The effects of Mg, Al, and Fe on diagenetically induced phase changes in biogenic opal-A : implications for diatom silica [delta]¹⁸O values as a paleoenvironmental proxy

Advisors: Justin P. Dodd.Committee members: Mark Frank; Lee Sunderlin.Includes bibliographical references.Includes illustrations.Oxygen isotope values (δ¹⁸O) preserved in biogenic silica (opal-A) are commonly used to reconstruct paleoenvironmental conditions; however, diagenesis and the chemistry of host sediments may alter initial δ¹⁸O values. During sedimentation and diagenesis, opal-A dissolves and reprecipitates in the form of opal-CT and microcrystalline quartz, which may partially or completely overwrite the initial δ¹⁸O values of opal-A. Previous studies have suggested that the presence of metal cations in host sediments impede the rate of transition from opal-A to opal-CT during diagenesis, but none have evaluated this effect on the δ¹⁸O values. Based on the results from this study, the δ¹⁸O values and temperature of the diagenetic water are controlling factors for oxygen isotope variation of sedimentary diatom silica regardless of pore water chemistry or phase transitions. The mechanism responsible for the oxygen isotope variation is likely due to 1) opal-A' precipitation or 2) exchange of oxygen atoms between opal-A and water during structural condensation. Since the amount of equilibrated silica varied for each unique experiment, the time and temperature that is required to complete the reequilibration of the diatom silica with surrounding water may be impacted by the chemistry of the pore water. Although the initial δ¹⁸O values of diatom silica are not preserved in the sediment at the higher end of diagenic temperatures (>100 °C), the opportunity to use biogenic silica as a proxy for paleoenvironmental conditions may still exist if the relationship between diatom silica and diagenic water is better constrained and applied to natural settings.M.S. (Master of Science