Keywords

Introduction

Different processes are responsible for climate and atmospheric circulation forcing, and their relevance depends on the specific period analyzed and the frequency of climate change considered. Viewing the Earth’s climate as a global system, Frakes (1999) described the evolution of climate throughout the past 600 Ma. He highlighted the complex interactions between the carbon cycle, continental distribution, tectonics, sea-level variation, ocean circulation, and temperature change as well as other processes. Valdes (2000) provided an overview of climatic forcing mechanisms and explored their possible role in Phanerozoic climate variations.

Continental drift and orography are important for low-frequency processes, those involving changes over hundreds of millions of years. Glacial eras characterized times when continental masses were located at the pole due to the capacity of land to support and retain ice sheets. During such periods polar ice caps can maintain themselves and grow through the snow and ice accumulation during repeated annual cycles. Warm climate with Earth free of permanent ice sheets characterized the times when the oceans dominated the polar and subpolar regions of the world and continental masses were located in tropical and subtropical latitudes (Crowley and North 1999).

Forcing through atmospheric greenhouse gases is another important factor of climatic change. Two hundred and fifty million years ago, global CO2 concentrations were approximately 2,000 ppmv (i.e., 3–8 times higher than today; Royer 2006), producing pronounced warming and enhancing seasonal monsoon circulation. Both forcing processes were present especially during the Triassic and Jurassic periods. These times were characterized by a Gondwana supercontinent without significant landmasses located at the southern polar region, together with exceptionally high CO2 concentrations. Furthermore, sulfur dioxide (SO2) is the most voluminous chemically active gas emitted by volcanoes, and the major volcanic eruptions have usually formed sulfuric acid aerosols in the lower stratosphere that cooled the Earth’s surface. According to Ward (2009), major volcanic activity in Silicic Volcanic Provinces (SVP) has typically preceded an increase in glaciation and a subsequent decrease in sea level throughout the last 600 Ma.

In this chapter, the influence of the different forcing processes in the Southern South America climate and atmospheric circulation during the Gondwana supercontinent are described.

The paleoatmospheric circulation taken from climatic modeling scenarios through the ages is reviewed and compared with proxy data. Detailed and updated reference information on the relevant topics analyzed in this chapter is provided as well.

Note that the ages indicated for each geological period cited in this chapter correspond to those presented at the US Geological Survey Geologic Names Committee (2010), as Divisions of Geologic Time—major chronostratigraphic and geochronological units.

Forcing Description

Paleoclimatic differences from the Triassic to the Cretaceous periods are due principally to changes in the land–ocean distribution. The Pangaea supercontinent was the largest landmass in the Earth’s history (Fig. 1). According to plate tectonics and continental drift theory, the Pangaea supercontinent began breaking up about 225–200 Ma ago and rifted into two major landmasses during the Jurassic, Laurasia to the north and Gondwana to the south, roughly separated by the equator.

Fig. 1
figure 1

World map from the Early Triassic (upper left panel) to the Late Cretaceous (lower right panel) (Adapted from Scotese (2012; http://www.scotese.com/earth.htm))

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During the Middle to Late Jurassic, new oceanic gateways were formed with, in particular, the opening of the proto-Atlantic Ocean (the so-called Hispanic Corridor) (Fig. 1), connecting the Pacific Ocean to the western Tethys Ocean (Ziegler 1988; Scotese 2001, 2012; http://www.scotese.com/earth.htm). The reorganization of the Tethys–Atlantic oceanography was triggered by the opening and deepening of the Hispanic Corridor and produced paleoclimatic changes (Rais et al. 2007).

This single continent stretched in a north–south direction across every part of the zonal atmospheric circulation, thereby producing an extraordinary effect on global paleoclimate (e.g., Dubiel et al. 1991; Sellwood et al. 2000; Valdes 1993; Sellwood and Valdes 2006).

Although the continents have not always been in their present positions, the most extreme land of Southern South America, Patagonia, has always been within latitudes actually affected by the westerlies belt area. Iglesias Llanos et al. (2006) suggested that Patagonia shifted between the Earliest and Late Jurassic from latitudes ∼50° S to around 30° S affecting its climate (for more details see Volkheimer et al. 2008).

