One common assumption in interpreting ice-core CO₂ records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO₂ diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO₂ associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO₂ diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca²⁺ ion concentrations to show that substantial CO₂ diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO₂ in ice is ~4 × 10⁻²¹ mol m⁻¹ s⁻¹: Pa⁻¹ at -23°C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO₂ record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of ~930-950 m (~60-70 kyr) indicate that smoothing of the CO₂ record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified
