Estimating The Flux Of Rare Earth Elements And Neodymium Isotopes To The Coastal Ocean Via Submarine Groundwater Discharge
The dissertation is comprised of three manuscripts presenting rare earth element (REE) and neodymium (Nd) isotopic analyses for the groundwaters, surface waters, sediments, and bedrocks of two estuaries along the eastern coast of the United States: Indian River Lagoon, Florida, and Pettaquamscutt Estuary, Rhode Island. This research was performed to understand the behavior of REEs in subterranean estuaries, the REE SGD fluxes, and the Nd isotopic composition of SGD. The selection of these sites offers contrasting geology (carbonate/sand matrix aquifer versus glacial till aquifer sourced from granitoids) and contrasting subterranean estuary structure. In the first site, the flux of REEs to the Indian River Lagoon, FL is comprised of a nearshore source of terrestrial SGD displaying a HREE-enriched fractionation pattern, and LREE- and MREE-enriched sources that originate from the reductive dissolution of Fe (III) oxide/hydroxides in the subterranean estuary and transported by bioirrigation to the overlying lagoon. The ÎµNd(0) value the Indian River Lagoon groundwater is much more radiogenic than those of the surface water and sediments which could be due to the use of fertilizers in adjacent communities. The surface waters Nd isotopic composition appears to be a mixture of weathering of the Anastasia Formation and dissolution of eolian-transported Saharan Dust. In contrast at the second site, phosphate minerals control the surface and groundwaters of the Pettaquamscutt estuary, RI. The weathering of apatite and precipitation of secondary REE phosphate minerals most likely produce the MREE-enriched fractionation patterns of the Pettaquamscutt groundwaters. The further precipitation of the secondary REE phosphates in the surface waters of the Pettaquamscutt yields HREE-enriched fractionation patterns. The radiogenic Nd isotopic ratios of the Pettaquamscutt waters relative to the bedrock further suggest that apatite is the source of REEs. The Nd flux of SGD for both sites is roughly equal to the respective river fluxes; however, the Nd flux of SGD to the Pettaquamscutt is approximately 3 times greater than the SGD flux to the Indian River Lagoon. More research is needed in both environments to evaluate the impact of SGD on the Nd isotopic budget of the oceans.