A comparison of marine Fe and Mn cycling: U.S. GEOTRACES GN01 Western Arctic case study

Dissolved iron (Fe) and manganese (Mn) share common sources and sinks in the global ocean. However, Fe and Mn also have different redox reactivity and speciation that can cause their distributions to become decoupled. The Arctic Ocean provides a unique opportunity to compare Fe and Mn distributions because the wide Arctic continental shelves provide significant margin fluxes of both elements, yet in situ vertical regeneration inputs that can complicate scavenging calculations are negligible under the ice of the Arctic Ocean, making it easier to interpret the fate of lateral gradients. We present here a large-scale case study demonstrating a three-step mechanism for Fe and Mn decoupling in the upper 400 m of the Western Arctic Ocean. Both Fe and Mn are released during diagenesis in porewaters of the Chukchi Shelf, but they become immediately decoupled when Fe is much more rapidly oxidized and re-precipitated than Mn in the oxic Chukchi Shelf water column, leading to Fe hosted primarily in the particulate phase and Mn in the dissolved phase. However, as these shelf fluxes are transported toward the shelf break and subducted into the subsurface halocline water mass, the loss rates of all species change significantly, causing further Fe and Mn decoupling. In the second decoupling step in the shelf break region, the dominant shelf species are removed rapidly via particle scavenging, with smallest soluble Fe (sFe 1000 km offshore with the prevailing current into the low-particle waters of the open Arctic, cFe and dMn appear conserved, while pFe, dFe, and sFe are very slowly removed with variable log-scale distances of transport: pFe ≪ dFe