Carbon to phosphorus ratios in sediments: Implications for nutrient cycling

Abstract

Defining the transformations of phosphorus (P) in sediments is critical to assessing the geochemical behavior of sedimentary P and the behavior of reactive P (Preactive= Poxide‐associated + Pauthigenic + Porganic) relative to organic carbon (Corganic). This study includes the determination of P components and Corganic concentrations on samples from sites in the California Current region (Ocean Drilling Program (ODP) leg 167, sites 1010, 1011, 1012, 1014, 1016, and 1021), in the Benguela Current region (ODP leg 175, sites 1082, 1084, and 1085), and on Blake Nose in the western Atlantic (ODP leg 17 1B, sites 1050 and 1052). Sample depths range up to 650 m, and ages range up to 65 Ma. Our data demonstrate the transformation of labile forms of P (Porganic and Poxide‐associated) to Pauthigenic throughout the sediment column in all redox states and at all sedimentation rates investigated, until a substantial portion of reactive P is in the authigenic form (>80%). (C/P)organic ratios are always greater than or equal to the Redfield Ratio in all of the sediments we studied. Sediments with Corganic concentrations ≤2 wt % have Corganic/Preactive ratios less than the Redfield Ratio, and Corganic/Preactive ratios increase linearly with Corganic, consistent with degradation of Corganic with age. In general, Corganic/Preactive ratios for sediments with Corganic concentrations ≥2 wt %, from a variety of redox states and sedimentation rates, center around the Redfield Ratio, although some ratios are as high as 3 times the Redfield Ratio. However, Corganic/ Preactive ratios (up to 400) do not approach the high ratios observed for (C/P)organic ratios (up to 4500). Because of the efficient transfer of labile forms of P to Pauthigenic, Corganic/Preactive ratios better describe the geochemical behavior of sedimentary P.