Australia: The Land Where Time Began

A biography of the Australian continent 

End-Triassic Mass Extinction – Marine Anoxia During the End Triassic Indicated by an Enormous Sulphur Excursion

In the debate over the cause of the mass extinction event at the close of the Triassic the role of oceanic anoxia has been proposed. In this paper He et al. presented carbonate-associated sulphate δ³⁴S data from sections that span the Late Triassic-Early Jurassic transition which document synchronous large positive excursions on a global scale over an interval of ~50 Kyr (thousand years). It was demonstrated by biogeochemical modelling that the perturbation of the sulphur isotope is explained best by a 5-fold increase in global burial of pyrite, which is consistent with development of marine anoxia on a large scale on the margin of Panthalassa and the northwestern European shelf. This burial of pyrite event coincides with the loss of Triassic taxa that was observed in the sections that were studied. It was also indicated by modelling results that the ocean surface concentration of sulphate was low (<1 millimolar), a common feature of many deoxygenation events in the Phanerozoic. He et al. propose that the scarcity of sulphate preconditions oceans for the development during intervals of rapid warming by increasing the flux of the benthic methane and the result of oxygen demand.

The end-Triassic mass extinction event (ETME) is one of the largest biological crises that is known from the Phanerozoic and is regarded is 1 of the “Big 5” (Dunhill et al., 2018). This extinction has been linked to large amounts of volcanism that took place during the emplacement Central Atlantic Magmatic Province (CAMP) and the environmental effects that were associated with it (Ruhl et al., 2011). Including among these effects were global warming and anoxia of the ocean. It is suggested by existing evidence that there was widespread basinal anoxia on the northern margin of Panthalassa on Pangaea and that intense shelf euxinia also became widespread in the latest Triassic-Earliest Jurassic of Western Europe, though some of these conditions developed about 150 kyr (thousand years) following the onset of the ETME (Richoz et al., 2012; Kasprak et al., 2015; Jaraula et al., 2013; Wignall et al., 2007). An increase in the anoxic deposition of through the Triassic-Jurassic boundary is suggested by additional findings from seawater δ²³⁸U in the Lombardy Basin of western Tethys (Jost et al., 2017). Clear evidence for widespread anoxia in the latest Rhaetian in other oceans has been found, however, that coincides directly with the onset of the ETME has not been recorded, with the result that its role as the cause of the marine component on the ETME has remained questionable (Wignall et al., 2010).

Carbonate-associated sulphate (CAS) in bulk marine carbonate and biogenic calcite has been used widely in the reconstruction of primary seawater sulphate S isotope composition during major redox perturbations of the surface of the Earth system (Newton et al., 2011; Owens et al., 2013; Gill, Lyons & Jenkyns, 2011; Luo et al., 2010; He et al., 2019). Variation in the fluxes and isotopic compositions of riverine sulphate sources and marine pyrite burial dynamically control seawater sulphate δ³⁴S. Sulphate removed from the oceans via gypsum precipitation does not impart a isotopic fractionation, though the global sulphate reservoir is made smaller, and therefore more isotopically susceptible to changes in other fluxes (Garrels & Lerman, 1981). A primary redox-a sensitive pathway in the marine sulphur cycle is represented by the production and burial of pyrite, which drives a large offset between the sulphur isotopic composition of seawater sulphate and sedimentary pyrite pools, therefore may control variations in the S isotope composition of oceanic sulphate (δ³⁴S_CAS) through time. It seems that large and rapid global-scale S isotope perturbations, as well as the small ocean sulphate reservoirs that are needed to produce them, are a feature of major deoxygenation events of the Phanerozoic (Newton et al., 2011; Owens et al., 2013; Gill, Lyons & Jenkyns, 2011; Luo et al., 2010; He et al., 2019). Direct records of changes in the marine sulphate pool, and therefore impacts on the global sulphur cycle, have not been documented, though there is some evidence in the sedimentary pyrite isotope record that suggests the regional development of marine anoxia at the ETME (Jaraula et al., 2013; Williford, Foriel, Ward & Luo, 2018). In this paper He et al. report 3 open marine CAS-δ³⁴S_CAS profiles from Sicily [Mount Sparagio section (MS)], Northern Island [Cloghan Point section (CP)], and British Columbia [Black Bear Ridge section (BBR)]. These derive from both Tethyan and Panthalassan locations; the first 2 sections have archived well preserved, shallow water, peritidal, micritic , and shelly limestones and shell materials (Todaro et al., 2017; Simms, 2007); and the last section is comprised of open-shelf, organic-rich, and bivalve-rich, marly limestone (Wignall et al., 2007; Sephton et al., 2002). The Norian to lower Hettangian are spanned by this section and record the major losses of the ETME (Wignall et al., 2007; Todaro et al., 2017; Simms, 2007; Sephton et al., 2002). They provide, therefore, a window into the possible links between the ecosystem response and the marine redox variations in oceans of the Late Triassic across a broad area.

