Investigating Subantarctic¹⁴C Ages of Different Peat Components: Site and Sample Selection for Developing Robust Age Models in Dynamic Landscapes

Abstract

Precise radiocarbon (14C) dating of sedimentary sequences is important for developing robust chronologies of environmental change, but sampling of suitable components can be challenging in highly dynamic landscapes. Here we investigate radiocarbon determinations of different peat size fractions from six peat sites, representing a range of geomorphological contexts on the South Atlantic subantarctic islands of the Falklands and South Georgia. To investigate the most suitable fraction for dating, 112 measurements were obtained from three components within selected horizons: a fine fraction <0.2 mm, a coarse fraction >0.2 mm, and bulk material. We find site selection is critical, with locations surrounded by high-ground and/or relatively slowly accumulating sites more susceptible to the translocation of older carbon. Importantly, in locations with reduced potential for redeposition of material, our results show that there is no significant or systematic difference between ages derived from bulk material, fine or coarse (plant macrofossil) material, providing confidence in the resulting age model. Crucially, in areas comprising complex terrain with extreme relief, we recommend dating macrofossils or bulk carbon rather than a fine fraction, or employing comprehensive dating of multiple sedimentary fractions to determine the most reliable fraction(s) for developing a robust chronological framework.

Identification of Icelandic tephras from the last two millennia in the White Sea region (Vodoprovodnoe peat bog, northwestern Russia)

“…Contamination with older carbon, as has often been observed in peat records (e.g. Camill et al ., 2009; Amesbury et al ., 2012; Thomas et al ., 2019), appears to be responsible for the age reversal, particularly when considering the topography of the Vodoprovodnoe bog. The bog is situated on a low‐gradient marine terrace that is bounded by a relatively steep slope (~10°) to the west (Romanenko and Shilova, 2012).…”

“…This slope may have been the source of redeposited older carbon and, by extension, the anomalously old age at 33–34 cm depth of Core 2014‐1P. The effect of local geomorphology on 14 C determinations through translocation of older carbon downslope as a result of erosion has previously been shown (Thomas et al ., 2019). Further support for such a scenario comes from the comparison of the peat‐accumulation rates in a neighbouring core from the Vodoprovodnoe bog.…”

A continental perspective on the timing of environmental change during the last glacial stage in Australia

“…The methods employed here incorporate the chronological uncertainty associated with age-depth model development and interpolation between dated depths, however no assessment is made of the quality of the radiometric dates used. Identification of anomalous radiometric ages and accurate quantification of individual age determinations requires an increase in the density of dated layers (Blaauw et al, 2018) and dating of multiple sediment fractions (Martin et al, 2019; Thomas et al, 2019). The presence of sedimentary hiatuses in records used in this compilation may currently be masked by the limited number of radiometric ages analysed from some sites (Supplementary Fig.…”

“…Many of the proxy records incorporated in this synthesis were developed before current radiocarbon dating methods were available and are based on sparse and imprecise radiocarbon ages. The precision and accuracy of individual chronologies can be influenced by the number of radiocarbon ages obtained and the type of material dated (i.e., bulk sediment, plant macrofossils and microfossils, or chemical fractions) (Martin et al, 2019; Thomas et al, 2019). Many factors, including poor preservation of organic materials, high costs, and access to radiocarbon laboratories are limitations for achieving dense and robustly dated records (Blaauw et al, 2018).…”

RADIOCARBON PROTOCOLS AND FIRST INTERCOMPARISON RESULTS FROM THE CHRONOS 14CARBON-CYCLE FACILITY, UNIVERSITY OF NEW SOUTH WALES, SYDNEY, AUSTRALIA

“…For peat and lake sediments, terrestrial macrofossils are generally considered to be the most reliable material for 14 C sample selection within a sediment matrix (Lowe and Walker 2000;Bronk Ramsey et al 2012;Thomas et al 2019). Terrestrial macrofossils are carefully selected with tweezers and cleaned of surrounding sediment (under a low-power microscope if necessary).…”