Aerosol Deposition and Snow Accumulation Processes From Beryllium-7 Measurements in the Central Arctic Ocean: Results From the MOSAiC Expedition

We use a tracer method involving the cosmogenic radioisotope beryllium-7 (half-life = 53.3 days) to follow the deposition of aerosols and the fate of snow on the MOSAiC ice floe during winter and spring 2019-2020. When examined alongside data from earlier studies in the Arctic Ocean that covered summer and fall, Be-7 inventories indicate a summertime peak for aerosol Be-7 deposition fluxes coinciding with seasonal minima boundary-level aerosol concentrations, which suggests that deposition fluxes are primarily controlled by precipitation. This conclusion is supported by the linear relationship between Be-7 fluxes and precipitation rates derived from data from the MOSAiC and SHEBA expeditions. Inventories of Be-7 within the snow column exhibited evidence of significant redistribution. Be-7 deficits, relative to the flux, were observed in areas of level sea ice while excess Be-7 was found associated with deformed ice features such as pressure ridges, leading to the following estimates for the distribution of snow on the ice floe in May 2020: 75-93% of the snow mass is found on deformed sea ice with the remainder on level ice. Furthermore, uncertainties associated with measurements of Be-7 concentrations within the ocean mixed layer would allow for losses of snow through open leads of up to approximately 20% of the flux. Our snow distribution estimates agree with data from repeat snow depth transect measurements. These results suggest that Be-7 can be a useful tool in studying snow redistribution. Beryllium-7, a radioactive isotope with a half-life of 53.3 days, is formed in the atmosphere, attaches to aerosol particles, and is deposited on the earth's surface through wet and dry processes. In this project, we measured Be-7 concentrations in aerosol particles, as well as its deposition and distribution on an ice floe. The project was part of the MOSAiC expedition, in which German Icebreaker Polarstern was moored to ice in the Arctic Ocean and drifted for a full year. Aerosols are an important source of trace elements to the central Arctic Ocean, and our results give a clearer picture of the seasonal cycle of aerosol deposition there. Forthcoming results from a concurrent study will utilize our deposition rates to deduce fluxes of biologically important and pollutant trace elements. Regarding snow distributions, we estimate that at least 75% of snow mass was in areas of deformed sea ice in early May. This is a result of winter storms that sweep snow from level ice to ridge flanks. Since snow acts as an insulator and it reflects sunlight, high degrees of snow redistribution may have important consequences, particularly if ridging increases as sea ice thins, as some scientists have suggested. Be-7 inventories suggest that Arctic Ocean aerosol deposition peaks in summertime and is largely controlled by precipitation Estimates of snow mass redistribution based on Be-7 data are consistent with estimates from repeat snow depth transects Be-7 depth profiles reflect the complex history of snow accumulation processes on an ice floe