Abstract
Volcanic eruptions are thought to be a key driver of rapid climate perturbations over geological time, such as global cooling, global warming, and changes in ocean chemistry. However, identification of stratospheric volcanic eruptions in the geological record and their causal link to the mass extinction events during the past 540 million years remains challenging. Here we report unexpected, large mass-independent sulphur isotopic compositions of pyrite with Δ33S of up to 0.91‰ in Late Ordovician sedimentary rocks from South China. The magnitude of the Δ33S is similar to that discovered in ice core sulphate originating from stratospheric volcanism. The coincidence between the large Δ33S and the first pulse of the Late Ordovician mass extinction about 445 million years ago suggests that stratospheric volcanic eruptions may have contributed to synergetic environmental deteriorations such as prolonged climatic perturbations and oceanic anoxia, related to the mass extinction.
Highlights
Volcanic eruptions are thought to be a key driver of rapid climate perturbations over geological time, such as global cooling, global warming, and changes in ocean chemistry
We report sulphur isotope data (δ34S, Δ33S and Δ36S) of pyrite from two sedimentary successions from South China, and we use a characteristic signal of large Δ33S anomalies, resulting from stratospheric photochemical reactions[16,17,18,19,20] to constrain the nature of volcanic eruptions during the Late Ordovician
At or above the ozone layer, the sulphur ejected from volcanic emissions will be exposed to UV radiation, acquiring a S-isotope mass-independent fractionation (S-MIF) signal with non-zero Δ33S value[16,17,18,19,20,29,30]
Summary
Origin of the sulphur isotope mass-independent fractionation signal. Almost all physical, chemical and biological processes fractionate S-isotopes by the relative mass differences of each isotope, producing Δ33S values that are near-zero[27]. A plot of δ34S–δ33S for all pyrites that have a non-zero S-MIF at Honghuayuan follows a highly-correlated array: δ33S = 0.5174 × δ34S + 0.7461 (R2 = 0.9975, p < 0.01, n = 13) This array parallels the mass fractionation line (MFL), indicating that the mass-dependent process of microbial sulphate reduction (MSR), which would produce sulphide to form pyrite, started from a sulphate pool with a SMIF composition (Fig. 3). Multiple S-isotope data from thermally mature oil shales and organic carbon-enriched sedimentary rocks with burial temperature higher than 200°C still show low δ34S of pyrite or organic-bound sulphide with mass-dependent Δ33S signals ranging from −0.055‰ to 0.073‰; these have been attributed to retain the sulphur isotopic composition acquired during MSR and/or microbial sulphur disproportionation[39,40]. Our large Δ33S anomalies are comparable to the Δ33S found in ice core sulphate and attributed to stratospheric volcanism, which likely resulted from
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