Distribution and characterization of marine iron-rich particles

• Main Author: Von Der Heyden, Bjorn Phillip
• Other Authors: Roychoudhury, Alakendra N.
• Format: Others
• Language: en_ZA
• Published: Stellenbosch : Stellenbosch University 2013
• Subjects: Colloids; Biogeochemical cycles; Dissertations > Earth sciences; Theses > Earth sciences
• Online Access: http://hdl.handle.net/10019.1/85822

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.