@phdthesis{vanderVeen2021,
  author    = {van der Veen, Iris},
  title     = {Defining moisture sources and (palaeo)environmental conditions using isotope geochemistry in the NW Himalaya},
  doi       = {10.25932/publishup-51439},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-514397},
  school      = {Universit{\"a}t Potsdam},
  pages     = {152},
  year      = {2021},
  abstract  = {Anthropogenic climate change alters the hydrological cycle. While certain areas experience more intense precipitation events, others will experience droughts and increased evaporation, affecting water storage in long-term reservoirs, groundwater, snow, and glaciers. High elevation environments are especially vulnerable to climate change, which will impact the water supply for people living downstream. The Himalaya has been identified as a particularly vulnerable system, with nearly one billion people depending on the runoff in this system as their main water resource. As such, a more refined understanding of spatial and temporal changes in the water cycle in high altitude systems is essential to assess variations in water budgets under different climate change scenarios. However, not only anthropogenic influences have an impact on the hydrological cycle, but changes to the hydrological cycle can occur over geological timescales, which are connected to the interplay between orogenic uplift and climate change. However, their temporal evolution and causes are often difficult to constrain. Using proxies that reflect hydrological changes with an increase in elevation, we can unravel the history of orogenic uplift in mountain ranges and its effect on the climate. In this thesis, stable isotope ratios (expressed as δ2H and δ18O values) of meteoric waters and organic material are combined as tracers of atmospheric and hydrologic processes with remote sensing products to better understand water sources in the Himalayas. In addition, the record of modern climatological conditions based on the compound specific stable isotopes of leaf waxes (δ2Hwax) and brGDGTs (branched Glycerol dialkyl glycerol tetraethers) in modern soils in four Himalayan river catchments was assessed as proxies of the paleoclimate and (paleo-) elevation. Ultimately, hydrological variations over geological timescales were examined using δ13C and δ18O values of soil carbonates and bulk organic matter originating from sedimentological sections from the pre-Siwalik and Siwalik groups to track the response of vegetation and monsoon intensity and seasonality on a timescale of 20 Myr. I find that Rayleigh distillation, with an ISM moisture source, mainly controls the isotopic composition of surface waters in the studied Himalayan catchments. An increase in d-excess in the spring, verified by remote sensing data products, shows the significant impact of runoff from snow-covered and glaciated areas on the surface water isotopic values in the timeseries. In addition, I show that biomarker records such as brGDGTs and δ2Hwax have the potential to record (paleo-) elevation by yielding a significant correlation with the temperature and surface water δ2H values, respectively, as well as with elevation. Comparing the elevation inferred from both brGDGT and δ2Hwax, large differences were found in arid sections of the elevation transects due to an additional effect of evapotranspiration on δ2Hwax. A combined study of these proxies can improve paleoelevation estimates and provide recommendations based on the results found in this study. Ultimately, I infer that the expansion of C4 vegetation between 20 and 1 Myr was not solely dependent on atmospheric pCO2, but also on regional changes in aridity and seasonality from to the stable isotopic signature of the two sedimentary sections in the Himalaya (east and west). This thesis shows that the stable isotope chemistry of surface waters can be applied as a tool to monitor the changing Himalayan water budget under projected increasing temperatures. Minimizing the uncertainties associated with the paleo-elevation reconstructions were assessed by the combination of organic proxies (δ2Hwax and brGDGTs) in Himalayan soil. Stable isotope ratios in bulk soil and soil carbonates showed the evolution of vegetation influenced by the monsoon during the late Miocene, proving that these proxies can be used to record monsoon intensity, seasonality, and the response of vegetation. In conclusion, the use of organic proxies and stable isotope chemistry in the Himalayas has proven to successfully record changes in climate with increasing elevation. The combination of δ2Hwax and brGDGTs as a new proxy provides a more refined understanding of (paleo-)elevation and the influence of climate.},
  language  = {en}
}
@phdthesis{Menges2020,
  author    = {Menges, Johanna},
  title     = {Organic Carbon Storage, Transfer and Transformation in the Himalaya},
  school      = {Universit{\"a}t Potsdam},
  pages     = {179},
  year      = {2020},
