highland Complex (HC) in Sri Lanka are considered in detail and compared with similar occurrences in East Antarctica, India, Madagascar and Africa. The variations seen in the Highland Complex of Sri Lanka imply that carbonate deposition was younging from west to east, spanning apparent ages from Mesoproterozoic to Neoproterozoic. In the case of East Antarctica, such variations are within the Neoproterozoic, whereas in southern India, Madagascar and Mozambique they have a broader age range possibly from the Paleoproterozoic to Mesoproterozoic. There is also clear evidence that some carbonates were deposited in an open ocean surrounding volcanic islands in the Mesoproterozoic. Shale-normalized REE patterns have typical signatures of open ocean deposition in a passive continental margin with variable continental input in platforms nearby to island arcs. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other parameters such as (Pr/Yb)SN, (Pr/Tb)SN, and (Tb/Yb)SN were used to evaluate relative enrichments of the LREE, MREE and HREE fractions that are characteristic of ambient seawater. Pronounced La, and Y anomalies with minor Eu and Gd anomalies and correlations of REE parameters and anomalies with carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr initial ratios and εNd values are evaluated for meta-carbonate rocks in the Proterozoic collision zone. The εNd values and Sr initial ratios suggest that basins in the western Mozambique Ocean that separated the East Gondwana from West Gondwana received contributions from Archean continental crust and ambient seawater, whereas the eastern Mozambique Ocean had REE contributions from specific cratonic continents in passive margins or from continental/volcanic island arcs in active margins.
... A number of explanations have been proposed for the largeamplitude negative anomalies observed throughout the Ediacaran period as well as the aforementioned oxidation event that likely spawned Ediacaran metazoans. These include the destabilization of methane hydrates (Kennedy et al., 2001;Jiang et al., 2002Jiang et al., , 2006, sulphate reduction of dissolved organic carbon (DOC) during progradation (Rothman et al., 2003;Fike et al., 2006;Jiang et al., 2007), deep-marine seawater depleted in δ 13 C mixing and upwelling to the surface during deglaciation (Kaufman et al., 1991;Grotzinger and Knoll, 1995;Shields, 2005), diagenesis associated with eustatic sea-level changes (Derry, 2010, Oehlert et al., 2012Swart and Kennedy, 2012), an increase in the precipitation of CaCO3 Macdonald et al., 2013) and the release of stored carbon from organic-rich host rocks (Lee et al., 2015) (Cochrane et al., 2019). ...
... This transition in sampling medium is supported by studies that illustrate bulk carbonate and brachiopod data from identical sections show broadly similar δ 13 Ccarb values (e.g., Cramer et al., 2010a). Of course most of the Precambrian stratigraphic record lacks skeletal carbonate components altogether and bulk carbonate must be used for analysis (e.g., Schidlowski et al., 1975;Knoll et al., 1986;Kaufman et al., 1991;Derry et al., 1992;Buick et al., 1995;Hoffman et al., 1998;Halverson et al., 2005;Swanson-Hysell et al., 2010;Hodgskiss et al., 2019). ...
The ¹³C/¹²C value of dissolved inorganic carbon (DIC) in the ocean has varied through time and can be determined from the marine carbonate record as changes in δ¹³Ccarb. These variations provide insight into global carbon cycle dynamics, as well as relative age information (chronostratigraphy) that can be used to correlate sedimentary successions globally. The global carbon cycle includes both short- and long-term components, and their interactions dominate the isotopic record presented in this chapter. The partitioning and sequestration of carbon between organic and carbonate rock reservoirs, and their fluxes to and from the ocean–atmosphere–biosphere system, drive secular changes in the δ¹³C of DIC in the oceans that are ultimately recovered from the stratigraphic record. The pre-Cenozoic data presented here utilize bulk carbonate data for compilation, but a wide range of materials has been analyzed in the literature to produce previous composites. Care must be taken to consider what materials have been analyzed in comparing global carbon isotope records from the literature.
... Consequently, δ 13 C values from ancient dolomites are widely used as archives of the global carbon cycle in Earth history. Impressive stratigraphic reproducibility of δ 13 C values and detailed textural studies in dolomites from Namibia (Halverson et al., 2002;Kaufman et al., 1991) and Svalbard (Halverson et al., 2004) support the common assumption that δ 13 C values are rock-buffered and preserve the carbon isotopic composition of the precursor carbonate minerals. Together with other geologic and geochemical evidence ( 87 Sr/ 86 Sr, biostratigraphy, etc.), δ 13 C values have also been used to correlate Paleozoic carbonate strata worldwide (Saltzman and Sedlacek, 2013). ...
Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However, chemostratigraphic studies of the Neoproterozoic Tsagaan-Olom Group in southwestern Mongolia exhibit 3-12‰ differences in carbon isotopes between stratigraphically equivalent limestone and dolomitized successions. To understand the origins of this geochemical variation, we conducted detailed geological mapping, petrographic, isotopic (δ¹³C, δ¹⁸O, δ44/40Ca, and δ²⁶Mg), and fluid inclusion analyses of carbonates in the Taishir, Ol, and Shuurgat formations of the Tsagaan-Olom Group. Stratigraphic and textural constraints distinguish fabric retentive early dolomitization (Dolomitization Event 1) that occurred in the Taishir and Ol formations during and soon after deposition of the ca. 635 Ma Marinoan cap dolomite of the Ol Formation and fabric destructive dolomitization that occurred after the deposition of the younger Shuurgat Formation (Dolomitization Event 2), either prior to or during early Cambrian foreland basin formation. The dolomitizing fluids moved through porous stratigraphic units bound by impermeable shale of the Khongor and Zuun-Arts formations. Dolomitization homogenized the isotopic records of all Neoproterozoic Tsagaan-Olom Group dolomites; undolomitized successions show more extreme negative excursions from more positive background values. The salinity and isotopic values of seawater, the dolomitizing fluids, and the original platform carbonate were estimated from fluid inclusion data and a numerical model of diagenetic dolomitization. These results demonstrate that both early and late dolomitization can have a profound effect on carbon isotopic records of carbonates; however, with a multi-isotope approach the original carbon isotopic composition of platform-top carbonate can be distinguished from that of seawater and other dolomitizing fluids.
... Treptichnus pedum first occurs within the fine-grained sandstone of VF2 (Wilson et al., 2012) (Fig. 13). The first occurrence of T. pedum within the Nomtsas Formation coupled with no record of pronounced negative excursion of δ 13 C carb from this interval (Kaufman et al., 1991;Grotzinger et al., 1995b;Jensen and Runnegar, 2005) has led to the suggestion that the Ediacaran-Cambrian boundary is contained in the unconformity at the base of the Nomtsas Formation (e.g., Germs, 1995;Grotzinger et al., 1995b;Jensen and Runnegar, 2005) (Fig. 13). However, a U-Pb zircon date of 538.18 ± 1.24 Ma (Grotzinger et al., 1995b) for an ash-bed at the base of the valley fill deposit and age recalibration of a sample from a volcanic ash-bed within the Spitskopf Member indicating a U-Pb zircon age of 540.61 ± 0.67 Ma (Grotzinger al., 1995b) suggested that the Ediacaran-Cambrian boundary is placed well below the valley incision (recalibration in Schmitz, 2012) (Fig. 13). ...
