SEDIMENT ACCUMULATION RATES QUANTIFIED WITH COSMOGENIC NEON-21 ON A GLACIAL MORAINE, BLOODY CANYON, CA

In this project, we demonstrate that cosmogenic neon-21 produced in quartz is effective in quantifying sedimentation rates in relatively young landforms with fairly high accumulation rates. On the east side of the Sierra Nevada in Bloody Canyon, CA, we dug a total of four soil pits at the toe of a Tahoe-aged moraine (c. 42-50 ka). From each pit, we collected a vertical series of bulk sediment samples. We purified quartz from these samples, and measured cosmogenic Ne-21 using noble gas mass spectrometry at the Berkeley Geochronology Center. The concentration of Ne-21 with depth in the soil pits depends on the amount of any Ne-21 inherited prior to deposition of the moraine, that which was accumulated during erosion and transport down the moraine, and the accumulation rate of sediment at the pit site. There is also a portion of Ne-21 that is created nucleogenically due to Uranium and Thorium in the quartz. We also measured Ne-21 from two pits dug at this site in 2005 for which we have Be-10 data. Comparing the Ne-21 concentrations to the Be-10 concentrations from these two older pits suggests that the nucleogenic portion of these measurements is 5-6 Matoms/gram-quartz. Preliminary results from the four pits dug in 2015 show that all four sites are accumulating sediment because the concentration of cosmogenic Ne-21 increases with depth at all four sites. Two pits do show profiles that could be vertically mixed, so these two accumulation rates are not well constrained. Preliminary analysis suggest that the sediment accumulation rates that best predict the measured concentrations of Ne-21 range between 5-20 cm/ka. The two pits measured along the same topographic contour show striking similar rates, and the three pits along a topographic profile show increasing accumulation rates downslope, which is expected for their locations. Our measurements of Ne-21 will be compared to model predictions for local and nonlocal sediment transport process, which predict varying rates of landscape evolution. These results demonstrate the effectiveness of cosmogenic Ne-21 as a tracer for determining sedimentation rates, which is useful because this isotope is both less expensive and requires less sample preparation than techniques that utilize cosmogenic isotopes like Be-10, Al-26, or Cl-36.