What drives the millennial and orbital variations of δ18Oatm?

The isotopic composition of atmospheric oxygen (δ18Oatm) is a complex marker that integrates changes in global sea-level, water cycle, and biosphere productivity. A strong signature of orbital precession has been identified leading to the use of low-resolution measurements of δ18Oatm to date ice core records. However, the drivers of these δ18Oatm variations are still poorly known. Here, we combine records of millennial and orbital scale variations on the NorthGRIP, Vostok, and EPICA Dome C (EDC) ice cores to explore the origin of δ18Oatm variations. We show that, superimposed on the dominant precession signal, millennial δ18Oatm variations record systematic decreases during warm phases of the Dansgaard–Oeschger events and systematic increases during the cold phases.We show that at both timescales δ18Oatm is strongly related to the monsoon activity itself influenced by precessional and millennial shifts in InterTropical Convergence Zone (ITCZ). Then, we show that despite its simplicity, the Dole effect defined as the difference between δ18Oatm and δ18O of global sea-level enables one to remove the obliquity signal within the δ18Oatm record and is a good indicator of hydrological cycle and biosphere productivity. Finally, we compare the δ18Oatm records to past changes in atmospheric composition recorded in ice cores and conclude that δ18Oatm responds much more than CH4 to precession signal, in contrast with earlier views. Similarities observed at orbital timescales between CO2 and δ18Oatm reveal a stronger coupling than previously thought between the carbon and the oxygen cycles.