Geokjemi & Geobiologi

Detecting microbial sulfate reduction coupled to anaerobic methane oxidation in hydrothermal sediments with stable isotopes (S, Fe) and trace elements

Prosjektbeskrivelse / Project description
Microbial sulfate reduction was probably one of the earliest metabolisms to evolve on Earth and represents an important component of the modern marine and terrestrial carbon cycle. The presence and activity of sulfate reducers in modern sediments can be detected using molecular biology techniques, geochemical analyses of pore fluids, rate measurements using radioactive sulfate, and stable isotopes measured in sulfide minerals that are produced by sulfate reduction. However, the detection of past microbial activity in the rock record is more complicated and requires robust tracers for microbial sulfate reduction, that ideally also provide information on the type of electron donors (molecular hydrogen, organic matter or methane) used by ancient sulfate reducers.

Recent work has suggested that a combination of stable sulfur (S) isotopes, iron (Fe) isotopes and trace elements measured in pyrite from methane seep sediments can be used to identify microbial sulfate reduction coupled to anaerobic methane (CH4) oxidation (AOM), a process that involves an association between sulfate reducers and anaerobic methanotrophs (ANME). Identifying this specific pathway is important for our understanding of carbon cycling in the present and past. For example, sulfate-driven anaerobic methane oxidation in anoxic swamps prevents the release of the greenhouse gas methane to the atmosphere. In addition, it has been suggested that the coupling between sulfate reduction and methane oxidation was involved in the rise of oxygen in the atmosphere more than 2.5
billion years ago.

In this project, we will test the use of the recently proposed proxy for SRB-AOM in anoxic hydrothermal sediments, which represent a different type of environment than methane seeps and may be more representative of settings on the early Earth. The project will use samples collected from the low-temperature Loki’s Castle hydrothermal barite field in the Norwegian-Greenland sea, which have been previously studied for their microbiology and where anaerobic methanotrophs have been identified. The student will have to separate pyrite minerals from the sediments, perform multiple sulfur and iron isotope analyses on the samples and dissolve samples for trace element analyses. Iron and trace element work will be done at the University of Bergen, and the sulfur isotope work will be done in collaboration with prof. Harald Strauss at the University of Münster in Germany and will involve a short stay abroad.

Foreslåtte emner i spesialiseringen (60 stp) // Proposed course plan during the master's degree (60 ECTS):
Geobiology GEOV344, 10 ECTS
Geomicrobiology GEOV245, 10 ECTS
The Geochemical Toolbox (GEOV342), 10 ECTS
Selected Topics in Geoscience GEOV300, 5 ECTS
Environmental Geochemistry GEOV243, 10 ECTS
Data analysis in Earth Science GEOV302, 10 ECTS

Særskilte krav for opptak til prosjektet / Prerequisites
Background in geochemistry or geobiology, and a strong motivation for a laboratory-based project. The student must be willing to stay abroad for a short period (1-3 weeks) to do sulfur isotope analyses with prof. Harald Strauss.

Felt- lab- og analysearbeid
Separation of sulfides from sediments. Conversion of sulfides into Ag2S for sulfur isotope analysis. Dissolution of sulfides into strong acids for Fe isotope and trace element analyses. Work in the clean laboratory to prepare samples for Fe isotope analyses. Measurement of sulfur isotopes by gas-source mass spectrometry (Germany) and iron isotopes by multi-collector ICP-MS (Bergen).