Furthermore, the warm Mesozoic Era (230–65 Ma ago) was likely associated with high levels of CO2 (Fig. 2) that became stabilized around 2,000 ppm. The Early Jurassic to Late Cretaceous times at 184–66.5 Ma were characterized by very high CO2 levels (6,000 ppm), though with cool pulses lasting less than 3 Ma (Frakes 1999). Thereafter, CO2 levels oscillated between very high (∼2,000 ppm) and low (500 ppm) values (Royer 2006).

Fig. 2
figure 2

Atmospheric CO2 and continental glaciation 400 Ma to present. Plotted CO2 records represent five-point running averages from each of the four major proxies (Adapted from Royer 2006). Also plotted are the plausible ranges of CO2 from the geochemical carbon cycle model GEOCARB III (Adapted from Berner and Kothavala 2001). All data have been adjusted to the Gradstein et al. (2004) timescale

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A major expansion of Antarctic glaciations starting around 35–40 Ma was likely a response, in part, to declining atmospheric CO2 levels from their previous peak in the Cretaceous (∼100 Ma) (DeConto and Pollard 2003). Furthermore, atmospheric CH4 concentration, land surface albedo, and ocean heat transport may all have played major, but not mutually exclusive, roles (Valdes 2000). Another important factor was the opening of the Drake Passage, between 49 and 17 Ma (Scher and Martin 2006).

Another important influence on climatic change is volcanic activity. It has been suggested that increased volcanic and seafloor-spreading activities during the Jurassic period released large amounts of carbon dioxide—a greenhouse gas—and led to higher global temperatures. In Fig. 3 the data of the major volcanic activity in Silicic Volcanic Provinces (SVP) is presented, together with the average δ18O (thick black line) (per million year = 1.45‰—[per mil]) which represent times of extensive glaciation or icehouse climate (below the solid back line) or warmer times of limited glaciation or greenhouse climate (above the solid back line) and sea-level oscillations (dotted line). The Gondwana supercontinent began breaking up in the Late Jurassic; the opening of the Atlantic Ocean is coincident with the start of major SVP. According to Ward (2009), the SVPs, as far back as they have been mapped, typically occurred at the onset of decrease in sea level most likely associated with increased glaciation.

Fig. 3
figure 3

The times of major volcanic activity in Silicic Volcanic Provinces (SVP). The shaded gray areas show the δ18O proxy for tropical sea-surface temperature (From Veizer et al. 1999). Values below the black line show times of glaciation (“icehouse world”), whereas over such line, the values show times of little of no glaciation (“greenhouse world”). The dotted black line shows sea level (Haq et al. 1987; Ross and Ross 1987, 1988; Adapted from Ward 2009)

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Climate Models of Paleoatmospheric Circulations

Climate models provide a framework within which existing data can be interpreted and appropriate hypotheses proposed. They are particularly valuable for regions with limited data and provide ways to help interpret local paleoclimate records. Paleoatmospheric circulations are normally inferred using three-dimensional general circulation models (GCMs) of the ocean and atmosphere. These models predict the fluid flow on a rotating sphere heated by solar radiation (more information in Crowley and North 1999; Trenberth 1992; Huber et al. 2000; Poulsen 2008, among others). All the different GCM simulations for the Gondwana supercontinent (Moore et al. 1992a, b; Chandler et al. 1992; Valdes and Sellwood 1992; Kutzbach and Gallimore 1989) showed strong seasonal alteration of summer monsoon lows and winter monsoon highs.

Early Triassic to Early Jurassic

In the Middle Permian (Wordian; at ∼265 Ma ago), the Pangaean supercontinent was surrounded by a Panthalassa super-ocean, and the CO2 level was around 1,000+ ppm and remained quite high until the Early Triassic (Royer 2006). Most of Early Jurassic atmospheric simulations show the dominance of monsoonal circulation conditions along the eastern part of Pangaea (Scotese and Summerhayes 1986; Crowley et al. 1989; Kutzbach and Gallimore 1989; Kutzbach et al. 1990; Fawcett et al. 1994; Barron and Fawcett 1995).

The sensibility of the Wordian climate to changes in greenhouse gas concentrations, high-latitude geography, and Earth orbital configurations was analyzed by Winguth et al. (2002) using a coupled atmosphere–ocean model. The 1× CO2 concentration (present level) would more likely be associated with a glacial climate, whereas high CO2 (8× CO2) concentrations simulate very warm conditions, even in high latitudes.

