Climate of the ~angea'

fudith Totman Parrish Department of Geosciences, Gould-Simpson Bldg., University of Arizona, Tucson, AZ 85721 USA

ABSTRACT Numerous climate models predict that the geography of the supercontinent Pangea was conducive to the establish- ment of a llmegamonsoonalJ1circulation. In general, geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the . Pangea in the Late had widespread peat formation in what is now central and eastern and and relatively dry conditions on the Colorado Plateau. The equatorial of the became drier through the end of the Carboniferous. By the , the equatorial region of Pangea was dry, and indicators of aridity and rainfall seasonality became more widespread. Wind directions from Colorado Plateau eolian are consistent with an increasing influence of monsoonal circulation at this time. In the Triassic, climate in the Colorado Plateau region became relatively wet, though still seasonal, and the few eolian sandstones indicate a major shift in wind direction at that time. In addition, sedimenta- tion in , which was in relatively high latitudes, took on a much drier and more seasonal character. These two events support the hypothesis that the Pangean monsoon was at maximum strength during the Triassic. In the Early , the Colorado Plateau region became arid again, but climate apparently became wetter in eastern Laurussia and . Finally, drying occurred in Gondwana and southern , indicative of the breakdown of the Pangean monsoon.

Introduction The supercontinent Pangea represented an excep- presence of a warm seaway, Tethys, to act as a tional phase in 's paleogeographic history, a source of moisture would have maximized sum- maximum of continental aggregation (Valentine mer heating. and Moores 1970) that began in the Carboniferous In the past decade, increasing attention has been with the collision of Laumssia and Gondwana and paid to the geography and climate of the supercon- culminated in the Triassic with the addition of Ka- tinent. Much of this work has concentrated on spe- zakhstan, , and parts of China and south- cific parts of the Pangean interval, which com- eastern (Smith and Briden 1977; Smith et al. prises the Late Carboniferous through Middle 1973; McElhinny et al. 1981; Klimetz 1983; Ziegler Jurassic Periods. The purpose of this paper is to et al. 1983). In the Triassic, exposed extended consolidate some of this work into a progress re- from about 85ON to 90° (Ziegler et al. 1983). Al- port on Pangean climatic studies. During the next though epeiric seaways were present, sea level was several , such efforts as "Project Pangea," the relatively low through much of what might be newest project under the Global Sedimentary Ge- called the "peak" Pangean interval-Permian and ology Program, will significantly increase our un- Triassic-and the area of exposed land was great derstanding of Pangea. (Vail et al. 1977). More importantly, most of this This paper is organized in four sections. First, exposed land area was in a contiguous mass. given is a brief history of Pangean paleogeographic First, it is expected that the continent would evolution. This is not intended to be an exhaustive have had an extraordinary effect on global climate. review of various paleogeographic models, but sim- It cut across, and therefore would have disrupted, ply to provide the basis for the discussion on cli- nearly every part of the zonal circulation. In addi- mate. Second, a review is presented of work prior to tion, the great size of the exposed land, the pres- 1982. Beginning in that , the dynamical under- ence of large portions in low mid-latitudes, and the pinnings for the evolution of Pangean climate were treated with global climate models. Prior to 1982, 1 Manuscript received August 17, 1992; accepted November only one paper limited to the Permian attempted 24, 1992. to apply climate models to the understanding of

[The Journal of , 1993, volume 101, p. 215-2331 @ 1993 by The University of Chicago. All rights reserved. 0022-1376/93/10102-001$1.00


