CRYOGENIC BRINE FORMATION IN THE ROSS SEA RECORDED BY DIATOM SILICA OXYGEN ISOTOPE VALUES FROM A PLIOCENE MARINE SEDIMENT CORE

Oxygen isotope (δ¹⁸O) values of sedimentary diatom silica have been increasingly used to reconstruct past water temperatures and changes in ocean circulation in marine environments. The use of diatom δ¹⁸O values as a paleoenvironmental proxy is especially significant in the Southern Ocean, where diatoms represent a majority of the biologic sedimentary record and carbonate materials are not well preserved. Previous studies have shown that early diagenetic processes may significantly alter oxygen isotopes in diatom silica; therefore, the δ¹⁸O values are potentially re-equilibrating with and recording the δ¹⁸O value of the sediment-water interface/sediment pore waters. The Andrill-1B (AND-1B) sediment core from the Ross Sea, Antarctica, recovered a mid to late Pliocene (~4.68 to 3.44 Ma) age diatomite unit with diatom δ¹⁸O values that range from +28 to +35.5‰. The silica-water fractionation relationship for marine diatoms of Juillet-Leclerc and Labeyrie (1987) and standard marine δ¹⁸O water values of ~0‰ result in calculated sea surface temperatures of 24°C, which is unrealistically high for the Ross Sea and suggests a δ¹⁸O water value that deviates from average ocean water. Using a projected temperature range for the Pliocene Ross Sea of 0 to 10°C, the calculated δ¹⁸O water values are -7 to -16‰. These values are too low to reflect surface water values, even during a warm Pliocene environment. An alternate hypothesis is that the diatom silica δ¹⁸O values reflect isotopic equilibration with cryogenically formed hypersaline pore fluids. Low δ¹⁸O values (-6 to -11‰) and high-salinity pore waters documented in the Andrill-2A sediment core (~50 km west; Frank et al., 2010) are in agreement with our calculated δ¹⁸O water values. The large range of diatom δ¹⁸O values suggests variable bottom water conditions in the Ross Sea during the mid to late Pliocene warm period that are likely the result of changes in the amount of cryogenic brine inputs and mixing with open ocean waters. These results have significant implications for reconstructing and predicting Antarctic bottom water inputs and formation along with ocean circulation.