『Abstract
A detailed geochemical and microbiological study of a 〜2 m sediment
core from the inactive Alvin mounds within the TAG hydrothermal
field was conducted to examine, for the first time, the role of
prokaryotes in subsurface weathering of hydrothermal sediments.
Results show that there has been substantial post-depositional
remobilisation of metal species and diagenetic overprinting of
the original high-temperature hydrothermal minerals, and aspects
have involved prokaryotic processes. Prokaryotic enumeration demonstrates
the presence of a population smaller than the average for deep
sea sediments, probably due to the low organic carbon content,
but not inhibited by (and hence adapted to) the metal rich environment.
There was a small but significant increase in population size
associated with the active redox boundary in an upper metal sulphide
layer (50-70 cm) around which active metal remobilisation was
concentrated (Cu, Au, Cd, Ag, U, Zn and Pb). Hence, subsurface
prokaryotes were potentially obtaining energy from metal metabolism
in this near surface zone. Close association of numbers of culturable
Mn and Fe reducing prokaryotes with subsurface Fe2+
and Mn2+ pore water profiles suggested active prokaryotic
metal reduction at depth in core CD102/43 (to 〜175 cm). In addition,
a prokaryotic mechanism, which is associated with bacterial sulphate
reduction, is invoked to explain the U enrichment on pyrite surfaces
and Zn and Pb remobilisation in the upper sediment. Although prokaryotic
populations are present throughout this metalliferous sediment,
thermodynamic calculations indicated that the inferred low pH
of pore waters and the suboxic/anoxic conditions limits the potential
energy available from Fe(II) oxidation, which may restrict prokaryotic
chemolithotrophic biomass. This suggests that intense prokaryotic
Fe oxidation and weathering of seafloor massive sulphide deposits
may be restricted to the upper portion of the deposit that is
influenced by near neutral pH and oxic seawater unless there is
significant subsurface fluid flow.』
1. Introduction
2. Geological setting: Metalliferous sediments at TAG
3. Sampling and methods
4. Results
4.1. Mineralogy and solid phase compositions
4.2. Pore water compositions
4.3. Prokaryotic abundances and activities
5. Discussion
5.1. Sediment redox conditions
5.2. Sediment pH conditions
5.3. The role of prokaryotes in sediment diagenesis
5.4. Fe(II) oxidation - microbially mediated or abiotic?
5.5. The role of prokaryotes in trace metal diagenesis
6. Conclusions
Acknowledgments
References