Modeling the isotopic composition of Antarctic snow using backward trajectories: simulation of snow pit records

The quantitative interpretation of isotope records (d18O, dD, and d excess) in ice cores can benefit from a comparison of observed meteorology with associated isotope variability. For this reason we studied four isotope records from snow pits in western Dronning Maud Land (DML), Antarctica, covering the period 1998–2001. Timing and magnitude of snowfall events on these locations were monitored using sonic height rangers. For the distinguished snowfall events we evaluated the isotopic composition of the moisture during transport by combining backward trajectory calculations with isotopic modeling, using a Rayleigh-type distillation model (MCIM). The initial isotope ratio of the moisture was determined from monthly mean isotope fields from a general circulation model (ECHAM4). The trajectory analysis showed that the southern Atlantic Ocean is the major moisture source for precipitation in DML. Modeling results along the trajectories revealed that most of the isotopic depletion occurred during the last day of the transport. Finally, a diffusion model was applied to describe the diffusion in the firn layer such that the modeled isotopes could be compared with the observed isotope records. The resulting modeled isotope profiles were mostly in good agreement with the observed seasonal variability in the snow. However, at low temperatures (especially on the Antarctic interior), magnitude of the total distillation was underestimated. Regarding the d excess parameter, our results show a large influence of advection height on the final value of d excess in precipitation. This in turn points to the importance of the vertical structure of d excess over the oceanic source region, which obscures the classical interpretation of this parameter in terms of temperature and relative humidity in the moisture source region.