A 2000-year leaf wax-based hydrogen isotope record from Southeast

Quaternary Science Reviews 148 (2016) 44e53

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A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a centennial timescale Kweku A. Yamoah a, *, Akkaneewut Chabangborn a, b, Sakonvan Chawchai a, b, Frederik Schenk a, Barbara Wohlfarth a, Rienk H. Smittenberg a, ** a b

Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden Department of Geology, The Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 December 2015 Received in revised form 4 July 2016 Accepted 6 July 2016

Limited understanding of the complex dynamics of the tropical monsoon exists, partly due to inadequate paleo (hydro)-climate proxy data from monsoonal regions. This study presents a 2000-year long record of hydrogen isotope values of leaf wax (dDwax) from a sedimentary sequence recovered from Lake Pa Kho, Northern Thailand. Evaluation of present day rainfall patterns and water isotope data indicates that dDwax ~ o-Southern Oscillation (ENSO) dynamics. reflects the amount of rainfall and is also influenced by El Nin Over the last 2000 years, wettest conditions occurred between ca. 700 AD and ca. 1000 AD, whereas the driest intervals lasted from ca. 50 BCE to ca. 700 AD and from ca. 1300 AD to ca. 1500 AD. Further investigations to establish the spatiotemporal variability of ENSO within the wider tropical Asian-Pacific realm over centennial timescales revealed a low-frequency-tripole pattern between mainland SE Asia (MSEA), the tropical West Pacific, and the central-eastern Pacific, with a wetter than normal MSEA during ~ o-like climate conditions. This pattern stands in contrast to the annual event where El Nin ~ o cause El Nin drier conditions in MSEA. We hypothesize that on centennial timescales the land-sea contrast, which drives monsoon intensity in MSEA, is modulated by the latitudinal shift of the Walker circulation and associated ENSO dynamics. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Plant wax dD Plant wax d13C Hydroclimate Mainland Southeast Asia Pacific Walker Circulation ~ o-Southern Oscillation El Nin

1. Introduction Most parts of Asia are characterized by the Asian monsoon system (Wang et al., 2014), which consists of three sub-systems: the Indian Ocean monsoon (IOM), the Western North Pacific monsoon (WNPM) and the East Asia monsoon (EAM) (Wang et al., 2014). The Asian monsoon system is the largest and most dynamic of all the monsoonal systems (Wang, 2009; Wang et al., 2014) with their subsystems showing seasonally distinct strength and feedback mechanisms due to geographical location and differences in topography (Wang et al., 2014). Processes governing Asian monsoon dynamics are complex and comprise various interconnected mechanisms. Many studies have

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.A. Yamoah), rienk.smittenberg@ geo.su.se (R.H. Smittenberg). http://dx.doi.org/10.1016/j.quascirev.2016.07.002 0277-3791/© 2016 Elsevier Ltd. All rights reserved.

~ o-Southern Oscillation (ENSO), a component shown that the El Nin of the Pacific Walker Circulation (PWC), modulates the Asian summer monsoon; shifts in the PWC are associated with El Nino and La Nina events (Neelin et al., 1998; Glantz, 2001; McPhaden et al., 2006). ENSO dynamics also influence the movement and strength of the South Pacific Convergence Zone (SPCZ) (Vincent et al., 2011; Haffke and Magnusdottir, 2013), the largest rainfall band that modulates precipitation patterns in the southern hemi~ o-like conditions sphere (Trenberth, 1976; Vincent, 1994). El Nin displace the SPCZ northeastward while La Ninea-like conditions drive it to the southwest (Folland et al., 2002; Juillet-Leclerc et al., 2006). Indeed, the expansion of the SPCZ in the South Pacific has ~ a-like conditions (Linsley been attributed to long-term mean La Nin et al., 2006). The dynamics of the Asian monsoon are also strongly associated with the movement and strength of the Intertropical Convergence Zone (ITCZ) (Gagan et al., 2004; Marriner et al., 2012; Wang et al., 2014), which is a band of deep convection across the entire

