Novel Insights to the Factors Controlling Monsoon Precipitation in Southwestern US From a mid-Pleistocene Lake Sediment Record, Valles Caldera, Northern New Mexico

Monsoon precipitation is vital for the arid regions of Southwestern United States, but currently there are insufficient quantitative paleoclimate data for parameterizing climate models. We studied an 82-m lacustrine sediment core from Valles Caldera, Northern New Mexico. The basal age of the core is 552 ±3 kyr determined by dating of a tephra layer. Multiple proxies including, TOC, δ13C, C/N ratios, magnetic susceptibility and pollen taxa revealed three glacial and two interglacial periods, spanning Marine Isotope Stages (MIS) 10 through 14. In order to quantitatively elucidate the hydrological history of the region during these stages, we measured the concentrations and δD values of higher plant leaf waxes throughout the core. δD values of leaf waxes show pronounced glacial-interglacial variations throughout the core, with interglacials (MIS11 & 13) showing 40-60 permil higher values than glacials. We interpret such systematic hydrogen isotopic changes as changes in δD values of summer precipitation. The higher values during the interglacials indicate reduced summer monsoon intensity as a result of precipitation amount effect. Given that winter precipitation would also have been lower during interglacials as a result of the retreat of the jet stream northward, interglacial periods must be characterized by significantly drier conditions than glacial periods throughout the seasonal cycles. Superimposed on the glacial-interglacial hydrogen isotopic changes are distinctive ~40 permil cyclic fluctuations of δD values, with a periodicity similar to the precessional cycles, suggesting solar insolation as an important driver for summer monsoon intensity in the Southwestern United States.