Early Holocene vegetation, human activity and climate from Sarawak, Malaysian Borneo

Quaternary International 249 (2012) 105e119

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Early Holocene vegetation, human activity and climate from Sarawak, Malaysian Borneo C.O. Hunt a, *, R. Premathilake a, b a b

School of Geography, Archaeology & Palaeoecology, Queen’s University Belfast, Belfast BT7 1NN, UK Postgraduate Institute of Archaeology, University of Kelaniya, 407, Bauddhaloka Mawatha, Colombo 7, Sri Lanka

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 29 April 2011

A 40 m core from Loagan Bunut, Malaysian Borneo, yielded a high-resolution early Holocene (11.3e6.75 ka) sequence of marginal-marine deposits. Palynological analysis showed relatively stable fire-regulated lowland forest through this time, with the local development and regression of mangrove vegetation. A general trend of rising rainfall and thus strengthening North East monsoonal circulation linked to the migration of the mean position of the ICTZ was interrupted by what may be episodes of drier climate and weakening monsoonal activity at 9250e8890, 7900 and 7600e7545 cal. BP. Magnetic susceptibility peaks suggest marked short-term ENSO-style activity superimposed upon this record. Repeated markers for open and disturbed habitats, plus occasional imported and probably-cultivated taxa, point towards human impact from the earliest Holocene on the wet tropical forest at Loagan Bunut. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction This paper reconstructs early Holocene vegetation and climate history at Loagan Bunut, Sarawak, Malaysian Borneo, based on analysis of pollen, organic microfossils, palynofacies and magnetic susceptibility, to throw light on several unresolved issues. These are: the nature of early Holocene vegetation in Borneo and the impact of human activity and climate on that vegetation. The West Pacific Warm Pool, with its high convective activity, is regarded as the ‘Boiler Box’ for the World’s climate and events there have repercussions widely in tropical and subtropical environments (De Deckker et al., 2002; Gagan et al., 2004; Partin et al., 2007). The area, however, is considerably under-researched and there are very few continuous high-resolution Early Holocene sequences (Hope et al., 2004). In particular, Borneo, the largest island in the region, is poorly studied, with only the stalagmite isotopic record of Partin et al. (2007), which shows a general rise in precipitation from the Last Glacial Maximum into the Holocene, covering this interval. In contrast, the late Holocene in Borneo is better known from palynology (e.g. Anderson and Muller, 1975; Anshari et al., 2001, 2004; Hope et al., 2005; Yulianto et al., 2005; Hunt and Rushworth, 2005b).

* Corresponding author. E-mail address: [email protected] (C.O. Hunt). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.04.027

Investigations of later Quaternary environmental changes in island South East Asia. (e.g. Flenley, 1985; Stuijts et al., 1988; Ellison, 1989; Heaney, 1991; Maloney, 1992; van der Kaars and Dam, 1995; Kamaludin and Azmi, 1997; Urushibara-Yoshino and Yoshino, 1997; Yakzan and Hassan, 1997; Sarnthein and Wang, 1999; Van der Kaars et al., 2000; Anshari et al., 2001; Suparan et al., 2001; Dam et al., 2001; Van der Kaars et al., 2001; Maxwell and Liu, 2002; White et al., 2004; Wust and Bustin, 2004; Bird et al., 2005; Simanjuntak, 2006; Hunt et al., 2007; Partin et al., 2007; Piper et al., 2008; Cannon et al., 2009) suggest that precipitation was reduced substantially during much of the Last Glacial, including the Last Glacial Maximum (LGM), and probably rainforest cover in many areas was diminished or thinned as a result of low precipitation. The transition from the Last Glacial Maximum to the Holocene seems to have been accompanied by steady rather than rapid climate change and the Younger Dryas is not strongly marked across much of this area (Partin et al., 2007, but see Maloney, 1996; Steinke et al., 2001; Turney et al., 2005). There is little published evidence, however, on the development of vegetation following the climate changes of the Last GlacialeHolocene transition in Borneo. There is a widespread conception that the rainforests of Borneo were effectively primeval and virtually unaffected by human activity (e.g. Morley, 2000) before recent times; although alternative views have been expressed (e.g. Gibbs, 1914), they have received little attention. Recently, however, one of us suggested that from ca. 48,000 BP during the Late Pleistocene, afforestation during warm stages was accompanied by biomass burning to

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provide forest-edge and regenerating habitats suitable for important food plants (Hunt et al., 2007). Similar biomass burning has been widely suggested elsewhere in island SE Asia (e.g. Van der Kaars et al., 2000). As yet, no studies have examined the Early Holocene vegetation record in Borneo, and therefore whether this biomass-burning behaviour persisted into the Holocene e or if any other anthropogenic modification of vegetation occurred is presently unknown. In the later Holocene, most published records are from raised bogs or other peatlands (e.g. Anderson and Muller, 1975; Anshari et al., 2001, 2004; Hope et al., 2005) which are very nutrient-deficient, low in biodiversity and thus unattractive for human activity of this sort. There is therefore a need for substantial high-resolution studies to provide a full understanding of Early Holocene environmental and vegetation change and variability in Borneo and other parts in Island South East Asia and of the impact of human activity. This study is a contribution towards resolving these issues. 2. Materials and methods This paper describes a high-resolution early Holocene record from Loagan Bunut, in the Malaysian state of Sarawak (3
470 -3
440 N, 114
130 -114
150 E; Fig. 1), in Borneo, the world’s third largest island. This region has some of the highest sea surface temperatures in the world (Partin et al., 2007). Mean annual temperature at sea level is 27
C, but diurnal variation is 6e10
C, which is very significant compared with the seasonal variation of mean temperature of 1e2
C. The average daily temperature at Loagan Bunut

ranges from 23
C during the early hours of the morning to 32
C during the day (Murtedza et al., 2006). In lowland Sarawak, precipitation is high from the North East Monsoon. The South West Monsoon is usually drier, although rainfall occurs throughout the year. Thus, high rainfall can be expected from November to February, with a drier period from June to August. The rainfall, which at Loagan Bunut averages about 3300e4600 mm per year, is determined to a great extent by the North East Monsoon (Murtedza et al., 2006). Loagan Bunut is the only large freshwater lake in Sarawak. It lies about 40 km inland from the shores of the South China Sea, within the Loagan Bunut National Park, which was established in 1990. Loagan Bunut lies on the floodplain of the Sungai Tinjar, a tributary of the Baram River. The Tinjar is still tidal until just below the confluence with the Sungai Bunut, which drains Loagan Bunut (Fig. 1). It occupies part of a flat-floored, probably rifted, valley system which is bounded by Oligocene-Miocene and MiocenePliocene prodeltaic and turbiditic formations (Hunt et al., 2006). The buried valley beneath Loagan Bunut is over 40 m deep and infilled with Holocene sediments. In general, the area of the National Park is characterised by an undulating landscape with relatively low hills. The tallest peak, Bukit Pajek, is 132 m asl. in elevation. The water level of the lake has a normal surface elevation of ca. 14 m asl., but is dependent on the water level of the local rivers, the Sungai Tinjar, Sungai Teru and Sungai Bunut (Murtedza et al., 2006). Loagan Bunut is bordered by several major wetland ecosystems, including raised bogs and swamp forests, with lowland forest, cut over in 1977e78, on the

Fig. 1. Location of Loagan Bunut in North-western Borneo.

