Related to what goff et al. (2012) has to say about the
Eltanin asteroid impact is an interesting paper published
online in Geology. It is:

Gao, C., J. H. McAndrews, X. Wang, J. Menzies, C. L. Turton,
B. D. Wood, J. Pei, and C. Kodors, 2012, Glaciation of North
America in the James Bay Lowland, Canada, 3.5 Ma. geology.
First published online September 4, 2012 [geology.gsapubs.org]

This provides evidence of glaciation in North America of
continental glaciations that are of a magnitude that is likely
comparable to Pleistocene glaciations started about 3.5
million years ago. This glaciation coincides with the
documented intensification of glaciation in the northwest
Pacific region about 3.45 million years ago. Thus, "Ice Ages"
were already in process a million years before the Eltanin
asteroid impact and it may have simply only accentuated,
if anything at all, climatic changes that had already started.

Best wishes,

Paul H.

"The past is never dead. It's not even past."
William Faulkner, Act 1, Scene III, Requiem for a Nun (1951)