Evolution of chemical peak shapes in the Dome C, Antarctica, ice core

[1] Interpretation of the chemical layers measured in ice cores requires knowledge of processes occurring after their deposition on the ice sheet. We present evidence for the diffusion of soluble ions in the top 350 m of the Dome C ice core, Antarctica, that helps in explaining the unexpectedly broad volcanic peaks observed at depth. A windowed-differencing operation applied to chemical time series indicates a damping of the signals over the past 11,000 years, independent of minor climatic variation, for sulfate and chloride, but not sodium. This implies a diffusive process is transporting both sulfate and chloride ions while the sodium ions remain fixed. We estimate the effective diffusivity in the core to be 4.7 x 10(-8) m(2) yr(-1) for sulfate and 2.0 x 10(-7) m(2) yr(-1) for chloride. These values are not high enough to significantly disrupt chemical interpretation in this section of core, but could be significant for older ice. The temperature of this section of ice (-53degreesC) implies that the predominantly acidic sulfate (and possibly chloride ions) will exist in the liquid phase while the sodium may be solid. We propose and develop two new mechanisms that could explain the observed solute movement. One involves the diffusion of solute through a connected vein network driven by liquid concentration imbalances instigated by the process of grain growth. The other considers a system of discontinuous veins where grain growth increases connectivity between isolated vein clusters allowing the spread of solute. In both mechanisms, the effective diffusivity is governed indirectly by grain growth rate; this may be a significant factor controlling effective diffusion in other cores.

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