H₂S events in the Peruvian oxygen minimum zone facilitate enhanced dissolved Fe concentrations

Area of Science:

• Marine chemistry
• Biogeochemical cycles
• Oceanography

Background:

• Oxygen minimum zones (OMZs) in Eastern Boundary Upwelling Systems exhibit high dissolved iron (DFe) due to sediment supply.
• Variations in DFe within the Peruvian OMZ suggest factors beyond dissolved oxygen (O2) control iron cycling.

Purpose of the Study:

• To investigate the factors controlling dissolved iron (DFe) concentrations in the anoxic waters of the Peruvian oxygen minimum zone (OMZ).
• To understand the role of sulfide and nitrate/nitrite in iron speciation and DFe accumulation.

Main Methods:

• Analysis of dissolved iron (DFe) concentrations.
• Measurement of dissolved oxygen (O2), nitrate, nitrite, and hydrogen sulfide concentrations in seawater samples from the Peruvian shelf.
• Characterization of iron speciation, including aqueous iron sulfide complexes.

Main Results:

• Sediment-derived reduced iron (Fe(II)) is the primary source of DFe in the anoxic Peruvian water column, reaching up to 200 nmol L-1.
• Lowest DFe concentrations were found in anoxic waters with high nitrate/nitrite, indicating Fe(II) oxidation and removal.
• Highest DFe levels occurred in sulfidic waters with low nitrate/nitrite, where aqueous iron sulfide (FeS_aqu) stabilized Fe(II), reducing removal.

Conclusions:

• Iron sulfide complexation significantly enhances dissolved iron (DFe) availability in sulfidic OMZ waters.
• The frequency of sulfidic events, which increase Fe availability, may rise with projected OMZ expansion.
• Understanding these processes is crucial for predicting future ocean biogeochemistry and ecosystem responses.