Isotope Excursion Research Articles

Interpretation of abnormally negative carbon isotope excursions in Eifelian–Givetian (Middle Devonian) sediments of South China and implications for paleo-environmental variation

Many negative carbon isotope excursions (NCIEs) have been employed to interpret global biogeochemical events in Earth history, but they are also influenced by local depositional settings and diagenetic alteration, thus making it difficult to link them to global events. Like the NCIEs in Middle Devonian sediments of North America, similar NCIEs were also observed in the age-equivalent sediments of South China, and therefore it is necessary to further address the logical significance of these intercontinentally comparable NCIEs. Here, we present major and trace elements as well as δ15N, δ13Ccarb, and δ13Corg records across the Middle Devonian Eifelian–Givetian transition in the intra-platform basin of South China. The changes in stable isotopes (δ13Ccarb, δ13Corg, δ18Ocarb, and δ15N) and their relationships may jointly indicate limited influence of diagenetic processes. Our abnormally negative δ13Ccarb values (−7 to −5‰), even lighter than the mantle input end-member value (∼ −5‰), cannot be easily explained by dramatic changes in the δ13C values of global dissolved inorganic carbon (DIC) reservoir. In a restricted platform basin, enhanced nutrient input may stimulate high paleo-productivity, resulting in the depletion of DIC concentrations in surface waters. CO2 is then replenished via invasion from the atmosphere, and intense evaporative conditions may promote the large isotopic fractionation of CO2 invasion owing to hydration reactions of aqueous CO2. Our NCIEs are most likely related to the local 13C-depleted values of platform DIC owing to CO2 invasion associated with large isotopic fractionation due to enhanced evaporation and primary productivity. The “productivity-anoxia feedback” mechanism can explain the coupled changes in our terrestrial clastics input, productivity, and redox proxies. By combining redox proxies and C and N isotopic records, we further propose a model for local carbon and nitrogen cycling in a restricted and evaporative intra-platform basin. Our study highlights local depositional controls on NCIEs, and these local factors should be considered before NCIEs are applied to understand the global carbon cycle perturbations during the Middle Devonian.

Cyclostratigraphic calibration of the ca. 1.56 Ga carbon isotope excursion and oxygenation event recorded in the Gaoyuzhuang Formation, north China

Recent studies of the upper Gaoyuzhuang Formation (Fm) (ca. 1600–1540 Ma) in north China have suggested that Earth underwent an oxygenation event associated with a negative δ 13 C carb excursion and this event may have led to the expansion of eukaryotic life. However, the duration of this negative δ 13 C carb excursion, which is crucial to understand its origin, remains uncertain. Here, we report a floating astronomical timescale based on Milankovitch cycles extracted from the carbonate-dominated strata of the Member 3 of the Gaoyuzhuang Fm in north China, in which the oxygenation event is recorded. High-resolution magnetic susceptibility, anhysteretic remnant magnetization and saturation isothermal remnant magnetization series were obtained from the stratigraphic sections in the Jixian and Yanqing regions. Rock magnetic experiments suggest that the magnetic parameters are dominated by the detrital materials whose flux was likely controlled by orbitally forced climatic changes. Our results for the first time reveal well-constrained eccentricity, obliquity, and precession cycles from the Gaoyuzhuang Fm and demonstrate that the ca. 1560 Ma negative δ 13 C carb excursion and associated transient oxygenation event indicated by I/(Ca + Mg) had durations of ~1.7 Myr and ~6.2 Myr, respectively. The ca. 1560 Ma δ 13 C carb excursion is comparable with the short-term Neoproterozoic excursions in respect of duration, rather than with the long-term ones such as the ca. 574–567 Ma Shuram excursion. The similarity may indicate a common mechanism in origin of the short-term excursions in the Mesoproterozoic and Neoproterozoic time, which deserve testing by more investigations. • Cyclostratigraphic analyses revealed Milankovitch cycles from the ca. 1.56 Ga Gaoyuzhuang Formation • Durations of the δ 13 C carb excursion and the oxygenation event in the ca. 1.56 Ga are 1.7 Myr and 6.2 Myr, respectively • The duration of this δ 13 C carb excursion are comparable to the short-term ones in the Neoproterozoic

