The Lateglacial/early Holocene sediments from the ancient lake at Hasseldala Port, southern Sweden provide an important archive for the environmental and climatic shifts at the end of the last ice age and the transition into the present Interglacial. The existing multi-proxy data set highlights the complex interplay of physical and ecological changes in response to climatic shifts and lake status changes. Yet, it remains unclear how microorganisms, such as Archaea, which do not leave microscopic features in the sedimentary record, were affected by these climatic shifts. Here we present the metagenomic data set of Hasseldala Port with a special focus on the abundance and biodiversity of Archaea. This allows reconstructing for the first time the temporal succession of major Archaea groups between 13.9 and 10.8 ka BP by using ancient environmental DNA metagenomics and fossil archaeal cell membrane lipids. We then evaluate to which extent these findings reflect physical changes of the lake system, due to changes in lake-water summer temperature and seasonal lake-ice cover. We show that variations in archaeal composition and diversity were related to a variety of factors (e.g., changes in lake water temperature, duration of lake ice cover, rapid sediment infilling), which influenced bottom water conditions and the sediment-water interface. Methanogenic Archaea dominated during the Allerod and Younger Dryas pollen zones, when the ancient lake was likely stratified and anoxic for large parts of the year. The increase in archaeal diversity at the Younger Dryas/Holocene transition is explained by sediment infilling and formation of a mire/peatbog.

The lack of semi-quantitative paleo data from mainland Southeast Asia, especially from the emergent Sundaland, creates a marked difficulty in following the past Asian monsoon variability. The published pollen records from the adjacent sites to the Sundaland were selected in this study and evaluated for the relationship between taxa and plant functional types (PFTs), and subsequently between PFTs and biome, to eventually reconstruct the semi-quantitative temporal temperature and precipitation profile. In order to comprehend the Asian monsoon modification between 18.5 and 11 ka BP, the derived pollen based temperature and precipitation records were further analyzed together with other selected speleothem records from the Asian monsoon region, with reference to the Greenland and Antarctic ice cores. The warmer temperature in the Southern Hemisphere caused a southward shift of the mean position of the Intertropical Convergent Zone (ITCZ) and weakened the summer monsoon in the Asian monsoon region between 18.5 and 15 ka BP. The Northern Hemisphere temperature played an important role in the Asian monsoon modification between 15 and 13.5 ka BP, where the warmer Northern Hemisphere conditions strengthened the summer monsoon intensity in the Asian monsoon region and moved the mean position of the ITCZ to the north. However, the opposing precipitation pattern between the East and the West Indian Ocean suggested the potential influence of the Walker circulation on the Sundaland climate from 13.5 to 11 ka BP.

The seafloor sediments of Spathi Bay, Milos Island, Greece, are part of the largest arsenic-CO2-rich shallow submarine hydrothermal ecosystem on Earth. Here, white and brown deposits cap chemically distinct sediments with varying hydrothermal influence. All sediments contain abundant genes for autotrophic carbon fixation used in the Calvin-Benson-Bassham (CBB) and reverse tricaboxylic acid (rTCA) cycles. Both forms of RuBisCO, together with ATP citrate lyase genes in the rTCA cycle, increase with distance from the active hydrothermal centres and decrease with sediment depth. Clustering of RuBisCO Form II with a highly prevalent Zetaproteobacteria 16S rRNA gene density infers that ironoxidizing bacteria contribute significantly to the sediment CBB cycle gene content. Three clusters form from different microbial guilds, each one encompassing one gene involved in CO2 fixation, aside from sulfate reduction. Our study suggests that the microbially mediated CBB cycle drives carbon fixation in the Spathi Bay sediments that are characterized by diffuse hydrothermal activity, high CO2, As emissions and chemically reduced fluids. This study highlights the breadth of conditions influencing the biogeochemistry in shallow CO2-rich hydrothermal systems and the importance of coupling highly specific process indicators to elucidate the complexity of carbon cycling in these ecosystems.

This paper investigates the possible social responses to changes in the strength of the southwest monsoon in northeastern Thailand during the currency of the Angkor civilisation. These assessments are based on hydrogen and carbon isotope records of leaf waxes (delta D-wax and delta 13C(wax)) from a 2000-year-long wetland sequence of Pa Kho in northeastern Thailand, a region that formed the northern boundary of the Angkor Kingdom. Our data indicate anthropogenic flooding of the Pa Kho wetland through the control of water through dam construction from c. AD 1300 in response to the fluctuating strength of monsoon rains. delta D-wax, a proxy for regional hydroclimate variability, corroborates pre-existing evidence that increased summer monsoon rains, which supported the expansion of the agrarian economy, aided the rise of the Angkorian Empire whereas extreme drought contributed to its demise. Interestingly, our delta D-wax record shows already a gradual decreasing monsoon intensity from c. AD 1000 onwards, although Angkor's prosperity reached its peak at c. AD 1200. We suggest that the complex hydrological system established under royal patronage at Angkor provided a resilient buffer against short-term monsoon fluctuations. The long-term decline in monsoon rains over a similar to 300-year period, combined with ongoing urbanisation, may have stretched the hydrological systems to their limit. We suggest that this was a major factor that contributed to the demise of Angkor in the mid-15th century.

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.

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 (delta D-wax) from a sedimentary sequence recovered from Lake Pa Kho, Northern Thailand. Evaluation of present day rainfall patterns and water isotope data indicates that delta D-wax reflects the amount of rainfall and is also influenced by El Nino-Southern Oscillation (ENSO) dynamics. 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 El Nino-like climate conditions. This pattern stands in contrast to the annual event where El Nino cause 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.

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 (δD_wax) in Thailand was shown to relate to the amount of precipitation and the extent of the El Niño Southern Oscillation.  Higher values of δD_wax 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 δD_wax 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. δD_wax-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.

This article integrates palaeoenvironmental and archaeological sequences covering the mid- to late-Holocene in Northeast Thailand. The former reveal the fluctuating intensity of the Asian summer monsoon, leading to periods of higher moisture availability with intervals of relative aridity. The latter are founded on a series of new radiocarbon determinations that provide a basic chronological framework, from the initial Neolithic settlements by rice farmers (c. 3700 cal. BP) to the end of the prehistoric Iron Age around 1300 cal. BP. By dovetailing the two, we find that periods of relative aridity occurred during the later Iron Age as an agricultural revolution witnessed water control measures, plough and irrigated rice cultivation and a marked rise in social elites. The correlation between climatic and cultural changes is found to continue into the period of the Angkorian state. Rather than cause a decline and/or abandonment of late Iron Age settlements, we find that the environmental stress caused by a weaker summer monsoon was met by a strong social response and by adaptations that generated a transition into early socially hierarchic polities.

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.

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 (δ¹³C_wax) 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 δ¹³C_wax and in δ¹³C of bulk organic matter (δ¹³C_bulk). 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 δ¹³C_bulk. 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.