Journal of Asian Earth Sciences 69 (2013) 166–176

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Journal of Asian Earth Sciences

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Paleoclimate variability in central Taiwan during the past 30 Kyrsreflected by pollen, d13CTOC, and n-alkane-dD records in a peatsequence from Toushe Basin

1367-9120/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2012.12.005

⇑ Corresponding author. Tel.: +886 2 33662929.E-mail addresses: hcli1960@ntu.edu.tw (H.-C. Li), liewpm@ntu.edu.tw (P.-M.

Liew), seki@pop.lowtem.hokudai.ac.jp (O. Seki), fu5710@gmail.com (L.-C. Wang),tqlee@earth.sinica.edu.tw (T.-Q. Lee).

Hong-Chun Li a,⇑, Ping-Mei Liew a, Osamu Seki b, Tz-Shing Kuo c, Kimitaka Kawamura b, Liang-Chi Wang a,Teh-Quei Lee d

a Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROCb Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, 8 Sapporo, Hokkaido 060-0819, Japanc Department of Earth Sciences, National Cheng-Kung University, Tainan 70101, Taiwan, ROCd Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan, ROC

a r t i c l e i n f o a b s t r a c t

Article history:Available online 20 December 2012

Keywords:C and H stable isotopeOrganic carbon contentPeat sequence of central TaiwanPaleoclimateLate Pleistocene

A total of 233 samples from the upper 16 m of the Toushe peat core retrieved in central Taiwan were mea-sured for TOC and d13CTOC values. From these samples, 17 selected samples with large d13CTOC fluctuationswere analyzed for n-alkane and dD of the C27 and C29 n-alkanes. Combining with the detailed high-reso-lution pollen and geochemical records, this study reveals more detailed climatic variations in terms of tem-perature and precipitation as well as abrupt climatic events during the past 30 Kyrs. Before the Last GlacialMaximum (LGM), climate was cold and damp with predominantly woodland vegetation in Toushe Basin,and turned to cold and dry after 25 Kyr BP. Climatic conditions there were the worst during LGM overthe past 30 Kyrs, especially around 23 and 18 Kyr BP when the woodland was diminished and C4 grasswas dominated. Although short durations of relatively wet conditions could be found at 17, 16 and14 Kyr BP, cold and dry climates were prevailing during 29.5–28, 24–22, 17–15 and 13–11.5 Kyr BP, corre-sponding to Heinrich (H) Events 3, 2, 1, and Younger Dryas (YD), respectively. During the early Holocene,dry climate occurred at �11, �10, 9.7–9.2 and �8 Kyr BP; whereas wet condition appeared at 10.3, 9.8,9–7.5 Kyr BP. In the middle Holocene, climate kept warm and moderate wet in the first half period, butmany dry events existed in the second half following a cold and dry event at 6 Kyr BP. After a sharply warmpeak at 5.2 Kyr BP, the climate in Toushe turned to cold quickly, and tree/shrub vegetation disappearedcompletely with the replacement of C3 grasses. In the late Holocene, climate was relatively wetter with pre-dominant C3 grasses in the basin. Our climatic interpretations based on the peat records agree well with theGreenland ice core and Chinese speleothem records on millennium time scales during the last glacial per-iod. Dry climates corresponding to weakening of the East Asian Summer Monsoon (EASM) during the Hein-rich events and Younger Dryas in central Taiwan and eastern China demonstrate the climatic forcing onsuch long time scales in concert with regional monsoon climate. However, the discrepancies exist betweenour peat record and the Dongge/Hulu stalagmite record on: (1) the age of H2; (2) climate intensities of LGMand H1; and (3) wetness condition during Holocene. These observations call for further study on high-res-olution climatic changes especially on moisture budget in the East Asian monsoonal region.

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1. Introduction in Earth orbital parameter, solar insolation, ice volume and ocean

The East Asian Monsoon (EAM) is an important climate systemand strongly affects precipitation and temperature in the Asiancontinent. Previous studies from loess, marine, lake and speleo-them records have shown that the paleomonsoon variability onmillennial or longer time scales has been influenced by changes

circulation (e.g., An, 2000; Ding et al., 1995; Wan et al., 2011;Wang et al., 2005, 2008; Wei et al., 2003; Xiao et al., 1999). TheEast Asian Summer Monsoon (EASM) variations depicted by theChinese speleothem d18O records show a strong coupling to insola-tion’s precession component (23-Kyr period) and link to the highlatitude temperature changes (e.g., Cheng et al., 2006; Johnson,2011; Wang et al., 2001). Many climatic features such as Heinrichcold events, Dansgaard–Oeschger (D–O) warm events and YoungerDryas (YD) have been found in Chinese speleothem d18O records(e.g., Wang et al., 2001; Li et al., 2011 and reference therein). Theseclimatic episodes are often described as cold/dry and warm/wet

H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176 167

climate anomalies. In Pausata et al. (2011), the model resultshowed that a sudden cooling in the Northern Hemisphere causedby an increase in North Atlantic sea-ice during the last glacial per-iod could weaken the Indian monsoon and rainfall over the Indianbasin. Consequently, moisture supply to the eastern China from theIndian monsoon was isotopically heavier, resulting in heavier d18Ovalues in the Chinese speleothem records during Heinrich events.Therefore, they argued that the heavy d18O values correspondingto Heinrich events in the speleothem records actually ‘‘reflectchanges in the intensity of Indian rather than East Asian monsoonprecipitation’’. However, few records have been found to interpretsolely precipitation change in the EASM region during the last gla-cial period, and paleorecords about Heinrich events and D–Oevents besides these speleothem records are not common. Thus,it is critical to search a rainfall proxy record in the EASM regionback to the last glacial period. Taiwan is such an ideal place whoseclimate is dominated by the East Asian monsoon.

