Abstract
Changes in the major element and related isotope profiles in porewaters
of organic-rich sediments suggest that various microbial processes using
a succession of electron acceptors are in play during the
remineralization of organic matter. Of the electron acceptors, sulfate
is by far the most abundant and bacterial sulfate reduction (BSR) is
responsible for most organic matter remineralization in sediments. In
addition, nearly all the methane produced during methanogenesis below
the sulfate minimum zone is oxidized anaerobically through sulfate
reduction (anaerobic methane oxidation (AMO)). In places where AMO
occurs, recent studies have demonstrated that the majority of the
sulfate is reduced by methane. This results in linear diffusive profiles
of sulfate concentrations over tens and even hundreds of meters. Oxygen
isotopes in marine sulfate (δ18OSO4) from porewater profiles from
ODP leg 175 were measured to better understand microbial sulfur cycling
and the coupling between sulfate reduction and methane oxidation. In
these sites, sulfate concentrations are depleted with depth, mainly
through AMO. The δ18OSO4profiles show a rapid increase near the
top of all sites from seawater values of 9% to maximums between 22 and
25%. The δ18OSO4 remains enriched and constant (between 22 and
25%) through the rest of the core as sulfate is continually depleted,
then decreases at the bottom of the core as sulfate is consumed in the
zone of AMO. The δ18OSO4 increase at the top of the cores is
difficult to explain without significant rates of sulfate reduction,
yet reoxidation rates must approach 100 percent because of the lack of
depletion in sulfate concentrations and lack of change in sulfur
isotopes. This suggests that sulfate is recycled in the system. The
isotopic decrease in δ18OSO4 into the zone of AMO in all cores
indicates that isotopically heavier sulfate is preferentially reduced
during sulfate reduction associated with methane oxidation.
Original language | English |
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Title of host publication | American Geophysical Union, Fall Meeting 2005 |
Pages | B31A-0978 |
State | Published - 2005 |
Externally published | Yes |
Keywords
- 0414 Biogeochemical cycles
- processes
- and modeling (0412
- 0793
- 1615
- 4805
- 4912)
- 0448 Geomicrobiology