Palaeoenvironments of insular Southeast Asia during the Last Glacial Period: a savanna corridor in Sundaland?

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Palaeoenvironments of insular Southeast Asia during the Last Glacial Period: a savanna corridor in Sundaland? Michael I. Birda,, David Taylorb, Chris Huntc a

School of Geography & Geosciences, University of St Andrews, St Andrews Fife KY16 9AL, Scotland, UK b Department of Geography, Trinity College, Dublin 2, Ireland c Division of Geographical Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, England Received 20 August 2004; accepted 7 April 2005

Abstract Consideration of a range of evidence from geomorphology, palynology, biogeography and vegetation/climate modelling suggests that a north-south ‘savanna corridor’ did exist through the continent of Sundaland (modern insular Indonesia and Malaysia) through the Last Glacial Period (LGP) at times of lowered sea-level, as originally proposed by Heaney [1991. Climatic Change 19, 53–61]. A minimal interpretation of the size of this corridor requires a narrow but continuous zone of open ‘savanna’ vegetation 50–150 km wide, running along the sand-covered divide between the modern South China and Java Seas. This area formed a land bridge between the Malaysian Peninsula and the major islands of Sumatra, Java and Borneo. The savanna corridor connected similar open vegetation types north and south of the equator, and served as a barrier to the dispersal of rainforest-dependent species between Sumatra and Borneo. A maximal interpretation of the available evidence is compatible with the existence of a broad savanna corridor, with forest restricted to refugia primarily in Sumatra, Borneo and the continental shelf beneath the modern South China Sea. This savanna corridor may have provided a convenient route for the rapid early dispersal of modern humans through the region and on into Australasia. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction The shallow epicontinental seas surrounding the islands of the Indonesian archipelago, often called the ‘Maritime Continent’, is currently a region of significance to global climate and ocean circulation. These seas comprise part of the Indo-Pacific Warm Pool (De Deckker et al., 2002) and are the warmest on earth, with temperatures averaging 28 1C or more (Yan et al., 1992). The region is a major source of latent heat to the atmosphere and acts as a major driver of both the Hadley circulation and ENSO oscillations associated with the Walker circulation. The major heat source to the atmosphere migrates seasonally from the Tibetan Corresponding author. Tel.: +44 1334 463 928; fax: +44 1334 463 949. E-mail address: [email protected] (M.I. Bird).

0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2005.04.004

plateau in July, through the Sundaland region to the West Pacific in January (McBride, 1998). Rainfall is high (generally 42000 mm) and modulated by the seasonal reversal of winds associated with the East Asian (northeast) and Australasian (southwest) monsoons. Thus the surface ocean waters in the region are not only warm, but also generally of low salinity (De Deckker et al., 2002). These warm, low-salinity waters are transferred from the South China Sea and Pacific Ocean to the Indian Ocean via the Indonesian throughflow, a number of narrow channels between the southern- and easternmost islands of Indonesia (Schneider and Barnett, 1997). The hot and humid conditions that pertain throughout the region mean that the islands of the maritime continent were largely covered by closed lowland rainforest before the considerable deforestation that has occurred in recent times. These forests are gradually

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replaced to the north and south by deciduous forest types and savanna woodlands (Wikramanayake et al., 2001). There is no other area in the tropics where the contrast between the modern distribution of land and sea with their distributions during the Last Glacial Period (LGP; Oxygen Isotope Stages 2–4), and in particular the Last Glacial Maximum (LGM; 20,000 years ago) is so marked, or where these differences could potentially have had such large impacts on global climate (De Deckker et al., 2002). Glacio-eustatic depression of sea level by
120 m at the LGM fully exposed the Sunda shelf joining mainland Southeast Asia to Sumatra, Java, Borneo and (possibly) Palawan. The Gulf of Thailand was exposed, as was the very broad continental shelf east of Malaysia and north of Borneo, substantially reducing the size of the South China Sea. This exposed continent has been called ‘Sundaland’ (Molengraaff, 1921). The magnitude of the changes in palaeogeography that have occurred in the region since the LGM present a challenge to models that aim to deduce the climate of the LGM and thereby the vegetation of the LGM. The emergence of Sundaland meant that the surface area of ocean water available for evaporation in the IndoPacific Warm Pool was substantially reduced and the flow of water between the Pacific and Indian Oceans restricted to the deep-water channels east of Borneo and Bali. There are currently two general alternatives for the vegetation (and therefore climate) of Sundaland at the LGM. The first has been most clearly articulated by Heaney (1991) who postulated a wide ‘savanna corridor’ extending down the Malaysian Peninsula and across the now flooded region between Borneo and Java, flanked east and west by tropical forest. Palawan and the western Philippines are also considered to have been savanna covered in this scenario. The second possibility is that a belt of tropical rain forest extended right across Sundaland from east to west, possibly diminished in north–south extent over the modern latitudinal range of tropical forest. This scenario has been advocated on the basis of some pollen records (e.g. Sun et al., 2000; Hope et al., 2004) and predicted by a range of vegetation models for the LGM (e.g. Prentice et al., 1993; Crowley and Baum, 1997; Otto et al., 2002). Determining which of these divergent possibilities is the more correct is important for two reasons. The first of these bears upon the causes, development and maintenance of modern biogeographic patterns, and also relates to the possible routes available for early human dispersal through the region and on into Australasia during the LGP (Stringer, 2000; Barker et al., 2001; Turney et al., 2001; Bird et al., 2004). The second reason is that the sheer size of Sundaland at the LGM (similar to Europe) means that it potentially

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stored a significantly greater amount of terrestrial carbon in soil and vegetation than it does today. The type and distribution of vegetation inferred to have been growing in the region at the LGM will therefore have an impact on estimates of global terrestrial storage of carbon and carbon-isotopes at the LGM (e.g. Bird et al., 1994; Otto et al., 2002). The purpose of this paper is to review and assess the available evidence for the distribution of terrestrial environments across Sundaland through the LGP, with emphasis on the LGM, to determine whether it is possible to distinguish between the main competing scenarios described above.

