MONO LAKE-AN ENIGMA WRAPPED IN A FRUSTULE

As one of the oldest lakes in the US, Mono Lake has long been of interest to paleolimnologists, but late glacial to middle Holocene sediments have been difficult to obtain due to the presence of coarse tephra layers erupted form nearby Mono Craters and the disruption of the lake floor by the uplift of Paoha Island about 300 years ago. Cores collected in 2010 (BINGO-MONO10-4A; 38.00^o N, 119.13^o W, 2.8 m water depth) and 2015 (UWI-MONO15-1C/1D; 37.99^o N, 1119.12^o W, 18 m water depth) provide a sediment record spanning the last ~16,000 years.

Based on the age model established for the 2010 core, most of the latest Pleistocene diatom flora is comprised of freshwater and brackish epiphytic species indicating a shallow environment with substantial aquatic macrophytes along the margin of the lake. Coincident with the deposition of a thick tephra layer (~438-430 cm), this assemblage was replaced by a freshwater eutrophic planktic assemblage dominated by Stephanodiscus spp., indicating deeper conditions (>15 m), which continued into the Holocene. Stephanodiscus spp. gradually decreased in abundance during the early Holocene as benthic species increased in number. The lack of epiphytic species may be due either to increased depth or changes in water chemistry that limited the growth of the aquatic macrophytes that supported the epiphytic flora during the Pleistocene. Beginning in the middle Holocene, conditions in the lake became less stable, leading to greater variability in the composition of the diatom flora. Diatom species tolerant of higher salinity increased in abundance in the early Holocene at about the same time that heterotrophic taxa (those that contain cyanobacterial endosymbionts) exhibited a decrease in numbers.

Preliminary analyses of the upper 500 cm of the 2015 core have identified an assemblage dominated by poorly to well-preserved Stephanodiscus spp. The remainder of the assemblage includes epiphytic, freshwater tychoplanktic, and freshwater and brackish benthic taxa. The abundance of Stephanodiscus spp. suggests that the lake was sufficiently deep during this time to maintain a planktic flora.