Evidence of Late Quaternary Fires from Charcoal and Siliceous Aggregates in Lake Sediments in the Eastern U.S.A.

The late-glacial transition to the Holocene, 15,000–11,600 cal yr BP, is an enigmatic period of dynamic global changes and a major extinction event in North America. Fire is an agent of disturbance that transforms the environment physically and chemically, and affects plant community composition. To improve understanding of the linkages between fire, vegetation, and climate over the late glacial and Holocene in the eastern U.S., I analyzed lake-sediment cores for charcoal and indicators of wood ash, and compared results to existing pollen records. A new microscopic charcoal record from Anderson Pond, Tennessee revealed high fire activity from 23,000–15,000 cal yr BP when conifers dominated, and during the Mid-Holocene Warm Period (8000–5200 cal yr BP), when hardwoods dominated. Macroscopic charcoal analysis of sediments from Pigeon Marsh, Georgia showed high fire activity from 16,500–14,500 cal yr BP, below a major hiatus. Jackson Pond, Kentucky and Cahaba Pond, Alabama had low macroscopic charcoal concentrations during the late glacial; largest charcoal peaks occurred around 5000 cal yr BP at Jackson Pond, and from 1370–640 cal yr BP at Cahaba Pond.

Thin sections were prepared for cores from the four southeastern U.S. sites and from Swift and Slack Lakes, Michigan, and analyzed together with nitrogen isotopes and element data from XRF. Thin sections showed the presence of siliceous aggregates, a unique grain type, in sediments from five sites. These grains are rare, occurring in only three periods, around 19,250, 14,000 and 12,400 cal yr BP. In laboratory experiments, I produced siliceous aggregates from wood ash with simulated rain, and found their formation requires silt, but not high acidity. On the landscape, siliceous aggregates form after fires in wood ash by the action of water. The alkaline pH of the wet ash dissolves phytoliths, and amorphous silica nucleates around silt-sized quartz grains. Then aggregates are transported into lake sediments. My research demonstrates that siliceous aggregates are a new proxy for wildfires in paleoenvironmental records. The wildfire-derived siliceous aggregates in cores examined from the eastern U.S. are contemporaneous with combustion signals in Greenland ice cores, suggesting widespread late-glacial fire events.