A locally calibrated, late glacial Be-10 production rate from a low-latitude, high-altitude site in the Peruvian Andes
Well-dated records of tropical glacier fluctuations are essential for developing hypotheses and testing proposed mechanisms for past climate changes. Since organic material for radiocarbon dating is typically scarce in low-latitude, high-altitude environments, surface exposure-age dating, based on the measurement of in situ produced cosmogenic nuclides, provides much of the chronologic information on tropical glacier moraines. Here, we present a locally calibrated Be-10 production rate for a low-latitude, high-altitude site near Quelccaya Ice Cap (similar to 13.95 degrees S, 70.89 degrees W, 4857 m asl) in the southeastern Peruvian Andes. Using an independent age (12.35 +0.2, -0.02 ka) of the late glacial Huancane Ha moraines based on thirty-four bracketing radiocarbon ages and twelve Be-10 concentrations of boulders on the moraines, we determine a local production rate of 43.28 +/- 2.69 atoms gram(-1) year (at g(-1) yr(-1)). Reference Be-10 production rates (i.e., production rates by neutron spallation appropriate for sea-level, high-latitude sites) range from 3.97 +/- 0.09 to 3.78 +/- 0.09 at g(-1) yr(-1), determined using scaling after Lal (1991) and Stone (2000) and depending on our assumed boulder surface erosion rate. Since our boulder surface erosion rate estimate is a minimum value, these reference production rates are also minimum values. A secondary control site on the Huancane Mb moraines suggests that the Be-10 production rates are at least as low as, or possibly lower than, those derived from the Huancane Ha moraines. These sea-level, high-latitude production rates are at least 11-15% lower than values derived using the traditional global calibration dataset, and they are also lower than those derived from the late glacial Breque moraine in the Cordillera Blanca of Peru. However, our sea-level, high-latitude production rates agree well with recently published, locally calibrated production rates from the Arctic, New Zealand, and Patagonia. The production rates presented here should be used to calculate Be-10 exposure ages in low-latitude, high-altitude locations, particularly in the tropical Andes, and should improve the ability to compare the results of studies using Be-10 exposure-age dating with other chronological data. (C) 2013 Elsevier B.V. All rights reserved.