The Ediacaran–Cambrian transition was a time of marked biological and sedimentary changes, representing a significant evolutionary breakthrough in Earth’s history. Numerous explanations have been posited for these dramatic changes. One view emphasizes an extensive period of widespread continental denudation during the Neoproterozoic followed by extensive reworking of the basement during the early Cambrian, resulting in the formation of what is known as the “Great Unconformity”. Geologic events leading to the formation of this unconformity have been hypothesized as an environmental trigger for the Cambrian explosion. Here we review seventeen key Ediacaran-Cambrian successions around the globe with a focus on their sedimentary facies and sequence-stratigraphic architecture. Based on this information, we explore the relationship of the Ediacaran–Cambrian boundary to this unconformity, underscoring a more nuanced scenario. We consider each section in a chemostratigraphic and chronostratigraphic framework, and consider the hypothesized origin of this unconformity at each locale—for example, whether it was eustatic in origin and therefore of global stratigraphic significance or whether it resulted from local tectonics, implying marked inter-basin diachronism. Our systematic review shows that significant diachronism was involved in the generation of the Great Unconformity, suggesting that the sea-level fall reflects overprint of local tectonics on pure eustasy. In most places, the Great Unconformity is actually a composite sequence boundary resulting from successive eustatic episodes of sea-level rise and fall overprinted by multiple tectonic events of subsidence and uplift. Thus this surface should not be used for inter-basin correlations. Although the general proximity and common co-occurrence of the Ediacaran–Cambrian boundary with a SB, in places resulting in fluvial valley incision, has proved challenging for biostratigraphic sampling, by mapping this unconformity and its associated facies, there are new opportunities. In particular, paleontological work in terminal Ediacaran deposits located in interfluve positions may be promising because these areas may have preserved thick shallow-marine strata below the SB. Such strata could hold additional clues to help fill in gaps in our understanding of the Ediacaran roots of the Cambrian explosion.
... Sequence stratigraphy and δ 13 C carb chemostratigraphy are well established [16][17][18][19] , and detailed mapping of individual members enables parasequence level correlation across distances of up to 100 km 19 . Sediments were deposited in two sub-basins, the Zaris and the Witputs, that deepened to the northwest and southwest, respectively, with distance from a basement forebulge, the Osis Arch (Fig. 1b) 12 . ...
The late Ediacaran witnessed an increase in metazoan diversity and ecological complexity, marking the inception of the Cambrian Explosion. To constrain the drivers of this diversification, we combine redox and nutrient data for two shelf transects, with an inventory of biotic diversity and distribution from the Nama Group, Namibia (~550 to ~538 Million years ago; Ma). Unstable marine redox conditions characterised all water depths in inner to outer ramp settings from ~550 to 547 Ma, when the first skeletal metazoans appeared. However, a marked deepening of the redoxcline and a reduced frequency of anoxic incursions onto the inner to mid-ramp is recorded from ~547 Ma onwards, with full ventilation of the outer ramp by ~542 Ma. Phosphorus speciation data show that, whilst anoxic ferruginous conditions were initially conducive to the drawdown of bioavailable phosphorus, they also permitted a limited degree of phosphorus recycling back to the water column. A long-term decrease in nutrient delivery from continental weathering, coupled with a possible decrease in upwelling, led to the gradual ventilation of the Nama Group basins. This, in turn, further decreased anoxic recycling of bioavailable phosphorus to the water column, promoting the development of stable oxic conditions and the radiation of new mobile taxa.
... Shuram/Wonoka excursion has been identified globally (Calver, 2000;Condon et al., 2005;Fike et al., 2006;Grotzinger et al., 1995;Kaufman et al., 1991;Narbonne et al., 1994), suggesting it records a primary oceanic event (figure 1.8). If primary, the excursion is extraordinary because its magnitude and duration require explanations that differ from the modern carbon cycle, and it indicates a major perturbation to the carbon cycle and an increase in the redox potential of the ocean during the evolution of early macroscopic animal life. ...
There is a long-standing interest in the relationship between the rise of early Metazoans and changes in the redox structure of the oceans. As such, there is a need for reliable geochemical proxy archives that record palaeo-redox. Before we can use proxies in deep time we must understand their application in modern environments, and ensure pristine seawater signals can be extracted effectively. We investigate the sulfur cycle in the modern ocean, using new data from minor sulfur isotopes to constrain the proportional pyrite burial flux - a key control on atmospheric oxygen regulation through time - to between 20 and 35%. Ce anomalies in rare earth element patterns record redox information, and we develop the leaching methods for extracting pristine signals from carbonates. We suggest that a partial leach in nitric acid reduces the risk of contamination. We apply multiple redox proxies (Fe-speciation, TOC, carbon isotopes, CAS-pyrpaired sulfur isotopes and Ce anomalies) to terminal Ediacaran carbonates from the Nama Group, Namibia, to reconstruct the redox structure of the Nama Group and its relationship to the distribution of biomineralising Metazoans. We generate a holistic redox model that distinguishes between anoxic, intermediate and fully oxygenated waters, including identification of manganous conditions using novel observations of positive Ce anomalies. We distinguish between spatial and temporal variability in redox using nine sections from variable relative water depths. Dynamic redox conditions are associated with small, monospecific communities of Metazoans in short-lived horizons. Metazoans are largely absent from low oxygen manganous waters, whereas fully oxic waters host large, complex Metazoan communities. We suggest that redox exerted an important control on the ecological structure of terminal Ediacaran Metazoan communities.
... The Nama Group was deposited in two inter-connected sub-basins, the southern Witputs and the northern Zaris, separated by a zone of depositional thinning representing the "Osis Arch" paleo-bathymetric high (Germs, 1974). Unrestricted connection to the open ocean has been suggested because of the preservation of δ 13 C excursions in timeequivalent sections, and normal marine rare earth element signatures Kaufman et al., 1991;Tostevin et al., 2016b;. The Zaris Formation (Kuibis Subgroup) in the Zaris sub-basin was sampled at Zebra River Farm, and the Urusis Formation (Schwarzrand Subgroup) in the Witputs sub-basin was sampled at Swartkloofberg Farm and at Swartpunt Farm. ...
... Scherer, 1977). The δ 18 O values of marine carbonate are sensitive to diagenetic alternation, as alteration in meteoric fluids and in burial stage would lead to a significant decrease of δ 18 O values (Brand and Veizer, 1981;Frank et al., 1997;Kaufman et al., 1991;Knauth and Kennedy, 2009;Li and Jones, 2017;Veizer et al., 1999). Thus, the dataset indicates (1) primary deposition of laminae in meteoric water, or (2) diagenetic alteration of laminae precipitated from seawater in meteoric water or recrystallization during the burial process. ...
