Climate change in low latitudes

Coinciding with the end Eocene cooling event, low-latitude climates also changed substantially, becoming significantly cooler and drier. This was particularly the case in Southeast Asia, where both palynological and lithological evidence suggests that everwet climates became of very limited extent, except, perhaps in the areas of Assam and Myanmar, where Oligocene coals yield rainforest leaf floras (Awasthi and Mehrota, 1995). There may also have been small refugia in other areas, such as the southeast margin of Sundaland (Morley, unpublished). The terminal Eocene event resulted in numerous extinctions across the tropics—for example, of Nypa from Africa and South America (Germeraad et al., 1968). However, the impact of this event was probably felt less in South America than other areas, since several taxa persisted there into the Neogene, such as mauritioid and other palm lineages. In general, equatorial floras began to take on an increasingly modern aspect during the course of the Oligocene.

A detailed pattern of climate change for the Oligocene is forthcoming from the Indonesian West Natuna Basin, which contains thick deposits of latest Eocene to Oligocene freshwater lacustrine and brackish lagoonal, followed by Neogene paralic, deposits that yield a rich palynomorph succession (Morley et al., 2003). Sediments were sourced primarily from the paleo Chao Phraya/Pahang catchments (Figure 1.8) and pollen data probably reflect vegetation change on a catchment rather than local scale. The latest Eocene and earlier Oligocene are characterized by pollen assemblages rich in Gramineae and with the very limited representation of "wet climate" elements, such as pollen of peat swamp trees, suggesting a warm, but seasonally dry climate (Figure 1.9). However, the mid- and Late Oligocene contains four maxima of temperate gymnosperms, which include Abies, Picea, and Tsuga, associated with Alnus, and also with Pinus and Poaceae (seasonal climate elements) and some pollen of rainforest taxa, followed by acmes with rainforest elements correlating with periods of higher relative sea level. These assemblages suggest alternating cool, seasonal, followed by warm, seasonal climates. The maxima of temperate gymnosperms suggest that cool climate oscillations brought freezing temperatures to tropical mountains, and consequently relatively cool lowland climates must also have been present.

Southward dispersal of montane elements during times of cool climate/low sea level

Sediment catchment area for W Natuna Basin Present day

Natuna Basin

I I Evergreen

Monsoon forest with rain forest patches

22 Kyr ■ Lr montane refugia | | Lowland refugia

Land exposed during LGM

Figure 1.8. Present day distribution of megathermal (and tropical) rainforests in Southeast Asia, and probable distribution at c. 22cal. yr bp showing positions of rainforest refugia. The shoreline at c. 22 cal. yr bp is also shown, together with the position of the catchment that fed the Malay/West Natuna Basins, and Natuna Basin, modified from Morley (2000a) using current palynological data (especially Morley et al., 2004), and taking account of the mammalian data of Meijaard (2003) and Bornean generic diversity data of Slik et al. (2003).

I I Evergreen

Monsoon forest with rain forest patches

22 Kyr ■ Lr montane refugia | | Lowland refugia

Land exposed during LGM

Figure 1.8. Present day distribution of megathermal (and tropical) rainforests in Southeast Asia, and probable distribution at c. 22cal. yr bp showing positions of rainforest refugia. The shoreline at c. 22 cal. yr bp is also shown, together with the position of the catchment that fed the Malay/West Natuna Basins, and Natuna Basin, modified from Morley (2000a) using current palynological data (especially Morley et al., 2004), and taking account of the mammalian data of Meijaard (2003) and Bornean generic diversity data of Slik et al. (2003).

Previously, the high representation of montane gymnosperms in the Southeast Asian area has been interpreted as reflecting a source from high mountains (Muller, 1966, 1972), but geological data suggest an inverse relationship between phases of mountain building and the general abundance of temperate elements (Morley, 2000b), emphasizing that most abundance variation within montane gymnosperms is climatic. The four cool climate intervals in Natuna coincide roughly, but not precisely, with the Oligocene cooler climate episode indicated from benthic isotope data (Miller et al., 1987; Zachos et al., 2001), and can be approximately tied to positive oxygen isotope excursions in the high-resolution oxygen isotope curve of Abreu and Anderson (1998).

Grass pollen also shows a series of maxima through the Oligocene of West Africa (Morley, 2000a, p. 140), reflecting similar drier and wetter periods, but without evidence for temperature change.

1.6 EARLY AND EARLIEST MIDDLE MIOCENE, RETURN OF GREENHOUSE CLIMATES

1.6.1 General trends

The latest Oligocene/earliest Miocene was characterized by globally warmer climates but with some cooler episodes (Zachos et al., 2001). The highest global temperatures

14fl g

30 f

"SA"

Climate trends Climate trends Niger Delta Natuna (8°N) (5°N)

Cool Warm Seasonal Everwet Cool Warm Seasonal Everwet

Figure 1.9. Summary of Oligocene to Pliocene climatic change in relation to sea level change, suggested from Natuna Basin palynological studies, together with Middle Miocene to Pliocene climate cycles for the Niger Delta. Sequence nomenclature follows Morley et al. (2003) for Oligocene to Early Miocene and Morley (2000a, Figure 7.13) for Middle Miocene to Pliocene. Timescale used is that of Berggren et al. (1995).

Tropical Forest Extent Africa Miocene

Figure 1.10. Distribution of closed canopy megathermal rainforests during the Middle Miocene, coinciding with the Miocene 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 1.10. Distribution of closed canopy megathermal rainforests during the Middle Miocene, coinciding with the Miocene 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.

were at the beginning of the Middle Miocene (mid-Miocene climatic optimum), although CO2 levels remained stable over this period (Pearson and Palmer, 2000).

The renewed warming in the Early and earliest Middle Miocene once again resulted in the expansion of moist megathermal forests poleward of subtropical high-pressure zones, although this time for only a short period (Figure 1.10). In the northern hemisphere, mangrove swamps with Rhizophora and rainforests with Dacrydium extended northward to Japan (Yamanoi, 1974; Yamanoi et al., 1980), Symplocos and Mastixia diversified in southern and central Europe (Mai, 1970), and megathermal elements extended along the eastern seaboard of North America (Wolfe, 1985). In South Africa, palm-dominated vegetation became widespread at two successive time intervals (Coetzee, 1978), and in southeast Australia the combination of warmer climates and northward drift once again resulted in the development of megathermal forests as far south as the Murray Basin (McPhail et al., 1994). Climates in India again became moist, and as a result many elements of the Malesian flora spread to the Indian Plate, with well-preserved macrofossils in the Siwaliks (Awasthi, 1992).

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