PHOSPHORUS ACCUMULATION RATES IN A MIOCENE LOW-OXYGEN BASIN - THE MONTEREY FORMATION (PISMO BASIN), CALIFORNIA

Phosphorus (P) limits oceanic productivity on long time scales, and th erefore determining changes in oceanic P accumulation through time is important for modeling oceanic paleoproductivity. We determined P conc entrations and accumulation rates at Shell Beach, California, a sectio n of phosphatic-rich marine sedimentary rocks which includes the wides pread Phosphatic and Siliceous Facies of the Monterey Formation. Phosp horus concentrations had wide ranges in the major lithologies found in these facies, with values in phosphatic shales ranging from 0.36 to 3 .9 wt%, compared to 0.02 to 0.42 wt% in dolomitic and siliceous strata . Phosphorus accumulation rates (in mumol P cm-2 yr-1 for all values r eported here) showed significant variability with both sample litholog y and position in the section. Phosphorus accumulation rates were much higher in phosphatic shale strata (1.5-29) than in dolomitic and sili ceous strata (0.1-3.1) in both facies. Furthermore, P accumulation rat es were generally higher in phosphatic shale strata of the upper Silic eous Facies (3.1-29) compared to the lower Phosphatic Facies (1.5-14). Mean P accumulation rates, calculated using linear sedimentation rate models for each facies and observations of the relative amount of pho sphatic shale, dolomite, and siliceous strata, are comparable between the portions of Phosphatic and Siliceous Facies studied. This indicate s that the two facies, with their faunal and sedimentological characte ristics considered to reflect major oceanic and climatic changes durin g the middle Miocene, are similar in terms of P burial. Total P accumu lation rates are also comparable to other phosphogenic Miocene and mod em environments. We modeled variable sedimentation rates to examine th e sensitivity of mean P accumulation rates to the linear sedimentation rate assumption. Increasing shale sedimentation rates to values above those estimated by linear models yields higher mean P accumulation ra tes in both facies, though they remain comparable in terms of net P bu rial. Modeling also revealed that, for increased shale sedimentation r ates in each facies, siliceous sedimentation must be about three times faster than dolomitic strata sedimentation.