The nature (depositional or diagenetic) of laminated crusts associated with pisolites is still in debate, which hampers the interpretation of sea-level change on carbonate platforms. The Ladinian Yangtze platform margin, formed by the Longtou Formation, contains abundant laminated crusts associated with pisolites, providing an opportunity to decode their formation in response to sea-level variations. There, individual laminae are formed of isopachous radiaxial fibrous calcite (RFC) crystals. The RFC displays strong undulose extinction and orange-red or dull red cathodoluminescence. The pisolites (vadoids) coexisting with the laminated crusts are characterized with inverse graded bedding, stalactitic cements, and red luminescence or non-luminescence. The laminated crusts and associated pisolites have δ¹⁸O values ranging from −10.56‰ to −3.83‰ and from −7.12‰ to −6.33‰, respectively, which are more negative than that of the Ladinian normal seawater. They also share similar rare earth element and yttrium (REE + Y) distribution patterns, with enriched heavy rare earth element (HREE), enriched Y element, true negative Ce anomalies, and Y/Ho ratios between 34 and 51. Detailed petrographic and geochemical analyses indicate that the laminated crusts and associated vadoids formed in caves in response to alternating meteoric water and seawater, with the fluctuating water table resulting in their mixed deposition. The preservation of laminated crusts and vadoids in the Longtou Formation implies that they formed during a period of overall sea-level transgression, which was punctuated by sea-level regression and allowed formation of cave fillings. Our results suggest that laminated crusts present in ancient carbonate successions may indicate the positions of subaerial exposure related to sea-level regressions.
In this study, Pediastrum has been observed in the Anjihaihe Formation (E2+3a) shale through organic petrology studies. Additionally, analyses of total organic carbon and rock pyrolysis (Rock–Eval) revealed that the E2+3a shale predominantly comprises oil-bearing type I and II kerogen at a low-maturity stage. The organic matter (OM) in the shale is primarily contributed by Pediastrum, indicating excellent original hydrocarbon generation potential. Hydrous pyrolysis was conducted on a Pediastrum-rich sample, and the liquid product was analyzed using gas chromatography–mass spectrometry (GC–MS) of the saturated fraction. It was discovered that n-C27 might serve as a characteristic biomarker indicating Pediastrum development. During warm and humid climatic conditions, rivers transport significant amounts of freshwater and terrigenous organic matter (OM) into the lake, leading to an increase in felsic minerals and a relative decrease in carbonate minerals. This, in turn, reduces water salinity, resulting in a relative increase in the abundance of Pediastrum. Conversely, during earlier hot and arid climatic conditions, the input of terrigenous OM decreases, and evaporation leads to increased salinity. When the water’s salinity exceeds the “salinity critical point”, the Pediastrum content decreases. Finally, this study presents a formation and evolution model for the E2+3a shale in the Junggar Basin.
The way towards the stage subdivision of the Vendian starts with the assemblage of a complete composite geological section of this interval of the Earth’s history, a succession of geological bodies each reflecting a certain ecosystem state. A Vendian composite section of the Siberian Platform, which is a succession of regional to sub-global scale geological bodies reflecting particular states or unidirectional transformations determined by episodic expansion of relatively oxygen-rich environments onto the shelf and alternating with large-scale oceanic anoxia and euxinia events is proposed as a reference for constructing the stage subdivision of the Vendian. The redox instability had to be accompanied by changes in nutrient availability and could not but affect the course of macroevolution and macroecology. The geological record of the Vendian of the Siberian Platform is also marked by episodic increase in alkalinity of the World Ocean. At least five such alkalinity events could be provisionally identified in the composite section of the Vendian of the Siberian Platform. A hypothesis is proposed suggesting that the alkalinity events could control the appearance and disappearance in the geological record of a distinctive suite of sedimentary structures in carbonates, the discrete nature of early diagenetic cementation of aluminosilicoclastic sediments, and fossilization of soft-bodied organisms in the Vendian.
Global correlation of negative excursions in the δ¹³C values of shallow water carbonates have been used to interpret and identify major environmental changes through geologic time, such as the glaciations of the Neoproterozoic. As a result of their global reproducibility and similar geometry, these signals have predominantly been interpreted as being representative of the original δ¹³C values of dissolved inorganic carbon in the oceans, which in turn reflect major changes in the global carbon cycle. In this paper δ¹³C values from cores drilled in Enewetak Atoll (Pacific) and the Great Bahama Bank (Atlantic), are shown as examples of how similarly correlating variations in global δ¹³C values, can arise from diagenetic processes. It is further shown that while the profiles of the changes in δ¹³C values may appear synchronous, as a result of varying rates of deposition and subsidence, that the actual timing of the two changes can be quite different. By comparison of the geometry and global correlation of δ¹³C values measured in this study, it is proposed that the δ¹³C values of ancient sequences, may have resulted from non-synchronous diagenetic processes, rather than true changes in the global carbon system. This emphasizes the need for precise chronological control when interpreting sediment records, which is not present during many older geological time periods, in order to constrain the origin and correlation of carbon isotope variations.
In this paper, the authors have summarized and analyzed the petrology and isotope and trace element characteristics of host rocks and calcite cements in the Yingshan and Yijianfang formations of Lower-Middle Ordovician in the Shunbei area of the Tarim Basin. The result successfully reveals four types of limestones and four types of calcite cements, such as the sparry grainstone (SG), the microcrystalline grainstone (MG), the micrite (MC), the silicified limestone (SL), the calcite coexisting with silicon in SL (C1), the calcite which develops in fractures or between the broken siliceous breccias (C2), the calcite which fills between the broken particles (C3), and the calcite filling the fractures of all the host rocks and cements (C4). The high U/Th and Sr/Ba ratios of all the calcite cements indicate that the fluids that formed them were of comparatively higher salinity in burial environments, and the differentiation of their Sr-isotopes, Fe, and Mn contents clearly shows two types of diagenetic fluids: the fluid forming C2 and C3 is the inherited seawater closely related to the host rocks, and the fluid forming C1 and C4 is the silica-rich hydrothermal fluid from the lower strata and flowed through the possible clastic strata.