  abstract  = {The transfer of particulate organic carbon from continents to the ocean is an important component of the global carbon cycle. Transfer to and burial of photosynthetically fixed biospheric organic carbon in marine sediments can effectively sequester atmospheric carbon dioxide over geological timescales. The exhumation and erosion of fossil organic carbon contained in sedimentary rocks, i.e. petrogenic carbon, can result in remineralization, releasing carbon to the atmosphere. In contrast, eroded petrogenic organic carbon that gets transferred back to the ocean and reburied does not affect atmospheric carbon content. Mountain ranges play a key role in this transfer since they can source vast amounts of sediment including particulate organic carbon. Globally, the export of both, biospheric and petrogenic organic carbon has been linked to sediment export. Additionally, short transfer times from mountains to the ocean and high sediment concentrations have been shown to increase the likelihood of organic carbon burial. While the importance of mountain ranges in the organic carbon cycle is now widely recognized, the processes acting within mountain ranges to influence the storage, cycling and mobilization of organic carbon, as well as carbon fluxes from mountain ranges remain poorly constrained. In this thesis, I employ different methods to assess the nature and fate of particulate organic carbon in mountain belts, ranging from the molecular to regional landscape scale. These studies are located along the Trans-Himalayan Kali Gandaki River in Central Nepal. This river traverses all major geological and climatic zones of the Himalaya, from the dry northern Tibetan plateau to the high-relief, monsoon dominated steep High Himalaya and the lower relief and abundant vegetation of the Lesser Himalayan region. First, I document how biospheric organic matter has accumulated during the Holocene in the headwaters of the Kali Gandaki River valley, by combining compound specific isotope measurements with different dating methods and grain size data, and investigate the stability of this organic carbon reservoir on millennial timescales. I show, that around 1.6 ka an eco-geomorphic tipping point occurred leading to a destabilization of the landscape resulting in today's high erosion rates and the excavation of the aged organic carbon reservoir. This study highlights the climatic and geomorphic controls on biospheric organic carbon storage and release from mountain ranges. Second, I systematically investigate the spatial variation of particulate organic carbon fluxes across the Himalaya along the Kali Gandaki River, using bulk stable and radioactive isotopes combined with a new Bayesian modeling approach. The detailed dataset allows the distinction of aged and modern biospheric organic carbon as well as petrogenic organic carbon across the Himalayan mountain range and the investigation of the role of climatic and geomorphic factors in their riverine export. The data suggest a decoupling of the particulate organic carbon from the sediment yield along the Kali Gandaki River, partially driven by climatic and geomorphic processes. In contrast to the suspended sediment, a large part of the particulate organic carbon exported by the river originates from the Tibetan part of the catchment and is dominated by petrogenic organic carbon derived from Jurassic shales with only minor contributions of modern and aged biospheric organic carbon. These findings emphasize the importance of organic carbon source distribution and erosion mechanisms in determining the organic carbon export from mountain ranges. In a third step, I explore the potential of ultra-high resolution mass spectrometry for particulate organic carbon transport studies. I have generated a novel and unprecedented high-resolution molecular dataset, which contains up to 103 molecular formulas of the lipid fraction of particulate organic matter for modern and aged biospheric carbon, petrogenic organic carbon and river sediments. First, I test if this dataset can be used to better resolve different organic carbon sources and to identify new geochemical tracers. Using multivariate statistics, I identify up to 10² characteristic molecular formulas for the major organic carbon sources in the upper part of the Kali Gandaki catchment, and trace their transfer from the surrounding landscape into the river sediment. Second, I test the potential of the molecular dataset to trace molecular transformations along source-to-sink pathways. I identify changes in molecular metrics derived from the dataset, which are characteristic of transformation processes during incorporation of litter into soil, the aging of soil material, and the mobilization of the organic carbon into the river. These two studies demonstrate that high-resolution molecular datasets open a promising analytical window on particulate organic carbon and can provide novel insights into the composition, sourcing and transformation