δ13Ccarb Excursion Research Articles

Potential causes of depleted δ13Ccarb excursions in Ordovician marine carbonates, Ordos Basin, China

Variations in the stable carbon isotope signature of marine carbonate rocks (δ13Ccarb) are utilized as an indicator for changes in depositional environment and tracers of diagenetic alteration. Various petrologic (textural changes) and geochemical indicators (trace elements) have been used to evaluate the effect of diagenetic alteration on the δ13Ccarb signature. However, how diagenetic processes involving interactions between organic matter and minerals impact the δ13Ccarb records of marine carbonate rocks has not been investigated thoroughly. In this study, systematic stable carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotope analyses were conducted on marine carbonates from the Middle Ordovician Majiagou Formation in the Ordos Basin, China to evaluate secondary overprint of the primary δ13Ccarb record. 496 carbonate samples from the gas reservoir (potential gas source rock) were selected from one well located in the natural gas exploration target—mid-east part of the Ordos Basin. A depleted δ13Ccarb excursion (i.e., a shift to more negative values) occurred in the 100 m-thick anhydrite-bearing carbonate sequence of this formation. The δ13Ccarb and δ18Ocarb values for the whole rock vary from −10.9‰ to 1.8‰ and −11.5‰ to −4.0‰, respectively. The calcite veins and vug-filling calcites also have depleted carbon isotope signature ranging between −21.1‰ and −4.4‰. The secondary calcites observed under the microscope were generated by the conversion from isotopically depleted organic matter and resulted in the depleted δ13Ccarb signature. The commonly occurring thermochemical sulfate reduction in the study area and the degradation of potentially enriched carboxylic acid salts in anhydrite-bearing carbonate setting are major factors contributing to the depleted δ13Ccarb values. The conversion from organic carbon to inorganic carbon through organic matter-involved diagenetic processes plays a significant role in explaining the genesis of depleted δ13Ccarb excursions in deeply buried marine carbonate sequences.

Carbon isotope (δ13Ccarb) stratigraphy of the Lower-Upper Ordovician of the Yangtze Platform, South China: Implications for global correlation and the Great Ordovician Biodiversification Event (GOBE)

We present new high-resolution carbonate carbon isotope data (δ13Ccarb) from four sections in the Yangtze Platform, South China, ranging in age from Tremadocian to early Katian. The data have five distinct characteristics: 1) A prominent δ13Ccarb increase from −2.79‰ to 0.13‰ at the base of the Ordovician within the Tremadocian Cordylodus angulatus-Rossodus manitouensis Zone. 2) A positive δ13Ccarb excursion from −2.56‰ to 0.58‰ in the Paltodus deltifer Zone during the late Tremadocian. 3) A gradual increase in δ13Ccarb from −1.16‰ at the top of the Oepikodus communis Zone to a peak of ~0.74‰ in the Baltoniodus triangularis Zone at the Floian-Dapingian boundary. 4) A moderate rise in δ13Ccarb from 0.59‰ to 1.24‰ in the middle Darriwilian Dzikodus tablepointensis Zone. 5) A distinct positive δ13Ccarb excursion from 0.86‰ to 2.32‰ at the base of the Amorphognathus superbus-Hamarodus europaeus Zone during the early Katian. These excursions can be correlated with C-isotopic trends identified in age-equivalent successions in South China, Argentine Precordillera, North America, and Central Sweden, demonstrating their potential for global correlations and providing a composite, high-resolution chemostratigraphic framework for sedimentary records of the Lower-Upper Ordovician in South China. Besides, our data demonstrate a temporal coincidence between the long-term rise in δ13Ccarb and the Great Ordovician Biodiversification Event, suggesting that enhanced organic matter burial and accompanied climatic cooling and oxygenation may have played an important role in promoting the increases in biodiversity during the Ordovician.

