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

摘要

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.