As a subtropical mountain island, Taiwan is generally humidwith warm–wet summers and mild and relatively dry winters.The moisture sources are chiefly from western Pacific and SouthChina Sea. Owing to its strongly tectonic activities and heavy rain-falls, high erosion rate and sedimentation rate in the landscape ofTaiwan result in rare long climatic archives. Undisturbed naturallakes in Taiwan are generally small and shallow with freshwaterin mountain ranges (Tsukada, 1966). Sediment records retrievedfrom these lakes are often younger than the past glacial period,and the 39.6-m long core from the Toushe Basin in central Taiwanis probably the only record continuously covering the past 96 Kyrs(Liew et al., 2006b). The materials from this peat sequence can beused for detecting climatic conditions during the Heinrich and D–Oevents, YD and LGM periods. Although the detailed pollen recordsfrom the peat core have been published, the interpreted climaticvariations based on the pollen records were unable to pin downHeinrich events (Liew et al., 2006b). In addition, pollen identifica-tion is difficult to distinguish C3 and C4 types of grass, e.g., bothScirpus articulatus (C3 type) and Cyperus sp. (C4 type) are belongto family Cyperaceae. Gramineae family has also both C3 and C4

Fig. 1. Location map and modern climatic conditions of the studying area (From Liew

species (Chmura and Aharon, 1995). Therefore, interpretation ofclimatic conditions especially on decadal-to-centennial scales dur-ing Holocene was limited with pollen records only. With d13CTOC

information, we should be able to shed the light on change inC3/C4 plant ratio. In this study, we attempt to use high-resolutiond13CTOC records plus the pollen records of the peat core during thepast 30 Kyrs for reconstruction of vegetation changes in further de-tail and discuss climatic conditions for these changes. The resultsenable us to understand the fore-mentioned climatic regimes andtheir forcing factors.

2. Background and previous results

Toushe Basin (23�490N, 120�530E) is located in central Taiwan,with an elevation of 650 m and an area of 1.75 km2 (Fig. 1). Accord-ing to the meteorological data from the nearby Sun-Moon Lake Sta-tion (altitude 1014 m), the mean annual rainfall and temperatureat the present in the area are 2300 mm and 21 �C, respectively.The average temperatures for the coldest and warmest monthsare 14 �C and 25 �C. Therefore, climate of this area belongs to hu-mid subtropical monsoon. Modern vegetation in the study area be-longs to the subtropical evergreen Lauro-Fagaceae forest (Liewet al., 2006a). The basin was occupied by a lake with �80 m thicksediments before the last glacial. The lake became a peat bog bythe early last glacial and dried out around �1.7 Kyr BP (Liewet al., 2006b). A 39.6 m-long core was taken from the Toushe peatbog in 1999.

The 39.6 m-long core was dated by 31 14C dates on plant re-mains from the upper 20.5 m, with good stratigraphic order andsmall uncertainties (Figs. 2 and 3). According to these 14C datesand extrapolating to the bottom of the core with linear sedimenta-tion rate obtained by an age-depth relationship of Depth(m) = 0.3912 � Age (Kyr) + 2.261 (R2 = 0.9454), this 39.6 m-longcore contains �96-Kyr depositional history which is the longestlake record so far in Taiwan. For this study, we only present the re-sults for the upper 16-m part of the core.

et al., 2006b). The start symbol denotes the drill core site of the peat sequence.

168 H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176

In the earlier chronology reconstruction, Liew et al. (2006b)used a chronology which was determined by linear sedimentationrates between each adjacent age pairs up to the oldest 14C date andextrapolating to the older part by the above linear equation. Thischronological reconstruction is very close to the polynomial func-tion to find the best fitting on all the dates throughout the upper20.5 m (Fig. 2). Thus, in the present study we adopt the previouschronology used by Liew et al. (2006b).

Climatic and paleoenvironmental reconstructions in the study-ing area have been intensively studied before, but mainly based onpollen records. Tsukada (1967) obtained a core from the nearbySun-Moon Lake and interpreted briefly vegetation and climate con-ditions during the last glacial period. Liew et al. (1998) and Kuoand Liew (2000) published the simplified pollen diagram of theupper 17 m of the Toushe peat sequence. After detailed studieson the surface pollen assemblages of the natural forests and recon-struction of plant functional types (PFTs) and biomes (biomization)with known climatic conditions in this region (Chung, 1994; Liewand Chung, 2001; Su, 1984; Liew et al., 2006a,b). Liew et al. (2006b)provided a high-resolution pollen record for the entire 39.6-m peatsequence and described vegetation and climate changes during thepast 96,000 yrs.

For the upper 16 m of the peat sequence, Liew et al. (2006b)classified the pollen assemblages into seven zones (Fig. 3) andshowed relatively temperature changes based on the vegetationbiomes. The climatic conditions deciphered by the pollen assem-blages with the previous chronology are: (1) Cold and dry climateswere dominated in the Marine Isotope Stage 2 (MIS2); (2) a warm–wet episode reflected by the peak of monolete spores at 15 Kyr BPcorresponding to the Bølling; (3) cold conditions showed by in-creased Salix between 13.0 and 12.5 Kyr BP; (4) relatively coldand dry referred by an Ilex peak at 12.1 Kyr BP and rise in Grami-neae (>35%) at 11.8–11.6 Kyr BP corresponding to the late half ofthe Younger Dryas; (5) subtropical conditions continue to prevailfrom 11.5 Kyr BP onward, but abrupt cold episodes indicated bySalix peaks at about 11.0, 9.6–9.4, 7.9, 7.1, 5.2 and 5.0, 4.0 and3.7 Kyr BP; (6) warm condition during 7.3–6.8 Kyr BP (except7.2 Kyr BP), 6.2–5.8 Kyr BP; and (7) the monolete spore peaks at10.7–10.3, 10.0–9.7, 9.5–8.8, 6.9–6.8 and 2.9–1.8 Kyr BP indicatingwet episodes. The detailed pollen analyses will help us to

Fig. 2. Chronology of the peat sequence with sedimentary stratigraphy.

understand the carbon isotope behaviors in the total organic car-bon (TOC).

3. Samples and methods

Samples for this study were from the upper 16 m of the 39.5-m-long core taken from Toushe peat bog, spanning the past �30 Kyrs(Fig. 3). This part of the core reveals mainly peaty sediments inter-calated with thin layers of clay or gyttja (<5 cm in general). Mostsamples in the core are rich in plant remains and contain negligiblecarbonate. Except a 20-cm gap between 55 and 75 cm depths, the16-m long segment was sub-sampled at 1–2-cm intervals, so that atotal of 902 samples were obtained from this part. We have se-lected 233 samples for measurements of total organic carbon(TOC) and d13CTOC. All samples were dried by a vacuumed freezedrier, then grinded into powder for well mixing. About 0.5 mg ofsample powder was weighted and wrapped into a tin cup, thenmeasured d13C by using EA-IRMS in the Department of Earth Sci-ences at the National Cheng-Kung University (NCKU), Taiwan.

The sample tin cups were placed in an auto-sampler equippedwith the Flash 1200 Elemental Analyzer (EA) which is connectedwith a Finnegan Delta XP plus isotopic ratio mass spectrometer(IRMS) via a Conflo-III. The samples were dropped into an oxida-tion furnace with a continuous He flow. All organic carbons weretransferred into CO2 under 1040 �C. The CO2 gas was carried byHe flow passing through a gas chromatograph (GC) column, thenentered the Conflo-III device and IRMS. The sample CO2 gas wasmeasured against the calibrated reference CO2 gas for its carbonisotopic ratios eight times. The external precision of d13C analysisis ±0.10‰ based on our standard measurements. One workingstandard runs with every five samples to monitor the instrumentalcondition. The d13C value of the samples is reported in standard d-notation relative to the Vienna Pee Dee Belemnite (VPDB) scale at25 �C.

In the meantime, TOC analysis is given by the EA/IRMS whenmeasuring d13CTOC. The calibration curve between peak area (orintensity) of 12C16O2 (mass 44) and TOC content is made by mea-surements on standard urea. As the peat samples contain highTOC, the analytical uncertainty is relatively low (RSD <1%). Wehave done duplicated analyses on many samples to check the dataquality. The d13CTOC and TOC results are presented in Fig. 3.

We attempt to use dD of plants grown above water and belowfor environmental and climate reconstruction. Thus, we selected17 samples with large fluctuations of d13CTOC for analyses of lipidbiomarkers (alkanes) and their dD. This work was done in the labat the Institute of Low Temperature Science, Hokkaido University,Japan. Alkanes were extracted from homogenized dry sedimentsfollowing the method described in Seki et al. (2010). Alkanes werequantified using an HP5890 gas chromatography (GC) equippedwith an on-column injector. dD values of the C27 and C29 n-alkanes were determined with a GC–isotope ratio mass spectrom-eter consisting of a HP 6890 GC connected to a Finnigan MAT DeltaPlus via a combustion interface (GC–C–IRMS). The precision of dDanalysis is ±1.5‰. The dD value of the samples is reported instandard d-notation relative to SMOW. Results of n-alkane dD areshown in Fig. 8.

4. Result and discussion

4.1. Pollen

Although the pollen records were interpreted before in the pre-vious publications (e.g., Kuo and Liew, 2000; Liew et al., 2006a andLiew et al., 2006b), it is difficult to see correlations among TOC,d13CTOC and vegetation. In order to find vegetation influence on

Fig. 3. Profiles of simplified major pollen assembly, vegetation zone, reconstructed forest on the 1st axis of detrended correspondence analysis (DCA), d13CTOC and TOC in thepeat sequence. The colors in the biomes denote relative temperature with an order from warm to cold: red>yellow>green>blue. (For interpretation of the references to colourin this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Profiles of major arboreal pollen family percentages which are in favor of cold conditions (AP cold%) in the Toushe peat sequence.

H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176 169

d13CTOC, we re-exam the pollen raw data sets. In the Toushe peatcore, 89 genus types of pollen were identified (Kuo and Liew,2000). However, many pollen genus types have very low abun-dance. We ignore the pollen genus whose total pollen counts areless than 300 in the upper 16 m (175 samples). After this filtering,

only Pinus, Ilex, Alnus, Quercus, Cyclobalanopsis, Symplocos, Salix,Castanopsis, Liquidambar, Ulmus, Ligustrum, Glochidion, Mallotus,Artemisia, Polygonum, Umbelliferae, Cyperaceae and Gramineae re-mained. Among these 18 genus types, Pinus, Ilex, Salix, Alnus, Quer-cus, Cyclobalanopsis and Symplocos, are arboreal pollen

Fig. 5. Profiles of major arboreal pollen family percentages which are in favor ofwarm conditions (AP warm%) in the Toushe peat sequence.

