Glendonites as a paleoenvironmental tool: Implications for early Cretaceous high latitude climates in Australia

Glendonites, calcite pseudomorphs after the metastable mineral ikaite (CaCO 3. 6H(2)O), occur in the Late Aptian interval of the Bulldog Shale in the E romanga Basin, Australia and in other Early Cretaceous basins at high paleo latitudes. Ikaite precipitation in the marine environment requires near-fre ezing temperatures (not higher than 4 degrees C), high alkalinity, increase d levels of orthophosphate, and high P-CO2. The rapid and complete transfor mation of ikaite to calcite at temperatures between 5 and 8 degrees C provi des an upper limit on the oxygen isotopic composition of the pore waters: - 2.6 < delta(2) < -3.4 parts per thousand SMOW. If it is assumed that these pore waters are representative of the shallow Eromanga Basin, the calculate d delta(w) can be used to reassess belemnite fossil oxygen isotopic paleote mperatures-temperature recorded by fauna living in the basin at the time of ikaite precipitation. Data previously reported as 11 to 16 degrees C (assu ming delta(w) = 0.0 parts per thousand SMOW) yield paleotemperatures rangin g from -1 to 5 degrees C, squarely in the range of ikaite stability. The lo w delta(w) indicates hyposaline conditions, most likely caused by mixing hi gh latitude meteoric waters with seawater. The O-18 depleted, low temperatu re waters suggest that the region was at least seasonally colder than previ ously accepted. Copyright (C) 1999 Elsevier Science Ltd.