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Inconsistencies BetweenPangean Reconstructions andBasic Climate ControlsClinton M. Rowe,1* David B. Loope,1 Robert J. Oglesby,1Rob Van der Voo,2 Charles E. Broadwater1

The supercontinent Pangea dominated our planet from the Permian into the Jurassic.Paleomagnetic reconstructions have been used to estimate the latitudinal position of Pangeaduring this 100-million-year period. Atmospheric circulation, recorded by eolian sandstones in thesouthwestern United States, shows a broad sweep of northeasterly winds over their northernmostextent, curving to become northwesterly in the south: This evidence is consistent withpaleomagnetic reconstructions of the region straddling the equator in the Early Permian but is atodds with its northward movement to about 20°N by the Early Jurassic. At least one of thefollowing scenarios must be true: The latitude based on paleomagnetism is incorrect; theinterpretation of how winds shaped the dunes is mistaken; the basic climate controls in the Jurassicwere different than those of today; or the paleogeographic reconstructions available areinsufficient to adequately reproduce the wind fields responsible for dune formation.

From the Permian into the Jurassic—100million years of Earth history—the super-continent Pangea dominated our planet.Detailedmaps of this pole-to-pole landmass, withpaleomagnetic data from each continental block,were published in the 1980s, establishing paleo-magnetism as the method par excellence ofestimating paleolatitude (1). Geologic work de-lineating the extent of climate-sensitive sedimen-tary rocks (e.g., coal, glacial till, evaporites, andcarbonate sediments) has often helped to reduceuncertainties in the quality of paleomagnetic dataand has sometimes led the way to importantreinterpretations (2, 3).

Atmospheric circulation is driven by thelatitudinal distribution of solar heating, which ismodified by the rotation of the planet and thedistribution of landmasses. Eolian dunes conclu-sively provide direct evidence of atmosphericcirculation (4, 5). Opdyke and Runcorn (6) werethe first to recognize the planetary-scale windsthat deposited the Late Paleozoic eolian cross-strata of Wyoming and northern Utah. The mark-edly consistent dip directions of Early Permianthrough Early Jurassic cross-strata in the westernUnited States (7, 8) are accurate indicators ofpaleowind direction (Fig. 1), because they weredeposited by transverse dunes that migrateddownwind by repeated avalanching (9). We haveextended the trade-wind explanation for theWyoming rocks southward by interpreting eolianstrata of the Colorado Plateau (southern Utah andArizona) as records of northwesterly winds lying

south of the trades in Pangea’s monsoonal windregime (10) (Fig. 2).

The Paleozoic formations that contain themost abundant evidence of life in the ancientdunefields (Cedar Mesa and Esplanade sand-stones) were deposited by the northwesterlywinds; the formations deposited farther north inthe trade-wind belt (i.e., Tensleep,Weber, Casper,andDeChelly) contain comparatively fewer signsof life. These wind and biotic patterns persistedthrough the Early Jurassic (11). During the entireinterval, the dominant winds in the north camefrom the northeast (NE), curving to the northwest(NW) over the southern portion of the outcrops.Jurassic dunes in the south were seasonallydrenched with rain, whereas weaker southeast(SE) winds reversed the dry-season slip faces(12).

Conventionalwisdom, largely based on paleo-magnetic evidence, places the plateau along orjust north of the equator during the EarlyPermian, which then moves north through theTriassic to lie near 20°N by the Early Jurassic, ascalculated from published mean poles. Bestestimates for the global mean poles for ~200million years ago vary between 70°N, 96°E and66°N, 91°E (in North American coordinates),with the variations being minor and dependingon methodology (13). These mean poles yieldpaleolatitudes for a location in SE Utah (40°N,110°W) ranging between 17°N and 28°N.We donot know of any published paleomagneticsyntheses that placed the area south of theequator during the Early Jurassic. Yet the con-stancy of the wind regime indicated by the dipdirections of the cross-strata suggests that thesand dunes and the plateau stayed within thesame climatic zone during the entire time span.Furthermore, the inferred northward migration

