The effect of sediment lithology on oxygen isotope composition and phase transformation of marine biogenic opal

Biogenic amorphous opal (biogenic opal-A; SiO2·nH2O) in the form of diatoms, radiolaria, and sponge spicules, matures to more stable thermodynamic silica phases of opal-CT and microquartz chert upon accumulation and burial in deep sea sediments. Both temperature and lithology of the sediments (i.e., amounts of clay relative to calcium carbonate) influence the time and depth of maturation. Given that δ18O of silica reflects the δ18O of the water and temperature in which silica forms, it is possible to trace pathways of maturation by measuring δ18O of the different silica phases. We measured δ18O of biogenic opal-A, opal-CT, and microquartz chert from carbonate rich sediments to characterize the effects of lithology on the silica maturation. We have identified a set of cores with calcium carbonate rich and clay poor lithologies from ODP sites 1049–1053 in which these phases and phase transformations occur. The mineralogical phases are characterized by X-ray diffraction (XRD) and the purity of the samples with Scanning Electron Microscope/Energy Dispersive X-ray spectroscopy (SEM/EDS). The δ18O of biogenic opal-A is ~41 to 45‰, the δ18O of opal-A' (a transition silica phase from opal-A to opal-CT) is ~38 to 43‰, the δ18O of opal-CT is ~37 to 45‰, and δ18O of two microquartz cherts is ~38‰. δ18O of opal-A' and opal-CT indicate maturation at low temperature and with shallow depths in the sediments out of silica equilibrium with local porewater δ18O and temperature. In contrast, δ18O of two microquartz cherts reflect formation in isotope equilibrium with local porewater δ18O and temperature. Such scenarios of silica phase transformations appear to be unique to geographical locations with a low geothermal gradient and where sediments are composed of high calcium carbonate to clay ratio, either leading to a shallow and young maturation of biogenic opal-A or a delay by as much as several hundreds of meters and tens of millions of years. Heavy δ18O of porcellanites (siliceous sedimentary rocks with opal-CT mineralogy) and potentially also cherts through geologic time must reflect formation at shallow depths within the sediments, a maturation scenario influenced by simultaneous depositions of marine calcium carbonates.