Dating glacier ice of the last millennium by quantum technology

First insights into the age of the giant ice deposits in the Eisriesenwelt cave (Austria)

Frozen water is the most widespread type of ice present in ice caves and forms ice stalagmites and stalactites as well as floor ice, which is often several meters thick. Organic macroremains are commonly rare in this type of cave ice, which makes it difficult to establish a chronology and severely limits the use of such ice deposits as paleoenvironmental archives. Here, the chronology of such ice deposits in the inner part of the glaciated Eisriesenwelt, one of the world's largest ice caves located in the European Alps of Austria, is determined by a combination of radiocarbon and 230Th dating of cryogenic calcite. The data suggest that this cave ice has formed over the last three millennia, with a marked increase in the average accumulation rate during the thirteenth century, coinciding with the onset of the Little Ice Age in the Alps. Data from a second site closer to the entrance suggests that large parts of this tourist cave were likely ice-free during the Medieval Warm Period and that a substantial part of the ice is probably a relic of the Little Ice Age. The current warming has already penetrated deeper into the cave than during the Medieval Warm Period, although air exchange during the warm season is restricted by a door at the cave entrance.

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.

Contemporary mass balance on a cold Eastern Alpine ice cap as a potential link to the Holocene climate

Alpine cold ice caps are sensitive indicators of local climate. The adequate interpretation of this information in an ice core requires detailed in situ glaciological and meteorological records, of which there are few. The Weißseespitze summit ice cap (3499 m) presents an ideal case to compare past and present climate and mass balance, with limited ice flow, but close to 6000 years locked into about 10 m of ice. First-ever meteorological observations at the ice dome have revealed that over 3 years of observation most of the accumulation took place between October and December and from April to June. In the colder winter months, between January and March, wind erosion prevents accumulation. Melt occurred between June and September, ice was only affected during short periods, mainly in August, which caused ice losses of up to 0.6 m (i.e. ~ 5% of the total ice thickness). Historical data points at a loss of of 34.9 ± 10.0 m between 1893 and 2018 and almost balanced conditions between 1893 and 1914. The local evidence of ice loss lays the basis for the interpretation of past gaps in the ice core records as past warm/melt events.

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%.