Negative δ13Corg Research Articles

Sedimentary facies controlled biogeochemical process of biotic extinction and turnover across the Cambrian SPICE event

The Steptoean positive carbon isotope excursion (SPICE) event, one of the largest carbon cycle perturbations in the Cambrian, coincides with shallow-shelf-fauna extinction and plankton revolution (critical transition of plankton). The event is globally documented, but biogeochemical responses of these biotic evolutions in varying facies environments are not well understood. Here high-resolution paired δ18Ocarb, δ13Ccarb and δ13Corg datasets from varied paleodepth environments in the Tarim Basin, NW China reveal facies-dependent signatures of the event, with globally synchronous patterns but notable intra-basinal variability. Shallow marine facies record the end-Marjuman extinction with a distinct negative δ13Corg excursion prior to the event, while the transitional facies region marks twice positive δ13C excursions corresponding to an asynchronous plankton revolution from shallow and deep areas during the event. The varying isotope responses are interpreted in the context of primary productivity and redox conditions, with deeper basins recording more 13C enriched signals (i.e., higher δ13C) due to greater organic matter preservation under anoxic conditions, compared to the platform area. The biotic extinction, the planktonic revolution and the interaction of organisms along the shallow to deep marine depth gradient were reflected by the significant isotopic shifts recorded during the event, suggesting depth-dependent biogeochemical processes that shaped marine ecosystems.

Carbon, iron and sulfur records of lacustrine paleo-environments during the middle Eocene in eastern China

It is widely recognized that anoxic conditions facilitate the preservation of organic carbon in marine sediments. However, the specific geological factors that lead to the development of such conditions in paleo-lakes are less well understood. Owing to their smaller size, paleolakes could experience more frequent and stronger changes in geochemical conditions than oceans. Such changes, such as volcanism, hydrothermal fluids, or ocean transgressions, can also strongly affect the lacustrine organic carbon burial thereby complicating sediment diagenesis record. Here, we used total organic carbon content (TOC), organic carbon isotope (δ13Corg), iron speciation, and pyrite sulfur isotope (δ34Spy) data to establish relationships between organic carbon preservation and anoxic conditions in fine-grained sediments from the middle Eocene lacustrine depositional environments from the Shahejie Formation of the Jiyang Depression, Bohai Bay Basin, eastern China. The results reveal TOC between 1 % and 10 %, highly-reactive iron to total iron ratios >0.38, and most TOC to total sulfur ratios exceeding 2. These data indicate that the organic-rich shales of the Shahejie Formation were formed as a result of high primary productivity during the warm and humid middle Eocene period, coupled with the efficient preservation of organic matter in anoxic bottom waters. Negative δ13Corg and δ34Spy excursions recorded in the Shahejie Formation indicate water column conditions to have been influenced by transient volcanic eruptions. Positive δ13Corg and negative δ34Spy excursions may have been caused by hydrothermal fluids input whereas δ34Spy values approaching 20 ‰ suggest frequent marine transgressions. In particular, despite potential inputs of S into the paleolake by volcanism, hydrothermal fluids, or marine transgressions, bacterial sulfate reduction efficiently depleted the sulfate pool to have created ferruginous geochemistry water conditions for the effective preservation of organic carbon in sediments. Our results establish a direct link between lacustrine shale geochemical signatures and geological phenomena that impact its sedimentation.

Sedimentary Expressions of the Early Jurassic Jenkyns Event in an Inland Lacustrine System in the Yin'gen–Ejinaqi Basin, North China

AbstractThe Jenkyns Event, more widely known as the Toarcian Oceanic Anoxic Event (T‐OAE), is marked by globally distributed negative carbon‐isotope excursions, widespread oxygen depletion, and large‐scale organic carbon burial, which indicate major climate/environmental perturbations in Earth's surface systems during the Early Jurassic. Although extensive research has been conducted in European continental settings, particularly in the western peri‐Tethys regions, the impacts of this event beyond Europe remains largely unexplored. Here, a multiapproach study including investigations into the spore‐pollen assemblages, pyrite framboids, clay minerals, total organic carbon (TOC) levels, and organic carbon isotope (δ13Corg) levels in a lacustrine borehole section (MED1) from the Yin'gen–Ejinaqi Basin, North China, provides evidence of the occurrence of the Jenkyns Event and its extensive sedimentary responses in the eastern Tethys terrestrial systems. Two distinct spore‐pollen assemblages have been identified in MED1 (drilling depth: 982.4 m to 1267.5 m), with the Cycadopites–Protopinus–Osmundacidites assemblage in the lower part (1267.5 m to 1132.9 m) indicating a middle Early Jurassic age and the Classopollis assemblage in the upper part (1132.9 m to 985.7 m) suggesting a Toarcian age. Framboidal pyrite data suggest more anoxic conditions during the deposition of black mudstone and shale intercalations in the lower part of the Classopollis assemblage (1132.9 m to 1066.9 m), which combined with organic carbon enrichment and negative δ13Corg excursions, are considered the paleoenvironmental response to the Jenkyns Event in the study area. Furthermore, the evolution of vegetation groups changed from plant groups characterized by bisaccate and cycad pollen, as well as fern spores, to vegetation groups represented by Cheirolepidiaceae pollen across the Jenkyns Event, as evidenced by spore‐pollen data, together with the clay mineral assemblage change characterized by a notable increase in illite at the expense of kaolinite, suggests that while a subtropical‐temperate climate persisted, a change toward warmer and drier conditions most likely occurred in the early Toarcian in the study area. In contrast to the humidification evidenced in many coastal settings, this aridification trend in the Yin'gen–Ejinaqi Basin aligns with the conditions in many inland areas. It is hypothesized that the underlying cause of these divergent changes may be linked to certain patterns of spatially variable water availability on land, potentially driven by extremified hydrological conditions.

