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Understanding hydrological processes is of fundamental importance for the Vietnamese national food security and the livelihood of the population in the Vietnamese Mekong Delta (VMD). As a consequence of sparse data in this region, however, hydrologic processes, such as the controlling processes of precipitation, the interaction between surface and groundwater, and groundwater dynamics, have not been thoroughly studied. The lack of this knowledge may negatively impact the long-term strategic planning for sustainable groundwater resources management and may result in insufficient groundwater recharge and freshwater scarcity. It is essential to develop useful methods for a better understanding of hydrological processes in such data-sparse regions. The goal of this dissertation is to advance methodologies that can improve the understanding of fundamental hydrological processes in the VMD, based on the analyses of stable water isotopes and monitoring data. The thesis mainly focuses on the controlling processes of precipitation, the mechanism of surface–groundwater interaction, and the groundwater dynamics. These processes have not been fully addressed in the VMD so far. The thesis is based on statistical analyses of the isotopic data of Global Network of Isotopes in Precipitation (GNIP), of meteorological and hydrological data from Vietnamese agencies, and of the stable water isotopes and monitoring data collected as part of this work.
First, the controlling processes of precipitation were quantified by the combination of trajectory analysis, multi-factor linear regression, and relative importance analysis (hereafter, a model‐based statistical approach). The validity of this approach is confirmed by similar, but mainly qualitative results obtained in other studies. The total variation in precipitation isotopes (δ18O and δ2H) can be better explained by multiple linear regression (up to 80%) than single-factor linear regression (30%). The relative importance analysis indicates that atmospheric moisture regimes control precipitation isotopes rather than local climatic conditions. The most crucial factor is the upstream rainfall along the trajectories of air mass movement. However, the influences of regional and local climatic factors vary in importance over the seasons. The developed model‐based statistical approach is a robust tool for the interpretation of precipitation isotopes and could also be applied to understand the controlling processes of precipitation in other regions.
Second, the concept of the two-component lumped-parameter model (LPM) in conjunction with stable water isotopes was applied to examine the surface–groundwater interaction in the VMD. A calibration framework was also set up to evaluate the behaviour, parameter identifiability, and uncertainties of two-component LPMs. The modelling results provided insights on the subsurface flow conditions, the recharge contributions, and the spatial variation of groundwater transit time. The subsurface flow conditions at the study site can be best represented by the linear-piston flow distribution. The contributions of the recharge sources change with distance to the river. The mean transit time (mTT) of riverbank infiltration increases with the length of the horizontal flow path and the decreasing gradient between river and groundwater. River water infiltrates horizontally mainly via the highly permeable aquifer, resulting in short mTTs (<40 weeks) for locations close to the river (<200 m). The vertical infiltration from precipitation takes place primarily via a low‐permeable overlying aquitard, resulting in considerably longer mTTs (>80 weeks). Notably, the transit time of precipitation infiltration is independent of the distance to the river. All these results are hydrologically plausible and could be quantified by the presented method for the first time. This study indicates that the highly complex mechanism of surface–groundwater interaction at riverbank infiltration systems can be conceptualized by exploiting two‐component LPMs. It is illustrated that the model concept can be used as a tool to investigate the hydrological functioning of mixing processes and the flow path of multiple water components in riverbank infiltration systems.
Lastly, a suite of time series analysis approaches was applied to examine the groundwater dynamics in the VMD. The assessment was focused on the time-variant trends of groundwater levels (GWLs), the groundwater memory effect (representing the time that an aquifer holds water), and the hydraulic response between surface water and multi-layer alluvial aquifers. The analysis indicates that the aquifers act as low-pass filters to reduce the high‐frequency signals in the GWL variations, and limit the recharge to the deep groundwater. The groundwater abstraction has exceeded groundwater recharge between 1997 and 2017, leading to the decline of groundwater levels (0.01-0.55 m/year) in all considered aquifers in the VMD. The memory effect varies according to the geographical location, being shorter in shallow aquifers and flood-prone areas and longer in deep aquifers and coastal regions. Groundwater depth, season, and location primarily control the variation of the response time between the river and alluvial aquifers. These findings are important contributions to the hydrogeological literature of a little-known groundwater system in an alluvial setting. It is suggested that time series analysis can be used as an efficient tool to understand groundwater systems where resources are insufficient to develop a physical-based groundwater model.
