Glacial–interglacial organic carbon record from the Makassar Strait, Indonesia: implications for regional changes in continental vegetation

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*Correspondin

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USA.

0277-3791/$ - see

doi:10.1016/j.qua

Quaternary Science Reviews 23 (2004) 17–27

Glacial–interglacial organic carbon record from the Makassar Strait,Indonesia: implications for regional changes in continental vegetation

Katherine Visser1, Robert Thunell*, Miguel A. Goni

Department of Geological Sciences, University of South Carolina, Columbia SC 29208, USA

Received 24 February 2003; accepted 31 July 2003

Abstract

Recent studies convincingly show that climate in the Western Pacific Warm Pool and other equatorial/tropical regions was

significantly colder (by B3–4�C) during glacial periods, prompting a reexamination of the late Pleistocene paleoenvironments ofthese regions. This study examines changes in continental vegetation during the last two deglaciations (Terminations I and II) using

a sediment core (MD9821-62) recovered from the Makassar Strait, Indonesia. Evidence based on the lignin phenol ratios suggests

that vegetation on Borneo and other surrounding islands did not significantly change from tropical rainforest during the last two

glacial periods relative to subsequent interglacial periods. This supports the hypothesis that the winter monsoon increased in

strength during glacial periods, allowing Indonesia to maintain high rainfall despite the cooler conditions.

r 2003 Elsevier Ltd. All rights reserved.

1. Introduction

There is a growing consensus that tropical climateshave varied significantly on both glacial–interglacial andmillennial time-scales, and that the tropics may play amore important role in controlling climate variability onthese timescales than previously thought (Cane, 1998;Cane and Clement, 1999; Pierrehumbert, 2000). Whilethe CLIMAP Project members (1981) initially estimatedthat sea surface temperatures (SST) in the tropicswere less than 2�C cooler than present during the lastglacial maximum (LGM), more recent studies from boththe Atlantic (Guilderson et al., 1994, 2001; N .urnberget al., 2000) and Pacific (Patrick and Thunell, 1997; Mixet al., 1999; Lea et al., 2000; Kienast et al., 2001; Stottet al., 2002; Visser et al., 2003) report that glacial SST intropical regions were 3–6�C cooler than present. Theserevised marine temperatures are more in line withobservations that point to a steepening of the atmo-spheric lapse rate and a depression of snow lines in thetropics (Webster and Streeten, 1978), as well as d18Odata from tropical high elevation ice cores (Thompson

g author. Tel.: +1-803-777-7593.

s: [email protected] (R. Thunell).

ess: US Geological Survey, Woods Hole, MA 02543,

front matter r 2003 Elsevier Ltd. All rights reserved.

scirev.2003.07.001

et al., 1995, 1998) that indicate a 5–6� cooling over thecontinents during the last glacial period.An important question to be answered is how did

continental vegetation in tropical regions respond tosuch large-scale changes in climate? For the Indonesianregion, previous studies of glacial–interglacial changesin vegetation, based primarily on pollen and d13CTOC(in an attempt to pinpoint a change from C3 to C4vegetation) data, suggest that during glacial periodsgrasslands (i.e., C4 grasslands) dominated the Indone-sian (as well as the tropical) terrestrial landscape (vander Kaars, 1991; van der Kaars et al., 2000; Hope, 2001).This explanation requires the glacial climate to havebeen arid, most likely due to an increase in the length ofthe dry season, rather than the humid environment thattypifies this region today (van der Kaars, 1991; van derKaars et al., 2000; Gasse, 2000; Moss and Kershaw,2000; Behling, 2002). However, a recent study illustratesthat the link between the increased occurrence ofPoaceae (grass) pollen and increased aridity may over-simplify glacial climate conditions due to the influenceof numerous factors other than aridity in controlling thedistribution of this vegetation (Bush, 2002). Emergingevidence suggests that many tropical regions were notsignificantly drier during glacial periods (Stuijts et al.,1988; Schlunz et al., 1999; Colinvaux and de Oliveira,2000; Colinvaux et al., 2000; Sun et al., 2000; Cowling

