Carbon to phosphorus ratios in sediments: Implications for nutrient cycling

Defining the transformations of phosphorus (P) in sediments is critical to assessing the geochemical behavior of sedimentary P and the behavior of rea ctive P (P-reactive = Poxide-associated + P-authigenic + P-organic) relativ e to organic carbon (C-organic). This study includes the determination of P components and C-organic concentrations on samples from sites in the Calif ornia Current region (Ocean Drilling Program (ODP) leg 167, sites 1010, 101 1, 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 (O DP leg 171B, sites 1050 and 1052). Sample depths range up to 650 m, and age s range up to 65 Ma. Our data demonstrate the transformation of labile form s of P (P-organic and Poxide-associated) to Pauthigenic throughout the sedi ment 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 R atio in all of the sediments we studied. Sediments with C-organic concentra tions less than or equal to2 wt % have C-organic/P-reactive ratios less tha n the Redfield Ratio, and C-organic/P-reactive ratios increase linearly wit h C-organic consistent with degradation of C-organic with age. In general, C-organic/P-reactive ratios for sediments with C-organic concentrations gre ater than or equal to2 wt %, from a variety of redox states and sedimentati on rates, center around the Redfield Ratio, although some ratios are as hig h as 3 times the Redfield Ratio. However, C-organic P-reactive ratios (up t o 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 P-au thigenic C-organic/P-reactive ratios better describe the geochemical behavi or of sedimentary P.