This doctoral thesis demonstrates that important aspects of hydrological processes can be understood by statistical analysis of stable water isotope and monitoring data. The approaches developed in this thesis can be easily transferred to regions in similar tropical environments, particularly those in alluvial settings. The results of the thesis can be used as a baseline for future isotope-based studies and contribute to the hydrogeological literature of little-known groundwater systems in the VMD.
Li and B in ascending magmas: an experimental study on their mobility and isotopic fractionation
(2022)
This research study focuses on the behaviour of Li and B during magmatic ascent, and decompression-driven degassing related to volcanic systems. The main objective of this dissertation is to determine whether it is possible to use the diffusion properties of the two trace elements as a tool to trace magmatic ascent rate. With this objective, diffusion-couple and decompression experiments have been performed in order to study Li and B mobility in intra-melt conditions first, and then in an evolving system during decompression-driven degassing.
Synthetic glasses were prepared with rhyolitic composition and an initial water content of 4.2 wt%, and all the experiments were performed using an internally heated pressure vessel, in order to ensure a precise control on the experimental parameters such as temperature and pressure.
Diffusion-couple experiments were performed with a fix pressure 300 MPa. The temperature was varied in the range of 700-1250 °C with durations between 0 seconds and 24 hours. The diffusion-couple results show that Li diffusivity is very fast and starts already at very low temperature. Significant isotopic fractionation occurs due to the faster mobility of 6Li compared to 7Li. Boron diffusion is also accelerated by the presence of water, but the results of the isotopic ratios are unclear, and further investigation would be necessary to well constrain the isotopic fractionation process of boron in hydrous silicate melts. The isotopic ratios results show that boron isotopic fractionation might be affected by the speciation of boron in the silicate melt structure, as 10B and 11B tend to have tetrahedral and trigonal coordination, respectively.
Several decompression experiments were performed at 900 °C and 1000 °C, with pressures going from 300 MPa to 71-77 MPa and durations of 30 minutes, two, five and ten hours, in order to trigger water exsolution and the formation of vesicles in the sample. Textural observations and the calculation of the bubble number density confirmed that the bubble size and distribution after decompression is directly proportional to the decompression rate.
The overall SIMS results of Li and B show that the two trace elements tend to progressively decrease their concentration with decreasing decompression rates. This is explained because for longer decompression times, the diffusion of Li and B into the bubbles has more time to progress and the melt continuously loses volatiles as the bubbles expand their volumes.
For fast decompression, Li and B results show a concentration increase with a δ7Li and δ11B decrease close to the bubble interface, related to the sudden formation of the gas bubble, and the occurrence of a diffusion process in the opposite direction, from the bubble meniscus to the unaltered melt. When the bubble growth becomes dominant and Li and B start to exsolve into the gas phase, the silicate melt close to the bubble gets depleted in Li and B, because of a stronger diffusion of the trace elements into the bubble.
Our data are being applied to different models, aiming to combine the dynamics of bubble nucleation and growth with the evolution of trace elements concentration and isotopic ratios. Here, first considerations on these models will be presented, giving concluding remarks on this research study. All in all, the final remarks constitute a good starting point for further investigations. These results are a promising base to continue to study this process, and Li and B can indeed show clear dependences on decompression-related magma ascent rates in volcanic systems.
Stable isotopes in precipitation: Modelling intra-event variations using meteorological parameters
(2017)
The short-term variability of the isotopic composition of precipitation in Golm, Germany was assessed and modelled. Isotopic data (D/H and 18O/16O) on intra-event timescales as well as meteorological data from a weather station and a micro rain radar was used. After data preparation and the combination of all three data sets, a multivariate linear regression analysis was conducted. This was done for four different isotopic response variables and for the entire data set as well as for the two subsets Summer and Winter. The used response variables are the δ18O values as the difference to the corresponding event-based mean and as the difference to the median, and the deuterium excess values as the difference to both the mean and the median. The models were evaluated by comparing the modelled values with the observed ones. This showed that the observations could not be reproduced in a satisfactory way. Therefore, several suggestions on how to possibly improve the methods and thus the modelling results are given in the end.
