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

Abstract : 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 δ 56Fe values increase from -1.16‰ at 115cm depth to +0.2‰ at 7cm depth due to isotope fractionation in three distinct lithostratigraphic zones. The deepest zone contains orange sands with elevated Fe-oxide contents (0.2wt.%) that dissolve through diagenetic Fe-oxide reduction and elevate Fe concentrations in porewaters (100 to 300μM/l). This reaction causes porewater δ 56Fe values to be ~1‰ lighter than the sediment δ 56Fe values. An intermediate zone contains white Fe-poor sands, with Fe-oxide contents <0.1wt.% and dissolved Fe concentrations <20μM/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 δ 56Fe values that are as much as 1.8‰ lighter than sediment δ 56Fe values. The uppermost zone contains organic carbon and Fe-sulfide rich black sediments with low dissolved Fe (<1μM/l) and elevated porewater sulfide (up to 600μM/l) concentrations. Precipitation of isotopically light Fe-sulfides increases the porewater δ 56Fe values as much as 0.68‰ more than corresponding sediment δ 56Fe values. The near-surface Fe-sulfide precipitation delivers to the lagoon dissolved Fe with slightly positive δ 56Fe 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.