PHOSPHATE REMOVAL BY OCEANIC HYDROTHERMAL PROCESSES - AN UPDATE OF THE PHOSPHORUS BUDGET IN THE OCEANS

We present a compilation of dissolved phosphate and solid-phase P data from the oceanic crust to evaluate the effects of hydrothermal proces ses on the oceanic budget of P. Concentrations of phosphate in fluids that emanate from ridge-axis hydrothermal systems are less than that i n bottom seawater. The extent of removal in these fluids is at least 3 0% and in some hydrothermal systems dissolved phosphate is removed com pletely from the circulating fluid. Evidence for the removal of phosph ate in each of six ridge flank hydrothermal systems is based on system atic variations in porewater profiles of phosphate and speeds of porew ater flow. The extent of removal is >80% in these ridge-flank systems. These removal processes are recorded in the basaltic crust as an incr ease in P concentration that coincides with an increase in extent of a lteration and content of ferric iron. Phosphate also is removed in hyd rothermal plumes by coprecipitation with Fe oxyhydroxide particles, wh ich eventually deposit on the seafloor. Each of these hydrothermal pro cesses results in a flux of P into the oceanic crust. Bottom seawater flow through ridge-axis hydrothermal systems removes at most 0.4% of t he preindustrial dissolved riverine flux of P, while ridge flanks remo ve at least 5%, but less than 50% of the dissolved riverine flux, cons istent with P data from Deep Sea Drilling Project (DSDP) Sites 417 and 418. Removal of phosphate by coprecipitation with Fe-rich particles i n hydrothermal plumes along ridge axes accounts for 18-33% of the diss olved riverine flux. Thus, hydrothermal systems remove about 50% of th e preindustrial dissolved riverine flux of phosphate. We have included these new estimates in a revised budget for oceanic P.