The relationship between stable isotopes in Antarctic firn and the meteorological conditions at the deposition site.

The study of ice cores from the Greenland and Antarctic ice sheets is one of the most successful methods in climatological research. To be able to understand a possible climatic change and the possible anthropogenic element in it, a thorough knowledge and understanding of the climatic changes of the past is required. One of the crucial ice core properties for climate studies is the stable oxygen isotope content of the ice, delta 18O. It is fairly well correlated to the annual mean air temperature of the deposition site, although it depends in a complex way on the source and distance to the source of precipitation and fractionation processes during the moisture transport to the deposition site of the snow. In spite of these complicated physics a linear relationship between the mean annual 18O content of snow and the mean annual air temperature at the deposition site is found. However, there is increasing evidence that the delta 18O content of the ice is dependent on several other factors apart from the temperature, e.g. sea ice extent (which changes the distance to the moisture source), seasonal distribution of accumulation, and strength of the inversion layer. Unfortunately, most drilling sites are situated in remote areas, for which no meteorological measurements are available, which makes calibration by direct comparison of observed temperatures and 18O contents impossible. Also little is known about the atmospheric circulation and precipitation processes in these areas. But, especially with regard to the planned European deep drilling in Dronning Maud Land in the framework of EPICA (European Project for Ice Coring in Antarctica), there is an urgent need to gain more insight into the atmospheric conditions and transport mechanisms which lead to the stable isotope profiles observed in the cores. At the German wintering base "Neumayer" (7037` S, 822`W, establ. in 1981) on Ekström Ice Shelf (Dronning Maud Land), extensive glaciological studies have been carried out continuously for the last 17 years. Weekly readings of accumulation stakes were complemented by sampling snow pits and shallow firn cores as well as freshly fallen snow, whose isotope contents were analysed. Neumayer is also a meteorological observatory, for which routine observations of all important meteorological parameters including upper air soundings are available. To the applicant`s knowledge, Neumayer is the only base with such a long-term high time resolution accumulation series, and whereas for most drilling sites in Antarctica neither independent accumulation measurements nor meteorological data are available, here we have the major advantage of finding detailed accumulation stake measurements, snow pits, shallow firn cores, surface snow samples, and meteorological measurements (including radiosondes) at the same place. This gives us the unique possibility of a comprehensive glacio-meteorological study. The suggested project aims at an improved understanding of the relationship between stable isotopes in firn/ice and the meteorological conditions of the drilling/sampling site. The research activities within the proposed project can be devided into two interrelated parts: 1. Investigation of the relationship between stable isotopes in firn and the meteorological parameters measured at the deposition site, i.e. accumulation, 18O-temperature relationship, especially the dependence on temperature at the surface, at the lifting condensation level, and during snow fall events 2. Investigation of the synoptic conditions which caused the observed temperatures and delta 18O values. Of special interest here is the use of a trajectory model in order to get more information about the source of precipitation and the transport processes of water vapour into and within the Antarctic atmosphere. Both studies will be done for surface snow samples and pit/core data separately, since redistribution of snow after deposition due to wind influence and water vapour diffusion within the snowpack tend to complicate the direct comparison of surface snow samples and snow pits and cores, respectively. Usually the amplitude of seasonal variations of delta 18O is considerably higher in the surface snow samples than in the cores. Therefore a parallel treatment of these data sets as well as an investigation of the correlation between the two is planned. The investigation will be carried out in close cooperation with the Alfred-Wegener-Institute of Polar and Marine Research (AWI), Bremerhaven, Germany. Part of the surface snow samples and cores are yet to be analysed, which will be done at AWI, where cold rooms and a mass spectrometer for the isotope analysis are available. Also the interpretation of the data will be done in close collaboration with Dr. H. Oerter of the Glaciology Division of AWI, who is also responsible for the field data collection in Antarctica.

The study of ice cores from the Greenland and Antarctic ice sheets is one of the most successful methods in climatological research. To be able to understand a possible ongoing climatic change and the possible anthropogenic element in it, a thorough knowledge and understanding of the climatic changes of the past is required. One of the crucial ice core properties for climate studies is the ratio of stable oxygen isotopes (different types of oxygen molecules) of the snow and ice, the so-called delta-18-O. This ratio is fairly well correlated to the annual mean air temperature of the deposition site, although it depends in a complex way on the source and distance to the source of precipitation and physical processes during the moisture transport to the deposition site of the snow. However, for a correct interpretation of the delta-18-O profiles of the cores in terms of temperature, we need to gain more insight into the atmospheric conditions and transport mechanisms that lead to the stable isotope ratio observed in the cores. The main problem is here, that ice cores are usually taken in remote areas, for which no meteorological data are available. At the German Antarctic base "Neumayer", extensive glacio- meteorological studies have been carried out during the past two decades. Weekly readings of snow height were complemented by sampling snow pits and shallow firn cores as well as freshly fallen snow, whose isotope contents were analysed. Neumayer is also a meteorological observatory for which routine observations of all important meteorological parameters including radiosonde (weather balloons) measurements are available. Thus here we could study the relationship between stable isotopes in firn/ice and the meteorological conditions of the drilling/sampling site, the latter meaning both the prevailing weather at the site and the general weather situation and thus the atmospheric transports. To study the transports, a computer model was used that calculated the transport paths of an air particle, so-called trajectories, for five days backwards, thus yielding information about the origin of precipitation which is important for the relationship between temperature and stable isotopes. In the frame of the European ice core project EPICA, an ice core is taken on the high plateau about 550km SSE of Neumayer. For the interpretation of this core our study will be helpful. It was found that, at Neumayer, the distribution of snowfall during the year has a large influence on the annual mean delta-18-O value (e.g. a higher contribution of winter snow to the annual mean leads to a too low delta-18-O value and v. v.) This can be important for the interpretation of deep ice cores at the transition between an ice age to a warmer climate, since systematic changes in the atmospheric circulation might have changed precipitation patterns which results in misinterpretations of the delta-profile in the core. The relationship between temperature and isotopes is also influenced by the origin and transport paths of the moist air. The calculated trajectories were divided into five different classes. The best correlation between temperature and delta-18-O was found for short trajectories and those ones that were situated over sea ice or the continent. The worst correlation was seen for long trajectories from the NW that led above open water for longer time periods, since above the ocean, more evaporation and condensation processes occur that alter the isotope ratio. This is important especially for the EPICA core, since weather situations with such long transports of warm, humid air from the NW can bring a large percentage of the yearly snowfall at the drilling site, which can cause errors in the temperature interpretation.