The simulated climate with 4× CO2 present level (Fig. 4) usually agrees well with sediments and phytogeographical patterns and has a significantly warmer polar climate than the present day, owing to increased flux of heat from ocean to atmosphere in high latitudes. The simulated ocean conditions is characterized by the following: a strong westward equatorial current which is blocked by islands at the eastern Tethys Sea, a warm poleward-directed currents along the Tethys coast and east coast of Gondwana, a cold equatorward current along the east coast of Angara, large meridional overturning circulation cells with deep water formation at both poles, and a warm deep ocean with low current speed in the eastern equatorial Panthalassa. The simulations suggest that a moderate climate over South Gondwana could be reproduced by an atmospheric CO2 concentration of at least four times present levels and/or the existence of a south polar seaway that further increases the heat flux to the atmosphere from the ocean.

Fig. 4
figure 4

Water mass classification according to Ziegler et al. (1998) derived from the climate simulation with 4× CO2 concentrations (Winguth et al. 2002)

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The climate of the Late Permian and Triassic times (∼250–200 Ma) has been described in terms of strong continental climate conditions and monsoons. The paleoclimate conditions were simulated by Kutzbach and Gallimore (1989) using idealized Pangaean continent with changes in CO2 and solar luminosity. In spite of the various limitations of the simulations carried out by these authors, the resulting scenarios show the mechanism whereby the large Pangaean continent produces the conditions needed for massive summer and winter monsoonal circulations and cross-equatorial flows. The presence of the large continent also helps to produce the high summer temperatures and evaporation rates and the long overland trajectories that deplete the available atmospheric moisture. Acting together, these factors generate the large arid region in the continental interior and in the equatorial zone. The presence of the large continent also helps to produce the very cold winters of middle and high latitudes and the very hot summers of the tropics.

Sellwood and Valdes (2006) simulated the climate responses to the Mesozoic changes in geography. For the Triassic period, the Central Pacific (Panthalassa) is modeled as having a super, and semipermanent, El Niño system.

Figure 5 shows the biome zones according to the precipitation, temperature, and evaporation model simulated. Southwestern Gondwana is winter–wet. The eastern parts of Gondwana are moister than the western parts, being generally wetter during the summer months (particularly December, i.e., a modeled summer–wet biome/climate). The balance between evaporation and precipitation reflects these seasonal changes. Southern Gondwana is in balance, or has an excess of evaporation, from November through February (summer dryness). But from March to October, the southern parts of the continent in particular have an excess of precipitation (winter–wet). This embraces the months of winter darkness for the southern polar area.

Fig. 5
figure 5

Following Walter (1985), the biome zones for the Late Triassic based on the model predicted temperatures and precipitation are shown. At the right, the key to Walter’s (1985) biome zones (Adapted from Sellwood and Valdes 2006)

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In theEarly Jurassic three-dimensional model simulation of Chandler et al. (1992), the atmospheric composition, solar radiation, and orbital parameters were set at present-day values. The resulting major features include warm surface-air temperatures, extreme continental aridity in the low and middle latitudes of western Pangaea, and monsoons which dominate along the midlatitude coast of Tethys and Panthalassa and also affect conditions deep into the continental interiors in some regions. They also experimented with variations in topography, sea-surface temperature, vegetation, and CO2 content and discovered that none of the scenarios yielded continental interiors in which winter temperatures remained above freezing everywhere. The most important outcome of the simulations is that the warm climate of the Early Jurassic could be maintained without additional forcing from CO2. The results show that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 Ma. The mega-monsoons are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography. The simulated sea-level pressure (not shown) and surface winds (Fig. 6) shown in the Southern Hemisphere, as in the Northern Hemisphere, pressure centers localized above high topography regions, the winter high-pressure cells being shifted poleward from the summer lows. During winter the winds are generally light and below freezing surface-air temperature extends across most of the southern Pangaea, although seasonally averaged coastal temperatures remain above freezing at all latitudes. During summer, monsoon circulations are responsible for increases in Southern Hemisphere precipitation. Cyclonic circulation about the subtropical low in the southwestern Pangaea causes surface winds over the western Tethys Ocean to flow southward, where they invade upon Pangaea in the region of the Indian subcontinent. Winds from the polar oceans, despite their cooler source, also cause prominent summer precipitation maximum over southeastern Pangaea, primarily as a result of the uplift of air over coastal mountains. This monsoon circulation causes great seasonal variations in the Southern Hemisphere precipitation fields.

