Seasonal signals of trace elements and stable isotopes in snow and firn at Kohnen station, Antarctica

alexander.weinhart [ at ]


Ice cores provide several environmental archives that give us insights into the history of the climate of the earth. Stable water isotopes can be used for long term temperature trends during Holocene and young Pleistocene while trace elements indicate seasonal patterns on short term and glacials on long term scales. Nevertheless, syn- and postdepositional processes influence the originally deposited signal of those proxies. As there is lack of continuous data-based annual accumulation distribution in Antarctica, it is still not clear, how single species are deposited in snow and how the signal can be interpreted. Especially the temporal variability of deposition dependent on seasonal accumulation is a fact that needs to be unterstood. Therefore, in this master thesis we try to explain, how the deposition of proxies is coupled with accumulation and show implications for an interpretation of distinct proxy signals. For this purpose, snow profiles with a length of 50 cm were taken at four different locations, which are along a path of 40 m. The main site was sampled 41 times within a period of 53 days, while the other three locations were used as reference for spatial variance. With that setup, especially the temporal resolution was in the focus of interest. The liner were cut into distinct samples of 1 cm (0-30 cm depth) and 2 cm (30-50 cm depth) and analyzed on several trace elements (Na+, Cl-, NO3 -, SO42-, Ca2+, Mg2+ and MSA) using a Dionex IC 2100 ion chromatograph. δ18O and δ2H measurements were conducted using cavity ring-down spectroscopy (CRDS) and an Picarro analyzer. Study area is the EPICA drill site Kohnen (75°0‘ S; 0°4‘ E) in Dronning Maud Land, Antarctica with an accumulation rate of 64 mm w.e. per year and upward tendency towards higher values. We performed a time-depth-correlation taking recent ablation stake measurements into account and plotted the isotopic and aerosol record on the basis of an estimated accumulation distribution. We used a value of 0.5 cm a-1 snow per winter month which is, based on density data from the snow liner, about 1.83 mm w.e. The validity of the temporal correlation was done by conservative aerosol records of sea salt components Na+ and Cl-. In general, the case study reveals poor correlation between isotopic record and surface temperature on the small temporal and special scale. We observed a lack of summer signal and high dependence on short term climate fluctuations like high precipitation events during extended winter season. Nevertheless, those small scale variations cannot be seen in the isotopic record as smoothing exceeds the temporal resolution. 2H-excess in uppermost snow layers and the trend of δ18O versus δ2H confirms high influence of evaporation and sublimation effects on the snow surface that alter the real deposition signal. The allocation of summer and winter to ice core data therefore seems to be based on a high noise influence. Furthermore we showed that local and short time intervals can drastically influence the deposited tracer signals. Wind erosion by strong wind events or squalls may highly disturb the continuous stratigraphy of snow and firn. Finally, we state the urgent need of continuous accumulation distribution in Antarctica to verify patterns shown in this thesis and carry out further observations in higher resolution.

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Weinhart, A. (2016): Seasonal signals of trace elements and stable isotopes in snow and firn at Kohnen station, Antarctica , Master thesis, Universität Bremen.

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