ALBERT

Description: Foraminiferal oxygen isotopes from deep-sea sediment cores suggest that a rapid expansion of the Antarctic ice sheet took place in the Middle Miocene around 13.9 million years ago (Ma). The origin for this transition is still not understood satisfactorily. Among the proposed causes are a drop in the partial pressure of atmospheric carbon dioxide (pCO2) in combination with orbital forcing. An additional complication is the large uncertainty in the magnitude and age of the reconstructed pCO2 values and the low temporal resolution of the available record in the Middle Miocene. We used an ice sheet-climate model with an energy and mass balance module to assess variations in ice-sheet volume induced by pCO2 and insolation forcing and to better constrain atmospheric CO2 in the Middle Miocene. The ice-sheet sensitivity to atmospheric CO2 was tested in several scenarios using constant pCO2 forcing or a regular decrease in pCO2. Small, ephemeral ice sheets existed under relatively high atmospheric CO2 conditions (between 400–450 ppm), whereas more stable, large ice sheets occurred when pCO2 is less than 400 ppm. Transitions between the states were largely CO2-induced, but were enhanced by extremes in insolation. In order to explain the Antarctic glaciation in the Middle Miocene as documented by the oxygen isotope records from sediment cores, pCO2 must have decreased by approximately 150 ppm in about 30 ka, crossing the threshold pCO2 of 400 ppm around 13.9 Ma. Forcing the ice sheet-climate model with cyclic pCO2 variations at a period of 100 ka and amplitudes of approximately 40 ppm generated late Pleistocene glacial-interglacial like ice-volume variations, where the ice volume lagged pCO2 by 11–16 ka.