STABLE AND CLUMPED ISOTOPE STUDY OF AUTHIGENIC CARBONATES FROM THE KOOTZNAHOO FORMATION, ALASKA, AND IMPLICATIONS FOR STUDY OF PALEOGENE CLIMATE AND HYDROLOGY

Oxygen isotope ratios of authigenic carbonates have long been used as a proxy for those of local precipitation and/or groundwater in terrestrial settings, and these values can in turn be used to study climate and hydrology of the past. The nature of this proxy relation, however, is dependent on the temperature of carbonate formation, which is often assumed to reflect that at the surface. In this study we use clumped isotope ratios of authigenic carbonate from the Paleogene-aged Kootznahoo Formation of Alaska to estimate the temperature of formation and thus provide a more accurate estimate of the oxygen isotope ratio of precipitation that can then be used to investigate past environments.

Samples of carbonate cement and spar from the Kootznahoo formation have carbon and oxygen isotope ratios that fall into two distinct groups. These are interpreted to represent mixing trends between marine and meteoric end-member fluids, with the former having oxygen isotope ratios of ~-24 and ~-15 ‰ if a formation temperature of 10°C, as has been inferred for high latitudes during the Paleogene, is assumed. Clumped isotope analyses indicate, however, that carbonate formation occurred at high temperatures between ~50 and 120°C, and suggest that oxygen isotope ratios of meteoric waters were ~-14 and ~-5 ‰ for the two sample groups.

In general, the large differences in estimated oxygen isotope ratio of precipitation obtained using these different formation temperatures result in a significantly different paleohydrogical and paleoclimatic interpretations of the data. More importantly, this work suggests that it may not always be valid to assume that carbonates formed under surface conditions. The reason for the existence of two distinct groups of isotopic data remains unclear, although it may reflect changes in the global or regional hydrological cycle over time.