Abstract
The end-Triassic extinction is one of the major Phanerozoic mass extinctions and it appears to have been linked to coeval rapid and severe environmental change, thought to be triggered by volcanism in the Central Atlantic Magmatic Province (CAMP). However, direct stratigraphic evidence to substantiate this linkage and to help develop scenarios for the cascade of events is still scarce. Mercury is an increasingly widely used proxy to trace the volcanic activity associated with large igneous provinces (LIPs) in distal sedimentary sections, but so far Hg records are available from only a handful of Triassic–Jurassic boundary (TJB) sections. One of the few well-studied marine sedimentary sections with a continuous sedimentary record across the TJB is located at Csővár (Hungary) and it exposes an extended succession of carbonates deposited in an intraplatform basin on the western Tethyan shelf. Previously, this section yielded one of the first convincing records of carbon isotope excursions (CIEs) across the TJB, albeit from low-resolution sampling. Here we report a new, high-resolution δ13Ccarb curve, supplemented with Hg measurements. A series of successive negative carbon isotope excursions (termed NCIE-1 to 6) attests to carbon cycle perturbations in the TJB interval. Four excursions appear significant after statistical smoothing. Of these, NCIE-3 exhibits the highest amplitude and is biostratigraphically constrained to the topmost Triassic, hence reliably correlated with the initial CIE, a globally recognised excursion closely preceding the TJB, and coincident with the end-Triassic extinction (ETE) horizon. The Hg concentration data provide the longest record available to date from a single section across the TJB. It shows very low values below NCIE-3 that are interpreted as the pre-volcanic background, followed by a prominent Hg peak that is nearly coincident with the most significant carbon isotope spike (NCIE-3). The slight lag suggests that onset of a major extrusive phase of CAMP (marked by a significant rise in Hg) closely followed the very onset of carbon cycle perturbation at that time (expressed by an abrupt change in the δ13Ccarb signal), possibly from biogenic methane release. Subsequent and recurring smaller Hg psuggest a pulsatory nature of prolonged volcanic activity. Organic content in the section is consistently low and sedimentary Hg concentrations are therefore normalized against Fe content, a reliable proxy in the lack of significant lithological changes. The maximum sedimentary Hg concentration at Csővár is greater than that in any other TJB section, although not unprecedented if other events are considered. Three hypotheses are explored to explain the high values; i) the hit-or-miss model could suggest that deposition of the sampled beds was fortuitously coincident with major eruptions, ii) the presence and preservation of cryptotephra could account for the unusually high sedimentary Hg enrichment, and iii) changes in the proportion of Hg-carrier phases throughout the studied succession, e.g. from magnetite to pyrite dominance, could have enhanced the potential of Hg capture and deposition. Collectively, the new data provide direct stratigraphic and geochemical evidence for the link between CAMP volcanism and carbon cycle perturbations and strengthen the case of their causal relationship with the end-Triassic extinction.
Highlights
Large igneous provinces (LIPs) were commonly synchronous with, and are suspected to be the main trigger of, cascades of environmental changes and concomitant mass extinctions that fundamentally influ enced the Phanerozoic history of the Earth system (Wignall, 2005; Bond et al, 2014)
The subsequent reali zation that complex and intertwined environmental and biotic changes are best reflected in perturbations of the carbon cycle, was evidenced by discoveries of major negative carbon isotope excursions (NCIEs) across the Triassic–Jurassic boundary (TJB) interval (Pálfy et al, 2001; Ward et al, 2001; Hesselbo et al, 2002)
How ever, controversies exist, and alternative interpretations have been proposed as causes for the CIEs, especially concerning the immediate cause for the CIE correlated with the end-Triassic mass extinction (ETE)-horizon, on whether sub surface intrusion-related gas release, explosive-magmatic gas release, or enhanced biogenic methane gas release was more potent (Beerling and Berner, 2002; Ruhl et al, 2010a, 2010b; Ruhl and Kürschner, 2011; Davies et al, 2017; Capriolo et al, 2020)
Summary
Large igneous provinces (LIPs) were commonly synchronous with, and are suspected to be the main trigger of, cascades of environmental changes and concomitant mass extinctions that fundamentally influ enced the Phanerozoic history of the Earth system (Wignall, 2005; Bond et al, 2014). There is a significant NCIE, observed in marine and terrestrial sedimentary archives, across both hemispheres, which suggests a major perturbation of the carbon cycle This anomaly is widely used as a global correlation tool, commonly referred to as the initial CIE (e.g. Pálfy et al, 2001; Ward et al, 2001; Hesselbo et al, 2002). An additional nomenclatural issue concerns the use of Spelae Zone, which is no longer an accepted zone and is not a biostratigraphic unit, the younger two CIEs both belong to the Tilmanni Zone Due to these controversies, we use the more widely established correlation scheme as advocated by Korte et al (2019)
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