The grandest of them all: the Lomagundi–Jatuli Event and Earth's oxygenation

The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope ( δ 13 C carb ) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ 13 C carb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ 13 C carb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ 13 C carb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ 13 C carb value of +1.5 ± 2.4‰, comparable to those settings for most of Earth history. In contrast, the 13 C-rich values that are the hallmark of the LJE are limited largely to nearshore-marine and coastal-evaporitic settings with mean δ 13 C carb values of +6.2 ± 2.0‰ and +8.1 ± 3.8‰, respectively. Our findings confirm that changes in δ 13 C carb are linked directly to facies changes and archive contemporaneous dissolved inorganic carbon pools having variable C-isotopic compositions in laterally adjacent depositional settings. The implications are that the LJE cannot be construed a priori as representative of the global carbon cycle or a planetary-scale disturbance to that cycle, nor as direct evidence for oxygenation of the ocean–atmosphere system. This requires rethinking models relying on those concepts and framing new ideas in the search for understanding the genesis of the grandest of all positive C-isotope excursions, its timing and its hypothesized linkage to oxygenation of the atmosphere. Supplementary material : C–O isotope data, figure S1, tables S1–S5 and the dataset for δ 13 C carb values are available at https://doi.org/10.6084/m9.figshare.c.5471815

A sequential record of the Llandovery δ13Ccarb excursions paired with time-specific facies: Anticosti Island, eastern Canada

The Silurian climate was volatile and punctuated by multiple short-lived climate events, each associated with biotic crisis and perturbations to the global carbon cycle. In total, seven positive carbon isotopic excursions are globally recognized in the Silurian with the large excursions of the Wenlock-Ludlow interval in the Baltic succession garnering the most research; these excursions are recorded in association with cyclic facies changes and the enigmatic resurgence of microbial carbonates. Climate models, hypothesizing cycles of alternating humid and arid climate states, have been developed to explain these paired lithologic-isotopic fluctuations; however, little work has been done to test these models on lower Silurian strata. To test for the presence of δ13Ccarb excursions and the stratigraphic motif of proposed climate events, we developed a high-resolution integrated stratigraphic framework for the upper Hirnantian to mid-Telychian (Upper Ordovician to Llandovery) carbonate-dominated succession of Anticosti Island, by examining ~450 m of strata from a recent drill core, supplemented by ~120 m of outcrop. Four facies assemblages and three time-specific facies are identified and organized into three orders of superimposed transgressive-regressive cycles. High-resolution isotopic curves reveal the presence of four global positive carbon isotope excursions all sequentially recorded in the study interval; the HICE (peak of +5‰), Early Aeronian (peak of +2‰), Late Aeronian (peak of +6‰), and Valgu (peak of +3.5‰) excursions. Common cyclic stratigraphic trends are recognized with these δ13Ccarb excursions, where each excursion is associated with a shift from shale-rich to shale-poor facies, an intermediate-order transgression, and a minor δ18O excursion. These repetitive lithologic-isotopic patterns in the Llandovery succession of Anticosti Island are linked to cyclic fluctuations between low-latitude humid and arid climate states. In addition to these patterns, microbial-rich carbonates are associated with the Late Aeronian and Valgu δ13Ccarb excursions on Anticosti Island; this suggests a genetic link between the elevated δ13C values and the proliferation of calcified cyanobacteria and associated microfossils. This integrated stratigraphic model linking carbon excursions with climate and carbonate factory shifts provides an excellent framework for future studies, using a wide variety of geochemical systems, to better understand the processes that drove environmental and biodiversity changes through the Llandovery.

Feedback Between Carbon and Nitrogen Cycles During the Ediacaran Shuram Excursion

The middle Ediacaran Period records one of the deepest negative carbonate carbon isotope (δ13Ccarb) excursions in Earth history (termed the Shuram excursion). This excursion is argued by many to represent a large perturbation of the global carbon cycle. If true, this event may also have induced significant changes in the nitrogen cycle, because carbon and nitrogen are intimately coupled in the global ocean. However, the response of the nitrogen cycle to the Shuram excursion remains ambiguous. Here, we reported high resolution bulk nitrogen isotope (δ15N) and organic carbon isotope (δ13Corg) data from the upper Doushantuo Formation in two well-preserved sections (Jiulongwan and Xiangerwan) in South China. The Shuram-equivalent excursion is well developed in both localities, and our results show a synchronous decrease in δ15N across the event. This observation is further supported by bootstrapping simulations taking into account all published δ15N data from the Doushantuo Formation. Isotopic mass balance calculations suggest that the decrease in δ15N during the Shuram excursion is best explained by the reduction of isotopic fractionation associated with water column denitrification (εwd) in response to feedbacks between carbon and nitrogen cycling, which were modulated by changes in primary productivity and recycled nutrient elements through remineralization of organic matter. The study presented here thus offers a new perspective for coupled variations in carbon and nitrogen cycles and sheds new light on this critical time in Earth history.