of riverine particulate organic carbon. Collectively, these studies advance our understanding of the processes contributing to the storage and mobilization of organic carbon in the Central Himalaya, the mountain belt that dominates global erosional fluxes. They do so by identifying the major sources of particulate organic carbon to the Trans-Himalayan Kali Gandaki River, by elucidating their sensitivity to climate and geomorphic processes, and by identifying some of the transformations of this material on the molecular scale. As a result, the thesis demonstrates that the amount and composition of organic carbon routed from mountain belts is a function of the dynamic interactions of geologic, biologic, geomorphic and climatic processes within the mountain belt. This understanding will ultimately help in answering whether the build-up and erosion of mountain ranges over geological time represents a net carbon source or sink to the atmosphere. Beyond this, the thesis contributes to our technical ability to characterize organic matter and attribute it to sources by scoping the potential of high-end molecular analysis.},
  language  = {en}
}
@phdthesis{Kochlik2019,
  author    = {Kochlik, Bastian Max},
  title     = {Relevance of biomarkers for the diagnosis of the frailty syndrome},
  doi       = {10.25932/publishup-44118},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-441186},
  school      = {Universit{\"a}t Potsdam},
  pages     = {IV, 99},
  year      = {2019},
  abstract  = {Frailty and sarcopenia share some underlying characteristics like loss of muscle mass, low muscle strength, and low physical performance. Imaging parameters and functional examinations mainly assess frailty and sarcopenia criteria; however, these measures can have limitations in clinical settings. Therefore, finding suitable biomarkers that reflect a catabolic muscle state e.g. an elevated muscle protein turnover as suggested in frailty, are becoming more relevant concerning frailty diagnosis and risk assessment. 3-Methylhistidine (3-MH) and its ratios 3-MH-to-creatinine (3-MH/Crea) and 3 MH-to-estimated glomerular filtration rate (3-MH/eGFR) are under discussion as possible biomarkers for muscle protein turnover and might support the diagnosis of frailty. However, there is some skepticism about the reliability of 3-MH measures since confounders such as meat and fish intake might influence 3-MH plasma concentrations. Therefore, the influence of dietary habits and an intervention with white meat on plasma 3-MH was determined in young and healthy individuals. In another study, the cross-sectional associations of plasma 3-MH, 3-MH/Crea and 3-MH/eGFR with the frailty status (robust, pre-frail and frail) were investigated. Oxidative stress (OS) is a possible contributor to frailty development, and high OS levels as well as low micronutrient levels are associated with the frailty syndrome. However, data on simultaneous measures of OS biomarkers together with micronutrients are lacking in studies including frail, pre-frail and robust individuals. Therefore, cross-sectional associations of protein carbonyls (PrCarb), 3-nitrotyrosine (3-NT) and several micronutrients with the frailty status were determined. A validated UPLC-MS/MS (ultra-performance liquid chromatography tandem mass spectrometry) method for the simultaneous quantification of 3-MH and 1-MH (1 methylhistidine, as marker for meat and fish consumption) was presented and used for further analyses. Omnivores showed higher plasma 3-MH and 1-MH concentrations than vegetarians and a white meat intervention resulted in an increase in plasma 3-MH, 3 MH/Crea, 1-MH and 1-MH/Crea in omnivores. Elevated 3-MH and 3-MH/Crea levels declined significantly within 24 hours after this white meat intervention. Thus, 3-MH and 3-MH/Crea might be used as biomarker for muscle protein turnover when subjects did not consume meat 24 hours prior to blood samplings. Plasma 3-MH, 3-MH/Crea and 3-MH/eGFR were higher in frail individuals than in robust individuals. Additionally, these biomarkers were positively associated with frailty in linear regression models, and higher odds to be frail were found for every increase in 3 MH and 3-MH/eGFR quintile in multivariable logistic regression models adjusted for several confounders. This was the first study using 3-MH/eGFR and it is concluded that plasma 3-MH, 3-MH/Crea and 3-MH/eGFR might be used to identify frail individuals or individuals at higher risk to be frail, and that there might be threshold concentrations or ratios to support these diagnoses. Higher vitamin D3, lutein/zeaxanthin, γ-tocopherol, α-carotene, β-carotene, lycopene and β-cryptoxanthin concentrations and additionally lower PrCarb concentrations were found in robust compared to frail individuals in multivariate linear models. Frail subjects had higher odds to be in the lowest than in the highest tertile for vitamin D3 α-tocopherol, α-carotene, β-carotene, lycopene, lutein/zeaxanthin, and β cryptoxanthin, and had higher odds to be in the highest than in the lowest tertile for PrCarb than robust individuals in multivariate logistic regression models. Thus, a low micronutrient together with a high PrCarb status is associated with pre-frailty and frailty.},