appears to be inconsistent with basic qualitativeconcepts of climate: NE winds changing south-ward to northwesterlies would be out of placenear 20°N but would fit well near and just southof the equator, where NWwinds represent cross-equatorial flow induced by a strong summermonsoonal circulation in the southern hemi-sphere (SH). The wind regimes recorded by therocks are consistent with paleogeographic recon-structions and climate simulations (14) of theLate Permian, with the Colorado Plateau strad-dling the equator, but are very much at odds witha northward movement to about 20°N by theEarly Jurassic. Given our understanding of basicclimate controls, as supported by climate modelresults, the wind regime that formed the eoliansandstones of the Colorado Plateau must haveoccurred on or south of the equator rather than atthe generally accepted paleomagnetic latitude ofabout 20°N (15).

An earlier attempt at paleogeographic inter-pretation (10) was partially based on the windregime simulated by a Jurassic climate model(16) and on paleomagnetic data collected fromColorado Plateau strata (17–19). The climatemodel showed trade winds during December,January, and February turning to become west-erly a few degrees north of the equator. Thiscorresponded reasonably well to estimates ofpaleolatitude for the Colorado Plateau (17, 20)but did not fit well with paleomagnetic data fromthe eastern United States (which placed the centerof the plateau at about 18°N) nor with the Pangeanreconstruction shown by the Paleomap Project(21). Moreover, the Jurassic climate model hadextremely low resolution (8° in latitude by 10° inlongitude) andwas simplistic by current standards.More recently, geophysicists have called attentionto the importance of sediment compaction topaleomagnetic interpretations, especially for rocksin which the paleomagnetic signal is carried bydetrital hematite. Comparisons of paleomagneticdata from such sedimentary rocks with those fromigneous rocks (which do not compact) indicatethat sedimentary rocks are likely to yield paleo-latitudes that are too low (22, 23). Because thesedimentary rocks from the plateau that providedthe evidence for a low-latitude position containdetrital hematite, most paleomagneticists wouldnow favor the higher-latitude interpretation for theplateau, thereby aggravating the discrepancy withour earlier climate-based interpretation.

In an attempt to resolve the discrepancy be-tween winds that appear to have been constantover 100My and the paleomagnetic evidence fornorthward movement of the Colorado Plateau,we made a series of climate simulations for theEarly Jurassic using the latest version of theCommunity Climate System Model (24, 25)from the National Center for Atmospheric Re-search. We made four model simulations withatmospheric CO2 at present-day levels; two ofthese runs used “conventional wisdom” for EarlyJurassic paleogeography based on Paleomap

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1Department of Geosciences, University of Nebraska,Lincoln, NE 68588–0340, USA. 2Department of GeologicalSciences, University of Michigan, Ann Arbor, MI 48109–1005, USA.

*To whom correspondence should be addressed. E-mail:crowe1@unl.edu

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(21), and two were made with Paleomap-derivedpaleogeography shifted 30° southward along thenominal 150°E meridian to put the ColoradoPlateau under a favorable SH wind regime. Foreach pair of runs with a given paleogeography,one was made with prescribed sea surfacetemperatures (SSTs) for the Early Jurassicderived from Chandler et al. (16) and the otherwas made with SST computed via a slab oceanmodel component. We conducted additional

model runs using the unshifted paleogeographywith constant 100-m topography and with uni-form vegetation to exclude these factors as majorinfluences on atmospheric circulation. Finally, toinvestigate the impact of increased CO2, weconducted two additional model runs using theshifted paleogeography, the slab ocean model,and, respectively, four times and eight timespresent-day CO2 levels. All model runs were atT31 (3.75° in latitude by 3.75° in longitude)resolution, and the last 10 model years wereaveraged to produce climatologies for analysis.