Discussion

Latest Triassic anoxia, enhanced burial of pyrite, and low marine sulphate

The positive swings that have been observed in the S isotopic compositions of seawater sulphate in the latest Triassic could have been driven by an increase in the net burial of sedimentary pyrite under of expanded anoxic/euxinic conditions. Enhanced microbial sulphate reduction (SR) resulting from these conditions leads to an enhanced burial flux of pyrite on the continental shelves and slopes when available iron and organic matter is in sufficient supply. Elevated burial fluxes on a global scale would drive the seawater sulphate δ³⁴S to values that are more positive, because pyrite is depleted in the heavier ³⁴S. The oxidative biotic pathway of the global sulphur cycle may also have the potential to drive seawater Sulphate δ³⁴S enrichment to some extent by microbial sulphide oxidation by some sulphide-oxidising microorganisms (Pellgrin et al., 2019). The contribution of this oxidative metabolic pathway to the sulphur of the oceans has remained unclear, however, and there is no obvious mechanism by which it could have driven a prolonged positive S isotope excursion in the sulphate inventory of the global seawater. It may be possible on a larger scale to drive S isotope variations by altering the rates of weathering of continental pyrite and gypsum; a cession of pyrite weathering and a switch to an isotopically heavy riverine flux might be represented here by a geologically sudden increase in seawater δ³⁴S.

A time-dependant sulphur cycle single-box model was applied, in order to investigate the response of seawater δ³⁴S to variations of the oceanic inventory of sulphate and the degree of change in the net pyrite burial flux. It was assumed by the model that the isotopic composition of pyrite and gypsum weathering fluxes remain constant, and then experiments alter the input and output of pyrite through either weathering or burial. A substantial increase in the model of the burial of pyrite by approximately a factor of 5 and a very small marine reservoir of sulphate (<1 mM) is required to replicate the magnitude and timing δ³⁴S shift. The version of the model that was used in this study fixes the isotopic enrichment of buried pyrite to 30‰ more negative than contemporaneous seawater sulphate, but the expansion of euxinia may have increased this enrichment factor; therefore, they also experiment with a scenario in which this is increased to 40‰ during the event (Owens et al., 2013). There is s very similar requirement of this experiment for a large increase in the burial of pyrite and very low concentrations of seawater sulphate. Note that in the modelling approach of He et al. it is the size, direction and duration of change that are the important foundation. As a similar sized isotope excursion is present in all records, Differences in regional sulphate baselines have no impact on the conclusions obtained from the modelling work. Replicating the change in δ³⁴S by reducing the weathering rate of pyrite, while maintaining the same gypsum weathering flux, is much more difficult and requires a complete cessation of pyrite weathering and extremely low levels of oceanic sulphate (~0.1 mM). Even then, the shape of the excursion is not readily reproducible, as the very low concentrations of sulphate means that the system recovers rapidly from the perturbation.

