DISSOLVED SILICA CONCENTRATIONS AND REACTIONS IN PORE WATERS FROM CONTINENTAL-SLOPE SEDIMENTS OFFSHORE FROM CAPE-HATTERAS, NORTH-CAROLINA, USA

The pore water concentrations of dissolved silica in sediment cores fr om the continental slope offshore from Cape Hatteras, North Carolina, varied from 150 to almost 700 mu M with depth in the top 40 cm of sedi ment. Sediment cores from 630 to 2010 m depth had very similar profile s of dissolved silica in their pore waters, even though these cores ca me from regions greatly different in slope, topography, sedimentation rate, and abundance of benthic macrofauna. Cores from 474 to 525 m wer e more variable, both with respect to pore water dissolved silica prof iles, and with respect to sediment texture. Experiments indicate that both the rate of dissolution of silica and the saturation concentratio n decrease as sediment depth below the sediment-seawater interface inc reases. These data are consistent with depletion of a reactive silica phase in surface sediment, which may be radiolarian tests, or the alte ration of biogenic silica to a less reactive form over time. Experimen tal results suggest that the pore water dissolved silica concentration in sediments below the top few centimeters may be higher than the sed iments could now achieve. The flux of dissolved silica out of these se diments is estimated to be 15 mu moles cm(-2) yr(-1).