Erosion, weathering, and sedimentation

Abstract

This chapter explains how a variety of nuclides have been applied to catchments throughout the world. One of the most exciting new approaches for quantifying the rate at which catchments erode is the measurement of in situ produced cosmogenic nuclides. The commonly applied nuclides for erosion rate measurements are 3 He, 10 Be, 26 A1, and 36 C1. Use of such nuclides was restricted to determining denudation rates of exposed bedrock outcrops. It appears that samples have generally been collected from outcrops standing above the surrounding landscape. These protrusions of bedrock may erode more slowly because they shed water rapidly, thus reducing the efficacy of chemical weathering. Geomorphologists have used sediment deposits or suspended particle loads in rivers to evaluate erosive processes in catchments. Linking this approach to fluxes of anthropogenic radionuclides such as 137 Cs or natural 210 Pb opens up a new avenue for understanding processes such as particle formation via weathering, soil formation, denudation, transport, and sedimentation. There are some factors that must be considered when attempting to use Sr isotopes to identify solute sources or quantify mineral weathering rates or processes at the catchment scale. First, 87 Sr/ 86 Sr observed in streamflow probably does not reflect current weathering in the catchment but rather a partial integration of the weathering history and the evolution of the cation exchange pool. Second, Sr release must be distinguished from mineral dissolution as a bulk mass transfer process in cases where Sr may be preferentially lost from the mineral relative to more tightly bound cations. Third, an understanding of the emplacement history of catchment soil substrates, for example, moraine vs. alluvium; residuum vs. colluvium; fractured vs. massive bedrock, may be critical to confirming the weathering reactions inferred from the Sr isotopes.