THE OXYGEN ISOTOPIC RECORD OF LATE PENNSYLVANIAN TO MID-PERMIAN CLIMATE CHANGE IN NORTH AMERICA AND THE RUSSIAN PLATFORM

To evaluate the isotopic record of climate change in the Late Paleozoic, we have compiled new and published oxygen isotopic data for the late Carboniferous through Middle Permian (310 – 260 Ma) based on the measurements of more than 1000 brachiopod shells. The oxygen and carbon isotope records are predominantly from the U.S. mid-continent (Texas, Oklahoma, Kansas) and the Russian Platform (Moscow Basin, Urals), two regions that provide well-preserved marine fossils spanning a broad time interval. To screen new specimens for diagenesis, shells are thin-sectioned and photographed in plane light and cathodoluminescence. Nonluminescent shell areas with clear crystals and preserved microstructures are microsampled (< 150 ug) from billets using a dental drill or pick. Oxygen isotopic compositions of U.S. mid-continent shells average -2.3 ± 0.7‰ VPDB (N = 676), while Russian Platform data average -2.6 ± 1.3‰ (N = 253). These mean values equate to paleotemperatures of 26º and 28ºC if modern seawater δ¹⁸O values (0‰ SMOW) are assumed. Oxygen isotope values for Kansas and Oklahoma remain relatively constant from the late Pennsylvanian and Early Permian, whereas Texas values decrease about 1‰. Neither record shows the ¹⁸O enrichment expected with Asselian glacial maximum, either because these sediments were deposited during interglacial (perhaps ice-free) conditions, or because freshwater input and restricted circulation have modified the isotopic composition of seawater in the epicontinental seas. δ¹⁸O increases in the late-early to mid Permian, opposite the trend expected during this time of glacial contraction, and correlating with evidence for excess evaporation. The Russian Platform record shows a δ¹⁸O maximum during the glacial maximum in the Asselian (Korte et al., 2005, Palaeogeog., Palaeoclim., Palaeoecol. 224, 333-351). New data for the Kungarian, a time of low ice volume, show some of highest δ¹⁸O values in the Permian, likely due to aridification. These results highlight the need to develop proxies to identify the effects of restricted circulation on the isotopic composition of epicontinental seawater.