INTERPRETING OLD SURFACE EXPOSURE AGES IN GLACIAL VALLEYS: AN EXAMPLE FROM THE PERUVIAN ANDES

Surface exposure dating using in situ cosmogenic radionuclides provides a long-awaited means of directly dating glacial deposits that predate the last glacial cycle. Although the potential benefits of longer chronologies are obvious, the greater uncertainties associated with older cosmogenic ages may be less readily apparent. We illustrate the challenges of developing and interpreting a long chronology using our data from the Peruvian Andes. We used the cosmogenic radionuclides (CRNs) ¹⁰Be and ²⁶Al to date 140 boulders on moraines in valleys bordering the Junin Plain (11° S 76° W) in central Peru. Our chronology spans multiple glacial cycles and includes several zero-erosion exposure ages greater than 1 million years. The presence of surfaces with such old exposure ages indicates that long-term rates of boulder erosion are relatively low (on the order of 0.3-0.5 meters per million years). Interpreting the chronology of moraines for glaciations that predate the last glacial cycle is complicated by the need to consider loss of CRNs through boulder erosion, change in effective production rate through time due to surface uplift in this tectonically active region, introduction of younger surfaces through boulder exhumation resulting from moraine degradation, and inheritance of CRNs from previous exposure intervals. As an illustration, we recalculate exposure ages using our boulder erosion rates and published surface uplift rates, both of which increase the exposure ages. For example, a zero-erosion exposure age of 400,000 years increases by about 20% when boulder erosion and surface uplift are included in the calculations. Our results serve to emphasize both the challenges involved in interpreting old surface exposure ages and the rewards of having chronological data, even with uncertainties, when reconstructing the paleoclimate of a region.