Carbonyl sulfide in air extracted from a South Pole ice core: a 2000 year record

In this study, we present carbonyl sulfide (COS) measurements from an ice core drilled near South Pole, East Antarctica (SPRESSO). The samples are from 135-291 m, with estimated mean COS ages ranging from 278 to 2155 years before present (defined as 2000 C.E.). When combined with the previous records of COS from Antarctic ice cores and firn air, the current data provide a continuous record of COS extending beyond the last two millennia. The general agreement between ice cores, firn air, and modern air measurements supports the idea that polar ice is a valid archive for paleoatmospheric COS. The average COS mixing ratio of the SPRESSO data set is (331 +/- 18) ppt (parts per trillion in mol/mol, +/- 1 sigma, n=100), excluding 6 outliers. These data confirm earlier firn air and ice core measurements indicating that the late 20th century COS levels of 500 ppt are greatly increased over preindustrial levels and represent the highest atmospheric levels over the past 2000 years. The data also provide evidence of climate-related variability on centennial time-scales, with relative maxima at the peaks of Medieval Climate Anomaly and Little Ice Age. There is evidence for a long-term increasing trend in COS of 1.8 ppt per 100 years. Further ice core studies will be needed to determine whether this trend reflects secular variability in atmospheric COS, or a slow post-depositional chemical loss of COS in the ice core.