CALCIUM ISOTOPIC CONSTRAINTS ON THE ORIGIN OF THE MID-DARRIWILIAN CARBON ISOTOPE EXCURSION (MDICE)

The Middle Ordovician Darriwilian Stage (~469 – 458 Ma) records a ~2‰ positive carbon isotope shift in bulk carbonate known as the MDICE (Mid-Darriwilian Carbon Isotope Excursion). Although studies have shown that the MDICE is a globally synchronous event, the link between the MDICE and changes in the global carbon cycle remains unclear. This is largely due to possible local processes such as diagenesis that can decouple δ¹³C signals recorded in shallow marine carbonate rocks from the global dissolved inorganic carbon (DIC) reservoir. Here, we use paired measurement of δ¹³C, δ^44/40Ca, and Sr/Ca from a stratigraphic section at Meiklejohn Peak in southwest Nevada (USA) to constrain the potential for local processes in decoupling the recorded δ¹³C signals from the global carbon cycle. Measured variations in δ^44/40Ca and Sr/Ca in this section at Meiklejohn Peak can in part be explained by changes in primary mineralogy and diagenesis. However, these processes are unable to account for the entire shift in δ¹³C (MDICE). While this study cannot completely deconvolve local signals from the recorded δ¹³C, we argue that the MDICE at least in part reflects a primary signal of increase organic carbon burial that may relate to changes in primary productivity and nutrient delivery during increased basaltic weathering associated with the Taconic uplift. The proposed link between tectonic uplift and increase in primary productivity strengthens the notion that tectonic processes played a significant role in modulating changes in the global carbon cycle and climate during the Ordovician Period.