Sulfur cycle in buried evaporites

Abstract

Sulfur isotopes are potent indicators of the way in which sulfur behaves chemically during diagenesis. We have studied sulfur isotope ratios (34S/32S) from a number of minerals and compounds across the Permian-Triassic boundary in the Khuff Formation, Abu Dhabi. The δ34S in dissolved marine sulfate increased by 10‰ from the Late Permian to the Early Triassic. Despite precipitation of gypsum from Permian and Triassic seawater and its subsequent dehydration to anhydrite at depths greater than about 1000 m, the primary marine stratigraphic sulfur isotope variation has been preserved in anhydrite in the Khuff Formation. A combination of biostratigraphic and absolute age data show that this 10‰ shift occurred over <2 m.y. Gypsum dehydration to anhydrite has not involved significant isotopic fractionation or diagenetic redistribution of material in the subsurface. The sulfur isotope variation across the Permian-Triassic boundary is also present in elemental sulfur and H2S, at depths greater than 4300 m, formed by reaction of anhydrite with hydrocarbons via thermochemical sulfate reduction. This demonstrates that sulfate reduction has not led to isotope fractionation. It also demonstrates that significant mass transfer has not occurred, at least in the vicinity of the Permian-Triassic boundary, even though elemental sulfur and H2S are both fluid phases at depths greater than 4300 m. Thus, despite two major diagenetic processes that converted the sulfur in gypsum into elemental sulfur and H2S by 4300 m burial and the potentially mobile nature of some of the reaction products, the primary differences in sulfur isotopes have been preserved in the rocks and fluids. All reactions occurred in situ; there was no significant sulfur isotope fractionation, and only negligible sulfur was added, subtracted, or moved internally within the system.