Oxygen and hydrogen isotope compositions of paleosol phyllosilicates: Differential burial histories and determination of Middle-Late Pennsylvanian low-latitude terrestrial paleotemperatures

The clay mineralogy, chemistry, and stable hydrogen and oxygen-isotope compositions were measured from 20 phyllosilicate samples representing 11 Pennsylvanian-age paleosol profiles taken from three cores in the Illinois basin in order to assess their utility as proxies of low-latitude terrestrial paleotemperatures. The majority of the samples are mineralogical mixtures of illite-smectite (I/S), kaolinite, and rarely discrete illite. Samples from a shallowly-buried locality in the northern part of the basin are dominantly composed of smectite-rich I/S, with variable amounts of kaolinite, and no discrete illite. Samples from deeply-buried, interior parts of the basin are composed of illite-rich I/S, variable amounts of kaolinite, and discrete illite. These phyllosilicate mixtures have delta O-18(V-SMOW) and delta DV-SMOW values that range from 17.2 parts per thousand to 23.0 parts per thousand and -56 parts per thousand to -27 parts per thousand, respectively. Assuming that the phyllosilicates preserve a record of isotopic equilibrium with Pennsylvanian meteoric waters, these oxygen and hydrogen isotope values correspond to crystallization temperatures ranging from 22 +/- 3 degrees C to 55 3 degrees C. The clay mineralogy, phyllosilicate delta O-18 and delta D) values and calculated crystallization temperatures of 44 degrees C to 55 degrees C from deeply buried localities in the interior of the basin are not consistent with a pedogenic origin. Instead, these trends are considered to be the result of diagenetic recrystallization of pedogenic minerals in response to greater depths of burial (by similar to 1.5 to 3 km) in the southerly, basin-center localities, as well as an interval of middle Permian elevated heat flow associated with magmatic intrusions in the southern part of the basin. Phyllosilicate mineralogy, delta O-18 and delta D values, and calculated phyllosilicate crystallization temperatures from a shallowly buried, northern locality in the Illinois basin are consistent with a pedogenic origin, and reveal a long-term warming trend from an average temperature of 23 +/- 3 degrees C in the lower Desmoinesian to an average temperature of 32 +/- 3 degrees C in the Missourian. This temperature change is coincident with a significant change in the composition of wetland vegetation in Euramerica, which has been attributed to a shift in low-latitude Pennsylvanian climate towards warmer and drier conditions in the Late Pennsylvanian. This study reveals the presence of a dynamic Late Paleozoic paleoequatorial icehouse climate characterized by significant low-latitude temperature variability unprecedented on modern Earth. (C) 2013 Elsevier B.V. All rights reserved.