A Combined Carbon and Hydrogen Isotope Approach to Reconstruct the SE Asian Paleomonsoon

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A combined carbon and hydrogen isotope approach to reconstruct the SE Asian paleomonsoon

Impacts on the Angkor Civilization and links to paleolimnology

Kweku Kyei Afrifa Yamoah

Department of Geological Sciences Stockholm University, 106 91 Stockholm

To my parents and family

© Kweku Kyei Afrifa Yamoah, Stockholm University, 2016 ISSN 1101-1599 ISBN 978-91-7649-378-6 Cover picture: painted by Daria Anfilogova Printed in Sweden by Holmbergs, Malmo, 2016 Distributor: Department of Geological Sciences

Abstract

Changes in monsoon patterns not only affect ecosystems and societies but also the global climate system in terms of heat energy and humidity transfer from the equator to higher latitudes. However, understanding the mechanisms that drive monsoon variability on longer timescales remains a challenge, partly due to sparse paleoenvironmental and paleoclimatic data.

This thesis, which contributes new hydroclimate data sets for the Asian monsoon region, seeks to advance our understanding of the mechanisms that contributed to Southeast Asian summer monsoon variability in the past. Moreover, it explores how past climatic conditions may have impacted societies and ecosystems. In this study lake sediment and peat sequences from northeastern and southern Thailand have been investigated using organic geochemistry, and more specifically the stable carbon and hydrogen isotopic composition of specific biomarkers (n-alkanes, botryococcenes, and highly branched isoprenoids).

The hydrogen isotopic composition of leaf waxes (δDwax) in Thailand was shown to relate to the amount of precipitation and the extent of the El Niño Southern

Oscillation. Higher values of δDwax can be interpreted as reflecting relatively dry climatic conditions, whereas lower values relate to wetter conditions.

The hydroclimate reconstruction for northeastern Thailand, based on the sedimentary record of Lake Kumphawapi, suggests higher moisture availability between ca. 10,700 cal. BP and ca. 7,000 cal. BP likely related to a strengthened early Holocene summer monsoon. Moisture availability decreased during the mid-Holocene, but seems to have increased again around 2,000 years ago and has fluctuated since. The high-resolution Lake Pa Kho peat sequence, which allows for a sub-centennial reconstruction of moisture availability, indicates that the wettest period occurred between ca. 700 and ca. 1000 CE whereas driest intervals were from ca. 50 BCE to ca. 700 CE and from ca. 1300 to ca. 1500 CE. Hydroclimate comparison of Pa Kho’s

δDwax record with other paleoclimate records from the Asian-Pacific region suggests that El Niño-like conditions led to Northeastern Thailand being wetter, whereas La Niña-like conditions led to drier conditions.

Regional hydroclimate variability also greatly influenced the Angkor Civilization, which flourished between ca. 845 and ca. 1450 CE. The shift from drier to wetter conditions coincided in time with the rise of the Angkor Civilization and likely favored the intense agriculture needed to sustain the empire. The gradual decline in moisture availability, which started after ca. 1000 years CE, could have stretched the hydrological capacity of Angkor to its limit. It is suggested that Angkor’s population resorted to unconventional water sources, such as wetlands, as population growth continued, but summer monsoon rains weakened.

The 150-year long record of Lake Nong Thale Prong in southern Thailand offers insights into decadal-scale hydroclimatic changes that can be connected to the instrumental record. δDwax-based hydroclimate was drier from ~1857 to 1916 CE and ~1970 to 2010 CE and wetter from ~1916 to 1969 CE. Drier climatic conditions between ~1857 and 1916 CE coincided with oligotrophic lake waters and a dominance of the green algae Botryococcus braunii. Higher rainfall between ~1916 and 1969 CE concurred with an increase in diatom blooms while eutrophic lake water conditions were established between ~1970–2010 CE.

Keywords: Holocene, summer monsoon, biomarkers, hydrogen isotopes, lake sediment, peat, Thailand.

Sammanfattning

Förändringar i monsunmönster påverkar inte bara lokala ekosystem och samhällen, utan i hög grad också klimatet på global nivå, eftersom att den storskaliga atmosfärs- cirkulationen transporterar både värmeenergi och fuktig luft från ekvatorn till högre latituder. Betydande utmaningar kvarstår dock för att uppnå en djupare förståelse av de mekanismer som driver monsunens variabilitet i ett längre tidsperspektiv, vilket till viss del kan förklaras av bristen på paleoklimatologiska och paleomiljödata i monsun- påverkade områden.

Denna avhandling bidrar med nya paleo-klimat-rekonstruktioner från den asiatiska monsunregionen, med syfte att öka förståelsen för de mekanismer som bidragit till Sydostasiens monsunvariabilitet under Holocen. Arbetet undersöker också hur tidigare klimatförhållanden kan ha påverkat samhällen och ekosystem i regionen. Sjösediment och torvlagerföljder från nordöstra och södra Thailand har analyserats med geokemiska metoder, med fokus på kol- och väte-isotopsammansättningen (δD, δ13C) i utvalda biomarkörer (n-alkaner, botrycoccenes och ”highly branched” isoprenoider).

Väteisotopsammansättningen i fossila lipider från terrestra växter (δDwax) i sjösediment från Thailand visade sig vara relaterad till mängden nederbörd samt omfattningen av ENSO (El Niño Southern Oscillation). Högre värden av δDwax kan tolkas som en återspegling av relativt torra klimatförhållanden medan låga värden indikerar våtare förhållanden.

En rekonstruktion av nordöstra Thailands hydroklimat, baserad på analyser av fossila sediment från Lake Kumphawapi, tyder på fuktigare förhållanden mellan c. 10 700 och c. 7000 cal. BP, vilket sannolikt är relaterat till en förstärkt sommar-monsun under tidig Holocen. Tillgången på fukt avtog i mitten av Holocen, men verkar ha ökat igen runt 2000 cal. BP . En torvlagerföljd från Lake Pa Kho har använts för att skapa en rekonstruktion av hydro-klimatet under sen-Holocen, med 10-100-års upplösning. Denna serie indikerar att den våtaste perioden inträffade mellan c. 700 och c. 1000 e.Kr, och de torraste perioderna inträffade mellan c. 50 f.Kr och c. 700 e.Kr, samt 1300-1500 e.Kr. Datat från Pa Khos, tillsammans med andra tillgängliga

arkiv från asiatiska Stilla Havs-regionen, tyder på att El Niño-liknande förhållanden historiskt sett har lett till att nordöstra Thailand blivit våtare, medan La Niña-liknande förhållanden resulterat i att regionen blivit torrare.

Variationer i det regionala hydroklimatet hade stor påverkan på Angkor- civilisationen, som blomstrade mellan c. 845 och c.1450 e.Kr. Växlingen från relativt torrare till blötare förhållanden, sammanfaller i tid med uppgången av Angkor- civilisationen. Förändringen i klimatet kan ha gynnat det intensiva jordbruk som krävdes för att upprätthålla riket. Den gradvisa minskningen i fukttillgång startade c. 1000 år e.Kr., och kan ha bidragit till att tänja Angkors hydrologiska kapacitet till sin yttersta gräns. I takt med att Angkors befolkning ökade och monsunregnen försvagades kan befolkningen förslagsvis ha börjat utnyttja mer okonventionella vattenresurser såsom våtmarker.

Ett 150 år långt sjö-sedimentarkiv från Lake Thale Prong i södra Thailand erbjuder inblick i hydroklimatvariationer med decennium-upplösning som kan kopplas till instrumentellt uppmätta klimatdata. Rekonstruktionen som baseras på δDwax indikerar torra förhållanden från ~1857 till 1916 e.Kr. och ~1970 till 2010 e.Kr. och våta förhållanden från ~1916 till 1969 e.Kr. De torrare klimatförhållandena mellan ~1857 och 1916 e.Kr. sammanföll med oligotrofiskt sjövatten och dominans av den gröna algen Botryococcus braunii. Kraftigare nederbörd mellan ~1916 och 1969 e.Kr. sammanföll med att blomningen av diatoméer ökade. Eutrofiska förhållanden etablerades i sjön mellan ~1970-2010 e.Kr.

List of papers

This doctoral thesis consists of a summary and five appended papers.

Paper I Yamoah, K.K.A., Chabangborn, A., Chawchai, S., Väliranta, M., Wohlfarth, B. and Smittenberg, R.H., Large variability in n-alkane δ13C values in Lake Pa Kho (Thailand) driven by wetland wetness and aquatic productivity. Organic Geochemistry (in press).

Paper II Yamoah, K.K.A., Chabangborn, A., Chawchai, S., Schenk, F., Wohlfarth, B. and Smittenberg, R.H., A 2000-year leaf wax-based stable carbon and hydrogen isotope record from Southeast Asia, suggests low frequency ENSO-like teleconnections on a centennial timescale. Quaternary Science Reviews (in revision).

Paper III Yamoah, K.K.A., Higham, C.F.W., Wohlfarth, B., Chabangborn, A., and Smittenberg, R.H., Gradual dwindling of rainfall over a ~300-year period pushed Angkor Civilization to its limit of adaptation (Manuscript).

