The sulfur cycle in marine sediments exerts a major control on the redox state of the ocean and atmosphere. The overall driver in the sulfur cycle is the microbial mediated reduction of sulfate to sulfide. Using the highly sensitive radio-label technique to make direct measurement of sulfate reduction rates (SRR), we know that more than 90% of the sulfate turnover (not to be confused with total sulfate flux) can be typically be attributed to oxidation of compounds other than methane , and that only a small fraction of the sulfate reduced becomes buried as pyrite. 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.

Chemical species and pathways in the marine sulfur cycle .
Figure adapted 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.

Coupling of sulfur, iron, phosphorus and carbon cycles in marine sediments

In most marine sediments the cycling of sulfur is tightly coupled with the speciation and turnover of iron and manganese solid phases. For instance, the reaction between the product of sulfate reduction, sulfide, and ferric iron in the form of metal oxides or bound in silicate minerals leads not only to the formation of Fe-sulfide minerals and complexes, but also to other higher oxidation state sulfur compounds.
Closely linked to the speciation and turnover of C, S and Fe compounds in marine sediments and the benthic boundary layer, is the fate of phosphorus (P). As P is a key biolimiting element in aquatic environments, understanding the interactions between the P, C, Fe and S cycles is paramount.

A part of this research is undertaken in the context of our collaboration with MARUM in projects GB2 (with doctoral student S. Sokoll) and GB3. A recently developed tool for examining P biogeochemistry is these systems is the use of 33P radiotracer on both near-shore sites (see Goldhammer et al., Nature Geoscience, 2010) and open-ocean oligotrophic sediments (MarMic Master project, J. Marshall).

Oceanography and biogeochemistry of seafloor and sub-seafloor ecosystems

The surface and near-surface seafloor represents a plate of high microbial abundance, high microbial diversity, high microbial activity separating the vast deep biosphere habitat from the deep ocean water masses that dictate climate. This zone is characterised by a close interaction between microbes, which make up around 90% of the seafloor benthic biomass, and the less abundant (macro-)fauna. The latter, however, have a strong impact on functioning and diversity of seafloor habitats.

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.

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 329 to the South Pacific Gyre and other drilling and coring expeditions to the North Atlantic and South Atlantic, as well as extensive research in the Baltic and Black Seas.

For more detailed information and links go to:

Drilling ship JOIDES Resolution

JOIDES Resolution leaving Pte Delgado, Azores (photo: T. Ferdelman)

(Photos: T. Ferdelman)