Earth's climate varies continuously across space and time, but humankind has witnessed only a small snapshot of its entire history, and instrumentally documented it for a mere 200 years. Our knowledge of past climate changes is therefore almost exclusively based on indirect proxy data, i.e. on indicators which are sensitive to changes in climatic variables and stored in environmental archives. Extracting the data from these archives allows retrieval of the information from earlier times. Obtaining accurate proxy information is a key means to test model predictions of the past climate, and only after such validation can the models be used to reliably forecast future changes in our warming world. The polar ice sheets of Greenland and Antarctica are one major climate archive, which record information about local air temperatures by means of the isotopic composition of the water molecules embedded in the ice. However, this temperature proxy is, as any indirect climate data, not a perfect recorder of past climatic variations. Apart from local air temperatures, a multitude of other processes affect the mean and variability of the isotopic data, which hinders their direct interpretation in terms of climate variations. This applies especially to regions with little annual accumulation of snow, such as the Antarctic Plateau. While these areas in principle allow for the extraction of isotope records reaching far back in time, a strong corruption of the temperature signal originally encoded in the isotopic data of the snow is expected. This dissertation uses observational isotope data from Antarctica, focussing especially on the East Antarctic low-accumulation area around the Kohnen Station ice-core drilling site, together with statistical and physical methods, to improve our understanding of the spatial and temporal isotope variability across different scales, and thus to enhance the applicability of the proxy for estimating past temperature variability. The presented results lead to a quantitative explanation of the local-scale (1–500 m) spatial variability in the form of a statistical noise model, and reveal the main source of the temporal variability to be the mixture of a climatic seasonal cycle in temperature and the effect of diffusional smoothing acting on temporally uncorrelated noise. These findings put significant limits on the representativity of single isotope records in terms of local air temperature, and impact the interpretation of apparent cyclicalities in the records. Furthermore, to extend the analyses to larger scales, the timescale-dependency of observed Holocene isotope variability is studied. This offers a deeper understanding of the nature of the variations, and is crucial for unravelling the embedded true temperature variability over a wide range of timescales.
The global carbon cycle is closely linked to Earth’s climate. In the context of continuously unchecked anthropogenic CO₂ emissions, the importance of natural CO₂ bond and carbon storage is increasing. An important biogenic mechanism of natural atmospheric CO₂ drawdown is the photosynthetic carbon fixation in plants and the subsequent longterm deposition of plant detritus in sediments.
The main objective of this thesis is to identify factors that control mobilization and transport of plant organic matter (pOM) through rivers towards sedimentation basins. I investigated this aspect in the eastern Nepalese Arun Valley. The trans-Himalayan Arun River is characterized by a strong elevation gradient (205 − 8848 m asl) that is accompanied by strong changes in ecology and climate ranging from wet tropical conditions in the Himalayan forelad to high alpine tundra on the Tibetan Plateau. Therefore, the Arun is an excellent natural laboratory, allowing the investigation of the effect of vegetation cover, climate, and topography on plant organic matter mobilization and export in tributaries along the gradient.
Based on hydrogen isotope measurements of plant waxes sampled along the Arun River and its tributaries, I first developed a model that allows for an indirect quantification of pOM contributed to the mainsetm by the Arun’s tributaries. In order to determine the role of climatic and topographic parameters of sampled tributary catchments, I looked for significant statistical relations between the amount of tributary pOM export and tributary characteristics (e.g. catchment size, plant cover, annual precipitation or runoff, topographic measures). On one hand, I demonstrated that pOMsourced from the Arun is not uniformly derived from its entire catchment area. On the other, I showed that dense vegetation is a necessary, but not sufficient, criterion for high tributary pOM export. Instead, I identified erosion and rainfall and runoff as key factors controlling pOM sourcing in the Arun Valley. This finding is supported by terrestrial cosmogenic nuclide concentrations measured on river sands along the Arun and its tributaries in order to quantify catchment wide denudation rates. Highest denudation rates corresponded well with maximum pOM mobilization and export also suggesting the link between erosion and pOM sourcing.
