Development of a high-resolution record of Great Basin climate change during MIS 5, 6, and 7

As evinced by the cycles of the large paleolakes Lahontan and Bonneville, the currently semi-arid to arid Great Basin region of the United States has experienced significant changes in climate and water balance in the past. Although there has been substantial research concerning these changes for times around and since the last glacial maximum, relatively little is known about the region's previous climate and water balance history. There is a clear need for a long-term record for earlier glacial and interglacial periods. Here, we present some of our initial results from Lehman Cave (39.01°N, 114.22°W), a well-decorated, active cave located on the edge of the Bonneville Basin in the Great Basin National Park, for times correlating with large portions of Marine Oxygen Isotope Stages (MIS) 5, 6, and 7. We initially surveyed a suite of speleothems to obtain times and durations of growth phases to aid in choosing samples appropriate for more extensive analysis. Approximately eighty 2 to 10 mg samples with a mean ²³⁸U of 400 ppb, representing most of the major growth phases of this suite of 20 speleothems, were analyzed for preliminary uranium-thorium dates. A subset of eight of these stalagmites grew collectively over large portions of MIS 5 and 6 (an interval that includes the Little Valley lake cycle of the Bonneville Basin) as well as a substantial portion of MIS 7. This record includes several significant periods of contemporaneous growth, including: 81.5 to 103 ka, which corresponds to the interval between Dansgaard-Oeschger events 21 and 23; 204 to 207 ka; and 118 to 132 ka, an interval including the beginning of the last interglacial period and the end of Termination II and Heinrich Stadial 11. We have yet to identify growth phases between 103 to 118 ka, 134 to 137 ka, and 164 to 169 ka. Initial stable isotope results indicate a shift of approximately +3‰ δ¹⁸O and +5‰ δ¹³C around 131 × 2.5 ka, which agrees within error with the findings of Shakun et al. (2011) of a T-II consistent with the Milankovitch orbital theory of climate change. Detailed confirmation of this shift awaits higher resolution dating and stable isotope analysis. Additionally, for two of the samples so far, we have discovered fluorescent banding observable with confocal microscopy. These bands, which range in width from approximately 5 to 100 μm, may be annual in nature. If this is so, counting and characterization of these bands may represent an opportunity to investigate the timing and nature of past climate events in the Great Basin on an annual to decadal scale. Shakun, J. D. et al. (2011). Milankovitch-paced Termination II in a Nevada speleothem? Geophysical Research Letters, 38(18), L18701.