PETROLOGY AND STABLE ISOTOPE GEOCHEMISTRY OF CALCITE-CEMENTED CONCRETIONS IN NEOGENE TERRESTRIAL SEDIMENTS, SOUTH-CENTRAL ALASKA

Neogene (~12 to ~2 Ma) sedimentary sequences in the Tanana and Cook Inlet Basins of south-central Alaska were deposited during a period of climate cooling caused by global change and local tectonic effects. Changes in sediment sources and regional drainage patterns during this interval were driven by the onset of active uplift of the modern Alaska Range beginning ~6 Ma. Concretions formed by precipitation of calcite cement from shallow groundwater and soil water record the stable isotopic systems of carbonate buffered by meteoric water and carbon from different organic degradation pathways. Bulk organic carbon shows very little variation in d13CPDB with samples from the Tanana Basin interval averaging -25.98 permil and Cook Inlet Basin coal samples averaging -26.15 permil consistent with a C3 source for the bulk of the organic matter. Carbonate from Tanana Basin concretion cement has average d13C PDB of -14.87 permil consistent with derivation of carbonate from direct oxidation of organic matter in the sediment. Carbonate from the Cook Inlet Basin has average d13C PDB of +3.45 permil consistent with derivation of CO2 from fermentation of the organic matter that formed the coal in the basin. Tanana Basin average d18O PDB in concretion calcite is -22.46 permil; average Cook Inlet Basin concretion calcite is -20.31 permil. Modern waters in springs from two sampling sites near Healy and Homer Alaska have d18OSMOW water of -18.6 permil and -14.68 permil, respectively. Based on published mean annual temperature estimates for southern Alaska of 5 to 10 degrees C through the late Neogene intervals studied, the oxygen isotope values of carbonate cements in concretions from the two basins indicate that meteoric waters were more depleted than modern southern Alaska waters or that temperatures of calcite precipitation were higher than the mean annual temperatures reported from floristic analyses.