Paper IV Chawchai, S., Yamoah, K.K.A., Smittenberg, R.H., Kurkela, J., Väliranta, M., Chabangborn, A., Blaauw, M., Fritz, S.C., Reimer, P.J. and Wohlfarth, B. 2015. Lake Kumphawapi revisited – The complex climatic and environmental record of a tropical wetland in NE Thailand. The Holocene. DOI: 10.1177/0959683615612565.

Paper V Yamoah, K.K.A., Callac N., Chi Fru E., Wiech, A., Wohlfarth, B., Chabangborn, A., and Smittenberg, R.H., 2016. A 150-year record of phytoplankton community succession controlled by hydroclimatic variability in a tropical lake, Biogeosciences Discussions. DOI: 10.5194/bg-2015-633.

I have designed and written papers I-III and V of this thesis. The co-authors contributed with comments on the manuscripts. For Paper IV, I provided the biomarker dataset, wrote the section on biomarkers and commented on other parts of the paper. Where, in the remaining part of the text, reference is made to paper I-V, then this refers to the publications listed above unless otherwise specified. Elements of papers I-III can be found in the licentiate thesis published earlier as part of the PhD studies (Yamoah et al., 2015).

Author Paper I Paper II Paper III Paper IV Paper V contributions Fieldwork, A. Chabangborn A. Chabangborn A. Chabangborn A. Chabangborn K.K.A. Yamoah stratigraphy and B. Wohlfarth, S. Chawchai S. Chawchai S Chawchai A. Chabangborn sub-sampling L. Löwemark B. Wohlfarth B. Wohlfarth Wohlfarth, B. S. Chawchai S. Fritz S. Fritz S. Fritz S. Fritz B. Wohlfarth. S. Chawchai L. Löwemark L. Löwemark L. Löwemark S. Fritz W. Klubseang W. Klubseang W. Klubseang S. Inthonkaew W. Klubseang W. Klubseang M. Väliranta Pb-210 dating K.K.A. Yamoah Radiocarbon P.J. Reimer P.J. Reimer P.J. Reimer P.J. Reimer dating B. Wohlfarth B. Wohlfarth B. Wohlfarth B. Wohlfarth Age-depth S. Chawchai S. Chawchai S. Chawchai M. Blaauw K.K.A Yamoah model M. Blaauw M. Blaauw M. Blaauw Compound K.K.A Yamoah K.K.A Yamoah K.K.A Yamoah K.K.A. Yamoah K.K.A. Yamoah specific A. Wiech biomarker analysis Compound K.K.A Yamoah K.K.A Yamoah K.K.A Yamoah K.K.A Yamoah K.K.A. Yamoah specific isotope analysis of carbon and hydrogen Bulk S. Chawchai S. Chawchai S. Chawchai S. Chawchai K.K.A. Yamoah geochemistry Plant M. Väliranta M. Väliranta macrofossils DNA analysis N. Callac Archaeology C.F.W. Higham Figures and K.K.A. Yamoah K.K.A. Yamoah K.K.A. Yamoah B. Wohlfarth K.K.A. Yamoah tables A. Chabangborn F. Schenk Data K.K.A. Yamoah K.K.A. Yamoah K.K.A. Yamoah B. Wohlfarth K.K.A. Yamoah interpretation R.H. R.H. K.K.A. Yamoah E. C. Fru Smittenberg Smittenberg R.H. N. Callac Smittenberg R.H. M. Väliranta Smittenberg Writing K.K.A. Yamoah K.K.A. Yamoah K.K.A. Yamoah B. Wohlfarth K.K.A. Yamoah R.H. R.H. R.H. K.K.A. Yamoah E.C. Fru Smittenberg Smittenberg Smittenberg R.H. Smittenberg

Table of contents

Pages 1. Introduction 1 2. Background 3 2.1. Study area 3 2.2. Climate influence on Angkor Civilization 6 2.3. Lipid biomarkers as a paleoclimatic tool 8 2.3.1. Lipid biomarker proxies 8 2.3.2. Stable isotopes analysis 9 2.3.3. Stable carbon isotopic composition of leaf waxes 9 2.3.4. Stable hydrogen isotopic composition of leaf waxes 10 2.4. Previous studies from Southeast Asia 13 3. Thesis objectives 14 4. Materials and Methods 15 5. Summary of papers: results and discussions 17 5.1. Paper I 17 5.2. Paper II 19 5.3. Paper III 22 5.4. Paper IV 25 5.5. Paper V 26 6. Synthesis and concluding remarks 28 7. Future perspective and ongoing work 31 8. Acknowledgments 33 9. References 34 Appendices Paper I: Large variability in n-alkane δ13C values in Lake Pa Kho (Thailand) driven by wetland wetness and aquatic productivity Paper II: A 2000-year leaf wax-based stable carbon and hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a centennial timescale Paper III: Gradual dwindling of rainfall over a ~300-year period pushed Angkor Civilization to its limit of adaptation Paper IV: The complex climatic and environmental record of a tropical wetland in NE Thailand Paper V: A 150-year record of phytoplankton community succession controlled by hydroclimatic variability in a tropical lake

1. Introduction

Climate change will affect rainfall patterns in tropical monsoon regions (Stocker et al, 2014), which will have a large impact on ecosystems and society. Moreover, the monsoon is considered a primary component of the global climate system in terms of heat energy and humidity transfer from the equator to higher latitudes (e.g. Webster et al., 1998; An et al., 2000; Trenberth et al., 2000; Cook and Jones, 2012; Zheng et al., 2014). Monsoon rains affect a region where more than half of the world´s population lives (Maher and Hu, 2006; Cook et al., 2010). Significant changes in precipitation patterns can lead to drought or flooding, which can have large consequences for the financial sector, agriculture, environment, property, social life and public health (Cook and Jones, 2012). Studies have shown that changes in summer monsoon intensity on decadal to centennial timescales can have influenced human migrations and social changes (Weiss and Bradley, 2001; Haug et al. 2003; Lieberman and Buckley, 2012; Buckley, et al. 2010, 2014). This is particularly evident in Southeast Asia where shifting rainfall conditions may have contributed to the rise or/and demise of the Angkor Civilization (Buckley, et al. 2010, 2014; Cook et al., 2010; Lieberman and Buckley, 2012; Day et al., 2012).

The Asian monsoon system, which is the largest and most dynamic of all monsoon systems, consists of three sub-systems: the Indian Ocean Monsoon (IOM), the Western North Pacific Monsoon (WNPM) and the East Asian Monsoon (EAM) (Fig. 1) (Wang, 2009; Wang et al., 2014). These three subsystems are distinct in their strength and response mechanisms due to geographical location and differences in topography (Wang et al., 2014). Although mechanisms such as the Indian Ocean Dipole (IOD) (Weller and Cai, 2014) and the Atlantic Meridional Overturning Circulation (AMOC) (Yu et al., 2009) modulate Asian summer monsoon variability, the annual movement and strength of the Intertropical Convergence Zone (ITCZ) (Gagan et al., 2004; Marriner et al., 2012; Wang et al., 2014) and the El Niño- Southern Oscillation (ENSO) (Trenberth et al, 2002; Goswami and Xavier, 2005) play major roles. The movement of the ITCZ has been well studied on annual through millennial timescales in the entire tropical region (e.g. Philander et al., 1996; deMenocal, 2000; Trenberth et al, 2002; Dykoski et al., 2005; Wang et al., 2005; Broccoli et al., 2006; Sachs et al., 2009; Tierney et al., 2010; Schneider et al., 2014).

In contrast, the dynamics of ENSO on longer timescales have barely been studied in detail over the Southeast Asian region. Understanding and predicting the spatiotemporal variability associated with the complex dynamical behavior of the Asian summer monsoon is thus still an open question. Paleoclimate model simulations are indispensible to understand these complexities but these need to be tested against instrumental measurements and paleo-proxy data. However, instrumental observations for Southeast Asia rarely go beyond 50 years and only few paleoclimate proxy data exist that reflect ENSO dynamics in Southeast Asia. This PhD thesis seeks to contribute to our understanding of the important mechanisms that influenced Southeast Asian summer monsoon variability and its relation to ENSO and the ITCZ. In addition, the relationship between climate variability and the Angkor Civilization (Khmer Empire), which governed greater parts of Southeast Asia and existed from ca. 845 to ca. 1450 Common Era (CE)1∗ (Welch, 1998; Evans et al., 2007), is explored. The objectives of this study were achieved through the investigation of sediment and peat records preserved in lacustrine and wetland environments in Thailand, in particular by use of organic geochemical techniques to obtain qualitative and semi-quantitative paleoclimate data sets.

∗ Footnote: The ages expressed in calendar years before present (cal. BP), where present is 1950 CE, has been maintained throughout the thesis for clarity. However, AD/BC are referred to as the Common Era (CE) and the preceding years referred to as Before the Common Era (BCE) in the thesis summary.

