A Complicated Record of Late Pleistocene and Holocene Lake Level and Salinity Variability at Mono Lake, California

Abstract:

Due to the potential for a long and detailed record of Sierra Nevada paleohydrology, Mono Lake is of interest to paleolimnologists. However, that interest has been frustrated by the difficulty of obtaining sediment cores due to the disruptive effects of local volcanic activity, impenetrable tephra layers, and difficulty in dating the sediments. Recent coring efforts, however, have included the collection of a shallow water (BINGO-MONO10-4A; 38.00^o N, 119.13^o W, 2.8 m water depth, Early through Late Holocene) and deep water (UWI-MONO15-1C/1D; 37.99^o N, 119.12^o W, 18 m water depth, late Pleistocene through Middle Holocene) cores, which when combined, provide a record spanning the last 16,000 years.

The sediments of the shallow-water core suggest a complicated record which includes multiple transgression-regression cycles, deposition affected by turbidites, pycnoclines, and reworking, whereas variations in the preservation quality and composition of the diatom assemblage indicate changing salinity and nutrient loads. Although located in the same embayment on the western side of the lake, the deep-water core provides evidence of a more stable depositional environment. Intra-sample variation in preservation in samples from both cores is evidence of a stratified water column.

Freshwater eutrophic planktic Stephanodiscus comprise as much as 97% of the diatom assemblage in the deep-water core, with abundances higher during late glacial and Younger Dryas highstands; while the genus is most abundant (<70%) in Early Holocene sediments in the shallow-water core. Although undated, likely late Pleistocene sediments in the shallow-water core are dominated by epiphytic species which may explain the high C_org:N_total values in the sediments of the deep-water core. A variety of benthic taxa, including epiphytic species attached to aquatic macrophytes become increasing abundant in the Early and Middle Holocene sediments of the deep-water core, but are only a major component in isolated samples. The abundance of freshwater species decreases rapidly near the end of the Early Holocene in the shallow-water core and remains low for the remainder of the Holocene, indicated the possibility of seasonal variations in salinity related to lake level changes.