Benthic foraminiferal oxygen and carbon stable isotopes (δ18O and δ13C) represent an important tool to reconstruct changes in ocean circulation through time. In our group, we apply these proxies to investigate the evolution of ocean circulation in the North Pacific and Atlantic Oceans from the middle Eocene to the earliest Oligocene. This is a very interesting time as Earth transitioned from the warmhouse of the middle Eocene to the coolhouse of the early Oligocene, with lower temperatures, lower concentrations of CO2 in the atmosphere, the presence of permanent ice-sheets in East Antarctica, and episodic glaciations in the Northern Hemisphere.
For example, using benthic foraminiferal δ18O and δ13C we provided evidence that the opening and deepening of the Drake Passage (which isolated Antarctica from South America) and the Tasman Gateway (which isolated Antarctica from Australia) impacted ocean circulation in the Atlantic. In particular, we showed that a modern-like ocean circulation began in the late middle Eocene (~38.5 Ma), with the development of a bipolar ocean circulation characterized by thermal differentiation between the deepwater mass originating in the Southern Ocean (Southern Component Water – SCW) and the deepwater mass originating in the North Atlantic (Northern Component Water – NCW) (see figure below from Borrelli et al., 2014).
Over geological time scales, the formation of calcium carbonate (CaCO3) at the surface of the ocean and its burial at the seafloor are important components of the global carbon cycle because of their impact on ocean alkalinity, dissolved inorganic carbon, and CO2 concentration in the atmosphere.
In our lab, we investigated Eocene CaCO3 accumulation rates at several sites located at different paleodepths in the Pacific, Atlantic, Indian and Southern Oceans during the middle and late Eocene. This time preceded the increase in pelagic carbonate accumulation in the deep sea reported ~33.9 Ma (‘deepening’ of the carbonate compensation depth – CCD) . Our data revealed that CaCO3 accumulation was highly heterogeneous in different ocean basins and at different paleodepths, emphasizing the importance of processes such as surface CaCO3 production, as well as ocean ventilation, in influencing accumulation and burial of carbonates ~42-34 Ma (see figures below from Borrelli et al., 2021). We are now expanding our records into the Oligocene and Miocene.