C.O. Hunt, R. Premathilake / Quaternary International 249 (2012) 105e119

hills. Recent floral survey in the raised bog forests identified 97 flowering plant species and members of 42 fern families (Tawan et al., 2006). Initially, lithostratigraphical investigations were carried out using a piston corer. Twenty four cores were taken along two transects; one from the margin of the peat swamp forest towards the middle of the lake along the northeast side; and a second, parallel to the first, on the south and west side of the lake (Fig. 2). Sediments were described in the field (Hunt et al., 2006). Deep sequences from the middle of the lake were sampled using a Livingstone auger and then a powered percussion auger once it became clear that the deposits were too deep to be sampled by hand. Cores of 40 m and 38.5 m were taken using the percussion auger. The cores were wrapped in polythene for transport to the laboratory and refrigerated as soon as possible after collection.

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Detailed sediment descriptions were made in the laboratory on the basis of macroscopic and microscopic observations and the deposits were grouped into five sedimentary units (Table 1). Samples were taken from the 40 m core at 1e10 cm intervals for the first 10 m of the sequence. Below 10 m, the sample interval was generally at 30e40 cm intervals. Eighty one sub-samples for the analysis of pollen and spores were treated by the method described by Hunt (1985). Volume specific samples (3 cm3) were boiled with 5% potassium hydroxide and sodium pyrophosphate solution for 40 min. Coarse particles from the resultant suspension were removed using a 150 mm sieve. The suspension was sieved on a 6 mm nylon sieve to remove fines and solutes. Silt and fine sand were removed by swirling on a clock-glass. The retained fraction was washed several times and stained with safranine and mounted in Gurr Aquamount. Pollen counting took place under 400,

Fig. 2. Location of boreholes at Loagan Bunut (after Hunt et al., 2006).

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Table 1 Lithology of the Loagan Bunut core. Depth (m)

Lithology

0.00e0.33 0.33e0.87 0.87e0.96 0.96e1.00 1.00e1.25 1.25e1.56 1.56e1.64 1.64e1.84

Clay, brownish grey, 10YR4/1 Silty-clay, brownish grey, 10YR5/1 Organic mud with wood, black, 10YR2/1 Organic mud with leaves, brownish grey, 10YR4/1 Silty-clay, organic, bright yellowish brown, 10YR7/6 Organic mud, with wood, black, 10YR2/1, 7/1 Silty-clay, organic, greyish brown, 10YR5/2 Silty-clay, organic, with wood and leaves, dull yellowish brown, 10YR4/3 Silty-clay, organic, with wood and leaves, yellowish brown, 10YR5/1) Silty-clay, organic, with wood and leaves, pale yellow, 10YR8/3, 2.5Y8/4 Organic mud with wood, dull yellow orange, 10YR7/4 Organic mud with wood and leaves, brown, 10YR4/6 Organic mud with wood, light yellowish orange, 10YR8/3, 8/4 Silty-clay, organic, with wood, brownish grey, 10YR5/1, 6/1, 4/1 Slightly organic mud, brownish grey, 10YR4/1, 5/1 Slightly organic mud, dull yellowish brown, 10YR5/3, 5/4, 4/3 Slightly organic mud, brownish grey, 10YR4/1, 5/1 Slightly organic mud, greyish yellowish brown, 10YR4/2, 5/2 Slightly organic mud, brownish grey, 10YR4/1, 5/1 Slightly organic mud, brownish grey, 10YR4/1, 5/1, dull yellowish brown, 10YR4/3 Slightly organic mud, dull yellowish brown, 10YR4/3, brownish grey, 10YR4/1 Slightly organic mud, bright yellowish brown, 10YR6/8 Slightly organic mud, brownish grey, 10YR4/1, light yellowish brown, 10YR6/8 Slightly organic mud, dull yellowish brown, 10YR5/4 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1, bright yellow brown, 10YR6/8 Slightly organic mud, brownish grey, 10YR4/1, brownish black, 10YR3/1 Slightly organic mud, brownish black, 10YR3/1 Slightly organic mud, yellowish brown, 10YR5/8 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1, brownish black, 10YR3/1 Slightly organic mud, brownish grey, 10YR4/1, yellowish brown 10YR5/8 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1, yellowish brown, 10YR5/8 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1, yellowish brown, 10YR5/8 Slightly organic mud, yellow orange, 10YR/7/8, brownish grey, 10YR4/1 Slightly, organic mud and sand, greyish yellow, 2.5Y4/1 Slightly organic mud, brownish grey, 10YR4/1 Slightly organic mud, brownish grey, 10YR4/1, yellowish brown, 10YR5/6

1.84e2.53 2.53e3.00 3.00e4.00 4.00e6.00 6.00e9.50 9.50e10.00 10.00e13.26 13.26e13.52 13.52e17.00 17.00e17.39 17.39e19.00 19.00e19.60 19.60e20.50 20.50e20.88 20.88e22.00 22.00e22.25 22.25e26.50 26.50e27.30 27.30e27.90 27.90e28.50 28.50e28.70 28.70e29.00 29.00e29.30 29.30e33.50 33.50e33.65 3365e34.00 34.00e34.40 34.40e35.50 35.50e35.70 35.70e38.00 38.00e38.29 38.29e38.31 38.31e39.25 39.25e40.00

while  1000 phase contrast was used for critical identifications. In total, between 250 and 800 pollen grains (excluding spores and all other organic particulates) were counted in each sample. Pollen and spores were identified using two reference slide collections: one at the Palaeoecology Unit, Queen’s University, Belfast, collected by Dr B. Maloney, and other from the Postgraduate Institute of Archaeology, University of Kelaniya, Sri Lanka. In addition, the work of Erdtman (1952); Huang (1972, 1981); Harland (1977, 1983); Van Geel et al. (1981); Cole (1992); Matsuoka (1992); Van Geel and Grenfell (1996); Reille (1998); Pearson (1999); Rochon et al. (1999); Vasanthy and Grard (2007) were also used. Unidentified pollen and spores make up 1e15% of the count at any given level. Microscopic charcoal particles with a diameter >25 mm and other

palynofacies (e.g. dinoflagellates, foraminifera, algae, diatoms, fungal spores and hyphae, vesicular arbuscular micorrhyza (VAMs), testate amoebae and silicoflagellates) were counted on the same slide as used for pollen and spore analyses, as were thermally mature materials, (which are a by-product from biomass burning), amorphous matter resulting from the decay of organic material, and various types of subfossil plant materials (e.g. gum, cuticles) following the palynofacies method of Hunt and Coles (1988). Volume specific susceptibility (k) was measured using a Bartington Instrument System MS2 with a MS2C sensor in order to the describe concentrations of the magnetic component in whole cores. Measurements for mass specific (c), low frequency, high frequency and frequency dependent susceptibility values were made on selected levels where high peaks and more uniform trends occurred in the k stratigraphy in order to describe concentration and grain size variations of the magnetic mineral components (Table 2). These variations seem to reflect small-scale layering in the core and thus to reflect natural erosion and deposition processes. Humans may have enhanced the degree of erosion, but if the pollen diagrams do not clearly indicate the presence of humans in the near vicinity, the degree of erosion should be interpreted in terms of natural processes. Pollen and spore, microfossil and summary diagrams were constructed using the TILIA program (Grimm, 1991e1993). Assemblage zones were established according to CONISSconstrained cluster analysis, which operates on the incremental sum of squares (Grimm, 1991e1993), and are shown as dendrograms at the right-hand side of the pollen diagram. The basic sum includes pollen and spores from trees, shrubs, herbs, lianes and pteridophytes. Ecological groupings were based on Flora of Sarawak and several other references (Ashton, 1988; Kitayama, 1992; Soepadmo and Wong, 1995; Soepadmo et al., 1996). In addition to the generic and where possible the specific name, the family names of all pollen and spore taxa are shown in the pollen diagrams, for ease of reference. One of the problems with working with the palynology of Borneo is the huge diversity of species within palynologically-recognisable taxa. Families such as the Ericaceae (including palynologically-recognisable genera such as Rhododendron, Gaultheria and Vaccinium) and the Fagaceae (including the palynologically-indistinguishable Castanopsis and Lithocarpus) occur from sea level to high on the highest mountain, Mount Kinabalu (Kitayama, 1992; Heads, 2003), and have considerable ecological plasticity. In these cases, it is plausible that taxa are Table 2 Characteristics of selected magnetic susceptibility peaks in the Loagan Bunut core, with comparative intervening samples. Depth /cm 3842 3817 3386 3356 2915 2903 2883 2865 2848 2812 2721 2659 1983 1924 1733 1706 977 968