The second part of this thesis focusses on the applicability of stable isotope records such as plant wax n-alkanes in sediment archives as qualitative and quantitative proxy for the variability of past Indian Summer Monsoon (ISM) strength. First, I determined how ISM strength affects the hydrogen and oxygen stable isotopic composition (reported as δD and δ18O values vs. Vienna Standard Mean Ocean Water) of precipitation in the Arun Valley and if this amount effect (Dansgaard, 1964) is strong enough to be recorded in potential paleo-ISM isotope proxies. Second, I investigated if potential isotope records across the Arun catchment reflect ISM strength dependent precipitation δD values only, or if the ISM isotope signal is superimposed by winter precipitation or glacial melt. Furthermore, I tested if δD values of plant waxes in fluvial deposits reflect δD values of environmental waters in the respective catchments.
I showed that surface water δD values in the Arun Valley and precipitation δD from south of the Himalaya both changed similarly during two consecutive years (2011 & 2012) with distinct ISM rainfall amounts (~20% less in 2012). In order to evaluate the effect of other water sources (Winter-Westerly precipitation, glacial melt) and evapotranspiration in the Arun Valley, I analysed satellite remote sensing data of rainfall distribution (TRMM 3B42V7), snow cover (MODIS MOD10C1), glacial coverage (GLIMSdatabase, Global Land Ice Measurements from Space), and evapotranspiration (MODIS MOD16A2). In addition to the predominant ISM in the entire catchment I found through stable isotope analysis of surface waters indications for a considerable amount of glacial melt derived from high altitude tributaries and the Tibetan Plateau. Remotely sensed snow cover data revealed that the upper portion of the Arun also receives considerable winter precipitation, but the effect of snow melt on the Arun Valley hydrology could not be evaluated as it takes place in early summer, several months prior to our sampling campaigns. However, I infer that plant wax records and other potential stable isotope proxy archives below the snowline are well-suited for qualitative, and potentially quantitative, reconstructions of past changes of ISM strength.
Two of the most controversial issues concerning the late Cenozoic evolution of the Andean orogen are the timing of uplift of the intraorogenic Puna plateau and its eastern border, the Eastern Cordillera, and ensuing changes in climatic and surface-process conditions in the intermontane basins of the NW-Argentine Andes. The Eastern Cordillera separates the internally drained, arid Puna from semi-arid intermontane basins and the humid sectors of the Andean broken foreland and the Subandean fold-and-thrust belt to the east. With elevations between 4,000 and 6,000 m the eastern flanks of the Andes form an efficient orographic barrier with westward-increasing elevation and asymmetric rainfall distribution and amount with respect to easterly moisture-bearing winds. This is mirrored by pronounced gradients in the efficiency of surface processes that erode and re-distribute sediment from the uplifting ranges. Although the overall pattern of deformation and uplift in this sector of the southern central Andes shows an eastward migration of deformation, a well-developed deformation front does not exist and uplift and associated erosion and sedimentary processes are highly disparate in space and time. In addition, periodic deformation within intermontane basins, and continued diachronous foreland uplifts associated with the reactivation of inherited basement structures furthermore make a rigorous assessment of the spatiotemporal uplift patterns difficult.
This thesis focuses on the tectonic evolution of the Eastern Cordillera of NW Argentina, the depositional history of its intermontane sedimentary basins, and the regional topographic evolution of the eastern flank of the Puna Plateau. The intermontane basins of the Eastern Cordillera and the adjacent morphotectonic provinces of the Sierras Pampeanas and the Santa Bárbara System are akin to reverse fault bounded, filled, and partly coalesced sedimentary basins of the Puna Plateau. In contrast to the Puna basins, however, which still form intact morphologic entities, repeated deformation, erosion, and re-filling have impacted the basins in the Eastern Cordillera. This has resulted in a rich stratigraphy of repeated basin fills, but many of these basins have retained vestiges of their early depositional history that may reach back in time when these areas were still part of a contiguous and undeformed foreland basin. Fortunately, these strata also contain abundant volcanic ashes that are not only important horizons to decipher tectono-sedimentary events through U-Pb geochronology and geochemical correlation, but they also represent terrestrial recorders of the hydrogen-isotope composition of ancient meteoric waters that can be compared to the isotopic composition of modern meteoric water. The ash horizons are thus unique recorders of past environmental conditions and lend themselves to tracking the development of rainfall barriers and tectonically forced climate and environmental change through time.
