Atmospheric controls on water stable isotopes in Antarctica

In order to understand the climate system and to assess possible future climate change, we have to be able to explain the past climate changes. Valuable information about paleoclimate comes from deep ice cores (up to 3km length, 800.000 years old) from Greenland and Antarctica. From air bubbles in the ice we can reconstruct the former constitution of the atmosphere (greenhouse gases). Various physical properties of the ice yield additional information. Former temperatures are derived from measurements of the so-called stable isotopes of water. Isotopes are different types of atoms of the same element, e.g. there are three types of oxygen: the normal oxygen (O16) and two more varieties, which are heavier than the normal one and thus have slightly different physical properties, O17 and O18. Likewise there is normal and heavy hydrogen, the latter being called deuterium. The ratio of these different water isotopes in the ice is related to the temperature at the drilling site. Until recently, only the ratio of O18 to O16 was measured. New measuring techniques have been developed that now enable us to also measure O17, which occurs only in very small quantities. Those instruments are relatively small and light and thus suitable for field expeditions. They can be used to measure not only the isotope ratio of the ice and snow, but also continuously measure the isotope ratio of water vapour in the field. The relationship between stable isotopes in the ice and the temperature at the drilling site is fairly complex and thus cannot easily be determined from ice core data alone. The isotope ratio does not only depend on air temperature but also on the conditions under which the snow fell that later was transformed into ice. To gain a better understanding of this relationship we thus investigate the atmospheric processes involved in the formation of precipitation under present conditions because here an abundance of meteorological data, including upper-air measurements from radiosondes, is available. In this study we will conduct continuous measurements of the stable isotope ratio of water vapour at the German Antarctic wintering base Neumayer, which is situated in coastal Antarctica on a so-called ice shelf, a floating glacier. Additionally, a 36-year series of ratios of fresh snow samples exists here. The data will be compared to various computer models and related to the meteorological conditions at Neumayer. We aim at a better understanding of the relationship between air temperature and stable isotope ratios of snow. This will reduce error possibilities in the interpretation of ice core data. The results of our study will lead to a more exact reconstruction of the temperature of the past from polar ice cores.

The project aimed at an improvement of interpretation of deep Antarctic ice cores, which store information about the climate of the past for the last 800.000 years. Understanding the climate of the past is a precondition for a full understanding of the complex climate system. This is necessary before we can assess the climate of the future. Various variables are being investigated in the ice cores. Air bubbles in the ice yield information about the former constitution of the atmosphere and the greenhouse gas concentration. In particular, the air temperature of the past can be derived from properties of the ice, namely the so-called stable isotopes. Isotopes are different types of the same atom, which have a different weight and thus behave slightly different, e.g. hydrogen and oxygen atoms, which form the water molecule. Both occur in a lighter and a heavier version. The ratio of the different isotopes changes during evaporation and condensation processes. Since ice is former snowfall, the processes involved during formation of precipitation are crucial for a correct temperature interpretation of the cores. New measuring techniques allow us to measure not only the isotope ratios of the snow in a laboratory, but also continuously measure the isotope ratios of water vapour. In the austral summer of 2017/18 measurements of the isotope ratios of water vapour were accompanied by a comprehensive snow sampling programme at Neumayer Station, coastal East Antarctica, so that parallel isotope data from water vapour and snow can be studied using the meteorological data of the station. Of additional interest was the study of temperature inversions. Inversions are frequent in Antarctica, meaning that the air temperature increases rather than decreases with height, the latter being the normal case. These inversions can be very strong and complicate the temperature derivation from ice cores. A first 25-year climatology of inversions was created for Neumayer, which statistically analysed various inversion features for different inversion heights depending on the prevailing weather. No previous study of Antarctic inversions covers such a long time period. The isotope study was hampered by various factors including weather (which has to be expected for Antarctic field work), problems with the sample analysis etc. Covid-19 also delayed some of the work, thus the analysis of the data is still ongoing. The stable isotope ratio is generally related to air temperature, however, cases were found where the change was not parallel. Possible reasons for this are studied using observational as well as model data to analyse the atmospheric flow. If the air comes from relatively low latitudes, the isotope ratios are different from cases when the precipitation was formed close to the coast. This has to be considered when interpreting the ice cores.