Abstract
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 210Pb analysis from eight lake sediment cores collected from the middle Yangtze reaches, southeast China. Unsupported 210Pb activities generally declined exponentially with mass depth in the eight cores. The chronologies and sedimentation rates for the sediment cores were calculated using different 210Pb-based mathematical models. The 137Cs 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 210Pb 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 The SCP-corrected CRS (constant rate of supply) model performs better than other models, based on validation using historical documents in the Yangtze basin.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 137Cs chronomarker might be erroneous due to catchment-driven 137Cs inputs from soil erosion and post-depositional diffusion. Both SCPs and 137Cs 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 210Pb 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.
Highlights
Dated lacustrine sediments are crucial archives of past environmental changes, in rapidly developing countries where long-term monitoring data are limited
The constant rate of supply model (CRS) assumes a constant flux of 210Pbex to sediment surface and that the initial 210Pbex concentration is inversely proportional to the sedimentation rate (Appleby and Oldfield, 1978; Córdoba et al, 2017; Oldfield et al, 1978)
This study describes the establishment of chronologies for short sediment cores collected from eight shallow lakes of the middle Yangtze reaches
Summary
Dated lacustrine sediments are crucial archives of past environmental changes, in rapidly developing countries where long-term monitoring data are limited. The constant rate of supply model (CRS) assumes a constant flux of 210Pbex to sediment surface and that the initial 210Pbex concentration is inversely proportional to the sedimentation rate (Appleby and Oldfield, 1978; Córdoba et al, 2017; Oldfield et al, 1978). The assumptions and prerequisites of the commonly used models (e.g. constant initial concentration of 210Pbex and constant sedimentation rate) are not always fulfilled in nature, and different models may produce inconsistent dating results and be more suited to particular environmental circumstances (Baskaran et al, 2015; Córdoba et al, 2017; Lan et al, 2018; Mabit et al, 2014; Tylmann et al, 2013, 2016; von Gunten et al, 2009)
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