Riedel, Carsten, Theilen, Friedrich and Scholten, Jan, eds. (2000) R/V POSEIDON Cruises 252/253 (1999). . Institute for Geosciences, Departments of Geophysics and Geology/Palaeontology , Kiel, 100 pp.

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Abstract

Preface
The cruises 252 and 253 of RV "Poseidon"
were performed for a common project on the investigation of hydrothermal activities in the area of the Tjoernes-Fracture-Zone north of Iceland.
The main objectives were focussed on the invest
igation of the structure and morphology of hydrothermal vents. Multichannel reflection seismic investigations were performed during the first leg as a pre-site survey for the subsurface structures beneath recent hydrothermal vents and the seismic characterisation of the seafloor, whereas the
second leg was dedicated to the investigation of the morphology in the vent areas using cores and the research submersible "JAGO". The main strategy of the combination of a geophysical
and a geological / geochemical cruise leg was
to illustrate the relationships between heat
sources in the subsurface and the formation of vent fields at the sea floor in order to get a more comprehensive understanding of the dynamic
processes of fluid circulation and the formation
of the uppermost sediments.
Two reflection seismic sections show clearly
the position of a magma chamber beneath a vent
field thus indicating the energy source which
is driving the thermal fluid circulation in the overlying sediments that showed quite clearly when cores were heaved on board and they were
still boiling. The combined interpretation of the data shows a clear correlation between
phase reversals in seismic response
and high gas accumulations as well as anhydrite
in cores and expected seismic reflections coefficients of this sampled material. Thus
combination of geophysical and geological
methods have prooven that reflection seismic
investigations can reveal sea floor characterisics and their spatial distributions
after calibration by groundtruthing in a very effective way.
For future cruises a complete bathymetric
survey should be included in the research
programme as many subsurface features are imaged in the bottom morphology of the sea floor.

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