Atmospheric carbonyl sulfide during the last 2,000 years from analysis of air extracted from Antarctic ice cores

Carbonyl sulfide (OCS) is a long-lived sulfur gas that contributes to the formation of stratospheric sulfate aerosol during periods of volcanic quiescence. In this study, we present OCS measurements from an ice core drilled near the South Pole Remote Earth Science and Seismic Observatory (SPRESSO). The SPRESSO core was dry-drilled to a depth of ~300 m and the deepest sample is from 291 m, at which depth we estimate the OCS gas age to be about 2,150 years before present. When combined with the previous records of OCS from Antarctic ice cores and firn air (Montzka et al., JGR 2004), the current data provide a continuous record of OCS extending beyond the last two millennia. The general agreement between ice cores, firn air, and modern air measurements suggests that polar ice is a valid archive for paleoatmospheric OCS. The average OCS mixing ratio of the SPRESSO data is 336∓28 ppt (∓1σ, n=106). The new data reveal variability on centennial time-scales as well as a long-term increasing linear trend of 1.8 ppt per hundred years. OCS levels reach relative highs at the peaks of Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), which stand out as periods of variability during the general climatic stability of the late Holocene. The OCS peak coincident with the height of the MCA measures about 350 ppt and appears to be a result of a long term increasing trend that correlates with increasing temperatures between 600 C.E. and 1100 C.E. The following 400 years is characterized by stabilization of and a subsequent drop in OCS. OCS starts to increase again around 1500 C.E., approaching 400 ppt at the peak of the LIA to reach the highest levels prior to the 20th century. The SPRESSO data do not contradict prior estimates of the impact of anthropogenic emissions on the OCS burden that pushed the tropospheric OCS levels to about 500 ppt during late 20th century.