Marine Anoxia Research Articles

A Carboniferous apex for the late Paleozoic icehouse

Abstract Icehouse climate systems occur across an abbreviated portion of Earth history, constituting c. 25% of the Phanerozoic record. The Late Paleozoic Ice Age (LPIA) was the most extreme and longest lasting glaciation of the Phanerozoic and is characterized by periods of acute continental-scale glaciation, separated by periods of ice minima or ice-free conditions on the order of <10 6 years. The late Paleozoic glaciogenic record of the Paraná and Kalahari basins of southern Gondwana form one of the largest, best-preserved and well-calibrated records of this glaciation. In the Carboniferous, the eastern and southern margins of the Paraná Basin and the Kalahari Basin were characterized by subglacial conditions, with evidence for continental and upland glaciers. In the latest Carboniferous, these basins transitioned from subglacial reservoirs to ice-free or ‘ice distal‘ conditions evidenced by the widespread deposition of marine deposits juxtaposed on subglacial bedforms. High-precision U–Pb zircon chemical abrasion thermal ionization mass spectrometry geochronological constraints from volcanic ash deposits in the deglacial marine black shales of the Kalahari Basin and from fluvial and coal successions, which overlie marine deposits in the Paraná Basin, indicate subglacial evidence in these regions is constrained to the Carboniferous. The loss of ice in these regions is congruent with a late Carboniferous peak in p CO 2 and widespread marine anoxia in the late Carboniferous. The permeant retreat of glaciers in basinal settings, despite an early Permian p CO 2 nadir, highlights the influence of short-term perturbations on the longer-term CO 2 record and suggests an ice threshold had been crossed in the latest Carboniferous. A definitive driver for greenhouse gases in the LPIA, such as abundant and sustained volcanic activity or an increased biological pump driven by ocean fertilization, is unresolved for this period. Lastly, the proposed Carboniferous apex for the high-latitude LPIA record is incongruent with observations from the low-latitude tropics where an early Permian peak is proposed.

Regional conditions cause contrasting behaviour in U-isotope fractionation in black shales: Constraints for global ocean palaeo-redox reconstructions

The U-isotope system is a well-established palaeo-redox proxy that potentially constrains the global extent of marine anoxia during average as well as extreme redox events throughout Earth's history. A typical archive that forms underneath a reducing water column and acts as an intense U sink is organic-rich black shale. However, the degree to which black shale archives reflect the marine U-isotope signature is not well understood because U-isotope fractionation between U(VI)-bearing seawater and U(IV)-bearing black shales may vary as a function of local environmental conditions. Here, we present a combination of U-isotope and elemental concentration datasets, supported by a complementary Mo-isotope record, for the Furlo sedimentary section in Marche–Umbria, Italy and interrogate the combined systematics to unravel the mechanisms controlling the U-isotope fractionation factor between black shales and ambient seawater. We examine black shales deposited before and during Oceanic Anoxic Event 2 (OAE 2), which was one of the most extreme climatic perturbations of the Mesozoic Era that took place around the Cenomanian–Turonian boundary (Late Cretaceous, c. 94 Ma). The results of this study show that the U-isotope signature in the black shales deposited before OAE 2 was controlled by different mechanisms than the U-isotope ratios recorded in black shales deposited during OAE 2, with both stratigraphic intervals likely influenced by local environmental conditions. Probable local environmental changes include increased U reduction associated with biomass at or above the sediment–water interface and varying dissolved hydrogen sulphide concentrations in the water column and sediment. The overall results of this study confirm that black shales are a highly complex archive for U-isotope studies of past oceanic redox conditions, due to the sensitivity of the U-isotope fractionation mechanism to local environmental conditions, which are difficult to constrain. We propose the application of a ∆238Ushale-seawater of 0.6 ± 0.1 ‰ to black shale records deposited under locally constant euxinic conditions at non-restricted settings.