Fig. 1. Wind climatological map (850-mb level at 1500 m altitude) superimposed on precipitation for the season of June-August showing the Asian monsoon sub-systems. The arrows represent the wind direction (vector) whiles the colour bar shows the rainfall intensity. Wind vectors are from the NCEP/NCAR reanalysis (modified from Wang et al., 2003).

2. Background

2.1. Study Area

Thailand (between approx. 5° - 20°N and 97° - 105°E) (Fig. 2) is located at the crossroad of three sub-systems of the Asian monsoon (Buckley et al., 2014). Home to about 65 million people, Thailand is amongst the leading exporters of rice (Dawe, 2002) with agriculture contributing ~9% of Thailand’s total gross domestic product (GDP) (Lee, 2015). Thailand’s hydroclimate is mainly influenced by the IOM. Between May and October, the southwest monsoon trade winds bring warm moist air from the Indian Ocean, which then precipitates over land (Thai Meteorology Department, 2011). During the winter period from November to January, the northeast monsoon trade winds mainly dominate bringing cold and dry air from the Siberian anticyclone over Thailand. February to April are considered transitional

3 periods between the winter and summer monsoon seasons (Thai Meteorology Department, 2011).

Thailand’s hydroclimate is sensitive to the movement of the mean position of the ITCZ (Thai Meteorological Department, 2011) and to ENSO variability (Singhrattna et al., 2012). The ITCZ stretches to southern Thailand in May and brings along southwesterly winds from the Indian Ocean. It moves northward from June to early July, reaching southern China, and then back to Thailand in August (Thai Meteorology Department, 2011). It has been argued that ENSO dynamics lead to increased rainfall during La Niña events and to decreased rainfall during El Niño years (Singhrattna et al., 2012). Tropical cyclones also cause high precipitation over Thailand because these pick up moisture in the Bay of Bengal and the South China Sea. Mean annual precipitation for Thailand is about 1500 mm and mean air temperatures range between 22°C and 25°C from November to February, and between 27°C and 30°C from March to October (Klubseang, 2011).

Thailand is geographically divided into four main regions (Ogawa et al., 1961): (1) the Northwestern Highlands, which are characterized by high mountain ranges that extend between Myanmar and Thailand; (2) the Khorat Plateau in the northeastern part, which has flat-topped peaks (up to 1600 m a.s.l.) at its southern and western sides, whereas the northern and eastern sides drain to the Mae Kong River; (3) the Central Plain, located to the south includes deltas and alluvial plains, which often are flooded during rainy seasons; (4) the Peninsular region, which stretches as a long belt southward towards Malaysia.

Three lakes were used for this study (Fig. 2): Lake Kumphawapi (KMP) and Lake Pa Kho (LPK) are situated on the Khorat Plateau and Lake Nong Thale Pron (NTP) is located in the Peninsular region. KMP is the largest lake in Thailand and located at approximately 17⁰11´N, 103⁰02´E and at 170 m above sea level. It is a shallow lake and has a size of about 28 km2 with a maximum water depth of 4 m and a pH of 6.8. Penny (1999, 1998) described the vegetation in and around Kumphawapi as dominated by grasses (Poaceae including Phragmites sp.) and sedges (Cyperaceae). Other taxa that were identified include Nymphoides indicum, Persicaria attenuata, Eichhornia crassipes, Ipomoea aquatica, Saccharum spp., Salvinia cucullata,

Ludwigia adscendens, L. octovalis, Nelumbo nucifera, Nymphaea lotus and Typha angustifolia.

Neighboring, LPK is located at 17°06'70"N, 102°56'17"E and at approximately 186 m above sea level. This small (~1km2) and shallow lake (<2 m water depth) was a wetland prior to dredging and damming in the 1990s (Penny, 2001). The vegetation of the Pa Kho wetland prior to dredging was dominated by grasses, sedges and ferns with floating communities, including Typha angustifolia, Nelumbo nucifera, Persicaria attenuate, Alocasia macrorhiza, Ipomoea aquatica, Ludwigia adscendens, Sagittaria sagittifolia, Jussiaea linifolia, Nepenthes thorii, Alternanthera sessilis, Eupatorium odoratum, Hewittia sublobata, Nymphoides indicum, and Physalis angulata. The current vegetation around KMP and PK includes paddy fields, sugar cane and Eucalyptus plantations (Klubseang, 2011).

NTP is a shallow (<7 m water depth), small (~210 m2) sinkhole located at 17˚ 11`N, 99˚ 23`E and at ~60 m above sea level (Snansieng et al., 1976). The lake has been eutrophic over the last 50 years (Yamoah et al., 2016) and supports aquatic plants. Vegetation of the region consists of tropical rain forest supported by high rainfall and high soil humidity (Wipanu, 2006). Diversity of tropical plant species from families such as Leguminosae, Annonaceae, Moraceae, Rubiaceae, Euphorbiaceae, Sapindaceae, Apocynaceae, Burseraceae, and Guttiferae characterizes the rain forest. Also, high diversity from the family Myrtaceae and Dipterocarpaceae dominates parts of this region (Santisuk, 2006). Different species of palm such as Borassodendron machadonis, Johannesteijsmannia altifrons, Orania sylvicola, Oncosperma horrida, Licuala elegans, L. distans, Arenga westerhoutii, Livistona speciosa, Caryota spp., Calamus spp., Daemonorops spp., and Korthalsia spp. is also very common in the region (Santisuk, 2006).

Fig. 2. Location and topography of Lakes Pa Kho and Kumphawapi on the Khorat Plateau of northeast Thailand, and of Lake Nong Thale Prong on the Thai Peninsula.

2.2. Climate influence on Angkor Civilization

Human migrations and social changes have been influenced by changes in climate patterns (Weiss and Bradley, 2001; Haug et al., 2003; Burke et al., 2009; Lieberman and Buckley, 2012). The Angkor Civilization (also known as the Khmer Empire) is a prominent example of how climate impacted on a society that was dependent on rainfall (Penny et al., 2007; Cook et al., 2010; Buckley et al., 2010, 2014). The Angkor Civilization existed between ca. 845-1450 CE and ruled large parts of

Southeast Asia including northeastern Thailand, Cambodia and Vietnam (Evans et al., 2007). The Angkorians made great advances in temple constructions and agriculture, which was supported by a complex hydrologic system to conserve water (Evans et al., 2007; Kummu, 2009). The massive infrastructure including temples and extensive road networks built by the Angkor people were to a large extent dependent on their agrarian based economic system (Hall, 1985). This, of course, was in turn dependent on sufficient water coming with the each summer monsoon (Fletcher et al., 2008). Angkor’s economy was reliant on rice cultivation. Farming took place all year round using water stored in the reservoirs during the dry seasons (Tully, 2005). Water was also important within their religion (Hall, 1985; Fletcher et al., 2008).

The growth of the civilization supported by the massive infrastructural development led to the exploitation of ever more arable land (Jacob, 1978; Evans et al., 2007), which caused extensive deforestation, topsoil degradation and erosion (Coe, 2003). Several hypotheses exist to explain the waxing and waning of the Angkor Civilization. Some authors have argued that since the economic system of Angkor was largely agrarian-based, consistent summer monsoon rains contributed immensely to its expansion (Lieberman and Buckley, 2012). Similarly, it has been put forward that a decrease in summer monsoon rains and extreme droughts contributed to their demise (Cook et al., 2010; Buckley et al., 2010, 2014; Day et al., 2012). Subsequently, Buckley et al. (2014) have argued that the lack of maintenance, coupled with extensive rainfall, caused the collapse of the hydrological infrastructure, which was the backbone of Angkor’s society. Other researchers have attributed the demise of Angkor to the social structure, politics and environmental over-exploitation (Briggs, 1951; Coe, 2003; Stone, 2006).

Overall, wrong decisions, a vulnerable landscape and heavy dependency on water, could have made ancient societies, such as the Khmer, vulnerable to shifts in hydroclimatic conditions. These problems are akin to the present day of over- exploitation of the earth's natural resources, combined with potential rapid climate change related to global warming. The relationship between the Southeast Asian summer monsoon variability and the rise and fall of Angkor’s Civilization could be an important analogue for our understanding of how environment and climate change can influence an entire civilization.

2.3. Lipid biomarkers and their isotopic composition as a paleoclimatic tool

2.3.1. Lipid biomarker proxies

Lipid biomarker proxies are organic molecules occurring in geological materials (Brassell, 1992) that can be linked to a particular source organism, group of organisms and /or to a particular process (Peters et al., 2005, Castañeda and Schouten, 2010). Some biomarkers are ubiquitous and are produced by many organisms but others are produced by a specific genus or species (Castañeda and Schouten, 2010). Although many more biomarkers exist, this thesis has focused on hydrocarbons only, which are relatively straightforward to purify for subsequent isotope analysis: n- alkanes (C19-C35), C25 highly branched isoprenoids (HBIs) and botryococcenes.

