Dissolved cadmium and cadmium stable isotopes in the western Arctic Ocean

Abstract

Abstract The Arctic Ocean is a region of great scientific interest, particularly considering the rapid rate at which temperature and sea-ice coverage have changed over the past decade and the projections under various climate scenarios. In this context, the marine nutrient-like element cadmium (Cd) and its stable isotope ratios (δ114Cd) provide valuable insights into the modern physical and biological processes in the Arctic. However, few data on the Arctic are available because of the difficulty of accessing this region. Here, we present measurements of dissolved Cd and δ114Cd, and leachable particulate Cd, in the Western Arctic Ocean during the U.S. GEOTRACES GN01 cruise. Broadly, the Arctic Ocean reflects mixing between Pacific and Atlantic end-members. Waters from the Bering Sea just outside the Arctic in the North Pacific have the lightest deep water δ114Cd values yet reported and some of the highest Cd concentrations (∼+0.17‰; ∼1.00 nmol kg−1), reflecting the buildup of isotopically light Cd along the ocean conveyor belt. Conversely, waters in the deep Arctic have the lowest Cd concentrations and highest δ114Cd of any deep ocean (∼0.2 nmol kg−1; ∼+0.5‰) reflecting input of waters from the North Atlantic. More subtle features of mixing can also be observed, for example within the Arctic near the Bering and Chukchi Shelves, where a tongue of high-Cd, low δ114Cd Pacific water lies below the surface. In the surface water, we observe low Cd and higher δ114Cd, reflecting dilution by low-Cd sea ice melt as well as biological uptake in the surface Arctic and over the Bering shelf as waters flow in from the Pacific. Surface water δ114Cd decreases towards the North Pole under the permanent ice zone due to mixing with North Atlantic waters. Other processes of note inferred from our data include Cd input from Bering and Chukchi shelf sediments and an unexplained mechanism causing high δ114Cd found in intermediate waters over the Chukchi Abyssal Plain. Finally, we observe waters in the deep Arctic with lower Cd/PO4 than observed anywhere else in the global ocean, perhaps reflecting differences in North Atlantic circulation and nutrient cycling during the Little Ice Age when the waters were formed, an input of dissolved PO4 originating from PO4 scavenged onto sedimented Fe oxyhydroxide particles, or an input of dissolved PO4 from sedimentary weathering of glacial minerals. Our data provide a means for better understanding biogeochemical cycling in the modern Arctic as well as a baseline for comparison with future biogeochemical change.