Manganese, iron, and sulfur cycling in Louisiana continental shelf sediments

- Response of sediments to hypoxia varied between stations on the Louisiana continental shelf.
- Sulfate reduction rates increased during hypoxia at one station but not another.
- Sediment Fe(III) and MnO concentrations were important in controlling sulfate reduction rates.
- Metal oxides more important in OM decomposition on this shelf than earlier estimated.

Sulfate reduction is considered the primary pathway for organic carbon remineralization on the northern Gulf of Mexico Louisiana continental shelf (LCS) where bottom waters are seasonally hypoxic, yet limited information is available on the importance of iron and manganese cycling in the region. Sedimentary manganese, iron, and sulfur cycling were investigated on the LCS using a combined chemical analysis and sediment diagenesis modeling approach. Three stations situated 320 km across the LCS along the 20 m isobath were sampled up to five times between the spring of 2006 and summer of 2007. Bottom water oxygen levels at the stations ranged from 203 mmol m−3 in spring to 2.5 mmol m−3 in summer. Porewater Mn and Fe2+ concentrations (up to 275 and 300 μmol L−1, respectively), sulfate reduction rates (1.0-8.4 mmol m−2 d−1), and the fraction of total oxalate extracted iron obtained as Fe(II) (0.25-0.52) differed between station and season. Sediments at station Z02 on the eastern LCS, south of Terrebonne Bay, had higher organic matter content and sulfate reduction rates than sediments at Z03, 160 km further west. Sulfate reduction rates were higher in summer than spring at station Z02 but not at Z03 where porewater Mn and Fe concentrations were highest in summer. Porewater Fe2+ concentrations, solid phase oxalate-extractable Fe concentrations, and sediment incubation experiments suggested iron reduction at Z03 may account for 20% or more of organic carbon remineralization. LCS Fe(III) concentrations decreased and sulfate reduction rates increased in model simulations by lowering interfacial dissolved oxygen levels and increasing the rates of organic matter deposited on the sediment surface. Results from this study demonstrate that LCS sedimentary metal oxide cycling may be more important in organic carbon mineralization pathways than previously recognized.