TERRESTRIAL PALEOCLIMATOLOGY OF THE MID-CRETACEOUS GREENHOUSE I: CROSS-CALIBRATION OF PEDOGENIC SIDERITE & CALCITE d18O PROXIES AT THE HADLEY CELL BOUNDARY

We are investigating the position of the Hadley Cell boundary (HCB) in North America during a period of greenhouse warming forced by high pCO₂. Our interpretations hinge on the paleolatitudinal distribution of mid-Cretacous pedogenic carbonates. A N-S transect of the Cretaceous Western Interior Basin provides evidence for location of the HCB near 30 °N paleolatitude. Ufnar et al. (2002; Palaeo-3 188:51-71) outlined the distribution of Albian pedogenic siderites from 34 °N (central Kansas) to 75 °N (North Slope Alaska) paleolatitudes, all recording terrestrial paleoclimates with positive precipitation-evaporation balances expected north of the HCB. Our oxygen isotope mass balance model of the North American Cretaceous paleolatitudinal gradient in meteoric sphaerosiderite lines led us to postulate up to a 220% increase in mid-latitude precipitation rates over those of today. Calcic paleosols of the Aptian-Albian Antlers Formation of southeastern Oklahoma (29.5 °N paleolatitude) and exposure surfaces in marine carbonates of the Albian Glen Rose and Walnut formations of central Texas (25 °N paleolatitude) record terrestrial paleoclimates with negative precipitation-evaporation balances that are expected within the descending arm of the Hadley Cell. Diagenetic investigations of meteoric phreatic calcites in pedogenic nodules from the Antlers Formation yield meteoric calcite lines (MCLs) with d¹⁸O values of –8‰ VPDB. Fine-grained pedogenic calcites from these nodules have d¹⁸O values that were enriched by evaporation by up to 7.5‰. At a zonal average MAT of 27.7 °C, evaporative enrichments of these magnitudes suggest evaporative losses of 30-50% of the vadose soil water. The Glen Rose and Walnut formations yield MCL values of -4 and -6‰ VPDB respectively, and evince a flattening of the calculated paleoprecipitation d¹⁸O gradient into the subtropics, defined by d¹⁸O_water=-0.003x²–0.0249x–2.3276, where x=degrees paleolatitude and r²=0.96. Our new data from 25-34 °N are consistent with the extrapolated model results of Ufnar et al. (2002).