Carbon Isotope Excursion Research Articles

Volcanism and carbon cycle perturbations in the High Arctic during the Late Jurassic – Early Cretaceous

Large perturbations in the global carbon cycle recorded as carbon-isotope (δ13C) excursions (CIEs) in both organic carbon and carbonate records have been linked to volcanism during the emplacement of Large Igneous Provinces (LIPs). This link is based primarily on the purported temporal coincidence between CIEs and LIP emplacement. Mercury (Hg) concentration in sedimentary rocks has been used as a regional to global tracer of large-scale volcanic activity, yet few studies have been undertaken on Upper Jurassic – Lower Cretaceous sediments from Boreal localities compared to those for Tethyan (northern mid-latitude) successions. This has limited our understanding of the regional-to-global spatial impact of volcanic activity during this period. This study examines the Hg record as a proxy for volcanism, and the δ13C records from organic matter (δ13Corg) of CIEs from the uppermost Jurassic to Lower Cretaceous (Callovian – Aptian) successions from Axel Heiberg and Spitsbergen in the Canadian Arctic and Svalbard archipelagos, respectively. This interval includes three regional- to global CIEs. These sections show no significant variation in the ratio of Hg to total organic carbon (TOC) across the Boreal-wide Volgian negative CIE (Volgian Isotopic Carbon Excursion, “VOICE”), which has not been associated with LIP volcanism. The examined successions spanning this interval all show some influence from changing environmental or post-burial parameters, however, which could have (partially) overprinted a volcanic signal. Despite some problems in stratigraphically constraining the Weissert Event, increased Hg/TOC ratios are observed across this interval, which may be partially driven by volcanism associated with the emplacement of the Paraná-Etendeka Traps. A spike in Hg/TOC is observed immediately prior to the negative peak of the Aptian Oceanic Anoxic Event (OAE1a) CIE, supporting recent evidence of a pulse of High Arctic Large Igneous Province (HALIP) volcanic activity preceding this oceanic anoxic event.

Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion

The early to mid-Paleoproterozoic Lomagundi-Jatuli Excursion (LJE) is ostensibly the largest magnitude (approximately +5 to +30‰), longest duration (ca. 130–250 million years) positive carbon isotope excursion measured in carbonate rocks in Earth history. The LJE has been attributed to large nutrient fluxes, an increase in the size of the biosphere, a reorganization of the global carbon cycle, and oxygenation of the atmosphere. However, significant debate remains about its genesis, synchroneity, global-versus-local extent, and role in atmospheric oxygenation. Here we review existing models and mechanisms suggested for the LJE and analyze a compilation of ∼9,400 δ13Ccarb and associated contextual data. These data call into question the interpretation of the LJE as a globally synchronous carbon isotope excursion and suggest that any model for the LJE must account for both the absence of a clearly defined initiation and termination of the excursion and a facies-dependent expression of 13C-enrichment. ▪The Lomagundi-Jatuli Excursion (LJE) continues to challenge current understandings of the carbon cycle.▪Understanding this excursion is critical for reconstructing biogeochemical cycles and atmospheric oxygenation through Earth history.▪Some evidence indicates local rather than global changes in δ13CDIC and raises the possibility of asynchronous, local excursions.▪Resolving whether the LJE was globally synchronous or asynchronous is essential for discriminating between different models.

Ordovician strata of the Indian subcontinent

Abstract Ordovician rocks of the Indian Tethyan Himalaya contain a conspicuous angular unconformity between mostly marine Cambrian and overlying terrestrial Ordovician strata, which is a record of the Kurgiakh Orogeny. This tectonic event is traceable across the Tethyan Himalaya from Pakistan to Bhutan. The Pin Formation in the Spiti Valley provides a high-resolution account of the marine depositional history, palaeontology and isotope geochemistry of Late Ordovician events. The middle (Takche) member is late Katian and the upper Mikkim Member is lower Silurian (Llandovery), based on an ozarkodinid conodont fauna. The Pin Formation records the Boda event, the last warming interval prior to Hirnantian glaciation. The δ 13 C carb chemostratigraphic data allow precise global correlation, and recognition of the Paroveja positive excursion, the last major excursion of the Katian. The Mikkim Member records a ‘lower HICE’ (Hirnantian isotopic carbon excursion) of the Katian–Hirnantian boundary interval. Palaeontological data indicate that there are no known fossils diagnostic of any Ordovician ages older than the Katian Stage in India. Evidence of Ordovician sedimentary rocks in the Lesser Himalaya is intriguing, but presently equivocal. The widespread absence of pre-Katian strata on the Indian subcontinent is due to erosion associated with the Kurghiak orogeny and delayed onlap onto topographically high areas.

Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction

Land plants can adjust the concentration of protective ultraviolet B (UV-B)-absorbing compounds (UACs) in the outer wall of their reproductive propagules in response to ambient UV-B flux. To infer changes in UV-B radiation flux at Earth's surface during the end-Permian mass extinction, we analyze UAC abundances in ca. 800 pollen grains from an independently dated Permian-Triassic boundary section in Tibet. Our data reveal an excursion in UACs that coincide with a spike in mercury concentration and a negative carbon-isotope excursion in the latest Permian deposits, suggesting a close temporal link between large-scale volcanic eruptions, global carbon and mercury cycle perturbations, and ozone layer disruption. Because enhanced UV-B radiation can exacerbate the environmental deterioration induced by massive magmatism, ozone depletion is considered a compelling ecological driver for the terrestrial mass extinction.

Carbon isotopic composition and organic petrography of thermally metamorphosed Antarctic coal: Implications for evaluation of δ13Corg excursions in paleo-atmospheric reconstruction

Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13Corg excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13Corg). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13Corg in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% Rmax). These highly altered coals and cokes can have random vitrinite reflectances (Rr) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with Rr > 2%. Values of δ13Corg range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13Corg and the level of organic maturity, organic carbon (Corg) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13Corg studies.

Astrochronology and carbon-isotope stratigraphy of the Fengcheng Formation, Junggar Basin: Terrestrial evidence for the Carboniferous-Permian Boundary

The Carboniferous–Permian Boundary (CPB) is a major stratigraphic boundary and a marker for global stratigraphic correlation. However, the delimitation and global correlation of the CPB is hindered by the erosion during the Late Paleozoic Ice Age. Several carbonate platforms show a negative carbon isotope excursion (NCIE) in response to the CPB, which has great potential for the delimitation of this boundary. However, the response of terrestrial profiles to the CPB is unclear, and the global synchroneity of this NCIE is controversial. We studied a high paleolatitude section to evaluating the feasibility of the NCIE for delimitating CPB. We used astrochronological analysis of the 340 m Gamma Ray log of the Fengcheng Formation in Mahu Sag, Junggar Basin, northwest China, where the CPB may have been recorded. We constructed a high-resolution chronostratigraphic framework for the Fengcheng Formation, which precisely defines the chronostratigraphic attribution of the Fengcheng Formation as the Gzhelian–Asselian, and confirms that the CPB is recorded in the Fengcheng Formation. In addition, a global correlation of carbon isotope profiles revealed a negative excursion in terrestrial carbon isotope profiles across the CPB, which is consistent with the characteristics of NCIEs in several marine profiles. The similarity of the NCIE records supports their global synchrony and confirms that they can be used to delimit the CPB in terrestrial sections. Combining the astronomical time scale and the carbon isotope profile, we constrain the position of the CPB to the depth range of 4694.84–4703.56 m, with 4698.9 m as a preliminary estimate of the CPB. This new astronomical timescale and the stratigraphic position of the CPB enable the revision of the chronostratigraphic attribution of the terrestrial strata in the Junggar Basin. Overall, our results contribute to studies of the Carboniferous–Permian global carbon cycle, high-latitude glaciation, eustatic fluctuations, global volcanism, and their potential relationships.

Carbonate�Corganic decoupling during the first Neoproterozoic carbon isotope excursion

