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

Margaret B. Williams,Murat Aydin,Eric S. Saltzman,Cheryl Tatum

doi:10.1029/2006gl029142

Margaret B. Williams, Murat Aydin + Show 2 more

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https://doi.org/10.1029/2006gl029142

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Abstract

Methyl chloride (CH3Cl) is a naturally occurring ozone‐depleting gas with a complex biogeochemical cycle involving tropical vegetation, soils, biomass burning and the oceans. This study presents CH3Cl measurements in air extracted from a 300 m ice core from South Pole, Antarctica, covering the time period from 160 BC to 1860 AD. The data exhibit an increasing trend of 3 ppt (parts per trillion) over 100 years and higher frequency variations that appear to be climate‐related. CH3Cl levels were elevated from 900–1300 AD by about 50 ppt relative to the previous 1000 years, coincident with the warm Medieval Climate Anomaly (MCA). CH3Cl levels decreased to a minimum during the Little Ice Age cooling (1650–1800 AD), before rising again to the modern atmospheric level of 550 ppt. These variations most likely reflect changes in tropical and subtropical conditions, and raise the possibility that a warmer future climate may result in higher tropospheric CH3Cl levels.

Highlights

[3] CH3Cl has been measured in Antarctic firn air (Siple Dome, South Pole, Law Dome, Dronning Maud Land) [Aydin et al, 2004; Butler et al, 1999; Kaspers et al, 2004; Trudinger et al, 2004]
Air trapped in a South Pole ice core sample represents a mixture of ages encompassing several decades, because of the diffusion of air in the firn column prior to lock-in [Battle et al, 1996; Butler et al, 1999]
The ice core was sampled at roughly 1 meter intervals, corresponding to 12– 15 years in mean gas age

Summary

Full Article

A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability. Measurements of CH3Cl 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]. The samples cover a 2000 year range in mean gas age from 160 BC to 1860 AD, greatly extending the atmospheric history for CH3Cl. Methodology is similar to that described previously for trace gas measurements from the Siple Dome ice core [Aydin et al, 2002, 2004; Saltzman et al, 2004]

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