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Sources of Ga, In, Zr and Hf to the coastal waters of the California current system Lanthier, Kira

Abstract

A method developed for the determination of picomolar levels of Ga, In, Zr and Hf in seawater is used to study the sources of these hydrolysis-dominated elements to the coastal surface waters of the California Current System (CCS), focussing mainly on the Columbia River and upwelling regions along the western coast of the United States. This thesis provides the first detailed measurements of Ga, In, Zr and Hf in the CCS. A resin extraction/preconcentration step featuring Chelex-100 was used to achieve -1000- fold concentration factors. Metals were quantified by flow injection-ICP/MS following addition of a Rh internal standard. Metal recoveries for Ga, In, Zr and Hf were 89, 97, 38 and 36%, respectively. Detection limits for one liter samples were 0.24, 0.012, 6 and 0.25pmol/kg, respectively, Ga, Zr and Hf being blank-limited while In is limited by sensitivity. Although this procedure was well suited for In, the data acquired was not usable as mass-115 was not corrected for isobaric Sn interferences, which were found to be significant. Results from this study revealed that the Columbia River is a significant source of Ga, Zr and Hf to CCS surface waters, with maximum values ([Ga]:=49pM; [Zr]=86pM; fHf]=6pM) corresponding to the lowest salinity waters sampled (S=21). Ga and Zr traces were still detectable as far south as the Oregon-California border ([Ga]~30pM; [Zr]~50pM) therefore these two metals are good tracers of the Columbia River effluent. Dissolved metal/salinity plots of Ga and Zr show evidence of removal in these coastal waters, with -30 and 50%, respectively, of the dissolved Ga and Zr in the plume core (S~21) being removed in the region sampled. Estimated minimum river concentrations for Ga (150±30pM) and Zr (240±150pM) were comparable to other previously reported rivers ([Ga]=68-250pM; [Zr]=300-1834pM). Upwelling of deeper waters is also a moderate source of Ga (7-14pM), Zr (24-65pM) and Hf (0.6-1.5pM) to the background levels of the CCS ([Ga]=5-8pM; [Zr]=16-34pM; [Hf]=0.3- 0.6pM). Particularly for Ga and possibly also for Zr, concentrations in the upwelled waters were highest in areas overlying wider (>10km) continental shelves, revealing shelf sediments as a significant source of metals to these bottom waters. Zr/Hf molar ratios of all samples from this study (Zr/Hf~44±40) indicate that the previously-observed marine fractionation between these two elements (Zr/Hf~100-300) does not arise from processes originating in coastal environments.

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