[muawiyah]

I think the closing of the Isthmus was about 7.5 million years ago and that coincides with the development of the present ice sheet on that continent.

The North American deal is more recent ~ 3 million years or so ~ and it moved into position to support year round ice and also to restrict the flow of warmer water into the Arctic ocean basin.

[colorado tanker]

I read several papers online and the consensus seems to be the Isthmus closed 3.5 to 2.5 million years ago. I found an outlier calling it at 5 million years ago, but they have to resort to some exotic speculation as to how the closing could first cause the Pliocene warm and then cause the Holocene glacial cycles. It makes most sense to me that the closing coincided with the ending of the Pliocene warm. A couple of papers make the excellent point that this likely was not a sudden occurrence, but likely for a time was intermittent, open during interglacial periods of high sea levels and closed during glacial lows.