7. Oxygen Isotope Composition of Dissolved Sulfate in Deep-Sea Sediments: Eastern Equatorial Pacific Ocean¹

Ruth E. Blake,² Alexander V. Surkov,² Michael E. Böttcher,^{3, 4} Timothy G. Ferdelman,³ and Bo B. Jørgensen³

ABSTRACT

High-resolution analyses of the oxygen isotope ratio (¹⁸O/¹⁶O) of dissolved sulfate in pore waters have been made to depths of >400 meters below seafloor (mbsf) at open-ocean and upwelling sites in the eastern equatorial Pacific Ocean. ¹⁸O values of dissolved sulfate (¹⁸O-SO₄) at the organic-poor open-ocean Site 1231 gave compositions close to modern seawater (+9.5 vs. Vienna-standard mean ocean water, providing no chemical or isotopic evidence for microbial sulfate reduction (MSR). In contrast, the maximum ¹⁸O values at Sites 1225 and 1226, which contain higher organic matter contents, are +20 and +28, respectively. Depth-correlative trends of increasing ¹⁸O-SO₄, alkalinity, and ammonium and the presence of sulfide indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations at these sites. Although sulfate concentration profiles at Sites 1225 and 1231 both show similarly flat trends without significant net MSR, ¹⁸O-SO₄ values at Site 1225 reveal the presence of significant microbial sulfur-cycling activity, which contrasts to Site 1231. This activity may include contributions from several processes, including enzyme-catalyzed equilibration between oxygen in sulfate and water superimposed upon bacterial sulfate reduction, which would tend to shift ¹⁸O-SO₄ toward higher values than MSR alone, and sulfide oxidation, possibly coupled to reduction of Fe and Mn oxides and/or bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at Sites 1225 and 1226 ( values between 42 and 79) likely reflect concurrent processes of kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. The oxygen isotope ratio of dissolved pore water sulfate is a powerful tool for tracing microbial activity and sulfur cycling by the deep biosphere of deep-sea sediments.

¹Blake, R.E, Surkov, A.V., Böttcher, M.E., Ferdelman, T.G., and Jørgensen, B.B., 2006. Oxygen isotope composition of dissolved sulfate in deep-sea sediments: eastern equatorial Pacific Ocean. In Jørgensen, 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/116/116.htm>. [Cited YYYY-MM-DD]

²Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven CT 06520-8109, USA. Correspondence author: ruth.blake@yale.edu

³Department of Biochemistry, Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany.

⁴Present address: Liebniz Institute for Baltic Seas Research, Seestrasse 15, D-18119 Rostock, Germany.

Initial receipt: 28 August 2004
Acceptance: 3 November 2005
Web publication: 1 May 2006
Ms 201SR-116