Geochemical characteristics of phosphorus in surface sediments from the continental shelf region of the northern South China Sea

Forty surface sediment samples were collected from the northern South China Sea (NSCS) during March–April 2014 to study the geochemical forms and characteristics of phosphorus (P), including exchangeable or loosely adsorbed P (Ex-P), Fe-bound P (Fe-P), authigenic P (Ca-P), detrital apatite P (De-P), and organic P (OP), which were separated and quantified through a sequential extraction procedure. Total P (TP) in surface sediments ranged from 5.10 to 18.79μmolg−1, and inorganic P (IP) was the dominant form of P. De-P and Ca-P were the main chemical forms of IP in surface sediments. Terrestrial inputs, biological processes, hydrodynamic conditions, Fe oxides, particle size and organic matter degradation were responsible for the geochemical characteristics of different P species in surface sediments of the NSCS. Organic carbon to OP (OC/OP) ratios ranged from 84 to 595 with an average of 151 in surface sediments, which indicates that organic matter deposited on the NSCS shelf can be ascribed to predominantly terrestrial sources. Fe/P ratios indicate that Fe oxides in most surface sediments of the NSCS had elevated capacity for the retention of particulate P. Bioavailable P represented a significant proportion of the sedimentary P pool, and accounted for 17.9–55.7% of TP in surface sediments of the NSCS.