Volcanic and orbitally forced carbon release during the Middle Eocene Climatic Optimum

The drivers of the Middle Eocene Climatic Optimum (MECO) remain enigmatic. Here we report a high-resolution terrestrial MECO record from the Bohai Bay Basin, eastern China. The record shows episodic Hg enrichments and a volcanogenic Δ199Hg signature during the MECO, and an abrupt chemical weathering enhancement and negative δ13Corg excursion (CIE) during the MECO peak warmth. A high-resolution age model constrains the MECO to ca. 40.59−40.18 Ma with the onset in the first ∼320 k.y. Peak warmth and the CIE initiated at 40.27 Ma, corresponding to 405 k.y. and 100 k.y. eccentricity maxima, and lasted for ∼90 k.y., reminiscent of the early Eocene hyperthermals. Our findings suggest that episodic volcanism contributed to gradual atmospheric pCO2 rise, leading to the long duration of the MECO onset. Orbital forcing at 40.27 Ma, on top of CO2 forcing, caused the peak warmth and a positive carbon cycle feedback. The regional increase in weathering is inconsistent with evidence for weakened global continental weathering, suggesting strong spatial heterogeneity in the weathering response to MECO warming.

Isotopic composition of carbon (δ13C) and nitrogen (δ15N) of petrologically different Tertiary lignites and coals

This study investigates the carbon (δ13Corg) and nitrogen (δ15N) isotopic composition of tertiary lignites and coals from six sedimentary basins: Velenje, Mura-Zala, and Zasavje in Slovenia; Sokolov in Czech Republic, Barito in Indonesia; and Istria in Croatia. The aim is to investigate the correlation between the fine detrital (fD) component and δ13C and δ15N in Velenje lignite samples. Additionally, we aim to evaluate the biogeochemical processes of organic substances during their deposition in all analyzed samples, calculate their δ13CCO2 values and compare the analyzed values of δ13C and δ15N to those reported in the literature. Thirty-two samples were analyzed, predominantly from the Velenje ortho-lignite (Pliocene), with additional lignites and coals from the Pannonian to Paleocene epochs for comparison. Carbon isotopic composition (δ13Corg) ranged from -27.9 to -23.6 ‰, and nitrogen isotopic composition (δ15N) ranged from 1.8 to 7.4 ‰. The fine-detrital lithotypes of the Velenje ortho-lignite exhibited the most negative δ13Corg values due to anaerobic bacterial activity in an intramontane alkaline lake environment influenced by the carbonate hinterland. Moreover, gelification processes affected fine-detrital organic matter more than larger wooden pieces. Terbegovci, Hrastnik meta-lignites, and Barito sub-bituminous coal also displayed low δ13Corg values, indicating limited gelification, while variations in the δ15N values suggested differences in mineralization. The Velenje xylitic lithotypes have higher δ15N values, indicating a more intense mineralization under aerobic conditions. Raša ortho-bituminous coal, deposited in a brackish environment, displayed the highest δ13Corg values and a wide range of δ15N values due to fluctuating water tables in a paralic carbonate platform environment. The lowest δ15N value was observed in the Sokolov Basin lignite coal, indicating minimal mineralization and low bacterial activity. The isotopic composition of CO2 in air (δ13Cair), which was calculated using the δ13C values in lignites and coal, ranged from -8.4 to -3.4 ‰, with Velenje lignite displaying the minimum value and Raša coal showing the maximum value. The determined δ13C and δ15N values of the coal and lignite samples in this research fall within the typical range of world coals.

