Paleobotanical Evident for “Pluvial” Intervals in the Western Pangean Tropics During the Early Permian

April 29th, 2008

The transition from the Pennsylvanian to the Permian in the tropics of western Pangea was marked by a general trend toward increased temperature and decreased soil moisture, based on geophysical indicators, such as paleosol morphologies and oxygen isotopes. Vegetation tracked these changes and there is a 1:1 correspondence of species pools with climate proxies: floras dominated by spore-producing plants and primitive seed plants characterize wetter-cooler conditions, with floras dominated by more derived seed plants characterizing drier-warmer conditions. Taxa characteristic of wet habitats, particularly tree ferns and sphenopsids, continue to appear sporadically during periods that geophysical indicators suggest were dry-warm, possibly reflecting persistent wet sites on otherwise more xeric landscapes. However, during the middle Artinskian, parts of the Waggoner Ranch Formation of north-central Texas are characterized by the repeated recurrence of tree-fern dominated floras within an interval that includes xeric, seed-plant dominated floras and physical indicators of warm-dry climates. There are only minor, but noteworthy, overlaps between these two species pools. In several instances, the wet floras occur in channel-form deposits suggesting short, “pluvial” periods that did not leave significant paleosol records. Considering the close association of floral composition and climate, it can be inferred that there were fluctuations in soil moisture and possibly temperature that permitted the short-term spatial expansion of wetland vegetation during the Early Permian, probably from populations persisting locally in sites marginal to water bodies. These intervals of climate oscillation further suggest that glacial-interglacial cycles, similar to those of the Pennsylvanian SubPeriod, characterized Permian glaciations as well.

Kristopher Rhodes, ’08 Mount Vernon, IA
Major: Geology
Bill DiMichele
National Museum of Natural History, Smithsonian Institution
Dan Chaney
National Museum of Natural History, Smithsonian Institution
Neil Tabor
Southern Methodist University

Sponsor: Benjamin Greenstein

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