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Abstract
The rifting zone of Bransfield Strait, Antarctica, is tectonically and geologically unique. It is a back-arc basin that was opened by extensional forces associated to roll-back subduction after cessation of spreading activity of the Phoenix Ridge, and the transtension of the westward ending of Scotia-Antarctica Plate boundary. The Bransfield Rift/Ridge is still active generating volcanism or magma rise to force hydrothermal activity. During the ANT-XXV/4 cruise onboard R/V “Polarstern”, standard CTD and beam transmission measurements were done to determine temperature anomaly and turbidity. Water sampling was performed to determine δ3He and to find thermophilic microorganisms to examine the Orca seamount hydrothermal activity. A temperature anomaly of ~0.08 °C, a pick of turbidity, and high value of δ3He (>10%) were found inside Orca seamount. Results are consistent with a hydrothermal flux coming from the seamount. The report of the first observation of thermophilic and hyperthermophilic microorganisms in cold deep Antarctic waters is part of this study. Inside Orca seamount these microorganisms were found at three different depth levels close to the bottom. We suggest that the fluid migration from the volcano resulted from recent magmatic activity and provided the required elemental nutrients for microbial growth. Besides some thermophiles were found outside the seamount in a small quantity close to the seafloor. These would probably be related to subsidiary structures of the Orca seamount, or were transported by currents from other active volcanic sites as Deception Island. The finding of these thermophilic and hyperthermophilic microorganisms raise questions about the dispersal and their resistance in these extreme environments.
Highlights
The seaf loor hydrothermal activity has a great impact on the chemistry of the oceans and is responsible for extensive alteration of the oceanic crust (Herzig and Hannington, 1995)
The profiles of temperature (T) and salinity (S) for stations A (Orca) and B show similar patterns and represent the typical hydrographic characteristics of the area (Fig. 3A) (e.g., Gordon and Nowlin, 1978; Wilson et al, 1999; García et al, 2002; Sangrà et al, 2011). Both stations show a flow of warmer and saltier water between 200 and 450 m below the sea level (b.s.l.) that corresponds to the Modified Circumpolar Deep Water (CDW), which centres around σθ=27.75 (Fig. 3B)
This flow has its maximum in T and S at ~400 m b.s.l., which coincides with the oxygen minimum, and it is more marked at the oceanographic variables for A Station (Orca) Station profiles, indicating weaker influence of the Southern Ocean waters near the slope of the King George Island
Summary
The seaf loor hydrothermal activity has a great impact on the chemistry of the oceans and is responsible for extensive alteration of the oceanic crust (Herzig and Hannington, 1995). Typical submarine hydrothermal vents support unusual ecosystem communities where primary production, which is the basis for the local food chain, depends on chemosynthetic microorganisms (Baross and Deming, 1983). This phenomenon is not unique to mid-ocean ridges since it occurs in volcanic arcs, back-arc spreading centers and intra-plate volcanoes (e.g., German et al, 2000). Every mentioned site has its own associated tectonics, geodynamics and geological characteristics that produce distinctive geophysical and geochemical signatures, which are reflected in the variability in composition, size or volume, spatial distribution, frequency and transience of the deep-sea hydrothermal vents (Lupton et al, 1998; Baker et al, 2001; Hey et al, 2004; Hannington et al, 2005). Subduction and extension processes occur simultaneously and there is currently no clarity in their geological evolution
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