K.A. Yamoah et al. / Quaternary Science Reviews 148 (2016) 44e53

tropical region (Wang et al., 2014). This rainfall band is as a result of the convergence of trade winds due to intense heating by the sun along the equator (Philander et al., 1996). Changes in insolation dictate the movement of the ITCZ across the lower latitudes (Wang, 2009; Wang et al., 2014) and also drives the landesea thermal contrast, which in turn, impacts on the strength of ENSO and its teleconnections (Kumar et al., 1999). The southward shift of the ITCZ also interacts with the northward shift of the SPCZ bringing along enormous rainfall in the Western Pacific as a result of moist air feeding into the SPCZ (Vincent, 1994). Understanding the mechanisms associated with Asian monsoon dynamics has focused mainly on inter-annual timescales (e.g. Krishna Kumar et al., 1999; McPhaden et al., 2006; Vincent et al., 2011; Haffke and Magnusdottir, 2013; Schneider et al., 2014), while the complexities over sub-millennial timescales have still not been explored in detail. It has for example been suggested that the movement of the mean position of the ITCZ modulates monsoon variability on decadal to sub-millennial timescales (e.g. Oppo et al., 2009; Sachs et al., 2009; Zheng et al., 2014). In contrast, a recent synthesis argues that shifts in the mean position of the ITCZ cannot explain the symmetric latitudinal variability observed across the West Pacific on decadal to centennial timescales (Yan et al., 2015). The proposed mechanism for this latter hypothesis is that less radiative forcing (as was the case during the Little Ice Age, 1400e1850 AD) leads to smaller land-sea contrasts; this leads to drier conditions at the outer zones of the ITCZ/monsoon region and to increased precipitation in the central zone that surrounds the Indo-Pacific Warm Pool (IPWP) (Yan et al., 2015). Other studies have also argued that zonal changes in the Pacific Walker Circulation (PWC), and attendant ENSO teleconnections might contribute significantly to tropical hydroclimate variability on centennial timescales, apart from the ITCZ (Yan et al., 2011; Konecky et al., 2013). There are thus still many uncertainties regarding the interrelations of ENSO and monsoon dynamics on centennial timescales. Moreover, climate model simulations would be necessary to better understand the mechanisms underlying the cumulated effects of ENSO on centennial timescales over the Asian-Pacific region. However, observational data that are needed to test these models, rarely, date back beyond 100 years ago. A spatially welldistributed network of well-dated proxy records of past climate is, therefore, critical but such a network remains far from complete. This study forms part of the effort to resolve this deficiency, by presenting and evaluating a 2000-year long hydroclimatic proxy record from Lake Pa Kho (LPK) in northeast Thailand, consisting of the carbon and hydrogen isotopic composition of plant leaf waxes (d13Cwax and dDwax). 2. Study area Lake Pa Kho (LPK) (17 50 54.97400 N and 102 560 24.92600 E) is located on the Khorat Plateau at approximately 186 m above sea level in the northeastern part of Thailand (Fig. 1). An earlier study described LPK’s stratigraphy as peat overlying peaty gyttja and detritus gyttja (Chawchai et al., 2015a). The artificial lake was a wetland before dredging and damming in the 1990s (Penny, 2001; Chawchai et al., 2015a). The climate of the study area is characterized by southwest summer monsoonal winds (MayeOctober) originating from the Indian Ocean, and northeast winter monsoonal winds (NovembereApril) that emanate from the high atmospheric pressure areas in Mongolia and China. Tropical cyclones also contribute substantially to the annual precipitation in Thailand. On average three tropical cyclones annually reach Thailand, with the highest storm frequency occurring in October (Thai Meteorological Department,

