Pore water chemical constraints on petrophysical shifts following biosilica diagenesis
Shahab Varkouhi, Nicholas J. Tosca, Joseph A. Cartwright, Zixiao Guo, Pooria Kianoush, Agata Jurkowska
Abstract
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.