Biostratigraphically significant palynofloras from the Paleocene–Eocene boundary of the USA

Pollen and spores were recovered from the Paleocene Fort Union Formation and Paleocene–Eocene Willwood Formation of the Bighorn Basin (BHB), northwestern Wyoming, USA. In many local stratigraphic sections in the BHB, the base of the Eocene has been identified by the characteristic negative carbon isotope excursion (CIE) that marks the beginning of the Paleocene–Eocene Thermal Maximum (PETM). The palynotaxa from outcrop samples were examined using light microscopy (LM) and scanning electron microscopy (SEM). Seven new species are formally described (Tricolpites vegrandis, Rousea spatium, Striatricolporites astutus, Striatopollis calidarius, Friedrichipollis geminus, Retistephanocolporites modicrassus and Retistephanocolporites pergrandis). The temporal and geographic distributions of many of these palynotaxa suggest that hotter and more seasonally dry climates facilitated their northward range shifts during the PETM from the tropics or subtropics of the USA. For the temperate palynotaxa, the hotter and seasonally dry conditions resulted in local extirpation. A re-evaluation of the palynostratigraphic schemes established for the Paleocene–Eocene boundary confirms that the first appearance of Platycarya platycaryoides denotes the Paleocene–Eocene boundary in the Rocky Mountains region. A new Striatopollis calidarius Subzone, associated with early Wasatchian (Wa) Wa-0 and Wa-R faunas, is also recognized for CIE body localities in the BHB.

Volcanism driven Pliensbachian (Early Jurassic) terrestrial climate and environment perturbations

Initial breakup of Pangaea and opening of the Central Atlantic Ocean during the Early Jurassic resulted in widespread volcanism and was accompanied by significant changes in atmospheric composition, climate and environment of the Earth's surface system. Although profound changes in global geochemical cycles, palaeoclimate, and palaeoenvironments during the early part of the Toarcian Stage have been extensively studied and attributed to the emplacement of the Karro-Ferrar Large Igneous Province (LIP), similar changes in other parts of the Early Jurassic are less well known, especially in terrestrial settings. We studied Pliensbachian aged terrestrial strata from the Dameigou section in the Qaidam Basin of the Qinghai-Tibet Plateau to elucidate climate and environmental changes and their potential driving mechanisms. Three significant climate and environment perturbations were recognized by high-resolution mineralogical and geochemical analyses, with each characterized by peaks in kaolinite composition that indicate relatively warm and humid climates. Each kaolinite peak is accompanied by proxy evidence that indicates anoxic redox conditions, high paleoproductivity, and strong continental weathering prevailed, and coincides with negative carbon isotope excursions and peaks in Hg/TOC ratios that record three episodes of concurrent volcanism. The three episodes of volcanism are correlated with global events and suggest volcanism as the driving mechanism for major global climate and environment perturbations during the Pliensbachian. Each interval of volcanism released massive quantities of greenhouse gases and Hg into the atmosphere, leading to negative carbon isotope excursions and mercury anomalies in sedimentary strata. Global climatic warming induced by volcanism resulted in an enhanced hydrological cycle and can be correlated with more humid climatic conditions, intensified continental weathering and organic matter burial, lake expansion, and eutrophication. Such changes have important negative feedback on the global carbon cycle by increasing consumption of CO 2 in lacustrine settings, which in the late Pliensbachian led to the restoration of similar climates and environments that occurred pre-volcanism in the study area. • High-resolution Pliensbachian terrestrial climate and environmental change record. • Volcanism identified from negative carbon isotope excursions and Hg/TOC peaks. • Kaolinite, C-values, V/Cr ratios, and P/Al ratio peaks associated with volcanism. • Three Pliensbachian volcanic events coincide with climate and environment changes. • Volcanism driving environmental and climate change in terrestrial setting.