Manganous water column in the Tethys Ocean during the Permian-Triassic transition

Ocean anoxia was one of the key killing mechanisms responsible for the end-Permian mass extinction (~252 Ma). However, the temporal evolution and the triggering mechanisms of the end-Permian anoxia are controversial, with the current view being that the water column deoxygenation was a spatially and temporally heterogeneous event. Here, we use cerium-anomalies, uranium contents and rare earth element and yttrium (REY) compositions measured on the carbonate fraction of samples from two marine sections in Armenia and South China to constrain the evolution of end-Permian marine anoxia. In particular, we attempt to identify intervals of manganous redox conditions (i.e., Mn2+-rich water column). These are characterized by the reduction of Mn(IV)-oxides at a redox potential lower than of nitrate and higher than of Fe(III)-oxide reduction. Most of our samples yield REY patterns with seawater characteristics suggesting the dominance of hydrogenous REYs. While decreasing U concentrations in the C. yini Zone suggest an overall intensification of global marine anoxia in the latest Permian, Ce-anomalies shift from negative to positive above the C. yini Zone, implying a deterioration in the oxygen content of the local water columns. Highly positive Ce-anomalies (Ce/Ce* > 1.3) are firstly reported for the P-T interval, indicating enrichment of Ce in the water column and the establishment of manganous redox conditions at both locations. This change coincided globally with climatic warming, the onset of the marine extinction interval as well as locally with the disappearance of sponge spicules and radiolarians (South China) and the appearance of microbialites (Armenia).

An overview of the connection between Earth’s climate evolution and mass extinction events

Authors present a brief review of the potential impact of climate change on biodiversity throughout the history of the Earth. Studying paleoclimate is difficult because it uses proxies that occurred millions of years ago, and there is an intrinsic uncertainty associated with that. However, the climate of the past and the evolution of life itself are related to each other. The current discussion goes through the different geological eras, emphasizing the Phanerozoic Eon, where terrestrial conditions allowed life to flourish. Recent studies seem to support the argument that the five great mass extinctions are related to warm climate modes produced by intense volcanism that generate changes in the concentrations of greenhouse gases and marine anoxia. This should be one more alert for humanity to implement effective measures to counteract the current global warming trend before the consequences on ecosystems are more serious.

Pyrite Sulfur Isotope Systematics Suggest Low Marine Sulfate Levels across the Ediacaran–Cambrian Transition

The Ediacaran–Cambrian (E–C) transition was a critical period in Earth’s history, and it was characterized by variable but potentially significant increases in atmospheric oxygen levels (pO2). Sulfate is an essential oxidant and plays a key role in regulating Earth’s surface redox conditions. However, there is uncertainty regarding seawater sulfate levels during the E–C transition. To address this issue, organic carbon (δ13Corg) and pyrite sulfur isotope (δ34Spy) analyses of the Heyu section (the Dengying Formation) in the Sichuan Basin, South China, were carried out. The δ13Corg varies between −36.4‰ and −27.4‰, and δ34Spy ranges from −6.6‰ to 27.3‰. The TOC and pyrite content ranges from 0.13 wt.% to 4.28 wt.% and from 0.01 wt.% to 0.94 wt.%, respectively. The marine sulfate concentration was modeled using a one-dimensional diffusion–advection–reaction model (1D-DAR). The modeling results indicate that the seawater sulfate level remained at a relatively low level of ~2–6 mM. The increased pyrite burial at extensive marine anoxia during the terminal Ediacaran period was likely a driver of such a low sulfate level.

Carbon isotope of the Early Cretaceous sediments from the West Pacific and the Sulu orogenic belt: Implying the global atmospheric pCO2 changes during the Early Aptian Oceanic Anoxic Event 1a

Multiple lines of evidence revealed that enormous CO2 was input into the ocean-atmosphere system during Oceanic Anoxic Event 1a (OAE 1a), which led to a series of environmental perturbations (e.g. temperature rising, enhanced weathering, promoted primary productivity and marine anoxia). Among them, increased carbon isotope fractionation (Δ13Ccarb-org) between the carbonate (δ13Ccarb) and organic carbon (δ13Corg) in the sediments is one of the most important indicators to the enhanced atmospheric pCO2. However, only a few research reported high-resolution paired carbon isotopes during OAE 1a, which were almost limited around the western Tethys. Here, we report new paired carbon isotopes of the Early Cretaceous sediments from the ODP site 866A in the western Pacific and the Lingshan Island section in the Sulu orogenic belt. The δ13Ccarb curves from both sections correlate well with OAE 1a, in which a positive δ13Ccarb excursion is identified between the C3–C6 segments. The Δ13Ccarb-org displays a ∼7–11‰ positive shift coinciding with the δ13Ccarb excursion, indicating a remarkable increase and a gradual decrease in CO2 in water and atmosphere, which further support significant atmospheric pCO2 changes during OAE 1a on a global scale. The sediments also show a few variations in carbon isotope records during OAE 1a between different locations, which may reflect different influences from local environments, and the deviations of stratigraphic correlation and diagenesis cannot be ruled out either.

