Radiocarbon dating of vegetation horizons, illustrated by an example from the Holocene coastal plain in the northern Netherlands

Radiocarbon dating of vegetation horizons (fossil A0/A1-soil horizons occurring within the younger Holocene marine deposits) is subject to contamination by old carbon which forms part of the sedimentary matrix. Samples of recent mud from several locations in The Netherlands give apparent radiocarbon ages of up to 5900 years BP, demonstrating that a significant aging effect may occur. Notably the age of the organic residue, left after alkali pretreatment of the sample of vegetation horizons, is affected by this contamination. A substantial part of the total amount of the organic matter in these vegetation horizons consists of humic substances. Since the larger part of these humic substances has actually been formed in situ by humification of fresh organic material, it is to be expected that radiocarbon dating of humic substances (dissolved in the alkaline extracts) gives a better indication of the true age of the vegetation horizons than the dates from the (apparently too old) residues. This hypothesis is compared to geological evidence in the coastal plains of the northern Netherlands and of Suriname. (Auth.)