Shift In δ13Ccarb Research Articles

Involvement of Hg-bearing methane seeps in forming Ediacaran cap carbonate in South China

During the Cryogenian-Ediacaran transition, an extremely cold climate transitioned into a warm climate which was accompanied by the worldwide precipitation of cap carbonates showing striking negative carbonate carbon isotope (δ13Ccarb) anomalies. High mercury (Hg) contents up to 180 ppb have been reported from the Doushantuo cap carbonates, but the origin of Hg anomalies and its relationship with cap carbonate precipitation remain unclear. To address these issues, we measured Hg concentrations and isotopes along with total organic carbon (TOC) content and δ13Ccarb from the basal Ediacaran carbonates at the Jiulongwan section of western Hubei Province, South China. The results show that in laterally persistent intervals, Hg spikes (as high as 77 ppb) co-occur with decrease of Δ199Hg values (as low as 0.07‰), increase of δ202Hg values (−1.47‰), and conspicuous negative δ13Ccarb shifts (as low as ca. -17‰). Such Hg isotopic signature with high Hg concentration are comparable to those of natural gas hydrates and thus are ascribed to be an involvement of Hg-bearing methane seeps in precipitation of cap carbonate after melting of ice-sheets of the Marinoan glaciation (latest Cryogenian). Anaerobic oxidation of methane from hydrates via sulfate reduction may have produced bicarbonate (HCO3−), which was transformed into cap carbonate when combining with alkaline ions transported from continent to seawater via weathering process. This study provides the first evidence for Hg-bearing methane oxidation during cap carbonate precipitation.

Local and global controls on carbon and sulfur isotope records in the Middle Ordovician Epeiric sea successions of the Taebaeksan Basin, Korea

Although the sedimentary carbonate carbon isotope record is generally considered a reliable archive of long-term changes in the global carbon cycle, the influences of local variability in carbon cycling and diagenesis frequently overwhelm its global controls, particularly in shallow platform environments. During the Middle Ordovician period, the middle Darriwilian isotopic carbon excursion (MDICE) has been identified worldwide, including the Taebaeksan Basin in the Sino-Korean Block. However, in the eastern part of the Taebaeksan Basin, the positive δ13Ccarb trend associated with the MDICE was disrupted by negative δ13Ccarb anomalies, indicating the alternating prevalence between local and global controls. By comparing paired carbon and sulfur isotope signatures, we demonstrate the involvement of two distinct mechanisms in this local event: a gradual decrease in δ13Ccarb closely parallels that of organic carbon, and a negative δ13Ccarb peak, superimposed on the long-term δ13Ccarb decrease, coincides with the positive sulfur isotope shift in pyrite, without a corresponding change in δ13Corg. The latter indicates the formation of 13C-depleted authigenic carbonate during early diagenesis, associated with microbial sulfate consumption, whereas post-depositional processes cannot fully account for the coupled negative shifts in δ13Ccarb and δ13Corg values. Instead, this simultaneous divergence from the MDICE event may be due to limited exchange of dissolved inorganic carbon with open ocean water, coupled to an increase in terrestrial runoff, as shown by changes in the clay mineral assemblage. While generally aligning with sedimentological and paleontological investigations regarding the formation of restricted environments in the eastern part of the Taebaeksan basin, this interpretation challenges the previously presumed timing and duration of platform restriction. Our findings highlight that the spatial variability of isotopic records may compromise their fidelity as global proxies but offer valuable insights into the evolution of depositional environments and platform geometry, often obscured by sporadic outcrop exposure.

