Published radioisotopic (K/Ar, 40 Ar/ 39 Ar, and Rb/Sr) and astronomical ages for the Eocene-Oligocene boundary are essentially consistent at ca. 33.8 ± 0.1 Ma, but the 40 Ar/ 39 Ar ages have been calculated relative to an outdated age of 27.83–27.84 Ma for the Fish Canyon Tuff sanidine dating standard. Application of a revised age of 28.02 Ma, or the new astronomically calibrated age of 28.201 Ma, leads to significant discrepancies, while others are eliminated. In particular, the astronomically tuned ages of ca. 33.79 Ma at Ocean Drilling Program (ODP) Site 1218 and of 33.90–33.95 Ma at Massignano–Monte Cagnero are now in good agreement with recalculated (alternative) 40 Ar/ 39 Ar sanidine ages for the boundary as derived from the volcanic ignimbrite complex in New Mexico and for the Persistent White Layer (PWL) ash bed in North America, which is supposed to closely correspond to the boundary. This mutual consistency suggests that the tuning is correct at the scale of the 400 k.y. eccentricity cycle. Evidently, additional single-crystal 40 Ar/ 39 Ar sanidine dates from the tuffs in North America and independent checks on the astronomical tuning and the intercalibration between the astronomical and 40 Ar/ 39 Ar dating methods are needed to definitively solve the problem of the numerical age of the Eocene-Oligocene boundary. It is anticipated that such analyses and tests will be carried in the coming years as part of the international Earthtime initiative and associated projects to significantly improve the geological time scale. Clearly, an accurate and precise dating of the Eocene-Oligocene boundary is crucial if we are to unravel the underlying cause of the major climate transition associated with it.

We present an overview of the Eocene-Oligocene transition from a marine perspective and posit that growth of a continent-scale Antarctic ice sheet (25 × 10 6 km 3 ) was a primary cause of a dramatic reorganization of ocean circulation and chemistry. The Eocene-Oligocene transition (EOT) was the culmination of long-term (10 7 yr drawdown and related cooling that triggered a 0.5‰–0.9‰ transient pre-scale) CO 2 cursor benthic foraminiferal δ 18 O increase at 33.80 Ma (EOT-1), a 0.8‰ δ 18 O increase at 33.63 Ma (EOT-2), and a 1.0‰ δ 18 O increase at 33.55 Ma (oxygen isotope event Oi-1). We show that a small (~25 m) sea-level lowering was associated with the precursor EOT-1 increase, suggesting that the δ 18 O increase primarily reflected 1–2 °C of cooling. Global sea level dropped by 80 ± 25 m at Oi-1 time, implying that the deep-sea foraminiferal δ 18 O increase was due to the growth of a continent-sized Antarctic ice sheet and 1–4 °C of cooling. The Antarctic ice sheet reached the coastline for the first time at ca. 33.6 Ma and became a driver of Antarctic circulation, which in turn affected global climate, causing increased latitudinal thermal gradients and a “spinning up” of the oceans that resulted in: (1) increased thermohaline circulation and erosional pulses of Northern Component Water and Antarctic Bottom Water; (2) increased deep-basin ventilation, which caused a decrease in oceanic residence time, a decrease in deep-ocean acidity, and a deepening of the calcite compensation depth (CCD); and (3) increased diatom diversity due to intensified upwelling.