Re-Os geochronology for the Cambrian SPICE event: Insights into euxinia and enhanced continental weathering from radiogenic isotopes

Abstract

AbstractThe late Cambrian Steptoean positive carbon isotope excursion (SPICE) represents a major perturbation to the global carbon cycle and was associated with trilobite extinctions and expansion of anoxic and/or euxinic water masses during episodes of eustatic sea-level change. We present a new Re-Os age together with Os and Nd isotope stratigraphy and major- and trace-element data from the Alum Shale Formation (Scania, Sweden). The Re-Os age of 494.6 ± 2.9 Ma is from the interval of peak δ13Corg values, providing the first radiometric age constraint for this Cambrian carbon isotope excursion, interpreted as a possible pre-Mesozoic ocean anoxia event, and the timing of biomere-level extinctions. The Os isotope chemostratigraphic profile can be explained by an increase in terrigenous weathering prior to the SPICE, potentially driven by sea-level fall, and in agreement with enhanced nutrient supply, primary productivity, and organic matter burial as the driver of the SPICE event. Post-SPICE, the Os isotopes become increasingly unradiogenic; however, invariant εNd(t) values argue against a change in provenance and instead support a decrease in the continental weathering flux, possibly related to eustatic sea-level rise.