Ichnological response to the Middle Eocene Climatic Optimum (MECO) in the Bartonian deposits of Kutch Basin, India

The Middle Eocene Climatic Optimum (MECO) represents a short-lived interval of global warming that disrupted the long-term cooling episode extending from the middle to late Eocene time. Biostratigraphically the MECO corresponds to the upper interval of planktic foraminiferal zones E11–E12 and the shallow benthic zone 17. Both the planktic and benthic foraminiferal zones are well-documented within the Bartonian mixed siliciclastic‑carbonate sequence of the Kutch Basin in western India. This case study aims to investigate the impact of MECO on the endobenthic community using ichnology as a proxy. The studied sequence comprises the underlying Harudi Formation, consisting of the marine green and grey shale and bioclastic limestone beds, and the overlying Fulra Limestone characterized by benthic foraminifera-rich limestones. The Harudi Formation is bereft of trace fossils except in one bioturbated firmground and two bioeroded hardground horizons. The MECO corresponds to the thin carbonate packstone, the middle green shale, and the Nummulites obtusus rudstone occurring at the middle to upper part of the Harudi succession, established earlier by a concurrent negative δ13Corg isotopic excursion. The shelly limestone or bivalve coquina bed deposited immediately prior to the beginning of the MECO consists of a monospecific firmground ichnofabric characterized by Balanoglossites triadicus. Out of two bioeroded beds, the lower one coincides with the earliest MECO interval and is marked by a brown carbonate hardground colonized with two ichnospecies of Gastrochaenolites. The upper bioeroded horizon corresponding to the end of the MECO is characterized by a series of top- and side-bioeroded (with Gastrochaenolites, Entobia, and Oichnus) carbonate nodules containing multigenerational septarian cracks. The upper hardground demarcates the top of the Nummulites obtusus coquina bed and the starkest basin-wide sediment starvation, making it a major condensed horizon that developed following a steady glacio-eustatic rise during the warming event. The MECO prompted the subsequent deposition of shallow-marine platform carbonates of the Fulra Limestone by accelerating a sharp rise in the sea-level and proliferation of diverse Larger Benthic Foraminifera in a warm oligotrophic seawater.

Palaeoenvironmental changes in the Late Triassic lacustrine facies of the Ordos Basin of Northwest China were driven by multistage volcanic activity: Implications for the understanding the Carnian Pluvial Event

The Carnian Pluvial Event (CPE) induced an intense carbon-cycle disturbance and biological crisis at a global scale. The eruption of the Wrangellia Large Igneous Province (W-LIP) is considered a possible triggering mechanism; however, this cannot fully explain the significant differences in environmental perturbations across different regions. In this paper, we present detailed records of petrography, mercury geochemistry, carbon and sulfur isotopes, trace elements, and provenance data for the Triassic lacustrine facies of the Ordos Basin in the North China Plate, which provide insights into the important effects of volcanic activities, other than the W-LIP, on palaeoenvironment during the CPE interval. Massive emissions of SO2, CO2, and pyroclastic deposits resulting from these volcanic activities triggered four humid episodes, each characterized by lake-level rise, negative δ13Corg excursion, a transition from mudstones to organic-rich shales facies, as well as algal bloom within a time span of 180 kyrs. The establishment of an age framework in the Ordos Basin enables comparative analyses of environmental evolution during the Carnian age across the North China Plate, the western Tethys domain, and the South China Plate. During the period of global climate perturbation driven by the W-LIP eruption, we conclude that short-term localized volcanic activities may have further induced high frequentcy environmental perturbations, which influenced biological shifts and increase organic carbon burial. These short-term volcanic activities with different occurrence times and frequencies complicate local-scale palaeoenvironmental evolutions, which may be an important reason contributing to significant differences in environmental perturbations during the CPE interval across different regions of the globe.

Chemo- and bio-stratigraphic constraints on Cretaceous-Paleocene biotic turnover in the southern Tethys low-oxygen margin, Egypt

The late Cretaceous-Danian (K-Pg) organic-rich sequence is a global archive that witnessed abrupt paleo-condition changes during a high sea-level. In Egypt, this sequence is present in the Duwi and Dakhla formations as an E-W belt from the Red Sea to the Western Desert. We analyzed elemental and isotopic distributions in an age-constrained core from the Quseir area to understand paleoclimatic and paleoenvironmental factors influencing its deposition. Calcareous nannofossil and the chemical index of alteration (CIA) confirm a global cooling trend during the late Cretaceous, with two warming episodes indicating early phases of the Deccan volcanism. The Danian stage experienced a warm climate and intense chemical weathering. Mo-U enrichment factors covariation suggest deposition under oxygen-deficient conditions with evident watermass restrictions. High-productivity-indicating taxa were more abundant in Danian strata than in late Cretaceous strata, consistent with organic carbon characteristics. The δ13COrg-δ15N data show temporal heterogeneity due to organic matter type, redox conditions, preservation, stratigraphic condensation, and recycling of isotopically-light CO2. The negative δ13COrg shift around the K-Pg transition is globally correlative. Dynamic pyrite δ34S distribution resulted from variation in riverine influx, degree of pore-water openness, sulfate reduction rate, and diagenetic disproportionation of sulfur intermediates. Generally, radiogenic 187Os/188Os values were observed that were seemingly influenced by continental weathering and subsequent runoff influx of 187Os/188Os under a greenhouse climate. A positive 187Os/188Os shift to 3.68 (187Os/188Osi of 0.74) occurred at the onset of the Dakhla organic-rich spike. However, without a precise correlative stratigraphic framework, a global or regional seawater isotopic shift cannot be confirmed. We suggest that temporal variations in organic matter composition, redox, watermass conditions, and isotope fractionation of the K-Pg sequence in the study area were driven by discrepancies in climate, sea-level, and tectonic uplift. Further integrated approaches are needed to explore the geospheric drivers of this organic-rich sequence in time-equivalent sections.

