Stable isotopes of carbonate and organic carbon from two Upper Devonian sections in Alberta, Canada, indicate a “heavy carbon event” across the Frasnian-Famennian (F-F) boundary and a negative δ13C shift at the boundary. At Cinquefoil Mountain, δ13C values in carbonate are anomalously high (3.5‰ to 4.5‰ PDB [Peedee belemnite]) within an interval (10 m below to 23 m above the F-F boundary) representing about 1 m.y. (two standard conodont zones), compared to background values of ∼1‰ above and below this interval. Superimposed on this positive excursion is a marked δ13C decrease of about 1‰ at precisely the F-F boundary. At Medicine Lake, a similar δ13C excursion in organic carbon, from a background of −29‰ to a high of −26.5‰, is recorded across the F-F boundary. The δ34S data for iron sulfide in these sections show unusually high values (as much as 15‰ to 20‰ CDT [Cañon Diablo troilite]) in the F-F boundary interval. These data, together with those from other areas, indicate a period of major global changes (e.g., greenhouse and marine anoxia) that took place well before and after the sudden biomass killing event at the F-F boundary. Existing evidence suggests that the F-F mass extinction was probably caused by a large extraterrestrial impact on Earth at a time when the global ecosystem was already severely stressed.