Oxygen Isotope Composition of Dissolved Sulfate in Deep-Sea Sediments: Eastern Equatorial Pacific Ocean

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. δ18O values of dissolved sulfate (δO-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 δ18O values at Sites 1225 and 1226, which contain higher organic matter contents, are +20‰ and +28‰, respectively. Depth-correlative trends of increasing δO-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, δO-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 enzymecatalyzed equilibration between oxygen in sulfate and water superimposed upon bacterial sulfate reduction, which would tend to shift δ18OSO4 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 1Blake, R.E, Surkov, A.V., Bottcher, M.E., Ferdelman, T.G., and Jorgensen, B.B., 2006. Oxygen isotope composition of dissolved sulfate in deep-sea sediments: eastern equatorial Pacific Ocean. In Jorgensen, B.B., D’Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201, 1–23 [Online]. Available from World Wide Web: . [Cited YYYYMM-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, MaxPlanck-Institute for Marine Microbiology, Celsiusstrasse 1, D28359 Bremen, Germany. 4Present address: Liebniz Institute for Baltic Seas Research, Seestrasse 15, D18119 Rostock, Germany. Initial receipt: 28 August 2004 Acceptance: 3 November 2005 Web publication: 1 May 2006 Ms 201SR-116 R.E. BLAKE ET AL. OXYGEN ISOTOPE COMPOSITION 2 observed at Sites 1225 and 1226 (e 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.