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et al., 2001; Baker et al., 2001a, b; Taylor et al., 2001;Bush et al., 2002). For example, Sun et al. (2000) suggestthat humid glacial conditions in the Indonesian region,due in part to the increased strength of the wintermonsoon, allowed tropical rainforest to vegetate theexposed Sunda Shelf during the LGM.An increased awareness of the importance of the

tropics in the global climate system has prompted areexamination of the paleoenvironments of this region.The overall objective of this study is to use an organicgeochemical approach to evaluate the response of theterrestrial ecosystem in the Indonesian region to climatechange during the last two glacial–interglacial transi-tions. Specifically, a study of the lignin phenol ratiospreserved in a core from the Makassar Strait, Indonesiawas carried out in order to examine changes interrestrial vegetation on Borneo and Sulawesi associatedwith Terminations I and II.

2. Modern climatological setting

Sediment core MD9821-62 was collected from thesouthern end of the Makassar Strait, between Borneo tothe west and Sulawesi to the east (Fig. 1). This straittrends north–south and is bounded by a wide, shallowshelf (B200m water depth) on its western boundary anda narrow, shallow shelf on its eastern boundary. During

Fig. 1. Map of the Indonesian region showing the location of core MD9821-6

area shallower than the present day 120m isobath and represents seafloor that

http://ingrid.ldgo.columbia.edu/SOURCES/.WORLDBATH/.bath/.

glacial low stands of sea level (�120m), large portionsof the shallow shelves emerged; however, water was notrestricted from flowing through the strait (as was thecase for the South China Sea; Fig. 1).The seasonal shifting of the Indonesian low-pressure

cell produces a monsoonal system that greatly influencesthe climatology and surface ocean circulation in thisarea. In the Indonesian region, the northwest monsoonoccurs during the boreal winter (December–February)and is characterized by winds from the west/northwestand high amounts of precipitation. Winds originatefrom the high-pressure cell over Asia, travel towards thesouth/southeast collecting moisture from the ocean untilthey eventually hit the Inter-Tropical Convergence Zone(ITCZ), located as far south as northern Australia(Roberts and Phipps, 1988; Soegiarto, 1992). Thisresults in heavy precipitation over the Indonesian regionduring this time of the year. The northwest monsoonends in March as the Asiatic high-pressure cell weakens(Roberts and Phipps, 1988; Roberts et al., 1988).The southeast monsoon (June–August) is character-

ized by strong winds from the south/southeast as a resultof the high-pressure cell having migrated to a positionover Australia (Roberts and Phipps, 1988). The windsoriginating over northern Australia are dry and cool asthey pass over Indonesia; however, as they near theITCZ, centered over Southeast Asia, they rise andrelease their moisture as rain, causing the heaviest rains

2 (4�41.330S, 117�54.170E; 1855m water depth). The black region is the

would have been exposed during the last glacial maximum. Data from:

ARTICLE IN PRESSK. Visser et al. / Quaternary Science Reviews 23 (2004) 17–27 19

in Southeast Asia (north of Indonesia) during thesummer (Soegiarto, 1992; Barmawidjaja et al., 1993).The present day vegetation on Borneo and Sulawesi is

characterized by tropical lowland and montane rainforests (i.e., angiosperms; MacKinnon, 1997; van derKaars et al., 2001). The distribution of vegetation in theregion is dictated by the monsoon-controlled seasonalrainfall patterns. On Sulawesi, the rainfall amounts arenot drastically reduced during the ‘dry’ season; only onthe tip of southern Sulawesi is there a dry season thatlasts forB3–4 months (van der Kaars et al., 2001). Theaverage rainfall for Borneo and Sulawesi is greater than3000mm/yr (van der Kaars and Dam, 1995).