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corresponding to trees under cool/cold conditions. We name thisgroup of pollen as AP cold, and its percentage distribution shownin Fig. 4. In this group, Alnus was dominant before 25,000 yr BPand disappeared after 11,000 yr BP. Ilex and Salix became dominantafter 16,000 yr BP (Fig. 4). The rest AP cold pollen are less impor-tant. Pollen assemblage of Castanopsis, Liquidambar, Ulmus, Ligu-strum, Glochidion and Mallotus represents trees under relativelywarm condition, which is named as AP warm. The percentage dis-tribution of AP warm is plotted in Fig. 5. In the warm AP assem-blage, Castanopsis and Ligustrum are common, whereas the restwarm AP are less important, except that Glochidion showed astrong peak during 5,300–5,100 yr BP (Fig. 5).

When we compare AP%, AP cold% and AP warm%, it is clear thatthe variation of total arboreal pollen percentage is mainly con-trolled by the variation of AP cold% (Fig. 6a). There is a strong cor-relation between AP% and AP cold% (Fig. 6b). This relationship mayimply that temperature change strongly affects the growth eleva-tion of the tree taxa which prefer in cold environments. The AP

Fig. 6. (a) Variations of AP%, AP cold% and AP warm% during the past 30 Kyrs. (b

warm%, on the other hand, did not vary significantly before Holo-cene and became dominant after then. The variations of AP cold%and AP warm% are good indicators of temperature change.

Artemisia, Polygonum, Umbelliferae, Cyperaceae and Gramineaeare herbaceous pollen, which accounted for more than 50% duringthe most Holocene period. Low abundances occurred before25,000 yr BP and during 14,000–10,500 yr BP (Fig. 7). Cyperaceaeand Gramineae are the major herbaceous pollen, and Artemisia isless important during the Holocene. In General, higher Gramineae%in the pollen assemblage represents drier condition, whereasmonolete spore peak indicates wetter condition (Liew et al.,2006a, 2006b). In Fig. 3, monolete spore% and Cyperaceae% havesimilar trends which are generally opposite to trend of Gramineae%.The variation of NAP% is mainly influenced by the variation ofCyperaceae% which is favorable in wet condition (Fig. 7). Gramineaehad low abundance before 24,000 yr BP and during 17,000–11,000 yr BP, and showed opposite trends to that of Cyperaceaeduring Holocene (Fig. 7). Therefore, we may use variations ofCyperaceae% and Gramineae% as an indicator of moisture change.As mentioned before, both Cyperaceae and Gramineae families haveC3 and C4 grass types, so that their interpretations for climatic con-ditions are not straight forward. An evidence for changes in C3/C4grass ratio is needed, which calls for d13C analysis.

4.2. TOC

TOC and C/N in many freshwater lake studies have been used asproxies for lake productivity and organic source hence paleocli-mate conditions (e.g., Hartmann and Wünnemann, 2009;Schwanghart et al., 2009; Selvaraj et al., 2012). However, for a lakewhich contains relatively low TOC may have complicated sourcesand processes of organic matter, so that interpretations of TOC var-iation in such a lake core in terms of paleoclimate change should becaution. For the Toushe peat core, the TOC contents are very high,ranging from 3.2 wt.% to 58.1 wt.% with an average of 26 wt.%(Fig. 3). And, the organic carbon was mainly from plant remains,so that the source and process of organic matter were relativelysimple. On the other hand, plant materials in nature contain littlenitrogen. All the samples we measured have very low mass 28intensity (<200 mv) during the EA/IRMS analyses even if we usedlarge sample size, yielding <0.1 wt.% of total nitrogen. Such low

) Correlations between AP% and AP cold%, and between AP% and AP warm%.

Fig. 7. Profiles of major herb pollen family percentages (NAP%) in the Toushe peat sequence.

H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176 171

total nitrogen contents are not reliable. Hence, we are not able toobtain C/N from the samples.

According to the previous study, the 39.6-m long core containstwo major parts: peaty sediments above 32.5 m depth and claysediments below (Liew et al., 2006b). Therefore, for the upper32.5 m, a peat bog in Toushe Basin was the depositional environ-ment, indicating a relatively shallow water depth and a low spillingpoint of the peat bog. The feature of continuously dominated peatdeposits with a few thin (less than 5 cm in general) layers of clay orgyttja in the upper 16 m of the core illustrates that the environ-mental deposition of the Toushe peat bog was shallow water dur-ing the past 30 Kyr BP.

Fig. 3 exhibits the comparison of TOC with the major pollengenus. Although the pollen assemblages changed from tree/shrub-dominated vegetation to herb-dominated vegetation at 7-m depth, the TOC does not show an increasing or decreasing trendbetween the two segments. Thus, both woody and grassy plantscan produce high TOC remains, implying that the TOC variationwas not caused directly by changes in vegetation type. For exam-ple, a dry climate might cause a decline of forest leading to a de-crease of TOC from wooden plants, but the grass productionmight increase to elevate the TOC under the same climate. In addi-tion, for the shallow peat bog in Toushe Basin, clay and detritus canbe derived from surface erosion from surrounding area by waterand wind. Therefore, the TOC variation in the Toushe peat core isdifficult to be explained with climate change. Hence, the TOC re-cord is not a good indicator of vegetation and climate changes inthe Toushe peat core, so that we do not use it for climaticreconstruction.

