GLOBAL CHANGES IN SUBAERIAL WEATHERING INTENSITY DURING THE EARLY TRIASSIC

Changes in chemical weathering intensity in continental environments during the end-Permian and Early Triassic crisis are not well understood. Recent research has demonstrated an extreme warming event during this interval (Sun et al., 2012, Science 338:366). Thermal maxima of other ages (e.g., the PETM) are known to have resulted in greatly intensified chemical weathering, so a similar intensification might be expected for the Early Triassic. In this study, we investigated variations in the chemical index of alteration (CIA = Al₂O₃/(Al₂O₃+K₂O+Na₂O); Nesbitt and Young, 1982, Nature 299:715) of marine sediments in 20 Permian-Triassic sections as a function of age and paleolatitude. (We use a modified form of the CIA equation that discards the CaO term of the denominator, which can lead to anomalies in carbonate-rich successions.) Preliminary results for several sections show rapid changes in CIA during the end-Permian crisis. Sections at Guryul Ravine, India and Selong, Tibet exhibit Late Permian CIA of 0.76-0.78 and 0.78-0.80, respectively. Both sections exhibit a rapid decrease to <0.60 followed by a rapid increase to ~0.90 during the crisis before stabilization in the Early Triassic. The Black Ridge West section in east-central Greenland exhibits a stepwise shift from 0.65 to 0.70 at the Permian-Triassic boundary. In general, boreal sections exhibit lower CIA—both before and after the crisis—than sections in warm tropical regions, reflecting a latitudinal gradient in weathering intensity, the scale of which will be constrained by further work. Ultimately, detailed analyses of this type will provide important insights into global-scale patterns of climate change during major geo-events.