Asynchronous δ13Ccarb and δ13Corg records during the onset of the Mulde (Silurian) positive carbon isotope excursion from the Altajme core, Gotland, Sweden

High-resolution paired analyses of δ13Ccarb and δ13Corg from a new drill core from Gotland, Sweden, demonstrate asynchronous positive change in the carbon isotope records during the onset of one of the major Silurian biogeochemical events known as the Mulde Event or “Big Crisis”. The detailed carbon isotope record presented here provides Δ13C (the difference between δ13Ccarb and δ13Corg) and allows the calculation of changes in organic carbon burial (forg) throughout the late Wenlock. The paired data suggest a ~ 38% increase in forg during the peak of the positive δ13Ccarb excursion and the high-resolution record reveals several short-lived inflections in Δ13C that have not been previously identified. When combined with sedimentological and sequence stratigraphic data from multiple paleocontinents, the new data presented here provide strong evidence for a transient global decrease in CO2, in support of previous interpretations of regression and global cooling coinciding with the Mulde Extinction Event.

First record of lower – Middle Ordovician (Tremadocian – Dapingian) carbon isotope (δ13Ccarb) chemostratigraphy in the Canning Basin, Western Australia; calibrated with geochronology/biostratigraphy and implications for global correlations

New stable carbon isotope data (δ13Ccarb and δ18Ocarb) from the Lower–Middle Ordovician is presented from petroleum exploration well Olympic 1 in the Canning Basin, Western Australia. Olympic 1 continuously cored 319.53 m of sedimentary strata, from the basal part of the Willara Formation through the majority of the Nambeet Formation. This cored section comprises carbonate and siliciclastic deposits that constitute the primary fill sequence of the Canning Basin. Previous studies report CA-IDTIMS dates between 479.37 ± 0.16 Ma and 470.18 ± 0.13 Ma from bentonite beds within the Nambeet Formation and lower Willara Formation in Olympic 1. Four conodont biozones ranging from the Paroistodus proteus conodont Biozone (upper Tremadocian) to the Jumudontus gananda Biozone (upper Floian to lower Dapingian) have been identified from the same interval. The new carbon isotope data from the Olympic 1 core are superimposed on this robust framework to provide context for Tremadocian to Dapingian stable isotope trends for Australia.Three positive δ13Ccarb excursions are identified in the Olympic 1 δ13Ccarb isotope curve, which are correlated to the global generalized δ13C curve. The highest of these occurs in the basal Willara Formation, immediately above a CA-IDTIMS radioisotopic date of 470.18 ± 0.13 Ma within the upper Floian to lower Dapingian J. gananda conodont Biozone. The second falls in the upper Nambeet Formation, immediately above a middle Floian CA-IDTIMS radioisotopic date of 472.82 ± 0.13 Ma, and within the Oepikodus communis conodont Biozone. The oldest positive excursion occurs in the lower Nambeet Formation, immediately above a CA-IDTIMS radioisotopic date of 479.37 ± 0.16 Ma, and within the upper Tremadocian P. proteus conodont Biozone.Correlation of the Olympic 1 δ13Ccarb curve to Lower-Middle Ordovician δ13Ccarb curves from the Great Basin USA, Argentine Precordillera, Western Newfoundland and South China confirms the global distribution of dissolved inorganic carbon preserved in the geological record. This integrated record may be a useful reference for constraining the timing of major oceanographic events, and a key to establishing better links between the sedimentary successions of Western Australia and the larger Ordovician world.

Validating the deep time carbonate carbon isotope records: effect of benthic flux on seafloor carbonate

It is a consensus that marine carbonate archives the isotopic composition of seawater dissolved inorganic carbon (DIC, δ13Csw), the largest active C reservoir in the hydrosphere. Carbonate carbon isotope (δ13Ccarb) excursions have been used to reflect perturbations of the global carbon cycle and related environmental change. However, the deep time δ13Ccarb records indicate faster and more pronounced perturbations of the carbon cycle compared to the present day. Here, we report δ13Ccarb and elemental compositions of Late Paleozoic carbonate sections from South China, showing negative correlations between δ13Ccarb and Fe2+ content of carbonate (Fecarb). We suggest that, because Late Paleozoic carbonate was mainly produced by benthic carbonate-secreting organisms, δ13Ccarb recorded the isotopic composition near the seafloor, where benthic flux derived from anaerobic organic matter degradation delivers both Fe2+ and 13C-depleted DIC from porewater. The binary mixing between seawater and benthic flux would result in the deviation of δ13Ccarb from δ13Csw. The negative correlation implies that δ13Ccarb is influenced by benthic flux and is affected by the seafloor redox and sedimentation rate. The deep time spatially heterogeneous and temporally oscillatory δ13Ccarb records in the basin-scale could be alternatively attributed to the variations of local environmental factors rather than a δ13Csw depth-gradient. Thus, the seafloor carbonate precipitation is continuously affected by diagenetic reactions in sediments, suggesting that δ13Ccarb recording the seawater DIC composition is conditional. Our study urges that the interpretation of δ13Ccarb should also consider the sedimentary process and depositional environment of marine carbonate.