Every model configuration yielded a strongmonsoon that is hemispherically symmetric [thatis, with strong low pressure in the summerhemisphere and strong high pressure in thewinter hemisphere (Fig. 3)]. Low-latitude windsover western Pangea during SH summer curvefrom northeasterly trades in the northern hemi-sphere (NH) to northwesterlies after crossing theequator. These simulated winds are physicallyreasonable and consistent with trade-wind flowin the winter hemisphere and strong monsoonalheating in the summer hemisphere. This is theonly wind regime with the large, concave-to-the-east, north-to-south sweep in agreement with thewinds recorded by the eolian sandstones of thewestern United States. This is also consistentwith current concepts of the atmospheric generalcirculation and would indicate a low-latitude SHlocation for the Colorado Plateau during theJurassic. Furthermore, every model configurationyielded the necessary countervailing SE windsduring SH winter, which are consistent with thekey characteristics of the southernmost paleo-dunes. In addition, every simulation exhibitsseasonally dry periods for the region of theColorado Plateau, which is consistent with therock record (12). Finally, although the atmo-spheric concentration of CO2 for the EarlyJurassic is uncertain, the model results obtainedto date show that the winds are insensitive to arange of CO2 values from the present-day

concentration (355 parts per million) to eighttimes that amount. However, increased atmo-spheric CO2 does change modeled precipitation,with possible impacts on the migration of thedunes (fig. S1).

Results from our simulations (Fig. 3), likethose of the low-resolution model (16), show thatalthough the trade winds turned while north ofthe equator in eastern Pangea, these winds turned5° to 10° south of the equator in western Pangea.Thus, we could infer that this was the positionoccupied by the Colorado Plateau (1 or 2 gridpoints south of the placement in the earlierstudy). Our model would then indicate that theJurassic sandstones were deposited between10°S and 20°S, not 10°N to 20°N as suggestedby paleomagnetism-based reconstructions.Assuming that winds inferred from the sand-stones should correspond to the winds shownby the model, either the climate model is in-correct or the northward movement of theColorado Plateau occurred much later thansuggested by current paleogeographic recon-structions. This result is independent of (i)whether topography representative of the Juras-sic is imposed, (ii) if the geography is shiftedsuch that the Colorado Plateau is centered at10°S, and (iii) if the geography is robust re-gardless of whether SSTs are prescribed orcalculated via a slab ocean model.

If we assume that the winds inferred from thepaleodunes are correct, the climate model resultsand the paleomagnetism-based paleogeographicreconstructions cannot both be correct, thoughboth can be wrong.We are left with the followingpossibilities: (i) The paleomagnetism-based paleo-geographic reconstructions are incorrect inmoving the Colorado Plateau from around theequator to about 20°N between the EarlyPermian and the Early Jurassic; (ii) The EarlyJurassic climate model results are incorrect insimulating a pattern of northeasterly trade windsnorth of the equator, arcing just south of the

Fig. 1. The eolian sandstones of the ColoradoPlateau (dark shading) reach an aggregate of 2500min thickness and span about 140 My of Earthhistory. The tails of the black “tadpoles” (circleswith extending lines) point in the mean dipdirection of dune cross-strata and, therefore, pointin the downwind direction of the paleowinds thatdrove the dunes. From the base of the Cedar MesaSandstone (Ss) (bottom) to the top of the TempleCap Ss, the rocks show northwesterly winds(interpreted as the belt of tropical westerliessituated south of the trade winds). The DeChellySs is somewhat anomalous, showing northeasterlywinds. Northern exposures of the Coconino Ss,however, record northeasterly winds (interpretedas trade winds); to the south, it records winds fromthe NW. West-to-east variations in rock strata aredepicted across the diagram; vertical lines showmajor unconformities (gaps in the rock record).Fm, xxxx xxxx xxxx; Mbr, xxxx xxxx xxxx; Ls, xxxxxxxx xxxx. [Modified from Blakey et al. (26) andPeterson (7)]

