The Himalaya-Tibetan Plateau orogenic system is an exclusive example for a continent-continent collision. It evolved from the collision of the Indian and Eurasian continents in the Paleogene. Several scientists have been engaged for decades studying its evolution. There are still challenging open questions and competing evolution models. Our group is involved in research work unraveling mysteries of plate tectonics and mountain building processes. An important tool is palaeomagnetism. Magnetic minerals in rocks record the direction of the Earth’ field and gives information about north-south movements and block-rotations of continents or smaller tectonic units. Geochronology and biostratigraphy enable us to determine the age of remanence acquisition and thus the time span during which these motions have happened.
The Tübingen group has conducted various projects in the area of the Himalayan mountain range and the Tibetan Plateau within the last two decades. Our present activities are part of the extensive multidisciplinary research programme TiP (www.tip.uni-tuebingen.de), which is operated as a German–Chinese cooperation. In our work we collaborate with different sections of geosciences, e.g. geochronology, stratigraphy, petrology, structural geology and remote sensing.
Small-circle methods provide some new ways to analyse palaeomagnetic remanences. They especially provide a geometrically alternative estimate of the palaeomagnetic field direction (cross-checking tilt correction and fold tests) and they are the key to the tectonic interpretation of synfolding remanences.
Once paleofield direction and remanence age are known (from tilt correction, E-W tilted remanences, small circle intersections or an APWP), a remanence can be tilted back to reach its expected inclination. In this way, the vertical-axis rotation is determined also for synfolding remanences (Waldhör 1999 and Waldhör et al. 2001) and fold geometries can be reconstructed for the time of remanence acquisition.
Global climate change and its political and economic implications, makes evident why this research field is very important. In order to understand and evaluate recent processes, unraveling of the natural palaeoenvironmental evolution is a crucial factor. Various geologic archives on different time scales are studied for this purpose based on various parameters (isotopes, fossils, pollen, etc.).
In our working group, we focus on using magnetic parameters as proxies for the palaeoclimate of the last million years. A particular goal is to understand the development of the Asian monsoon circulation in this time frame and aridification processes as a response to surface uplift of the Tibetan Plateau. The advantage of using magnetic proxies in paleoclimate research is the fast and nondestructive measurement procedure and therefore a potentially high resolution. Magnetic proxies used in this context are magnetic susceptibility, isothermal remanence, anhysteretic remanence, and derived ratios (e.g. S-ratio). For interpretation, there is the need of calibration with direct climate proxies such as pollen, δ18O or TOC, which are provided by a network of cooperating working groups within the coordinated programmes TiP and CAME.
Besides marine sequences and loess deposits, lacustrine sediments are particularly suitable archives for studying magnetic proxies.They were formed with relatively constant sedimentation rates and consist of predominantly fine-grained material. Magnetostratigraphy and cyclostratigraphy are important methods for dating such sequences. Additional time constraints (e.g. from radiometric dating methods such as U-Th) for one or more stratigraphic layers of the sequence are sometimes necessary in order to acquire absolute age markers for allowing a correlation of the profile with the Global Polarity Time Scale (GPTS). Our current studies in the field of palaeoclimate reconstruction are conducted in a framework of a Sino-German cooperation and are part of both, the DFG Priority Programme TIP (Tibetan Plateau: Formation - Climate - Ecosystems) and the BMBF Programme Central Asia: Monsoon Development & GeoEcosystems (CAME). The focus of our work within these larger project frameworks lies on the Qaidam Basin in the northeastern part of the Tibetan Plateau. A first drilling campaign, conducted in 2008, provided the sediment core SG-1 with a recovery rate of >93% and 940 m length. This, basically undisturbed sequence, could be correlated with an age between 0.1 and 2.77 Ma. The second core SG-1b (723 m and 93% recovery rate), drilled in a nearby anticline structure in 2010, comprises the interval between 1.6 and 7.3 Ma. Further, the possibility to acquire a complete sequence from present down to 15 Ma is currently assessed in a pilot study. For more in-depth information, it is referred to the TIP- and CAME homepages.
