The application of a new rapid pollen extraction method for robust radiocarbon dating of wetland archives
Dr Haidee Cadd1, Mr Bryce Sherbourne-Higgins1, Dr Lorena Becerra-Valdivia3, Ass. Prof John Tibby7, Dr Cameron Barr7, Dr Matt Forbes6, Ass Prof Tim Cohen2, Dr Jonathan Tyler7, Dr Marcus Vandergoes8, Dr Alex Francke7, Mr Richard Lewis7, Ass Prof Lee Arnold7, Dr Geraldine Jacobson9, Dr Chris Marjo3, Prof Chris Turney3
1University Of Wollongong, WOLLONGONG, Australia, 2Centre of Excellence in Australian Biodiversity and Heritage (CABAH), WOLLONGONG, Australia, 3Chronos 14-Carbon-Cycle Facility, Sydney, Australia, 4University of New South Wales, Sydney, Australia, 5Oxford Radiocarbon Accelerator Unit, University of Oxford, Oxford, United Kingdom, 6KCB Australia, Brisbane, Australia, 7University of Adelaide, Adelaide, Australia, 8GNS Science New Zealand, Wellington, New Zealand, 9Australia Nuclear Science and Technology Organisation, Sydney, Australia
Wetland sediments provide valuable natural archives of environmental change, but it can be challenging to develop robust geochronological frameworks. In many sequences, identifiable terrestrial macrofossils are sparse, reducing the confidence in radiocarbon (14C) dating. An alternative approach for capturing changes in atmospheric 14C is the use of terrestrial microfossils. We present the first application of the filtration and floatation method developed at the Chronos 14-Carbon Cycle Facility. We 14C date pollen microfossils from two wetland sediments in eastern Australia and compare these to ages obtained from different organic sediment fractions (n=56). The pollen concentrate 14C ages obtained provide robust 14C chronologies for both sequences. For the Holocene record from Lake Werri Berri near to Sydney, pollen 14C ages are consistent with 14C ages on bulk sediment and humic acids (n=14), whilst residual Stable Polycyclic Aromatic Carbon (SPAC) 14C ages (n=4) are significantly younger. Welsby Lagoon, near Brisbane, captures a record spanning the late Pleistocene and the pollen concentrate 14C ages (n=21) provide a stratigraphically coherent sequence back to 50 ka BP. 14C ages determined from humic acids (n=6) are consistent with pollen concentrate 14C ages, while the >100 µm fractions (n=7) do not overlap with either fraction, and are significantly younger.
We use the newly developed pollen concentrate ages and previously published 14C and OSL ages to compare the outputs from three commonly used Bayesian age-depth modelling software; Oxcal, Bacon and Undatable. We compare sedimentation and total organic carbon (%) data from the two records using the mean age-depth models and 500 age-depth model iterations, representing the modelled uncertainty. The three age modelling packages produce variable results for both mean and uncertainty determinations. Given the potential risk of contamination from different sources of carbon, intensive dating of sedimentary sequences and application of Bayesian modelling substantially improves the identification of outliers and generation of robust age models.
Dr Haidee Cadd is an early career researcher who graduated from her PhD in 2020. Haidee is a Palaeoecologist and palaeoclimatologist with an interest in understanding past fire and vegetation dynamics across Australia.