Authigenic P formation and reactive P burial in sediments of the Nazaré canyon on the Iberian margin (NE Atlantic)

Abstract

Profiles of different forms of sedimentary phosphorus were measured at four sites at the Iberian margin (NE Atlantic), which were chosen on the basis of differences in depositional environment: a shelf site (113 m), a mid-slope station (1387 m) and two stations at the head (396 m) and base (3097 m) of the Nazaré Canyon. The sediment was sequentially extracted for Fe-bound P, carbonate fluorapatite (CFA)+biogenic P+CaCO 3-bound P and detrital Ca-P, and non-sequentially for total P and inorganic P, where the difference between total and inorganic P was assumed to be organic P. Measurements of organic carbon and nitrogen, citrate–dithionite–bicarbonate- and citrate–ascorbate–bicarbonate-extractable Fe, carbon oxidation rates, Fe reduction rates, phosphate effluxes and sedimentation rates were used to quantify the cycling and burial of phosphorus at each site. CFA formation was observed only in sediments of the Nazaré canyon, where enhanced rates of organic matter decomposition and Fe reduction provided the necessary conditions. The concentrations of reactive P were similar at all sites, but the sediment accumulation rates differed greatly. The high bulk accumulation rate at the base of the canyon results in a reactive P burial rate exceeding those estimated for continental margins including the Iberian margin by an order of magnitude. The estimated P burial efficiency is smallest at the slope station (3–24%), reasonably high (63–86%) at the shelf and head of the canyon stations and extremely high (>97%) at the base of the canyon. We propose that local depositories such as those at the base of canyons may be key sites for reactive P burial.