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Quaternary Science Reviews 30 (2011) 3413e3422

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Quaternary Science Reviews

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Variations in monsoonal rainfall over the last 21 kyr inferred from sedimentaryorganic matter in Tung-Yuan Pond, southern Taiwan

Tien-Nan Yanga,*, Teh-Quei Leea, Philip A. Meyersb, Sheng-Rong Songc, Shuh-Ji Kaod, Ludvig Löwemarke,Rou-Fei Chena,f, Huei-Fen Cheng, Kuo-Yen Weic, Cheng-Wei Fanh, Liang-Jian Shiaua,g, Hong-Wei Chiangc,Yue-Gau Chenc, Min-Te Cheng

a Institute of Earth Sciences, Academia Sinica, Taipei 115, TaiwanbDepartment of Earth and Environmental Sciences, The University of Michigan, Ann Arbor, MI 48109-1005, USAcDepartment of Geosciences, National Taiwan University, Taipei 106, TaiwandResearch Center for Environmental Changes, Academia Sinica, Taipei 115, TaiwaneDepartment of Geological Sciences, Stockholm University, Stockholm 10691, SwedenfDepartment of Geology, Chinese Culture University, Taipei 111, Taiwang Institute of Applied Geosciences, National Taiwan Ocean University, Keelung 202, TaiwanhDepartment of Earth and Environmental Sciences, National Chung Cheng University, Chia-Yi 621, Taiwan

a r t i c l e i n f o

Article history:Received 17 January 2011Received in revised form28 August 2011Accepted 30 August 2011Available online 19 September 2011

Keywords:Atomic TOC/TN ratioEast Asian summer monsoonHolocene Thermal OptimumPaleorainfall intensityTaiwanYounger Dryas

* Corresponding author. Tel.: þ886 2 27839910x41E-mail address: tnyang@earth.sinica.edu.tw (T.-N.

0277-3791/$ e see front matter Crown Copyright � 2doi:10.1016/j.quascirev.2011.08.017

a b s t r a c t

Changes in paleorainfall intensity linked to the strength of the East Asian (EA) summer monsoon since21 cal kyr BP are inferred from the organic matter contents of a 15-m sediment core from Tung-YuanPond in southern Taiwan. High total organic carbon/total nitrogen (TOC/TN) values in association withincreased TOC content suggest that more soil-derived material containing terrestrial organic matter (OM)was delivered to the lake during periods of increased runoff associated with extensive precipitation thatresulted from intensified summer monsoons, whereas low values indicate OM possessing a dominantalgal origin during weakened summer monsoons. Rainfall intensity in terms of the proportion ofterrestrial OM was high in four periods: the last deglaciation (w17.2 to w12.2 ka), the early Holocene(w10.6 to w8.6 ka), the middle Holocene Thermal Optimum (w7.7 to w5 ka) and the late Holocene(w4.2 to w2 ka), whereas it was low in the intervening time periods. High TOC/TN values coincide withpeak values of summer insolation, and thus the strongest EA summer monsoon during the early andmiddle Holocene; small drops in these ratios correspond to increasing and decreasing solar radiation inthe deglacial period and the late Holocene, respectively. The four intervals with low TOC/TN ratios, aswell as episodic drops of the ratios during the deglaciation and the early and late Holocene areconcordant with the late last glacial (w21 to w17.2 ka), the Oldest (w14.8 ka), the Older (w13.3 ka) andthe Younger Dryas (w13 to w11.5 ka), the 8.2 cold event (w8.6e7.7 ka) and a late Holocene cold event(w5e4.2 ka), and suggest a weakened EA summer monsoon during these times. Moreover, highfrequency hydrological variability occurred during the early Holocene, heavy rainfall persisted during themiddle Holocene, and precipitation intensity generally diminished after w5 ka. The Tung-Yuan Pondsediment record indicates that the TOC/TN ratio can be used as a paleorainfall intensity proxy to tracevariations in the EA summer monsoon strength in other small lakes.

Crown Copyright � 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Unpredictable variations in precipitation, which can lead eitherto flooding or to limited water availability, create serious challengesfor human society, especially for the dense populations in easternand southeasternAsia. It is therefore important tobetterunderstand

2; fax: þ886 2 27839871.Yang).

011 Published by Elsevier Ltd. All

modern and past rainfall variations in order to improve predictionsof the impacts of future climate changes. Toward this end, manystudies of the monsoon-related changes in precipitation in easternAsia have been performed (e.g., Feng et al., 1999; An, 2000; Wanget al., 2001; Xiao et al., 2006; Chen et al., 2007; Zhou et al., 2007;Yang and Scuderi, 2010). These studies generally reveal past mois-ture patterns that were paced by the orbital evolution of solaractivity. In addition, some other studies describe the multi-decadalrainfall variation in Asia and North America from lacustrine records

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T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e34223414

during the middle Holocene (e.g., Meyers, 2002; Zheng et al., 2007;Kirby et al., 2010). In view of increasing global warming, it is timelyto improve our understanding of the nature of past episodes ofintense precipitation in eastern Asia, where the East Asian (EA)summer monsoon produces heavy rainfall.