2. The extent of Sundaland The boundary of ice-age Sundaland is approximated by the 120 m isobath (Fig. 1). It is easily defined to the south and west by the deep waters of the Indian Ocean, and included what are now small island chains west of Sumatra such as the Mentawai Islands. To the east, Sundaland is separated from the biogeographically distinct region of Wallacea by deep-water channels that have ensured that no land bridge has ever existed between the two. This boundary corresponds with Huxley’s Line, running between Bali and Lombok in the south, Borneo and Celebes/Sulu archipelago on the equator and between Palawan and the rest of the Philippines in the north. It is possible that a narrow channel (the Balabac Strait) was maintained between Palawan and Borneo throughout the LGP, but the fauna and flora of Palawan are more closely associated with those of Sundaland than the Philippines. The continental shelf that forms the northeastern boundary of Sundaland is now entirely submerged, but includes a substantial portion of the modern South China Sea inshore of the 120 m isobath. The northern boundary of Sundaland is difficult to delineate as the region is now mostly submerged and cannot be defined in bathymetric terms. A phytogeographic transition between Indochinese and Sundaic floras occurs north of the modern Thailand–Malaysian border at
91N, possibly associated with Neogene seaways separating the two regions in Miocene and early Pliocene times (Hughes et al., 2003). This latitude therefore marks the most appropriate latitude for the northern boundary of Sundaland, extending east across the now flooded continental shelf to the south of the Mekong delta (Fig. 1).

3. Early Sundaland Though it is not the major focus of this paper, the Neogene geological and biogeographic evolution of the

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Fig. 1. Sundaland at the Last Glacial Maximum, showing the modern distribution of land in dark grey and the additional land exposed during the LGM in light grey (adapted from Voris, 2000). The northern boundary of Sundaland defined by 91N latitude shown as a dashed line. Possible lakes are marked by the letter L, the mouths of the major Molengraaff Rivers are indicated by letters as follows: 1—South Sunda River; 2—North Sunda River; 3—Siam River; 4—Malacca River. Major locations discussed in the text are also shown.

region does provide information of relevance to assessing the trajectory of climate and vegetation of Sundaland in more recent times. Detailed reviews of the subject have been published elsewhere (e.g. Sartono, 1973; Metcalfe, 1998; Hall, 1998, 2001; van den Bergh et al., 2001). The relative areas of land and sea on Sundaland have changed dramatically during the Neogene in response to the collision between the Indian-Australian and Eurasian plates and in response to changes in global sealevel. Sundaland consists of a stable core of continental crust of Palaeozoic age that has been augmented in size by tectonism and volcanism associated with subduction along the southern margin of the continent, with episodes of uplift and subsidence affecting other areas. The existence of fluvial sediments at depths in excess of 200 m testifies to periods when relative sea-level was considerably lower than at any time in the Quaternary (e.g. Batchelor, 1979), while periods of higher relative sea-level in the late Pliocene may have flooded a much larger proportion of Sundaland than is the case at present (Woodruff, 2003; Turchyn and Schrag, 2004).

In the late Pliocene and early Pleistocene, both flora and fauna had to negotiate a constantly changing matrix of available land bridges and favourable habitats in order to disperse through Sundaland. There is no evidence of mammals on Java prior to 2.4 million years ago, but after that time intermittent land bridges allowed colonization to occur (van den Bergh et al., 2001). The available evidence suggests that climate in the region was comparatively dry. Thick boulder beds, braided river and alluvial fan sediments of the ‘Older Sedimentary Cover’, indicative of a more arid, seasonal climate, were deposited widely in Malaysia and Indonesia during the Late Pliocene and early Pleistocene (Verstappen, 1975, 1997; Batchelor, 1979, 1988). The presence of faunas characteristic of open woodlands in the vertebrate fossil record of the early Pleistocene on Java (van den Bergh et al., 2001) also support this conclusion, as this would have required a connected tract of open vegetation from the Asian mainland to Java. Recent work suggests that the central Javan region where the earliest Homonid fossils have been found

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emerged at 1.970.02 Ma (Bettis et al., 2004), only slightly earlier than the oldest age of 1.8 Ma proposed for the arrival of Homo erectus in the region (Swisher et al., 1994). Despite the majority of evidence pointing to drier, more seasonal climates in the late Miocene and early Pliocene, the phylogeography of Lithocarpus, a rainforest tree genus with limited tolerance to seasonal variation in moisture levels, indicates a continuous presence in at least isolated pockets in parts of Sundaland from the Neogene (Cannon and Manos, 2003). The relative extent of tropical forest cover may have gradually increased through the Pleistocene and evidence of this is provided by the late arrival of orang utans on Java. This primate requires a forest environment and its exclusion from Java until the latter part of oxygen isotope stage 5 (110–70 ka; van den Bergh et al., 2001) suggests that no suitable combination of available land bridge with contiguous forest cover had been available between the Asian mainland and Java prior to that time. The available evidence therefore suggests that a ‘savanna corridor’ allowed the migration of openvegetation-adapted species (including Homo erectus) from Asia through Sundaland into Java well into the Pleistocene. The evidence also suggests that at least at interglacial times in the later Pleistocene, tropical forests may well have expanded across much of the sub-aerial extent of Sundaland.

