Distribution and Diagenesis of Phosphorus in the Deep-Sea Sediments of the Central Indian Basin

Abstract

A regional solid-phase phosphorus (P) speciation study in the Central Indian Basin (CIB) was carried out to elucidate the spatial distribution of sedimentary P species and P diagenesis in an oligotrophic setting. The results show that P enrichment is in the order of biogenic (Pbio) > authigenic (Pauth) > iron bound (PFe) > detrital (Pdet) > organic (Porg) P. The higher concentrations of Pbio are found in siliceous oozes, which are attributed to high biological productivity in the overlying waters compared to pelagic clay region. High Pauth and Pdet contents with low molar Corg/Preact ratios in the pelagic/red clays indicate the deposition of calcium fluorapatite and refractory material from the atmospheric input. The oxygenated bottom water promotes adsorption of P onto iron oxyhydroxides making the PFe an important sink. Remobilization of P within the sediments is limited because of well-oxygenated conditions and an efficient adsorption by iron oxyhydroxides and clay-sized sediments. A twofold increase in Ptotal and P-species is observed in hydrothermally altered, ferruginous sediments from a seamount flank suggesting an important role of hydrothermal processes in P cycling. The calculated P accumulation rate in the present study ranges between 0.6 and 11.7 μmol·cm−2·kyr−1. The burial flux of P for the entire CIB (5.7 × 106 km2) is 0.01 × 1010 mol P per year which accounts for 0.05% of the global flux. The benthic P fluxes from the seawater to the sediments in the area range between 0.0093 and 0.133 μmol·cm−2·kyr−1 indicating that the CIB sediments are an important sink for P.