The europium anomaly of seawater: implications for fluvial versus hydrothermal REE inputs to the oceans

Seafloor hydrothermal vent fluids are characterized by rare earth element (REE) enrichments and large positive Eu anomalies; however, this input is not recorded by seawater. Hydrothermal sediments from the southeast Pacific (DSDP Site 598) were examined to address this discrepancy between the observed and predicted REE pattern of seawater. “Scavenging indices”, which reflect long-term REE scavenging by iron-rich hydrothermal precipitates, were calculated for each of eight REE's. Comparisons between these indices and the REE content of seawater indicate that the intensity of REE scavenging by hydrothermal precipitates is proportional to the concentration of REE's in the fluid phase. We predict that REE's are rapidly scavenged from hydrothermal solutions, and that Eu is removed disproportionately faster relative to its neighbors. Eu-anomaly values for Site 598 and other hydrothermal sediments support this prediction: a vent signature is recorded in sediments closest to the ridge axis and the Eu-anomaly values become more seawater-like with increasing distance and scavenging. This implies that the REE content of seawater cannot be used to constrain hydrothermal versus fluvial fluxes to the oceans.