Geochemical behavior of Si and Li in a siliciclastic subterranean estuary

Shaily Rahman, University of Colorado Boulder, Geological Sciences, Ft Walton Beach, United States and Jonathan B. Martin, University of Florida, Geological Sciences, Gainesville, United States
Silicate mineral weathering and reverse weathering reactions are posited to exert long–term controls on atmospheric CO2, acting as the Earth’s “Thermostat.” Stable lithium (Li) isotopic compositions (7Li/6Li) in streams and in marine foraminiferal paleorecords are promising proxies for these reactions in the modern ocean and in reconstructing past climate. The main modes of lithium inputs to the ocean are assumed to be via river waters, hydrothermal vents, and refluxing of subduction fluids. Though much remains unknown about the magnitude and form of Li sinks in the ocean, the major outputs are predicted to be via low–temperature basalt alteration and incorporation into marine authigenic aluminosilicate clays (MAACs). Marine submarine groundwater discharge (SGD), made up of seawater circulating through subterranean estuaries (STE), is assumed to play a negligible role in the marine Li cycle. Here we report seasonal Si and Li concentrations as well as Li isotopic compositions in a siliciclastic coastal aquifer along the east coast of Florida, the Indian River Lagoon. Conservative mixing plots show net dissolved silica (DSi) enrichment and, at the same time, net dissolved Li uptake in the STE. Brackish porewaters had δ7Li compositions ranging from 25 to 37‰, ~1 up to 13‰ heavier than surface lagoon waters. Li concentrations in brackish SGD ranged from 2 to 21µM, whereas Li concentrations in surface lagoon waters were ~19-20µM. Trace element data and Li/Si molar ratios point to mineral dissolution as the main source of dissolved silica. We posit that MAACs are forming in this coastal aquifer, drawing down some fraction of dissolved silica and Li in recirculating seawater and fractionating Li isotopic compositions toward heavier values. This process may occur in other aquifers of differing lithology and warrants further study to evaluate the potential role of reactions in STEs on the global Si and Li cycles.