  language  = {en}
}
@phdthesis{Stoessel2018,
  author    = {St{\"o}ßel, Daniel},
  title     = {Biomarker Discovery in Multiple Sclerosis and Parkinson's disease},
  school      = {Universit{\"a}t Potsdam},
  pages     = {135},
  year      = {2018},
  abstract  = {Neuroinflammatory and neurodegenerative diseases such as Parkinson's (PD) and multiple sclerosis (MS) often result in a severe impairment of the patient´s quality of life. Effective therapies for the treatment are currently not available, which results in a high socio-economic burden. Due to the heterogeneity of the disease subtypes, stratification is particularly difficult in the early phase of the disease and is mainly based on clinical parameters such as neurophysiological tests and central nervous imaging. Due to good accessibility and stability, blood and cerebrospinal fluid metabolite markers could serve as surrogates for neurodegenerative processes. This can lead to an improved mechanistic understanding of these diseases and further be used as "treatment response" biomarkers in preclinical and clinical development programs. Therefore, plasma and CSF metabolite profiles will be identified that allow differentiation of PD from healthy controls, association of PD with dementia (PDD) and differentiation of PD subtypes such as akinetic rigid and tremor dominant PD patients. In addition, plasma metabolites for the diagnosis of primary progressive MS (PPMS) should be investigated and tested for their specificity to relapsing-remitting MS (RRMS) and their development during PPMS progression. By applying untargeted high-resolution metabolomics of PD patient samples and in using random forest and partial least square machine learning algorithms, this study identified 20 plasma metabolites and 14 CSF metabolite biomarkers. These differentiate against healthy individuals with an AUC of 0.8 and 0.9 in PD, respectively. We also identify ten PDD specific serum metabolites, which differentiate against healthy individuals and PD patients without dementia with an AUC of 1.0, respectively. Furthermore, 23 akinetic-rigid specific plasma markers were identified, which differentiate against tremor-dominant PD patients with an AUC of 0.94 and against healthy individuals with an AUC of 0.98. These findings also suggest more severe disease pathology in the akinetic-rigid PD than in tremor dominant PD. In the analysis of MS patient samples a partial least square analysis yielded predictive models for the classification of PPMS and resulted in 20 PPMS specific metabolites. In another MS study unknown changes in human metabolism were identified after administration of the multiple sclerosis drug dimethylfumarate, which is used for the treatment of RRMS. These results allow to describe and understand the hitherto completely unknown mechanism of action of this new drug and to use these findings for the further development of new drugs and targets against RRMS. In conclusion, these results have the potential for improved diagnosis of these diseases and improvement of mechanistic understandings, as multiple deregulated pathways were identified. Moreover, novel Dimethylfumarate targets can be used to aid drug development and treatment efficiency. Overall, metabolite profiling in combination with machine learning identified as a promising approach for biomarker discovery and mode of action elucidation.},
  language  = {en}
}
@phdthesis{Genderjahn2018,
  author    = {Genderjahn, Steffi},
  title     = {Biosignatures of Present and Past Microbial Life in Southern African Geoarchives},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-410110},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XI, 166, xxii},
  year      = {2018},
  abstract  = {Global climate change is one of the greatest challenges of the 21st century, with influence on the environment, societies, politics and economies. The (semi-)arid areas of Southern Africa already suffer from water scarcity. There is a great variety of ongoing research related to global climate history but important questions on regional differences still exist. In southern African regions terrestrial climate archives are rare, which makes paleoclimate studies challenging. Based on the assumption that continental pans (sabkhas) represent a suitable geo-archive for the climate history, two different pans were studied in the southern and western Kalahari Desert. A combined approach of molecular biological and biogeochemical analyses is utilized to investigate the diversity and abundance of microorganisms and to trace temporal and spatial changes in paleoprecipitation in arid environments. The present PhD thesis demonstrates the applicability of pan sediments as a late