Climatic Control of Biomass Burning During the Last Glacial-Interglacial Transition

Sedimentary charcoal and pollen records were used to test the hypothesis that an extraterrestrial impact at the beginning of the Younger Dryas Chonozone (YDC, 12.9 to 11.7 ka) caused widespread biomass burning in North America. Comet-theory proponents argue that continental-scale wildfires were triggered by the ET impact and are evidenced by carbon spherules, charcoal and soot found at archaeological sites across the continent. We examined charcoal accumulation rates and pollen-inferred vegetation changes in lake-sediment records during the 5000-year interval surrounding the YDC to look for evidence of continental-scale burning. None of the study sites used in the analysis are associated with archaeological sites and many are in remote, high-elevation locations where impacts from human-caused burning was probably minimal. All the records show evidence of sporadic fires throughout the late-glacial period, and together show a trend of increasing biomass burning until the beginning of the YDC, little increase during, and then increasing biomass burning at the end of the YDC. Three of fifteen of the highest-resolution records (i.e. < 50 years per sample), in which individual fire episodes are registered as charcoal peaks, show large fires around the beginning of the YDC, but the strongest evidence for widespread, synchronous fire activity during any 100-yr interval occurs at 11.7 ka, the end of the YDC. Among the potential controls of biomass burning, climate emerges as the most parsimonious explanation for the abrupt increase in biomass burning accompanying the abrupt warming at the end of the YDC through the influence of temperature on biomass productivity (and hence fuels), and fire-promoting environmental conditions. The association between increased biomass burning and the abrupt warming at the end of the YDC is replicated in the response of a composite global biomass burning record to the 20 Greenland Interstadial events during the past 80,000 years. In conclusion, we find no evidence for widespread or synchronous wildfires in North America at the beginning of the YDC. Rather, there is strong evidence that abrupt warming leads to high fire activity at continental and larger scales.