The Radiocarbon Ages of Different Organic Components in the Mires of Eastern Australia

Abstract

Radiocarbon (14C) dating is widely used to determine the age of organic material in palaeoenvironmental research. Here we compare 14C dates (n=17) resulting from macro-charcoal (>250 μm), short-lived plant macrofossils and pollen-rich residues isolated from two mire environments in eastern Australia. In most samples we found that short-lived plant macrofossils were the youngest organic component, the charcoal samples most often fell into the middle and the pollen-rich residues consistently returned older dates than the other samples. Although pollen-rich residues have been widely used for 14C dating in Australasia we suggest some caution in their use, perhaps because in our fire-prone environments these samples often also contain fine charcoal and other oxidative resistant organic matter that is older than the surrounding sediment matrix. The macro-charcoal samples also often returned older calibrated ages compared to short-lived plant macrofossils from the same depth, although this difference was relatively small (<245 years). Our results demonstrate that 14C dating of short-lived plant macrofossils are likely to yield more accurate chronologies and we advocate their routine use in palaeoenvironmental research when they are available.

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

“…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).…”

“…Inter-site comparisons have also been inhibited by the relatively recent adoption of age-depth modelling, limiting our ability to identify coherent regional and continental climate changes. A more accurate and realistic estimation of the true timing of events may be possible through the objective use of selection criteria, application of consistent and up-to-date age modelling approaches, and recognition of the associated errors and uncertainties surrounding mean age-depth chronologies (Blaauw et al, 2018; Blaauw et al 2020; Martin et al, 2019).…”

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

“…A key concern is the vertical penetration of roots, resulting in inclusion of "younger" carbon in the samples being dated (Kilian et al 2000;Brock et al 2011). For example, root penetration can reach up to 2 m in depth and can therefore bias the radiocarbon determination of the peat unit by several thousand years (Martin et al 2018). When dating bulk peat samples, removal of visible roots can reduce the influence of this "younger" carbon, resulting in older ages.…”

“…Dating short-lived terrestrial plant remains ensure that the assimilated atmospheric CO 2 is likely to be near-contemporaneous with the terrestrial environment. However, misidentification of root material for plant macrofossils can result in significant errors (Martin et al 2018), so extreme caution must be taken. For highly humified peat, from which it is difficult to select specific macrofossils, or in the absence of visible macrofossils, it is possible to date either the whole peat sample (termed a "bulk" sample), or specific fractions of the peat, which may be separated physically or chemically.…”

“…For instance, some work has found that the incorporation of bulk peat dates appeared to introduce no significant systematic biases into large datasets (Blaauw et al 2004;Holmquist et al 2016). A study from Australia found that 14 C dating of short-lived plant macrofossils resulted in consistently younger ages than from both pollen concentrate material and charcoal, with the macrofossils thought to be potentially closer to the age of the deposition of the sediment (Martin et al 2018). In addition to separate size fractions, different chemical fractions may also provide divergent age determinations (Shore et al 1995;Turetsky et al 2004); vertical transport of humic acids in peat columns is potentially variable due to site-specific conditions, and may be arrested in acidic environments due to lower humic acid solubility (Wüst et al 2008).…”