Spatiotemporal evolution of late Neoproterozoic marine environments on the Yangtze Platform (South China): inking continental weathering and marine C-P cycles

Abstract The geodynamic reorganization of major cratonic blocks during the assembly of Gondwana in the late Neoproterozoic caused a (bio)geochemical evolution of marine habitats that witnessed fluctuating seawater oxygenation and nutrient fluxes. Previous studies of carbonates deposited on the Yangtze Platform (South China) have shown their use as reliable archives of paleo-redox and bio-productivity changes intrinsically linked to continental weathering fluxes and water mass cycling. Despite its complex submarine morphology, only a few carbonate-bearing sections comprising deeper depositional environments of the Yangtze Platform have been evaluated. Here we report temporal, lateral, and vertical variations of stable C and radiogenic Sr, Nd, and Os isotopic compositions together with trace element and rare earth and yttrium (REY) systematics in carbonates from the Doushantuo Formation representing different paleo-marine environments of the Yangtze Platform, including I) inner shelf, II) restricted intra-shelf basin, III) slope and IV) deep-water basin sections to trace the extent and evolution of paleo water masses. Variations in shale-normalized (subscript SN) Ce anomalies argue for variably oxidizing atmosphere/ocean conditions. The combination of REY SN systematics with carbonate associated P enrichments defines three distinct local water masses from which the carbonates precipitated: i) fully oxidized shallow-water, ii) nutrient-poor intra-shelf basin waters, and iii) organic carbon-rich slope and deep-waters with temporal evolution to higher dissolved p O 2 . Negative carbon isotope excursions in the carbonates such as during the putative Shuram equivalent Doushantuo Negative Carbon Excursion (DOUNCE) correlate with less negative Ce SN anomalies and ambient seawater shifts to more radiogenic initial 87 Sr/ 86 Sr and 187 Os/ 188 Os, but unradiogenic 143 Nd/ 144 Nd compositions, arguing for coupling of redox and weathering trends. Our comprehensive geochemical study highlights the vertical and temporal variations of physicochemical water mass properties recorded in late Neoproterozoic Yangtze Platform carbonates until the emergence of the first metazoans. Highlights • Intensive geochemical study spanning four paleo-depths on the Yangtze Platform • First carbonate Osmium isotopic compositions from the Late Neoproterozoic • Os, Nd and Sr isotope variations in concert with influx of terrigenous solutions towards the end of the Doushantuo Fm • REY systematics and P enrichments reveal significantly different water masses along the passive margin of the Yangtze craton • Changes in C-P cycles during Shuram anomaly linked to intensified terrigenous solution flux

Dynamic sulfur and carbon cycles related to microbial sulfate reduction and volcanic activity during the Hirnantian glaciation in the Upper Yangtze Basin, South China

Parallel positive excursions of organic carbon (δ13Corg) and pyrite sulfur (δ34Spy) isotopes occurred globally during the Hirnantian glacial period. However, the reasons for these isotope excursions and their relationship with paleoenvironmental dynamics are not fully understood. This study presents a high-resolution geochemical investigation of the Tianlin section of the Upper Yangtze Basin, South China. The total organic carbon (TOC), iron speciation, trace elements, δ13Corg, and δ34Spy were analyzed and compared with published results from the other four sections in the same basin. A negative relationship was observed for the sulfate reduction index (SRI) with TOC and organic carbon accumulation rate. This suggests that the abundance of sedimentary organic matter was not the main factor inhibiting microbial sulfate reduction (MSR). The significant positive correlations of SRI with pyrite and δ34Spy showed that MSR promoted the formation of pyrite and that the availability of sulfate was the main limiting factor for the δ34Spy change. The negative excursions of δ13Corg and δ34Spy during the pre-glacial and post-glacial periods were coupled with dense segments of bentonite, indicating that volcanism was the main driving force for the negative excursions of C and S isotopes. Moreover, the sulfate input of rivers, restored at the post-glacial period, was the main reason for the continued negative excursion of S isotopes. Our results emphasize the key role of MSR and volcanic activity in C and S isotope excursions and their corresponding environmental changes in semi-restricted basins during the Hirnantian glaciation.

