Tiny plants challenge greenhouse plan.

Evidence from an algal graveyard has thrown a wrench into investigations of the perplexing climate changes of the last ice age. By extension, those same findings raise important questions about a controversial proposal to combat global warming by seeding the Antarctic Ocean with iron.

A decade ago, scientists discovered that carbon dioxide levels dropped by about 30 percent during the last ice age, thereby lowering Earth's greenhouse effect and helping to keep the planet locked in the deep freeze. To explain such atmospheric alterations, oceanographers suggested that microscopic algae in Antarctic waters proliferated during the ice age, drawing carbon dioxide out of the atmosphere and storing it in the deep sea.

Not so, say Richard A. Mortlock and his colleagues at Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y. Using sediment cores from the Antarctic Ocean floor, they studied the glassy remains of ice-age algae called diatoms. Contrary to expectations, the scientists found that Antarctic diatoms fared poorly during the glacial age, they report in the May 16 NATURE.

"These results are disconcerting because they all but demolish one potential regulatory mechanims for atmospheric [CO.sub.2], one which has been considered quite powerful by many geochemists and paleoceanographers," comments Wolfgang H. Berger of the Scripps Institution of Oceanography in La Jolla, Calif.

The seemingly esoteric topic of Antarctic al;ae took on headline status last year when an oceanographer suggested enlisting these tiny plants to slow global warming. John H. Martin of the Moss Landing (Calif.) Marine Laboratory proposed that adding extra iron to the Antarctic Ocean would stimulate algal growth, causing the plants to absorb millions of tons of carbon dioxide from the atmosphere each year (SN:1/26/91, p. 63). He reasoned that the same iron-supplement scenario occurred naturally during the last ice age.

The new data challenge Martin's theory about the ice age, but that doesn't necessarily scuttle the entire proposal, says Berger. Some laboratory evidence indicates that adding iron to seawater does stimulate the growth of algae, regardless of what happened in the past. Scientists are now considering an ocean experiment to test Martin's proposal.