Globally limited but severe shallow-shelf euxinia during the end-Triassic extinction

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic period (~201.4 million years ago). The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions. However, the global-scale pattern of anoxic conditions across the end-Triassic event is not well constrained. Here we use the sedimentary enrichment and isotopic composition of the redox-sensitive element molybdenum to reconstruct global–local marine redox conditions through the extinction interval. Peak δ98Mo values indicate that the global distribution of sulfidic marine conditions was similar to the modern ocean during the extinction interval. Meanwhile, Tethyan shelf sediments record pulsed, positive δ98Mo excursions indicative of locally oxygen-poor, sulfidic conditions. We suggest that pulses of severe marine de-oxygenation were restricted largely to marginal marine environments during the latest Triassic and played a substantial role in shallow-marine extinction phases at that time. Importantly, these results show that global marine biodiversity, and possibly ecosystem stability, were vulnerable to geographically localized anoxic conditions. Expanding present-day marine anoxia in response to anthropogenic marine nutrient supply and climate forcing may therefore have substantial consequences for global biodiversity and wider ecosystem stability. While global ocean redox patterns during the end Triassic were similar to today, pulses of localized anoxia were probably linked to mass extinctions on continental shelves, according to analysis of molybdenum records.

One of the most severe extinctions of complex marine life in Earth's history occurred at the end of the Triassic period (~201.4 million years ago).The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions.However, the global-scale pattern of anoxic conditions across the end-Triassic event is not well constrained.Here we use the sedimentary enrichment and isotopic composition of the redox-sensitive element molybdenum to reconstruct global-local marine redox conditions through the extinction interval.Peak δ 98 Mo values indicate that the global distribution of sulfidic marine conditions was similar to the modern ocean during the extinction interval.Meanwhile, Tethyan shelf sediments record pulsed, positive δ 98 Mo excursions indicative of locally oxygen-poor, sulfidic conditions.We suggest that pulses of severe marine de-oxygenation were restricted largely to marginal marine environments during the latest Triassic and played a substantial role in shallow-marine extinction phases at that time.Importantly, these results show that global marine biodiversity, and possibly ecosystem stability, were vulnerable to geographically localized anoxic conditions.Expanding present-day marine anoxia in response to anthropogenic marine nutrient supply and climate forcing may therefore have substantial consequences for global biodiversity and wider ecosystem stability.
The Triassic/Jurassic boundary interval (TJB, ~201 million years ago) is marked by one of the largest extinctions of complex marine life in Earth's history: the end-Triassic mass extinction event (ETME) 1 .The ETME has been closely associated with supraregional volcanism from the Central Atlantic Magmatic Province, which has been linked to atmospheric carbon injection as evidenced through Triassic-Jurassic negative carbon isotope excursions 2 .Central Atlantic Magmatic Province activity is also thought to have caused Triassic-Jurassic marine acidification and marine de-oxygenation 2,3 .Studies of Triassic-Jurassic marine biomarkers and δ 34 S (where the delta notation (δ) is defined as the difference in isotopic ratio between a sample and a reference standard) indicate that locally sulfidic conditions were prominent within marginal marine surface waters of the Tethys and Panthalassa oceans around the TJB [3][4][5][6][7][8] .Oxygen-poor conditions have been further identified from blooms of prasinophycean algae 3,9 , a negative excursion in carbonate uranium isotopes 10,11 , widespread Early Jurassic black-shale deposition 9,12 , elemental redox proxies 3,13 and iron speciation data 14 .Therefore, recent studies suggest that marine redox change may have played an important role in end-Triassic marine extinction phases.However, most existing Late Triassic redox studies are at a local, basinal or sub-regional scale and do not provide information about the global-scale distribution of marine redox conditions.