The EAmonsoon system consists of thewet summer and the drywinter monsoons, which are associated with air masses originatingfrom the tropical Pacific and Mongolia in warm and cold seasons,respectively (Fig. 1). The EA monsoon system has significantimpacts primarily on the amount and distribution of precipitationin eastern Asia on both local and regional scales and, in turn, isimpacted by a number of factors. Intensification of the summermonsoon has resulted from strengthened insolation since the lastglacial maximum (LGM), and it has triggered an increase inprecipitation over much of eastern Asia (Yi et al., 2003; Chan andZhou, 2005; Herzschuh, 2006; Kubota et al., 2010; Zhou, 2011).Based on instrumental data, the EA monsoon is evidently alsoinfluenced by the inter- tomulti-decadal climatic modulation of theEl Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscil-lation (PDO) (e.g., Zhou et al., 2006; Zhou, 2011). Moreover, the EAsummer monsoon displays interdecadal precipitation anomalies inEast China (Jiang et al., 2008).

It has been recently documented by Webster et al. (2005) thatboth the frequency and intensity of tropical cyclone increase insynchrony with global warming. In addition, the interannual vari-ability of tropical cyclones rainfall is primarily modulated by ENSOevents (Kubota and Wang, 2009). Furthermore, Fedorov et al.(2010) describe a positive feedback between tropical cyclones andthe upper-ocean circulation in the tropical Pacific Ocean that mayhave been essential for maintaining warm, El Niño-like conditionsduring the early Pliocene (w 5e3 Ma). It is likely that more frequentand more intense episodic typhoons resulted in the NorthwesternPacific Ocean from the increase of insolation in terms of the EAsummer monsoon since the last glacial maximum (LGM).

In this paper, a continuous record of hydrological variations intropical southern Taiwan for the interval between 21 ka and 2 ka isreconstructed from changes in the organic matter source compo-sition of a core of lake sediment. Lakes generally have highersedimentation rates and greater contributions of land-derivedmaterials than the oceans, so short-term processes that affectorganic matter delivery and burial are amplified in their sedimentrecords (Meyers, 2003). The atomic ratio between total organic

Fig. 1. (A) Location of Tung-Yuan Pond (TYP) on Taiwan Island. Origins and the flow paths ofThe thick dash line designates the northernmost extent of the Asian summer monsoon. DataGeomorphology of the watershed. The drilled site of core TYP-B is indicated as an asterisk

carbon and total nitrogen (TOC/TN) of organic matter is littleaffected by decomposition and can be a reliable identifier of organicmatter (OM) sources despite some small early diagenetic alter-ations (Meyers, 1994). TOC/TN ratios larger than 20 and rangingbetween 4 and 10 are indicative of OM produced by vascular landplants and lacustrine algae, respectively. It is therefore reasonablefor a mixture of both organic end-member sources to yield sedi-ment TOC/TN ratios between 10 and 20. Based on studies of Meyers(1994) and Lou and Chen (1997), Yang et al. (2011) suggested thatthe TOC/TN ratio of 10 can be used as the boundary value betweenthe two end-members to distinguish settling particles in YuanyangLake, Taiwan, derived from autochthonous (<10) and allochtho-nous (>10) organic matter sources. They employed this distinctionto show a high correlation between greater rainfall and increasingamounts of pedogenic matter delivered to the lake. An averageTOC/TN value of 12 over two years shows that settling particles aredominated by allochthonous materials, implying that the TOC/TNproxy might be useful for reconstructing episodically enhancedrainfall in the geological past. We therefore reasoned that it mightbe feasible to reconstruct hydrological variability in terms ofprecipitation amounts and intensity since the last glacial usingTOC/TN values to identify changes in the dominant sources oforganic matter in lacustrine sediments.

The purposes of this study are: (1) to test whether the TOC/TNratio can be applied as a reliable rainfall proxy for deliveringpedogenic material into Tung-Yuan Pond, southern Taiwan, assuggested by the study of Yang et al. (2011); (2) to reconstruct therecord of paleoprecipitation variations over the last 21,000 years insouthern Taiwan; and (3) to reconstruct the evolution of the EAmonsoon in terms of moisture transport in the study area since thelast glaciation.

2. Study area

Situated at the border between the Southeast Asian continentand the northwestern Pacific Ocean, Taiwan Island is under theinfluence of the EA monsoon (Fig. 1). A major climate characteristicin Taiwan is the Pacific typhoon season, with tropical cyclonesusually forming during late summer and early autumn (Chen et al.,2007). Episodic tropical cyclone-related precipitation results inenhanced runoff and high sediment discharge in Taiwanese rivers(Chen et al., 2006; Galewsky et al., 2006; Goldsmith et al., 2008).

winter and summer monsoons are indicated by open and dashed arrows, respectively.from marine core MD98-2181 (MD81) (Stott et al., 2004) are compared in the study. (B).

Table 1Radiocarbon dates and corresponding calibrated ages in sediment core TYP-B fromTung-Yuan Pond.

