Drilling discoveries in the Pacific.

Drilling discoveries in the Pacific

JOIDES Resolution, the drillship of theinternational Ocean Drilling Program (ODP), spent the last several months of 1986 cruising around the eastern equatorial Pacific. During Legs 111 and 112, it deepened the ocean's deepest drill hole and retrieved the most continuous sedimentary record ever collected by ODP scientists from a coastal margin. It also helped to erode traditional thinking about water circulation through ocean crust and sediments, as well as scientists' concept of what happens when an oceanic plate plunges beneath a continent.

One of the principal goals of the drillingprogram has been to unearth the structure of oceanic crust. Over the last few decades, some clues have come from ophiolites, or slices of oceanic crust that have been pushed up onto continents. These suggest that the crust is made of three principal layers: the topmost "pillow basalts,' which were formed when extruded lava was cooled by seawater; a "sheeted-dike complex' consisting of a tangled mass of feeders that brought the lava toward to the surface; and gabbros, or coarse-textured basalts that crystallized under pressure and were never extruded.

But because ophiolites were probablydeformed and altered during their journey onto land, scientists have wanted to sample oceanic crust directly. That is why in 1979 they began to drill Hole 504B into 6-million-year-old crust on the southern flank of the Costa Rica ridge--off the west coast of South America and to the northeast of the Galapagos Islands. On three previous legs, researchers had drilled through sediments and pillow lavas and into the sheeted-dikes layer, to a total depth of 1,350 meters below the seafloor.

Because of technical difficulties duringLeg 111, which lasted from mid-August to mid-October, JOIDES Resolution drilled only 212 meters deeper into the sheeted-dike complex. But seismic profiles of the crust made from the bottom of the hole reveal reflectors, or boundaries between rock layers, that are 100 and 400 meters down. This suggests that the gabbros layer is within reach of one or two more drilling expeditions.

"The most significant accomplishmentin Hole 504B was undoubtedly the well logs [records of measurements taken as instruments are dropped down the hole],' says Russell B. Merrill, a staff scientist on Leg 111. "With this kind of logging, we can see what was missing in the core samples.'

So far, says Merrill, the logging anddrilling results are consistent with the ophiolite model of the crust.

The most exciting scientific results ofLeg 111 came from holes drilled a few kilometers away from 504B, in one of the areas most scrutinized by scientists studying the heat flow and convection of water through the crust and sediments. In recent years, researchers have recognized that the hydrothermal vents at the spreading ridges--where rising lava churns out new seafloor--play an important role in the exchange of chemicals and heat between the oceans and the crust (SN: 12/20&27/86, p.389). As the crust ages and is covered by thick sediment layers, however, this exchange is cut off.

"We know a fair bit about the ridges andabout the very old crust, but we don't know much about the area in between,' says Leg 111 scientist Michael J. Mottl of the University of Hawaii at Manoa in Honolulu. "We believe the ridge flanks are of equal or perhaps more importance' for exchanging chemicals and heat than are ridge axis vents, he says.

In the mid-1970s the drilling programchose the Costa Rica flank as a site for the study of flank hydrothermal processes. By the time Leg 111 came around, scientists had already discovered that the study area contained two high and two low heat-flow bands aligned parallel to the ridge axis and that calcium and magnesium levels in the pore waters of the topmost sediments change rapidly with depth in high-heat-flow areas. This suggested that in these areas, water from the crust was carrying heat and chemicals up through the sediments.

On Leg 111, scientists drilled throughthe sediments and to the crust in a high-heat-flow zone and confirmed that water was slowly moving up the sediment column. They also drilled into a heat-flow low and measured changes in the chemical concentrations of the pore water along the hole. These suggested that water was flowing down in this area.

Mottl estimates that the rate of flow is afew millimeters per year through the 275 meters of sediments. That's surprising result, he says, because previous thinking held that no water escapes from the crust through sediments thicker than 150 meters.

Mottl thinks a small amount of thewater circulating through the crust escapes into the sediments and, finally, into the ocean. This water is replaced by seawater that finds its way down through the sediments and into the crust.

