STABLE ISOTOPE TEMPERATURE RECONSTRUCTIONS FROM PERMO-PENNSYLVANIAN PEDOGENIC MINERALS: INSIGHT INTO TERRESTRIAL CLIMATE DURING MELTDOWN OF THE LATE PALEOZOIC ICEHOUSE

Mineralogic, chemical, oxygen and hydrogen isotope compositions of 15 pedogenic phyllosilicate and 8 hematite samples from Permo–Pennsylvanian-age paleosols are presented from the eastern shelf of the Midland basin, Texas (Western Equatorial Pangea). The δD values of the phyllosilicates range from –69‰ in Upper Pennsylvanian to –55‰ in Lower Permian samples, whereas δ¹⁸O values range from 19.5‰ in Upper Pennsylvanian to 22.7‰ in Lower Permian samples. Considering available temperature-dependent fractionation equations, the combined oxygen and hydrogen isotope compositions of the phyllosilicates correspond to crystallization temperatures in the soil ranging from 22±3°C in Upper Pennsylvanian paleosols to 35±3°C in Lower Permian paleosols.

Paleopedogenic hematite δ¹⁸O values range from –0.4‰ in Upper Pennsylvanian to 3.7‰ in Lower Permian samples, and they exhibit a similar stratigraphic trend to δ¹⁸O values measured from coexisting paleopedogenic phyllosilicate. At five stratigraphic levels that span the Permo-Pennsylvanian boundary, hematite δ¹⁸O values were used in conjunction with coexisting phyllosilicate δ¹⁸O values to calculate an oxygen isotope pair paleotemperature estimate of mineral co-precipitation. The oxygen isotope difference between coexisting hematite and phyllosilicate (Δ¹⁸O) ranges from 18.7‰ to 20.1‰, and corresponds to temperatures from 24±3°C in the Late Pennsylvanian to 35°C±3°C in the Early Permian. Significantly, the (1) single-mineral phyllosilicate and (2) phyllosililcate-hematite oxygen isotope pair calculations provide paleotemperature estimates that are, within analytical uncertainty, indistinguishable from one another. These results suggest that ancient pedogenic systems can retain their original isotopic compositions and represent important paleoenvironmental archives. In particular, this stratigraphic trend of estimated temperatures suggests that Early Permian surface temperatures may have been up to 10 °C warmer than those of the latest Pennsylvanian, an interval of time that is believed to overlap with breakdown of the Gondwanan icesheet. The relationship of this paleotemperature record to regional ecosystem change, floral extinction and atmospheric CO₂ will be discussed.