Carbon Isotope Excursion Research Articles

Integrated conodont biostratigraphy and stable isotope chemostratigraphy from the Tremadocian (Lower Ordovician) of the Yangtze Platform, South China

Tremadocian (Lower Ordovician) carbonate rocks were collected from the Xiangshuidong section in the Hubei Province, South China, and analyzed for conodont biostratigraphy. A total of 24 species belonging to 16 genera were recovered from the Nantsinkuan and Fenhsiang formations. Three conodont zones are recognized based on this material, i.e., the Rossodus manitouensis Zone, Colaptoconus quadraplicatus - Paltodus pristinus Assemblage Zone, and the Paltodus deltifer Zone, in ascending order. A biostratigraphic gap, corresponding to the Cordylodus lindstromi and Cordylodus angulatus zones, implies a major hiatus spanning the Cambrian/Ordovician boundary interval on the Yangtze Platform. A remarkable conodont faunal turnover is evident within the uppermost Rossodus manitouensis Zone and Colaptoconus quadraplicatus - Paltodus pristinus Assemblage Zone, corresponding to the early Tremadocian Base Stairsian Mass Extinction event (BSME). This turnover took place at a stratigraphic interval corresponding to the rising limb of the Top Skullrockian Isotopic Carbon Excursion (TSICE). According to the herein revised conodont biozonation of the Xiangshuidong section, a small but distinct positive shift in δ 238 U values begins in the lower part of the Colaptoconus quadraplicatus - Paltodus pristinus Assemblage Zone and contemporaneously with the TSICE. This shift suggests sustained improved oxygenation of the sea-floor environment in the early Tremadocian, post-BSME. There are no apparent δ 238 U changes before the BSME.

Positioning the North China Block (NCB) in the Cambrian: Constraints From the Integrated Bio‐, Chemo‐ and Magnetostratigraphy

AbstractThe Paleozoic paleogeographic and tectonic evolution of the North China Block (NCB) are poorly constrained due to the polarity ambiguity of its early Paleozoic paleopoles. Here we present magnetostratigraphic results of two upper Cambrian limestone‐dominated marine successions in the NCB that span the Steptoean positive carbon isotope excursion (SPICE), a global carbon‐cycle perturbation concomitant with trilobite extinctions. Stepwise thermal demagnetization reveals three‐component magnetizations with the high‐temperature component being a primary remanence as substantiated with paleomagnetic and rock magnetic analysis. A composite polarity sequence across the SPICE is established for the NCB and is correlated with coeval polarity sequence of Siberia, allowing to assign a reversed polarity for the NCB during the SPICE. This inter‐continental correlation, together with paleobiogeographic affinities, supports a southern hemispheric low‐latitude location adjacent to the west of East Gondwana for the NCB during the late Cambrian, thus constraining further paleogeographic‐tectonic reconstructions around East Gondwana during the Paleozoic.

Shallow marine redox fluctuations of Yangtze Platform during the Cisuralian-Guadalupian transition: Implications for Kungurian Carbon Isotope Excursion event

Shallow marine redox fluctuations of Yangtze Platform during the Cisuralian-Guadalupian transition: Implications for Kungurian Carbon Isotope Excursion event

Morphological Evolution and Extinction of Eodiscids and Agnostoid Arthropods

The temporal range of eodiscids and agnostoid arthropods overlaps with several early Paleozoic geological events of evolutionary significance. However, the responses of agnostids to these events and how the perturbations associated with them (both abiotic and/or biotic) may have impacted agnostids remain uncertain. To address this uncertainty, we employ geometric morphometrics to reconstruct morphospace occupation for agnostids, thereby elucidating their evolutionary response to geological events during the early Paleozoic. The results indicate that maximum morphospace occupation was reached by Cambrian Series 2 and then declined soon thereafter. Subsequent reductions in agnostid morphospace occupation coincide not only with significant abiotic changes and associated extinction events, such as the Botoman–Toyonian Extinctions (BTEs), the Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), the Drumian Carbon Isotope Excursion (DICE), and the Steptoean Positive Carbon Isotope Excursion event (SPICE), but also with major evolutionary episodes, such as the Great Ordovician Biodiversification Event (GOBE). These repeated and periodic declines in agnostid morphological diversity following Cambrian Series 2 suggest that the extinction of agnostids reflects the culmination of an episodic reduction in morphological occupancy for agnostids rather than a singular, sudden event. Accordingly, it cannot be tied to a single cause, either abiotic or biotic.

