G05_06

The inbuilt age of charcoal fragments in a sand-bed stream, Macdonald River, NSW, Australia

Wood R¹,  King F³, Chen Q⁴, Esmany R¹, Schneider L¹, Dotte-Sarout E⁵, Fryirs K⁶, Fallon S¹, Gillespie R¹, Blong R⁷

¹University Of Oxford, Oxford, United Kingdom, ²Australian National University, Canberra, Australia, ³La Trobe University, Melbourne, Australia, ⁴Independent scholar, Independent scholar, Independent scholar, ⁵University of Western Australia, Perth, Australia, ⁶Macquarie University, Sydney, Australia, ⁷Risk Frontiers, Sydney, Australia

Charcoal in fluvial and lacustrine environments can have a considerable inbuilt age, confounding efforts to approximate sedimentation age with radiocarbon dating. Carbon is sequestered in tree rings during growth, and may be hundreds of years older than the charring event (‘old wood effect’). Charcoal is then transported and stored at various positions along hillslopes before reaching the valley floor where it may be stored in floodplains and other riverine landforms. 

To examine the extent of the inbuilt age, we redated charcoal that was first dated using conventional methods by Blong and Gillespie (1978) in the Macdonald River, NSW, Australia. In that study, charcoal was sieved and four size fractions of bulk charcoal dated. The smaller fragments had a greater age than the larger fragments.

Here, 31 individual charcoal fragments from the 2-3 mm size fraction, (SUA-618, 1050-670 calBP) were dated. Only two date to the time of collection, and the oldest was 1700-1590 calBP. It is clear that large numbers of individual charcoal fragments need to be dated to obtain the correct age of deposition when radiocarbon dating charcoal from fluvial and lacustrine environments. To assess whether it is possible to select charcoal with the least inbuilt age prior to dating, we characterized the taphonomic and dendrological features of the dated charcoal fragments. The impact of inbuilt age on Bayesian modeling in OxCal was assessed, and a revised Charcoal Outlier model is proposed for dating charcoal from lacustrine and fluvial settings.

Blong and Gillespie, 1978, Nature, 271, 739-741