Distribution, source and chemical speciation of phosphorus in surface sediments of the central Pacific Ocean

Abstract

The abundance of five forms of phosphorus (P) in surface sediments from the central Pacific Ocean (4.5–15ºN, 154–143ºW) was determined using a sequential extraction procedure (SEDEX) to examine the distribution and source of different P species. Total P (TP) concentrations ranged from 13.2 to 119µmol-P/g with an average of 48.6±27.4µmol-P/g. Within the TP pool, total inorganic P (TIP) concentrations varied from 11.1 to 121µmol-P/g, while total organic P (TOP) concentrations ranged from undetectable to 4.8µmol-P/g. Inorganic P was generally the predominant form in surface sediments, comprising on average up to 93% of sedimentary TP, leaving <16% as TOP. Among the five P species, the authigenic or CaCO3-bound P and detrital P were the two major P species (comprising on average 43.4±13.5% and 45.7±14.8% of TP, respectively), followed by the refractory organic P, representing 6.7±2.4% of TP. Fe-bound P accounted for 3.3±1.3% of TP, and exchangeable or adsorbed P made up less than 1% of TP. The spatial distribution of different sedimentary P species showed that higher concentrations of detrital P and Fe-bound P were both found at around 11°N, suggesting similar sources for these two P species. Much of the detrital P was derived from atmospheric sources in the study area, where heavy rainfall in the intertropical convergence zone between 3°N and 11°N has been widely reported. Compared with other marine environments, the central Pacific Ocean had relatively higher detrital P, but lower abundance of adsorbed-P and Fe-bound P. These unquine results suggested that most of the labile P could have been released into the water column during its settling from the surface to the seafloor, or that atmospheric inputs of refractory P were an important source for sedimentary P, accounting for an average of 63% of the TP, in the central Pacific Ocean. High proportions of authigenic P in deep-sea sediments, on the other hand, implied that oceanic sediments are an important sink for reactive P species. Relatively lower OC/Org-P and OC/Preact ratios suggested a higher sedimentary burial for Org-P and/or attested the importance of detrital P derived from atmospheric sources in the central Pacific Ocean.