Quaternary geo-archive based on microbial signature lipid biomarkers, such as archaeol, branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) as well as phospholipid fatty acids (PLFA). The microbial signatures contained in the sediment provide information on the current or past microbial community from the Last Glacial Maximum to the recent epoch, the Holocene. The results are discussed in the context of regional climate evolution in southwestern Africa. The seasonal shift of the Innertropical Convergence Zone (ITCZ) along the equator influences the distribution of precipitation- and climate zones. The different expansion of the winter- and summer rainfall zones in southern Africa was confirmed by the frequency of certain microbial biomarkers. A period of increased precipitation in the south-western Kalahari could be described as a result of the extension of the winter rainfall zone during the last glacial maximum (21 ± 2 ka). Instead a period of increased paleoprecipitation in the western Kalahari was indicated during the Late Glacial to Holocene transition. This was possibly caused by a southwestern shift in the position of the summer rainfall zone associated to the southward movement of the ITCZ. Furthermore, for the first time this study characterizes the bacterial and archaeal life based on 16S rRNA gene high-throughput sequencing in continental pan sediments and provides an insight into the recent microbial community structure. Near-surface processes play an important role for the modern microbial ecosystem in the pans. Water availability as well as salinity might determine the abundance and composition of the microbial communities. The microbial community of pan sediments is dominated by halophilic and dry-adapted archaea and bacteria. Frequently occurring microorganisms such as, Halobacteriaceae, Bacillus and Gemmatimonadetes are described in more detail in this study.},
  language  = {en}
}
@phdthesis{Grewe2016,
  author    = {Grewe, Sina},
  title     = {Hydro- and biogeochemical investigations of lake sediments in the Kenyan Rift Valley},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-98342},
  school      = {Universit{\"a}t Potsdam},
  pages     = {110},
  year      = {2016},
  abstract  = {Die Seen im kenianischen Riftsystem bieten die einmalige Gelegenheit eine große Bandbreite an hydrochemischen Umweltbedingungen zu studieren, die von S{\"u}ßwasserseen bis hin zu hochsalinen und alkalinen Seen reichen. Da wenig {\"u}ber die hydro- und biogeochemischen Bedingungen in den darunterliegenden Seesedimenten bekannt ist, war es das Ziel dieser Arbeit, bereits existierende Datens{\"a}tze mit Daten aus der Porenwasser- und Biomarker-Analyse zu erweitern. Zus{\"a}tzlich wurden reduzierte Schwefelkomponenten und Sulfatreduktionsraten in den Sedimenten bestimmt. Mit den neu gewonnenen Daten wurde der anthropogene und mikrobielle Einfluss auf die Seesedimente untersucht sowie der Einfluss der Wasserchemie auf den Abbau und den Erhalt von organischem Material im Sediment. Zu den untersuchten Seen geh{\"o}rten: Logipi, Eight (ein kleiner Kratersee in der Region Kangirinyang), Baringo, Bogoria, Naivasha, Oloiden und Sonachi. Die Biomarker-Zusammensetzungen in den untersuchten Seesedimenten waren {\"a}hnlich; allerdings gab es einige Unterschiede zwischen den salinen Seen und den S{\"u}ßwasserseen. Einer dieser Unterschiede war das Vorkommen eines mit β-Carotin verwandten Molek{\"u}ls, das nur in den salinen Seen gefunden wurde. Dieses Molek{\"u}l stammt wahrscheinlich von Cyanobakterien, Einzellern die in großer Anzahl in salinen Seen vorkommen. In den beiden S{\"u}ßwasserseen wurde Stigmasterol gefunden, ein f{\"u}r S{\"u}ßwasseralgen charakteristisches Sterol. In dieser Studie hat sich gezeigt, dass Bogoria und Sonachi f{\"u}r Umweltrekonstruktionen mit Biomarkern besonders gut geeignet sind, da die Abwesenheit von Sauerstoff an deren Seegr{\"u}nden den Abbau von organischem Material verlangsamt. Andere Seen, wie zum Beispiel Naivasha, sind aufgrund des großen anthropogenen Einflusses weniger gut f{\"u}r solche Rekonstruktionen geeignet. Die Biomarker-Analyse bot jedoch die M{\"o}glichkeit, den menschlichen Einfluss auf den See zu studieren. Desweiteren zeigte diese Studie, dass sich Horizonte mit einem hohen Anteil an elementarem Schwefel als temporale Marker nutzen lassen. Diese Horizonte wurden zu einer Zeit abgelagert, als die Wasserpegel sehr niedrig waren. Der Schwefel wurde von Mikroorganismen abgelagert, die zu anoxygener Photosynthese oder Sulfidoxidation f{\"a}hig sind.},
  language  = {en}
}
@phdthesis{Aichner2009,
  author    = {Aichner, Bernhard},
  title     = {Aquatic macrophyte-derived biomarkers as palaeolimnological proxies on the Tibetan Plateau},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-42095},
  school      = {Universit{\"a}t Potsdam},
  year      = {2009},
