High-resolution speleothem record of climate variability during the late Pleistocene from Spring Valley Caverns, Minnesota

Abstract

Understanding how the Earth's oceanic and atmospheric systems responded to abrupt climatic forcings in the past is crucial in determining potential effects of anthropogenic climate change. This is of particular importance in the mid-continental United States, an agricultural hub that produces much of the world's corn and soybeans. High resolution paleorecords in the region remain sparse, restricting the predictability of global climate models and limiting our knowledge of atmospheric teleconnections across North America. This study analyses the growth and stable isotopic concentration of late Pleistocene speleothem samples from Spring Valley Caverns, located in SE Minnesota. Timing of growth for six samples was determined using 230Th dating and confocal microscopy, recording ages between 114 - 29 ky BP. This places all sample growth within the last glacial period. Sample growth largely correlates with warm conditions in the North Atlantic, indicating elevated temperature and moisture availability. Growth during MIS4 may be due to enhanced anti-cyclonic activity over the Laurentide Ice Sheet, resulting in anomalous southerly moisture transport. High resolution stable isotope analysis was completed for two samples, showing both long term trends and short term variability. Samples show a steady decrease in d13C values from 104 - 29 ky BP, suggesting a transition from C4 to C3 dominated ecosystems consistent with global cooling. However, d18O values climb steadily throughout this period. This is indicative of drier conditions and an increase in Gulf of Mexico sourced moisture in the region. Short term variability from 64 - 44 ky BP shows impressive correlation to global paleorecords. North Atlantic cold events are shown as an increase in d18O values, and North Atlantic warm events as a decrease d18O values. As d18O and d13C generally covary during this period, elevated d18O values suggest regional aridity. The transport of Gulf of Mexico summer moisture into the region appears significantly susceptible to changes in North Atlantic climate, most likely through large scale atmospheric systems such as the North Atlantic Oscillation and Pacific-North America teleconnection. The results of these records agree with a previous study of Spring Valley Caverns from the Holocene (Dasgupta, 2008). This signifies that large scale dynamics present during the last glacial period continue to affect regional climate, and thus these results may prove useful not only in improving our understanding of abrupt climate events during the late Pleistocene but in predicting impacts on the region from anthropogenic warming.