Reconstruction of sedimentation changes under anthropogenic influence in a medium-scale estuary based on a decadal chronological framework

Abstract

The present study uses sedimentary records from the Yalu River Estuary (YRE) over the past five decades to establish a regional age framework and investigate the influence of anthropogenic activity. Ten sediment core samples collected from various locations in the estuary were analysed. Core-averaged sediment accumulation rates calculated based on the 210Pb and 137Cs profiles of these samples were in the range 1.09–2.83 and 0.94–3.11, respectively. Accordingly, a regional age framework with major environmental events was established assuming the deposition rate in a sub-section was constant. The age framework showed that most of the sediments in the YRE were deposited from the 1960s, and a grain size analysis also evidenced that the sedimentary environment experienced dramatic change after 1963. Changes in sedimentary environment in the estuary due to human activity (e.g. dam construction, deforestation and large mining activities) were also discussed: (1) the mean grain size at sites near the main channel became finer after 1963 due to the contemporaneous reduction in sediment discharge caused by upstream reservoir construction; (2) abnormal 137Cs signals in the records during the 1980s probably reflected a concurrent deforestation, and the 137Cs signals in the 1990s were related to mixing of sediments caused by mining activities. Results of this study demonstrate that sedimentation processes in a medium-scale estuary can be greatly changed under anthropogenic influence. The feasibility of using 137Cs time markers (especially auxiliary time markers in 1975 and 1986) in a dynamic estuary is confirmed in this study. Furthermore, the regional age framework established in this study can provide a chronological reference for future studies in the YRE and adjacent areas.