Negative δ13C Excursion Research Articles

First multi-proxy chronostratigraphy of the lower Cambrian Byrd Group, Transantarctic Mountains and correlation within East Gondwana

Antarctica and Australia were sutured together at the equator during the major pulse of animal biodiversification associated with the Cambrian radiation. However, the lack of detailed systematic chemostratigraphic and biostratigraphic sampling of lower Cambrian sedimentary successions from Antarctica has significantly impeded precise age determination and correlation with Cambrian strata on other palaeocontinents. This study is the first to present integrated, simultaneously sampled biostratigraphic and chemostratigraphic (δ13C isotopes) data from the same measured stratigraphic sections through the lower Cambrian Byrd Group in the Transantarctic Mountains. Shelly fossil assemblages (brachiopods, tommotiids, molluscs, bradoriids, trilobites) from the Holyoake Range and Churchill Mountains facilitate direct correlation with the Dailyatia odyssei Zone of South Australia (Cambrian Stages 3–4), and trilobites provide strong correlation between the Starshot Formation and the Cymbric Vale Formation in western New South Wales. A new ID-TIMS radiometric date of 514.96 ± 0.16 Ma from a tuff in the lower Cymbric Vale Formation is similar to dates from tuff beds in the Third Plain Creek Member of the Mernmerna Formation in the Flinders Ranges, providing an important absolute-age tie point between these lower Cambrian successions. Chemostratigraphic data from the upper Shackleton Limestone in the Holyoake Range capture a negative δ13C excursion that can be correlated to negative values within the multi-peaked MICE (cycles V–VIII in Siberia). Integrated faunal and chemostratigraphic data indicate a Cambrian Stages 3–4 age, giving robust chronostratigraphic context for the upper Shackleton Limestone–Holyoake Formation–Starshot Formation succession for the first time, permitting reconstruction of the depositional history of the lower Cambrian of Antarctica and global correlation of Byrd Group strata.

Integrated record of the Late Lutetian Thermal Maximum at IODP site U1508, Tasman Sea: The deep-sea response

The Late Lutetian Thermal Maximum (LLTM) was a transient and brief global warming event recorded in the middle Eocene, at 41.52 Ma. The biotic response to the LLTM has been documented at only a few marine sites so far. Here, we present the first record of deep-sea benthic foraminiferal assemblage changes during the LLTM in the southwest Pacific at International Ocean Discovery Program Hole U1508C (1609 m water depth) in the Tasman Sea. The LLTM coincides with a negative excursion in bulk sediment δ13C (0.47‰) and benthic foraminifera δ13C (0.36‰), with changes in the relative abundance of benthic foraminiferal species and in the deep-water organic geochemistry. The decrease in diversity of the assemblages indicates environmental stress during the event, potentially linked to oxygen deficiency, as evidenced by the occurrence of dysoxic taxa (e.g. Lenticulina spp., Turrillina brevispira). Although calcareous taxa dominate, the presence of corrosion-resistant species and poorly preserved foraminiferal tests suggest slightly CaCO3-corrosive bottom waters, but no dissolution was evident. We suggest the shallowing of the thermocline and enhanced water column stratification at this site during the LLTM.

Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

Multiple abrupt warming events ("hyperthermals") punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ13C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation's sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ13C and δ18O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ13C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ13C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ13C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ13C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

Paleoenvironmental and Paleoecological Dynamics of the U.S. Atlantic Coastal Plain Prior to and During the Paleocene-Eocene Thermal Maximum

Abstract We studied the rapid paleo-environmental changes and the corresponding biotic responses of benthic foraminifera of a shallow shelf site during the late Paleocene and the Paleocene-Eocene Thermal Maximum (PETM). The PETM is globally characterized by a negative δ13C excursion in marine and terrestrial sediments. Isotope data from the Atlantic Coastal Plain from the South Dover Bridge core, Maryland, show an additional small δ13C excursion just below the base of the PETM: the “pre-onset excursion” (POE). The benthic foraminiferal and coupled grain-size record of the late Paleocene indicates a well-oxygenated, current-dominated environment with a stable, high food supply. During the POE, bottom currents become subdued and finer-grained sediment accumulation increased. These changes are partially reversed after the end of the POE. Before the PETM the river influence increases again, food supply becomes more pulsed and the benthic taxa, typically connected to the PETM, start to appear in those gradually warming conditions. During the PETM, the environment shifts to a river-dominated one, with strongly reduced currents. The low-diversity PETM fauna thrives under episodic low-oxygen conditions, caused by river-induced stratification, while the Paleocene assemblage nearly vanishes from the record. Gradually the environment begins to recover, the grain size shows an uptick in bottom currents and pre-PETM foraminifera become more abundant again, indicating increased oxygen levels and a more stable food supply. While the overall environmental shifts at South Dover Bridge fit within the observations across the shelf, the POE related insights are so far unique. Our bathymetric reconstructions show an outer neritic paleodepth (∼100 m) during the Paleocene, with a modest sea level rise in the core phase of the PETM, which is subsequently reversed during the recovery phase.

