Abrupt Cooling at the Eocene-Oligocene boundary

The Earth has gone through significant climate change over the last 100 million years going from the mid-Cretaceous “greenhouse” to the late Cenozoic icehouse. Roughly 33.7 million years ago, at the Eocene-Oligocene boundary (EOB), the world would begin to develop permanent ice sheets, particularly the East Antarctic. This accompanied the so-called Oi-superglacial driven by a reduction in atmosphere CO2 concentrations.

_{IPCC AR4 Chapter 6, Figure 6.1}

In paleoclimate studies using stable isotopes, oxygen is of primary interest, along with deuterium in ice core research. The oxygen isotopic composition of seawater is the most commonly used metric of deep-time paleotemperatures. Oxygen isotopes from benthic (bottom-dwelling) foraminiferal calcite obtained from deep-sea cores generally provide good representations of global variations as they are isolated from geographic, and seasonal variations, unlike surface records. The oxygen isotopic composition of a sample is often expressed as a departure of the ¹⁸O/¹⁶ ratio from an arbitrary standard.

S= 10³*[(¹⁸O/¹⁶O)_sample – (¹⁸O/¹⁶O)_standard]/¹⁸O/¹⁶O_standard

In which resulting values are in “per mil” (%_o) units. Positive values represent higher ratios in the sample (more ¹⁸O) and negative values represent lower ratios (and therefore isotopically lighter). During glacial times, the preferential removal of isotopically light oxygen to form ice sheets leads to an increase in the ¹⁸O/¹⁶O ratio.

In pre-Quaternary climates, benthic (bottom-dwelling) ¹⁸O/¹⁶O records do not discriminate between ice volume and cold temperature. Despite this climate transition at the EOB, this dilemma of cooling vs. ice-sheet growth and sea-level fall reconstructed from oxygen isotope values jhas not been resolved in any satisfying manner. Some studies have showed ice sheet growth at the EOB unaccompanied by cooling, something that is certainly counter-intuitive.

In a recent paper in Science, the authors Liu et al. use two independent temperature-proxies (called the alkenone unsaturation index (U^K’₃₇) and tetrather index (TEX₈₆) from 11 different locations in the ocean to determine what part of the oxygen isotope changes is from ice-volume and what part is from temperature. They can then subtract their results from the ¹⁸O/¹⁶O record to resolve the component that has to do with ice sheet growth. Further insight on this is given in the Perspective article by Penn State’s Lee Kump.

Both temperature proxies show high-latitude cooling a little over 33 million years ago. Model simulations run by the authors show ~5°C of high-latitude cooling consistent with their proxy data, global-mean surface cooling of 4.4°C, with slightly less ocean floor cooling than surface changes. The authors warn these values may be underestimates (for the model, the inability to account for the full range of ice sheet feedbacks, and for the proxies, low resolution data, sediment gaps, and poor core recovery.)

There’s large uncertanties, as the authors show Antarctica became roughly 40 to 120% of its modern size during the EOB transition, and and 30 to 100% of Antarctic ice volume during the LGM. There was no significant Northern Hemispheric glaciation at this time. The transition is consistent with reductions of greenhouse gas concentrations because of the globally synchronous changes at the time. In their model, a range of atmosphere CO2 concentrations appropriate for the Late Eocene and Early Oligocene conditions was applied. The reduction of atmospheric CO2 levels necessary to explain the full range was more than CO2 proxy estimates suggest, which Lee Kump interprets as meaning we’re not including some important variable effecting long-term sensitivity estimates (which I discussed in The Uncloudy Cretaceous).