Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models

Abstract

A number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates [1]. The in situ radionuclides can be used for a high-resolution tomography of the erosional history of an exposed surface; two stable nuclides (³He, ²¹Ne) and five radionuclides (¹⁰Be, ²⁶Al, ³⁶Cl, ¹⁴C, ³⁹Ar) having half-lives in the range of ~300-1.5 × 10⁶ yr half-life are measurable in many rock types. A prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions; altitude, latitude, irradiation geometry and shielding. Relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons [2]. Absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible, as was done in the case of several cosmic ray produced isotopes in the atmosphere [3]. Based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for ~11,000 yrs were sampled, the absolute production rates of ¹⁰Be and ²⁶Al in quartz have been accurately estimated for mountain altitudes in Sierra Nevada [4]. The absolute production rates of ¹⁰Be and ²⁶Al in quartz can therefore be estimated fairly accurately for any given latitude and altitude. Some measurements of ¹⁴C in rocks of low erosion rate [5] similarly allow an estimate of its production rate. Attempts made to measure the in situ production rates of ³He in rocks have not yet led to a convergent production rate. In view of the importance of knowing the production rates of isotopes of He, Ne and Ar, I present here theoretical estimates of their production rates based on available cross-section data. I discuss the information that can be extracted from the study of the in situ nuclides in rocks. Useful parameters characterizing the exposure history of a rock surface are: (1) the effective surface exposure age; and (2) the time-averaged erosion rate. The implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered.