Environmental changes in the Pearl River Estuary (China) as reflected by light stable isotopes and organic contaminants

Abstract

A dated surface sediment core and surface sediment samples from the Pearl River Estuary (PRE) were analyzed for the spatial distribution and the temporal evolution of light stable isotope (C, N, and S) signals, besides organic contaminants (PAHs, DDTs, HCHs and PCBs). The sediment core was recovered from about 6.5 m water depth and was deposited with a sedimentation rate of about 4.5 mm/a. The stable carbon ( 13C/ 12C) and nitrogen ( 15N/ 14N) isotope signatures of organic carbon and total nitrogen, and the sulfur ( 34S/ 32S) isotope signature of reduced inorganic sulfur (TRIS) have been analyzed by combustion stable isotope-ratio-monitoring mass spectrometry to follow changes in the sources of organic matter and changes in the corresponding sedimentary element cycles within the past about 120 years. Carbon and sulfur isotope signals are depleted in the heavier isotopes compared to surface water inorganic carbon and seawater sulfate, respectively. Nitrogen isotope ratios, on the other hand, display an enrichment of the heavier isotope when compared to atmospheric air. Both, the carbon and nitrogen isotope signals indicate a mixture of different organic matter sources. The downcore variations can be related to by changes in the relative mixing proportions of OM besides minor effects due to eutrophication. A conservative evaluation of carbon isotope signatures ( δ 13C between − 23.8 and − 25.7‰) considering a control by changing mixing proportions, yields a mixture of 20 to 49% marine organic matter as one end-member, with up to 80% derived from terrestrial sources. The relative enhancement of reactive organic matter starts before the sixties of the last century. Nitrogen isotope variations in the sediment core indicate that changes in the nitrogen cycle took place in the early seventies, likely due to enhanced eutrophication in the PRE. The highest values of pesticides and polychlorinated biphenyls (PCBs) were found in the sediment layer dated for 1980. The areal investigations of surface sediments indicate higher contributions of marine organic matter with enhanced distance from the coast line. Relatively heavy stable isotope data are only found in coarse-grained surface sediments, probably due to enhanced nitrate penetration and benthic denitrification, or local pollution. The sulfur isotopic composition of TRIS reflects the dissimilatory activity of bacteria upon early diagenetic oxidation of organic matter using dissolved sulfate as final electron acceptor. Before the sixties of the 20th century, the activity (metabolic rates) of sulfate-reducing bacteria seems to have been increased due to enhanced input of metabolizable organic matter, indicating changes in the depositional environment and the benthic sulfur cycle. The temporal variabilities of C and N isotope ratios and organic contaminants as reflected in the sediment core are compared to spatial changes in surface sediment samples, indicating an increasing contribution of marine organic matter with increasing distance from the coast. Application of a binary carbon isotope mixing model yields proportions between 35 and 77% contributions from non-marine organic matter. Due to the low organic carbon content the values of organic pollutants in surface sediments are relatively low in comparison the other investigations in the Pearl River Estuary area.