Department of Earth System Science

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.