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Leg 121

July 18, 1988
COLLEGE STATION, TX
-- In the eastern Indian Ocean, west of Australia, lies a mysterious underwater mountain range named Broken Ridge. The ridge was once a long, narrow island before sinking below the sea surface, burying with it the secrets of its origin, the record of ancient shell littered beaches and evidence of the mass extinction of plant and animal life that occurred more than 60 million years ago.

In the same region is a second linear subterranean mountain range named Ninetyeast Ridge. This ridge records part of the Indian Ocean's tectonic history by showing how India moved from its original attachment to Antarctica to its present Southern Asian location.

During May and June, scientists on board JOIDES Resolution, drill ship for the Ocean Drilling Program (OD), recovered from these two ridges cores totaling more than a mile of sediment and basalt, material that will enable them to reconstruct this region's geologic history.

Ninetyeast Ridge extends for 5,000 kilometers along the 90th meridian from the Bay of Bengal to a latitude equal to Australia's southern coast, a distance greater than that between Boston and Seattle. It terminates at the east-west trending submerged mass of Broken Ridge.

Cores of sediment and rock from Broken Ridge revealed that about 45 million years ago, geologic forces associated with rifting lifted the range above the sea surface, forming a long, narrow island. Scientists knew that Broken Ridge once had been part of Kerguelen Plateau, a giant underwater formation now lying southwest. They did not know, however, what type of rifting forces separated the ridge from the plateau. Rifting could have been either active or passive. If active, molten rock rising from deep within Earth separated the ridge and plateau. Rut if forces pulled the weakened crust apart, it would be an instance of passive rifting.

Drilling results clearly show that the rifting was passive. Samples retrieved from the sedimentary cover of Broken Ridge reveal that about 45 million years ago a portion of the Kerguelen Plateau split away to the south along a deep fault or chasm in the seafloor causing the opposite end of the plate, bearing Broken Ridge, to upturn. When the plateau slumped, it created a see-saw effect, lifting the opposite edge bearing Broken Ridge above sea level. Since that time, the eroded crest of Broken Ridge has been sinking slowly back into the sea; it now lies 3000 feet beneath the sea surface.

By drilling on the edge of this once-exposed island, the scientists recovered evidence of an old beach littered with the shells of ancient mollusks. These fossils mark the resubmergence of Broken Ridge after the rifting event. The cores retrieved from this ridge also show that hundreds of species of plankton (organisms that drift with the water) suddenly died out on Broken Ridge at the end of the Cretaceous period (about 66 million years ago), part of a worldwide mass extinction.

Evidence from this site is particularly valuable because the sediment at Broken Ridge accumulated rapidly, giving scientists an expanded record of how species evolve in response to newly created ecological niches.

At the Ninetyeast Ridge scientists cored through an extensive sequence of sediments, lava flows and thick piles of volcanic ash. These lava flows were erupted from and represent the volcanic trail of a hot spot located near Kerguelen Island. Hot spots originating deep below Earth's mantle melt their way to the surface allowing hot magma to well up and form volcanoes. As a crustal plate such as the one carrying India passes over a hot spot, it takes the volcano's cone with it, leaving a trail of extinct volcanoes in its wake. Scientists on Leg 121 will be able to date the Indian plate's movements studying samples from the recovered lava flows.

The northernmost site drilled on Ninetyeast Ridge lies in the Bay of Bengal, a huge fan of accumulated sediments carried from the Himalayas to the ocean by the Ganges and Brahmaputra rivers. Samples from this site contain two important environmental records. Scientists can study these sediments to learn how the Indian and Asian monsoon patterns have changed through time and how the eroded material from the Himalayas responded to worldwide glaciation.

Evidence of the Indian Ocean's ancient climates will help scientists better understand the history of wind patterns, ocean circulation and cycles of laciation in the Southern Hemisphere and how they comPare with the climate history of the Northern hemisphere.

Co-chief scientists for the cruise were Dr. John Peirce of Petro Canada, Calgary, Alberta, and Dr. Jeff Weissel of Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York. Dr. Elliott Taylor, Texas A&M University, College Station, was the ODP staff scientist.

(Note: JOIDES institutions are: University of California at San Diego, Scripps Institution of Oceanography; Columbia University, Lamont-Doherty Geological Observatory; University of Hawaii, Hawaii Institute of Geophysics; University of Miami, Rosenstiel School of Marine and Atmospheric Science; Oregon State University, College of Oceanography; University of Rhode Island, Graduate School of Oceanography; Texas A&M University, Department of Oceanography; University of Texas at Austin, Institute of Geophysics; University of Washington, College of Ocean and Fishery Sciences; and Woods Hole Oceanographic Institution.

Non-U.S. members are Department of Energy, Mines, and Resources, Earth Sciences Sector, Canada; European Science Foundation Consortium for the Ocean Drilling Program, Belgium, Denmark, Finland, Iceland, Italy, Greece, the Netherlands, Norway, Spain, Sweden, Switzerland and Turkey; Bundesanstalt fur Geowissenschaften und Rohstoffe, Federal Republic of Germany; Institut Francais de Recherche pour l'Exploitation de la Mer, France; University of Tokyo, Ocean Research Institute, Japan; and Natural Environment Research Council, United Kingdom.)



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