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

Abstract

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.

A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate‐controlled variability

“…Measurements of CH 3 Cl in a shallow ice core from Siple Dome, Antarctica covering the past 300 years suggest that the twentieth century increase was part of a more complex, preexisting pattern of variability [ Aydin et al , 2004]. Limited measurements of CH3C1 in Deep ice from Dome Fuji, Antarctica [ Saito et al , 2007] imply that atmospheric levels were relatively constant throughout the Holocene. The firn air and ice measurements published to date suggest that the Antarctic ice archive contains a record of atmospheric CH 3 Cl that is not severely overprinted by local effects or in situ production and loss.…”

Methyl chloride in a deep ice core from Siple Dome, Antarctica

“…During last glacial period (33–65 kyr BP) about 50% of the Ca +2 was of terrestrial origin. Saito et al [2007] did not observe a correlation between dust levels and CH 3 Cl during the Holocene when Ca +2 levels were below 0.6 μ mol l −1 . This may suggest a threshold in dust input above which in situ production of CH 3 Cl becomes detectable above the atmospheric background.…”

“…By contrast, the Siple Dome glacial data prior to the LGM and the glacial/interglacial transition (33 -65 kyr BP) are comparable to, or slightly lower than Holocene levels, with a mean of 456 ± 46 ppt. Saito et al [2007] demonstrated a strong positive correlation between the glacial CH 3 Cl levels at Dome Fuji and the levels of Ca +2 in the enclosing ice. This is convincing evidence that there is ''excess'' CH 3 Cl in the Dome Fuji core, resulting from in situ production, most likely related to chemical or biological reactions involving terrigenous dust and/or associated organic material.…”

“…Studies of air extracted from dry‐drilled, shallow ice cores at South Pole and Siple Dome suggest that ambient CH 3 Cl is also preserved during the process of pore close‐off and burial, at least on time scales of 100–2,000 years [ Aydin et al , 2004; Williams et al , 2007]. Saito et al [2007] recently reported the first CH 3 Cl measurements in a deep ice core from Dome Fuji. They found near‐modern CH 3 Cl mixing ratios in Holocene ice and elevated levels in glacial ice.…”

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

“…Recently, 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 Butler et al, 1999;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.…”