Ice voyage into Antarctic past.

Icy voyage into Antarctic past

Protected by a Danish support ship that nudged aside threatening icebergs, scientists on the most recent leg of the Ocean Drilling Project spent two months in the waters off Antarctica pulling up sediment cores from the ocean bottom. Their cores are leading researchers to revamp scientific theories about the Antarctic's prehistoric shift from a temperate region laced with rivers and forests to its present state as the icebox of the world.

Results from previous drilling projects have suggested that ice sheets started to spread over the eastern end of the Antarctic continent in the early part of the Oligocene period, 35 million years ago. But the sediments from Leg 119 indicate that extensive ice sheets were already covering East Antarctica by 37 million years ago and must have started growing before that time. Moreover, the scientists found evidence that tentatively suggests the ice may date back as far as 42 million years ago.

Part of the evidence comes in the form of glacial till, or sand and pebbles ground up by the action of glaciers. When this till appears in the sedimentary record, it alerts scientists to the presence of ice on the continent, says John Barron, of the U.S. Geological Survey in Menlo Park, Calif., who was co-chief scientist of Leg 119, which ended in February. As well, scientists are examining sections from more northern drill sites that document changes in currents at the ocean bottom. Caused by the cooling effect of the ice, these currents provide indirect information about Antarctic ice.

Barron and his colleagues also found signs that the edge of the ice shelf near Prydz Bay has advanced and retreated over the eons by as much as 140 kilometers. This frequent melting and refreezing supports a controversial theory that sea levels have often risen and dropped through time (SN: 3/7/87, p.154).

As an added bonus, the Leg 119 scientists pulled up a unique section of rock from the Cretaceous-Tertiary boundary, formed 65 million years ago, that will allow paleontologists to trace the history of marine life as it slowly rebounded from the mass extinctions for which that boundary is famous. What excites the researchers about this 15-centimeter-thick section is that the sedimentary rocks in it are well layered. In many marine sections of the boundary, ancient worms and other animals have made a mess of the stratigraphic record by rearranging the layers before the sediments could harden into rock.

Analysis of this newly found section aboard the drillship JOIDES Resolution indicates that tiny plants called calcareous nannoplantkton recovered slowly from almost complete extinction at the boundary. Land-based studies of the section should tell more about the evolution of new life forms, as well as help explain what kinds of climatic stress were responsible for the mass extinctions (SN: 3/12/88, p.164).