Depositional environments and carbon isotope excursions of the Middle Oxfordian (Transversarium Zone) sediments in the Central Saharan Atlas, Southwestern margin of the Tethys

This study examines the Middle Oxfordian sediments of the Theniet Et-Temar Formation in the Central Saharan Atlas (NW Algeria), which was located in the southwestern part of the Tethys Ocean during the Late Jurassic. Using microfacies analysis and δ13Ccarb isotopes, this research aims to elucidate the sedimentological and isotopic signatures of these sediments.Ten microfacies types were identified and grouped into four facies associations: nearshore (FA1), inner lagoon (FA2), shoal bar (FA3), and outer lagoon (FA4). Sequence stratigraphy, employing the T-R sequence concept, delineates the evolutionary stages of the studied succession into prograding deltaic front facies (LST1), retrogradational open marine facies (TST2), and deltaic plain facies within the highstand systems tract (HST2).Stable carbon isotopes from three sections namely, El Bayadh, Ghassoul, and Sidi Tifour of the Middle Oxfordian (Transversarium Zone) provide insights into carbon cycle perturbations and paleoenvironmental conditions. There are two δ13Ccarb excursions in the studied sections. A major positive excursion in the El Bayadh, Ghassoul and Sidi Tifour sections shows similar records in both southern (Saudi Arabia, Iraq) and northern Tethys regions (France, Poland, Scotland, Spain, Switzerland, and Uzbekistan). The negative δ13Ccarb shifts with values of −2.77‰ at the top of the Ghassoul section and −2.28‰ in the middle member of the Sidi Tifour section are coupled with more negative δ18Ocarb values suggesting alteration of isotopic values due to meteoric diagenesis.The variations in carbon isotopes suggest synchronous changes in the global marine dissolved inorganic carbon pool, potentially influenced by sea level fluctuations, climatic changes, and/or Jurassic dysoxic to anoxic events.

Paleoenvironmental variations during the Late Cambrian: Implications from Zn isotopes, I/(Ca+Mg) ratios, and other elemental proxies

The Upper Cambrian at the Martin Point section (∼110 m-thick) in western Newfoundland is a part of the Cow Head Group and partially spans the coeval Tuckers Cove and the Martin Point members of the Shallow Bay and Green Point formations, respectively. These units comprise alternating shale, minor conglomerate beds, and limestone rhythmites of a toe-of-slope apron. Earlier studies of the C-isotope profile, based on the lime mudstone interbeds, documented global negative δ13Ccarb shifts (from old to young: NL1, NL2, HERB) correlated with the lowermost Proconodontus posterocostatus, Proconodontus muelleri and base of Eoconodontus notchpeakensis conodont zones, respectively (post-Steptoean Positive Carbon Isotope Excursion NL1 and NL2 and Hellnmaria-Red Tops Boundary-HERB). Samples were extracted from the most preserved spots of micritic to near-micritic grain size (≤4 μm−10 μm). Fabric retention confirms petrographic preservation and insignificant correlations (R2 ≤ 0.1) of diagenetic proxies (e.g., Sr) with their environmental counterparts support the preservation of at least near-primary geochemical signatures. The paleoenvironmental proxies (TOC, Zn, P, Cu, Al/Ti, Mn, Fe, V, Mo) support dominant dysoxic conditions. The δ66Zn values (0.09–0.73 ‰ JMC Lyon) and (I/(Ca + Mg)) ratios (0.02–0.48 μmol/mol) vary consistently throughout the studied events. The minor decreases in δ66Zn values seem to denote inhibition of bioproductivity in a dysoxic water column, consistent with warm and humid climate during general transgressive settings and shoaling of organic-rich water into the shallow environment. The low I/(Ca + Mg) ratios (<0.5 μmol/mol) reinforce this interpretation, suggesting general dysoxic settings throughout the entire section.

Calcium isotopes reveal shelf acidification on southern Neotethyan margin during the Smithian-Spathian boundary cooling event