Fig. 2. Dip directions of cross-strata of Late Paleozoic (left) and Early Jurassic (right) eolian sandstones.Tails of “tadpoles” point downwind; each represents scores of measurements. Large gray arrows show theflow of trade winds (top arrows) and tropical westerlies (bottom arrows). Paleomagnetic data indicate that,during the Permian, Utah was near the equator but that, by the Jurassic, it had drifted to 20°N. [Modifiedfrom Peterson (7)]

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equator into monsoonal-driven northwesterlies;or (iii) The circulation patterns inferred for thedunes represent an extreme climate state that isfar removed from the mean state that we havesimulated thus far.

The paleomagnetic reconstructions appear tobe tightly constrained by data (23); so, if paleo-magnetic interpretations are indeed the cause ofthis conundrum, a basic assumption of thepaleomagnetism model would have to be ques-tioned; given the consistency of Early Jurassicpaleomagnetic poles from the Atlantic-borderingcontinents (13) and the convincing paleolatitudedeterminations from the Newark sediments (23),we are not ready to argue that the paleomagneticfieldmodel must be wrong. However, the climatemodel results appear to be very robust based onour understanding of fundamental climaticcontrols. Indeed, the basic qualitative nature ofthese results could have been inferred just fromthis understanding, without the use of a climatemodel. Our modeling work to date suggests thatatmospheric CO2 is not likely to have a sub-stantial impact on the orientation and migrationof paleodunes, in large part because of the spatial

and seasonal homogeneity of this gas (whichmeans that it warms or cools more-or-lessuniformly around the globe). The most relevantpotential climate control that we have not yetconsidered is changing orbital states. Orbitalcyclicity is known to strongly affect both thelatitudinal and seasonal distribution of solarradiation and could therefore affect the monsoon-al circulations that prevailed over Pangea. Fur-thermore, the amplitudes and periodicities ofobliquity, precession, and eccentricity cycles inthe past could have differed from those at present.

During the past two decades, climate modelshave been increasingly used to simulate atmo-spheric circulations of the past. Slipface orienta-tions of modern dunes coincide well with thepattern of modern circulation (4), and we haveshown that the dip directions of avalanche-dominated eolian cross-strata can, in conjunctionwith model simulations, be used to accuratelydelineate ancient atmospheric circulation. Pastclimate reconstructions are important in their ownright, and they have previously provided strongconstraints on paleogeographic reconstructionsbased on paleomagnetic data. Although proxy

records such as coal and evaporite deposits aresubject to uncertain interpretation of paleocli-matic conditions (e.g., temperature, precipitation,and evaporation), wind directions preserved ineolian sediments relate directly to the atmospher-ic circulation. Furthermore, atmospheric circula-tion patterns can be a more profound indicator ofa given climatic state than temperature and pre-cipitation alone. Because the atmospheric generalcirculation is strongly linked to latitude, this in-formation has great value as an independent checkon magnetism-based paleogeographies.

References and Notes1. E. Irving, Proc. Natl. Acad. Sci. U.S.A. 102, 1821

(2005).2. R. Van der Voo, Paleomagnetism of the Atlantic, Tethys,

and Iapetus Oceans (Cambridge Univ. Press, Cambridge,1993).

3. P. H. Heckel, B. J. Witzke, in The Devonian System:A Palaeontological Association International Symposium,M. R. House, C. T. Scrutton, M. G. Bassett, Eds.(Palaeontological Association, London, 1979), vol. 23,pp. 99–123.

