Oxygen isotopes of phosphatic compounds—Application for marine particulate matter, sediments and soils

The phosphate oxygen isotopic composition in naturally occurring particulate phosphatic compounds (y18Op) can be used as a tracer for phosphate sources and to evaluate the cycling of phosphorus (P) in the environment. However, phosphatic compounds must be converted to silver phosphate prior to isotopic analysis, a process that involves digestion of particulate matter in acid. This digestion will hydrolyze some of the phosphatic compounds such that oxygen from the acid solution will be incorporated into the sample as these phosphatic compounds are converted to orthophosphate (PO4 3 ). To determine the extent of incorporation of reagent oxygen into the sample, we digested various phosphatic compounds in both acid amended with H2 O (spiked) and unspiked acid and then converted the samples to silver phosphate for y18Op analysis. Our results indicate that there is no isotopic fractionation associated with acid digestion at 50 8C. Furthermore, we found that reagent oxygen incorporation is a function of the oxygen to phosphorus ratio (O:P) of the digested compound whereby the percentage of reagent oxygen incorporated into the sample is the same as that which is required to convert all of the P-compounds into orthophosphate. Based on these results, we developed a correction for reagent oxygen incorporation using simple mass balance, a procedure that allows for the determination of the y18Op of samples containing a mixture of phosphatic compounds. We analyzed a variety of environmental samples for y 18Op to demonstrate the utility of this approach for understanding sources and cycling of P. D 2005 Elsevier B.V. All rights reserved.