The long-chain C27 to C35 n-alkanes are leaf waxes typically produced by higher plants (Eglinton and Hamilton, 1963, 1967; Rieley et al., 1993) whereas the C23 to C25 n-alkanes are typically attributed to (submerged) aquatic macrophytes (Ficken et al.,

2000; Baas et al., 2000). Aquatic algae or bacteria are the primary producers of C17 to

C21 n-alkanes. However, it is important to note that these classifications are based on the dominant carbon chains in these different groups, and that there is always an overlap between the carbon chain length distributions produced by different plant types.

Botryococcenes and C25 HBIs are both algal lipid biomarkers (Zimmerman and Canuel, 2000; Coolen et al., 2004; Smittenberg et al., 2005). Lipids are an integral part of cell membranes aiding in structural support and as storage compounds (Jungblut et al., 2009). Botryococcenes are known specific biomarkers for the green algae Botryococcus braunii (Metzger et al., 1988; Smittenberg et al., 2005; Zhang et al., 2007) and have been used as a proxy for oligotrophic conditions (Smittenberg et al., 2005; Souza et al., 2008; Waldmann et al., 2014). C25 HBIs are a valuable indicator of diatom-derived organic matter inputs to sediments (Damsté et al., 2004; McKirdya et al., 2013).

2.3.2. Stable isotope analysis

Stable isotope analysis has become a popular tool in the study of paleoclimatic and paleoenvironmental reconstructions. Stable isotope ratios, which are expressed as delta (δ) values, represent the relative deviation of the isotopic ratio (R) in a sample from a standard and is calculated based on the following equation:

δX = (Rsample/Rstandard – 1) × 1000 (1) 13 where X is the heavier isotope (e.g., C), Rsample is the ratio of heavy and light isotope in the sample, and Rstandard is the ratio of heavy to light isotope in a standard. The internationally accepted standards for carbon and hydrogen are Vienna Pee Dee Belemnite (VPDB) and Vienna standard mean ocean water (VSMOW), respectively. δ18O values are commonly reported relative to either VPDB in carbonates or VSMOW in water samples.

2.3.3. Stable carbon isotopic composition of leaf waxes

13 The stable carbon isotopic composition of leaf waxes (δ Cwax) has been used to track organic matter sources but also to differentiate between plant types (Bush and McInerney, 2013; Tipple et al., 2013). Plants can be grouped into two main types, C3 and C4, based on their physiology and carbon fixation pathways (Westhoff and Gowik, 2004, 2010). Each pathway fractionates differently against carbon and hence 13 changes in the relative abundance of C3 and C4 plants can cause variability in δ Cwax even when atmospheric CO2 remains relatively constant. This feature is used 13 regularly in paleoclimate research where shifts to lower δ Cwax values represent C3 13 plants whereas higher δ Cwax values reflect a higher contribution of C4 plants (e.g. 13 Russell et al., 2014). δ Cwax variability can however also be due to environmental conditions such as aridity (Polissar and Freeman, 2010; Douglas et al., 2012). Submerged aquatic plants (macrophytes) living in wetlands utilize dissolved inorganic carbon (DIC) as their carbon source, and hence changes in the isotopic composition of DIC affects the leaf waxes produced by these wetland plants. Highly 13C-depleted methane, combined with carbon recycling mechanisms, may affect the δ13C content DIC (Nichols et al., 2010; McClymont et al., 2010). During wetter conditions and greater stratification of the water column, more methane-derived DIC remains in the

9 water resulting in depleted δ13C values of DIC (Pancost et al., 2000; van Breugel et al., 2005; Nichols et al., 2010; McClymont et al., 2010). Other alternative mechanisms that have been suggested to cause variability in δ13C values of organic matter include: a saline habitat (Wooller et al., 2007) and a switch in carbon source - from CO2 to HCO3 by macrophytes (Gichuki et al., 2001; Kahara and Vermaat, 2003; Street-Perrot et al., 2004; Kristen et al., 2010).

2.3.4. Stable hydrogen isotopic composition of leaf waxes

The stable hydrogen isotopic composition of leaf waxes (δDwax) has emerged as an important proxy for past hydroclimate variability. However, it is important to first constrain the relationship between hydroclimate variability and hydrogen isotopes of precipitation (δDprecip) as a first step in the mechanism that drives δDwax variability (e.g. Moerman et al., 2013).

Stable water isotopes (δD and δ18O) are important tracers used for understanding the effects of environmental factors, such as temperature, latitude, precipitation amount, and rainout effect within the hydrologic cycle (Dansgaard 1961; Rozanski et al. 1993; Gat 2000). For instance, the relationship between δD and δ18O has been used extensively to determine the source of water and the effect of temperature by comparing it with the isotopic composition of meteoric waters, which generally fall on a global meteoric water line (GMWL) (Craig, 1961). Isotopic values below the GMWL would suggest evaporative enrichment waters. Although the GMWL is a global best-fit line, it may not accurately represent a given area due to differences in evaporative enrichment of precipitation in different regions. Therefore, a meteoric water line referred to as local meteoric water line (LMWL) can be calculated for a particular region and used as the baseline for precipitation inference. To determine the LMWL for Thailand, stable isotope data of rainfall from Bangkok, Thailand was retrieved from the Global Network of Isotopes in Precipitation (GNIP) database and the best-fit line of a plot of δD against δ18O was used as the LMWL (Paper II) (Fig 3a).

The hydrogen isotopic composition of precipitation (δDprecip) in the tropics is mainly influenced by the amount effect and not temperature (Dansgaard, 1964; Rozanski et

10 al., 1993). This is also the case for Thailand, where the amount of precipitation shows an inverse relationship with the δDprecip such that higher precipitation relates to lower average δDprecip values and vice versa (Paper II) (Fig. 3b and 3c). However, this relationship is not obvious on monthly timescales, which can be attributed primarily to the sources of moisture. Precipitation in Thailand is influenced by atmospheric circulation emanating from the Indian Ocean, the Pacific Ocean and the South China Sea (Singhrattna et al, 2012). These atmospheric variables could impact the overall isotopic composition of rainfall in Thailand based on the isotopic composition of the sources of precipitation via the atmospheric variables. Over the last 2000 years, atmospheric circulation from the Indian Ocean has not changed (Sarkar et al., 2015). On the other hand, variation in the atmospheric circulation of the Pacific Ocean, however, is likely to have changed the isotopic composition of δDprecip. In particular, ENSO can alter the atmospheric circulation with as much as 2‰ for δ18O (i.e. ~16 ‰ for δD) (Ishizaki et al., 2012). El Niño leads to a decrease in annual rainfall amount in

Thailand with a corresponding increase in δDprecip values, whereas during a La Niña year annual rainfall amount increases with decreasing δDprecip values (Paper II) (Figs. 3b and 3c).

a b 60 GMWL -25 y=0.017x-17.29 Fig. 3. Climatic controls Rainfall data R2=0.535 40 Thailand, GNIP LMWL on δD of precipitation in 20 -35 Thailand. a) Plot of δD 0.0 against δ18O; b) cross plot -20 -45 between rainfall δD and -40 annual precipitation H ‰ VSMOW

2 -60 -55 -80 amount; c) Bangkok D of precipitation (‰ VSMOW)

-100 -65 rainfall δD (red line) and -15 -10 -5 0 5 10 1000 1400 1800 2200 annual precipitation 18 Annual precipitation amount (mm) O ‰ VSMOW amount (blue line). The c Years (CE) strong 1988 La Niña and 1969 1976 1986 1990 1995 1999 2003 2007 2014 Annual precipitation amount (mm) 1997/98 El Niño events Very strong are indicated as reference -63.5 El Nino Year 2500 (data retrieved from the -53.5 2000 Global Network of -43.5 Isotopes in Precipitation -33.5 1500 (GNIP) database, http:// -23.5 https://nucleus.iaea.org). Strong 1000 La Nina Year D of precipitation (‰ VSMOW) -13.5 Modified from paper II. 500 Several studies have shown that δDwax values of higher terrestrial plants reflect the source water available

11 to the plant during growth, which in turn also links to precipitation (Sessions et al., 1999; Sachse et al., 2004; Chikaraishi and Naraoka, 2005; Sachse et al., 2006).

However, certain controls can bias the δDwax values and these therefore need to be accounted for. It has been shown that evaporative enrichment in soils (Smith and Freeman, 2006) and transpiration in higher plants (Kahmen et al., 2013) lead to a modification of the δDwax values. Additionally, δDwax can also be influenced by variations in biosynthetic fractionation effects of the same plants under different conditions, or by changes in the relative contribution of leaf wax lipids from different plants that may fractionate to a lesser or larger degree against deuterium. Different environmental conditions and changes in vegetation can thus disturb the primary isotopic source water signal (Douglas et al., 2012; Sachse et al., 2012).

To summarize, δDwax values found on Thailand's vegetation are influenced foremost by the amount of precipitation, which is modulated by ENSO and evapotranspiration.

Overall, more positive δDwax values can be interpreted as a signal of dryness/decreased rainfall, whereas lower δDwax values correspond to wetness/increased rainfall (Fig. 4) (Paper II).

Fig. 4. Conceptual model of factors affecting the hydrogen isotopic composition of leaf waxes (δDwax) in Thailand. δDprecip is the δD of precipitation in Thailand, which is mainly influenced by the rainfall amount and the El Niño Southern Oscillation (ENSO). The precipitation is the source water available to plants for biosynthetic processes.