Formation of Magnetic Minerals Related to Hydrocarbon Contamination
The occurrence of hydrocarbons in soils was shown to be associated with a change of soil magnetic properties (e.g. Morris et al. 1994; Hanesch and Scholger 2002). The effect has been observed in various environments with hydrocarbon contaminations but systematic studies to investigate the determining processes and factors and the time frame for generation of a measurable magnetic signal are lacking. It is suspected that geo-microbiological processes involving microbially catalysed redox transformation of iron minerals are responsible for these changes. Iron(II)-oxidizing and iron(III)-reducing bacteria can both produce/precipitate and remove/dissolve magnetic minerals under certain conditions (Lovley et al., 2004; Kappler and Straub, 2005). Some iron(III)-reducing microbes can use hydrocarbons as carbon source to reduce Fe(III) to Fe(II) and under certain conditions even magnetite is formed (Lovley et al., 1989; Lovley and Anderson, 2000.
The proposed feasibility study aims at systematically simulating oil spills in a laboratory frame and monitoring changes in magnetic mineralogy of contaminated soil and sediment samples using both, geomicrobiological and environmental magnetic expertise. Depending on the initial concentration of organisms, on the concentration of available iron and carbon (hydrocarbons) and on the necessary growth conditions for the organisms, the reactions can take up to several weeks. Therefore the time frame of this project is set for 4 months and in particular investigating the kinetics of the processes is the main focus of this study.
To allow magnetic measurements without sub-sampling and destroying the sample and to insure small amounts of contaminated material for disposal, small sample sizes (20-50 ml) will be used. To guarantee comparable and reproducible results between the different experimental set ups, homogenous samples are required.
Vertical susceptibility profiles: its use in magnetic pollution screening
Within the last 10-20 years, environmental magnetism became an active and successful field, recently also for detecting anthropogenic pollution history as well as modern industrial pollution in soils and sediments.
Magnetic methods are a promising tool for fast and cost-effective delineation of ‘anomalous patterns’ (screening) before selecting suitable geochemical sampling sites, or for quickly performing repeated measurements (monitoring). Measurements of magnetic susceptibility (MS) on surface are the fastest technique. A main problem is the unknown natural background. The suggested proposal aims to use vertical sections of MS from the uppermost 30-40 cm as a key to allow discrimination of the anthropogenic and natural signal from the shape of the curves. Translocation processes and chemical/mineralogical alterations, which depend on the pedons, have to be taken into account. The anthropogenic contribution will be semi-quantified by integrating the curve above the background signal. To preserve the benefit of a fast measurement tool, a newly developed ultra-shallow MS logging device will be used which enables data acquisition in situ. The reliability of in-situ MS-logs and the correlation with pollutants (mainly heavy metals) will be studied systematically and will end up in a synthesis for an optimum application of vertical MS profiles acquired in-situ, and also for better judging the quality of surface MS measurements. Cooperation with a research group in China accompanies the study (based on the DFG-NSFC agreement).
Projejt TASK mAGpROX (Pollution) (see also TASK final report 2012 )
MagProx demo-video (link youtube). Please click on picture to start vid
(note: there is currently no active project in this field in the geophysics group)
GPR techniques and Geotomographyaulic properties
High resolution geotomography and 3-D GPR techniques to characterize Quarternary gravel deposits and their hydraulic properties
Quarternary sand and gravel deposits characterise one of the most important aquifer type in South Germany and in many other parts of the world. To understand and model groundwater flow and contaminant transport in these high heterogenous systems a detailed image of the subsurface is needed. Techniques must be developed which lead to high resolution characterisation of sedimentological structures and their hydraulic properties in the regions of interest. Geophysical methods and especially their combination with hydrogeological data and sedimentological knowledge seem to be a promising tool for a better understanding of such deposits and their hydraulic response.
Applied geophysical methods mainly include surface GPR measurements and tomographical imaging. On the picture below you can see our GPR equipment during profiling in the Neckar valley. GPR, which is a time domain electromagnetical technique similar to reflection seismics, provides high resolution structural information of the shallow subsurface. Therefore, it is an ideal tool for the 3-D visualization of different sedimentological units in unconsolidated deposits.
Geotomography is similar to the tomographical imaging techniques known from medicine. It represents a possibility to obtain and correlate physical properties of the geological medium - e.g. seismic or EM velocity distribution - between boreholes. The calculated tomogramms can be interpreted in view of sedimentological units and hydraulic properties. On the picture below, you can see our Borehole Radar System with 100 MHz antennas during field work.