COLLABORATIVE RESEARCH: A Detailed Test of Cosmogenic Radionuclide Dating of Moraines: Pinedale and Bull Lake Moraines, Fremont Lake, Wyoming

点击翻译按钮获取中文摘要

This research is a detailed test of cosmogenic radionuclide exposure dating based principally on measurements of 1OBe, 26Al, and 14C in quartz from boulders which lie on moraine crest of known or accepted ages at the Pinedale Glaciation type locality, Fremont Lake, Wyoming. The purpose is to determine the limits, accuracy, precision, and resolution of the cosmogenic dating technique. We will adher to a stringent sampling strategy which minimizes errors due to high erosion rate (i.e. fire-induced surface spallation), pre-exposure, cosmic radiation shielding, unsuitable boulder surface geometries, and disturbed exposure, cosmic radiation shielding, unsuitable boulder surface geometries, and disturbed exposure histories due to redeposition or burial of the boulder. In the first year we will determine the reliability and precision of the technique to date a moraine by sampling multiple boulders on single moraine ridges, and multiple samples from selected boulders. Based on direct dating, regional correlations to dated stratigraphic equivalents, and correlation to the high resolution oxygen isotope chronostratigraphy they Pinedale (Stage 2) and Bull Lake (Stage 6) deposits can be aged bracketed at 14-30 ka and 130-160ka. We will date 30 samples from each of the Pinedale and Bull Lake terminal moraines at Fremont Lake. In the second year, armed with a measure of the method's precision, we will date samples from each of 6 nested Pinedale recessional moraines in order to determine its resolution. If the ages of the outermost to innermost moraines yield progressively younger ages this will imply that the technique will be able to directly measure rates to alpine or continental glacial retreat. We will also date 10 boulders from a suspect Stage 4 moraine which lies between the Pinedale and Bull Lake deposits in order to establish whether or not Stage 4 (Early Pinedale) deposits are preserved at the Pinedale type locality. Finally, we will make a useful cross check of dates produced from our method with dates using F. Phillips' 36C1 technique on duplicate samples from 10 boulders which yield the best dates from Year 1.