MULTIPLE ISOTOPE COSMOGENIC PALEOALTIMETRY

A theoretical cosmogenic paleoaltimeter has been developed which makes use of the exponential decay of cosmic ray flux in both atmosphere and rock. The ratio of concentrations of two cosmogenic isotopes produced by cosmic ray particles with different attentuation pathlengths can, under sufficiently general conditions, be independent of erosion and changes in the primary cosmic ray flux rate. Additionally, burial of the paleosurface by deposition can be modeled with the addition of a third isotope. One of the principal difficulties in successfully designing such a paleoaltimeter lies in finding a pair or trio of suitable isotopes whose cosmogenic components are both measurable and distinguishable from any non-cosmogenic components. Several stable or long-lived radioactive cosmogenic isotopes, such as ²¹Ne and ³He, are produced primarily through neutron spallation, with abundant information available about their production mechanisms and terrestrial cosmogenic production rates. However, studies of terrestrial production of isotopes from other cosmic ray particles are extremely limited. Recent experimental evidence (Renne et al., 2001) indicates that both ³⁸Ar and ³⁶Ar are produced cosmogenically, and theoretical consideration of muon-induced isotopic production of ³⁸Ar, produced from muon capture in K or Ca, indicates muon capture is significant. For instance, the sea level ³⁸Ar production rates in sanidine and fluorite from muons are theoretically calculated at ~10 ³⁸Ar atoms g (KAlSi3O8)^-1 yr^-1 and ~20 ³⁸Ar atoms g (CaF2)^-1 yr^-1, suggesting significant production in exposed surfaces. The experimental and theoretical support for the existence of in situ cosmogenic neutron- and muon-produced 36,38Ar, as well as the wide geological applicability of the multiple isotope technique, suggests that this method has the potential to become both a viable and widely-used paleoaltimeter.