Seasonality of dissolved rare earth elements in the lower Mississippi River

Abstract

Dissolved rare earth element (REE) concentrations were determined in a 27‐month time series of the lower Mississippi River. Overall, the results agree with limited previous investigations; that is, 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. This seasonal investigation reveals significant temporal variations in the river's REE chemistry. In particular, large (approximately fivefold) variations in light REE concentrations are observed. The light REEs follow a seasonality similar to particle‐reactive trace elements. Also, the Ce anomaly shows a corresponding seasonality with greatest fractionation when dissolved 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 compatible with previous observations of dissolved trace elements in the lower Mississippi River which suggested the importance of redox processes within the river system in controlling seasonal concentration variability for many of these elements. Additionally, speciation modeling indicates that carbonate complexes likely dominate the solution chemistry of REEs in the lower Mississippi. The observed behavior of the dissolved REEs is likewise compatible with this speciation.