What caused the Big Freeze?

After many years of Scientists stating that a huge comet or asteroid started the big freeze, there are now many doubts.

Studies of material at various sites where the big freeze would have occurred have led to Scientists finding that only 3 of 29 sites studied actually have material linked to a huge impact such as a comet. The remaining sites have material which is either much younger or older than first thought. Great debating material within this article, particularly for G&T pupils and at A-Level.

http://www.cbsnews.com/news/questions-raised-over-theory-that-an-asteroid-or-comet-caused-the-big-freeze/