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 Pi 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 +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 μmol) of porewater P_ito silver phosphate (Ag₃PO₄) 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 Ag₃PO₄. Samples as low as 200 μg 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 (δ¹⁸O_P) during micro extraction. Porewater data on δ¹⁸O_P has been obtained from two sediment cores of the Moroccan margin. The δ¹⁸O_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 δ¹⁸O_P to study microbial processes in the subseafloor and at the sediment water interface.