Micro radiocarbon dating of the particulate organic carbon fraction in Alpine glacier ice: method refinement, critical evaluation and dating applications

• Main Author: Hoffmann, Helene Margarethe
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2016
• Subjects: 530; 530 Physics; 550; 550 Earth sciences; ice core
• Online Access: https://archiv.ub.uni-heidelberg.de/volltextserver/20712/; https://archiv.ub.uni-heidelberg.de/volltextserver/20712/1/Dissertation_Helene_Hoffmann.pdf; https://doi.org/10.11588/heidok.00020712; https://nbn-resolving.org/urn:nbn:de:bsz:16-heidok-207128

This work focuses on radiocarbon dating of the Particulate Organic Carbon fraction (POC) of Alpine glacier ice. This organic carbon fraction was chosen, because the initially preferred Dissolved Organic Carbon fraction (DOC) was found to be biased by incorporation of in situ produced 14C at high Alpine sites. Concentrations of POC in high Alpine ice are generally very low, ranging on average between 10 and 50 µgC per kg ice. Thus, POC 14C dating of Alpine ice cores, where available masses for dating are not exceeding a few hundred grams of ice, becomes a large challenge. Within this work, a unique sample preparation system (REFILOX), with a process blank reproducibly at a total carbon mass of (1.5 +/- 1.0) µgC or even below detection limit for combustion at 340 °C, has been deployed. This system was complemented by the setup of a gas ion source for direct CO2 AMS (Accelerator Mass Spectrometry) 14C measurements. Together, these novel sample preparation systems allow reliable dating of samples down to 2 µgC with a relative error not exceeding 10% for ages up to 15 000 years BP. For comprehensive age interpretation,POC age biasing effects such as input of old Saharan dust or sediment material have been investigated. The Saharan dust consistently featured an age range of 2200-2800 years BP and the sediment samples showed ages up to more than 20 000 years BP. POC combustion temperature investigations revealed that such biases by incorporated aged material can be avoided best by combustion at a temperature not higher than 340 °C. A first application of the method to ice block samples from Titlis glacier (Swiss Alps, 3020 m a.s.l.) yielded a consistent basal ice age of almost 5000 years BP. Additionally, 16 samples from an ice core drilled at the high Alpine glacier saddle Colle Gnifetti (Swiss Alps, 4500 m a.s.l.) have been POC micro-radiocarbon dated and revealed a so far not fully explained systematic age offset to younger ages than proposed by the annual layer counting based working chronology.