Hochaufgelöste basale Eiseigenschaften von sedimentären und nicht-sedimentären hochalpinen Gletschern

• Antragsteller: Dr. Pascal Bohleber
• Fachliche Zuordnung: Mineralogie, Petrologie und Geochemie
• Förderung: Förderung von 2014 bis 2017
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 249850554

• Erstellungsjahr: 2017

Zusammenfassung der Projektergebnisse

Ice cores drilled at polar ice sheets and cold mountain glaciers offer reconstructing temperature-related and bio-geochemical signals stored in cold glacier ice. However, for the lowermost and thus oldest ice it is so far not sufficiently understood to which extent ice deformation and ice-bedrock interaction can distort these meteoric signals. Here, cold basal layers of alpine glaciers are a promising target, since covering a broader spectrum of basal ice properties, while being much easier to access compared to their km-thick polar counterparts. Accordingly, this project investigated a wide range of basal ice recovered from from Colle Gnifetti, a high Alpine drilling site and Chli Titlis, a lower cold based summit glacier. Colle Gnifetti (CG, 4500 m asl, Monte Rosa massif) offers millennial ice core records with the interesting paleo-climate period compressed into thinned bottom layers, however. The reconstruction of atmospheric signals beyond the instrumental era from Colle Gnifetti ice cores is so far hampered by dating uncertainties and interpretation deficits related to depositional noise and potentially distorted basal layer signals. These sub-cm basal layers were approached here by novel application of laser ablation ion coupled plasma mass spectrometry (LA-ICP-MS) offering up to three orders of magnitude larger depth resolution than ever before. The results from laser ablation were compared to datasets obtained from conventional ice core impurity analysis. By this means it was demonstrated that LA-ICP-MS can identify sub-cm annual impurity layers beyond the resolution of state-of-the-art analyses. Using LA-ICP-MS for counting highly thinned annual layers it became possible to construct the first age scale of a CG ice core that is fully based on annual layer counting over the last millennium. The resulting age scale is further corroborated by micro-14 C analysis and provided an essential foundation for novel interpretation of the CG time series over the last millennium. Being an unexpected challenge to this project, 14 C dates revealed a confined shift towards younger ages approximately 10 m above bedrock, before continuing to increase in age with depth. This age discontinuity is likely due to glaciological processes and is currently under further investigation. No further evidence of a stratigraphic disturbance was detected, including the lowermost sections above bedrock. High resolution annual layer counting and 14 C dates remain in agreement also for the lowermost 10 m, indicating annual layers as thin as 1 mm. Specifically targeting basal ice, this project also included a pilot study on using the LA-ICP-MS technique to investigate the location of impurities, and to explore the resulting imprint of ice microstructure on the high-resolution signals. As a whole, the results of this project are an important addition to fully demonstrating the potential of the high-resolution impurity analysis by LA-ICP-MS for investigating highly thinned sections of alpine and also polar ice cores. Chli Titlis glacier (CT, 3030 m asl, Swiss Alps) offers direct access to the ice-bedrock interface in an ice tunnel. A pioneering study more than 25 years ago demonstrated the presence of ice frozen to bedrock and, a phenomenon also known from CG and other mountain ice cores, a distinct depletion in the stable water isotope signal just above bedrock. Obtaining new samples by chain saw this project demonstrated that cold basal ice still exists at CT today, despite of ongoing warming and recent years with negative mass balance. Stable water isotope analysis reproduces the anomaly signal found in the previous study, thus indicating almost stagnant basal ice conditions. The basal ice at CT shows distinct differences with respect to CG regarding i) being much closer to the melting point and ii) a crystal structure being characterized by very large grains (up to 10 mm2 ), an order of magnitude larger than basal ice at CG. Moreover, micro-14 C analysis of the samples indicates that CT has likely been ice-covered for roughly the last 5000 years. Based on the success of this investigation, its approach will further contribute to a more systematic investigation on the past and future fate of Alpine summit glaciers in the Eastern Alps.

Projektbezogene Publikationen (Auswahl)

• (2018) A New Sample Preparation System for Micro- 14 C Dating of Glacier Ice with a First Application to a High Alpine Ice Core from Colle Gnifetti (Switzerland). Radiocarbon 60 (2) 517–533
  Hoffmann, H., Preunkert, S., Legrand, M., Leinfelder, D., Bohleber, P., Friedrich, R. and Wagenbach, D.
  
• (2018) Temperature and mineral dust variability recorded in two low-accumulation Alpine ice cores over the last millennium. Clim. Past (Climate of the Past) 14 (1) 21–37
  Bohleber, P., Erhardt, T., Spaulding, N., Hoffmann, H., Fischer, H. and Mayewski, P.
  
• (2015). New LA-ICP-MS cryocell and calibration technique for submillimeter analysis of ice cores. Journal of Glaciology 61, 233-242
  Sneed, S., Mayewski, P., Sayre, W.G., Handley, M., Kurbatov, A., Taylor, K., Bohleber, P., Wagenbach, D., Erhardt, T. and Spaulding, N.
  
• (2016). Exploring ice core drilling chips from a cold Alpine glacier for cosmogenic radionuclide (10Be) analysis. Results in Physics 6, 78-79
  Zipf, L., Merchel, S., Bohleber, P., Rugel, G., and Scharf, A.
  
• (2016). Ultra-High Resolution Snapshots of Three Multi-decadal Periods in an Antarctic Ice Core. Journal of Glaciology 62, 31-36
  Haines, S. A., Mayewski, P. A., Kurbatov, A. V., Sneed, S. B., Maasch, K. A., Bohleber, P. D., Spaulding, N. E. and Dixon, D. A.
  
• (2017). Investigating cold based summit glaciers through direct access to basal ice: A case study constraining the maximum age of Chli Titlis glacier, Switzerland
  Bohleber, P., Hoffmann, H., Kerch, J., Sold, L., and Fischer, A.
  
• (2017). Next-generation ice core technology reveals true minimum natural levels of lead (Pb) in the atmosphere: Insights from the Black Death. GeoHealth 1, 211-219
  More, A. F., Spaulding, N. E, Bohleber, P., Handley, M., J., Hoffmann, H., Korotkikh, E. V., Kurbatov, A., V., Loveluck, C., P., Sneed, S., B., McCormick, M. and Majewski, P.