Apparent Diachroneity of Calcareous Nannofossil Datums During the Early Eocene in the High‐Latitude South Pacific Ocean

AbstractThe late Paleocene to early Eocene interval is characterized by a series of carbon perturbations that caused transient warming (hyperthermal) events, of which the Paleocene‐Eocene Thermal Maximum (PETM) was the largest. These hyperthermals can be recognized in the pelagic sedimentary record as paired negative δ13C and δ18O excursions, in addition to decreased calcium carbonate and increased iron content caused by carbonate dissolution. However, current data are predominantly sourced from the equatorial‐to subequatorial regions. Here we present a new high‐latitude late Paleocene—early Eocene record, recovered during International Ocean Discovery Program (IODP) Expedition 378 on the Campbell Plateau off New Zealand, in the southwest Pacific Ocean. To construct an age model, we correlated our chemostratigraphic and biostratigraphic data to existing astronomically‐tuned age models from Walvis Ridge (South Atlantic Ocean) and Demerara Rise (equatorial Atlantic Ocean). Our results indicate that the Site U1553 composite section spans ∼7 million years of the latest Paleocene to early Eocene (50.5–57.5 Ma), and preserves many of the early Eocene hyperthermals; including a PETM interval that is more expanded than elsewhere in this region. However, construction of the age model also revealed discrepancies between the chemostratigraphic and biostratigraphic tie points used for correlation. This is likely due to latitudinal diachroneity in the calcareous nannofossil biostratigraphic datums, which are primarily based on low‐to mid‐latitude assemblages. Therefore, our study highlights the need to establish a revised calcareous nannofossil biozonation that is more appropriate for high‐latitude age models.

Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution

High resolution stable isotopic (C and O) and geochemical studies of Paleoproterozoic carbonates were carried out in the Umra sub-basin of the Aravalli Supergroup, Rajasthan, India. All the sections, Umra-Dedkiya (UM), Umra-Jampa (BD), and Matoon (MU and GU), have yielded characteristic Lomagundi type positive values, with δ13Ccarb > 4 ‰ VPDB (maximum up to 6.86 ‰). Samples of two sections (UM and BD) have also yielded several distinct negative δ13Ccarb values (up to −6.14 ‰ VPDB). Geochemical validation of trace element and REE data indicate both these types of excursions to be of pristine character, with no signature of organic-remineralisation or hypersaline depositional condition. Mineralogical assemblages lack hypersalinity indicator minerals, such as Na-tourmaline, Na-Albite or scapolite, in the positive δ13C bearing carbonate samples. This supports the inference regarding the observed positive δ13C excursions mimicking the global Paleoproterozoic Lomagundi event. Chemostratigraphic correlation within the basin and with one of the global strato-type Paleoproterozoic carbonate unit from Paso Severino Formation, Uruguay, indicate interruptions in the initial part of the Lomagundi episode marked by several negative δ13C excursion events before achieving a stable positive δ13C character. This phenomenon is also evident from variation in paleoproductivity proxies, such as TOC, TN, Ba and P values, consistent with δ13C fluctuations. Mathematical model calculations have corroborated such a conclusion. All these observations motivated us to present a modified version of the global Paleoproterozoic δ13Ccarb curve and a six stage basin evolution model to explain the evolution of δ13C vis-à-vis variation in atmospheric oxygen.

