Carbonate-Associated Sulfate from the Precambrian-Cambrian Transition in the Western Great Basin, USA

The isotopic composition of carbonate lattice-bound seawater sulfate can be an important indicator of redox conditions of the ancient oceans during critical times in Earth history. Here, we present a record of δ³⁴S for carbonate-associated sulfate from Precambrian-Cambrian boundary sections in western North America. In eastern California, samples were collected at high stratigraphic resolution from the Reed and Deep Spring Formations (White-Inyo succession) and the Wood Canyon Formation (Death Valley succession, a more nearshore facies). Both successions consist of mixed siliciclastic and carbonate strata, presenting a unique opportunity to link the δ³⁴S_CAS and previously reported δ¹³C from the carbonates with the first appearance of Treptichnus pedum (the trace fossil used to correlate the base of the Cambrian, restricted to siliciclastic strata and commonly absent from chemostratigraphic studies). 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 bilaterians and the first biomineralizing organisms. CAS concentrations of <250 ppm are consistent with low seawater sulfate concentrations, which suggests that the reservoir was likely more responsive to the effects of oxygenation and/or bioturbation, which were both increasing throughout this interval. Using sulfate as a proxy for changing atmospheric and oceanic conditions, we gain insight into how oxygenation of the oceans played a role in the Cambrian radiation and can possibly discern the cause(s) of the global Precambrian-Cambrian boundary δ¹³C excursion.