Label

Peak: C Peak: F Peak: G Peak: H Peak: I Peak: J Peak: M Peak: O Peak: P

k (vol. specific)

c (mass specific)

0.5 118.0 4.4 52.6 3.3 23.4 0.7 16.7 1.2 31.1 3.0 26.2 3.9 56 0.9 29.1 0.0 10.9

7.77 263 1.66 72.94 8.75 132.85 0 7.77 10 50 6 15.85 7 16.25 6.25 0.5 10 11.11

LF

HF

Freq. Dep%

15.6 701.8 22.9 161.2 29.9 428.6 21.2 24.3 28.6 356 18.3 386.6 41.2 296.1 58.7 21.5 15.8 15.8

14.4 700.9 24 160 30 425 21.3 25 29.8 357.2 18.8 388.9 43.8 293.9 59.9 22.1 16.7 97.1

7.69 0.13 4.8 0.74 0.33 0.84 0.47 2.88 4.2 0.34 2.73 0.59 6.31 0.74 2.04 2.79 5.7 0.62

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arbitrarily assigned to the ecological group where these taxa are most frequently found e in the case of the Ericaceae the high montane flora and in the case of the Fagaceae the lower montane flora e although it must be conceded that they will also have been present elsewhere. Thus the Ericaceae are also well-represented in kerangas (heath forest on low-nutrient, well-drained substrates such as old river gravels), in the centres of large raised bogs and even as epiphytes in lowland forest and mangroves (Heads, 2003), Castanopsis has species commonly found on levees on lowland rivers while some Lithocarpus spp. occur in regenerating lowland forest and others are found in the early part of raised bog successions. Similarly, it must be noted that many raised bog taxa (for instance Palaquium and Santiria) are also common in kerangas vegetation and are also found in lower montane forest vegetation (Kitayama, 1992; Soepadmo and Wong, 1995; Soepadmo et al., 1996). It is unlikely in these cases that the pollen source is montane or kerangas vegetation, given the proximity of the massive Baram valley raised bogs, however. The percentages for non-pollen and spore palynomorphs (palynofacies) were calculated as other palynofacies/other palynofacies þ basic sum. Radiocarbon dating was undertaken on seventeen selected small roundwood or bark samples. Four rangefinder samples were dated at the Beta Analytical, Florida, USA, and fourteen samples were dated at the Radiocarbon Laboratory in the CHRONO Centre, Queens’s University Belfast. All dates were done using the AMS technique (Table 3). Ages are stated 2s using a normalisation of d13C ¼ (26e30.7) per mil against PDB and a half-life (T1/2) of 5570 years. Calibration was made according to Stuiver and Reimer (1993) using Calib 5.0.2. The age model (Fig. 3), inferred from the radiocarbon dated levels, was used to date intermediate levels and to compute accumulation rates. For this purpose, linear interpolations were performed even though a number of lithological boundaries appear between individual dates. The uppermost age for the deposits of Unit 4 was extrapolated linearly. A hiatus is inferred between this date and the sediments of the uppermost horizon, Unit 5, which can be assigned to the period of logging in the basin in 1977e78. The wood samples separated from the core for dating all appeared to have been in-situ and there is no reason from the sequence of the dates to infer that the sequence was unstratified. It is likely that no carbon from unknown sources (e.g. carbonate beds, or other bedrock) has been incorporated within these samples. Possible source of errors, e.g. percolation of humic acids, bioturbation and groundwater fluctuation effects are likely to be negligible in these fine-grained, well-stratified, rapidly-

109

accumulating sediments. There is no vegetation adjacent to the coring site at the present day. The lithostratigraphy of the site makes it apparent that the sedimentary units sampled in this borehole extend laterally over several hundred metres (Hunt et al., 2006). Thus, while most of the deposits sampled here accumulated, forest vegetation was most probably several tens to hundreds of metres from the deposition site, so the risk of direct contamination from roots and rootlets is generally likely to have been slight. Thus, the seventeen radiocarbon dates on wood samples from the sequence provide good chronological control. Dates are in correct stratigraphic order apart from two locations where dating inversions are apparent (Fig. 3). The dating inversions at 15.40 and 20.00 m may be explained by root penetration, re-working or old wood effect. These two dates have been disregarded only after considering possible options when the construction of the agedepth model was adopted. 3. Lithostratigraphy The sequence can be divided into five distinct lithological units based on colour, grain size variations and organic content (Table 1). Unit 1 (40.00e38.29 m) mainly consists of brownish grey slightly organic silty-clay. It shows bedding 2e4 cm thick. A thin layer of sand occurs within this unit. Unit 2 (38.29e10.00 m) consists of brownish grey slightly organic silty-clay, with bedding 1e4 cm thick, sometimes picked out by yellowish brown and brownish black colour bands. Unit 3 (10.00e9.50 m) consists of brownish grey organic silty-clay with wood. Unit 4 (9.50e0.87 m) consists of yellowish brown organic mud, with minor organic silty-clay layers. Unit 5 (0.87e0.00 m) is brownish grey, very compact silty-clay.

Table 3 Radiocarbon dates of wood samples from the Loagan Bunut core. Depth(m) Lab no.

14

3.34 4.50 5.40 6.92 9.02 10.00 13.20 15.40 18.96 20.00 21.22 26.30 28.40 30.00 33.33 37.80 40.00

6240 6393 6545 6849 7128 7320 7513 7384 7980 8440 8099 8302 8627 9170 9229 9326 9869

UBA9454 UBA9455 UBA9456 UBA9457 UBA9458 Beta201451 UBA9459 UBA9460 UBA9462 Beta201452 UBA9463 UBA9461 UBA9464 Beta211453 UBA9465 UBA9466 Beta211454

C age  yr BP d13C                 

24 25 26 26 26 50 26 26 43 50 25 36 29 50 28 27 50

26.5 26.9 26.4 28.6 26.0 31.6 30.4 29.4 29.1 27.9 25.6 29.3 30.7 26.7 30.1 28.4 29.3

Material Cal. Age  2s (BP) Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood Wood

7027e7253 7267e7417 7423e7498 7616e7732 7931e8006 8009e8295 8214e8391 8072e8319 8650e8999 9316e9535 8994e9111 9140e9434 9533e9663 10234e10490 10278e10497 10430e10648 11199e11396

Fig. 3. Age-depth model using calibrated 14C dates (Table 3). The curve is divided into phases (Ph1ePh5). The highest accumulation rate is in Ph2 (43.33 mma1). Low accumulation rates occur in Ph1 (2.89 mma1) and Ph3 (2.09 mma1). A hiatus follows Ph5, before the deposition of the post-1977 clay.

