Results of the East Pacific Rise Research in the MODE'94
From this June to November, the Japan Marine Science and Technology
Center (President Mitsugu Ishizuka) conducted international joint diving
expeditions in the Central Atlantic and the Eastern Pacific sea areas with the
"Shinkai 6500" research submersible, This series of research cruise was named
MODE'94 (Mid Oceanic Ridge Diving Expedition), Based on the joint research
agreement, the Woods Hole Oceanographic Institute participated in the MODE'94
researchers conducted in the Atlantic, and as a part of the "international
joint research for the elucidation of the energy and mass flux" sponsored by
the Science and Technology Agency's Special Coordination Founds for Promoting
Science and Technology, many Japanese institutions participated in the MODE'94
researchers conducted in the Eastern Pacific. In this project, it was planned
to comprehensively conduct geological, geophysical and geochemical surveys,
along the mid-oceanic ridges where oceanic crusts were formed. The plan was
also a part of the InterRidge Plan. By late August, two research phases in the
Atlantic were successfully completed. Also in the East Pacific Rise, the
researches were started on September 14, and all of the research phases were
successfully ended on November 29. This report will focus on the results of the
two research phases conducted in the East Pacific Rise, funded by the Special
1. Research objectives
1) Measuring the total amount of the thermal and mass flux brought to the
earth surface layer by the hydrothermal activities and under standing the flux
behaviors through the elucidation of the chemical composition and biological
features of the hydrothermal fluid, plume fluid and hydrothermal activities on
the chemical composition of the sea water. The items to be analyzed are helium
isotope ratio, methane, carbon dioxide, main constituent, infinitesimal metal,
bacteria concentration, organic matter, DNA, amino acid and radioactive
2) Creating a detailed topographical map of the vicinity of the ridge axis by
using the narrow multi-beam. Comparing the created map with the one created by
"Melville" research submersible using the sea bream 2000, and analyzing the
3) Actually observing the oceanic floor from the research submersible and
mapping the oceanic floor topography, volcanic topography and hydrothermal
areas with a video/camera.
4) Observing natural earthquakes. Elucidating the ridge's subterrestrial
structure by seismic reflection measurement of the artificial earthquakes
caused by the air gun.
5) Estimating the age when the rocks and ores were generated and presuming the
6) Observing the long-term and short-term crust thermal flux.
7) Measuring the microbic biomass flux.
2. Researched sea areas
The southern part of the East Pacific Rise (one of the mid-oceanic ridges,
where Pacific oceanic floors are newly formed), which is located in the area
extending in latitude 14 to 20 S, is known as the world-fastest oceanic floor
spreading axis, This time, the research was conducted targeting this area. It
is estimated that the oceanic floor is spreading east and west by 15 cm or more
than 15 cm a year, which means 0.5 mm or more than 0.5 mm a day. Based on the
fact, it was expected that there would be extremely strong hydrothermal
activities in this area. Not many researchers, however, had been conducted in
the area, because it was very far from the developed countries. The top of this
area's ridge was at the water depth of about 2600 to 2700 m and it was known
that there was very thin magma's relationship with the hydrothermal activities.
3. Past researches
In this research, the points of the diving expeditions were decided based
on the pre-research results of the "Melville" research submersible, and
multi-field diving expeditions were systematically conducted. The "Melville"
navigation was conducted from November to December 1993, chartering the
"Melville" belonging to the Scripps Institution of Oceanic Research, as a part
of the researches funded by the Special Coordination Funds. As a result of the
research navigation, it was revealed that there were about 26 hydrothermal
plumes (abnormal hydromass with the diameter of 0 to 5 km and thickness of 400
to 500 m, with abnormality in temperature of 0.1 degree C. or lower) on the
ridge extending as long as 520 km from south to north. The existence of there
hydrothermal plumes showed that there were many hydrothermal activities (active
chimneys such as black smokers) along with the ridge axis. The hydrothermal
plumes could be roughly divided into those containing much sulfur and those
containing much iron.
4. Research activities
1) Phase Three
For the 36 days from September 14 (Port Balboa in Panama) to October 19,
1994 (Port Valparaiso in Chile), the Phase Three research was conducted, and 15
dives were made in the southern part of the East Pacific Rise, 4000 km off the
west Chile (in latitude 14 to 18 S and longitude 113 W), The research was
jointly conducted with the Pacific Marine Environmental Laboratory (PMEL) of
the U.S. National Oceanic and Atmospheric Administration (NOAA). Tetsuro Urabe
of the Geological Survey of Japan served as the Chief Scientist in Phase Three,
and nine Japanese scientists participated in the research from the Geological
Survey of Japan and the Bioengineering Research Institute of MITT Agency on
Industrial Science and Technology, the Japan Marine Science and Technology
Center, the Hydrographic Department of the Maritime Safety Agency, and from
several universities. From the United States, three scientists including John
Lupton (NOAA/PMEL) participated in the research phase.
