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 particlereactive 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. sampling and filtration (0.4 :m Nuclepore polycarbonate) protocols were followed as described previously (Shiller 1997). REE’s were determined by ICP-MS (Finnigan MAT Element 2) using a desolvating microconcentric nebulizer (CETAC Aridis). The desolvating nebulizer minimizes oxide formation which can cause severe interferences with ICP-MS analysis of REE’s. Some samples were quantified using standard additions. In other instances, the samples were spiked with enriched isotopes of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb, extracted using an organo-phosphate chelator bound to a polymer substrate, and quantified by isotope dilution. 3 RESULTS AND DISCUSSION In general, analyses performed by standard additions agreed well with those performed by isotope dilution/extraction. We did note, however, an unresolved interference on Gd-157 analyses performed without extraction. Eu analyses without extraction tended to suffer from a BaO interference of approx. 2 pM due to the high concentrations of Ba in Mississippi River water. Our results agree well with previous single sample REE determinations from the lower Mississippi River (Goldstein & Jacobsen 1988, Sholkovitz 1995). In particular, Sholkovitz’ August 1993 sample was obtained in the same time period as our time series and shows dissolved REE concentrations nearly identical to our samples collected in July and August 1993. As has been observed previously, the data show a pronounced heavy REE enrichment in river water as compared with a shale composite. In our time series, the light REE’s show an annual cycle similar to that previously observed for Mn, Fe, Zn and Pb (i.e., highest concentrations during the winter). Concentrations of light REE’s can vary seasonally by a factor of 5 or more. In contrast, the heavy REE’s show much less seasonal variability (less than 2-fold concentration variability) and are even slightly negatively correlated with the light REE concentrations. This results in the “light-toheavy” ratio (e.g., shale-normalized Nd/Yb) showing a seasonal pattern similar to that of the light REE’s (e.g., Figure 1). The Ce anomaly (ratio of observed Ce concentration to that predicted from the concentrations of its neighboring elements) is typically 0.35, indicating preferential Ce removal. The Ce anomaly also varies seasonally, with higher values in winter when light REE concentrations are highest. No Gd or Eu anomaly was observed. The time series included samples collected when waters from record floods on the Missouri and upper Mississippi Rivers reached our sampling location at Baton Rouge. However, no effect was observed on the seasonal patterns of REE concentrations or ratios. The REE observations described here are in accord with and lend confidence to previous interpretations of seasonal variability of dissolved trace elements in the lower Mississippi River (Shiller 1997). The light REE’s, being the more particle-reactive part of the series, show a seasonality similar to that of other particle-reactive trace elements. And REE’s, like many other previously determined trace elements, seem to respond more strongly to biogeochemical effects than to hydrological events such as floods. Cerium shows its least fractionated behavior relative to its neighbors during winter, indicating either less oxidation (and scavenging removal) or greater reduction of this element at this time of year. This is in accord with the observations of other transition elements (such as V and Mo) indicating greater reduction during winter. (Note that this reduction most likely occurs upstream in stratified lakes and reservoirs, the river itself never being oxygen-depleted.) Nonetheless, the Ce anomaly shows less relative variation than Ce concentrations, suggesting that adsorption of Ce(III) may be a more important factor in controlling dissolved Ce than oxidation.