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The sequence can be regarded as showing a general progression from marginal-marine to non-marine. Unit 1 is likely of subtidal origin. It passes into tidal flat deposits of Unit 2. Unit 3 probably reflects the increasing isolation of the basin from the sea, with development of mangrove swamp. Unit 4 is lagoonal-lacustrine. Unit 5 reflects inwash of hillslope materials during and after logging of the basin in 1977e78. 4. Sediment accumulation In general, apparent accumulation rates are remarkably high throughout the sequence. Depending upon the accumulation rates, the sequence can be divided into five main phases, (Fig. 3). Low accumulation rates occur in Phase 1 (2.89 mma1) and Phase 3 (2.09 mma1) between 11300 and 10540 cal. BP and 10360e9600 cal. BP respectively. These two phases were interrupted by a phase with the highest accumulation rate (Phase 2: 43.33 mma1), between 10540 and 10360 cal. BP. Phase 4 (9600e8150 cal BP) has an accumulation rate of 12.71 mma1 while Phase 5 (8150-ca.7250 cal. BP) has an accumulation rate of 6.52 mma1. It is suggested that there is a hiatus at this site after Phase 5, until the deposition of the silty-clays of Unit 5 following logging in 1977e78, although deposits of mid and late Holocene age are known from other boreholes around the basin (Hunt et al., 2006). 5. Palynology A total of 232 pollen, 22 spore, 2 foraminiferal, 21 dinoflagellate and 20 algal, chlorophyte and bacillariophyte taxa were identified. In general, most of the pollen taxa occur at relatively low frequencies. Preservation is generally good throughout the sequence. The pollen and spores are shown in Fig. 4, other microfossils are shown in Fig. 5 and selected palynofacies types are shown in Fig. 6. Pollen assemblage zones (PAZ) are defined in Table 4 and the sequence is summarised in Fig. 7. 5.1. PAZ LB-1 In PAZ LB-1 (40.00e31.00 m: 11,295e10,370 cal. BP), the Rhizophoraceae, Sonneratiaceae and Nypa cf. fruticans indicate the presence of both front (near-marine) and back mangroves. A relatively high-diversity assemblage of marine dinoflagellates including Protoperidinium spp., Brigantedinium spp., Bosedinia sp. and Bitectatodinium sp., plus foraminifera, indicate that significant tidal influence affected the coring site at this time. Several freshwater algae (e.g. Botrycoccus sp., Pediastrum sp., Spirogyra spp.) and dinoflagellates (e.g. Saeptodinium sp.) suggest freshwater input. Pollen from submontane/montane taxa (e.g. Perrottetia sp., Syzygium spp., Podocarpus spp., Gaultheria spp., Schefflera spp. and Rhododendron sp.) are present. These may have been transported by the Tinjar River from distant uplands into the tidal water body. Active river transportation is also evident from the presence of riparian taxa such as Phoenix sp., and Elaeocarpus spp., which are riverine taxa. The marine taxa suggest that the present lake area was part of a marine water body. Taxa such as Lophopetalum cf. multinervium, Gluta sp., Pandanus sp., Campnosperma cf. auriculatum, Blechnum sp. suggest nearby freshwater swamp. The Dipterocarpaceae and Celastraceae suggest lowland forest environments associated with a relatively humid tropical climate. Kerangas (areas of acid-tolerant vegetation often found on old river terraces) are suggested by Myrica cf. esculenta and Casuarina sp. Pollen indicators for raised bog such as Thymelaceae and Durio sp. are rather common. Disturbance indicators can be seen at this early stage including significant occurrences of Macaranga spp., Albizzia sp., Acacia sp., and several taxa from the

Calamoidea subfamily. Open-ground taxa are apparent, including Poaceae (including scabrate Oryza-type), Amaranthaceae, Asteraceae, Polygonaceae and Caryophyllaceae. This is paralleled by the occurrence of VAMs (which are root symbionts), other soil fungal types and recycled palynomorphs which are all indicators of soil erosion in this non-pedogenic environment (cf. Hunt, 1994). These, with pollen records from Metroxylon sp. and Eugeissonia sp. (sagotype), and the cultivated plant, Murraya cf. paniculata suggest a clear signal of human-induced disturbance and possibly management of the environment, which is also supported by the phytolith record from this core (Hunt et al., unpublished data), which shows high values for wild rice phytoliths, many burnt, in this zone. The high (20%) thermally mature (charred) material is likely to result from natural or anthropogenic biomass burning and possibly forest clearance, given that taphonomic work (Hunt and Rushworth, 2005a) found very low (<5%, often <1%) thermally mature matter in undisturbed dryland and swamp forest and that similarly high values of thermally mature matter are associated with clearance episodes in mangroves at Niah (Hunt and Rushworth, 2005b). 5.2. PAZ LB-2 In PAZ LB-2 (31.00e25.50 m: 10,370e9250 cal BP), the disappearance of front mangrove and reduction of back mangrove taxa can be regarded as a diminution of marine influence at the sample site as the sedimentary pile aggraded seawards, but the presence of marine dinoflagellate cysts point to continuing marine influence. The freshwater algae include common zygnemataceous algae (Zygnema sp., Spirogyra spp. and Meugotia sp.), which are all benthic, suggesting a shallow water body at this time. The reduction in Castanopsis/Lithocarpus spp. suggests that riverine influence diminished in this zone. In the lowermost part of the zone, the count of submontane/montane pollen taxa (e.g. Perrottetia sp, Rhododendron sp., Gaultheria sp.) decrease, perhaps responding to the weakening of river transportation from inland to the deposition site. Continued riverine influence is, however, shown by presence of taxa such as Pometia and Elaeocarpus. The raised bog species Gonystylus cf. bancanus show high values (up to 60%), suggesting spread of peat-forming environments close to the coring site. The significant reduction in values of Lophopetalum cf. multinervium, Blechnum sp., Pandanus sp., Korrdersiodendron sp., Gluta sp. and Cyperaceae suggests that the expansion of raised bog taxa was at the expense of freshwater swamps. Pollen values for lowland forest species are relatively low, as are droughttolerant kerangas elements (Myrica cf. esculenta and Casuarina sp.). The expansion of raised bog taxa and contraction of kerangas elements may reflect an episode of very humid climate. Continued disturbance can be seen throughout the zone, as evidenced by the pollen from the disturbed habitat species such as Macaranga spp., Celtis sp., sago (Metroxylon sp.), open habitat taxa (Asteraceae, Plantago sp., Poaceae). The percentage values for thermally mature material are maintained at the level of the previous zone. These together are suggestive of human activity through the zone. 5.3. PAZ LB-3 In PAZ LB-3 (25.50e19.00 m: 9250e8830 cal. BP), back mangrove elements (Sonneratia cf. caseolaris, Nypa cf. fruticans and Lumnitzera racemosa) increase gradually, especially from 9135 cal. BP. This implies that sea-level rise encroached on the site while sedimentation slowed. Continuing marine influence is shown by marine microfossil assemblages characterised by low numbers but moderate diversity. The non-marine algae are frequent and dominated by benthic taxa such as the Zygnemataceae and Microcystis,

Fig. 4. part 1. First part of the pollen diagram for Loagan Bunut, showing mangrove and freshwater swamp taxa. part 2. Second part of the pollen diagram for Loagan Bunut, showing raised bog taxa. part 3. Third part of pollen diagram for Loagan Bunut showing raised bog, riparian and lowland forest taxa. Pollen sum is based on total pollen and spores. part 4. Fourth part of radiocarbon dated pollen diagram for Loagan Bunut showing submontane/montane, kerangas and disturbed habitat taxa. part 5. Fifth part of radiocarbon dated pollen for Loagan Bunut diagram showing disturbed habitat, open habitat, cultivated and catholic taxa. Dendrogram based on CONISS-constrained cluster analysis is included at the righthand side of the diagram. part 6. Sixth part of radiocarbon dated pollen diagram for Loagan Bunut showing spore taxa.

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Fig. 4. (continued).

