Megathermal Rainforests During The Early Tertiary Period Of Greenhouse Climate

At the beginning of the Tertiary, megathermal rainforests were thus established in three parallel latitudinal zones (Figure 1.4). In the northern hemisphere, Northern Megathermal (termed "Boreotropical" in Morley, 2000a) mesic and monsoonal forests extended from North America and Europe, to East Asia, Southern Megathermal forests were present in mid-latitude South America, Australasia and southern Africa, and equatorial forests of the Palmae province were well-developed in northern South America, Africa, India and probably Southeast Asia (Morley, 2000a). The Paleocene saw global temperatures rise dramatically (Figure 1.5), due to increased atmospheric C02 (Pearson and Palmer, 2000). At the Paleocene-Eocene boundary, megathermal forests were thus at their most extensive (Figure 1.6), more or less reaching the polar regions, as far as 60°N in Alaska (Wolfe, 1985), and with Nypa swamps at 57°S in Tasmania (Pole and McPhail, 1996). At this time, intermittent land connections from North America to Europe via Greenland, from South America to Australasia via Antarctica and with a filter dispersal route between the Americas (Hallam, 1994; Morley, 2003) megathermal plants were able to disperse globally in a manner seen at

Tertiary Period
Figure 1.4. Closed canopy megathermal rainforests first became widespread during the Paleocene (Morley, 2000a). Paleogeography and paleocoastlines from Smith et al. (1994). Occurrences of evaporites and bauxites from Boucot et al. (in press). Dotted lines are floristic province boundaries.
Oxygen Isotope Foraminifera

Figure 1.5. Generalised oxygen isotope curve for benthonic (bottom dwelling) foraminifera through the Cenozoic (from Zachos et al., 2001). The ratio of leO to lsO for benthonic foraminifera provides a proxy for high-latitude surface marine temperatures (Hudson and Anderson, 1989), and therefore is a guide to global temperature trends: Oi = glacial interval at beginning of Oligocene; Mi = glacial at beginning of Miocene. The temperature scale was computed for an ice-free ocean, and thus applies only to the pre-Oligocene period of greenhouse climates.

Figure 1.5. Generalised oxygen isotope curve for benthonic (bottom dwelling) foraminifera through the Cenozoic (from Zachos et al., 2001). The ratio of leO to lsO for benthonic foraminifera provides a proxy for high-latitude surface marine temperatures (Hudson and Anderson, 1989), and therefore is a guide to global temperature trends: Oi = glacial interval at beginning of Oligocene; Mi = glacial at beginning of Miocene. The temperature scale was computed for an ice-free ocean, and thus applies only to the pre-Oligocene period of greenhouse climates.

no other time—with, for instance, members of the family Bombacaceae spreading from North America to Europe, on the one hand, and via South America and presumably Antarctica to Australia and New Zealand, on the other (Morley, 2000a; 2003). In the Middle and Late Eocene, subsequent to the thermal maximum, climate oscillations resulted in the successive expansion and contraction of megathermal forests in mid-latitudes, as recorded for North America by Wolfe (1977).

Climate During The Tertiary Period

Figure 1.6. Distribution of closed canopy megathermal rainforests during the Late Paleocene/ Early Eocene thermal maximum (Morley, 2000a). Paleogeography and paleocoastlines from Smith et al. (1994). Occurrences of evaporites and bauxites from Boucot et al. (in press). Dotted lines are floristic province boundaries.

Figure 1.6. Distribution of closed canopy megathermal rainforests during the Late Paleocene/ Early Eocene thermal maximum (Morley, 2000a). Paleogeography and paleocoastlines from Smith et al. (1994). Occurrences of evaporites and bauxites from Boucot et al. (in press). Dotted lines are floristic province boundaries.

The nature of the vegetation that characterized mid-latitudes at the time of the thermal maximum has been widely studied, with classic fossil localities in Europe, such as the London Clay (e.g., Reid and Chandler, 1933; Chandler, 1964; Collinson, 1983) and Messel in Germany (Collinson, 1988), North America (Wolfe, 1977; Manchester, 1994, 1999), South America (Wilf et al., 2003) and Australia (Christophel, 1994; Greenwood, 1994), but the character of equatorial vegetation at the time of the thermal maximum remains unclear. There has been some discussion as to whether mid-latitude areas experienced a "tropical" climate at this time (Daley, 1972; Martin, 1992). Most authors logically conclude that climates at this time were different from any present day climates. The critical factors were lack of frosts and absence of a water deficit. Summer-wet climates in Indochina and Mexico are probably the closest modern analogs, not surprisingly, in areas where many Boreotropical elements are relict (Morley, 2000a, Figure 11.9).

Very few studies demonstrating ecological succession from low latitudes from this critical period of the Paleocene-Eocene thermal maximum have been published. Reference has been made to "reduced global climate gradients'' based on oxygen isotope analysis of calcareous foraminiferal tests (e.g., Shackleton and Boersma, 1983), but current evidence shows that low-temperature estimates from the equatorial zone are erroneous and due to diagenetic effects. Recent sea surface estimates based on very well-preserved microfossils from the equatorial zone suggest Eocene sea surface temperatures were at least 28-32°C (Pearson et al., 2001; Zachos et al., 2003). Evidence from paleofloras suggests that there was a marked vegetational zonation from the equator to mid-latitudes (Morley, 2000a)—for instance, equatorial and South Africa were characterized by very different floras at this time, indicating a climatic zonation from mid- to low latitudes and current Eocene sea surface estimates are in line with those expected by modelling climates from vegetational data.

