Abstract:
The ratio of organic carbon (C (sub org) ) to organic phosphorus (P (sub org) ) in marine organic matter is about 106, but extremely high ratios (above 1000) have been reported for several low-oxygen marine sediments (e.g., Ingall, Van Cappellen and Jahnke), spurring suggestions of preferential P release from low oxygen sediments during organic matter degradation. If widespread, this additional P input would be a substantial factor in oceanic nutrient budgets, ocean productivity, and climate. To test this suggestion, we determined C (sub org) contents and performed extensive P geochemistry from Ocean Drilling Program samples recently collected from the anoxic Saanich Inlet in British Columbia (ODP Leg 169S Sites 1033 and 1034). These sites reveal clear laminations indicating persistent anoxia since at least 10,500 ybp. C (sub org) values from these sites range from 1-2.5 wt%, and reveal a slight decrease with depth. A sequential extraction was used to characterize P geochemistry, separating P that is organically-bound, bound to reducible oxyhydroxides, and incorporated in authigenic mineral forms. Clear downward trends in P geochemistry are observed: (1) P (sub org) decreases rapidly from high values at the surface (about 5 umol/g) to very low values at depth (1 umol/g at 40m), (2) oxyhydroxide-bound P increases rapidly at the surface (offsetting the rapid initial decrease in P (sub org) ) then decreases with depth, and (3) authigenic P increases with depth. The net result of these geochemical changes is that the total reactive P pool (the sum of the above three components) remains constant with depth. Organic matter degradation does indeed release P (sub org) preferentially compared to C (sub org) , but the released P (sub org) is incorporated into several different geochemical pools and is eventually buried in an authigenic form (this phenomenon has been observed in other margin and deep sea sediments). The C (sub org) :P (sub org) ratio increases rapidly with depth, starting at 150 at the surface and increasing to over 900 at depth (40m). This increase, however, does not indicate P release from these anoxic sediments back to the water column, but rather reveals the intricacies of P diagenesis and cycling in marine sediments.