LATE QUATERNARY PALEOHYDROLOGY RECORD FROM MONO LAKE, CALIFORNIA, USA

Mono Lake, located at the base of the Sierra Nevada escarpment on the western edge of the Great Basin, may have been in existence for more than 750,000 years. However, the depositional history of the basin is marked by the disruptive effects of local volcanic activity, impenetrable tephra layers, and downslope sediment transport, resulting in large gaps in our understanding of this hydrological archive.

Recent coring efforts collected shallow water (BINGO-MONO10-4A; 2.8 m water depth, Holocene) and deep water (UWI-MONO15-1C/1D; 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, and 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. The deep-water core provides evidence of a more stable environment.

Stephanodiscus, a freshwater, eutrophic planktic genus, comprises 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 contemporaneous sediments in the shallow-water core are dominated first by epiphytic species followed by Stephanodiscus during the Younger Dryas and Early Holocene. A variety of benthic taxa, including epiphytic species, become increasing abundant in the Early and Middle Holocene sediments of the deep-water core, but are a major component only in isolated samples. The abundance of planktic freshwater species decreases rapidly near the end of the Early Holocene and remains low in the shallow-water core. They are replaced by a salt-tolerant benthic assemblage. Heterotrophic Epithemia spp. are present in low numbers throughout the record but are more common in the Holocene. Intervals of poor preservation often associated with an increase in sediment grain size and the abundance of fossil taxa are more common in the Middle and Late Holocene and may be associated with subaqueous slumping resulting from rapid changes in lake level.