STABLE ISOTOPE DATA FROM THE EARLY PALEOGENE OF ALASKA: EVIDENCE FOR GROUNDWATER MIXING, TOPOGRAPHIC RELIEF, AND METHANE FORMATION

The Paleogene Chickloon Formation of Alaska is made up of a succession of sand/siltstones, carbonaceous shales, conglomerates, and coals. Oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) of authigenic carbonate from it provide information regarding groundwater hydrology, paleotopography, and carbon dynamics in high-latitude regions during greenhouse climate states. These inferences can be made because authigenic carbonate forms in equilibrium with groundwater, which has a source in surface water, and thus its chemistry provides a proxy for reconstructing surface conditions. This information is important because high-latitude Paleogene terrestrial deposits are rare and thus relatively little is known about these regions.

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.