Dramatic changes in the carbonate-hosted barium isotopic compositions in the Ediacaran Yangtze Platform

Barium (Ba) isotopic compositions (δ138Ba) have been advocated as a novel paleo-productivity proxy, because of the strong control of biological productivity on the distribution of the Ba isotopic composition in the modern oxic oceans. However, the ocean was dominantly anoxic throughout the majority of the Earth’s history, and the biogeochemical cycle of Ba in anoxic oceans may have been different from the case in oxic ones. It is not clear whether δ138Ba can be used to trace biological productivity in ancient oceans. The Ediacaran Period was marked by a dramatic evolution in the climate, atmospheric and oceanic oxygen level, marine carbon cycle, and biosphere, which have been documented in multiple successions of the Ediacaran sedimentary rocks. This provides an opportunity to investigate variations in biogeochemical Ba cycle during the crucial period. In this study, we find large δ138Ba variations in carbonate rocks from the Ediacaran Doushantuo and Dengying Formations in the Yangtze Gorges area, South China. We suggest that detrital contamination and potentially post-depositional alteration cannot explain the δ138Ba variations. The post-Marinoan carbonate rocks record generally negative carbonate-hosted δ138Ba (δ138Bacarb) around −0.3‰, reflecting the existence of a replete and homogeneous reservoir of light Ba isotopes in the postglacial ocean. Although the Shuram/Wonoka correlative carbonate rocks have documented a prominent negative δ13Ccarb excursion, the δ138Bacarb merely shows minor fluctuations between 0.00‰ and 0.35‰, possibly due to local anoxia/euxinia inhibiting the biogeochemical cycle of Ba. The late-Ediacaran carbonate rocks show a gradual increase in δ138Bacarb systematically with enhanced negative cerium anomalies, suggesting the establishment of the modern-ocean-like cycle of Ba in shallow waters of the Yangtze Platform, which may result from the replacement of cyanobacteria by eukaryotic macroalgae as the dominant primary producer. We suggest that the biogeochemical Ba cycle in ancient marine system is strongly controlled by local redox conditions, and thus Ba isotopic compositions could be utilized to trace variations in paleo-productivity, especially in oxic environments.

Middle to Late Ordovician carbon isotope chemostratigraphy of the Lower Yangtze Platform: Implications for global correlation

Middle to Late Ordovician carbonates were collected in South China (Anhui Province) from the Kuniutan, Datianba, and Pagoda formations in the Dingxiang and Daling sections and analysed for high‐resolution carbon isotope chemostratigraphy. An increase by 0.4‰ in δ13Ccarb is observed in the upper part of the Kuniutan Formation (Eoplacognathus pseudoplanus Biozone), representing the rising limb of the Middle Darriwilian Isotopic Carbon Excursion (MDICE). In the upper part of the Datianba Formation, positive shifts of ca. 1‰ are recorded in the studied section which compare with the carbon shifts reported in North America and Baltoscandia. A pronounced positive δ13Ccarb excursion of ca. 1.5‰ from the lower part of the Pagoda Formation can be correlated with the global Guttenberg Isotopic Carbon Excursion (GICE). The documentation of three carbon isotope shifts in this study provides new evidence for changes in the global carbon cycle from Darriwilian to Early Katian which could have been influenced by changes in global climate and primary productivity.

