Rare earth element behavior in springs and streams on a basaltic island: San Cristóbal, Galápagos

Abstract

Dissolved Rare Earth Element (REE) fractionation mechanisms in freshwater environments have been parameterized by laboratory experiments and measured in major world rivers. To investigate the relationship between such observations and small-scale natural systems, we analyze REE contents in water samples from groundwater springs and streams on San Cristóbal Island, Galápagos. The island is comprised of a single lithology (Ocean Island Basalt) and exhibits a steep precipitation gradient on the windward slope over a short spatial extent, which results in a wide range of physicochemical properties and regolith weathering intensities. This allows us to investigate primary controls of REE fractionation without the influence from varying lithology. Samples show a wide range of measured pH, as well as concentrations of Mn and Fe. Local basalt-normalized REE patterns exhibit a wide range ([Nd/Yb]b = 0.19–1.21 and [Ce/Ce*]b = 0.65–2.22) for the small areal extent of the watersheds (~13 km2). Comparisons between data from 0.45 μm to 0.02 μm filtered samples demonstrate the importance of colloidal-sized (0.02–0.45 μm) metal species in aqueous solution. Our results suggest that two mechanisms are responsible for the observed REE fractionation in this catchment: pH-driven changes in aqueous complex stability by organic or anionic complexes and the preferential adsorption of heavy rare earth elements (HREE) onto Fe- and Mn- oxyhydroxides at low pH. This contrasts with moderate enrichment of light rare earth elements (LREE) observed at higher pH. These results suggest that several proposed REE fractionation mechanisms established previously in laboratory experiments can be observed in a natural watershed system, despite being obscured in larger world rivers. The homogenous lithology and strong climate gradient of San Cristóbal island allow for the observation of REE fractionation mechanisms which are usually obscured in larger world rivers despite being predicted in laboratory experiments, such as a positive Ce anomaly associated with HREE enrichment in low pH waters.