Constraints on the late Ediacaran sulfur cycle from carbonate associated sulfate

Abstract

We report new sulfur isotope compositions (δ34S) in carbonate associated sulfate (CAS) and pyrite from the lower Nama Group, Namibia (∼550 to <547Ma), and use these data to interrogate terminal Ediacaran sulfur cycle dynamics. Our extraction method utilizes an improved pre-leaching procedure that reduces the likelihood of contamination from matrix-bound sulfur. Data generated with the improved extraction method show CAS δ34S as much as 12‰ higher (34S-enriched) than previously reported which suggests a reevaluation of the phenomenon of ‘superheavy’ pyrite. The average δ34S of seawater sulfate increases from 30 to 38‰ in the lower Nama Group, and we correlate this rise in δ34S among contemporaneous marine basins. Global seawater sulfate δ34S >35‰ is highly unusual in Earth history, and in the terminal Ediacaran is best explained by a high pyrite burial flux. Pyrite in the Nama Group is close in isotopic composition to coeval sulfate, but the sulfur isotope fractionation between sulfate and pyrite varies widely among different studied basins, suggesting highly heterogeneous redox and depositional conditions.