Laboratorycodea

Depth(cm)

14C age(yr BP)

2s cal. agerange (yr BP)

Sampletypeb

Calibrationdata set

NTU-4179 162.5 2170� 40 2186.5� 131.5 Wood INTCAL04.14cNTU-4176 178.5 2190� 60 2191� 148 Wood INTCAL04.14cNTU-4180 266.5 2640� 40 2783� 61 Wood INTCAL04.14cNTU-4184 468.5 4560� 60 5183.5� 146.5 Wood INTCAL04.14cNTU-4195 470.5 4610� 60 5263� 210 Wood INTCAL04.14cNTU-4181 532.8 5640� 50 6408� 103 Wood INTCAL04.14cNZA-22545 836. 5 8336� 40 9361.5� 107.5 Woodc INTCAL04.14cNZA-29500 868.5 8260� 35 9228.5� 103.5 PD INTCAL04.14cNZA-29434 980.5 9229� 35 10,386� 118 PD INTCAL04.14cNZA-22546 982.6 9257� 40 10,418� 149 Wood INTCAL04.14cNZA-29498 1134.5 10,384� 40 12,234� 162 PD INTCAL04.14cNTU-4185 1140.5 10,400� 90 12,324.5� 334.5 Wood INTCAL04.14cNZA-29501 1180 11,941� 45 13,816.5� 112.5 PD INTCAL04.14cNTU-4196 1296 13,200� 110 15,653.5� 440.5 Wood INTCAL04.14cNTU-4187 1335 14,200� 110 16,966.5� 496.5 Wood INTCAL04.14cNZA-30428 1420.5 19,356� 70 23,018� 376 BS INTCAL04.14c

a Prefix NTU and NZA indicates the National Taiwan University and New Zealand,respectively.

b Organic materials of wood, plant debris (PD), and bulk sediment (BS) are usedfor dating.

c A reworked-wood is indicated by the anomalously old 14C age, and this samplewas discarded in the age versus depth model.

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e3422 3415

Furthermore, in a two-year observation on the settling particle fluxin subalpine Yuanyang Lake, northern Taiwan, Yang et al. (2011)show that heavy precipitation induced by typhoon events signifi-cantly increased the annual accumulation rate as a consequence ofan increased denudation rate of the lake catchment. Lee and Liew(2010) and Lee et al. (2010) used pollen assemblages to infer thatclimate in southern Taiwan followed the solar-radiation trend sincethe last glaciation: cold during the LGM and the early deglaciation,warming starting in the last termination, highest temperaturesduring the Holocene Thermal Optimum (HTO), and cooling againbeginning in late middle Holocene. However, an accompanyingunderstanding of past precipitation fluctuations as a consequenceof moisture transportation from the tropical northwestern Pacific islacking.

In southern Taiwan, average monthly air temperatures ininstrument records dating from 1897 have seasonally variedbetween 20.5 and 28.0 �C. The annual pattern of average monthlyrainfall, ranging from 20.3 to 510.1 mm, exhibits an obvious sea-sonality with low and high precipitation during cold and warmintervals, respectively (Wang et al., 1994). Enhanced precipitationresulting from tropical cyclones that occur in summer and early fallcontributes more than 85% of the annual rainfall in southern Tai-wan (Wang et al., 1994). The precipitation record of the past 100years for southern Taiwan, however, displays a decreasing trend,whereas temperature has become warmer (Hsu and Chen, 2002).

Tung-Yuan Pond (22�100 N, 120�500 E; 360 m above sea level),2�104 m2 in area and <2 m in water depth, is located in thenorthern edge of a hilly basin in southern Taiwan (Fig. 1). Thedrainage area covers around 94�104 m2 and ranges from 360 m to500 m a.s.l. (Fig. 1). The lake is host to some aquatic macrophytes,while the surrounding terrestrial area is covered by lowland ever-green broad-leaved forest (Hsieh et al., 1996). Changes in thisvegetation were inferred from the pollen content of the sedimentsof Tung-Yuan Pond by Lee and Liew (2010) and Lee et al. (2010) andused in their reconstructions of the post-glacial climate of southernTaiwan. The substrate rocks are mainly shale, siltstone and argillite(Ho, 1986). It was an undisturbed lake for the last 21,000 years,except for the most recent 70 years (Chen, 2000). The lake levelbecame artificially managed starting in the early twentieth centuryto provide a water reservoir for a nearby village, and the processmixed the upper meter or so of sediments. For conservativereasons, this work is limited to the naturally archived records below160 cm.

3. Materials and methods

3.1. Core sediment and organic samples

Thew14.5 m-long core TYP-B, which consists of mud layers andinterbedded peat-like layers, was taken from the center of Tung-Yuan Pond in 2004 (Fig. 1). Two-cm thick sediment samples werecollected at 2e5 cm intervals and stored at 4 �C until analysis. Analiquot of each sample was freeze-dried, ground, acidified toremove carbonate, and then subjected to analysis of total organiccarbon (TOC) and nitrogen (TN), and the organic carbon isotopecomposition (d13Corg), following the procedures described in Chenet al. (2009).

To constrain both end-members of the conservative TOC/TNratio of total organic matter, both aquatic and pedogenic substancesare also studied. Particulate organic matter (POM) suspended in thelake surface water was collected every month from July 2004through June 2005 and filtered immediately using a precombusted(450 �C) Whatman glass fiber filter (GF/F, 0.7 mm in pore size,47 mm in diameter), as described in Yang et al. (2007). Five surfacesoils were sampled and specimens of 5 species of common

terrestrial vascular plants were harvested during the collection ofPOM. Organic samples were freeze-dried and decarbonated beforemeasurements of TOC and TN, and d13Corg.

3.2. Age determination

Organic carbon from fifteen samples of charcoal obtained fromwood pieces and plant debris and from one bulk sediment near thecore bottom was used for conventional and AMS 14C dating at the14C Dating Laboratory of the Department of Geosciences, NationalTaiwan University, and the Rafter Radiocarbon Laboratory, Instituteof Geological and Nuclear Sciences, New Zealand, respectively(Table 1). Both types of radiocarbon dates were calibrated usingCALIB Rev 5.0 (Stuiver et al., 2005).

