Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Pore water chemistry (total dissolved CO₂, NH₄, NO₃, NO₂, PO₄, Si(OH)₄, Ca, Mg, Fe, Mn, SO₄, H₂S and F, and titration alkalinity), solid phase chemistry (C_org, P_org, C_TOT, N_TOT, F, Si_OPAL and S_II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20-40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C_org), very high porosity (> 0.96 ml cm^-3) and very low dry bulk density (< 0.1 g cm^-3). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μM, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation. The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a "testable" model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.