Rare earth element behavior in springs and streams on San Cristobal Island, Galapagos


Dissolved Rare Earth Element (REE) fractionation mechanisms in freshwater samples have been parameterized by lab experiments and measured in major world rivers. To confirm whether results from controlled lab experiments are reciprocated in small-scale natural systems, I analyzed REE contents in water samples from groundwater springs and streams on San Cristóbal Island, Galápagos. The island is comprised of essentially a single lithological unit (Ocean Island Basalt) and exhibits a steep hydrological gradient on the windward slope over a short spatial extent which results in a wide range of physicochemical and dissolved mineralogical properties of water. This allows for the investigation of factors exerting primary control on REE fractionation without the influence of changing lithology. Samples show a wide range of pH (5.5-8.2) associated with changes from Mn-dominated dissolved fractions to Fe-dominated as pH increases. Local basalt-normalized REE patterns exhibit a wide range (La/Ybb = 0.41-1.78 and Ce/Ce*b = 0.58-2.12) for the small areal extent of the watershed (~ 13 sq. km). Comparisons between “dissolved” load (<0.45μm) and “soluble” load (<0.02μm) data demonstrate the importance of colloidal-sized metal species in aqueous solution. The difference of REE results between the "dissolved" load data are attributed to contrasting fractionation from opposing aqueous mineralogical systems (Mn vs. Fe) present across the watershed, consistent with laboratory studies.Bachelor of Scienc

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