Glacial controls on iron and manganese concentration and lability in an Arctic fjord

Abstract. Retreating glaciers are altering sediment and micronutrient fluxes to coastal waters, yet the magnitude and environmental impacts of change remain unclear. Here we examine how contrasting glacier systems in an Arctic fjord control the export and speciation of dissolved and particulate iron (Fe) and manganese (Mn) by comparing locations influenced by marine-terminating and land-terminating glaciers. Data was collected from two glacierized bays in Hornsund, a southern Svalbard fjord, during research cruises conducted in 2022 and 2023. Highly reactive ascorbate‑extractable particulate Fe was a major component of the Fe pool in both systems, exceeding dissolved Fe even beyond the inner coastal zone, indicating that reactive particulate Fe is transported farther offshore than previously assumed. The marine-terminating glacier influenced bay exhibited substantially higher concentrations of reactive particulate Fe and Mn, elevated dissolved Mn, and sedimentation rates one to two orders of magnitude greater than in the bay with a land‑terminating glacier. These differences might be a result of higher subglacial discharge, a larger glacier cover with enhanced chemical weathering, and a higher abundance of freshly weathered mineral surfaces in the marine-terminating glacier influenced bay. In contrast, the input of suspended particulate matter and reactive micronutrient phases from the land‑terminating system was lower, as advanced glacial retreat has led to stabilisation of proglacial sediments and the glacier is smaller. Based on this dataset, coastal micronutrient inputs are likely to decline as glaciers continue to retreat, due to falling glacier discharge and supply of reactive sediment enriched in reactive micronutrients.