GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 282-1
Presentation Time: 9:00 AM-6:30 PM


LONSDALE, Mary C.1, AHM, Anne-Sofie C.2, HIGGINS, John A.2 and SMITH, Emily F.1, (1)Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N Charles St, Olin Hall, Baltimore, MD 21218, (2)Department of Geosciences, Princeton University, Washington Road, Guyot Hall, Princeton, NJ 08544

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 (δ44/40Ca) and trace element (Sr/Ca, Mn/Ca) proxies with δ13Ccarb and δ18Ocarb 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 δ13Ccarb and δ18Ocarb 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 δ44/40Ca with δ13C values, as well as positive covariation of Sr/Ca ratios with δ13C values and Mn/Ca ratios with δ44/40Ca values. Where δ44/40Ca 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 δ13Ccarb values approaching 0‰. In contrast, where δ44/40Ca values trend towards -0.6‰ and correlate with low Sr/Ca ratios, we interpret resetting of the primary δ13Ccarb 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.