"The story we have now is one of themore complete and satisfying stories about thermal process on a midocean ridge flank,' he says. "This is the first place we have ever found and verified that [an exchange] is taking place. The next question will be to find whether this is a very unusual situation for a ridge flank or relatively common.'

While Leg 111 focused on crust that hadbeen created relatively recently, one purpose of Leg 112 was to study the zones where oceanic crust is destroyed as it plunges into the mantle beneath continents, in a process called subduction. Leg 112 holes were drilled on the continental margin of South America, under which the Nazca plate is subducting. Using the classic model for subduction, scientists expected that sediments carried on the Nazca plate would be added, or accreted, to the South American continent. But seismic and other studies have suggested that this has not always been the case.

Leg 112 confirmed that instead of beingbuilt up, the continental margin had experienced a period of subsidence, or sinking. The JOIDES Resolution brought up fossils of animals that had lived in very shallow waters as long ago as 40 million years but that are now buried about 4,000 meters beneath the sea surface. ODP researchers suspect that the subsidence of the margin was caused by the movement of the downgoing Nazca plate, which eroded the front and bottom of the South American plate, causing it to sink.

"We've now raised the level of questioningfrom whether or not massive erosion at the front of the continent takes place to what the mechanism of that erosion is,' says co-chief scientist Roland von Huene at the U.S. Geological Survey in Menlo Park, Calif.

Leg 112 researchers also found thatstarting about 12 million years ago, sediments began to accrete in the classic way after a 2-kilometer-high ridge plunged beneath the margin. But they don't understand what caused this change from subsidence to accretion.

What scientists are beginning to realize,according to von Huene, is that erosion in subduction zones is more common than previously supposed. "We've now finding that there is a real spectrum between accretion and erosion, and that erosion is quite important,' he says.

In addition to its significance intectonics investigations, the South American margin is an important place for studying past oceanic and climatic conditions. In particular, the coasts of Peru and Ecuador are renowned for their upwelling of cold, nutrient-rich waters, which support a rich fishing industry. (El Ninos, changes in ocean and wind patterns that disrupt world weather patterns, were named by South American fishermen, whose livelihood is threatened when El Ninos suppress this upwelling.)

On Leg 112, scientists collected a nearlycontinuous record of the sediments and fossils left by plants and animals living at the South American margin over the last 7 million years. The cores "contain a tremendous potential for very detailed studies of the climate and oceanographic history of Peru' with an unprecedented resolution, says co-chief scientist Erwin Suess at Oregon State University at Corvallis.

Suess says some sediment layers arelaminated, or composed of finer layers. These layers alternate with nonlaminated ones. He believes that contrary to scientists' expectations, the laminated sediments were deposited during glacial periods, when sea levels were low and the winds that blow westward from Peru intensified, displacing surface water and resulting in a period of enhanced upwelling. With more upwelling, algae and diatoms flourished. When these organisms sank to the bottom, they consumed the oxygen that sustains bottom-dwelling animals, which normally churn up laminated sediments. Previously, says Suess, scientists had correlated laminated sediments with high sea level.

The most surprising find of Leg 112 wasthe discovery of water in the sediments that was twice as salty as normal seawater. Suess suspects that about 10 million years ago the outer shelf was lifted and isolated from the oceans, leaving a salty layer of evaporites--sedimentary rocks that formed when seawater evaporated from a basin--that is now buried deep in the margin. While ODP scientists were able to detect the salt that is now seeping upward from a salt deposit, they did not encounter the salt source itself.

With summer now under way in theSouthern Hemisphere, the JOIDES Resolution, on Leg 113, is in the Weddell Sea, in the Antarctic, studying the past ocean circulation around Antarctica as well as the tectonic processes that separated Antarctica from Australia and South America millions of years ago.

On future cruises, it's possible thatODP scientists will be joined by their Soviet colleagues. Both the U.S. National Science Foundation (NSF) and the Soviet Union have indicated a desire for the Soviets to pay the $2.5 million annual fee and become ODP's seventh member. But according to an official at NSF in Washington, D.C., there is some opposition in the Reagan administration to Soviet membership. In fact, an NSF delegation's scheduled trip to the Soviet Union to sign the ODP membership agreement has been postponed, he says, apparently because of "administration infighting.'