Carbon and sulfur cycling during the mid-Ludfordian anomaly and the linkage with the late Silurian Lau/Kozlowskii Bioevent

The Ludfordian (late Silurian) records one of the largest perturbations in the Phanerozoic marine carbon cycling, the mid-Ludfordian positive δ13Ccarb excursion (MLCIEcarb), which occurred in the aftermath of the Lau/Kozlowskii Bioevent (LKB). There are still controversies regarding origin of the MLCIE and its possible causal link(s) with respect to the preceding LKB. Surprisingly, the isotope fractionation of carbon between inorganic and organic carbon pools during the MLCIE was not investigated in detail. To constrain better this parameter of marine C cycle, its linkage to the LKB, and local sulfur cycle, we analysed high-resolution δ13Ccarb, δ13Corg, and δ34Spy records in the Prague Basin (peri-Gondwana) representing one of the most complete records of the MLCIEcarb in the world. Evaluation of these high-resolution δ13Corg and δ13Ccarb records reveals their tight coupling, which in turn indicates the photosynthetic origin of the MLCIEorg via links to primary production and changing rates of organic carbon burial. On the other hand, our data also reveal a short-term decoupling of the δ13Corg and δ13Ccarb records preceding the MLCIE which may have been related to a purported increase in atmospheric pCO2 levels or anoxic event. The peak of the Δ13C anomaly exactly coincides with the LKB and, therefore, for the first time suggests a tight link between the change in marine carbon cycling and the bioevent. Subsequent rapid drop of the Δ13C values (and by inference also atmospheric pCO2) fits well with a rapid cooling and globally documented regression reported for this time interval. Finally, the documented strong coupling of δ13Ccarb and δ34Spy records during the MLCIE points to an intensive microbial sulfate reduction which should increase carbonate alkalinity input, thus contributing to formation of authigenic carbonates and an increase in δ13C values of DIC (and anticipated seawater carbonate saturation). The coupling of δ13Corg and δ34Spy values suggests a global enhancement of organic carbon and pyrite burial likely driven by an expansion of euxinic (anoxic and sulfidic water column) conditions in the oceans. Present study of C and S isotope variations during the MLCIE reveals for the first time a tight link among carbon and sulfur cycling, global cooling, and reorganization of faunal communities and fits well with purported seawater carbonate hypersaturation at the onset and during MLCIE.

Decoupling δ13Ccarb and δ13Corg at the onset of the Ireviken Carbon Isotope Excursion: Δ13C and organic carbon burial (forg) during a Silurian oceanic anoxic event

Paired records of δ13Ccarb and δ13Corg across the Llandovery-Wenlock boundary demonstrate asynchronous behavior during the onset of the Ireviken Biogeochemical Event (IBE). The extremely high-resolution data produced from the Altajme Core, drilled from Gotland, Sweden, capture a negative excursion in δ13Corg during the initiation of the Ireviken Extinction Event (IEE) and prior to the onset of the Ireviken positive δ13Ccarb Excursion (ICIE). The record of carbon isotopic changes through this interval illustrate that both Δ13C (the difference between δ13Ccarb and δ13Corg) as well as the relative flux of organic carbon burial (forg) vary in unique ways and at different times during the progression of the IBE. Both process-oriented variables within the global carbon cycle (Δ13C and forg) track a series of events that help to demonstrate potential causative mechanisms of both the extinction and carbon cycle perturbation. The sequence of events demonstrated here largely mirror the cascade of events that took place during the Cretaceous Oceanic Anoxic Event 2 (OAE2) and a detailed comparison between the two events is provided here for the first time. The unique insight into the IBE presented in this work results primarily from the novel, nearly Neogene-scale resolution of the paired isotope data, which demonstrates the critical importance of high-resolution chemostratigraphic research to evaluating ancient perturbations to the Earth-life system. Additional data sets of equal or greater resolution through this interval will be critical to evaluate the global synchroneity of these short-lived events during the IBE, and similar high-resolution studies of other Paleozoic biogeochemical events may shed light on potentially similar causative mechanisms.

Integrated bio-chemostratigraphy of Lower and Middle Triassic marine successions at Spiti in the Indian Himalaya: Implications for the Early Triassic nutrient crisis

