THOSE PESKY MICROBES: BIOTIC INFLUENCE ON GEOCHEMICAL HETEROGENEITY IN THE MODERN GREAT SALT LAKE CARBONATE RECORD

Paleoenvironmental reconstructions using geochemical signals recorded in carbonates are frequently based on limited spatial sampling. Are assumptions of lateral heterogeneity reliable, particularly in lacustrine settings? How do the active microbial communities present in the GSL water column and sediments influence sediment geochemistry? To investigate this, we measured carbon, oxygen, and clumped isotope compositions (δ­¹³C, δ¹⁸O, Δ₄₇) of coeval Great Salt Lake sediments from sediment cores collected from four different Great Salt Lake South Arm shoreline sites in 2018. In addition, we characterized the microbial community at different depths in the cores via 16S rRNA gene amplicon sequencing. We found that while oolitic sediments provided time-averaged records of lake chemistry with little evident microbial influence, more time-sensitive facies (including carbonate mud and microbialites) displayed substantial temporal and spatial variability in δ¹³C (and, in one case, possible disequilibrium Δ₄₇) driven by microbial activities including photosynthesis, sulfur cycling, and fermentation. Organisms identified by 16S sequencing were also implicated in substantial variability in carbonate saturation laterally, by facies, and with depth. Our study highlights the importance of carefully considering facies (and microfacies) and spatial environmental and biological heterogeneity when selecting and interpreting samples for geochemical analysis and paleoenviornment reconstructions.