Seasonal variations of accumulation and the isotope record in ice cores: a study with surface snow samples and firn cores from Neumayer station, Antarctica

At the German wintering base Neumayer, an intensive glacio-meteorologicalprogrammehasbeen carried out during the last two decades. A completemeteorological dataset and data from surface snow samples, snow pits, firn cores and weekly accumulation measurements from a stake array are available.We first investigated the attenuation of the seasonal ̄O signal due to water-vapour diffusion in the snowpack. A comparison of surface snow samples and firn cores of different ages shows that only onethird of the seasonal ̄O signal of the surface snow samples remains in the cores after the first year. No further significant change in the amplitude of the seasonal ̄O signal is found later. Changes in the seasonal distribution of accumulation can lead to a bias in ice-core properties. This is studied on a short time-scale, using high-time-resolution data of accumulation, stable-isotope ratios and air temperature. Mean annual ̄O values from firn cores are not well correlated to annual mean air temperatures. However, the correlation is improved considerably by calculating an annual mean air temperature using monthly mean temperatures weighted by monthly accumulation. At Neumayer, it is mainly the cyclonic activity in late winter/early spring that determines whether and how the core data are biased.