Bipolar deep-water formation during the climatic warmth of the early-middle Eocene

The Atlantic meridional overturning circulation (AMOC) is a major component of global ocean circulation and through the distribution of heat, salt, and nutrients exerts a fundamental influence on global and regional climates. However, there is limited understanding of AMOC stability or its sensitivity, under acute climatic warmth that is marked for Earth’s future. To tackle this important gap in our understanding, the climatic warmth of the Eocene (~34-56 Ma) offers a unique opportunity and setting to investigate existence, structure, stability, and operation of AMOC. These fundamental gaps in our knowledge and understanding limit the ability to ground-truth ocean model simulations of past warm climates, and thus also diminish our confidence in the capabilities of these models to predict ongoing changes to our oceans.

Here, we present the first reconstruction of the early-middle Eocene AMOC using a meridional transect of Atlantic and Southern Ocean drill sites. Across sites, detailed chemostratigraphic correlations provide a common, high resolution age model spanning a 500 kyr interval (46.7-47.2 Ma). During this interval, high-resolution (~10 ka) carbon (δ¹³C), oxygen (δ¹⁸O), and neodymium (εNd) proxies were used to determine ocean ventilation state, temperature and salinity, and deep-water mass flow pathways. We find an early-middle Eocene AMOC, which consisted of bipolar deep-water formation forming two major cells, a southern and a northern cell. We will discuss characteristics of these water mass cells and their origin and operation using δ¹³C, δ¹⁸O, and εNd isotopic signatures. Evidence of deep-water mass formation in the North Atlantic is supported by sedimentological evidence from Hohbein et al. (2012) and Boyle et al., (2017), suggesting deep Nordic seas overflows at ~49 Ma and deep-water current flow forming contourite drifts on the Newfoundland Ridges at 47.8 Ma respectively.

Ocean circulation modelling of intervals of past extreme warmth, such as DeepMIP, provide understanding into potential ocean structures that could have existed during the early-middle Eocene. The most common feature of model predictions is a global meridional overturning circulation with strong deep convection in the Southern Ocean and no deep convection in the North Atlantic (Zhang et al., 2022). This study provides compelling evidence to bolster the Southern Ocean findings, yet a major data-modelling discrepancy exists within the North Atlantic, where most current model simulations don’t reproduce the proxy derived deep northern cell. This could point to non-CO₂ boundary conditions, such as North Atlantic bathymetry and gateways, as a cause of this discrepancy. Further proxy and modelling work is warranted to resolve the temporal extent of deep-water convection in the North Atlantic during the Eocene.

References

Boyle et al., 2017. Marine Geology, 385, 185–203.

Hohbein et al., 2012, Geology, 40, 3, 255–258.

Zhang et al., 2022, Paleoceanography and Paleoclimatology, 37, 3, 1–22.