STABLE ISOTOPE DATA FROM THE EARLY PALEOGENE OF ALASKA: EVIDENCE FOR GROUNDWATER MIXING, TOPOGRAPHIC RELIEF, AND METHANE FORMATION
Evidence for groundwater mixing comes in the form of a curvilinear array of δ13C and δ18O. This array falls between two hypothetical end-member waters, one associated with anoxic swamp water, and the other with river water. δ18O of these end-members are influenced by hydrological recharge. Low δ18O end-member values are associated with fluvial sandstones, and are interpreted to reflect recharge by precipitation falling on highlands adjacent to the Chickaloon depocenter. High δ18O end-member values are associated with organic-rich mire/swamp deposits, and are interpreted to reflect recharge by precipitation from low elevation air masses. Different δ13C end-members are interpreted to reflect differences in cycling of surface carbon. Lower δ13C of the fluvial deposits are consistent with oxidation of organic matter in well-drained soils, while much higher δ13C (up to +13‰) suggest that methane formation took place in anoxic swamps.
To get waters with very different δ18O values, a large topographic feature, such as a mountain range, could have been present in the area. Based on the isotopic data, that range would have had a paleotopographic elevation of a minimum of ~2.2 km. In the case of carbon, high δ13C values like those from the Chickaloon swamp deposits are also found in authigenic carbonate from other high paleolatitude Paleogene strata, suggesting that methane formation may have been an integral part of high latitude terrestrial systems, and that the production of such methane — a powerful greenhouse gas — is one reason for the existence of greenhouse climates during the Paleogene.