Ecosystem responses of two Permian biocrises modulated by CO2 emission rates

Carbon dioxide (CO2) emissions and associated climate change are thought to have caused a number of widespread marine anoxia and mass extinction events in the geologic past. However, how marine ecosystems respond to different CO2 emission patterns remains an important unresolved question. The geologic records of the Permian Period, which witnessed two mass extinctions associated with volcanic eruption (thus CO2 emissions) but with vastly different biological responses, provide a unique window to address this issue. Here, we present a long-term uranium isotope (δ238U) record using marine limestones covering the latest Early Permian through Middle to Late Permian. The δ238U values show two episodes of low values in the middle Capitanian and late Changhsingian, indicating two periods of expansion of marine anoxia during the Permian Period. We use a uranium isotope mass balance model to quantify the anoxic seafloor areas, and we further use a carbon cycle model (LOSCAR, Long-term Ocean Sediment Carbon Reservoir) based on observed δ13C of marine carbonates, sea surface temperature records, and ocean surface pH data to quantify the carbon emission rates across the two biocrises. The uranium isotope mass balance model reveals that the anoxic seafloor area is three times larger during the end-Permian mass extinction (EPME, covering ∼35% of the seafloor areas) than that during the end-Guadalupian event (EGE, covering ∼10% of the seafloor areas). The CO2 emission rates across the two biocrises modeled from the LOSCAR model show that the carbon emission rate across the EPME was at least five times faster than that during the EGE, with the best-fit δ13C values of the input sources ranging from −8 to −12‰, indicating a predominant volcanic CO2 source during the EPME, and close to −25‰ during the EGE. Comparing model results and observed proxy data led to the suggestion that the more severe ecosystem responses during the EPME, including higher extinction rate and larger extent of seafloor anoxia, are closely linked to the faster carbon emission rates compared to the EGE.

Increases in reef size, habitat and metacommunity complexity associated with Cambrian radiation oxygenation pulses

Oxygenation during the Cambrian Radiation progressed via a series of short-lived pulses. However, the metazoan biotic response to this episodic oxygenation has not been quantified, nor have the causal evolutionary processes been constrained. Here we present ecological analyses of Cambrian archaeocyath sponge reef communities on the Siberian Platform (525–514 Ma). During the oxic pulse at ~521–519 Ma, we quantify reef habitat expansion coupled to an increase in reef size and metacommunity complexity, from individual within-community reactions to their local environment, to ecologically complex synchronous community-wide response, accompanied by an increase in rates of origination. Subsequently, reef and archaeocyath body size are reduced in association with increased rates of extinction due to inferred expanded marine anoxia (~519–516.5 Ma). A later oxic pulse at ~515 Ma shows further reef habitat expansion, increased archaeocyath body size and diversity, but weaker community-wide environmental responses. These metrics confirm that oxygenation events created temporary pulses of evolutionary diversification and enhanced ecosystem complexity, potentially via the expansion of habitable space, and increased archaeocyath individual and reef longevity in turn leading to niche differentiation. Most notably, we show that progression towards increasing biodiversity and ecosystem complexity was episodic and discontinuous, rather than linear, during the Cambrian Radiation.

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

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

Aluminium-normalised trace-element paleoredox proxies and their application to the study of the conditions of Burgess Shale-type preservation

