Determination of geochronology and sedimentation rates of shallow lakes in the middle Yangtze reaches using Pb-210, Cs-137 and spheroidal carbonaceous particles

摘要

Accurate chronologies for recent sediments of shallow lakes in the Yangtze floodplain are critical to calibrate proxy records for reconstructing environmental changes during the past century. This study presents the results of detailed Pb-210 analysis from eight lake sediment cores collected from the middle Yangtze reaches, southeast China. Unsupported Pb-210 activities generally declined exponentially with mass depth in the eight cores. The chronologies and sedimentation rates for the sediment cores were calculated using different Pb-210-based mathematical models. The Cs-137 chronomarker (i.e. the 1963 fallout peak) and the spheroidal carbonaceous particle (SCP) chronomarker (i.e. the start of the rapid increase in 1970 CE) were selected to validate the Pb-210 dating. Sedimentation rates derived from different models were validated using historical data including lake area, arable land area, sediment discharge and reservoir volume in the Yangtze basin. The SCP-corrected CRS (constant rate of supply) model performs better than other models, based on validation using historical documents in the Yangtze basin. The Cs-137 chronomarker might be erroneous due to catchment-driven Cs-137 inputs from soil erosion and post-depositional diffusion. Both SCPs and Cs-137 are susceptible to inputs from catchment soil erosion, but SCPs show no apparent degradation and post-depositional changes in lake sediments. The SCP profile provides a relatively reliable chronomarker, which can be used for validating Pb-210 chronologies in these floodplain lakes. Generally, sedimentation rates in the eight lakes were & 0.2 g cm(-2) yr(-1) before the 1930s, and then increased to a peak in the 1960s. Afterwards, sedimentation rates decreased and remained low after the 1980s.