Distribution and characterization of marine iron-rich particles

[摘要] ENGLISH ABSTRACT: This thesis investigates questions surrounding the role that iron-rich colloids (nominally sizedbetween 0.02 μm and 0.2 μm) and particulates (>0.2-0.45 μm) play in the context of the greater ironbiogeochemical cycle. To this end, this study complements a review of reported size-fractionatediron (Fe) measurements with chemical and mineralogical data derived from synchrotron-based xraymeasurements. From an extensive literature review, the global surface ocean colloidal iron (cFe)pool is found to be highly dynamic, frequently exhibiting seasonal trends and nutrient-likebehaviour. Spatial variability in surface ocean colloidal iron concentration is primarily a function oftotal iron supply, although the concentration and strength of iron-binding ligands, and inorganicthermodynamic constraints are additional influential factors. The size-fractionated study of colloidalFe has rendered considerable evidence pointing towards direct or indirect biological utilization ofthis cFe pool; however, a more complete understanding of cFe-biological interaction necessarilyrequires better knowledge of cFe chemistry and mineralogy.To address these issues, this thesis documents the development of a novel x-ray microscopy andspectroscopy technique for determining the Fe speciation of individual Fe-rich particles underenvironmental conditions. Variations in the peak splitting in iron L3-edge XANES (X-rayAbsorption Near-Edge Structure) spectra reflect changes in the local coordination environmentsurrounding the metal centre. Specifically, the energy splitting ( ΔeV) and intensity ratio of the splitpeaks at the L3-edge vary as a function of the Fe valence state, the number and chemistry ofcoordinating ligands and polyhedral distortion effects; and combinations of the two parameters arefound to be characteristic of individual Fe minerals.To understand Fe speciation, the Δ eV versus intensity ratio plot was successfully applied to a variety of environmental Fe particles (greater than 20 nm diameter) collected from two oceanbasins; the Southern Ocean and the south western Pacific Ocean. Speciation differences in Feparticles collected from the Southern Ocean show distinct compositional trends between the coastsof South Africa and Antarctica, with different Fe pools associated with the different oceanographicfrontal zones. Despite the oxygenated nature of the seawater sampled, the presence of significantparticle-hosted Fe(II) was observed in both the Southern Ocean at high latitudes, and at samplingsites proximal to the Kermadec Ridge in the Pacific Ocean. Ferrous iron particles at the latter studyarea were shown to be strongly associated with carbon functional groups, notably alcohol andcarboxamine moieties. These findings, relating to particle chemical differences and associations with organic matter, have significant implications for our understanding of particle behaviour, theirsurface interactions and the role that they play in primary productivity and global elemental cycles.

[发布机构] Stellenbosch University