Modern and last glacial maximum sea surface δ18O derived from an Atmospheric General Circulation Model

The past isotopic contents of sea surface waters (δs) cannot be measured directly, although they would be a good indicator of ocean circulation through their relationship with sea surface salinities. A method of δs reconstruction is proposed based on the use of outputs from an Atmospheric General Circulation Model including a full isotopic model. Using the outputs of the NASA/GISS isotopic GCM and a simple box model, we have established that there is a strong correlation (r2 = 0.75) between atmospheric fluxes and δs measured in the frame of GEOSECS program, indicating that δs is largely governed by atmospheric fluxes. This justifies the use of the present-day statistical relationship reflecting essentially the strong atmospheric forcing on sea surface water, for different conditions to those prevailing during the Last Glacial Maximum. For this period, over subtropical areas, lower isotopic compositions are obtained in the Atlantic Ocean whereas higher values are obtained in the Indian and Pacific oceans, thus reducing the isotopic contrast between the Atlantic and Pacific oceans. The Pacific and Indian oceans show a similar isotopic pattern, with the tropical δs values accentuated by a marked decrease in the equatorial zone. However, whereas the predictions from the Atlantic and Indian oceans exhibit good agreement with proxy data derived from foraminifera, important discrepancies exist in the Pacific Ocean.