4. Paleogeography of Sundaland since the last interglacial The extent of Sundaland since the last interglacial period is tied closely to the global changes in sea-level that have occurred during this period (e.g. Hanebuth et al., 2000; Voris, 2000; Fig. 1). The relationship between relative sea-level in Sundaland and glacio-eustatic changes in ocean volume is not straightforward, because the landmass of Sundaland itself has changed its elevation in response to diachronous changes in water loading as the land was progressively flooded by sealevel rose or emerged as sea-level fell (e.g. Lambeck and Chappell, 2001). This uncertainty, plus the variable thickness of marine sediments deposited following the most recent sea-level rise, make the prediction of past coastline positions uncertain, particularly in the large areas of low relief that characterize much of the Sunda shelf. The bathymetry of the Sunda Shelf suggests that Sumatra would be connected by a land bridge to the Malaysian peninsula at times when sea-level dropped to 30 m below the present level. This is complicated by the possibility that the Malaysian Peninsula has been subsiding, and Bird et al. (in review) has argued that

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the land bridge south of the Malaysian Peninsula across the Straits of Singapore to the Riau Archipelago was never submerged prior to the Last Interglacial, and since that time had been submerged only for short periods in Stages 5a, 5c and 5e, and since the beginning of the Holocene. Borneo would have become connected when sea-level dropped to 40 m and Java would have become connected to Sumatra when sea-level dropped below 50 m. Submerged shorelines in this depth range at 45, 36 and 30 to 33, 28 m and 18 to 22 m have been identified (Tjia, 1970) and some of these may relate to any of the several interstadial sea-level highstands or lowstands following the Last Interglacial and during the LGP. Of particular note is the existence of a flat widespread surface at 20 to 30 m depth, interpreted as a marine abrasion terrace, identified in the vicinity of both Banka and Karimata Islands, that is, along the axis of the land bridge to Borneo (Aleva et al., 1973). The eustatic sealevel curve of Lambeck and Chappell (2001), even allowing for the uncertainties identified above, suggests that this surface was cut during one (or more) of the highstands that occurred in later OIS-5. Therefore, it seems likely that land bridges from the Malaysian Peninsula to Sarawak and Borneo emerged intermittently from 110 to 85 ka and thereafter Borneo and Sumatra were continuously connected as a single landmass to the Malaysian Peninsula except for a possible brief period around 70 ka when the land bridge to Borneo may have been temporarily severed. Java would not have become fully connected to Sumatra until slightly later (around 80 ka), if the comparatively deep, narrow channel through the Sunda Straits existed at that time. However, Wohletz (2000) has complicated this simple interpretation of the bathymetry with the identification of the remains of an ancient ‘proto-Krakatau’ caldera 50 km in diameter in the modern Sunda Straits, suggesting that the Straits were formed by the explosive eruption of ‘protoKrakatau’ in the 6th century AD. Other authors have cited ancient Javanese texts that indicate a major eruption in the 5th century (e.g. Judd, 1889). A much earlier timing for the eruption of ‘proto-Krakatau’ might also be possible, as several thick ash layers up to 5 cm in thickness and of unknown origin occur in sediments of 56–74 ka in age in a core off the modern Sunda Straits (Gingele et al., 2002). Either way, Sumatra and Java may have been joined as a single mountain chain and island prior to this eruption and therefore the timing of emergence of a land bridge between the two may not be tied to the modern bathymetry of the Sunda Straits. From shortly after 30 ka to the end of the LGM at
20 ka, Sundaland was sub-aerially exposed to its maximum extent. This large continent, about the size

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of Europe, had roughly double the current land area, but the coalescence of many modern islands into a single landmass reduced the length of coastline by about 50% (Dunn and Dunn, 1977). As is the case today, the highland areas of Sundaland at the LGM occurred as a long arc of volcanic mountains fringing the southern and southeastern margin of the continent in Sumatra, Java and Bali, as extensive highlands in central and northern Borneo, and down the spline of the Malaysian peninsula. The great majority of the land area of Sundaland, particularly on the exposed continental shelf, was of low relief, generally less than a few hundred metres. This large lowland region was drained by several major rivers, first recognized by Molengraaff (1921). The largest southand east-flowing East Sunda River drained southern Borneo and Northern Java along the floor of the modern Java Sea, debouching in to the ocean north of Bali. The North Sunda River drained northeastern Borneo, the northern Java Sea and Southern Sumatra, debouching into the South China Sea southeast of Natuna Island. The Siam River system drained the eastern Malaysian Peninsula, the area north of a low divide running between the Natuna and Anambas Islands, and the modern Gulf of Thailand, running into the South China Sea north of Natuna Island. A smaller catchment drained tributaries from western Malaysia and Eastern Sumatra to the Indian Ocean via the Malacca Straits River System (Mollengraaff, 1921; Voris, 2000). The sediments/soils on the exposed continental shelf derived from the marine sediments deposited during the last interglacial and these can be inferred to some degree from the modern distribution of marine sediments. Emery et al. (1972) collated 33,000 determinations of bottom sediment type for the Java Sea up to a latitude approximately coincident with the southern tip of the Malaysian Peninsula. These results demonstrate that sand dominates the bottom sediments along the submarine divide between the Java Sea and South Shina Sea, from the tip of the Malaysian peninsula south to Bangka Island, and east from Belitung Island to the western coast of Borneo. Along this line, fine material is winnowed from the sand by currents between the South China and Java Seas. Elsewhere, the bottom sediments are a mixture of mud and ‘mud and sand’ (Emery et al., 1972). Sea-level fall in the LGP would have led to the emergence of extensive sandplains along the axis of the land bridges from the Malaysian peninsula to Sundaland. Further sea-level fall would have exposed progressively larger areas of mud and sand, and wave/storm action is likely to have led to the reworking of the sand and the development of long arcuate series of beach ridges separated by low muddy, poorly drained swales. An analogous modern environment might be the

‘permatang’ coast of eastern peninsula Malaysia, where Holocene sea-level fall has progressively exposed muddy marine and marginal marine sediments, upon which have been built a complex series of sandy ridges and sandplains (Teh, 1993). In addition, several currently submerged closed basins, north of Java and off the modern coast of Borneo, may have been large freshwater lakes at the LGM, flooded progressively as sea-level rose following the LGM (Fig. 1).