2) Phase Four
For the 36 days from October 25 (Port Valparaiso in Chile) to November 29,
1994 (Papeete in French territory), the Phase Four research was conducted and
15 dives were made in the southern part of the East Pacific Rise, 4000 km off
the west of Chile (in latitude 14 to 20 S and longitude 112 to 113 W), as in
Phase Three. Kantaro Fujioka, Senior Scientist at the Japan Marine Science and
Technology Center served as the Chief Scientist for Phase Four. From Japan, ten
members participated in the research in total, as from the Japan Marine Science
and Technology Center, the Geological Survey of Japan of the MITT Agency of
Industrial Science and Technology, Hydrographic Department of the Maritime
Safety Agency and from several universities. From the United States, four
scientists including Gary Matheus (NOAA/PMEL) participated.
5. Outline of the research results
1) Results of Phase Three
(1) Detection of high-temperature and low-temperature hydrothermal activities
directly under the plume.
as a result of the dive made directly under the plume, strong thermal
activities were detected directly under the plume, and the result of the
pre-research conducted in the previous year was backed up. The dive was
conducted at four points, and high-temperature hydrothermal activities such as
black smokers were detected at the two points, RM28 and RM29, Low-temperature
hydrothermal activities were also detected in a wide range at the point RM23.
It was revealed that the chemical composition of the plume was clearly
distinguished, depending whether it originated from high-temperature black
smoker or from wide-range, low-temperature spring water. That is to say, the
plume containing much sulfer originates from low-temperature hydrothermal
fluid, while the plume containing much iron originates from high-temperature
hydrothermal fluid. As characteristic of the low-temperature site, many living
creatures such as shrimps, crabs, shellfish and seaflowers were seen, while in
the high-temperature site, only limited number of living creatures were
(2) Amount of released flux more than 10 times larger than usual ridges.
The high-temperature hydrothermal fluid seemed to boil under the ground,
and the fluid with salt concentration of one fourth or one and half times of
that of the sea water was detected. The former seemed to be the re-condensed
vapor, while the latter seemed to be the erupted high-concentration salt water.
The low-temperature hydrothermal fluid contained thiosulfuric acid and it was
presumed that the independent nutritive bacteria obtained energy by oxidizing
the sulfer. Because of this, many living creature groups could survive in the
area and the high bacteria concentration was observed in the plume. By
analyzing the results comprehensively, it became possible to calculate the
quantity of heat and other substances released from the internal earth. For
example, it was revealed that the amount of released iron per unit length of
the ridge in the southern part of the East Pacific Rise was ten or more than
ten times as large as that of the northern part of the East Pacific Rise. Also
much amount of other chemical elements, carbon dioxides and methanes was
released and it was revealed that the heat and other substances were released
most in the area on the earth.
(3) Elucidation of the subterrestrial structure by the detailed geophysical
The subterrestrial structure of the ridge was elucidated by the use of six
submarine seismographs and sound sources (air-gun oscillators). The data is now
being processed and detailed analysis results will be reported soon.
2) Results of Phase Four
(1) Detection of new hydrothermal chimneys and collection of the hydrothermal
endmember, plume, deposit and living creature sample.
Based on the results of Phase Three, 15 dives were made directly under the
hydrothermal plume by "Shinkai 6500", As a result, new hydrothermal chimneys
were detected at eight sites in total, and the full set required for the
estimation of flux was obtained around the chimney area, including the
hydrothermal fluid, plume, hydrothermal deposit, chimney and living creature
sample. The chimney at RM04, which was located at the north end of the research
sea area, was an example of the smoker containing much iron, with the highest
temperature of 374 degrees C., and it was different from the chimneys observed
at other sites.
(2) Collection of information on the topography, geology and rocks of the wide
From the information on the topography, geology and rocks of the wide area
along the ridge axis of the East Pacific Rise (in latitude 14 to 18 18 22' S),
detailed data was obtained for each segment. Especially, much information was
obtained on the vertical direction (east-wast) to the ridge axis, and it was
really useful for understanding the history of the fast spreading axis.
(3) Successful installation of the long-term observation system.
The long-term thermal flux measuring instrument (cable type and flat
type) and short and long-term thermal flux measuring instrument (flat type)
were successfully installed on the ridge axis. These instruments accurately
measured the amount of the thermal flux released from the internal earth and
they were very useful for elucidating the thermal flux. Besides, seven
submarine seismographs were installed on the oceanic floor and the
subterrestrial structure of the ridge was elucidated by the use of sound
sources (air gun oscillators).
(4) Wide-range research by "Yokosuka"
With the observation system mounted on the mother ship "Yokosuka", The
geophysical observation was conducted in a wide range around the East Pacific
Rise and the neighboring areas. Areas as long as 7,000 miles were observed in
total, and the topography, gravity and geomagnetism were studied. The area with
no geophysical observation data (in latitude 18 45' to 20 S) in the central
axis part of the East Pacific Ridge (in latitude 14 to 20 S) was also covered
in the observation. These data are very useful for understanding the
mid-oceanic ridge's subterrestrial and temperature structures.