Effects of firn ventilation on geochemistry of polar snow
Neumann, Thomas A., 1973-
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The effects of air motion in firn on geochemical species is examined. Stable isotope data from Taylor Mouth, Antarctica are compared with data from nearby Taylor Dome and are found to be significantly different. Three possible reasons for the difference between these two records are presented: (1) the standard stable isotope - surface temperature relationship, (2) mixing precipitation from several sources and (3) post-depositional isotopic change.A two-dimensional model to estimate condensation and sublimation rates of water in firn based on firn-ventilation theory is presented. This model is used to investigate preservation characteristics of chemical species in snow and quantify the effects of post-depositional water-vapor motion. Results demonstrate the sensitivity of sublimation and condensation rates in the firn to hand-to-hand diffusion, grain size, wind speed and temperature.The sensitivity of stable isotope ratios to firn ventilation quantify what other investigators have suggested: isotopic exchange in the upper few meters is more rapid than can be explained by existing models, isotopic equilibration with atmospheric vapor is an important component of isotopic exchange and ventilation enhances exchange by creating regions of relatively rapid sublimation and condensation in the firn.The concentration of irreversibly deposited species (e.g. non-sea-salt-sulfate) is decreased by condensation in pore spaces and increased by sublimation of surrounding ice grains. Results suggest that concentration changes induced by firn ventilation are on the order of 3%, making it unlikely that this effect can be detected in field data.The concentration of reversibly-deposited species in snow (e.g. H 2O2) is generally decreased by sublimation of the surrounding ice grain. Condensation increases the diffusion path from the grain interior to the surface and can scavenge species which exist as trace gasses from pore-space air. Results suggest that firn ventilation has little effect on H2O 2 concentration in firn, but may be more important to the preservation of other reversibly-deposited species.In summary, results show that firn-ventilation induced water vapor motion is a key component of isotopic change in the firn, and may be important for reversibly-deposited geochemical species; but is probably not important for irreversibly-deposited geochemical species.
- Geophysics