3.3. Measurements of TOC, TN, TC, and d13Corg

Analyses of TOC, TN and d13Corg of organic matter-containingsamples followed the procedures described in Ku et al. (2005)and Chen et al. (2009), respectively. In brief, an aliquot of decal-cified sample was wrapped in a tin capsule and then combustedusing a ThermoQuest EA1110 elemental analyzer to measure theTOC and TN concentrations, thus yielding the atom/atom TOC/TNratio. For measuring total carbon (TC) content, an aliquot of groundbulk sediment, without carbonate removal, was wrapped in a newtin capsule and processed in the same way. The precisions of themeasurement are�0.2 wt%, 0.3 wt%, and�0.6 wt% for TOC, TN, andTC, respectively.

To determine organic carbon isotopic ratios, another aliquot ofeach decalcified sample was wrapped in a new tin capsule, com-busted to generate CO2 in the elemental analyzer, and then deliv-ered on-line into a Finnigan Delta Plus mass spectrometer. Theresults are expressed in d-notation relative to the internationalstandard VPDB (Coplen, 1996). The d13Corg value is calculated fromthe measured carbon isotope ratios of the sample and standardgases as:

d13Corg (&)¼ [(Rsample� Rstandard)/Rstandard]� 103

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e34223416

where d13Corg is given in parts per thousand, or per mil (&), ofthe difference between the 13C content of the sample and that ofthe standard, and R is the mass 45/44 ratio of the sample or stan-dard gas. The precision of d13Corg measurement is �0.08&.

3.4. Measurements of n-alkanes

Molecular biomarkers, e.g., n-alkanes, can be used to comple-ment organic matter source information from the TOC/TN ratio andother bulk source proxies (e.g., Meyers, 2003; Mead et al., 2005). Inparticular, the aquatic proxy ratio (Paq) compares the mid-chain n-alkanes (C23, C25) to the long-chain n-alkanes (C29, C31) in sedi-ments to differentiate vascular plant sources between aquaticmacrophytes and terrestrial plants (Ficken et al., 2000).

Paq¼ (C23þ C25)/(C23þ C25þ C27þ C29)

In a study on tropical modern plants, Ficken et al. (2000)established that Paq< 0.1 corresponds to terrestrial plants, 0.1e0.4to emersed macrophytes and 0.4e1 to submerged/floatingmacrophytes.

Approximately 1.5w4 g freeze-dried sediment subsamples wereextracted with a mixture of dichloromethane and methanol (6:4 V/V) in an accelerated solvent extraction DIONEX ASE 200. Thesurrogate standard of 5a-cholestane (Fluka) was added to eachsample before extraction. The extracts were concentrated andreduced in volume to approximately 1 ml by a TurboVap (CaliperLifesciences) under a stream of nitrogen. Extract concentrates werefractionated using a silica gel column. The n-alkane fraction waseluted with 3w4 ml hexane and reduced in volume to 1 ml. Aninternal standard of squalene (Fluka) was added to the concen-trated 1-ml hexane fractions prior to instrumental analysis. All thesamples were analyzed on a Hewlett Packard 6890 gas chromato-graph coupled to a Hewlett Packard 5973 flame ionization detector.A HP-5MS column was used (30 m� 0.25 mm i.d. with 5% phenyland 95% methylpolysilarylene). The n-alkanes were identified andquantified by using calibration standards that contained knownconcentrations of 5a-cholestane, squalene, and all of the n-alkanes(Dr Ehrenstorfer, Reference Materials) of interest in this study (n-C10wn-C35). The concentrations reported in this study were cor-rected by the recoveries of the standards.

4. Results and discussion

4.1. Lithology and sedimentation rate of the TYP-B core

The sediments that have accumulated in the Tung-Yuan Pondbasin since the end of the last glacial maximum exhibit alternationsof layers of gray-green or dark brown muds and dark gray mudsthat contain vascular plant debris. The latter layers have beendescribed as “peat” by Lee and Liew (2010) and Lee et al. (2010)because of their richness of plant debris. Generally, the lightercolored mud layers coincide with low concentrations of TOC, whilethe plant-debris laden darker sections contain up to 30% TOC(Fig. 2). 14C dating on dispersed organic carbon extracted from bulksediment might contain fossil carbon and thus yield a significantlyolder age than the true depositional age (Kao et al., 2008). Weconsequently exclude the bulk sediment-14C date from the bottomand the reworked-wood-14C date from 863.5 cm (Table 1), and weignored one date obtained from the upper sediments (w160 cm)that were disturbed by human interference. The age versus depthmodel of core TYP-B shown in Fig. 2B indicates continuous sedi-mentation in the deepest part of the lake basin from w21,000 to2000 calibrated (cal) years before the present (0 cal yr BP¼ 1950AD). Hereafter, only calibrated kyr age before present (ka) is used in

this work. Sedimentation rates were higher during the Holocene(w0.8 mmyr�1) than after the last glacial maximum(w0.5 mmyr�1) and during Termination I (Fig. 2C). Each 2-cm thicksample has a mixing signature between 25 yrs and 45 yrs duringand prior to the Holocene, whereas the average time resolution fordiscrete samples is about 60 yrs and 155 yrs in both the sameintervals, respectively. Consequently, this sediment record hasadequate time resolution to reveal multi-decadal variations inprecipitation.

