THE EFFECTS OF MG, AL, FE AND CA ON DIAGENETICALLY INDUCED PHASE CHANGES IN BIOGENIC OPAL-A: IMPLICATIONS FOR DIATOM SILICA AS PALEOENVIRONMENTAL PROXY

Oxygen (δ¹⁸O) variations in biogenic opal (e.g. diatom frustules, sponge spicules) have been shown to be reliable proxies of paleoenvironmental conditions; however, diagenetic processes may alter these values by several permil. In marine sediments, a primary concern is the effect of opal phase changes on the chemical and structural composition of the diatom silica. For example, oxygen isotope values of opal in the Monterey Formation, California, are well correlated with the phase change from opal-A to opal-CT and microcrystal quartz as a result of increasing geothermal temperatures with depth (Murata et al., 1997). Additionally, a number of studies have suggested that the presence of metallic cations (e.g. Mg, Al, Fe) in host sediments may affect the rate of phase transitions from opal-A to opal-CT during diagenesis. However, the timing and oxygen isotope fractionation associated with opal phase changes is not well constrained. Opal-A from marine diatom silica was subjected to elevated temperatures (150-200°C) in acid digestion vessels for 4-6 weeks to initiate opal-CT phase changes. During the series of 10 experiments, magnesium carbonate (MgCO₃), iron chloride (FeCl₂), aluminum chloride (AlCl₃), magnesium chloride (MgCl₂) and calcium carbonate (CaCO₃) were added individually to pure diatom silica to constrain the effects of Mg, Fe, Al and Ca on the timing of opal-CT formation and quantify the structural and isotopic (δ¹⁸O) changes that occur during these transitions. Preliminary results from this study suggest that host sediments effect δ¹⁸O values of diatom silica during diagenesis. Although the incorporation of Al and Fe into opal-A during sedimentary processes appear to have little effect on the δ¹⁸O values, the presence of Mg during diagenesis promotes the formation of Opal-CT and overwrites the initial δ¹⁸O values of the diatom opal. An experiment conducted with enriched deionized water and without a cation source demonstrates that pore water chemistry can cause variability in the δ¹⁸O values of sedimentary opal.