Marine sediment pore-water profiles of phosphate δ18O using a refined micro-extraction

Phosphorus cycling in the ocean is influenced by biological and geochemical processes that are reflected in the oxygen isotope signature of dissolved inorganic phosphate (P-i). Extending the P-i oxygen isotope record from the water column into the seabed is difficult due to low P-i concentrations and small amounts of marine porewaters available for analysis. We obtained porewater profiles of P-i oxygen isotopes using a refined protocol based on the original micro-extraction designed by Colman (2002). This refined and customized method allows the conversion of ultra-low quantities (0.5-1 mu mol) of porewater P-i to silver phosphate (Ag-3 PO4) for routine analysis by mass spectrometry. A combination of magnesium hydroxide co-precipitation with ion exchange resin treatment steps is used to remove dissolved organic matter, anions, and cations from the sample before precipitating Ag-3 PO4. Samples as low as 200 mu g were analyzed in a continuous flow isotope ratio mass spectrometer setup. Tests with external and laboratory internal standards validated the preservation of the original phosphate oxygen isotope signature (delta O-18(P)) during micro extraction. Porewater data on delta O-18(P) has been obtained from two sediment cores of the Moroccan margin. The delta O-18(P) values are in a range of +19.49 to +27.30%. We apply a simple isotope mass balance model to disentangle processes contributing to benthic P cycling and find evidence for P-i regeneration outbalancing microbial demand in the upper sediment layers. This highlights the great potential of using delta O-18(P) to study microbial processes in the subseafloor and at the sediment water interface.