4.2. Organic matter sources for Tung-Yuan Pond sediment

Sedimentary materials in the Tung-Yuan Pond originate fromboth aquatic and pedogenic sources, which can be distinguished bycomparisonwith TOC/TN and d13Corg end-member values. The TOC/TN values we used to identify algal input are based on (1) POMcollected monthly from the surface layer of the lake, which rangefrom 4.7 to 10.2, (2) marine phytoplanktonw7 (Redfield, 1934), (3)both freshwater and marine algae, which typically have valuessmaller than 10 (Meyers, 1994), and (4) surface soils from thewatershed range from 9.3 to 12.3. A mixed signature of the TOC/TNindicates that the sedimentary OM originates variably from bothalgae and land vascular plants (Fig. 3). The TOC/TN values below 9in our sedimentary materials consequently can be attributed toalgal origins, though it might also reflect some contribution frominorganic ammonium retention on minerals in the sediments(Müller, 1977). Our assumption that values larger than 9 are fromsurrounding terrestrial OM input is based on our measurements ofleaves of vascular plants collected in southern Taiwan, which rangefrom 10.7 to 35.6 (Table 2), as well as terrestrial plants cited to havevalues larger than 20 (Meyers,1994; Lou and Chen,1997; Yang et al.,2011). In addition, a two-year sediment trap collection of particlessettling in Yuanyang Lake, Taiwan, shows an average value of TOC/TN of 12 for the total sedimentary mass dominated by allochtho-nous materials (Yang et al., 2011). Moreover, Jasper and Gagosian(1990) recognized OM in marine sediments with high TOC/TNratios as land-derived, while low ratios indicated an algal origin.

4.3. Paleohydrologic proxy variations

We use the three organic matter proxies of TOC and TNconcentrations and TOC/TN atomic ratios to reconstruct the post-glacial paleohydrological history of Tung-Yuan Pond. TOC and TNconcentrations and TOC/TN ratios generally varied together overthe last 21 kyr, suggesting dramatic fluctuations in the relativecontributions of organic matter from algae and vascular plants(Fig. 4). Furthermore, TOC also varied in concert with TC and TOCpredominantly contributed to the total carbon component. Inde-pendent analyses on clayminerals also reveal that the core containsonly a trace amount of carbonate (H.F. Chen, personal communi-cation). It is therefore reasonable that the soils derived from thesiltstone, argillite and slate surrounding the lake have evidentlycontributed only a minor amount of inorganic carbon to the lakesediments. The concentration of TOC ranges from 0.27% to 30.4%,and it varies with the lithological units: high TOC exists in the darkpeaty mud layers, while TOC is low in the gray mud layers (Fig. 4).Based on its lithology and its TOC concentrations, the sediment corecan be divided into twelve downcore intervals that relate toprecipitation fluctuations (Fig. 4). The variations in moisturedelivery are supported by TOC/TN values that range between 3.3and 35.2, indicating sediments derived from either aquatic and landsources as a consequence of changes betweenweaker and strongerprecipitation through time, respectively.

The TOC concentration can also provide important informationabout variations in organic matter delivery because high TOC

Fig. 2. (A) Lithology and percent of total organic carbon (TOC, %), (B) a plot of depth versus cal. age, and (C) sedimentation rate of core TYP-B. Twelve intervals are divided accordingto both lithology and TOC. The age model for the core is formulated by interpolation between the fifteen levels of sediment against their mean calibrated 14C dates shownwith dots,except for the bottom by extrapolation. Error bars shown in (B) indicate 2-sigma range of ages.

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e3422 3417

concentrations are often evidence of organic matter derived fromland plants, which evidently is washed into this lake whenprecipitation is high (e.g., Lou and Chen, 1997; Meyers et al., 2006;Selvaraj et al., 2007). We therefore interpret the correspondencebetween high TOC concentrations and high TOC/TN values asevidence that large inputs of soil material accompanied by organicmatter generated by land plants were delivered into the lake duringperiods of enhanced rainfall. In contrast, little land plant materialwas delivered when rainfall was low. By applying the same

approach, Xiao et al. (2006) suggested TOC as a paleoprecipitationproxy for reconstructing monsoonal precipitation in north-centralChina over the Holocene.

Three other lines of evidence lead us to conclude that pastperiods of intensified runoff repeatedly delivered importantamounts of land-derived OM to Tung-Yuan Pond. First, local rainfallis concentrated during MayeSeptember in association withepisodic typhoon events that contribute w85% of the annualprecipitation in southern Taiwan (Wang et al., 1994). Second,

Fig. 4. Multi-proxies for reconstructing paleohydrological change in southern Taiwan.(A), TOC (%); (B), TN (%); (C), TOC/TN; and (D), TC (%) from core TYP-B for the last 21 ka.Fifteen 14C dates and their respective 2-sigma errors are also presented. Six timeintervals highlighted with color follow the high TOC content see Fig. 2. Intervals for thelast glacial maximum (LGM), BøllingeAllerød (BA), Younger Dryas (YD) and Preboreal(PB) (Stuiver et al., 1995), and Holocene Thermal Optimum (HTO) (Jansen et al., 2009)are represented. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Fig. 3. A plot of d13Corg versus TOC/TN for various organic materials: particulate organicmatters (POM, dots), terrestrial plants (TP, diamonds), soils (triangles), and sediments,which are grouped into two categories, origins of algae (SA, open circles) and of TP(STP, stars) by value of TOC/TN< 9 and TOC/TN> 9, respectively.