Warming and Carbon Injection at the Paleocene‐Eocene Boundary: Bayesian Modeling Supports Synchroneity

AbstractWe evaluate lead‐lag relationships between warming associated with the Paleocene‐Eocene thermal maximum (PETM) and the attendant carbon isotope excursion (CIE) recorded in sedimentary archives from the mid‐Atlantic continental margin. Cores from the New Jersey coastal plain (NJCP) provide thick (up to 15 m), rapidly deposited shelf PETM sections recording a 6–8°C warming based on the TEX86 paleothermometer. Updip cores from Medford, NJ provide the most expanded CIE onset, associated with a gradual lithologic change from uppermost Paleocene glauconitic sands to the lowermost Eocene kaolinitic Marlboro Clay. The relative thickness of these transitional sediments and the CIE onset allow for high‐resolution stable isotopic (bulk sediment, foraminifera, and organic carbon) and lipid biomarker analyses. The TEXH86‐derived temperature records from Medford cores show no warming prior to the CIE onset, with the carbon isotopic decrease in phase with temperature. Previous studies of more downdip PETM sections on the NJCP suggested that warming preceded the CIE, consistent with a thermal trigger for carbon release. Re‐evaluation of published records and Bayesian analysis of TEXH86‐derived temperature and δ13C from five NJCP sites do not indicate a statistically discernible lead or lag of carbon injection and warming. Though we can not preclude that a lead or lag exists, our analysis suggests that a synchronous CIE onset and warming is the most likely scenario on the NJ paleoshelf.

The record of the Early Late Paleocene Event (ELPE, 59.5 Ma) in shallow-water Tethys Himalayan carbonates (Zongpu Formation, South Tibet)

ABSTRACT The Early Late Paleocene Event (ELPE, 59.5 Ma) was a short-lived climatic perturbation accompanied by prominent biotic changes. Although the ELPE has been widely recognized from pelagic and continental archives, shallow-marine records remain scarce and poorly documented. To constrain the pattern and magnitude of the ELPE and associated environmental changes, we here present a detailed sedimentologic, biostratigraphic, and stable-carbon-isotope study of upper Paleocene platform carbonates continuously exposed in the Jiajin section of southern Tibet. Two distinct negative carbon-isotope excursions (CIEs) are documented: the first one occurred during shallow benthic zone (SBZ) 3 with a magnitude of ∼ 1.0‰, the second one began near the SBZ3–SBZ4 boundary with a magnitude of ∼ 1.5‰. Carbonate microfacies dominated by packstones with rotaliids and/or dasycladacean algae testify to an open shallow-marine environment. Neither a significant change in paleo–water depth nor evidence of early diagenetic dissolution are documented across the ELPE. The microfacies change observed during the ELPE, characterized by a decrease in the abundance of larger benthic rotaliid foraminifera relative to calcareous green algae, is related to environmental perturbations and increased nutrient supply. The changed trophic level may have resulted from intensified continental weathering promoted in turn by global warming. Compared with biocalcification crises observed in deep-water sedimentary records, shallow-water carbonate production remained relatively unaffected by changes in ocean-water chemistry.

New Re-Os Geochronological Data from the Upper Doushantuo Formation: Age Constraint on the Shuram Excursion and Implication for the Ediacaran Fluctuated Continental Weathering.

