METHANE FLUXES FROM MID-CRETACEOUS WETLAND SOILS: INSIGHTS GAINED FROM CARBON AND OXYGEN ISOTOPIC STUDIES OF SPHAEROSIDERITES IN PALEOSOLS

Pedogenic sphaerosiderites (mm-scale spherulites of FeCO₃) form in wetland soils from reducing groundwaters, and are commonly associated with coal-bearing strata. Stable isotopic studies of sphaerosiderites in Albian (mid-Cretaceous) siliciclastic mudstone paleosols of the North American Western Interior Basin show pronounced paleolatitudinal gradients, with d¹⁸O values decreasing and d¹³C values increasing at higher latitudes (data currently extend from 34° to 75°N paleolatitude). The mid-Cretaceous latitudinal d¹⁸O gradient is isotopically lighter and steeper than that calculated for the modern, and we have argued (Ludvigson et al., 1998, Geology 26:11:1039-1042; White et al., 2001, Geology 29:4:363-366) that these data indicate a more active hydrologic cycle in the Albian. Oxygen isotope mass balance modeling results suggest Albian mid-latitude precipitation rates ranging up to 2000-3000 mm/yr between 40-60°N, and might in part account for the abundance of thick mid-latitude coals during this time. Extremely light d¹³C values in sphaerosiderites (£ -30 ‰ VPDB) suggest formation from fluids with DIC derived from oxidation of biogenic methane, and are confined to paleolatitudes less than 40°N. Extremely heavy d¹³C values in sphaerosiderites (³ 0 ‰ VPDB) suggest precipitation from fluids with DIC controlled by primary methanogenesis, and are confined to paleolatitudes greater than 45°N. Maximum d¹³C values (³ +10 ‰ VPDB) in pedogenic siderites occur in coal-bearing strata from North Slope, Alaska at a paleolatitude of about 75°N. The noteworthy trend towards heavier d¹³C values in high latitude sphaerosiderites suggests that methane fluxes from mid-Cretaceous wetlands was principally a high latitude phenomenon. Voluminous methane emissions from high latitude wetlands could suggest that regional concentration of this greenhouse gas acted as a positive feedback mechanism for sustaining polar warmth during the “Greenhouse World” of the mid-Cretaceous.