EVIDENCE FOR PRECIPITATION VARIABILITY DURING MIS 11 AS RECORDED BY DIATOM SILICA δ18O VALUES FROM THE VALLES CALDERA, NEW MEXICO

Diatom frustules are nearly ubiquitous in lacustrine sediment archives and δ¹⁸O values of diatom silica from lacustrine environments are a powerful paleoprecipitation proxy. We present diatom silica δ¹⁸O values from a middle-Pleistocene core (VC-3) collected from the Valles Caldera as part of the GLAD5 drilling project. During interglacial Marine Isotope Stage 11 (MIS 11), diatom silica δ¹⁸O values record ~12‰ of variability. Reconstruction of mean annual temperature from MBT/CBT ratios indicates 2-3ºC of variation during MIS 11, which is insufficient to explain variations in the δ¹⁸O_diatom values. Instead, much of the variation in the VC-3 core diatom silica δ¹⁸O values reflects changes in the δ¹⁸O values of the lake water, which we attribute to two primary causes. First, variations in the δ¹⁸O_diatom values on the order of ~8‰ occur due to changes in basin hydrology. High Ca (calcite) concentrations in the VC-3 core between 379.52 and 385.31 kya indicate a closed basin environment, in which more evaporation resulted in greater oxygen isotope fractionation and higher δ¹⁸O values than during open basin conditions. Secondly, within both environments, variations in the δ¹⁸O_diatom values of 4 to 5‰ are attributed to changes in paleoprecipitation source and amount. We interpret these variations as changes in relative amount of summer (i.e. monsoon) and winter (i.e. westerly storm) precipitation. Under open basin conditions thermal stratification results in bottom temperatures that are nearly constant at 4ºC, which allows for the reconstruction of variations in paleowater δ¹⁸O values without the conflating influence of fluctuating water temperature and variable δ¹⁸O values that occur at the surface. MIS 11 is a potential pre-industrial analog to the present interglacial climate; therefore, such extreme variations in precipitation as recorded by the δ¹⁸O_diatom values in the VC-3 core have significant implications for present and future precipitation in the southwestern United States.