4.3. d13CTOC

d13C of total organic carbon (d13CTOC) is a useful tool for studiesof paleovegetation and paleoclimate (e.g., Diefendorf et al., 2008;Hong et al., 2001, 2009; Malamud-Roam and Ingram, 2004; Selv-araj et al., 2012; Xiao et al., 2002; Zhou et al., 2005). In general, sta-ble carbon isotopic composition of TOC in lake deposits isinfluenced by various organic carbon sources including plants,

organisms and black carbon. Factors including vegetation type,moisture supply, temperature, CO2 and sunlight availability,hydrochemistry, organic detritus, etc. all can affect d13CTOC in lakesediments. Therefore, interpretation of d13CTOC for such a mixtureas a climatic proxy is not a simple task. Fortunately, peat is mainlya mixture of plant remains, and its d13CTOC chiefly reflects percent-age of different vegetation types and their d13C values.

It is well known that three major vegetation types in nature andtheir d13C ranges. C3 plants with the Calvin cycle in their photosyn-thetic pathway have d13C values ranging �20‰ to �35‰ with anaverage of �26‰. C4 plants go through the Hatch-Slack cycle fortheir photosynthetic pathway and have d13C values ranging�9‰ to �16‰ with an average of �14‰ (Smith and Epstein,1971; Lerman, 1974; Osmond et al., 1982; Ehleringer and Rundel,1989; Schleser, 1995; Bi et al., 2005). CAM plants owing to theirbiochemical and physiological features show large range of d13Cvariations between �10‰ and �33‰ (Smith and Epstein, 1971;Bender et al., 1973). Trees and shrubs normally belong to C3 plantswhich are generally in favor of cold and wet climatic settings. C3grasses are dominated in cold and wet environment, and C4grasses are prevailing in warm and dry conditions (because theHatch-Slack pathway helps C4 plants to avoid drought stress) (Raj-agopalan et al., 1999; Ehleringer et al., 1997; Marchese et al., 2006and references therein). This is why C4 and CAM plants are nor-mally dominated in desert and salt marshes environments (Chm-ura and Aharon, 1995); and the occurrence of C4 plants reduceswith increased altitude and latitude (Tieszen et al., 1979; Sageet al., 1999). In addition, previous studies have shown that carbonisotopic fractionation during the incorporation of CO2 by C3 plantsis a function of humidity, with lighter d13C value under wetter andcooler conditions (Schleser, 1995; Li et al., 1999). For instance, con-sidering only C3 species, the group mean d13C value is the highestfor deciduous trees (�24.55‰), and significantly lower for ever-green trees (�26.06‰) and herbs (�26.74‰) (Descolas-Gros andSchölzel, 2007). Liu et al. (2011) measured 325 C3 plant samplesfrom three climatic regions in China and found that the d13C rangesare �29.9‰ to �26.7‰ (in semi-humid region), �28.4‰ to�25.6‰ (in semi-arid region) and �28.0‰ to �25.4‰ (in arid

172 H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176

region). Loader and Hemming (2004) found a positive correlationbetween mean April–June temperature and pollen d13C for Pinussylvestris pollen collected from the European spatial study network.On the other hand, C4 plants have higher photosynthetic rates thanC3 plants at low atmospheric CO2 concentrations. Therefore, lowCO2 levels should favor C4 photosynthesis whereas high CO2 levelsshould favor C3 photosynthesis under changing atmospheric CO2

(Ehleringer et al., 1991).In summary, d13CTOC of peat samples is controlled by vegetation

type and climatic condition, considering: (1) temperate forest envi-ronment (colder than tropical/subtropical setting) provides higher(tree + shrub)/grass ratio, resulting lighter isotopic ratio; (2) intropical/subtropical vegetation setting, cooler and wetter condi-tions favor to C3 plant growth with lighter d13CTOC; and (3) loweratmospheric CO2 is favorable for C4 plant development, producingheavier d13CTOC.

For the upper 16 m peat core, the measured 233 d13CTOC valuesrange from �17.84‰ to �30.23‰, averaging �26.27‰ (Fig. 3). Thed13CTOC values indicate that vegetation type in the studying areawas mostly C3 plants during the past 30 Kyr BP. This agrees withthe common situation that bogs in general are damp places whereC4 plants occur less. In Fig. 3, the pollen analyses show that below7 m depth (prior to 10.38 Kyr BP) vegetation in Toushe area wasmainly temperate deciduous and conifer mixed forest. After then,vegetation changed to warm temperate forest and subtropicalevergreen forest with a short tropical evergreen forest period ca.5 Kyr BP. Tree and shrub were dominant during the last glacial ex-cept between 22.6 and 17.5 Kyr BP, whereas herbs were dominantin Holocene. However, the average d13CTOC values for the last gla-cial and Holocene have no significant difference. In addition, thed13CTOC has no correlation with the TOC (Fig. 3). Thus, what arethe major factors to influence the d13CTOC?

We plot the d13CTOC, Gramineae%, Cyperaceae%, AP%, and n-al-kane dD together in Fig. 8 and find that the d13CTOC change mimicsthe variation of Gramineae% in general. When Gramineae% is high,the d13CTOC is heavier (gray bars in Fig. 8). This is because althoughfamily Gramineae contains many C3 type species, the majority of

Fig. 8. Comparisons among d13CTOC, dD of C27 and C29 n-alkanes, AP%, Cyperaceae% and Ghave the same direction which is opposite to the scales for the other three. Gray bardisappearing of wooden plants, which has different cause from these dry episodes.