Biostratigraphy, carbon isotope and sequence stratigraphy of South Tethyan Valanginian successions in the Essaouira-Agadir Basin (Morocco)

A detailed stratigraphic analysis was carried out on Valanginian deposits of six Moroccan sections of the Essaouira-Agadir Basin (EAB, South Tethyan margin) in order to characterize the Weissert Event and propose correlations with the North Tethyan margin (south–east France Basin). The studied successions consist of alternating marlstone and limestone beds with sandy intercalations, and are well exposed and relatively rich in ammonites. Ammonite biostratigraphy and sequence stratigraphy were established for all successions, whereas calcareous nannofossil and carbon stable isotope analyses were performed for two and five sections, respectively. For each succession, an accurate ammonite zonal scheme allows to recognize the standard zonation established for the Mediterranean Province. A calcareous nannofossil zonation is provided for Zalidou and Aït Hamouch, considered as reference sections. The detailed biozonations allowed to ascribe an accurate age for most of the sedimentary discontinuities and depositional sequences identified in the EAB. East–west and north–south transects are established for the Valanginian depositional system. Despite some stratigraphic gaps, the Valanginian carbon isotope excursion (CIE) was recognized in most of the studied sections. The Moroccan successions are correlated with those of reference sections of south–east France i.e., the Vocontian Basin (Vergol-La Charce) and Provence Platform (Carajuan), using bio-sequence-chemo-stratigraphy. These inter-basin correlations allowed to evidence major “mid-Valanginian” and upper Valanginian eustatic regressions; the possible role of tectonics and glacio-eustacy is also discussed.

Reply to Cui: Comment on “On the origin of Shuram carbon isotope excursion in South China and its implication for Ediacaran atmospheric oxygen levels”

Reply to Cui: Comment on “On the origin of Shuram carbon isotope excursion in South China and its implication for Ediacaran atmospheric oxygen levels”

Stable Southern Hemisphere westerly winds throughout the Holocene until intensification in the last two millennia

The Southern Hemisphere westerly winds sustain the Southern Ocean’s role as one of Earth’s main carbon sinks, and have helped sequester nearly half the anthropogenic CO2 stored in the ocean. Observations show shifts in the vigor of this climate regulator, but models disagree how future change impacts carbon storage due to scarce baseline data. Here, we use the hydrogen isotope ratios of sedimentary lipids to resolve Holocene changes in Southern Hemisphere westerly wind strength. Our reconstruction reveals stable values until ~2150 years ago when aquatic compounds became more 2H-enriched. We attribute this isotope excursion to wind-driven lake water evaporation, and regional paleoclimate evidence shows it marks a trend towards a negative Southern Annular Mode – the Southern Ocean’s main mode of atmospheric variability. Because this shift is unmatched in the past 7000 years, our findings suggest that previously published millennium-long Southern Annular Mode indices used to benchmark future change may not capture the full range of natural variability.

Nearshore warm-water biota development in the aftermath of the Late Ordovician Mass Extinction in South China

The Late Ordovician Mass Extinction (LOME) occurred between two significant biotic diversifications: the Great Ordovician Biodiversification Event and the Silurian–Devonian Nekton Revolution. Upper Hirnantian carbonate deposits therefore are critical archives for understanding warm-water biotas, biodiversity resurgence and trophic chain development in the immediate aftermath of the LOME. In this study, we use microfacies analysis to investigate composition and biodiversity of the upper Hirnantian carbonates developed in northeastern Guizhou Province of China. These carbonates lay along the northern margins of the Dianqiangui landmass and are composed of intraclastic pack- to grainstone, mud- to wackestone, and bioclastic wacke- to packstone. Depositional age of the carbonates is estimated to be late Hirnantian, chiefly indicated by their coincidence with the Hirnantian Carbon Isotope Excursion and the brachiopods Cathaysiorthis and Eospirigerina. The development of carbonates containing diverse warm-water fossils at this transitional time reflects glacio-eustatic changes in the aftermath of the maximum Hirnantian glaciation and LOME, and the consequence of regional Kwangsian Orogeny in South China. Supposed autotrophic organisms in the carbonates are represented by calcimicrobes Allonema and Girvanella, and calcareous algae Amsassia, Dasyporella and Dimorphosiphon. Heterotrophic organisms are also abundant, including bryozoans (Atactoporella, Dekayia, Eridotrypa, Hallopora, Moorephylloporina and Ptilodictya?), trilobites (Dicranopeltis, Eoleonaspis, Mucronaspis (Songxites), Niuchangella and an aulacopleurid-like trilobite), rugose corals (Axiphoria, Eurogrewingkia, Meitanolasma and Grewingkia), tabulate corals (auloporid coral, Catenipora, Halysites, Paleofavosites and Propora), stromatoporoids (Camptodictyon, Ecclimadictyon and Cystostroma), conodonts (Ozarkodina and Walliserodus), benthic graptolites (Desmograptus), gastropods (at least two genera), and ostracods. Such a high biodiversity is evidence of a complex trophic web after the extinction pulses in South China. The biotic development of the warm-water benthic community might have been triggered by a short-lived localized favourable environment that facilitated the survival of warm-water biota during the Hirnantian, which is postulated to have pioneered further Silurian reef recovery in South China.

