CO₂ diffusion in polar ice: observations from naturally formed CO₂ spikes in the Siple Dome (Antarctica) ice core

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

Differentiating bubble-free layers from melt layers in ice cores using noble gases

International audienceMelt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt even