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The valley of Bishop Creek, which drains part of the eastern flank of the Sierra Nevada, California, contains an unusually well-preserved set of middle to late Quaternary moraines. These deposits have been mapped by previous investigators, but they have not been quantitatively dated. We used the accumulation of cosmogeulc 36C1 to assign a chronology to the maximal glacial positions mapped in the valley. Our results indicate that the terminal moraines mapped by previous investigators as Tahoe were all deposited between ca. 165 and ca. 135 ka, during marine isotope stage (MIS) 6. Moraines mapped as Tioga were deposited between 28 and 14 ka, during MIS 2. These can be subdivided into Tioga 1 (28-24 ka), Tioga 3 (18.5-17.0 ka), and Tioga 4 (16.0-14.5 ka) advances (no moraines dated to Tioga 2 [21-19 ka] were found, presumably because the Tioga 3 advance either overrode or fluvially eroded them). At 15.0-14.5 ka, the Tioga 4 glacier retreated abruptly to the crest of the range. This was followed by the brief and fairly minor Recess Peak advance at ca. 13.4 ka. No Holocene advances extended beyond the very restricted limits of ice during the Matthes (Little Ice Age) advance. All preserved terminal moraines at lower elevations were deposited during either the Tahoe or Tioga stades. The Tahoe terminal moraines are extensive and voluminous, whereas the Tioga moraines are relatively narrow and have small volumes. However, this notable difference may be more a result of idiosyncrasies in the local glacial history than the result of differences in the length or intensity of glaciation between the two glacial episodes. The history of glacial advances at Bishop Creek exhibits a strong correspondence to global climate cycles, and to paleoclimate events in the North Atlantic in particular.

Knowledge of the production history of cosmogenic nuclides, which is needed for geological and archaeological dating, has been uncertain. Measurements of chlorine-36/chlorine ([sup.36.Cl]/Cl) ratios in fossil packrat middens from Nevada that are radiocarbon-dated between about 38 thousand years ago (ka) and the present showed that [sup.36.Cl]/Cl ratios were higher by a factor of about 2 before ~11 ka. This raises the possibility that cosmogenic production rates just before the close of the Pleistocene were up to 50% higher than is suggested by carbon-14 calibration data. The discrepancy could be explained by addition of low-carbon-14 carbon dioxide to the atmosphere during that period, which would have depressed atmospheric radiocarbon activity. Alternatively, climatic effects on [sup.36.Cl] deposition may have enhanced the [sup.36.Cl]/Cl ratios., Cosmogenic nuclides, including [sup.14.C], [sup.36.Cl], [sup.10.Be], [sup.26.Al], and [sup.3.He], are widely used for geological dating(1), for studying geomorphic processes(2), and as geochemical tracers(3). An understanding of how the production rates [...]

The geologic record of mountain glaciations is a robust indicator of terrestrial paleoclimate change. During the last glaciation, mountain ranges across the western U.S. hosted glaciers while the Cordilleran and Laurentide ice sheets flowed to the west and east of the continental divide, respectively. Records detailing the chronologies and paleoclimate significance of these ice advances have been developed for many sites across North America. However, relatively few glacial records have been developed for mountain glaciers in the northern Rocky Mountains near ice sheet margins. Here, we report cosmogenic beryllium-10 surface exposure ages and numerical glacier modeling results showing that mountain glaciers in the northern Rockies abandoned terminal moraines after the end of the Last Glacial Maximum around 17-18 ka and could have been sustained by -10 to -8.5°C temperature depressions relative to modern assuming similar or drier than modern precipitation. Additionally, we present a deglacial chronology from the northern Rocky Mountains that indicates while there is considerable variability in initial moraine abandonment ages across the Rocky Mountains, the pace of subsequent ice retreat through the Lateglacial exhibits some regional coherence. Our results provide insight on potential regional mechanisms driving the initiation of and sustained deglaciation in the western U.S. including rising atmospheric CO2 and ice sheet collapse. [ABSTRACT FROM AUTHOR]