By using the maximum rate of change in δ³⁴S_CAS the maximum marine concentrations of sulphate can be estimated independently. An upper estimate for marine sulphate is given by the “rate method” (He et al., 2019; Algeo et al., 2015) model as about 0.2 to 1.1 for the interval through the Late Triassic-positive isotope excursion event. The lower end of these estimates of maximum is consistent with the calculations that were inferred from the sulphur cycle box model of He et al. The intervals that predate and those during the positive S isotope excursion event, therefore, appear to be characterised by a scarcity of oceanic sulphate when compared to a higher fluid inclusion based estimate of ≥13 mM during the Carnian. Though was about 20 Myr earlier (Horita, Zimmermann & Holland, 2002). In the later Triassic the development of a low sulphate ocean was likely to have been caused by the deposition of substantial amounts of evaporite. Minimum estimate of global halite deposition suggest there was a 16-fold increase from the Middle to Late Triassic, as is shown in global compilations for this interval (Hay et al., 2006). Contrasting with this, a low level of evaporite occurrence that followed the end-Permian extinction was experienced in the earlier part of the Triassic (Hay et al., 2006). In rift basins that were newly formed, that developed in an arid climate, as the breakup of Pangaea began, evaporites from the Late Triassic (Hay et al., 2006). E.g., when examined on a regional scale, the deposition of evaporite became widespread surrounding the North Atlantic rift (northeastern Grand Banks, Oranian meseta, and Western Europe) during the Late Triassic and subsequently peaked in the Earliest Jurassic (Holser et al., 1988).

The rapid expansion of anoxia during is facilitated by low sulphate

He et al. propose in this paper a conceptual model to link these observations. A major control over the balance between 3 biogeochemical pathways in marine sediments that are microbially mediated:

SR (SO₄^2- +2CH₂O → h₂S +2 HCO₃^-),

Methanogenesis (CH₃OO^- + H⁺ → VH₄ + CO₂ and CO₂ + 4H₂ → CH₄ + 2H₂O), and

the anaerobic oxidation of methane (AOM) (CH₄ + SO₄^2- → HCO₃^- + HS^- + H₂O).

Under conditions of high sulphate such as the modern ocean SR consumes large amounts of organic carbon, while methane is produced in the deeper sediment where sulphate has been depleted. AOM is fuelled by the overlying pore water which is rich in sulphate, and prevents the escape of methane, therefore limiting the consumption of oxygen in the bottom-water. Under conditions of low sulphate availability in contrast, the balance of processes that oxidise organic matter in marine sediments shifts in favour of methanogenesis, as widely occurs in freshwater sediments (e.g., lakes) (Wassmann & Thein, 1996; Jørgensen & Kasten, 2006), where the supply of sulphate is usually limited. The sulphur-methane transition zone is brought closer to the sediment-water interface (SWI) by lower concentrations of sulphate, and the amount of organic matter that is consumed by SR is reduced, and the amount of organic matter that is consumed is ultimately increased the flux of organic carbon to methanogens and limits the capacity for anaerobic oxidation of the remaining methane. The organic matter that has reached the zone of maximum methanogenesis will also have increased reactivity. A greater flux of methane from the sediment is the result of this, which leads to increased aerobic respiration close to the SWI, which increases the burden on the O₂ in the bottom water (Luo et al., 2010; et al., 2018; Hall et al., 2018; Wortmann, 2007).

