SULFUR AND CARBON ISOTOPIC TRENDS THROUGH THE EDIACARAN-CAMBRIAN TRANSITION AT CLAYTON RIDGE OF EASTERN NEVADA, USA

The transition from the Ediacaran to the Early Cambrian period is of particular interest because it contains the first appearances of several metazoan groups including the first biomineralizing organisms. Here, we present records of δ³⁴S, δ¹³C and carbonate-associated sulfate (CAS) concentration, which yield insight into the redox conditions of the oceans during this critical evolutionary interval. Samples were collected at high stratigraphic resolution from the Precambrian Reed Formation and the overlying Deep Spring Formation (White-Inyo Succession) at Clayton Ridge in eastern Nevada. The Deep Spring Formation consists of mixed siliciclastic and carbonate strata, providing a unique opportunity to link the δ³⁴S and δ¹³C from the carbonates with the first appearance of Treptichnus pedum, the trace fossil used to correlate the base of the Cambrian.

In addition to the data from Clayton Ridge, we present data from other localities in the White-Inyo and the Death Valley regions of eastern Nevada and western California. The Ediacaran-Cambrian strata in these areas have CAS concentrations less than 250 ppm, approximately an order of magnitude smaller than modern carbonates. These areas are also isotopically dynamic with respect to sulfide (-5 to +35‰) and sulfate (+23 to +37‰). This variability, particularly the heavy sulfide, is indicative of a small sulfate reservoir, and by proxy low concentrations of marine oxygen. The magnitude of the negative δ¹³C isotopic shift (-10‰) at the Ediacaran-Cambrian boundary is also indicative of an Ediacaran and early Cambrian ocean low in oxygen. These proxy data lend support to a non-actualistic, low oxygen marine environment during the evolution of early biomineralizers such as Cloudina, and the subsequent Cambrian radiation.