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2011). The mean annual precipitation of the study site is approximately 1455 mm, 88% of which falls between May and October. The average temperature ranges between ~22 and 25 C in winter and between 27 and 30 C in summer (Thai Meteorological Department, 2011). Earlier studies have suggested that Thailand’s hydroclimate is sensitive both to the movement of the mean position of the ITCZ (Thai Meteorological Department, 2011) and to ENSO variability (Singhrattna et al., 2012). 3. Materials and methods 3.1. Sample collection and age model Cores from LPK were obtained in January 2010 from a coring platform using a modified Russian corer (7.5 cm diameter, 1 m long sections). To achieve continuous sequences, sediment cores were taken with an overlap of 50 cm, wrapped in plastic and placed in PVC tubes for transport to the Department of Geological Sciences at Stockholm University. For this study, sediments from 2.00 to 3.50 m below the water surface were used. Depth below the water surface is used as a reference point instead of depth below the sediment surface because the sediment-water interface was lost during coring and its depth could not be determined. The sedimentary sequence was subdivided into five lithostratigraphic units (see Chawchai et al., 2015a for details on sampling and lithostratigraphy). The age model used in this study follows that of Chawchai et al. (2015a) and Yamoah et al. (2016). The age of the upper part between the last 14C date and the date of core recovery (2010) was interpolated, since no 210Pb and 137Cs dating could be carried out. 3.2. Compound-specific isotopes analyses Sampling for biomarker analysis was guided by the established chrono- and lithostratigraphy (Fig. 2). From 2.01 to 2.51 m sampling depth (1500e2000 AD) sub-samples were taken at 5 cm intervals, from 2.51 to 2.81 m (550e1500 AD) contiguous 1 cm samples were taken, and from 2.81 to 3.41 m (0e550 AD) sub-samples were again taken at 5 cm intervals. A detailed description of the biomarker analysis procedure used in this study is described in Chawchai et al. (2015b) as well as in Yamoah et al. (2016). Briefly, samples were freeze-dried and crushed to a fine powder before extraction. Squalane was added to the crushed sample prior to extraction as an internal hydrocarbon standard. Total lipids were extracted using a mixture of dichloromethane and methanol (DCM-MeOH, 9:1, v/v) and hydrocarbon fractions recovered by eluting the total lipid extract over deactivated silica gel (5%) with pure hexane. Identification and quantification were performed by gas chromatographyemass spectrometry (GCeMS) on a Shimadzu GCMS-QP2010 Ultra. Compound-specific isotope analysis was carried out in a similar way as described in the literature (e.g. Sessions et al.,1999; Chawchai et al., 2015b). A saturated fraction of n-alkanes was obtained by eluting the hydrocarbon fraction with hexane over a pipette column filled with 10% AgNO3-coated silica gel. The hydrogen isotopic composition (dD) of the n-alkanes was determined by gas chromatographyeisotope ratio monitoringemass spectrometry (GC-IRMS) using a Thermo Finnigan Delta V mass spectrometer interfaced with a Thermo Trace GC 2000 using a GC Isolink II and Conflo IV system. A reference standard (A4, provided by Arndt Schimmelmann, Indiana University, USA) was regularly analyzed to: (1) check instrument performance (2) calibrate the reference gas (H2) against which the samples were measured. The Hþ 3 factor was relatively stable with an average value of 3.9 ppm mV1. Triplicates were measured for all samples and are reported as mean values. The average standard

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Fig. 1. (A) Modified map from the Thai Meteorological Department, indicating monsoon winds (red and blue arrows mark the southwest and northeast monsoon, respectively) and the path of tropical cyclones (dashed purple lines) over Thailand. The black box indicates the location of the study area, Lake Pa Kho in Northeastern Thailand. (B) Topographical map of the study site, showing the location of the retrieved sequence (yellow dot). For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.

