TIME-TRENDS IN THE PATTERN OF OCEAN-ATMOSPHERE EXCHANGE IN AN ICE CORE FROM THE WEDDELL SEA SECTOR OF ANTARCTICA

The east coast of the Antarctic Peninsula is strongly influenced by air masses that have traversed the Weddell Sea zone. A continuous record of annual-average values for delta O-18, delta-D, Cl- and non sea-salt SO42- in snowfall deposited since 1795, has been obtained on an ice core drilled on Dolleman Island (70-degrees-35.2'S, 60-degrees-55.5'W). Chemical changes along the ice core seem to be linked to changes in the concentration of the ice cover in the marginal ice zone. In the period since 1956, these variations appear to be coupled to the atmospheric circulation, as indexed by the atmospheric pressure gradient across the marginal ice zone. The largest anomaly in the 200-year sequence occurs in the period 1820-1880, during the final stages of the Little Ice Age. Exceptionally high concentrations of Cl-, low concentrations of biologically-derived sulphate, and high deuterium excess suggest that at this time there was a dense, compacted marginal ice zone with cyclones tracking more frequently than normal across ocean areas to the north of the ice edge. During the past century, there bas been a marked decrease in deuterium excess of about 4 parts per thousand which implies that there has been a progressively increasing contribution to precipitation from moisture sources at lower temperature, probably from within the marginal ice zone. The implication is that there may have been significant weakening of the ice cover in this zone during the past century, despite satellite evidence which reveals no significant change in the position of the ice edge, at least since 1973.