The role of fire during the Eocene-Oligocene transition in southern South America
Wednesday, 17 December 2014: 9:45 AM
The geological record of wildfire, particularly across climate transitions, can help elucidate the complex relationships between climate, vegetation, and fire at long temporal scales. Across Eocene-Oligocene Transition (EOT), previous workers have proposed climate changes (drying and changes in seasonality) contemporaneous with the growth of the Antarctic ice sheet that would have changed the likelihood of wildfires in terrestrial ecosystems. We document short-lived changes in fire regime and plant community in Patagonia near the time of the EOT. Specifically, the concentration of magnetic oxide minerals in Eocene-Oligocene loessites from the Sarmiento Formation correlates with the fraction of burnt palm phytoliths as well as with the fraction of non-palm phytoliths. We interpret the magnetic mineral assemblage magnetite + maghemite ± hematite as pyrogenic, forming in reducing conditions at temperatures between 300 and 600°C. The disappearance of fire-related characteristics near the EOT is possible if seasonal drought was suppressed due to a northward shift in the westerlies – a process consistent with changes in modal particle sizes in the Vera Member. Although the transitory nature of the changes in fire regime remains a puzzle, these results imply a more important role for fire in structuring Eocene-Oligocene landscapes than previously thought.