Low oxygen seawater and major shifts in the paleoenvironment towards the terminal Ediacaran: Insights from the Portfjeld Formation, North Greenland

Earth’s environment during the diversification of metazoans in the Late Ediacaran was characterised, amongst other factors, by changing oxygen levels and an extreme negative carbon isotope (δ13C) excursion, the Shuram Excursion. The Portfjeld Formation, North Greenland, deposited in the Franklinian Basin of eastern Laurentia, presents a unique opportunity to investigate a previously geochemically unstudied succession of Ediacaran marine carbonates to better understand the environment at this time. The succession encompasses a δ13C excursion (from +4 to −8‰) comparable to the Shuram Excursion, as well as the Portfjeld Biota, fossils akin to the earliest putative metazoan life forms, the Weng’an Biota. We present a multiproxy geochemical study of 44 samples including rare earth element (and yttrium; REE + Y), chromium isotope (δ53Cr) and strontium isotope (87Sr/86Sr) compositions.The Portfjeld Formation reveals that low oxygen waters existed beneath oxic surface waters in a stratified early Franklinian Basin. Samples from an interval including bituminous cherty dolomites are enriched in cerium relative to its neighbouring REE+Y’s, expressed as positive cerium anomalies (Ce/Ce* = 1.15–1.38), as well the redox sensitive element molybdenum (EF(Mo) = 23–179). This indicates deposition in waters with active metal oxyhydroxide dissolution, which necessitates dissolved oxygen levels as low as <10 μM. This has potential implications for the habitable environment of early putative metazoan forms, as the Portfjeld Biota fossils are found in this interval.The δ53Cr composition of carbonates can be used to track redox related paleoenvironmental changes. A stratigraphic increase in the estimated authigenic δ53Cr composition (from +0.75 to +1.28‰) occurs throughout the nadir and recovery of the succession’s δ13C carbon excursion. This δ53Cr shift coincides with an increase in detrital element concentration and 87Sr/86Sr values. Therefore, in this instance the δ53Cr composition of the carbonates of the Portfjeld Formation appears to have responded to elevated on-land weathering, and thus nutrient input, which may have induced further fractionation associated with surface water Cr (VI) back reduction by primary producers, in line with modern ocean observations. These geochemical changes indicate that Ediacaran carbon isotope excursions may be tied to environmental shifts that eventually led to the dramatic increase in metazoan complexity that ensued.

Catastrophic event sequences across the Permian-Triassic boundary in the ocean and on land

This review forms the preface of a special issue dealing with Environmental stresses and biotic responses during the Palaeozoic-Mesozoic transition, and develops an improved understanding of the sequence of catastrophe events associated with the Permian-Triassic (P–Tr) transition in particular. The 16 papers that comprise this special issue mostly focus on the Paleo-Tethys and Panthalassia regions, and enable us to recognize seven distinct phases of biotic and environmental evolution that may be precisely correlated between marine and terrestrial environments. Phase 1 represents the normal marine environments and biota that existed at 252.104 Ma, prior to the onset of major environmental perturbation. Phase 2 is marked by the onset of a negative carbon (δ13Ccarb) isotopic excursion, and coincides with a significant decline in radiolarian diversity, the collapse of communities in shallow metazoan reefs and deep-water settings, and the loss of rainforests on land (including the disappearance of Gigantopteris flora). Phase 3 is characterized by a pulse of biodiversification on the eve of the end-Permian extinction (EPE). Phase 4 comprises the EPE event itself, dated to 251.941 Ma, and coincides with an extreme episode of widespread anoxia, biodiversity loss, community structural collapse, and volatile volcanic eruptions. Phase 5 is marked by the renewed onset of oxic conditions in all oceanic environments except those in microbialite-rich shallow platform settings where abundant pyrite framboids signal dysoxia. Phase 6 includes the earliest Triassic extinction (ETE) event, dated to 251.880 Ma, and associated with euxinic to anoxic conditions and ocean acidification. An extended phase of deoxygenation and acidification returned to the marine settings during Phase 7, possibly driving a calcium crisis. The terrestrial extinction event (Phase 2) probably commenced c. 60 kyrs (in South China) or 370 kyrs (in Sydney basin) before the marine P–Tr extinction, the latter consisting of two distinct events (Phases 4 and 6), spaced c. 60 kyr apart, each being marked by both a carbon isotope excursion and ecologic collapse. The environmental changes associated with these three extinction phases in the ocean and on land can be attributed to the effects of volcanic emissions (e.g., CO2, SO2, halogens and metals) during the eruption of the Siberian Traps Large Igneous Province and convergent margin volcanism related to Tethyan tectonics.

Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO2, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ199Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ66Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ66Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ66Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

Major sulfur cycle perturbations in the Panthalassic Ocean across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event

The early Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was characterized by marine deoxygenation and the burial of organic-rich sediments at numerous localities worldwide. However, the extent of marine anoxia and its impact on the sulfur cycle during the T-OAE is currently poorly understood. Here, stable sulfur isotopes of reduced metal-bound sulfur (δ 34 S pyrite ) and pyrite sulfur concentrations (S PY ) have been analyzed across the Pliensbachian-Toarcian boundary (Pl-To) and the T-OAE from the Sakahogi and Sakuraguchi-dani sections (Japan), which were deposited in the deep and shallow Panthalassic Ocean, respectively. Our data reveal marked positive δ 34 S pyrite excursions of >10‰ across both the Pl-To and the T-OAE at Sakahogi, coincident with increases in S PY , and a positive excursion of >20‰ at the onset of the T-OAE at Sakuraguchi-dani. Whilst the development of deep-water anoxic/euxinic conditions could have resulted in an enhanced burial of pyrite, and also partly contributed to the positive excursion of δ 34 S pyrite , variations in δ 34 S pyrite at Sakahogi were most likely controlled by elevated export production and/or preservation. On the shallow shelf generally low and highly variable S PY and the positive shift in δ 34 S pyrite were likely attributable mainly to elevated sedimentation rates, with redox playing only a minor role in controlling pyrite abundance. Our discovery of a positive δ 34 S pyrite excursion across the Pl-To at Sakahogi indicates a hitherto unrecognized perturbation to the deep-water sulfur cycle, potentially associated with increased seafloor organic matter flux and pyrite burial at this time, consistent with a transient interval of anoxia. • First pyrite S-isotope (δ 34 S pyrite ) data across the Toarcian OAE (T-OAE) • Positive pyrite S-isotope excursions across T-OAE from deep and shallow Panthalassa • Excursions mainly attributable to local depositional conditions. • Deep-water sulfur isotope excursion across Pliensbachian-Toarcian boundary

Stratigraphic and volcanic signatures of Miaolingian-Late Ordovician rift pulses in the Alborz Mountains, northern Iran

• The Cambrian-Ordovician succession of the eastern Alborz Mountain Range displays uplift episodes associated with erosion. • The SPICE chemostratigraphic shift marks the Miaolingian–Furongian boundary. • Alkaline to subalkaline magmatic activities are separated by quiescent intervals of up to 35 m.y. • Intraplate (continental rift) volcanism reflects OIB-type mantle sources . • Plume-related rift pulses are linked to a large igneous province about 600 km across. Recent reconnaissance geological mapping, identification of unconformities and chronostratigraphic dating of Miaolingian to Ordovician strata in the eastern Alborz Mountains have led to recognition of synsedimentary uplift episodes associated with block tilting and partial erosion of the Cambrian Mila and Ordovician Simeh Kuh, Qumes and Lashkarak formations. A chronostratigraphic revision of the Miaolingian-Late Ordovician fossil content is presented, and the Steptoean Positive Carbon Isotope Excursion (SPICE) chemostratigraphic shift, marking the Miaolingian–Furongian boundary, is constrained close to the Members 2/3 contact of the Mila Formation. Upper Ordovician basaltic lava flows embedded in the Abarsaj Formation and Devonian sills and dykes intruded in the Mila and Simeh-Kuh formations were geochemically analysed and compared with the voluminous lava eruptions recognized in the Katian-Llandovery Soltan-Maidan and Gorgan Schists formations, and in the ?Emsian-Eifelian Mighan Formation. They represent successive magmatic pulses of alkaline to subalkaline activities, separated by background (quiescent) intervals of up to 35 m.y., characterizing an intraplate field (continental rift) affinity compatible with OIB-like (ΔNb greater than 0) mantle sources. Successive Miaolingian-Mid Ordovician (volcanic-free) uplift episodes were related to lithospheric doming preceding the onset of stepwise plume-related rift pulses, which shaped a large igneous province with transects of about 600 km, linking the Damghan and Alestan domains of the eastern Alborz Mountains with South Kopet-Dagh and some blocks of Central Iran.