deviation for the samples was 4.4‰ whereas the long-term precision of the instrument evaluated routinely using standard n-alkanes mixtures was 4‰. The carbon isotopic composition (d13C) of the long chain n-alkanes was analyzed using the same instruments and approach as for dD. However, a CO2 reference gas was used for standard calibration against which the samples were measured. Triplicate analyses for each sample were made and are presented as mean values. The average standard deviation of samples was ca. 0.5‰, whereas the long-term precision, determined using the reference mixture, was 0.3‰. 3.3. Calculation of accumulated ENSO events based on modern day records In order to depict the typical precipitation patterns related to the PWC, we calculated a composite for precipitation anomalies ~ o and La Nin ~ a events from 1960 to during the five strongest El Nin 2011. We used monthly mean precipitation from NCEP/NCAR reanalysis data (Kalnay et al., 1996) to include ocean areas. In order to display the anomalies linked to ENSO states only, the long-term trends, which may not be linked to ENSO states were excluded. The de-trending is a standard method here defining the linear trend by means of linear regression fitted to the data using least squares. Based on the derived regression slope, the trend is subtracted for every grid point and time step before calculating the anomalies. The monthly anomalies were then calculated by subtracting the monthly means of the period 1960e2011. Based on a three month running sum, we selected all relevant months of the strongest ~ o Index (ONI) ENSO events (SI Table A.1) based on the Oceanic Nin (NOAA, 2014). The composite figures were finally calculated by ~ o and La Nin ~ a states. This averaging the data for all selected El Nin

calculation is comparable to Kiel Climate Model (KCM) simulations described in Latif et al. (2015) to show the impact of different ENSO states. 4. Results and discussion 4.1. Re-evaluation of age model Chawchai et al. (2015a) presented two possible age models for the Lake Pa Kho sequence: one assumed low accumulation rates around 2.63 m depth, and the other considered the presence of a hiatus. Chawchai et al. (2015a) argued that it was difficult to reconcile low accumulation rates and wetland conditions between 2.68 m and 2.63 m with enriched d13Cbulk values, and argued for the hiatus model. The assumption was that humid conditions would promote primary production leading to higher accumulation rates and vice versa. However, there was no lithological evidence for the existence of a hiatus. Lipid biomarker analysis has provided additional information about the mechanisms that influenced d13Cbulk variability, thus, reconciling the enriched d13Cbulk values with increased moisture availability on the wetland (Yamoah et al., 2016). Additionally, Yamoah et al. (2016) showed that higher moisture availability at LPK led to an increase in productivity of aquatic plants and a reduction of terrestrial vascular plant input. Since high annual precipitation and temperatures in the tropics favor extensive degradation of organic matter (Wüst, 1995), we thus suggest that the low accumulation rates between 2.68 m and 2.63 m depth were the result of a higher degradation rate of softtissue aquatic plants, combined with a smaller contribution of more resistant terrestrial plant organic matter. We therefore adopt the non-hiatus age model presented by Chawchai et al. (2015a) (Fig. 2).


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Fig. 2. The lithostratigraphy and age model used in this study (Chawchai et al., 2015a; Yamoah et al., 2016). (a) Lithostratigraphy, (b) age model (without hiatus), and (c) accumulation rates. The blue shapes show the calibrated 14C dates with two standard deviations, the dark grey shading indicates the likely age model and the dotted lines show the 95% confidence ranges of the age model (Chawchai et al., 2015a). For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.

4.2. Source and potential pre-aging of terrestrial n-alkanes Terrestrial derived n-alkanes can be pre-aged on land as part of soil organic matter, before being eroded and transported to a sedimentary sequence (e.g. Smittenberg et al., 2006; Douglas et al., 2014). As a consequence, any stable isotope record based on such nalkanes will be one that is naturally smoothed and attenuated to a smaller or larger degree. The extent of such a potential pre-aging and attenuation depends on many factors, which include the size, topography and hydrology of the catchment; together this determines the residence time in soils, erosion patterns, and transport time (Gierga, 2015). Before the 1990s, and prior to the establishment of an artificial lake, LPK was considered a wetland (Penny, 2001). Yamoah et al. (2016) concluded that local wetland vegetation forms the bulk of the total organic matter, and that of the long chain n-alkanes. This means that the organic material has predominantly been produced in situ, and should thus be of the same age as the plant macrofossils used for the 14C dating. Even if we assume that some of the n-alkanes were derived from the surrounding catchment of LPK, then these would likely have derived from surface runoff, and not from deep gully erosion that would access old soil layers, as the landscape is rather flat. In addition, some n-alkanes may have been deposited by aeolian transport

