Lower Groundwater 14 C Age by Atmospheric CO 2 Uptake During Sampling and Analysis

Uptake of atmospheric CO 2 during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO 2 can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated 14 C measurements. Kinetic isotope fractionation during atmospheric CO 2 uptake may also result in decrease of δ 13 C , leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured 14 C values of groundwater should not be used for age estimation without corresponding δ 13 C values, and historical 14 C data in the range of 1 to 10% modern Carbon should be re‐evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for 14 C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if 14 C measurements are made with an accelerator mass spectrometer.