Abstract
Abstract. In this study, we report measurements of CFC-12 (CCl2F2) in firn air and in air extracted from shallow ice cores from three Antarctic sites. The firn air data are consistent with the known atmospheric history of CFC-12. In contrast, some of the ice core samples collected near the firn-ice transition exhibit anomalously high CFC-12 levels. Together, the ice core and firn air data provide evidence for the presence of modern air entrapped in the shallow ice core samples that likely contained open pores at the time of collection. We propose that this is due to closure of the open pores after drilling, entrapping modern air and resulting in elevated CFC-12 mixing ratios. Our results reveal that open porosity can exist below the maximum depth at which firn air samples can be collected, particularly at sites with lower accumulation rates. CFC-12 measurements demonstrate that post-drilling closure of open pores can lead to a change in the composition of bubble air in shallow ice cores through purely physical processes. The results have implications for investigations involving trace gas composition of bubbles in shallow ice cores collected near the firn-ice transition.
Highlights
The air in polar firn and ice cores is a unique archive of the ancient atmosphere
The ice core mixing ratio of 0.7±0.9 ppt is consistent with the atmospheric history of CFC-12, which shows a rise from 0 ppt to 1 ppt between 1935 and 1945
Through an examination of firn air and ice core samples, this study presents evidence from three Antarctic sites for the presence of unexpectedly high CFC-12 levels in some ice core samples near the firn-ice transition region
Summary
The air in polar firn and ice cores is a unique archive of the ancient atmosphere. Extraction and analysis of this air has enabled reconstruction of paleo-atmospheric records of carbon dioxide (Petit et al, 1999; Luthi et al, 2008), methane (Brook et al, 1996), and nitrous oxide (Sowers et al, 2003), which have been validated by establishing consistency between gas records from multiple ice core sites. Firn air and ice core studies have been used to examine the atmospheric variability of less abundant trace gases such as methyl chloride, methyl bromide, and carbonyl sulfide (Aydin et al, 2008; Butler et al, 1999; Montzka et al, 2004; Saito et al, 2007; Saltzman et al, 2008; Sturges et al, 2001a, b; Trudinger et al, 2004; Williams et al, 2007) These measurements, which can potentially be extended to a range of low-level trace gases, allow for examination of the natural variability in atmospheric levels of these compounds as well as the influence of human activities on the trace gas composition of the atmosphere. We focus on the compositional integrity of the air bubbles trapped in shallow ice cores, using CFC-12 measurements as a tool to identify post-coring entrapment of modern air
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