Phosphate δ13Corg chemostratigraphy from the Gantour basin, Morocco: A proof of concept from the K–Pg transition to mid-Thanetian

The Late Cretaceous–early Paleogene interval is globally associated with transient to long-term changes in the stable carbon isotopic composition of marine carbonates (δ13Ccarb). Based on biostratigraphic reconstruction, this critical period of Earth's history is thought to coincide with the deposition of world heritage Paleocene phosphate deposits (phosphorites) in northwestern Morocco. However, the detailed stratigraphy of the Gantour basin, one of the most important Moroccan phosphate deposits, has not yet been constrained. For instance, the former “Montian” Stage has been used to tentatively approximate the Danian, whereas the succeeding Selandian Stage remains to be identified. Here, we develop a detailed organic carbon isotopic (δ13Corg) curve from phosphorus-rich horizons of the western Gantour sedimentary sequence in an attempt to constrain their stratigraphic placement and depositional age model. Upsection, these strata host long-term negative and positive δ13Corg trends that tend to correlate with global δ13Ccarb records of the Cretaceous–Paleogene and mid-Thanetian transitional boundaries. The data support the presence of Danian and Selandian rocks in the Gantour basin, which are succeeded by strata containing characteristic signatures of the well-known Cenozoic δ13C maximum at 58–57.5 Ma (the Paleocene Carbon Isotope Maximum). Our results shift the previously proposed Cretaceous–Paleogene transition in the Gantour basin further down into the older sediment C2M layer without interfering with recorded massive biological turnover in faunal diversity and abundance. Moreover, the refined stratigraphy suggests that the deposition of the Gantour phosphorites spanned ∼8.5 Myr. Our results confirm the utility of δ13Corg chemostratigraphy for dating and correlating phosphate-bearing deposits of the Tethyan province. They have important implications for deciphering Paleocene phosphogenesis, the co-evolution of associated vertebrate groups, and for prospecting phosphorus-rich mineral deposits.

Hydrothermally-altered coal from the Daqingshan Coalfield, Inner Mongolia, northern China: Evidence from stable isotopes of C within organic matter and C-O-Sr in associated carbonates

Igneous intrusions can bring hydrothermal fluids, metals, and ligands to the surrounding host sequences, changing their physical, textural and chemical features to some extent depending on the degree of thermal exposure. However, regarding hydrothermally-altered coals, previous studies have predominately focused on variations in coal quality, chemistry and mineralogy, despite the growing research on stable and radioactive isotopes within coal-bearing sequences.The Pennsylvanian coal from the Daqingshan Coalfield, Inner Mongolia, China, was thermally affected by a granitic intrusion in the eastern part of the coalfield during the Yanshanian Orogeny, between the Late Jurassic and Early Cretaceous. The investigated Hailiushu, Datanhao, and Adaohai mines in present study are distributed in Daqingshan Coalfield from west to east. The Adaohai Mine is located to the southwest of the intrusion point, at a distance of approximately 50 km. The Hailiushu and Adaohai mines are about10 km apart, and the Datanhao Mine is located between them. With decreasing distance to the intrusion, there is an increase in vitrinite reflectance values (from 0.84% through 1.17% to 1.58%) and a decrease in the volatile matter content (from 44.57% through 31.31% to 19.60%) in the studied three coal mines.The present study revealed that the westernmost Hailiushu coals have the most negative δ13Corg values (from −24.8 ‰ to −23.5 ‰; −24.3 ‰ on average). In contrast, with the distance from the intrusion decreases, the δ13Corg values of coals from the Datanhao mine range from −24.8 ‰ to −23.5 ‰ (−24.2 ‰ on average). The δ13Corg values of the easternmost Adaohai coals range from −23.8 ‰ to −22.8 ‰, (−23.3 ‰ on average), about 1 ‰ less negative than those in the Datanhao and Hailiushu coals. It is not just the maceral composition that causes the difference in organic carbon isotopes, but the loss of 12C-enriched volatiles due to the thermal effects associated with the igneous intrusion. This hypothesis was further supported by Rock-Eval analysis on the organic matter: the average Tmax of the Hailiushu, Datanhao, and Adaohai coals increases but generation of hydrocarbons and pyrolysable carbon decrease as approaching the intrusion. Moreover, fracture/cleat-filling carbonates are more abundant in the Adaohai coals than in the Datanhao and Hailiushu coals. The δ13CPDB (from −14.2 ‰ to 1.7 ‰) and δ18OPDB (from −16.9 ‰ to −7.0 ‰) of these vein carbonates all indicate a hydrothermal origin. Hydrothermal fluids derived from igneous intrusions is also responsible for the elevated 87Sr/86Sr ratios (0.711392–0.717643) in the carbonate mineral in the thermally altered coals.