5. The climate and vegetation of Sundaland during the LGP Evidence for the terrestrial environments likely to pertain in Sundaland over the LGP comes from a variety of sources. The available evidence for terrestrial environments is summarized in Table 1 and Fig. 2, and is discussed below: 5.1. Climate evidence from the surrounding oceans De Deckker et al. (2002) have reviewed the evidence for climate in the region at the LGM, deduced from deep-sea cores in the South China Sea and waters east and south of Sundaland. They conclude that a general increase in sea surface salinity in the Indo-Pacific Warm Pool resulted from a significant decrease in rainfall in the region, partly attributable to the large reduction in the area of ocean surface available for evaporation. These authors also conclude that sea surface temperatures decreased by 2 1C at the most, and that higher lapse rates explain the evidence for lower tree-lines and glaciers at the LGM. Wang et al. (1999a) concluded that long-term variations in SST since the Last Interglacial in the South China Sea immediately north of Sundaland did not exceed 3 1C, and that glacial conditions in the region were characterized by an intensified winter monsoon and weakened summer monsoon. They also infer from the uniformity of terrigenous (fluvial) sediment supply between glacial and interglacial periods that there was little change in precipitation on the adjacent exposed shelf of Sundaland. Gingele et al. (2002) also conclude from a study of a core off the Sunda Strait that glacial conditions in the region (particularly from 70 to 55 and 35 to 20 ka) were characterized by greatly strengthened winter monsoon and weakened summer monsoon. A reduction in the Indonesian Throughflow, coupled with the strengthened northwesterly winds associated with the East Asian winter monsoon, resulted in an intensification of the southeast-flowing Indian monsoon current along the southern coast of Sundaland.

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Table 1 Available sources of information regarding the nature of terrestrial environments on Sundaland during the Last Glacial Period No.

Reference

Details

Location

1

Meijaard (2003)

Various

2 3 4

Majid (1982) De Dapper and Debaveye (1986); De Dapper (1986) Cranbrook (2000)

5

Gathorne-Hardy et al. (2002)

6 7

Brandon-Jones (1996) Anshari et al. (2001)

8

van der Kaars (1991); Wang et al. (1999b)

9

van der Kaars et al. (2000)

Modern species indicate forest or nonforest at LGM Fossil fauna indicates dry deciduous forest Geomorphology indicates drier, seasonal climate at and/or before LGM Pongo and Presbytis indicate tall evergreen forest at 40 ka Termite distribution (includes ‘anomalous’ savanna site in northern Sumatra) Primate distributions Pollen core—local forest, but smaller drier swamp indicated by dates of 28.6 and 16.8 ka BP 10 cm apart and humic peat Marine pollen core—open vegetation in region at LGM Pollen core—increased open vegetation in late Glacial period Marine pollen core—tropical forest on exposed shelf Pollen core—open vegetation in late Glacial period Pollen core—open vegetation in late Glacial period Pollen core—cooler, not drier Pollen core—cooler, not drier Pinus savanna during penultimate glacial maximum Geomorphic evidence of more sparse vegetation and erratic rainfall during last glacial period (
27–60 ka) Pollen core—cooler locally forested but grass present regionally, dates of 23 and 8.6 ka separated vertically by 20 cm Pollen core—cooler, not drier Pollen core—undated ‘graminae phase’ beneath swamp forst, similar to peats dated to LGM by Page et al. (1999) Sheet flood deposits on shelf beneath Java Sea from Sumatera, Java, Kalimantan? Pollend core—cooler and open vegetation during LGM Locally forested but evidence of opening at LGM and also dates of 17.5 and 30.5 ka only 0.5 m apart, possible unconformity Open vegetation at the LGM Peat at 30 m below sea level interpreted as indicating peatlands prior to sea-level rise Marine pollen core—increased open vegetation in region at LGM Murine rodent phylogeography

10

Sun et al. (2000)

11

Cited from Flenley (1998)

12

van der Kaars (1998)

13 14 15

Stuijts et al. (1988) Stuijts et al. (1988) Morley (2000)

16

Thorp et al. (1990), Thomas et al. (1999); Thomas (2000)

17

Taylor et al. (2001)

18 19

Maloney and McCormac (1995) Morley (1981)

20

Emery et al. (1972)

21

Dam et al. (2001)

22

Hope (2001)

23 24 25

Caratini and Tissot (1988) Situmorang et al. (1993) cited in Meijaard (2003) van der Kaars et al. (2000)

26

Gorog et al. (2004)

Niah Cave, Sarawak peninsula Malaysia Niah Cave, Sarawak Various Various Lake Sentarum, Kalimantan, 30 masl

Timor Sea Rawa Danau, Java 90 masl South China Sea Misedor, E. Kalimantan Bandung, 660 masl, Java Situ Bayongbong, Java Danau di Atas, 1535 masl, Sumatra Near Kuala Lumpur N.W. Kalimantan

Nee Soon, Singapore, 5 masl

Pea Bullock, 140 masl, Sumatra Sebangau , S. Kalimantan

various, Java Sea Lake Tondano, Sulawesi,
600 masl Lake Wanda, Sulawesi, 445 masl

Mahakam Delta, Kalimantan Java Sea between Madua and Kalimantan Banda Sea Various

Numbers relate to Fig. 2.

Taken together there is a general convergence of opinion that sea surface temperatures in the region were reduced by not more that 2–3 1C during the LGM and that the East Asian Winter monsoon was strengthened while the Southeast summer monsoon was weakened. However, opinion diverges as to whether rainfall in the region was reduced during glacial times, with some

evidence interpreted as indicating no change, but other evidence interpreted as indicating a substantial reduction in precipitaion across much of the region. In this regard, it should be noted that the marine evidence gives little indication as to the distribution of precipitation across Sundaland. Thus, the continuous fluvial input during the LGM in the South China Sea inferred by

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Fig. 2. Distribution of locations for which there is some evidence for the nature of the terrestrial environment during the LGP. The evidence comes from geomorphology, biogeography and palynology, and the details for individual locations can be found in Table 1. Base map adapted from Voris (2000). The ‘savanna corridor’ proposed by Heaney (1991) is also shown.

