Sulfur isotope signatures in gypsiferous sediments of the Estancia and Tularosa Basins as indicators of sulfate sources, hydrological processes, and microbial activity

Abstract

In order to reconstruct paleo-environmental conditions for the saline playa lakes of the Rio Grande Rift, we investigated sediment sulfate sources using sulfur isotope compositions of dissolved SO 4 2 - ions in modern surface water, groundwater, and SO 4 2 - precipitated in the form of gypsum sediments deposited during the Pleistocene and Holocene in the Tularosa and Estancia Basins. The major sulfate sources are Lower and Middle Permian marine evaporites (δ 34S of 10.9–14.4‰), but the diverse physiography of the Tularosa Basin led to a complex drainage system which contributed sulfates from various sources depending on the climate at the time of sedimentation. As inferred from sulfur isotope mass balance constraints, weathering of sulfides of magmatic/hydrothermal and sedimentary origin associated with climate oscillations during Last Glacial Maximum contributed about 35–50% of the sulfates and led to deposition of gypsum with δ 34S values of −1.2‰ to 2.2‰ which are substantially lower than Permian evaporates. In the Estancia Basin, microbial sulfate reduction appears to overprint sulfur isotopic signatures that might elucidate past groundwater flows. A Rayleigh distillation model indicates that about 3–18% of sulfates from an inorganic groundwater pool (δ 34S of 12.6–13.8‰) have been metabolized by bacteria and preserved as partially to fully reduced sulfur-bearing minerals species (elemental sulfur, monosulfides, disulfides) with distinctly negative δ 34S values (−42.3‰ to −20.3‰) compared to co-existing gypsum (−3.8‰ to 22.4‰). For the Tularosa Basin microbial sulfate reduction had negligible effect on δ 34S value of the gypsiferous sediments most likely because of higher annual temperatures (15–33 °C) and lower organic carbon content (median 0.09%) in those sediments leading to more efficient oxidation of H 2S and/or smaller rates of sulfate reduction compared to the saline playas of the Estancia Basin (5–28 °C; median 0.46% of organic carbon). The White Sands region of the Tularosa Basin is frequently posited as a hydrothermal analogue for Mars. High temperatures of groundwater (33.3 °C) and high δ 18O(H 2O) values (1.1‰) in White Sands, however, are controlled predominantly by seasonal evaporation rather than the modern influx of hydrothermal fluids. Nevertheless, it is possible that some of the geochemical processes in White Sands, such as sulfide weathering during climate oscillations and upwelling of highly mineralized waters, might be considered as valid terrestrial analogues for the sulfate cycle in places such as Meridiani Planum on Mars.