The Smithian-Spathian transition of the late Early Triassic was a critical period of environmental and biological upheavals, following the end-Permian mass extinction. Changes in carbonate deposition during this period have been attributed to intensified upwelling along shelf margins, but relevant studies are scarce. Here, we present calcium isotopes of bulk marine carbonate (δ44/40Cacarb) from a Smithian–Spathian boundary (SSB) succession (Guryul Ravine section, Kashmir) on the southern margin of the Neo-Tethys. Our smoothed δ44/40Cacarb curve reveals a ∼ 0.2‰ negative shift (from ∼ − 1.1‰ to ∼ − 1.3‰) across the SSB, concurrent with a ∼ +10‰ shift in δ13Ccarb. While increased Ca isotopic fractionation could play a role, we specifically examine potential impacts due to changes in marine Ca fluxes. Using a Ca-cycle mass balance model, we explore scenarios of decreased carbonate burial flux (Fcarb), decreased riverine flux (Friv), and a combination of these processes. The modeling suggest that a pulse decrease in Fcarb by 40% over ∼0.06 Myr match the negative shift in δ44/40Cacarb at Guryul Ravine. We infer that this decrease was likely related to intensified upwelling of acidic deep seawater due to invigorated global-oceanic circulation during the SSB cooling event. We suggest that the regionally diverse excursions in δ44/40Cacarb in the Tethyan region could be attributed to spatially varied upwellings in the shelf margin. The upwelling of acidic and anoxic deep seawater may have driven the second-order extinction of ammonoids and conodonts at the beginning of the SSB cooling event.

High-resolution marine osmium and carbon isotopic record across the Aptian–Albian boundary in the southern South Atlantic: Evidence for enhanced continental weathering and ocean acidification

The Early Cretaceous Aptian–Albian boundary (AAB) interval (∼113 Ma) was marked by severe environmental perturbations, including Oceanic Anoxic Event (OAE) 1b, global warming, and a major species turnover of planktonic foraminifera. Most paleoenvironmental studies across the AAB have focused on Tethyan sedimentary sequences deposited at low latitudes, but, because of the scarcity of well-dated sedimentary sequences across the AAB at high latitudes, the global extent of these environmental perturbations remains uncertain. Deep Sea Drilling Project (DSDP) Site 511 was drilled on the Falkland Plateau, southern South Atlantic, and is one of the few sedimentary sequences at southern high latitudes to record an almost continuous AAB interval. Previous studies documented the planktonic foraminiferal changes across the AAB at DSDP Site 511, but chemostratigraphic information has been limited. We reconstructed an osmium and carbon isotopic stratigraphy to further constrain the position of the AAB. We also inferred the paleo-marine redox conditions at southern high latitudes by combining chemical and lithological information. We found an abrupt negative δ13Ccarb shift coincident with the first occurrence of Microhedbergella renilaevis, an event that marks the base of the Albian Stage. This negative δ13Ccarb shift has also been observed across the AAB interval in the Tethyan and Atlantic regions. The fact that the osmium isotopic ratios show a pronounced radiogenic shift after the AAB is consistent with osmium isotopic variations detected in the Tethys and Pacific regions. Because the Os shift corresponded to a warming event during OAE1b in the low-latitude Atlantic and Tethyan regions, we interpret this radiogenic osmium isotopic shift as a response to increased continental weathering caused by global warming. A large marcasite crystal in the AAB interval at DSDP Site 511 suggests that the southern Atlantic Ocean may have been oxygen-depleted with a low pH caused by contemporaneous global environmental perturbations.

Marine anoxia as a trigger for the largest Phanerozoic positive carbon isotope excursion: Evidence from carbonate barium isotope record

The mid-Ludfordian Lau carbon isotope excursion (Lau CIE) represents the largest positive carbon isotope excursion in the Phanerozoic (∼9‰), coincident with the biodiversity loss of many marine animal clades. Two main explanations for the Lau CIE are enhanced organic carbon burial via increased marine productivity and preservation-driven expansion of anoxia. While these two explanations are not mutually exclusive, the main driver of Lau CIE is yet to be constrained. Here, we resolve this longstanding debate using barium isotopes (δ138Ba) of marine carbonates deposited across the Lau CIE. Our δ138Ba data from the Kosov section (Czech Republic) record a large negative excursion in correlation to the positive shift in δ13Ccarb. We suggest that the observed negative shift in δ138Ba to values as low as −0.33‰ can be best interpreted as upwelling of isotopically light Ba from deeper waters due to pelagic barite dissolution under euxinic conditions. This hypothesis is consistent with results from barium concentration data as well as the results from the sulfate mass balance modeling that indicates a contraction in the seawater sulfate reservoir, with seawater sulfate concentrations decreasing from several mM ranges before the Lau CIE to less than 100 μM during Lau CIE. Taken together, evidence for a strong negative correlation between δ138Ba and δ13Ccarb suggests that shallow water anoxia, rather than enhanced marine productivity, was a primary driver of the Lau CIE that resulted in a notable decrease in the size of seawater sulfate reservoir.

