Towards 14C-dating of gases in ice cores – constraining the in situ cosmogenic 14C production rates by muons
Michael Dyonisius1,2, Vasilii Petrenko2, Andrew Smith3, Benjamin Hmiel2, Peter Neff2,6, Bin Yang3, Quan Hua3, Philip Place2, James Menking4, Sarah Shackleton5, Ross Beaudette5, Christina Harth5, Michael Kalk4, Heidi Roop2,6, Bernhard Bereiter5, Casey Armanetti4, Christo Buizert4, Jochen Schmitt7, Edward Brook4, Jeffrey Severinghaus5, Ray Weiss5, Joseph McConnell8
1Niels Bohr Institute, Physics Of Ice Climate And Earth, Copenhagen, Denmark, 2University of Rochester, Dept. Earth and Environmental Sciences, Rochester, USA, 3Australian Nuclear Science and Technology Organization, Sydney, Australia, 4Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, USA, 5Scripps Institution of Oceanography, San Diego, USA, 6University of Minnesota, Dept. of Soil, Water, and Climate, Minnesotta, USA, 7University of Bern, Climate and Environmental Physics, Bern, Switzerland, 8Desert Research Institute, Reno, USA
Radiocarbon dating of glacial ice has been a longstanding goal in ice core science. In glacial ice, ¹⁴C is incorporated mainly through trapping of ¹⁴C-containing atmospheric gases (¹⁴CO₂, ¹⁴CO, and ¹⁴CH₄). However, ¹⁴C in ice is also produced in situ, directly in the ice lattice from reactions with secondary cosmic rays. In situ ¹⁴C in ice mostly accumulates after bubble close-off (generally at firn depths between 50-120 m) because almost all of the in situ produced ¹⁴C in the firn column is lost to the atmosphere via diffusion. The in situ ¹⁴C at corresponding close-off depths of most ice core sites is generally dominated by production from deep penetrating muons. Understanding the muogenic ¹⁴C production rates is thus important to deconvolve the in situ cosmogenic and atmospheric ¹⁴C signals in ice cores. In this study, we use measurements of ¹⁴C in ancient ice (>50 kilo-annum before present, ka BP) from the Taylor Glacier ablation site, Antarctica to calibrate the muogenic ¹⁴C production rates. We find that literature values are overestimated by factors of 5.7 (3.6-13.9, 95% confidence interval) and 3.7 (2.0-11.9 95% confidence interval) for negative muon capture and fast muon interactions respectively. Furthermore, the partitioning between the in situ ¹⁴C species appears to be constant (¹⁴CO:¹⁴CO₂ ratio of 1:2, with small <0.2% contributions from ¹⁴CH₄). Our results allow for future ice core ¹⁴C studies to be potentially used for several applications, including absolute dating of gases and improving the ¹⁴C calibration curve in periods where high-resolution tree ring data are not available.
I am presenting part of my PhD work at the University of Rochester. Now I am a postdoc at University of Copenhagen, Niels Bohr Institute.