End-Permian terrestrial ecosystem collapse in North China: Evidence from palynology and geochemistry

The Permian-Triassic Mass Extinction (ca. 252 Ma; PTME) is the most severe biocrisis of the Phanerozoic in both the oceans and on land. The crisis saw the collapse of terrestrial ecosystems in low, mid and high latitudes. Although terrestrial plant losses have been implicated as a driver of concurrent changes in terrestrial sedimentary environments and facies (e.g., fluvial style and/or grain size), the relationship between extinction and environmental change in the North China Plate (NCP) remains uncertain due to a paucity of plant macrofossils. We explore the relationship between terrestrial environments and changes in plant communities using a combination of sedimentology, palynology, geochemistry, mineralogy and charcoal data from a terrestrial succession in the Yiyang Coalfield located in the southern NCP. Our multiproxy approach places the end-Permian Terrestrial Collapse (EPTC) at the base of bed 20, below the level of the main (marine) PTME at the top of bed 21. The EPTC manifested as a rapid loss of vegetation accompanied by climatic warming and frequent wildfires. The main PTME was accompanied by warming, spikes in the Chemical Index of Alteration (corrected CIA, CIA*) and sedimentary Ni (Ni/Al) concentrations, and a transition from arid floodplain to fluvial facies in the sedimentary record. Our results reveal a rapid increase in charcoal content, rapid decline of spore-pollen content, disappearance of plant macro-fossils, rapid decline in TOC content, the onset of a negative δ13Corg excursion, and a shift in lithology from grayish-green sandstone and mudstone to purplish-red mudstone, suggesting that wildfires induced by global warming during the early eruption phase of the Siberian Traps Large Igneous Province triggered terrestrial ecosystem collapse in the NCP prior to the PTME. Plant extinctions during the EPTC were accompanied by changes in sedimentology and environment, but there was no abrupt change in fluvial styles. Temporal coincidence suggests that shifts in end-Permian terrestrial ecosystems toward those tolerant of warmer and more environmentally stressed environments were driven by concurrent Siberian Traps volcanism.

The Emeishan large igneous province eruption triggered coastal perturbations and the Capitanian mass extinction: Insights from mercury in Permian bauxite beds

The Capitanian mass extinction occurring at the mid-Capitanian to the Guadalupian–Lopingian boundary (GLB; ca. 262–257 Ma) was temporally coincident with the eruption of the Emeishan large igneous province (LIP). Recent observation of Hg anomalies at the GLB at shelf and slope/basinal settings demonstrated a possible causal link between the Capitanian extinction and the Emeishan LIP eruption. However, Hg records of the GLB at coastal settings remain uninvestigated, limiting our knowledge of global impact of the Emeishan LIP and its role in the Capitanian mass extinction. Here we report new datasets of Hg concentrations, Hg isotopes and organic carbon isotopes in two coastal shallow-water sections in the Guangxi region, SW China. Elevated Hg/TOC ratios (up to 3330 and 221 ppb/wt%, respectively) and near-zero to positive Δ199Hg values (−0.04‰ to +0.26‰) were observed across the GLB in the two sections, indicative of extensive volcanic Hg input to the ocean via wet Hg(II) deposition. A negative δ13Corg excursion (2.5‰) coincides with widespread marine anoxia, peak Emeishan LIP magmatism, Hg/TOC anomalies, sea-level rise, and the Capitanian mass extinction. The temporal link between these phenomena suggests that the Emeishan LIP was a key driver of the climate/ocean dynamics, global ecosystems, and the formation of bauxite beds during the GLB.