Wang et al. (1999a) may have been the result of precipitation falling comparatively close to the LGM coastline, or may have been derived from central Borneo, drained initially westward into the interior of Sundaland by the Kapuas River and then north and northeast to the South China Sea via the South Sunda River, or from a regional maintenance of comparatively high precipitation throughout the LGP. 5.2. Evidence from geomorphology A range of geomorphic evidence suggests drier, more seasonal climates in Sundaland prevailed in Sundaland during glacial periods in the later Quaternary (Verstappen, 1975, 1997). This evidence is generally in the form of active slope pediment formation and coarse valley fill sediments deposited by braided stream systems, indicating more seasonal and arid environments, rather than the valley incision characteristic of rivers in humid tropical environments. Emery et al. (1972) reported seismic evidence of steep down-channel bedding in wide palaeochannels beneath the central Java Sea possibly

resulting from sheet-flood deposits across the shelf from the major highland sources of detrital sediment on Java and Kalimantan, also implying more seasonality in rainfall distribution. The interpretation of this type of evidence is generally that vegetation at the time of sediment deposition was relatively sparse savanna-woodland, allowing seasonally heavy rains to erode large quantities of sediment from upland regions, temporarily depositing it in river valleys during dry periods (Verstappen, 1997). Unfortunately much of this evidence is not dated, but in some cases there is evidence that such sediments were deposited during the LGP. De Dapper (1986) and De Dapper and Debaveye (1986) report slope pediments and stonelines at three locations in Peninsula Malaysia that are interpreted to have formed in a drier climate under open vegetation. The association between these deposits and ash layers attributed to the
70 ka Toba eruption and developed on river terraces associated with the pediments suggest that the ‘open vegetation’ phase of landform development relates to the LGP, or possibly the penultimate glacial period. In West Kaimantan,

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broad fan-like bodies of sand fringing bedrock relief of inferred colluvial origin, dated to 27–60 ka, suggest more open vegetation with seasonally heavy rainfall (Thorp et al., 1990; Thomas et al., 1999; Thomas, 2000). Situmorang et al. (1993) note the existence of peat deposits underlying the late Glacial and Holocene marine sediments in the Java Sea and suggest that these indicate that freshwater peat forests existed in the area prior to sea-level rise. Given that mangrove peats are commonly deposited in the intertidal zone as sea-level rises, it seems likely that the peats underlying the Java Sea derive from mangrove swamps and do not provide evidence of the pre-existing terrestrial vegetation. 5.3. Evidence from biogeography As discussed above, the early evolution of terrestrial environments on Sundaland from the perhumid forests of the Miocene to the drier more seasonal climates of the Pliocene to early Pleistocene are relatively well characterized. The drier climates and associated open vegetation facilitated the migration of large grazing animals such as stegodonts (and also H. erectus) through the equatorial core of Sundaland to Java and islands further east. Whether these conditions extended into the LGP is less clear. Evidence from the phylogeography of Lithocarpus (stone oaks) on Borneo for example suggests that the genus has been present on the island since the Late Eocene, have experienced little migration and never gone locally extinct (Cannon and Manos, 2003). Some animals, such as shrews, can rapidly colonize a wide range of habitats and therefore neither the distribution of such species not their genetic makeup can provide significant information on the past distribution of habitats (Ruedi, 1996). Interpretation is further complicated by the possibility that even if there was a substantial reduction in forest cover, a network of gallery forests along the main rivers draining Sundaland may have allowed for survival of forest-dependent species in areas surrounded by more open vegetation. Fossil evidence for the arrival of forest-dependent primates such as orang utans and gibbons in Java provides evidence that a continuous belt of forest existed through the region at a time presumed to be later than OIS-5 (van den Bergh et al. 2001) and fossil evidence of these species in sediments dated to
40 ka or earlier at Niah Cave in Borneo suggests at least local forest cover around the cave at that time (Cranbrook, 2000). Majid (1982) also assessed the fossil evidence from Niah Cave at the LGM and concluded that the area was covered by deciduous forest, indicating some reduction or increased seasonality in rainfall at this time, but insufficient to cause replacement of forest by a more open vegetation type. It is possible to obtain indirect biogeographic evidence of habitat distributions during the LGP in

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several ways. Brandon-Jones (1996) used the absence of some species of columbine primates from Sundaland and the disjunct distribution of another (Presbytis comata) to infer that these rainforest-dependent species either became extinct, or were forced into refugia in northern Sumatra, Western Java and northern Borneo at some time in the past. This interpretation implies a contraction of rainforest habitat to around these areas during the last two glacial periods. Gathorne-Hardy et al. (2002) used the modern distributions of termite species to infer forest refugia in Northern and Eastern Borneo, northern and western Sumatra and the Mentawai Islands, with most of peninsula Malaysia, western and southern Borneo, eastern and southern Sumatra and Java interpreted as being covered by savanna. Gorog et al. (2004) applied a phylogeographic approach to the genetics of three rainforest-dependent murine rodent species from locations on Borneo, Sumatra, Java and the Malaysian Peninsula. These authors found that there was a deep history of vicariant evolution between the various populations of these species on Sundaland. The existence of a land bridge in the Java Sea during glacial times should have enabled the mixing of these separate populations, so the fact that they have remained separate and evolved in isolation from one another indicates that the habitat on the exposed continental shelves, served as a barrier to the dispersal during glacial periods (i.e. was not forest), while flooding of the land bridges served as a barrier to dispersal during interglacial periods. A different approach was adopted by Meijaard (2003), who compared the distributions of forestdependent mammal species with species adapted to open vegetation on the many small islands on the Sunda Shelf. The study assmes that mammal populations representative of earlier habitats would have been ‘trapped’ on islands as they became separated from mainland areas by sea-level rise. Therefore, the earlier the separation the more likely an island would retain a relict group representative of the population frequenting the area in glacial times. Thus, the fact that no mammals characteristic of open environments are present on Natuna Island (Fig. 1), which was separated by sealevel rise soon after the LGM, can be taken as strong evidence that the region was forested during the LGM. Conversely, the absence of forest-dependent species such as mouse deer on the currently forested Bawean Island (Java Sea; Fig. 1), suggests that no forest was present in that region at the LGM. For the many islands that were not flooded until Holocene times the evidence is ambiguous, as forest-adapted species may have migrated to these islands along with forest vegetation, prior to flooding of the shallow land bridges. Using this approach, Meijaard (2003) identifies Natuna Island and the Mentawai Islands as likely to have been forested

