GEOTRACES intercalibration of neodymium isotopes and rare earth element concentrations in seawater and suspended particles. Part 2: Systematic tests and baseline profiles
Limnol. Oceanogr. Methods 10:252-269 (2012) | DOI: 10.4319/lom.2012.10.252
ABSTRACT: The large-scale collection of seawater samples and analysis of trace elements and isotopes by the GEOTRACES program call for careful comparison and examination of different methods for sample collection, treatment, and processing. Here we report results for different filter types, sampling bottles, and preconcentration methods for Nd from seawater. Neodymium isotope ratios and concentrations of unfiltered and filtered seawater using five different filter types agree within 43-56 ppm (2σ standard deviation, SD) and 10% (relative 2σ standard deviation), respectively. The filter choice therefore has no effect on the measured parameters at the studied locations in the western North Atlantic. This could well be different, however, in areas of high particle concentrations. We therefore strongly recommend filtration of seawater for Nd isotope and rare earth element concentration measurements. Dissolved 143Nd/144Nd ratios of seawater collected with Niskin bottles with internal nylon-coated stainless steel springs and trace metal-clean GO-FLO bottles were the same within 43 ppm (2σ SD). The Niskin bottles used here are therefore suitable for the collection of seawater samples for the analysis of 143Nd/144Nd ratios. Iron coprecipitation and complexation with HDEHP/H2MEHP using C18 cartridges for the preconcentration of Nd from seawater yielded the same 143Nd/144Nd ratios within 37 ppm (2σ SD). Both methods are therefore appropriate. Water column profiles of 143Nd/144Nd ratios and Nd concentrations are reported for the North Atlantic (BATS) and North Pacific (SAFe), and profiles of all rare earth elements are reported for BATS. These baseline profiles are recommended for continued quality control and intercalibration by future GEOTRACES cruises.