HOW CONTINENTAL MARGIN SEDIMENTS CONTROL THE WEATHERING FLUX OF PHOSPHATE TO THE OCEANS

Global biogeochemical cycle models routinely evaluate chemical weathering fluxes from continents to the oceans based solely on dissolved concentrations in rivers. Phosphorus (P) is delivered via rivers to the oceans in a variety of reactive dissolved and particulate phases. A compilation of benthic phosphate fluxes from continental margin and deep sea sediments reveals that the global efflux of phosphate from marine sediments is on the order of 30 times higher than the global river dissolved flux of P.

The total benthic flux can be apportioned between two P sources: allochthonous (i.e., derived from riverine reactive particulate P phases) and autochthonous (i.e., derived from marine reactive particulate sources). Calculations of the allochthonous flux show that 80-90% of the weathering flux of P into the oceans enters initially in reactive particulate form. This P is released as phosphate to the water column only subsequent to diagenesis, mainly in continental margin sediments. Inclusion of this input flux of P to the oceans results in a marine residence time on the order of 10,000-20,000 years. The active role that margin sediments play in mediating weathering fluxes requires a revision in the way global biogeochemical cycles are structured for elements such as P.