CARBON AND OXYGEN ISOTOPES IN GLENDONITES FROM THE EARLY CRETACEOUS AND PALEOCENE

Carbon and oxygen isotopic data from glendonites of Valanginian, Hauterivian, Middle Aptian and Paleocene Age from Svalbard and East Greenland display values that suggest formation in cold seeps, possibly similar to those described from the Sea of Okhotsk (Greinert et al., 2004). Carbon isotopic values are exclusively depleted relative to normal marine calcite. δ¹³C_carbonate (PDB) values range from -39 to –11 per mil suggesting that remineralized organic matter is the most likely dissolved inorganic carbon (DIC) source. Variable contributions of DIC derived from the oxidation of methane and normal marine source can account for the observed range of values. δ¹⁸O (PDB) values range from –1 to –10 per mil. Formation of ikaite (calcium carbonate hexahydrate), the precursor of glendonite requires temperatures at or below 0^o C. Precipitation of calcite in equilibrium with marine DIC and waters near 0oC would have δ¹⁸O of approximately 4 per mil. While the use of glendonites as a qualitative paleothermometer is not disputed, the use of oxygen isotope paleothermometry is not possible due to variability in the δ¹⁸O_water. The implied variations in the isotopic composition of water and DIC in glendonites could be explained by formation in cold seeps where the water source could be in part meteoric. Host sediments vary from carbonate concretions, clay shales to fine continential shelf sands but appear to be deposited in relatively near-shore environments where a ¹⁸O depleted water source from cold seeps is possible.