Aptian–Albian sequence stratigraphy, biostratigraphy, chemostratigraphy, and chronostratigraphy: Sonoran shelf and Tamaulipas Basin, Mexico

The Late Jurassic extensional Bisbee Basin in northwestern Mexico and southwestern United States lay between the Pacific magmatic arc and subduction zone and the cratonic Comanche shelf. Aptian to late Albian fluvial, deltaic, and shallow marine sediments and patch reefs of the Bisbee Group filled the basin. The upper Barremian(?)–lower Aptian Cerro de Oro Formation and its equivalent Rancho La Colgada Formation are overlain by the Morita, Mural, Cintura, and La Juana formations. The paralic Morita Formation and the marine Mural Limestone grade southeast into the marine Agua Salada and Lampazos formations, which correlate with deep-water facies in the Chihuahua trough and Tamaulipas Basin. Three late Aptian to middle Albian long-term depositional cycles were approximately 2.5 myr in duration.Foraminifers, calcareous algae, bivalves, rudists, corals, dinoflagellates and scarce ammonites compose the Mural Limestone biostratigraphy. First-occurrence datums of Aptian–Albian benthic Foraminifera are integrated with planktonic Foraminifera and rudists and define biostratigraphic zones that correlate with the Comanche shelf. In the Chihuahua trough and Tamaulipas Basin planktonic Foraminifera and nannofossils identify global zones. Stage boundaries on the Sonora shelf are correlated by a comprehensive biostratigraphic database.Carbon isotope chemozones correlate with Tethyan chemostratigraphy. The lower Aptian OAE 1a and uppermost Aptian–lowest Albian OAE1b chemozone are in the Lampazos section. The negative δ13C excursions and fossil evidence correlate with global Jacob, Kilian, Paquier, and Leenhardt levels. Low-oxygen water masses flooded the shallow shelf and even affected coral-rudist buildups.

Upper Carnian to Rhaetian (Upper Triassic) conodont and carbon-isotope stratigraphy of the Csővár borehole succession, Hungary

A total of 125 samples from core from the Csővár borehole (north-central Hungary) were investigated for conodont bio- and carbon-isotope chemostratigraphy. This 620 m thick continuous pelagic carbonate succession comprises strata from the upper Carnian to upper Rhaetian, including both, the Carnian-Norian and the Norian-Rhaetian stage boundaries. The mass occurrence of the Metapolygnathus communisti conodont group followed by the occurrence of Primatella gulloae mark the base of the Norian in the lowermost part of the section, which allows good correlation with the Pizzo Mondello section (Sicily, Italy). The lower Norian conodonts of the core belong predominantly to the genus Ancyrogondolella. Subdivision of the middle Norian interval is hampered by the lack of fully adult specimens. The upper Norian shows a greater diversity than the rest of the Norian, and the rich assemblages give an insight into the evolutionary transition from Misikella hernsteini to Mi. posthernsteini (sensu stricto). The first occurrence of Mi. ultima in the core changes the previous views of the thickness of the upper Rhaetian in the Csővár area and emphasizes the necessity of re-calibration of the isotope stratigraphic results of previous studies. The carbon-isotope results show distinct fluctuations and trends. Some of them are similar to literature data, others are different. It can be stated from the new data that a ∼ 2 ‰ long-term δ13C background decline throughout the Norian is most probably global in extent. Negative carbon-isotope excursions occur in Norian substages: (upper) Alaunian, lower Sevatian, middle Sevatian and upper Sevatian. Distinct positive δ13C excursions are present across the Alaunian-Sevatian boundary and the Norian-Rhaetian boundary, and the climax of the latter is located in the lower part of the posthernsteini Zone. This last isotope event is particularly important because at the Global Stratotype Section and Point (GSSP) candidate for the base of the Rhaetian stage at Pignola (Southern Apennines, Italy) a distinct negative δ13C excursion has been reported for this stratigraphic level.

Tethyan ocean acidification triggered the end-Triassic mass extinction: New Ca isotopic constraints from the Qiangtang Basin

Oceanic acidification has long been suggested as a potential environmental trigger of marine ecosystem collapse in the Earth’s history. However, evidence for oceanic acidification as an important controlling mechanism of the end-Triassic mass extinction is still limited. Here, the first coupled carbon- and calcium-isotope records across the Triassic-Jurassic transition are provided. The calcium isotope (δ44∕40Ca) profile from the Qiangtang Basin, eastern Tethys exhibits a prominent negative excursion of ∼0.3‰ at the Triassic-Jurassic (T-J) boundary, which is consistent with the onset of negative δ13C excursion, suggesting a possible relationship between large-scale volcanic eruption and calcium isotopic composition. The Ca isotope perturbation during the T-J boundary is interpreted to be triggered by a significant decrease in CaCO3 burial related to CO2-induced ocean acidification. The coupled δ13C and δ44/40Ca records reveal that the carbon cycle perturbations controlled oceanic acidification and decreased carbonate burial. This provides robust evidence for the causal link between oceanic acidification, carbon cycle perturbation in response to Central Atlantic Magmatic Provence (CAMP), and the T-J boundary mass extinction.

