Geochemical investigations of the late early Cambrian extinctions

Abstract

Following the radiation of animals in the early Cambrian, the reef-building archaeocyathids went extinct around 513 – 508 Ma and redlichiid and olenellid trilobites went extinct around 506 Ma. Both extinction events have been documented to occur with negative carbon isotope excursions (CIEs) that likely represent changes in the carbon cycle. The emplacement of the Kalkarindji Large Igneous Province (LIP) in Australia is hypothesized to have caused the extinctions and driven the carbon cycle perturbations represented by the CIEs. However, uncertainties in radiometric dating of the LIP and the regional variations of the CIEs make it difficult to establish a causal relationship. In this study, we conducted a multi-proxy geochemical analysis of the Shady Dolomite Formation in southwestern Virginia, USA, which represents carbonate deposition on the passive margin of Laurentia from early to middle Cambrian. While negative shifts in δ13Ccarb and δ13Corg resembling the two negative CIEs have been identified, incomplete drill core and pervasive alteration complicates this straightforward interpretation. In addition, iron speciation and mercury content analyses from the Flatwoods Member within the stratigraphic interval tentatively linked to the olenellid extinction suggest the occurrence of transient local euxinic (anoxic, sulfidic) conditions. These findings give support to the volcanic greenhouse scenario that drove bottom water anoxia and trilobite extinction and was initiated by the Kalkarindji LIP. However, such iron and mercury records could also reflect localized changes in seawater geochemistry independent of volcanic activity. To better understand the relationship between extinction events, CIEs, and the Kalkarindji LIP, further geochemical investigations of other Cambrian successions is needed.