A U/TH BASED DEGLACIAL HYDROCLIMATE HISTORY IN THE MONO BASIN, USA
We recently discovered a few visually clean, white, shiny, dense carbonate samples that represent a very small fraction of tufa deposits in the Mono Basin. Their very low thorium, but high uranium contents allow precise and reproducible U/Th age determinations. A highly resolved history of a minimum lake level through the last deglaciation can therefore be inferred based on sample locations and their ages.
Calcite coating along vertical fractures at Black Point indicates that the lake level already rose to ~2098 m asl by ~16.8 kyr BP. It climbed to at least ~2120 m at ~15.4 kyr BP, evidenced by calcite cements on conglomerates from the northwestern slope of the Cowtrack Mountains. There may have been some lake level fluctuations after that, but the level was ~2114 m at ~14.9-14.6 kyr BP, supported by both tabular carbonate layers and calcite coatings along vertical fractures found at the top of Black Point. Calcite coating samples from thinolitic tufa mounds at different terraces in the northern basin, combined with an erosion surface at ~2020 m, indicate a rapid lake level drop from ~2114 m to ~2007 m in 600 years or less between 14.6 to 14.0 kyr BP. The lake level rose again to ~2030 m before 13.0 kyr BP and remained relatively stable through 12.0 kyr BP, followed by a final drop to Holocene conditions.
Relative to the present lake level of ~1950 m, Mono Lake broadly stood high during Heinrich stadial 1 and Younger Dryas, when the climate was extremely cold over the North Atlantic, and Asian monsoon was much weakened. When the climate shifted from cold to warm during the transition between Heinrich stadial 1 and Bølling-Allerød interstadial, the lake level however dropped significantly by ~180 m/kyr. The U/Th ages on the tufa samples therefore not only establish a highly resolved chronology of hydroclimate history in the Mono Basin, but also put the lake level oscillations in a global context.