The Triassic–Jurassic boundary interval (TJB, ~ 201 Ma) 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 provincial volcanism from the Central Atlantic Magmatic Province (CAMP) which has been linked to atmospheric carbon injection as evidenced through Triassic–Jurassic negative carbon isotope excursions 2. CAMP activity is also thought to have caused Triassic–Jurassic marine acidification and marine de-oxygenation 2, 3. Studies of latest Triassic marine de-oxygenation indicate that locally sulfidic conditions were prominent within marginal marine surface waters of the Tethys and Panthalassic oceans around the TJB 3–8. Oxygen-poor conditions have been further identified from blooms of prasinophycean algae 3, 9, a negative excursion in carbonate U isotopes 10, widespread Early Jurassic black shale deposition 9, 11, elemental redox proxies 3, 12, and Fe speciation data 13. 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 scale or sub-regional scale, and do not provide information about the global-scale distribution of marine redox conditions. The unconstrained areal extent of globally sulfidic conditions during the ETME limits understanding of the role of spatial redox change in the 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 14, 15. Under oxic depositional conditions Mo typically exhibits low sedimentary enrichments and isotopically light compositions, due to (Mn)oxyhydroxide adsorption, with compositions ~ 3‰ lighter than coeval seawater 15–17. 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 adsorped onto oxyhydroxides 18, 19. Generally well-oxygenated global marine conditions are therefore reflected by isotopically heavy δ98Mo seawater values due to oxyhydroxide adsorption acting as the primary vector of Mo burial 20. Whereas poorly oxygenated global marine conditions are represented by isotopically light δ98Mo seawater values as global oxyhydroxide burial declines 20. However, studies of δ98Mo from across the TJB and ETME remain undocumented.
Here we use the sedimentary enrichment (MoEF) and isotopic composition of Mo to examine the link between marine de-oxygenation and extinction during the ETME. We obtained material from the Carnduff-2 core (Northern Ireland), Hebelermeere-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, 21–23. We identify little correlation between proxies for detrital sediment input and δ98Mo or MoEF and therefore interpret stratigraphic variations in Mo as a function of local redox conditions (see supplementary information). The samples analysed in this study are therefore ideally suited to explore the relationship between marine extinction and de-oxygenation on the Tethyan shelf during the TJB.
Oxygenated global oceanic conditions prevailed during the Triassic–Jurassic transition
The δ98Mo of coeval seawater (δ98MoSW) must be resolved to determine global redox conditions. However, in order to determine δ98MoSW, sedimentary δ98Mo values must be corrected for isotopic fractionation, with the isotopic fractionation and sedimentary enrichment of Mo being dependent on local redox conditions 20. The maximum δ98Mo compositions of Upper Triassic mudstones in the studied cores are ~ 1.6‰ (Carnduff-2: 1.56‰; Hebelermeere-2: 1.63‰; Fig. 2). These δ98Mo values are obtained from sampling levels where trace metal distributions are indicative of non-euxinic depositional conditions (see supplementary information) 12. Non-euxinic conditions coinciding with upper-bound Carnduff-2 δ98Mo values are further supported by oxic–anoxic iron speciation values from a correlative horizon within the Larne Basin 13. Molybdenum sulfides forming in sedimentary porewaters underlying a non-euxinic water column are fractionated by a minimum of ~ 0.7‰ relative to coeval seawater 24, 25, with fractionation likely exceeding 0.7‰ within the Larne Basin on account of local redox conditions 12, 13. Therefore, Late Triassic δ98MoSW was likely > 2.3‰, similar to modern day seawater 26. A Late Triassic δ98MoSW equal to or greater than the modern ocean is consistent with sulfidic conditions covering no more than ~ 0.05–0.1% of the Late Triassic seafloor, similar or even less than in the modern-day 15.
The average upper bound δ98Mo throughout the basal Jurassic of the Carnduff-2 core is 1.47 ± 0.58‰ (n = 3) and characterises horizons with TM distributions that are not indicative of localised euxinia 12. Predominantly non-euxinic conditions from the basal Jurassic of the Larne Basin are further supported by iron speciation data 13. Therefore, basal Jurassic δ98MoSW was likely > 2.2%, also similar to the Late Triassic and modern global ocean. The persistence of a similar δ98MoSW from the Late Triassic through to the basal Jurassic indicates that there was no significant long-term change in global marine redox conditions across the Triassic–Jurassic boundary interval, and that the basal Jurassic global open ocean remained generally well oxygenated. Our conclusion of a globally oxygenated open ocean during the basal Jurassic is further supported by pyrite framboid data from an open ocean Panthalassa site 27.
