Post-coring entrapment of modern air in some shallow ice cores collected near the firn-ice transition: evidence from CFC-12 measurements in Antarctic firn air and ice cores

摘要

In this study, we report measurements of CFC-12 (CCl₂F₂) in firnair and in air extracted from shallow ice cores from three Antarctic sites.The firn air data are consistent with the known atmospheric history ofCFC-12. In contrast, some of the ice core samples collected near thefirn-ice transition exhibit anomalously high CFC-12 levels. Together, theice core and firn air data provide evidence for the presence of modern airentrapped in the shallow ice core samples that likely contained open poresat the time of collection. We propose that this is due to closure of theopen pores after drilling, entrapping modern air and resulting in elevatedCFC-12 mixing ratios. Our results reveal that open porosity can exist belowthe maximum depth at which firn air samples can be collected, particularlyat sites with lower accumulation rates. CFC-12 measurements demonstrate thatpost-drilling closure of open pores can lead to a change in the compositionof bubble air in shallow ice cores through purely physical processes. Theresults have implications for investigations involving trace gas compositionof bubbles in shallow ice cores collected near the firn-ice transition.