Fe Isotope Fractionation Across the Oxic – Ferruginous Redox Transition

Fe isotope results from selectively leached oxic – ferruginous marine sediments from the Potter Cove Estuary (King George Island, Antarctica) show that downcore Fe isotope variability in reactive Fe-(oxyhydr)oxides is almost exclusively the result of fractionation of surface-bound reactive Fe. The downcore trend in δ⁵⁶Fe is comparable to that observed in porewater but offset to slightly higher values. This suggests isotopic exchange between particles and solution. Across the ferruginous – oxic redox boundary δ⁵⁶Fe values increase towards the oxic sediment surface as Fe is removed from porewater by oxidation and precipitation. This trend is principally similar to observations of δ⁵⁶Fe in particulate and dissolved iron across ferruginous – oxic boundaries in the water column of bottom anoxic marginal seas. Decreasing δ⁵⁶Fe along with decreasing dissolved Fe concentration across this redox boundary suggest a principal preference of light isotopes during particle formation by oxidation and precipitation at marine conditions in the sediment as well as in the water column. This is contrary to previously published laboratory experiments performed under non-marine conditions, which are commonly referred to when interpreting iron isotope profiles. When Fe isotopes are used to determine sources of dissolved Fe in the open ocean, the potential modification of δ⁵⁶Fe in dissolved iron during particle formation and during exchange with Fe bound on particle surfaces must be considered.