Provenance of loess in the central Great Plains, U.S.A. based on Nd-Sr isotopic composition, and paleoenvironmental implications

Abstract

Loess of the central Great Plains, U.S.A., records intervals of Quaternary aeolian dust accumulation at rates among the highest known worldwide. This study used Nd-Sr isotopic analysis to investigate the provenance of Middle Pleistocene to Holocene loess in western and central Nebraska, U.S.A., essential information for understanding the paleoenvironmental conditions that allowed such rapid accumulation to occur at some times and not others. Nd-Sr isotopic data suggest that dust from unglaciated Great Plains landscapes has been the primary component of loess accumulated at both very high and low rates since the Middle Pleistocene. However, loess isotopic compositions all require an additional minor source with higher 87Sr/86Sr and lower εNd, most likely debris from Precambrian rocks in the Rocky Mountains, carried to the Great Plains by the Platte River system. The contribution from this secondary source—probably including glacially eroded sediment—was greater in Marine Isotope Stage (MIS) 3 and early MIS 2, but decreased somewhat just after the last glacial maximum (LGM) at the Bignell Hill section. Earlier research showed that the highest accumulation rates in that section also occurred after the LGM; thus our results likely indicate increased dust emission from unglaciated landscapes at that time. Greater plant moisture stress in a warming climate with still-low CO2 could have played a role in that increased dust production, along with lagged vegetation response; however, a strengthened westerly component of surface winds is the most straightforward explanation.