METEORIC SPHAEROSIDERITE LINES FROM THE ATOKAN (LANGSETTIAN) CROOKED FORK GROUP, EASTERN TENNESSEE: A FIRST-CUT COMPARISON BETWEEN PALEOEQUATORIAL PRECIPITATION RATES FROM THE CARBONIFEROUS ICEHOUSE AND CRETACEOUS GREENHOUSE WORLDS

Pedogenic siderites formed during gley overprinting of vertic paleosols in the Atokan Crooked Fork Group of eastern Tennessee (Driese and Ober, 2005, JSR 75:997-1010). On the basis of chemical index of alteration (CIA-K), mean annual precipitation (MAP) rates were estimated at 1300 mm/yr, similar to modern tropical humid environments (ibid.). The occurrence of sphaerosiderites and microcrystalline siderites in well-contrained micromorphologic contexts from the Crook Fork paleosols opens opportunity for examination of Meteoric Sphaerosiderite Line (MSL; Ludvigson et al., 1998, Geology 26:1039-1042) δ¹⁸O values from paleoequatorial sites (paleolatitude < 10º S) in the Early Carboniferous Icehouse World. We compare these Early Pennsylvanian data to a new MSL dataset from the Early Cretaceous (Albian) Caballos Formation of Colombia, a paleoequatorial site from the Cretaceous Greenhouse World. Latitudinal transects of the δ¹⁸O values of modern MAP have a pronounced sag centered at the equator from an “amount effect” distillation of ¹⁸O produced by high precipitation rates along the seasonally-migrating Inter-Tropical Convergence Zone. Higher precipitation rates are associated with lower δ¹⁸O values, and thus lower precipitation rates are associated with higher δ¹⁸O values. Atokan MSL δ¹⁸O values from the Crooked Fork Gp range from -7.1 up to -3.6‰ VPDB (n = 27, grand mean = -5.9‰ VPDB). Cretaceous MSL δ¹⁸O values from the Caballos Formation range from -6.9 up to -3.9‰ VPDB (n = 58, grand mean = -4.5‰ VPDB). Our first-cut comparison of the Pennsylvanian icehouse and Cretaceous greenhouse paleoequatorial MSL values is that they are quite similar, but that Pennsylvanian MSL δ¹⁸O values might be slightly lower. These initial empirical results counter a widely-held expectation among deep-time paleoclimatologists that paleoequatorial precipitation rates were much higher during greenhouse episodes.