MID-CRETACEOUS PRECIPITATION RATES IN N. SLOPE, ALASKA DETERMINED FROM PALEOSOL SIDERITE OXYGEN ISOTOPE VALUES

Siderite-bearing paleosols of the Nanushuk Group of the N. Slope Alaska provide a critical high paleolatitude (75° N) meteoric d¹⁸O proxy record for paleoclimatic reconstructions and models of "greenhouse-world" precipitation rates. Siderite d¹⁸O compositions were determined from four paleosol horizons in the National Petroleum Reserve Grandstand # 1 Core, and the values range between -17.6‰ and -14.3‰ (PDB) with standard deviations generally less than 0.6‰ in individual horizons. The d¹³C compositions are much more variable, ranging from -4.6‰ to +10.8‰ (PDB). A covariant d¹⁸O vs. d¹³C trend in one horizon probably resulted from mixing of marine-meteoric phreatic fluids during siderite precipitation.

Groundwater compositions calculated from siderite isotopic compositions and empirical paleotemperature estimates range from -23.0‰ to -19.5‰ (SMOW). Minor element analyses show that the siderites are impure, having enrichments in Ca, Mg, Mn, and Sr. Compositions range from 79.16 to 93.70 mol % FeCO₃, with Ca⁺² substitutions up to 6.54 mol%, Mg⁺² up to 12.55 mol%, Mn⁺² up to 12.09 mol%, and Sr⁺² not exceeding 0.07 mol%. The minor element substitutions, and Mg/Fe and Mg/Ca + Mg ratios also suggest influence of marine fluids upon siderite precipitation. The paleosols developed in coal-bearing delta plain facies, and are interpreted to be analogous to modern inceptisols. The paleosols are characterized by: gleyed colors; rare root traces; abundant siderite; abundant organic matter; rare clay and silty-clay coatings and infillings; some preservation of primary sedimentary stratification; and a lack of ferruginous oxides and mottles. The pedogenic features suggest that these were poorly-drained, reducing, hydromorphic soils. Model derived estimates of precipitation rates for the Late Albian of the N. Slope, AK (485-626 mm/yr) are consistent with precipitation rates necessary to maintain modern peat forming environments. This information reinforces the mutual consistency between empirical paleotemperature estimates and isotope mass balance models of the hydrologic cycle, and can be used in future GCM modeling experiments of "greenhouse-world" climates to constrain high latitude precipitation rates in simulations of ancient worlds with decreased equator-to-pole temperature gradients.