CLIMATE INFLUENCE ON ATMOSPHERIC SCALING OF COSMOGENIC NUCLIDE PRODUCTION IN ROCKS

This work examines the influence in cosmogenic nuclide production rates due to changes in the atmospheric density distribution during a glaciation. The normal formulations used to adjust production rates for spatial differences due to geomagnetic latitude and atmospheric depth use a standard atmosphere model. J. Stone (2000, JGR, 102, 23752-23759) showed that for exposure ages requiring more precise scaling adjustments it is necessary to account for quasi-stationary atmospheric pressure anomalies as revealed from 30 years of averaged barometry. In places such as Antarctica, scaled production rates calculated using the standard atmosphere model deviate more than 5% from Stone's atmospheric model. However, to determine production rates for samples with exposure durations that include one or more glaciations, scaling should include the effects of both a redistribution of atmospheric pressure and the compression of atmosphere due to cooling. Because the magnitude of these deviations varies spatially and temporally, we calculate the production rates as a function of atmospheric pressure derived from a CCM3 simulation that reproduces Stone's present day measured global pressure records within 1%. To calculate the influence of production rates due to climate change at 20 ka and determine the percentage deviation from the modern-day scaled production rates, we use a paleoclimate simulation that incorporates the University of Maine Ice Sheet Model global ice cover. For sea level sites away from paleo-glaciers, the effects will be minimum. Without considering atmosphere compression, production rates for surfaces with 20 kyr of exposure at high altitude (3 km) overestimate the production rate by >3%. For longer durations, the effect will be greater. Places near large ice volume changes show higher sensitivity, particularly where high katabatic winds develop. This climate effect on atmospheric shielding may explain some of the small remaining disparities in production rate calibrations from sites at different altitudes and proximity to paleo-ice sheet margins.