Improving mercury systematics with molybdenum and vanadium enrichments: New insights from the Cambrian-Ordovician boundary

The Cambro-Ordovician interval marks a significant transition from extinction to bio-diversification in deep time. However, the relationship of bio-transition to volcanism, commonly characterized by mercury (Hg) systematics in sedimentary records, has not been examined. We present the first Cambro-Ordovician Hg systematics from the Scandinavian Alum Shale. Our results show pronounced Furongian Hg enrichments, coupled with positive Δ¹⁹⁹Hg, Δ²⁰⁰Hg, and Δ²⁰¹Hg values and negative Δ²⁰⁴Hg values that we ascribe to atmospheric Hg transport over long-distances, while Early Ordovician Hg anomalies, characterized by near-zero mass-independent isotope values, indicative of submarine source. Our findings are supported by two new proxies: molybdenum-Hg and vanadium-δ²⁰²Hg co-variations, demonstrating Hg systematics were strongly influenced by changes in source and depositional conditions. Constrained by a synchronous atmospheric-tectonic-oceanic model, we hypothesize Furongian subaerial volcanism contributed to global extinction and oceanic anoxia, whereas Early Ordovician submarine volcanism concurrent with ocean water upwelling promoted the nascent bio-diversification.