MODERN AND LAST GLACIAL MAXIMUM SEA-SURFACE DELTA-O-18 DERIVED FROM AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

The past isotopic contents of sea surface waters (delta(s)) cannot be measured directly, although they would be a good indicator of ocean ci rculation through their relationship with sea surface salinities. A me thod of delta(s) reconstruction is proposed based on the use of output s from an Atmospheric General Circulation Model including a full isoto pic model. Using the outputs of the NASA/GISS isotopic GCM and a simpl e box model, we have established that there is a strong correlation (r (2) = 0.75) between atmospheric fluxes and delta(s) measured in the fr ame of GEOSECS program, indicating that delta(s) is largely governed b y atmospheric fluxes. This justifies the use of the present-day statis tical relationship reflecting essentially the strong atmospheric forci ng on sea surface water, for different conditions to those prevailing during the Last Glacial Maximum. For this period, over subtropical are as, lower isotopic compositions are obtained in the Atlantic Ocean whe reas higher values are obtained in the Indian and Pacific oceans, thus reducing the isotopic contrast between the Atlantic and Pacific ocean s. The Pacific and Indian oceans show a similar isotopic pattern, with the tropical delta(s) values accentuated by a marked decrease in the equatorial zone. However, whereas the predictions from the Atlantic an d Indian oceans exhibit good agreement with proxy data derived from fo raminifera, important discrepancies exist in the Pacific Ocean.