3. Glacial–interglacial paleoceanographic/paleoclimatic

changes in the Indonesian region

Numerous studies suggest that the boreal SoutheastAsian winter monsoon intensified during glacial periods,due perhaps to increased cooling of the Asian continentas a result of increased global albedo (e.g., Wang andWang, 1990; Huang et al., 1997a; Wang et al., 1999; Sunet al., 2000). Evidence for a stronger glacial wintermonsoon is found in Chinese loess sequences (An et al.,1991), Chinese lake sediment records (Huang et al.,1997b; Hodell et al., 1999), and deep-sea records (Huanget al., 1997b; Wang et al., 1999; Sun et al., 2000). As aresult of the increased winter monsoon, the northernreaches of the SE Asian monsoon (e.g., the northernSouth China Sea) were drier and colder, whereas theIndonesian region (and the southern South China Sea)was colder but as wet as or wetter than today (Pelejeroet al., 1999; Sun et al., 2000). Data from the Ontong-Java Plateau also suggests that sea surface salinities inthis region decreased during glacial periods due to anincrease in precipitation, as a result, perhaps, ofincreased water vapor transport into the Western Pacificby stronger glacial trade winds (Lea et al., 2000).In addition to these changes in the monsoon system,

recent stable isotope and Mg/Ca paleotemperaturestudies on core MD9821-62 indicate that glacialtemperatures (MIS 6 and 2) in the Makassar StraitwereB4�C cooler than those of the ensuing interglacialperiods (MIS 5e and 1) (Visser et al., 2003). Asmentioned previously, these relatively large glacial–interglacial SST changes in the Indonesian region arecorroborated by snowline reconstructions from tropicalAustralasia (Webster and Streeten, 1978).Some palynological studies suggest that parts of

Indonesia and northern Australia were drier duringthe LGM, as evidenced by an increase in grass pollen atthis time (e.g., Caratini and Tissot, 1988; van der Kaars,1991; Barmawidjaja et al., 1993; van der Kaars andDam, 1997; Moss and Kershaw, 2000; Dam et al., 2001).However, many of these studies acknowledge that the

pollen deposited in Indonesian Seas may have beentransported from nearby regions (e.g., northern Aus-tralia, southern China) that were more arid during theLGM. Other palynological studies report that theIndonesian region was colder, but not drier duringglacial periods (Stuijts, 1984; Stuijts et al., 1988). Sunet al. (2000) illustrated the highly variable nature of theregion, showing that the northern shelf of the SouthChina Sea experienced drier, colder conditions, while theSunda shelf in the southern South China Sea (adjacentto Borneo) was continuously vegetated by a tropicalhumid rainforest during the LGM. They show thatwhile the whole region cooled, the northern parts weremore arid and the southern parts were at least as moistas they are today. Similar evidence for cooler and humidconditions in the equatorial region of South Americaduring the LGM are now becoming widely accepted(Colinvaux and de Oliveira, 2000; Colinvaux et al., 2000;Baker et al., 2001a, b; Kastner and Goni, 2003),suggesting a global distribution of this climatic regime.

4. Materials and methods

4.1. Sample material

The sediment core used for this study was recoveredduring the IMAGES IV (International Marine GlobalChanges Study) cruise in June 1998 through theIndonesian archipelago and the easternmost IndianOcean. This study utilizes core MD9821-62 (4�41.330S,117�54.170E, 1855m water depth; Fig. 1) that is 55.33mlong and has an average sedimentation rate of B25 cm/kyr. This core is composed of homogeneous hemipelagicmud throughout, containing no visible turbidites orvolcanic ash layers. One centimeter thick samples(B10 cc) were collected every 10 cm over the upper40m of the core. A gap in the sediment, from 2230 to2330 cm, caused by stretching of the sediment core uponretrieval from the ocean floor was deleted from thestratigraphic sequence.The samples were freeze dried and approximately

two-thirds of each sample was wet sieved through a>63 mm sieve, dried at 50�C and further sieved to retainthe 250–355 mm size fraction for stable oxygen isotopeanalyses and radiocarbon dating. The remaining bulksample was ground to a homogeneous, fine powder in anagate pestle in order to be used for organic carbon,nitrogen, and lignin analyses.