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consistent with rather shallow, sunny water. There is a significant occurrence of riparian pollen including Elaeocarpus spp., Pometia sp., Drypetes sp., Mesua sp., Typha sp. and Malvaceae. Pollen and spores from submontane/montane (Gaultheria sp., Rhododendron sp. and Lygodium-types) re-appear suggesting riverine transport from the interior was reaching the deposition site. The raised bog species Gonystylus cf. bancanus abruptly reduces while freshwater swamp forest taxa (Lophopetalum cf. multinervum and Barringtonia cf. racemosa) show slightly increasing values. This is accompanied by a general increase of lowland forest elements up to the middle part of the zone. The kerangas taxa Casuarina sp., Myrica cf. esculenta are more common. The kerangas species are drought-tolerant and it is possible that their expansion and the decline of raised bog taxa could be related to a less humid climate. There may have been increased anthropogenic pressure in the vicinity of the site, indicated by a general increase in values of disturbed habitat taxa, especially Metroxylon sp., Macaranga spp., Lepidocaryum-types, Arecoideae/Raveneae, Nenga cf. macrocarpa, Albizzia sp. and Acacia sp. The zone has relatively high pollen values for open habitat taxa (Amaranthaceae, Poaceae, Caryophyllaceae). Charcoal and other thermally mature matter remains high suggesting continued forest burning. 5.4. PAZ LB-4 In PAZ LB-4 (19.00e14.00 m: 8830e8375 cal. BP), back mangrove species (Sonneratia cf. caseolaris, Nypa cf. fruticanus) dominate. Front mangrove species appear, with low values of Rhizophora sp., Brugiera sp. and Avicennia sp., probably reflecting parent plants at some distance from the coring site. The occurrence of marine and non-marine microfossils does not change greatly, however, suggesting a continued marine connection and shallow, sunny waters. Raised bog taxa remain broadly unchanged. Lowland forest taxa become less frequent during the upper part of the zone, as do riparian, freshwater swamp, and kerangas species. The decline in drought-tolerant kerangas elements relative to raised bog species may suggest a rise in humidity during the zone. Possible anthropogenic activity can be seen throughout the zone with the presence of disturbed habitat taxa such as Lepidocaryum-types, Albizzia sp., Acacia sp, Metroxylon sp. and Macaranga spp. and open habitat types such as Asteraceae, Poaceae, and Polygonaceae. Thermally mature material and charcoal remain high. 5.5. PAZ LB-5 In PAZ LB-5 (14.00e9.00 m: 8375e7940 cal. BP), the large increase in pollen values of Rhizophora sp. and Brugiera sp. and a decline in Sonneratia sp. suggest a strong development of front mangroves near to the coring site. Total marine microfossils rise and their diversity is high during the zone also suggesting strong marine influence, but close to the end of the zone diversity falls and a bloom of Corrudinium sp may be indicative of partial isolation from the sea and a more lagoonal environment. This is also suggested by high Microcystis, which also indicates shallowing of the water body. The occurrence of freshwater algae and of riparian pollen indicates continued riverine input, however. Freshwater swamp, raised bog, and lowland forest taxa are relatively unchanged. The low counts for kerangas species suggest that the climate was rather humid. The disturbed habitat taxa (Macaranga sp., Lepidocaryum-type, Aranga sp., Caryota sp.), open habitat pollen (Cyperaceae and Poaceae) and VAMs increase towards the top of the zone, suggesting perhaps greater human impact on the forested landscape than previously. Charcoal and thermally mature materials continue to be important, suggesting anthropogenic or natural fires in the catchment.

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5.6. PAZ LB-6 In PAZ LB-6 (9.00e6.00 m: 7940e7545 cal. BP), front mangrove species decrease, but back mangrove taxa remain important. Total marine microfossil counts decrease and foraminifera are not present from this time onward. The marine taxa that persist are of low diversity and show the characteristics of bloom assemblages and thus probably reflect relative isolation from the sea and quasilagoonal conditions. Tidal influence is thus likely to have started to decrease at the coring site. Freshwater algae and dinoflagellates decrease towards the top of the zone. The predominance of taxa such as Mycrocystis sp. and the Zygnemataceae suggest rather shallow water. Testate amoebae peak, suggesting in situ accumulation (cf. Bobrov, 2007) and shallow water. Freshwater swamp taxa (Pandanus sp., Calophyllum sp., Garcinia cf. parvifolia/miquelii, Palaquium sp. and Sapotaceae) slightly increase. Raised bog taxa rise and then decline (Alstonia sp., Gonystylus sp., Parishia sp, Glochidion sp. and Shorea sp.) while kerangas taxa peaks at the beginning and end of the zone, suggesting a shortlived humid event flanked by dryer conditions. Riparian (Elaeocarpus spp., Casearia sp., Drypetes sp., Malvaceae) and submontane/ montane rise slightly suggesting perhaps a little more riverine input to the deposition site. There is a short-lived peak of lowland forest taxa at the top of the zone. Disturbed habitat taxa (Acacia sp. and Korthalsia sp.) and open habitat taxa (Amaranthaceae, Polygonaceae, Asteraceae, Cyperaceae and Poaceae) are relatively frequent in this zone and pollen of probably-cultivated Cucurbitaceae occur at 7875 cal. BP. Charcoal, thermally mature materials, VAMs are also common. It is tentatively suggested that these together are indicative of increasingly intense human activity. 5.7. PAZ LB-7 In PAZ LB-7 (6.00e2.00 m: 7545e6910 cal. BP), front and back mangrove taxa decrease. Several blooms of the marine dinoflagellate cyst Corrudinium sp. suggest increasingly isolated, lagoonal conditions. The algal assemblage, dominated by Microcystis sp., with low Zygnemataceae (which require sunlight to produce spores) plus some testate amoebae, may suggest rather shallow, perhaps shaded water. Riparian and montane/submontane species are relatively constant, suggesting continued riverine input from the hinterland. Raised bog taxa rise slightly at the beginning of the zone and kerangas taxa are very low, suggesting that this is a humid period. Notably, there is a rise in disturbed-habitat (especially Lepidocaryum-type and Macaranga spp.) and open habitat (Amaranthaceae, Asteraceae, Poaceae, Polygonaceae and Caryophyllaceae) taxa with major peaks at the end of the zone, while microcharcoal and thermally mature matter are common. This may reflect increasing human impact on the environment. 5.8. PAZ LB-8 In PAZ LB-8 (2.00e1.00 m: 6910e6735 cal. BP) mangroves disappear, indicating relative isolation of the water body from marine influence, although the continued occurrence of marine dinoflagellate cysts suggests that a residual marine connection was still extant. The appearance of planktonic freshwater algae (Botrycoccus sp. and Pediastrum sp.) indicates deepening water, although sunny, shallow water nearby is suggested by the presence of Zygnemataceae. The continuing presence of submontane/momtane taxa and riparian species suggests that there was still a riverine connection to the hinterland. Raised bog taxa remain common, but kerangas species are not represented, suggesting very humid conditions. High counts for

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Fig. 5. Radiocarbon dated marine and freshwater microfossil diagram for Loagan Bunut. Sum based on total marine and freshwater microfossils.

