Biotic and abiotic alteration of hydrothermal sulphides at the seafloor


Mueller, Moritz (2009) Biotic and abiotic alteration of hydrothermal sulphides at the seafloor. University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science, Doctoral Thesis , 208pp.

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Description/Abstract

When active venting has ceased, reduced minerals in hydrothermal mounds and sediments
continue to provide an inorganic energy source for chemolithotrophic microbes.
This research focuses on the nature of microbially-mediated metal transformations in
hydrothermal sediments during sulphide alteration and their impact on the ultimate
fate of hydrothermal sulphides on the seafloor. The core studied is more seawater
altered than other cores studied at TAG and provides an insight into the bacterial
and archaeal communities as well as the geochemical processes taking place in highly
altered metalliferous sediments.
This study combines geochemical approaches with microbiological and organic
biomarker measurements within the suboxic transition zone of sulphidic sediments
to characterise the reactions and microbial communities present. This integrated
approach demonstrates that (a) there is biogeochemical zonation within the sediment
sequence with distinct microbial communities present at the sulphide-oxic seawater
transition, (b) the microbes identified are associated with Fe and S redox cycling, (c)
Marinobacter sp. are dominant at the sulphide interface.
There is a significant shift in the microbiological community across the redox transition
zone in these sulphidic sediments. The microbial assemblage of the suboxic transition
zone is dominated by Bacillus sp. which are microaerophilic whereas the sulphide
layer assemblage is dominated by Marinobacter sp. which are Fe oxidisers. Based
on biomarker assemblages and genetic analyses, archaeal (and to a certain degree
bacterial) communities are comparable to other hydrothermal settings despite the low
biomass present. Processes inferred to be important in this sediment include the S, Fe
and N cycle, all potentially coupled to the release and uptake of a range of transition
metals.
Significant recycling of redox active species occurs in the suboxic transition zones present in the sediment core. Uranium concentrations are low compared with
other less altered sulphidic hydrothermal sediments, and U is associated with the
upper regions of the suboxic transition zone and is associated with enrichment of a
suite of other transition metals (e.g. Cu, Mo, As and V).
Massive electrodes were constructed from CuFeS2 (from Ireland and TAG) and
FeS2 (from TAG) and studied in oxygenated artificial seawater under circumneutral
conditions using electrochemical methods. The results can be explained by existing
hypotheses about pyrite and chalcopyrite oxidation and their oxidation products. The
oxidation status of Cu in covellite could be identified as Cu+1. Impurities found (as
expected because natural samples were used) have no effect on the electrochemical
behaviour of the electrodes.
The impact of Marinobacter sp. on sulphide alteration was studied in detail using
an electrochemical approach. The results demonstrate that Marinobacter aquaeolei
enhances the rates of oxidation. Marinobacter species seem to be of special importance
for weathering reactions on the seafloor and in hydrothermal settings like the TAG
area.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
G Geography. Anthropology. Recreation > GC Oceanography
Divisions: University Structure - Pre August 2011 > School of Ocean & Earth Science (SOC/SOES)
ePrint ID: 69029
Date Deposited: 14 Oct 2009
Last Modified: 27 Mar 2014 18:48
URI: http://eprints.soton.ac.uk/id/eprint/69029

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