Abstract

Over the last two decades, cosmogenic nuclides have yielded a wealth of quantitative information about erosion in diverse landscapes. Although applications of cosmogenic nuclides are now becoming routine, some of their limitations and complications have not been fully explored. Here we elaborate on the consequences of chemical erosion on the buildup of cosmogenic nuclides in saprolite, soils, and stream sediment. Chemical erosion near the surface (in soil) complicates interpretation of cosmogenic nuclides to the extent that it preferentially enriches (or depletes) minerals harboring cosmogenic nuclides. For quartz, the target mineral of choice in the vast majority of erosion rate studies, selective dissolution of relatively soluble minerals leads to enrichment in soils, because the overall erosion rate of quartz is lower than the soil, on average. Thus, 10Be, 26Al and 21Ne concentrations in quartz are not representative of erosion of the soil as a whole unless differential chemical erosion is taken into account. Chemical erosion introduces an additional complication if it occurs below the depths of cosmic ray penetration and thus does not influence the buildup of nuclides in minerals. Although the complications of chemical erosion have been recognized in the literature, their effects have rarely been taken into account in cosmogenic nuclide measurements of landscape erosion. Here we derive a general expression, termed the “chemical erosion factor” or CEF, which corrects for biases introduced by chemical erosion in saprolite and differential weathering of target minerals in soil. The CEF differs from the “quartz enrichment factor” of previous work in that it is not specific to quartz; it can also be applied to relatively soluble minerals, including olivine and magnetite. Using data from diverse climatic settings, we calculate CEFs ranging from 1.04 to 1.93 for cosmogenic nuclides in quartz. This implies that biases introduced by chemical erosion can be as large as a factor of two in intensely weathered soils and saprolite. We suggest that quantifying the CEF is crucial to accurate cosmogenic nuclide measurements of erosion in many landscapes.

Citation

Riebe, C.S., Granger, D.E. (2011): Effects of Chemical Erosion on Cosmogenic Nuclide Buildup in Soils, Saprolite and Sediment. Fall meeting, American Geophysical Union, December 2011. Abstract EP41B-0606. .