MORAINE DEGRADATION RATES QUANTIFIED WITH COSMOGENIC BE-10 FROM SOIL PROFILES IN BLOODY CANYON, CA

We quantified the amount and rate of moraine degradation from two moraines in the eastern Sierra Nevada, CA, by analyzing the concentration of in-situ cosmogenic Be-10 from bulk sediment collected in vertical profiles from hand-dug soil pits. Glacial moraines are frequently dated to establish the timing of glaciations. However, it is clear that moraine degradation through geomorphic processes complicates our ability to accurately date these surfaces. Hillslope models suggest that moraines evolve through time as slope processes move sediment from the crests of moraines and redeposit this material at the bases of the moraine slopes. This results in the crest lowering and the toe growing, with both features becoming more rounded over time. Most models suggest that moraines change quickly at first, with the rates of change slowing down over time as the slopes and curvature become more relaxed. To verify these model predictions, we analyzed the concentration of in-situ produced Be-10 from bulk sediment samples collected from two moraines with different ages in CA. The two study sites are the Tioga 3 (ca. 18 ka) and the Mono Basin moraine (ca. 92,000-141,000 ka) in Bloody Canyon, CA. On each moraine, we dug soil pits on the crest, the mid-point, and the toe of the moraine profile. For each pit, we collected and analyzed three sediment samples collected in a vertical profile: one from the surface (1-4 cm), one as deep as we could dig the pit (65-120 cm), and one intermediate sample (30-60 cm). From the bulk sediment sample, quartz minerals were isolated from the 300-500 um size fraction and analyzed for the concentration of cosmogenic Be-10. From the concentration of Be-10 in these depth profiles we can quantify the erosion and burial rate of sediment on these moraines and extrapolate a total amount of erosion and accumulation. Preliminary results for the Tioga 3 moraine indicate that the crest is lowering at a rate of 5.0 cm/ka, while the toe is accumulating sediment at a rate of 5.6 cm/ka. On the older Mono Basin moraine, the crest is lowering at a rate of 3.2 cm/ka while the toe is accumulating sediment at a rate of 1.7 cm/ka. These results are consistent with the main ideas captured by hillslope models that the crests are lowering while the toes are growing, and that the rates of change are faster initially and do slow down over time.