lithosphere and to determine the conditions of isostatic equilibrium that the
lithospheric plate had undergone. This data would be contributed to the international geodetic database for an area of the World where, up to now, no in situ
gravimetric data had been available. These measurements were also completed by
others made along two submersible transects carried out at more than 3500 m
depths in the vicinities of the two major volcanoes, Adams and Bounty (Dives 7
and 10).
The first gravity transect was made with Jacques Dubois as the observer on the
northeastern limit of the hotspot and the abyssal plain, at more than 3500 m depth,
starting at the base of the Bounty volcano. The second transect dive was done by
Jérome Amman, an engineer at the IPGP (Institut de Physique du Globe, Paris), in
the southern part of the hotspot at 3958 m depth. Continuous gravimetric measurements were carried out over a distance of 7.5 km.
There were three additional goals for our dives: (1) to determine the nature of
any hydrothermal activity, (2) to carry out gas and fluid sampling of the hydrothermal vents and (3) to use a new sampling probe device in order to detect living
organisms in a hydrothermal system.
Dives on Adams and Bounty Volcanoes
The Adams Volcano, which is the tallest edifice of the Pitcairn hotspot, consists of
two summit cones at less than 100 m from sea level. The summit is covered with
algae on top of a coral bed that is mixed with volcanic cinders and hydrothermal
sediment. Three dives were made on this edifice with the submersible Nautile
along the western flank of the volcano, from a depth of 3000 m up to the summit at
less than 100 m form the surface. The dives have revealed that the volcano is still
active.
I did not know that I was scheduled to make the first and the last dive of the
Polynaut cruise. I was very pleased and honored to learn that my colleagues and
Jacques Dubois, the chief scientist, had decided that I would make these dives. As
usual, the briefing for the next morning’s dive was made in the presence of the
captain, the chief scientist, the head pilot (Jean-Paul Justiniano from La Seynes in
Toulon), and all the scientific party. On the newly made map of the area, I pointed
to my target area for the dive. This dive was the first of a series of dives that were
planned to visit the flank of Adams in order to make a stratigraphic section. The
first dive was to start at 2500 m depth, near the base of the volcano.
As soon as we arrived on the bottom on August 28 at 19:29 GMT (geographic
Meridian Time), which corresponded to 11:29 local time, pilot Jean-Paul
Justiniano stabilized the Nautile, or in other words, put the submersible into a state
of neutral buoyancy, in order to take a gravity measurement. The seafloor at
2470 m depth was covered with sediment that was partially dusted by volcanic
ash. The presence of animals, revealed by the worm burrows in the sediment,
suggested that no recent volcanism had occurred. The gravimeter was placed
332
9 Hotspots
lithospheric plate had undergone. This data would be contributed to the international geodetic database for an area of the World where, up to now, no in situ
gravimetric data had been available. These measurements were also completed by
others made along two submersible transects carried out at more than 3500 m
depths in the vicinities of the two major volcanoes, Adams and Bounty (Dives 7
and 10).
The first gravity transect was made with Jacques Dubois as the observer on the
northeastern limit of the hotspot and the abyssal plain, at more than 3500 m depth,
starting at the base of the Bounty volcano. The second transect dive was done by
Jérome Amman, an engineer at the IPGP (Institut de Physique du Globe, Paris), in
the southern part of the hotspot at 3958 m depth. Continuous gravimetric measurements were carried out over a distance of 7.5 km.
There were three additional goals for our dives: (1) to determine the nature of
any hydrothermal activity, (2) to carry out gas and fluid sampling of the hydrothermal vents and (3) to use a new sampling probe device in order to detect living
organisms in a hydrothermal system.
Dives on Adams and Bounty Volcanoes
The Adams Volcano, which is the tallest edifice of the Pitcairn hotspot, consists of
two summit cones at less than 100 m from sea level. The summit is covered with
algae on top of a coral bed that is mixed with volcanic cinders and hydrothermal
sediment. Three dives were made on this edifice with the submersible Nautile
along the western flank of the volcano, from a depth of 3000 m up to the summit at
less than 100 m form the surface. The dives have revealed that the volcano is still
active.
I did not know that I was scheduled to make the first and the last dive of the
Polynaut cruise. I was very pleased and honored to learn that my colleagues and
Jacques Dubois, the chief scientist, had decided that I would make these dives. As
usual, the briefing for the next morning’s dive was made in the presence of the
captain, the chief scientist, the head pilot (Jean-Paul Justiniano from La Seynes in
Toulon), and all the scientific party. On the newly made map of the area, I pointed
to my target area for the dive. This dive was the first of a series of dives that were
planned to visit the flank of Adams in order to make a stratigraphic section. The
first dive was to start at 2500 m depth, near the base of the volcano.
As soon as we arrived on the bottom on August 28 at 19:29 GMT (geographic
Meridian Time), which corresponded to 11:29 local time, pilot Jean-Paul
Justiniano stabilized the Nautile, or in other words, put the submersible into a state
of neutral buoyancy, in order to take a gravity measurement. The seafloor at
2470 m depth was covered with sediment that was partially dusted by volcanic
ash. The presence of animals, revealed by the worm burrows in the sediment,
suggested that no recent volcanism had occurred. The gravimeter was placed
332
9 Hotspots