In-Situ Radiocarbon Production by Neutrons and Muons in an Antarctic Blue Ice Field at Scharffenbergbotnen: A Status Report

Abstract

ABSTRACT. In the radiocarbon accelerator mass spectrometry ( 14 C AMS) analysis of gases obtained in a dry extraction from a 52-m Antarctic ice core, we observed 14 CO 2 and 14 CO concentrations decreasing with depth. The concentrations are explained in terms of in-situ production by neutrons and captured muons in ablating ice. The ratio of the 14 CO 2 concentration to that of 14 CO has been found to be constant at 1.9 ± 0.3. The ablation rates obtained of 42 ± 18 cm.yr −1 and 40 ± 13 cm.yr −1 for the neutron and muon components, respectively, are about three times higher than observed from stake readings. The discrepancy may point to an incomplete extraction of the dry extraction method. Using the constant ratio in 14 CO 2 and 14 CO concentrations we correct for the in-situ component in the trapped 14 CO 2 and deduce an age of 10,300 ± 900 BP for the ice core.

The 18O isotope exchange rate between firn air CO2 and the firn matrix at three Antarctic sites

“…Inside the firn air column, OH levels are zero, and from this point of view, firn air should preserve the atmospheric 14 CO signal. However, 14 C is not only formed in the atmosphere by 14 N(n, p)→ 14 C but also is formed in ice through the spallation of 16 O (which is ubiquitously present) by muons from cosmic rays [e.g., Lal and Jull , 1994; van der Borg et al , 2001; van der Kemp et al , 2002]. Thus, up to the penetration depth of muons in ice at the site under consideration, 14 C is formed.…”

“…Thus, up to the penetration depth of muons in ice at the site under consideration, 14 C is formed. The presence of the in situ produced 14 C component has been demonstrated [e.g., van der Borg et al , 2001; van der Kemp et al , 2002] in the form of 14 CO and 14 CO 2 extracted from polar ice. The in situ produced 14 C component is partly released into firn air (in form of 14 CO + 14 CO 2 ), and the degree of that release depends on many factors.…”

In situ cosmogenic radiocarbon production and 2‐D ice flow line modeling for an Antarctic blue ice area

“…Several previous studies have used 14 C concentrations in combination with equation (12) to estimate ablation rates in glaciers and BIAs [e.g., Lal et al , 1990; Van Roijen et al , 1995; Van der Borg et al , 2001]. Our modeling results show that neglecting the flow history of the ice parcels can result in an error of up to 25% where topography is rugged (black dashed line in Figure 7b); the proximity of many Antarctic BIAs to mountain ranges and nunataks [ Bintanja , 1999; Sinisalo and Moore , 2010] suggests that caution is warranted when estimating ablation rates from radiocarbon data.…”

“…We first consider neutron spallation reactions [ Lal et al , 1990, 2000], negative muon capture [ Van der Borg et al , 2001; Van Der Kemp et al , 2002; Heisinger et al , 2002a], and fast muon reactions [ Heisinger et al , 2002b; Nesterenok and Naidenov , 2009]. The neutron and muon fluxes incident on the glacier surface are attenuated in the ice, giving a production rate P ( z ) that falls off with depth following where P i 0 is the surface production rate, Z the overburden in g cm −2 , z the depth in cm, Λ i the absorption mean free path in g cm −2 , and ρ the density of the medium (for ice we use ρ = 0.92 g cm −3 ).…”

“…The opposite is also true: where ablation rates are high, activity will be low. By this principle, measurements of 14 C have been used to estimate BIA ablation rates [e.g., Lal et al , 1990; Van Roijen et al , 1995; Van der Borg et al , 2001]. It has also been suggested that 14 C in ice core samples can be used to infer past changes in accumulation rate [ Lal et al , 1990, 2000].…”