In this paper we study the Lilang Super Group in the Spiti area, Indian Himalaya to understand environmental changes in the aftermath of the end-Permian mass extinction. We focus on the Mikin and Kaga Formations, which span the Induan to Ladinian stages of the Lower and Middle Triassic. These strata formed on the southern mid-palaeolatitude margin of East Gondwana and are interpreted as condensed, mixed carbonate – siliciclastic ramp deposits that deepened distally. Carbon isotope ratios of carbonate (δ13Ccarb) show an increase from −2.6 to 0‰ from the Griesbachian substage to the Dienerian-Smithian transition, followed by a negative shift to −3‰ in the Smithian substage and a large positive excursion from −3.0 to 3.5‰ across the Smithian-Spathian boundary. A short negative shift to −1.0‰ occurred in the early Spathian substage, and is followed by a positive trend from ~−1.0 to 1.0‰ in the Middle Triassic, with several minor excursions occurring during the Aegean substage. Carbon isotope ratios of total organic carbon (δ13Corg) co-vary with δ13Ccarb, suggesting that both proxies represent the original isotopic signatures. Sedimentary and palaeontological evidence, as well as trace metal geochemistry (Mo/Al ratio), indicate anoxic conditions developed from the late Griesbachian to the Dienerian substages, and dysoxic to probably fully oxic conditions from the late Dienerian substage onward. Anoxic conditions only very briefly reoccurred in the late Smithian substage. Ti/Ca and Zr/Al ratios suggest a consistently high terrestrial input in the first three substages of the Early Triassic, followed by a decrease across the Smithian–Spathian boundary. Thus, in the Spiti area, the positive δ13Ccarb excursion across the Smithian–Spathian boundary is recorded in generally well‑oxygenated sediments and coincides with a decrease in terrestrial input. While evidence for enhanced weathering is lacking, observations are strongly at odds with studies postulating eutrophication as a universal kill mechanism for the Smithian–Spathian crisis. The strata in the Spiti area contain an Early Triassic gap in phosphorite deposition indicating a ~ 5 Myr waning of coastal upwelling in an otherwise persistent (>100 Myr) upwelling zone of the East Gondwana margin. The phosphorite gap suggests low P availability, low rates of organic matter degradation, and a reduction in nutrient exchange between deep and surface water masses. Altogether, the phosphorite gap, the paucity of sessile and filtering fauna in oxygenated waters, and low organic carbon burial rates indicate a collapse in marine productivity in the aftermath of the end-Permian mass extinction.

Mercury isotopes track the cause of carbon perturbations in the Ediacaran ocean

Abstract The unusual carbon biogeochemical cycling that occurred in the Ediacaran (ca. 635–541 Ma) ocean may have been critical for ocean oxygenation and Ediacaran life evolution. However, the triggers of the peculiar Ediacaran carbonate carbon isotope (δ13Ccarb) excursions are not well understood. Because mercury (Hg) has a strong affinity for organic carbon, we measured Hg isotope compositions (δ202Hg and Δ199Hg) from Ediacaran marine sediments of South China and South Australia to better understand the causes of the Ediacaran δ13Ccarb excursions. During two local positive δ13Ccarb excursions in the Doushantuo Formation, gradually decreasing Δ199Hg and increasing δ202Hg trends are primarily ascribed to enhanced terrestrial input. Decreasing δ202Hg in both the Doushantuo and Wonoka Formations during the latter part of the Shuram negative δ13Ccarb excursion demonstrates the significant, and potentially widespread, contribution of upwelling Hg associated with dissolved organic carbon (Hg-DOC) from the open deep ocean. New Hg isotope data also suggest that a large Hg-DOC reservoir may have been present in the open ocean continuously from the early to mid–late Ediacaran Period (ca. 635–551 Ma), buffering ocean oxygenation at that time.

Reply to ‘Uppermost Cambrian carbon chemostratigraphy: the HERB and undocumented TOCE events are not synonymous’

AbstractIn a recent communication on carbon isotope chemostratigraphy of the uppermost Cambrian strata, it was claimed that the Top of Cambrian Excursion (TOCE) is (1) an undocumented negative δ13Ccarb excursion; (2) ambiguously defined; (3) deliberately fictitious or, in the authors’ words, a ‘nihilartikel’; and (4) not synonymous with the Hellnmaria–Red Tops Boundary (HERB) Event. As the authors who have been involved in much of the discussion surrounding the TOCE since its introduction and in subsequent clarification, we wish to emphasize that the recent communication overlooks the fact that the TOCE is in fact a well-documented and clearly defined negative δ13Ccarb excursion, and that the term ‘HERB Event’ was originally used informally, without definition or reference data, for a negative δ13Ccarb peak, a peak later shown to occur within the TOCE excursion. Nearly a decade after the TOCE was named, the concept of the HERB Event was modified from a negative δ13Ccarb peak to a negative δ13Ccarb excursion, making it conceptually synonymous with the TOCE excursion. The recently published commentary is misleading and replete with misconceptions, as we discuss here.

