Limited dechlorination of sea-salt aerosols during the last glacial period: evidence from the European Project for Ice Coring in Antarctica (EPICA) Dome C ice core

Chloride (Cl-) and sodium (Na+) in ice cores originate mainly from sea salt, and one would thus expect the Cl-/Na+ ratio to reflect the seawater ratio. However, at Dome C, a low-accumulation site in East Antarctica, this is not the case in present-day snow. Instead, a Cl- excess relative to Na+ is observed in surface snow, and within a few meters depth the Cl- concentration decreases, and the Cl-/Na+ ratio becomes significantly lower than the seawater ratio. Aerosol studies at coastal Antarctic sites have shown that the reaction of sea-salt aerosols with nitric and sulphuric acid leads to the formation of HCl that eventually escapes the sea-salt aerosol. The observed decrease in Cl- concentrations in the uppermost snow layers is due to reemission of HCl from the snow. Postdepositional loss of HCl depends among other factors on the accumulation rate at the site, with lower accumulation rates leading to larger losses. During the Last Glacial Maximum (LGM) the Cl-/Na+ ratio is relatively stable and close to the seawater ratio, despite the even lower accumulation rate during that time. The likely explanation for this conflicting observation is that high levels of dust neutralized nitric and sulphuric acids during the LGM which in turn reduced the formation of HCl from sea-salt aerosol. With less or no HCl formed, postdepositional loss would be prevented, keeping the Cl-/Na+ ratio close to that of sea water