Bibliographic record
Abstract
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 paleo-thermometers to reconstruct sea surface temperatures (SSTs) from the southern Labrador Sea (Ocean Drilling Program – ODP Site 647) across the EOT. We find a permanent cooling step of ~3 °C (from 27 to 24 °C), between 34.9 Ma and 34.3 Ma, which is ~500 kyr prior to Antarctic glaciation. This step in SST values is asynchronous across Atlantic sites, signifiying considerable spatiotemporal variability in SST evolution. However, it is part of an overall cooling observed across sites in the North Atlantic (NA) in the 5 million years bracketing the EOT. Such cooling is unexpected in light of proxy and modelling studies suggesting the startup or strengething of the Atlantic Meridional Overturning Circulation (AMOC) before the EOT, which would warm the NA, although parallel Eocene CO2 decline on the decent into the Oligocene icehouse might counter this feedback. Here we show, using a published modelling study, that a reduction in atmospheric CO2 from 800 to 400 ppm is not sufficient to produce the observed cooling, if combined with NA warming from an AMOC startup, simulated here through Arctic closure from the Atlantic. Possible explanations of the apparent discrepancy are discussed and include uncertainty in the SST data, paleogeography and atmospheric CO2 boundary conditions, model weaknesses, and an earlier AMOC startup that just strengthened at the EOT. The results highlight the remaining uncertainty in many aspects of data and modelling results which need to be improved before we can draw robust conclusions of the processes acting before and across the EOT.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.011 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.004 | 0.003 |
| Open science | 0.002 | 0.002 |
| Research integrity | 0.012 | 0.007 |
| Insufficient payload (model declined to judge) | 0.659 | 0.522 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".