Molar-tooth structure (MTS) is an unusual carbonate fabric that is composed of variously shaped cracks and voids filled with calcite microspar. Despite a century of study, MTS remains enigmatic because it juxtaposes void formation within a cohesive yet unlithified substrate with the penecontemporaneous precipitation and lithification of void-filling carbonate microspar. MTS is broadly restricted to shallow marine carbonate strata of the Mesoproterozoic and Neoproterozoic, suggesting a fundamental link between the formation of MTS and the biogeochemical evolution of marine environments. Despite uncertainties in the origin of MTS, molar-tooth (MT) microspar remains a popular target for geochemical analysis and the reconstruction of Precambrian marine chemistry. Here we review models for the formation of MTS and show how detailed petrographic analysis of MT microspar permits identification of a complex process of precipitation and diagenesis that must be considered when the microspar phase is used as a geochemical proxy. ▪ Molar-tooth fabric is an enigmatic structure in Precambrian sedimentary rocks that is composed of variously shaped cracks and voids filled with carbonate microspar. ▪ Time restriction of this fabric suggests a link between this unusual structure and the biogeochemical evolution of marine environments. ▪ Petrographic analysis of molar-tooth fabric provides insight into fundamental processes of crystallization.
Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Among the three cratons (North China, Yangtze, and Tarim) in China, the Tarim Craton during Precambrian is the least known for its evolution due to its broad Phanerozoic cover. The affinity of the Tarim Craton with Gondwana supercontinent during the Cryogenian-Ediacaran interval has long been debated. In this contribution, we report carbon and oxygen isotopes of the Ediacaran-Cambrian siliciclastic‑carbonate from successions in the northwestern Tarim Craton. The results exhibit one pronounced negative and one positive carbon isotopic excursions (CIE) in the Ediacaran sections (N3 and P1), and a distinct negative CIE at the base of the Cambrian (N4). With the constraints of geochronological data, we proposed that the Ediacaran-lower Cambrian chemostratigraphy of the northwestern Tarim basin correlates well with the lower Ediacaran and lower Cambrian from Yangtze, Oman, Australia, Namibia, and Canada basins. N3 in the Tarim Craton possibly represents a new contribution to the Shuram-Wonoka anomaly globally. With these data, together with previously published tectonostratigraphic and paleogeographic reconstructions, we propose that the Tarim Craton may have been involved in the Pan-African Orogeny during the assembly of Gondwana, and was positioned close to the Yangtze, Oman, and Namibia cratons along the northern periphery of the Gondwana supercontinent
The dolomite explosion in the terminal Ediacaran period has great geological significance for understanding the paleoenvironmental evolution of the Precambrian-Cambrian transition. However, dolomite origins remain controversial. Abundant dolomites in the Ediacaran Dengying Formation in the northern Yangtze Block provide a window for solving the problem. In this paper, the genesis of dolomites and the paleoenvironment of the terminal Ediacaran were studied by facies, trace elements (including rare earth elements, REEs), and carbon, oxygen, and strontium isotopes of the Dengying Formation in the Xichuan area of South Qinling, Northern Yangtze Block. The results indicate that the Dengying Formation was deposited on a carbonate platform, and four types of dolomite (i.e., micro-crystalline dolomite, fine-medium crystalline dolomite, brecciated dolomite, and saddle dolomite) are identified. According to the differences in the ∑REEs, δ13C, Z value and 87Sr/86Sr values in different types of dolomite, it is concluded that micro-crystalline dolomite mainly formed in the early diagenetic stage, fine-medium crystalline dolomite in the middle diagenetic stage, and saddle dolomite in the late diagenetic stage, while brecciated dolomite formed in the epigenetic stage. Moreover, it is determined that the dolomitization models for the Dengying Formation include the seepage-reflux, mixed water, burial, and hydrothermal dolomitization models. In addition, micro-crystalline dolomite has a Sr/Ba value far greater than 1, the Z value is usually greater than 125, δEu is positive, V/(V + Ni) is less than 0.74, and δ13C is positive, indicating that the Dengying Formation dolomite was deposited in a shallow water environment with high salinity, weak oxidation, and dry and warm climate. By comparing the characteristics of dolomite in other parts of the world in the same period, it indicates that the global climate warmed up, the ocean appeared oxygenation process, and extensive retrogression events occurred in the Precambrian-Cambrian boundary.
In the brackish water sedimentary environment of the Junggar Basin in Xinjiang, the organic-rich shale and shale oil formation with relatively low maturity, is characterized by relatively high abundance of C28 regular steranes. However, the biogenic source of organic matter and favorable sedimentary environment for the type of source rock has been thoroughly classified. In this paper, organic geochemical and petrological, analyses of the Lucaogou Formation shale in the Jimusaer Sag, Junggar Basin in the north of Xinjiang Uygur Autonomy Region were carried out. The source rocks in the Lucaogou Formation are mainly composed of two types of maceral including lamalginite (cyanobacteria) and telalginite (green algae), with the former predominantly fueling the high C28 regular sterane content. Petrological, organic and inorganic geochemical analyses suggest that the lamalginite was developed in low-salinity water while the telalginite was developed in high-salinity water. Based on geochemical and organic petrological characteristics, both the lamalginite and telalginite have good hydrocarbon-generating potential, but in the lower thermal evolution stage of the Lucaogou Formation source rock, telalginite have higher hydrocarbon-generating rate.
Cloudina is a globally distributed Ediacaran metazoan, with a tubular, funnel-in-funnel form built of thin laminae (ca. 1–10 μm). To what degree local environmental controlled morphology, and whether early diagenesis controlled the degree of calcification of Cloudina, is debated. Here we test these hypotheses by considering assemblages from four, coeval localities from the Upper Omkyk Member, Nama Group, Namibia, from inner ramp to mid-ramp reef across the Zaris Subbasin. We show that sinuosity of the Cloudina tube is variable between sites, as is the relative thickness of the tube wall, suggesting these features were environmentally controlled. Walls are thickest in high-energy reef settings, and thinnest in the low-energy, inner ramp. While local diagenesis controls preservation, all diagenetic expressions are consistent with the presence of weakly calcified, organic-rich laminae, and lamina thicknesses are broadly constant. Finally, internal ‘cements’ within Cloudina are found in all sites, and pre-date skeletal breakage, transport, as well as syn-sedimentary botryoidal cement precipitation. Best preservation shows these to be formed by fine, pseudomorphed aragonitic acicular crystals. Sr concentrations and Mg/Ca show no statistically significant differences between internal Cloudina cements and botryoidal cements, but we infer all internal cements to have precipitated when Cloudina was still in-situ and added considerable mechanical strength, but may have formed post-mortem or in abandoned parts of the skeleton.
Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of
Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims
the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous
distally tapered foreslope carbonate wedge of the same age is called Swakop Group.
Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust
cuts out the facies transition to the platformal Otavi Group.
Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655
Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern
Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing
continental margin. No hint of a western margin is evident in Otavi Group, suggesting a
transform margin to the west, kinematically consistent with S−N plate divergence. Rift related
peralkaline igneous activity in southern Swakop Group occurred around 760 and
746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop
Group is impoverished in rift-related igneous rocks.
Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere
imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic
division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2)
glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian,
661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial
early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic
fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This
study aimed to deconstruct the carbonate succession for insights on the nature of
Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate
fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764
pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information).