  abstract  = {The Tibetan Plateau is the largest elevated landmass in the world and profoundly influences atmospheric circulation patterns such as the Asian monsoon system. Therefore this area has been increasingly in focus of palaeoenvironmental studies. This thesis evaluates the applicability of organic biomarkers for palaeolimnological purposes on the Tibetan Plateau with a focus on aquatic macrophyte-derived biomarkers. Submerged aquatic macrophytes have to be considered to significantly influence the sediment organic matter due to their high abundance in many Tibetan lakes. They can show highly 13C-enriched biomass because of their carbon metabolism and it is therefore crucial for the interpretation of δ13C values in sediment cores to understand to which extent aquatic macrophytes contribute to the isotopic signal of the sediments in Tibetan lakes and in which way variations can be explained in a palaeolimnological context. Additionally, the high abundance of macrophytes makes them interesting as potential recorders of lake water δD. Hydrogen isotope analysis of biomarkers is a rapidly evolving field to reconstruct past hydrological conditions and therefore of special relevance on the Tibetan Plateau due to the direct linkage between variations of monsoon intensity and changes in regional precipitation / evaporation balances. A set of surface sediment and aquatic macrophyte samples from the central and eastern Tibetan Plateau was analysed for composition as well as carbon and hydrogen isotopes of n-alkanes. It was shown how variable δ13C values of bulk organic matter and leaf lipids can be in submerged macrophytes even of a single species and how strongly these parameters are affected by them in corresponding sediments. The estimated contribution of the macrophytes by means of a binary isotopic model was calculated to be up to 60\% (mean: 40\%) to total organic carbon and up to 100\% (mean: 66\%) to mid-chain n-alkanes. Hydrogen isotopes of n-alkanes turned out to record δD of meteoric water of the summer precipitation. The apparent enrichment factor between water and n-alkanes was in range of previously reported ones (≈-130 per mille) at the most humid sites, but smaller (average: -86 per mille) at sites with a negative moisture budget. This indicates an influence of evaporation and evapotranspiration on δD of source water for aquatic and terrestrial plants. The offset between δD of mid- and long-chain n-alkanes was close to zero in most of the samples, suggesting that lake water as well as soil and leaf water are affected to a similar extent by those effects. To apply biomarkers in a palaeolimnological context, the aliphatic biomarker fraction of a sediment core from Lake Koucha (34.0° N; 97.2° E; eastern Tibetan Plateau) was analysed for concentrations, δ13C and δD values of compounds. Before ca. 8 cal ka BP, the lake was dominated by aquatic macrophyte-derived mid-chain n-alkanes, while after 6 cal ka BP high concentrations of a C20 highly branched isoprenoid compound indicate a predominance of phytoplankton. Those two principally different states of the lake were linked by a transition period with high abundances of microbial biomarkers. δ13C values were relatively constant for long-chain n-alkanes, while mid-chain n-alkanes showed variations between -23.5 to -12.6 per mille. Highest values were observed for the assumed period of maximum macrophyte growth during the late glacial and for the phytoplankton maximum during the middle and late Holocene. Therefore, the enriched values were interpreted to be caused by carbon limitation which in turn was induced by high macrophyte and primary productivity, respectively. Hydrogen isotope signatures of mid-chain n-alkanes have been shown to be able to track a previously deduced episode of reduced moisture availability between ca. 10 and 7 cal ka BP, indicated by a 20 per mille shift towards higher δD values. Indications for cooler episodes at 6.0, 3.1 and 1.8 cal ka BP were gained from drops of biomarker concentrations, especially microbial-derived hopanoids, and from coincidental shifts towards lower δ13C values. Those episodes correspond well with cool events reported from other locations on the Tibetan Plateau as well as in the Northern Hemisphere. To conclude, the study of recent sediments and plants improved the understanding of factors affecting the composition and isotopic signatures of aliphatic biomarkers in sediments. Concentrations and isotopic signatures of the biomarkers in Lake Koucha could be interpreted in a palaeolimnological context and contribute to the knowledge about the history of the lake. Aquatic macrophyte-derived mid-chain n-alkanes were especially useful, due to their high abundance in many Tibetan Lakes and their ability to record major changes of lake productivity and palaeo-hydrological conditions. Therefore, they have the potential to contribute to a fuller understanding of past climate variability in this key region for atmospheric circulation systems.},
  language  = {en}
}
@phdthesis{Kristen2009,
  author    = {Kristen, Iris},