Pangean climate (Nairn and smithwick 1976). comm.),so climatic conclusions herein are prelim- Nevertheless, this early work already outlined the inary until the paleogeography is better known. cardinal feature of Pangean paleoclimate what In general, Pangea came into being during the Kutzbach and Gallimore (1989) were to call the Carboniferous with the collision of Gondwana and 'megam~nsoon.'~Third, I discuss monsoonal cir- Laurussia. By the Permian, the continent was culation, drawing on current understanding of the largely assembled, except for the numerous small dynamics of the Asian monsoon and showing why fragments that would eventually constitute most such a system probably occurred with a Pangean of Asia (McElhinny et al. 1981; Nie et al. 1990). geography. Fourth, is a summary of current model- Most of the continental area in the Late Carbonif- ing results, and finally, with the climatologic erous (306 Ma) and Late Permian (255 Ma, Scotese framework in place, the geological evidence for the and Golonka 1992; table 1)was south of the equa- evolution of Pangean climate. Because most stud- tor, about 58% and 61%, respectively. These fig- ies of Pangean paleoclimate have concentrated on ures are similar to Parrish's (1985). specific intervals, this section is arranged chrono- During the Triassic and Early Jurassic, Pangea logically. was nearly symmetrical about the equator (Parrish 1985; table 1). By the Early Jurassic (195 Ma, Scotese and Golonka 19921, about 56% of the land Paleogeographic Evolution of Pangea was in the Northern Hemisphere. In addition, epei- Published reconstructions of the major continental ric seaways formed as sea level rose during the Jur- plates during the Pangean interval have been rela- assic, dividing the areas of exposed land, especially tively stable since at least the late 1970s (e.g., Zo- in the Northern Hemisphere. By the , nenshayn and Gorodnitskiy 1977; Scotese et al. Pangea was breaking up, first in the Central Atlan- 1979; Smith et al. 1981; and subsequent publica- tic and then by the separation of Laurasia and Gon- tions). The major differences have been in the dwana. placement of the blocks that make up China and The large area of exposed land, its symmetry southern . Eastern Tethys was interpreted about the equator, and the relative positions of the as having been completely open throughout the exposed land and Tethys created the paleogeo- Pangean interval (Scotese et al. 1979),but more re- graphic conditions favorable to formation of the cently as having been blocked by a continuous line megamonsoon described by Dubiel et al. (1991)and of large islands and shallow continental shelves Kutzbach and Gallimore (1989).These conditions (Scotese and Golonka 1992).Interpretations of the were optimal in the Triassic. placement of these blocks probably would not af- fect the gross features of atmospheric circulation Early Work on Pangean Climate over the continent but certainly would have an ef- fect on predictions of the oceanography. In a landmark work linking the then-new concept Apart from the placement of the small circum- of polar wanderinglcontinental drift and paleocli- Tethyan plates, the major proposed change in mate, Briden and Irving (1964)compared the mod- Pangean geographic reconstructions is in the orien- ern and paleolatitudinal distributions of climati- tation of the continent during the Early Jurassic. Until recently, nearly all reconstructions showed the western coastline as parallel to longitude along Table 1. Percentages of Total Exposed Land Area in most of its length. This orientation places most of the Northern and Southern Hemispheres During the the Eurasian part of the continent into low and Pangean Interval mid-latitudes. Recent paleomagnetic data of Ba- Northern Southern zard and Butler (1991),however, rotates the North Time (Ma) Hemisphere Hemisphere American part of the continent a few degrees coun- tercloclzwise. If northern Pangea were still a single, Late Carboniferous (277) 32 68 coherent continental plate at that time, Bazard and Late Permian (255) 38 62 Early Triassic (237) 45 55 Butler's (1991)rotation would result in a major dis- Late Triassic (216) 45 55 placement of the eastern portions of the continent Early Jurassic (195) 56 44 to the north. This would most strongly affect inter- pretations of the climate of the eastern Eurasian Note. Measurements were made with a planimeter from Moll- part of Pangea (discussed in Jurassic climates). weide equal-area projections of paleogeography by Scotese and Golonka (1992).The dates refer to the dates of the reconstruc- However, the coherence of northern Pangea at that tions given by them, and the paleogeography represents an aver- time is now being questioned (Nie Shangyou, pers. age for at least the corresponding geologic age, if not epoch. Journal of Geology CENTENNIAL SPECIAL ISSUE 217 cally significant rocks. Although they used the time), however, the faunal diversity appeared to be paleoclimatic data as a test of continental recon- completely random, and the faunas thus appeared structions based on paleomagnetic data, their re- to offer evidence against . sults gave some hint of the patterns that were to Robinson (1973)was the first to describe explic- become clearer as continental reconstructions be- itly the climate of Pangea as monsoonal. She based came more independent and reliable. Briden and this description on the global distribution of red Irving (1964)started with the assumption that the beds, evaporites, and coals in Triassic roclzs. She distributions of climatically controlled sediments confirmed Briden and Irving's (1964)finding that were similar in the past to their distributions to- these paleoclimatically significant roclzs were not day, that is, that the general zonal pattern charac- distributed in simple latitudinal zones and also teristic of much of modern circulation and climate found that the roclzs formed an odd geographic over most of the earth's surface would have existed mixture. Although this mixture was at least partly throughout earth history. With this assumption, an artifact of plotting data for the entire period on the distributions of most rock types, and particu- one map, Robinson (1973) suggested that the cli- larly carbonates, made more sense when plotted mate was strongly seasonal, and used the term against paleolatitude than against present latitude. 'monsoon" to describe it. Modern reef carbonates, for example, occur be- The concept of a Pangean monsoon has appeared tween 30' north and south. Plotted against present elsewhere in different forms. Daugherty (1941),for latitude, ancient reef carbonates ranged between example, described the flora and environment of 40's and 70°Npeaking at about 50° latitude; the Triassic red bed Chinle Formation of the Colo- plotted against paleolatitude, they ranged between rado Plateau as "savannah." Much of the earth's 20° and 50°Npeaking at 10° and 30°N present savannah occurs within the region influ- Unlike carbonates, evaporites and eolian sand- enced by the Asian monsoon (Espenshade and Mor- stones did not fit the paleolatitudinal reconstruc- rison 1978; Rumney 1968) and always in areas of tions (Briden and Irving 1964). Modern evaporites strongly seasonal rainfall. and sand seas tend to occur in the "dry belts" cen- One of the most interesting outgrowths of the tered on 30' north and south (Borchert and Muir fascination with the Pangean interval has been the 1964; McKee 1979). Plotted against modern lati- debate on the climatic significance of (e.g., tudes, their ancient counterparts showed a similar Van Houten 1964). The Permo-Triassic especially distribution. Plotted on the continental recon- has long been regarded among geologists as an structions, however, evaporites and eolian sand- unusual time in earth history because red beds stones clustered near the equator (Briden and and evaporites were exceptionally widespread trying 1964; see also Parrish et al. 1982; Ziegler (Schwarzbach 1963; Vollzheimer 1967, 1969; et al. 1979).A breakdown of the plots by time into Waugh 1973; Turner 1980; Glennie 1987; Enos four intervals-lower and upper , Meso- 1993; and many others). Evaporites accumulated zoic, and -revealed that and faster, over a greater area, in the Triassic than at Cenozoic deposits plotted where expected when re- any other time (Gordon 1975). As paleoclimati- stored to their paleolatitudes but that Paleozoic de- cally significant data have gradually accumulated posits, particularly those of the upper Paleozoic, and put into the context of , the did not (Briden and Irving 1964). Parrish et al. strangeness of the Pangean interval has only been 11982) showed further that the upper Paleozoic pat- emphasized. tern extended into the lower Mesozoic. Although Red beds have been attributed to environments Briden and Irving (1964)were using the paleocli- ranging from desert to marine (Ziegler and McKer- matic data as a test of the paleomagnetic data, they row 19751, and eventually Van Houten (1982)was did not change their reconstructions for the late moved to point out that not all red beds are alike. Paleozoic based on the discrepancy between the The present consensus is that the reddening of al- two data sets. They recognized, instead, that the luvial and fluvial deposits (as opposed to sablzha anomaly might have been due to some different or marine deposits; Van Houten 1982)occurs with climatic process. alternating wet and dry climates. is leached The distribution of Permian marine faunas pre- from easily weathered iron-bearing minerals dur- sented similar problems for Stehli (1970). Plotted ing the wet cycle. Hematite, which is relatively on modern base maps, Permian tropical faunas stable, is then precipitated during the dry cycle formed a regular latitudinal diversity gradient, sim- (Walker 1967, 1976; Bown and Kraus 1981).In red ilar to that observed in modern marine faunas. beds ranging from desert sandstones (Walker 1967; Plotted against paleolatitude (as understood at the Van Houten 1982) through red paleosols (Bown 218 JUDITH TOTMAN PARRISH

1979; Bown and Kraus 1981; Kraus and Bown 1986) stron pressure gradient H to even bauxites, the reddening is dependent upon seasonality of rainfall (Bardossy and Aleva 1990). Thus red beds are formed along a climatic gradient, with seasonal rainfall in a mostly dry environment mH weak pressure gradient L at one extreme and seasonal rainfall in a mostly land - ~~~~~~~~N wet environment at the other. Calibration may 40Â 30Â 20' 10' O0 10' 20' 30' show that red beds can be useful as paleoclimatic indicators (Gyllenhaal 1991 and Bardossy and Aleva 1990).

Monsoonal Circulation and the Pangean Megamonsoon H L H land - The Asian monsoon is a complex system that has ~~~~~~~~N been the object of intense study for hundreds, if 40Â 30" 20' 10" 0" 10' 20' 30' not thousands, of years (G. Kutzbach 1987; Warren Figure 1. Schematic diagrams of meridional circulation 1987); the following discussion of modern mon- in the Asian summer (top) and winter (bottom)mon- soons is thus highly simplified. Recent work has soons (adapted from Webster 1987).H = high pressure, shown that hypotheses about relatively subtle fea- L = low pressure. tures of ancient monsoonal circulation can be tested in the Quaternary and Holocene records (Rossignol-Striclz1985; J. E. Kutzbach 1987; Davis commences in the middle and upper troposphere. 1988). However, the pre-Quaternary record is Monsoonal circulation is possible with simply a much more poorly resolved, and most current ef- cross-equatorial difference in sensible heating forts are directed toward testing the general hy- (Young 1987, figure 2G) although it would be rela- pothesis of monsoonal circulation during the tively weak and shallow (that is, occupying only Pangean interval. If the hypothesis is supported, a the lower few kilometers of the atmosphere, Web- future task will be to attempt predictions of the ster 1987). Sensible heat is about one-tenth the subtler features of the Pangean monsoon. magnitude of latent heat (Webster 1981) released General Monsoonal Circulation and the Asian Mon- when precipitation forms. The so-called heat low soon. The word "monsoon" is derived from the centered over northwest and Pakistan, which Arabic word for season (Webster 1987). Reference remains arid through the summer monsoon, is is made, therefore, to the winter monsoon as well maintained and intensified by high-level subsi- as the summer (alternatively, Southwest) mon- dence of air warmed by the release of latent heat soon, particularly in Asia. The meridional circula- over southwestern and northeastern India (Ramage tions in the Asian summer and winter monsoons 1971; Das 1986, figure 3)and blocks the northward are illustrated diagrammatically in figure 1. The penetration of moist air in that region (Krishna- summer monsoonal circulation in Asia is con- murti and Ramanathan 1982). Although surface trolled by four factors: (1)the sensible heating of pressures are low (e.g., Riehl 1978, his figure 10.4), the land surface of Asia; (2) the thermal cross- net radiation over the region indicates strong cool- equatorial contrast between the Indian Ocean and ing (Winston and Krueger 1977)from the surface. Asia; (3)the supply of moist air from the Indian (2)Cross-equatorial contrast. This effect is best Ocean and release of latent heat over the conti- developed in the modern summer Asian monsoon. nent; and (4)the effect of the as a Circulation in the southern Indian Ocean is high-altitude heat source. strongly affected by the circulation in the northern (1) Sensible heating. Initiation of the summer hemisphere (Ramage 1971; Webster et al. 1977). monsoonal circulation occurs when Asia begins to The subtropical high-pressure cell (called the Mas- warm up in the spring (Ramage 1966); this is called carene High in many papers) has an annual cycle sensible heating. This warming occurs because the unlike any other. Typically, a subtropical high- net radiative flux changes from strongly negative pressure cell is centered over the eastern half of the (more heat radiated to space than absorbed at the ocean basin it occupies, and its intensity is greatest surface) to weakly negative or positive (Webster during the summer, the season of maximum ther- 1987).With the warming, the surface air rises and mal contrast between ocean and adjacent conti- southward return flow toward the cold hemisphere nents. The southern Indian Ocean high, however, Journal of Geology CENTENNIAL SPECIAL ISSUE

