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 delta Fe-56 values increase from -1.16 parts per thousand at 115 cm depth to +0.2 parts per thousand 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 mu M/l. This reaction causes porewater delta Fe-56 values to be similar to 1 parts per thousand lighter than the sediment delta Fe-56 values. An intermediate zone contains white Fe-poor sands, with Fe-oxide contents <0.1 wt.% and dissolved Fe concentrations <20 mu M/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 delta Fe-56 values that are as much as 1.8 parts per thousand lighter than sediment delta Fe-56 values. The uppermost zone contains organic carbon and Fe-sulfide rich black sediments with low dissolved Fe (<1 mu M/l) and elevated porewater sulfide (up to 600 mu M/l) concentrations. Precipitation of isotopically light Fe-sulfides increases the porewater delta Fe-56 values as much as 0.68 parts per thousand more than corresponding sediment delta Fe-56 values. The near-surface Fe-sulfide precipitation delivers to the lagoon dissolved Fe with slightly positive delta Fe-56 values, averaging about +0.24 parts per thousand 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. (C) 2012 Elseviel B.V. All rights reserved.