Inhaltszusammenfassung

This work is a study of rock varnishes collected from different locations worldwide and from several environments. The focus was set on the microanalytical investigation of i) the geochemistry of different varnishes and adjacent collected mineral dust, ii) the structures of different varnishes, and iii) signs for a definite proof for a biogenic compound necessary for the formation of rock varnish. To this end, a combination of different cutting-edge analytical techniques was utilized to inves...This work is a study of rock varnishes collected from different locations worldwide and from several environments. The focus was set on the microanalytical investigation of i) the geochemistry of different varnishes and adjacent collected mineral dust, ii) the structures of different varnishes, and iii) signs for a definite proof for a biogenic compound necessary for the formation of rock varnish. To this end, a combination of different cutting-edge analytical techniques was utilized to investigate the rock varnishes. Rock varnish is a black, micrometer thin, sedimentary crust on top of rock surfaces. It occurs on almost all lithologies and can be found for instance in deserts, in caves, on mountains and plateaus, in large cities, Antarctica, at waterfronts, and possibly even on Mars. Rock varnish consists of a mixture of dust grains as its main component, and a Mn-rich matrix material as cement component. A biogenic contribution to the genesis has been suggested. Rock varnish is a phenomenon that has defied more than 200 years of intermittent scientific examination. Methods and instruments used in this study were amongst others fs/ns LA-ICP-MS, fs LA-MC-ICP-MS, solution ICP-MS, portable XRF, stationary XRF, EPMA, NanoSIMS, FIB- and Microtome-slicing, STXM-NEXAFS, SEM, Raman spectroscopy, EDX, XRD, and TEM. More than 150 individual rock samples were investigated for their chemical composition by fs LA-ICP-MS, and on each sample about 10-30 measurements were conducted to obtain enough statistical accuracy since the crusts are very inhomogeneous. On the basis of the results, several samples were selected that were additionally investigated by other methods mentioned above, to receive further information about, e.g., nano- and microstructures, element distributions, in-situ oxidation state distributions, and isotope ratios. Mineral dust samples were collected and investigated by fs LA-ICP-MS and XRD, due to their role as potential element and material source for rock varnish. Additionally, STXM-NEXAFS measurements, Raman spectroscopy, extensive PCR, qPCR, and NGS studies, EPR measurements, light microscopic investigations, δ13C measurements, as well as fluorescence microscopy analyses on DAPI, DRAQ5, and OsO4 fixed thin sections were performed in the search for proof of a biogenic genesis of rock varnish. It was found that rock varnishes from disparate environments differ, and that the term rock varnish can thus be subdivided and categorized. It was possible to analyze the trace element composition of the microstructure of varnish without contamination from the host rock. Images of nano- and microstructures were obtained for a large set of varnishes and for varnishes from different environments. It was furthermore possible to determine and locate Mn oxidation states at the nanometer scale within single samples and by this show the reduction of the manganese matrix around cavities, a sign for previous redox reactions. The results indicate that the airborne dust is the major contributor to the genesis of varnishes found in deserts, vehicle emissions the main source of varnish on facades in urban areas, and water the source of crusts within river splash zones. No indicator for a possible biogenic origin was found in any varnish sample, even though a large variety of tests was performed. However, one would expect to find organisms capable of oxidizing Mn, at least in the most recently formed layers, or on the surface of rock varnish. Thus, an abiogenic genesis of rock varnish seems most plausible. In addition to the measurements, technical developments were made, such as the development of a new microanalytical reference material for Mn-rich rocks, a calibration for portable XRF measurements of thin Mn-rich coatings, and a measurement technique that allows the measurement of femtogram amounts of powdered material (such as dust) with high precision and accuracy. By providing these technical developments and a broad variety of information about different varnish types, as well as details about individual samples, significant cornerstones are now given on the basis of which future investigations can proceed with the aim to disclose the secret of the genesis of rock varnish.» weiterlesen» einklappen

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DDC Sachgruppe:
Geowissenschaften