Quantification of Long-term Rates of Bedrock Weathering and Soil Production Using Terrestrial Cosmogenic Nuclides: Principles, Methodology, Current Research Status, and Perspectives

Abstract

This paper reviews the methodology and applications of terrestrial cosmogenic nuclides as a tool for quantifying rates of geomorphic processes. The review starts from systematics in the production of cosmogenic Be-10 and Al-26 in quartz, and Cl-36 in calcite, and then describes the basic modeling of the accumulation of those nuclides under varying denudation rates. Procedures for sample preparation and nuclide measurement using accelerator mass spectrometry are also summarized. Recent research reveals denudation rates of bare rock surfaces for both silicates and carbonates, as well as soil production rates from saprolite beneath the soil layer on hillslopes. The empirical formulation of soil production rates as a function of soil thickness enables us to test hypothetical transport laws of soil particles through a combined analysis with topographic parameters of hillslopes. Chemical processes contributing to soil production and denudation have been quantified with a coupled approach using cosmogenic nuclide analysis and geochemical mass balance method. However, linkages across climate conditions, element leaching, and denudation rates are still debated because of timescale discrepancies between soil and saprolite formation. Climate seems to affect soil production indirectly by reducing the mechanical strength of saprolite resulting from chemical weathering of bedrock. A theoretical framework is presented for modeling saprolite weakening and denudation, which connects bedrock weathering, erodibility of uppermost saprolite, soil production and transport with steady-state topography of hill-noses.