Wastelands of tropical Pangea: High heat in the Permian

We study the sedimentary record to gain perspective on the range of environmental conditions through Earth’s history. Lithology, internal organization, fossil content and composition are used to reconstruct depositional environments, guided by a uniformitarian-approach: physical laws that command Earth systems today have not changed through time. Most Phanerozoic continental sediments indicate surface conditions within the range of modern Earth. This is intuitively obvious, for the fossil record documents vertebrate and plant life on land at least since the Silurian (ca. 440 Ma), and environmental conditions never went outside the limits where these can survive. Occasionally the sedimentary record preserves evidence for conditions too extreme for complex life, outside the modern range. Extraordinarily harsh climatic conditions are documented by Zambito and Bennison (2013, p. 587 in this issue of Geology), who use fl uid inclusion homogenization temperatures (T h ) in uppermost Lower Permian (ca. 270 Ma) halite beds of the Nippewalla Group (Kansas, United States) as proxies for surface air temperatures. The region was then in the tropics. Average T h values rise from those typical for the modern tropics (~26 °C) near the base of the Nippewalla Group, to ~40–45 °C in its lower and middle parts, then return to typical values of 21–33 °C toward its top. The zone of peak average T h includes maximum values of >70 °C, and diurnal variability of >30 °C, both more extreme than recorded on Earth today. Evidence for extremely high surface temperatures during deposition of the Nippewalla Group provides a better understanding of some of the peculiar aspects of the Permian terrestrial record in western tropical Pangea, but also presents paradoxical paleoclimate problems, which can be appreciated only within the broader context of Permian–Carboniferous (P-C) sediments, soils, and plant and animal fossil assemblages. During the late Paleozoic, the continents were grouped into two large landmasses: Laurasia, moving southward on the Northern Hemisphere, and Gondwana, moving northward on the Southern Hemisphere. At ~340–320 Ma (Early Carboniferous), they collided near the equator to form Pangea (Scotese et al., 1979). Around this time, climate cooled, with continental ice-sheet development documented by proxies collected at high paleolatitudes (near-fi eld) and low paleolatitudes (far-fi eld). There may have been multiple centers of ice-sheet growth, and retreats of variable extent and duration (Isbell et al., 2012). Beginning in the mid-Carboniferous (ca. 327 Ma), ice sheets grew from small centers, reached their acme in the Late Carboniferous–Early Permian (ca. 303–290 Ma), then shrunk again to small ice centers until the end of the “ice house” in the Late Permian (ca. 260 Ma; Fielding et al., 2008). Comparisons between near- and far-fi eld glacial indicators and associated paleoclimate patterns are hampered by a lack of accurate correlations. Long-term, “classical” Pennsylvanian–Permian sedimentary indicators of paleoenvironments and paleoclimate in terrestrial strata indicate that (1) in the Pennsylvanian, Euramerica had predominant humid, ever-wet “swampy” environments in which vast coal deposits formed; (2) dryer and seasonal, fl uvial-dominated, depositional conditions prevailed in the Early Permian, when the red-bed deposits across the central United States formed; and (3) in the late Early Permian, deposits formed under arid eolian-dominated conditions, with locally arid climates that included wet sabkha and playa depositional environments now exposed in the High Plains and Mountain West of the United States. Not all paleotropical basins preserve the entire lithostratigraphic trend, but long-term