Iron isotope fractionation in a sulfide-bearing subterranean estuary and its potential influence on oceanic Fe isotope flux

We trace pathways of Fe reactions in the Indian River Lagoon (Florida, USA) subterranean estuary using Fe isotopes to provide new constraints on Fe-isotopic fractionation in a sulfide-bearing subterranean estuary. Porewater δ56Fe values increase from − 1.16‰ at 115 cm depth to + 0.2‰ at 7 cm depth due to isotope fractionation in three distinct lithostratigraphic zones. The deepest zone contains orange sands with elevated Fe-oxide contents (0.2 wt.%) that dissolve through diagenetic Fe-oxide reduction and elevate Fe concentrations in porewaters (100 to 300 μM/l). This reaction causes porewater δ56Fe values to be ~ 1‰ lighter than the sediment δ56Fe values. An intermediate zone contains white Fe-poor sands, with Fe-oxide contents < 0.1 wt.% and dissolved Fe concentrations < 20 μM/l. This zone is a sink for dissolved Fe through adsorption of isotopically heavy dissolved Fe(II) onto mineral surfaces. This adsorption results in porewater δ56Fe values that are as much as 1.8‰ lighter than sediment δ56Fe values. The uppermost zone contains organic carbon and Fe-sulfide rich black sediments with low dissolved Fe (< 1 μM/l) and elevated porewater sulfide (up to 600 μM/l) concentrations. Precipitation of isotopically light Fe-sulfides increases the porewater δ56Fe values as much as 0.68‰ more than corresponding sediment δ56Fe values. The near-surface Fe-sulfide precipitation delivers to the lagoon dissolved Fe with slightly positive δ56Fe values, averaging about + 0.24‰, via submarine groundwater discharge (SGD). Iron-sulfide precipitation in sulfide-containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy Fe to the coastal oceans.