The largest negative carbon-isotope excursion in geological history has been reported by several studies of the upper Doushantuo Formation of South China, which has been correlated to the middle Ediacaran-Shuram excursion (SE). Due to a scarcity of radiometric age constraints on the excursion in South China, however, global correlations and comparisons of this event remain a debate. Here, we present Re-Os and carbon isotope data on organic-rich sediments obtained from a drill-core sample in the Chengkou area, the northeastern margin of the Yangtze Platform, and South China. The Re-Os geochronology yields a depositional age of 568 ± 15 Ma (Model 3, MSWD = 1.9, n = 13; 2σ), indicating a middle-late Ediacaran age for the upper Doushantuo Formation. This is supported by a negative δ13Ccarb excursion, which can be reliably correlated to the SE sequences. This age is consistent with the Re-Os radioisotopic dates bracketing the Shuram peaks in Northwest Canada and Oman. A compilation of 187Os/188Os and 87Sr/86Sr isotope ratios as well as the contents of redox-sensitive elements (RSE) from organic-rich sediments deposited between 635 and 540 Ma shows that the radiogenic 187Os/188Os ratios (>1.0) associated with enhanced oxidative weathering occurred at ca. 635, 580, and 560 Ma. As a result, accelerated influxes of nutrients stimulated primary productivity, promoting organic carbon burial and leading to ocean oxygenation. Additionally, elevated continental weathering could have delivered high fluxes of oxidants (e.g., sulfates) to oceans, resulting in transient ocean oxygenation. Corresponding to elevated radiogenic Os and Sr isotope ratios, the significant RSE enrichments at these three times indicate the presence of large marine RSE reservoirs and an oxygenated ocean. Therefore, the Re-Os age and initial Os isotope composition of organic-rich shale can be a sensitive tool for constraining the time interval of enhanced continental weathering and resulting pulses of ocean oxygenation during the Neoproterozoic era.

Pulsed atmospheric oxygenation recorded by Mo isotopes and Ce anomalies during Lomagundi Event from Jingshan Group marble of North China Craton

The Lomagundi Carbon Event (LE), the large, long-lived Paleoproterozoic positive carbon isotope excursion, is traditionally associated with a significant increase in atmospheric oxygen. However, the magnitude and extent of atmosphere–ocean oxygenation during this critical period of Earth’s history remain poorly constrained. Here, we present molybdenum isotope data and Ce anomaly values of Paleoproterozoic Jingshan Group marble samples deposited coincident with the peak of LE. Analyzed samples are characterized by near-modern seawater δ98Mocarb values (maximum = 2.13 ‰ ± 0.05), crustal Ce anomalies (average = 1.03), and δ98Mocarb values (average = 0.41 ‰). The great variation of Mo isotope values displayed by samples recovered from a thin stratigraphic interval likely reflect the existence of a small Mo reservoir during LE. We suggest that Mo removal from poorly oxygenated oceans under ferruginous conditions was responsible for the contraction of the Mo oceanic reservoir. Mo concentrations and isotope values of deposits coincident with this time interval appear to have been controlled by iron and manganese oxides shuttle. Placed in the context of Earth’s oxygenation history, our findings suggest that O2levels of the atmosphere–ocean system remained much less (1 % PAL) than the present level with occasional episodes of increased or pulsed atmospheric oxygen during the peak of LE.

Palaeoenvironmental interpretation and isotope stratigraphy of the Campanian–Danian successions in the Zagros Foreland Basin (Kabir Kuh, Lurestan, Iran)

The successions of the Gurpi Formation and the lower Pabdeh Formation were examined through analyses of lithology, sequence stratigraphy, stable isotope stratigraphy, and ichnofossils in the Jahangir Abad and Abhar sections of the Kabir Kuh anticline within the Lurestan Zone. Stratigraphic correlation with the well-dated Gandab section, located within the same anticline, indicates a late Campanian to early Danian age for these successions. This correlation further confirms the presence of notable carbon isotope excursions associated with the Campanian-Maastrichtian Boundary Event (CMBE) and Mid-Maastrichtian Event (MME). Field observations, along with sedimentological and paleontological analyses, suggest that the Kabir Kuh area was situated within the fore-bulge of the Zagros foreland basin during the Late Cretaceous. Changes in lithology, fossil content, and ichnofacies indicate a significant decrease in water depth at the end of the Campanian and the beginning of the Maastrichtian. This interval contains shallow marine macrofossils, including oysters (mainly Lopha), benthic foraminifers, and fossil debris traces, indicative of a high-energy environment. In contrast, shallow-water facies are not recorded within the middle and late Maastrichtian sequences. These sequences are composed of pelagic mud-dominated facies containing deep-sea ichnofossils, indicating a rapid increase in water-depth during the middle–late Maastrichtian. The observed late Campanian to early Maastrichtian shallowing is associated with the development of the Zagros fore-bulge, while the subsequent deepening and significant sediment deposition during the mid to late Maastrichtian are attributed to the subsidence of the basin. This research re-evaluates and refines the existing sequence stratigraphic and sedimentary models of the Zagros foreland basin, offering new perspectives on its geological evolution.

