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 Coordination Funds.

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 nuclide.

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 topographical changes.

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 generation causes.

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 observed.

(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 measurement.

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 area.

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.

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