The paleo-ecological settings of Burgess Shale-type (BST) fossils have been the subject of extensive investigations, particularly with regard to the role of anoxia in their exceptional preservation. Our understanding of the complexity of both paleontological and geochemical environmental proxies has evolved with increased knowledge of niche communities and marine geochemical and biogeochemical processes. Accordingly, dated approaches should be reevaluated. The commonly applied paleoredox proxies V/Cr, Ni/Co, and V/(V + Ni), as well as the redox proxy V/Al2O3 and productivity proxy Ba/Al2O3, were all tested on the Wheeler Formation (Utah, USA) for which a fluctuating oxycline environment has been well established based on paleontological features. Subsequently, these proxies were applied to three additional BST fossil-bearing deposits: the Emigsville Member of the Kinzers Formation (Pennsylvania, USA), the Burgess Shale (British Columbia, CA) and the Emu Bay Shale (South Australia, AU). Their host shales lack perceptible V of marine origin, indicative of oxic conditions, in contrast to the Wheeler and brine-related strata from the Burgess Shale which include additional marine input of V in some beds, consistent with varying redox conditions. The black shales of the Longs Park Member (Kinzers) have elevated V/Al2O3 indicative of anoxic conditions. Molybdenum concentrations are relatively low in most samples, but minor variations tend to correlate with V/Al2O3 patterns. Ba/Al2O3 indicates high productivity with deposition of BST fossils in the Wheeler Formation, and diagenetic remobilisation of Ba owing to sediment anoxia in the Burgess Shale. Our findings show that redox proxy V/Al2O3 can be effectively applied to BST samples, whereas Ba/Al is a reliable proxy for low to moderate paleoproductivity levels and may be useful as a secondary or tangential proxy for redox conditions. Taphonomic processes leading to BST preservation occur in a variety of environments characterised by oxyclines, matgrounds, variable sedimentation rates, hypersalinity and/or anoxia. KEY POINTS V/Al2O3 and Ba/Al2O3 are effective proxies for anoxia and productivity in BST deposits. BST preservation can occur in a variety of depositional environments. Marine anoxia is not required for BST preservation.

Dynamic ocean redox conditions during the end-Triassic mass extinction: Evidence from pyrite framboids

The end-Triassic (∼201 Mya) records one of the five largest mass extinction events of the Phanerozoic. Extinction losses were coincident with large igneous province volcanism in the form of the Central Atlantic Magmatic Province (CAMP) and major carbon isotope excursions (CIEs), suggesting a link between these phenomena. Marine anoxia has been implicated as a causal factor in the crisis, but there remains some uncertainty regarding the role of marine redox changes in marine extinction phases because both intensity and duration of marine anoxia are poorly constrained. We employ high resolution pyrite framboid size-frequency analysis at two Triassic-Jurassic (Tr-J) boundary sections: Kuhjoch in Austria (the Tr-J Global Boundary Stratotype Section and Point; GSSP) and St. Audrie's Bay in England (former GSSP candidate) in order to further evaluate the role of marine anoxia in the end-Triassic mass extinction (ETME). The St. Audrie's Bay section records predominantly anoxic conditions punctuated by weakly oxygenated (dysoxic) conditions through the Tr-J transition, even during shallow-water intervals. Kuhjoch experienced both anoxic and dysoxic conditions during the ETME but became better oxygenated near the Tr-J boundary. Marine anoxia is therefore implicated in the extinction at both locations. A similar redox history is known from the Central European Basin, Western Tethys and Panthalassa, where marine anoxia developed in the lead up to the ETME prior to reoxygenation around the Tr-J boundary. • High-resolution pyrite framboids size distribution from two European Triassic-Jurassic boundary sections. • Global dynamic ocean redox evolution through the Triassic-Jurassic boundary is reconstructed. • Oxygen restriction occurred in extremely shallow waters and was likely one of the key causes of the crisis.

Mediterranean–Black Sea gateway exchange: scientific drilling workshop on the BlackGate project

Abstract. The MagellanPlus workshop “BlackGate” addressed fundamental questions concerning the dynamic evolution of the Mediterranean–Black Sea (MBS) gateway and its palaeoenvironmental consequences. This gateway drives the Miocene–Quaternary circulation patterns in the Black Sea and governs its present status as the world's largest example of marine anoxia. The exchange history of the MBS gateway is poorly constrained because continuous Pliocene–Quaternary deposits are not exposed on land adjacent to the Black Sea or northern Aegean. Gateway exchange is controlled by climatic (glacio-eustatic-driven sea-level fluctuations) and tectonic processes in the catchment as well as tectonic propagation of the North Anatolian Fault Zone (NAFZ) in the gateway area itself. Changes in connectivity trigger dramatic palaeoenvironmental and biotic turnovers in both the Black Sea and Mediterranean domains. Drilling a Messinian to Holocene transect across the MBS gateway will recover high-amplitude records of continent-scale hydrological changes during glacial–interglacial cycles and allow us to reconstruct marine and freshwater fluxes, biological turnover events, deep biospheric processes, subsurface gradients in primary sedimentary properties, patterns and processes controlling anoxia, chemical perturbations and carbon cycling, growth and propagation of the NAFZ, the timing of land bridges for Africa and/or Asia–Europe mammal migration, and the presence or absence of water exchange during the Messinian salinity crisis. During thorough discussions at the workshop, three key sites were selected for potential drilling using a mission-specific platform (MSP): one on the Turkish margin of the Black Sea (Arkhangelsky Ridge, 400 m b.s.f., metres below the seafloor), one on the southern margin of the Sea of Marmara (North İmrali Basin, 750 m b.s.f.), and one in the Aegean (North Aegean Trough, 650 m b.s.f.). All sites target Quaternary oxic–anoxic marl–sapropel cycles. Plans include recovery of Pliocene lacustrine sediments and mixed marine–brackish Miocene sediments from the Black Sea and the Aegean. MSP drilling is required because the JOIDES Resolution cannot pass under the Bosporus bridges. The wider goals are in line with the aims and scope of the International Ocean Discovery Program (IODP) “2050 Science Framework: Exploring Earth by Scientific Ocean Drilling” and relate specifically to the strategic objectives “Earth's climate system”, “Tipping points in Earth's history”, and “Natural hazards impacting society”.