Carbon reservoir perturbations induced by Deccan volcanism: Stable isotope and biomolecular perspectives from shallow marine environment in Eastern India.

The Deccan Traps in Western India is hypothesized to have caused significant fluctuations in climatic condition and organic matter (OM) productivity across the Cretaceous-Paleogene Boundary (K/PgB). The periodic release of large amounts of volatiles into the atmosphere is thought to drive these changes. Yet, direct impact of volcanism on the carbon cycle and ecosystem remains relatively unconstrained. For the first time, we attempt to trace changes in both marine and terrestrial carbon reservoirs from pre- and intervolcanic sedimentary units (infra- and inter-trappeans respectively) from Rajahmundry, ~1500km SE of main eruption sites in Western India. Molecular level characterization of OM and stable isotope composition of carbonates (δ13 Ccarb ), bulk OM (δ13 Corg ), and n-alkane (δ13 Calk and δDalk ) have been analysed to provide a chemo-stratigraphic framework. In Rajahmundry, high CO2 concentration estimated from infra-trappean carbonate nodule is synchronous with the onset of the Deccan Traps and the Late Maastrichtian warming episode. Impact of the warming event is reflected in Rajahmundry from a major shift in the terrestrial ecosystem. Marine OM production also seems to have been low throughout the infra-trappean. A steady decrease in δ13 Ccarb values, increase in mortality rates and dwarfism in invertebrates immediately below the first volcanic units in Rajahmundry suggest stressed conditions from eruption in the western part of India ~40-60kyrs prior to K/PgB. A significant increase in heterotrophic activity is observed after the volcanic deposits in Rajahmundry that seems to have controlled the marine carbon reservoir for a maximum of ~200kyrs after the boundary. Advent of pteridophytes, increase in carbon content and positive shifts in δ13 Ccarb and δ13 Calk values in the upper inter-trappean units mark the onset of recovery in terrestrial and marine environments. Overall, our results suggest significant perturbations in the carbon reservoir as a consequence of the Deccan eruption.

Geochemistry of lower Ordovician microbialites on the Yangtze Platform, South China: Implications for oceanic oxygenation at the onset of the GOBE

The causes of the Great Ordovician Biodiversification Event (GOBE) remain a topic of debate. Recent studies indicate that global and regional oceanic oxygenation was a plausible trigger of the GOBE. The Ce anomaly of rare earth and yttrium (REY) elements in shallow marine carbonates is thought to be a powerful indicator of marine redox conditions of local waters. In this study, we analyzed REY, major and trace element compositions, and C and O isotopes of the Lower Ordovician microbial-dominant reefs on the Yangtze Platform, South China. We evaluate which carbonate components within the reefs most reliably record the seawater REY composition. The elemental correlations show that the REY elements of oncoids, stromatoporoids, sponges and carbonate infillings were affected by clastic influence, diagenetic processes, and the vital effects of carbonate secreting organisms; whereas the microbialites REY were partially modified by synsedimentary terrigenous and little diagenetic modification. Such introduction of synsedimentary terrigenous REY increased ΣREY and flattened the REY patterns of the microbialites, but the redox signals (Ce anomalies) of input source of the water column might still preserved in microbialites formed in situ. Although the magnitude of Ce anomalies is small, variation might still reflect authentic redox history of primary local seawater. In the middle Tremadocian, microbialites have positive Ce anomalies which were coeval to positive δ13Ccarb shifts. The positive δ13Ccarb shifts were documented from other sections of central Yangtze Platform, North American and Argentina, which might indicate global anoxic event. In the late Tremadocian to early Floian, microbialites have more visible negative Ce anomalies, which might indicate regional oceanic oxygenation event. The negative Ce anomalies provide direct local-geochemical evidence of oceanic oxygenation in the late Tremadocian-early Floian on the Yangtze Platform of South China, which might facilitate the first acme of biodiversification of local faunas predating the main pulse of the GOBE on the other palaeoplates.

