Using ice core measurements from Taylor Glacier, Antarctica, to calibrate in situ cosmogenic <sup>14</sup>C production rates by muons
<p>Cosmic rays entering the Earth's atmosphere produce showers of secondary particles such as protons, neutrons, and muons. The interaction of these particles with oxygen-16 (<span class="inline-formula"><sup>16</sup>O</span>) in minerals such as ice and...
| Main Authors: | , , , , , , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-02-01
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| Series: | The Cryosphere |
| Online Access: | https://tc.copernicus.org/articles/17/843/2023/tc-17-843-2023.pdf |
| Summary: | <p>Cosmic rays entering the Earth's atmosphere produce showers of secondary particles such as protons, neutrons, and muons. The interaction of these particles with oxygen-16 (<span class="inline-formula"><sup>16</sup>O</span>) in minerals such as ice and quartz can produce carbon-14 (<span class="inline-formula"><sup>14</sup>C</span>). In glacial ice, <span class="inline-formula"><sup>14</sup>C</span> is also incorporated through trapping of <span class="inline-formula"><sup>14</sup>C</span>-containing atmospheric gases (<span class="inline-formula"><sup>14</sup>CO<sub>2</sub></span>, <span class="inline-formula"><sup>14</sup>CO</span>, and <span class="inline-formula"><sup>14</sup>CH<sub>4</sub></span>). Understanding the production rates of in situ cosmogenic <span class="inline-formula"><sup>14</sup>C</span> is important to deconvolve the in situ cosmogenic and atmospheric <span class="inline-formula"><sup>14</sup>C</span> signals in ice, both of which contain valuable paleoenvironmental information. Unfortunately, the in situ <span class="inline-formula"><sup>14</sup>C</span> production rates by muons (which are the dominant production mechanism at depths of <span class="inline-formula">>6</span> m solid ice equivalent) are uncertain. In this study, we use measurements of in situ <span class="inline-formula"><sup>14</sup>C</span> in ancient ice (<span class="inline-formula">>50</span> ka) from the Taylor Glacier, an ablation site in Antarctica, in combination with a 2D ice flow model to better constrain the compound-specific rates of <span class="inline-formula"><sup>14</sup>C</span> production by muons and the partitioning of in situ <span class="inline-formula"><sup>14</sup>C</span> between CO<span class="inline-formula"><sub>2</sub></span>, CO, and <span class="inline-formula">CH<sub>4</sub></span>. Our measurements show that 33.7 % (<span class="inline-formula">±11.4 <i>%</i></span>; 95 % confidence interval) of the produced cosmogenic <span class="inline-formula"><sup>14</sup>C</span> forms <span class="inline-formula"><sup>14</sup>CO</span> and 66.1 % (<span class="inline-formula">±11.5 <i>%</i></span>; 95 % confidence interval) of the produced cosmogenic <span class="inline-formula"><sup>14</sup>C</span> forms <span class="inline-formula"><sup>14</sup>CO<sub>2</sub></span>. <span class="inline-formula"><sup>14</sup>CH<sub>4</sub></span> represents a very small fraction (<span class="inline-formula"><0.3 <i>%</i></span>) of the total. Assuming that the majority of in situ muogenic <span class="inline-formula"><sup>14</sup>C</span> in ice forms <span class="inline-formula"><sup>14</sup>CO<sub>2</sub></span>, <span class="inline-formula"><sup>14</sup>CO</span>, and <span class="inline-formula"><sup>14</sup>CH<sub>4</sub></span>, we also calculated muogenic <span class="inline-formula"><sup>14</sup>C</span> production rates that are lower by factors of 5.7 (3.6–13.9; 95 % confidence<span id="page844"/> interval) and 3.7 (2.0–11.9; 95 % confidence interval) for negative muon capture and fast muon interactions, respectively, when compared to values determined in quartz from laboratory studies (Heisinger et al., 2002a, b) and in a natural setting (Lupker et al., 2015). This apparent discrepancy in muogenic <span class="inline-formula"><sup>14</sup>C</span> production rates in ice and quartz currently lacks a good explanation and requires further investigation.</p> |
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| ISSN: | 1994-0416 1994-0424 |