Complete separation of rare earth elements from small volume seawater samples by automated ion chromatography: method development and application to benthic flux

A new Ion Chromatography–Inductively Coupled Plasma Mass Spectrometry (IC–ICP-MS) method is presented for measuring individual rare earth elements (REEs) in small volume (5 ml) seawater samples, with the goal of measuring pore water REEs. The method is shown to be accurate and reproducible for small volumes of a seawater reference standard within the determined precisions. The method has reasonable yield (75%), and, more importantly, is shown to not internally fractionate individual REEs. After describing the method, data from bottom, overlying and interfacial pore waters from four sites (three California margin, one Chile margin) are presented. These data show that there is a REE flux from sediments into bottom water. This flux ranges from 10−11 to 10−8 mol cm−2 ky−1 at these sites and is, therefore, on the same order as riverine input to the oceans. Benthic chamber samples support these flux estimates. When pore water and benthic chamber water are normalized to PAAS, the REE patterns can be characterized using three descriptors: the magnitude of the Ce anomaly (always negative), a “MREE bulge” and “HREE enrichment.” However, when normalized to bottom water, LREE>HREE and there is a significant positive Ce anomaly. These features generally support current understanding: that the REEs in the ocean are strongly influenced by organically driven processes. A key proviso is that the fluctuations in the Ce anomaly should be considered to be the net result of changes in dissolved La and Pr, and not mainly a function of Ce oxidation.