| Summary: | Methyl chloride (CH3Cl) is a naturally occurring ozone-depleting substance and a significant component of the atmospheric chlorine burden. In this study CH3Cl was analyzed in air bubbles from the West Antarctic Siple Dome deep ice core with gas ages ranging from about 65 kyr BP to the Late Holocene. CH3Cl levels were below the modern Antarctic atmospheric level of 530 ppt in glacial ice (456 ± 46 ppt, 33–65 kyr BP) and above it during the early Holocene (650–700 ppt, 10–11 kyr BP). For most of the Holocene, CH3Cl levels were 500–550 ppt, with good agreement between CH3Cl levels in this core and in the Dome Fuji ice core (Saito et al., 2007). Several late Holocene ice core samples (<2 kyr BP), show evidence of enrichment in CH3Cl relative to South Pole ice core samples of overlapping gas age. The Siple Dome record suggests that CH3Cl levels in the glacial Southern Hemisphere atmosphere were about 16% lower than those during the mid-late Holocene.
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