| Summary: | Thesis (PhD)--Stellenbosch University, 2013. === ENGLISH ABSTRACT: This thesis investigates questions surrounding the role that iron-rich colloids (nominally sized
between 0.02 μm and 0.2 μm) and particulates (>0.2-0.45 μm) play in the context of the greater iron
biogeochemical cycle. To this end, this study complements a review of reported size-fractionated
iron (Fe) measurements with chemical and mineralogical data derived from synchrotron-based xray
measurements. 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-like
behaviour. Spatial variability in surface ocean colloidal iron concentration is primarily a function of
total iron supply, although the concentration and strength of iron-binding ligands, and inorganic
thermodynamic constraints are additional influential factors. The size-fractionated study of colloidal
Fe has rendered considerable evidence pointing towards direct or indirect biological utilization of
this cFe pool; however, a more complete understanding of cFe-biological interaction necessarily
requires better knowledge of cFe chemistry and mineralogy.
To address these issues, this thesis documents the development of a novel x-ray microscopy and
spectroscopy technique for determining the Fe speciation of individual Fe-rich particles under
environmental conditions. Variations in the peak splitting in iron L3-edge XANES (X-ray
Absorption Near-Edge Structure) spectra reflect changes in the local coordination environment
surrounding the metal centre. Specifically, the energy splitting ( ΔeV) and intensity ratio of the split
peaks at the L3-edge vary as a function of the Fe valence state, the number and chemistry of
coordinating ligands and polyhedral distortion effects; and combinations of the two parameters are
found 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 ocean
basins; the Southern Ocean and the south western Pacific Ocean. Speciation differences in Fe
particles collected from the Southern Ocean show distinct compositional trends between the coasts
of South Africa and Antarctica, with different Fe pools associated with the different oceanographic
frontal zones. Despite the oxygenated nature of the seawater sampled, the presence of significant
particle-hosted Fe(II) was observed in both the Southern Ocean at high latitudes, and at sampling
sites proximal to the Kermadec Ridge in the Pacific Ocean. Ferrous iron particles at the latter study
area were shown to be strongly associated with carbon functional groups, notably alcohol and
carboxamine moieties. These findings, relating to particle chemical differences and associations with organic matter, have significant implications for our understanding of particle behaviour, their
surface interactions and the role that they play in primary productivity and global elemental cycles. === AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek kwessies met betrekking tot die rol wat ysterryke kolloïede (van nominale
groottes tussen 0.02 μm en 0.2 μm) en partikels (>0.2-0.45 μm) in die konteks van die groter
ysterbiogeochemiese siklus speel. Vir hierdie doel bou die studie voort op ’n oorsig van aangemelde
grootte-gefraksioneerde yster- (Fe-)metings met behulp van chemiese en mineralogiese data wat uit
sinchrotrongebaseerde x-straalmetings verkry is. Na aanleiding van ’n uitvoerige literatuurstudie,
blyk die globale poel kolloïdale yster (cFe) op die see-oppervlak hoogs dinamies te wees en toon dit
dikwels seisoenale tendense en voedingstofagtige gedrag. Ruimtelike veranderlikheid in die cFekonsentrasie
op die see-oppervlak is hoofsaaklik ’n funksie van totale ystervoorsiening, hoewel die
konsentrasie en sterkte van ysterbindende ligande sowel as anorganiese termodinamiese beperkings
ook ’n invloed kan hê. Die grootte-gefraksioneerde studie van kolloïdale Fe het beduidende bewyse
opgelewer wat op die direkte of indirekte biologiese benutting van hierdie cFe-poel dui. Tog verg ’n
vollediger begrip van cFe- biologiese interaksie noodwendig meer kennis van die chemie en
mineralogie van cFe.
Om hierdie kwessies te ondersoek, dokumenteer hierdie tesis die ontwikkeling van ’n innoverende
X-straalmikroskopie- en X-straalspektroskopietegniek om die Fe-soortvorming van individuele Feryke
partikels in omgewingsomstandighede te bepaal. Variasies in die pieksplitsing van yster-L3
rand-XANES- (“X-ray absorption near-edge structure”-)spektra weerspieël veranderlikheid in die
lokale koördinasie-omgewing rondom die metaalkern. In die besonder wissel die energiesplitsing
( eV) en intensiteitsverhouding van die splitsingspieke by die L3-rand na gelang van die Fevalensietoestand,
die getal en chemie van koördinasie-ligande, en poliëdriese distorsie-effekte, en
kombinasies van die twee parameters blyk kenmerkend van individuele Fe-minerale te wees. Om Fe-soortvorming te verstaan, is die stipping van Δ eV versus intensiteitsverhouding suksesvol
toegepas op ’n verskeidenheid Fe-omgewingspartikels (groter as 20 nm in deursnee) wat uit twee
oseaankomme – die Suidelike Yssee en die suidwestelike Stille Oseaan – bekom is. Soortverskille
in Fe-partikels wat uit die Suidelike Yssee bekom is, toon kenmerkende samestellingspatrone tussen
die kus van Suid-Afrika en Antarktika, en verskillende Fe-poele word met die verskillende
oseanografiese frontsones verbind. Ondanks die suurstofhoudende aard van die seewatermonsters,
is beduidende Fe(II) in partikels opgemerk in die Suidelike Yssee by hoë breedteliggings sowel as
op studieterreine naby die Kermadec-rif in die Stille Oseaan. Ysterhoudende partikels van
laasgenoemde studieterrein het ’n sterk verband met koolstof- funksionele groepe, veral alkohol en karboksamien, getoon. Hierdie bevindinge met betrekking tot die chemiese verskille tussen
partikels en die verband met organiese materie het beduidende implikasies vir ons begrip van
partikelgedrag, die oppervlak-interaksies van partikels, en die rol wat dit in primêre produktiwiteit
en globale elementsiklusse speel.
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