Iron isotopic composition of seawater within the oxygen minimum zone over the shelf and the ocean basin in the tropical Atlantic Ocean

The world’s ocean’s oxygen minimum zones (OMZs) play an important role in the cycling of redox sensitive trace metals, such as iron (Fe), which is an essential micronutrient for marine primary productivity. The elevated dissolved Fe concentrations found in OMZs may be the result of Fe remineralisation originated from high amounts of sinking organic matter from the productive surface waters, combined with the upwelling of high Fe – low oxygen rich waters from the deep ocean. However, the processes (redox, Fe-L) leading to these observations are still poorly quantified.

Here we present isotope data of Fe in the dissolved fraction (dFe, < 0.2 µm) of seawater collected from the eastern tropical Atlantic Ocean along a transect that covers the continental shelf, slope and ocean basin, along ~12 N (GEOTRACES cruise GA06). In this region the hypoxic OMZ (<70 µmol/kg) is located between ~200 and ~800 m depth. Elevated dFe concentrations (up to 6 nM) in suboxic bottom waters over the shelf display δ⁵⁶Fe values down to -0.3 ‰ (relative to IRMM-14), suggesting input of Fe from reducing sediments. Within the OMZ (O₂ down to 44 µmol/kg) moving offshore, the δ⁵⁶Fe is always relatively lower than adjacent water masses, which may indicate transport of Fe from the shelf to the open ocean and/or remineralisation of Fe from sinking particles. Surface waters, that receive atmospheric dust deposition, display higher δ⁵⁶Fe values up to +0.48 ‰, suggesting isotopic fractionation occurring during dust dissolution.

We demonstrate that Fe isotopes can be used as a tool to identify different Fe supply sources (e.g., atmospheric vs. benthic) to the ocean, and to identify certain processes that may stabilise or remove Fe from the dissolved fraction in seawater.