Chemical pretreatments for improving the radiocarbon dates of peats and organic silts in a gley podzol environment: Grahams Terrace, North Westland

Abstract

Radiocarbon dates obtained for peats and organic silts from the high rainfall/leaching environment of North Westland have been shown to conflict with known stratigraphic evidence by yielding anomalously young dates.

This study examines the effectiveness of 14 chemical pretreatment methods, including classical humus fractionation of soil organic matter, hydrolysis with mineral acids (6 M HC1 and 70% HNO₃), and extraction of lipids and cellulose, in removing young carbon contaminants from peat and organic silt samples prior to radiocarbon assay. The criterion chosen for judging the effectiveness of a treatment in removing young contaminants is an increase in ¹⁴C age, with the assumption that contamination is largely due to young carbon.

Chemical pretreatments were undertaken on peat and organic silt samples taken immediately above (5–0 cm) and below (0–5 cm) a layer of Kawakawa Tephra found at Grahams Terrace in North Westland. Radiocarbon dates, obtained by accelerator mass spectrometer (AMS), showed that most of the pretreatments and their extracts improved the ¹⁴C ages compared with untreated samples. The hydrolysis of peat and organic silt samples with 70% HNO₃ and dating the resultant residue produced significantly increased ¹⁴C dates (c. 6000 years) with respect to untreated samples.

The major conclusion from this study is that existing radiocarbon dates of 12 000 yr B.P. and older, on peats and organic silts from gley podzol environments, are contaminated by younger, less stable, more mobile carbon, such as fulvic and humic acid fractions.

Keywords:

• Grahams Terrace
• North Westland
• radiocarbon dating
• gley podzol
• Kawakawa Tephra
• peats
• organic silts
• chemical pretreatment
• soil fractionation