Each glaciation began and ended abruptly, and each was followed by anomalously thick
‘catch-up’ depositional sequences that filled accommodation space created by synglacial
tectonic subsidence accompanied by very low average rates of sediment accumulation. Net
subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times
shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1,
despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin
subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional
disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift
basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are
dominated by massive diamictite, and the associated fine-grained laminated sediments
appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No
marine ice-grounding line is required in the 110 Sturtian measured sections in our survey.
In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine
ice grounding zone, which became the dominant repository for glacial debris eroded from
the upper foreslope and broad shallow troughs on the Otavi Group platform, which was
glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct
glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial
disconformity that underlies the main lowstand grounding-zone wedge, which includes a
proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall,
since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is
that of an early ice maximum and subsequent slow recession and aggradation, due to
tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified
diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial
cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan
sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite
(16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features
including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The
overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic
cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence
stratigraphy is laid out over ≥0.6 km of paleobathymetric relief.
Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick
Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup
(~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and
could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates
746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The
negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope
excursions) in NE Laurentia.
Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two
heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW
Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is
observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand
wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent
to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform
values of +3.0−3.5‰.
Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep
negative excursion associated with the post-Marinoan depositional sequence and heavy
values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi
Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner
platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop
Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative
CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered
diagenesis (dolomitization), driven by geothermal porewater convection that sucks
seawater into the proximal foreslope of the platform. This hypothesis provocatively implies
that CIEs originating in epi-platform waters and shed far downslope as turbidites are
decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the
altered proximal Swakop Group values closer to DIC of modern seawater.
Carbonate sedimentation ended when the cratonic margins collided with and were
overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko
orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from
overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle,
the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick
mass slides were displaced gravitationally toward both trenches, prior to orogenic
shortening responsible for the craton-rimming fold belt.
Sedimentary sequences preserve the records of changes in major controls of sedimentation namely, tectonics, climate, relative sea level and sediment production and preservation. The potential to characterize these changes in spatial and temporal scales has led to the development of the branch of chemostratigraphy. Chemostratigraphic study of sedimentary sequences commenced from recognizing identical/contrasting geochemical features across major geochronological boundaries, and evolved into one of the essential tools in exploration, characterization, and well development strategies. Chemostratigraphy incorporates applications on continuous, real-time geochemical mapping and direction of lateral drilling, and machine learning, among others. As the sedimentary systems operate on a variety of temporal scales that range from few hours (tidal cycles) to few tens of millions of years, within which many perturbations such as catastrophic and diagenetic events take place, that lead to unique geochemical signature which can be correlated at appropriate spatial and temporal scales. The application of chemostratigraphic technique in hydrocarbon exploration and reservoir characterization has gained momentum in recent years, particularly with the advent of developments in analytical instrumentation. This has also led to the integration of a variety of data from field sedimentary structures, mineralogy, major, trace and isotopic chemical compositions of whole rock, selected components of rocks, organic and inorganic components of oil and gas, etc., for reservoir characterization more accurately than ever. The geochemical fingerprinting of oil and gas reservoir components plays a major role in the identification of source rocks, discrimination of oil families, characterization of reservoir, source, and seal segments in petroleum systems. Future trends indicate the relevance and growing applications of machine learning techniques, artificial intelligence in real-time assessment, monitoring and planning of hydrocarbon exploration and production.
Atlas of Fossils contains descriptions and images of archaeocyaths, sponge spicules, hyolithozoans, molluscs, trilobites, bradoriids, brachiopods, 'calcified algae' (renalcids), Aulophycus, and various calcareous and phosphatic problematic fossils from 19 type sections of the Lower Cambrian stages of the Siberian Platform (the middle Aldan and Lena rivers' area).
Describes important principles of sedimentary geochemistry: concentrations and activities, equilibrium, adsorption, incorporation of trace elements and partition coefficients, and stable isotope fractionation. The techniques covered in this chapter are X-ray fluorescence, atomic absorption spectrometry, inductively-coupled plasma optical emission and mass spectrometry, electron microbeam analysis, neutron activation analysis and stable isotope (C.O.S.) analysis. Applications are described to the study of provenance and weathering, the deduction of environmental parameters, diagenesis and pore fluid chemistry, and elemental cycling. -from Editor
Considers carbonate diagenesis and the use of stable isotopes in recognizing the relative influence of meteoric or marine waters, different sources of dissolved carbonate, and changes in pore fluids and the extent of rock-water interactions throughout the diagenetic history of a particular sediment and sedimentary basin. Also presents the basic concepts of trace element partitioning and textural changes during carbonate diagenesis in order to emphasize that stable isotopic analysis alone cannot provide for a unique solution to the chemistry of natural waters during diagenetic events. -from Editor
This study examines the oxygen and carbon isotopic distribution in the brachiopods, crinoids, rugose corals, and matrix/cement from the Mississippian Burlington Limestone (Iowa and Missouri) and the Silurian Read Bay Formation (Arctic Canada). The isotopic data can be reconciled with textural and trace element observations only if the 18O content of ancient oceans differed from that of the present oceans. This postulated secular variation in the 18O content of seawater requires a separate evaluation of the Mississippian and Silurian carbonate components. Examination of the 18O content of the least-altered low-Mg calcite fossil components of the two formations suggests that the delta18O of Mississippian and Silurian seawater was lighter than the delta18O of present day ocean water. The delta13C values are bimodally distributed. This distribution is independent of geologic age, inferred original mineralogy and degree of diagenetic alteration.-from Authors
Both carbon system measurements and derived products can be used in ocean circulation studies to infer water mass transport. In this paper we give a brief overview of the global CO2 survey effort; then we give a few examples of how carbon data can be used to examine ocean transport. Finally, we discuss the estimates of anthropogenic CO2 from the global survey and the relation between anthropogenic tracers and ocean circulation. In the 1990s, several programs, including the Joint Global Ocean Flux Study (JGOFS), the World Ocean Circulation Experiment (WOCE), and NOAA's Ocean-Atmosphere Carbon Exchange Study (OACES), conducted major field campaigns that included water column measurements of at least two inorganic carbon parameters in the major ocean basins. Working with international investigators, we have been striving to synthesize these data into a unified consistent data set. The synthesis effort began with the Indian Ocean (IO). The IO survey marked the culmination of the global CO2 survey effort. Over 20,000 water samples collected between December 1994 and July 1996 were analyzed for both total dissolved carbon dioxide (TCO2) and total alkalinity (TA) using coulometric and potentiometric techniques as outlined in the methods handbook prepared by the CO2 survey science team (DOE, 1994). Based on the analysis of certified reference materials (CRMs) provided by A. Dickson of Scripps Institution of Oceanography and a post cruise analysis of the data, the accuracy of the TCO2 and TA measurements was estimated to be ±2 and ±4 µmol kg -1 , respectively. These data were combined with the French INDIGO and CIVA 1 results to provide the most comprehensive data set ever assembled for the IO. Between 1991 and 1996, 24 survey cruises in the Pacific had measurements of at least two carbon parameters. The Pacific Ocean survey involved investigators from 15 different laboratories and 4 countries. On average, carbon samples were analyzed on one third of the total water samples collected on these cruises. TCO2 was measured on all 24 cruises, but the second carbon parameter varied depending on the carbon group responsible for the cruise. An extensive assessment of the Pacific data quality is described by Lamb et al. (2001). Several approaches aimed at assessing cruise-to-cruise differences in the deep waters were used to help evaluate the data. These approaches involved a range of assumptions, but adjustments were only proposed for cruises where several lines of evidence indicated that an adjustment was necessary. TCO2 adjustments were proposed for three cruises to give an overall estimated accuracy of 3 µmol kg-1. TA adjustments were proposed for five cruises to give an overall estimated accuracy of 5 µmol kg-1.
