THE INFLUENCE OF GLACIER COVER ON IRON AND MANGANESE CYCLING IN PATAGONIAN FJORDS (Invited Presentation)

Fjords are dynamic interface zones between fresh and marine waters and are 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) and manganese (Mn) play important roles in the carbon cycle due to their importance as micronutrients for marine biota, complexation and association with macronutrients (e.g. organic C and P), and their 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 Fe 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 Fe inputs downstream is debated due to rapid removal from surface water at low salinities. Furthermore, little information is available on Mn concentrations, speciation and cycling in glacial meltwaters and downstream fjords.

The coastline of Chilean Patagonia is dominated by fjords, spanning over 14 degrees of latitude, and receiving freshwater inputs from pristine rivers draining hydrological catchments 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 complementary suite of surface water measurements (including size fractionated concentrations, reactive particulate phase concentration, Fe isotope measurements, electron microscopy, organic matter composition and synchrotron X-ray absorption spectroscopy) from 4 Patagonian fjord systems and 33 rivers, spanning a large latitudinal range and with catchments of no to high glacial cover, to elucidate the importance of glacial inputs in fjord Fe and Mn cycling and export further offshore. We highlight the importance of particulate and organic carbon complexed phases in sustaining high surface water concentrations of Fe and Mn, and contrasting behaviour of Mn and Fe from freshwater input to the ocean.