Understanding the production and retention of in situ cosmogenic 14C in polar firn.

• Main Authors: Hmiel, B, Petrenko, VV, Dyonisius, M, Smith, AM, Schmitt, J, Buizert, C, Place, P, Harth, CM, Beaudette, R, Hua, Q, Yang, B, Vimont, I, Kalk, M, Weiss, RF, Severinghaus, JP, Brook, EJ, White, JWC
• Format: Conference Object
• Language: English
• Published: AGU Fall Meeting, 12-16 Dec 2016, San Francisco, USA. 2019
• Subjects: Drill cores; Antarctic regions; Carbon 14; Greenland; Ice; Cosmic ray spectrometers; Antarctic; Bomb Peak; Antarc*; ice core
• Online Access: http://apo.ansto.gov.au/dspace/handle/10238/9062; https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/184811

Radiocarbon in CO2, CO and CH4 trapped in polar ice is of interest for dating of ice cores, studies of past solar activity and cosmic ray flux, as well as studies of the paleoatmospheric CH4 budget. The major difficulty with interpreting 14C measurements in ice cores stems from the fact that the measured 14C represents a combination of trapped paleoatmospheric 14C and 14C that is produced within the firn and ice lattice by secondary cosmic ray particles. This in situ cosmogenic 14C component in ice is at present poorly understood. Prior ice core 14C studies show conflicting results with regard to the retention of in situ cosmogenic 14C in polar firn and partitioning of this 14C among CO2, CO and CH4. Our study aims to comprehensively characterize the 14C of CO2, CO, and CH4 in both the air and the ice matrix throughout the firn column at Summit, Greenland. We will present preliminary measurements of 14C in Summit firn air and the firn matrix, along with initial interpretations with regard to in situ cosmogenic 14C retention. Preliminary results from firn air indicate a 14CO increase with depth in the lock-in zone resulting from in situ production by muons, as well as a lock-in zone 14CO2 bomb peak originating from nuclear testing in the late 1950s and early 1960s. A decrease in 14CH4 with depth is observed in the lock-in zone that is in agreement with observations of increasing atmospheric 14CH4 over the past several decades. We observe that only a small fraction of in-situ produced 14CO, 14CH4 and 14CO2 is retained in the firn matrix. Additionally, we describe progress in the development of a field-portable sublimation apparatus for extraction of CO2 from firn and ice for 14C measurements.