4.2. Analytical techniques

Oxygen isotope analyses were conducted on shells ofthe planktonic foraminifer species Globigerinoides ruber

picked from the 250–355 mm size fraction. Approxi-mately 60–70 mg of sample was analyzed using a VG

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Table 1

AMS 14C and calendar year ages for core MD9821-62

Sample

depth

(cm)

Measured 14C age

(14C yrBP)a,bCalibrated 14C age

(cal yr BP)cCalibrated 1s agerange (cal yr)d

11 910740880

903–818

141 23307402400

2463–2348

341 36207404040

4078–3974

521 69707407820

7862–7772

661 954074010,800

10,838–10,583

698 10,59074012,619, 12,477,

12,383e

12,653–12,329

861 14,89074017,710

17,957–17,440

992 17,74075021,000

21,303–20,653

1111 22,58076027,290

27,436–27,136

1271 28,430711033,910

34,056–33,756

aAll 14C dates were determined on G. ruber.b 14C ages were corrected for an oceanic reservoir effect of �400

years (Bard, 1988).c 14C ages were converted to calendar years using the CALIB

software, version 4.3 (Stuiver and Reimer, 1993; Stuiver et al., 1998).d1s range encloses 68.3% of the area of probability (Stuiver and

Reimer, 1993).eThe average of these 3 calendar ages was used (12,490 cal yr BP).

K. Visser et al. / Quaternary Science Reviews 23 (2004) 17–2720

Optima stable isotope ratio mass spectrometer (IRMS)equipped with an automated carbonate system. Thesamples were reacted in a common acid bath of 100%phosphoric acid at 90�C. The evolved CO2 gas was thenanalyzed on the IRMS. Oxygen and carbon isotopicdata are reported in delta notation relative to the(Vienna) Pee Dee Belemnite (VPDB) standard via ourworking standard, Burgess Ferry. The long-term stan-dard reproducibility for oxygen (d18O) isotopes based onreplicate measurements of reference standard is70.07%.Organic carbon content was determined using

8–12mg of carbonate free bulk homogenized sediment.To remove the carbonate, the sediments were sonicatedwith 7.5ml of phosphoric acid (1M) for 8–9minand filtered through a glass fiber filter (retentiono1 mm) with deionized water. The filter was dried(at 50�C), folded into a tin disk and analyzed bycombustion on a Perkin Elmer 2400 CHN ElementalAnalyzer. Every 10th sample was run in triplicateand cystine standards were used to ensure preciseand accurate results, respectively. The mean precisionwas 70.06% organic carbon, based on triplicateanalyses.Total carbon and nitrogen contents were determined

on the same CHN analyzer. Approximately 8–11mg ofuntreated bulk sediment was place in a tin capsule andanalyzed. Triplicate analyses performed on every tenthsample show the mean precision to be 70.06% totalcarbon and 70.02% total nitrogen.Twenty samples (5 each from marine isotope stages 1,

2, 5e, and 6) were analyzed for lignin phenol concentra-tions using the alkaline CuO oxidation technique ofGoni and Montgomery (2000). Briefly, alkaline CuOoxidations were carried with oxygen-purged 2N NaOHat 150�C for 1.5 h using a microwave digestion system.After the oxidation, recovery standards (ethyl vanillin,trans-cinnamic acid) were added and the solution wasacidified to pH 1 with concentrated HCl. Samples werethen extracted with ethyl acetate. Extracts were evapo-rated to dryness under a stream of N2. The CuOreaction products were re-dissolved in pyridine andderivatized with bis trimethylsilyl trifluoroacetamide(BSTFA)+1% trimethylchlorosilane (TMCS) to silylateexchangeable hydrogens prior to injection in the gaschromatograph. Gas chromatographic analyses weredone in order to quantify the following cupric oxidationproducts: lignin-derived vanillyl, syringyl, and cinnamylphenols. The identities of all CuO reaction productswere confirmed by gas chromatography-mass spectro-metric analyses. Two samples were analyzed in triplicateand the standard deviation was calculated on thesesamples to estimate the precision of the measurements.The average of the standard deviation of the twosamples is: vanillyl: 0.021; syringyl: 0.033; cinnamyl:0.0089; syringyl/vanillyl: 0.128; cinnamyl/vannillyl: 0.41;

[Ad/Al]v (vanillic acid/vanillin): 0.086; [Ad/Al]s (syringicacid/syringealdehyde): 0.127.

