Climate history: blowing in the wind.

In composing a picture of the earth's past climate, scientists assemblea montage of data, drawn largely from fossils in deepsea sediments. These data can be linked to ocean circulation, which the plays a part in climate models.

The atmosphere also plays a role in climate, but until recently there was no direct way of chating its history. In the Feb. 15 SCIENCE, David K. Rea of the University of Michigan in Ann Arbor and co-workers present a marker of past wind intensity: dust, once carried by winds from the land and now buried in oceanbottom deposits. "The size of the dust gives us an indication of how fast the winds were blowing," says Rea. "Stronger winds carry larger grains." Moreover, the amount of dust, he says, is a measure of how arid land was -- a lot of a dust means dry continents.

The researchers studied dust from four drill holes in the Pacific Ocean, where winds have left dust from China, the Gobi Desert and Central America. Overall, the dust data agree with the prevailing view of climate over the last 70 million years for that region. Variations in the amount of dust deposited are in sync with the ebb and flow of ice ages. For the Pacific, however, the data indicate that glacial periods overthe last few million years were more humid than interglacial times -- just the opposite of what's been suggested in other parts of the world. Variations in grain size over this time also mesh with the periods associated with the earth's orbital moon, providing Rea's group with the first documentation that atmospheric circulation, like other climate indicators (SN: 11/10/79, p.324), responds to orbital forces.

Another important finding was that grains deposited 65 million years ago were relatively large, implying strong winds during the late Cretaceous -- a period usually thought to be marked by sluggish ocean circulation and warm climate.