2.4. Previous studies from Southeast Asia

The mechanisms that drive Asian summer monsoon intensity are still widely debated due to asynchronous palaeoclimatic trends as a result of sub-regional climatic differences (Cook and Jones, 2012) or differences in the response of paleo-proxies to climatic changes (Wang et al., 2003). Additionally, the spatial distribution of paleoclimatic records from the tropical Asian region is concentrated to China, which is mainly influenced by the EASM, whereas the Indian sub-continent is characterized by the ISM. Mainland Southeast Asia is a region sensitive to changes in the strength of the EASM, ISM and WPNSM and thus an important region to understand Asian summer monsoon intensity and variability. Yet only a limited number of data sets of different type and time integral exist in this region (e.g. Buckley et al., 2010, 2014; Muangsong et al., 2014; Proske et al., 2011; Wohlfarth et al., 2012; Chawchai et al., 2013, 2015a). This may lead to a skewed interpretation of Asian Monsoon intensity and variability in the past. There is clearly a need to acquire more paleoclimate proxy data for mainland Southeast Asia to further constrain the factors influencing summer monsoon variability and also to fully understand past environmental controls.

Paleoclimatic and paleoenvironmental studies in Thailand have focused on tree-ring records (e.g. Buckley et al., 2010, 2014; Cook et al., 2010) and lake sedimentary sequences (e.g. Penny and Kealhofer, 2005; Penny, 1999, 2001; Wohlfarth et al., 2012; Chawchai et al., 2013, 2015a, b). Buckley et al., (2010, 2014) presented the Monsoon Asia Drought Atlas (MADA), which is based on high-resolution tree ring studies covering the last millennium, to elucidate the role of drought in the demise of Angkor’s civilization. Lake sediments can be investigated using analyses such as pollen stratigraphy, plant macrofossil analyses, organic, and inorganic geochemistry (e.g. Penny and Kealhofer, 2005; Penny, 1999, 2001; Wohlfarth et al., 2012; Chawchai et al., 2013, 2015a, b), allowing unlocking the complexities associated with the paleoclimatic and paleoenvironmental dynamics. Relying on just one or two analyses could lead to misinterpretation. For instance, Kealhofer and Penny (1998) and Penny and Kealhofer (2005) used pollen and phytolith records from Lake Kumphawapi to infer past vegetation changes in northeastern Thailand. Subsequently, Penny (2001) presented a detailed pollen diagram for Lake Pa Kho covering the past

40,000 years. However none of these investigations realized that major hiatuses were present in both records (Wohlfarth et al., 2012; Chawchai et al., 2013).

This thesis forms part of the Thailand Monsoon Project, which aims to reconstruct the response of tropical/subtropical aquatic ecosystems to shifts in monsoon intensity and variability during the past 25,000 years. Chawchai (2014) applied a variety of biological, geochemical and geophysical methods to sequences from Lakes Kumphawapi and Pa Kho, to reconstruct the environmental and hydroclimatic history of Thailand during the Holocene. Based on multi-proxy analysis of multiple sediment sequences from Lake Kumphawapi, Wohlfarth et al. (2012), Chawchai et al. (2013) and Chawchai (2014) suggested that the lake formed some time before 10,000 cal. BP in response to a strengthened summer monsoon. The gradual lake level lowering and subsequent infilling of the basin (7000-6600 cal. BP) was interpreted as a sign of a weakening of the summer monsoon (Wohlfarth et al., 2012; Chawchai et al., 2013). The presence of multiple hiatuses in the two studied Kumphawapi sequences and a relatively poor late Holocene chronology did not make it possible to discuss mid- and late Holocene lake status changes in detail (Wohlfarth et al., 2012; Chawchai et al., 2013). Based mainly on plant macrofossil analysis and bulk geochemistry, a high- resolution record of hydrological changes was generated from the 2000-year long peat sequence of neighboring Lake Pa Kho (Chawchai et al., 2015a). This study suggests a strengthened summer monsoon between 2120-1580 cal. BP, 1150-980 cal. BP, and after 500 cal. BP; and a weakening of the summer monsoon between 1580-1150 cal. BP and 650-500 cal. BP (Chawchai et al., 2015a).

3. Thesis objectives

The overall objective of this thesis is to reconstruct Southeast Asian paleohydrology from lake and wetland deposits using organic geochemical techniques in order to improve our understanding of the mechanisms that control Southeast Asian summer monsoon variability on decadal to millenial timescales. Specific objectives include:

1) Constraining the organic matter input into LPK, and NTP, to provide a robust

paleoclimate background for the interpretation of δDwax as a proxy for rainfall (Paper I and V, respectively).

2) Generating leaf wax δ13C and δD records to understand vegetation changes and their relationship to precipitation (Paper II and III). 3) Understanding how summer monsoon variability influenced Angkor societies (Paper III). 4) Constraining the environmental and hydrological conditions of northeastern Thailand using compound-specific δD analysis (Paper IV). 5) Generating lipid biomarker data and quantitative polymerase chain reaction (qPCR) to understand aquatic ecosystem dynamics in response to climate variability (Paper V).

4. Materials and Methods

Peat and lake sediment sequences were retrieved from the deeper parts of LPK, KMP and NTP in January 2009, 2010 and 2012 using a modified Russian corer (7.5 and 10 cm diameter, 1 m length) and maintaining a 50 cm overlap between the 1 m core segments. Surface samples from Nong Thale Pron were obtained using a HTH gravity corer. The long cores (>1 m) were preliminarily described in the field, wrapped in plastic and placed in PVC tubes for onward transportation to the Department of Geological Sciences at Stockholm University where the cores were kept in storage at 4°C until they were subsampled (see Chawchai et al., 2014 for details). The surface cores were sub-sampled every 1 cm in the field and stored in plastic bags before being transported to Stockholm University, where they were stored in the freezer (see paper V for detailed description).

The most detailed description of the methods used in this thesis are given in paper IV (Chawchai et al, 2015b) but were also already described in Yamoah (2015). In brief, sediments and peat samples were freeze-dried and ground to fine powder. The total lipid extracts were recovered using a mixture of dichloromethane and methanol (DCM–MeOH, 9:1, v/v), which was then added to the samples for extraction using a microwave system. The hydrocarbon fraction was obtained by eluting hexane over 6- mL glass solid phase extraction tubes filled with silica gel. The hydrocarbon fraction was then analysed by gas chromatography–mass spectrometry (GC–MS). Biomarkers such as n-alkanes, C25 highly branched isoprenoid and botryococcenes were identified based on a combination of retention time and comparison with mass spectra from the

15 literature. Quantification was typically performed using an internal standard (squalane) added to the sample immediately after extraction. In principle, the total ion current was used for the quantification of all compounds; in most cases baseline separation was achieved. An equal response factor for the various compounds was also assumed (Woszcyck et al., 2011). Although this does induce an absolute error (i.e. the reported concentrations may not be entirely accurate), this does not affect the precision, nor does it affect the observed relative changes in the concentrations of the analyzed biomarkers within the sedimentary sequences.

For the isotopic analysis of the n-alkanes, further purification of the hydrocarbon fraction was done. Fractions containing almost exclusively n-alkanes were obtained by eluting the hydrocarbon fraction over a pipette column filled with 10% AgNO3- coated silica gel with hexane. The hydrogen and carbon isotopic composition of the n- alkanes was analysed by gas chromatography–isotope ratio monitoring–mass spectrometry (GC-IRMS) on a Thermo Finnigan Delta V mass spectrometer interfaced with a Thermo Trace GC 2000 using a GC Isolink II and Conflo IV system (Details of the methodology including GC program, identification and quantification can be found in the papers accompanying the thesis, especially paper IV and II). The figure below shows a workflow diagram for the methods (Fig. 5).

Sample

Freeze dried and crushed

Total lipid extract

Extracted using microwave extractor with dichloromethane and methanol (9:1) Column chromatography Solid phase chromatography

using SiO2 Hydrocarbon fraction

GC-MS Column chromatography Solid phase chromatography

using AgNO3-impregnated Silica gel GC-IRMS n-alkanes

Fig. 5. Workflow diagram for biomarker and compound specific isotope analysis.

5. Summary of papers: results and discussion

5.1. Paper I (Yamoah, K.K.A., Chabangborn, A., Chawchai, S., Väliranta, M., Wohlfarth, B. and Smittenberg, R.H., A large variability in δ13C values in Lake Pa Kho (Thailand) is driven by wetland wetness and aquatic productivity. Organic Geochemistry, in-press).

