Radiocarbon dating of alpine ice cores with the dissolved organic carbon (DOC) fraction

• Main Authors: L. Fang, T. M. Jenk, T. Singer, S. Hou, M. Schwikowski
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-03-01
• Series: The Cryosphere
• Online Access: https://tc.copernicus.org/articles/15/1537/2021/tc-15-1537-2021.pdf
• ISSN: 1994-0416; 1994-0424

<p>High-alpine glaciers are valuable archives of past climatic and environmental conditions. The interpretation of the preserved signal requires a precise chronology. Radiocarbon (<span class="inline-formula">¹⁴</span>C) 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 <span class="inline-formula">¹⁴</span>C 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 WIO<span class="inline-formula">¹⁴</span>C 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; <span class="inline-formula">¹⁴</span>C dating of the two fractions yielded comparable ages, with WIO<span class="inline-formula">¹⁴</span>C 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 <span class="inline-formula">¹⁴</span>C contributing to the DOC resulting in a bias towards younger ages and (ii) incompletely removed carbonates from particulate mineral dust (<span class="inline-formula">¹⁴</span>C-depleted) contributing to the WIOC fraction with a bias towards older ages. The estimated amount of in-situ-produced <span class="inline-formula">¹⁴</span>C 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, DO<span class="inline-formula">¹⁴</span>C 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 <span class="inline-formula">98±2</span> %, 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 WIO<span class="inline-formula">¹⁴</span>C dating. With this study we demonstrate that using the DOC fraction for <span class="inline-formula">¹⁴</span>C dating not only is a valuable alternative to the use of WIOC but also benefits from a reduced required ice mass of typically <span class="inline-formula">∼250</span> <span class="inline-formula">g</span> to achieve comparable precision of around <span class="inline-formula">±200</span> 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.</p>