Timing and Provenance of Volcanic Fluxes Around the Permian‐Triassic Boundary Mass Extinction in South China: U‐Pb Zircon Geochronology, Volcanic Ash Geochemistry and Mercury Isotopes

AbstractAnomalous mercury (Hg) contents recorded near the Permian‐Triassic boundary (PTB) are often linked to Siberian Traps Large Igneous Province (STLIP) volcanism and the Permian‐Triassic boundary mass extinction (PTBME). However, mounting evidence indicates that the relation between STLIP volcanism and Hg “anomalies” is not straightforward. This study focuses on the timing and provenance of volcanic fluxes around the PTBME in South China. We constrain carbon isotope (δ13C) and Hg concentration and isotope records by utilizing high‐precision U‐Pb zircon ages from two expanded deep‐water marine sections spanning the Late Permian to Early Triassic in the Nanpanjiang Basin. Results reveal two episodes of Hg enrichment. The oldest episode predates the onset of a large negative δ13C excursion, which is documented to be older than 252.07 ± 0.130 Ma. The second episode occurred between 251.822 ± 0.060 and 251.589 ± 0.062 Ma, coinciding with the nadir of the δ13C excursion. Volcanic ash geochemistry and Hg isotope compositions suggest that mercury was mainly sourced from subduction‐related volcanic arc magmatism in the Tethys region, which peaked between 251.668 ± 0.079 and 251.589 ± 0.052 Ma. These results are compatible with suggestions that regional arc volcanism contributed to the causes of the PTBME in South China and provide evidence that Hg anomalies close to the PTB are not a reliable stratigraphic marker for the PTB extinction event. This study demonstrates that relations between volcanism, environmental perturbations and mass extinction during the Permian‐Triassic transition are better resolved with the aid of high‐precision U‐Pb zircon ages.

A Record of the Snowball Earth Events? The Parecis Basin May Encompass Three Major Neoproterozoic Glaciations on the Amazonian Craton

The Parecis Basin is an intracratonic basin located at the south-southeast portion of the Amazonian craton. Its stratigraphic units comprise siliciclastic and carbonate rocks that potentially record intense paleoenvironmental changes related to the Sturtian (∼717–660 Ma), Marinoan (∼657–635 Ma), and Gaskiers (∼580 Ma) glaciations. We present novel X-ray fluorescence geochemistry, C, O and Sr isotopic data from carbonate rock samples retrieved from the stratigraphic well 2ANP 0006 MT. The results show two negative excursions of δ13C here correlated to post-Sturtian and Marinoan glaciations, recorded in the Salto Magessi and Mirassol d’Oeste carbonate units, respectively. Both negative excursions were followed by δ13C positive incursions. At the base of the section, Salto Magessi carbonate presents unradiogenic Sr ratios (87Sr/86Sr from 0.70655 to 0.70776) typical of Cryogenian interglacial carbonate rocks found elsewhere. The negative δ13C excursion from Mirassol d’Oeste is associated with radiogenic 87Sr/86Sr ratios (up to ∼0.7186), possibly related to continental influx from the intense erosion generated by the Marinoan deglaciation. Upward, the Nobres Formation displays preserved primary sulfate minerals (gypsum and anhydrite) and isotopically heavy oxygen compositions which point to an evaporative environment and support the scenario of enhanced oxidative weathering during early to middle Ediacaran. Finally, despite the absence of cap carbonate, diamictites related to the Serra Azul Formation occur overlying the Nobres Formation and are associated with the Gaskiers event. Therefore, the 2ANP 0006 MT may record three major glacial events from the Neoproterozoic, representing a promising section for further studies concerning chemostratigraphic and paleoenvironmental reconstructions during the Neoproterozoic Era.

