| 1. |
- Afrifa, Yamoah Kweku Kyei, 1984-
(författare)
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Reconstruction of the Southeast Asian hydro-climate using biomarkers and their hydrogen isotopic composition
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Southeast Asia is characterized by a monsoonal climate, with distinct wet and dry seasons. This has great impact on societies, agriculture and infrastructures. Despite the critical importance to understand the mechanisms that influence the variability of the Asian Monsoon, there is scarcity of both historical and paleoclimate proxy data from Southeast Asia. For this reason, two lakes from Thailand, Lake Pa Kho (LPK) and Lake Nong Thale Pron (NTP), which are located in the northeastern and southern part of Thailand, respectively, were cored. The region also offers the opportunity to study the potential influence of climate on the Angkor civilization. Overall, this project seeks to improve our understanding of the mechanisms that contribute to Asian monsoon variability and how the variability influenced Angkor Civilization. Here I present results on a 2000-years sediment record from LPK. The most important part of the work presented here consists of compound-specific hydrogen isotope ratios (δD), which are used to infer past changes in the hydrological cycle of Southeast Asia. This approach is based on the premise that δD of lipid biomarkers from plants, algae and microorganisms deposited in sediments reflects the δD of their source water, which in turn is influenced by local hydrology. A rapid increase in precipitation is inferred from ca. 700 to ca. 850 AD, after a long dry phase. The inferred shift to wet conditions likely contributed to the rise of the Angkor Civilization, by boosting agriculture. However, gradual drying occurred at around 900 AD until the 19th century. This long-term decline in precipitation, favoring ever more frequent occurrences of severe droughts, likely also contributed to the demise and fall of the Angkor, around 1400 AD. Comparison with other hydroclimate proxy records revealed that wet conditions in tropical SE Asia corresponded to a dry Western Pacific, wet conditions in the East Pacific, and vice versa - a pattern that can be explained by opposing centers of convection and subsidence. Moreover, our tropical record also appears to be anti-correlated with the subtropical East Asian Monsoon, possibly caused by rainout effects along moisture trajectories. These long-term rainfall shifts closely match patterns observed during periods of strong El Niño, and suggests a central role for Pacific Walker circulation as a driver of centennial-scale hydroclimatic change. Besides these results from LPK oriented towards reconstructing hydroclimate, I also present some initial results concerning the evolution of the plant community of LPK, based on compound specific 13C analysis, as well as first biomarker results from NTP.
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| 2. |
- Ahmed, Engy, et al.
(författare)
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Archaeal community changes in Lateglacial lake sediments: Evidence from ancient DNA
- 2018
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 181, s. 19-29
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Tidskriftsartikel (refereegranskat)abstract
- The Lateglacial/early Holocene sediments from the ancient lake at Hässeldala 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 Hässeldala 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 Allerød 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.
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| 3. |
- Björck, Svante, et al.
(författare)
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A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial
- 2019
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 15:6, s. 1939-1958
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4-18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37ĝF S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3-4 ĝC, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas..
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| 4. |
- Callac, Nolwenn, et al.
(författare)
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Modes of carbon fixation in an arsenic and CO2-rich shallow hydrothermal ecosystem
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322.
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Tidskriftsartikel (refereegranskat)abstract
- 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 iron-oxidizing 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.
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| 5. |
- Chawchai, Sakonvan, et al.
(författare)
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Lake Kumphawapi revisited – a synthesis of Holocene environmental and climatic changes for NE Thailand
- 2016
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 26:4, s. 614-626
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- De Jonge, Cindy, et al.