Interactions of the Westerlies and Asian Summer Monsoon Since the Last Deglaciation in the Northeastern Qinghai‐Tibet Plateau

AbstractThe Asian summer monsoon (ASM), controlled by tropical ocean‐atmosphere systems, and the westerlies, directly linked to high‐latitude climates, interact in mid‐latitude East Asia. Located at a climate junction, the northeastern Qinghai‐Tibetan Plateau (QTP) is a sensitive zone for the interaction of the westerlies and ASM. In the current work, we present a new loess data set from the northeastern QTP since the last deglaciation, including grain size, stable isotope (δ13Corg and δ18Ocarb), total organic carbon, total nitrogen, magnetic susceptibility, and mineral composition. The PMIP3‐CMIP5 multi‐model ensemble and TraCE‐21ka transient simulation are used synthetically to analyze climate mechanisms. All proxies suggest instable glacial climate and stable interglacial climate in the northeastern QTP. The coarser grain size during the last deglaciation compared with the Holocene reflects intensified westerlies controlled by the weakening of Atlantic Meridional Overturning Circulation and existing ice sheets. More positive δ18Ocarb values during abrupt cooling events [Heinrich event 1 (H1) and the Younger Dryas (YD)] indicate the effect of a weakened monsoon triggered by the low‐latitude solar insolation and sea surface temperature. Therefore, the anti‐phase pattern between the ASM and westerlies is the primary reason for differences in climate stability. Moreover, low temperature favors the predomination of C3 plants and the development of permafrost, leading to more negative δ13Corg values and fine grain size during the H1 and YD. Taken together, the anti‐phase relationship between the westerlies and ASM emphasizes the complicated dynamic characteristics of atmospheric systems in the mid‐latitudes and the challenges for future predictions.

Ecological adaptation of marine floras and faunas across the Early Jurassic Toarcian Oceanic Anoxic Event – A case study from northern Germany

We present new, high resolution bio- and chemostratigraphic data (calcareous nannofossils, ammonites, δ13Corg) of the lower Toarcian from an outcrop (Hondelage) in northern Germany. The three data sets, which cover lithostratigraphically the uppermost Amaltheenton-Formation (upper Pliensbachian) and the lower part of the Posidonienschiefer-Formation (lower Toarcian), allow a calibration of the different stratigraphic schemes in ultra high resolution. They further shed light on the ecologic conditions controlling the assemblage composition and evolutionary patterns of calcareous nannofossils, ammonites and gastropods.The distinctive negative δ13Corg excursion (negative CIE, Jenkyns Event), which is part of the early Toarcian Oceanic Anoxic Event (T-OAE), falls lithostratigraphically into the lower part of the laminated black shales of the Posidonienschiefer facies. The lower part of the Jenkyns Event is marked by the first occurrences of the calcareous nannofossil species Carinolithus superbus crassus and Diductius constans. These two events are biostratigraphically assigned to the Tiltoniceras antiquum ammonite Biohorizon (uppermost Dactylioceras tenuicostatum ammonite Zone). The top of the Jenkyns Event has been assigned to the Cleviceras exaratum ammonite Biohorizon (Harpoceras serpentinum ammonite Zone) of the lower Toarcian. The last occurrence of the nannofossil species Biscutum grande lies in the middle part of the Jenkyns Event.Calcareous nannofossils, ammonite faunas and gastropods are discussed with respect to the environmental and climatic changes, which occurred across the Pliensbachian/Toarcian boundary interval and throughout the Jenkyns Event. The extreme impoverished nannofossil and ammonite assemblages of the uppermost Pliensbachian are interpreted in the context of a cooling, primarily affecting the calcareous shelled primary producers. The onset of the Jenkyns Event in the lower Toarcian post-dates this cooling and is related to a subsequent warming. This climatic shift caused a bloom of diverse opportunistic nannofossil and ammonite taxa in the lower Toarcian. Nannofossil abundance patterns were controlled by an increased nutrient availability. The rapid evolution of the ammonite genera Tiltoniceras, Eleganticeras and Cleviceras (family Hildoceratidae), which started in the lower part of the Jenkyns Event, is potentially related to their adaptation to a shallow water dwelling habitat. The mass occurrence of phylogenetically first holoplanktonic gastropods in the lower part of the Jenkyns Event is interpreted in the context of impoverished ocean water oxygenation, which in turn triggered the evolution of a new ecological strategy.

Intensive peatland wildfires during the Aptian–Albian oceanic anoxic event 1b: Evidence from borehole SK-2 in the Songliao Basin, NE China

The Cretaceous has been considered a “high-fire” world accompanied by widespread by-products of combustion in the rock record. The mid-Cretaceous oceanic anoxic event 1b (OAE1b) is marked by one of the major perturbations in the global carbon cycle characterized by deposition of organic-rich sediments in both marine and terrestrial settings. However, our understanding is still limited on changes in wildfire activity during OAE1b period. Here, we carried out a comprehensive analysis, including organic carbon isotope (δ13Corg), total organic carbon (TOC), coal petrology, trace elements, and pyrolytic polycyclic aromatic hydrocarbons (pyroPAHs), of coal seams of the middle Aptian to early Albian Shahezi Formation from borehole SK-2 in Songliao Basin, Northeast China. Two negative δ13Corg excursions in the Shahezi Formation can be corresponded with the 113/Jacob and Kilian sub-events of OAE1b. Moreover, the intensive peatland wildfires have been identified during the sub-event periods of OAE1b based on the co-occurrence of high abundance of charcoal and pyroPAHs at that time. In addition, Sr/Ba, Sr/Cu and Sr/Rb ratios demonstrate that enhanced peatland wildfires were controlled by dryer climate conditions owing to episodic northward migration of arid zones in East Asia related with rising global temperature during the sub-events of OAE1b. The climate-driven extensive wildfire activity in the mid-latitude terrestrial ecosystems can be a contributing factor for OAE1b through the increased flux of nutrients fuelling primary producers in the lake and marine environments and leading to more speculative anoxia to allow the deposition of organic-rich sediments. Our results provide essential understanding of the importance of wildfires in driving mechanism of oceanic anoxic events (OAEs) in Earth's history.

