Pore water chemical constraints on petrophysical shifts following biosilica diagenesis

Silica diagenesis commonly induces marked petrophysical changes to biosiliceous sediments across the sub-seafloor opal-A to opal-CT transition zone (TZA/CT). Integration of pore water thermodynamics with textural and mineralogical data reveals that sediments in the TZA/CT drilled at ODP Site 794 are sharply compacted via opal-A dissolution and matrix collapse. The porosity decline is linked to silica solubility through pore water saturation states with opal-A and opal-CT. A dissolved silicon (DSi) drop near the depth of the petrophysical shift implies that fluid expulsion removes the DSi produced by opal-A dissolution. Production of opal-CT from the pore fluid that is thermodynamically equilibrated with solubility of this silica mineral phase however slightly impacts the anomalous compaction at the TZA/CT.