IS MERCURY FROM “BACKGROUND” VOLCANISM PRESERVED IN THE SEDIMENTARY RECORD? A TEST USING FLUVIO-LACUSTRINE STRATA FROM THE UPPER TRIASSIC OF ARGENTINA

High concentrations of mercury (Hg) in the sedimentary record are an emerging proxy for large igneous province (LIP) volcanism, based on the observation that extant volcanism is a major source of Hg to the modern natural environment. Previously, most geologic studies of Hg have focused only on very large eruptions (such as LIPs), which are assumed to produce greater Hg flux that is more likely to be capable of affecting the global Hg inventory, and therefore be widely preserved in the geologic record. However, these assumptions have not been empirically tested in deep time, so there is little understanding of how smaller-scale “background” eruptions might affect the sedimentary Hg record, particularly at local to regional scales. To examine the effect of local volcanism on Hg concentration in sediments, we studied Upper Triassic fluvio-lacustrine strata in the Ischigualasto-Villa Unión rift basin of northwestern Argentina. Here, the lacustrine Los Rastros Formation (Carnian) and overlying fluvial Ischigualasto Formation (upper Carnian-lower Norian) preserve multiple tuff horizons (<1 cm to >1 m thick) derived from local volcanic eruptions. From both formations, we analyzed samples of the tuffs as well as underlying and overlying siliciclastic sediments (claystone, siltstone, and sandstone) for Hg concentration, total organic carbon content (TOC), and organic carbon stable isotope (δ¹³C_org) values. These data reveal multiple spikes in Hg concentration (normalized against TOC) and dynamic changes in the sources of organic carbon. Consequently, these investigations provide important new insights into whether LIP eruptions are the only volcanic events that can perturb the Hg cycle (as largely assumed), or whether smaller-scale “background” volcanism must also be considered when interpreting Hg anomalies in the non-marine sedimentary record.