Résumé
Combined measurements of delta O-18, delta O-17, and delta D in ice cores, leading to d excess and O-17 excess, are expected to provide new constraints on the water cycle and past climates. We explore different processes, both in the source regions and during the poleward transport, that could explain the O-17 excess increase by 20 per meg observed from the Last Glacial Maximum (LGM) to Early Holocene (EH) at the Vostok station. Using a single-column model over tropical and subtropical oceans, we show that the relative humidity at the surface is the main factor controlling O-17 excess in source regions. Then, using a Rayleigh-type model, we show that the O-17 excess signal from the source region is preserved in the polar snowfall, contrary to d excess. Evaporative recharge over mid and high latitudes and delta O-18 seasonality in polar regions can also affect the Vostok O-17 excess but cannot account for most of the 20 per meg deglacial increase from LGM to EH. On the other hand, a decrease of the relative humidity at the surface (rh(s)) by 8 to 22% would explain the observed change in O-17 excess. Such a change would not necessarily be incompatible with a nearly unchanged boundary layer relative humidity, if the surface thermodynamic disequilibrium decreased by 4 degrees C. Such a change in rh(s) would affect source and polar temperatures reconstructions from delta O-18 and d excess measurements, strengthening the interest of O-17 excess measurements to better constrain such changes.