| Summary: | 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 ...
|
|---|