CHANGING SEAWATER CARBON ISOTOPE COMPOSITIONS AT THE EDIACARAN-CAMBRIAN BOUNDARY PRESERVED DUE TO EARLY MARINE DIAGENESIS IN SOUTHWESTERN LAURENTIA

A negative carbon isotope excursion is recognized among Ediacaran-Cambrian boundary strata from the western margin of Laurentia. While this excursion, termed the BAsal Cambrian carbon isotope Excursion (BACE), is not precisely biostratigraphically constrained to the Ediacaran-Cambrian boundary outside of Laurentia, it has been used globally to correlate strata, construct age models, and interpret environmental change during a critical interval in the evolutionary history of animals. Here, we assess the diagenetic history of the BACE in southwestern Laurentia to evaluate the traditional interpretation of the BACE as a record of perturbation to seawater dissolved inorganic carbon (DIC) values.

We present data from three stratigraphic sections in Nevada, California, and Mexico, where the BACE has previously been recognized in Ediacaran-Cambrian boundary strata. We compare carbon isotope (δ¹³C) records from these sections with stable isotope (δ^44/40Ca, δ²⁶Mg) and elemental (Sr/[Ca+Mg]) records, which we interpret as proxies for early marine diagenesis. We find that δ¹³C and δ^44/40Ca values do not covary across all sections, but that each section predominantly preserves seawater-buffered δ¹³C and δ^44/40Ca records. From these results, we interpret the BACE not as a product of early marine diagenesis, but as a record of changing DIC δ¹³C values during early marine diagenesis. We note that the most negative δ¹³C values preserved are among the most extensively seawater-buffered and suggest that a seawater-buffered δ¹³C record is more reflective of ancient DIC δ¹³C compositions than a sediment-buffered δ¹³C record.