Non-evaporitic gypsum formed in marine sediments due to sulfate-methane transition zone fluctuations and mass transport deposits in the northern South China Sea

Abstract

Gypsum (CaSO4·2H2O) of non-evaporitic origins has been observed in marine sedimentary environments over the past decade. Our understanding of related diagenetic processes has improved by analyzing the sulfur and oxygen isotopic compositions of gypsum, but, a comprehensive study of the isotopic composition of authigenic gypsum precipitated in methane-rich marine environments has not been performed to elucidate their implications for the sulfur cycle (e.g., sulfide oxidation). Furthermore, there are no studies of authigenic gypsum associated with submarine mass transport deposits, which are common sedimentary features on continental slopes. In this study, we analyzed the sulfur and oxygen isotopic compositions of gypsum and the sulfur isotopic composition of pyrite in drill cores collected from methane hydrate-bearing sites GMGS2–08, GMGS2–16, and GMGS4-W02B in the northern South China Sea to determine the formation mechanisms of authigenic gypsum. The stable sulfur isotopic analyses of gypsum and co-existing pyrite revealed that sulfate produced by pyrite oxidation contributed to gypsum precipitation. The low oxygen isotopic composition values of gypsum (−2.6 to 4.5‰ Vienna Standard Mean Ocean Water) suggest that pyrite is oxidized by metal oxides under anaerobic conditions, incorporating water oxygen, whose isotopic composition was modified by gas hydrate formation, into the produced sulfate. As pyrite oxidation also releases protons (H+) into the porewater, calcium concentrations may be elevated by the dissolution of carbonate minerals. At sites GMGS2–08 and GMGS2–16, authigenic gypsum was precipitated within paleo sulfate-methane transition zones, indicating that gypsum formation is probably associated with the downward migration of the sulfate-methane transition zone, which causes anaerobic pyrite oxidation at its original site of formation. However, at site GMGS4-W02B, most of the gypsum was distributed in a mass transport deposit characterized by a high abundance of reworked foraminiferas between the two paleo sulfate-methane transition zones. Given that seawater sulfate contributes more (>85%) than pyrite oxidation to the gypsum sulfate at site GMGS4-W02B, it is believed that authigenic gypsum formation is associated with mass transport deposits, which can rapidly trap overlying seawater sulfate within the pore space of the newly deposited sediments, combined with the sulfate and calcium from pyrite oxidation, leading to a local gypsum supersaturation.