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at the LGM, whereas islands in the Java Sea, as well as islands east and north of Borneo and along the western margin of the Malaysian Peninsula, as likely to have been covered by open vegetation. 5.4. Evidence from palynology Several marine cores from the Sundaland region provide constraints on the changing nature of the terrestrial vegetation on the adjacaent land surface, although interpretation is complicated by the possibility that changing wind speeds and directions may have changed the source regions for the pollen in the past. Van der Kaars (1991), Wang et al. (1999b) and Van der Kaars et al. (2000) have obtained long records of vegetation change from cores south and east of Sundaland. The results of these studies suggest a consistent pattern of increases in pollen derived from open savanna-woodland environments in glacial periods, particularly OIS 4 and 2, and increases in forest pollen during interglacial periods. The data suggest contraction of the southern forest margin in Sundaland and the commensurate expansion of open vegetation types during the LGP, although the results do not allow an assessment of how far the forests of Sundaland may have retreated. Sun et al. (2000) has provided pollen records back to
30 ka from two cores in the South China Sea, immediately offshore from the points at which both the South and North Sunda Rivers reached the ocean during the LGM. Pollen from the LGP is dominated by lowland and montane rainforest taxa and these researchers concluded that a strengthened winter (northeast) monsoon picked up moisture across the South China Sea, resulting in increased rainfall on the northern coast of Sundaland and the expansion of lowland tropical forest on the exposed Sunda shelf north of Borneo. One complication with this interpretation is that most of the pollen may be fluvially-derived and hence there may be an over-representation of taxa from gallery forests close to the river, even if more open vegetation dominated away from the rivers. Pollen records from terrestrial sites have produced a fairly consistent picture of vegetation during the LGP for parts of Sundaland, hampered by the paucity of records from the northern margin and the region of the ‘savanna corridor’ of Heaney (1991), running down the spine of peninsula Malaysia and through the Java Sea. Pollen records from highland sites in Java and Sumatra suggest cooling at the LGM, but the maintenance of forest cover (Stuijts et al., 1988; Maloney and McCormac, 1995). Records from lowland sites in Java, southern Borneo and the islands east of Sundaland suggest either the existence of open vegetation, with an abundance of Graminae pollen (Caratini and Tissot, 1988; Van der Kaars, 1998; Van der Kaars et al., 2000; Dam et al., 2001)

or the local maintenance of forest cover within a regionally drier climate with more open vegetation (Hope, 2001). Morley (1981) reported an undated ‘Graminae phase’ in sediments beneath swamp forest in southern Borneo, and Page et al. (1999) have reported a single date of 18,300750 BP for the base of a peat sequence in an adjacent area, suggesting that the ‘Graminae phase’ of Morley (1981) may relate to open swamp vegetation in the region in LGM to early Holocene times. Pollen records from two more northerly lowland locations suggest the maintenance of local forest cover at the LGM, at Lake Sentarum in western Borneo (Anshari et al., 2001) and Nee Soon swamp in Singapore (Taylor et al., 2001). In the case of Nee Soon, the pollen taxa present at the LGM would be consistent with more open vegetation present away from the immediate swamp location. A feature of both these records is that the record covering the LGM is either highly compressed or missing. In the case of Lake Sentarum dates of 28.6 and 16.8 ka BP are separated by
10 cm and the peat in this interval is humified and compact. At Nee Soon, dates of 23 and 8.6 ka are separated vertically by 20 cm in adjacent cores. The original record from Lake Sentarum has been augmented by pollen data on several additional cores, and these tend to confirm the maintenance of local forest cover, but detailed interpretation is hampered by several reversals in the radiocarbon chronologies for the cores and the existence of several hiatuses in deposition (Anshari et al. (2004). In both cases, and also in the case of the Lake Wanda record of Hope (2001), the results would be consistent with a substantial decline in, or cessation of, peat accumulation as a result of lowered rainfall around the LGM. This scenario might parallel that suggested by Ledru et al. (1998), who re-examined the chronologies of seven pollen records from the Amazon Basin. These authors concluded sediments from the LGM were either absent or represented by very narrow intervals at all seven locations, indicative of drier regional climates either before or after the LGM. There are, amazingly, no pollen records of terrestrial vegetation change extending into the LGP from either lowland or highland localities anywhere on the Malaysian Peninsula. Some indication of the type of vegetation that might have occupied the region is provided by Morley (2000) who notes that vegetation during the penultimate Glacial Maximum (OIS-6) near Kuala Lumpur was an open savanna dominated by pines and grasses.

