Soil Dating by Fractional Extraction of Humic Acid

Abstract

The addition of organic materials derived from the upper soil layer yields, for samples at greater depth, younger 14C dates than the date of deposition. To find a criterion for the contamination with younger carbon in a soil sample, we examined the radiocarbon concentrations in two humic acid fractions and humin taken from the same sample. The humic acid extracted from a soil sample was divided into two fractions HA1 and HA2. HA1 is the first fraction extracted by 30 minutes' heating with 2 percent NaOH solution, and HA2 is the second fraction extracted by 2 hours' heating with 2 percent NaOH solution after the extraction of HA1. The residue was assumed as the humin (HM). Many of the peat or soil samples taken from the layer just above the nonpermeable layer contain appreciable amounts of organic materials transported from the upper layer after the sedimentation of the deposits. For the limited number of cases tested here, there is a trend in which the contaminants are selectively extracted by HAl or HA2. When the soil samples are contaminated the ages of the HA1 and HA2 fractions appeared to differ widely in most cases. Agreement between the HA1, HA2, and HM ages may be used as a criterion for the reliability of the soil dating.