during the dry season, but these would be derived directly from living plants and thus not be pre-aged at all. To conclude, pre-aging of n-alkanes before deposition is not expected to have significantly altered, smoothed or attenuated our stable isotope record. 4.3. Controls on dD of precipitation in Thailand The stable isotopic compositions of tropical precipitation (d18Oprecip and dDprecip) are mainly influenced by the amount of rainfall (amount effect) (Dansgaard, 1964; Rozanski et al., 1993) or/ and moisture source (Aggarwal et al., 2012; Moerman et al., 2013). Isotope and rainfall data for Bangkok, Thailand, obtained from the International Atomic Energy Agency e Global Network of Isotopes in Precipitation database (IAEAeGNIP; International Atomic Energy Agency, 2014), show an inverse relationship between dDprecip and the amount of precipitation on annual timescales (Fig. 3a and b). This inverse relationship suggests a dominant role of the “amount effect” on dDprecip values in Thailand where an increase in precipitation leads to more negative dDprecip values, whereas more positive values relate to decreasing rainfall. In addition, precipitation in Thailand is influenced by ENSO dynamics (Ishizaki et al., 2012; Singhrattna et al., 2012). ENSO activity associated with rainfall intensity can alter the isotopic values of the water vapor within the

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~ a and the strong 1997/ Fig. 3. a) Bangkok rainfall dD (red line) and annual precipitation amount (blue line) showing the general correlation between the two. The strong 1988 La Nin ~ o events are indicated as reference. b) Cross-correlation plot between rainfall dD and annual precipitation amount (data was retrieved from GNIP Database). For inter98 El Nin pretation of the references to colour, the reader is referred to the web version of this article.

atmospheric circulation as much as 16‰ for dD (i.e. ~2‰ for d18O) (Ishizaki et al., 2012). Therefore, dDprecip in Thailand reflects the amount effect and ENSO dynamics (Fig. 3a). 4.4. Controls on dDwax values in Lake Pa Kho

dDwax values of higher terrestrial plants are strongly correlated to the dD values of the source water available to plants during growth, i.e. dDprecip (Sessions et al., 1999; Chikaraishi and Naraoka, 2003; Sachse et al., 2004, 2006; Hou et al., 2008). However, factors such as soil evaporation (Smith and Freeman, 2006) and transpiration in higher plants (Kahmen et al., 2013) can modify this primary control. Additionally, dDwax can be influenced by: (1) large scale changes in vegetation (Sachse et al., 2012), (2) variations in biosynthetic fractionation effects of the same plants under different environmental conditions (Douglas et al., 2012), and (3) potential shifts between either C3 or C4 plant types. An earlier study on the same core has shown that the relative distribution of all the n-alkane homologues remained generally unchanged throughout the last 2000 years (Yamoah et al., 2016). This suggests that no large-scale vegetation changes between C3 and C4 vegetation have occurred through time with respect to plant sources in and around LPK. Subsequently, Yamoah et al. (2016) found that the d13C values of the n-alkane homologues exhibited different ranges of magnitude (Fig. 4a). The d13C values of C27-C31 nalkanes, which varied between 30‰ and 33‰, were predominantly derived from C3 plant types whereas d13C values of C33-C35 n-alkanes show a much wider variability from 34‰ to 22‰ (Fig. 4a).

The large variability observed in the d13C values of C33-C35 nalkanes has been attributed to differences in carbon source, and linked to the wetness of the wetland (Yamoah et al., 2016). Lower moisture availability on the wetland was characterized by lower d13C values (~30‰), whereas greater moisture availability corresponded to higher d13C values (~20‰) (Yamoah et al., 2016). The moisture availability on the wetland can either be as a result of direct precipitation or by artificially flooding the wetland. These would lead to an increase in d13C values of C33-C35 n-alkane. In line with these arguments, we infer human influence on the wetland from ca. 1500 AD onwards based on the d13C and dD values of C33C35 n-alkanes: high d13C values of C33-C35 n-alkanes (~25‰), which reflect moisture availability on the wetland, remained relatively stable whereas the dD values of the C33-C35 n-alkanes initially show wetter period from ca. 1500e1600 AD and then a drying trend afterwards (Fig 4a and b). Therefore, the susceptibility of the wetland to inundation only affects the d13C and not the dD values of C33-C35 n-alkanes. Despite the differences in magnitude of the d13C values of the nalkane homologues (C27-C35), the dD values on the other hand are very similar both in terms of their absolute magnitudes and patterns of variability (Fig. 4a and b), which indicates that irrespective of plant type, they all record the same primary isotopic signal from environmental water in a similar way. It does appear that shifts in the relative contribution of different C3 plants does not exert a significant influence on the dD values of the long chain n-alkanes, which thus primarily reflect changes in source water dD. However, since the C27-C31 n-alkanes show the lowest spread in their d13C values (Fig. 4a), we suggest that C27-C31 n-alkanes homologues are better recorders of precipitation signal than C33-C35 n-alkanes. This


