Antarctic Ice Sheet dynamics during the Late Oligocene and Early Miocene: climatic conundrums revisited

Abstract The Oligocene-Miocene transition (OMT) is one of the most enigmatic periods (~23.3–22.9 Ma) in Earth’s Cenozoic climate history. It is characterised in deep-sea benthic foraminiferal (δ18O) records by an increase of up to +1‰ spanning a 200–300 kyr interval associated with global cooling and growth in Antarctic ice volume to as much as 120% of present day (Mi-1 glaciation). The Mi-1 glaciation was then terminated by a −1.2‰, δ18O decrease that occurred in less than 100 kyrs, implying rapid Northern Hemisphere-style ice retreat, and potentially continental-scale deglaciation of Antarctica. Antarctic margin ocean drill core records and seismic reflection profiles display evidence of ice-proximal glacimarine deposition or glacimarine erosion, and imply continent-wide advance of the ice sheet terminus into the marine realm during the Mi-1 glaciation. This major transient glaciation occurred within a ~400 kyr-duration eccentricity cycle and appears to be coupled with an orbitally-paced perturbation of the carbon cycle. Atmospheric CO2 reconstructed from geological proxy records imply a long-term decrease during the Oligocene from about 500 to less than 300 ppm. Atmospheric CO2 declining below a threshold (~400 ppm), together with an extreme cold orbital configuration, enabled widespread seasonal sea-ice formation and the development of extensive marine-based ice margins around Antarctica. Atmospheric CO2 concentration appears to rebound rapidly following the Mi-1 glaciation, with some proxy estimates as high as 1000 ppm by the earliest Miocene. The OMT challenges our current understanding of orbitally-paced, ocean-atmosphere carbon exchange and associated feedbacks in the climate system. Prior to the OMT, between ~27 and 24 Ma, a trend towards lower δ18O values suggested an extensive period of global warmth, polar ice volume decrease and global sea-level rise. This is in contrast with widespread evidence from Ross Sea drill cores that show cooling of near surface ocean and land temperatures, and glacial advance calving ice bergs at the coastline. However, on the Wilkes Land margin cooling begins later, after ~25 Ma. Here, we summarise new evidence of the relative influences of tectonics, atmospheric carbon dioxide, ocean dynamics and orbital forcing on the evolution of the Antarctic Ice Sheet (AIS) during the Late Oligocene and across the OMT. We revisit the longstanding conundrums and provide some insights for the future (in)stability of the Antarctic Ice Sheet.