You are right. When Penatuba (sp?) erupted, there is historical evidence of atmospheric and climate change. I'm not so sure about forest fires.
I will try to re-state my point to include this point. It is much more unlikely that CO2 increases were a result of volcanoes or forrest fires following temperature increases. Volcanoes aren't going to respond to fluctuations on the order of 10 degrees farhenheit in land temperature. Forrest fires are not going to lead to hundreds of years of increasing CO2 concentrations. When using the ocean as a CO2 source, the math works out very well and can explain the large (and always present) time lag as well as the rate of change.
Actually the biggest problem with volcanoes is that they emit much more aerosol than CO2 (I believe). If I am not mistaken, volcanoes have had a net cooling effect in history because of their large aerosol release..which as a fact...leads to cooling (they reflect incoming sunlight, increasing the earth's albedo). It was this same aerosol effect that was responsible for much of the "ice age" scare of the late 70s ... as aeorosol emissions (like sulfur from coal-fired plants) spiraled up out of control.