Trace and rare earth elements geochemistry of sedimentary rocks in the Ediacaran-Cambrian transition from the Tarim Basin, Northwest China: Constraints for redox environments

Spatiotemporal variations in the redox conditions during the Ediacaran-Cambrian (E-C) transition are closely related to the extinction of Ediacaran biota and the Cambrian explosion. However, the detailed variations in the ocean redox gradients remain a controversial issue. In this study, a large dataset of major and trace elements was obtained from the leachates of carbonate rocks sampled from three sections of the E-C transition in the Tarim Basin, northwest China. The Ce anomaly in the late Ediacaran Qigebulage Formation exhibited a regular gradient from anoxic conditions at its base to oxic conditions at its top, suggesting a gradually oxidizing environment. This could have been a beneficial factor that led to the diversification of late Ediacaran fauna. In addition, significantly negative carbon isotope excursions within the E-C boundary of the three sections studied were identified at the base of the Yuertusi Formation. This is coincident with the extinction of Ediacaran biota due to an anoxic sedimentary environment. The base of the Yuertusi Formation was deposited under anoxic or/and euxinic conditions and intense hydrothermal activity during the early Cambrian. The upper layers of this formation exhibited significant Ce anomalies following a general increase in the oxidation level, which reflected an environment transitioning from anoxic/euxinic to oxic following the early Cambrian transgression. A significantly negative δ13Ccarb excursion was evidence of the Redlichiid-Olenellid Extinction Carbon isotopic Excursion event recorded at the boundary of the Cambrian Series 2 and Cambrian Miaolingian. From the Cambrian Series 2 Xiaoerbulak Formation to the Cambrian Miaolingian Awatage Formation, carbonate rocks were deposited in deep water conditions and an anoxic environment. Redox fluctuations could be ascribed to widespread early Cambrian transgressions and tectonic activity, such as the formation of the Tianshan Ocean during the early Cambrian, and the accompanying volcanic activity in the Tarim Basin. Marine redox fluctuations coupled with the paleontological records indicate that the increased concentration of oxygen in the water promoted evolutionary transitions and accelerated the diversification of species during the E-C transition.

New age constraints on the duration and origin of the Late Ordovician Guttenberg δ13Ccarb excursion from high-precision U-Pb geochronology of K-bentonites

AbstractPerturbations to the global carbon cycle as recorded in the isotopic compositions of marine deposits have been commonly associated with major shifts in the climate and/or biologic activity, including mass extinctions. The Late Ordovician Guttenberg isotopic carbon excursion (GICE) is a large, globally correlative positive shift (∼3‰) in the carbon isotopic composition of marine carbonates (δ13Ccarb), but its driving mechanism(s) remains ambiguous. This is in large part due to uncertain correlations among Late Ordovician records, as well as complex and poorly constrained temporal relationships of abundant K-bentonite (altered volcanic ash) marker beds deposited in this time interval. Here, we provide new, high-precision U-Pb zircon geochronology by chemical-abrasion–isotope-dilution–thermal ionization mass spectrometry for K-bentonites bounding the GICE in the North American Midcontinent, including robust 206Pb/238U ages (reported with 2σ analytical uncertainty) for two important regional markers: the Deicke (453.35 ± 0.10 Ma) and Millbrig (453.36 ± 0.14 Ma) K-bentonites. The new data from these K-bentonites directly constrain the duration of the GICE to less than 400 k.y. at two well-studied locations in eastern Missouri, United States. The abruptness of the GICE precludes relatively gradual tectonic mechanisms as possible drivers of the excursion and suggests more rapid environmental drivers, such as changes in eustatic sea level associated with pre-Hirnantian glacial activity.

Global or regional? Constraining the origins of the middle Bambuí carbon cycle anomaly in Brazil

The Ediacaran-Cambrian Bambuí Group in Brazil records an anomalously positive excursion in carbonate carbon isotopes (δ13Ccarb) with a sustained plateau of ca. +15‰ (aka the Middle Bambuí Excursion–MIBE). Considering that the δ13Ccarb signals in Ediacaran-Cambrian seawaters do not typically exceed +6‰, the MIBE therefore represents a profound carbon cycle anomaly in Earth’s history. Although intensive studies have been done on the Bambuí Group, origins of the MIBE remain enigmatic. In order to better constrain the biogeochemical carbon and sulfur cycles during the MIBE, high-resolution chemostratigraphic analysis was conducted for both the plateau (i.e., Lagoa do Jacaré Formation) and the recovery part (i.e., lower Serra da Saudade Formation) of the MIBE. Chemostratigraphic profiles reveal remarkably different values in δ13Ccarb, δ13Corg, δ18Ocarb, and δ34Spyrite between these two studied MIBE intervals. The new data show that the plateau of the MIBE is characterized by coupled higher δ13Ccarb, higher δ13Corg, and higher δ34Spyrite signals compared with the recovery part of the MIBE. Based on multiple lines of sedimentological, geochemical, and model evidence, we propose that the possibilities of enhanced organic carbon burial and porewater methanogenesis are insufficient to explain the MIBE. Instead, local or regional controlling factors, including water-column methanogenesis, low-sulfate conditions, and enhanced carbonate recycling in a restricted basin may have played a role, independently or in unison, in generating this profound positive δ13Ccarb excursion. Therefore, the MIBE may reflect a regional event, instead of a global carbon cycle anomaly. We caution against the use of the MIBE in chemostratigraphic correlations on a global scale or any other attempt to infer global carbon cycling at that time. The biogeochemical landscape of the late Ediacaran-Cambrian basins and ocean margins may be more heterogeneous than previously thought.

