Early diagenesis of silica in sandy North sea sediments: quantification of the solid phase

Abstract

The suitability of selected alkaline leaching techniques for the quantification of reactive solid phase silica in sandy North Sea sediments was assessed. A single leach in a 2 M Na 2CO 3 solution for 5 h at 85°C resulted in an overestimation of the biogenic silica fraction owing to an increased leachout of silica from clays. Sequential leaching in 0.1 M Na 2CO 3 at 85°C for 5 h yielded two to ten times lower contents. Levels down to 3 μmol g −1 dry sediment were determined. The obtained contents ranged from below 10 μmol g −1 for the erosion dominated stations of the Dogger Bank proper to contents exceeding 20 μmol g −1 for locations north and south of the Dogger Bank. Dissolution of natural sediments in silica-poor seawater allowed for a characterization of the most labile pool of silica. Marked differences between the individual stations in readily soluble silica contents were only observed in the upper layers. The stations at the border of the area had maximal contents exceeding 1 μmol g −1, as compared with below 0.2 μmol g −1 for the others. The consistency between the results of the dissolution experiments, pore-water data and exchange fluxes indicates a control of the pore-water silica concentration by the availability of reactive solid silica in the sediments studied.