Paleoenvironmental evolution during the Early Eocene Climate Optimum in the Chicxulub impact crater

The Early Eocene represents the warmest climate during the Cenozoic Era punctuated by transient warming events. Here, we apply for the first time biomarker and stable isotope data to describe the depositional and environmental changes at the actual Chicxulub impact crater [International Ocean Discovery Program (IODP) Site M0077A] during the Early Eocene (∼56–48 Ma) including the Early Eocene Climate Optimum (EECO; ∼53–49 Ma). TEX86-based reconstructions indicate elevated sea surface temperature around 37°C for the entire succession, punctuated by warmer and cooler periods (37.8°C, 35.7°C, respectively). Biomarker and δ13C-org data of the core sediments reveal a shift from an organic-lean lower to an organic-rich upper section of the Early Eocene interval. Embedded in the lower section is a prominent organic-rich black shale layer. The lower section, directly overlying strata deposited upon the Paleocene-Eocene Thermal Maximum (PETM), is organically lean with low biomarker concentrations indicating low primary productivity. In the upper section, higher concentrations of algal and bacterial biomarkers are ascribed to high primary productivity and nitrogen fixation in an epipelagic zone, potentially supported by upwelling. The organic-rich black shale layer exhibits a negative δ13C-org isotope excursion ascribed to recycling of 13C-depleted organic matter via upwelling attributed to a transient hyperthermal event.

Terrestrial record of carbon-isotope shifts across the Norian/Rhaetian boundary: A high-resolution study from northwestern Sichuan Basin, South China

A series of biotic crises and turnovers have been recorded at the Norian/Rhaetian boundary (NRB), together with a putative perturbation to the global carbon cycle in the form of one or more negative carbon-isotope shifts. Despite this evidence, the full validation of Norian/Rhaetian climate changes has not been possible. In particular, no clear record of Norian/Rhaetian environmental change has yet been recovered from a terrestrial setting. The eruption of the Angayucham Large Igneous Province has been considered as a possible cause for the putative NRB negative carbon-isotope shifts. Hg as a volcanism proxy has been well-studied in many ancient marine and terrestrial sediments, but to date no Hg record has been obtained across the NRB. In this study, we focus on a terrestrial Norian-Rhaetian section (Xujiahe section) located north of Guangyuan in the northwestern Sichuan Basin, which has been correlated to the NRB using cyclostratigraphy and magnetostratigraphy. A long-lived negative trend in δ13Corg, superimposed with four negative shifts, is recognized across the NRB interval. This can be correlated to the uppermost part of the third negative δ13Corg shift (named S3) recognized in profiles from marine sites in Europe. Contemporaneous Hg concentration, Hg/Al and Hg/TOC anomalies in the Xujiahe section were possibly caused by large-scale volcanic activity. The large amplitude changes in δ13Corg (ca. −5.2‰) that we observe are suggested to be associated with local wildfire and enhanced coarse sediment input, indicating a significant environmental change during the NRB interval. Similar paleoclimatic perturbations have been noted across other extinction events (e.g., end-Permian mass extinction event) as a result of the emplacement of large igneous provinces, or because of felsic volcanism.

Geochemical and carbon isotopic studies of carbonaceous phyllites from Dharwar craton, India – Reconstruction of Precambrian depositional environment

