of the St. Paul’s Rocks Fracture Zone: (1) The Intra-Transform Ridge (ITR)
segments and, (2) The sigmoidal shaped St. Peter and St. Paul’s Rocks (SPPR)
massif on which the islets are constructed (Fig. 8.15a, b). We started with our
exploration of the ITR, due to logistical reasons.
Diving in the Intra-Transform Ridges (ITR) of St. Peter
and Paul’s Rocks
The Intra-Transform Ridge (ITR) spreading centers represent short spreading ridge
segments interrupted by East–West trending transform faults (\230 km) within
the main St. Paul F.Z. On a regional scale, this reflects deep-seated colder convective systems than those existing to the north and south of the Equator, within a
band of about 440 km (between 2°S and 2°N) in the Equatorial Atlantic. The three
short Intra-Transform Ridge (ITR) segments at 25°27
0 W, 26°10
0 W and 27°42
0 W
were seen on satellite altimetry data (Fig. 8.15a). The two intra transform ridges
located at 25°27
0 W (ITR A) and 27°42
0 W (ITR C) were chosen for investigation
because we had time to dive on them, while still remaining within our short
schedule at sea (Hekinian et al. 2000). The short (\20 km in length) spreading
segments show axial valleys about 4600–4700 m deep, are less than 2 km wide,
and are bounded by walls that are 2000 m high. Like other major Mid-Atlantic
Ridge segments, the Saint Peter’s and St Paul’s ITR have a general N 340°(NNW)
trending orientation and sedimented nodal basins that are more than 5000 m deep
(27°42
0 W and 25°27
0 W) at their spreading ridge segment ends (Fig. 8.15b).
AT 7 pm on the evening of December 23, 1997, we arrived on the first site of
our dives, on the ITR system near 0°43
0 N and 25°27
0 W. Our first task was to try a
new system of deep towed camera at a depth of 2750 m, in order to test the electric
coaxial cable and the quality of the images. After 5 h on the bottom, the system
had stopped working. Since we did not have any spare parts on board, the engineer
(Cavarec) had to improvise the best he could in order to make the device work.
This first lowering of the deep-towed camera enabled us to see the bottom landscape before the first dive.
The next morning, I was scheduled to make the first dive. I could see that my
colleague scientists were pleased with my decision because they did not have to
miss the Christmas Day’s special meal. But, the steward (Michel, who was also
called ‘‘Aldo’’) had given an order for the diving team to also be supplied with a
better menu to take to the bottom.
The first task was to explore the depression, or Rift valley, and to find the type
and the extent of any volcanic activity. As usual, on the morning of December 24,
we were ready at 08h00. I went up the ladder on the side of the Nautile and down
into the sphere where Olivier Cipriani, the co-pilot, was finishing his last check on
the instruments. Then Max Dubois, the pilot, sat on the edge of the airlock, waiting
for the Nautile to be pulled to its launching position on the back deck of the
Diving in the Equatorial Atlantic
287
segments and, (2) The sigmoidal shaped St. Peter and St. Paul’s Rocks (SPPR)
massif on which the islets are constructed (Fig. 8.15a, b). We started with our
exploration of the ITR, due to logistical reasons.
Diving in the Intra-Transform Ridges (ITR) of St. Peter
and Paul’s Rocks
The Intra-Transform Ridge (ITR) spreading centers represent short spreading ridge
segments interrupted by East–West trending transform faults (\230 km) within
the main St. Paul F.Z. On a regional scale, this reflects deep-seated colder convective systems than those existing to the north and south of the Equator, within a
band of about 440 km (between 2°S and 2°N) in the Equatorial Atlantic. The three
short Intra-Transform Ridge (ITR) segments at 25°27
0 W, 26°10
0 W and 27°42
0 W
were seen on satellite altimetry data (Fig. 8.15a). The two intra transform ridges
located at 25°27
0 W (ITR A) and 27°42
0 W (ITR C) were chosen for investigation
because we had time to dive on them, while still remaining within our short
schedule at sea (Hekinian et al. 2000). The short (\20 km in length) spreading
segments show axial valleys about 4600–4700 m deep, are less than 2 km wide,
and are bounded by walls that are 2000 m high. Like other major Mid-Atlantic
Ridge segments, the Saint Peter’s and St Paul’s ITR have a general N 340°(NNW)
trending orientation and sedimented nodal basins that are more than 5000 m deep
(27°42
0 W and 25°27
0 W) at their spreading ridge segment ends (Fig. 8.15b).
AT 7 pm on the evening of December 23, 1997, we arrived on the first site of
our dives, on the ITR system near 0°43
0 N and 25°27
0 W. Our first task was to try a
new system of deep towed camera at a depth of 2750 m, in order to test the electric
coaxial cable and the quality of the images. After 5 h on the bottom, the system
had stopped working. Since we did not have any spare parts on board, the engineer
(Cavarec) had to improvise the best he could in order to make the device work.
This first lowering of the deep-towed camera enabled us to see the bottom landscape before the first dive.
The next morning, I was scheduled to make the first dive. I could see that my
colleague scientists were pleased with my decision because they did not have to
miss the Christmas Day’s special meal. But, the steward (Michel, who was also
called ‘‘Aldo’’) had given an order for the diving team to also be supplied with a
better menu to take to the bottom.
The first task was to explore the depression, or Rift valley, and to find the type
and the extent of any volcanic activity. As usual, on the morning of December 24,
we were ready at 08h00. I went up the ladder on the side of the Nautile and down
into the sphere where Olivier Cipriani, the co-pilot, was finishing his last check on
the instruments. Then Max Dubois, the pilot, sat on the edge of the airlock, waiting
for the Nautile to be pulled to its launching position on the back deck of the
Diving in the Equatorial Atlantic
287