Inconsistent mercury records from terrestrial upland to coastal lowland across the Permian-Triassic transition

Abstract

The enrichment of mercury (Hg) in sediments has been increasingly utilized as a proxy for large-scale volcanism. However, a causal link between these two remains ambiguous, as Hg enrichments can be caused by many non-volcanic factors, e.g., changes in local depositional environments or terrestrial runoffs. In this study, we present high-resolution Hg concentration and isotope records of two drill cores, one located in a coastal lowland (HK-1) and the other in a terrestrial upland (TK-1), spanning the Permian–Triassic boundary (PTB). Interestingly, the coastal lowland core exhibits significant Hg enrichments over a stratigraphic horizon of approximately 200 meters (corresponding to a duration of ∼2 Myrs), with dominating negative Δ199Hg values. In contrast, the terrestrial upland core primarily displays background Hg levels with near-zero Δ199Hg values. Given the spatial proximity of the two cores, separated by only about 70 km, we interpret these discrepant Hg signatures to indicate that terrestrial upland underwent minimal influence from terrestrial and/or atmospheric depositions rather than volcanic input, while the coastal lowland was primarily sourced from enhanced terrestrial influxes, depositing under oxygen-deficient conditions. This interpretation is supported by the high concentrations of organic carbon and sulfur, substantial pyrite layers, and positive correlations between Hg and chalcophile elements, such as Mo, Se, and Co, in the HK-1 core. Our study highlights the need to examine multiple Hg enrichment factors in sedimentary records before comprehensively establishing a robust causal link with volcanic activities. A thorough evaluation of multiple lines of evidence, including Hg isotopes, local environmental proxies, and sedimentology investigations, should be conducted first, particularly necessary for the non-marine sediments.