Changes in productivity associated with algal-microbial shifts during the Early Triassic recovery of marine ecosystems

Abstract The recovery of marine ecosystems in the aftermath of the Permian-Triassic mass extinction was accompanied by significant carbon-cycle perturbations, as reflected in large-amplitude global excursions in Lower Triassic carbonate carbon isotope records. In the present study, we generated paired carbonate carbon (δ13Ccarb), organic carbon (δ13Corg), and nitrogen (δ15N) isotope records along with molar C/N ratios for a composite section in the Chaohu area of Anhui Province (northern Yangtze Platform, South China) that spans the entire Lower Triassic. These records document concurrent changes among multiple proxies related to marine plankton community composition, productivity rates, and nutricline structure, providing unparalleled insights into changes at the base of the marine trophic web during the Early Triassic recovery interval. Changes in carbonate-organic carbon isotopic differences (Δ13Ccarb-org) and C/N ratios indicate a general shift from anoxygenic photoautotrophy to eukaryotic algal productivity during the Early Triassic. The prevalence of prokaryotic photoautotrophs in the Griesbachian to Smithian was due to frequent environmental disturbance, whereas the reestablishment of eukaryotic algae as dominant primary producers in the Spathian reflects a general amelioration of marine environments at that time. Positive δ13Ccarb excursions and brief spikes toward higher Δ13Ccarb-org and C/N ratios around the Induan-Olenekian boundary and Smithian-Spathian boundary record transient shifts toward improved conditions and temporary rebounds of algal productivity. A negative δ15N shift was associated with decreasing δ13Ccarb, indicating that marine productivity was closely linked to N-fixation intensity owing to a general nutrient-N deficiency. Synchronous fluctuations of δ13Ccarb and δ13Corg through the Olenekian reflect changes in the δ13C of oceanic dissolved inorganic carbon, but the more limited variation in δ13Corg may have been due to concurrent changes in algal-microbial assemblages and, thus, net photosynthetic fractionation of carbon isotopes.

Transient and stepwise ocean oxygenation during the late Ediacaran Shuram Excursion: Insights from carbonate δ238U of northwestern Mexico

The Ediacaran Shuram Excursion (SE) was a key episode in the history of atmospheric and oceanic oxygenation that has also been linked to contemporaneous bioevolutionary events. However, key aspects of the redox state of Ediacaran oceans during the SE remain in doubt. Here, the uranium isotope compositions (δ238U) of marine carbonates from Sonora, northwestern Mexico were measured to investigate ocean oxygenation during the SE. We found that the SE in Sonora is characterized by a shift toward higher δ238U values (from −0.7‰ to −0.3‰), consistent with a major oxygenation event that has now been documented in widely separated coeval upper Ediacaran sections including the Doushantuo Formation in South China, the Bol'shoy Patom Formation in Siberia, and the Johnnie Formation in California. In the Sonora Succession, this δ238U shift exhibits a ~0.8-Myr lag relative to the δ13Ccarb excursion that defines the SE. To account for these observations, we propose a two-step oceanic oxygenation model for the SE: (1) initial oxidation of an aqueous reductant (e.g., dissolved organic carbon) that resulted only in a shift of δ13Ccarb (from +5‰ to −3‰); and (2) subsequent oxidation of methane or particulate organic matter that caused a continuation of the negative shift in δ13Ccarb (from −3‰ to −12‰) as well as a positive excursion of δ238U due to progressive oceanic oxygenation. This transient period of enhanced oceanic oxygenation of the late Ediacaran ocean, which may have promoted the evolution of early metazoans, terminated after ~5.0 to 6.5 Myr.