HOLOCENE FLUCTUATIONS OF MONO LAKE, CALIFORNIA IN THE CONTEXT OF REGIONAL PALEOCLIMATIC CHANGE

Mono Lake is a closed-basin lake in the eastern Sierra Nevada of California, and is sensitive to climatic changes affecting the mountain snowpack, which also supplies much of the water for California’s municipal and agricultural needs. Extensive efforts to recover cores of the Holocene sediments have been challenged by coarse tephras from the Mono Craters, and by disruption of the lake floor by the uplift of Paoha Island ~300 years ago. The core BINGO-MONO10-4A-1N (herein BINGO10/4A; 2.8 m water depth), records nearly all of the Holocene in variable proportions of detrital, volcanic, and authigenic sediment. Both the 1350-year and 600-year old tephras are present in BINGO10/4A, as well as several older, as-yet unidentified tephras. The core is located on the down-dropped side of the Sierran front, so sudden deepenings could be either lake level rise or faulting. However, evidence of shallower conditions, as well as fluctuations requiring short-lived lake level changes, likely indicates a climatic control.

Following the dramatic changes of the deglacial period,when lake level dropped more than 150 m following the 2155 m highstand, laminated muds indicate relatively deep conditions during the early Holocene (10,660 – 8000 cal BP) at this site, similar to many records across the region during that period. The middle and late Holocene units contain greater and more variable amounts of authigenic carbonate, indicating a shallower lake and thus lower effective moisture. Over the period 8 -7 ka the lake became generally shallower and more variability is observed in the BINGO10/4A core. This corresponds to the development of the middle Holocene arid period, the time of maximum summer insolation in the northern hemisphere, and most northerly position of the ITCZ. The driest part of the middle Holocene ended at ~5300 cal BP, with the onset of deeper conditions. Two dry events at ~4800 and ~4100 cal BP punctuated the deeper period, which culminated at Mono Lake in the late Holocene highstand between ~3600 and ~3050 cal BP. Beginning at ~3050 cal BP, drought steadily lowered the lake to its lowest late Holocene level, during the Late Holocene Dry Period, dating here to ~3050-2100 cal BP. This drought likely caused a minor hiatus in the BINGO 10/4A core, and was similar in magnitude to the droughts of the Medieval Climate Anomaly.