Scientists at Columbia University's Lamont-Doherty Earth Observatory report that new images of the Caribbean seafloor--the most comprehensive and detailed ever obtained--will help solve a number of long-standing geological puzzles:

• Did the Caribbean seafloor form where it is now, or was it once a tongue of the Pacific Plate that was separated by the Panamanian isthmus when North and South America converged?

• Do volcanic plumes from earth's mantle periodically burst through the seafloor, dramatically affecting the face of the planet and life on it?

• How did now-extinct deep currents in the Pacific Ocean circulate millions of years ago?

The Observatory's research vessel, the Maurice Ewing, last winter collected high-resolution images of seafloor sediments that originated from land. Such deposits--never before detected in the Caribbean--should provide a telltale marker to pinpoint precisely how fast and how far the Caribbean Plate has moved over millions of years, said John Diebold and Neal Driscoll, co-leaders of the Ewing expedition. The two are geophysicists at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y.

The answer will solve whether the Caribbean seafloor was created by a massive volcanic burst from the mantle beneath the Caribbean, or whether it was born near the hotspot that created the Galapagos Islands and migrated hundreds of miles eastward over millions of years.

The Ewing images also revealed previously undetected deposits and erosional patterns formed by deep currents that flowed millions of years ago, but no longer do. The patterns will divulge how eastern Pacific Ocean currents flowed long before the isthmus closed. Such records were once thought forever lost as the Pacific seafloor was pushed back into the planet's interior by the movement of earth's crustal plates.

The Ewing expedition, supported by the National Science Foundation, provided more than 3,000 nautical miles of high-resolution, deep-penetrating seismic images.

"The Ewing expedition was terribly productive and provides such an improvement over existing data in the Caribbean," said Thomas Donnelly, a geologist at the State University of New York, Binghamton, who has proposed an international Ocean Drilling Program's expedition for early 1997 in the Caribbean. The Ewing data has spurred scientists to revise the goals and strategy of that proposed ODP leg.

The Ewing data also comes at a time of renewed scientific interest in the Caribbean. In the winter of 1996, ODP had already planned a drilling expedition in the Caribbean and the French submersible Nautile also will explore the region.

For its fall meeting in December in San Francisco, the American Geophysical Union has scheduled a special session on the "Origin, Tectonics and Stratigraphic Development of the Caribbean Plate," to be chaired by Drs. Diebold and Driscoll.

The Ewing data "gives scientists their first potential to test the two competing hypotheses on the origin and history of the Caribbean, which has remained an enigma despite 40 years of investigation by geologists, geophysicists and even by drilling," Dr. Driscoll said.

The Lamont-Doherty scientists used sound waves that penetrate and reflect off subseafloor rock and sediments. By recording the reflections with many detectors and measuring the speed and amplitude of the reflected seismic energy, the scientists uncover patterns and compositions of the rock layers.

Over the decades, "many hundreds of miles of seafloor profiles have been accumulated in the Caribbean, but very few of these were acquired with systems powerful enough to reliably and consistently image the entire crustal sequence," said Dr. Diebold, who himself acquired seismic images of the Caribbean in the mid-1970's for his doctoral thesis. The new Ewing images provide substantially more detail.

"The difference is like looking at a picture of a hand and being able to discern not only fingers but fingerprints," Dr. Diebold said.

The images that most surprised the scientists were of large deposits of seafloor sediments that originated from land. Previously it had been thought that Caribbean sediments only came from debris falling from the ocean surface.

The Lamont-Doherty scientists hypothesize that the sediments were poured into the Caribbean by a river, possibly an ancient incarnation of the Orinoco River. That large river, which now flows west to east and drains into the Atlantic, may once have flowed north before the rise of the Andes Mountains diverted it, they say.

By tracing the seafloor sediments back to their source on the South American continent, the scientists can determine where and when the sediments originally were deposited on the ocean bottom. By comparing where those sediments are located today, the scientists can calculate how far and fast the Caribbean plate beneath the sediments has moved. If the sediments are near the place where they were originally deposited, the plate below them could not have moved hundreds of miles eastward from the Pacific.

The new images also open entirely new windows on another research frontier: Large Igneous Provinces, or LIPs. These rapid and massive outpourings of volcanic rock onto the seafloor, which are seen elsewhere on earth, may be caused by plumes of magma that rise occasionally from deep within the earth. Scientists are beginning to suspect that such widespread submarine volcanism may be an important factor in shaping the face of the earth and also may be correlated with several abrupt mass extinctions of life on the planet.

The Ewing's seismic images revealed unusually thick, smooth oceanic crust, immediately adjacent to thin, rough crust. The finding has led Drs. Diebold and Driscoll to theorize that the Caribbean seafloor may have thinned, extended and cracked as a plume of hot magma rose from the mantle some 90 million years ago. The mantle magma seeped between faults near the seafloor, spreading a smooth layer over the rough, cracked blocks of the original oceanic crust.

Finally, the newly uncovered records of bottom currents in the Caribbean will allow scientists to reconstruct how the sea evolved. They may also provide the only preserved record of how the currents in the eastern Pacific flowed before the plate convergence that closed the gateway between the ocean and the sea some 3 million to 9 million years ago. Patterns left on the Pacific seafloor were erased as the eastern part of the Pacific plate has been pushed below the South American Plate.

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