(författare)
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The impact of soil chemistry, moisture and temperature on branched and isoprenoid GDGTs in soils : A study using six globally distributed elevation transects
- 2024
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Ingår i: Organic Geochemistry. - 0146-6380 .- 1873-5290. ; 187
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Tidskriftsartikel (refereegranskat)abstract
- Glycerol dialkyl glycerol tetraethers (GDGTs) are microbial membrane-spanning lipids that are produced in a variety of environments. To better understand the potentially confounding effect of soil chemistry on the temperature relationship of branched GDGTs (brGDGTs), isoprenoid GDGTs (isoGDGTs) and GDGT-based proxies MBT’5ME and TEX86, soils from 6 elevation transects (mean annual air temperature 0 – 26 ℃, n = 74) were analyzed. Corroborating earlier work, the MBT’5ME index correlates well with mean annual air temperature in the low pH (pH < 7), non-arid soils under study (r = 0.87, p < 0.001). However, a clear over-estimation of reconstructed temperature in the lowest pH (<3.5) soils is observed, explained by the correlation between brGDGT Ia and free acidity. TEX86 also shows a significant correlation with mean annual air temperature (r = 0.45, p < 0.001), driven by temperature dependent concentration changes of isoGDGTs 3 and cren’. However, an overarching correlation with P/E values dominates concentration changes of all supposed Thaumarchaeotal isoGDGTs lipids (GDGT1-3, cren and cren’), implying a potential impact of soil moisture on TEX86 values. In addition to identifying the impact of these confounding factors on the temperature proxy, GDGT ratios that can be used to constrain changes in soil chemistry, specifically exchangeable Ca2+, sum of basic cations, exchangeable Fe3+ and sum of soil metals are proposed (0.53 < r2 < 0.68), while existing ratios for soil moisture availability are tested for the first time in a dataset of non-arid soils. While the impact of soil chemistry on GDGTs may complicate the interpretation of their temperature proxies, our proposed GDGT ratios can potentially be used to constrain a subset of soil chemistry changes through time.
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| 7. |
- Fischer, Benjamin M. C., et al.
(författare)
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Mojito, Anyone? An Exploration of Low-Tech Plant Water Extraction Methods for Isotopic Analysis Using Locally-Sourced Materials
- 2019
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Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- The stable isotope composition of water (delta O-18 and delta H-2) is an increasingly utilized tool to distinguish between different pools of water along the soil-plant-atmosphere continuum (SPAC) and thus provides information on how plants use water. Clear bottlenecks for the ubiquitous application of isotopic analysis across the SPAC are the relatively high-energy and specialized materials required to extract water from plant materials. Could simple and cost-effective do-it-yourself MacGyver methods be sufficient for extracting plant water for isotopic analysis? This study develops a suite of novel techniques for plant water extraction and compares them to a standard research-grade water extraction method. Our results show that low-tech methods using locally-sourced materials can indeed extract plant water consistently and comparably to what is done with other state-of-the-art methods. Further, our findings show that other factors play a larger role than water extraction methods in achieving the desired accuracy and precision of stable isotope composition: (1) appropriate transport, (2) fast sample processing and (3) efficient workflows. These results are methodologically promising for the rapid expansion of isotopic investigations, especially for citizen science and/or school projects or in remote areas, where improved SPAC understanding could help manage water resources to fulfill agricultural and other competing water needs.
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| 8. |
- Gierga, Merle, et al.
(författare)
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Long-stored soil carbon released by prehistoric land use : Evidence from compound-specific radiocarbon analysis on Soppensee lake sediments
- 2016
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 144, s. 123-131
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Tidskriftsartikel (refereegranskat)abstract
- Compound-specific radiocarbon (C-14) analyses allow studying the fate of individual biomarkers in ecosystems. In lakes with small catchments, terrestrial biomarkers have the potential to be used for the dating of sediments that lack the traditionally targeted terrestrial macrofossils, if the specific organic compounds are deposited soon after production. On the other hand, if the biomarkers have been stored for a significant amount of time in the soils of the catchment before transported to the lake, their age can be used to reconstruct changes in average residence time of organic material on land through time. Here we present a study based on compound-specific C-14 analysis of the sedimentary record of Lake Soppensee, Switzerland, targeting long-chain n-alkanes of exclusive terrigenous origin, and comparing them with sediment ages obtained by high-resolution macrofossil dating. Additionally, we measured C-14 ages of bulk organic matter and carbonate samples to assess the hard water effect. Prior to 3100 cal BP n-alkanes had about the same age as the sediment or they were slightly older, indicating that the vast majority of the terrestrial organic carbon transported to the lake had a short residence time on land. In the samples younger than 3100 cal BP an increasing offset is observed, indicating liberation of old buried soil organic matter that must have accumulated over the previous millennia. Our results indicate that as long as stable ecosystem conditions have prevailed, the distribution and isotopic composition of the n-alkanes can be used as environmental proxies in small catchments with limited surface runoff, confirming a few earlier studies.