<p><b>The Neoproterozoic Era (1000�C539 Ma) experienced repeated negative carbon isotope excursions, representing the largest negative excursions in Earth history and their origins remain enigmatic. One salient observation is that unlike younger Phanerozoic carbon isotope excursions that almost exclusively exhibit covariation in the carbon isotopes of carbonate and organic carbon, about half of Neoproterozoic excursions are expressed in carbonate carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>carb</b></sub><b>) but their organic carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>org</b></sub><b>) lack the same anomaly. Documenting which excursions exhibit carbonate�Corganic coupling or decoupling may provide a possible clue as to the enigmatic dynamics of the anomalous Neoproterozoic carbon cycle. We report here that a newly recognized and the first Neoproterozoic carbon isotope anomaly also exhibits decoupling. Thus, the ca. 940 Ma Majiatun anomaly now represents the oldest-known decoupled Neoproterozoic excursion��300 m.y. earlier than previously thought. Ruling out altogether or mostly merely localized artifacts such as diagenesis, changes in water depth, and contamination of detrital organic matter, we interpret this new evidence of carbonate�Corganic ��</b><sup><b>13</b></sup><b>C decoupling in the context of other anomalies through time. Although the remineralization of a large pool of dissolved organic carbon (DOC) was proposed to explain late Neoproterozoic ��</b><sup><b>13</b></sup><b>C decoupled anomalies, our earlier evidence of decoupling suggests this pool was much more ancient. Our new data also require the temporal oscillation between decoupling and coupling of the Neoproterozoic carbon cycle, implying a dynamic diminution of the large Precambrian DOC pool reflective of the rising but equally variable oxygenation of the oceans and rise of animal life at this time.</b></p>

Organic matter variations in the deep marginal basin of the Western Tethys and links to various environments in isotopic Albian–Cenomanian Boundary Interval

In the latest Albian–lowermost Cenomanian, the palaeoenvironments along the northern margin in the Carpathian domain of the Western Tethys were characterized by deep basins with floors below the calcium compensation depth, which were filled with flysch and hemipelagic sediments. On the other hand, the terrestrial areas surrounding the Western Tethys to the north were covered by a diverse flora of gymnosperms and experienced an increase in the diversity of angiosperms; the latter was characterized by a rapid spread in most terrestrial environments. The flysch/hemipelagic succession of the Silesian Basin (the Polish part of the Outer Carpathians) contains a sedimentological record of organic-rich accumulation that is related to the carbon isotope excursion, known as the Albian–Cenomanian Boundary Interval (ACBI); its older part corresponds to Oceanic Anoxic Event 1d (OAE1d). Organic matter (OM) records from these noncalcareous sediments are characterized by the occurrence of both terrigenous and marine organic compounds, and an increase in their accumulation intensity occurred during OAE1d, similar to other Western Tethyan areas and their northern surroundings. Quantitative analyses of the OM compounds using petrographic and chemical methods indicate that the majority were redeposited from land and estuarine environments; however, the OAE1d strata (Alternans Level) and the lower part of the post-OAE1d black shale succession are also enriched in OM of marine origin. This enrichment is interpreted as the result of enhanced surface water productivity, most likely related to redistribution of nutrients from coastal areas during the sea level rise of the KAl8 eustatic cycle. OAE1d was also the period in which the degradation of OM took place under reducing conditions in the bottom waters (although they were not anoxic), which was possibly caused by poor ventilation. A high proportion of terrigenous/estuarine OM compounds redeposited in the Silesian Basin is characteristic, especially for the post-ACBI black and dark grey shales. A large amount of this OM type has undergone relatively rapid degradation as it has settled from the shallows through a well‑oxygenated water column in the epi-, meso- and bathypelagic zones. Some terrigenous OM came from forest fires that occurred in the catchment area and supplied material to the Silesian Basin.

Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event associated with a large injection of carbon into the ocean-atmosphere system, as evidenced by a diagnostic carbon isotope excursion (CIE). Evidence also suggests substantial hydrologic perturbations, but details have been hampered by a lack of appropriate proxies. To address this shortcoming, here we isolate and measure the isotopic composition of hydroxyl groups (OH−) in clay minerals from a highly expanded PETM section in the North Sea Basin, together with their bulk oxygen isotope composition. At this location, we show that hydroxyl O- and H-isotopes are less influenced than bulk values by clay compositional changes due to mixing and/or inherited signals and thus better track hydrologic variability. We find that clay OH− hydrogen-isotope values (δ2HOH) decrease slowly prior to the PETM and then abruptly by ∼8‰ at the CIE onset. Coincident with an increase in relative kaolinite content, this indicates increased rainfall and weathering and implies an enhanced hydrologic cycle response to global warming, particularly during the early stages of the PETM. Subsequently, δ2HOH returns to pre-PETM values well before the end of the CIE, suggesting hydrologic changes in the North Sea were short-lived relative to carbon-cycle perturbations.