Figure 2. Idealized geographic con- ditions of monsoon circulation (adapted from Young 1987).A: Lati- tude-parallel equatorial continent, no monsoonal circulation. B: Equa- torial ocean, northern summer (cf. central Pacific);ITCZ = Intertropi- cal Convergence Zone. C: Latitude- normal equatorial continent (cf. Pangea); note reversal of western equatorial flow. D: High-latitude continent, weak monsoon resulting from sensible heat contrast (cf. Pangea). E: Low-latitude continent, stronger monsoonal circulation re- sulting from sensible heat (cf. Pangea). F: Low-latitude continent with mountains, very strong mon- soonal circulation resulting from sensible heating and latent heat re- lease (cf.southern Asia, Tibetan Pla- teau).G: Wholly terrestrial Earth (cf. Mars), monsoonal circulation. H: Equatorial barrier to easterly flow (cf.]. I: Latitudinal barrier be- tween warm and cool parts of conti- cold nents (cf. Tibetan Plateau).

occupies the expected position only during Janu- rial barrier (Africa in the Asian monsoon, the cen- ary-March. During the height of the Asian sum- tral portion of the continent in the Pangean mon- mer monsoon, the cell is shifted to the west; it is soon) enhances cross-equatorial flow (figure 2H). also at its strongest, despite the season (southern A consequence of the cross-equatorial flow is winter; see, for example, Knox 1987, his figure that equatorial East Africa is much drier than 13.1). Clearly the intensity and position of the would be expected in a zonal climate. Equatorial southern Indian Ocean subtropical high-pressure and the eastern East Indies have cell, at least during the northern summer, are con- annual rainfall patterns more typical of zonal cli- trolled in large part by the climate in the north mate. In a zonal climate, the equatorial easterlies (Webster et al. 1977). This illustrates the impor- carry warm, very moist air over the land, which is tance of a cross-equatorial pressure contrast in dis- released as abundant rainfall. Equatorial East Af- rupting zonal circulation and enhancing the effect rica, by contrast, receives relatively little rainfall, of the monsoon. Some workers, e.g. Young (19871, and most of that occurs in the spring and fall. This consider the cross-equatorial flow a fundamental is because the equatorial easterlies are diverted by component of monsoonal circulation. An equato- the monsoon, northward in the summer and southward (but less strongly) in the winter; most of the rainfall occurs during the transition from NW India NE India winter to summer circulation and back. The high- est rainfall in Africa is in the western equatorial . This also is attributed to the summer monsoon over Asia. The thermal low over the Sa- hara, which is especially strong in the summer, is intense enough to reverse equatorial flow (Das 1986). Figure 3. East-west circulation in the Asian summer (3)Latent heating. Moist processes are very im- monsoon (Das 1986).The winter monsoon counterpart portant to the strength of the Asian monsoon, par- to this circulation is between Asia and the eastern Pa- ticularly the summer monsoon (Webster 1981).In cific (sink)and the region of New Guinea (source].Heavy a dry climate system, the heating of the air at the line at the bottom symbolizes theland surface. ground surface-sensible heating-forces the air 220 JUDITH TOTMAN PARRISH to rise, creating a relatively weak inflow of air at dominated by sensible heating in the NM simula- ground level. The strength and duration of the cir- tion and by latent heating in the M simulation, culation depends entirely on continued heating at but it should be noted that the modeled circulation the ground surface. In a moist system, however, patterns were similar everywhere but over the pla- the is self-propagating. Once the air teau itself (Hahn and Manabe 1975, figure 4.5). starts to rise and precipitation forms, the energy of (c)The Somali jet, a concentration of northward the system is increased further by the release of moving streamlines parallel to the Somalian coast, latent heat. is predicted by the M but not the NM simulation. The source of moisture for the Asian monsoon A possible effect is that precipitation in the adja- is the Indian Ocean. Air flowing into the monsoon cent region of Africa would be higher in the ab- at the surface originates in the high-pressure cell sence of the Tibetan Plateau, whereas in the pres- over the southern Indian Ocean. As that air is ent system, precipitation in eastern equatorial drawn northward, it is heated and picks up addi- Africa is low and limited to spring and fall. How- tional moisture. ever, the differences between the M and NM simu- (4)High-altitude heat source. The Tibetan Pla- lations in precipitation and the position of the teau enhances the summer monsoonal circulation ITCZ in that region are small. by functioning as a heat source high in the atmo- The Tibetan Plateau, besides contributing to the sphere. The ground surface of the plateau lies on deep penetration of heat into the atmosphere, also average about 5 km above sea level and almost 4 isolates southern Asia from the interior. The ther- km above the average elevation of the surrounding mal significance of the plateau in winter is appar- lowlands. Heating of the ground surface increases ently not great (Murakami 1987b), but the plateau the altitude of warm isotherms, so that the temper- is effective as a mechanical barrier to air flow. Al- ature at 6000 m on the plateau is much warmer though the emphasis has been on the pooling of than the temperature of air at the same altitude cold air north of the plateau and the role of the over the lowlands. The effect on circulation is to plateau in cold surges and cyclogenesis (Murakami intensify the low pressure that forms with heating 1987b, and references therein), the barrier effect of of the land surface in general. the plateau may also work in the other direction, Although the cross-equatorial thermal contrast isolating the interior of the continent from the is the primary cause of the monsoon (Young 1987; ameliorating influences of the Indian Ocean. The Murakami 19870; Webster 1987), the Tibetan Pla- pooling of cool air north of the plateau must also teau is important to the strength of the monsoon, play some minor role in facilitating the heating of as has been discussed at length in the literature southern Asia in the summer. (Flohn 1968; Hahn and Manabe 1975; Das 1986; General Monsoonal Circulation and the Pangean Murakami 1987b).For example, Hahn and Manabe Monsoon. The controls on Asian monsoonal circu- (1975)modeled the Asian monsoonal system with lation-sensible heating, cross-equatorial contrast; and without mountains (M and NM cases, respec- latent heating, and, possibly, high-altitude heat ef- tively). The major differences they noted in the re- fects-can be applied to Pangea. Because of the sults of the simulations were the following: great size of the continental area in mid-latitudes, (a)The center of the low-pressure cell over Asia a strong monsoonal circulation would be expected is far to the northeast in the NM case, over north- for Pangea. The continent was large, but more im- eastern China. Even though the low pressure was portantly, divided by the equator, with the two as intense as the observed and simulated M low halves flanking Tethys (Hansen, in Parrish 1982; pressure, the important effect was to decrease the Parrish and Curtis 1982; Parrish et al. 1979). The pressure gradient over southern Asia. shape of Pangea enhanced the seasonally alternat- (b)Partly because of the smaller pressure gradi- ing circulation that would have occurred in both ent, latent heat release and, therefore rainfall, did hemispheres by maximizing the pressure contrast. not penetrate far into the continental interior in The pressure contrast would have caused air to the NM simulation. Summer and winter rainfall flow across the equator in opposite directions dur- would have been confined to the coastal regions ing the year. and desert-like conditions for the interior, as in In the Pangean monsoon, differential heating be- Australia today (Webster 1981) and was predicted tween Tethys and the summer hemisphere would for Pangea by Parrish et al. (1982).The distribution have been similar to that occurring during the of latent and sensible heating in Asia is similar for summer Asian monsoon. The interhemispheric the two models except in the vicinity of the Ti- thermal contrast would have been much greater, betan Plateau. The overall effect is that Asia is however, because both hemispheres were land Journal of Geology CENTENNIAL SPECIAL ISSUE 22 1