U-Pb zircon geochronology and paleocurrent reconstructions of conglomerate sequences in the Humahuaca Basin of the Eastern Cordillera at 23.5° S suggest that the basin was an integral part of a largely unrestricted depositional system until 4.2 Ma, which subsequently became progressively decoupled from the foreland by range uplifts to the east that forced easterly moisture-bearing winds to precipitate in increasingly eastward locations. Multiple cycles of severed hydrological conditions and drainage re-capture are identified together with these processes that were associated with basin filling and sediment evacuation, respectively. Moreover, systematic relationships among faults, regional unconformities and deformed landforms reveal a general pattern of intra-basin deformation that appears to be linked with basin-internal deformation during or subsequent to episodes of large-scale sediment removal. Some of these observations are supported by variations in the hydrogen stable isotope composition of volcanic glass from the Neogene to Quaternary sedimentary record, which can be related to spatiotemporal changes in topography and associated orographic effects. δDg values in the basin strata reveal two main trends associated with surface uplift in the catchment area between 6.0 and 3.5 Ma and the onset of semiarid conditions in the basin following the attainment of threshold elevations for effective orographic barriers to the east after 3.5 Ma. The disruption of sediment supply from western sources after 4.2 Ma and subsequent hinterland aridification, moreover, emphasize the possibility that these processes were related to lateral orogenic growth of the adjacent Puna Plateau. As a result of the hinterland aridification the regions in the orogen interior have been characterized by an inefficient fluvial system, which in turn has helped maintaining internal drainage conditions, sediment storage, and relief reduction within high-elevation basins.
The diachronous nature of basin formation and impacts on the fluvial system in the adjacent broken foreland is underscored by the results of detailed sediment provenance and paleocurrent analyses, as well as U-Pb zircon geochronology in the Lerma and Metán basins at ca. 25° S. This is particularly demonstrated by the isolated uplift of the Metán range at ~10 Ma, which is more than 50 km away from the presently active orogenic front along the eastern Puna margin and the Eastern Cordillera to the west. At about 5 Ma, Puna-sourced sediments disappear from the foreland record, documenting further range uplifts in the Eastern Cordillera and hydrological isolation of the neighboring Angastaco Basin from the foreland. Finally, during the late Pliocene and Quaternary, deformation has been accommodated across the entire foreland and is still active. To elucidate the interactions between tectonically controlled changes in elevation and their impact on atmospheric circulation processes in this region, this thesis provides additional, temporally well-constrained hydrogen stable isotope results of volcanic glass samples from the broken foreland, including the Angastaco Basin, and other intermontane basins farther south. The results suggest similar elevations of intermontane basins and the foreland sectors prior to ca. 7 Ma. In case of the Angastaco Basin the region was affected by km-scale surface uplift of the basin. A comparison with coeval isotope data collected from sedimentary sequences in the Puna plateau explains rapid shifts in the intermontane δDg record and supports the notion of recurring phases of enhanced deep convection during the Pliocene, and thus climatic conditions during the middle to late Pliocene similar to the present day.
Combined, field-based and isotope geochemical methods used in this study of the NW-Argentine Andes have thus helped to gain insight into the systematics, rate changes, interactions, and temporal characteristics among tectonically controlled deformation patterns, the build-up of topography impacting atmospheric processes, the distribution of rainfall, and resulting surface processes in a tectonically active mountain belt. Ultimately, this information is essential for a better understanding of the style and the rates at which non-collisional mountain belts evolve, including the development orogenic plateaus and their bordering flanks. The results presented in this study emphasize the importance of stable isotope records for paleoaltimetric and paleoenvironmental studies in mountain belts and furnishes important data for a rigorous interpretation of such records.
