Changes in sedimentary environment based on the geochemical composition of bottom sediments in Lake Shinji, southwest Japan

Abstract

The geochemical composition of bottom sediments collected in 2016 from Lake Shinji, southwest Japan, was examined to evaluate the sedimentary environment. The distribution of elevated levels of Zn, MnO and P₂O₅ show considerable accumulation of these elements in the lake. This data set was compared with published measurements of samples collected in 2006 to document changes in the lake environments and input by human activities. Zn, Pb and P₂O₅ contents are similar in 2016 and 2006 samples based on variation ranges and mean values in box plots. Considerable change can be observed in MnO and TS (total sulfur) content suggesting reducing conditions in bottom sediments. Cu content also shows increased values in 2016 samples, compared to 2006, which is suggestive of relatively reducing conditions, lowered sedimentation rates, and inputs from Lake Nakaumi. This is due to higher Cu content in the bottom sediments of Lake Nakaumi related to historical Cu mining in the Nakaumi catchment. This history can be seen in the variation of Cu content in cores from Lake Nakaumi.