Multi-elemental tracers in the Amerasian Basin reveal interlinked biogeochemical and physical processes in the Arctic Ocean Upper Halocline

Key Points:  

● Chemical tracer relationships to salinity help define the role of Pacific water, rivers, sediments on the biogeochemistry of the halocline.  

● Dissolved Mn, V, Cd, Cu, Cd and Co are scavenged by MnO2; scavenging in the halocline is likely an important sink for these constituents. 

● Age of halocline waters (10-20 years) is consistent with circulation of the Beaufort Gyre. 

Abstract: The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient-rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient-poor) Atlantic-derived water below. Biogeochemical signals entrained into the Upper Halocline Layer result from Pacific Waters modified by sediment and river influence on the shelf. 

In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi-tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf-influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). 

Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, DOC), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation.