Temporal and spatial variations in signatures of sedimented organic matter in Lingding Bay (Pearl estuary), southern China

The concentration, accumulation rate, the ratio of total organic carbon to total nitrogen, and δ13C value of sedimented organic carbon from three 210Pb-dated sediment cores were studied in Lingding Bay, a large estuary of the Pearl River in southern China. It was found that terrestrial organic matter contributed significantly to the Lingding Bay sediments, and that hydrodynamic conditions greatly influenced the sources of organic matter, and hence carbon isotope distributions. Specifically, river-transported terrestrial organic matter is the main type of organic matter in core ZJ-6, located near the northwestern river outlets and overlaid mainly by flowing river water. Aquatic organic matter is the main input for the core ZJ-7 in the east of the bay, where the river current is weak. In core ZJ-8 in the southwest of the bay, where seawater and river water interact, organic matter is characterized by mixture of terrestrial and aquatic origin. TOC/TN behaviors vary with organic matter sources. In core ZJ-6, TOC/TN ratios are depressed due to considerable proportion of inorganic nitrogen, and samples with lowest TOC contents are the most depressed. In core ZJ-7 and ZJ-8, the preferential degradation of nitrogen-rich organic matter may result in misleading TOC/TN source signature. Sedimented organic matter had a low accumulation rate and stable δ13C before about the 1950s, but a high accumulation rate and variable δ13C since the 1950s. Rapid economic development in the region of Pearl River delta, including Hong Kong and Macao, since the 1950s may give rise to the change.