Methane Sulphonic Acid in East Antarctic Coastal Firn and Ice Cores and Its Relationship with Chlorophyll-a and Sea Ice Extent in the Southern Ocean

Abstract: The seasonal retreat of sea ice in the austral spring and summer around Antarctica has a significant effect on phytoplankton activity, mainly due to light availability, meltwater input of dissolved iron, and surface water stratification. Phytoplankton produce dimethylsulfoniopropionate, the precursor to the climate-cooling gas dimethyl sulphide, which is ventilated to the atmosphere and oxidised to methane sulphonic acid (MSA). MSA is preserved in firn and ice cores from both the Arctic and Antarctica. Attempts to reconstruct sea ice conditions in different regions of Antarctica with the help of MSA records from ice cores have had varying success, highlighting the often-regional relationship between ice core MSA and sea ice. This study uses MSA records from three firn cores and one ice core drilled on Fimbul Ice Shelf in Dronning Maud Land, East Antarctica, to investigate the relationship to satellite-derived sea ice extent (SIE) in five sectors of the Southern Ocean. Chlorophyll-a concentrations, serving as a measure of phytoplankton biomass, are correlated to the MSA records to further test the MSA – SIE relationship. The firn cores are named after the ice rise where they were drilled: Kupol Ciolkovskogo (KC), Kupol Moskovskij (KM), and Blåskimen Island (BI). The ice core is named S100. The results show that there is a significant, yet weak positive correlation between summer MSA in the KM core and winter SIE in the Weddell Sea Sector. There is also a significant, weak positive correlation between summer MSA in the BI core and summer chlorophyll-a concentrations in the Weddell Sea Sector. There are no significant correlations between MSA in the low-accumulation KC or S100 cores and SIE or chlorophyll-a concentrations. Furthermore, the two high-accumulation core sites in this study, BI and KM, do not display the same relationship between MSA and SIE or MSA and chlorophyll-a, which is likely due to very local wind patterns. Surface winds on Fimbul Ice Shelf are easterly or north-easterly which results in a more coastal influence at the KM site compared to the BI site, likely introducing the differences observed when comparing the two MSA records. More research aimed at evaluating the meteorological conditions that prevail at the core sites is needed to further assess the use of the MSA records from the high-accumulation ice rise cores BI and KM as proxies for SIE in the Weddell Sea region, but in their current state these MSA records are not suitable to use for sea ice reconstruction.