Disentangling environmental and diagenetic δ18O and δ13C signals from marine carbonates deposited under warm climate conditions during the early Danian

Paleoceanographic reconstructions are usually based on the chemical composition of foraminiferal tests, and pristine calcite preservation is crucial to support reliable interpretations. Therefore, prior knowledge about the structure and chemical composition of microfossils calcite is important to assess whether geochemical results reflect environmental and/or post-burial signals. Here, we present the first microstructural study of early Danian benthic foraminiferal calcite, and evaluate environmental signals and post depositional biases in benthic and planktonic foraminiferal and bulk carbon (δ13C) and oxygen (δ18O) isotope records at Ocean Drilling Program (ODP) Site 1262 (South Atlantic Ocean). We focused our study on the first ∼350 kyr that followed the Cretaceous-Paleogene (K-Pg) boundary, encompassing the Dan-C2 carbon cycle perturbation. Benthic and planktonic δ13C records show the expected surface-to-bottom gradient, suggesting the preservation of original environmental signals. However, benthic and planktonic δ18O records show strongly overlapping and scattered values, suggesting diagenetic alteration of isotopic signals. We identified the occurrence of authigenic carbonates, such as girvasite and siderite, in benthic foraminiferal tests, which increased Mg, Mn, Ba and Fe contents of tests calcite. Our results support that thermodynamic changes occurred in tandem with the decrease of bottom and pore water oxygenation to favor precipitation of authigenic carbonate phases, which altered isotopic fractionation of δ18O. Nevertheless, bulk sediments clearly depict the negative δ13C and δ18O excursions characteristic of the DanC2, with remarkably low values, characterizing the record of a surface ocean environmental signal. This low δ13C and δ18O were mostly recorded by highly-abundant calcispheres (calcareous dinocysts), suggesting that this group of microfossils has a high potential for surface ocean paleoceanographic reconstructions.

Multiproxy analyses of paleoenvironmental and paleoceanographic changes during the Danian-Selandian in East Central Sinai: An integrated stable isotope and planktic foraminiferal data

Forty-three planktic foraminifera samples from the Themed section (East Central Sinai; Egypt) spanning the Zone Parvularugoglobigerina eugubina (Pα) to the Subzone Acarinina subsphaerica (P4b) have been studied. Data from δ13C, δ18O, and planktic foraminifera-based species diversity, depth habitat, preference for warm and cool surface waters, and nutrients (oligotrophic, mesotrophic, and eutrophic conditions) are used to infer paleoenvironmental changes throughout the Danian‒Selandian duration. Based on quantitative multivariate analyses (hierarchical cluster and principal component), three distinct intervals were recognized, Interval 1 (Pα‒P1b), Interval 2 (P1c‒P3a), and Interval 3 (P3a‒P4b). Interval 2 is further subdivided into three subintervals, 2a (part P1c), 2b (part P1c), and 2c (P2‒P3a). Two δ13C events are identified, Dan-C2 and Latest Danian Event (LDE) and elaborated concerning paleoenvironmental changes. During the earliest Danian planktic foraminiferal Pα Zone, moderately shallow and eutrophic conditions prevailed with cool surface waters and a shallow thermocline. Comparable conditions were still prevailing during P1a‒P1b, but with slightly deeper and mesotrophic conditions and a somewhat deeper thermocline and reduced stratification. P1b‒P1c exhibits a major shift from Eoglobigerina to Subbotina‒Parasubbotina with cooler surface waters and moderate mesotrophic conditions. For Subzone P1c (upper part), slightly mesotrophic conditions were inferred, whereas for P2‒P3a (lower part), surface water warming and thermocline shallowing events have inferred with increased oligotrophic conditions. The Latest Danian Event (mid-P3a) is marked by a dramatic negative δ13C excursion, warm waters, increased mesotrophic conditions, and enhanced stratification. The dominance of Morozovella, Acarinina, and Igorina specify warm and oligotrophic conditions for subzones P3b‒P4b.