Smithian–Spathian carbonate geochemistry in the northern Thaynes Group influenced by multiple styles of diagenesis

AbstractThe Smithian–Spathian boundary interval is characterised by a positive carbon isotopic excursion in both δ13Ccarb and δ13Corg, concurrent with a major marine ecosystem reorganisation and the resurgence of microbialite facies. While these δ13C records have been traditionally interpreted as capturing global carbon cycle behaviour, recent studies have suggested that at least some excursions in early Triassic δ13C values may incorporate influences from authigenic or early diagenetic processes. To examine the mechanistic drivers of Smithian–Spathian boundary geochemistry, the carbonate geochemistry of a core from Georgetown, Idaho (USA), was analysed using a coupled δ44/40Ca, δ26Mg and trace‐metal framework. While the δ13C record in the Georgetown core is broadly similar to other Smithian–Spathian boundary sections, portions of the record coincide with substantial shifts in δ44/40Ca, δ26Mg and trace‐metal compositions that cannot feasibly be interpreted as primary. Furthermore, these geochemical variations correspond with lithology: The δ13C record is modulated by variations in the extent of dolomitisation, and the diagenetic styles recognised here coincide with individual lithostratigraphic units. A primary shift in local sea water δ13C values is inferred from the most geochemically unaltered strata, from ca 3‰ in the middle Smithian to ca 5‰ in the early Spathian, although the timing and pathway through which this occurs cannot be readily identified nor extrapolated globally. Therefore, the Georgetown core may not directly record exogenic carbon cycle evolution, showing that there is a need for the careful reconsideration of the Smithian–Spathian boundary—and more broadly, Early Triassic—geochemical records to examine potential local and diagenetic influences on sedimentary geochemistry.

Molecular compositional variation of organic matter deposited on the East Tasman Plateau during the Paleocene–Eocene Thermal Maximum

Molecular compositions in sediments collected by Ocean Drilling Program Leg 189 at Site 1172 in the Tasmanian Gateway, Australia have been characterised by gas chromatography–mass spectrometry to assess the impact of the Paleocene–Eocene Thermal Maximum (PETM) on organic matter origins and evolution. Total organic carbon (TOC) and sulfur contents reach the lowest values immediately before the initiation of the negative carbon isotope excursion (CIE) in the shallow marine environment on the East Tasman Plateau. The TOC content increases slightly while the sulfur content increases more substantially during deposition of the CIE, suggesting the occurrence of low-oxygen environments during the PETM event. The studied section is thermally immature, as shown by the dominance of biological configuration biomarkers such as 17β(H),21β(H)-hopanes (ββ) and hopenes. Changes in relative abundance of compound classes and isomer ratios are largely caused by the variation of source inputs and depositional conditions. Very low abundance of marine steroids during the CIE, but very enriched hopanoids, indicate low marine productivity, in accord with substantial remineralisation of organic matter in the bottom waters. The ββ-hopanes are the most abundant hopanoid series, followed by the neohop-13(18)-ene and hop-17(21)-ene series, further verifying the immaturity of the organic constituents in the samples. Hopane ratios (ββ/(βα + αβ) and βα/(βα + αβ) with different carbon numbers are systematically higher in sediments deposited during the CIE than those formed pre- and post-CIE, suggest increased bacterial inputs. Higher C30/C29 and C30/C31 hopanoid ratios in the CIE sediments are consistent with a greater contribution from in situ bacterial organic matter, rather than continental soil erosion by flooding. This is supported by the higher C27–32 hopanoids/(C27 + C29 + C31n-alkanes) ratio and sulfur content during the CIE. A relatively lower TOC content in the sediments formed during the PETM is inferred to have been caused by dilution by both enhanced detrital input and intensified bacterial consumption. This study demonstrates that the biomarker approach can play an important role in the identification of organic matter origin and characterisation of depositional environment for PETM sequences.