Progressive expansion of seafloor anoxia in the Middle to Late Ordovician Yangtze Sea: Implications for concurrent decline of invertebrate diversity

The Great Ordovician Biodiversification Event (GOBE) achieved its peak during the Middle Ordovician, likely in association with climatic cooling and a rise of atmospheric O2. However, unstable redox states developed widely in contemporaneous epeiric seas, challenging previous assumptions about sustained oceanic oxygenation driven by deep-ocean ventilation in the aftermath of Ordovician cooling. Here, we investigate two Middle-Upper Ordovician shale-dominated successions from intra-shelf basin and slope settings of the Yangtze Sea, South China. Negative shifts in bulk δ15N and productivity proxies reveal temporal changes in the pattern of nutrient cycling under moderate-to-high productivity conditions, which were likely sustained by increased upwelling of cold and nutrient-rich deep waters in sync with contemporaneous climatic cooling. As a result, marine anoxia likely expanded in deep waters of the Yangtze Sea, as reflected in enrichments of highly reactive iron and redox-sensitive metals (Mo, U). Although moderate Mo abundances (25-50 ppm) and δ98/95Mo values (+0.4 to +0.8‰) within the euxinic interval suggest that bottom waters had low sulfide concentrations, the δ98/95Mo values are comparable with those of coeval euxinic facies (∼+0.6‰). Furthermore, our modeling results show that low δ98/95Mo can be produced when sulfidic sinks dominate Mo removal, suggesting that Middle-Late Ordovician oceans may have been less oxygenated than previously assumed, at least for the continental seas of South China, Baltica and Laurentia. These environmental changes may provide a new explanation for the ∼50% decline of invertebrate species following the peak of GOBE. Our findings thus link the progressive expansion of marine anoxia and climatic cooling to a profound biotic change during the Mid-Late Ordovician Period.

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

What triggered the Late Ordovician mass extinction (LOME)? Perspectives from geobiology and biogeochemical modeling

Recent studies have described an oceanic anoxic event during the Hirnantian (HOAE) and linked this event to the Late Ordovician mass extinction (LOME). However, the extent and duration of the HOAE remain under debate, as do questions about how oceanic anoxia impacted marine ecosystems. For this study, we investigated two previously unstudied sections adjacent to the open ocean in South China for iron speciation data and sulfur isotope signatures of pyrite and carbonate associated sulfate. Combined with published results, these data provide a view of both local and global oceanic redox landscapes during this interval. Previous work has challenged the idea that increased oxygen solubility under colder temperatures was the critical control of glacial marine oxygenation. Here we describe oxygenated Hirnantian shallow-water environments impacted by the effects of glacio-eustatic variation and deep-water anoxia induced by an enhanced biological pump. Decreased isotopic fractionation between seawater sulfate and sedimentary pyrite has the potential to reveal a period of low oceanic sulfate levels and helps to constrain the duration and style of oceanic redox evolution during the HOAE. Moreover, our modeling work suggests another possibility that the Hirnantian sulfur cycle shifted less than +3‰ within 1 Ma as a result of decreased silicate weathering, increased volcanism, and enhanced pyrite burial. Correlations with data for marine faunal diversity are somewhat inconsistent with previous assertions that oceanic anoxia triggered the LOME. Nonetheless, cool-water tolerant animals (especially within benthos) may have suffered from oxygen-deficient seawater and lethal metal levels within their limited ecospace much later in the middle of the Hirnantian. Recovery or proliferation of some portions of the marine ecosystem coincided with oscillating redox conditions at the end of glaciation, including the deep-water Anji sponge-dominated benthic faunas. Our findings contribute new insights into the processes operating during the LOME and, more generally, the evolutionary interactions between OAEs and mass extinctions over Earth history. • Oxygenated shallow waters and persistently anoxic deep waters characterized marine redox landscape during the glaciation. • Modeling hypothesis proposed alternate possibilities for the odd δ 34 S record. • Glaciation initiated the Late Ordovician mass extinction, and marine anoxia cut in later as a major trigger.