Oceanic redox conditions during the terminal Cambrian extinction event

Marine animal diversity during the late Cambrian was reduced by a series of extinctions that have generally been attributed to oceanic anoxic events associated with positive carbon isotope excursions. Here we present carbon and uranium isotope ratios (δ13C and δ238U values) as proxies for global carbon cycle and oceanic redox conditions, respectively, from carbonate rocks of the Wa'ergang section, South China. The dataset spans an interval that includes the last major negative δ13C excursion (TOCE) of the Cambrian Period. The TOCE is a globally documented event, recovery from which corresponds to the terminal Cambrian extinction event. The δ13C and δ238U values covary through the section, shifting initially to lower values, with δ238U falling below the modern open-ocean seawater value from the start to the middle of the profile, followed by a shift to higher values towards the end of the Cambrian. Neither the co-occurrence of δ13C and δ238U negative excursions, nor the association of rising δ238U with extinction have been commonly reported. Here we argue that robust positive coupling of δ13C and δ238U relates to the existence of extensive intermediate reducing settings (from low-O2 suboxia to intermittent anoxia) during the late Cambrian alongside low atmospheric pO2 and a greenhouse climate. Similarly, a stepwise increase in the δ238U baseline in carbonates across the Ediacaran−Cambrian boundary is consistent with the growing importance of an intermediate reducing sink through that interval. We propose further that divergent trends in lower and upper ocean redox conditions could have driven the parallel isotope excursions. An expansion of intermediate reducing conditions, rather than persistent anoxic euxinia, is consistent with the recovery of δ13C and δ238U to higher values, as well as the presence of benthic fauna and shoreward extension of deeper-water fauna that may have had a greater tolerance against hypoxia.

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.

Collapse of Late Permian chert factories in the equatorial Tethys and the nature of the Early Triassic chert gap

Permian chert successions were geographically extensive, spanning from the palaeoequator to the northern high latitudes. Large-scale chert production was abruptly terminated in the latest Permian, resulting in a multi-million-year “chert gap” in the Early Triassic. In order to constrain the tempo of chert production changes and understand their nature, we combine proxy data with Si box model analyses and focus on the Talung Formation of South China—the most representative Upper Permian siliceous unit in the equatorial Tethys. Two deepwater sections from the northern margin of the Yangtze Platform were investigated, showing that bedded cherts had already become less common in the Clarkina changxingensis conodont zone. The waning of chert production coincided with water column deoxygenation and an increase in carbonate and siliciclastic components in the late Changhsingian. The final collapse of the chert factory in South China predated the negative δ13C excursion and climate warming but coincided with a sharp decrease in primary productivity. Together with Si box model output, we suggest that warming-induced expansion of the oceanic dissolved silica inventory (and decrease in burial efficiency) alone cannot maintain a multi-million-year chert gap. Instead, a loss of siliceous biomass during the end-Permian crisis is the primary cause of the Early Triassic chert demise.

New Early Triassic marine δ13C record from the northeastern Yangtze Platform: Implications for contemporaneous temperature changes and volcanic eruptions

The Early Triassic records the largest inorganic carbon isotope excursions of the Phanerozoic. The causes of these enormous δ13C shifts remain unclear. Here, we present a new high-resolution marine carbonates δ13C record from the Yashan section in the northeastern Yangtze Platform of South China. The δ13C profile shows two significant positive excursions (with maximum δ13C values of ∼ +6‰ and ∼ +9‰, respectively) at the Induan–Olenekian boundary and Smithian–Spathian boundary, which is similar to what is recorded in the shallow platform in Nanpanjiang Basin and in the upper Yangtze Platform area. Negative δ13C excursions during the Griesbachian–early Dienerian, Smithian and middle Spathian, are coincident with warming intervals and accounted for by coeval peaks of volcanic activity (e.g., the release of CO2 from the Siberian Traps and arc volcanism). Enhanced marine primary productivity, as a final effect of massive carbon release, could account for the two positive δ13C excursions, with minor influence of local effects on the magnitude of the shifts. Although the age of volcanism after the Permian–Triassic mass extinction is still not constrained, more lines of evidence support a volcanism-temperature-weathering-productivity mechanism of δ13C perturbations during the Early Triassic.