(Crowley et al. 1989). The pressure contrast over Pangea might be compared with the seasonal pres- sure contrast over Asia today. The pressure con- trast between winter and summer over Asia is more than 36 mb (Espenshade and Morrison 1978). S. Pangea eq. N. Pangea Model results for Pangea! for example those of Kutzbach and Gallimore (1989)1show a pressure subsidence in contrast of about 25 mb. middle The interhemispheric thermal contrast alone latent heat release might have been enough to drive a monsoonal cir- culation over Pangea! as on Mars (figure 2G; Ha- berle 1986; Young 1987). In additionl the expanse of warm! equatorial ocean that the air would have had to cross (Tethys)is comparable to the width of I the Indian Ocean crossed by the modern air massl S. pangea eq. /N. pangea so the latent energy in the two systems might be expected to have been comparable. dry continental interior By comparison with the collision of India and Laurasia! Pangea may have had mountains equiva- lent to the Tibetan Plateau where Gondwana and alternate S. Laurussia collided. These mountains may have had Hemisphere an effect on the development of Pangean climate in circulation the Carboniferous (Rowley et al. 1985).In addition! Hay et al. (1982) proposed that these mountains were as high as 4 lzml not as high as the Tibetan Plateau! but comparable to the Colorado Plateau! which today generates a weak monsoonal circula- I tion. A mountain chain that probably resembled S. Pangea eq. N. Pangea the Andes lay along the northern margin of Tethysl formed during northward movement and subduc- tion of the paleo-Tethyan plate (the present Tien Figure 4. Simplified alternative scenarios for meridio- Shan and Nan Shan mountain ranges; Sengor 1979; nal circulation over Pangea and Tethys, northern sum- Klimetz 1983). These mountains might have en- mer. Significant zonal circulations are obscured in these hanced the release of latent heat much as the West- diagrams. A: Full monsoonal circulation similar to the present Asian summer monsoon. B: Modified mon- ern in southwestern India do now (Ramage soonal circulation with latent heat release confined to 1966). In addition! the mechanical effects of the coastal mountain ranges. C: "Normal1' summer zonal Tibetan Plateau during the winter monsoon might circulation as favored! for examplel by Hay et al. (1982). have been duplicated by the mountain barrier Heavy lines symbolize the land surfaces and the in- along the northern margin of Tethys. tervening thin line symbolizes the surface of Tethys. Despite the aspects of Pangean paleogeography favorable to monsoonal circulationl the absence of over the heat low centered in Palzistanl intensi- regions equivalent to the Tibetan Plateau cannot fying the aridity in that region (Ramage 1966; Das be dismissed out of hand in considerations of 1986). The mountains along the northern margin Pangean circulation. Two possible models! full of Pangea might have created similar conditionsl monsoonal and modified monsoonall can be con- restricting rainfall to the coastal regions (as pre- structed for meridional circulation during the dicted by Parrish and Curtis 1982). Pangean interval (bearing in mind that significant Models of Pangean Climate. Based on the basic zonal motions are obscured in these diagrams).The principles of atmosphericl including monsoonall full monsoonal circulation (figure 4A) is equivalent circulation outlined in the previous sectionsl the to the circulation during the Asian summer mon- beginnings of a theoretical basis for the hypothesis soon. The modified monsoonal circulation (figure of Pangean monsoonal circulation were established 4B) is analogous to the situation that obtains in using conceptual climate models of: the general the Arabian Sea during the Asian summer mon- hypothesis (Parrish et al. 1982; Parrish et al. 1986)! soon. Air warmed by the release of latent heat over the development of the monsoon (Nairn and the Western Ghats in southwestern India subsides Smithwick 1976; Rowley et al. 1985; Parrish 222 IUDITH TOTMAN PARRISH