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

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.

Constraining marine anoxia under the extremely oxygenated Permian atmosphere using uranium isotopes in calcitic brachiopods and marine carbonates

The redox chemistry change in ancient oceans has profoundly shaped the evolutionary trajectories of animals. Uranium isotopes (δ238U) in marine carbonate sediments have widely been used to place quantitative constraints on the oxygenation state of the oceans through geological history. However, syndepositional and post-depositional diagenesis impose a positive and variable δ238U offset in the carbonate sediments relative to contemporaneous seawater, leaving uncertainties on quantification of anoxic seafloor areas in the past. Studies from modern settings suggest that Low-Magnesium Calcite (LMC) in articulate brachiopod shells are diagenetic resistant materials that may faithfully record the δ238U value of ancient seawater. However, this notation has not yet been validated in geological records. Here, we test this hypothesis by analyzing paired brachiopod shells and the host carbonate rocks from several Permian stratigraphic successions in South China. Forty-nine articulate brachiopod fossil shells and twenty-six host carbonate rocks from the Early Permian to the Late Permian were investigated. We performed a rigorous screening to monitor the diagenetic process that might have influenced the δ238U of the shells using a scanning electronic microscope, cathodoluminescent microscopy, and major and trace elements. We estimate that 53% of the shells preserve the primary seawater δ238U signals. Using the screened δ238U values of shells and bulk carbonates, we suggest there were four episodes of expanding marine anoxia during the Permian that are temporally coincident with periods of volcanism. The first two oceanic anoxic events occurred at a time when atmospheric pO2 levels were predicted to be higher than today. The last two events are temporally coincident or immediately preceding the end-Guadalupian and the end-Permian mass extinctions, respectively, confirming previous suggestions that marine anoxia was a critical factor in driving marine animal extinctions.

Continental weathering and recovery from ocean nutrient stress during the Early Triassic Biotic Crisis

Following the latest Permian extinction ∼252 million years ago, normal marine and terrestrial ecosystems did not recover for another 5-9 million years. The driver(s) for the Early Triassic biotic crisis, marked by high atmospheric CO2 concentration, extreme ocean warming, and marine anoxia, remains unclear. Here we constrain the timing of authigenic K-bearing mineral formation extracted from supergene weathering profiles of NW-Pangea by Argon geochronology, to demonstrate that an accelerated hydrological cycle causing intense chemical alteration of the continents occurred between ∼254 and 248 Ma, and continued throughout the Triassic period. We show that enhanced ocean nutrient supply from this intense continental weathering did not trigger increased ocean productivity during the Early Triassic biotic crisis, due to strong thermal ocean stratification off NW Pangea. Nitrogen isotope constraints suggest, instead, that full recovery from ocean nutrient stress, despite some brief amelioration ∼1.5 million years after the latest Permian extinction, did not commence until climate cooling revitalized the global upwelling systems and ocean mixing ∼10 million years after the mass extinction.

Marine siliceous ecosystem decline led to sustained anomalous Early Triassic warmth

In the wake of rapid CO2 release tied to the emplacement of the Siberian Traps, elevated temperatures were maintained for over five million years during the end-Permian biotic crisis. This protracted recovery defies our current understanding of climate regulation via the silicate weathering feedback, and hints at a fundamentally altered carbon and silica cycle. Here, we propose that the development of widespread marine anoxia and Si-rich conditions, linked to the collapse of the biological silica factory, warming, and increased weathering, was capable of trapping Earth’s system within a hyperthermal by enhancing ocean-atmosphere CO2 recycling via authigenic clay formation. While solid-Earth degassing may have acted as a trigger, subsequent biotic feedbacks likely exacerbated and prolonged the environmental crisis. This refined view of the carbon-silica cycle highlights that the ecological success of siliceous organisms exerts a potentially significant influence on Earth’s climate regime.