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 Delta 199Hg, Delta 200Hg, and Delta 201Hg values and negative Delta 204Hg 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-delta 202Hg 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. The late Cambrian-Early Ordovician interval is a crucial time that bridges the Cambrian extinction and Great Ordovician Bio-diversification events. The former is associated with 50% decrease in genera, whereas the latter displays threefold increase in species. Volcanism is associated with extinction and bio-development events throughout Earth's history. Prior works investigated potential biogeochemical controls that could have supported the Cambro-Ordovician bio-transition, but none explored the role of volcanism. We, for the first time, examine Hg abundance ratios and isotopes in the Scandinavian Alum Shale core across this boundary. Two novel molybdenum-Hg and vanadium-delta 202Hg models are proposed to improve our interpretation of the geochemical records about the effects of volcanism on environmental changes during this enigmatic transition. Constrained by a synchronous atmospheric-oceanic-tectonic model, our results demonstrate that late Cambrian subaerial volcanism contributed to oceanic anoxia and extinction, whereas Early Ordovician submarine volcanism and water upwelling led to the subsequent bio-radiation. Mercury is associated with organic matter in carbonaceous Alum shale deposited under sulfidic conditions Late Cambrian-Early Ordovician mercury was released by volcanism that also triggered major environmental change Mercury mass independent fractionation isotopes suggest late Cambrian subaerial volcanism but Early Ordovician submarine source