ASSESSING ABIOTIC AND BIOTIC PROCESSES CONTRIBUTING TO SULFATE FORMATION IN ANTARCTICA: ASTROBIOLOGICAL IMPLICATIONS FOR MARS

The McMurdo Dry Valleys in Antarctica are often used as a terrestrial analog for understanding geochemical and aqueous processes that led to the formation of sulfate minerals on Mars. However, spatially limited δ³⁴S analyses on Antarctic soils, bedrock, and saline lakes/ponds do not allow for adequate constraint of all potential sulfate contributions. The main goal of this study was to characterize and quantify contributions of sulfates from atmospheric deposition, sulfide weathering and microbial sulfate reduction (MSR) in four distinctive lowland thaw zones of the dry valleys. Studied sites include Don Juan Pond (DJP) and South Fork within Wright Valley as well as Blood Falls and Lake Fryxell within Taylor Valley. Bedrock, soils and surface water samples were analyzed for δ³⁴S and δ¹⁸O of sulfate and δ³⁴S of sulfide and H₂S. Measured δ³⁴S and δ¹⁸O values in the soils and glacial tills of South Fork showed wide ranges, from +14.8 to +21.5 ‰ and -8.6 to -5.9 ‰, respectively, implying varied sources and proportions of sulfate from atmospheric deposition and sulfide weathering. Conversely, the sulfate from the DJP brine and shallow sediment and the Lake Fryxell water exhibited some of the highest δ³⁴S values ranging from +36.5 to +37.7 and +41.4 to +71.6 ‰, respectively. It has been shown previously that viable sulfate-reducing bacteria (SRB) are present in Lake Fryxell and this is in good agreement with our measurements for δ³⁴S of sulfate. MSR is also supported by variable δ³⁴S of H₂S measured in Lake Fryxell water (-14.3 to +27.4 ‰). Generally, our results suggest there could be active MSR at the DJP site. However, no microbial activity has been found in DJP to date. Additionally, there was no H₂S or biogenic sulfides in the surface DJP brine or sediments. Therefore, we infer contemporary MSR might be occurring in the subsurface. Alternatively, the MSR-altered brine may have formed in the past and/or been transported to DJP from another location in the South Fork catchment. Further analysis of the DJP sediments with depth could reveal if active MSR is occurring under current conditions. If SRB can function in extreme environments such as DJP, where chaotropic solutes are abundant and water activity is low, then sulfur metabolism could be possible in the subsurface of Mars where brines with similar characteristics may persist.