Si isotope ratio of radiolaria across Triassic–Jurassic transition in a pelagic deep-sea bedded chert (Inuyama, Japan)

The end-Triassic extinction event (ETE) marks one of the “Big five” mass extinction events of the Phanerozoic, yet the timing and nature of environmental changes on a global scale remain elusive. Here we report a mm-scale high-resolution δ30Si profile of sea surface-dwelling radiolaria, preserved as moulds, spanning the end-Triassic radiolarian turnover interval of the deep-sea succession at the Katsuyama section, Inuyama, Japan. The δ30Si of the Triassic-Jurassic radiolaria between − 0.6 ± 0.5‰ and 2.6 ± 0.3‰ overlap with available radiolarian δ30Si data for Triassic to Cenozoic, including modern samples. The mass balance of Si with dominant radiolaria up to 90% in chert supports negligible impact of diagenesis on δ30Si of radiolarian moulds. The cm-scale δ30Si variations up to 2 ‰ are overall associated with changes in silica contents. Here we hypothesize that the δ30Si of radiolarian reflect radiolarian productivity. We detected negative silicon isotope excursions within and above the end-Triassic radiolarian turnover interval, named here the SIE 1 and 2. The first SIE of 2 ‰ is detected from 0 mm to 10 mm above the base of purple chert interval, probably linked with the initial massive volcanism in the Central Atlantic magmatic provinces (CAMP) and associated deep-sea acidification and the onset of the end-Triassic radiolarian turnover. Subsequent positive SIE up to 2 ‰ ca. 10 mm above the first negative SIEs may record recovery of biosiliceous productivity, which could may be related with the first appearance of several Jurassic radiolarian taxa within the radiolarian turnover interval. We also detected data that is suggestive of SIE 2 in the lowermost dusky red chert bed, suggesting similar changes in productivity. These negative SIEs occurred within ~ 1 mm interval, implying the occurrence of kyr- or shorter-scale drastic environmental perturbations across the end-Triassic radiolarian turnover interval.

Lacustrine redox variations in the Toarcian Sichuan Basin across the Jenkyns Event

The Early Jurassic Jenkyns Event (~183 Ma) represents a major environment perturbation event, characterized by the negative carbon isotope excursion during the early Toarcian. Reconstruction of redox conditions, especially in the mega-lakes, across the Jenkyns Event is of significance to understand the biogeochemical dynamics on land at the time. Here, we report iron speciation and trace metal data from the two drill cores (LQ104X and X3 wells) with different depositional environments in the lacustrine Sichuan Basin of SW China. Results show that the water column in the Sichuan Basin experienced a significant shift of redox conditions in the Toarcian. Frequent development of anoxic-ferruginous conditions interspersed with significant euxinic episodes across the Jenkyns Event. Conversely, during the pre/post-Jenkyns Event interval, dominantly oxic conditions, with short-lived anoxic-ferruginous or euxinic conditions, developed in the water column of the Sichuan Basin. We then proposed three possible mechanisms interdependent in nature that could have induced the water-column redox variability in Toarcian lakes: (1) lake stratifications, (2) enhanced methane release from the lake floor under low sulfate conditions that exhausted bottom water oxygen, and (3) lake eutrophication. We further anticipated that a close link between redox shift and ecological stress in the Sichuan Basin across the Jenkyns Event.

