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High latitude meteoric ¹⁸O compositions: Paleosol siderite in the Middle Cretaceous Nanushuk Formation, North Slope, Alaska

David F. Ufnar^,1, Greg A. Ludvigson^,2, Luis A. González^,3, Robert L. Brenner^#,4 and Brian J. Witzke^,5

¹ Department of Geology, University of Southern Mississippi, Box 5044, Hattiesburg, Mississippi 39406, USA
² Iowa Geological Survey Bureau, Iowa City, Iowa 52242, USA and Center for Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa 52242, USA
³ Department of Geology, University of Kansas, Lawrence, Kansas 66045-7613, USA
⁴ Department of Geoscience, University of Iowa, Iowa City, Iowa 52242-1379, USA
⁵ Iowa Geological Survey Bureau, Iowa City, Iowa 52242, USA

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 "greenhouse- world" precipitation rates. Siderite ¹⁸O values were determined from four paleosol horizons in the National Petroleum Reserve Alaska (NPR-A) Grandstand # 1 Core, and the values range between –17.6 and –14.3 Peedee belemnite (PDB) with standard deviations generally less than 0.6 within individual horizons. The ¹³C values are much more variable, ranging from –4.6 to +10.8 PDB. A covariant ¹⁸O versus ¹³C 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 –23.0 to –19.5 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 "greenhouse- world" climates to constrain high latitude precipitation rates in simulations of ancient worlds with decreased equator-to-pole temperature gradients.

Key Words: siderite • oxygen isotopes • Cretaceous • paleosols • carbon isotopes • paleoclimatology

This article has been cited by other articles:

				T. White, B. Witzke, G. Ludvigson, and R. Brenner Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence Geology, January 1, 2005; 33(1): 13 - 16. [Abstract] [Full Text] [PDF]

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