A Tibetan ice core covering the past 1,300 years radiometrically dated with ³⁹Ar

Ice cores from alpine glaciers are unique archives of past global and regional climate conditions. However, recovering climate records from these ice cores is often hindered by the lack of a reliable chronology, especially in the age range of 100 to 500 anni (a) for which radiometric dating has not been available so far. We report on radiometric ³⁹Ar dating of an ice core from the Tibetan Plateau and the construction of a chronology covering the past 1,300 a using the obtained ³⁹Ar ages. This is made possible by advances in the analysis of ³⁹Ar using the laser-based detection method atom trap trace analysis, resulting in a twofold increase in the upper age limit of ³⁹Ar dating. By measuring the anthropogenic ⁸⁵Kr along with ³⁹Ar we quantify and correct modern air contamination, thus removing a major systematic uncertainty of ³⁹Ar dating. Moreover, the ⁸⁵Kr data for the top part of the ice core provide information on firn processes, including the age difference between the ice and its enclosed gas. This first application of ³⁹Ar and ⁸⁵Kr to an ice core facilitates further ice cores from nonpolar glaciers to be used for recovering climate records of the Common Era, a period including pronounced anomalies such as the Little Ice Age and the Medieval Warm Period.