The isotopic ratio of atmospheric O2, Δ18Oatm, deduced from ice cores, displays large fluctuations during climatic cycles. These are caused principally by changes in the oxygen isotopic ratio of sea-water, Δ18Osw, and changes in the biosphere and in the hydrological cycle. As both Δ18Oatm and Δ18Osw coincide closely over the last 135 kyr BP, it is generally believed that Δ18Oatm is driven mainly by Δ18Osw. Here we focus on the major discrepancy, which arises between those two signals during the prior isotopic glacial stage 6, around 175 kyr BP, discrepancy which calls into question the role of changes in Δ18Osw as the driving mechanism for Δ18Oatm. We present arguments, based on the occurrence and pollen content of Mediterranean sapropels, for another source of the Δ18Oatm change: insolation at low latitudes via hydrosphere/biosphere activity.¿ 1997 American Geophysical Un