A DOM regulation model for dolomite versus calcite precipitation in the Ediacaran ocean: Implications for the “dolomite problem”

The Ediacaran Period (∼635–539 Ma) witnessed the largest negative carbonate carbon isotope (δ13Ccarb) excursion in Earth history (to −12‰ VPDB), known as the Shuram Excursion (SE), which has been attributed to oxidation of a massive dissolved organic matter (DOM) reservoir, yielding highly 13C-depleted oceanic dissolved inorganic carbon (DIC). Limestone and dolostone are intimately interbedded in the Ediacaran Doushantuo Formation (South China) as well as in coeval successions globally, suggesting frequent shifts between calcite and dolomite precipitation in the Ediacaran ocean. In this study, we compiled paired Mg/(Mg + Ca) and δ13Ccarb data for 991 well-preserved (i.e., minimally altered) carbonate-rich samples from 17 SE-associated sections with a wide global distribution. δ13Ccarb compositions vary among the dolomite-dominated (−10‰ to +7‰), calcite-dominated (mainly in −10‰ to −5‰), and mineralogically mixed samples (with a bimodal distribution of −10‰ to −5‰ or +4‰ to +8‰). Inspired by the recent DOM catalyzation hypothesis and the highly-stratified redox model as well as the large DOM reservoir hypothesis for the Ediacaran ocean, we propose a “DOM regulation model” to account for controls on the precipitation of dolomite versus calcite in the Ediacaran ocean and, thus, for observed Mg/(Mg + Ca)-δ13Ccarb patterns. In this model, calcite was precipitated under low-DOM conditions, which existed in oxic surface waters and the nearshore part of a mid-depth DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, buffered by atmospheric CO2, whereas the latter had δ13CDIC decreasing from +4‰ to −10‰ due to increasing contributions of DOM oxidation. On the other hand, dolomite precipitation was catalyzed by high-DOM conditions, which existed in distal anoxic deep waters and the offshore part of the DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, ultimately buffered by atmospheric CO2, whereas the latter had δ13CDIC increasing from −10‰ to +4‰ due to decreasing contributions of DOM oxidation. Our model is well supported by coexisting redox data. Our study highlights a possible link between massive dolomite formation and the existence of a large DOM reservoir in the Ediacaran ocean, providing an alternative perspective on the origin and widespread distribution of dolomite in deep-time marine systems (i.e., the “dolomite problem”).

Finding a VOICE in the Southern Hemisphere: A new record of global organic carbon?

Variations in the carbon isotopic composition of carbonate and organic carbon (δ13Ccarb and δ13Corg) are generally used to record perturbations in the global carbon cycle, which are in turn closely linked to changes in climate. However, because of climate gradients on Earth, assignment of the “global” signal in ancient records is not straightforward. Here, we report the δ13C values of organic material in the Upper Jurassic to Lower Cretaceous sedimentary record of the Vaca Muerta Formation, situated in the Neuquén Basin, Argentina, which show similar patterns to those observed in several northern latitude basins. This record of δ13C values in the organic material differs from those measured in the early Atlantic Ocean, a record previously considered to be representative of the global values of organic carbon. As a result of the global synchronicity observed in the δ13C values of organic material from both northern and southern latitudes, we suggest that these patterns may represent the global record of δ13C values in organic material rather than those measured in the proto−Atlantic Ocean. The δ13C values of the organic components show a slight initial decrease of ∼2‰ in the early Tithonian (149−145 Ma) and then another decrease of ∼2‰ before reaching a minimum of −30.29‰ in the late Tithonian (145−143 Ma), followed by a gradual increasing trend throughout the Berriasian (143.1−137.7 Ma). The early Valanginian (137.7−135.5 Ma) was marked by a more substantial increase in δ13C values up to −23.46‰. These changes mirror those seen in Northern Hemisphere locations during the Late Jurassic and Early Cretaceous, where this perturbation has been termed the Volgian isotopic carbon excursion (VOICE). This difference in the Late Jurassic and Early Cretaceous δ13C values between the early Atlantic Ocean and the Neuquén Basin is interpreted to be the result of the climate gradient at the time, which was characterized by more humid conditions in high latitudes compared to dry conditions in the Atlantic Ocean basin.

