AIR PRESSURE AND COSMOGENIC ISOTOPE PRODUCTION
John O. Stone
Journal of Geophysical Research 105, 23,753-23,759. (2000)
Abstract
The cosmic ray flux increases at higher altitude as air pressure and
the shielding effect of the atmosphere decrease. Altitude-dependent scaling
factors are required to compensate for this effect in calculating cosmic
ray exposure ages. Scaling factors in current use assume a uniform relationship
between altitude and atmospheric pressure over the Earth's surface. This
masks regional differences in mean annual pressure and spatial variation
in cosmogenic isotope production rates. Outside Antarctica, air pressures
over land depart from the standard atmosphere by ±4.4 hPa (1 sigma)
near sea level, corresponding to offsets of ±3-4% in isotope production
rates. Greater offsets occur in regions of persistent high and low pressure
such as Siberia and Iceland, where conventional scaling factors predict
production rates in error by ±10%. The largest deviations occur
over Antarctica where ground level pressures are 20-40 hPa lower than the
standard atmosphere at all altitudes. Isotope production rates in Antarctica
are therefore 25-30% higher than values calculated by scaling Northern
Hemisphere production rates with conventional scaling factors. Exposure
ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides
at levels close to saturation, may be millions of years younger than published
estimates.