Bulk organic δ13C and C/N as indicators for sediment sources in the Pearl River delta and estuary, southern China

Abstract

Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon cycle. This paper presents bulk d13C and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of d13C and C/N as indicators for sources of organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine and marine suspended particulate organic carbon (POC) from both summer and winter. Results show that the average d13C of estuarine surface sediment increases from 25.0 1.3& in the freshwater environment to 21.0 0.2& in the marine environment, with C/N decreasing from 15.2 3.3 to 6.8 0.2. In the source areas, C3 plants have lower d13C than C4 plants (29.0 1.8& and 13.1 0.5& respectively). d13C increases from 28.3 0.8& in the forest soil to around 24.1& in both riverbank soil and mangrove soil due to increasing proportion of C4 grasses. The d13CPOC increases from 27.6 0.8& in the freshwater areas to 22.4 0.5& in the marine-brackish-water areas in winter, and ranges between 24.0& in freshwater areas and 25.4& in brackish-water areas in summer. Comparison of the d13C and C/N between the sources and sink indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that bulk organic d13C and C/N analysis can be used to indicate sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments derived from anthropogenic sources such as human waste and organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated d13C of sedimentary organic matter due to some agricultural products such as sugarcane, C3 plants are still the dominant vegetation type in this area, and the bulk organic d13C and C/N is still an effective indicator for sources of organic matter in estuarine sediments.