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, 12, 13. Such conditions have been further identified here on the basis of δ98Mo and MoEF data with redox conditions varying according to both site and stratigraphy. The Schandelah-1 and Carnduff-2 cores both exhibit positive δ98Mo shifts that directly coincide with the main extinction interval, as denoted by the “initial” negative carbon isotope excursion (CIE) 2, 28, 29 (Fig. 2). The increase in δ98Mo (and MoEF) during the main extinction interval indicates an increased availability of reduced sulfur [HS-], likely due to the shoaling of the sulfate reduction zone within sedimentary porewaters up to the sediment-water interface 24, 25, 30. Relatively high MoEF alongside low δ98Mo around the base of the main extinction interval within the Hebelermeere-2 core also indicates increased [HS−], likely due to the expansion of the sulfate reduction zone into the water column, with the burial of intermediate thiomolybdate species at H2S concentrations < 11µM 19, 31. Isotopically light values are inconsistent with oxide adsorption given the elevated TS (%) at this horizon 6. Weakly sulfidic conditions around the base of the initial CIE at Cloghan Point (Northern Ireland) and St. Audrie’s Bay (Somerset, UK) are also suggested through positive δ34S excursions 4, 7, and low sulfate within Late Triassic seawater of the Tethyan shelf has been interpreted through δ34S data 7.
Pulsed de-oxygenation during the end-Triassic and earliest Jurassic
Multiple pulses of marine deoxygenation were prevalent on the Tethyan shelf during the latest Triassic and earliest Jurassic. Low to moderate MoEF as well as isotopically heavy δ98Mo 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 24, 25, 30. Similar shoaling of the sulfate reduction zone is also observed within the Carnduff-2 and Hebelermeere-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, 32 (Fig. 3). δ98Mo 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 MoEF decreasing or remaining low. Isotopically light δ98Mo and low MoEF are suggestive of a regional shift to more oxygenated conditions with high Mo fractionation likely being associated with Fe-Mn oxide formation and organic uptake 16, 33.
Multiple further positive shifts in δ98Mo and MoEF 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 δ98Mo and MoEF shifts indicative of shoaling of the sulfate reduction zone and low-sulfur, oxygen poor (ferruginous) conditions. The very low enrichment of elements sensitive to reduction (U) and variable enrichment of elements sensitive to HS− availability (Mo, Cu, Zn) from this horizon within the Schandelah-1 core further supports ferruginous conditions (see supplementary information), as does overall low sulfate on the Triassic–Jurassic Tethyan shelf interpreted from δ34S data 7. 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 based on eccentricity modulated precession timescales in the Bristol Channel Basin (St. Audrie’s Bay) with laminated organic-rich black shales forming every precession cycle 34. Pulses of marine redox change have also been reported from the Larne Basin, largely coinciding with the disappearance of infaunal bivalve taxa 12. We similarly note a close relationship between redox pulses and Late Triassic extinction phases (Fig. 3).
Localised marine de-oxygenation as a driver of extinction during the ETME
The coincidental pulsed nature of marine de-oxygenation and end-Triassic marine extinction phases strongly suggests a causal relationship (Fig. 3, Fig. 4). However, the persistence of broadly oxygenated global marine conditions through the Triassic–Jurassic boundary interval inferred from our new Mo isotope datasets suggests that these pulses of marine de-oxygenation were largely limited to marginal marine environments. Geographically localised anoxic conditions on the Tethyan shelf are further supported by carbonate U isotope data 10. We suggest that geographically localised marine de-oxygenation, within Late Triassic marginal marine environments, therefore had major implications for Late Triassic biodiversity and ecosystem stability.
Our inference of geographically localised marine de-oxygenation is consistent with recent studies indicating elevated weathering and erosion rates in Tethyan Shelf localities during the Late Triassic 35, 36. Localised marine de-oxygenation may have been driven by high run-off from the Late Triassic continents triggering eutrophication and stratification, with increased run-off being driven by a warming climate and the collapse of forest ecosystems 37. Open marine environments will have been significantly less affected by such perturbations and consequently the open oceans may have remained a refugia for marine life.
Modern, marginal marine environments are likely also particularly sensitive to changes in marine redox whilst also being some of the most biodiverse oceanic environments on Earth 38–41. Therefore, anthropogenically driven environmental change, including the expansion of marine anoxia and enhanced marine nutrient supply, may result in geographically localised, marine de-oxygenation, which could have major consequences for future marine biodiversity and ecosystem stability 42, with particularly severe consequences for marginal marine corals, mangroves, and coastal fishes 1, 40, 41.