1993~1)~the monsoonal maximum (~ariishand widespread. Seasonality may be expressed as sedi- Peterson 1988)! and the breakdown of the mon- mentary cyclesl biological cycles (e.g.! growth rings soon (Parrish 1993b). Subsequentlyl the so-called in trees and shellsJ Dubiel et al. 1991)!or in certain "Fujita-Ziegler" model! a parametric! semi- types of paleosols. Seasonal cycles have been ob- quantitative model (Gyllenhaal et al. 19911, was served in roclzs of all ages, but for each age! depos- applied to Permian paleogeography by Patzkowsky its showing seasonal cycles are relatively uncom- et al. (1991). Finally! several different numerical mon. This suggests one of two things! either (a) models have been applied to Pangean paleocli- most sedimentary systems are relatively insensi- mates that support the interpretation of a mon- tive to seasonality or (b)seasonality is commonly soon-dominated circulation system (Crowley et al. or in most places not strong enough to overcome 1989; Kutzbach and Gallimore 1989; Chandler et the other controls on sedimentation to impose cy- al. 1992; Valdes and Sellwood 1992). The Pangean clicity. A strong! full monsoonal system would interval is the only time period other than the have created a strong seasonal cycle over much of for which global climate has received Pangea! and it might be expected that (a)sedimen- treatment by seven completely different models tary systems that might not otherwise have re- and approaches (two conceptual models! a para- flected seasonality would have had a seasonal cycle metric model! and four different numerical mod- imposed on them! or (b)the wide distribution of els), so comparing the models is especially inter- strong seasonality would have created more oppor- esting. tunities for seasonality to be expressed! or both. What all model predictions have in common is (2)The equatorial region! particularly along the strong seasonality. Nairn and Smithwiclz (1976)! eastern continental margin! will be dry relative to Parrish (1982)!Patzkowsky et al. (1991)!and Crow- the high-latitude regions of Pangea or to the conti- ley et al. (1989)all treated the Permian. The princi- nental equatorial region at most other times in pal difference between the conceptual model pre- earth history. At its greatest intensityl the mon- dictions of Nairn and Smithwiclz (1976)and Parrish soon might have been strong enough to reverse (1982) were in the intensity of the seasonalityl equatorial flow, and in that event the western which was less in Nairn and Smithwiclzls (1976) equatorial region of Pangea will be relatively predictions. Crowley et a1.l~(1989) models explic- humid. itly predicted temperature! and they obtained a (3) Climatic belts will not be zonal. Although cold-month mean temperature of -30° and a some asymmetry exists on even the smallest conti- warm-month mean temperature of + 25OC for nents! particularly if the isolatitudinal coastal re- southern Pangea. The temperatures reported by gions are compared (Parrish and Ziegler 1980)! the Crowley et al. (1989) were similar to those ob- proposed monsoonal circulation over Pangea tained by Kutzbach and Gallimore (19891, using a would have completely disrupted zonality. Instead, global circulation model for a schematic Pangea the equatorial region would be expected to have approximating Triassic paleoge~graphy~and Chan- been dry! particularly in the east! and rainfall dler et al. (1992)using a realistic Early Jurassic pa- would have been concentrated in belts paralleling leogeography. The Late Jurassic models of Valdes the northern and southern coasts of Tethys and and Sellwood (1992) show less extreme seasonal- along the western high mid-latitude coasts (i.e.! in ity! not surprising given that Pangea had by that a position equivalent to southern British Colum- time split in two. The winds predicted by Kutz- bia). Terrestrial organismsl especially plants, bach and Gallimore (1989)were qualitatively com- should reflect the loss of zonality. parable to those predicted from the conceptual cir- (4)The conditions described above should reach culation models (Parrish 1982; Parrish and Curtis a maximum in the Triassic. The hypothesis of 1982). Patterns of barometric pressure and! in- Pangean monsoonal circulation is dependent on directlyl evapotranspirationl obtained using the the size of Pangea and the cross-equatorial contrast Fujita-Ziegler model (Patzlzowsky et al. 1991)were between its northern and southern halves! and that similar to those predicted by Parrish (1982) and contrast would be expected to be maximal when Kutzbach and Gallimore (1989). the contrasting continental areas are about equal Implications and Predicted Effects of Monsoon Cli- in size. Thus! the strongest period of monsoonal mate. If Pangea had a strong monsoonal circula- circulation is expected to have been in the later tion! the expected effects on the distribution of cli- part of the Triassic! when the land area was divided matically significant rocks are as follows: in half by the equator (table 1; Parrish 1985; Par- (1) Evidence of seasonality will commonly be rish et al. 1986). strong and rocks containing such evidence will be The major distinction between the full and mod- Journal of Geology CENTENNIAL SPECIAL ISSUE 223 ified monsoonal models (figure 4) would be the mate from the presence of coal. Peat formation is penetration of moisture into the continental inte- only secondarily related to rainfall (Gyllenhaal rior. In the modified monsoon model! rainfall 19911, and is mostly due to high groundwater ta- would be most abundant in the coastal regions! bles (McCabe 1984; Ziegler et al. 1987; McCabe whereas in the full monsoon model! seasonal rain- and Parrish 1992). However! the refinement of fall would be expected some distance into the con- coals as climatic indicators has not progressed far, tinental interior! perhaps 600- 1200 kml the ap- and a reevaluation of all Pangean coals for their proximate distance from the heads of the Bengal paleoclimatic significance is not feasible at this Gulf and Arabian Sea! respectivelyl to the Himala- time. In most cases mentioned below! changes in yan mountain front. the distribution of coal are paralleled by changes in The prediction that the monsoon would have other climatic indicators! thus strengthening the reached its maximum development in the Triassic interpretations. implies that the climate system would have under- gone some evolutionary changes. As the monsoon Development of the Monsoon-Late developed, equatorial Pangea would have become Carboniferous and Permian. progressively drier. Drying would have started in The greatest extent of continental glaciation in the the west and extend eastward. The western equato- Carboniferous began toward the end of the West- rial regions of Pangea would have become arid first phalian Age (Late Carboniferous; Veevers and Pow- as the continent consolidated and sea level fell be- ell 1987). At the end of the Carboniferousl peat cause those areas would have been downwind of formation on Pangea itself began to shift from low the equatorial easterlies carrying moisture from latitudes to high latitudes (Ziegler et al. 1979, Tethys. As the longitudinal extent of the exposed 1981; Parrish et al. 1982)!although peat formation land at the equator increased, the downwind re- continued in the microcontinents that now consti- gons would have become drier. As the monsoon tute China. This shift may marlz the beginning of developed and increasingly disrupted zonal circula- large-scale poleward transport of moisture (Manabe tion! aridity would have progressed eastward as the and Wetherald 1980; Brass et al. 1982; Parrish et equatorial easterlies were diverted north or south al. 1982; Parrish 1993~).Major supporting evidence into the summer hemisphere. As the monsoon for the monsoon hypothesis and, indeed, the evi- strengthened! however! the summer lows could be- dence that prompted Robinson (1973) to suggest come strong enough to draw moisture from the monsoonal circulation in the first place! is the west, as occurs in Africa today. A gradient from widespread aridity in Pangea. wet in the west to dry in the east would mark the In central North America, the trend from wet in monsoon maximum. In addition, the climate of the Early Carboniferous to dry in the Early Per- Pangea should have evolved with increasing aridity mian was not monotonic (Phillips et al. 1985). in the low- and mid-latitude continental interiors Rowley et al. (1985) suggested that the develop- and with poleward expansion of relative aridity ment of the monsoon was retarded by the high- and strong seasonality of rainfall. that resulted from the collision of Gondwana It should be noted, however! that aridity can be and Laurasia and that lay along the equator. If brought about by increases in temperature! and these highlands were Himalaya-scale, they would within limitsl a warming trend can make the cli- have had the same effect as the Tibetan Plateau! mate! as recorded in the geologic record, drier! in- intensifying latent heat release. However, in the dependent of changes in the precipitation regime. Carboniferous, the mountains would have intensi- Climatic indicators of relative humidity such as fied the normal equatorial low pressure. The strong paleosols and plants are controlled by the balance effect of these highlands might have countered the between precipitation and evaporation (which is effectsof the increasing size of Pangea and symme- principally dependent on temperature)! not by pre- try of the continent about the equator! and the al- cipitation alone. Higher temperatures increase ternating wet and dry periods leading into the Per- evapotranspirationl and the net water balance is mian might reflect a system poised between two recorded in the geologic record. climatic states! zonal! with the strongest low pres- sure on the equator, and monsoonall with the Geological Evidence of the Pangean strongest low in low mid-latitudes in the summer Megamonsoon hemisphere. Eventually, as Pangea grew and Throughout the following discussion, reference is moved north, this effect might have subsided. At made to coals as wet-climate indicators but! in the same time! global climate was warming fact, caution must be exercised in interpreting cli- (Diclzins 1977, 1979, 1983, 1985; Berner 1990) and 224 JUDITH TOTMAN PARRISH

