Carbonyl sulfide during the late Holocene from measurements in Antarctic ice cores (Invited)

Carbonyl sulfide (COS) is the most abundant sulfur gas in the troposphere with a global average mixing ratio of about 500 parts per trillion (ppt) and a lifetime of 3 years. It is produced by a variety of natural and anthropogenic sources. Oceans are the largest source, emitting COS and precursors carbon disulfide and dimethyl sulfide. The most important removal process of COS is uptake by terrestrial plants during photosynthesis. Interest in the atmospheric variability of COS is primarily due to its potential value as a proxy for changes in gross primary productivity of the land biosphere. Ice core COS records may provide the long term observational basis needed to explore climate driven changes in terrestrial productivity and the resulting impacts, for example, on atmospheric CO₂ levels. Previous measurements in a South Pole ice core established the preindustrial COS levels at ~30% of the modern atmosphere and revealed that atmospheric COS increased at an average rate of 1.8 ppt per 100 years over the last 2,000 years [Aydin et al., 2008]. We have since measured COS in 5 additional ice cores from 4 different sites in Antarctica. These measurements display a site-dependent downcore decline in COS, apparently driven by in situ hydrolysis. The reaction is strongly temperature dependent, with the hydrolysis lifetimes (e-folding) ranging from thousands to hundreds of thousands of years. We implement a novel technique that uses ice and heat flow models to predict temperature histories for the ice core samples from different sites and correct for the COS lost to in situ hydrolysis assuming first order kinetics. The 'corrected' COS records confirm the trend observed previously in the COS record from the South Pole ice core. The new, longer record suggests the slow increase in atmospheric COS may have started about 5,000 years ago and continued for 4,500 years until levels stabilized about 500 years ago. Atmospheric CO₂ was also rising during this time period, suggesting the atmospheric levels of both trace gases might have changed as a response to a long-term decline in terrestrial productivity during the late Holocene.