MONO LAKE SEDIMENTS: A RECORD OF RECENT ENVIRONMENTAL CHANGE

Mono Lake is a closed-basin, hypersaline soda lake at the eastern base of the Sierra Nevada, California. From 1941 to about 1994, the lake underwent drastic water level changes due to anthropogenic water diversions that decreased its volume by half and increased its salinity two-fold. The lake’s calibrated sedimentary record provides an opportunity to evaluate the geochemical and biological fingerprints of changing environmental conditions over time. We analyzed a 70 cm sediment core collected near the center of the lake. Based on sedimentation rates previously determined for Mono Lake, we estimate this core to represent the last 116 years of deposition. 16S rRNA gene sequencing analysis suggested that Cyanobacteria are the most prevalent bacterial species. In SEM images, pennate diatom frustules were found to be well-preserved and much more concentrated below 10 cm depth than above. Fatty acid (FAME) biomarkers for diatoms and algal sterols were identified throughout the core, as well as large amounts of phytol, with ratios of phytol/C18 FAME often greater than 200. Consistent with observations of dark green color throughout, the unusually high abundance of phytol suggests a higher preservation potential under alkaline conditions. While the δ¹³C_org ranged between -17.5 and -20‰ in the lower 52 cm of the core, δ¹³C_org is as low as -28‰ in the top 10cm. We interpret this significant shift in δ¹³C_org to reflect a replacement of the diatom-dominated primary productivity of the lake, likely by Picocystis, an algal species that dominates today and has an unusually low d¹³C value of -32.5‰. Because this event dates to about 23 years ago and is thus roughly in sync with the highest reported salinity, 88 g/L in 1995, we hypothesize that this ecological shift was instigated by hypersaline conditions. δ¹⁵N_org varied between +10 and +13‰ throughout the core - these unusually high values in δ¹⁵N_org could be explained by NH₄ dissociation and subsequent NH₃ volatilization, processes enhanced under high pH conditions. This might also offer an explanation for the scarcity of bioavailable nitrogen in Mono Lake. We find that the sedimentary record of Mono Lake enables us to outline preservation of recent environmental change as well as characteristic fingerprints that could be used explore similar lake systems in the rock record.