Lower Groundwater 14C Age by Atmospheric CO2 Uptake During Sampling and Analysis

Uptake of atmospheric CO2 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 CO2 can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated C-14 measurements. Kinetic isotope fractionation during atmospheric CO2 uptake may also result in decrease of delta C-13, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured C-14 values of groundwater should not be used for age estimation without corresponding delta C-13 values, and historical C-14 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 C-14 analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if C-14 measurements are made with an accelerator mass spectrometer.