39Ar dating with small samples provides new key constraints on ocean ventilation

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.

Fast atom-trap analysis of 39Ar with isotope pre-enrichment

We demonstrate fast analysis of ³⁹Ar/Ar at the 10^-16 level using a mass spectrometer for isotope pre-enrichment and an atom trap for counting. An argon gas sample first passes through a dipole mass separator that reduces the dominant isotope ⁴⁰Ar by two orders of magnitude while preserving both the rare tracer isotope ³⁹Ar and a minor stable isotope ³⁸Ar for control purposes. Measurements of both natural and enriched samples with atom trap trace analysis demonstrate that the ³⁹Ar/³⁸Ar ratios change less than 10%, while the overall count rates of ³⁹Ar are increased by one order of magnitude. By overcoming the analysis-speed bottleneck, this advance will benefit large-scale applications of ³⁹Ar dating in the earth sciences, particularly for mapping ocean circulation.

An atom trap system for 39Ar dating with improved precision

Cosmogenic ³⁹Ar dating is an emerging technique in dating mountain glacier ice, mapping ocean circulation, and tracing groundwater flow. We have realized an atom-trap system for the analysis of the radioactive isotope ³⁹Ar (half-life = 269 years) in environmental samples. The system is capable of analyzing small (1-5 kg) environmental water or ice samples and achieves a count rate of 10 atoms/h for ³⁹Ar at the modern isotopic abundance level of 8 × 10^-16. By switching frequently between counting ³⁹Ar atoms and measuring the stable and abundant isotope ³⁸Ar, drift effects in the trapping efficiency are largely suppressed, leading to a more precise measurement of the isotope ratio ³⁹Ar/³⁸Ar. Moreover, cleaning techniques are developed to alleviate cross-sample contamination, reducing the background ³⁹Ar count rate down to <0.5 atoms/h. These advances allow us to determine the ³⁹Ar age in the range of 250-1300 years with precisions of <20%.

An electromagnetic separation system for the enrichment of 39Ar

An isotope enrichment system for ³⁹Ar has been developed at the Institute of Modern Physics, which is designed to increase the abundance of ³⁹Ar in the incident sample gas. With intense Ar⁺ beams produced by a 2.45 GHz electron cyclotron resonance ion source and a high mass resolution spectrometer system, Ar isotopes are evidently separated on the target plane and selectively collected by an Al target. The separated Ar isotopes have been identified on the target plane, which is consistent with the simulations. According to the recent cross-checked results with atom trap trace analysis, a high enrichment factor of ³⁹Ar has been successfully achieved. This paper will present the design and test results of this system.

Dating glacier ice of the last millennium by quantum technology

Radiometric dating with ³⁹Ar covers a unique time span and offers key advances in interpreting environmental archives of the last millennium. Although this tracer has been acknowledged for decades, studies so far have been limited by the low abundance and radioactivity, thus requiring huge sample sizes. Atom trap trace analysis, an application of techniques from quantum physics such as laser cooling and trapping, allows us to reduce the sample volume by several orders of magnitude compared with conventional techniques. Here we show that the adaptation of this method to ³⁹Ar is now available for glaciological applications, by demonstrating the entire process chain for dating of alpine glacier ice by argon trap trace analysis (ArTTA). Ice blocks as small as a few kilograms are sufficient and have been obtained at two artificial glacier caves. Importantly, both sites offer direct access to the stratigraphy at the glacier base and validation against existing age constraints. The ice blocks obtained at Chli Titlis glacier at 3,030 m asl (Swiss Alps) have been dated by state-of-the-art microradiocarbon analysis in a previous study. The unique finding of a bark fragment and a larch needle within the ice of Schaufelferner glacier at 2,870 m asl (Stubai Alps, Austria) allows for conventional radiocarbon dating. At both sites the existing age information based on radiocarbon dating and visual stratigraphy corroborates the ³⁹Ar ages. With our results, we establish argon trap trace analysis as the key to decipher so far untapped glacier archives of the last millennium.