About 98% of buried organic carbon in the ocean is stored in marginal sediments of the continents in the modern system (Jørgensen & Kasten, 2006). About 20% on average of the global organic carbon flux (~191 Tmol C/year to the sea floor is processed by SR and about 3-4% is converted to methane, which gives an annual methane flux from the seafloor of ~5.7 to 7.6 Tmol methane/year (Jørgensen & Kasten, 2006; Bowles et al., 2014; Egger et al., 2018). The rate of SR will be reduced by a similar amount, if it assumed that there is a drawdown in the concentration of oceanic sulphate by about 97%, which is the modern value, to 1 mM, and that the excess organic matter will all be used by methanogens (i.e., they now process about 22-23% of the organic carbon), then the methane flux would rise to ~42 to 49 Tmol CH₄/year. According to He et al. this calculation is conservative, as any increase in the reactivity of the organic matter as it reaches the methanogenic zone is not taken into account. Also, AOM suppression under these conditions of low sulphate would make it easier to reach the water column and consume free O₂. Further modelling is required in order to make calculations that are more detailed on the expected impact of low sulphate conditions of oxygen demand in the water column, but this is beyond the scope of this study, though it was demonstrated by the calculations of He et al. that there is clear potential for at least 6- or 7-fold increase in elevation of the methane flux at SWI and a concomitant increase in the global consumption of benthic O₂. Note that these elevated demands on bottom water O₂ exist where concentrations of sulphate are low prior to any additional drivers from the release of volcanic O₂.

It is not simple to find evidence for elevated the aerobic oxidation of methane under condition s of low sulphate in the sedimentary record because of the resulting dissolved inorganic carbon (DIC) flux, while it is large when considered in the context of the uptake of dissolved O₂, it is small when compared to the abundance of ocean DIC, especially when oxidation takes place in the water column as proposed. Carbonate cements which are isotopically depleted form from pore water are features of the sedimentary record, and therefore do not provide definitive evidence. It is likely that calcifying organisms that live at the SWI provide the best archive for recording this process, evidence for which has been recognised in high-latitude bivalves from the Late Cretaceous (Hall et al., 2018).

A key feature of the conceptual model of He et al. is that poor conditions are established prior to volcanic perturbation, probably by widespread deposition of evaporite. Previously, the link between the expansion of marine anoxia driven by large igneous province (LIP)-driven warming and extinction events via decreased solubility of O₂ in warmer waters and increased productivity and the demand for O₂ that is driven by increased weathering fluxes of nutrients from land, and the recycling of phosphorus once euxinic water conditions have been established (Hall et al., 2018). The oceans swill be predisposed to the rapid expansion of anoxic conditions via these mechanisms through the higher oxygen demand of the bottom-water of a steady-state Earth system with a small marine sulphate reservoir. Also, it is likely that a low sulphate ocean will impose some additional feedbacks once warming has been limited: Methane production will increase with the temperature of the sediment, because the rate of methanogenesis is highly temperature sensitive (Poulton et al., 2015), a situation that is amplified by the reduced depth go the methanogenic zone under low sulphate conditions. Increased production of marine organic matter will increase the delivery of organic matter and its reactivity to the methanogenic zone in sediments, which again adds to increased methane fluxes across SWI and the consumption of oxygen from methane oxidation. As anoxic conditions expand the burial of pyrite will increase, which would apply downwards pressure on concentrations of marine sulphate, though increased sulphate weathering from land may counter this. Methane release to the atmosphere may also be promoted elevated production of global marine methane, and thereby contribute to warming trends that are released by large-scale CO₂ release, though much of the produced methane is likely to be oxidised in the water column. The explanation of why anoxic conditions were severe under low sulphate conditions may be that these additional feedbacks and why not all LIP-driven warming events lead to widespread depletion of oxygen.

Marine anoxia and mass extinction

Geochemical evidence, in the form of enrichment of elements that are redox-sensitive (Mg and Mo), and of nitrogen isotope fluctuations, suggest that there was a major intensification of the oxygen minimum zone (OMZ) in the Panthalassa Ocean during the Triassic-Jurassic transition (Wignall et al., 2010), though anoxia may not have developed on the floor of the deep ocean at that time (Wignall et al., 2010). There is tangible for this where the OMZ impinged on the western margin of the Pangaean supercontinent, which led to extensive deposition of black shale in Western Canada (Wignall, Zonneveld et al., 2007; Ward et al., 2001). The latest shelf seas of Western Europe, euxinia also became extensive, during as well as at the termination of the mass extinction phase (Richoz & van de Schootbrugge, 2012; Jaraula et al., 2013). A possible measure of ocean redox conditions with negative excursions of δ²³⁸U values, which signified enhanced reduction from U (VI) to U (IV) (Jost et al., 2017), is provided by uranium isotope data from marine carbonates. When seen at the onset of a mass extinction, such a signal suggests a major increase in the area of anoxic deposition that lasted foe ~50 ka (Jost et al., 2017).

