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

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 210 Pb analysis from eight lake sediment cores collected from the middle Yangtze reaches, southeast China. Unsupported 210 Pb activities generally declined exponentially with mass depth in the eight cores. The chronologies and sedimentation rates for the sediment cores were calculated using different 210 Pb-based mathematical models. The 137 Cs chronomarker (i.e. the 1963 fallout peak) and the spheroidal carbonaceous particle (SCP) chronomarker (i.e. the start of the rapid increase in 1970 AD) were selected to validate the 210 Pb 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 The SCP-corrected CRS (constant rate of supply) model performs better than other models, based on validation using historical documents in the Yangtze basin.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 137 Cs chronomarker might be erroneous due to catchment-driven 137 Cs inputs from soil erosion and post-depositional diffusion. Both SCPs and 137 Cs are susceptible to inputs from catchment soil erosion, but SCPs showno apparent degradation and post-depositional changes in lake sediments. The SCP profile provides a relatively reliable chronomarker, which can be used for validating 210 Pb chronologies in these floodplain lakes. Generally, sedimentation rates in the eight lakes were less than 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.

The rapid increases in microplastics in urban lake sediments

“…According to the SCPs-corrected CRS data (Fig. 2), the average deposition rate at the sampling site reached 0.95 cm•year −1 , which is generally consistent with the deposition rates in other lakes in the middle and lower reaches of the Yangtze River 36 . Yang et al 41 (2004) measured a deposition rate of 0.87 cm•year −1 in the same lake, but the sampling site of this study was closer to land than the sampling site investigated in Yang et al Therefore, the deposition rate of 0.95 cm•year −1 was reasonable.…”

“…The SCPs were also detected at the bottom of the sediment. Based on the history of coal power generation in the study area 36 , the bottom sediment was deposited after the 1950s 36 , and the result was consistent with the 137 Cs time marker. The bottom sediment was set to 1959, and the entire sedimentary column data were corrected based on this time marker.…”

“…However, this had caused great potential disturbances to lake sediments because of the frequent human activities such as dredging, prospecting, tunnel construction, etc., consequently, brought the uncertainty of date results. The CRS model assumes the sediment of lakes is mainly originate from the atmospheric deposition 36 . Although there is no river remittance in Donghu Lake, this assumption may be not corresponding with the sediment character of Donghu Lake with the consideration of frequently human activities, urban construction and expansion surrounding Donghu Lake.…”

“…SCPs are very stable in sediments and do not easily migrate. SCPs can be used to correct the age of the constant rate of supply (CRS) 36 . The experimental procedure followed the improved method provided by Rose 54,55 .…”

“…Thus, the 1963 137 Cs time marker 58 and the SCP time marker 59,60 were used to correct the CRS model. The calculation and correct method were provided by Chen et al (2019) 36 .…”