Unexpected Problems in AMS ¹⁴C Dating of Fen Peat

Abstract

Four fen peat sequences in northern Finland were dated by the accelerator mass spectrometry (AMS) radiocarbon method in order to study past peatland dynamics and carbon accumulation patterns. Initially, plant macrofossils were used for dating. However, the dates were severely disordered, with marked inversions in all sequences. In one 140-cm peat core, for example, all ages fell within a ~1000-yr time window. Following these unreliable results, a few bulk peat samples were dated to help assess if any of the plant macrofossil-derived dates were reliable. Bulk dates did not help to solve the problem. This study evaluates the possible sources of error but is unable to single out one clear cause. It is probable that many factors related to the fen environment, such as flooding and root intrusion, may have contributed to the errors. Peat plant macrofossils and bulk peat samples are considered to be reliable dating materials, but the examples given herein highlight the difficulties that can be associated with AMS dating of peat samples.

A microstratigraphic reevaluation of the Florisbad spring site, Free State Province, South Africa: Formation processes and paleoenvironment

“…We do note, however, that Unit E contains MSA artifacts and therefore must be older than 40 ka. In order to fit the stratigraphy to the radiocarbon dates of Units B and C, Kuman and Clarke () introduced at the level of Unit D a long temporal gap, for which we do not see sedimentary evidence in terms of erosion, which should account for the much older age of Unit E. In addition, the presence of outliers among the 14 C dates, the possibility of contamination from roots of modern plants growing in the topsoil and the inherent difficulties in radiocarbon dating of peat (e.g., Shore, Bartley, & Harkness, ; Väliranta et al., ) indicate that great caution should be applied in the evaluation of the available data. For instance, Kuman and Clarke (, p. 110) dated Unit C to 12 ka, whereas Scott and Nyakale (, p. 500) dated underlying Unit D to 8 ka, in both cases using radiocarbon.…”

Holocene fen–bog transitions, current status in Finland and future perspectives

“…It should be noted, however, that age is not a very strong succession phase determinant; peatlands in Lapland had already initiated during the early Holocene period, but have remained as fens since mainly because of climate (e.g. Mäkilä and Moisanen, 2007; Väliranta et al, 2014). Climate envelope models suggest that temperature is the most important individual climate parameter to define the current distribution of bogs and fens (Parviainen and Luoto, 2007).…”

Palynological evidence from a sub-alpine marsh of enhanced Little Ice Age snowpack in the Marrakech High Atlas, North Africa

“…Unexpectedly young ages in the order of centuries or more have been previously reported in sedge peats Fig. 5 Pollen diagram showing select pollen types, NPPs, and microcharcoal data from Oukaïmeden plotted against core depth (cm) from temperate and high-latitude settings (Head et al 2007;Väliranta et al 2014). Similar problems may be occurring in the OUK case due to a combination of highly seasonal moisture regime and topogenous setting leading to sub-surface lateral groundwater flow through the deposit (promoting both mobility of humic acids as well as creating an attractive moisture reservoir for deeper rooting plants).…”

“…Certainly, the problem of rootlet intrusion from overlying sediment will also be most critical in a sub-recent deposit due to the extremely high 14 C content of plant matter which formed in the second half of the 20th century, since the percentage of modern carbon (pMC) averaged 140 pMC over this interval and peaked at >190 pMC in the 1960s. Modest contributions of intrusive rootlets (<20% of the sample) could therefore result in apparent modern ages for samples of ~ 1,000 years age where 89 pMC is anticipated (Väliranta et al 2014); this appears the most likely explanation for reported values near 100 pMC at 68-69 cm, 52-53 cm, 40-41 cm, 38-39 cm, 34-35 cm, 32-33 cm and 26-27 cm depth (Table 3), all occurring within the rooting zone depth range of mid-to late 20th century plants. Similarly, high pMC values 20-21 cm and 14-15 cm depth may also reflect enrichment in 14 C of the dense upper root mat zone of plants which grew in the aftermath of 20th century bomb-testing (Hua et al 2013).…”