Cover Image

HARDBACK
$47.00



View/Hide Left Panel

lion. Scripps immediately set about acquiring a suitable vessel and recruiting the managerial and technical staff required to operate the scientific parts of the program, now called the Deep Sea Drilling Project (DSDP). Scripps, in 1967, sub-contracted with Global Marine, Inc., to supply the drill vessel, which Global Marine christened Glomar Challenger, in honor of the great exploring vessel of the nineteenth century. Scripps equipped it with laboratories for the preliminary study and curation of core samples. By the middle of 1968 the ship was ready to sail, staffed by Global Marine ship and drilling crews and by Scripps technicians. Scientific parties were recruited for each eight-week leg from the entire scientific community, foreign and domestic. The contractual terms required that Scripps take from JOIDES its scientific advice on the definition of objectives for each leg, the general track of the vessel, the shipboard measurements to be made, the curation of the cores, and down-hole measurements. JOIDES also recommended shipboard scientists to DSDP. JOIDES advised, Scripps managed as prime contractor, and NSF monitored—and paid.

The First 18 Months: Validating the Promise

The Deep Sea Drilling Project would not have been possible had it not been that the main features of the bathymetry of the oceans were known from echo sounding during and after World War II. In addition, a near-global web of seismic reflection profiles had already been collected, mainly by Lamont ships; and magnetometer surveys showing anomaly patterns had been made. The new plate tectonic hypothesis was the hottest topic in Earth science. There were big ideas to put to the test and there was a way to pick good sites for the testing.

What was hoped for and what was accomplished during the first 18 months? A quick overview of the major scientific achievements shows why nobody wanted to stop drilling at the end of that time. JOIDES planners had by now devised a nine-leg plan of drilling: a beginning leg in the Gulf of Mexico, partly to explore one of the Sigsbee Knolls (a group of buried salt domes); then one leg each across the North and South Atlantic, mainly to date oceanic crust; a leg in the Caribbean; then five legs in the Pacific, including a north-south transect of the thick pile of pelagic sediments close to the Equator; and a long loop westward to explore the possibly very old crust farthest from the active East Pacific Rise spreading ridge. Then to the home port of Long Beach to end the project.

The ship went first to the Atlantic, mainly to test the sea-floor-spreading hypothesis. The first leg, led by Ewing, drilled into the caprock of a salt dome in 3,572 m of water, where geophysical evidence suggested the crust might be oceanic rather than continental. The drilling did not settle the question of the depth of water during salt accumulation. The drilling did make JOIDES aware of the risk of encountering uncontrollable hydrocarbons, and so planners created a Safety and Pollution Panel to screen proposed sites for their risk potential.

The following leg, across the North Atlantic, ran into serious problems with hard chert layers in the lower Cenozoic sediments, and reached basaltic basement at only three sites. Calcareous sediments at depths below the present compensation depth for calcite (about 4,500 m) suggested subsidence of the seafloor. The age distribution of basement was shown to be consistent with spreading from the Mid-Atlantic Ridge, but could not be considered a good test of the hypothesis.

Leg 3, across the South Atlantic from Dakar to Rio, was a blockbuster. The main objective was no less than a rigorous test of the then-new hypothesis of seafloor spreading. J. Heirtzler had identified magnetic anomalies on both sides of the Mid-Atlantic Ridge along a transect across the South Atlantic and had estimated the ages of the anomalies by extrapolating back into the Late Cretaceous the radiometric ages of magnetic reversals in Neogene lava flows on land, assuming uniform spreading rates. Two geophysicists, Art Maxwell and Dick von Herzen, were designated as co-chief scientists. Drilling showed a near-perfect match between magnetically predicted basement ages and paleontologically determined ages of basal sediments, and for this reason alone the leg was a triumph. Seafloor spreading leaped from hypothesis to ruling theory at a single bound. But also among the scientific party were two geologists, Ken Hsü and Jim Andrews, who persuaded their co-chiefs to take lots of sediment cores on their way to the crucial contact between sediments and basement—the single core that some geophysicists wanted from a hole. In the long sequences of near-continuous cores, Hsü and Andrews recognized changes in the degree to which calcareous fossils were preserved from destruction by dissolution at the seafloor. Their data provided the basis for others to reconstruct the history for the South Atlantic of fluctuations in the depth of complete calcite dissolution, the calcite compensation depth (CCD). Quantitative paleoceanography was now a discipline, and drilling was the way toward writing a paleoceanographic history, back to about 180 million years ago and for all the world ocean.

The final Atlantic leg, in the western South Atlantic and in the Caribbean, reconnoitered a diverse array of problems, solving none of them, but whetting appetites for more focused work, especially in the Caribbean. Reconnaissance legs—and there were a number of them in the early part of DSDP—open up problems but don't generally solve them.

In the Pacific, two big questions lay open to the drill: What was the history of pelagic sedimentation in the equatorial high-productivity zone, and what was the age of oceanic lithosphere in the western Pacific, far from the active East Pacific Rise spreading ridge? In the eastern Pacific, planners laid out a three-leg, north-south transect, from about 41°N to 30°S, but the results of the first of these, from 41°N to 14°N, showed that dissolution on the seafloor had destroyed most



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement