The sulfur isotope composition of carbonate-associated sulfate in Mesoproterozoic to Neoproterozoic carbonates from Death Valley, California

We have analyzed the concentration and sulfur isotope composition of trace sulfate in carbonate from three Proterozoic formations in Death Valley, California. Trace sulfate concentrations for the Crystal Spring Formation and Beck Spring Dolomite, which were deposited in the late Mesoproterozoic and mid-Neoproterozoic and are not associated with glacial sediments, range from 0 to 144 ppm with δ³⁴Ssulfate values spanning 11.0‰-27.4‰. Within these formations, stratigraphic shifts in δ³⁴Ssulfate of up to ∼9‰ occur over <50 m. Trace sulfate concentrations for the Noonday Dolomite, which was deposited in the late Neoproterozoic and directly overlies glacial sediments associated with the "snowball Earth" events, range from 2 to 272 ppm with δ³⁴Ssulfate values varying between 15‰ and 35‰. The ∼17‰ δ³⁴Ssulfate increase at the base of the Noonday Dolomite is similar in magnitude and rate to the >20‰ positive δ³⁴S shifts recorded in Neoproterozoic postglacial carbonates from Namibia. The results indicate that the sulfur cycle behaved differently in the late versus early Neoproterozoic as a possible consequence of severe late Neoproterozoic glacial events. Furthermore, based on δ ³⁴Ssulfate patterns and carbonate-associated sulfate concentrations recorded in the Crystal Spring and Beck Spring formations, we speculate that late Mesoproterozoic to mid-Neoproterozoic oceanic sulfate concentrations were ∼10% of modern values (e.g., ∼3 mM).