Analysis of hydroclimate in the Great Basin during the LGM from clumped isotope measurements at paleolakes
Friday, 19 December 2014
In the Great Basin, paleoshoreline reconstructions indicate that lacustrine highstands correspond with Pleistocene glacial maxima. However, uncertainties remain regarding the magnitude of temperature change between glacial and interglacial episodes, and it is difficult to estimate precipitation and evaporation trends from geomorphic records alone. Hence, geochemical and biologic proxies have been applied to better constrain temperature, precipitation, and primary productivity during both lacustrine highstands and lowstands. Carbonate clumped isotope analyses of lacustrine materials provide new constraints on summertime water and air temperatures. The work presented here is an update of ongoing investigations at Lake Bonneville, Lake Manix, and Surprise Valley. These systems were actively transgressing during, and slightly after, the Last Glacial Maximum. Multiple phases of carbonate have been evaluated, including aragonitic shells of lacustrine gastropods and bivalves, as well as marls, and calcite cements. Given what is known about the season of growth for these materials in mid-latitude lakes, clumped isotope measurements should record warm season hydrographic conditions. Biogenic aragonite in mollusk shells is largely precipitated during the April through October interval, when food is abundant and water temperatures are most conducive to organismal development. Accumulation of carbonate muds (i.e. marl) and cements is also typically restricted to warm months, when lake waters reach carbonate saturation. Clumped isotope results are also used to identify the 18O/16O composition of water at the time of mineral precipitation, a proxy which tracks evaporative enrichment and/or moisture source. Results in this work are being calibrated by comparison to clumped isotope measurements of modern lacustrine samples, from sites where water and air temperatures are known. Water paleotemperatures are applied to estimate summertime and mean annual air temperatures using lake-atmospheric transfer functions. These results are also being compared to climate model simulations for the Last Glacial Maximum interval.