Diagenetic controls of phosphorus in outer continental-shelf sediments from the Gulf of Mexico

Abstract

A sequential extraction scheme was used to isolate organic P and inorganic P associated with various forms of Fe, Mn and major cations in two low-carbonate cores from the outer Texas shelf, in which sulfate reduction is minor. Although total extractable P does not change significantly with depth, there is a strong shift in the phase associations. At the surface and the subsurface oxidation maximum of both cores, inorganic P is associated mainly with oxidized Fe. Oxidation of organic matter by reduction of the hydrous Fe-oxides causes a rapid decrease in both the hydrous-Fe-oxide-related P and organic P in the more reducing parts of the cores. The P released during these reactions becomes associated with Mg, Al, K and reduced Fe in one or more highly reactive [1 M acetic acid (HOAc)-soluble] phases with an average Mg:K:Fe:P molar ratio of ∼ 16:6:6:1. Although the exact nature of the HOAc-soluble P phase is unknown, it appears to control pore-water concentrations of P, precluding formation of struvite or vivianite. ( C P ) org ratios in both cores tend to increase with depth in the sediment column, which suggests preferential breakdown of organic P. The shallower station 19 (32 m) has an average molar ( C P ) org ratio of 190, whereas station 9 (112 m) has an average ratio of 253. Accumulation rates of hydrogenous P at station 19 are between 40 and 140 μg P cm −2 yr. −1, while rates at station 9 are 20 μg P cm −2 yr. −1. These rates are 5–35 times greater than average accumulation rates of P along ridge crests, where high concentrations of P are associated with hydrogenous Fe. Flux calculations indicate ∼ 0.2 μg P cm −2 yr. −1 of dissolved reactive P is released from sediments at the deeper station, while 6–55 μg P cm −2 yr. −1 is released from the sediments at the shallower station.