Date of Award

Summer 2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

Nora Noffke

Committee Member

Donald Swift

Committee Member

Desmond Cook

Committee Member

Robert M. Hazen

Abstract

The current Mars Exploration Rover Program (MER) is one of NASA's most successful missions. The aim of the MER is to explore for possible life on the surface of Mars. My thesis developed new methods of how to detect and to identify microbial mats in siliciclastic sediments (modern and ancient), and to make recommendations on the applicability of MISS as biosignatures. Predominantly, I employed instrumentation portable by future rovers. To search for life on other planets, we need to have information on how this life might look. Most astrobiological studies therefore focus on Earthly analogues of life and its habitats. 'Microbially induced sedimentary structures (MISS)' are formed by benthic cyanobacteria in shallow-marine settings by the interaction of the microbiota with physical sediment dynamics. MISS are found exclusively in siliciclastic sedimentary environments, setting them apart from stromatolites that form in chemically controlled settings. Because the MISS occur since the earliest Archean, the structures constitute important biosignatures. It is noteworthy that we can study both modern as well as fossil examples. In order to compare fossil and modem MISS, I used rock material from the 2.9 Ga old Archean Pongola and Witwatersrand Supergroups, South Africa, that record former tidal and shelf settings. I also investigated modern examples on Fishermans Island, located on the coast of Virginia at the mouth of the Chesapeake Bay. To describe and to quantify various microtextures such as bacterial filaments, extracellular polymeric substances, mat fabrics, etc., I coupled morphometric and geochemical analyses. Optical microscopy and confocal scanning laser microscopy (CSLM) revealed that the morphology and structure of the microtextures of both Archean and modern material are very similar. Raman spectroscopy and electron microprobe analyses on the fossil material indicate their mineralogical and elemental composition. I distinguished alternating bands of iron hydroxides, titanium oxides, and carbon, as well as mica with striae of titanium oxides and carbon, that reflect the mineralization of the organic components of ancient microbial mat fabrics. These microfossils are the result of in situ biomineralization of organic material right after burial during diagenesis and also possibly during recrystallization in the course of post-diagenetic and tectonic overprinting of the host rock. Carbon isotope analyses on the fossil filament-like textures show δ13C values between -22.0-24.2+/-0.5%, typical for preserved organic matter. The morphological and geochemical results, support the presence of cyanobacteria in rocks of at least 2.9 Ga age, and provide Earthly analogs for use on Mars.

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DOI

10.25777/z3wn-m591

ISBN

9780549739081

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