Jane...I posted a little below on this...with a reference from an article from the journal Nature...but not the article itself. I work with Jim Kennett on another project and told him about the black layer on Santa Rosa Island. I suggested he talk to his son, who is an archeologist who has worked on the islands, and that is what he did, and results from this black layer are part of their story. But, I don't have any particular opinion or insight on whether an impact caused the ice cap to break up, leading to a big meltwater surge and messing with ocean circulation: this is probably the most controversial and hard to prove part. I'm not going to the meeting.
Chris