Gramineae are C4 type (Chmura and Aharon, 1995; Li et al.,2005). In the meantime, AP% has opposite trends to Gramineae%and Cyperaceae% especially when AP% is >40% (Fig. 8). Tree andshrub are C3 plants with the lightest d13C value. Therefore, whenAP% increases and Gramineae% decreases, the d13CTOC becomeslighter; and vice versa. Furthermore, two periods in Fig. 9 needto be paid attention: (1) Between 17,500 and 13,500 yr BP, and(2) after 6000 yr BP. In the first period, AP% was strongly fluctu-ated, but the d13CTOC did not follow the fluctuation and kept rela-tively light values. The reasons for such a phenomenon are: (1)Gramineae% was low during this period; (2) Cyperaceae% and AP%had opposite trends.

Although family Cyperaceae can contain both C3 and C4 plants,Wu (2009) investigated the Cyperaceae in Taiwan and found thatC4 plants accounted for only 35%. Previous studies also concludedthat C3 type Cyperaceae species were dominant in mountainouswater environment (e.g., Tieszen et al., 1979; Chmura and Aharon,1995; Rajagopalan et al., 1999; Malamud-Roam and Ingram, 2004;Larridon et al., 2011). Hence, family Cyperaceae in Toushe peat corecontains mainly C3 grass plants. In the peat bog, grass and tree/shrub were competing plants. When tree and shrub retreated,grasses became dominant, resulting in peat development; and viceversa. In the case when AP% decreased (retreat of tree/shrub), theCyperaceae% increased, so that the enrichment of d13CTOC due to de-crease of AP% would be turnover by increase of Cyperaceae%. Such aphenomenon was also clearly showed during Holocene especiallyafter 6,000 yr BP. Nevertheless, the d13CTOC record of the Toushepeat core serves as a good indicator of changes in C3/C4 plant ratio,being lighter d13CTOC reflecting enhanced C3/C4 plant ratio.

4.4. dD of n-alkane

Although the use of D/H ratio in n-alkanes is a relatively newapproach in paleoclimate studies, the large variation range of D/H ratio in plants has advantage to address environmental changes(Sternberg, 1988; Sessions et al., 1999; Xie et al., 2000). Correlationof n-alkane dD to climatic condition and the mechanism of their

ramineae% in the Toushe peat core. Note that the scales for d13CTOC and Gramineae%s denote dry climatic episodes. The shaded zone in the top of the core indicates

Fig. 9. Climate interpretations reflected by the pollen and d13CTOC records of the peat sequence and comparisons with GRIP ice core and Chinese speleothem records. Shadedbands indicate cold periods. The dot symbols on the bottom of the plot denote 14C dates. LH, MH, EH, YD, H1, H2, H3, LGM and Pre-LGM are the periods discussed in the text.

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isotopic fractionation under different environments are not wellknown. Previous studies have shown that hydrogen isotope ratiosof plants are strongly influenced by environmental condition andbiochemical process, especially hydrological cycle (meteoric wateravailability and isotopic differences) (e.g., Hassan and Spalding,2001; Li et al., 2001). In general, dD and d18O of plants are posi-tively correlated with the isotopic signals in precipitation (Saureret al., 1997). However, Loader and Hemming (2004) found thatstrongly positive correlation between dD in precipitation fromthe local GNIP (Global Network of Isotopes in Precipitation) sta-tions and pollen dD, but negative correlation between d18O in pre-cipitation and pollen d18O for raw Pinus sylvestris pollen collectedfrom the European spatial study network. Although the cause ofthe opposite correlations is unclear, one hypothesis is that discrep-ancy of the biochemical fractionations involved in the respectivephotosynthetic pathways for H and O (Loader and Hemming,2004). Nevertheless, dD of plant components is positively corre-lated with the dD of precipitation in the European area, which in

turn, is positively correlated with air temperature (Dansgaard,1964).

In a recently study, Bi et al. (2005) measured dD and d13C of al-kanes on 26 plants including C3, C4 and CAM types from Guang-dong of China. They found that both dD and n-alkane chainlength distribution are not diagnostic discriminators for C3, C4and CAM plants, in contrast to d13C (Bi et al., 2005). More interest-ingly, they demonstrated that dD and d13C of n-alkanes from C4plants revealed a strongly negatively linear relationship whilethese for trees showed a positive relationship. Therefore, althoughwe are currently unable to fully understand the mechanism of thedD of n-alkanes responding to climatic conditions, a positive corre-lation between n-alkane dD with the rain dD, as well as the nega-tive relationship between dD and d13C of alkanes for C4 plantshave been observed.

Fig. 8 exhibits that a negative correlation between the dD of C27and C29 n-alkanes and d13CTOC exists. Molecular distribution ofn-alkane indicate that sources of n-alkane deposited in the peat

174 H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176

samples are terrestrial plant (input of aquatic plant is small). Theanti-correlations of the dD and d13CTOC were pronounced whenGramineae was dominated, implying the Gramineae was mainlyC4 type. Furthermore, the lighter dD of C27 and C29 n-alkanes re-flects colder air temperature. Hence, heavier d13CTOC, abundantGramineae, and anti-correlation of the dD and d13CTOC all pointedout arid and cold climates during 4800–4900, 9,350–9,610,17,200–17,900 and 20,830–22,760 yr BP (Fig. 8).

From the discussion on detailed pollen assemblage, d13CTOC anddD of C27 and C29 n-alkanes in the Toushe peat core, we are able touse AP cold% and AP warm% as well as dD of C27 and C29 n-alkanesto reflect temperature change, Gramineae% and Cyperaceae% asindicators of wetness change, and d13CTOC for variation of C3/C4plant ratio.

