Common Methods Used in Charcoal Analysis Destroy Charcoal Formed at Low Temperature

Abstract:

The use of an oxidant (H₂O₂ or NaOCl) is a common method for the concentration of charcoal in organic-rich sediments, with the assumption that non-pyrogenic carbon is bleached and easily removed or disregarded and charcoal is unaffected. This study describes the effects that ‘bleach’ (sodium hypochlorite ~4% NaOCl) has on laboratory produced charcoal formed at temperatures and charring intensities (CI) common in wildfires. To do this, a series of charcoal samples were produced under conditions ranging from low intensity/low severity to those simulating high intensity/high severity fire: charcoal was quantified prior to and after 24 hours in 4% bleach. Charcoal produced in conditions approximating low intensity fire showed nearly complete chemical digestion (~96% lost by area) after 24 hours in bleach, whereas charcoal produced to simulate high intensity fire was much more resistant with an average of ~27% lost. Our results indicate that methods commonly used in charcoal analysis degrade charcoal formed in low intensity fire and confirm that the temperature threshold at which more labile charcoal becomes resistant (as polyaromatic structures become more dominant) to chemical digestion is ~ 400-450°C. These results have serious implications for the investigation of past fire regimes and suggest that methods using oxidants could be biasing the record towards high-intensity, high-temperature fires and removing evidence of low-intensity, low-temperature burns. As an example, the loss of charcoal from low intensity/severity fires could obfuscate anthropogenic signals, especially where low intensity fires are used as a tool for manipulating resources or for landscape management.