6. Sulfur Isotope Fractionation by the Deep Biosphere within Sediments of the Eastern Equatorial Pacific and Peru Margin1

Michael E. B÷ttcher,2, 3 Timothy G. Ferdelman,2 Bo B. J°rgensen,2 Ruth E. Blake,4 Alexander V. Surkov,4 and George E. Claypool5


Fifty-seven interstitial water samples from six sites (Ocean Drilling Program Sites 1225–1229 and 1231) in the eastern equatorial Pacific Ocean and the Peru margin were analyzed for the stable sulfur isotopic composition (34S/32S) of dissolved sulfate along with major and minor ions. With the exception of Site 1231, sulfate from the interstitial fluids (34S values as much as 89 vs. the SF6-based Vienna-Canyon Diablo troilite standard) is found at depth to be enriched in 34S with respect to modern seawater sulfate (34S = ~21), indicating that microbial sulfate reduction (MSR) took place to different extents at all investigated sites. Deeper sediments at Sites 1228 and 1229 are additionally influenced by diffusion of a sulfate-rich brine that has already undergone sulfate reduction. The intensity of MSR depends on the availability of substrate (organic matter), sedimentation conditions, and the active bacterial community structure. Formation of isotopically heavy diagenetic barite at the sulfate–methane transition zone is expected at Sites 1227 (one front), 1229 (two fronts), and probably Site 1228. At Site 1231, the constant sulfur isotopic composition of sulfate and concentrations of minor pore water ions indicate that suboxic (essentially iron and manganese oxide based) diagenesis dominates and no net MSR occurs.

1B÷ttcher, M.E., Ferdelman, T.G., J°rgensen, B.B., Blake, R.E., Surkov, A.V., and Claypool, G.E., 2006. Sulfur isotope fractionation by the deep biosphere within sediments of the eastern equatorial Pacific and Peru margin. In J°rgensen, B.B., D'Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/201_SR/109/109.htm>. [Cited YYYY-MM-DD]

2Department of Biogeochemistry, Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany. Correspondence author: michael.boettcher@io-warnemuende.de

3Present address: Leibniz Institute for Baltic Seas Research, Seestrasse 15, D-18119 Rostock, Germany.

4Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven CT 06520-8109, USA.

58910 West Second Avenue, Lakewood CO 80226, USA.

Initial receipt: 26 July 2004
Acceptance: 2 November 2005
Web publication: 11 April 2006
Ms 201SR-109