Extensive anoxia after the end-Triassic mass extinction: uranium isotope evidence from the Triassic-Jurassic boundary section at Csővár

The end-Triassic mass extinction (ETE) ranks as one of the ’Big Five’ biotic crises in Earth history. The processes that led to the ecosystem collapse are thought to have been triggered by the volcanism of the Central Atlantic Magmatic Province (CAMP). However, there is an ongoing debate about which environmental effect was the main trigger for the extinction. Our research aimed to produce a new uranium isotope dataset from the Triassic-Jurassic boundary section of Csővár and to carry out Earth system modelling to understand the role of anoxia in driving the extinction and/or delaying the subsequent biotic recovery.

The uranium isotope ratio (δ²³⁸U) is a novel paleoredox proxy as its application dates back only a few years. The main advantage of the method is that δ²³⁸U measured in limestone is a global proxy, i.e. it provides information on the redox conditions of the whole ocean rather than that of the local basin. It can be used to reconstruct the proportion of the global seafloor that was under anoxic conditions during the deposition of the studied sediment. Our δ²³⁸U measurements were performed on the NEPTUNE Plus™ MC-ICP-MS instrument at the Institute for Nuclear Research (ATOMKI) in Debrecen. The obtained data represent only the second δ²³⁸U dataset from the Triassic-Jurassic boundary worldwide.

The studied Csővár section is suitable for uranium isotopic analyses as the deposition took place in an oxic environment and was continuous across the boundary interval, as proven by biostratigraphy of multiple fossil groups and cyclostratigraphy. The section is of international importance as it was among the first sections in the world where the TJB event was recognized in the carbon isotope record.

We detected a major negative uranium isotope anomaly immediately below the Triassic-Jurassic boundary, which is a global signal and indicates widespread marine anoxia. This anomaly coincides with the previously detected carbon isotope anomaly and Hg peaks, which are associated with the volcanism of the CAMP and mark the extinction horizon. Our results support the hypothesis that volcanism indirectly induced anoxia in the ocean, which may have played a role in triggering the marine ETE.

Using the geochemical data (δ¹³C, Hg, δ²³⁸U) and the astrochronological age constraints of the section, we modelled the coupled behaviour of carbon, phosphorus and uranium cycles after volcanic carbon emissions. The model allowed us to estimate when the anoxic conditions were the most severe in the ocean. Our results suggest that anoxia did not reach its maximum extent during the extinction but only about 200-250 kyr later, when approximately 13% of the global ocean floor may have been depleted in oxygen. This delayed peak of anoxia is probably the result of the later, extrusive phase of the CAMP marked by the prominent Hg peak of the section. Our geochemical and modelling results suggest that marine anoxia played a key role in hindering the biotic recovery after the end-Triassic extinction.

 

This research was supported by the National Research, Development and Innovation Fund (Project No. K135309).