Article
https://doi.org/10.1038/s41561-023-01303-2from the Carnduff-2 core (Northern Ireland), Hebelermeer-2 core (west Germany) and Schandelah-1 core (north Germany), which preserve lithological records of marine marls, sandstones and organic-rich shales deposited on the Tethyan shelf before, during and after the ETME (Fig. 1).All three sites have undergone previous stratigraphic study and include detailed biotic records for correlation 6,[23][24][25] .The samples analysed in this study are therefore ideally suited to explore the relationship between marine extinction and de-oxygenation on the Tethyan shelf across the TJB.We identify little correlation between proxies for detrital sediment input and δ 98 Mo or Mo EF and therefore interpret stratigraphic variations in Mo concentrations mainly as a function of local redox conditions (Supplementary Fig. 4).

Limited euxinia during Triassic-Jurassic transition
The δ 98 Mo of coeval seawater (δ 98 Mo SW ) must be resolved to determine global redox conditions.However, to determine δ 98 Mo SW , sedimentary δ 98 Mo values must be corrected for isotopic fractionation, with the isotopic fractionation and sedimentary enrichment of Mo being dependent on local redox conditions 21 .The maximum δ 98 Mo compositions of Upper Triassic mudstones in the studied cores are ~1.6‰(Carnduff-2: 1.56‰; Hebelermeer-2: 1.63‰; Fig. 2).These δ 98 Mo values are obtained from sampling levels where trace-metal distributions are indicative of The unconstrained areal extent of globally sulfidic conditions during the ETME limits understanding of the spatial complexity of redox change as a driver of Late Triassic marine extinctions.
The isotopic composition of molybdenum (Mo) in organic-rich sediments has been widely used to reconstruct the oxygenation of both local and global ancient marine environments 15,16 .Under oxic depositional conditions, Mo typically exhibits low sedimentary enrichments and isotopically light compositions, due principally to (Mn)oxyhydroxide adsorption, with compositions ~3‰ lighter than coeval seawater [16][17][18] .Under reducing conditions, Mo exhibits higher sedimentary enrichments due to the formation of thiolated (poly)molybdate species that have smaller isotopic offsets from seawater than Mo adsorbed onto oxyhydroxides 19,20 .Generally well-oxygenated global marine conditions are therefore reflected by isotopically heavy δ 98 Mo seawater values due to oxyhydroxide adsorption acting as the primary vector of Mo burial 21 .However, isotopically light Mo burial can also occur due to intermediate thiomolybdate formation 22 .High sulfidic global marine conditions, by contrast, are represented by isotopically light δ 98 Mo seawater values as global oxyhydroxide burial declines 21 , but no observations of δ 98 Mo evolution exist from across the TJB and ETME.
In this Article, we use the sedimentary enrichment (Mo EF ) and isotopic composition of Mo to examine the link between marine de-oxygenation and extinction during the ETME.We obtained material suboxic depositional conditions (Supplementary Figs.1-3) 13 .Suboxic conditions coinciding with upper-bound Carnduff-2 δ 98 Mo values are further supported by oxic-ferruginous iron speciation values from a correlative horizon within the Larne Basin 14 .Molybdenum sulfides forming in sedimentary porewaters underlying a non-euxinic water column are fractionated by a minimum of ~0.7‰ relative to coeval seawater 26,27 , with fractionation probably exceeding 0.7‰ within the Larne Basin because of local redox conditions 13,14 .Burial of Mo under ferruginous conditions would lead to sediment compositions with similar isotopic offsets from δ 98 Mo SW and so would not alter this conclusion 28 .Therefore, Late Triassic δ 98 Mo SW was probably >2.3‰, similar to or greater than modern-day seawater 29 .A Late Triassic δ 98 Mo SW equal to or greater than the modern ocean is consistent with sulfidic conditions covering no more than ~0.05-0.10% of the Late Triassic seafloor, similar to or even less than in the modern day 16,29 .The average upper-bound δ 98 Mo throughout the basal Jurassic of the Carnduff-2 core is 1.47 ± 0.58‰ (n = 3) and characterizes horizons with trace-metal distributions that are indicative of localized suboxia 13 .Predominantly non-euxinic conditions from the basal Jurassic of the Larne Basin are further supported by iron speciation data 14 .Therefore, basal Jurassic δ 98 Mo SW was probably >2.2‰, also similar to the Late Triassic and modern global ocean.The persistence of a similar δ 98 Mo SW from the Late Triassic to the basal Jurassic suggests that, unlike uranium isotope records 10,11 , there was no substantial change in the global Mo cycle across the TJB and that sulfidic anoxia in the Early Jurassic global open ocean remained spatially limited.Our conclusion is further supported by pyrite framboid data from an open ocean Panthalassa site 30 .