6. Terrestrial environments in Sundaland and vegetation modelling There is reasonable evidence for at least parts of Sundaland as to the general nature of the terrestrial

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environment during the LGP and the LGM in particular. The climate was generally 2–3 1C cooler and rainfall was reduced at least regionally. Kershaw et al. (2001) concluded that rainfall was reduced by 30–50% over much of Sundaland, though this does not provide any indication regarding the regional distribution of rainfall, and the same authors also conclude that the rainfall reduction was not sufficient to have a major influence on the extent of the core area of tropical rainforest. The probable strengthening of the East Asian winter monsoon and weakening of the Australasian summer monsoon may have also resulted in an increased seasonality of rainfall over much of the region. There appears to be broad agreement between the various proxy indicators discussed in previous sections, that mountainous regions in Java and Sumatra probably maintained a continuous forest cover throughout the LGP, along with probable areas of forest on the Mentawai Islands, in northeastern and southern Borneo, and on the Sunda shelf around Natuna Island. It also seems probable that open vegetation expanded its range across much of lowland Java, across the Java Sea to the southern and eastern coasts of Borneo. In the north, there is evidence that open vegetation extended down the Malaysian peninsula, into eastern lowland Sumatra and possibly into western Borneo. The evidence in the north is less conclusive, due to the paucity of available information, and there is virtually no direct information regarding vegetation history in now flooded northern Java Sea and southern South China Sea areas, or from Palawan in the northeast. A reasonable inference for the core of Sundaland might be that the rainshadow effect of the high mountains of Java and Sumatra, coupled with the very reduced size of the South China Sea, and differences in the relative strengths of the East Asian and Australasian monsoons at the LGM may have resulted in a significant reduction of, and increased seasonality in, rainfall in the centre of Sundaland and allowed the development of a ‘savanna corridor’ between the forested cores of Java/Sumatra and Borneo. Given the paucity of information from much of the Sundaland region, another means of testing the possibility of a savanna corridor lies in the modelling of vegetation distributions under LGM conditions. Several attempts have been made to model the global distribution of vegetation at the LGM, driven by a number of available Atmospheric General Circulation Models with or without coupled models of ocean circulation. These models generate a climate field for LGM conditions and a vegetation model predicts the type of vegetation for the combination of climate and soil conditions at each point on the surface of the earth exposed at the LGM. Examples of the application of coupled climate-vegetation models to predicting vegetation types, carbon and

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carbon-isotope stocks globally at the LGM can be found in Friedlingstein et al. (1992), Prentice et al. (1993), Crowley and Baum (1997), Francois et al. (1999), Kaplan et al. (2002a), Otto et al. (2002) and Crucifix et al. (2005). There are several climate simulations available to drive the vegetation models, and these produce a range of climate fields for the LGM. For the Sundaland region, they usually suggest a temperature decrease of a few degrees and can predict both increases and decreases in precipitation in different areas. This variability, plus the relatively coarse resolution of the models limits the direct comparison between the model output and observations; nevertheless several general features emerge from model results for the Sundaland area. The modelling approach generally predicts the widespread maintenance of either humid tropical forest or deciduous forest across the equatorial zone of Sundaland (e.g. Otto et al., 2002; Crucifix et al., 2005). Most models predict a varying degree of contraction of the northernmost limit of forest vegetation and a corresponding southward expansion of open vegetation types. The degree to which the southern/southeastern forest boundary contracts northwards varies considerably between models. For example, the simulations of Francois et al. (1999) and Otto et al. (2002) predict variable but comparatively small contractions of the southern forest boundary at the LGM, though more recent simulations do predict expansion of savannas into Java, the Java Sea and southern Borneo (L. Francois, pers. comm.). In order to attempt to generalize the results of the many simulations available, an overlay was made of the predicted distributions of predominantly open vegetation from 11 simulations from the Biome 4 dynamic global vegetation model driven by six predictions of LGM climate (Fig. 3). This provides an indication of the strength of agreement between simulations as to the distribution of open vegetation predicted by the Biome 4 dynamic vegetation model. There is strong agreement between the simulations that open vegetation expanded from the south towards the equator up to
21S and from the north down to
51N across the Sunda shelf. There is also strong agreement that open vegetation covered the western margin of the Malaysian Peninsula into Sumatra as well as at least northern Palawan. The prediction of open vegetation in northern Sumatra is at variance with some of the biogeographic evidence of primate refugia, but is supported by a savanna-like termite population at Ketambe in northern Sumatra that was considered anomalous by Gathorne-Hardy et al. (2002) given other evidence for continuous forest cover in the region. This might be explained if forest cover persisted in the mountains, while open vegetation expanded across lowland regions.

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Fig. 3. The average distribution of open vegetation types predicted for the Sundaland region at the LGM by the Biome-4 vegetation model coupled to a range of climate models, as discussed in Kaplan et al. (2002a, b). White areas correspond to areas where no model predicted an open vegetation type, black areas indicate areas where all 17 simulations predicted a more open vegetation type. The approximate modern limits of closed tropical forest north and south of the equator are shown by dashed lines.

No single simulation predicts a continuous corridor of open vegetation from north to south through Sundaland, but taken together, it can be seen that several models do converge on predicting open vegetation south of Peninsula Malaysia in the area of the likely land bridges in the Java Sea. One attribute that is not well represented in any of the vegetation simulations for continental shelf areas is soil texture. This variable is usually used in conjunction with temperature and rainfall estimates to ascribe a value to soil moisture available for plant growth (e.g. Otto et al., 2002; Kaplan et al, 2002b). As the nature of soil at the LGM on the flooded continental shelves of Sundaland cannot be determined, a mixed soil of sand and clay is assumed in the models. This is very unlikely to be the case because most coastal and shallow marine processes tend to sort particles into either sand, or mud, but not a homogeneous mixture of both. In addition no current

globle models are capable of resolving the effects of the comparatively local variations in soil type that may be of relevance to the concerns of this study. As discussed in preceding section, the land bridges above 40 m depth in the northern Java Sea are covered by thick sand and would have been covered with sand when the land bridges were exposed. In the Straits of Malacca, linear, mobile sand waves up to 20 m high and 50 km long are common between the Straits of Singapore and the town of Malacca on the Malaysian coast. While these sands may have been redistributed during the various sea-level rises and falls of the glacial period, they would have remained as discrete bodies redistributed by coastal processes over time. Likewise, in the Java Sea, the progressively lower sea-level highstands and lowstands in OIS-3 and OIS-4 would have left bands of long, shore parallel ‘Permatang’-like bodies of sand on the progressively exposed floor of the Java

