Ice-core record of methyl chloride over the last glacial-Holocene climate change

Methyl chloride (CH₃Cl) concentration was measured in air trapped in a deep ice core from Dome Fuji, Antarctica covering the last glacial-present interglacial (Holocene) change. The record shows that the CH₃Cl concentration was relatively constant, being similar to the present levels, during the pre-industrial Holocene. In contrast, the CH₃Cl concentration was significantly high and variable in the last glacial period, possibly due to impurity-related production of CH₃Cl in ice sheet. Under the assumption that the production was the sole cause of the excess CH₃Cl, the atmospheric CH₃Cl concentration during the last glacial was estimated using simultaneously measured calcium data for the ice core to have been enhanced by 30% compared with the pre-industrial Holocene concentration. Because the major sink of CH₃Cl was stronger during the last glacial than during the Holocene, the enhancement of CH₃Cl during the last glacial was likely due to the glacial period source being enhanced.