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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
Stockholm University, Faculty of Science, Department of Geological Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2016.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 363
Keyword [en]
Holocene, summer monsoon, biomarkers, hydrogen isotopes, lake sediment, peat, Thailand
National Category
Geosciences, Multidisciplinary
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-128736ISBN: 978-91-7649-378-6 (print)OAI: oai:DiVA.org:su-128736DiVA: diva2:916386
Public defence
2016-05-20, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:30 (English)
Opponent
Supervisors
Projects
Monsoon project
Funder
Swedish Research Council, 621-2011-4916Swedish Research Council, 348-2008-6071Swedish Research Council, 621-2008-2855
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Manuscript.

Available from: 2016-04-27 Created: 2016-04-01 Last updated: 2017-02-24Bibliographically approved
List of papers
1. Large variability in n-alkane δ13C values in Lake Pa Kho (Thailand) driven by wetland wetness and aquatic productivity
Open this publication in new window or tab >>Large variability in n-alkane δ13C values in Lake Pa Kho (Thailand) driven by wetland wetness and aquatic productivity
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2016 (English)In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 97, 53-60 p.Article in journal (Refereed) Published
Abstract [en]

Understanding past climate and environmental conditions depends largely on accurate interpretations of proxy records from a range of environments, including tropical wetlands and lakes. Lipid biomarker analysis can provide important information about the sources of the accumulated organic material, and thus about the environmental information contained therein. Here we use n-alkane distributions and stable carbon isotopes of leaf waxes (δ13Cwax) to identify the sources of organic matter (OM) of a 2000-year long sediment/peat record from Lake Pa Kho (LPK) in northeastern Thailand, and to constrain the mechanisms that cause shifts in δ13Cwax and in δ13C of bulk organic matter (δ13Cbulk). Our results show three main sources of OM: terrestrial plants, aquatic macrophytes and algae. The δ13C values of the long chain n-alkanes, show two distinct groups: C27–C31 and C33–C35n-alkanes, where the δ13C values of C33–C35n-alkanes reflect that of δ13Cbulk. Lower moisture availability on the wetland, known from other sedimentary evidence, was characterized by low carbon isotope values typically seen for C3 plants, whereas greater moisture availability corresponded to higher δ13C values (around –20‰) of C33–C35n-alkanes, resembling a typical C4 plant signal. However, various lines of evidence argue against large shifts between C3 and C4 plant input. Instead, we suggest that the high δ13C values were indirectly caused by higher aquatic productivity during periods of greater moisture availability, decreasing dissolved CO2, but increasing bicarbonate availability caused by higher pH. This caused the dominant macrophytes (e.g., Potamogeton spp.) to shift their carbon source from CO-2 to bicarbonate, which has much higher δ13C values. Our results show that the environmental context should be taken into account when interpreting n-alkane δ13C variability as a paleo-environmental/climatic signal as this contains several important variables that need to be disentangled and explained.

Keyword
δ13Cwax, wetland, carbon fractionation, late Holocene, Lake Pa Kho, Northeastern Thailand
National Category
Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-128991 (URN)10.1016/j.orggeochem.2016.04.008 (DOI)000376532900006 ()
Funder
Swedish Research Council
Available from: 2016-04-11 Created: 2016-04-11 Last updated: 2017-11-30Bibliographically approved
2. A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a sub-millenial timescale
Open this publication in new window or tab >>A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a sub-millenial timescale
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(English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457XArticle in journal (Refereed) Submitted
National Category
Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-128992 (URN)
Available from: 2016-04-11 Created: 2016-04-11 Last updated: 2017-11-30Bibliographically approved
3. Human adaptation to changing climate: an insight from Angkor Civilization
Open this publication in new window or tab >>Human adaptation to changing climate: an insight from Angkor Civilization
(English)Manuscript (preprint) (Other academic)
National Category
Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-129478 (URN)
Available from: 2016-04-22 Created: 2016-04-22 Last updated: 2016-04-26Bibliographically approved
4. Lake Kumphawapi revisited – a synthesis of Holocene environmental and climatic changes for NE Thailand
Open this publication in new window or tab >>Lake Kumphawapi revisited – a synthesis of Holocene environmental and climatic changes for NE Thailand
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2016 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 26, no 4, 614-626 p.Article in journal (Refereed) Published
Abstract [en]

Kumphawapi, which is Thailand’s largest natural freshwater lake, contains a >10,000-year-long climatic and environmental archive. New data sets (stratigraphy, chronology, hydrogen isotopes, plant macrofossil and charcoal records) for two sedimentary sequences are here combined with earlier multi-proxy studies to provide a comprehensive reconstruction of past climatic and environmental changes for Northeast Thailand. Gradually higher moisture availability due to a strengthening of the summer monsoon led to the formation of a large shallow lake in the Kumphawapi basin between >10,700 and c. 7000 cal. BP. The marked increase in moisture availability and lower evaporation between c. 7000 and 6400 cal. BP favoured the growth and expansion of vegetation in and around the shallow lake. The increase in biomass led to gradual overgrowing and infilling, to an apparent lake level lowering and to the development of a wetland. Multiple hiatuses are apparent in all investigated sequences between c. 6500 and 1400 cal. BP and are explained by periodic desiccation events of the wetland and erosion due to the subsequent lake level rise. The rise in lake level, which started c. 2000 cal. BP and reached shallower parts c. 1400 cal. BP, is attributed to an increase in effective moisture availability. The timing of hydroclimatic conditions during the past 2000 years cannot be resolved because of chronological limitations.

Keyword
archaeology, Holocene, hydroclimate, multi-proxy lake sediment study, plant wax δD, Thailand
National Category
Earth and Related Environmental Sciences
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-126155 (URN)10.1177/0959683615612565 (DOI)000372866100011 ()
Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2017-11-30Bibliographically approved
5. A 150-year record of phytoplankton 1 community succession controlled by hydroclimatic variability in a tropical lake
Open this publication in new window or tab >>A 150-year record of phytoplankton 1 community succession controlled by hydroclimatic variability in a tropical lake
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2016 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 13, no 13, 3971-3980 p.Article in journal (Refereed) Published
Abstract [en]

Climate and human-induced environmental change promotes biological regime shifts between alternate stable states, with implications for ecosystem resilience, function and services. While this has been shown for recent microbial communities, the long-term response of microbial communities has not been investigated in detail. This study investigated the decadal variations in phytoplankton communities in a ~150 year long sedimentary archive of Lake Nong Thale Prong (NTP), southern Thailand using a combi nation of DNA and lipid biomarkers techniques. Reconstructed drier climate from ~1857-1916 Common Era (CE) coincided with oligotrophic lake water conditions and dominance of the green algae Botryococcus braunii, producing characteristic botryococcene lipids. A change to higher silica (Si) input ~1916 CE, which was related to increased rainfall concurs with an abrupt takeover by diatom blooms lasting for 50 years. Since the 1970s more eutrophic conditions prevailed, which was likely caused by increased levels of anthropogenic phosphate (P), aided by increased lake stratification caused by somewhat dryer conditions. The eutrophic conditions led to increased primary productivity consisting again of a Botryococcus sp., though this time not producing the botryococcene lipids. Moreover, Cyanobacteria became dominant. Our results indicate that a combined DNA and lipid biomarker approach provides an efficient way to allow tracking centennial-scale hydroclimate and anthropogenic feedback processes in lake ecosystems.

National Category
Environmental Sciences
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-129481 (URN)10.5194/bg-2015-633 (DOI)000381099900009 ()
Available from: 2016-04-22 Created: 2016-04-22 Last updated: 2017-11-30Bibliographically approved

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