THE SEQUENCE OF EVENTS SURROUNDING TERMINATION-II AND THEIR IMPLICATIONS FOR THE CAUSE OF GLACIAL-INTERGLACIAL CO2 CHANGES

Events surrounding Termination II, as preserved in the Vostok ice core , provide a number of clues about the mechanisms controlling glacial t o interglacial climate change. Antarctic temperature and the atmospher e's CO2 content increased together over a period of similar to 8000 ye ars. This increase is bounded by a drop in dust flux at its onset and by a drop in the delta(18)O of trapped air at its finish. A similar la g between dust flux and foraminiferal delta(18)O is seen in a Southern Ocean marine record, suggesting chat the delta(18)O in air trapped in Vostok ice is a valid proxy for ice volume. The synchronous change of atmospheric CO2 and southern hemisphere temperature thus preceded the melting of the northern hemisphere ice sheets. This observation, coup led with the fact that nutrient reorganization in the North Atlantic o ccurs with or after the sea level rise, eliminates many scenarios prop osed to explain the CO2 rise, including those which rely on sea level change, conveyor-related nutrient redistribution, or North Atlantic co oling. Southern Ocean scenarios become the front but the most popular mechanism, iron fertilization, has two problems in explaining the CO2 rise before Termination Ig. First, much of the dust demise occurs prio r to the change in CO2, so if iron is the villain, a threshold value o f its supply must be called upon above which productivity does not con tinue to increase. Second the CO2 rise continues for some 4-5 kyr afte r the dust flux has fallen to close to zero. These problems may be sol ved if the increased iron supply in dust caused higher rates of nitrog en fixation during the glacial periods. In this case the residence tim e of oceanic nitrate of a few thousand years would enable decreasing p roductivity to be a global rather than a local phenomenon and would ex plain the slow rampup of atmospheric CO2.