Sulfur cycling at two Mars-relevant saline lakes underscores the importance of dynamic metabolic substrate concentrations for isotopic biosignatures

Abstract

The presence of magnesium sulfate minerals (Mg-sulfates) within the sulfate-bearing unit of Gale Crater on Mars may coincide with a major planetary drying transition at ∼ 3.5 Ga that would have had negative consequences for surface habitability. Here, we investigate the formation conditions and biosignature potential of lacustrine Mg-sulfates by contrasting the sulfur biogeochemistry of two saline lakes of the Interior Plateau of British Columbia, Canada: Basque Lake 2 and Goodenough Lake. Basque Lake 2 is characterized by magnesium-sulfate-chloride-type hypersalinity with evaporative Mg-sulfate precipitation. It exhibits invariant dissolved sulfate isotope ratios (δ34S) throughout its porewater profiles. This uniformity is best explained by the lake’s extremely high sulfate-to-sulfide ratio (>1900:1 on average), which nullifies the isotopic influence of dissimilatory microbial sulfate reduction (MSR) on the sulfate reservoir. In contrast, Goodenough Lake is alkaline sodium-carbonate-chloride type with unambiguous evidence of temporally and spatially dynamic sulfur cycling, as indicated by high porewater sulfate δ34S and low chromium-reducible sulfide δ34S values, as well as porewater sulfide concentrations that vary across field seasons and meter-scale distances. At both lakes, the upper bound on MSR appears related to physiological limitations of sulfate reducing bacteria. Specifically, Basque Lake 2 waters reach inhibitory salinity levels, while Goodenough Lake porewaters can approach toxic sulfide concentrations. Importantly, these contrasting systems indicate that the sulfate isotopic signature of MSR is masked in Mg-sulfate-precipitating lakes, challenging the use of δ34S values in Mg-sulfate minerals to interpret the ancient sulfur cycle and/or paleohabitability on Mars. However, abundant sulfate with high δ34S values that co-occurs with mineral sulfides exhibiting low δ34S values in a low-temperature environment would match the MSR profile of Basque Lake 2 and defy an abiotic explanation. In the context of evaluating potential biosignatures, isotopic systems defined by sufficient, dynamic metabolic substrate reservoirs are preferable to substrate-replete conditions.