A study of the palynological succession through the Venezuelan Guasare, Mirador and Misoa formations by Rull (1999) provides a glimpse of the evolutionary and ecological changes that characterized the Late Paleocene to Early Eocene thermal maximum onset in northern South America. A conspicuous ecological change took place at the Paleocene-Eocene boundary. The Late Paleocene flora is similar to other low-latitude pollen floras of similar age, such as that from Pakistan (Frederiksen, 1994), emphasizing its pantropical character, whereas the Early Eocene palynoflora is geographically more differentiated, owing to a high proportion of restricted elements caused by the extinction of Paleocene taxa and the incoming of new components. The incoming of new Eocene taxa was gradual (or possibly stepped), and diversities increase in a manner that parallels global temperature estimates. At a detailed level several palynocycles could be defined, both in terms of assemblage and diversity changes, suggesting cyclic forcing mechanisms controlling vegetation changes. This study clearly suggests that vegetation change at low latitudes at the beginning of the thermal maximum was as pronounced as at mid-latitudes. A substantial temperature increase most likely accounted for the vegetation change recorded.

Some recent studies suggest that Early and Middle Eocene low-latitude climates were moisture-deficient or strongly seasonal in some areas. A well-dated Middle Eocene leaf flora from Tanzania, about 15° S paleolatitude, suggests the presence of wooded, rather than forest vegetation with near-modern precipitation estimates for this area (Jacobs and Heerenden, 2004). The plant community was dominated by caesalpinoid legumes and was physiognomically comparable to miombo woodland. Data from a very thick Early and Middle Eocene succession from southwest Sulawesi in Indonesia indicates alternating phases of dry climate (possibly reflecting periods of low sea level), in which Restionaceae were prominent members, and wetter climate, dominated by palms (Morley, unpublished).

1.5 MIDDLE EOCENE TO OLIGOCENE CLIMATES 1.5.1 General trends

From the Middle Eocene through to Late Eocene global climates show an overall cooling, with a further rapid temperature decline at the end of the Eocene (Miller et al., 1987; Zachos et al., 2001) following which mid-latitude northern hemisphere climates mostly became too cold to support megathermal vegetation. The decline in global temperatures is associated with a major build-up of polar ice, initially over Antarctica; and consequently sea levels fell globally, especially during the Oligocene.

With cooler temperatures in mid-latitudes, megathermal rainforests underwent a major retraction to low latitudes (Figure 1.7). This retraction was particularly pronounced in the northern hemisphere, with megathermal forests virtually disappearing from most of the North American continent (Wolfe, 1985) and becoming much more restricted in Europe, some elements possibly being maintained along the Atlantic

Eocene Paleogeography

Figure 1.7. Distribution of closed canopy megathermal rainforests during the Oligocene, following the terminal Eocene cooling event (Morley, 2000a). Paleogeography and paleo-coastlines from Smith et al. (1994). Occurrences of evaporites and bauxites from Boucot et al. (in press). Dotted lines are floristic province boundaries.

Figure 1.7. Distribution of closed canopy megathermal rainforests during the Oligocene, following the terminal Eocene cooling event (Morley, 2000a). Paleogeography and paleo-coastlines from Smith et al. (1994). Occurrences of evaporites and bauxites from Boucot et al. (in press). Dotted lines are floristic province boundaries.

coast as a result of warm currents. Northern hemisphere megathermal forest species had to disperse equatorward or face extinction. Their success at southward dispersal was related to the different tectonic setting in each of the three main areas. Because there was a continuous land connection from East Asia to the equatorial zone, many Boreotropical elements were able to find refuge in the forests of Southeast Asia. The Boreotropical relicts included many so-called primitive angiosperms, and as a result there is a concentration of such taxa in that area, especially in the rainforest refugia of southern China and Vietnam (e.g., Magnoliaceae, Trochodendron). This area has also provided a refuge for many Boreotropical gymnosperms, such as Cunninghamia, Glyptostrobus, and Metasequoia.

With respect to North America, Northern Megathermal elements may have been able to find refuge along the southern margin of the North American Plate, but could not disperse to the equatorial zone until the formation of the Isthmus of Panama in the Pliocene (Burnham and Graham, 1999). As a result, many more Northern Megathermal elements are likely to have become extinct in the Americas than in Southeast Asia. Many of those that did survive, and have parallel occurrences in Southeast Asian forests, are now extant as the amphi-Pacific element of van Steenis (1962).

For Europe, the east-west barriers of Tethys, the Alps, and the Sahara combined to limit equatorward dispersal to Africa to just a few taxa; hence, there are barely any true Northern Megathermal elements in present day African rainforests (Tiffney, 1985; Morley, 2001).

In the southern hemisphere, the end Eocene cooling event had a negative impact on the Southern Megathermal forests of South Africa and southern South America. However, the northward drift of the Australian Plate at the time of the period of major mid-Tertiary climate decline, allowed most Australian Southern Megathermal elements to survive this event. Today, the concentration of primitive angiosperm elements in the rainforests of northeast Australia is testament to reduced Australasian climate stress during the period of mid-Tertiary global cooling. The isolation of Australia and associated continental fragments has resulted in opportunities for many primitive elements to survive in this area compared with elsewhere. The concentration of primitive angiosperms in the area from "Assam to Fiji", which Takhtajan (1969) termed his "cradle of the angiosperms", has nothing to do with angiosperm origins, but is the response of these groups to finding refugia in a tectonically active global plate-tectonic setting during the period of mid- to Late Tertiary climate decline.

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Responses

  • alan
    Why are there so many primitive angiosperms in the rain forests of asia australasia?
    9 years ago
  • rosarmosario
    What is megathermal rainforest?
    9 years ago

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