Rare earth element cycling in a sandy subterranean estuary in Florida, USA

Rare earth element (REE) analyses were performed on 77 water samples and 5 sediment samples from the Indian River Lagoon system along the east coast of Florida. Groundwater samples were collected using nine multi-level piezometers installed along a shore-normal transect of the sandy subterranean estuary. Surface water samples were collected from Eau Gallie River and Crane Creek, the water column of the Indian River Lagoon above each piezometer, and the coastal Atlantic Ocean at Canova Beach. The REE concentrations of lagoon surface and subterranean estuary waters indicate that geochemical reactions within the subterranean estuary play a substantial role in the local REE cycle. The submarine groundwater discharge (SGD) flux of REEs calculated by using a modified form of the 1-dimensonal vertical-flow equation is comprised of a nearshore heavy REE (HREE) enriched advective source chiefly composed of terrestrial SGD, and a light REE (LREE) and middle REE (MREE) enriched source that originates from reductive dissolution of Fe(III) oxides/oxyhydroxides in the subterranean estuary and subsequent transport by bioirrigation to the overlying lagoon. The total SGD flux of REEs reveals that this sandy subterranean estuary is a source for light and middle REEs and a sink for the heavy REEs to coastal waters. The SGD Nd flux is estimated at 7.69 ± 1.02 mmol day-1, which is roughly equivalent to the effective local river flux to the Indian River Lagoon. Although our re-evaluated SGD flux of Nd to the Indian River Lagoon is lower than an earlier estimate, it represents a substantial input to the coastal ocean and thus provides additional insights into the global oceanic Nd budget and the “Nd Paradox”.