Sulfur in polar ice and snow : Interpretations of past atmosphere and climate through glacial archives
Abstract: Snow contains information on the atmosphere it is deposited from. This information is stored in polar ice sheets (Antarctica and Greenland), which are unique geochronological archives of past climate and atmospheric composition. On time scales from annual to glacial cycles, this thesis deals with the signals of sulfur compounds in these archives. The objectives are to interpret the content and origin of sulfur aerosols in the atmosphere and their interactions with climate.By using sulfur isotopic signals, the sources of sulfate were apportioned from two shallow Antarctic ice cores covering the last 1200 years of deposition. Marine phytoplankton was shown to be the predominant source. A surprisingly stable sulfate flux signal over the last eight glacial cycles was obtained from the Antarctic EPICA Dome C ice core, which with a predominant marine biogenic sulfate origin implies that production of sulfur aerosols by phytoplankton neither forced nor was sensitive to glacial/interglacial shifts in climate.Methanesulfonate (MS-) originates solely from phytoplankton, but the record in Antarctic ice cores is biased by variations in atmospheric dust concentrations. In contrast to Antarctic conditions, enhanced dust concentrations on Greenland increase the deposition of sulfate but do not affect MS-. Thus, MS- remains in Greenlandic ice cores as a potential proxy record of marine biogenic sulfur aerosol production. Northern and central Greenland ice core signals of MS- displayed systematically different responses to climate variations during the last glacial period, possibly due to different portioning of air mass origin.Surface snow analyses indicate that sulfate on Greenland is at present mainly deposited as a salt in association with dust particles. On an Antarctic site where sulfate and possibly MS- are present as acids, their concentrations in the surface snow are suggested to be a result of a redeposition process altering the signals of the original snow fall.
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