Nd and Sr isotopic measurements from a variety of marine chemical precipitates provide insight into processes controlling the chemistry of seawater during the Proterozoic. Nd isotope measurements on banded iron formations and phosphorites, and Sr isotope measurements on carbonates indicate that during both the Early and Late Proterozoic, hydrothermal input to the oceans was a significant fraction of the total input to ocean chemistry. Isotopic data from Early Proterozoic clastic sediments show systematic differences from coeval chemical sediments. These differences become less marked toward the end of the Proterozoic. This implies a higher hydrothermal water to river water flux ratio during the Early Proterozoic. The significant changes in seawater isotopic composition during the Proterozoic reflect the transition from mantle dominated Archean oceans to a typically modern system. Isotopic and mass balance constraints from banded iron formations indicate that oxidation of hydrothermal Fe2+ in seawater could have been an important control on the redox state of Proterozoic ocean-atmosphere system. The present isotopic balances of carbon and sulfur could have been decoupled as a result.
The Upper Proterozoic Virgin Spring and Sour Dough limestones occur within the glaciogenic Kingston Peak Formation. The Virgin Spring limestone is black, thinly laminated, and consists of microscopic spheroids and scattered ooids, contained within coarse neomorphic spar. An original aragonite mineralogy is inferred and this is supported by high contents of strontium (up to 3650ppm). Thin sandstone beds occur within the limestone, and their features suggest a storm origin. The Sour Dough Limestone has been metamorphosed and deformed but shows similar sedimentary structures and a high Sr content. Both limestones are interpreted as deeper-water, outer-shelf deposits, and their sedimentation is related to a sea-level rise through deglaciation.-from Author
Organic-rich laminated limestones are a major facies of the Late Precambrian Biri Formation, deposited around the margins of the Sparagmite basin of southern Norway. The laminated limestones consist of an alternation of calcitic laminae and clay laminae. Trace element and stable isotope analyses confirm a marine origin for the carbonate and suggest that the original precipitate was aragonite. The organic matter reaches 3% (weight loss on ignition) and apparently is finely dispersed within the sediment. Both cm-thick limestone beds of probable storm origin, and thin, disrupted and folded horizons of slump origin occur within the laminated facies. The organic-rich laminated limestones occur at several horizons and are interbedded with carbonate grainstones of shallow-water origin. Basinwards, they pass into light to dark grey to green, organic-deficient mudrocks with resedimented and slumped units.The facies context of the organic-rich laminated limestones suggests deposition in an outer shelf setting at times of higher sea-level stands. The transgressive events, possibly induced by changes in glacial ice cover of near or distant landmasses, led to increased organic productivity through seasonal blooms of phytoplankton. Preservation of the organic matter in the sediments resulted from the development of anoxic or at least oxygen-depleted bottom waters. Some physical constriction of water circulation in the Sparagmite basin may have contributed towards the anoxic events.The development of Precambrian organic-rich ‘black shales’ in the Sparagmite basin of southern Norway does show that potential hydrocarbon source rocks were formed in the pre-Phanerozoic, in situations comparable to some Mesozoic and Tertiary oil source rocks.
Using the U-Pb and Rb-Sr methods, age measurements were made on 29 whole rock and 24 mineral samples from the Middle Proterozoic rocks of the Orange River Group and the granitoids of the Vioolsdrif igneous suite in the NW Cape Province and southern SW Africa. The samples are briefly described; a short account is given of the geological setting and the stratigraphic relationship in this geologically complex area, characterized by multiphase deformation and metamorphism. The age patterns are also complex and show the effects of the approx. 1100 m.y. Namaqua and approx. 500 m.y. Pan-African metamorphic events. Metalavas of the lower De Hoop Subgroup in the Orange River Group yield a Rb-Sr whole rock isochron age of 1000 +- 30 m.y. The high initial 87Sr/86Sr ratio of 0 722 +- 0 02 indicates resetting at that time. Analyses of Vioolsdrif granitoid zircons from six localities give two U-Pb ages of 1830 +- 30 m.y. and 1730 +- 20 m.y. The latter age was obtained for only one sample (a composite zircon and three fractions). This sample comes from close to the boundary between the Richtersveld Province and the Namaqua Province and U-Pb data for sphene and apatite give an approx. 1100 m.y. age. Reservations are expressed about the validity of the younger 1730 +- 20 m.y. zircons age, although it agrees closely with the 1740 +- 40 m.y. whole rock Rb- Sr age for granitoid samples from the Klein Helskloof area. It is noted that the age pattern for the Vioolsdrif suite closely resembles that of the Fransfontein granitic rocks in northern SW Africa. This may indicate that these two regions belong to the same crustal entity.-Authors
The most recent orogenic event in South-West Africa-the Damaran Episode-is reflected by consistent radiometric ages in the general range 450-550 (570) million years (hereinafter m.y.), yielded by regional metamorphic and granite-pegmatite minerals from orogenically deformed Upper Proterozoic rocks in two structurally continuous zones: (1) a northeast-southwest trending segment from the coast to the Kalahari Desert; and (2) a roughly north-south trending segment along the coast. The preponderance of ages within this range from correlative sequences of deformed and metamorphosed rocks in central and southern Africa, and in crystalline rocks associated with deformed correlatives in western Africa, demonstrates the regional extent of this episode. In addition, similar age patterns have been obtained from zones of gneisses, such as the Mozambique and Zambesi Belts of eastern and central Africa, in which Upper Proterozoic rocks have not been widely recognized. The tectono-thermal kinship of the zones of deformed Upper Proterozoic rocks and of the Mozambique and Zambesi Belts is indicated by the similarity of their age patterns and by their apparent geographical and structural continuity. Together these regions define a sinuous system of mountain chains-the Damarides-which, in part at least, " emerged" in Cambro-Ordovician times from an Upper Proterozoic-Lower Paleozoic mobile zone. In southern Africa this mobile zone separated and partly encircled two stable areas, termed the Congo and Kalahari Cratons, which have been stable since the Kibaran Episode ca. 1100 m.y. ago. The evidence of known orogenic intrusives and late-or syn-orogenic pegmatites and migmatites, suggests that the main pulse of the Damaran Episode took place 500 to 550(570) m.y. ago. In addition to the Damaran Episode, there is evidence that the Upper Proterozoic-Lower Paleozoic mobile zone was affected by an older episode, the Katangan Episode. This identification is based largely on U-Th-Pb ages from Katanga Province and on a limited number of similar Rb-Sr, U-Th-Pb, and K-Ar ages from western Africa and parts of the Mozambique Belt. The significance and extent of the effects of the Katangan Episode are still uncertain; its relationship to the Damaran Episode may, however, be similar to that of the Acadian and Taconian orogenies in the northern Appalachians. The evidence favors the retention of the term "Katangan Episode" for tectono-thermal events reflected by ages in the range 580-680 m.y.
