Test of AMS ¹⁴C dating of pollen concentrates using tephrochronology

Abstract

Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior (2003), shows that AMS 14 C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k 14 C yr Kawakawa Tephra, which provided an independent test of the 14 C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured 14 C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10-50 mm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments.

The archaic and puzzling record of Lake Xere Wapo, New Caledonia

“…The paired dating of wood and pollen-rich samples from these layers was carried out to resolve this issue. Vandergoes and Prior (2003) and a follow up study Newnham et al (2007) highlighted the difficulties of dating the organic fraction in old sediments from several lakes and swamps in a high rainfall area of southern New Zealand. They concluded that it was the accompanying fine organic fraction in some pollen concentrates that caused young carbon contamination, or in some cases, the incomplete removal of humic acids that may have entered the older sediments through root penetration.…”

A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand

“…Galway tarn (43 24 0 30 00 S, 169 52 0 24 00 E, 130 m asl) is a small kettle lake formed within pre-LGM moraines (Newnham et al, 2007a). Sediment coring revealed 5.5 m of water and soft sediment overlying 4.3 m of stiff undisturbed sediment.…”

“…Calibrated eruption age in bold. (Newnham et al, 2007a). Newnham et al (2007a) obtained mean ages from pollen and organic concentrates directly above and below the KOT (AE1 sd) of 21,000 AE 170 14 C yr BP (above, n ¼ 6) and 21,585 AE 180 14 C yr BP (below, n ¼ 6) (Lowe et al, 2008).…”

“…Growing suspicion about whether the adopted age of KOT is accurate has arisen from (i) detailed radiocarbon chronologies from LGM lake sediments (e.g. Newnham et al, 2007a;Vandergoes et al, in this issue), and (ii) OSL ages of KOT in loess Grapes et al, 2010aGrapes et al, , 2010b. Here we present results from recent sampling and dating of carbonised wood within the ignimbrite, intact and in situ plant remains overwhelmed by distal tephra-fall deposits, and organic material from undisturbed lake sediment enclosing the tephra layer.…”

New constraints on late Holocene eustatic sea-level changes from Mahé, Seychelles

“…There are other approaches that researchers have taken to dating sediments with multiple carbon sources, and it may be that by using a particular size fraction of organic material (as described above) or pollen concentrates in the future, we could resolve the other reversals seen in the bulk sediment ages and extend the existing record further back in time (e.g. Vandergoes and Prior 2003;Newnham et al 2007;Li et al 2014;Woodroffe et al 2015).…”