POREWATER PH AND AUTHIGENIC PHASES FORMED IN THE UPPERMOST SEDIMENTS OF THE SANTA-BARBARA BASIN

In this paper porewater and solid phase analyses are used in combinati on with in situ O-2 and pH microelectrode measurements to characterize early diagenetic processes in the uppermost sediments of the Santa Ba rbara Basin, California. Rapid reduction of dissolved oxygen, nitrate, solid phase manganese and iron, and dissolved sulfate is observed. Be tween sediment depths of 0 and 2 cm, reductive solubilization of ferri c iron phases releases Fe, adsorbed phosphate, and fluoride to the por ewaters and contributes to a sharp increase in porewater pH. Between 2 and 4 cm, sulfate reduction rates peak, pH levels off, and acid volat ile sulfides and pyrite become the dominant forms of solid phase iron. Saturation state calculations, which depend largely on pH, indicate t hat the porewaters of the Santa Barbara Basin become saturated with re spect to carbonate fluorapatite and calcite within the first 0.25 mm o f the sediment and are highly supersaturated by and below 2 cm. In spi te of this result, porewater evidence of phosphate and fluoride remova l into a solid phase is observed only in the first similar to 5 cm of some cores, whereas dissolved Ca profiles suggest dispersed calcite pr ecipitation throughout the sediment column. This finding is interprete d as an indication of the nonsteady state nature of the surface reacti ons that may, given sufficient nucleation sites and time, lead to carb onate fluorapatite genesis in anoxic sediments. Finally, microelectrod e pH profiles from two other basins in the California Borderlands are presented. These demonstrate that the porewaters of the Santa Barbara Basin are more alkaline than those of other basins. This outcome is at tributed to the lack of particle mixing and a unique interplay between Fe liberation and FeS precipitation reactions in the Santa Barbara Ba sin.