Carbon dioxide retention and carbon exchange on unsaturated Quaternary sediments

Retention of CO{sub 2} on three air-dried and partly water-saturated glacial and eolian sediments was measured at 20{degree}C for a range in PCO{sub 2} that commonly occurs in unsaturated zones. Ratios of the relative losses of CO{sub 2} and {sup 14}CO{sub 2} from a surrogate atmosphere overlying the sediments were 1:1 for the dry condition. For the wet condition, those relative losses were generally {le}1:2, indicating bicarbonate-ion formation and C-isotope exchange. Mass losses of CO{sub 2} per surface area of sediment were similar for dry and wet conditions; however, CO{sub 2} losses for the wet condition were 8 to 17 times greater than losses predicted by calcite equilibria. Occurrence of this comparatively large reservoir of immobile, exchangeable C in unsaturated zones can cause alteration of the C-isotope composition of soil CO{sub 2} and of dissolved inorganic C in interstitial water, and needs to be considered when modeling {sup 14}CO{sub 2} movement in the unsaturated zone or when interpreting radiocarbon ages of infiltrating water.