Gallium: A New Tracer of Pacific Water in the Arctic Ocean

Determining the proportions of Atlantic and Pacific Ocean seawater entering the Arctic Ocean is important both for understanding the mass balance of this basin as well as its contribution to formation of North Atlantic deep water. To quantify the distribution and amount of Pacific and Atlantic origin seawater in the western Arctic Ocean, we used dissolved Ga in a four-component linear endmember mixing model. Previously, nutrients, combined in their Redfield ratios, have been used to separate Pacific- and Atlantic-derived waters. These nutrient tracers are not conservative in practice, and there is a need to find quantities that are conserved. Dissolved Ga concentrations show measurable contrast between Atlantic and Pacific source waters, shelf-influenced waters show little impact of shelf processes on the dissolved Ga distribution, and dissolved Ga in the Arctic basins is conserved along isopycnal surfaces. Thus, we explored the potential of Ga as a new parameter in Arctic source water deconvolution. The Ga-informed deconvolution was compared to that generated with the NO3:PO(4)relationship. While distributions of the water masses were qualitatively similar, the Ga-based deconvolution predicted higher amounts of Pacific water at depths between 150 and 300 m. The Ga-based decomposition yields a smoother transition between the halocline and Atlantic layers, while nutrient-based solutions have sharper transitions. A 1-D advection-diffusion model was used to constrain estimates of vertical diffusivity (K-z). The Ga-based K(z)estimates agreed better with those from salinity and temperature than the nutrient method. The Ga-based approach implies greater vertical mixing between the Pacific and Atlantic waters.