What drives the millennial and orbital variations of δ18O atm?

Abstract

The isotopic composition of atmospheric oxygen ( δ 18O atm) 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 δ 18O atm to date ice core records. However, the drivers of these δ 18O atm 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 δ 18O atm variations. We show that, superimposed on the dominant precession signal, millennial δ 18O atm 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 δ 18O atm 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 δ 18O atm and δ 18O of global sea-level enables one to remove the obliquity signal within the δ 18O atm record and is a good indicator of hydrological cycle and biosphere productivity. Finally, we compare the δ 18O atm records to past changes in atmospheric composition recorded in ice cores and conclude that δ 18O atm responds much more than CH 4 to precession signal, in contrast with earlier views. Similarities observed at orbital timescales between CO 2 and δ 18O atm reveal a stronger coupling than previously thought between the carbon and the oxygen cycles.