disturbed (Lepidocaryum sp., Macaranga sp, Celtis sp.), open habitat (Poaceae, Cyperaceae), cultivated (Mangifera cf. indica) taxa with microcharcoal and thermally mature material suggest anthropogenic activity in the vicinity of the site. 6. Magnetic susceptibility The magnetic susceptibility profile is generally low, but shows clear cyclic behaviour which correlates with rhythmic colour and texture changes in the core, the magnetic peaks being generally slightly siltier and paler than the troughs, which are slightly darker and less silty to hand texturing. A series of major peaks, labelled here AeP correspond with slightly coarser sediment. Near-identical LF and HF peak susceptibility values (Table 2 and Fig. 8), low percentage frequency dependent susceptibility values (less than 7.69), and general LF susceptibility peak values less than 1000 SI units indicate very low occurrence of ultrafine grains with SP (superparamagnetic) domains (these are produced by bacteria or other biogenic or internal processes). An exception is at the 9.68 m level (O). Thus, most probably, multi domain, pseudo single domain crystals predominate at those peaks. This suggests that ferromagnetic (magnetite, maghematite) minerals are primary and possibly iron sulphides are secondary minerals. Relatively high mass specific

susceptibility (c), volume specific susceptibility (k), LF and HF susceptibility values also indicate high proportions of ferrimagnetic minerals in those peaks. It is likely that sedimentary processes may have supported the concentration of ferimagnetic grains as the primary mineral in the peaks in the sequence. The anomalous peak, P, has a mineralogy consistent with formation by biogenic processes. In those parts of the core between the peaks, minus values (between 0.5 and 10) in c indicate dominantly diamagnetic materials, like water, quartz, organic matter and clay minerals such as kaolinite (cf. Hunt et al., 2006). Values in k, LF and HF susceptibility are also low in these areas. Major peaks in magnetic susceptibility occur throughout the core but are concentrated in the lower part. Peaks A and B occur ca. 11,090e10,955 cal. BP. The very high peak C lies at ca. 10,745e10,610 cal. BP. The lowermost boundary of this event corresponds to a lithological change from organic silty-clay with sand to organic silty-clay. The uppermost boundary of this event corresponds with the beginning of a rise in sedimentation rate (Fig. 9). Between 38.00 and 35.70 m (ca. 10,610e10,470 cal. BP), there are no major peaks in the magnetic stratigraphy. This part of the core corresponds to a period of rapid accumulation (Fig. 9). Three peaks

Fig. 6. Palynofacies diagram showing selected types of particulate organic matter for Loagan Bunut. Sum based on total particulate organic matter.

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Table 4 Definition of pollen assemblage zones for the early Holocene sequence at Loagan Bunut. Assemblage biozone, depth, ages and defining taxa

Characteristics

LB1, 40.00e31.00 m, 11297e10370 cal. BP, Lophopetalum Mallotus Calopyhllum Blechnum Polypodiaceae LB-2, 31.00e25.50 m, 10370e9250 cal. BP, Gonystylus Mallotus Diospyros Asplenium

Characterised by Lophopetalum (10%), Mallotus (5%), Calopyhllum (3e4%), Gonystylus (1%), Madhuca (1%), Oncosperma (1e2%), Euonymus (1e2%), Kookona (1%), Ligustum (1%), Macaranga (1%), Podocarpus (1e2%), Sonneratia (1e2%), Calamus (1%), Albezzia (1%), Metroxylon (<1%), Pandanus (<1%), Gultheria sp (<1%), Campanula, Lithocarpus/Castanopsis, Myrica cf. esculenta, Casuarnia, Elaeocarpus, Poaceae, Amaranthaceae, Asteraceae, Polygonaceae, Caryophyllaceae, Cyperaceae, Blechnum (25%), Polypodiaceae (20e30%), Asplenium (15%), Vittariaceae/Lindsaeaceae (12%), Lycopodiaceae (4e5%), Syngramma, Davilliaceae, Cyathaceae (4%), and Pteris. Organic-walled marine microfossils include Brigantedinium, Bitectatodinium. Freshwater fossils include Microcystis, Cyanobacteria, Spyrogyra, Zygnema, Chlorophyacae, Concentricystes sp. Also present are fungal microfossils, microcharcoal (20e25%), thermally mature (15e20%), plant-derived tissue and recycled palynomorphs. Characterised by Gonystylus (40%), Mallotus (4e5%), Diospyros (2e3%), Oncosperma (1e2%), declining Lophopetalum (<1%), decreasing Pandanus, Campnosperma, Lithocarpus/Castanopsis, Cyperaceae, Calopyhllum (<1%), Barringtonia (<1%), Madhuca (1%), Calamus (1%), Planchonella, Palaquium, Euonymus, Loeseneriella, Mastixia, Ligustum, Podocarpus, Metroxylon Macaranga, Elaeocarpus, Plantago sp., Poaceae, Asplenium (20%), Polypodiaceae (15%), Lycopodiaceae (5e7%), Vittariaceae/Lindsaeaceae (10%), Cyathaceae (4%), decreasing Blechnum (3e5%) and Pteris. Marine microfossils include Bitectatodinium, Protoperidinium and other occasional taxa. Freshwater microfossils include Spyrogyra, Zygnema, Chlorophycae, Concentricystes sp. Fungal microfossils, microcharcoal (15e18%), thermally mature (10e12%), plant-derived tissue and recycled palynomorphs occur. Characterised by Mallotus (7e8%), Barringtonia (3e5%), Calamus (4%), Oncosperma (3e4%), Pometia (3%), Calopyhllum (1e3%), Ligustrum (1%), Sonneratia (1%), Metroxylon (1%), Lophopetalum, Loeseneriella,declining Gonystylus (<4%), Sonneratia (1%), Diospyros, Glochidion, Chisocheton, Planchonella, Palaquium, Madhuca, Nypa, Euonymus Mastixia, Podocarpus, Dacrydium, Gultheria, Eugeissona, Macaranga, Acacia, Drypetes, Elaeocarpus, Malvaceae, Poaceae, Nenga, Cyperaceae, Asplenium (20%), Polypodiaceae (15%), Stenochlaena, Lycopodiaceae (5e6%), Cyathaceae (4%), Davilliaceae, Pteris, decreasing Blechnum (1e2%). Organic-walled microfossils include Brigantedinium, Microcystis, Cyanobacteria, Spyrogyra, Zygnema, Chlorophycae, Concentricystes sp. Fungal microfossils, microcharcoal (15e18%), thermally mature (10e12%), plant-derived tissue and recycled palynomorphs occur. Characterised by Sonneratia (20e25%) Mallotus (3e4%), Calamus (2e3%), Palaquium (1%), Rhizophora (1%), declining Gonystylus (1e2%), decreasing Barringtonia, Stemonurus, Urticularia, Amoora, Calopyhllum, Planchonella, , Madhuca, Oncosperma, Nypa, Euonymus Loeseneriella, Mastixia, Ligustum, Podocarpus, Gultheria, Metroxylon, Macaranga, Acacia, Lepidocaryum, Pometia, Poaceae, Polypodiaceae (25%), Asplenium (15%), Stenochlaena Lycopodiaceae (5%), Cyathaceae (4e5%), Dennstaedtiaceae, Selaginellaceae, Pteris and decreasing Blechnum. Organic-walled microfossils include Brigantedinium, Microcystis, Cyanobacteria, Spyrogyra, Chlorophycae, Concentricystes sp. Fungal microfossils, microcharcoal (15e18%), thermally mature (10e12%), plant-derived tissue and recycled palynomorphs occur. Characterised by Brugiera (30-20%), Rhizophora (13e15%), Mallotus (3e4%) declining Sonneratia (8%), decreasing Barringtonia, Pometia (2%), Glochidion (1%), Ilex (1%), Amoora (1%), Madhuca (<1%), Oncosperma (1%), Podocarpus (1%), Macaranga (1%), Nypa, Euonymus, Mastixia, Calamus, Lepidocaryum, Caryotoideae, Poaceae, Cyperaceae, Asplenium (10e14%), Polypodiaceae (10e15%), Lycopodiaceae (5%), Vittariaceae/Lindsaeaceae (5%), Cyathaceae (4%), Stenochlaena, Davalliaceae, Selaginellaceae, and Pteris. Organic-walled microfossils include Bosedinia, Operculodinium, Microcystis, Spyrogyra, Chlorophycae, Concentricystes sp. Fungal microfossils, microcharcoal (20e25%), thermally mature (14%), plant-derived tissue and recycled palynomorphs occur.