Evolution is sometimes envisioned as a dynamic process played out on an inert or passive planetary platform; however, if the geological record contains a message for evolutionary biology, other than the fact of the fossil record itself, it is that the Earth’s surface is in a continual state of change. On various time scales, climates fluctuate, reservoirs and fluxes of biogeochemical cycles vary, sea levels rise and fall, and oceanic water masses form and decay, while continents grow, split apart, drift, and recombine. At the same time, organisms that compete with, prey upon, provide food for, or live in symbiotic association with any given taxon of interest radiate, undergo morphological and physiological change, and become extinct. This is the context of evolution, and paleontological efforts to understand evolutionary events of the past require that we attempt to root those events in the context of ongoing biological and environmental change.
Mathematical modeling of the diagenetic behavior of the trace element Sr in conjunction with oxygen isotopic changes during the diagenesis of limestone substantiates recent suggestions that experimentally determined distribution coefficients for calcite do not apply under actual diagenetic conditions. An analogous situation probably exists with respect to dolomite. Most ancient dolomite is trace-element-depleted relative to Holocene analogs. Present, quantitative interpretation of absolute isotopic or trace element values of dolomite is tenuous. Because recrystallization is neither completely open nor completely closed-system, chemical information from early events is incompletely erased by later ones. -from Author
Cathodoluminescence (CL) intensities of calcite cements from the Mississippian Lake Valley Formation are compared with Mn and Fe electron microprobe analyses of the same calcites. CL intensities of these calcites show excellent positive correlation with Mn concentrations at approximately constant Fe/Mn ratio and within narrow ranges of Fe contents. Our data suggest that Fe/Mn ratios may control the maximum CL intensity of calcite. -from Authors
For over ten years, the difficult problem of the Precambrian-Cambrian boundary has been intensely studied by a Working Group of the IUGS Subcommission on Precambrian Stratigraphy together with IGCP Project 29. Candidates for the stratotype boundary have been narrowed to localities in China, Newfoundland, and Siberia. The author describes key sections in Siberia, Kazakhstan and Mongolia, including one of the main boundary candidates on the Aldan River of W Siberia, in the Ulakhan-Sulugur section at the base of the Aldanocyathus sunnaginicus zone of the Tommotian stage.-from Author
Late Precambrian to Early Palaeozoic rocks, exposed in the NW Cape Province (S Africa) and southern SW Africa, comprise the mainly sedimentary Nama and Gariep groups, the Kuboos-Bremen line of intrusives and the Richtersveld and Bremen Igneous Complexes. Known stratigraphic relationships between these rock-units are enumerated, and the stratigraphic history of the area is described briefly. In this paper we present new Rb-Sr (mainly whole-rock) and U-Pb zircon age-determinations, involving a total of 145 samples, on these rocks. A complementary paper is concerned with Middle-Proterozoic rocks from the same area (Welke et al. 1979). Three bodies were studied from the Kuboos-Bremen line of plutons. Ages of approx. 500 m.y. (possibly a minimum age), approx. 520 m.y. and approx. 550 m.y. were obtained, and multiphase intrusion is indicated. Data for the post-Nama younger Bremen intrusives establish a minimum age of 518 +- 15 m.y. (and probably 553 +- 13 m.y.) for the Nama group Felsic dykes that post-date the Richtersveld Igneous Complex underlie the Nama Group, but reliable data on these dykes was not obtained either by extensive Rb-Sr data points from widely separated dyke samples; data for groups of samples collected across the width of individual dykes each defined a separate line on an isochron plot. Both events seem confined to a belt extending down the W coast, and it is suggested that the approx. 700 m.y. event is related to Gariepian (i.e. Pan-African) orogenic deformation and that the approx 500 m.y. event is related to subsequent thermal acticity.-Authors
Demonstrates the application of stable isotope geochemistry to the origin of sedimentary dolomite and diagenesis in clastic systems - clay mineral authigenesis, quartz, carbonate, and zeolite cementation. Case studies on diagenesis illustrate the mode of application of stable isotopic analyses. -from Editor
Detrital white micas from molasse sediments of the Nama Group were dated by the K/Ar method. The Kuibis Subgroup and the underlying pre-Damara Sinclair Sequence contain detrital muscovite with ages of 1100-1000 Ma and thus cannot be derived from the Damara Orogen, but prove to have originated from a basement probably affected by the Kibaran orogeny. Muscovite size-fractions of 250-100 μm from the upper Nama Group (upper Schwarzrand and Fish River Subgroups) yielded K/Ar ages 670-570 Ma. Because these sediments are regarded as molasse series of the Damara Orogen, these K/Ar ages can be interpreted as cooling ages of a first, early tectono-thermal event in the Damara Orogen. A later Damaran metamorphic overprint, which peaked at about 530 Ma, is not documented in the detrital content of the Nama Group at the present level of erosion, because detrital micas with K/Ar ages over 600 Ma, have been found up to the uppermost Fish River Subgroup. It can be concluded that, at least, two tectono-metamorphic events followed by uplift and erosion, have taken place during the development of the Damara Orogen.Thermal alterations of the Nama sediments were detected by means of illite crystallinity and K/Ar dating of clay size-fractions. These data, as well as the almost exclusive occurrence of the 2M mica polytype in these size-fractions point to a diagenetic to very low grade metamorphic alteration of the area investigated at about 530-500 Ma and younger. The deposition of the upper Nama Group is younger than the 570 Ma detrital white micas but occurred before the 530-500 Ma thermal alteration of the Nama deposits. The deposition of the lower Nama Group probably took place between 635 and 570 Ma.
The constancy of the 13C/12C ratio in marine
carbonate over the last half-billion years provides evidence that an
amount of organic carbon comparable to that presantly contained in the
earth's sedimentary reservoir must have existed in early Paleozoic time.
