Abstract:
A critical climate and oceanographic transition occurred during the late middle Eocene to mid-Oligocene ( approximately 38-28 Ma), marked by global cooling and development of continental-scale Antarctic ice sheets. Progressive development of the Antarctic Circumpolar Current (ACC) and reorganization of global ocean circulation accompanied this climate transition. The ACC "engine" began to develop in the middle Eocene with shallow flow through the Drake Passage, followed by rapid deepening of the Tasman gateway (late Eocene to early Oligocene), and more gradual deepening of the Drake Passage through the remainder of the Oligocene. The ACC is a dominant feature of present-day ocean circulation and climate, influencing the strength of meridional overturning circulation, transition depth from surface to deep ocean, gas exchange rate between atmosphere and deep ocean, and global surface heat distribution. Here, we present high-resolution benthic foraminiferal d (super 18) O and d (super 13) C records with Mg/Ca data from Atlantic Slope Project corehole 5 (ASP-5; approximately 600 m paleodepth, southeast US continental slope) and d (super 18) O and d (super 13) C records from ODP Site 1053 ( approximately 1500-1750 m paleodepth; Blake Nose), with comparisons to published isotopic records (Cramer et al. 2009). We show that strengthening of interbasinal deepwater thermal differentiation during the early Oligocene was accompanied by the development of a significant carbon isotopic (d (super 13) C) offset between mid-depth ( approximately 600 m) and deep (>1000 m) waters in the western North Atlantic. We interpret the d (super 13) C offset to indicate development of low O (sub 2) conditions associated with vertical stratification of nutrients, analogous to the modern low O (sub 2) zone ( approximately 700-1000 m deep) that results from ventilation by Antarctic Intermediate Water. These records show that the engine of modern ocean overturning circulation, the ACC, gradually, but significantly, impacted global ocean circulation even while it was in the early development stage in the late middle Eocene to mid-Oligocene.