LB-3, 25.50e19.00 m, 9250e8829 cal. BP Mallotus Barringtonia Calamus Oncosperm Asplenium LB-4, 19.00e14.00 m 8829e8375 cal. BP Sonneratia Mallotus Gonystylus Calamus Polypodiaceae LB-5, 14.00e9.00 m 8375e7938 cal. BP Brugiera Rhizophora Mallotus Polypodiaceae LB-6, 9.00e6.00 m, 7938e7545 cal BP, Brugiera Sonneratia Mallotus LB-7, 6.00e2.00 m, 7545e6908 cal BP Lepidocaryum Mallotus Sonneratia Rhizophora Brugiera Polypodiaceae LB-8, 2.00e1.00 m, 6908e6734 ca. BP Auxtrobuxus Shorea Mallotus Cyperaceae

Characterised by decreasing Brugiera (20-10%), Sonneratia (6%), Rhizophora (<5%), Mallotus (4%), Alstonia, Shorea, Glochidion, Stemonurus, Canthium, Urticularia, Gonystylus, Ilex, Eugenia, Parishia, Calophyllum, Pandanus, Palaquium, Sapotaceae, Oncosperma, Nypa, Podocarpus, Ericaceae, Korthalsia, Calamus Caryota, Macaranga, Elaeocarpus, Lithocarpus/Castanopsis, Pometia (<1%), Cyperaceae, Poaceae, Ipomoea, Cucurbitaceae, Polypodiaceae, (10%), Cyathaceae (2e3%), Vittariaceae/Lindsaeaceae (5%), Blechnum. Organic-walled microfossils include Bosedinia, Microcystis, Spyrogyra, Concentricystes sp. Increasing testate amoebae. Microcharcoal (25%), thermally mature (20%) and plant-derived tissues occur. Characterised by increasing Lepidocaryum (20%), fluctuating RhizophoraeBrugiera (1e6%), Sonneratia (5%), Mallotus (2e3%), Urticularia (1e2%), Macaranga (1e2%), Pometia (1e2%), Durio (1%), Shorea (1%), Glochidion (1%), Amoora (1%), Ilex (1%), Baccaurea, Gonystylus (<1%), Calophyllum (1%), Oncosperma, Nypa, Euonymus, Mastixia, Podocarpus, Gultheria, Symplocos, Calamus, Eugeissona, Asplenium (10%), Polypodiaceae (20%). Marine and freshwater microfossils include Corrudinium (<1%), Bitectatodinium (<1%), Microcystis (2e3%), Concentricystes sp. Decreasing testate amoebae. Microcharcoal (30%) thermally mature (30%), plant-derived tissue occur.

Characterised by Cyperacae (3%), Mallotus (7e8%) Austrobuxus (5%), Shorea (2e4%), Hopea (2e3%), Macaranga (4%), Urticularia (1%), Gonystylus (1%), Podocarpus, Mesua, Calamus, Korthalsia, Polypodiaceae (10%), Asplenium (5e10%), Belchnum (2e5%). Marine and freshwater microfossils include Bosedinia, Bitectatodinium (<1%), Botrycoccus (2%) and Pediastrum sp. (15%). Increasing testate amoebae. Microcharcoal (10e15%) thermally mature (20e25%), plant-derived tissue occur.

(D and E and F) are then evident. They can be assigned to dates of 10,470e10,460, 10425e10410 and 10,400e10,395 cal. BP respectively and occur during the phase of rapid accumulation (Fig. 9). The uppermost boundary of peak F corresponds to the uppermost boundary of PAZ LB-1. Between 33.50 and 29.30 m (ca. 10,395e10,030 cal. BP), there are no high magnetic peaks. This episode occurs during the beginning of a period of relatively slow deposition (Fig. 9). Between 29.30 and 26.46 m (ca. 10,030e9310 cal. BP) there are four moderately high magnetic peaks. These peaks can be assigned to dates of 9980e9885 cal. BP (G), 9740e9640 cal. BP (H),

9640e9540 cal. BP (I) and 9345e9315 cal. BP (J). This episode corresponds approximately with an episode of high humidity, indicated by the palynology and to low and rising sedimentation rates (Fig. 9). Between 26.46 and 22.23 m (ca. 9310e9100 cal. BP), a more uniform pattern in magnetic susceptibility can be seen, with relatively low-amplitude cycles of change. This corresponds to a period of high accumulation rates and of relatively dry climate (Fig. 9). There is a marked change in the profile starting at 22.23 m and persisting to 17.00 m: 9100e8645 cal. BP. The mean susceptibility rises and there are four relatively high peaks at 9095e9085 cal. BP (K), 9020e8970 cal. BP (L), 8890e8830 cal. BP (M) and

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Fig. 7. Radiocarbon dated summary diagram for Loagan Bunut plotted against the age model derived from Fig. 3. Pollen and spores are based on a sum of total pollen and spores. Non-pollen is based on a sum of total particulate organic matter. Pollen and spore sum is included at the right-hand side of the diagram.

8685e8645 cal. BP (N). The accumulation rate is high and the climate starts relatively dry but becomes more humid (Fig. 9). Between 17.00 and 13.54 m (8645e8335 cal BP) the trend of relatively high susceptibility continues, but the peaks become less marked. The top of this part of the core and peak O occurs at 8330e8310 cal. BP, which corresponds to the upper boundary of the PAZ LB-4. The climate was humid during this time and the accumulation rate was high (Fig. 9). Magnetic susceptibility then starts to decline gently and the amplitude of cyclic changes becomes generally less between 13.54 and 10.00 m (ca. 8335e8150 cal. BP), although the climate remained humid and the sedimentation rate was still high (Fig. 9). The remainder of the Early Holocene part of the core, between 10.00 and 0.87 m (ca. 8150e6725 cal. BP) is characterised by relatively low levels of magnetic susceptibility and shows lowamplitude and diminishing cyclic change. Peak P near the base of the unit was probably formed by biogenic processes. This unit corresponds to the highest levels of exposed ground and disturbed habitat indicators in the pollen diagram, to continued high accumulation rates and to a fluctuating climatic regime (Fig. 9), but the sediments are finer grained and more organic-rich than those lower in the core.

progression towards a wetter climate overall (Fig. 9). This general progression towards a wetter climate parallels the results of Partin et al. (2007), but the variability between stalagmites in that study does not enable the detailed pattern shown in the present work to be tested. Some of the major magnetic susceptibility peaks in the core

7. Discussion The palaeoecological history at the Logan Bunut sampling site is the complex result of several interacting processes (Fig. 9). These include (1) changes in rainfall associated with the Intertropical Convergence Zone (ITCZ), (2) eustatic sea-level change and neotectonic activity (3) changes in tropical lowland vegetation. The analysed sequence can be divided into parts, dated to ca. 11,300e8200 cal. BP and ca. 8200e6725 cal. BP. The sequence starts very early in the Holocene (ca. 11,300 cal. BP). By this time, the dry, cool Late Pleistocene climate (Morley, 1982; Partin et al., 2007; Hunt, unpublished data) had already become humid and warm in Borneo. The Holocene climate does not appear to have been completely uniform e there were a series of small changes in the vegetation involving the relative frequency of the raised bog (wet) and kerangas (drought-tolerant) taxa, which can tentatively be interpreted as evidence for changing precipitation patterns, with the possibility of relatively drier phases before 10,370 and at 9250e8890, 7900 and 7600e7545 cal. BP and a gradual

Fig. 8. Magnetic susceptibility (k) curve for Loagan Bunut. Lettering corresponds to peaks in susceptibility discussed in the text.

