A potential barrier to deep Antarctic circumpolar flow until the late Miocene?

Scientists have found geologic evidence which casts doubt on one of the conventional explanations for how Antarctica's ice sheet formed.

The Antarctic circumpolar current (ACC), an ocean current flowing clockwise around the entire continent, isolates Antarctica from warmer ocean water to the north, thereby helping to maintain the ice sheet. For several decades it has been surmised that the onset of a complete ACC played a critical role in the initial glaciation of the continent about 34 million years ago.

Now, dredging in the central Scotia Sea near Antarctica reveals the remnants of a now-submerged volcanic arc that formed from 28 to 12 million years ago and may have formed a physical barrier to the formation of the ACC until after 12 million years ago.

This work bolsters an alternate hypothesis which claims that dropping levels of atmospheric carbon dioxide around 34 million years ago was the primary trigger for the formation of the Antarctic ice sheet. Indeed, ocean temperatures globally fell at that time.

Hence the onset of the ACC may be related to the well documented descent of the planet into a much colder 'icehouse' glacial state rather than to the initial glaciation of Antarctica.