this would have resulted in not onli increasing Campos 1973; Veevers and Powell 1987). Faunas aridity but also poleward expansion of the rela- dominated by the cold-water bivalve! Ezzrydesma tively arid regions because! globallyl the evapo- (Dickins 1978! 1985)occur in southern Gondwana transpiration rate would have increased. (Dickins 1985; Veevers and Powell 1987). Recent numerical climate modeling results Evaporites and! particularlyl red beds spanned a (Kutzbach et al. 1993) show that the topographic slightly greater latitudinal range in the Late Per- effect suggested by Rowley et al. (1985)may indeed mian than in the Early Permian (Assereto et al. have operated. These models further suggest that 1973; Rocha-Campos 1973; Solzolova et al. 1973; the rainfall resulting from the topography may Waugh 1973; Nairn and Smithwiclz 1976; Witzke have been seasonal on either side of the moun- 1990; Ziegler 1982; Enos 1993). Coals were less tains. A test of these models would include seeking widespread in both Asia and southern Gondwana evidence for seasonality within the coal-bearing se- (Nairn and Smithwick 1976; Veevers and Powell quences in the adjacent lowlandsl a climatic pat- 1987; Enos 1993).Only in Australia did peat forma- tern already suggested by Cecil (1990). tion apparently retain its original extent (Brake1 Increases in aridity during the Permian are sug- and Totterdell 1990; J. T. Parrish! M. T. Bradshaw gested by changes from sedimentary indicators of et al.! unpub. data). Monsoonal floras were less relatively humid climate! such as coalsl to indica- widespread! and desert floras more widespread tors of dry climatel such as eolian sandstones and than in the Early Permian (Kremp 1980; Ziegler evaporite~~especially in the equatorial regions 1990). The overall trend in the Permian! then! ap- (e.g.! Gordon 1975; Ziegler 1982; Witzke 1990). pears to have been toward increasing aridity! either Redbeds also became more widely distributed dur- through an absolute reduction of rainfall! an in- ing the Permian and into the Triassic (Waugh crease in evapotranspirationl or an increase in sea- 1973). sonality of rainfall. The simplest explanation is Eolian sandstones of the Colorado Plateau gen- that evapotranspiration increased owing to global erally show north-northeasterly or northeasterly warming following the disappearance of large-scale flow during the Permian (Parrish and Peterson glaciationl and that the resulting drying was en- 1988). Had the circulation associated with the hanced by increasing seasonality as the Pangean monsoon flow predominated! flow would have monsoon strengthened. been northwesterly (Parrish and Peterson 1988). However! the Queantoweap ! Nevada The Monsoon Maximum-Triassic and Utah (Early Permian) does show northwesterly flow (Parrish and Peterson 1988; Peterson 1988! As Pangea moved north and the land area was dis- figure 10).This suggests that the monsoon contin- tributed more evenly on either side of the equator ued to vary in strength before reaching its maxi- (Parrish 1985)! seasonality would have become mum in the Triassic. stronger and the equatorial region and mid-latitude As the South Pole became marine in the Early continental interiors drier until the Triassic. In the Permian! significant glaciation ceased (Veeversand Triassic! the monsoonal circulation is predicted to Powell 1987). At the same time! extensive evapo- have been at maximum strength because the ex- rites and eolian sandstones were being deposited posed land area was large and divided in half by across the Northern Hemisphere (Nairn and the equator (Parrish 1982; Parrish and. Curtis 1982; Smithwick 1976; Glennie 19721 1983; Ziegler Parrish and Peterson 1988). The monsoon might 1982; Witzke 1990)! and red beds and evaporites have been strong enough to draw moisture along were deposited across Gondwana (Rocha-Campos the equator from the west. 1973; Nairn and Smithwick 1976; Ziegler 1982). The Triassic is the time of greatest evaporite Plants typical of desert and summer-wet (1.e.)mon- formation worldwide [Gordon 1975)! but on the soonal) climates were widespread in low latitudes Colorado Plateau! in westernmost equatorial in the Permian (Kremp 1980; Ziegler 1990). Pangea! the Triassic was the wettest part of the Peat swamps formed on low-latitude islands and Pangean interval! shifting from evaporite and eo- in high northern and southern latitudes on Pangea lian deposition in the Late Carboniferous and Per- (Rocha-Campos 1973; Nairn and Smithwick 1976; mianl to fluvial red bed deposition in the Triassic! Veevers and Powell 1987; Enos 1993).For the most and back to eolian deposition in the Jurassic (e.g./ part! the coal deposits in southeastern Gondwana Peterson 1988). The few eolian sandstones depos- (Australia and India) occur stratigraphically above ited on the Colorado Plateau during the Late Trias- the glacigenic deposits! which are widespread in sic and earliest Jurassic show a 90' change in wind the earlier part of the Ear1.y Permian (Rocha- direction to westerly or northwesterly from the Journal of Geology CENTENNIAL SPECIAL ISSUE 225 predominantly northerly or northeasterly winds of nomy! the presence of large trees! thin organic-rich most of the rest of the Pangean interval (Peterson swamp deposits! and cuticles (Ash 1967! 1978). At 1988). Parrish and Peterson (1988) suggested that the other extreme to Ash's views! Gottesfeld (1972) the proposed Pangean monsoonal circulation be- regarded the climate of the Chinle as predomi- came strong enough at this time to influence circu- nantly arid. Gottesfeldls (1972) interpretation is lation on the western side of Pangea and draw supported by evidence of at least periodic drying! moisture along the equator from the west. The Tri- and depended partly on the interpretation of red assic Chinle Formation is a fluvial system with beds as indicators of arid conditions. lakes! although it does contain some evidence of The invertebrate! vertebrate! and ichnofaunas of aridity (Dubiel et al. 1991). the Chinle Formation are diverse (Robinson 1915; In studies of the fluvial deposits in the Chinle Camp 1930; Colbert 1952; Breed 1972; Tasch Formationl Blakey and Gubitosa (1983! 1984) and 1978; Dubiel 1987; Dubiel et al. 1987; Murray DeLuca and Eriksson (1989)agree that climate had 1987; Miller and Ash 1988; Hasiotis and Mitchell been warm and seasonal with respect to rainfall. 1989; Parrish 1989; Kietske 1989; Good 1989). DeLuca and Erikssonls (1989) study showed that Among the invertebrates! only the molluscs have perennial and ephemeral streams coexisted in the been studied in detail (Good 1989; Dubiel et al. Chinle! precisely the pattern observed in regions 1991).Dubiel et al. (1991)interpreted growth band- with abundant! but highly seasonal! rainfall. Bla- ing in the shells of these molluscs to reflect sea- key and Gubitosa (1984) proposed that the sur- sonal variations in growth rate. The vertebrate rounding highlands were humid in order to supply fauna consists of both aquatic and terrestrial the perennial streams with water. Rainfall in the forms. Parrish (1989)used the taphonomy of verte- source area need not be constant to maintain pe- brate fossils and the presence of burrows rennial streams! so long as total annual precipita- and teeth in the Chinle to support hypotheses that tion is high, A wet source region is not incon- these animals lived and died in an environment sistent with the monsoon hypothesis because characterized by seasonal rainfall. Dubiel et al. mountains usually are wetter and total annual (1987! 1991) and Parrish (1989) suggested that the rainfall in a monsoon system! though extremely deep (>2m) lungfish and crayfish burrows may in- seasonal! is also very high compared with other dicate a high range and repeated cycles of water climatic regimes (Dubiel et al. 1991). table fluctuation owing to highly seasonal precipi- Recent! interdisciplinary studies of the deposi- tation. At present! lungfish live in monsoonal envi- tional systems and paleobiological communities in ronments! and the trace fossil Scoyenia also com- the Chinle basin have provided evidence for a mon- monly occurs in shallow lake deposits subject to soonal paleoclimate (Dubiel et al. 1991)!but an in- periodic drying (Ekdale et al. 1984). teresting aspect of Chinle deposition is the contro- Although the Chinle represents a climatic versy over interpretations of the flora. The first full anomaly on the Colorado Plateau during the description of the major elements of the fossil flora Pangean interval! the Triassic record is not com- of the Chinle was published by Daugherty (1941). plete. The only other Triassic unit is the Middle Subsequentlyl voluminous and detailed work by Triassic Moenkopi Formation! which Stewart et al. Ash (1967-1989) revealed the full extent and diver- (1972)described as a "typical red bed unitl'' com- sity of the flora! which includes pterophytesl sphe- prising alluvial fanl floodplainl sabkha! and marine nophytesl cycadophyte~~and coniferophytes. In ad- facies (Stewart et al. 1972; Baldwin 1973).The sab- dition to the major groups! some taxa of uncertain kha facies of the Moenkopi contains abundant pri- affinity also have been described. Daugherty (1941) mary gypsum! and salt crystal casts and mudcracks and Ash (1975! 1977! 1987! 1989) believed that are common throughout the unit! especially in the Chinle flora grew in a warm climate with abun- lower part (Baldwin 1973). dant rainfall. However! Daugherty (1941) also The wetter conditions suggested by Triassic red noted that growth rings preserved in the perminer- beds and associated rocks are consistent with a re- alized logs were evidence of some sort of seasonal- versal of equatorial flow into the strong monsoon ity in the climate (see also Dubiel et al. 1991)! and system created by Pangea (Parrish and Peterson that the Chinle flora compared well to modern sa- 1988)! and are unlikely to be the result of global vannahs. By contrast! Ash (1967! 1972a, 1978; Ash cooling. No independent evidence exists for global and Creber 1990) favors warm to hot temperatures cooling at this time! and the region was in low and everwet (see also Ziegler 1990) conditions latitudes throughout the late Paleozoic and early based principally on comparisons with the habitats Mesozoic. Indeed! Berner (1990)modeled substan- of nearest living relatives! general leaf physiog- tially higher C02levels for the Triassic relative to 226 JUDITH TOTMAN PARRISH the Permian (about 4 x present C02versus 1.5-2 that rather than acting as a rainshadow, the x present C02, respectively). In addition, global highlands reinforced the monsoon. temperature change is poorly expressed in low lati- The Triassic is poorly preserved worldwide and tudes, implying that any temperature change was is best studied in North America. The geology in likely to have been too small to account for the other parts of the is consistent with a mon- dramatic change in style of sedimentation. The soonal climate. Redbeds, with local occurrences of strong seasonality associated with monsoonal cir- evaporites and eolian sandstones, are widespread culation may explain the contradictory climatic in- in South America in the Triassic (Volkheimer terpretations of units such as the Chinle Forma- 1967, 1969; Rocha-Campos 1973; Parrish and Pe- tion (Dubiel et al. 1991). terson 1988), and occur in Triassic continental Wetter climates also occurred farther east in rocks in China (Enos 1993), and South Africa. Al- western Europe (Simms and Ruffell 1990; Simms though the Triassic record is especially sparse in and Ruffell 1989). Simms and Ruffell (1989, 1990) Australia, Triassic rocks there are commonly red attributed this change to rifting, but it might also beds with fluvial deposits similar to those of the be evidence for increased strength of the monsoon, Chinle. The presence of such deposits in Australia, or a combination. In discussing the wide distribu- where they replaced Permian deposits indicative tion of evaporite deposits in the Pangean interval, of much wetter climates, notably the widespread Gordon (1975, p. 681) offered the explanation that Permian coals, is further evidence of the strength Pangea ". . . provided an unusually wide expanse of the monsoon in the Triassic (1. T. Parrish, M. T. of tropical and subtropical land with a minimum Bradshaw, and others, unpub. data). of marine influence." Hay et al. (1982)noted that neither the zonal model of the distribution of cli- The Breakdown of the Pangean matic belts nor the Koppen and Wegener (1924, Monsoon- Jurassic cited in Frakes 1979) modified zonal model could explain sedimentation in the proto-Atlantic rift. The Colorado Plateau region became arid again by Hay et al. (1982)suggested, in contrast to the mon- the Jurassic, as indicated by widespread deposition soon model (although not necessarily in conflict of eolian sandstones and sabkha deposits (Blakey with it), that aridity in the equatorial region re- et al. 1988). Earliest Jurassic eolian sandstones of sulted partly from a rainshadow effect of the rift the Colorado Plateau recorded northwesterly system highlands. The northern and eastern end of winds, consistent with the predicted summer mon- the rift system lay, in their reconstruction, at soonal circulation (Parrish and Peterson 1988) and about 20¡N and was drier throughout the interval with the pattern indicated by the few Triassic eo- studied (LateTriassic-Early Jurassic).Farther to the lian sandstones. Later in the Early Jurassic, how- west and south, ephemeral lakes and playas, remi- ever, the sandstones recorded a divergence of niscent of the East African rift lakes, formed the northerly winds to the southeast and southwest. principal deposits. This general paleogeographic This could represent either a geographic boundary distribution was maintained throughout the inter- between the summer monsoonal circulation to the val, although the evaporites became more wide- east and the "normal" summer subtropical high- spread, attributed by Hay et al. (1982)to northward pressure cell to the west or an alternation of mon- drift of the continent. It should be noted, however, soonal and "normal" circulation. Either case is that in their reconstruction (Hay et al. 1982, their consistent with weakening summer monsoonal figures 5 and 6), the whole of northern Africa lies circulation during the Early Jurassic (Parrish and along the equator to the east of the rift system. In Peterson 1988). Data from eolian normal zonal circulation, it is unlikely that the sandstones show no consistent wind field on the equatorial easterlies could retain enough moisture Colorado Plateau, and by the Late Jurassic, the across that continental expanse to create the wet zonal westerlies are well expressed in the eolian conditions described for the windward sides of the sandstones. The disorganized vectors in the Middle highlands (Hay et al. 1982 apparently assumed no Jurassic sandstones might have recorded a transi- heating of the air over northern Africa [p. 261 prior tional climatic state, from monsoonal to zonal to its arrival at the rift system). In a monsoonal (Parrish and Peterson 1988), either because the circulation, however, the effects of topography, as winds were relatively weak and variable or because pointed out by Hay et al. (1982), are profound, as monsoonal and zonal circulation alternated rap- seen in the contribution of the Tibetan Plateau to idly. Further study of the Middle Jurassic eolian the Asian monsoon. The increase of wetter- sandstones might make it possible to distinguish climate indicators on both sides of the rift suggest between these two possibilities. Journal of Geology CENTENNIAL SPECIAL ISSUE 227