Fig. 6
figure 6

Simulated global distribution of Early Jurassic wind vectors (a) surface wind; hatched line indicates approximate position of ITCZ for the South Hemisphere summer (Dec–Jan–Feb) and South Hemisphere winter (Jun–Jul–Aug) (Adapted from Chandler et al. 1992)

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The surface oceanic circulation during the Early Jurassic Panthalassa oceans modeled by Arias (2008) is shown in Fig. 7. The proposed conceptual paleoceanographic model was based on fundamental physical–oceanographic principles, a global paleogeographic reconstruction, the atmospheric wind paleo-patterns, and paleobiogeographic data. The resulting Panthalassic oceanic circulation pattern presents approximately hemispherical symmetric structure of two large subtropical gyres that rotates clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. During the Northern Hemisphere summer, the surface Tethyan Ocean circulation is dominated by monsoonal westerly directed equatorial surface currents that reached its western corner and drove them to the north, along the northern side of the Tethys Ocean, and in opposite direction during the winter.

Fig. 7
figure 7

Schematic representation of the surface oceanic circulation in the Panthalassa Ocean for the Northern Hemisphere (a) winter monsoon and (b) summer monsoon: Panthalassa Equatorial Counter Current (PECC); North Panthalassa Gyre (NPG) and South Panthalassa Gyre (SPG) or South Central Gyre (Adapted from Arias 2008)

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Middle and Late Jurassic

In the Middle to Late Jurassic the Hispanic Corridor (Fig. 1), connecting the Pacific Ocean to the western Tethys Ocean, affected the paleo-oceanic currents permitting a globe-circling, mainly west-flowing current system in low latitudes (Winterer 1991; Rais et al. 2007) and consequently the atmospheric circulation mainly in the tropical regions.

Moore et al. (1992a, b) used a GCM to obtain two Kimmeridgian/Tithonian (∼154.7–145.6 Ma) paleoclimate seasonal simulations, with geologically inferred paleotopography: one simulation used a CO2 concentration of 280 ppm (preindustrial level) and the other used 1,120 ppm. Increasing the CO2 content fourfold, it warms virtually the entire planet.

The simulations show greatest warming over the higher-latitude oceans and a smaller one over the equatorial and subtropical regions (Fig. 8). During the Southern Hemisphere summer (Fig. 8c), latitudes north of 30° S were characterized by the influence of the Intertropical Convergence Zone (ITCZ), while southwestern South America was dominated by the movement of the Southern Panthalassa ocean semipermanent anticyclone, which shifted to the south, affecting the southwest margins of Gondwana, that is, Patagonia. The monsoon lows were centered poleward of the western Tethys Sea, near 35° S. This location is just east of the region of summer maximum temperature. The wind pattern over Patagonia for June to August was opposite to that during summer. During winter the Panthalassa subtropical high was centered near 25° S. South of it, a subpolar low, centered near 60° S, dominated the south Panthalassa Ocean. These high- and low-pressure systems along with the continental winter monsoon high, which was located poleward of the western Tethys Sea and was centered near 40° S, were responsible for the winds from the northwest (Fig. 8d), forcing advection of wet and warm air over Patagonia. Furthermore, in the middle troposphere the axes of the Southern Hemisphere midlatitude storm track are located over 60° S. These factors are related to high positive net precipitation where precipitation exceeds evaporation. The model-simulated precipitation rate in the region is ≥5 mm/day (Valdes 1993) in concordance with the coal localities that are proxies for wet environments (Scotese 2012).

Fig. 8
figure 8figure 8

Latest Jurassic–Earliest Cretaceous: (a) Paleolatitude and paleogeography (Adapted from Scotese 2012); (b) Paleogeographic map of the southwestern Gondwana continent (Adapted from Scherer and Goldberg 2007) and maps of simulated Kimmeridgian/Tithonian surface wind vectors using 1,120 ppm CO2 for (c) December, January, and February; (d) June, July, and August (Adapted from Moore et al. 1992a)

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Moore et al. (1992a) proposed that the Jurassic warmth suggested by climate proxies may be explained by elevated atmospheric CO2. In contrast, Chandler et al. (1992), for the Early Jurassic simulation, suggested that the resulted warm SSTs in energy balance without high atmospheric CO2 imply that a warm Jurassic climate could have been the product of enhanced poleward heat transport through the oceans.

