A time series of dissolved rare earth elements in the lower Mississippi River

Dissolved rare earth element (REE) concentrations were determined in a 27-month time series of the lower Mississippi River. Overall our results agree with limited previous investigations; i.e., the river shows enrichment of heavy REEs relative to light REEs and also has a significant Ce anomaly. However, the previous investigations relied on only single samples from the river. Our seasonal investigation reveals significant temporal variations in the river's REE chemistry. In particular, we observe significant (5-fold) variations in light REE concentrations. The light REE's follow a similar redox-related seasonality in particle-reactive trace elements. Also, the Ce anomaly shows a corresponding seasonality with greatest fractionation when concentrations are lowest. The time series encompassed a period of extreme flooding in the U.S. Midwest. However, this event did not appear to affect dissolved REE concentrations. The results of this study are in accord with previous observations of transition elements in the lower Mississippi River which suggested the importance of redox processes within the river system in controlling seasonal concentration variability.