COUPLED GEOCHEMICAL AND MICROBIOLOGICAL CHARACTERIZATION OF NON-CARBONATE FIRMGROUNDS FROM A MODERN SODA LAKE, WALKER LAKE, NEVADA

Non-carbonate firmgrounds are known to enhance the preservation of endolithic borings and ichnofabrics and are commonly attributed to microbial processes, but their formation is poorly understood. Modern firmgrounds are present along the west shoreline of Walker Lake, Nevada, USA, a sodium bicarbonate lake. A thin layer of cohesive clay-like material comprises the firmgrounds, localized at the sediment-water interface. In this study, geochemical and molecular tools were used to characterize the firmgrounds in order to understand biotic and abiotic contributions to formation.

Firmground associated bacteria and archaea were characterized using 16S rRNA gene sequencing. 16S results revealed communities with large portions of unclassified microbes and candidate phyla, as well as taxa associated with soda lakes. However, no correlation between community structure and sample depth nor location was observed. Additionally, firmground community composition showed no significant difference from that of nearby non-firmground sediment communities. Metagenomic sequencing indicated community potential for S cycling, As reduction, and heavy metal detoxification. Notably, multiple genes for Fe-scavenging siderophores were found.

Lake and pore water cation, sulfur species, iron, and inorganic δ¹³C data indicate that physical processes dominate these important chemical systems. In general, sodium and sulfate concentrations decrease with depth beneath the firmgrounds and can be interpreted as simple mixing of lake and ground waters, despite metagenomic evidence for sulfur cycling. Fe²⁺ concentrations are vanishingly low in both lake water and pore waters.

Through combined geochemistry and molecular biology, our data suggest that physiochemical parameters are largely responsible for firmground geochemical profiles, where the elemental profiles likely reflect mixing between lake water and local ground water. Whereas there is no evidence indicating microbial control on firmground formation, further analyses are required to understand the microbial role in biogeochemical cycling and sediment diagenesis in Walker Lake firmgrounds, which can then aid in the interpretation of firmgrounds in the rock record.