Terrestrial rather than volcanic mercury inputs to the Yangtze Platform (South China) during the Ordovician-Silurian transition

Abstract

The Hg proxy has been used to infer a close relationship between volcanism and the ‘Big Five’ Phanerozoic mass extinctions. However, because of conflicting interpretations of Hg data, this relationship remains in debate for the Late Ordovician mass extinction (LOME). In this study, three sections (Datianba, Shuanghe and YY1), located in shallow to deep shelf settings, were chosen for investigation of the Hg cycle on the Yangtze Platform during the Ordovician-Silurian transition (OST). Our results show that sulfur-normalized Hg concentrations (Hg/TS, TS: total sulfur) are the best proxy for assessing Hg anomalies in these sections due to Hg being hosted mainly by sulfide minerals. The Hg/TS profiles of the three study sections exhibit no enhanced Hg loading of a potential volcanic origin. Differing Al-Hg relationships among the study sections imply different Hg sources, with dominantly terrigenous Hg at Datianba and hydrogenous (seawater-derived) Hg at Shuanghe and YY1. This inference is supported by contrasting near-zero to weakly negative Δ199Hg values at Datianba versus positive Δ199Hg values at Shuanghe. These findings demonstrate the influence of terrigenously sourced Hg on Upper Ordovician sediments of the Yangtze Platform, and, in conjunction with the non-correlatability of Hg anomalies between the study sections and their lack of a stratigraphic relationship to bentonites, they do not support a relationship between the LOME and large igneous province (LIP) or arc volcanism in the South China region during the OST.