The shift from monsoonal to zonal circulation interpreting Early Jurassic climates is further com- does not appear to have had a major impact on plicated by uncertainty regarding the position and climate in the Colorado Plateau region, which con- coherence of Pangea at that time, as discussed at tinued to be relatively dry (Parrish 1993b). Evapo- the beginning of this paper. Cooling could be re- ritic lakes are represented by the Todilto Lime- gional owing to the northward movement of north- stone and Pony Express Limestone Members of the eastern Pangea as the continent rotated counter- Wanakah Formation (Middle Jurassic; Condon and clockwise (Bazard and Butler 1991). However, this Peterson 1986) and the Brushy Basin Member of would not explain the distribution of coals in Gon- the (Upper Jurassic; Lake dwana (whether or not Pangea was coherent or T'oo'dichi', Turner-Peterson and Fishman 1986; fragmented) nor would it explain the southward Turner and Fishman 1991; see also Bell 1986). movement of the Boreal Realm faunas. Clearly, a However, fossil wood of Jurassic age is common in number of lines of research must be pursued re- the Morrison Formation in Utah (C. E. Turner pers. garding Early Jurassic climates before these prob- comm.; J. T. Parrish, unpub. data), indicating that lems can be sorted out. the water table was high enough to permit tree A dramatic shift in climate at the Middle-Late growth, and in some respects, the Morrison resem- Jurassic boundary in southern Laurasia is indicated bles the Chinle Formation. Given the predicted by changes in the distributions of not only coals, evolution of the Pangean megamonsoon, evidence but also evaporites, red beds, and (Vakhra- for seasonality in the Morrison should be less than meyev and Doludenko 1977; Parrish et al. 1982; in the Chinle; a comparison between the Morrison Vakhrameyev 1982; Hallam 1984).By the Late Jur- and Chinle Formations could throw light on the assic, coal deposition in eastern Laurasia was re- problem of interpreting seasonality rainfall from placed by evaporite deposition between 20Âand 40' the geologic record. north, consistent with zonal circulation and the Laurasia was relatively dry during the Late Tri- breakdown of the northern monsoon (Parrish et al. assic, but on the evidence of the onset of wide- 1982; Parrish 1988). The rifting of Laurasia from spread coal deposition in eastern Laurasia, appears Gondwana removed the principal forcing factor of to have become wetter during the Early Jurassic, the monsoon, the Pangean geography. Once the in contrast to the pattern observed on the Colorado monsoon broke down, the mechanism of moisture Plateau. Hallam's (1984) data indicate that the transport to those latitudes would have ceased to Early Jurassic pattern in Laurasia persisted into the exist. The maximum southward extension of Bo- Middle Jurassic. A possible explanation implied by real Realm ammonites at the end of the Middle Parrish et al. (1982)is that evaporation was reduced Jurassic (Taylor et al. 1984)suggests that the drying because of cooler temperatures brought about by could not be the result of global warming, although the more northerly position of Laurasia in the the coolest polar temperatures in the north may Early Jurassic. If global temperature was dropping actually have been earlier in the Middle Jurassic, as well, the shift could be explained simply by as evidenced by the presence of glacial-marine sedi- changing the precipitation-evaporation balance ments, diamictes ("pebbly argillites"), and glen- with a change in temperature and no changes in donites (Brandt 1986; see discussion in Parrish the northward transport of moisture by the mon- 1993b). soon. Berner's (1990)CO, curve would suggest that The shift to drier climate in southern Laurasia global temperature was more likely to have is also reflected in changes in the distribution of warmed at this time. However, evidence exists for Classopollis, the pollen of the conifer family cooling, at least in the higher latitudes. Cooling Cheirolepidiaceae, which is interpreted to have during the Early Jurassic is suggested by the appear- been drought-tolerant (Alvin et al. 1978; Alvin et ance of the Boreal Realm ammonite fauna in the al. 1981; Doyle and Parrish 1984). This shift has Northern Hemisphere (Taylor et al. 1984), which been documented with high resolution and appears is commonly interpreted to represent cooler water, to have taken place during late Bathonian- and of glendonites in high latitudes (Kemper 1987). Callovian time (Vakhrameyev 1982). In addition, peat formation was widespread in mid- Although not as well resolved, climatic changes latitudes in Gondwana (Volkheimer 1967). in the Southern Hemisphere appear to have Rising sea level during the Early Jurassic, cou- roughly paralleled those of the Northern Hemi- pled with favorable basin settings in eastern Lau- sphere. This indicates that although Gondwana is rasia, may explain the onset of widespread peat for- likely to have had its own monsoonal circulation, mation there, and peat formation was in lower that circulation was probably much weaker than latitudes there than in Gondwana. The problem of the Pangean monsoon. In the Early Jurassic, abun- 228 JUDITH TOTMAN PARRISH