The overarching goal of this dissertation is to provide a better understanding of the role of wind and water in shaping Earth’s Cenozoic orogenic plateaus - prominent high-elevation, low relief sectors in the interior of Cenozoic mountain belts. In particular, the feedbacks between surface uplift, the build-up of topography and ensuing changes in precipitation, erosion, and vegetation patterns are addressed in light of past and future climate change. Regionally, the study focuses on the two world’s largest plateaus, the Altiplano-Puna Plateau of the Andes and Tibetan Plateau, both characterized by average elevations of >4 km. Both plateaus feature high, deeply incised flanks with pronounced gradients in rainfall, vegetation, hydrology, and surface processes. These characteristics are rooted in the role of plateaus to act as efficient orographic barriers to rainfall and to force changes in atmospheric flow.
The thesis examines the complex topics of tectonic and climatic forcing of the surface-process regime on three different spatial and temporal scales: (1) bedrock wind-erosion rates are quantified in the arid Qaidam Basin of NW Tibet over millennial timescales using cosmogenic radionuclide dating; (2) present-day stable isotope composition in rainfall is examined across the south-central Andes in three transects between 22° S and 28° S; these data are modeled and assessed with remotely sensed rainfall data of the Tropical Rainfall Measuring Mission and the Moderate Resolution Imaging Spectroradiometer; (3) finally, a 2.5-km-long Mio-Pliocene sedimentary record of the intermontane Angastaco Basin (25°45’ S, 66°00’ W) is presented in the context of hydrogen and carbon compositions of molecular lipid biomarker, and oxygen and carbon isotopes obtained from pedogenic carbonates; these records are compared to other environmental proxies, including hydrated volcanic glass shards from volcanic ashes intercalated in the sedimentary strata.
There are few quantitative estimates of eolian bedrock-removal rates from arid, low relief landscapes. Wind-erosion rates from the western Qaidam Basin based on cosmogenic 10Be measurements document erosion rates between 0.05 to 0.4 mm/yr. This finding indicates that in arid environments with strong winds, hyperaridity, exposure of friable strata, and ongoing rock deformation and uplift, wind erosion can outpace fluvial erosion. Large eroded sediment volumes within the Qaidam Basin and coeval dust deposition on the Chinese Loess plateau, exemplify the importance of dust production within arid plateau environments for marine and terrestrial depositional processes, but also health issues and fertilization of soils.
In the south-central Andes, the analysis of 234 stream-water samples for oxygen and hydrogen reveals that areas experiencing deep convective storms do not show the commonly observed patterns of isotopic fractionation and the expected co-varying relationships between oxygen and hydrogen with increasing elevation. These convective storms are formed over semi-arid intermontane basins in the transition between the broken foreland of the Sierras Pampeanas, the Eastern Cordillera, and the Puna Plateau in the interior of the orogen. Here, convective rainfall dominates the precipitation budget and no systematic stable isotope-elevation relationship exists. Regions to the north, in the transition between the broken foreland and the Subandean foreland fold-and-thrust belt, the impact of convection is subdued, with lower degrees of storminess and a stronger expected isotope-elevation relationship. This finding of present-day fractionation trends of meteoric water is of great importance for paleoenvironmental studies in attempts to use stable isotope relationships in the reconstruction of paleoelevations.
The third part of the thesis focuses on the paleohydrological characteristics of the Mio-Pliocene (10-2 Ma) Angastaco Basin sedimentary record, which reveals far-reaching environmental changes during Andean uplift and orographic barrier formation. A precipitation- evapotranspiration record identifies the onset of a precipitation regime related to the South American Low Level Jet at this latitude after 9 Ma. Humid foreland conditions existed until 7 Ma, followed by orographic barrier uplift to the east of the present-day Angastaco Basin. This was superseded by rapid (~0.5 Myr) aridification in an intermontane basin, highlighting the effects of eastward-directed deformation. A transition in vegetation cover from a humid C3 forest ecosystem to semi-arid C4-dominated vegetation was coeval with continued basin uplift to modern elevations.