Publication Date:
2022-05-25
Description:
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 268 (2008): 110-123, doi:10.1016/j.epsl.2008.01.010.
Description:
The subsurface biosphere in the basaltic ocean crust is potentially of major
importance in affecting chemical exchange between the ocean and lithosphere. Alteration
of the oceanic crust commonly yields secondary pyrite that are depleted in 34S relative to
igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures
of biological fractionation, previous interpretations of sulfur isotope fractionation in altered
volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur
species formed during basalt alteration. Here, we report results for multiple-S isotope
(32S,33S,34S) compositions of altered basalts at ODP Site 801 in the western Pacific and
provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ
ion-microprobe analyses of secondary pyrite in basement rocks show a large range of δ34S
values, between –45‰ and 1‰, whereas bulk rock δ34S analyses yield a more restricted
range of –15.8 to 0.9‰. These low and variable δ34S values, together with bulk rock S
concentrations ranging from 0.02% up to 1.28% are consistent with loss of magmatic
primary mono-sulfide and addition of secondary sulfide via microbial sulfate reduction.
High-precision multiple-sulfur isotope (32S/33S/34S) analyses suggest that secondary
sulfides exhibit mass-dependent equilibrium fractionation relative to seawater sulfate in
both δ33S and δ34S values. These relationships are explained by bacterial sulfate reduction
proceeding at very low metabolic rates. The determination of the S-isotope composition of
bulk altered oceanic crust demonstrates that S-based metabolic activity of subsurface life in
oceanic basalt is widespread, and can affect the global S budget at the crust-seawater
interface.
Description:
Alt's contribution was supported by NSF OCE-0424558
and OCE-0622949. Rouxel's contribution was supported by NSF OCE-0622982 and Frank
and Lisina Hoch Endowed Fund. Ono thanks Agouron Institute and NSF OCE-0753126 for
funding. This research used samples and/or data provided by the Ocean Drilling Program.
The ODP is sponsored by the US National Science Foundation (NSF) and participating
countries under the management of Joint Oceanographic Institutions (JOI).
Keywords:
Sulfur isotopes
;
Seafloor weathering
;
Deep biosphere
;
Oceanic crust
;
Sulfur cycle
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
