Sulfur isotope signatures in gypsiferous sedimentsof the Estancia and Tularosa Basins as indicators of sulfatesources, hydrological processes, and microbial activity

摘要

In order to reconstruct paleo-environmental conditions for the saline playa lakes of the Rio Grande Rift, we investigatedsediment sulfate sources using sulfur isotope compositions of dissolved SO_4~(2-) ions in modern surface water, groundwater, andSO_4~(2-) precipitated in the form of gypsum sediments deposited during the Pleistocene and Holocene in Tularosa andEstancia Basins. The major sulfate sources are Lower and Middle Permian marine evaporites (δ34S of 10.9-14.4ie), butthe diverse physiography of the Tularosa Basin led to a complex drainage system which contributed sulfates from varioussources 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 LastGlacial Maximum contributed about 35-50% of the sulfates and led to deposition of gypsum with δ34S values of -1.2ieto 2.2ie which are substantially lower than Permian evaporates. In the Estancia Basin, microbial sulfate reduction appearsto overprint sulfur isotopic signatures that might elucidate past groundwater flows. A Rayleigh distillation model indicatesthat about 3-18% of sulfates from an inorganic groundwater pool (δ34S of 12.6-13.8ie) have been metabolized by bacteriaand preserved as partially to fully reduced sulfur-bearing minerals species (elemental sulfur, monosulfides, disulfides) with dis-tinctly negative δ34S values (-42.3ieto -20.3ie) compared to co-existing gypsum (-3.8ie to 22.4ie). For the Tularosa Basinmicrobial sulfate reduction had negligible effect on δ34S value of the gypsiferous sediments most likely because of higherannual temperatures (15-33 °C) and lower organic carbon content (median 0.09%) in those sediments leading to more efficientoxidation of H2S and/or smaller rates of sulfate reduction compared to the saline playas of the Estancia Basin (5-28 °C; med-ian 0.46% of organic carbon). The White Sands region of the Tularosa Basin is frequently posited as a hydrothermal analogue for Mars. High temper-atures of groundwater (33.3 °C) and high δ180(H20) values (1.1ie) in White Sands, however, are controlled predominantly byseasonal evaporation rather than the modern influx of hydrothermal fluids. Nevertheless, it is possible that some of the geo-chemical processes in White Sands, such as sulfide weathering during climate oscillations and upwelling of highly mineralizedwaters, might be considered as valid terrestrial analogues for the sulfate cycle in places such as Meridiani Planum on Mars.