Carbonaceous shales formed in anoxic, deep marine environment are depocenters of prolific microbial life and rich sources of organic matter. In the present study, geochemical and organic carbon isotopic (δ13Corg) signatures of carbonaceous phyllites from Archean Chitradurga, Gadag, Sandur, Shimoga greenstone belts and Proterozoic Cuddapah basin of Dharwar Craton were studied to understand the Precambrian depositional environment by delineating the similarities and differences in the Archean and Proterozoic samples. Both Archean and Proterozoic carbonaceous phyllites are fine grained siliceous in nature with depleted CaO, MgO and alkalies. The REE concentrations in most of the Archean samples are characterized by conspicuous positive Europium anomalies while the Proterozoic counterparts display feebly positive ratios indicating hydrothermal source. The Archean samples possess elevated Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Weathering (PIA) values reflecting extreme weathering conditions while the Proterozoic carbonaceous phyllites are confined to moderate weathering conditions. Cr/Th, La/Sc, Th/Sc and Co/Th indicate mafic provenance and the Cr-Zr relationship suggests deep sea depositional setting for both Archean and Proterozoic carbonaceous phyllites. The Archean carbonaceous phyllites indicate humid to semi-humid paleoclimate conditions while the Proterozoic samples are restricted to humid palaeoclimatic conditions which is evident from Sr/Cu ratios. V/(V + Ni) ratios indicate fluctuating dysoxic to euxinic redox conditions for the Archean samples while the Proterozoic samples display euxinic conditions. The predominantly negative δ13Corg values of Chitradurga (−38.5‰ to −14.2‰ VPDB), Gadag (−24.3‰ to −8.06‰ VPDB), Sandur (−24.8‰ to −29.2‰ VPDB), Shimoga (−24.6‰ to −11.8‰ VPDB) and Cuddapah (−30.19 to −13.98‰ VPDB) suggest the role of primitive life forms through biological processes during the deposition of organic matter.

Terrestrial records of the early Albian Ocean Anoxic Event: Evidence from the Fuxin lacustrine basin, NE China

The early Albian Oceanic Anoxic Event 1b (OAE 1b) is well documented in the Tethys, Pacific and North Atlantic, but few studies have evaluated whether or not terrestrial records of OAE 1b exist. In order to identify terrestrial records of the early Albian OAE 1b and to infer possible driving mechanisms, an integrated multi-proxy study from the late Aptian to Albian in the Fuxin lacustrine basin was conducted, including thick, organic-rich black mudstones, total organic carbon (TOC), organic carbon isotopes (δ13Corg), mercury concentration (Hg) and results from pyrolysis analyses (S2, Tmax and HI). Results show three distinct short-term negative δ13Corg excursions corresponding with relatively high TOC values, which could be counterparts of the Kilian, Paquier and Leenhardt sub-events of the early Albian OAE 1b. Atmospheric CO2 concentration (pCO2) recovered from C3 plant δ13Corg compositions indicates an increasing trend in Unit C during the early Albian, and there are three short-term increases of pCO2 corresponding to the three sub-events of OAE 1b at this time interval. We infer that a trend of increasing pCO2 during the Kilian sub-event in the study area is closely related to volcanism. Continental weathering calculated using chemical weathering indices (CIA, WIP and MIA(O)) show an increasing trend during the OAE 1b interval, likely resulting from warmer and more humid conditions. Mixed sources of terrestrial plants and lacustrine plankton demonstrated by pyrolysis analyses (HI vs. Tmax and S2 vs. TOC), indicate a terrestrial contribution to the organic-rich sediments of the Kilian, Paquier and Leenhardt sub-events of OAE 1b. We suggest that a CO2-forced greenhouse effect during the early Albian might have triggered the relatively warm and humid palaeoclimatic conditions, and intensified chemical weathering that combined to create high nutrient and organic matter levels that were flushed into lakes contributing to eutrophication and anoxia in lacustrine and in contemporaneous oceanic systems.

Planktic foraminifera of the upper Barremian–Aptian black shale intervals from the Chaqiela section (Gamba, southern Tibet): Biostratigraphic and palaeoenvironmental implications

Marine sedimentary intervals of the upper Barremian–upper Aptian record the profound changes in global plate configurations and the ocean-climate system. Black shale lithofacies of the Lower Cretaceous crop out extensively in the Tethyan Himalaya region of southern Tibet, and in the Gamba area they are assigned to the Gambadongshan Formation. By using 3–5% hydrofluoric acid, well-preserved planktic foraminiferal tests have been recovered for the first time from black shales in the Chaqiela section of the Gambadongshan Formation. The detailed foraminiferal biostratigraphic work allowed recognition of four upper Barremian to upper Aptian planktic biozones: Hedbergella excelsa Interval Zone, Globigerinelloides ferreolensis Taxon-Range Zone, Hedbergella infracretacea Partial-Range Zone, and the Paraticinella rohri Taxon-Range Zone. Total organic carbon (TOC) and organic carbon isotope (δ13Corg) records were generated for the Chaqiela section. The C-isotope profile and the planktic foraminiferal data enable the identification of Oceanic Anoxic Event 1a (OAE 1a) at the lower part of the Globigerinelloides ferreolensis Taxon-Range Zone. Diagenetic processes, lower sea surface temperature, and the regional hydrologic cycle were probably the main factors for the development of OAE 1a in the Chaqiela section. The relatively higher δ13Corg values and smaller size of carbon isotopic fluctuations in the Chaqiela section may represent the local expression of OAE 1a in the Gamba area. A long-term negative δ13Corg excursion and a decreased trend of planktic foraminiferal abundance relative to benthic foraminifera, in the upper part of the Gambadongshan Formation may suggest a lower sea-level during the late Aptian.