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 < 0.02 µm) being least subject to loss, while colloidal Fe (0.02 µm < cFe < 0.2 µm), dissolved Mn (dMn), and non-lithogenic particulate Fe (pFexs) are all lost at similarly rapid rates. In the third decoupling step, once these species are swept >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 < sFe. To assess the role of physicochemical speciation on these trends, we observed that Fe(II) was a small (∼7%) fraction of total dFe in the upper 400 m of the Arctic, even over the shelf (∼2%). Also, colloidal contribution to dFe was very low (∼20%) in the open Arctic, in contrast to dFe in the North Atlantic, which is composed much more by colloids (≥50%). Throughout the Western Arctic Ocean, Fe and Mn are thus decoupled as a result of distinct oxidation kinetics and different scavenging rates within high- and low-particle regimes. As the “scavengers of the sea”, the relative distribution of particulate Fe and Mn phases across the Arctic Ocean shelf and slope, respectively, will play an important role in determining the distribution and ultimate sediment burial site for other scavenging-prone trace elements. Additionally, we suggest that the future effects of climate change, including loss of sea ice that could impact the formation of the halocline, might change distributions of Fe and Mn species in the future Western Arctic.