Evolution of rare earth element and eNd compositions of Gulf of Mexico seawater during interaction with Mississippi River sediment

A closed-system batch reaction experiment was conducted for 270 days to evaluate the effects of inter-action between Gulf of Mexico (GOM) seawater and Mississippi River sediments on the system's dis-solved rare earth elements (REE) concentrations and neodymium isotopic compositions (eNd). This study specifically focuses on geochemical reactions involving silicic sediments derived from weathering of the North American continent as they affect the REEs and eNd of seawater along continental margins, in contrast to previous studies that investigated the influence of basaltic rocks and sediments on REEs and eNd in the ocean. Our results show that the dissolution of labile phases of lithogenic Mississippi River sediments leads to an approximately 100-fold increase in dissolved REE concentrations within the first 33 days of the experiment. Secondary mineral precipitation appears to lower the REE concentrations between days 33 and 270 of the experiment, although seawater REE concentrations remain elevated compared to initial values. The two-way elemental transfer involving dissolution and precipitation results in a net increase by a factor of 24 +/- 12 (mean +/- 1r) in the dissolved REE concentrations by the end of the experiment (i.e., day 270). The dissolved REE concentration maxima observed after 33 days of the experiment represent the mobilization of approximately 0.37 % of the REE content of the opera-tionally defined exchangeable fraction of the riverine sediments. The eNd values of the reactive lithogenic components were-9.77 and-9.95, which are similar to the GOM value of-9.81 +/- 0.36. Because of the similarity between eNd values, changes in the seawater Nd isotope value throughout the experiment were subtle (mean +/- std, reacted seawater eNd of-9.87 +/- 0.17). The highest REE concentrations coin-cided with the most radiogenic eNd (-9.65 & PLUSMN; 0.23; day 33), which suggests that REE concentrations and eNd compositions of the GOM may be buffered by fluxes from sediments in the system. Our results are comparable to previous studies involving basaltic rocks and/or sediments of basaltic composition in that they demonstrate that silicic, river sediments are highly reactive in marine environments with regard to REE mobilization. The experimental results further suggest that boundary exchange plays an important role in influencing the eNd of seawater along continental margins dominated by large river systems, although the impacts of boundary exchange will be most profound where ambient seawater and river sediments have distinct Nd isotopic compositions (e.g., basaltic, or Precambrian shield material). Finally, our results indicate that the eNd value of GOM seawater is largely controlled by the lithogenic sediment delivered to the basin by the Mississippi River.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study conducted a closed-system batch reaction experiment to evaluate the effects of interaction between Gulf of Mexico seawater and Mississippi River sediments on dissolved rare earth elements (REE) concentrations and neodymium isotopic compositions (eNd). The results showed that the dissolution of labile phases of the river sediments significantly increased the dissolved REE concentrations in seawater, while secondary mineral precipitation lowered the concentrations. The study also highlighted the importance of river sediments in influencing REEs and eNd in seawater along continental margins.