A Pulsed Oxygenation in Terminal Paleoproterozoic Ocean: Evidence From the Transition Between the Chuanlinggou and Tuanshanzi Formations, North China

AbstractThe mid‐Proterozoic (ca. 1.8–0.8 Ga) witnessed a period of stable carbon cycling and long stasis in eukaryotic evolution, which was commonly ascribed to the persistently low oxygen levels in the atmosphere‐ocean system. Recently, several pulsed marine oxygenations were identified from different continents, and presumed to have facilitated the short‐term diversification of eukaryotes in the early Mesoproterozoic Era. To test this hypothesis, an integrated study of iodine species [I/ (Ca + Mg)], paired carbon isotopes (δ13Ccarb and δ13Corg), and elemental abundances was conducted on the fossil‐bearing Tuanshanzi Formation (∼1.64–1.63 Ga), North China. The near‐zero I/(Ca + Mg) ratios (0.00 ± 0.01 μmol/mol) coupled with low δ13Corg (−30.2 ± 1.4‰) in the strata below the fossil‐bearing interval suggests anoxic conditions in shallow seawater. An increase of I/ (Ca + Mg) up to ∼1.54 μmol/mol coupled with a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13Corg in the fossil‐bearing interval point to an increased oxygen concentration. Above the fossil‐bearing interval, I/ (Ca + Mg) decrease to &lt;0.5 μmol/mol, suggesting deoxygenation and a return to anoxic to suboxic conditions. Combined with relevant data from other mid‐Proterozoic sections of North China and Australia, these data likely indicate dynamic redox conditions in the shallow seawaters of this period, with multiple pulses in oxygenation in otherwise anoxic conditions. These pulsed oxygenations may have facilitated short‐term proliferation and diversification of early eukaryotes in shallow seawaters, but the overall low oxygen levels have impeded their continuous development and the rise to ecological dominance until late Neoproterozoic.

Calibrating the late Smithian (Early Triassic) crisis: New insights from the Nanpanjiang Basin, South China

Abstract The biotic recovery following the Permian/Triassic boundary mass extinction was influenced by several secondary extinctions during the Early Triassic, of which the late Smithian crisis is the most severe known for some nekto-pelagic organisms such as ammonoids. The Smithian-Spathian transition is characterized by successive global biotic and environmental changes, including a dramatic positive carbon isotopic excursion, oceanic anoxia and a cooling event beginning in the late Smithian. However, the tempo, modalities and the causal relationships among these various events remain poorly constrained. Here we synthesized paleontological, sedimentological, and geochemical data from three sections (Motianling, Nafang, and Shanggang) within the Nanpanjiang Basin, representing platform slop to basinal deposits spanning the Smithian/Spathian boundary. High-resolution analyses of thin sections show a sudden decrease in richness and abundance of fossil grains at the middle/upper Smithian boundary in all three sections, coinciding with the onset of a positive shift in δ13Ccarb. It also slightly precedes the beginning of the late Smithian cooling. At Shanggang, a change in dominance from bivalve-rich to ostracod-rich strata is also recorded at the middle/upper Smithian boundary. Overall, our results therefore indicate that a first major biotic crisis and turnover happened during the beginning of the late Smithian in the Nanpanjiang Basin, rather than around the Smithian/Spathian boundary. Complementary analyses on pyrite framboid size indicate that these observed biotic changes are concurrent with oxygen depletion in the studied sections, supporting the hypothesis that oceanic anoxia also played an important role in the late Smithian crisis in combination with climate cooling and oceanic acidification.