4. D. G. Blumberg, R. Greeley, J. Clim. 9, 3248 (1996).5. R. Sridhar et al., Science 313, 345 (2006).6. N. D. Opdyke, S. K. Runcorn, Bull. Geol. Soc. Am. 71,

959 (1960).7. F. Peterson, Sediment. Geol. 56, 207 (1988).8. Peterson (7) reports consistency values (vector mean

strength) for the dip directions of eolian sandstones fromwestern United States paleowind indicators. These factorsare high (above 0.80) for nearly all of the Pennsylvanianthrough Early Jurassic sandstones and many are wellabove 0.90. The lowest factors (0.67 and 0.75) are forthe most areally extensive unit (the Navajo Sandstone)and, based on the location of this sandstone formation,likely reflect key geographic differences in the ancientwind field (that is, the arc connecting the trades andtropical westerlies) that are fundamental to our results.

9. G. Kocurek, Annu. Rev. Earth Planet. Sci. 19, 43 (1991).10. D. B. Loope, M. B. Steiner, C. M. Rowe, N. Lancaster,

Sedimentology 51, 315 (2004).11. D. B. Loope, C. M. Rowe, J. Geol. 111, 223 (2003).12. D. B. Loope, C. M. Rowe, R. M. Joeckel, Nature 412, 64

(2001).13. J. Besse, V. Courtillot, J. Geophys. Res. 107, 10.1029/

2000JB000050 (2002).14. F. Fluteau, J. Besse, J. Broutin, G. Ramstein, Palaeogeogr.

Palaeoclimatol. Palaeoecol. 167, 39 (2001).15. Paleomagnetic evidence indicates that (relative to the

North American craton) the Colorado Plateau has beenstructurally rotated clockwise about 6° since the LateCarboniferous (20). Counter-clockwise rotation of thepaleowind vectors (derived from Permian through EarlyJurassic rocks) to correct for this deformation would makethe wind vectors in the southern Colorado Plateau evenmore westerly and makes placement of Jurassic strata inthe subtropical trade-wind belt even more anomalous.

16. M. Chandler, D. Rind, R. Ruedy, Bull. Geol. Soc. Am. 104,543 (1992).

17. M. B. Steiner, in Mesozoic Paleogeography of the West-Central United States, M. W. Reynolds, E. D. Dolly, Eds.(Rocky Mountain Paleogeography Symposium 2, Societyof Economic Paleontologists and Mineralogists, xxxxxxxxx,xxxxxxxxxx, 1983), pp. 1–11.

18. S. R. May, R. F. Butler, J. Geophys. Res. 91, 11519(1986).

19. D. R. Bazard, R. F. Butler, J. Geophys. Res. 96, 9847(1991).

20. M. B. Steiner, Tectonics 22, 1020 (2003).21. C. R. Scotese, PALEOMAP; www.scotese.com/jurassic.htm.22. L. Tauxe, Earth Planet. Sci. Lett. 233, 247 (2005).23. D. V. Kent, L. Tauxe, Science 307, 240 (2005).24. W. D. Collins et al., J. Clim. 19, 2122 (2006).25. B. L. Otto-Bliesner et al., J. Clim. 19, 2526

(2006).

D

A B

F

C

E

Fig. 3. Simulation results for NH winter [xxxxx xxxxx xxxxx (DJF), left column] and NH summer [xxxxxxxxxx xxxxx (JJA), right column] for unshifted paleogeography with prescribed SST (A and B), shiftedpaleogeography with prescribed SST (C and D), and shifted paleogeography with slab ocean (E and F).Sea-level pressure (hPa) is contoured at 4-hPa intervals, and winds at the lowest model level are given asvectors, with a 10 m s–1 reference vector shown at bottom. The Colorado Plateau (outlined in red) islocated within the area.

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26. R. C. Blakey, F. Peterson, G. Kocurek, Sediment. Geol. 56,3 (1988).

27. This work was supported by a grant from NSF to D.B.L.and C.M.R. The authors thank C. R. Scotese for providingtopographic data for his Jurassic paleogeography

reconstruction. Computing support was provided by theNational Center for Atmospheric Research and theUniversity of Nebraska Research Computing Facility.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/[vol]/[issue no.]/[page]/

DC1Fig. S1

18 June 2007; accepted 12 October 200710.1126/science.1146639

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