MERCURY ENRICHMENTS COINCIDENT WITH TRILOBITE EXTINCTION AT THE CAMBRIAN SERIES 2 – 3 BOUNDARY; AN IMPRINT OF THE KALKARINDJI LIP? (Invited Presentation)

In the western Great Basin, USA, the Cambrian Series 2 – Series 3 boundary is marked by the extinction of olenellid trilobites and a negative inorganic carbon isotope excursion, the Redlichiid Olenellid Extinction Carbon isotope Excursion (ROECE). These events are proposed to coincide with the emplacement of the Kalkarindji large igneous province (LIP) in Australia. Here we use Hg chemostratigraphy as a proxy for volcanism to test the hypothesis that the olenellid extinction and Kalkarindji emplacement were synchronous. At Oak Springs Summit, eastern Nevada, the olenellid extinction is clearly recorded in the basal C-Shale Member of the Pioche Formation. At Emigrant Pass in eastern California, the olenellid extinction horizon is constrained to lie within a succession of silty marls in the Pyramid Shale Member of the Carrara Formation. We report a series of sedimentary mercury (Hg) enrichments below and within the extinction interval at both localities. In the Pioche Formation, background Hg concentrations are <5 ppb and spikes range between 15-52 ppb. In the Carrara Formation, background levels are again <5 ppb with spikes between 100-270 ppb. Because sedimentary organic matter is considered to be a major sink for Hg²⁺ we also present Hg concentrations normalised to Hg/TOC (total organic matter). Our results show Hg enrichments in rocks with low TOC (typically 0.4 – 0.01 wt % TOC), and we discuss the possibility that Hg is bound to clay minerals rather than organic matter in the marl-dominated successions. In the Pioche Formation at Oak Springs Summit, the extinction horizon is stratigraphically well constrained and correlates directly with a Hg spike. Presently, the most precise dating of the Kalkarindji LIP yields a zircon U-Pb age of 510.7 ± 0.6 Ma, which is indistinguishable within uncertainty from the age of the Cambrian Series 2 – Series 3 boundary and the coincident olenellid extinction. We suggest that the observed Hg spikes in the Pioche and Carrara formations record Kalkarindji volcanism, supporting the temporal coincidence of LIP emplacement and olenellid extinction.