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Fig. 4. Age profiles of (a) d13C of n-alkane homologues (C27-C35 n-alkanes) and the bulk organic matter (Chawchai et al., 2015a,b; Yamoah et al., 2016), and (b) dD of n-alkane homologues (C27-C35 n-alkanes). Error bars are omitted for clarity. The average standard error of measurement for d13C and dD are ±0.5 and ± 4.4‰, respectively.

conclusion is substantiated by the fact that no correlation exists between the dD and d13C values of the C27-C31 n-alkanes (R2 < 0.1) (SI Fig. 2). In other words, dD values of C27-C31 n-alkanes vary independently from d13C values. Therefore, we use dDwax to refer to the weighted average of C27-C31 n-alkane dD values in the remainder of the text. Given the discussions above, dDwax values in our record are influenced by the amount effect, ENSO dynamics and the extent of evapotranspiration, which all work in the same direction: drier climatic conditions lead to more positive dDwax values and wetter conditions to more negative values.

4.5. Hydroclimate reconstruction for northern Thailand Based on the hydrogen isotopic data set, the inferred hydroclimatic conditions over northeastern Thailand during the last 2000 years show alternating wet and dry periods (Fig. 5a). During the first 750 years of the investigated time interval (between ca. 50 BCE and ca. 700 AD), climate appears to have been more variable after ca. 500 AD as compared to the period before. A distinct shift towards more negative dDwax values between ca. 700 AD and ca. 800 AD, and gradually less negative dDwax values after that, signal a relatively fast change from drier to wetter conditions, and a gradual return to drier conditions from ca. 800 AD to ca. 1300 AD. Significant variability, but overall drier conditions, characterise the time interval between ca. 1300 AD and ca. 1500 AD. Inferred hydroclimatic conditions for the last 500 years vary between slightly wetter around 1600 AD and slightly drier between ca. 1600 and 1900 AD. It is interesting to note that higher climatic variability precedes the main climatic shifts from dry to wet. Around 1900 AD, there is a marked drop in dDwax values. To what extent this is a climatic signal showing wetter conditions, or related to human impact, would need further investigation.

4.6. Centennial scale effects of ENSO on Thailand Comparisons of the new LPK record with other paleoclimate records from the broader Asian-Pacific region can improve understanding of the main driving mechanisms that rule the hydroclimate of the region, and particularly of MSEA. dDwax from LPK (Fig. 5a) shows an opposite trend compared to the d18Oostracode record from Cattle Pond on Dongdao Island (South China Sea) (Fig. 5b) (Yan et al., 2011) over the last ca. 1000 yr. The shorter dDdinosterol curve from Palau (ca. 1500e2000 AD) (Fig. 5c) (Smittenberg et al., 2011) depicts the same long-term trend as the record of Dongdao Island. Further south in the Indonesian region, a d18Oseawater record (Fig. 5d) from Makassar Strait (Oppo et al., 2009) can be interpreted as reflecting a signal of isotopically depleted runoff entering the ocean, and the dDwax record of Lake Lading on East Java (Konecky et al., 2013) (Fig. 5e) shows similar long-term trends towards more humid conditions over the last millennium. All of these paleoclimate records from the maritime tropical West Pacific, between 16 N and 8 S, and east of 110 E, indicate opposite long-term trends compared to the new dDwax data set from LPK. The movement of the mean position of the ITCZ and SPCZ could explain the negative correlation between records from NE Thailand and Indonesia, but not the opposite trends observed between Dongdao Island and our site. This opposite pattern argues against shifts in the mean position of the ITCZ as the only or main cause of hydrological changes on multi-decadal and centennial timescales in the tropical Asian-Pacific region. An alternative explanation lies in a low-frequency longitudinal shift of the PWC, i.e. an ENSO-like behavior of the Pacific oceanatmosphere system, as has been proposed by Yan et al. (2011) and Konecky et al. (2013). Since stable water isotopes (dD and d18O of precipitation) and rainfall amount data show a strong influence of ENSO on rainfall patterns in Thailand over annual timescales (Ishizaki et al., 2012) (Fig. 3), the dDwax record could

