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

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 ³⁹ Ar dating of an ice core from the Tibetan Plateau and the construction of a chronology covering the past 1,300 a using the obtained ³⁹ Ar ages. This is made possible by advances in the analysis of ³⁹ Ar using the laser-based detection method atom trap trace analysis, resulting in a twofold increase in the upper age limit of ³⁹ Ar dating. By measuring the anthropogenic ⁸⁵ Kr along with ³⁹ Ar we quantify and correct modern air contamination, thus removing a major systematic uncertainty of ³⁹ Ar dating. Moreover, the ⁸⁵ Kr 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 ³⁹ Ar and ⁸⁵ Kr 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.