Evidence for Oligocene–Middle Miocene abyssal circulation changes in the western North Atlantic

Abstract

Previous studies of benthic foraminiferal isotopic composition have demonstrated that a latest Eocene–earliest Oligocene benthic foraminiferal δ18O increase occurred in the Pacific, Southern and Atlantic Oceans1–9. A Middle Miocene δ18O increase has been noted in the Pacific, Southern and South Atlantic Oceans1–3,7,10,11 and tentatively identified in the North Atlantic12,13. Due to the incomplete nature of the North Atlantic stratigraphical record14,15, however, the Oligocene to Middle Miocene isotopic record of this ocean is poorly understood. In the modern ocean, the North Atlantic and its marginal seas has a critical role in abyssal circulation, influencing deep- and bottom-water hydrography as far away as the North Pacific16–18. We now report oxygen isotope measurements on Oligocene to Middle Miocene (12–36 Myr BP) benthic foraminifera in the western North Atlantic which show two periods of enriched 18O values: early Oligocene and early Middle Miocene. These enriched intervals are interpreted as resulting, in part, from the build-up of continental ice sheets. The Oligocene to Middle Miocene δ13C record shows three cycles of enrichment and depletion of large enough magnitude to be useful for time–Stratigraphical correlations. Within the biostratigraphical age resolution, δ18O and δ13C records correlate with records from other oceans, helping to establish a useful Tertiary isotopic stratigraphy. An Atlantic–Pacific δ13C contrast of 0.3–0.9‰ during the latest Oligocene to Middle Miocene (12–26 Myr BP) indicates North Atlantic deep and bottom-water production analogous to modern North Atlantic deep water (NADW).