Characterization of phosphorus species in sediments from the Arabian Sea oxygen minimum zone: Combining sequential extractions and X-ray spectroscopy

The bulk phosphorus (P) distribution in sediment samples from the oxygen minimum zone of the northern Arabian Sea was determined using two methods: sequential chemical extraction (the ‘SEDEX’ procedure) and X-ray absorption near-edge structure (XANES) spectroscopy of the phosphorus K-edge. Our results show good agreement between iron (Fe-)associated P and calcium phosphate minerals (Ca-P) determined by both methods. Furthermore, we find that SEDEX exchangeable P likely represents loosely Fe-bound P, and that the SEDEX detrital fraction may consist partly of polyphosphate, i.e. microbially synthesized intracellular phosphate. Below productive waters with relatively high sedimentary organic matter and P contents, polyphosphates may represent an important P sink that is not easily identified by chemical sequential extraction. This study highlights the value of SEDEX as a generally accurate and fast P speciation technique (especially for Fe- and Ca-P) and, for the first time, demonstrates the possibilities of P K-edge XANES as a bulk P speciation tool for marine sediments.