UC San Diego

Marine carbonates display a marked color transition from dark gray in the early Paleozoic to predominantly white and tan in the Mesozoic. Previous studies have interpreted this transition to reflect a fundamental change in the redox state of marine sediments around ~200 Ma, with a reduction in bacterial sulfate reduction (BSR) and the emergence of active bioturbators being suggested as factors in this change. This study aims to assess the hypothesis that the change in color was primarily driven by changes in the rate of BSR, in which the loss of abundant gray carbonates was associated with a decline in pyrite deposition in marine sediments. Here, the bulk geochemistry of marine carbonates was analyzed using a handheld XRF Tracer. These rock samples include ~ 679 Paleozoic and Mesozoic carbonates that had previously been characterized for their color, depositional environment, and bio-stratigraphic age. Exploratory statistical methods, including Random Forest, PCA, and RDA, were then used to identify correlations between carbonate chemistry, color, age, and depositional facies to identify whether pyrite was the dominant colorant in black and dark gray carbonates. Based on the resulting models, the most dominant directions of geochemical variation are between carbonate-pure limestone and terrigenous material-rich marls. Rock color is not strongly associated with sulfur content or other evidence of hypoxia. The statistical analysis does not support the hypothesis that pyrite abundance controls carbonate color, thereby implying that other processes outside of changes in BSR were the driving mechanisms behind the observed changes.