Feasibility of reconstructing paleoatmospheric records of selected alkanes, methyl halides, and sulfur gases from Greenland ice cores

Abstract

Seven short‐lived atmospheric trace gases were measured in 25 ice core samples from Summit, Greenland. Samples were selected from contemporaneous sections of fluid‐ and dry‐drilled ice cores to examine what effects using n ‐butyl acetate as the drill fluid would have on the measurements. The gases include three light alkanes, C 2 H 6 , C 3 H 6 , and n ‐C 4 H 10 ; two methyl halides, CH 3 Cl and CH 3 Br; and two sulfur compounds, OCS and CS 2 , with gas ages from 125 to 325 years before present. Alkane levels are comparable to measurements in modern Arctic air, although C 2 H 6 exhibits greater variability than expected compared with C 3 H 6 and n ‐C 4 H 10 . These results are not consistent with the idea that the alkanes are primarily of anthropogenic origin, suggesting that the ice cores may not truly record a paleoatmospheric signal with respect to these gases. The CH 3 Br results are consistent with previous observations of “excess” CH 3 Br in Greenland firn air. In situ production processes appear to overwhelm the paleoatmospheric signal of this gas. CH 3 Cl exhibits the same effect to a lesser extent. OCS levels are similar to those in Antarctic ice cores and appear to reflect paleoatmospheric levels. CS 2 results are similar to the limited database of modern atmospheric measurements. Only C 3 H 8 and n ‐C 4 H 10 exhibit clear evidence of contamination because of the presence of the drill fluid. The results indicate that it is possible to analyze many trace gases in fluid‐ and dry‐drilled ice samples. However, it appears that in situ production may significantly alter the levels of some trace gases in Greenland ice cores.