DIAGENETIC TRANSFORMATION OF BIOGENIC SILICA: TEMPERATURE AND LITHOLOGICAL IMPACTS ON DIAGENESIS OF AMORPHOUS SILICA IN MARINE SEDIMENT

The diagenetic transformation of disorganized unstable amorphous biogenic silica (opal-A) (e.g., diatom tests) to authigenic silica phases (opal-CT and quartz) within the oceanic crust is a global process. This transformation significantly impacts physical properties of the crust, decreasing porosity, increasing permeability, and causing kilometer scale compaction that alter the shape of ocean basins. Classic laboratory experiments and outcrop observations have provided a foundation for understanding that biosilica diagenesis is a dissolution-precipitation reaction strongly influenced by temperature. Studies particularly found that increasing heat accelerates reaction rate and crystallization. Host-rock lithology has also been found to impact the rate of reaction.

Despite these findings, inaccuracies remain in models used to determine when transformation occurs in the sediment based on temperature gradients. They largely stem from model parameters being determined experimentally. Therefore, this work aims to evaluate conditions under which opal-CT precipitates in a natural setting. Using legacy cores provided by the International Ocean Discovery Program (IODP), we test the relationship between opal-CT maturity, temperature, and host-rock composition in deep-sea cores from the Indian Ocean at Broken Ridge (IODP 121, Site 752B, ‘cold’ site) and Exmouth Plateau (IODP 122, Site 762C, ‘warm’ site). At both sites the major lithological constituent is calcareous ooze, though the geothermal gradient and thermal history of site 752B are about half that of Site 762C.

The rate of crystallization measured for Site 762C is comparable to that of an additional ‘hot’ site, the Guaymas Basin (IODP 385, Site U1545) which is mainly composed of diatom oozes. Preliminary results suggest that ordering of opal-CT may not be controlled by absolute in situ temperatures and that host rock composition, specifically the presence of carbonates, might not influence the rate of crystallization.