Fjords are dynamic interface zones between fresh and marine waters and are hypothesised to be hotspots of carbon burial. However, the importance of fjords as biogeochemical reactors remains uncertain, in part because they are relatively understudied compared to other aquatic critical zones. Specifically, few fjords are studied in the context of trace element cycling. Iron (Fe) in particular plays an important role in the carbon cycle due its importance as a micronutrient for marine biota, its complexation and association with macronutrients (e.g. with organic C, and P), and its influence for carbon burial in sediments (e.g. the “Rusty Carbon Sink”). Glaciers are major contributors to fjord freshwater and sediment budgets, and have been postulated to be a significant source of iron to downstream environments. Turbid glacial meltwaters carry elevated concentrations of labile (nano)particulate and dissolved (<0.45 µm) Fe that may be directly or indirectly available to biota, and tidewater glaciers drive a “meltwater pump”, upwelling fjord bottom waters to the surface. However, the impact of these meltwater inputs downstream is debated due to rapid removal of iron from surface water at low salinities.
Fjords dominate the coastline of Chilean Patagonia, spanning over 14 degrees of latitude, and include freshwater inputs from pristine rivers draining regions of variable glacial cover. The region therefore provides a natural laboratory to test hypotheses relating the importance of glacial cover on fjord biogeochemical cycles. Here we combine a diverse suite of surface water and benthic measurements (including size fractionated concentrations, reactive particulate phase concentration, Fe isotope measurements, microscopy and organic matter molecular composition) from 4 Patagonian fjord systems and 33 rivers, spanning a large latitudinal range and variable glacial cover, to elucidate the importance of glacial inputs in fjord Fe cycling and export further offshore. We highlight the importance of particulate and organic carbon complexed iron in sustaining high surface water concentrations, and large benthic fluxes with a distinctive Fe isotopic signature.