More gaps than record! A new look at the Pliensbachian/Toarcian boundary event guided by coupled chemo-sequence stratigraphy

The Pliensbachian/Toarcian boundary (Pl/To) event precedes by ca. 1 Myr the onset of the Toarcian Oceanic Anoxic Event. It corresponds to a second order mass extinction associated with an outstanding collapse of shallow marine ecosystems at global scale. Yet, our knowledge about its exact driver(s) and unfolding is relatively ambiguous due to the numerous hiatuses present in the sedimentary record during this critical time interval. In this study, an integrated carbon isotope chemostratigraphy and sequence stratigraphy approach is applied to two case studies (the upper Pliensbachian in South-East France and the Pliensbachian–Toarcian transition in Morocco) to demonstrate how the major changes in sea-level and sedimentation supply accompanying the Pl/To event led to the formation of ubiquitous, often cryptic hiatal surfaces in the sedimentary record. Hence, as a consequence of strongly progradational stacking pattern during the latest Pliensbachian related to a global sea-level lowstand associated with cold greenhouse climate, proximal settings were characterized by bypass and/or erosion, inducing an incomplete record of the Spinatum chronozone in localities situated in the outer part of sedimentary basins. In the earliest Toarcian, the collapse of the neritic carbonate factory led to a halt of carbonate mud export into the basin, resulting in sediment starvation in most basins characterized by a carbonate-dominated sedimentation regime before the environmental perturbation. Only localities where vigorous siliciclastic sediment supply took over are likely to have a more complete sedimentary record of the immediate aftermath of the carbonate production collapse. This combination of causes explains the ubiquitous incompleteness of the record of the Pliensbachian/Toarcian transition in numerous European localities where the bulk of our current understanding about the Pl/To event derives from. A comparison between the two known most expanded and complete records of the Pliensbachian–Toarcian transition of the Llanbedr (Mochras Farm) core in Wales and Bou Oumardoul n'Imazighn section in Morocco shows that the onset of the environmental perturbations is associated with a positive carbon isotope excursion spanning the Pliensbachian/Toarcian boundary. This is followed by a negative carbon isotope excursion during the earliest Toarcian that coincides with the global collapse of neritic carbonate factory and an ample sea-level fall.

Mapping monstrosity: Malformed sporomorphs across the Smithian/Spathian boundary interval and beyond (Salt Range, Pakistan)

In the last 500 million years, Earth's biota experienced periods of crises with extinctions on a large scale and significant turnover events, one of them being the Permian-Triassic extinction event. The following Early Triassic was a critical time marked by a series of biological and environmental changes with a complex recovery pattern of marine faunas and ecosystems. Generally, animals and plants respond to stress by using different strategies that help them tolerate or recover from unfavourable environmental conditions. In this contribution, we quantitatively and qualitatively describe a variety of malformations of spores and pollen grains from the extensively studied Nammal section (Salt Range, Pakistan) across the Smithian/Spathian boundary interval (Early Triassic). High dominance of malformed sporomorphs is recorded throughout the studied interval, indicating stressful conditions for plants on the Indian margin during the Early Triassic. One of the highest abundances of malformed sporomorphs coincides with the spore spike and the negative carbon isotope excursion during the middle Smithian, emphasising this intense biotic stress. Either a cocktail of volcanic gases, acid rains, soil acidification, and heavy metal pollution as a consequence of a late pulse of the Siberian Trap large igneous province or climatic extremes might have been responsible for these malformations. Here teratology is used as a tool/means to assess the severity of biotic crises in the plant kingdom, indicating its potential value as a signal of ecological disturbance. • Fossil spores and pollen morphology as a proxy for ancient environmental pollution. • Sporomorphs identification schemes to quantify morphological change. • Evaluation of possible causes triggering morphological variation in sporomorphs.

No evidence for expansion of global ocean euxinia during the base Stairsian mass extinction event (Tremadocian, Early Ordovician)