  title     = {Investigations on rainfall variability during the late Quaternary based on geochemical analyses of lake sediments from tropical and subtropical southern Africa},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-32547},
  school      = {Universit{\"a}t Potsdam},
  year      = {2009},
  abstract  = {This thesis presents investigations on sediments from two African lakes which have been recording changes in their surrounding environmental and climate conditions since more than 200,000 years. Focus of this work is the time of the last Glacial and the Holocene (the last ~100,000 years before present [in the following 100 kyr BP]). One important precondition for this kind of research is a good understanding of the present ecosystems in and around the lakes and of the sediment formation under modern climate conditions. Both studies therefore include investigations on the modern environment (including organisms, soils, rocks, lake water and sediments). A 90 m long sediment sequence was investigated from Lake Tswaing (north-eastern South Africa) using geochemical analyses. These investigations document alternating periods of high detrital input and low (especially autochthonous) organic matter content and periods of low detrital input, carbonatic or evaporitic sedimentation and high autochthonous organic matter content. These alternations are interpreted as changes between relatively humid and arid conditions, respectively. Before c. 75 kyr BP, they seem to follow changes in local insolation whereas afterwards they appear to be acyclic and are probably caused by changes in ocean circulation and/or in the mean position of the Inter-Tropical Convergence Zone (ITCZ). Today, these factors have main influence on precipitation in this area where rainfall occurs almost exclusively during austral summer. All modern organisms were analysed for their biomarker and bulk organic and compound-specific stable carbon isotope composition. The same investigations on sediments from the modern lake floor document the mixed input of the investigated individual organisms and reveal additional influences by methanotrophic bacteria. A comparison of modern sediment characteristics with those of sediments covering the time 14 to 2 kyr BP shows changes in the productivity of the lake and the surrounding vegetation which are best explained by changes in hydrology. More humid conditions are indicated for times older than 10 kyr BP and younger than 7.5 kyr BP, whereas arid conditions prevailed in between. These observations agree with the results from sediment composition and indications from other climate archives nearby. The second lake study deals with Lake Challa, a small, deep crater lake on the foot of Mount Kilimanjaro. In this lake form mm-scale laminated sediments which were analyses with micro-XRF scanning for changes in the element composition. By comparing these results with investigations on thin sections, results from ongoing sediment trap studies, meteorological data, and investigations on the surrounding rocks and soils, I develop a model for seasonal variability in the limnology and sedimentation of Lake Challa. The lake appears to be stratified during the warm rain seasons (October - December and March - May) during which detrital material is delivered to the lake and carbonates precipitate. On the lake floor forms a dark lamina with high contents of Fe and Ti and high Ca/Al and low Mn/Fe ratios. Diatoms bloom during the cool and windy season (June - September) when mixing down to c. 60 m depth provides easily bio-available nutrients. Contemporaneously, Fe and Mn-oxides are precipitating which cause high Mn/Fe ratios in the light diatom-rich laminae of the sediments. Trends in the Mn/Fe ratio of the sediments are interpreted to reflect changes in the intensity or duration of seasonal mixing in Lake Challa. This interpretation is supported by parallel changes in the organic matter and biogenic silica content observed in the 22 m long profile recovered from Lake Challa. This covers the time of the last 25 kyr BP. It documents a transition around 16 kyr BP from relatively well-mixed conditions with high detrital input during glacial times to stronger stratified conditions which are probably related to increasing lake levels in Challa and generally more humid conditions in East Africa. Intensified mixing is recorded for the time of the Younger Dryas and the period between 11.4 and 10.7 kyr BP. For these periods, reduced intensity of the SW monsoon and intensified NE monsoon are reported from archives of the Indian-Asian Monsoon region, arguing for the latter as a probable source for wind mixing in Lake Challa. This connection is probably also responsible for contemporaneous events in the Mn/Fe ratios of the Lake Challa sediments and in other records of northern hemisphere monsoon intensity during the Holocene and underlines the close interaction of global low latitude atmospheric circulation.},
  language  = {en}
}