Debate may resume over volcano-climate link.

As huge volcanic eruptions darkened the skies over the northern Pacific Ocean 2.6 million years ago, temperatures dropped precipitously and the northern hemisphere drifted ever deeper into an ice age. Are these events directly related or merely coincidental?

Scientists who study the natural history of oceans set aside their debate on this question in the 1970s for lack of conclusive evidence. Now, extensive core samples gathered on leg 145 of the Ocean Drilling Program may prompt oceanographers to reexamine the connection between Pacific volcanism and the northern hemisphere's deep freeze, says David K. Rea, a marine geologist at the University of Michigan in Ann Arbor and co-chief scientist on the two-month cruise.

These new cores provide "a very exact definition of all the great changes that occurred 2.6 million years ago, which is the time when northern hemisphere glaciation really kicked in," Rea says.

Ship and crew returned in late September with core samples of soft, porous sediment pulled from the floor of the northern Pacific Ocean. Rea describes these sediments as having the consistency of soft cookie dough.

Since ordinary rotary core drills would have turned these soft sediments into soup, the crew used a technique called hydraulic piston coring, which collects fragile sediments virtually intact. This produced relatively continuous core samples that show important details in the sedimentary strata.

These high-quality cores offer a more detailed record of how the northern Pacific responded to climatic change and allow scientists to more accurately date individual events, Rea explains.

This geological record shows increasing glaciation at the time of the climate change, according to Rea's preliminary, unpublished report on the mission. A large number of volcanic eruptions coincided with this cooling trend.

The samples show for the first time that eruptions in the northern Pacific 2.6 million years ago were at least 10 times larger and more frequent than previous volcanic events recorded elsewhere in the sediments, Rea emphasizes. Could these eruptions have spewed enough ash into the stratosphere to reduce sunlight for long periods, kicking a preexisting cooling trend into a full-scale ice age?

"It's a hard thing to prove," replies James P. Kennett, a marine geologist at the University of California, Santa Barbara, and a participant in the previous debate. He notes, however, that the close connection between increased volcanism and climate change indicated in the new cores is significant. "The quality, detailed linkage is quite remarkable," he says.

Rea also stresses the difficulty of proving a cause-and-effect relationship. However, he points out, the 2.5-meter-thick layers of ash found in some of the new core samples seem like "pretty impressive evidence" of a connection between large-scale volcanic activity and the onset of northern glaciation.

"I and the others have to think about whether we want to reopen this whole discussion," Rea says.