Dissolved iron and manganese in the Canadian Arctic Ocean: on the biogeochemical processes controlling their distributions

Abstract: Dissolved iron (DFe) and manganese (DMn) are essential micronutrients involved in vital phytoplankton physiological pathways, and their deficit can limit primary production in otherwise nutrient-replete surface ocean waters. In this work we present the spatial distributions and biogeochemical cycling of these metals across the Canadian GEOTRACES transect in the Canadian Arctic Ocean during the summer and autumn of 2015. Surface concentrations are dominated by freshwater inputs showing a strong negative correlation with salinity, especially for DMn which behaves more conservatively than DFe. The highest surface concentrations were measured in the Canadian Arctic Archipelago (Fe: 0.401–1.91 and Mn: 4.33–9.54 nmol kg−1) and the Canada Basin (Fe: 0.225–0.479 and Mn: 3.93–7.02 nmol kg−1), regions highly influenced by riverine inputs, whereas the lowest values were found in the Labrador Sea (Fe: 0.106–0.362 and Mn: 0.450–1.09 nmol kg−1) where freshwater inputs diminished and phytoplankton uptake increased. Subsurface and deep water distributions for both metals are largely controlled by a complex balance between sources (advective inputs and organic matter remineralization) and removal processes. The subsurface peaks (∼100–300 m) observed in the Canada Basin (Fe: 0.541 ± 0.060 and Mn: 1.38 ± 0.42 nmol kg−1) and Baffin Bay (Fe: 0.753–1.03 nmol kg−1) were advected from the Chukchi Sea and the Canadian Arctic Archipelago respectively, where DFe and DMn are released from the benthic boundary layer in these shelf-dominated environments. Advective sources associated with the Arctic Circumpolar Boundary Current, rather than vertical fluxes of DFe and DMn in sinking particles, dominate metal distributions in the deep Canada Basin waters (>300 m). In the highly productive Baffin Bay and the Labrador Sea, organic matter remineralization is a notable source of DFe and DMn to deep waters. In the deepest waters (>1000 m), scavenging of DFe and DMn govern their vertical distributions; a pseudo-first order scavenging model explained the continuous removal of DMn in the Canada Basin, where the concentrations reach uniformly low concentrations (0.150 ± 0.004 nmol kg−1) after ∼400 years. Applying this DMn scavenging model we were able to estimate the age (120–190 years) of deep Baffin Bay waters, a topic of discussion for many years.