The isotopic composition of ice sheets is a temperature proxy and allows the reconstruction of past climates. However, the accuracy of the reconstructions is dependent on the knowledge about local processes affecting and altering the climatic signal. This thesis investigates the spatial and temporal variability of stable oxygen isotopes in firn cores from Dronning Maud Land, East Antarctica in order to infer reliable conclusions on the past climate in this region. In particular, different sampling, measuring and processing techniques can influence the resulting timeseries. The used sample types are samples from firn cores as well as samples from snow pits and trenches which differ in sample collection and storage. Individual snow samples have to be measured discretely whereas core segments can alternatively also be measured continuously in a CFA system. Both ways allow the determination of isotopic ratios in laser spectrometers; of which the raw data require a post-run correction and a calibration to international standards. In this thesis, discretly and continuously measured firn cores as well as samples from snow pits and snow trenches are analysed. For this, different post-run corrections schemes are compared. The results show that there are no significant discrepancies between the mentioned methods. Further on, eleven firn cores with horizontal distances up to 15km are analysed in a resolution of 50 cm to a depth of 30m covering the period from 1809 to 2012. Firstly, each core is analysed individually and mean annual accumulation rates are calculated. A general increase during the last 50 years is observed, although individual profiles deviate from the mean trend. Secondly, a stacked data set is developed to compensate for non-climatic effects and to improve the signal-to-noise ratio. Considering the last 50 years, we find a positive trend of 0.16 C/decade in the stacked isotopic timeseries. Although the imprint of climate signals in snow and firn is still not completely understood, the increase in isotopic ratios and the derived rise in temperature are robust due to the combination of several profiles. A comparison with a data set covering the period from 1000 to 1950 CE leads to the suggestion that the unusual high values during the last decades might be an indication for an anthropogenic influence on the climate.