Evolution of carbon isotope composition in atmospheric CO2 during the early Toarcian

The large negative carbon isotope excursions during the Toarcian Oceanic Anoxic Event (T-CIE) implicate the injections of 13C-depleted CO2 into the Earth's ocean-atmosphere system, with multiple climate-sensitive carbon reservoirs as possible carbon sources. The carbon isotopic composition of atmospheric CO2 (δ13Cco2) is an important parameter of the Earth's carbon cycle and is crucial for exploring the causes of carbon-isotope excursions and understanding the carbon cycle-climate feedbacks. Here we compile δ13C and δ18O data of brachiopod shells and belemnite rostra sampled from two well-known sections calibrated with high-resolution biostratigraphy in Peniche (Portugal) and Yorkshire (England). According to the equilibrium between δ13CDIC (carbon isotopic composition of oceanic dissolved inorganic carbon) and δ13Cco2, we reconstruct the evolution of δ13Cco2 from brachiopod and belemnite δ13C records from the late Pliensbachian to the early Toarcian. Our results indicate that the brachiopod-based δ13Cco2 trend shows a short negative carbon isotope excursion from −7 ‰ to −8 ‰ at the Pliensbachian–Toarcian boundary, followed by a gradual positive excursion from −8 ‰ to −5 ‰ in the Polymorphum Zone, which is interrupted by a ∼ 3 ‰ negative excursion during the T-OAE. After careful consideration of the vital fractionation and living habitats of the belemnites, the belemnite-based δ13Cco2 trend shows a similar trend of temporal variations to that from the coeval brachiopods with a clear negative CIE (∼2.2 ± 0.4 ‰) during T-OAE. The low atmospheric δ13Cco2 values relative to volcanic gas (−3.8 to −4.6 ‰) suggest that volcanogenic CO2 is unlikely to result in such negative carbon-isotopic excursion directly during the Early Toarcian. Using the carbon isotopic records of plant tissue from Portugal section, combined with our reconstructed δ13Cco2, we conclude that plant carbon isotopic discrimination (△δ13Cplant) varied from 15.7 ‰ to 24.3 ‰ during the early Toarcian, with a maximum occurring during the T-OAE, which should be a combined record of environmental factors.

Widespread upper-ocean deoxygenation in the Alpine-Mediterranean Tethys during the Toarcian Oceanic Anoxic Event

The early Toarcian (∼183 Ma) was characterized by pronounced climate warming associated with massive release of 13C-depleted carbon to the exogenic system, as evidenced by globally recognized negative carbon-isotope excursions (N-CIE) in biospheric carbon reservoirs. Global warming during this interval triggered a variety of environmental perturbations, of which large-scale marine deoxygenation (as indicated by the presence of widespread organic carbon-rich deposits) is arguably diagnostic and led to the naming of the interval in question as the Toarcian Oceanic Anoxic Event (T-OAE). Nevertheless, the spatial variability of water-column redox during the T-OAE is unclear because most sedimentological and geochemical methods used to infer marine redox are likely reflective of bottom-water and/or pore-water conditions. Here we report new I/(Ca + Mg) and Cerium (Ce)-anomaly data from two carbonate successions from northern Italy that encompass the T-OAE interval. Both successions were deposited in the Alpine-Mediterranean Tethys; one in a shallow-water platform setting and the other within a pelagic environment. Both successions record an abrupt drop in I/(Ca + Mg) values, coupled with positive excursions in Ce-anomaly records at the onset of the T-OAE N-CIE. The synchronized changes in marine iodate depletion and Ce enrichment suggest widespread and significant upper-ocean deoxygenation in the Alpine-Mediterranean Tethys. This redox pattern is attributed to an expanded oxygen minimum zone (OMZ) formed as a result of sluggish oceanic circulation under climate warming, augmented by enhanced dissolved oxygen consumption due to increased nutrient availability and the consequent eutrophication in both proximal and distal settings. Because reduced seawater dissolved oxygen [O2] would increase the ecological stress and constrict any potentially hospitable habitats, the broad synchroneity between biotic turnovers and upper-ocean deoxygenation in the Alpine-Mediterranean Tethys is compatible with a potential causal link.