Marine carbon and sulfur cycling across the Ediacaran-Cambrian boundary in Tarim Block and its implications for paleoenvironmental changes

The Ediacaran-Cambrian transition was a critical period of Earth history because of the coincidence among climate change, bioevolution, and tectonic activity. The nature of carbon and sulfur-cycling processes spanning the Ediacaran-Cambrian boundary remains debatable. This study considers high-resolution stable isotope chemostratigraphy of the Sugetbrak and Yutixi sections of the Keping area of the Tarim Basin, northwestern China. The sections display a pronounced negative δ13Ccarb shift (the Basal Cambrian Carbon Isotope Excursion) that spans the Ediacaran-Cambrian boundary. Deposits overlying the negative excursion are characterized by heavier δ13Ccarb values ranging from −7.8‰ to +4.1‰. The biostratigraphically calibrated δ13Ccarb profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution. The δ34Spy profiles of the studied sections display a wide distribution of values, ranging from −16.4‰ to 23.8‰. The variable δ34Spy signatures of these sections are partially controlled by microbial sulfate reduction rates, which are sensitive to local sulfate concentration, availability of organic matter and the marine redox state. The strong coupling between enhanced organic carbon and pyrite burial, the appearance of Cambrian-type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere-ocean realm with bio-events during the Ediacaran–Cambrian transition. The development of ecosystems across the Ediacaran–Cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in Earth's history and, in turn, affecting themselves.

Comparison of Ediacaran platform and slope δ238U records in South China: Implications for global-ocean oxygenation and the origin of the Shuram Excursion

The Ediacaran Shuram negative carbon isotope excursion (SE) records major paleoceanographic changes during the late Neoproterozoic, possibly linked to a global oceanic oxygenation event, yet its cause(s) remain uncertain. Earlier studies of the upper Ediacaran Doushantuo Formation in South China based on local redox proxies have documented strong spatial redox heterogeneity along shelf-to-basin transects, but variations of δ238U (a global redox proxy) have not yet been examined in deep-water SE carbonates. In this study, we examined δ238U variations through the SE in the upper slope Siduping section. Similar to platform SE sections, Siduping exhibits a shift toward higher δ238U values correlative with the peak of the SE (i.e., maximum negative δ13Ccarb), confirming inferences of global ocean oxygenation during the SE. This raises an apparent paradox, because a global negative carbon isotope excursion implies net oxidant consumption, requiring an ocean-based oxygenation mechanism. We hypothesize that an increase in the efficiency of phosphorus burial due to a plankton-driven shift from dominantly dissolved organic matter (DOM) cycling to greater particulate organic matter (POM) export depleted the ocean of nutrient phosphorus. By producing a steep redox gradient close to the sediment-water interface, we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light δ13Ccarb was precipitated in authigenic carbonate minerals. This scenario can account for δ238U differences between shallow-water and deep-water carbonates, which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow-water settings, generating both a larger negative δ13Ccarb shift and a larger early diagenetic δ238U offset.

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

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

New evidence for compaction-driven vertical fluid migration into the Upper Ordovician (Hirnantian) Guanyinqiao bed of south China

The Late Ordovician was a time of major climatic and sea-level changes linked to the Hirnantian Glaciation, marked by large positive excursions of δ13Ccarb and δ13Corg globally known as the Hirnantian carbon isotopic excursion (HICE). However, some Upper Ordovician sections in South China exhibit negative δ13Ccarb shifts within the Hirnantian-age Guanyinqiao Bed. We hypothesize that these unusual carbon-isotope patterns were due to influx of 13C-depleted dissolved inorganic carbon (DIC) from the underlying Wufeng and overlying Longmaxi black shales as a result of greater compaction of these fine-grained siliciclastic units compared to the coarse-grained carbonate Guanyinqiao Bed. The isotopically light DIC was generated through oxidation of organic matter by either microbial sulfate reduction (MSR) or methanogenesis (with subsequent anaerobic oxidation of methane, AOM) in the black shales. This hypothesis is supported by petrographic and geochemical evidence of vertical fluid migration within the Guanyinqiao Bed and across its contacts. In the Shiqiao drillcore, the Guanyinqiao Bed exhibits a V-shaped depression at its top that is connected to vertical channels filled with secondary carbonate cements having low δ13C values. In the Huanggexi section, both the base and top of the Guanyinqiao Bed are characterized by strong U enrichments, despite unambiguously oxic depositional conditions as inferred from other proxies (Fe speciation, Mo and V concentrations). These anomalous U enrichments resulted from oxidative remobilization of U along the Guanyinqiao-black shale contacts and subsequent transport into the Guanyinqiao Bed through compactional flow. This finding demonstrates the importance of recognizing remobilized trace metals in sedimentary successions in order to avoid potential errors in redox interpretations.

