The role of adsorption–desorption surface reactions in controlling interstitial Si(OH)4 concentrations and enhancing Si(OH)4 turn-over in shallow shelf seas

We report on rapid adsorption–desorption of Si(OH)4 on continental shelf sediments. Experiments were carried out with sandy, low-porosity sediments poor in biogenic silica collected at five stations in the southeastern North Sea and with one silty sediment sampled in the same area. Sorption isotherms for sediments from six stations and three depth layers revealed differences in the capacity of sediments to sorb Si(OH)4. The equilibrium concentration, the concentration for which neither adsorption nor desorption occurs, determined for core top sediments correlates with the HCl-extractable Fe content. Our results point to amorphous Fe-oxides as the main sorbing mineral phase. The comparison between Si(OH)4 equilibrium concentrations derived from isotherms and in situ pore water levels suggests a buffering of interstitial Si(OH)4 below 300 μM by surface reactions. Repeated exposure of particles to rapid variations of Si(OH)4 associated with biological and physical stirring of surface sediments, an ubiquitous process in shallow shelf seas, was simulated by resuspension experiments progressing from low to high (0.15–225 μM), respectively, high to low (225–0.15 μM) Si(OH)4. Time-series of release or uptake of Si(OH)4 by suspended sediments were adequately described by a model combining a kinetically controlled, reversible surface reaction, the equilibrium of which was derived from the sorption isotherms, with a first-order dissolution term for Si. Apparent saturation concentrations were below the solubility of biogenic silica. They ranged from 516 to 106 μM. Rate constants for both reactions were quantified and yielded characteristic time-scales extrapolated to in situ pore water conditions in the order of minutes (sorption) to several months (dissolution). Sorption thus appears as the rapid first response to drastic changes in Si(OH)4 concentrations typically associated with physical or biological reworking of surface sediments and percolation of bottom waters due to wave and current activity. We estimated the amount of Si(OH)4 added to bottom waters by a resuspension event. Desorption of Si from sediment particles following their resuspension from the seabed into bottom waters has the potential to enhance Si(OH)4 recycling in shallow shelf seas.