Measurement of bioactive trace metals (Cu and Zn) in the Southern Ocean : validation of sampling protocol and ICP-MS based analytical method

Cloete, Ryan (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: A method comprising an improved seawater collection protocol and subsequent Inductively Coupled Plasma Mass Spectrometry (ICP-MS) based analytical technique was validated through an intercalibration exercise performed with the University of Plymouth (UK), multiple cross-over stations and analyses of certified reference materials (SAFe, GEOTRACES and NASS-5). The commercially available seaFAST-pico preconcentration module was employed for the simultaneous extraction of a suite of trace metals (Mn, Fe, Ni, Cu, Zn, Co, Cd and Pb) from their seawater matrix prior to ICP-MS analysis. Extremely low detection limits (< 0.228 nmol/kg) combined with low blank values ensured quantitive recovery on ICP-MS and minimal interferences arising from alkali and alkaline earth metals (Na, K, Mg and Ca) present in the saline matrix. The results of the certified reference materials were in excellent agreement with their corresponding consensus values and validated the methods precision and accuracy. During ICP-MS analysis, repeatability and reproducibility were monitored through analysis of an internal Stellenbosch University (SU) TM4 control and various commercially available quality controls, the results of which further confirmed a high level of precision. The distribution of Dissolved Copper (DCu) and Dissolved Zinc (DZn) was investigated in the Atlantic sector of the Southern Ocean. DCu displayed typical nutrient type behaviour reflected by sub-nanomolar surface concentrations increasing steadily until maximum observed concentrations of 2 – 3 nmol/kg in the Antarctic Bottom Waters (AABW). DZn concentrations ranged between approximately 1 and 12 nmol/kg and exhibited characteristic nutrient-type behaviour although intermediate and deepwater distributions were more conservative compared to DCu. Local subsurface minima coincided with elevated levels of chlorophyll-a (chl-a) indicating biological utilisation by phytoplankton in the euphotic zone. Remineralisation of sinking organic matter, predominantly diatom frustules, from Antarctic Surface Water (AASW) resulted in deeper sub-surface maxima for DZn. The dominant supply of trace metals to surface waters south of the Antarctic Polar Front (APF) was advective upwelling of nutrient rich Upper Circumpolar Deep Water (UCDW) and AABW. Atmospheric inputs and melting ice accounted for minor surface influxes where there was a poor DCu/salinity correlation. Both trace elements displayed significant correlations with the macronutrient silica, evidence of their role in the biological cycle. An overall Cu:Si relationship of Cu (nM) = 0.011 Si (μM) + 0.851 (R2 = 0.85, n=98) was obtained for this study while the corresponding Zn:Si relationship was Zn (nM) = 0.043 Si (μM) + 1.021 (R2 = 0.80, n=98). The APF exerted a strong control over nutrient distributions separating low nutrient low chlorophyll (LNLC) subtropical waters to the north from high nutrient low chlororphyll (HNLC) waters to the south. Keywords: Southern Ocean, Dissolved Copper, Dissolved Zinc, GEOTRACES, seaFAST, ICP-MS

AFRIKAANSE OPSOMMING: 'n Metode wat bestaan uit 'n verbeterde seewater versameling protokol en die daaropvolgende Inductively Coupled Plasma Mass Spectrometry (ICP-MS) gebaseer analitiese tegniek is bekragtig deur 'n onderlinge oefening uitgevoer met die Universiteit van Plymouth (UK), verskeie ‘cross-over’ stasies en ontleding van gesertifiseerde verwysingsmateriaal (SAFe, GEOTRACES en NASS-5). Die kommersieel beskikbare seaFAST-pico preconcentration module is aangewend vir die gelyktydige onttrekking van 'n versameling van spoor metale (Mn, Fe, Ni, Cu, Zn, Co, Cd en Pb) van hul seewater matriks voor ICP-MS analise. Uiters lae opsporings perke (<0,228 nmol / kg) gekombineer met 'n lae leegwaardes verseker kwantitatiewe herstel op ICP-MS en minimale inmenging wat voortspruit uit alkalie en alkalie-aard metale (Na, K, Mg en Ca) teenwoordig is in die sout matriks. Die resultate van die gesertifiseerde verwysingsmateriaal was in 'n uitstekende ooreenkoms met hul ooreenstemmende konsensuswaardes en bekragtig die presisie en akkuraatheid van die metodes. Tydens ICP-MS analise, herhaalbaarheid en reproduseerbaarheid is gemonitor deur analise van 'n interne Universiteit Stellenbosch (US) TM4- beheer en verskeie kommersieel beskikbare kwaliteit beheer, die resultate waarvan 'n hoë vlak van akkuraatheid verder bevestig. Die verspreiding van opgeloste Koper (DCu) en opgeloste Zinc (DZn) is ondersoek in die Atlantiese sektor van die Suidelike Yssee. DCu vertoon tipiese tipe voedingstofagtige gedrag weerspieël deur sub-nanomolar oppervlak konsentrasies steeds toenemende tot maksimum waargenome konsentrasies van 2 - 3 nmol / kg in die Antarctic Bottom Waters (AABW). DZn konsentrasies wissel tussen ongeveer 1 en 12 nmol / kg en vertoon voedingstofagtige gedrag hoewel intermediêre- en diepwaterverspreidings meer konserwatief in vergelyking met DCu was. Plaaslike ondergrondse minima saamgeval met verhoogde vlakke van chlorofil-a (Chl-a) dui biologiese benutting deur fitoplankton in die euphotic sone aan. Remineralisasering van sink organiese materiaal, hoofsaaklik diatom frustules van Antarctic Surface Water (AASW) tot gevolg gehad vir ‘n dieper sub-oppervlak maksima vir DZn. Die dominante verskaffing van spoor metale om water oppervlak suid van die Antarctic Polar Front (APF) was advektiewe opwelling van voedingstofryke Upper Circumpolar Deep Water (UCDW) en AABW. Atmosferiese insette en smeltingsys is verantwoordelik vir klein oppervlak strome waar daar was 'n swak DCu / soutgehalte korrelasie. Beide spoorelemente vertoon beduidende korrelasies met die makro-silika, bewyse van hul rol in die biologiese siklus. 'N Algehele Cu:Si verhouding van Cu (nM) = 0,011 Si (μm) + 0,851 (R2 = 0.85, n = 98) is verkry vir hierdie studie, terwyl die ooreenstemmende Zn: Si verhouding was Zn (nM) = 0,043 Si (μM ) + 1,021 (R2 = 0.80, n = 98). Die APF oefen sterk beheer uit oor voedingstof verspreiding wat low nutrient low chlorophyll (LNLC) subtropiese waters in die noorde skei high nutrient low chlorophyll (HNLC) water na die suide. Trefwoorde: Suidelike Yssee, opgeloste Koper, opgeloste Zinc, GEOTRACES, seaFAST, ICP-MS

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