In other words, two feedback mechanisms are presumed to be primarily responsible for the very warm climate over Patagonia during the Jurassic: the elimination of sea and land ice that resulted from the warm polar sea-surface temperatures (SSTs) and the equatorward shift of Antarctica resulting in a decrease in surface albedo.

The modeled Kimmeridgian climate of Sellwood and Valdes (2006), between about 10° and 30° S, shows evaporation exceeding precipitation and a return to excess precipitation in the mid-to-high southern latitudes (Fig. 9). From fossil evidence, Jurassic plant productivity and maximum diversity were concentrated at midlatitudes, reflecting a migration of the zone of peak productivity from low to higher latitudes during greenhouse times (reviewed and modeled in Rees et al. 2000).

Fig. 9
figure 9

The Walter’s (1985) biome zones for the Late Jurassic (Kimmeridgian) based on the model predicted temperatures and precipitation (Adapted from Sellwood and Valdes 2006)

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Cretaceous

During the Late Jurassic to Early Cretaceous times, the separation between Gondwana and Laurasia was already well in progress, and the South Atlantic Ocean had already started to develop between those continents. India separated from Madagascar and raced northward on a collision course with Eurasia. North America was connected to Europe, and Australia was still joined to Antarctica. By the Late Cretaceous, the oceans had widened, and India approached the southern margin of Asia (see Scotese 2012).

The Early Cretaceous was a mild “icehouse” world. There was snow and ice during the winter seasons, and cool-temperate forests covered the polar regions. The Late Cretaceous was instead characterized by super greenhouse intervals of global warmth with ice-free continents. Globally averaged surface temperatures were 6–14 °C higher than at present (Barron 1983), and the temperature gradient between the poles and the equator was lower than today, that is, about 50 °C in the Northern Hemisphere and 90 °C in the Southern Hemisphere. The difference is largely due to adiabatic cooling reflecting the elevation of Antarctica. Frakes (1999) summarized the data on estimates of Cretaceous sea-surface and terrestrial temperatures.

There are four different assumptions concerning Cretaceous temperatures and meridional gradients: (1) tropical sea-surface temperatures were the same as today, but polar temperatures were warmer (5–8 °C) except when ice was present (0–5 º C); (2) the tropics were significantly cooler and midlatitudes warmer than today; (3) tropical sea-surface temperatures were 32–34 °C, with polar regions 10–18 °C; and (4) tropical sea-surface temperatures were about 42 °C and polar temperatures >18 °C. Furthermore, different hypothesis can explain the drastic warming and equable high latitudes during super-greenhouse intervals of the Cretaceous and Early Cenozoic. On the basis of coupled ocean–atmosphere model simulations of the Middle Cretaceous, Poulsen et al. (2003) hypothesized that the formation of an Atlantic Gateway could have contributed to the Cretaceous thermal maximum. Kump and Pollard (2008), by simulating the Middle Cretaceous using 4× CO2 from preindustrial atmospheric level, failed to produce results to explain the extreme high-latitude warmth implied by temperature proxy data. However, simulations with the combined increases in cloud droplet radii, which mainly affect cloud optical depth, and precipitation efficiency, resulted in a reduction in global cloud cover from 64 to 55 % with optically thinner clouds which reduced planetary albedo from 0.30 to 0.24. The ensuing warming was dramatic, both in the tropics and in high latitudes, where warming was augmented by surface albedo. In other words, warming was produced by albedo reduction due to diminishing cloud cover. Then, surface albedo feedback augmented warming in the tropics and high latitudes and produced almost the vanishing of snow and sea-ice cover, thus forcing less albedo and more intense warming. Otto-Bliesner et al. (2002) altered the models by the inclusion of high-latitude forest thus changing the paleogeography. These low-albedo forests warmed the high-latitude continents, which then transferred more heat to the high-latitude oceans, impeding sea-ice formation and warming coastal regions.

The scenarios for the Turonian (∼93.5–89.3 Ma) paleogeography were obtained by Floegel (2001) using GCM simulations with different orbital configurations and 1,882 ppm CO2 (5× AD 2,000 CO2, 7× preindustrial CO2) concentrations (Fig. 10). The simulated atmospheric circulation resulted in a much more complex circulation than that observed today. In this model, the tropical easterlies remain constant and strong throughout the year. However, at higher latitudes the circulation varies with the seasons. Due to the absence of polar highs during the winter, strong westerly wind belts develop between 50° S and the high-pressure zones at 30° S. Another important difference in Floegel’s scenarios lies in the strong trade winds, which developed during each hemispheric winter.

