The “sub-seafloor biosphere”, or those sediments and rock ecosystems that are reside well below the sediment-water interface, is one of the largest habitats on planet Earth. Globally, the largest reservoir of organic carbon resides in marine sedimentary deposits that are buried to depths of hundreds of meters below the seafloor. These sediments provide a habitat for bacterial populations whose total combined biomass is estimated to represent ca. 10% of the Earth's total surface biomass.
Principal challenges in studying the “deep biosphere” include efforts to: (1) quantify and understand the community structure of deeply buried microbial communities; (2) assess their metabolic activities; and (3) evaluate their quantitative impact on deep sediment and oceanic biogeochemical cycles. Recent insights into the into metabolic activities of the deep marine sedimentary biosphere have been gained through various scientific drilling expeditions, including ODP Leg 201, IODP Expedition 307 and IODP site surveys to the South Pacific Gyre and the North Atlantic.

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Sub-seafloor Science and Drilling

Drilling ship JOIDES Resolution sailing out of Pte Delgado, Azores at the end of IODP Expedition 307.

(Photo: T. Ferdelman)

Methane in marine sediments

The deposition of organic material in marine sediments and its burial below the sulfate zone is the basis for a microbiological production of vast amounts of methane. Controls on the distribution and turnover of methane in marine sediments are the themes of two projects: the EU-BMBFproject BONUS Baltic Gas, which focuses on the geophysical, geological, and (bio)geochemical controls on methane in European continental margin sediments.

Sulfur speciation and cycling

Most of the sulfide produced in surface marine sediments via microbial sulfate reduction is eventually oxidized back to sulfate via sulfur compounds of intermediate oxidation state in a complesx web of competing chemical and biological reactions. Improved handling, derivatization, and chromatographic techniques allow us to more closely examine the occurence and fate of sulfur intermediates such as elemental suflur, polysulfides, thiosulfate, tetrathionate, and sulfite. I have had a long standing interest in the chemistry and biogeochemistry of sulfur. In collaboration with J. Zopfi, H. Fossing, and B.B. Joergensen, we have examined the sulfur chemistry and cycling in a number of marine systems from the Chilean continental shelf, to Mariager Fjord (DK) and to North Sea sediments. Recent research has taken place in collaboration with sulfur specialist A. Kamyshny who has been busy exploring elemental sulfur and polysulfide distributions in a variety of marine habitats, e.g. the Wadden Sea, Black Sea, and Baltic Sea sediments. In addition to sulfur speciation, we employ 35S labeled compounds to explore rates and pathways of sulfur transformations.

Chemical species and pathways in the marine sulfur cycle (dashed lines indicate disproportionation reactions).

Figure is from: Zopfi, J., Ferdelman, T.G., and Fossing, H.. 2004. Distribution and fate of sulfur intermediates -- sulfite, thetrathionate, thiosulfate, and elemental sulfur -- in marine sediments, in Amend, J.P., et al., (eds) Sulfur biogeochemistry -- Past and present: Geol. Soc. America Special Paper 379, p. 97-116.