A First Look at Oxygen and Silicon Isotope Variations in Diatom Silica from a Pliocene Antarctic Marine Sediment Core

Coupled oxygen (δ¹⁸O) and silicon (δ³⁰Si) isotope variations in diatom silica (opal-A) are increasingly used as a proxy to reconstruct paleoenvironmental conditions (water temperatures, water mass mixing, nutrient cycling) in marine environments. Diatom silica is a particularly significant paleoenvironmental proxy in high latitude environments, such as the Southern Ocean, where diatom blooms are abundant and diatom frustules are well preserved in the sediment. The Andrill-1B (AND-1B) sediment core from the Ross Sea (Antarctica) preserves several Pliocene (~4.5 Ma) age diatomite units. Here we present preliminary δ¹⁸O and δ³⁰Si values for a diatomite subunit in the AND-1B sediment core. Initial isotope values for the AND-1B diatoms silica record relatively high variability (range δ¹⁸O: 36.3‰ to 39.9‰) that could be interpreted as large-scale changes in the water temperature and/or freshwater mixing in the Ross Sea; however, a significant concern with marine sediment of this age is isotope fractionation during diagenesis and the potential formation of opal-CT lepispheres. The effects of clay contamination on the diatom silica δ¹⁸O values have been addressed through sample purification and quantified through chemical and physical analyses of the diatom silica. The isotopic effects of opal-CT are not as clearly understood and more difficult to physically separate from the primary diatom silica. In order to better understand the isotope variations in the AND-1B diatoms, we also evaluated silicon and oxygen isotope fractionation during the transition from opal-A to opal-CT in a controlled laboratory experiment. Opal-A from cultured marine diatoms (Thalassiosira weissflogii) was subjected to elevated temperatures (150°C) in acid digestion vessels for ~4 weeks to initiate opal-CT precipitation. Quantifying the effects of opal-CT formation on δ¹⁸O and δ³⁰Si variations in biogenic silica improves our understanding of the use of diatom silica isotope values a paleoenvironmental proxy throughout the Cenozoic.