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.

Trace metal evolution of the Late Cretaceous Ocean

The Cenomanian-Turonian boundary (Late Cretaceous) witnessed the last spectacular manifestation of Mesozoic Anoxic Events (OAE 2, ∼94 Ma), marked by a prominent carbon isotope excursion (CIE) and burial of organic-matter-rich sediments under high atmospheric CO2 concentrations. But the Late Cretaceous generally was a time of profound environmental change. OAE 2 was preceded by other CIEs, including the Mid-Cenomanian Event (MCE), and was punctuated by a short re‑oxygenation and cooling event (the Plenus Cold Event, PCE). Extensive previous studies, including many trace metal studies, have focused on OAE 2, but there is still debate concerning the degree of drawdown of oceanic trace metal reservoirs during OAE 2, whether this drawdown is global or local, its causes and consequences for ocean ecology. Here, we present records of eight trace metals, over about 5 Myr of the Late Cretaceous, from the Tarfaya Basin in the proto-North Atlantic. The long records from a core preserving a continuous sedimentary succession allow us to set changes occurring across OAE 2 in the broader context of Late Cretaceous, including the lead up to OAE 2. Moreover, the multiple trace metal dataset allows us to broadly investigate the oceanographic setting in the context of recent studies of multiple trace metals in modern organic-rich sediments aimed at refining the proxies.Trace metal enrichments in these organic-rich sediments are discussed on three different timescales. Firstly, comparison of these Late Cretaceous sediments with modern organic-rich sediments are consistent with deposition in an open ocean upwelling margin in the Late Cretaceous, very like the modern Peru or Namibian Margin, although the deep proto-North Atlantic was probably partially restricted. Secondly, in common with previous studies, metal/TOC ratios often show sharp drops in the early part of OAE 2. Thirdly, however, this sharp drop occurs within a framework of pseudo-cyclical variations in metal/TOC, with a period of about 143 ±19 kyr (1 SD), that is a feature of these long records well before OAE 2, including across the MCE. Different metals respond differently to the perturbation in the early part of OAE 2 itself. Simple mass balance considerations suggest that trace metal drawdown with organic carbon must be at least partially compensated by changes in the rate of chemical weathering on the continents, as previously inferred from Li and Ca isotopes. Moreover, changes in the patterns of variation between different metals, as well as covariation of metal/TOC ratios and Os isotopes, hint at changes in the pattern of chemical weathering, most prominently in the contribution of mafic rocks to the chemical weathering flux.

Improved chronostratigraphy and fine-tuned timing for Late Triassic palaeoenvironmental changes in SW Britain using coupled magnetic polarity and carbon isotope stratigraphy

