Neodymium Isotope Geochemistry of a Subterranean Estuary

Ningfang Yang,Jaye E Cable,T Jade Mohajerin,Sidney R Hemming,David J Burdige,E Troy Rasbury,Karen H Johannesson,Christopher D White,Jonathan B Martin,Darren A Chevis

doi:10.3389/frwa.2021.778344

Abstract

Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.

Highlights

We hypothesized that submarine groundwater discharge (SGD) is an important, but poorly accounted for, source of rare earth elements (REE) and Nd isotopes to the ocean, and further that the SGD-flux of REEs may be sufficient to close the ocean REE budget and help resolve the Nd-paradox (Johannesson and Burdige, 2007)
The chief source of Nd to the Indian River Lagoon appears to be the result of biogeochemical reactions occurring in the underlying subterranean estuary whereby microbially facilitated reductive dissolution of ubiquitous Fe(III) oxides/oxyhydroxides that coat the Anastasia Formation sands and coquina release sorbed and/or co-precipitated REEs to the advecting submarine groundwater discharge (SGD)
Radiogenic terrestrial sourced SGD and waters from the surf zone at Canova Beach on the Atlantic side of the barrier island suggest either upward seepage of groundwaters from the underlying Upper Floridan aquifer that is composed of middle Eocene to lower Oligocene limestones and dolostones, and/or the phosphorite-bearing rocks of the Miocene Hawthorn, which confines the Upper Floridan aquifer

Summary

Introduction

We hypothesized that submarine groundwater discharge (SGD) is an important, but poorly accounted for, source of rare earth elements (REE) and Nd isotopes to the ocean, and further that the SGD-flux of REEs may be sufficient to close the ocean REE budget and help resolve the Nd-paradox (Johannesson and Burdige, 2007). Several other studies have investigated SGD fluxes of REEs to coastal waters (e.g., Duncan and Shaw, 2003; Prouty et al, 2009; Johannesson et al, 2011, 2017; Kim and Kim, 2011, 2014, 2015; Chevis et al, 2015a,b; Jiang et al, 2018; Paffrath et al, 2020). All these investigations have either inferred or demonstrated that SGD fluxes can be an important source of REEs to coastal waters. Numerous investigations have shown that SGD is an important flux of nutrients, carbon, and metals to the coastal ocean, and that the steep redox and salinity gradients that characterize subterranean estuaries are important “biogeochemical reactors” that can both remove solutes from solution or enhance fluxes of solutes to the ocean (Moore, 1999; Slomp and Van Cappellen, 2004; Charette and Sholkovitz, 2006; Dorsett et al, 2011; Johannesson et al, 2011; Gleeson et al, 2013; Suryaputra et al, 2015; Wang et al, 2015; Telfeyan et al, 2017, 2018; Santos et al, 2021)

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