4.3. Age model and stratigraphy

The age model for core MD9821-62 is based on 10accelerator mass spectrometry (AMS) 14C dates and theSPECMAP stacked d18O chronology developed byMartinson et al. (1987). The AMS 14C dates weredetermined on monospecific samples (4–10mg) ofthe planktonic foraminifer Globigerinoides ruber at theCenter for Accelerator Mass Spectrometry, LawrenceLivermore National Laboratory. The youngest eight 14Cages were converted to calendar ages using the calibra-tion program CALIB 4.3 (Stuiver and Reimer, 1993;Stuiver et al., 1998) and the oldest two 14C ageswere converted using the equation, �698.65E-3+1.2695(14Cc)–2.7039E-6(14Cc)2 (r2 ¼ 0:999; standarderror =7150 years), where 14Cc is the reservoircorrected 14C age (Guilderson et al., 2000; Table 1). Inorder to account for the difference in 14C compositionbetween the atmosphere and the surface ocean, areservoir correction of 400 years was subtracted fromthe 14C dates before converting them to calendar years

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Table 2

Tie points for first 160 kyr of the MD9821-62 core, based on the

Martinson et al. (1987) time scale

Sample depth

(cm)

Martinson age

(yr)

Marine isotope

stage

Error (yr)

1850 58,960 4 5560

2140 73,910 5 2590

2320 79,250 5.1 3580

2880 99,380 5.3 3410

3060 115,910 Btwn 5.4 and

5.51

6280

3320 125,190 5.53 2920

3370 129,840 6 3050

3470 135,100 6.2 4240

3610 142,280 6.3 5280

3900 157,000 End

Fig. 2. The Globigerinoides ruber d18O record, total organic carbon contenchronology for the upper part is based on 10 AMS radiocarbon dates (Table

d18O record to the SPECMAP d18O chronology (Martinson et al., 1987, Ta

K. Visser et al. / Quaternary Science Reviews 23 (2004) 17–27 21

(Broecker and Peng, 1982; Bard, 1988; Southon et al.,2003). The 14C ages range from 750 years (core top) to33,910 years (1271m). For ages older than 34 ka, theMD9821-62 G. ruber d18O record was graphicallycorrelated to the SPECMAP stacked d18O chronology(Martinson et al., 1987) using 10 tie points (Table 2).The d18O stratigraphy for core MD9821-62 (Fig. 2)matches that of other highly resolved cores from aroundthe world, with marine isotope stages (MIS) 1–6 beingwell developed. The entire time scale was constructedassuming constant sedimentation rates between radio-carbon dates and isotope tie points. Based on this agemodel, the average sedimentation rate for the core isB25 cm/kyr and the average sample spacing for the last160 ka is B400 years.

t (%) and (C/N)a ratio for core MD9821-62 plotted versus time. The

1). Below 34,000 years BP, the age model is based on correlating this

ble 2).

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5. Results and discussion

5.1. Organic matter sources: marine and terrestrial

The record of total organic carbon in core MD9821-62 shows a distinctive glacial-interglacial pattern(Fig. 2). The lowest TOC values occur during inter-glacial stages 5e and 1, while the last two glacial episodes(stages 2 and 6) are marked by high TOC values. Inaddition, there is a long term increase in TOC fromstage 5e to stage 2 that mirrors the increase in d18Ovalues that occurs during this time interval (Fig. 2). Dueto the close proximity of the study site to Borneo andSulawesi, the organic matter preserved in core MD9821-62 should be a mixture of both marine and terrestrialcarbon. With the data presently available, we are unableto quantify the proportions derived from each of thesesources and this precludes us from using the TOC recordto assess changes in surface water productivity in thisregion through time.Several lines of evidence suggest that the close

