A quick and low-cost technique to identify layers associated with heavy rainfall in sediment archives during the Anthropocene
Résumé
Long-term records are needed to investigate the impact of extreme events inthe current framework of global change. Sedimentary reconstruction with ahigh resolution remains difficult without conducting expensive, destructiveand/or time-consuming analyses. In this study, high resolution CT-scan profiles (0·6 mm resolution) were used to investigate their potential for detecting flood deposits induced by heavy rainfall events. This method was applied to a sediment core dated with fallout radionuclides–covering 120 year period–collected in a pond draining a small forested catchment (French Massif Central–Central France). Between 1960 and 2017, 28 layers were deposited. Seventy-six percent of these deposits were correlated to the occurrence of heavy rainfall (>50 mm) recorded during one or two consecutive days. The remainder of the deposits detected with the Computer Tomo-graphy (CT) scanner (n=5) were not correlated to weather events. They mainly occurred in response to landscape management operations (for example, afforestation works as a result of the major 1999 storm). This period was indeed characterized by an increase in the delivery of137Cs-enriched sediment, demonstrating a greater topsoil contribution to sediment during majorforest management operations. The intensity of detrital layers has significantly decreased throughout time after a major land use change that tookplace in 1948 and land abandonment. The frequency of heavy rainfall anassociated detrital deposits has nevertheless increased by 60% and 75%,respectively, between the years 1960 and 2017. These results outline the potential of CT-scan for reconstructing long-term flood deposits associated with heavy precipitation.
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