Devonian–Carboniferous Hangenberg Crisis in South China: Variations in Trace Elements, Strontium and Carbon Isotope Chemostratigraphy in the Nanbiancun Carbonate Section

AbstractThe global Hangenberg Crisis or Hangenberg Extinction is a mass extinction near the Devonian–Carboniferous boundary. Comprehensive research of petrology and geochemistry on the Devonian–Carboniferous boundary, as exposed in the Nanbiancun auxiliary stratotype section, South China, elucidates paleoenvironmental changes and controls on marine strontium (87Sr/86Sr) and carbonate carbon (δ13Ccarb) isotopes during the Hangenberg Crisis. The new 87Sr/86Sr data reveal a regression in the Middle Siphonodella praesulcata Zone, while the Hangenberg Extinction was occurring in South China. Moreover, the δ13Ccarb data records a negative excursion near the base of the Middle Siphonodella praesulcata Zone that may have been connected with the Hangenberg Extinction. A positive δ13Ccarb excursion, corresponding with the Upper Siphonodella praesulcata Zone, may reflect the effects of a vigorous biological pump. The magnitude of the Hangenberg Carbon Isotopic Excursion in peak δ13carb values and δ13Ccarb gradient in carbonate Devonian–Carboniferous boundary sections of the South China Craton during the Hangenberg Crisis, are a function of depositional water depth and distance from the shore. The carbon cycling during the Hangenberg Carbon Isotopic Excursion had a much stronger impact on oceanic surface waters than on the deep ocean and the δ13Ccarb gradient of local seawater was likely caused by enhanced marine productivity, associated with biological recovery in platform sediments during the Hangenberg Crisis.

Moderate levels of oxygenation during the late stage of Earth's Great Oxidation Event

The later stages of Earth's transition to a permanently oxygenated atmosphere during the Great Oxidation Event (GOE; ∼2.43–2.06 Ga) is commonly linked with the suggestion of an “oxygen overshoot” during the ∼2.22–2.06 Ga Lomagundi Event (LE), which represents Earth's most pronounced and longest-lived positive carbon isotope excursion. However, the magnitude and extent of atmosphere-ocean oxygenation and implications for the biosphere during this critical period in Earth's history remain poorly constrained. Here, we present nitrogen (N), selenium (Se), and carbon (C) isotope data, as well as bio-essential element concentrations, for Paleoproterozoic marine shales deposited during the LE. The data provide evidence for a highly productive and well-oxygenated photic zone, with both inner and outer-shelf marine environments characterized by nitrate- and Se oxyanion-replete conditions. However, the redoxcline subsequently encroached back onto the inner shelf during global-scale deoxygenation of the atmosphere-ocean system at the end of the LE, leading to locally enhanced water column denitrification and quantitative reduction of selenium oxyanions. We propose that nitrate-replete conditions associated with fully oxygenated continental shelf settings were a common feature during the LE, but nitrification was not sufficiently widespread for the aerobic nitrogen cycle to impact the isotopic composition of the global ocean N inventory. Placed in the context of Earth's broader oxygenation history, our findings indicate that O2 levels in the atmosphere-ocean system were likely much lower than modern concentrations. Early Paleoproterozoic biogeochemical cycles were thus far less advanced than after Neoproterozoic oxygenation.

Synchronous positive δ13Ccarb and δ13Corg excursions during 497–494 Ma: From a CO2 concentrating mechanism dominated photosynthesis?

Steptoean Positive Carbon Isotope Excursion (SPICE) event is featured particularly as the synchronous positive δ13Ccarb and δ13Corg excursions, which are recorded worldwide in the Late Cambrian (ca. 497–494 Ma). However, the cause of this event remains elusive. To better understand the underlying mechanism of the contemporaneous positive isotope excursions, here we present a series of geochemical data from two drilling wells of the Tarim Basin, China and some relevant data reported from worldwide sections. CO2 concentrating mechanisms (CCMs, e.g., utilized by cyanobacteria) developed in response to decrease in pCO2 and increase in pO2 during the SPICE event, in which the HCO3− enriched in 13C but depleted in 2H is utilized for photosynthesis, is proposed to result in the synchronous positive excursions of δ13Ccarb and δ13Corg coupled by a negative shift of δDorg as exemplified by the samples from the Tarim Basin. The SPICE event was followed by an increase in plankton, which could relate to triggering CCMs and maintaining the biodiversity of marine organisms during the event.