Paper I presents an in-depth study on the processes that affect δ13C values of n-alkane homologues from the sediment/peat sequence of Lake Pa Kho in order to constrain the environmental parameters necessary for a robust paleo-environmental and paleoclimatic interpretation. Algae, aquatic macrophytes and terrestrial plants were identified as the main contributors to the organic matter in LPK. The terrestrial contributors were divided into two main groups: C27-C31 and C33-C35 n-alkane- producing plants. δ13C values of the bulk organic matter and the n-alkane homologues show a large variation in 13C that is most apparent for the bulk organic matter and the

C33-C35 n-alkanes but much less pronounced for C27-C31 n-alkanes. This suggests that 13 plants that produced more of C33-C35 n-alkanes influenced the δ C values of the bulk organic matter more than the C27-C31 n-alkane-producing plants. Drier conditions 13 inferred from plant macrofossils correspond to lower δ C values of C33-C35 n-alkanes 13 13 and that of the δ Cbulk. Wetter conditions appeared to correspond with higher δ C values (around -20‰), resembling a typical C4 plant signal (Fig. 6). The typical 13 mechanism held responsible for shifts in δ Cwax found in the literature is a change in the relative contribution of C3 and C4 plant input. However, it is rather unlikely that nearly complete C3-C4 shifts would occur within a century, and back.

In paper I the argument is made that the most plausible mechanism that can cause an 13 increase in δ C values of C33-C35 n-alkanes and that of the bulk organic matter is a - shift in carbon source from dissolved CO2 to HCO3 . Chawchai et al. (2015a) showed that aquatic plant remains dominate at 2.22–2.73 m depth, indicating a wet period, and interpreted this as a sign of a strengthened summer monsoon. Higher rainfall would have kept the wetland replenished with both nutrients and oxygen, thereby stimulating aquatic productivity. This could have led to the depletion of dissolved

CO2 (e.g. Street-Perrott et al., 2004). Less dissolved CO2 shifted the system towards - 13 higher relative abundance of bicarbonate (HCO3 ) (e.g. Gichuki et al., 2001). δ C values of aquatic macrophytes, such as Potamogeton spp., can vary in a range similar to C4 plant values (Fry and Sherr, 1984; Gichuki et al., 2001; Kristen et al., 2010). Potamogeton spp were particularly abundant at 2.22–2.73 m depth (Chawchai et al., 2015a). In lake systems with relatively high pH values, Potamogeton spp. would - preferentially utilize HCO3 due to limited CO2 (Gichuki et al., 2001, Kahara and 13 Vermaat, 2003). The mechanism to explain the variability in δ C values of C33-C35 n- alkanes is thus one where an increase in aquatic productivity leads to a decrease in - dissolved CO2 and increase in HCO3 , and that the dominant wetland vegetation - shifted it's carbon source to isotopically heavy HCO3 , and that this also greatly influenced the δ13C values of the bulk.

Fig. 6. δ13C records of Lake Pa Kho’s peat sequence shown against age (average standard deviation = 13 13 0.5‰). The δ Cbulk and δ C values of C23-C35 n-alkanes homologues are color-coded (modified from Paper I).

5.2. Paper II (Yamoah, K.K.A., Chabangborn, A., Chawchai, S., Schenk, F., Wohlfarth, B. and Smittenberg, R.H., A 2000-year leaf wax-based stable carbon and hydrogen isotope record from Southeast Asia, suggests low frequency ENSO-like teleconnections on a centennial timescale. Quaternary Science Reviews (in revision)).

Paper II focuses on paleoclimate reconstruction using δDwax as a proxy for rainfall intensity in southeastern Thailand for the last 2000 years. However, since vegetation 13 changes can influence the δDwax values, it is important to first assess the δ C of the n- alkanes that would be used for the climate reconstruction. Two main reasons why δD of C27-C31 n-alkanes were used for the climate reconstruction of southeast Thailand

13 were: (1) δ C of the C27-C31 n-alkanes ranged between -30‰ to -33‰, which indicates that the plant types were mainly derived from C3 terrestrial plants (Rommerskirchen et al., 2006) and that there were no major changes in plant types throughout the entire 2000 years; (2) there was no correlation between δD and δ13C 2 values of C27-C31 n-alkanes (R =0.01), thus reinforcing the argument that vegetation has little or no influence on δDwax values.

Prior to the hydroclimate reconstruction, the controls on present day precipitation in Thailand were assessed, based on rainfall amount and stable water isotope data retrieved from the GNIP database. As described above, on annual timescales, δD of precipitation (δDprecip) in Thailand is mainly controlled by the rainfall intensity (amount of precipitation), where greater rainfall intensity leads to more negative values of δDprecip and more positive values relate to smaller rainfall intensity.

Additionally, δDprecip is influenced by El Niño and La Niña conditions, which leads to drier and wetter conditions, respectively (Fig. 3). These combine with evapotranspiration in the same direction for δDwax, which mainly reflects the δDprecip

(Sache et al., 2012). The δDwax shows that over the last 2000 years northeastern Thailand experienced a relatively wet period from ca. 700 CE to ca. 1000 CE. The driest periods were between ca. 50 BCE and ca. 700 CE and between ca. 1300 and ca. 1500 CE.

A comparison of the δDwax-based record from LPK to other paleoclimate proxies from the tropical Asian-Pacific region revealed a tripole pattern between Mainland Southeast Asia (MSEA), the tropical Western Pacific, and central-eastern Pacific. When MSEA and the East experience an increase in rainfall intensity, the tropical West Pacific receives less rain. The inverse relationship, where MSEA and central- eastern Pacific become relatively dry and tropical West Pacific experienced abundant rain, was also observed in the proxy records (Fig. 7). Using a composite ENSO reanalysis constructed for the last 50 years as an analogue, a similar pattern was observed between the proxy records and the instrumental records for cumulative El Niño events. No strong pattern emerged for the cumulative La Niña pattern.

Fig. 7. Comparison of (a) the Lake Pa Kho δDwax record (17˚N, 102°E) with similar proxy records from the tropical Asia-Pacific region. Shadings indicate empirical 95% uncertainty bounds based on analytical and age-model errors (see Muschitiello et al., 2015 for in depth technique); (b) ostracode δ18O record from Cattle Pond, Dongdao Island (16°N, 112°E) (Yan et al., 2011); (c) dinosterol δD from Palau (7˚N, 134˚E) (Smittenberg et al., 2011); (d) δ18O of sea water record from the Makassar Strait (4˚S, 119˚E) (Oppo et al., 2009); (e) leaf wax δD records Lake Lading, East Java (8˚S, 118˚E) (Konecky et al., 2013); (f) salinity record in practical salinity unit (PSU) from Washington Island (4°′N, 160°W) (Sachs et al., 2009); (g) botryococcene δD record from the El-Junco Lake, Galapagos (4°′N, 160°W) (Zhang et al., 2014); (h) dinosterol δD record from the El-Junco Lake, Galapagos (4°′N, 160°W) (Atwood and Sachse, 2014). Figure is modified from Paper II.

Fig. 8. A generalized mechanism of the Walker Circulation pattern during (a) an El Niño, and (b) a La Niña anomaly and it’s effect on the strength of the summer monsoon in Mainland Southeast Asia. The mechanism is underlain by a composite of mean precipitation anomaly patterns during the five strongest El Niño and La Niña events based on the reanalysis of the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) from 1960-2012 (Kalnay et al., 1996). The study site (Lake Pa Kho) is indicated as well as the locations of other paleohydrological records discussed in the text (paper II).

5.3. Paper III (Yamoah, K.K.A., Higham, C.F.W., Wohlfarth, B., Chabangborn, A., and Smittenberg, R.H., Gradual dwindling of rainfall over a ~300-year period pushed Angkor Civilization to its limit of adaptation (Manuscript)).

Paper III discusses the effect of climate on Angkor’s civilization, a civilization that existed between ca. 845 years and ca. 1450 years CE and extended over large parts of Southeast Asia. The 2000-year long wetland sequence of Lake Pa Kho presented the opportunity to discuss the waxing and waning of this ancient civilization in the context of changing hydroclimate conditions. Paper III combines the δ13C values of

C33-C35 n-alkanes and δD of C27-C35 n-alkanes to position Angkor’s civilization in a Southeast Asian hydroclimate context and to assess the impact of climate on the

22 agricultural successes that made Angkor so powerful. It also enabled tracking the impact that Angkor’s civilization had on the vegetation due to changing hydroclimatic conditions.

Reconstructed hydroclimate shows a rapid increase in rainfall amount from ca. 700 CE to ca. 1000 CE. The shift from drier to wetter conditions coincided in time with the rise of the civilization and likely favored the intense agriculture needed to sustain the empire. Favorable hydroclimate conditions, which supported the increase in rice cultivation, started to decline gradually after ca. 1000 CE even though the civilization reached its peak 200 years later at ca. 1200 CE. The establishment of the complex hydrological systems enabled the Angkor populace to adapt to dwindling summer monsoon rains for over two centuries. However, the continuous decline in summer monsoon rain intensity may have stretched the hydrological systems to their limit, leading to an eventual collapse of the delicate and complex hydrologic infrastructure. 13 One important aspect of Paper III is the combined use of δ C values of C33-C35 n- alkanes and of δD values of C27-C35 n-alkanes to elucidate the interaction between Angkor’s society and the environment amidst dwindling rainfall around 1300 CE.