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| 9. |
- Gierga, Merle, et al.
(författare)
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Purification of fire derived markers for mu g scale isotope analysis (delta C-13, Delta C-14) using high performance liquid chromatography (HPLC)
- 2014
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Ingår i: Organic Geochemistry. - : Elsevier BV. - 0146-6380 .- 1873-5290. ; 70, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Black carbon (BC) is the residue of incomplete biomass combustion. It is ubiquitous in nature and, due to its relative persistence, is an important factor in Earth's slow-cycling carbon pool. This resistant nature makes pure BC one of the most used materials for C-14 dating to elucidate its formation date or residence time in the environment. However, most BC samples cannot be physically separated from their matrices, precluding accurate C-14 values. Here we present a method for radiocarbon dating of the oxidation products of BC, benzene polycarboxylic acids, thereby circumventing interference from extraneous carbon. Individual compounds were isolated using high performance liquid chromatography (HPLC) and converted to CO2 via wet chemical oxidation for C-13 and C-14 isotope analysis. A detailed assessment was performed to identify and quantify sources of extraneous carbon contamination using two process standards with distinct isotopic signatures. The average blank was 1.6 +/- 0.7 mu g C and had an average radiocarbon content of 0.90 +/- 0.50 (FC)-C-14. We successfully analyzed the C-14 content of individual benzene polycarboxylic acids with a sample size as small as 20-30 mu g C after correcting for the presence of the average blank. The combination of delta C-13 and (FC)-C-14 analysis helps interpret the results and enables monitoring of extraneous carbon contribution in a fast and cost efficient way. Such a molecular approach to radiocarbon dating of BC residues enables the expansion of isotopic BC studies to samples that have either been too small or strongly affected by non-fire derived carbon.
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| 10. |
- Guelland, Kathi, et al.
(författare)
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Evolution of carbon fluxes during initial soil formation along the forefield of Damma glacier, Switzerland
- 2013
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Ingår i: Biogeochemistry. - : Springer. - 0168-2563 .- 1573-515X. ; 113:1-3, s. 545-561
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Tidskriftsartikel (refereegranskat)abstract
- Soil carbon (C) fluxes, soil respiration and dissolved organic carbon (DOC) leaching were explored along the young Damma glacier forefield chronosequence (7-128 years) over a three-year period. To gain insight into the sources of soil CO2 effluxes, radiocarbon signatures of respired CO2 were measured and a vegetation-clipping experiment was performed. Our results showed a clear increase in soil CO2 effluxes with increasing site age from 9 +/- A 1 to 160 +/- A 67 g CO2-C m(-2) year(-1), which was linked to soil C accumulation and development of vegetation cover. Seasonal variations of soil respiration were mainly driven by temperature; between 62 and 70 % of annual CO2 effluxes were respired during the 4-month long summer season. Sources of soil CO2 effluxes changed along the glacier forefield. For most recently deglaciated sites, radiocarbon-based age estimates indicated ancient C to be the dominant source of soil-respired CO2. At intermediate site age (58-78 years), the contribution of new plant-fixed C via rhizosphere respiration amounted up to 90 %, while with further soil formation, heterotrophically respired C probably from accumulated 'older' soil organic carbon (SOC) became increasingly important. In comparison with soil respiration, DOC leaching at 10 cm depth was small, but increased similarly from 0.4 +/- A 0.02 to 7.4 +/- A 1.6 g DOC m(-2) year(-1) over the chronosequence. A strong rise of the ratio of SOC to secondary iron and aluminium oxides strongly suggests that increasing DOC leaching with site age results from a faster increase of the DOC source, SOC, than of the DOC sink, reactive mineral surfaces. Overall, C losses from soil by soil respiration and DOC leaching increased from 9 +/- A 1 to 70 +/- A 17 and further to 168 +/- A 68 g C m(-2) year(-1) at the < 10, 58-78, and 110-128 year old sites. By comparison, total ecosystem C stocks increased from 0.2 to 1.1 and to 3.1 kg C m(-2) from the young to intermediate and old sites. Therefore, the ecosystem evolved from a dominance of C accumulation in the initial phase to a high throughput system. We suggest that the relatively strong increase in soil C stocks compared to C fluxes is a characteristic feature of initial soil formation on freshly exposed rocks.
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