PALEOMAGNETIC RECORD OF CLIMATE CONDITIONS DURING THE PERMO-TRIASSIC

The Permian-Triassic boundary has been interpreted as representing a shift to Triassic greenhouse conditions and increased weathering; this shift may be recorded in altered granites which occur at unconformity surfaces and contain Permian-Triassic secondary magnetizations in Europe and North America. Determining the origin of and how widespread this event was could have implications for understanding the unique paleoclimate conditions at the end of the Permian. Early Triassic age secondary magnetizations are documented by Ricordel et al. (2007) throughout mainland Europe. The magnetizations are held in authigenic hematite, which is precipitated during interaction of basement rock with weathering fluids. To test if this hypothesis extends into Scotland and North America, samples of basement rocks at unconformities were taken to determine if this magnetization is widespread. Samples of basement rocks with overlying Permo-Triassic (P-T) sediments were collected in Oklahoma, Colorado, and Scotland. The Long Mountain Granite in central Oklahoma is a reddened granite that becomes grey/green at depth. The red granite holds a stable magnetization in hematite; the age of the magnetization is ~295ma (paleopole: 44.9°N, 124.9°E). In west-central Scotland, reddened Dalradian schists that are overlain by late Permian sandstones also hold a stable magnetization in hematite. The magnetization was acquired ~270ma (paleopole: 52.7°N, 160.5°E). In western Colorado, reddened granitic basement rock in Unaweep Canyon was sampled. The basement is overlain by Triassic sandstones. The reddened granites do not contain stable magnetizations, despite hematite precipitation along joints in the granite, which indicate pervasive fluids penetrated the basement. These findings indicate that in mainland Europe, early Triassic magnetizations record a widespread weathering event; this event was not pervasive throughout Pangea. Local climatic variables (CO2 fluctuations) and local tectonics influences during the Permian may be responsible for the earlier secondary magnetizations recorded in Oklahoma and Scotland. The lack of secondary magnetization that is P-T in age in Colorado indicates that the climate conditions, and therefore weathering processes, at the P-T boundary may have been more localized and variable.