Gallium: A New Tracer of Pacific Water in the Arctic Ocean. Issue 7 (25th June 2020)

• Record Type: Journal Article
• Authors: Whitmore, Laura M.; Pasqualini, Angelica; Newton, Robert; Shiller, Alan M.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 7(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-06-25
• Subjects: GEOTRACES -- Arctic Ocean -- water mass tracer -- trace elements -- gallium; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015842
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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 :PO4 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 (Kz ). The Ga‐based Kz 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. Plain Language Summary: The Arctic Ocean is a key player in global ocean circulation and climate. The waters of the Arctic Ocean are influenced by a number of sources including seawater from both the Atlantic and Pacific oceans, sea‐ice formation and melting, and river input. Unraveling these source waters is important both for deciphering the inflow of nutrients and for other materials to the Arctic Ocean as well as understanding the basin's contribution to oceanic overturning circulation and the impact of ocean heat input to this basin on Arctic sea ice. Finding methods for unraveling these sources and how they may be altered by ongoing environmental change is thus an important research objective. Previous methods for distinguishing Pacific from Atlantic water in the Arctic Ocean have relied on nutrient‐based tracers that could be biased by poorly quantified biological processes. Herein, the authors demonstrate that gallium (Ga), a metal that primarily enters the ocean from dust, can be used in place of nutrient‐based tracers to yield a potentially more realistic distinction between the two ocean water sources. Key Points: Dissolved gallium can be used as a tracer to determine the percentage of Pacific‐ and Atlantic‐derived seawater in the Arctic Ocean The deconvolution using gallium predicts greater amounts of Pacific water between roughly 150 and 300 m than a nutrient‐based deconvolution Distribution of Pacific waters in the western Arctic is more vertically mixed than previously predicted with nutrient tracers