It was revealed by the δ³⁴S_CAS excursions in this study is that there is a similar link between the onset of mass extinction and an isotopic excursion that is driven by anoxia. The western Tethys (MS) is where the link is seen most clearly, where there is the sudden loss of magalodont bivalves and the foraminifera Triasina hantkeni at the onset of the positive shift (Todaro et al., 2017). Though there is no direct evidence for anoxia at this peritidal location, some contemporaneous anoxic sedimentary matrices that are found at a neighbouring site that was also connected to the western Tethys (Scopelitti et al., 2009). The extinction level is still recorded, though there is a hiatus in the Panthalassan section (BBR). This occurs in the dysoxic strata of the basal Fernie Formation, the site of the last disappearance of the Rhaetian conodonts, and it is coincident with the δ³⁴S_CAS excursion (Wignall et al., 2017). An earlier crisis at the end of the Norian that occurred several million years earlier than the end Triassic event, is marked by the extinction of the monotid bivalves at BBR (Wignall et al., 2007; Sephton et al., 2002). At CP the end-Triassic extinction can be seen, where several species of bivalve, which includes the Rhaetian marker Rhaetavicula contorta disappear from the fossil record at the base of the Cotham Member. At this level, the lack of limestones precludes the measurement of δ³⁴C_CAS, but the lowest data point that was obtained in this section, a short distance above, displays a strongly positive value. In summary, the major δ³⁴S_CAS that was found here is best explained by a major burial event of pyrite that was driven by a large-scale, increase in anoxia in the late Rhaetian. A-50 ka duration for the initial positive shift is suggested by the age model of He et al. for the MS section, a time span that is remarkable accord with the 50-ka for the main anoxia intensification during the latest Rhaetian based on the contemporary uranium isotope record (Jost et al., 2017). The gradual falling limb of the δ³⁴S_CAS excursion corresponds with the second phase of limited anoxia that extended into the Hettangian (Jost et al., 2017). The intensification of the Panthalassan OMZ and the deposition of black shales on the margin of Pangaea and in the shelf seas of Europe also occurred during this event. The shallowest water locations, such as MS, remained oxygenated. The coincidences of the δ³⁴S_CAS excursion with the extinction losses implicate anoxia as an important factor in the crisis.

Recurring OAEs punctuated the Late Permian and Mesozoic Era accompanied by hyperthermal events and enhanced weathering that coincide with the eruption of LIPs (Ruhl et al., 2011; Lou et al., 2010; Poulton et al., 2015). Large positive shifts of sulphur isotope in seawater sulphate provide that there is evidence of a greatly reduced marine sulphate reservoir and enhanced burial of pyrite for many of these OAEs (Newton et al., 2011; Owens et al., 2013; Gill, Lyons & Jenkyns, 2011; Luo et al., 2010). He et al. explain this generalised coincidence by a mechanism linkage between low dissolved sulphate, enhanced generation of methane in sediment, and consequent elevated consumption of bottom-water O₂. He et al. propose, therefore, that a low sulphate boundary condition prior to volcanically driven greenhouse warming events makes the expansion of anoxic conditions more likely, and that during the events associated feedbacks extend the geographic reach and intensity of anoxia. Many of these events are preceded by increased evaporite burial fluxes, which suggests that this is the mechanism for the removal of sulphate from the ocean (Hay et al., 2006; Wortmann & Chernyavsky, 2007; Mills et al., 2017). The development of widespread anoxia during rapid warming may, however, ultimately trace some of its origins to widespread rifting or other circumstances that form favourable conditions for deposition of evaporite.

He, T., et al. (2020). "An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction." Science Advances 6(37): eabb6704.