High-precision measurements of 14C in ice cores: results and future prospects

Measurements of 14C in carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO) from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric CH4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of 14C signals in ice is complicated by the fact that there is a poorly-understood in situ cosmogenic component in addition to the trapped atmospheric component. A new analytical system allowed 14C of CH4 in glacial ice to be measured for the first time and improved measurement precision for 14C of CO in ice by an order of magnitude over prior work. Measurements of 14C of CH4 in ablating Greenland ice suggested that wetlands were the likely main driver of the Younger Dryas - Preboreal rapid atmospheric CH4 rise ≈ 11,600 yr ago, but interpretation was complicated by what appeared to be an unexpected significant in situ cosmogenic 14CH4 component. Subsequent measurements in shallow firn at Greenland Summit and in 50-kyr-old ablating ice at Taylor Glacier, Antarctica ice definitively confirmed in situ cosmogenic 14CH4 production in glacial ice. The Taylor Glacier measurements also precisely quantified the in situ 14CH4 / 14CO ratio for muogenic 14C production (0.0078 ± 0.0001). The observed constancy of this ratio demonstrated that 14C of CO can be used to quantify the cosmogenic 14CH4 content, allowing for accurate reconstructions of the absolute paleo-atmospheric 14C of CH4 from glacial ice. Measurements in Greenland shallow firn clearly demonstrated that almost all in situ cosmogenic 14C is rapidly lost from the shallow firn to the atmosphere. This implies that 14C of CO2 at most ice core sites is dominated by the atmospheric component and, with a 14CO-based correction for the cosmogenic component, can likely be used for absolute dating of ice. Even given the rapid in-situ cosmogenic 14C loss in the firn, 14C of CO is still expected to be dominated by the cosmogenic component and is a promising tracer for past cosmic ray flux. © AGU 2012

Keywords

Drill cores, Ice, Cryosphere, Oceanography, Age estimation, Carbon 14, Greenhouse gases

Citation

Petrenko, V. V., Severinghaus, J. P., Smith, A. M., Schaefer, H., Riedel, K., Brook, E., Buizert, C., Baggenstos, D., Harth, C.M., Hua, Q., Orsi, A. J., Bauska, T. K., Schilt, A., Mitchell, L., Fain, X., Takeshita, Y., Lee, J. E., Brailsford, G., Franz, P. & Dickson, A. G. (2012). High-precision measurements of 14 C in ice cores: results and future prospects. Paper presented at the AGU Fall Meeting 2012, San Francisco, California, 3 to 7 December 2012. Retrieved from: https://abstractsearch.agu.org/meetings/2012/FM/C54B-01.html