Old Argument (Younger Dryas): We found iridium (rare on earth but abundant in meteorites), nanodiamonds and magnetic particles covering ancient tools and mammoth remains at sites which we believe are around 12,000 years old. Therefore, we believe a cosmic collision caused the 1,300 year deep-freeze.
New Argument: We performed radiocarbon dating on tools found at the 29 sites described in the Old Argument and found that only 3 of the 29 sites were around 12,000 years old. The tools at other sites were much older or younger. Therefore, the deep-freeze was probably not caused by a cosmic collision.