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e34223418

sedimentary input increases by a factor of four in the warm season(w2.1 g/m2/d) relative to cold periods (w0.5 g/m2/d) at YuanyangLake, a subalpine lake in northern Taiwan, largely as pedogenicmaterials eroded by episodic typhoon-induced strong rainfallintensity in summer and early autumn (Yang et al., 2011). Third, thehigh TOC concentrations and TOC/TN values are generally accom-panied by a higher sedimentation rate in the Tung-Yuan Pondsediment record (Fig. 2C). These high sedimentation events werelikely triggered by extensive precipitation, such as storms andtyphoons, when climate changed toward warmer intervals thatcorresponded with strengthened EA summer monsoon in theregion.

Low lake water level resulting from higher evaporation thanprecipitation (low effective precipitation) might have occurredduring times of less water vapor in the air in southern Taiwan in thepast. The Paq ratio of the mid-chain n-alkanes (C23, C25) to the long-chain n-alkanes (C29, C31), a complementary proxy to the land-derived composition containing TOC/TN ratio> 9, varies from0.05 to 0.23 (Fig. 5), indicating a fluctuating delivery of organicmatter generated from either terrestrial plants (Paq< 0.1) oremersed aquatic vascular plants (Paq¼ 0.1e0.4) since the lastdeglaciation (Ficken et al., 2000).

4.4. Variations of precipitation and the EA summer monsoon overthe past 21 ka

The evolution of paleoprecipitation and the EA summermonsoon since the LGM and the relations between these two

Table 2Elemental and isotopic compositions of representative land plants growing aroundTung-Yuan Pond. Plant carbon assimilation pathway is shown after its commonname.

Species (common name) TOC/TN(atomic)

d13C (&)

Bidens pilosa Linn. (Spanish needle, C3) 10.7 �29.8Leucaena glauca (L.) Berth (Lead tree, C3) 12.6 �30.7Acacia confusa Merr. (Taiwan hsiang shu, C3) 16.6 �29.1Miscanthus floridulus (Labill.) Warb. ex.

Schum. & Lauterb. (Giant Chinese silver grass, C4)32.3 �12.2

Casuarina equisetifolia Linn. (Horsetail pine, C3) 35.6 �30.9

climate features have been reconstructed by various workers (e.g.,Kutzbach et al., 1998; Wang et al., 2001; Yi et al., 2003; Zhou et al.,2005; Herzschuh, 2006; Liew et al., 2006a; Yi et al., 2006; Yanget al., 2008; Kubota et al., 2010; Lee and Liew, 2010; Lee et al.,2010). The EA summer monsoon strength increased at the end oflast glacial, maximized in the early Holocene and then declined tothe present. In concert with this variation, the local summer (June,July and August) solar radiation at 22� N increased initially from439W/m2 at 21 ka to a peak of 472W/m2 at 9 ka and then after-ward decreased to w450W/m2 in the last two millennia (Fig. 5C).In addition, thewinter (December, January and February) insolationgradient between low (10� N) and high (45� N) latitudes, which isan indicator of the potential of the Mongolian High in affecting thestrength of the EAwinter monsoon, displayed a pattern opposite tothe summer insolation; the highest and lowest difference of203 W/m2 and 191W/m2 existed at 19 ka and 9 ka, respectively(Fig. 5C). The EA winter monsoon results in a cold front associatedwith weak to moderate precipitation in contemporary northernTaiwan during the cold season (Hsu and Chen, 2002; Peng et al.,2010). It might be, however, that in the past strengthening of theEA winter monsoon induced more water vapor to reach southernTaiwan.

The TYP-B sediment record indicates a fluctuation of paleo-precipitation in terms of rainfall intensity index as inferred fromthe proportion of organic matter from terrestrial plants since theLGM. The proportion of terrestrial plant OC (OM-TP%) is calculatedby the equation:

(TOC/TN)BS¼ (TOC/TN)A� (1� TP%)þ (TOC/TN)TP� TP%

where the subscripts BS, A and TP represent bulk sediment,algae and terrestrial plants, respectively. The values of TOC/TN

Fig. 5. Paleoprecipitation variation from southern Taiwan combined with northern hemisphere insolation, records of humidity, percentage of spore and Cyperaceae pollen, low andhigh latitude water vapor transport for the past 21 kyr. Time-series records of (A) TP-based summer monsoonal rainfall intensity index. Intensified summer monsoon (ISM) isrepresented by red arrow. (B) Proxy ratio (Paq). Dotted line indicates the boundary value between aquatic- and terrestrial vascular plants, (C) summer insolation at 22� N (red line)and winter solar-radiation gradient between 10� N and 45� N, averaged through June, July and August (JJA) and December, January and February (DJF) (Laskar et al., 2004), (D) meaneffective moisture (MEM) from monsoonal central (dashed line) and southeastern Asia (solid line) (Herzschuh, 2006), (E) Spore (%) and (F) Cyperaceous pollen (%) of core TYP-B,southern Taiwan (Lee and Liew, 2010), (G) western tropical Pacific Ocean (WTP) sea surface temperature (SST) inferred from planktonic foraminifera of MD81 (Stott et al., 2004), (H)d18O values of sea water of WTP (Stott et al., 2004), and (I) d18O values of Greenland ice core GISP 2 (Robinson et al., 2006). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e3422 3419

values of algae and TP, based upon our data from this study, for thecalculation are 4 and 30, respectively. Our multiproxy paleo-precipitation record generally agrees with the conclusions ofprevious studies, and it reveals additional details about increasesand decreases in rainfall or rainfall intensity over the last 21 ka(Fig. 5).

