Understanding the ¹⁷O excess glacial-interglacial variations in Vostok precipitation

Combined measurements of δ¹⁸O, δ¹⁷O, and δD in ice cores, leading to d excess and ¹⁷O 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 ¹⁷O excess increase by 20 per meg observed from the Last Glacial Maximum (LGM) to Early Holocene (EH) at the Vostok station. Using a singlecolumn model over tropical and subtropical oceans, we show that the relative humidity at the surface is the main factor controlling ¹⁷O excess in source regions. Then, using a Rayleigh-type model, we show that the ¹⁷O excess signal from the source region is preserved in the polar snowfall, contrary to d excess. Evaporative recharge over mid and high latitudes and δ¹⁸O seasonality in polar regions can also affect the Vostok ¹⁷O 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 ¹⁷O 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°C. Such a change in rh_s would affect source and polar temperatures reconstructions from δ¹⁸O and d excess measurements, strengthening the interest of ¹⁷O excess measurements to better constrain such changes.