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

International audienceWe 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 (