Shelf Ecosystems Along the U.S. Atlantic Coastal Plain Prior to and During the Paleocene‐Eocene Thermal Maximum: Insights Into the Stratigraphic Architecture

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM) is the most pronounced global warming event of the early Paleogene related to atmospheric CO2 increases. It is characterized by negative δ18O and δ13C excursions recorded in sedimentary archives and a transient disruption of the marine biosphere. Sites from the U.S. Atlantic Coastal Plain show an additional small, but distinct δ13C excursion below the onset of the PETM, coined the “pre‐onset excursion” (POE), mimicking the PETM‐forced environmental perturbations. This study focuses on the South Dover Bridge core in Maryland, where the Paleocene‐Eocene transition is stratigraphically constrained by calcareous nannoplankton and stable isotope data, and in which the POE is well‐expressed. The site was situated in a middle neritic marine shelf setting near a major outflow of the paleo‐Potomac River system. We generated high‐resolution benthic foraminiferal assemblage, stable isotope, trace‐metal, grain‐size and clay mineralogy data. The resulting stratigraphic subdivision of this Paleocene‐Eocene transition is placed within a depth transect across the paleoshelf, highlighting that the PETM sequence is relatively expanded. The geochemical records provide detailed insights into the paleoenvironment, developing from a well‐oxygenated water column in latest Paleocene to a PETM‐ecosystem under severe biotic stress‐conditions, with shifts in food supply and temperature, and under dysoxic bottom waters in a more river‐dominated setting. Environmental changes started in the latest Paleocene and culminated atthe onset of the PETM, hinting to an intensifying trigger rather than to an instantaneous event at the Paleocene‐Eocene boundary toppling the global system.

Evidence for massive methane hydrate destabilization during the penultimate interglacial warming

The stability of widespread methane hydrates in shallow subsurface sediments of the marine continental margins is sensitive to temperature increases experienced by upper intermediate waters. Destabilization of methane hydrates and ensuing release of methane would produce climatic feedbacks amplifying and accelerating global warming. Hence, improved assessment of ongoing intermediate water warming is crucially important, especially that resulting from a weakening of Atlantic meridional overturning circulation (AMOC). Our study provides an independent paleoclimatic perspective by reconstructing the thermal structure and imprint of methane oxidation throughout a water column of 1,300 m. We studied a sediment sequence from the eastern equatorial Atlantic (Gulf of Guinea), a region containing abundant shallow subsurface methane hydrates. We focused on the early part of the penultimate interglacial and present a hitherto undocumented and remarkably large intermediate water warming of 6.8 °C in response to a brief episode of meltwater-induced, modest AMOC weakening centered at 126,000 to 125,000 y ago. The warming of intermediate waters to 14 °C significantly exceeds the stability field of methane hydrates. In conjunction with this warming, our study reveals an anomalously low δ13C spike throughout the entire water column, recorded as primary signatures in single and pooled shells of multitaxa foraminifers. This extremely negative δ13C excursion was almost certainly the result of massive destabilization of methane hydrates. This study documents and connects a sequence of climatic events and climatic feedback processes associated with and triggered by the penultimate climate warming that can serve as a paleoanalog for modern ongoing warming.

Terrestrial and marine organic matter evidence from a Cretaceous deep-sea chert of Japan: Implications for enhanced hydrological cycle during the Aptian OAE 1a

The early Aptian Oceanic Anoxic Event (OAE 1a; ca. 120 Ma) was accompanied by a massive eruption of the Ontong Java Plateau, doubling atmospheric pCO₂, and increasing sea surface temperature by 4–8 °C. In this study, we attempted a palynological analysis of the Hauterivian to Cenomanian deep–sea chert from the Goshikinohama site, and successfully obtained a variety of microremains, mainly represented by amorphous organic matter, aggregations of organic and inorganic particles, and cuticle fragments. One Hauterivian and many Aptian samples unexpectedly yielded terrestrial plant material, such as resins and wood fragments with tracheids, which represent the first record of terrestrial plant microremains from deep-sea pelagic chert. The occurrence of wood fragments within the negative and positive δ13C excursions during the OAE 1a can be correlated with an interval of increased atmospheric CO₂ level. We can also suggest that the occurrence of resins, mainly within the mudstone-rich chert intervals of the early Aptian negative δ13C peak and the middle-upper Aptian interval, are possibly related with periods with sea-level drops. Similar massive resin deposition events have been linked with intensified hydrological cycling during other abrupt climatic disruptions, such as the Carnian Pluvial Event, the Triassic−Jurassic boundary, and the Paleocene−Eocene Thermal Maximum. Therefore, an increase of terrestrial plant burial in deep–sea chert is likely linked with a massive plant discharge related with enhanced hydrological cycles, such as intensified storms and/or some sea–level change during the OAE 1a.