Pangean Paleoclimate By Erin Eastwood GEO 387H November 18, 2008Abstract The Pangean supercontinent existed for more than 100 million years and had a profound influence on Earth's climate and atmospheric circulation system. Proxy paleowind data from aeolian (wind-blown) deposits in the rock record and climate models indicate a monsoonal circulation system through the existence of Pangea. However, inconsistencies exist between studies and the exact reconstruction of the monsoonal system remains unclear. 1. Introduction Wind is integral to the Earth’s climate system and a major component of atmospheric circulation. Historically, however, paleowind information has been under-utilized as input data for ancient climate models. Aeolian (wind-blown) sand dunes are the most direct proxies for paleowinds at the Earth’s surface because sand dunes form in direct response to the transport of surface sediment by wind. Aeolian sand dunes provide direct evidence of atmospheric circulation and the migration of dunes masks short-term climate variation and produces paleowind information at a scale similar to global climate models (Blumberg and Greeley, 1996). Where the study of dune fields is only feasible by remote and satellite images, aeolian systems have been used to interpret the wind directions on Earth (Kocurek and Ewing, 2005; Beveridge et al., 2006), Mars (Tsoar et al., 1979; Malin et al., 1998) and Titan (Lorenz et al., 2006). Aeolian deposits are common in the ancient rock record and have been used for decades to infer dune migration direction (e.g., Reiche, 1938; Curray, 1956; Poole, 1962; Peterson, 1988). More detailed reconstructions of dune morphology and behavior from cross-strata have been used to reconstruct wind regime (Rubin and Hunter, 1983; Kocurek et al., 1991; Crabaugh and Kocurek, 1993). Changes in wind directions within seasonal (e.g. monsoonal; Hunter and Rubin, 1983; Erin Eastwood Pangean Paleoclimate Page 2Chan and Archer, 1999; Loope et al., 2001) and daily cycles (Hunter and Richmond, 1988) have been distinguished from aeolian cross-strata. 2. Pangea The supercontinent Pangea dominated our planet for over 100 million years of Earth’s history, from the Late Pennslyvanian to the Jurassic, with maximum extent during the Triassic. This single continent stretched latitudinally across every part of the zonal atmospheric circulation, thereby producing an extraordinary effect on the global paleoclimate (Dubiel et al., 1991). The Permian-Triassic interval has been noted as a unique and extreme paleoclimate interval due to the global occurence of red beds and evaporite deposits in numerous locations world-wide (Dubiel et al., 1991; Glennie, 1987). During the Triassic the supercontinent was approximately symmetric about the equator (Parrish and Peterson, 1988). Approximately 2500m of aeolian-deposited sandstones accumulated in the south-western United States during the time of the Pangea supercontinent, the majority of deposits occurring on the Colorado Plateau (Blakey et al., 1988 (Figure 1). The Colorado Plateau is a modern highland centered at the Four Corners region, or the geographic point where the states of Arizona, New Mexico, Colorado and Utah meet. Peterson (1988) compiled paleowind data from cross-strata within these eolian units, creating a dataset of dune migration directions spanning the Late Pennsylvanian through Jurassic periods from which paleowind directions can be inferred. Peterson's (1988) dataset is the largest paleowind data set covering any region or time in Earth’s history (Loope et al., 2004). Using this data set with reconstructed Pangean paleogeography, atmospheric circulation models can be used to predict the climate and atmospheric circulation Erin Eastwood Pangean Paleoclimate Page 3during the Pangean supercontinent. Please note that in this paper, all descriptions of wind directions (e.g. north-westerly winds) refer to paleo-coordinates unless otherwise noted. Figure 1. Paleogeographic map showing the Colorado Plateau and Four Corners region. Map by Ron Blakey. http://jan.ucc.nau.edu/~rcb7/ 3. Paleomagnetism The polarity of the Earth’s magnetic field has repeatedly reversed orientations through time. These reversals are recorded by iron-bearing minerals in igneous rocks that magnetize parallel to the orientation of the Earth’s current magnetic field. Rocks forming today (e.g. Atlantic Mid-Ocean Ridge basalts) contain magnetizations that match the current orientation of the Earth’s magnetic field with magnetic north aligned with the North Pole. However, older rocks contain magnetizations that are opposite, or reversed, from today’s orientation (Pitman and Heirtzler, 1966). Rocks forming today at different latitudes are concordant with the modern magnetic field. Likewise, Permian-aged rocks from various latitudes should all align with the Erin Eastwood Pangean Paleoclimate Page 4Permian magnetic field, as long as the continents are located in their correct paleogeographic position. The paleolatitudes of these rocks can be determined by rotating each continent from its current geographic position to one that aligns with the Permian magnetic field (Dott and Prothero, 1992) (Figure 2). Multiple samples from each continent are required to produce reasonable levels of accuracy, and the continent positions are adjusted until the paleo magnetic pole positions from multiple continents coincide in the same location (Dott and Prothero, 2002) (Figure 2). Figure 2. Diagram showing the method to calculate the paleo magnetic pole using multiple data points (same age) from different continents. From Dott and Prothero (2002). Recently some scientists have adopted the method of using detrital hematite minerals from sedimentary rocks to determine the magnetization and paleolatitude of rocks (Steiner, 2003). However, the accuracy of these paleomagnetic reconstructions are limited due to the sediment compaction that detrital hematite grains experience during sediment burial and lithification. Comparisons of paleomagnetic data from sedimentary rocks (which undergo Erin Eastwood Pangean Paleoclimate Page 5compaction) and from igneous rocks (which do not compact) indicate that sedimentary rocks will frequently yield paleolatitudes that are too low (Kent and Tauxe, 2005). For Pangean paleogeographic reconstructions, traditional paleomagnetism methods have reconstructed a steady