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| Titel |
Carbon fluxes from hydrothermal vents off Milos, Aegean Volcanic Arc, and the influence of venting on the surrounding ecosystem. |
| VerfasserIn |
Paul Dando, Stefano Aliani, Nike Bianchi, Hilary Kennedy, Peter Linke, Carla Morri |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093414
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| Publikation (Nr.) |
 EGU/EGU2014-8108.pdf |
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| Zusammenfassung |
| The island of Milos, in the Aegean Sea, has extensive hydrothermal fields to the east and
southeast of the island with additional venting areas near the entrance to and within the
central caldera. A calculation of the total area of the vent fields, based on ship and
aerial surveys, suggested that the hydrothermal fields occupy 70 km2, twice the
area previously estimated. The vents ranged in water depth from the intertidal to
300 m. As a result of the low depths there was abundant free gas release: in places
water boiled on the seabed. The stream of gas bubbles rising through the sandy
seabed drove a shallow re-circulation of bottom seawater. The majority of the water
released with the gas, with a mean pH of 5.5, was re-circulated bottom water that had
become acidified in contact with CO2 gas and was often diluted by admixture with the
vapour phase from the deeper fluids. The major component of the free gas, 80%,
was CO2, with an estimated total flux of 1.5-7.5 x 1012 g a-1. The methane flux,
by comparison, was of the order of 1010 g a.-1 Using methane as a tracer it was
shown that the major gas export from the vents was below the thermocline towards
the southwest, in agreement with the prevailing currents. Areas of hydrothermal
brine seepage occurred between the gas vents and occasional brine pools were
observed in seabed depressions. Under relatively calm conditions, many of the
brine seeps were covered by thick minero-bacterial mats consisting of silica and
sulphur and surrounded by mats of diatoms and cyanobacteria. The minerals were
not deposited in the absence of bacteria. Storms disrupted the mats, leading to an
export of material to the surrounding area. Stable isotope data from sediments and
sediment trap material suggested that exported POM was processed by zooplankton.
The combined effects of the geothermal heating of the seabed, the large gas flux,
variation in the venting and the effect of the brine seeps had a dramatic effect on the
surrounding ecosystem. The infaunal biomass and diversity were greatly reduced in the
vicinity of the vents (up to approximately 8 m away from the vents) and seagrass
cover was absent from the brine seeps. These changes were a consequence of high
sediment temperatures, hydrogen sulphide concentrations, high or low salinities
and sediment cementation. In contrast, the species diversity of the hard substrate
epibiota surrounding the vents was over twice as high as that at nearby areas of
equivalent water depth, even though vent-obligate fauna were absent. Sediment
cementation, caused by reactions with the minerals in the vent fluids, decreased the
habitat for sediment infauna, but enhanced bioconstruction by coralline algae and
corals, and hence habitat provision for epibenthic organisms. Since the water column
was typical of the oligotrophic Aegean, with low chlorophyll and phytoplankton
production, the diverse and enhanced benthic primary production, by cyanobacteria,
diatoms and chemoautotropic bacteria and archaea, in the mats over the hot seeps
was believed to provide a large diversity of nutritional niches for the epibenthos. |
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