5. Climatic reconstruction and comparison

Based on the high-resolution pollen and d13CTOC records withaccurate AMS 14C ages of the Toushe peat core, we will reconstructdetailed climatic conditions in terms of temperature and precipita-tion during the past 30 Kyrs in central Taiwan. This detailed cli-mate record will allow us to compare rapid climatic changessuch as Heinrich events, Younger Dryas, deglaciation, and Holoceneabrupt events with other global climatic records. Fig. 9 shows thecomparison of Toushe records with the GRIP ice core record(NorthGRIP Members, 2004) and Chinese speleothem records(Wang et al., 2001; Yuan et al., 2004).

5.1. Pre-Last Glacial Maximum (Pre-LGM)

Before 24,000 yr BP, Alnus was the dominant tree (Figs. 4 and 5),and grasses were generally in low abundance (Fig. 7), and thed13CTOC values were relatively light around �26‰ to �28‰ excepta few short enrichment excursions. Family Alnus tree prefers dampconditions and thrives in swamps and marshlands. The best cli-matic conditions for Alnus trees are cool and moist with full sun-light. With very high Alnus% and a decreasing trend from28,000 yr BP toward 24,000 yr BP, and very low Gramineae%, wemay conclude that the climatic condition during 28,000–24,000 yr BP was cold and relatively wet with C3 dominated vege-tation. A strong decrease in Alnus% and significant increase inGramineae% accompanied with enrichment of d13CTOC occurred be-tween 29,500 and 28,000 yr BP, reflecting an arid episode. This dryand cold episode matches the Heinrich event 3 (H3) (Heinrich,1988; Hemming, 2004) (Fig. 9). Cyperaceae% was also increasedin this interval, perhaps due to the retreat of Alnus trees, but notwetter condition. Nevertheless, the vegetation in Toushe Basin ap-peared to be mainly woodland pre-LGM.

5.2. LGM

The time interval of 24,000–17,500 yr BP was corresponding tothe Last Glacial Maximum. During this interval, Alnus% dropped to<20% and Ilex% increased. The increased Ilex% was probably associ-ated with woodland clearance as it took advantage of the diminish-ing competition from other forest trees. On the other hand,Gramineae% was strongly increased with strong enrichment ofd13CTOC (Fig. 9), indicating cold and dry climates. The most predom-inant feature is the two strongly positive d13CTOC excursionsaround 23 and 18 Kyr BP. The aridity caused diminishing of notonly woodland, but also C3 grasses. The depletions of d13CTOC be-tween these two positive excursions were due to increase in Cyper-aceae% and AP%. Therefore, even during the cold and dry LGM,three relatively wet episodes occurred at ca. 21.5, 20 and18.5 Kyr BP shown by peaks of Cyperaceae% (Fig. 9).

Heinrich event 2 (H2) was in the beginning of the LGM withages ranging from 24,000 to 22,000 yr BP corresponding to the coldperiod shown in GRIP ice core (Bond et al., 1992; Bond and Lotti,1995; Heinrich, 1988; Hemming, 2004; Vidal et al., 1999), andthe age of H2 in the Hulu Cave record was significantly offset withthe GRIP ice core record (Wang et al., 2001) (Fig. 9). During the H2period, Cyperaceae% was very low and Gramineae% was strongly in-creased with heavy d13CTOC values, reflecting dry climate in thestudying area. The slightly increased AP warm% seemed to matchslightly temperature increase shown in the ice core record. Ourpeat record supports the age of H2 to be 24,000–22,000 yr BP.The climatic condition during the LGM was the worst in the study-ing area over the past 30 Kyrs, which is different from the Chinesespeleothem records. The latter records show the heaviest d18O val-ues which reflect the worst climates during Heinrich event 1 (H1).The discrepancies on the age of H2 and the intensity of LGM con-dition between our peat record and the Chinese speleothem re-cords call for further investigation.

5.3. Between LGM and Holocene

After the LGM, Gramineae% in the peat core was sharply de-creased and kept low for the next 6000 yrs until 11,500 yr BP.The d13CTOC gradually decreased from �25‰ at 17,500 yr BP tothe minimum value of �30.23‰ at 11,000 yr BP, indicating C3plants occupied in the basin (Fig. 9). On the other hand, Cypera-ceae% (C3 dominated grass) and AP% (tree and shrub) were com-peting throughout 17,500 to 11,500 yr BP. Three peaks ofCyperaceae% at around 17, 16 and 14 Kyr BP corresponding to threeminimals in AP cold% and AP warm%, indicating relatively wet epi-sodes. High AP% at 16.5, 15 Kyr BP and from 13.5 to 11.5 Kyr BP re-flected the woodland returns in the basin (Fig. 9). During theHeinrich event 1 (H1), climate in Toushe Basin was not extremelydry, probably wetter and warmer than the conditions during theBølling-Ållerød interval (14.5–13 Kyr BP). Very high AP cold%(>60%) and very low Cyperaceae% (<5%) during the Younger Dryas(YD) period illustrate the cold and dry climates.

