High Latitude Meteoric Delta O-18 Compositions: Paleosol Siderite in the Middle Cretaceous Nanushuk Formation, North Slope, Alaska

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Geography and Geology


Siderite-bearing pedogenic horizons of the Nanushuk Formation of the North Slope, Alaska, provide a critical high paleolatitude oxygen isotopic proxy record of paleoprecipitation, supplying important empirical data needed for paleoclimatic reconstructions and models of "greenhouseworld" precipitation rates. Siderite delta(18)O values were determined from four paleosol horizons in the National Petroleum Reserve Alaska (NPR-A) Grandstand #1 Core, and the values range between -17.6parts per thousand and -14.3parts per thousand Peedee belemnite (PDB) with standard deviations generally less than 0.6parts per thousand within individual horizons. The delta(13)C values are much more variable, ranging from -4.6parts per thousand to +10.8parts per thousand PDB. A covariant delta(18)O versus delta(13)C trend in one horizon probably resulted from mixing between modified marine and meteoric phreatic fluids during siderite precipitation. Groundwater values calculated from siderite oxygen isotopic values and paleobotanical temperature estimates range from -23.0parts per thousand to -19.5parts per thousand standard mean ocean water (SMOW). Minor element analyses show that the siderites are impure, having enrichments in Ca, Mg, Mn, and Sr. Minor element substitutions and Mg/Fe and Mg/ (Ca + Mg) ratios also suggest the influence of marine fluids upon siderite precipitation. The pedogenic horizons 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 that developed in coal-bearing delta plain facies and are similar to modern Inceptisols. Model-derived estimates of precipitation rates for the Late Albian of the North Slope, Alaska (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 global circulation modeling (GCM) experiments of "greenhouseworld" climates to constrain high latitude precipitation rates in simulations of ancient worlds with decreased equator-to-pole temperature gradients.

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Geological Society of America Bulletin





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