Fig. 10
figure 10

Turonian (93.5 Ma) wind speed and pressure at sea level for (a) Dec, Jan, Feb and (b) Jun, Jul, Aug [m/s and hPa] (From Floegel 2001; © Sascha Floegel, IFM-GEOMAR, Kiel, Germany)

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In Floegel’s model, in the Southern Hemisphere summer (December to February), the polar region is under the influence of a low atmospheric pressure system, and therefore the westerly winds were weak and variable and may have even had a reverse direction. Also, the subtropical to polar frontal systems would not exist (Fig. 10). This would result in disruption of the mid- and high-latitude wind systems. In the winter (June to August), the southern polar region is under the influence of a high-pressure system and the westerlies are well developed (Hay et al. 2005; Hay 2008).

The Maastrichtian (65.5–70.6 Ma) paleowind scenario in Fig. 11 was obtained by Bush (1997) using an atmospheric–oceanic GCM, dynamically and thermodynamically coupled, with four times the present-day value of atmospheric CO2, as indicative of Cretaceous levels. Even with these relatively new results, it is necessary to be cautious with the adopted paleogeographic reconstruction for the Maastrichtian (Ziegler et al. 1982), which was used in the model, if the Drake Passage were considered as it was already open.

Fig. 11
figure 11

Annual mean wind stress vectors (in dynes per square centimeter) for the Late Cretaceous. Arrows are scaled as displayed in the bottom right (Adapted from Bush 1997)

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Some GCM simulations of the Cretaceous super-greenhouse also considered as a boundary condition that the Drake Passage was already open (Sewall et al. 2007; Poulsen et al. 2007; Kump and Pollard 2008; Zhou et al. 2008; among others), while others considered that the Drake Passage was still closed (e.g., Haywood et al. 2004). For instance, Bush and Philander (1997) had estimated a later age for the opening of the Drake Passage, ranging from 49 to 17 Ma ago; see also more details and additional references in Cavallotto et al. (2011).

The earliest connection between the Pacific and Atlantic oceans at the Drake Passage is still controversial but very important, because the gateway opening probably had a profound effect on the global circulation and climate (see Cavallotto et al. 2011). The Drake Passage influence on climate was studied by Sijp and England (2004) by means of three main model simulation experiments, all of them set up identically with the exception of bathymetric data. They either kept (a) the Drake Passage closed by a land bridge between the Antarctic Peninsula and South America, (b) the Drake Passage open to a maximum depth of 690 m, or (c) the Drake Passage open at its present-day depth, which was modeled as an uninterrupted through flow at a depth of ca. 2,316 m.

The modeled climate with the Drake Passage closed is characterized by warmer Southern Hemisphere surface-air temperature and little Antarctic ice. An increase in Antarctic sea ice and a significant cooling of the Southern Hemisphere would have taken place when the Drake Passage was open to a much shallower depth of 690 m. As the Drake Passage opened, the climate would have become mostly similar in the Southern Hemisphere to the conditions of the Drake Passage if open at 690 m.

Bush’s (1997) simulated pattern of annual mean wind stress (Fig. 11) resulted in a pattern similar to that of the present day with only some changes as a consequence of the modified continental–ocean distribution. The annual mean fields show the tropical–subtropical region circulation characterized by the ITCZ and the easterly winds occupying the latitudinal band from 30° S to 30° N. The Southern Hemisphere westerlies occupy the latitudinal band from 40° to 70° S and the midlatitude tropospheric jets dominate the atmospheric circulation over Patagonia (Fig. 11). The simulation shows no poleward shift of the midlatitude westerlies in response to an ice-free planet.

In the Bush and Philander’s (1997) simulation, using a coupled atmosphere–ocean GCM with quadrupled CO2, the annual mean Hadley circulation indicates a general reduction in strength of the Southern Hemisphere cells in the Cretaceous, with the south equatorial Hadley cell weakening by 20 %. Subsidence in the high southern latitudes decreased dramatically in the Cretaceous in response to the elimination of the Antarctic ice sheet, which in the present climate induces strong subsidence poleward of 75° S. As a consequence, the strengths of the middle- and high-latitude cells decrease. In addition, the separation of the North and South American continents allowed stronger northeasterly trades flowing from the Tethys basin into the Pacific Ocean basin, whereas the strength of the southeasterly winds off the western coast of South America remains approximately the same.