correspondence between the d18O and the TOC recordsmay be driven, at least in part, by a sea level controlledinput of terrestrial carbon. The atomic ratio of organiccarbon to nitrogen (C/N)a is one of a number of ways todifferentiate qualitatively between terrestrial or marinesources of organic matter (Muller, 1977). Terrestrialorganic matter typically has a ratio of greater than 20,while marine organic matter normally ranges between 6and 9 (Hedges and Oades, 1997; Kawahata et al.,2000b). Sediments collected from shallow sediment trapsnorth of New Guinea (B4�N, 136�E) have (C/N)a ratiosof 8–8.5 signifying a marine source for this material andgiving us a baseline value for marine organic matter inthis region (Kawahata et al., 2000a). For our core, the(C/N)a values range from B7 to 13 (Fig. 2). During theHolocene (MIS 1) and last interglacial (stage 5), (C/N)aratios were lower than during the last glacial maximum(MIS 2) and the penultimate glacial period (MIS 6)-8.5and 9.5, and 11 and 11.2, respectively (Fig. 2). Thehigher (C/N)a values during glacial MIS 2–4 and 6(average of 11.1) relative to interglacial MIS 1 and 5e(average of 9.0) suggest a slight increase in the input ofterrigenous organic carbon to our study site associatedwith the glacial lowering of sea level.The concentrations of lignin phenols in sediments

were also used to determine the presence of terrestrialorganic matter in the core. Lignin is found in the cellwalls of vascular plants (most of which are terrestrial)and is highly resistant to degradation (Goni and Hedges,1992; Hedges, 1992). For this reason, the totalconcentration of lignin phenols (the sum of vanillyl,syringyl and cinnamyl phenols) is a useful tracer ofterrigenous organic matter in marine environments. Thetotal lignin phenol concentrations (in units of mg/100mg sediment) in MD9821-62 are higher during

glacial periods relative to interglacial periods, suggestinga greater flux of terrigenous organic matter to the corelocation during these cold periods (Fig. 3). DuringMIS 2, average lignin concentrations (0.01170.004) areapproximately double those of the Holocene (0.00670.001). The average lignin concentrations in MIS 6(0.00770.001), the other glacial period we investigated,are 40% greater than those in MIS 5e (0.00570.001).Notably, the carbon normalized concentrations of ligninphenols (mg/100mg OC) exhibited comparable valuesduring both glacial and interglacial intervals. These dataindicate that although the total flux of terrigenousorganic matter were higher during glacial conditions, therelative contributions of marine versus terrigenoussources to the total organic matter present in thesesediments did not differ significantly between climaticstages. Most likely, the enhanced inputs of marinecarbon during glacial periods relative to interglacialperiods contribute to these observations. In all like-lihood, the increased input of terrigenous carbon duringglacial stages at our study site in the Makassar Strait isdue to enhanced erosion of exposed continental shelvesduring these times of lowered sea level.

5.2. Vegetation changes

Instead of using pollen assemblage data, we examinechanges in terrestrial vegetation on Borneo during thelast two interglacial and glacial periods using theconcentrations of different lignin phenols. Specifically,the ratios of the vanillyl (V), syringyl (S), and cinnamyl(C) phenols (specifically, S/V and C/V) derived fromcupric oxidation (Hedges and Ertel, 1982) can be used todifferentiate between different sources of terrestrialorganic matter, namely angiosperms versus gymnos-perms, and between nonwoody (leaves, grasses, needles)and woody tissue (Goni and Hedges, 1992; Goni, 1997).For example, elevated S/V ratios (0.6–4) indicate thepresence of angiosperms, while low S/V ratios (B0)indicate gymnosperm inputs, as these plants do notproduce syringyl phenols (Goni, 1997). Nonwoody andwoody tissues can be differentiated using the C/V ratio,since only nonwoody tissues produce cinnamyl phenols;nonwoody tissue have values of 0.1–0.8 and woodytissue have values less than 0.05 (Goni, 1997).In our core, total lignin concentrations are low and

dominated by the vanillyl and syringyl phenols (cinna-myl phenols are less than or equal to 0.03mg/100mgOC). The ratio of the syringyl to vanillyl phenols(S/V) ranges from 0.52 to 0.89, with an average of 0.73(Fig. 3). The cinnamyl: vanillyl ratio (C/V) ranges from0.0 to 0.18, with an average of 0.09 (Fig. 3). Thestandard deviation of the C/V ratios is quite large due tothe low concentrations of the cinnamyl phenol in thesediments. Two of the samples analyzed have almost nocinnamyl phenols. Overall, these results show no

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Fig. 3. Total lignin concentrations normalized to both 100mg organic carbon and 100mg sediment, and the lignin phenol ratios: syringyl/vanillyl

(S/V) and cinnamyl/vanillyl (C/V) ratios across Termination I (stages 1 (shaded) and 2) and Termination II (stages 5e (shaded) and 6). The error bars

are based on replicate analyses of the same sample.

