Hard Water Effect

A potential source of artifact contamination in radiocarbon dating. When material that is radiocarbon-dated has been buried, groundwater may have percolated into it. Groundwater frequently contains dissolved calcium carbonate, where it has passed through limestones. Such carbonate may crystallize within the sample to be dated. As a result, carbon from a source very much older than the sample may be included. Dates from material that has been contaminated in this way will be too old. Samples such as wood and charcoal may be treated with hydrochloric acid to dissolve away the crystallized carbonate, eliminating the problem. Shell samples, which are themselves made of calcium carbonate, cannot be so treated. If the hard water effect is suspected and corrections are not made, the dates should be reported as maximums only.

When material that is radiocarbon dated has been buried, groundwater may have percolated into it. Groundwater frequently contains dissolved calcium carbonate, where it has passed through limestones. Such carbonate may crystallize within the sample to be dated. As a result carbon from a source very much older than the sample may be included. Dates from material that has been contaminated in this way will be far too old. Samples such as wood and charcoal may be treated with hydrochloric acid to dissolve away the crystallized carbonate; this eliminates the problem. A real difficulty, however, arises with shell samples, which are themselves made of calcium carbonate. Any attempt to dissolve away the contaminant will result in destruction of the sample. This makes it hard to obtain reliable dates for shell.