Thermodynamic calculations indi e that magnetite is unstable under conditions of elevated dissolved silica concentrations In other cases silica-rich sediments.
In biogenic silica-rich sediments rapid recrystallization of diatom and radiolaria frustules can result in pore water dissolved silica concentrations up to 1250 μmol kg −1 Dixit et al. 2001 . Similarly the reactive nature of volcanic ash can also result in high pore water silica concentrations Siggurdson et al. 1997 .
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Cores of marine sediment from the Atlantic Ocean were maintained in seawater in the laboratory for 2 years and showed large sustained releases of dissolved silica to the overlying water. The releas
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 .
Recent estimates based on pore water studies and mass balance considerations put the average flux of dissolved silica from the sediments into the deep water at about 3 μmol cm -2 yr -1 . This flux if mixed uniformly in a bottom layer 100 m thick results in an anomaly increment of 0.3 μmol kg -1 yr -1 .
In sediments that experience a significant detrital input the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the
Abstract. Most of the silica dissolved in sea water comes from silica-rich interstitial waters of marine sediments and from rivers carrying the products of sub