K. Visser et al. / Quaternary Science Reviews 23 (2004) 17–27 23

significant differences between the lignin phenol com-positions of interglacial and glacial age sediments. Sincethe S/V and C/V ratios are comparable across bothterminations I and II, which have an age difference ofB100,000 years, it is unlikely that post-depositionaldegradation can explain the results obtained. Instead,these data suggest that the sources and composition ofthe lignin preserved in these sediments have not changedsignificantly over the age the core. The higher (C/N)avalues during glacial MIS 2–4 and 6 (mean=11.1)relative to interglacial MIS 1 and 5e (mean=9.0) (Fig. 2)are consistent with a slightly higher contribution ofterrestrial organic matter to the cores site during glacial,low sea level periods.The CuO products can be used to characterize the

extent of degradation of the lignin macromoleculepresent in sediment samples. Specifically, the ratios ofvanillic acid over vanillin (Ad/Al)v and syringic acid

over syringealdehyde (Ad/Ad)s have been shown toincrease as a result of the oxidative attack of lignin byfungi (Hedges et al., 1988; Goni et al., 1993, 1998).(Ad/Al)v ratios between 0.1 and 0.3 are indicative offresh plant tissue, whereas (Ad/Al)v and (Ad/Al)s ratiosgreater than 0.6 indicate that the material is highlydegraded (Goni, 1997). Such high elevated ratios areoften observed in humic substances and mineral soilsthat contain highly altered lignin remains (Goni et al.,1998). The vanillic acid:vanillin ratio (Ad/Al)v in coreMD2198-62 ranged between 0.40 and 0.82 (average:0.58) and the syringic acid: syringealdehyde ratio(Ad/Ad)s ranged between 0.50 and 1.07 (average: 0.67;Fig. 4). Hence, it appears that the lignin in our core ismoderately to highly degraded. Because the (Ad/Al)ratios remain fairly constant down core (Fig. 4), thedegradation of the lignin polymer is likely to haveoccurred on land, rather than after it was deposited on

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Fig. 4. A plot of acid: aldehyde ratios of syringyl (Ad/Al)s and vanillyl (Ad/Al)v phenols across Termination I (stages 1 (shaded) and 2) and

Termination II (stages 5e (shaded) and 6). Also plotted are the ranges for fresh/undegraded woods and highly degraded woods (Goni, 1997; Goni

et al., 1998).

K. Visser et al. / Quaternary Science Reviews 23 (2004) 17–2724

the ocean floor (Goni, 1997; Hu et al., 1999; Kastnerand Goni, 2003).In terms of climate-induced vegetation change, it is

critical to interpret the lignin phenol compositionsdescribed above in the context of previous studies.Presently, Borneo and Sulawesi are almost completelyvegetated by tropical lowland and montane humidevergreen rainforest, consisting almost entirely ofangiosperm trees (MacKinnon et al., 1997; van derKaars et al., 2001). In fact, grass pollen contributes lessthan 3% of the pollen in Mahakam delta surfacesamples (Caratini and Tissot, 1988), highlighting thepaucity of savanna vegetation in this region undermodern conditions. Investigations of glacial vegetationsuggest the expansion of montane forest (composed ofthe angiosperms Lithocarpus and Castanopsis) to loweraltitudes during glacial periods as a result of coldertemperatures (Barmawidjaja et al., 1993; van der Kaarset al., 2000). Furthermore, several pollen studiesconcluded that grasslands flourished at the expense ofrainforest vegetation in Indonesian during glacialperiods due to increased aridity and colder temperatures(van der Kaars, 1991, 1998; Barmawidjaja et al., 1993;van der Kaars and Dam, 1995, 1997; Dam et al., 2001).A plot of C/V versus S/V ratios (Fig. 5) can be used to