Understanding the synchronicity of global climatic, environmental, and biotic events around the Norian-Rhaetian boundary (NRB) is problematic because of major international differences in biochronology. We instead use magnetostratigraphic and global carbon isotopic changes to produce more precise global correlation. This work focusses on the base and top of the Rhaetian, with principal age control from a new late Norian to latest Rhaetian magnetostratigraphy from Lavernock (southern Wales) which can be directly correlated to the proposed NRB sections at Pignola Abriola (Italy) and Steinbergkogel (Austria). A disconformity exists in the Lavernock section in its late Norian part (Branscombe Mudstone Formation), but the NRB interval is largely complete. The magnetostratigraphy and a composite δ13Corg stratigraphy from three British sections, demonstrate synchronous changes in both terrestrial and marine records. This analysis indicates the older proposed definition of the NRB from Steinbergkogel is in the upper few metres of the Branscombe Mudstone Formation, while the younger NRB definition from Pignola Abriola is in the upper parts of the Blue Anchor Formation. The latest Rhaetian magnetostratigraphy from Lavernock records reverse magnetochrons UT26r and UT28r which closely pre-date and post-date the widely recognised Marshi and Spelae carbon isotope excursions, respectively. Magnetochrons UT28r and UT27r were previously recognised at St Audrie's Bay (SW England), with relationships to the Newark Supergroup which tightly constrain the first phase of CAMP eruptions to overlap the Spelae excursion. The carbon isotope excursions present in the Blue Anchor Formation lacustrine successions, demonstrate the likely atmospheric, and global spread of these perturbations.

The Paleocene‐Eocene Thermal Maximum Recognized in Distal Alluvial Fan Strata of the Sevier Fold‐And‐Thrust Belt (Southwestern Wyoming, USA)

AbstractFifty‐six million years ago a massive flux of exogenic carbon was rapidly released into Earth's oceans and atmosphere during the Paleocene‐Eocene Thermal Maximum (PETM). The warming associated with this release persisted for ∼200 kyr. It had severe impacts on ocean environments, global ecosystems, and the hydrologic cycle. Yet, regional patterns of response within continental settings are not well constrained. Here we describe the first recognition of the PETM in the Greater Green River Basin of western Wyoming, USA. We present new sedimentologic, chemostratigraphic, and paleontologic data from the Chappo Member of the Wasatch Formation in the Sevier fold‐and‐thrust belt. The Chappo Member contains lithofacies indicative of debris flows, ephemeral and incisional stream channels, and well‐drained overbank deposits, which, along with the geographic position of the study area, indicate a distal alluvial fan setting. Using pedogenic carbonates, we find a negative carbon isotope excursion (CIE) of ∼3.8‰ associated with the first appearance of Celtis endocarps stratigraphically above beds containing late Paleocene mammals. The CIE magnitude here is considerably smaller, compared to records from the Bighorn Basin, which we interpret to represent drier, better‐drained conditions. High baseline carbon isotope values suggest dry conditions in the study area, which likely resulted from well‐drained sediments associated with the distal alluvial fan. Low amounts of precipitation, as suggested by previous climate models, may have contributed to dryness. We also find sedimentologic evidence consistent with an enhanced hydrologic cycle during the PETM, as seen in other basins in the region.

High-resolution oceanic anoxic event 2 (OAE2) records from the north of eastern Tethys and evidence for short-term sea regression and wildfire at its early phase

Oceanic Anoxic Event 2 (OAE2), occurring at the Cenomanian-Turonian boundary (CTB; 93.9 Ma), is a representative global event of carbon cycling fluctuations and transient climate anomalies. OAE2 was initially discovered and intensively studied in the trans-Atlantic and western Tethys Oceans; however, research on the eastern Tethys Ocean, especially the vast northern regions, is inadequate. Further, there is still considerable controversy regarding the triggering mechanisms and early environmental changes of OAE2. In this study, we present high-resolution carbonate carbon isotopes (δ13Ccarb), total organic carbon, and mercury (Hg) concentration records from the northern part of the eastern Tethys Ocean during the OAE2 interval, sampled at the ZK20-1 drillcore section of the Kukebai Formation in the western Tarim Basin, China. Following the global classification scheme for carbon isotope excursion (CIE) at the CTB, the carbon isotope change curve of this study is classified into six stages (C1-C6), and three substages (C3a, C3b, C3c) in the C3 stage. Sedimentary features reveal a short-term sea-level regression in the Tarim Basin during the onset stage (C2) of OAE2, which likely involved vast regions of the eastern Tethys Ocean and even the Pacific region. The mercury (Hg) concentration data reveal increasing Hg levels in the lower part of the Kukebai Formation, mainly corresponding to the onset stage (C3) of the OAE2, but it is significantly lower than that in other typical volcanogenic Hg anomalies. We preliminarily speculated that the intensified wildfire activity at the onset stage of OAE2 potentially accelerate the Hg extracted from the terrestrial soil or organic matter into the offshore area. This is also supported by the evidence of wildfire and Hg anomalies from the Western Interior Seaway (WIS) of North America.

