Award: OPP-1753423

Atmospheric transport and deposition of aerosols is an important delivery mechanism of natural and contaminant trace elements (TEs) to the Arctic. Inputs of contaminant elements such as lead and mercury, and of biologically-essential elements such as iron and zinc have strong implications for the ecosystem. The assessment of these inputs to the remote central Arctic Ocean has been difficult, particularly for winter months, because of the limited opportunities to make aerosol chemistry and deposition measurements on a routine basis. As participants in the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) we were able to collect samples to assess aerosol TE fluxes and TE partitioning among the various catchments (seawater, sea ice and snow) in the Central Arctic Ocean for the period of October 2019 to May 2020. Because making direct measurements of wet and dry aerosol deposition is difficult and unreliable, we utilized Beryllium-7 (Be-7) as a tracer for analyzing TE atmospheric fluxes and catchment partitioning. Be-7 is a naturally occurring radionuclide that is created in the atmosphere by cosmic rays, and deposited on the ocean surface primarily through precipitation. The Be-7 method is a well established means of determining TE fluxes to the ocean surface, and involves the measurement of the isotope?s inventory in seawater, ice, and snow, as well the TE and Be-7 concentrations in aerosol particles. We collected and analyzed 26 aerosol filter samples (a roughly weekly sampling schedule) for Be-7 and TEs analysis. Catchment inventories were estimated from the following: 25 large volume (>1000L) seawater samples from the upper ocean, 20 ice core samples, and over 100 snow samples. In terms of numbers of samples and sampling frequency, this constitutes the largest Be-7 database of wintertime measurements for this area to date. As a result of our Be-7 analysis, as well as physical measurements (e.g. snow depth, ice thickness, etc), we were able to observe seasonal changes (winter to spring) in fluxes and partitioning of TEs among reservoirs (water/ice/snow). For example, we found that Be-7 seawater inventories decreased after freezeup according to the Be-7 decay rate. This strongly suggest that the upper water column remains isolated from atmospheric input beginning in late September, and that new aerosol deposition is partitioned exclusively in the snow cover until the spring melt season commences. Furthermore, a modeling study using our Be-7 data suggests that winter mixing rates in the upper water column are very low. Thus, mixed layer water is largely isolated from the atmosphere as well as from the deeper water column. We were also able to observe the effects of several significant weather events that redistributed snow (as well as Be-7 and TEs) from areas of level sea ice to deformed sea ice (ridges). Low precipitation rates and snow depths (~10cm on level sea ice) were observed during the winter months, and our Be-7 data suggest that aerosol deposition rates reached a minimum in late winter and early spring. Last Modified: 04/29/2022 Submitted by: Mark P Stephens