A Tremadocian (Early Ordovician, base Stairsian North American Stage) mass extinction event is recorded globally in rocks from several ancient continents and is accompanied by a globally correlated positive carbon isotope excursion (CIE; the largest during the Early Ordovician). In this study, elemental concentrations and uranium isotope compositions (δ238U) were measured for carbonate samples from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin to test the role of ocean anoxia/euxinia on the base Stairsian mass extinction event. Dysoxic/suboxic to oxic water columns and non-sulfidic porewaters are inferred for these sections based on multiple local redox proxies (e.g., Mo/Al, U/Al, Mo and U concentrations, Mo/U ratios).Carbonate δ238U data exhibit different δ238U trends in the three sections. The proximal Ibex area exhibits a small negative δ238U excursion (a magnitude of 0.2–0.3‰) and the Shingle Pass section has one sample with unusually low δ238U, in both cases within the middle of the CIE. Samples with the lowest δ238U in each section also have the highest Mn/Sr ratios, suggesting diagenetic alteration associated with adsorption of isotopically light U onto Mn oxides, followed by dissolution of Mn oxides by reducing porewaters and capture of the isotopically light U by sediments. The distal Shingle Pass and Meiklejohn Peak sections better preserve global ocean signals and the limited δ238U variations in these sections imply no significant changes in global seawater δ238U and thus no significant changes in the extent of global ocean euxinia. Given the non-sulfidic porewaters inferred for these carbonates, we apply a sediment–seawater δ238U offset of 0–0.27‰ (the 0.27 ± 0.14‰ isotopic offset observed for modern Bahamas carbonates with sulfidic porewaters is used as a maximum constraint) to the average carbonate δ238U from the two distal sections (−0.48 ± 0.09‰) to derive a coeval seawater δ238U of between −0.75 ± 0.09‰ and −0.48 ± 0.09‰. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area existed during deposition of these Tremadocian carbonates. Our study implies that a small biotic crisis during a small CIE was not associated with an expansion of global ocean euxinia, in contrast to major Phanerozoic mass extinctions that were commonly associated with significant expansion of ocean euxinia during large carbon perturbations (e.g., the Late Devonian). In addition, we propose that the post-SPICE Cambrian and Early Ordovician oceans were characterized by limited changes in ocean euxinia and only a few episodes of expanded ocean suboxia-anoxia during carbon perturbations. The “biomere” events within this period, including the base Stairsian event, were probably related to these episodes of expanded suboxic-anoxic oceans and/or other ecological stresses.

Facies Changes, Evolution of Biogenic Structures, and Carbon Isotope Stratigraphy of the Cambrian Series 2 to Miaolingian Transition on the Southern North China Craton

The Cambrian Series 2–Miaolingian transition is a pivotal period during Earth history, which witnessed the decline of biodiversity and the reduction in biomass, i.e., the redlichiid–olenellid trilobite extinction. The notable δ13C excursion (RECE) near the Cambrian Series 2–Miaolingian boundary in east Gondwana and China apparently corresponds with the redlichiid trilobite extinction. To better understand the causal mechanism of this biotic crisis, we report the carbon isotope stratigraphy and facies changes from Cambrian Series 2–Miaolingian transition of the Mantou Formation on the southern North China Craton. The carbon isotope excursions at the Cambrian Series 2–Miaolingian transition in the study area are 0.7‰ in the Chishanhe section and −0.2‰ in the Luoquan section, respectively, showing a weak negative excursion or even no negative excursion. The sedimentary environments in the study area gradually changed through time from a clastic tidal flat to a carbonate platform across the transition, which indicated a gradual rise in sea level, with anoxic conditions occurring predominantly before the RECE δ13C excursion. Microbially induced sedimentary structures and oncoids occurred widely at the top of Cambrian Series 2. Abundant metazoan trace fossils were preserved in the Miaolingian Series of the study area. The evolution of biogenic structures across the Cambrian Series 2–Miaolingian transition indicates the emergence of harsh environments associated with the proliferation of MISS and oncoids at the RECE horizon and the recovery of benthic metazoan fauna after the RECE biotic crisis.

Sedimentary model of the Ediacaran Doushantuo cap carbonates from the southeastern margin of the Yangtze Block: Evidence from carbon and oxygen isotopes and elemental geochemistry

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 δ13Ccarb 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.

Molecular record of the SPICE event in well YD2, Tarim Basin: Insights from catalytic hydropyrolysis of kerogens