The variability observed in the carbon isotopic signal has been interpreted as having 13 been caused by wetland wetness, where enriched (more positive) δ C values of C33-35 13 n-alkanes reflect wet periods, and dry periods lead to depleted δ C values of C33-35 n- alkanes and hence the bulk organic matter (Paper I). This also implies that the δ13C values of C33-35 n-alkanes, and by extension that of the bulk organic matter, are susceptible to water management practices. Evidence of anthropogenic activity from 13 ca. 1300 CE onwards is seen in LPK record by the divergence of δDwax and δ C values of C33-C35 n-alkanes (Fig. 9). δDwax is an indicator of past rainfall (paper II) 13 whereas δ C values of C33-C35 n-alkanes have been shown to reflect wetland wetness (paper I). The natural isotopic response of the wetland, where decreasing moisture 13 availability concur with lower δ C33-35 n-alkane values (paper I) is switched to one 13 where a decrease in rainfall is accompanied by higher and very stable δ C33-35 n- alkane values, suggesting an increase in moisture availability on the wetland. Flooding of the wetland would be the most probable explanation for the switch in the isotopic pattern. Natural flooding will typically be caused by intense rainfall and that

23 would reflect in the δDwax values. However, channeling rainwater to flood a wetland would not change the original hydrogen isotopic signature significantly, but would likely alter the carbon isotopic signal since it is susceptible to wetland inundation.

The timing of the anthropogenic influence on LPK at ca. 1300 CE is noteworthy since it was just about the time of the demise of Angkor. A growing population and a gradual decrease in rainfall for agricultural purposes would likely have coerced the Angkor people to resort to unconventional water sources such as wetlands.

Fig. 9. Archaeological periods in Northeast Thailand compared to Lake Pa Kho’s paleoclimate reconstructions for the past 2000 years (modified from Wohlfarth et al. (re-submitted)). (a) Archeological periods in northeast Thailand according to Higham (2014) and Higham et al. (2015); (b) 13 13 δ Cbulk values (Chawchai et al., 2015a); (c) δ C values of C33-C35 n-alkanes; (d) δD values of C27-C35 n-alkanes. Dotted green lines show the mean values of the different variables (Paper III).

5.4. Paper IV (Chawchai, S., Yamoah, K.K.A., Smittenberg, R.H., Kurkela, J., Väliranta, M., Chabangborn, A., Blaauw, M., Fritz, S.C., Reimer, P.J. and Wohlfarth, B. 2015. Lake Kumphawapi revisited – The complex climatic and environmental record of a tropical wetland in NE Thailand. The Holocene. DOI: 10.1177/0959683615612565).

Paper IV revisits previously published sedimentary records from Lake Kumphawapi (Kealhofer and Penny, 1998; Penny, 1998; Wohlfarth et al., 2012; Chawchai et al., 2013) and combines these with new data sets obtained on a third sequence (stratigraphy, chronology, hydrogen isotopes, plant macrofossil and charcoal records). This allowed re-evaluating Kumphawapi’s lake status changes and placing these in the context of Holocene changes in regional moisture availability.

The new and combined data set suggests that the lake formed sometime before 10700 cal. BP (Fig. 10), likely in response to a strengthened summer monsoon. Initially, the shallow lake may have been concentrated to the southern part of the Kumphawapi basin, but extended to the central and northern a few thousand years later (9900-7500 cal. BP). Higher moisture availability and lower evaporation during the early Holocene likely favored the growth and expansion of the aquatic and telmatic vegetation. This in turn increased the biomass, which led to overgrowing of the shallow lake, causing an apparent lake level lowering (7000-5900 cal. BP) and expansion of a wetland. All three investigated sedimentary sequences in Lake Kumphawapi show the presence of one or more hiatuses of variable length between 6500 and 1400 cal. BP. Periodic desiccation of the wetland has been brought forward to explain these hiatuses, but also erosion of underlying layers during the subsequent lake level rise around 1800 cal. BP. The late Holocene chronology of Kumphawapi’s sequences is poorly constrained, which hampers comparisons to other well-dated regional paleo-records. However the lake level rise could indicate higher moisture availability.

Stratigraphy Units Modelled age TOC (%) C/N δ13Cbulk (‰) δD wax (‰) (cal. BP) Clay Silt Sand 0 3b c. 600

50 3a c. 1400 c. 2300 compact c. 2700 c. 6500 2b 100 compact

sLB c. 7000 sLB 150 2a

c. 7600 200

1b 250

300 Depth (cm) below sediment-water interface sediment-water Depth (cm) below

c. 9600 350

400 1a

Mean value: 450 -140 ‰

500 c. 10,700 0 20 40 60 0 10 20 30 -16 -20 -24 -28 -80 -120 -160 -200

Legend Clay gyttja Peaty gyttja Gyttja clay Peat Gyttja or algae gyttja Silty gyttja clay

Hiatus Charcoal sLB Sharp lower boundary

13 Fig. 10. Lithostratigraphy, total organic carbon (TOC), C/N ratio, δ Cbulk values (Chawchai et al.,

2013) and δDwax values for Kumphawapi’s sediment sequence CP4. Error bars in the δDwax curve have been omitted for clarity (Chawchai et al., 2015b).

5.5. Paper V (Yamoah, K.K.A., Callac N., Chi Fru E., Wiech, A., Wohlfarth, B., Chabangborn, A., and Smittenberg, R.H., 2016. A 150-year record of phytoplankton community succession controlled by hydroclimatic variability in a tropical lake, Biogeosciences Discussions. DOI: 10.5194/bg-2015-633.)

There is a growing understanding of the factors that influence microbial communities in lake ecosystems (Thyssen et al., 2011; Wang et al., 2015). However, the broader linkages between different microbial groups and their response to past environmental conditions are poorly understood. This is partly due to the lack of suitable proxies that can constrain, capture and distinguish between the diverse parameters impacting on the microbial ecosystem structure. Paper V explores the combination of lipid biomarker analysis, quantitative polymerase chain reactions (qPCR) and sedimentary geochemistry to reconstruct phytoplankton community dynamics in response to changing hydroclimate conditions over the past 150 years using the sedimentary record of Lake Nong Thale Pron (NTP), in southern Thailand.

Reconstructed hydroclimate conditions in southern Thailand based on δDwax show a drier period between ~1857 and 1916 and between ~1970 and 2010 Common Era (CE) and a wet period between ~1916 and 1969 CE. The drier climate between ~1857 and 1916 CE concurs with the dominance of the green algae Botryococcus braunii. From ~ 1916 CE, HBIs, which proxy for diatoms, increases abruptly with Si/Ti ratios. Increase in Si/Ti ratios in NTP can be linked to runoff due to increase in rainfall intensity. Stable eutrophic state flourished between ~1969 and ~2008 CE, coinciding with a relatively drier conditions. This was not accompanied by the production of botryococcene lipids although an increase in Botryococcus genes was observed. Botryococcus braunii are classified into three main groups: A, B and C, but it is only the B group that produces botryococcene lipids (Eroglu et al., 2011). Therefore, another strain of Botryococcus braunii, which does not produce botryococcene lipids, likely became prevalent.

The cause of the eutrophic state of the lake is linked to higher levels of phosphorus, which is a limiting factor for aquatic cyanobacteria blooms (Paerl and Fulton, 2006; Paerl and Valerie, 2012). The source of the elevated phosphorus can most likely be related to the use of fertilizers in farming activities, which increased as a result of land reforms undertaken to expand agricultural activities (Gray, 1991; Puri, 2006). These may have accelerated the eutrophic state of the lake beyond the natural rate of nutrient enrichment. This study provide evidence that decadal climate variability can trigger changes in lake trophic states and promote regime shifts in dominant phytoplankton communities (Fig. 11) and primary productivity, and apparent carbon burial dynamics.

Fig. 11. Lake Nong Thale Pron depth and age profiles of lipid biomarkers (modified from Yamoah et al., 2016): (a) Botryococcenes, a proxy for Botryococcus braunii (B race); (b) C25 Highly branched

Isoprenoid (HBIs), a proxy for diatoms; (c) C17 n-alkane, a proxy for Cyanobacteria; and (d) δD of C27-

29-31 n-alkanes (δDwax), a proxy for rainfall amount. The blue line through the δDwax data set represents a polynomial fitted trendline and the shaded boxes represent units I, II and III. The last column shows the different trophic changes with time (paper V).

6. Synthesis and concluding remarks

Lipid biomarkers, and especially their hydrogen isotopic composition, for instance those of terrestrial leaf waxes (δDwax), constitute a powerful paleoclimatic tool to infer changes in regional moisture avalability in the tropics. A main advantage of δDwax over over other paleohydrologic proxies such as plant macrofossils, bulk sedimentary analysis and pollen studies, is that it accesses in a rather direct way the the close relation that exists between rainwater source, intensity, and its isotopic composition.

