Raman spectroscopy in Geobiology - Advances in detection and interpretation of organic signatures in rocks and minerals

Raman spectroscopy in Geobiology - Advances in detection and interpretation of organic signatures in rocks and minerals

Nadine Schäfer

Doctoral thesis

Date of Examination: 2013-04-12

Date of issue: 2014-01-21

Advisor: Reitner, Joachim Prof. Dr.

Referee: Reitner, Joachim Prof. Dr.

Referee: Thiel, Volker Prof. Dr.

Referee: Schmidt, Burkhard Dr.

Persistent Address: http://hdl.handle.net/11858/00-1735-0000-0022-5E02-B

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Abstract

English

In this thesis, Raman spectroscopy was applied to a broad range of rock types in order to test the ability of detecting and interpreting organic signals. In the field of geobiology Raman spectroscopy is relatively new, and has the great advantage of almost destruction free analyzing of organic and inorganic phases. However, this is complicated by the fact that in many samples vibrations of disordered carbon are the only detectable signals. The interpretation of these signals is not straight forward as the similarity between kerogenous and graphitic material is very high and also thermal maturity influences the carbon signature. Best results could be achieved in rock fractures from the Äspö Hard Rock Laboratory where the organic remains are relatively young, and therefore, still complex molecules could be detected and determined. However, in most other geobiological contexts this is not the case. Sometimes, when dealing with known substances, for the interpretation of the Raman spectra theoretical predictions can be consulted. This could be shown for the detection of melanin in various fossil systems. For the identification of cold seep carbonates the use of Raman spectroscopy turned out to be a good indicator for the discrimination between chemical identical carbonate phases. In the case of older fossil microbial mats or presumably organic material only signals of amorphous carbon can be detected. Here, the interpretation of the exact parameters of the D and the G band (intensity, full width at half height, band position) turned out to be very important for the classification of the carbon signals. Especially for ancient rock material (Archean) with a thermal history further information about the potential organic precursor material could be gained.

Keywords: Raman spectroscopy; Geobiology; deep biosphere; carbon signatures