4.4.1. Strengthening in rainfall intensity and the EA summermonsoon

Our OM-TP%-based rainfall intensity index record shows thatincreased precipitation happened in four major intervals since thelast deglacial period. Thefirst period is shownas higher TOC-TP% andTOC/TN ratios that initially appear at w17.2 ka together withincreasing summer insolation and persist untilw12.2 ka during thelate deglaciation (Fig. 5), a pattern that roughly agrees with anincreasing mean effective moisture (MEM) inferred from paleo-climatic records in central Asia (Herzschuh, 2006). However, then-alkaneproxy ratio (Paq) indicates that aquaticmacrophytesbecamesomewhat more abundant, probably responding to increases ofevaporationanddecreases of lakewaterdepth related to the summersolar-radiation increase in southern Taiwan. The increase of TP-inferred rainfall intensity also synchronized with the change toa smaller winter insolation gradient at 17.2 ka during the end of thelast glacial period. Themoisture signal is comparedwith d18O recordsof GISP 2, Greenland (Robinson et al., 2006) (Fig. 5). Enhancedprecipitation in the late deglacial period (w15.5 ka) is also suggestedby a large enrichment in d18O (w0.7&) and depletion in d18O(w1.6&) of sea surface water in the western tropical Pacific Ocean(WTP-SW) (evaporation effect) and of planktonic foraminifera fromthe equatorial Indian ocean (reservoir effect) (Stott et al., 2004;Tiwari et al., 2006), respectively. Moreover, the CCM1 simulation

also shows a slightly increased continental precipitation relative totoday from 0� N to 30� N at 16 ka and 14 ka (Kutzbach et al., 1998). Asignificant increase in rainfall intensity lasted until 12.2 ka, duringthe earlyYoungerDryas (YD), in southernTaiwan,which is consistentwith a pollen-based paleotemperature reconstruction from LakeSuigetsu, Japan (Nakagawa et al., 2005), and d18O values in a speleo-themfromSongjiaCave, China (Zhouet al., 2009). Themechanism forinitiation of the Younger Dryas is still debated. Whatever the differ-ence among the popular hypotheses e reorganization of the ocean’sthermohaline circulation (Broecker, 1997), reduced solar activity(Renssen et al., 2000), or extraterrestrial impact (Firestone et al.,2007) e the initiation location is either the Atlantic Ocean or NorthAmerica. Therefore, appearance of the abrupt climate change thatpunctuated glacial time might be reasonably delayed in the westernNorth Pacific for several centuries, according to the archive records ofTung-Yuan Pond, Lake Suigetsu and Songjia Cave. The period ofelevatedmoisture, however, was interrupted by two small decreasesin precipitation at w14.8 ka and w13.3 ka, which respectivelysynchronize with the Oldest Dryas and intra-Allerød cold events(Stuiver et al., 1995) and which correspond to drops in the TOC/TNpaleoprecipitation proxy shown in Fig. 5.

Two peaks in the content of OM-TP% between w10.6 ka andw8.6 ka record a period of important increase in rainfall thatoccurred concordantly with the early Holocene insolationmaximum (Fig. 5). The post-YD climate return to a wetter climateclearly agrees with the other paleoclimate proxies of MEM, the d18Orecords of WTP-SW and GISP 2 (Fig. 5), and the modeling result ofKutzbach et al. (1998). Furthermore, hydrologic instability duringthis interval is apparent from a period of diminished rainfall in thetime that separates the OM-TP% peaks. Percentage contributions ofspores and Cyperaceae pollen also indicate that the lake area might

T.-N. Yang et al. / Quaternary Science Reviews 30 (2011) 3413e34223420

have shrunk to provide habitats for aquatic macrophytes and toshorten the transport distance of allochthonous materials (Lee andLiew, 2010; Lee et al., 2010). The climate instability is also recordedin a sediment core from Lake Pichozero, southeastern Russia(Wohlfarth et al., 2004) and from the Changjiang (Yangtze River)Delta (Yi et al., 2003), and it is likely reflected in the depleted signalin d18O of WTP-SW during the same interval (Stott et al., 2004).

As summer solar radiation declined, the third period of strongrainfall is suggested by both the OM-TP% and Paq proxies during theHolocene Thermal Optimum between w7.7 ka and w5 ka. Ourrecords and other archives suggest that the middle Holoceneoptimum is characterized not only by an increase in temperature,but also in water vapor transport (Fig. 5). The climate in theChangjiang (Yangtze River) Delta region was warm and wet duringthis interval (Yi et al., 2003). This phenomenon is further evidencedby the residual d18O records of coral, indicating rainfall 20% higherthan that of today in southern Taiwan during this period (w6.7 ka,Shen et al., 2005) and of planktonic foraminifera in the equatorialIndian Ocean (Tiwari et al., 2006). The increase in water vaportransport during the HTO is evident also in the high lake levels inthe marginal areas of the East Asia summer monsoon (Yang et al.,2010). Nevertheless, the TOC-based peat formation was accompa-nied by an increase of Cyperaceae (Lee and Liew, 2010; Lee et al.,2010) at 6.3 ka, suggesting climatic variability in southern Taiwanduring the middle Holocene Thermal Optimum.

