@phdthesis{Rohrmann2015,
  author    = {Rohrmann, Alexander},
  title     = {The role of wind and water in shaping earth's plateaus},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-77938},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XXV, 157},
  year      = {2015},
  abstract  = {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.},
  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}
}