Estimation of inbuilt age in radiocarbon ages of soil charcoal for fire history studies

Radiocarbon age determinations of wood charcoal are commonly used to date p ast forest fire events, even though such ages should be greater than the fi re event due to the age of the wood at the time of burning. The difference in the C-14-derived age of charcoal and the time-since-fire (the "inbuilt a ge") may be considerable in some vegetation types and thus must be estimate d before interpreting fire dates. Two methods were used to estimate the pot ential range of inbuilt age of soil charcoal dated to determine ages of for est fires on the west coast of Vancouver Island (Canada). First, 26 C-14 ag es on charcoal in surficial soil were compared directly with ages of forest fire determined by tree-ring counts, suggesting inbuilt ages of 0-670 year s. Second, a simulation model that uses estimated fuel loads, fuel consumpt ion, charcoal production, and the ages of charred wood (time since wood for mation), suggests that the combination of slow growth rates and slow decay rates of certain species can account for inbuilt ages of more than 400 year s in this forest type. This level of inbuilt age is large enough such that the actual age of a fire may not occur within the 2 sigma confidence interv al of a calibrated charcoal C-14 age determination, and thus significantly affect the interpretation of fire dates. A method is presented to combine t he error of a calibrated C-14 age determination with the error due to inbui lt age such that the larger adjusted error encompasses the actual age of th e fire.