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Fig. 9. Synthesis of sedimentary, environmental and chronological information from the Loagan Bunut core. Dark areas in the susceptibility column indicate magnetic peaks.

may also reflect short-period climate events, although they may also simply reflect localised sedimentological phenomena. The mean position of the Intertropical Convergence Zone (ITCZ) has migrated across Sarawak during the Holocene (Gagan et al., 2004; Partin et al., 2007). ITCZ-induced high rainfall prevails during the North East Monsoon season. The connection between climate and vegetation changes reported in Fig. 9 can be regarded as results of variations in the North East Monsoon rains in response to seasonal shifts in the position of the ITCZ and its associated belt of high sea surface temperature and convection (cf. Partin et al., 2007). It is suggested that generally-rising rainfall through the Early Holocene reflects the migration of the mean position of the ITCZ towards central Sarawak. It is also interesting to note that this trend of rising humidity was interrupted by what are quite possibly dry episodes, which would reflect reversals of the movement of the mean position of the ITCZ and which (given the uncertainties in the dating) are plausibly local manifestations of events of regional or global significance. The event before 10,370 cal. BP, coinciding with magnetic susceptibility peak F could correlate with one of several short dry events between 11,000 and 10,000 BP at Dongge Cave, China (Dykoski et al., 2005). The event at 9250e8890 cal. BP, which coincides with magnetic susceptibility peaks K and L possibly reflects the period of cooling seen in the Ontong Java Plateau in ODP Hole 806B (Gagan et al., 2004), the dry episode centred at 9165  75 BP at Dongge Cave and the NGRIP event at 9.26 ka (Dykoski et al., 2005; Johnsen et al., 2001). Comparatively minor irregularities in the Loagan Bunut sedimentation rate would mean that the inferred drier episode at 7900 cal. BP was contemporaneous with the Dongge Cave dry episode at 8080  74 BP reported by Dykoski et al. (2005) and possibly the NGRIP 8.2 ka event. Only the event at 7600e7545 cal. BP is not seen at Dongge Cave, though it coincides with a major event in the North Atlantic (Bond et al., 2001). The magnetic susceptibility record (Fig. 8) described in this paper is consistent with a series of fining-upward sedimentary events on a variety of scales, at the sampling site. Whether these

have purely local significance is at present untested. Many of these events do not seen to correlate in any simple way with climate, sedimentation rate, relative sea-level behaviour, vegetation or fire history, although there is a quiet period in the record while the site was encroached upon and colonised by mangroves after 8000 cal. BP. It is therefore suggested that the major peaks in susceptibility may be the manifestation of major discharge events in the catchment. If this is the case, episodes of highest discharge occurred between ca. 10,370 and 8310 cal. BP. It is possible that these events may be linked in some way to the negative (La Niña) phase of ENSO oscillations, which should produce high rainfall over Borneo (Bush, 2003). Most authors suggest that intense ENSO events appear later in the Holocene (Bush, 2003; Gagan et al., 2004) but the data here suggests that similar events were already geomorphologically significant in the Early Holocene in Sarawak. The pattern of relative sea-level rise at Loagan Bunut is argued here to have controlled sedimentation, since high counts for benthic algae point to rather shallow water on site through most of this record. Interestingly, there seem to be phases of extremely rapid sea-level change at 10,540e10,360 and 9600e8150 cal. BP. These episodes do not coincide with the general pattern suggested by Fairbanks (1989) nor the rapid rises suggested by Cronin et al. (2007) and thus may reflect more local effects, perhaps caused by loading of the local crust by the flooding of the buried valley system which is now the Baram Delta and lower valley and the valley of the Tinjar River or by tectonic displacement. Although relative sea-level rise diminishes after 8000 cal BP at Loagan Bunut, the still-stand in sea-level rise identified in Singapore by Bird et al. (2010) between 8000 and 7400 BP is not present. It is thus possible that the stillstand in Singapore is the result of crustal levering (Milne and Mitrovica, 2008) as a response to the loading of the Sunda Shelf by the rising postglacial seas or that Loagan Bunut was undergoing displacement at this time. Moreover, the early Holocene sea-level related deposits at Loagan Bunut have a greater elevation than would be expected given their age, with truncation of the deposits w6500 cal BP. The present

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sedimentewater interface at the core site is ca. 12 m asl. and tidal influence in the nearby Tinjar river runs to ca. 8 m. Thus, the deposits at the end of the early Holocene record are 15e20 m above the elevation that would be predicted if the area were stable tectonically. It is thus suggested that the Loagan Bunut area was subject to considerable displacement, some time after 7000 years ago, and that this and the unusual accumulation rate curve may be an indicator of neotectonics in this graben and/or crustal levering caused by the loading of the Sunda Shelf by rising sea level (cf. Milne and Mitrovica, 2008). From the beginning of the Holocene record at Loagan Bunut, the full range of wet tropical lowland vegetation was established. Raised bog vegetation seems to have been poorly developed before 10,370 cal. BP, probably because it took time for peat bogs to become fully established and to have developed the topography and seral communities seen later in the Holocene (e.g. Anderson and Muller, 1975; Sabiham, 1990), but possibly also because rainfall and the groundwater table were relatively lower than at the present time, as a result of the sea-level lowstand. Other than the development of raised bogs after 10,370 cal. BP, there is no sign of the broad vegetational succession resulting from the sequential immigration and flourishing of major forest species seen in temperate Northern Hemisphere vegetation during the Early Holocene (e.g. Godwin, 1975). Part of this lack of broad succession is no doubt because species did not have to immigrate far from their glacial refugia. It is also probable, however, that this lack of succession was also because of the frequent and continuing disruption of the forest by fire. The fire may be linked, at least in part, to human activity, as has been suggested for forest phases in the Late Pleistocene in Borneo (Hunt et al., 2007). The regular appearance of open-ground and disturbed-habitat taxa, the frequent sago palm pollen and the occasional appearance of probably-domesticated species would support this suggestion. Although management of rainforests in Borneo by some indigenous peoples was reported at first European contact by Gibbs (1914: p. 10), human ‘management’ of Early Holocene forests in Borneo was previously unsuspected: this is clearly a hypothesis that should be tested further. 8. Conclusion This study confirms the general pattern of climate change in Borneo e and by extension the Pacific Warm Pool e suggested for the early Holocene by Partin et al. (2007). The hypothesis put forward here of four drier phases during the early Holocene is amenable to testing using high-resolution records, as is the suggestion of Early Holocene ENSO-style events. Further studies should also test the pattern of sea-level rise presented here and the hypothesis of neotectonic activity in what was thought to be a largely tectonically-stable island (Hutchison, 2005). Finally, this paper has set out unexpected evidence for the regular disruption of the wet tropical forest vegetation from the earliest Holocene by fire, possibly as the result of anthropogenic activity. This seems to have prevented large-scale successional development of the forest e effectively suggesting that humans were managing the forest ecosystem as has been suggested by Hunt et al. (2007) for the forest phases of the Late Pleistocene in Borneo and possibly practicing a form of arboriculture. There are suggestions that interior peoples were doing precisely this in Borneo before first western contact (Gibbs, 1914, p. 9e10), and it is possible that these practices may have had an extremely long prehistory. Acknowledgements This work was funded by the award of a British Academy large research grant LRG-41977 to COH. RP thanks Professor Nimal De

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