THE SIZE INHERITED AGE EFFECT ON RADIOCARBON DATES OF ALLUVIAL DEPOSITS: REDATING CHARCOAL FRAGMENTS IN A SAND-BED STREAM, MACDONALD RIVER, NSW, AUSTRALIA

Radiocarbon dates on charred plant remains are often used to define the chronology of archives such as lake cores and fluvial sequences. However, charcoal is often older than its depositional context because old-wood can be burnt and a range of transport and storage stages exist between the woodland and stream or lake bed (inherited age). In 1978, Blong and Gillespie dated four size fractions of charcoal found floating or saltating in the Macdonald River, Australia. They found larger fragments gave younger age estimates, raising the possibility that taphonomic modifications could help identify the youngest fragments. In 1978 each date required 1000s charcoal fragments. This study returns to a sample from the Macdonald River to date individual charcoal fragments and finds the inherited age may be more than 1700 years (mode 250 years) older than the collection date. Taphonomic factors, e.g., size, shape or fungal infestation cannot identify the youngest fragments. Only two fragments on short-lived materials correctly estimated the date of collection. In SE Australia, this study suggests that wood charcoal will overestimate the age of deposition, taphonomic modifications cannot be used to identify which are youngest, and multiple short-lived materials are required to accurately estimate the deposition age.

Automated summary

Radiocarbon dating of charred plant remains is commonly used for dating lake cores and fluvial sequences. However, the charcoal can be older than its depositional context due to various transportation and storage stages. This study revisited samples from the Macdonald River in Australia and found that the inherited age of charcoal fragments could be significantly older than the collection date. Taphonomic factors such as size, shape, and fungal infestation were not effective in identifying the youngest fragments. Only fragments from short-lived materials provided accurate estimation of the collection date. These findings suggest that wood charcoal overestimates deposition age in southeast Australia, and multiple short-lived materials are needed for accurate dating.