Radiocarbon dating of glacier ice

Abstract. High altitude glaciers and ice caps from mid-latitudes and tropical regions contain valuable signals of past climatic and environmental conditions as well as human activities, but for a meaningful interpretation this information needs to be placed in a precise chronological context. For dating the upper part of ice cores from such sites several relatively precise methods exist, but they fail in the older and deeper part, where plastic deformation of the ice results in strong annual layer thinning and a non-linear age-depth relationship. If sufficient organic matter such as plant, wood or insect fragments were found, radiocarbon (14C) analysis had thus been the only option for a direct and absolute dating of deeper ice core sections. However such fragments are rarely found and even then very likely not at the depths and in the resolution desired. About 10 years ago, a new, complementary dating tool was therefore introduced by our group. It is based on extracting the μg-amounts of the water-insoluble organic carbon (WIOC) fraction of carbonaceous aerosols embedded in the ice matrix for subsequent 14C dating. Meanwhile this new approach was improved considerably, thereby reducing the measurement time and improving the overall precision. Samples with ~ 10 μg WIOC mass can now be dated with reasonable uncertainty of around 10–20 % (variable depending on sample age). This requires about 100 to 500 g of ice considering the WIOC concentrations typically found in mid- and low-latitude glacier ice. Dating polar ice with satisfactory age precision is still not possible since WIOC concentrations are around one order of magnitude lower. The accuracy of the 14C WIOC method was validated by applying it to independently dated ice. With this method the deepest parts of the ice cores from Colle Gnifetti and Mt. Ortles glacier in the European Alps, Illimani glacier in the Bolivian Andes, Tsambagarav ice cap in the Mongolian Altai, and Belukha glacier in the Siberian Altai have been dated. In all cases a strong annual layer thinning towards bedrock was observed and the oldest ages obtained were in the range of 10 000 yrs. 14C WIOC-dating was not only crucial for interpretation of the embedded environmental and climatic histories, but additionally gave a better insight into glacier flow dynamics close to bedrock and past glacier coverage. For this the availability of multiple dating points in the deepest parts was essential, which is the strength of the presented WIOC 14C-dating method, allowing determination of absolute ages from principally every piece of ice.