The Effects of Mg and Al on Diagenetically Induced Phase Changes in Biogenic Opal-A: Implications for Biogenic Silica as Paleoenvironmental Proxy

Oxygen variations in biogenic opal (e.g. diatom frustules, sponge spicules) have been shown to be reliable proxies of paleoenvironmental conditions such as water temperature and salinity; 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 cultured marine diatoms (Thalassiosira weissflogii) was subjected to elevated temperatures (150°C) in acid digestion vessels for 4 – 6 weeks to initiate opal-CT phase changes. During the experiments, magnesium carbonate (MgCO₃) and aluminum chloride (AlCl₃) were added to pure diatom silica to constrain the effects of Mg and Al on the timing of opal-CT formation and quantify the structural and isotopic (δ¹⁸O) changes that occur during these transitions.