Rare earth elements and their isotopes in the ocean

The rare earth elements (REEs), comprising 15 lanthanide elements, are an extremely coherent group in terms of chemical behavior and have recently been subjected to intense investigation in the field of marine geochemistry. Since the GEOTRACES program began in 2006, comprehensive new techniques to determine REEs and their isotopes in ocean environmental samples have been developed, which have substantially reduced the sample volumes and detection limits compared with the methodology conducted in 1980s. Due to the development of new techniques, the distributions of REEs and their isotopes have been widely investigated in the world oceans, including the North Pacific, Southern Ocean, and Indian Ocean. REEs and their isotopes are commonly used for tracing various oceanic processes. For example, since REEs are quasi-conservative, their patterns and ratios have recently been used as water mass tracers to identify and quantify the mixing of various water masses. Also, the fractionations of REEs are useful for understanding marine biogeochemical processes. Anomalous behavior of Eu is often encountered in hydrothermal fluids. Ce anomalies arising from oxidation-reduction reactions as well as light/heavy REE ratios from their differences in particle reactivity have been used to investigate the particle/solution interactions of trace elements in seawater. In addition, recent investigations have indicated the enrichment of anthropogenic Gd anomalies in rivers and coastal waters, based on using Gd chelates as contrast agents in magnetic resonance imaging.