Both modeling analysis, Bush and Philander’s (1997) and Hotinski and Toggweiller’s (2003), suggest the possibility of intensified surface circulation. The stronger upper tropospheric westerlies and stronger lower tropospheric easterlies over the tropical Pacific Ocean suggest a “permanent El Niño” state, as Davies (2006) found by analyzing laminated sediments. The model results showed the global precipitation approximately 10 % higher than at present. The mean annual temperatures increased and the amplitude of seasonal cycle in near-surface temperatures diminished, consequently precluding the presence of year-round snow or ice in the simulation. In high latitudes, however, there are regions that seasonally drop below freezing.

Conclusions

A major transformation in the global paleogeography occurred between the Triassic and the Cretaceous, which involved opening of the Hispanic Corridor and formation of the Atlantic Ocean. These are the main factors that produced the climatic changes registered in this period. Other important factors were the variations in the greenhouse gases, especially in CO2 which is related to the volcanic activity and the heat transport through the oceans. Strong monsoon conditions dominated during the Mesozoic in the Gondwana supercontinent.

The major climate features of the Early Jurassic, as simulated by means of GCM, comprised warm surface-air temperatures, extreme continental conditions in the low and midlatitudes, and monsoonal environments, which basically dominated along the midlatitude coasts of the Tethys and the Panthalassa oceans. The sea-level pressures over the continent show high-pressure cells during winter and opposite low pressure in summer.

Over the oceans, a dipole of subpolar low-pressure cells and subtropical high-pressure zone is present. Despite the similarity between the hemispheric patterns, a substantial degree of asymmetry is caused by land–sea distribution and topographic differences. The strong differences between winters and summers generated a large annual cycle. During the Southern Hemisphere winter, faint insolation and strong heat loss caused a decrease of the temperature over the interior of the southern Pangaea, and consequently, the sinking cooled air could create a high-pressure belt over the Gondwana continent. At the same time during the Northern Hemisphere summer, the warm temperatures can also generate an extensive low-pressure zone over central Laurasia, northeastern Pangaea, (between 10° N and 30° N) that deepens above the higher elevations in the eastern part of the continent. In the Southern Hemisphere winter, the situation was exactly the inverse.

In the Cretaceous greenhouse world, the paleogeography was dramatically different from that of the Gondwana continent, which was by now breaking up and when the Atlantic Ocean was already well developed. Results from a coupled ocean–atmosphere model indicate that the opening of the Atlantic Equatorial Gateway could have caused a large-scale reorganization of the tropical climate and regional warming. This mechanism was considered as a cause of the Cretaceous Thermal Maximum. Both the oceanic and atmospheric circulation was affected. The westerly winds developed only seasonally. In the absence of persistent westerlies, the subtropical ocean gyres would have weakened leading to an ocean circulation dominated by eddies. As a result, the pycnocline would have been more diffuse than today, contributing to enhanced thermohaline circulation and global oceanic heat transport (Hay 2008).

A caution note must be highlighted about the very large differences in the response of the general circulation models to Cretaceous boundary conditions. For example, the GENESIS model (Bice et al. 2006) must have had much higher pCO2 than previously thought (∼12 × present-day levels) to match the warmest tropical paleotemperature. Otherwise, high atmospheric pCO2 levels are not required to achieve a very warm Cretaceous climate. With moderate pCO2 levels (∼3 × present-day levels), the Hadley coupled ocean–atmosphere model predicts a hot Cretaceous world dominated by latent heat (Markwick and Valdes 2002; Haywood et al. 2004). The NCAR model with moderate pCO2 levels (∼3 × present-day levels) predicts deep-ocean temperatures of 9–11 °C and surface temperatures that are 3–4 and 6–14 °C warmer than modern at low and high latitudes, respectively. Ocean heat transports in the model are diminished in the Northern Hemisphere and enhanced in the Southern Hemisphere (Otto-Bliesner et al. 2002). A similar spectrum of climate models used for the Cretaceous is being used to predict future climates, each of them with different sensitivity to increased greenhouse gas concentration.

In summary, the climate of the Gondwana supercontinent was quite varied and significantly different from present-day conditions in most of it. Very high mean annual temperatures and increased precipitation characterized the climate of the second half of the Mesozoic, which have had strong influence in the landscape and ecosystem evolution.