Timing and origin of the Lomagundi-Jatuli Event: Insights from U[sbnd]Pb geochronology, C-O-Fe isotopes and REE compositions from the Jingshan Group, North China Craton

The Lomagundi-Jatuli Event (LJE) represents perhaps the largest and longest positive carbon isotope excursion of Earth history. However, the synchroneity, scale, and linkage of the LJE to Earth's early history of atmospheric oxygenation remain controversial. Strata of the Paleoproterozoic Jingshan Group of the North China Craton that preserve the isotopic record of the LJE excursion in marble layers and abundant graphite deposits provide an opportunity to elucidate the significance of the LJE. Carbon isotopic values (δ13CV-PDB) of the LJE based on analysis of 20 samples of the Lugezhuang Formation, lower Jingshan Group, range from −0.8 to +9.6 ‰ and display positive co-variance with stable oxygen isotope values (δ18OV-PDB). The positive carbon isotope excursion is constrained to 2140.6 ± 8.5 Ma (MSWD = 0.82, n = 25) based on magmatic zircon UPb geochronology of biotite granulite. Variation of facies-dependent carbon isotope values, the presence of graphite deposits of the Douya Formation, upper Jingshan Group, and the absence of a Ce anomaly in PAAS normalized REE patterns of marble samples suggest that the positive carbon isotope excursion is not linked to a marked increase of organic carbon burial and associated significant atmosphere oxygenation. Elevated concentrations of iron and PAAS-normalized middle REE enrichment of analyzed Jingshan Group marble samples point to anoxic and ferruginous oceanic conditions during accumulation of Jingshan Group carbonate. A positive Eu anomaly (average = 1.58), low La (average = 0.23), (Nd/Yb)N (average = 1.27), and Y/Ho (average = 36.6) anomalies, and negative iron isotope values (average δ56Fe = −0.12 ‰) are consistent with accumulation of Paleoproterozoic Jingshan Group carbonate in a restricted marine setting that was affected by high temperature hydrothermal fluids. Enrichment of the studied samples in heavy carbon isotope suggests elevated bio-productivity in the restricted, redox stratified marine setting in which Jingshan Group carbonate accumulated. Thus, it is likely that the positive stable carbon isotope excursion associated with Jingshan Group strata as well as other contemporaneous isotope excursions are local signals that are linked to a global perturbation of the carbon cycle.

High-precision U–Pb geochronology of the Lundy igneous complex: implications for North Atlantic volcanism and the far-field Paleocene–Eocene ash record

The Lundy Island (Bristol Channel, UK) granite is a felsic expression of the southernmost igneous centre of the North Atlantic Igneous Province that emplaced millions of cubic kilometres of magma during the Paleogene. The granite's distinctive S-type, peraluminous, two-mica ± garnet ± tourmaline composition has led to the hypothesis that eruptions from the Lundy volcanic centre may be the source of thick felsic ash layers within the early Eocene Fur Formation (Denmark) that act as key marker horizons for the onset and duration of the Paleocene–Eocene Thermal Maximum. This paper presents high-precision zircon U–Pb emplacement ages of 57.24 ± 0.11/0.12/0.13 Ma for the granite and 55.970 ± 0.021/0.030/0.070 Ma for a felsic ‘lundyite’ dyke. Trace and rare earth element patterns indicate close similarities between late-stage Lundy activity and ash layer −33 in Denmark. This ash layer was deposited during the Paleocene–Eocene Thermal Maximum carbon isotope excursion, suggesting that the Lundy volcanic centre is likely to be the source of this key ash horizon and that magmatism at Lundy likely continued into the early Eocene.