SULFUR ISOTOPE SIGNATURES IN GYPSIFEROUS SEDIMENTS OF THE TULAROSA AND ESTANCIA BASINS AS INDICATORS OF SULFATE SOURCES AND THE LOCAL HOLOCENE HYDROLOGIC CYCLE

Pleistocene/Holocene gypsiferous lake sediments of the Tularosa Basin (White Sands area) and Estancia Basin were studied, using sulfur isotope methods, to try and identify primary sulfate sources and determine the hydrologic cycle during the Holocene tectonic evolution of these basins. Four sections of lake sediments taken from different sites in the White Sands area show wide variation in the δ 34 S values of sulfate minerals (from 2.2 to 13.8 ‰ vs. VCDT) suggesting different sulfate sources for and different sedimentary environments in the southern and northern parts of the study area. In the southern part, mixing process between sulfate-rich fluids originating from the dissolution of the Middle (10.9 to 12.3 ‰) and Lower (12.5 to 14.4 ‰) Permian strata is indicated by a steady increase of δ 34 S values (11.3 to 13.8 ‰) and linked to discharge of deeper-seated groundwater through fault-related fractures. This process was probably controlled by climate change and/or episodes of increased tectonic activity. Areas of groundwater discharge related to dissolution of Lower Permian strata were recognized based on alignments of gypsum-rich domes, visible on aerial photographs taken in October of 2007, with regional faults. The δ 34 S values of sediments from the northern part of the basin are dominated by sulfate (10.6 to 12.4 ‰) originating from the dissolution of the Middle Permian strata. However, three episodes of negative excursion of δ 34 S values (up to 2.2 ‰) suggest episodes of water influx with lower δ 34 S values that may be linked to the leaching of sulfate during nearsurface weathering of sulfides. Values of δ 34 S significantly higher (median 17.4 ‰) than those for sulfates derived from fluids that interacted with Lower Permian strata are found in sulfate-rich lake sediments from the Estancia Basin. This suggests the involvement of bacterial sulfate reduction processes. Higher organic carbon content in sediments from the Estancia Basin (median 0.46 %) compared to the White Sands area (median 0.07 %) is consistent with higher rates of bacterial sulfate reduction and the consequent overprinting of sulfur isotope signatures that would elucidate Holocene fluid flow paths.