Modelling the Isotopic Composition of Antarctic Snow Using Backward Trajectories: Simulation of Snow Pit Records

[1] The quantitative interpretation of isotope records (δ18O, δD, 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

Independent variations of CH4 emissions and isotopic composition over the past 160,000 years

During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition δ13C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the δ13C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO2 concentrations. We interpret this to reflect a climatic and CO2-related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable δ13C values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources—most notably tropical wetlands—must have responded simultaneously to climate changes across these periods