Seawater sources of Hg enrichment in Ordovician-Silurian boundary strata, South China

Abstract

The Late Ordovician mass extinction (LOME), the second-largest biodiversity crisis of the Phanerozoic, occurred in two discrete pulses, the causation of both of which has been linked to environmental stresses induced by the onset and termination of the Hirnantian Glaciation. Recent studies have inferred significant mercury (Hg) anomalies, mainly from the South China region, during the Ordovician-Silurian transition (OST) that links the LOME to massive volcanism–a hypothetical late Ordovician large igneous province (LIP). However, volcanism in South China during the OST was a dominantly regional phenomenon, even though coeval eruptions in Laurentia and Baltica were major events controlled by Caledonian tectonism. The potential linkage between regional arc or LIP volcanism and the LOME needs further investigation. Here, we examine Hg concentrations, Hg isotopes, and redox-sensitive elements in two deep-water successions (the YY2 and JY11 drillcores) of South China straddling the OST. Our analysis revealed no volcanic Hg anomalies and showed significant discordance between bentonite-bed distribution and the overall pattern of the OST redox evolution in South China. Also, Hg isotope data show relatively constant mass-independent fractionation (Δ199Hg = +0.13‰ ± 0.03‰), which implicates seawater sources of Hg enrichment during the OST. Hence, we propose that the LOME was caused not by massive volcanism, but rather by the expansion of anoxia triggered by long-term cooling and weathering P-induced marine eutrophication.