Carbon dioxide retention and carbon exchange on unsaturated Quaternary sediments

Retention of CO₂ on three air-dried and partly water-saturated glacial and eolian sediments was measured at 20°C for a range in, PCO₂ that commonly occurs in unsaturated zones. Ratios of the relative losses of CO₂ and ¹⁴CO₂ from a surrogate atmosphere overlying the sediments were 1:1 for the dry condition. For the wet condition, those relative losses were generally ⪯ 1:2, indicating bicarbonateion formation and C-isotope exchange. Mass losses of CO₂ per surface area of sediment were similar for dry and wet conditions; however, CO₂ 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 CO₂ and of dissolved inorganic C in interstitial water, and needs to be considered when modeling ¹⁴CO₂ movement in the unsaturated zone or when interpreting radiocarbon ages of infiltrating water.