The Steptoean Positive Carbon Isotope Excursion (SPICE) event is extensively documented in globally distributed carbonate and organic-rich successions of slope and platform environments during the late Cambrian (ca. 497–494 Ma). Despite numerous studies, its regional manifestation remains controversial because of the major uncertainty of the redox conditions in sedimentary paleoenvironments and its effect on the changes in the biota. Biogeochemical processes that occurred during the SPICE event are important for assessing whether the excursion is associated with the co-evolution of organisms and environments. In this study, we investigated the changes in plankton and paleoenvironment at the molecular level by combining kerogen elemental analysis, catalytic hydropyrolysis (HyPy), and gas chromatography-mass spectrometry (GC-MS) analyses of saturates and aromatics with published carbon isotopic data. The three fluorene series, gammacerane/C30hopane (G/C30H) and pristine/phytane (Pr/Ph) ratios demonstrated that the shallow shelf seawater changed significantly from relatively reducing to oxidizing and back to reducing conditions during the SPICE interval coupled with a regression-transgression cycle. The phytoplankton during the rise of the SPICE were mainly organic-walled, such as cyanobacteria, various acritarchs, and algae, as evidenced by terpanes, hopanes, and steranes. Significant decreases in phytoplankton diversity and abundance coincided with the peak of the SPICE, corresponding to the most oxidized and hypersaline periods during the SPICE event in the eastern Tarim Basin. Plankton assemblages diversified significantly with increasing living space due to the rise of sea-level during the post-SPICE period. Therefore, this study supports the idea that plankton groups changed dramatically with the environment, and its revolution followed the SPICE into the Great Ordovician Biodiversification Event.

Tracking depositional and geochemical variations in the Cambrian North China Platform: Insights from sedimentology, geochemistry, and C-O isotopic records

The Cambrian carbonate sequence in North China is of significant global and regional scientific importance due to its continuous and thick exposure and the data on the major environmental changes and biological events of the early Phanerozoic, which can be gleaned from it. While the well-exposed Cambrian outcrops have been utilized to unravel the processes governing these events, few studies have focused on the outcrops' spatio-temporal depositional patterns and geochemical variations. Therefore, our study address, these issues by integrating fieldwork with petrographic and geochemical analyses. In the North China Platform (NCP), the Cambrian carbonate sequence is predominantly composed of six third-order depositional sequences (~370–400 m thick) that can be divided into nine microfacies, representing three primary microfacies associations, namely tidal flat, lagoon, and shoal. In contrast to the standard carbonate sequence stratigraphy model and many other examples of Cambrian platforms, the deposition of oolitic grainstones in the NCP are overlain by the deep-water calcareous mudstone and shale during a period of slow fall of sea-levels. Such a particular style shows a similar trend to the Schlager model of a type III sequence boundary that formed between a highstand systems tract (HST) and a transgressive system tract (TST). Scanning electron microscope (SEM) observations show an abundance of microbial fossils, a nanosphere, and remnants of extracellular polymeric substances (EPS) inside the dark micrite of normal regressive deposits (oolitic grainstone), suggesting the significant role of microbial activity in the formation of ooids. The δ18O isotopic values range from −4.15 to −11.75‰ VPDB, portraying pervasive diagenetic alterations with a variable degree of diagenesis at different localities, and both marine- and burial-related diagenesis. In contrast, the δ13C VPDB values have a very narrow range, from −2 to +2‰ VPDB, suggesting little to no influence of meteoric diagenesis. At the same time, the slight depletion of δ13C isotope values may reflect the role of microbial processes during the genesis of ooids. This study highlights the importance of microbial activity and marine-related diagenetic overprints during the formation of carbonate facies in the Cambrian Period and provides evidence that carbon isotopic excursions observed during different major environmental and biological events in the NCP are related to primary signals and not altered by intensive meteoric diagenesis.

Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE 2; ca. ∼94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△εHfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼62°S) during the late Cretaceous. The reliability of △εHfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low △εHfclay values (−5.9 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in △εHfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced △εHfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ13C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments.

Carbon Sequestration of the Middle Miocene Sunda Shelf Facilitated Global Climate Change

AbstractLong‐term organic carbon (OC) burial on continental shelves has been widely recognized for regulating atmospheric CO2 (pCO2) and the global climate. However, quantitatively assessing shelf OC burial's role and process in prominent climate transitions is challenging. Using 367 drilling sites, we evaluated the impact of OC burial on the Sunda Shelf, the world's largest tropical shelf, on the middle Miocene carbon perturbation and climate change. Comparing the Miocene Climatic Optimum (MCO) and the Middle Miocene Climate Transition (MMCT) results demonstrated that OC was buried faster during the MMCT with an increment of 0.07–0.14 × 103 GtC/Myr. This would cause an additional 34.77–69.16 ppm pCO2 sequestration, contributing at least one‐sixth of the global pCO2 reduction. We found OC burial regulated the long‐standing Monterey carbon isotope excursion via negative and positive feedbacks during the MCO and MMCT, respectively. Expanded terrestrial carbon reservoirs and enhanced burial efficiency were key steps in the latter positive feedback.