Since the amount of organic carbon is a direct measure of the net amount
of oxygen produced by photosynthesis, this in turn implies no great
change in available oxygen over that time period. Finally, unless the
distribution of oxygen between the bound and molecular form has
undergone a large change, the oxygen content of the atmosphere must have
been comparable to its present value in early Paleozoic time.
Zusammenfassung Im Bereich der Südgrenze des Damara-Orogens wurden Alter und Intensität der Metamorphose mit Hilfe von K/Ar-Altersbestimmungen und Illitkristallinität bestimmt. Die Untersuchungen umfaßten folgende Einheiten:1.
den südwestlichen Teil des E-W streichenden Astes des Damara-Orogens;
2.
die Decken der Naukluft;
3.
die Nama-Group, die sich von nördlich der Naukluft bis zum Fish-River im Süden erstreckt (einschließlich des westlichen Teils der Dwyka-Formation).
Die Metamorphose der Naukluft-Decken sowie der unterlagernden Nama liegt in Bereich der höheren Anchibis unteren Epizone. Zwischen den Gesteinen der Naukluft-Decken und der südöstlich vorgelagerten gefalteten Nama besteht ein deutlicher Sprung zu niedrigerer Metamorphose. In Richtung SE nimmt der Grad der Metamorphose in der Nama kontinuierlich ab bis in den Bereich der Diagenese.In den drei oben erwähnten Einheiten wurden K/Ar-Altersbestimmungen an Hellglimmern durchgeführt. Zusätzlich wurden Muskowite aus dem Basement, das die Nama unterlagert, datiert. Die Muskowite aus dem Basement ergaben ein Alter von 1160 m. y. Bestimmungen an Hellglimmern des südlichen Damara, der Naukluft und der nördlichen Nama liegen auf zwei Isochronen mit Altern von 495 m. y. und 530 m. y. Das Alter von 530 m. y. datiert den Höhepunkt der Metamorphose und das Alter der Deformation in den synkristallin deformierten Gesteinen. Das Alter von 495 m. y. kann als Abkühlungsalter für die höher metamorphen Gesteine gedeutet werden oder zeigt eine Verjüngung durch eine zweite postkristalline Deformation in Teilen der Naukluft und Damara-Gesteine an. Dieses Alter von 495 m. y. wurde ebenfalls im Mylonit der Hauptüberschiebungsbahn der Naukluft-Decken gefunden und gibt den Zeitpunkt der Platznahme der Naukluft-Decken an.
New Rb-Sr whole rock age data are reported from two metasedimentary sequences of the Damara Supergroup in the central Pan African Damara belt of Namibia (South West Africa).
Calc-granofels rocks of the Karibib Formation (Swakop Group) near Usakos are dated at 665±34 Ma (λ87Rb=1.39×10−11 a−1) which is interpreted as reflecting a high-grade metamorphic event predating widespread granite intrusion.
Pyroxene-bearing feldspathic gneisses of the Khan Formation (Nosib Group) from the Khan-Swakop River area east of Swakopmund show incomplete homogenization at 474±16 Ma. In view of similar ages obtained on the nearby Rössing alaskite granite and on biotites from a variety of rock assemblages this age is interpreted as reflecting a second Damaran metamorphic event rather than a specific stage in a long cooling history as previously thought.
It is probable that the new ages characterize two distinct Pan-African tectono-metamorphic events previously named Katangan and Damaran episodes respectively (Clifford, 1967). The younger of these has affected large areas of south western Africa both within orogenic zones and on the neighbouring Kalahari Craton and may reflect crustal processes of sub-continental proportion during the closing stages of the Pan-African tectogenesis.
Recent findings about the late-Precambrian stratigraphic succession in the areas between the Orange River and the Klinghardt Mountains and south of Windhoek throw new light on the relationship between the Damara and Nama Systems.
In the Orange River area the formations of the Gariep Group are unconformably or pseudo-conformably overlain by the Nama System. In central and northern South West Africa a similar relationship is exhibited by the Nosib Group and overlying Damara System.
Pronounced similarities between the sedimentation and deformational history of both Nosib and Gariep indicate a probable correlation of the two groups and this result is also supported by radiometric ages.
It follows that Damara and Nama are most likely either completely or partly coeval. The latter may represent a shelf facies of the former geosynclinal succession.
The Gariep and Nosib deformation apparently started with the subsidence of their respective troughs and may have lasted from approximately 850 m. y. to 600 m. y. This tectonic event is older than the Damara Orogeny (550 to 450 m. y.) and may be time-equivalent to the Katanga Episode.
Two late-Precambrian glaciations were widespread in South West Africa and their sedimentary deposits are important for the correlation of Gariep/Nama with Nosib/Damara.
The dolomite beds of the Otavi Series are unusually rich in stromatolites and oncolites. Individual reefs contain varied composite forms dictated by local paleo-environment. It is thought that wide intertidal flats, shallow lagoons, and gently undulating continental shelf areas favored their extensive horizontal development. The algal growths from different stratigraphic units indicate that stable environmental conditions prevailed over long periods; an exception is the elongate Otavi Basin, where typical reef conditions developed on the north side.
Carbonate minerals are stained over a set period of time with alizarin red-S and potassium ferricyanide only if they will react with dilute hydrochloric acid solution, with which the stain is prepared. The rates of solution of carbonates in the acid control the intensity of color development. For calcite, the rate of solution varies with the optic orientation of the section. The speed of carbonate solution is changed if the acid concentration is altered, but only at concentrations of about 0.1 N is the optic orientation of calcite differentiated by the stain. Etching reduces thin section thickness and clarifies rock texture. Staining with alizarin red-S differentiates carbonate minerals into two groups. Aragonite, calcite, witherite, and cerussite, which dissolve rapidly in dilute hydrochloric acid, are stained, while dolomite, siderite, magnesite, and rhodochosite, which react much more slowly with the acid, remain unstained. The distribution of ferrous iron, as distinguished by staining with potassium ferricyanide, has proved to be highly significant in the genesis of cements. Ferrous iron can be introduced at any one stage in cementation, or repeatedly, forming zoned patterns. The paragenesis of zoned ferroan cements can be reconstructed after staining. Solution of the more soluble original constituents can sometimes be dated in relation to cementation. Ferroan calcite can be secondary in origin and is usually associated with replacement minerals.
Shelly faunas and phytoplankton from Lower Cambrian (Tommotian) strata within and outside the type area in Siberia indicate that the Tommotian Stage is younger than previously believed. Deposits referred to the Tommotian Stage in Siberia are, in part, time equivalent to the Talsy (Lukati) Horizon in the East European Platform, which has yielded trilobites and is correlated with the Schmidtiellus mickwitzi trilobite zone in Baltoscandia. Published micropaleontological data seem to indicate that the Yudomian Stage preceding the Tommotian may also belong to the Cambrian System. The demonstrated contemporaneity of Tommotian small shelly faunas with the trilobites could result in a revision of models dealing with the early radiation of metazoan phyla, speciation, and skeletonization.
