Iron Isotope Biogeochemical Cycling in the Western Arctic Ocean

The Arctic Ocean is unique, connecting the Atlantic and Pacific basins and being especially vulnerable to the impacts of a changing climate. Iron stable isotopes (δ⁵⁶Fe) provide a unique window into the biogeochemical cycling of Fe in the Arctic. Here we present the first seawater δ⁵⁶Fe for the Western Arctic Ocean, from the 2015 U.S. GEOTRACES GN01 transect. Samples analyzed for δ⁵⁶Fe include seawater dissolved (<0.2 μm), soluble (<∼0.003 μm), and leachable particulate phases. Several key processes were explored using Fe isotopes, each characterized by a distinct combination of δ⁵⁶Fe and Fe concentrations. Input of Fe from reducing continental shelf sediments was characterized by high dissolved Fe concentrations (6.18 ± 4.84 nmol kg⁻¹) and low δ⁵⁶Fe (−1.57 ± 0.66‰). Riverine Fe input observed in the Transpolar Drift Current was characterized by high Fe concentrations corresponding to a riverine end-member Fe concentration of 19 nM, and near-zero δ⁵⁶Fe (0.02 ± 0.23‰) that was similar to that of average crustal material. The deep Arctic was mostly characterized by low Fe concentrations (0.33 ± 0.14 nmol kg⁻¹) and slightly higher δ⁵⁶Fe (0.05 ± 0.30‰), except for samples taken near continental slopes that were affected by sedimentary Fe input with lower δ⁵⁶Fe, and in the Amundsen Basin which showed possible hydrothermal Fe input. Our data thus illuminate Fe biogeochemical cycling processes in the modern Arctic Ocean, and serve as a baseline for understanding how the Arctic Fe cycle responds to climate change.