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A major step in the long-term Cenozoic evolution toward a glacially driven climate occurred at the Eocene–Oligocene transition (EOT), ∼34.44 to 33.65 million years ago (Ma). Evidence for high-latitude cooling and increased latitudinal temperature gradients across the EOT has been found in a range of marine and terrestrial environments. However, the timing and magnitude of temperature change in the North Atlantic remains highly unconstrained. Here, we use two independent organic geochemical palaeothermometers to reconstruct sea surface temperatures (SSTs) from the southern Labrador Sea (Ocean Drilling Program – ODP Site 647) across the EOT. The new SST records, now the most detailed for the North Atlantic through the 1 Myrleading up to the EOT onset, reveal a distinctive cooling step of ∼3 ∘C (from 27 to 24 ∘C), between 34.9 and 34.3 Ma, which is ∼500 kyr prior to Antarctic glaciation. This cooling step, when compared visually to other SST records, is asynchronous across Atlantic sites, signifying considerable spatiotemporal variability in regional SST evolution. However, overall, it fits within a phase of general SST cooling recorded across sites in the North Atlantic in the 5 Myr bracketing the EOT. Such cooling might be unexpected in light of proxy and modelling studies suggesting the start-up of the Atlantic Meridional Overturning Circulation (AMOC) before the EOT, which should warm the North Atlantic. Results of an EOT modelling study (GFDL CM2.1) help reconcile this, finding that a reduction in atmospheric CO2 from 800 to 400 ppm may be enough to counter the warming from an AMOC start-up, here simulated through Arctic–Atlantic gateway closure. While the model simulations applied here are not yet in full equilibrium, and the experiments are idealised, the results, together with the proxy data, highlight the heterogeneity of basin-scale surface ocean responses to the EOT thermohaline changes, with sharp temperature con

Estimates of global mean near-surface air temperature (global SAT) for the Cenozoic era rely largely on paleo-proxy data of deep-sea temperature (DST), with the assumption that changes in global SAT covary with changes in the global mean deep-sea temperature (global DST) and global mean sea-surface temperature (global SST). We tested the validity of this assumption by analyzing the relationship between global SST, SAT, and DST using 25 different model simulations from the Deep-Time Model Intercomparison Project simulating the early Eocene Climatic Optimum (EECO) with varying CO2 levels. Similar to the modern situation, we find limited spatial variability in DST, indicating that local DST estimates can be regarded as a first order representative of global DST. In line with previously assumed relationships, linear regression analysis indicates that both global DST and SAT respond stronger to changes in atmospheric CO2 than global SST by a similar factor. Consequently, this model-based analysis validates the assumption that changes in global DST can be used to estimate changes in global SAT during the early Cenozoic. Paleo-proxy estimates of global DST, SST, and SAT during EECO show the best fit with model simulations with a 1,680 ppm atmospheric CO2 level. This matches paleo-proxies of EECO atmospheric CO2, indicating a good fit between models and proxy-data.