Variations in trace metal and total organic carbon concentrations in marine sediments from Hong Kong

Abstract

Four marine cores to depth of 2.8 m, from the Urmston Road‐Brothers Channel, Hong Kong, have been analyzed for the trace metals Cu, Zn, Cr, Ni, Mn and Pb, as well as total organic carbon and organochlorines. Most of these elements indicate enhanced concentrations due to anthropogenic inputs in the upper part of the sediment column, with an overall decrease with depth. The concentrations of these elements are lower than in surface sediment from offshore areas near the industrial and urban heart of Hong Kong. Mn and Zn appear to be distributed most evenly through the cores. However, the concentrations of the metals and total organic carbon content in sea bed samples are commonly lower than the adjacent underlying samples. This may be due to deposition of unpolluted sediment disturbed by dredging, diagenetic effects or through the sampling process. Although rank order of metal abundance is similar across cores, correlations between the metals are weak; the exceptions are chromium and copper. Concentrations for the lower part of the cores are indicative of the natural background levels for these marine sediments. The total organic carbon (TOC) content of the sediment lies between 0.5% and 0.9% with a general decrease with depth; TOC correlates most strongly with chromium and copper. Analysis of two organochlorine tracers of agricultural contamination indicated levels below the stated detection limits. Trace metal concentrations were slightly higher in the silt and clay fraction compared with the bulk sediment, but some mobilization of the analyte cations apparently occurred during the sieving process.