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 from the continental slope offshore from Cape Hatteras, North Carolina, varied from 150 to almost 700 μ,M with depth in the top 40 cm of sediment. Sediment cores from 630 to 2010 m depth had very similar profiles of dissolved silica in their pore waters, even though these cores came from regions greatly different in slope, topography, sedimentation rate, and abundance of benthic macrofauna. Cores from 474 to 525 m were more variable, both with respect to pore water dissolved silica profiles, and with respect to sediment texture. Experiments indicate that both the rate of dissolution of silica and the saturation concentration decrease as sediment depth below the sediment-seawater interface increases. These data are consistent with depletion of a reactive silica phase in surface sediment, which may be radiolarian tests, or the alteration of biogenic silica to a less reactive form over time. Experimental results suggest that the pore water dissolved silica concentration in sediments below the top few centimeters may be higher than the sediments could now achieve. The flux of dissolved silica out of these sediments is estimated to be 15 μmoles cm−2 yr−1.