Denudation rates of carbonate pinnacles in Japanese karst areas: Estimates from cosmogenic ~(36)Cl in calcite

摘要

This study attempted to quantify long-term subaerial denudation of bare carbonate rock surfaces by using in situ-produced cosmogenic ~(36)Cl in calcite. Carbonate samples were collected from the topmost 5 cm of exposed pinnacles at several non-glaciated karst areas in subtropical to subarctic regions in Japan. Concentrations of natural Cl and ~(36)Cl in calcite were determined by accelerator mass spectrometry with isotope dilution with ~(36)Cl-enriched carrier spike. The nuclide concentrations were of the order of 10~5-10~6 atom g~(-1), and converted to total denudation rates averaged over a 10~5 yr-timescale of chemical and physical processes acting on the karst surfaces. The denudation rates were 20-60 mm kyr~(-1), being lowest in the central part of Japan. This tendency indicates the alternation of relative importance of chemical (dissolution) and physical (frost shattering) processes in denudation, depending on site-specific factors such as mean temperature, annual precipitation, and freeze-thawing intensity.