MECHANICAL MIXING OF CLIMATE PROXIES BY SEDIMENT FOCUSING IN PYRAMID LAKE, NEVADA: A CAUTIONARY TALE
During the final period of Pleistocene glacial advance in the Sierras, Pyramid Lake spilled through passes into adjacent basins forming Lake Lahontan. The core record of this period is mostly heavily bioturbated with only occasional bands of poorly sorted siliciclastic sand deposited by ice melt. Sediments were dominated by a fine glacial flour mixed with minor subhedral to anhedral calcite crystals, and scattered pelagic diatoms. During periods of lake-level drop, there was a rapid transition to aragonite needles. An increase in ostracode shells, a mixture of benthic and pelagic diatoms, and a mixture of other carbonate minerals also accompany the change in mineralogy. The influx of the other materials is interpreted as older marginal sediment of the deeper lake washed into the shrinking lake.
Following the Lahontan highstand at about 15.2 ka, a precipitous drop in lake level formed a stratified lake with a saline lower water mass. Rising lake levels produced sub-mm laminae consisting of rhythmic layers of calcite, deep-water diatoms, and organic-rich mud. Falling lake levels resulted in aragonite precipitation replacing calcite and the inclusion within the clastic layers of a diversity of benthic and pelagic diatoms and a mixture of carbonate crystal types.
Although oxygen isotopes become heavier with falling lake levels, the degree of change is much less than expected. Furthermore, carbon isotopes show progressive discordance with oxygen isotopes in the shallower-water deposits, due to mixing of older carbon. Therefore sedimentation rates and reservoir effects on 14C dates are not constant, and the commonly used age models are not applicable. We believe this may be a pervasive but under-appreciated problem in closed-basin lakes.