dant fossil floras and widespread peat deposition of the sedimentation in Australia! which was in occurred! and humid conditions continued into the relatively high latitudes! took on a much drier and Middle Jurassic (Volkheimer 1967).In the Late Jur- more seasonal character than before or after. As assicl arid conditions were established in mid- indicated in figures 1 and 4) monsoonal circulation latitudesl resulting in extensive eolian sand deposi- generally involves only the low-latitude portions tion (Volkheimer 1967). Monsoonal circulation of the Earth's surface. If the changes in Australia would be expected to continue at a somewhat re- are attributable to the monsoonl the strength of duced strength in Gondwana! which was compara- the Pangean megamonsoon was great indeed. ble in size to modern Eurasia (Parrish and Curtis In the Early Jurassicl the Colorado Plateau re- 1982; Valdes and Sellwood 1992). gion again became arid and the eolian sandstones record zonal as well as monsoonal circulation. In eastern Laurussia! however! and in Gondwana! cli- Summary mate apparently became wetter! owing either to The starting point for climate on Pangea is Late global cooling orl in the case of Laurussia, rotation Carboniferousl which began with widespread peat of that portion of the continent into higher lati- formation (e.g.! Raymond et al. 1989) in what is tudes. Finallyl drying occurred in Gondwana and now central and eastern North America and Eu- southern Laurasia! and is interpreted as indicative ropel and relatively dry conditions on the Colorado of the breakdown of the Pangean monsoon. Plateau. As the period drew to a closel the equato- Details of the climatic developments outlined in rial regions became drier! although this progression this paper will inevitably change as more is learned was uneven. By the Permian! however! the equato- about Pangean geology and climatology. For exam- rial region of mainland Pangea was dryl and during ple! any new research into the climatic significance the Permian! indicators of aridity and seasonality of red beds will have a profound effect on our un- of rainfall became more widespread. This change derstanding of this time. However! at least two ma- was most likely driven by an increase in evapo- jor problems remain in the study of Pangean cli- transpiration owing to global warming following mates that could change substantially some of the the disappearance of large-scale glaciationl and an conclusions in this paper. One is the problem of enhancement of the resultant drying by increasing the timing of the monsoon maximum in the Trias- seasonality as the Pangean monsoon strengthened. sic, Triassic roclzs are relatively scarce and poorly Wind directions derived from Colorado Plateau eo- studied! particularly from a global perspective. The lian sandstones are consistent with an increasing other problem lies in uncertainties about the geog- influence of monsoonal circulation. raphy of Pangea during the Jurassicl that isl its In the Triassic! two changes occurred that sup- orientation and integrity. port the hypothesis that the monsoonal circulation attained its maximum strength in the Triassic. ACKNOWLEDGMENTS First, climate in the Colorado Plateau region be- came relatively wet! though still seasonac in the The author gratefully acknowledges reviews by J. Late Triassicl in contrast to the climate before and E. Kutzbach and two anonymous reviewers and the after. The few eolian sandstones indicate a major support of National Science Foundation Grants shift in wind direction at that time. Second, much EAR-8903549 and INT-8814477.

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