In the Toushe peat core, the records show dry and cold climatesduring H3, H2, H1 and YD periods, agreeing with the Chinese spe-leothem d18O records in general which interpret weak EASM undercold temperature of the Northern Hemisphere during Heinrichevents led to dry monsoonal climates (Wang et al., 2001). However,Pausata et al. (2011) argued that the heavier speleothem d18O sig-nals in Chinese speleothem records during the Heinrich intervalswere due to isotopically heavy moisture from Indian monsoonrather than rainfall decrease in eastern China. The moisture sourcefor central Taiwan has been mainly from western Pacific and SouthChina Sea which are also the moisture supply to eastern China. TheToushe record indicates that the climate was indeed drier duringthe period of Heinrich and YD events occurred. We believe thatboth decreasing precipitation and enriched precipitation d18O ap-peared during the cold Heinrich and YD events.

5.4. Holocene

The Holocene subtropical condition started from 11.5 Kyr BPand continued afterward. During the early Holocene (EH), the APwarm% increased significantly and AP cord% dropped to very lowexcept at 10.5 and 9 Kyr BP, reflecting warm temperature (Fig. 9).Gramineae% showed three peaks at �11, �10 and 9.2 Kyr BP,whereas Cyperaceae% was low, indicating dry climatic conditionsaround these three episodes. Between 9.7 and 9.2 Kyr BP, a strongpositive d13CTOC excursions occurred, demonstrating C4 dominatedvegetation under arid environmental condition. After a sharp de-crease in Gramineae% at 9 Kyr BP, the Gramineae% generally in-creased from 9 Kyr BP to about 8 Kyr BP with elevated

H.-C. Li et al. / Journal of Asian Earth Sciences 69 (2013) 166–176 175

Cyperaceae%. The peat development in the basin was prevailing andthe woodland was diminished during this interval. In the first halfof the middle Holocene (MH), climate was generally warm withmoderate moisture, shown by the relatively high AP warm%(Fig. 9). During the second half of the MH, strongly fluctuatedGramineae% might reflect unstable climates. For instance, coldand dry climate occurred at 6 Kyr BP, but turned to warm andwet within next 50 yrs. A sharp peak of Glochidion% at ca. 5.2 KyrBP pointed out a warm climate. After 5 Kyr BP, Toushe Basin wascompletely occupied with C3 grasses, showing increased Cypera-ceae% and decreased Gramineae% under relatively wet conditions.Unlike the GRIP ice core record and Chinese speleothem records,the Toushe peat record reveals that the Holocene climate wasstrongly fluctuated and the mean precipitation during the middleHolocene was even lower than that during late Holocene (LH).

6. Conclusions

Detailed analyses on the pollen assemblages and comparisonwith geochemical records in the Toushe peat sequence allow usto obtain that: (1) the total arboreal pollen percentage is mainlycontrolled by the variation of tree and shrub vegetation under coldconditions, namely AP cold% (such as Alnus%, Ilex% and Slix%). TheAP cord% and AP warm% reflect temperature changes. (2) Grami-neae and Cyperaceae are two major herb pollen contents, and theirpercentage variations can be used as wetness changes. (3) Thed13CTOC fluctuations mimic Gramineae% trends and indicatechanges in C3/C4 plant ratios. (4) A negative correlation betweenthe dD of C27 and C29 n-alkanes and d13CTOC exists, providing reli-ability of the d13CTOC record. (5) The TOC record in the peat se-quence is not a climatic proxy. With the new insights on thepollen assemblages and high-resolution d13CTOC record, a detailedclimatic reconstruction for Toushe Basin in the central Taiwan overthe past 30 Kyrs has been carried out.

The climate in Toushe Basin was cold and damp with woodlandvegetation during pre-LGM, and turned to cold and dry after 25 KyrBP. In Toushe Basin, climatic conditions during LGM were theworst over the past 30 Kyrs, especially around 23 and 18 Kyr BPwhen the woodland was diminished and C4 grass was dominated.Such a worst LGM condition was not presented in the Chinese spe-leothem records. During the deglaciation, short durations of rela-tively wet condition could be found at 17, 16 and 14 Kyr BP,which pointed out that climate in the studying area correspondingto H1 was not extremely dry.

Cold and dry climates were prevailing during 29.5–28, 24–22,17–15 and 13–11.5 Kyr BP, corresponding to Heinrich Events 3,2, 1, and Younger Dryas, respectively. Dry climates during theHeinrich events reveal weakening of the EASM. Our results donot agree with the interpretation of Pausata et al. (2011) which im-plied that the East Asian monsoon precipitation might not decreaseduring Heinrich events.

The Holocene climate in Toushe Basin was unstable and differ-ent from the North Atlantic climate shown by the GRIP ice core re-cord and the EASM climate shown by the Chinese speleothemrecords. During the early Holocene, dry climate was prevailing at�11, �10, 9.7–9.2 and �8 Kyr BP; whereas wet condition appearedat 10.3, 9.8, 9–7.5 Kyr BP. In the first half of the middle Holocene,climate kept warm and moderate wet. Many dry events appearedin the second half of the middle Holocene after a significant coldand dry event at 6 Kyr BP. An outstanding warm index appearedin the pollen assemblages at 5.2 Kyr BP, reflecting a warm climatein Toushe Basin. After 5 Kyr BP, the climate turned to cold quickly,and tree/shrub vegetation disappeared completely with thereplacement of C3 grasses in the basin. With the increased Cypera-ceae% and decreased Gramineae% as well as the light and depleting

trend of d13CTOC, climate became wetter from 4.5 Kyr BP toward1.7 Kyr BP in Toushe Basin.

Acknowledgments

HC Li thanks for funding support from National Science Councilof Taiwan Grant: NSC 98-3114-E-002-015, 99-2116-M-002-030and 100-3113-E-002 -009. We thank Mr. C.Y. Lee and other col-leagues for coring and pollen analysis.

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