In a business as usual, high emissions scenario, the IPCC projects CO2 levels to exceed 1000
ppm by the end of the century. The last time atmospheric CO2 surpassed this threshold was
∼50 million years ago during the early Eocene climatic optimum (EECO). The EECO presents
the warmest sustained temperatures of the Cenozoic, with global mean surface temperatures
(GMSTs) ∼14◦C higher than the preindustrial. As such the EECO is an increasingly utilised
time period for ground truthing climate models with real world data, to better understand
climate dynamics during a period of extreme warmth.
This thesis aims to provide constraints on deep ocean circulation during the EECO from a
combined model-data perspective. Neodymium isotope analyses of a total of 16 global DSDP,
ODP and IODP cores are used to determine regions of deep water formation, export and
water mass mixing, firstly at a low temporal resolution and then at orbital resolution. A total of
eight model simulations are carried out using a newly tuned version of the DeepMIP model
HadCM3BL, varying both CO2 level and orbital configuration. These results are utilised in
model-data comparisons of previously published sea surface temperature proxy data and the
newly informed Nd isotope inferred overturning structure.
Global Nd data suggest a circulatory regime driven by the southern hemisphere, with multiple,
discrete regions of deep water formation around Antarctica. This is consistent with model
outputs with a modern orbital configuration at both CO2 levels. Neodymium data furthermore
indicate the earliest onset of transient northern component water (NCW) formation off Baffin
Bay, and model outputs suggest that these intermittent pulses could be driven by transitions to
lower CO2 levels or minimum seasonality orbits, both associated with lower GMST. Although
models indicate a strong orbital control on ocean circulation, this is not evident in high
resolution Nd data.