Evidence for the loss of snow-deposited MSA to the interstitial gaseous phase in central Antarctic firn

Abstract

We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarctic ice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profiles exhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phase migration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probably to the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modify MSA concentration. It is proposed that, below the upper few metres where the communication with the free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later in air bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firn core do not take into account the MSA released to the gaseous phase, but this fraction is measurable in ice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmospheric MSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties.

Effect of accumulation rate on water stable isotopes of near‐surface snow in inland Antarctica

“…[] suggested that minima of nssSO 4 2− in the 1999 DF pit were formed in summer by sublimation‐condensation of water vapor between near‐surface snow and the atmosphere. Moreover, some studies have suggested that MSA maxima could move from summer to winter at high latitudes as a result of postdepositional modification [ Curran et al ., ; Delmas et al ., ]. This process involves removal of gaseous MSA from the snow layers and its return to the free atmosphere, where it may be partly redeposited in the snow layers or remains in the interstitial air [ Delmas et al ., ].…”

Atmospheric methane sulfonate and non‐sea‐salt sulfate records at the European Project for Ice Coring in Antarctica (EPICA) deep‐drilling site in Dronning Maud Land, Antarctica

“…However, the interpretation of MSA signals in ice cores is still ambiguous. This is due to the highly complex mechanisms controlling DMS emission from algae [ Andreae , 1990; Saltzman , 1995; Simó and Pedrós‐Alió , 1999; Simó and Dachs , 2002; Toole and Siegel , 2004], variable MSA yields from DMS photo‐oxidation [ Ayers et al , 1996; Koga and Tanaka , 1999; Gondwe et al , 2004], and finally post depositional losses and migration of MSA signals within annual firn layers [ Minikin et al , 1994; Pasteur and Mulvaney , 2000; Delmas et al , 2003; Weller et al , 2004].…”

Influence of large‐scale teleconnection patterns on methane sulfonate ice core records in Dronning Maud Land

“…This in principle holds for MS − as well and has been clearly observed for seasonally high acid layers in ice cores from the Filchner‐Ronne ice shelf [ Minikin et al , 1994]. In addition, ice core studies at Vostok showed decreasing MS − concentrations in the top layers and very low concentrations in the upper 50 m that have been attributed to a net loss of MS − from the firn column as well as loss due to improper storage [ Delmas et al , 2003].…”

“…As clearly shown [ Curran et al , 2002; Delmas et al , 2003; Legrand et al , 1996; Wagnon et al , 1999; Weller et al , 2004] MS − records are subject to diffusional smoothing and postdepositional net loss in the firn column, the latter being strongly dependent on snow accumulation rate. Because the snow accumulation in the past has been reconstructed in our ice cores a correction for loss process is in principle possible (e.g., according to Weller et al [2004]).…”

“…The use of MS − in ice cores as proxy for past production of DMS by algae, however, is hampered, by postdepositional changes in the snow pack. For low‐accumulation sites, postdepositional processes like diffusion, migration and loss of volatile species such as MS − via the gas phase become increasingly more important [ Curran et al , 2002; de Angelis and Legrand , 1995; Delmas et al , 2003; Legrand et al , 1996; Röthlisberger et al , 2002, 2003; Wagnon et al , 1999]. In fact, it may be questioned whether there is any atmospheric net signal left in the MS − records or at least whether the signal/noise ratio is high enough to achieve reliable results.…”