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Leg 117

November 13, 1987
-- For the first time scientists can reconstruct the origin and evolution of the southwest monsoon, the dominant atmospheric phenomenon that determines the fate of millions of Asians and Africans.

Scientists on board JOIDES Resolution, drill ship for the Ocean Drilling Program, recovered a record-breaking 4300 meters of sediment from the northwest Arabian Sea. The scientific expedition was one segment in an 18-month investigation of the geologic history of the Indian Ocean.

During September and October, 5 scientists from the United States, Canada, Japan France, the Federal Republic of Germany, the Netherlands, Sweden, Oman and the United Kingdom used their broad range of experience and diverse backgrounds to resolve questions about the monsoon's circulation system through time. Specifically they looked at how the changes in Earth's orbit around the sun and the Himalayan uplift have affected the monsoon's cycle during the past 6 million years.

To study the monsoon's history, they drilled for sediments from beneath the seafloor at 12 sites, one on the enormous submarine sediment pile known as the Indus Fan and 11 on the continental margin off Saudi Arabia. Sediment samples from these areas contain evidence of changes in climate, biologic productivity and structure and position of the continents through time.

Today, the Arabian Sea is one of the world's most fertile areas. The southwest monsoon literally blows away the top layer of water, causing cold, nutrient-rich waters from the bottom to rise to the surface. The phenomenon, called upwelling, is also found in other areas around the world where strong winds dominate like the northwest coast of Africa and the Peruvian coast of South America.

The history of upwelling lies in the microskeletons of tiny organisms buried deep within the layers of sediments. y examining these microscopic fossils on board the ship, scientists were able to place the onset of the monsoon cycle during the Miocene, about 6 million years ago. before that period, sediments revealed a low rate of biologic Productivity in the Arabian Sea.

Although proof of the monsoon's onset and evolution directly correlates to the history of upwelling, scientists had to turn to another geologic phenomenon to explain why the strength of monsoon-generated upwelling has varied considerably since the Miocene.

The Himalayan chain is a giant wrinkle on the face of Earth, caused by the collision of the northward-moving Indian subcontinent with the large piece of Earth's crust known as the Asian late. A computer-generated model suggested that the height of the Himalayas and the Tibetan Plateau plays an important role in the activity of the monsoon-generated pressure cells of central Asia and the Indian Ocean.

The mountain chain's uplift, which began 15-20 million years ago and continues today, has a history of fits and starts. Changes in the elevation of the Tibetan Plateau and the Himalayas, therefore, may play a leading role in controlling the nature of the monsoon.

The southwest monsoon dramatically dominates global climate. Today its force critically affects rainfall in Asia and Africa as well as regional atmospheric and oceanographic conditions. The almost three miles of sediment samples retrieved on this cruise will allow scientists to trace the monsoon's initiation and patterns of intensity during the last 6 million years.

Information gained by reconstructing the monsoon's evolution will help researchers better understand both human history and global climatic behavior on a scale of millions rather than thousands of years.

The cruise's second objective was to explore the chemical and sediment alterations of the Oman margin, the area immediately seaward of Oman. Results show that the composition of the water and sediment at the margin has altered drastically during the last 1.5 million years. Evidence of alternating cycles of oxygen-rich and oxygen-depleted waters suggest that a source of high oxygen content--perhaps the Red Sea--was closed off at one time, depleting the amount of oxygen in the Arabian Sea.

A second cause of these alternating cycles could also be directly connected to changes in the monsoon's intensity. When the monsoon is at its strongest, the upwelling water produces a high concentration of organic matter near the surface. When the monsoon is weak or dormant, the water near the surface is depleted in nutrients and biological productivity is low. Less organic matter in the surface waters means that less oxygen is consumed by oxidation of organic matter in the underlying water column.

Although scientists now have evidence of alternating oxygenation cycles, contained in the cores' striped patterns of laminated and non-laminated sediments, they have yet to pinpoint the cause. Large-scale changes in water circulation, caused by any number of undetermined geologic events, extreme shifts in monsoon intensity or a combination of these agents may someday explain this region's history of high and low organic productivity and preservation.

Co-chief scientists for the cruise were Dr. Warren Prell, Brown University, Providence, Rhode Island, and Dr. Nobuaki Niitsuma, Shizuoka University, Shizuoka, Japan. Dr. Kay-Chistian Emeis, Texas A&M University, College Station, was staff scientist.

The cruise, the 17th for the ODP, departed Colombo, Sri Lanka, on August 19 and arrived in Port Louis, Mauritius on October 17.

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