SIMS U-Pb Zircon Geochronological and Carbon Isotope Chemostratigraphic Constraints on the Ediacaran-Cambrian Boundary Succession in the Three Gorges Area, South China

The Ediacaran-Cambrian succession in South China records dramatic biological, oceanic and geochemical changes, but it is not well constrained geochronologically. This study reports a new SIMS U-Pb date of 543.4±3.5 Ma (MSWD=1.2) from a tuffaceous layer in the Zhoujiaao Section, and carbonate C-O isotopes in both Zhoujiaao and Sixi sections, Three Gorges area. This tuffaceous layer is present in the upper Dengying Formation (i.e., the Baimatuo Member) which is characterized by a stable δ13Ccarb plateau and the beginning of a negative δ13Ccarb shift near its upper boundary. In accordance with the existing biostratigraphic and chemostratigraphic data, this new date corroborates that the upper boundary of the Dengying Formation in South China is approximately equivalent to the Ediacaran-Cambrian boundary (ca. 541 Ma). This age also provides the minimum age of the last appearance of the Shibantan biota in the Three Gorges area, indicating that the terminal Ediacaran index fossils (e.g., Cloudina, Sinotubulites) are not reliable stratigraphic markers for further subdivision of the uppermost Ediacaran.

Reconstruction of nearshore chemical conditions in the Mesoproterozoic: evidence from red and grey beds of the Yangzhuang formation, North China Craton

ABSTRACT Mesoproterozoic red beds near ancient coasts have not aroused extensive interest. A new geochemical study of the alternating red and grey dolostones from the Yangzhuang Formation provides a better understanding of the redox conditions of nearshore sedimentary environments. In this contribution, whole-rock samples are characterized by positive correlations of rare earth elements (REE) vs. Th and FeT vs. Th and flat-type REE distribution patterns, indicating massive terrigenous input, which is considered to be inherited from felsic rocks. Relatively high (Femag+Feox)/FeT and Fe3+/Fe2+ ratios in red beds indicate more oxidized conditions in supratidal environments compared with the lower oxygen contents in intertidal environments. Under these two distinct chemical sedimentary conditions, acetic acid-leached red and grey samples both have HREE-depleted distributions, suggesting significant freshwater invasion. Moreover, limited terrigenous redox-sensitive elements (RSEs) can reach the coast where the red beds are deposited, whereas relatively high RSE enrichment factors originating from shallow oceans are recorded in grey beds. In the Mesoproterozoic, limited oxidative weathering, shallow seawater desalination, and low organic production occurred near the coast. Meanwhile, a prolonged period of low Mo and U availability preserved in carbonate minerals confirmed that marine oxygen levels failed to satisfy the deposition of offshore red beds. During the regression, potentially exposed sediments connected to atmospheric oxygen guaranteed the oxidation of iron and the formation of red beds, and these events were coupled with negative δ13Ccarb shifts in the Yanliao rift zone.

Middle–Upper Ordovician (Darriwilian–Sandbian) paired carbon and sulfur isotope stratigraphy from the Appalachian Basin, USA: Implications for dynamic redox conditions spanning the peak of the Great Ordovician Biodiversification Event

