Award: OCE-1433717

This project was part of the US GEOTRACES Arctic Expedition. GEOTRACES is an international program that investigates ocean-scale distributions of trace elements in order to improve understanding of the global ocean. Trace elements (chemicals present at very low quantities) are of interest because this diverse set of elements can provide insight into processes of global importance. For example, trace elements that are essential nutrients (e.g., iron) to marine algae provide information on the interplay between their supply, biological uptake and regeneration, which are key to understanding patterns of ocean biological production; these in turn contribute to our understanding of the global carbon cycle and Earth?s climate, and can be used to inform ecosystem-based management of fisheries. The US GEOTRACES Arctic Expedition took place in late summer 2015. We collected surface seawater samples at stations from the Bering Sea to the North Pole and back, and collected sea ice and snow from 6 sea ice stations (See map). Because this was a collaborative effort, in addition to collecting samples for our own trace element analysis, we provided samples to other participating scientists to expand the characterization of trace elements within the surface layer of the Arctic Ocean, with the goal of understanding how the sea ice environment influences trace element cycling in this region of the Arctic Ocean. The remoteness of the Arctic Ocean, and the difficulty in sampling for trace elements without contaminating them during the sampling process has resulted in very few direct measurements of trace elements within the sea ice covered Arctic. Our group designed and manufactured a custom-made plastic/titanium ice corer, processing tools and plastic melting chambers to avoid contamination during collection and processing of the samples. Tests prior to the expedition demonstrated our custom-made equipment was well suited for the job. We also assembled a pumping system that allowed for seawater collection in the freezing Arctic air temperatures. With the appropriate equipment in hand, the sampling portion of our project was successfully accomplished. Sea ice forms from seawater when the surface layer of the ocean reaches sufficiently low temperatures. As ice crystals form, they float to the surface and begin to coalesce into a thin layer of sea ice. Chemicals dissolved in the seawater (including trace elements) are not incorporated into the ice crystals, thus the surrounding seawater becomes very concentrated, forming a brine. Brine droplets and tiny suspended particles (also containing trace elements) get trapped in between ice crystals as these aggregate into ice floes. When sea ice forms in shallow regions, additional particles can be incorporated into sea ice because ice crystals forming at the bottom carry small sediment particles as they float to the surface. Once formed, ice floes thicken as more crystals attach at the bottom. Ice floes provide a surface where snow and dust can be deposited, adding to the trace element load of the floe. Ice floes move from their area of formation by currents and/or the wind, carrying along with them the trace elements they have accumulated. Some of the particles in the sea ice are living organisms (e.g., sea ice algae and bacteria), which can interact with the trace elements in the brine pockets and incorporated sediment. The sea ice itself and the snow on top begin to melt in late spring, with melting continuing through the summer. As ice warms, its brine pockets become more interconnected allowing for brine drainage, for some of the melted snow to infiltrate the top of the ice flow, and for seawater to infiltrate the bottom of the ice flow. Thus, sea ice trace metal content can also be influenced by area of formation, sea ice organisms, and seasonality. Our measurements demonstrate that in late summer sea ice contains trace elements as particles and in the dissolved form, and that the top and bottom layers of the ice tended to have greater trace element loads than the middle layers due to the influence of snow/dust from the top, and of seawater from below. Our sea ice environment data highlight some of the processes described above, and provides another piece to understanding the cycling of trace elements in the Arctic Ocean. Two graduate students (MS and PhD) and one technical staff member gained sea ice and trace element training through this project. Data from the project is part of Channing Bolt?s PhD thesis. Three Arctic native peoples translated sea ice vocabulary into their native language. An international intercomparison of custom-made sea ice sampling equipment was carried out. These efforts have resulted in capacity building in the field of chemical oceanography with a focus on sea ice trace element biogeochemistry. Last Modified: 12/30/2018 Submitted by: Ana M Aguilar-Islas