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Fig. 5. Comparison of hydroclimate proxy records from the tropical Asia-Pacific region. (a) LPK’s dDwax record (17 N, 102 E); shadings indicate the empirical 95% uncertainty bounds based on analytical and age-model errors (see Muschitiello et al. (2015) for in depth technique); (b) the ostracode d18O record from Cattle Pond, Dongdao Island (16 N, 112 E) (Yan et al., 2011); (c) dinosterol dD from Palau (7 N, 134 E) (Smittenberg et al., 2011); (d) a d18O of sea water record from Makassar Strait (4 S, 119 E) (Oppo et al., 2009); (e) leaf wax dD records derived from Lake Lading, East Java (8 S, 118 E) (Konecky et al., 2013); (f) a salinity record from Washington Island (4 0 N, 160 W) (Sachs et al., 2009); (g) a botryococcene dD record from the Galapagos (4 0 N, 160 W) (Zhang et al., 2014); and (h) the dinosterol dD record from the Galapagos (4 0 N, 160 W) (Atwood and Sachs, 2014). The location of the different sites is shown in Fig. 6.

reflect ENSO variability. To test this on centennial timescales, we compare the dDwax record from LPK with records from the central and eastern Pacific. Inferred salinity from Washington Island (4 N, 160 E) (Sachs et al., 2009) (Fig. 5f) that spans ca. 1000 to ca. 1700 AD broadly co-varies with the long-term trend seen in the LPK data set from ca. 1300 to ca. 1700 AD, to the extent that the increase in salinity corresponds to drier conditions. The dDbotryococcene record from El Junco Lake, Gal apagos (Fig. 5g) (Zhang et al., 2014) is also characterized by a dramatic change from positive dD values (i.e. dry conditions) around 700 AD to much more negative values (i.e. wet conditions) over the following centuries, followed by a long-term drying trend. The wettest period, however, was reached ca. 1250 AD, later than in LPK. Interestingly, a recent dDdinosterol data set from the same core (Fig. 5h) (Atwood and Sachs, 2014) shows in part the same trends but deviates significantly during the period 850e1250 AD. The authors of this study suggest that two complementary mechanisms are responsible for this: (i) a change in frequency and/ ~ o events (i.e. a shifting PWC) that would mainly or intensity of El Nin influence dDbotryococcenes and (ii) changes in the mean position of the ITCZ, which would be more responsible for variations in dDdinosterol.

The influence of ENSO on rainfall variability in Thailand via atmospheric circulation results in below-normal rainfall during El ~ o and above-normal rainfall during La Nin ~ a events (Shrestha and Nin Kostaschuk, 2005; Singhrattna et al., 2012). However, on centennial to millennial timescales, the proxy records reveal a low frequency tripole pattern between our site on MSEA, the maritime West Pacific, and the central-eastern Pacific, with an inverse relation between the hydroclimate in MSEA and the tropical West Pacific, and a positive correlation between MSEA and the central-eastern Pacific (Fig. 5). To investigate further, we compared the long-term pattern observed in the proxy records to the accumulated effects of precipitation anomalies using composites of the long-term mean of ~ o/La Nin ~ a events, anticipating that the longthe strongest El Nin term mean of the composite anomalies based on instrumental data mimics the long-term pattern observed in the proxy records. The composite mean precipitation anomaly patterns derived from ~ o and La Nin ~ a events over the reanalysis of the five strongest El Nin ~ othe last 50 years (Fig. 6a, b) can be interpreted as average El Nin ~ a-like states. These patterns reveal that MSEA belike and La Nin comes relatively wet compared to the western tropical Pacific ~ oelike states. during such El Nin