The Evans Ferry section from the Appalachian Basin of eastern North America has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the Great Ordovician Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) analyses were used in this carbonate-dominated locality from the Appalachian Basin to reconstruct the marine redox states during this key period of rapid biodiversification. This succession is one of the most expanded Sandbian Stage (Upper Ordovician) deposits known from North America, and it allows new high-resolution reconstructions of long-term, global carbon and sulfur cycle fluctuations. The integrated geochemical and sequence-stratigraphic investigation presented here from the Laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the Middle–Late Ordovician that may have been linked to global-marine paleoredox dynamics. Utilizing new conodont-based 87Sr/86Sr isotope stratigraphy along with previous conodont biostratigraphy, we can confidently correlate our stable isotope profiles to other carbonate successions globally. Carbonate facies indicate that this area experienced partial restriction from open-marine conditions during certain intervals in the Sandbian. These semi-restricted environments record carbon (δ13Ccarb) isotope trends that are secular in nature, as they can be correlated to other successions across Laurentia and Baltica. We identify several intervals when δ13Ccarb and δ34SCAS (carbonate-associated sulfate) trends are decoupled. These inverse stratigraphic trends are, at times, followed by parallel positive shifts in δ13Ccarb and δ34SCAS. Causal mechanisms for the observed decoupled δ13C and δ34S trends may include a wide variety of factors such as more closed-system, local biogeochemical processes associated in part with diagenesis, or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions. The positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments.

Negative δ13Ccarb shifts in Upper Ordovician (Hirnantian) Guanyinqiao Bed of South China linked to diagenetic carbon fluxes

The Late Ordovician Hirnantian glaciation coincided with a major perturbation of the global carbon cycle, characterized by large positive excursions of δ13Ccarb and δ13Corg known as the Hirnantian carbon isotopic excursion (HICE). Previous studies from South China have reported a positive shift in δ13Corg but ambiguous trends in carbonate carbon isotopes. To better define δ13Ccarb variation through the Ordovician-Silurian (O-S) transition in South China, we investigated multiple components (brachiopods, micritic matrix, and microspar cement) in five study sections representing shallow-shelf to deep-basin facies across the Yangtze Block. All sections exhibit negative δ13Ccarb shifts near the base of the Hirnantian stage, with small shifts in two sections (to between −2 and −1‰) and large shifts in three sections (mostly <−10‰, minimum −13.7‰). Brachiopods commonly exhibit the same δ13C compositions as micrite in a given sample, although some samples contain isotopically heavy brachiopods in a highly 13C-depleted matrix. Because the highly 13C-depleted carbonates are present in two widely separated areas between which only weakly 13C-depleted carbonates are found, there must have been multiple local diagenetic sources of 13C-depleted carbon. We infer that the highly negative δ13Ccarb values were linked to compactional influx of 13C-depleted dissolved inorganic carbon (DIC) into the Guanyinqiao Bed as a result of oxidation of organic matter through microbial sulfate reduction (MSR) or methanogenesis/anaerobic oxidation of methane (AOM) in black shales of the underlying Wufeng and overlying Longmaxi Formations. The results of the present study demonstrate the importance of careful diagenetic screening of C-isotope records in order to fully understand the origin of δ13Ccarb signals and to avoid potential errors in interpreting diagenetically overprinted records as primary marine carbonate events.

Depth-dependent δ13C trends in platform and slope settings of the Campbellrand-Malmani carbonate platform and possible implications for Early Earth oxygenation

The evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in Earth’s atmosphere during the Archean-Proterozoic transition period. The delay from the first biogenic production of oxygen and the permanent oxidation of Earth’s atmosphere during the early Paleoproteorozoic Great Oxidation Event (GOE) indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen. Here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates (δ13Ccarb from −1.6 to +0.4‰, relative to VPDB) and organic matter (δ13Corg from about −40 to −25‰, relative to VPDB) from the 2.58–2.50Gy old shallow-marine Campbellrand-Malmani carbonate platform (South Africa). This indicates an enhanced primary production and lateral transport of organic material offshore to greater depths as well as a change from an anaerobic to an aerobic ecosystem. Trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent, both supporting primary production and an increasing oxidation state of the platform interior. These restricted conditions allowed that the dissolved inorganic carbon (DIC) pool in the platform interior developed differently than the open ocean. This is supported by coeval carbonates from the marginal slope setting, which had a higher interaction with open ocean water and do not record a comparable shift in δ13Ccarb throughout the sequence. We propose that the emergence of stable shallow-water carbonate platforms in the Neoarchean provided ideal conditions for the evolution of early aerobic ecosystems, which finally led to the full oxidation of Earth’s atmosphere during the GOE.