ASSESSING ALTERATION OF EDIACARAN-CAMBRIAN CARBON ISOTOPE VALUES AT MOUNT DUNFEE, NV

A negative carbon isotope excursion unofficially marks the Ediacaran-Cambrian boundary and has been used for correlation and age calibration of carbonate-rich late Ediacaran to early Cambrian strata otherwise lacking appropriate biostratigraphic constraints. Here we evaluate the expression of a negative carbon isotope excursion at Mount Dunfee, NV, a locality at which this excursion has been previously identified and used to correlate to Ediacaran-Cambrian boundary sections globally. We pair Ca isotopes (δ^44/40Ca) and trace element (Sr/Ca, Mn/Ca) proxies with δ¹³C_carb and δ¹⁸O_carb from the Esmeralda Member of the Deep Spring Formation. These proxies have been shown to be diagnostic of carbonate mineralogy and early diagenesis on modern carbonate platforms, and, when combined with δ¹³C_carb and δ¹⁸O_carb records, reveal divergence from primary geochemical signatures (Higgins et al., 2018). We apply them here to assess the diagenetic history of Ediacaran-Cambrian carbonates at Mount Dunfee.

At Mount Dunfee, strata recording the basal Cambrian negative carbon isotope excursion show negative covariation of δ^44/40Ca with δ¹³C values, as well as positive covariation of Sr/Ca ratios with δ¹³C values and Mn/Ca ratios with δ^44/40Ca values. Where δ^44/40Ca values trend toward more negative values (-1.4‰) and correlate with high Sr/Ca ratios, we interpret neomorphism of primary aragonite under conditions that preserve primary geochemical signatures (sediment-buffered); such values are found in strata with δ¹³C_carb values approaching 0‰. In contrast, where δ^44/40Ca values trend towards -0.6‰ and correlate with low Sr/Ca ratios, we interpret resetting of the primary δ¹³C_carb values by a diagenetic fluid rich in isotopically light carbon. Together these data suggest that the stratigraphic variability in the Esmeralda Member is a result of local differences in the preservation of sediment chemistries rather than global changes in seawater chemistry.