Mercury as a proxy for volcanic emissions in the geologic record

Large igneous province (LIP) eruptions are increasingly considered to have driven mass extinction events throughout the Phanerozoic; however, uncertainties in radiometric age dating of LIP materials, along with difficulty in accurate age dating of sedimentary rocks that record the environmental and biological history of our planet, create inherent uncertainties in any linkage. As such, there is interest in using geochemical proxies to fingerprint periods of major volcanism in the sedimentary record (termed here LIP marks). The use of sedimentary mercury (Hg) contents has been suggested to be the best tool to accomplish this goal, and recent work is reviewed here. Studies to-date show that most extinction events, ocean anoxic events, and other environmental crises through the Phanerozoic have an associated sedimentary Hg anomaly. It remains unclear though if each Hg anomaly is truly a signature of massive volcanism, or if it is controlled by local or regional processes. As Hg has a strong affinity to organic matter (OM), normalisation with total organic carbon (TOC) has been used to assess anomalies. The measurement of TOC has been fraught with error throughout many studies, leaving some claimed Hg/TOC anomalies questionable. Normalisation by other elements that can affect Hg sequestration, such as Al and S, are less common but warrant further investigation. Stable isotope systematics of Hg have helped to further clarify the origin of Hg spikes, and clearly show that not all Hg anomalies are directly related to volcanism. Although a promising tool, the Hg proxy requires more refinement to accurately understand the nuances of an Hg anomaly in the rock record.