HOLOCENE HISTORY OF MONO LAKE FROM MULTIPLE SEDIMENT CORES

Great Basin lakes are sensitive recorders of climatic change, due to the arid climate and highly fragmented topography. Geomorphic and sedimentologic evidence of lake-level changes during the last ~2000 years and through the last glacial cycle in the Mono Basin has been studied extensively, but the history of the Basin between ~12,000 and ~2,000 years ago is poorly known. Geomorphic and outcrop evidence is removed or hidden in this interval, and while geochemical, sedimentological, and biological proxies in sediment cores can provide continuous records of hydroclimate, they can be challenging to interpret correctly.

In deep-lake sediments of the last glaciation in the Mono Basin, carbonate concentration is high when the lake level is high (Zimmerman et al., 2011, GSAB). Similarly, bands of fine-grained aragonite in Holocene deep-water cores (17 and 34 m water depth) likely indicate increased flux of Ca²⁺-bearing freshwater, perhaps mediated by stratification and overturn. In shallower cores (2-3 m water depth), high concentrations of carbonate apparently indicate regression and approach of shoreline processes to the core site, bringing coarse biogenic carbonate, aragonite flakes, and weathered tufa fragments. Some caution must be applied in the interpretation of facies changes in cores from the western embayment of Mono Lake, as even Holocene lake levels may have a component of tectonic offset.

Lake levels were highest during the early Holocene (~10,600-7,000 yr BP), where even the shallow cores are laminated mud. Between 8000 and 3000 yr BP the proportion of shallow carbonate layers increases, indicating an overall progression toward drier conditions, but with at least one sustained wet interval just after 3900 cal BP. This is followed by the lowest sustained lake indicated in the shallow cores, a >20 cm thick section of nearly pure coarse carbonate debris, possibly including a hiatus, and likely responsible for the Scholl terrace at 1940-1941 m elevation (Stine, 1990; PPP). This is followed by fine-grained unit indicating transgression by 2055 (+245/-160) cal yr BP. All of the shallow cores are capped by an interval of carbonate sand, likely the result of increasing lake level over the last 20 years.