Organic Iron-binding ligands in the Arctic, Antarctic and subtropical Regions

Abstract

Organic ligands determine the threshold of dissolved-Fe (DFe) concentrations in seawater. Understanding the processes controlling the dynamics of natural organic ligands is of fundamental importance towards a better understanding of the impact of global warming on the Fe cycle. However, there is a paucity of Fe speciation data to comprehensively assess the effects of global climate change on the biogeochemical cycle of Fe. Therefore, this thesis reports organic ligand properties in the oceanic regions that are vulnerable to global climate change and in an area subject to temporal variability. This thesis used an electrochemical approach, competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE-AdCSV). This method is an indirect method to determine organic ligand properties, the concentrations ([Lt] in nM equivalents of Fe (nM eq. Fe) and the conditional stability constant ("K" _"Fe'L" ^"cond" in M-1; it is shown in logarithmic value, log "K" _"Fe'L" ^"cond" ). The study of dissolved organic Fe-binding ligands in Fram strait reveals that the binding strength of organic ligands regulates the export of DFe from the largest glacier of northeast Greenland. A high [Lt] (up to ~3 nmol eq. Fe), yet with relatively weak binding strength, is found in the vicinity of the glacier terminus. These relatively weak ligands compete less efficient against scavenging and precipitation, relative to the stronger ligands. In the waters adjacent to the western Antarctic Peninsula, Fe-binding ligand concentrations and characteristics are investigated in different hydrographic regions. Under sea ice cover near the coast, organic ligand production is linked to ice-associated algae. Sediment-water interaction and resuspension result in high concentrations of DFe and ligands in upwelling deep waters over the continental slope. Towards the open ocean, phytoplankton blooms deplete nutrients and iron, while actively or passively producing organic ligands. The presence of fronts affects the distribution of the ligands, marking the border between shelf derived ligands and bloom-associated ligands offshore. The Hauraki Gulf in New Zealand, serves as a natural laboratory to study DFe speciation in a highly variable system, where changes of the microbial ecosystem can have a profound effect on DFe speciation. In the shelf regions, [Lt] was relatively high (~2.5 to 7.5 nM eq. Fe). The high [Lt] enabled relatively high DFe to persist on the shelf, and consequently, increased the potential export of DFe from the shelf to the open ocean, where Fe can be an important driver of primary productivity. This thesis reports the concentration, distribution, and characteristics of Fe-binding ligands in different ocean regions with distinct environmental circumstances, adding a valuable data set to expand our current knowledge about global Fe speciation. The study areas in Fram strait and the Northeast Greenland shelf, the Western Antarctic Peninsula, as well as the Hauraki Gulf, all have distinct hydrographic settings. The study areas in the high latitude oceans are particularly susceptible to changes in climate, and our results help provide the fundamental overview that is needed to predict the consequences of climate change on the Fe biogeochemical cycle in these globally important high latitude oceans.