Introduction |
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Hydrothermal chimneys are formed at tectonically active
sites on the sea floor. Cold sea water seeps into cracks
and crevices of the basal-tic rocks and is heated to
more than 300 °C in the sea bed. During this process,
trace elements are leached from the rocks and then transported
into the surrounding sea water when the hot solution
emerges from the sea floor. The change in temperature
causes the immediate precipitation of the less soluble
compounds; these form chimneys that may be several meters
high. It is assumed that the hydrothermal solutions are
one of the main sources of many heavy metals in sea water.
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| The research
ship "Sonne" |
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Little is known about the chemical valency and form
of bonding of most of the metals in the hydrothermal
solutions. Due to the presence of hydrogen sulfide these
solutions have reducing properties. After their release
they rapidly mix with the surrounding sea water, which
contains oxygen. As a result of this the valency and
configuration of any metals present can alter very quickly.
Oxidation by sea water components can occur. In addition
some of the metal ions form complexes very rapidly. The
presence of certain chromium species is used as an indicator
for the redox conditions prevailing in the hydrothermal
solutions and the surrounding sea water.
The various chromium species differ not only in their
chemical behavior, but also in their effects on the biosphere.
Cr(III) is more strongly adsorbed on particles and less
quickly taken up by sea organisms than Cr(VI), which
is present in sea water mainly as dissolved chromate.
As it is a very strong oxidizing agent, Cr(VI) also has
a considerably more toxic effect on organisms. There
is thus a direct connection between the degree of oxidation
of chromium and its biological availability.
The investigations described below were carried out
during a research expedition on board the research ship "Sonne" within
the framework of the HYFIFLUX II project. The sampling
sites were located in the North Fiji Basin in the Pacific
Ocean.
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Sampling |
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Sea water sampling was carried out by means of a probe
equipped with so-called Niskin bottles. Sea water samples
were taken at various depths down to approximately 2000
m.
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Positioning the sampling probe
equipped with
24 x 10 L Niskin bottles. |
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Sampling directly at the hydrothermal vents
was carried out with the aid of a special sampling system
(Hydro Bottom Station, HBS). This allows water samples
to be taken 15 cm below and 100 cm above the sea floor.
Thanks to the two sampling systems it was possible to obtain
a depth profile of the chromium concentration in the water
column above the tectonically active zones. |
| The HBS sampling
system for sampling directly at the hydrothermal vents. |
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Instruments and accessories
used |
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- The 746
VA Trace Analyzer with the 747 VA Stand was used
for the voltammetric analyses.
- The chromium determination was carried out at the
HMDE.
- An Ag/AgCl system with c(KCl) = 3 mol/L was used
as the reference electrode, a platinum electrode as
the auxiliary electrode.
- The 705
UV Digester was employed for the UV digestions.
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Summary |
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In the reducing environment at the hydrothermal
vents chromium is mainly present as Cr(III). The measured
concentrations range up to 2.5 µg/L. In contrast,
in normal sea water less than 0.5 µg/L chromium is
usually found, mainly as Cr(VI). The reduced form could
be detected up to 100 m above the sea floor. It could also
be shown that Cr(III) is then mostly complexed by organic
compounds or bound to particles. In normal sea water a
slow oxidation of Cr(III) to Cr(VI) occurs. |
