2,000‐year record of atmospheric methyl bromide from a South Pole ice core

Abstract

[1] This study reports measurements of methyl bromide (CH 3 Br) in air bubbles from a South Pole ice core, with gas ages covering the past two millennia. The air was extracted by mechanical shredding of the core under vacuum and the evolved gases were analyzed by gas chromatography with high resolution mass spectrometry and isotope dilution. These samples had estimated mean gas ages ranging from 160 BCE to 1860 CE. The mean CH 3 Br mixing ratio in the ice core samples was 5.39 ±.06 ppt (1s.e., n = 113). The CH 3 Br measurements from this core agree with those from a Siple Dome ice core for mean gas ages between 1671 and 1860 CE, where there is overlap between the cores. The data show no linear trend over the 2000 year period prior to industrialization. Together, Antarctic ice core and firn air measurements clearly demonstrate that the increase in atmospheric CH 3 Br during the twentieth century exceeds natural variability during the past 2000 years. There is evidence of centennial-scale variability in CH 3 Br on the order of ±10-20% that may indicate a natural climate sensitivity of the atmospheric levels of this ozone depleting substance. The analysis of CH 3 Br in additional Antarctic ice cores will be needed to confirm that the centennial-scale variability observed in this core represents a southern hemisphere atmospheric history.

Microbial metabolism directly affects trace gases in (sub) polar snowpacks

“…Using low-concentration metabolites and taking precautions against wind and photochemistry allows the unravelling of these small, variable biological signals from chemical and physical processes with far greater sensitivity than is possible with other parameters such as CO 2 . We calculate that, in an isolated environment, it would take approximately 50-100 years for the consumption and production of methyl halides to cause a 1 ppm deviation in carbon dioxide concentration within snowpack pore space.…”

“…This simple mechanism of glacial formation was described in the 1990s [1], and has been presented as a justification to use greenhouse gases (CO 2 , CH 4 ) entrapped in glacial ice as a proxy for atmospheric compositions (and hence, climate conditions) back in time. This same logic has been used to justify the quantification of shorter-lived, more reactive trace gases in ice cores including methyl bromide [2] and methyl chloride [3,4]. However, these methods rest on the assumption that the snowpack is quasi-sterile metabolically or, at least, that microbial production/consumption of these trace gases is not significant.…”

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

“…The WAIS-D and SPRESSO samples were extracted and analyzed using an updated procedure described in detail by Aydin et al (2007). These techniques were previously used to measure CH 3 Cl, CH 3 Br, and COS Aydin et al, 2008;Saltzman et al, 2008) and were applied unchanged during the analysis of WAIS-D and SPRESSO samples. Since the validity of our results hinges on the integrity of our methodology, we present a summary in the rest of this section.…”

“…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.…”

Speciation analysis of iodine and bromine at picogram-per-gram levels in polar ice

“…Volatile brominated hydrocarbon species have been previously detected [31] in Antarctic ice, but no specific focus was made on the presence of inorganic Br. While inorganic Br in polar ice is likely in the bromide form [5], iodine is mostly present in atmospheric depositions as iodide and iodate; the volatile iodine organic species photo-dissociate rapidly in the atmosphere to generate iodine atoms [32].…”