Geochemical sources, deposition and enrichment of heavy metals in short sediment cores from the Pearl River Estuary, Southern China

The Pearl River Estuary (PRE) is the largest estuary in the Southern China. Four short sediment cores (45–60 cm long) recovered within the PECAI project from the estuary have been analyzed for grain size and geochemistry to reveal geochemical source and deposition environment and to assess enrichment and pollution of heavy metals in the sediments. Four geochemical sources were identified as lithogenic, marine biogenic, anthropogenic and reductive deposit by the correlation analysis and principal component analysis. The representative elements and related elemental ratio reflected different depositional environments in the four cores, which are controlled by the hydrodynamic conditions in the PRE. Core IOW300 190 at the mid West Shoal maintained a constant sub-oxic environment with abundant fluvial deposits; core IOW300 480 at the mouth of the estuary formed an oxic environment influenced by the strong two-layer currents, while two cores (IOW300 020 and IOW300 630) at the East Shoal experienced an evident change from marine anoxic condition to a weak anoxic or a sub-oxic condition caused by increasing riverine discharges. According to the enrichment factors (EFs) of heavy metals (Co, Cr, Cu, Hg, Ni, Pb and Zn), there was no significant metal enrichment and contamination in the core sediments in spite of that some metals increased and was partly enriched in the uppermost or surface sediments. Hg was enriched in all four cores due to its non-point sources; enrichment of Cu and Pb in the core IOW300 020 was attributed to the proximity with the point sources of the pollutants. Core IOW300 630, with a constant sedimentation rate of 0.45 cm/a by Cs137 and Pb210 dating, exhibits that heavy metals began to increase in the 1960s and increased progressively in the 1980s and 1990s, responding to the rapid economic development in the Pearl River delta region in the last three decades.