Regional de-oxygenation during the ETME
Oxygen-poor conditions were present across Tethyan and Panthalassa marginal marine environments during the main extinction interval 5,7,13,14 .Such conditions have been further identified here on the basis of δ 98 Mo and Mo EF data with redox conditions varying according to both site and stratigraphy.The Schandelah-1 and Carnduff-2 cores both exhibit positive δ 98 Mo shifts that directly coincide with the main extinction interval, as denoted by the 'initial' negative carbon isotope excursion (CIE) 2,31,32 (Fig. 2).The increase in δ 98 Mo (and Mo EF ) during the main extinction interval indicates an increased availability of reduced sulfur [HS − ], probably due to the shoaling of the sulfate reduction zone within sedimentary porewaters up to the sediment-water interface 26,27,33 .Relatively high Mo EF alongside low δ 98 Mo values around the base of the main extinction interval within the Hebelermeer-2 core also indicates increased [HS − ] prevalence, probably due to the expansion of the sulfate reduction zone into the water column, with the burial of intermediate thiomolybdate species at H 2 S concentrations <11 µM (refs.20,22) (Supplementary Information section 1.4).Isotopically light values are inconsistent with oxide adsorption given the elevated total sulfur (%) at this horizon 6 .Sulfidic conditions around the base of the initial CIE at St Audrie's Bay (Somerset, UK) have been interpreted through a positive δ 34 S excursion 4 .
Low sulfate within Late Triassic seawater of the Tethyan shelf has been interpreted through δ 34 S data 7 .Low marine sulfate concentrations 7 are unlikely to have affected broad trends in δ 98 Mo observed here given the inter-site consistency in data and the argument that calculated sulfate concentrations in Late Triassic oceans (0.2-1.1 mM) were still in excess of those required for thiomolybdate formation up to and exceeding ~11 µM (refs.7,16) (Supplementary Information section 1.5).

Pulsed marginal marine euxinia during ETME
Multiple pulses of marine sulfidic anoxia were prevalent on the Tethyan shelf during the latest Triassic and earliest Jurassic.Low to moderate Mo EF as well as isotopically heavy δ 98 Mo in all three cores during deposition of the basal Westbury Formation and stratigraphically equivalent units are suggestive of shoaling of the sulfate reduction zone towards  the sediment-water interface 26,27,33 .Similar shoaling of the sulfate reduction zone is also inferred from the Carnduff-2 and Hebelermeer-2 cores during the deposition of the uppermost Westbury Formation and equivalent units.Both phases of porewater de-oxygenation coincided with photic-zone euxinia on the Tethyan shelf during the middle Rhaetian 6 (Fig. 3); the upper pulse also coincided with episodic photic-zone euxinia within the Bristol Channel Basin 8 and de-nitrification within the Central European Basin at the Mingolsheim and Mariental-1 sites 3,34 (Fig. 3).δ 98 Mo then decreases during the deposition of the lower Cotham Member and stratigraphically equivalent units, as seen within both the Carnduff-2 and Schandelah-1 cores, with Mo EF decreasing or remaining low.Despite previous studies interpreting broadly oxygenated depositional conditions during the basal Cotham Member, isotopically light δ 98 Mo within the Larne Basin may be associated with intermediate thiomolybdates and organic matter preservation, suggesting weakly oxygen-poor conditions 22,35 .
Multiple further positive shifts in δ 98 Mo and Mo EF are observed within the basal Jurassic of the Carnduff-2 core, suggestive of pulsed increases of [HS − ] caused through periodic shoaling of the sulfate reduction zone.Correlative horizons within the Schandelah-1 core contain δ 98 Mo and Mo EF shifts also indicative of variable shoaling of the sulfate reduction zone.Pulsed oxygen-poor conditions within the basal Jurassic of the Carnduff-2 and Schandelah-1 cores coincided with photic-zone euxinia within the Bristol Channel Basin 4,8 (Fig. 3).
Periodically pulsed anoxia during the Late Triassic and Early Jurassic has previously been noted on the basis of eccentricity modulated precession timescales in the Bristol Channel Basin (St Audrie's Bay), with laminated organic-rich black shales forming every precession cycle 36 .Pulses of marine redox change have also been reported from the Larne Basin, coinciding largely with the disappearance of infaunal bivalve 13 .We similarly note a close relationship between redox pulses and Late Triassic extinction phases (Fig. 3).

