It is not an increase in CO2 that is measured, but rather an increase in the amount of radiocarbon produced in the atmosphere, that then enters the carbon cycle.
The current favoured hypothesis to explain this event is a massive Solar proton Event or a series of smaller ones, not an impact.
On an additional note, i am privy to some interesting information relating to this event, but I am not at liberty to discuss it. It should hopefully become apparent within the next few months or so. Watch this space.......
Jonny
The path to good scholarship is paved with imagined patterns. - David M Raup