Another paleoprecipitation increase occurred from w4.2 tow2 ka and is equivalent in effectivemoisture and summer insolationto the deglacial increase. A small decrease of rainfall occurs atw3.0 ka, which is concordantwith a time of drought in Lake Edward,Africa (Russell and Johnson, 2005), suggesting the probability ofteleconnection between diminishing strength of the EA summermonsoon (Gupta et al., 2003; Wang et al., 2005) and of latitudinaldisplacement of the Intertropical Convergence Zone (ITCZ) (Russelland Johnson, 2005). Furthermore, the shoaling trend of the lakewater is suggestedbyboth largerPaq values and increasedCyperaceaepollen (Lee and Liew, 2010; Lee et al., 2010), which occurred morefrequently during the interval 3.5 ka and 2.2 ka.

4.4.2. Weakening in rainfall intensity and the EA summer monsoonOur record of OM-TP%-based paleoprecipitation intensity also

reveals four major periods of weak rainfall intensity atw21e17.2 ka,w12.2e10.6 ka,w8.6e7.7 ka, andw5e4.2 ka that areconcordant with numerical model and climatic archive data (Fig. 5)(Kutzbach et al., 1998; An, 2000; Wang et al., 2001; Yi et al., 2003;Herzschuh, 2006; Liew et al., 2006b; Robinson et al., 2006; Yi et al.,2006; Kubota et al., 2010; Lee and Liew, 2010; Lee et al., 2010). Theperiods of decreased rainfall intensity are at the same time as lowsummer insolation and a large radiation gradient between 10� Nand 45� N at the beginning of the last deglacial. The effectivemoisture inferred from paleoclimatic archives also revealsa concordant low level of monsoonal activity in Central Asia(Herzschuh, 2006). It is likely that the weakening in both precipi-tation and the summer monsoon is a response to low solar radia-tion and persistently southward shifting of the ITCZ during the lastglacial (Russell and Johnson, 2005).

The regional climate experienced a second important decreasein rainfall during the late YD and Preboreal, w12.2 to w10.6 ka,which is well confined by four radiocarbon dates. It is of signifi-cance that climate was cold as well as being little drier in Taiwan inthe late YD stadial, even though summer insolation was greaterthan that in the last glacial. However, Renssen et al. (2000) andFirestone et al. (2007) proposed that the start of the YD was trig-gered as solar activity reduced and by an extraterrestrial impact,respectively. The EA summer monsoon system and the shifting ofthe ITCZ therefore might be affected, too.

A third period with low rainfall intensity occurred around the8.2 ka cold event (w8.6e7.7 ka). Many climate proxy records revealan anomaly spanning 400e600 years and starting from about8.6 ka (cf. Rohling and Pälike, 2005). Finally, a drop in precipitationfrom w5 to 4.2 ka coincides with diminished moisture delivery inthe Changjiang (Yangtze River) Delta (Yi et al., 2003), and thecollapse of the Neolithic Culture of China (Wu and Liu, 2004; Wanget al., 2005).

5. Conclusions

Organic paleoprecipitation proxies measured in a well-datedsediment core from southern Taiwan indicate that the Tung-YuanPond is a sensitive and reliable recorder of past climate changes.Its multiproxy sedimentary record reveals details of low latitudeprecipitation variability in southeastern Asia corresponding to theevolution of the EA monsoon during the last 21 kyr. Precipitationintensity increased during four periods: the deglaciation (w17.2 tow13 ka), the early Holocene (w10.6 to w8.6 ka), the middleHolocene Thermal Optimum (w7.7 tow5 ka) and the late Holocene(w4.2 to w2 ka). The record also shows fluctuating hydrologicalchanges and stable enhanced rainfall during the early and middleHolocene, respectively. In contrast, diminished rainfall intensityoccurred in four time intervals: the Last Glacial Maximum and earlyTermination I (w21 to w17.2 ka), the Younger Dryas and earlyHolocene (w12.2 to w10.6 ka), w8.6 to w7.7 ka, and w5 tow4.2 ka, which are concordant with established cold events.

In summary, weak rainfall intensity events happened synchro-nously with a large winter insolation difference between high andlow latitudes and cold intervals at the last glacial, the YD and 8.2 kaevents. In contrast, periods with high precipitation intensityaccompanied relatively high values of summer solar radiation, andintervening cold intervals were lower. The variability of rainfallintensity likely responded to changes in strength of the EA summermonsoon, which was influenced by alteration of heat transfer fromlow latitudes northward to the high latitudes and latitudinaldisplacements of the ITCZ since the last glacial.

Acknowledgments

We appreciate our laboratory colleagues for sampling andmeasurements, and the TEC Data Center, IES, for graphic service.We are grateful to the two anonymous reviewers for their helpfuland constructive comments that improved the manuscript. Thiswork is supported by various grants: NSC-96-2811-M-001-037(TNY), NSC-96-2611-M-001-011 (TQL) and NSC-96-2627-M-002-011 (SRS) from the National Science Council of Taiwan, and theIntegrated Project, Asian Paleo-Environment Change (II): HoloceneHigh-Resolution Paleo-Climate Reconstruction in Taiwan and itsNeighboring Area (TQL), from the Academia Sinica. TNY and LL alsoacknowledge financial support from Swedish Research Council.

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