Localized marine euxinia as driver of extinction during ETME
The coincidental pulsed nature of sulfidic marine de-oxygenation and end-Triassic marine extinction phases strongly suggests a causal relationship (Figs. 3 and 4).However, the limited extent of marine sulfidic anoxia through the TJB inferred from our new Mo isotope datasets suggests that these pulses of marine de-oxygenation were limited largely to marginal marine environments.More persistent and widespread oxygen-poor conditions towards the end of the ETME have been suggested from carbonate δ 238 U data that have been modelled to suggest uranium reduction across ~8-20% of the global seafloor 10,11 .However, such anoxic conditions probably manifested within marginal marine and intermediate-depth Tethyan and Panthalassa environments [3][4][5]7 , with little evidence for open marine de-oxygenation 30 , and provide limited information on the extent of sulfidic conditions. Or new Mo isotope data add to these observations by showing that even if moderate de-oxygenation (enough to perturb the uranium cycle) took place near the ETME, there was little increase in the volume of waters containing dissolved sulfide.We suggest that geographically localized sulfidic marine de-oxygenation within Late Triassic marginal marine environments  Our inference of geographically localized marine de-oxygenation is consistent with recent studies indicating elevated weathering and erosion rates in Tethyan shelf localities during the Late Triassic 37,38 . Loalized marine de-oxygenation may have been driven by high run-off from the Late Triassic continents triggering eutrophication and stratification, with increased run-off driven by a warming climate, enhanced hydrological cycle and the collapse of forest ecosystems leading to enhanced soil erosion 39 .Marine de-oxygenation within Tethyan and Panthalassa marginal marine environments may also have been associated with the expansion of oxygen minimum zones 5,7 .Open marine environments would have been substantially less affected by weathering perturbations, with oxygen minimum zone expansion potentially being focused around intermediate-depth, marginal marine environments, and consequently the open oceans may have remained refugia for marine life.
Modern marginal marine environments are probably also particularly sensitive to changes in marine redox while also being some of the most biodiverse oceanic environments on Earth [40][41][42][43] .Therefore, anthropogenically driven environmental change, including the local expansion of marine anoxia and enhanced marine nutrient supply, may result in geographically localized marine de-oxygenation, which could have major consequences for future marine biodiversity and ecosystem stability 44 , with particularly severe consequences for marginal marine corals, mangroves and coastal fishes 1,42,43 .

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Fig. 4 |
Fig. 4 | Pulses of marine de-oxygenation and synchronous extinction during the latest Triassic and earliest Jurassic on the Tethyan shelf.a,b, Pre-initial CIE during the basal (a) and upper (b) Westbury Formation/Contorta beds.c, Initial CIE.d, Post-initial CIE.For data source(s), refer to Fig. 3. Palaeo-latitude as in Fig. 1. Figure adapted with permission from: palaeogeography, ref. 12, Wiley and ref. 23, Elsevier; extinction pictograms, ref. 32, Elsevier .

Overview of key stratigraphic, lithological and geochemical data for
Green dashed line represents minimum δ 98 Mo SW.Mo EF calculations detailed in Methods.