illustrate the possible vegetation sources that mayaccount for the lignin compositions measured in theanalyzed sediments. The high S/V ratios (0.52–0.89)during MIS 1, 2, 5e and 6 indicate that angiospermvegetation dominated this region over the last two

glacial and interglacial periods (Fig. 3), just as it doestoday. The C/V ratios (0.0–0.18) indicate that a similarmixture of woody and nonwoody material was trans-ported to the core location during MIS 1, 2, 5e and 6.The overall similarities in lignin compositions betweeninterglacial-and glacial-age sediments are a strongindicator of the lack of major vegetation change in thearea. In particular, we observe no evidence for a largeexpansion of savannas during glacial periods. Tropicalsavannas are enriched in C4 grasses, which would haveresulted in elevated C/V ratios. Instead, our resultsstrongly indicate that angiosperm tree vegetation was apersistent feature of the land masses adjacent to the coresite during glacial and interglacial periods of the last160 ka (Fig. 5). Unfortunately, the lignin compositionsobtained in this study do not allow us to differentiate thetypes of angiosperm trees that were present and thusconfirm the proposed glacial expansion of montanevegetation.The paired lignin phenol data provide evidence of

continuous angiosperm vegetation deposition at the corelocation arguing against a major vegetation change fromtropical rainforest to grasslands during glacial periods inthe Indonesian region (van der Kaars, 1991, 1998;Barmawidjaja et al., 1993; van der Kaars and Dam,1995, 1997; Dam et al., 2001), agreeing with studiesshowing evidence of continuous rainforest vegetation intropical regions (Stuijts et al., 1988; Schlunz et al., 1999;Colinvaux and de Oliveira, 2000; Colinvaux et al., 2000;Sun et al., 2000; Baker et al., 2001a, b; Cowling et al.,

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Fig. 5. A plot of cinnamyl:vanillyl (C/V) versus syringyl:vanillyl (S/V) phenol ratios for glacial and interglacial sediments. Also plotted are the values

of fresh angiosperm and gymnosperm woods and leaves/grasses (Goni, 1997; Goni et al., 1998).

K. Visser et al. / Quaternary Science Reviews 23 (2004) 17–27 25

2001; Taylor et al., 2001; Bush et al., 2002). The lack ofdrastic vegetation change evident from our results isconsistent with an increase in the intensity of the wintermonsoon during glacial periods, which would maintainhumid conditions in the area while still allowingtemperatures to decrease (Wang et al., 1999; Sun et al.,2000). Additional support for our explanation isprovided by a simulation of climate conditions duringthe last glacial maximum derived using a coupledatmosphere–ocean general circulation model, that pre-dicts an increase in rainfall in the Indonesian region atthis time (Bush and Philander, 1999).

6. Conclusions

1. The organic matter deposited at our core locationcontains a mixture of marine and terrestrial carbon.While the pronounced glacial–interglacial pattern intotal organic carbon content invariably contains

some surface water productivity signal, we concludethat the higher TOC values during the glacialepisodes is due to enhanced erosion of exposedcontinental shelves bordering the Makassar Strait.

2. There is no distinguishable difference in lignin phenolratios of glacial and interglacial samples and thus weconclude that the modern day/interglacial angios-perm rainforest vegetation on the islands of Borneoand Sulawesi persisted during the last two glacialperiods. This supports the idea that the wintermonsoon strength increased during glacial periods,allowing the area to maintain humid conditionsduring this time.

Acknowledgements

We would like to thank Heather Aceves for conduct-ing the lignin analyses, Eric Tappa for technicalassistance, and Mary Cathey and Stacey Russo for

ARTICLE IN PRESSK. Visser et al. / Quaternary Science Reviews 23 (2004) 17–2726

laboratory assistance. We thank the French PolarInstitute and the IMAGES Program for organizingand coordinating the 1998 cruise of the RV Marion

Dufresne to the Indonesian region. Oregon StateUniversity’s core facility is acknowledged for housingcore MD9821-62 (NSF grant: OCE97-12024). This workwas funded by NSF grant OCE-0094771 to R.Thunell.

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