Connect the dots in the ocean crust.

Connect the dots in the ocean crust

Until recently, scientists have been hard-pressed to get both the big picture and the detailed picture of the ocean floor at the same time. Seismic reflection profiling -- produced by sending sound waves through crustal layers -- paints a broad-brush outline of the seafloor structure. However, while seismic images cover vast areas of oceans, they don't reveal the chemistry or age of the crustal layers. For this kind of detailed information, scientists study cores from drill holes. But drill holes only dot the ocean floor over a mere fraction of the globe.

Now, with advancements in both drilling and seismic imaging technologies, scientists can correlate what they drill in isolated spots to what they seismically image over large areas. This will enable them to use seismic profiles to date and classify crustal layers far from drill holes.

In the Aug. 15 SCIENCE, Larry Mayer at Dalhousie University in Halifax, Nova Scotia, and his co-workers report that they used drill cores to link up and date a series of seismic reflectors (the boundaries between layers of different rock types) across the central equatorial Pacific Ocean.

Mayer directly measured some of the properties of core rocks retrieved during the Deep Sea Drilling Project's Leg 85, one of the first to produce complete, undisturbed cores. From the measured properties, Mayer produced a synthetic seismic profile that clearly matched the real seismic records. Previously, says Edward Winterer, one co-author at Scripps Institution of Oceanography in La Jolla, Calif., "No one had even come close to that because of the crudity of the technologies. We nearly went crazy."

The drill cores showed that the reflectors had formed when the amount of calcium carbonate being deposited and preserved on the ocean floor had changed. Because the influx of cold, carbonate-poor waters from the North Atlantic and Antarctica dissolve carbonate, the researchers think the reflectors were created during major changes in the circulation of Pacific bottom waters over the last 20 million years or so.

"We're now very hopeful that the reflection profile records, which we use conventionally to extend the detail information we get from drill holes, will become more readable, and readable in terms of the oceanography of long-ago oceans," Winterer concludes.