This thesis presents the first isotope/biomarker-based reconstructions of mainland Southest Asia, and constrains the environmental and paleoclimatic parameters of lakes Kumphawapi and Pa Kho, located in the northeastern Thailand, and lake Nong

Thale Pron in southern Thailand. To verify that δDwax can indeed be effective in its application as a paleoclimatic tool, both the source of precipitation and atmospheric variables that impact on the hydrogen isotopic composition of precipitation in the region were investigated. It was found that in Thailand, δD of precipitable water is influenced both by the amount effect, and by the El Niño Southern Oscillation

(ENSO): El Niño and La Niña lead to a decrease and increase in annual rainfall amount, respectively (paper II). Since δDwax largely tracks δD of precipitable water

(Sachse et al, 2012), higher δDwax values have been interpreted as dryness/decreased rainfall, whereas more negative values correspond to wetness/increased rainfall (paper II).

Hydroclimate reconstructions for northeastern Thailand based on δDwax over the last ten thousand years suggested higher moisture availability between ca. 10,700-7000 cal. BP, ca. 2000-1400 cal. BP and from ca. 700-1000 CE, whereas the driest intervals were from ca. 50 BCE to ca. 700 CE and ca. 1300 to ca. 1500 CE (papers IV and V). Higher moisture availability between ca. 7000-6400 cal. BP was used to elucidate a critical paleoenvironmental issue of Lake Kumphawapi’s infilling, in part coming to contradicting conclusions of earlier studies. The hydroclimate variability inferred from δDwax also allowed us to explore the mechanism influencing Southeast Asian hydroclimate over the last 2000 years by comparing the δDwax record from Lake Pa Kho with other paleoclimate records from the tropical-Asian region. This has led to the suggestion that an ENSO-like mechanism influences the spatiotemporal variability of the summer monsoon significantly on centenial to sub-millenial timescales. At these timescales, the overall cumulative effect of El Niño on decadal to centennial timescales renders most part of Mainland Southeast Asia wet, particularly Thailand, and relatively dry during La Niña events (paper II).

A lipid biomarker-based approach has also been used to re-interpret the carbon 13 isotopic composition of bulk organic matter (δ Cbulk) retrieved from Lake Pa Kho. A previous study (Chawchai et al., 2015a) supported by plant macrofossils interpeted 13 δ Cbulk values to reflect the contribution of terrestrial versus aquatic plants and algae. 13 An increase in δ Cbulk values was interpreted as reflecting more aquatic plants and a 13 decrease in δ Cbulk values as a greater terrestrial influence (Chawchai et al., 2015a). While this study provided some insight into the environmental conditions of Lake Pa 13 Kho, it still could not explain the mechanism that caused the increase in δ Cbulk values. There was not enough knowledge of what is contained in the bulk organic matter, making it difficult to constrain the environmental parameters controlling 13 organic matter input into Lake Pa Kho. The new δ Cwax dataset generated from Lake

Pa Kho indicated that the pronounced variability observed in the δ13C bulk values was 13 similar to that of δ C values of C33-C35 n-alkanes, and both expressed a much greater amplitude than other alkane homologues. The most viable explanation for this 13 puzzling data is one where increasing δ C values of C33-C35 n-alkanes are attributed - to changes in carbon source from (dissolved) CO2 to HCO3 by dominant macrophytes (e.g. Potamogeton spp.) (paper I).

13 The combination of δDwax and δ Cwax has been used to provide a combined record of moisture availability, and water management practices of the Angkor civilisation in Northeastern Thailand. Favourable hydroclimate conditions most likely contributed to the success of the empire, which hinged on rice production. A shift from drier to wetter conditions coincided in time with the rise of the Angkor Civilization and likely favored the intense agriculture needed to sustain the empire. Although there is a general consensus on the role of climate in the success and demise of Angkor civilization (Buckley et al., 2010, 2014; Cook et al., 2010; Lieberman and Buckley, 2012), there has not been any evidence of adaptability of the Angkor populace to changing climatic condition. The civilization reached its peak at ca. 1200 years CE about two hundred years after rainfall intensity had started to decline (paper III). The effect of dwindling rainfall intensity on agricultural activities was likely mitigated temporarily by the complex hydrological systems built to conserve water. Subsequently, the complex hydrological system was stretched to its limit and was likely not enough to support the large scale agriculture that Angkor was dependant upon. This led to the apparent encroachment of unconventional water bodies such as wetlands for agricultural purposes (paper III).

The ~150-year δDwax record from Lake Nong Thale Pron, southern Thailand, does not overlap with the δDwax records generated from Lakes Kumphawapi and Pa Kho, and in that sense cannot provide the initially envisioned spatio-temporal mapping of rainfall between the northeast and the south of Thailand. However, this record must be seen as an important first step towards that goal, because it can be connected to a much longer and continuous sediment record from the same lake, work that is currently in progress. The high-resolution 150-year record presented in the thesis can

30 directly be compared with instrumental data, thereby serving to evaluate the used climate proxies.

Reconstructed hydroclimate conditions for southern Thailand show a relatively drier period from ~1857-1916 and ~1970-2010 CE and relatively wetter conditions from ~1916-1969 CE (Yamoah et al., 2016). Additionally, the Lake Nong Thale Pron study presents an extensive dataset that combines lipid biomarkers and ancient DNA and is the first study ever in a tropical lake in SE Asia. The results are used to unravel trends in microbial community, and how these relate to changing environmental conditions, including rainfall variability. Drier conditions from~1857-1916 CE coincided with a dominance of the green algae Botryococcus braunii, indicating oligotrophic conditions. Subsequently, an increase in rainfall from ~1916-1969 CE led to increased runoff, which washed silica into the lake and favored diatom blooms. The decrease in rainfall intensity between ~1970 and 2010 CE coincided with more eutrophic conditions, indicated by a stark increase in cyanobacterial growth. This was however linked to an increase in anthropogenic Phosphorus input (Yamoah et al., 2016).

This dissertation presents a comprehensive combination of organic geochemistry and paleoclimatic inference. It has provided some new perspectives on tropical climate dynamics in Southeast Asia. In particular, it was found that ENSO-like variability appears to play an equally important role than shifts in the mean position of the ITCZ. Additionally, it has advanced understanding of the effect climate has had on the rise and demise of the great Angkor Civilization. Lastly, the power of combining lipid biomarkers and ancient DNA analysis is showcased, a technique that has the potential to open up new opportunities for multidisciplinary studies.

7. Future perspective and ongoing work

Notwithstanding the contribution made in this thesis towards reconstructing the hydroclimate and environment of Southeast Asia, proxy records are still rare. Moreover, some conclusions made in this thesis may need confirmation. In particular, future work could focus on modelling to understand the ENSO-like mechanism that has been proposed to influence the spatiotemporal variability of the summer monsoon on centenial to sub-millenial timescales.

Current ongoing work on a long continuous sediment core taken from Lake Nong Thale Pron extends into the last glacial-interglacial transition. Radiocarbon dates estimate the age of the core to be between between ca. 2000 and ca. 18,000 cal. BP. The following datasets have been generated for this core at 5 cm (centennial) resolution, but have not yet been evaluated and interpreted, and therefore have not been included in the thesis:

• n-alkane concentrations • Highly branched isoprenoids (HBIs) concentrations • Botryococcenes concentrations 13 • δD and δ C values of n-alkanes 13 • δD and δ C values of HBIs 13 • δD and δ C values of Botryococcenes

Glycerol dialkyl glycerol tetraether (GDGTs) abundances, which have a potential use as a proxy for past temperature (Weijers et al., 2007), have also been generated. These data, combined with other sedimentary data like plant macrofossil analysis, bulk 13 15 Carbon, Nitrogen contents and their isotopic composition (δ Cbulk and δ Nbulk), promises to become a highly relevant proxy record to gain more insight in the role tropical southeast Asia and the Indo Pacific Warm Pool have in the worldwide climate system and how this links to past climate variability.

8. Acknowledgements

First I would like to thank the Almighty God for making my PhD studies a reality. My sincere gratitude goes to my supervisors Rienk Smittenberg and Barbara Wohlfarth who gave me the opportunity to be part of the Thailand Monsoon project. I thank my colleagues on this project, Akkaneewut (Nut) Chabangborn, Sakonvan (Moo) Chawchai and Frederik Schenk for their tremendous discussions and inputs and also other members of the Thailand Monsoon project: Sherilyn Fritz, Ludvig Löwemark, Minna Väliranta, Wichuratree Klubseang, and Suda Inthongkaew for their contribution in terms of advice and fieldwork. A special thank you goes to Jayne Rattray, Alan Wiech, Christoffer Hemmingsson and Anna Hägglund for their assistance in the laboratory and discussions. A special thank you goes to the entire geochemistry group particularly Patrick Crill for his interest in my progress and wonderful support during my PhD. I am also grateful to Eve Arnold for her immense advice and encouragement. My appreciation goes to Hildred Crill for helping me to improve my writing skills and always insisting on sentence structure. Special thanks to Ernest Chi Fru and Nolwenn Callac for the amazing collaboration; they taught me everything about qPCR. I really appreciate sharing an office with Susanne Sjöberg, Engy Ahmed and Christoffer Hemmingsson; I really appreciate the countless discussions we’ve had. Last but not least, I would like to thank all my family Jennifer Afua Afrifa, Kwabena Afrifa, Maame Yaa and finally my mother Gladys Osaah for her support.

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