Extraction of 14C from Pore Water in Unsaturated Rock Using Vacuum Distillation

Abstract

We discuss a vacuum-distillation method for obtaining representative 14C samples from dissolved inorganic carbon in rock pore-waters. Distillation offers four advantages over current centrifugation and compression methods for obtaining pore-water carbon: 1) carbon recovery is possible from rocks that will not yield water by centrifugation or compression; 2) the mass required for 14C analysis can typically be obtained in a single extraction, eliminating the need for storing and combining multiple pore-water collections; 3) water and carbon are extracted and isolated simultaneously, reducing the number of required steps and the potential for contamination; and 4) distillation requires less equipment at lower cost than centrifugation or compression. In this study, isotopic fractionation resulting from incomplete recovery of carbon during distillation was too high for stable isotope applications, but was relatively minor for 14C applications. The lighter isotopes were favored in the recovered phase, resulting in samples depleted in 14C by a maximum of 4%. Mass balance calculations indicate that there may be a significant reservoir of carbon absorbed to mineral surfaces that is only partially removed by this method. Incorporation of adsorbed carbon into the recovered sample did not measurably alter the 14C activity.