TRACING CARBON TRANSFER FROM SOILS TO A CAVE USING STABLE CARBON ISOTOPES

The carbon isotope composition and concentration of CO₂ in Cave of the Bells, southern Arizona, and in the soil at several locations above the cave was measured in order to calibrate the carbon isotope composition of speleothem calcite as a record of vegetation at the surface. Gas samples were collected from soils in a seasonal streambed, a Pleistocene stream terrace and a Holocene stream terrace and from the atmosphere at several locations inside the cave. Gas samples were collected from soils on the eastern and western slope of the hill above the cave in a previous study. Samples were collected monthly during the course of one year. The δ¹³C values of CO₂ in the cave atmosphere varied only slightly in space and time, averaging -19.2±0.3‰. The δ¹³C values of CO₂ were more variable in the soils than in the cave: -17.8±1.3‰ in the streambed, -15.2±1.0‰ in the Pleistocene terrace, -16.1±0.7‰ in the Holocene terrace, -14.6±1.4‰ in the western slope and -15.2±1.6‰ in the eastern slope. The δ¹³C value of respired CO₂ (δ¹³C_r) was calculated for all gas samples from the measured δ¹³C values and CO₂ concentrations. The source of cave CO₂ was identified by comparing cave and soil δ¹³C_r values. The δ¹³C_r values of CO₂ in the streambed averaged -24.1±0.4‰, which is indistinguishable from the average δ¹³C_r value of CO₂ in the cave (-24.0±0.3‰). The δ¹³C_r values of CO₂ from the other soils were higher and more variable than those of the cave and streambed, averaging -20.1±1.0‰ in the Pleistocene terrace, -20.9±0.8‰ in the Holocene terrace, -21.4±2.5‰ in the western slope soil and -20.2±1.8‰ in the eastern slope soil. The difference between cave and terrace/slope δ¹³C_r values and the consistent similarity between cave and streambed δ¹³C_r values suggests that streambed soil rather than terrace/slope soil is the source of CO₂ in the cave. The preferential transfer of CO₂ to the cave from the streambed versus terrace or slope soils likely occurs because groundwater recharge, carrying dissolved inorganic carbon, is focused along the streambed. The bias of cave CO₂ toward a C₃ vegetation signal may be controlled by focused recharge, resulting from a higher proportion of C₃ plants growing in the streambed than on terraces and slopes, and may be reflected by the carbon isotope composition of calcite in speleothems from the cave.