Atmospheric oxygen 18 and sea-level changes

Past isotopic composition of atmospheric oxygen (delta(18)O(atm)) can be inferred from the analysis of air bubbles trapped in ice caps. The longest record covers the last 420 ka (thousand of years) at the Vostok site in East Antarctica. It shows a strong modulation by the precession and striking similarities, but also noticeable differences, with the deep-sea core oxygen 18 record from which changes in the oxygen content of sea-water (delta(18)O(sw)) and in sea-level can be derived. Indeed, delta(18)O(atm) is driven by complex fractionation processes occuring during respiration and photosynthesis. Both delta(18)O(atm) and its difference with respect to delta(18)O(sw) (the Dole effect) are influenced by factors such as the ratio of oceanic and terrestrial productivities which may have significantly changed between different climates. Also, the response time of delta(18)O(atm) to oceanic changes should be taken in consideration but this parameter itself depends on biospheric activity. We review the various aspects of the link between the delta(18)O(atm) and the delta(18)O(sw) signals. We also examine the approach followed by Shackleton (Science (2000)) for deriving sea-level change from the delta(18)O(atm) Vostok record, assuming that the phase between this record and insolation changes is constant and that the Dole effect is a fraction of the precessional component of the delta(18)O(atm) signal. Glaciological constraints on the Vostok chronology and the complexity of the Dole effect show that those two assumptions are quite probably too simplistic. (C) 2001 Published by Elsevier Science Ltd.