Abstract
Abstract. High-alpine glaciers are valuable archives of past climatic and environmental conditions. The interpretation of the preserved signal requires a precise chronology. Radiocarbon (14C) dating of the water-insoluble organic carbon (WIOC) fraction has become an important dating tool to constrain the age of ice cores from mid-latitude and low-latitude glaciers. However, in some cases this method is restricted by the low WIOC concentration in the ice. In this work, we report first 14C dating results using the dissolved organic carbon (DOC) fraction, which is present at concentrations of at least a factor of 2 higher than the WIOC fraction. We evaluated this new approach by comparison to the established WIO14C dating based on parallel ice core sample sections from four different Eurasian glaciers covering an age range of several hundred to around 20 000 years; 14C dating of the two fractions yielded comparable ages, with WIO14C revealing a slight, barely significant, systematic offset towards older ages comparable in magnitude with the analytical uncertainty. We attribute this offset to two effects of about equal size but opposite in direction: (i) in-situ-produced 14C contributing to the DOC resulting in a bias towards younger ages and (ii) incompletely removed carbonates from particulate mineral dust (14C-depleted) contributing to the WIOC fraction with a bias towards older ages. The estimated amount of in-situ-produced 14C in the DOC fraction is smaller than the analytical uncertainty for most samples. Nevertheless, under extreme conditions, such as very high altitude and/or low snow accumulation rates, DO14C dating results need to be interpreted cautiously. While during DOC extraction the removal of inorganic carbon is monitored for completeness, the removal for WIOC samples was so far only assumed to be quantitative, at least for ice samples containing average levels of mineral dust. Here we estimated an average removal efficiency of 98±2 %, resulting in a small offset of the order of the current analytical uncertainty. Future optimization of the removal procedure has the potential to improve the accuracy and precision of WIO14C dating. With this study we demonstrate that using the DOC fraction for 14C dating not only is a valuable alternative to the use of WIOC but also benefits from a reduced required ice mass of typically ∼250 g to achieve comparable precision of around ±200 years. This approach thus has the potential of pushing radiocarbon dating of ice forward even to remote regions where the carbon content in the ice is particularly low.
Highlights
For a meaningful interpretation of the recorded palaeoclimate signals in ice cores from glacier archives, an accurate chronology is essential
It is interesting to note that the average dissolved organic carbon (DOC) / water-insoluble organic carbon (WIOC) ratio at Belukha (2.5) is higher compared to the other sites
For DOC concentrations observed in this study, an initial ice mass of about 250 g was required, with about 20 %–30 % of the ice being removed during the decontamination processes inside the DOC set-up, yielding ∼ 200 g of ice available for final analysis
Summary
For a meaningful interpretation of the recorded palaeoclimate signals in ice cores from glacier archives, an accurate chronology is essential. Annual layer counting, supported by and tied to independent time markers such as the 1963 nuclear fallout horizon evident by a peak maximum in tritium or other radioisotopes or distinct signals from known volcanic eruptions in the past, is the fundamental and most accurate technique used for ice core dating. For ice cores from high-alpine glaciers this approach is limited to a few centuries only because of the exceptionally strong thinning. Ice flow models, which are widely used to retrieve full-depth age scales Nye, 1963; Bolzan, 1985; Thompson et al, 2006), fail in the deepest part of high-alpine glaciers due to the assumption of steady-state conditions and the complexity of glacial flow and bedrock geometry limiting realistic modelling of strain rates. This emphasizes the need for an absolute dating tool applicable to the oldest bottom parts of cores from these sites
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