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~ o, and (b) a La Nin ~ a anomaly and it’s effect on the strength of the summer monsoon in MSEA. Fig. 6. A generalized mechanism of the Walker Circulation pattern during (a) an El Nin ~ o and La Nin ~ a events based on the reanalysis of the National The mechanism is underlain by a composite of mean precipitation anomaly patterns during the five strongest El Nin Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) from 1960 to 2012 (Kalnay et al., 1996). The study site (Lake Pa Kho) and the location of other paleohydrological records discussed in the text are indicated.

The dynamical background behind this long-term pattern could be due to a land-sea driven mechanism (Fasullo, 2012). An enhancement of the land-sea contrast is likely due to a northeast~ o events, which causes dry ward shift of the SPCZ during El Nin conditions in the western tropical Pacific. Less convective activity over the sea, or even submergence of air, allows the summer monsoon to be expressed more fully over MSEA, thereby annulling ~ o in the broad western Pacific the generally drying effect of El Nin ~a border. On the other hand, the analogue of the composite La Nin events (Fig. 6b) does not show any tripole pattern as observed in the proxy observations. Mirroring the above argument, a south~ a years would increase westward shift of the SPCZ during La Nin precipitation in the Western Pacific, but decrease the land-sea contrast leading to weaker monsoon rains over MSEA. The cumu~ aelike conditions are, lative effect of the long-term mean La Nin however, not apparent in the instrumental records, most likely ~ a’s are less removed from present-day’s climatic because La Nin ~ o events. mean than El Nin

detailed picture of hydroclimatic changes during the past 2000 years. Relatively dry conditions characterized the intervals from 50 BCE to 700 AD and 1300 AD to 1500 AD; the wettest interval started ca. 700 AD and lasted until ca. 1000 AD. The most remarkable feature is the rapid shift from dry to wet conditions between 700e800 AD, and the long-term drying trend that spans over the ensuing millennium. Comparison of the LPK dDwax record with other isotope-based hydroclimatic records from the wider tropical Asian-Pacific region reveal a low-frequency tripole pattern between MSEA, the tropical West Pacific, and the central-eastern Pacific, with an inverse relation between the hydroclimate in MSEA and the tropical West Pacific, and a positive correlation of MSEA with the central and eastern Pacific. This pattern is similar to that of the composite ENSO pattern of the last 50 years. Based on the similarity between the proxy data and the instrumental record, we suggest that the overall cumulative effect of ENSO on decadal to centennial timescales renders MSEA, and particularly Thailand, ~ o-like conditions, and relatively dry relatively wet during El Nin ~ a-like climatic states. during La Nin

5. Conclusion Acknowledgments The dDwax record generated from the well-dated wetland sequence from Lake Pa Kho in northeast Thailand has provided a

This work was supported by Swedish Research Council research

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grants 621-2008-2855, 621-2011-4684 and 348-2008-6071 to BW, and 621-2011-4916 to RS. The Delta facility, funded by the Faculty of Science, Stockholm University, provided support for the isotope €wemark, Suda analyses. We wish to thank Sherilyn Fritz, Ludvig Lo Inthongkaew, Wichuratree Klubseang, Jayne Rattray, Anna €gglund and Francesco Muschitiello and all those who helped in Ha diverse ways to make this project a success. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2016.07.002. References Aggarwal, P.K., Alduchov, O.A., Froehlich, K.O., Araguas-Araguas, L.J., Sturchio, N.C., Kurita, N., 2012. Stable isotopes in global precipitation: a unified interpretation based on atmospheric moisture residence time. Geophys. Res. Lett. 39, 11. Atwood, A.R., Sachs, J.P., 2014. Separating ITCZ- and ENSO-related rainfall changes in the Gal apagos over the last 3-kyr using D/H ratios of multiple lipid biomarkers. 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