Relation between low latitude insolation and delta O-18 change of atmospheric oxygen for the last 200 kyrs, as revealed by Mediterranean sapropels

The isotopic ratio of atmospheric O-2, delta(18)O(atm), deduced from ice cores, displays large fluctuations during climatic cycles. These are caused principally by changes in the oxygen isotopic ratio of sea-water, delta(18)O(sw), and changes in the biosphere and in the-hydrological cycle. As both delta(18)O(atm) and delta(18)O(sw) coincide closely over the last 135 kyr BP, it is generally believed that delta(18)O(atm) is driven mainly by delta(18)O(sw). 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 delta(18)O(sw) as the driving mechanism for delta(18)O(atm). We present arguments, based on the occurrence and pollen content of Mediterranean sapropels, for another source of the delta(18)O(atm) change: insolation at low latitudes via hydrosphere/biosphere activity.