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Sea, separated by areas of exposed marine mud sediments. Even with the 42000 mm of precipitation that currently falls in the region, nutrient-poor sandy soils do not support the humid tropical rainforest, but an impoverished type of forest known as Kerangas (heath forest), characterized by a low continuous single-layered canopy. These forests today are common on the island of Belitung, on the likely land bridge, and also in western and southern Borneo on sandstone plateaus and Holocene beach ridges. Of possible importance to the dispersal of orang utans to Java in later OIS-5, is the observation that the species can inhabit this vegetation type (Wikramanayake et al., 2001). Kerangas is particularly sensitive to disturbance and if cleared or burned reverts to an open savanna of shrubs and trees over sparse grass and sedge known locally as ‘padang’. Another climate-independent modifier of vegetation type is atmospheric carbon dioxide and Crucifix et al. (2005) have shown that the low atmospheric carbon dioxide concentrations of the LGM conferred a competitive advantage upon C4 plants (grasses) compared to C3 plants (trees). This considerably widened the climatic range over which grasses and shrubs dominated, compared to modern conditions, and this would have further decreased the viability of closed forest on the sandy soils of the land bridge. Therefore, given the marginal capacity of nutrientpoor sands to support forest vegetation even under the humid climatic conditions and high carbon dioxide levels that pertain at present, it seems very likely that at least the sandy areas of exposed sea-floor on the land bridge south of the Malaysian Peninsula into the Java Sea were covered by open vegetation during the LGP. This may have been the case even if there was no reduction in rainfall and it is also conceivable that early humans may have contributed to the establishment and/ or maintenance of open vegetation in these regions by burning, as has been documented in other areas (e.g. Haberle et al., 2001; Anshari et al., 2004).

7. Conclusion: a savanna corridor in Sundaland An assessment of the evidence available from geomorphology, biogeography, palynology and vegetation modelling for insular Southeast Asia over the LGP suggests that there is relatively strong and consistent evidence supporting a northward expansion of open vegetation types from southern Sundaland towards the equator during the LGM. In contrast, evidence for the nature of palaeo-environments in the core of Sundaland and areas north of the equator is sparse and conflicting. However, consideration of the nature of sediments on the floor of the Java Sea suggests that a savanna corridor through the interior of Sundaland did exist, as

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originally proposed by Heaney (1991). This corridor connected the areas of open vegetation north and south of the Equator and separated forest areas of unknown extent to east and west of the corridor at times of lowered sea-level during the LGP. A minimal savanna corridor would have occupied sandplains and ridges from the Straits of Malacca in a belt 50–150 km wide on the land bridge defined by the high points between modern islands of Bangka, Belitung and Karimata (the current ocean-floor divide between the Straits of Malacca, the Java Sea and the South China Sea), and along the exposed sea-floor of the Java Sea to mainland Java (Fig. 4). This corridor of open vegetation must have been continuous enough throughout the LGP to have acted as a barrier to the dispersal of forest-dependent murine rodents between Borneo and Java/Sumatra at those times when these areas were connected by land (Gorog et al., 2004). A corridor of this size could not be resolved by current climate/ vegetation models and direct evidence of its existence would require the development of palaeo-environmental records from those areas along the putative corridor still above sea-level. If evidence from the biogeographic distribution of termites, primates and murine rodents is taken at face value, along with liberal interpretations of the other available geomorphic and palynological data, a ‘maximal’ savanna corridor could have occupied a wide area from the eastern fall of the Sumatran Highlands well into the interior of western Borneo, with forested areas reduced to the highlands of Sumatra and Java (and the western fall of those highland areas), as well as one or more forest refugia in northeastern and southern Borneo. In addition, forest cover may have largely been absent across most of Palawan and the western Philippines, but gallery forests could still have occupied valley areas along major river courses throughout the region. Climate/vegetation modelling on the whole provides only very limited support for this scenario and the palaeoenvironmental records with which to definitively test the plausibility of a ‘maximal’ savanna corridor (of similar scale to that proposed by Heaney, 1991) have yet to be developed for most of the critical region north of the equator and in the central Java Sea. The conclusion that a savanna corridor of undetermined extent did exist at times of lowered sea-level during the LGP bears on the possible pathways for early human dispersal through the region. H. sapiens, arriving along the Malaysian peninsula at any time between 45 and
60 ka, would have available an ‘inland coastal’ route, staying entirely or largely within similar open vegetation habitats to those that they had encountered previously along the Malaysian peninsula. From the head of the palaeo-Malacca River catchment near Singapore, the South China Sea was less than 100 km away and a sandy upland route of generally less than

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Fig. 4. Configuration and sediment cover of the land bridge exposed between Peninsular Malaysia, Borneo, Java and Sumatra at a sea-level 40 m below modern levels. Possible ‘savanna corridor’ routes for human migration are indicated by dashed lines. Base map and 40 m isobath adapted from Voris (2000), distribution of sediments adapted from Emery et al. (1972).

100 m local relief led south through open vegetation to the island of Belitung. From Belitung, a similar sandy open land bridge, with no major intervening rivers, provided access to the west coast of Borneo. H. sapiens completed this journey before 42 ka BP, the time by which they had arrived at Niah Cave (Barker et al., 2001; Fig. 4). From Belitung Island, a series of open beach ridges and sand plains, separated by clayey plains that may or may not have been covered by open vegetation bordered the Java Sea leading south to Java. H. sapiens were able to traverse this route, and make the several water crossings beyond Java, arriving in Australia before 46 ka BP (Turney et al. 2001).

Acknowledgements Colin Prentice and Louis Francois kindly provided access to the most recent output of their vegetation models for this study. Peter White provided a thoughtful review of an earlier draft of this manuscript.

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