Fe and Mn in sediments Their use as electron acceptors for Corg oxidation Mn cycling within the sediment column Fe cycling within the sediment column

Availability Riverine Particles Poorly crystalline oxides -- available -- Abundance of Fe oxides >> Mn oxides (almost always) -- Pelagic, deep-sea: low rain rates, little supply of either -- May be important in some margin & nearshore sites

Standard redox potentials for Mn(IV) oxides / Mn2+ and Fe(III) oxides / Fe2+ Bracketed by NO3-/N2 and SO42-/HS- Mn Fe - with ligands Fe Thamdrup, 2000

Fe and Mn: deep-sea To note: 1. Mn before Fe, as expected 2. Both Mn and Fe are removed from pore water above zone of production

Quantifying the contributions of Mn(IV) and Fe(III) reduction to Corg oxidation Reactions: Mn: Fe: Corg ox by MnO2 = xmn Corg ox by Fe2O3 = xfe

Calculating rates from dissolved Mn and Fe profiles is often difficult to do… NE Pacific, California margin Reimers et al., 1994 NE Atlantic margin Lohse et al., 1998

Mn and Fe for oxidizing Corg… results from solute profiles

An alternative. Sediment incubations e. g. , Canfield et al An alternative? Sediment incubations e.g., Canfield et al., 1993, Marine Geology 113, 27-40 Method: (1) Retrieve cores; section under nitrogen; seal sections in gas-tight bags; sample over time. (2) Measure sulfate reduction rates by 35SO42- technique Data:

Interpretation of Incubation results Difference between Total Cox and Cox by SO4 reduction ==> Important role for Fe(III) reduction Above dotted line: by O2 and NO3 Circles: SO4 reduction; hatched: metal oxide red.

Results from incubation studies Thamdrup, 2000 Summary of margin Site results: % of Corg ox O2 18±10% NO3 small Fe(III) 17±15% SO4 62±17% Mn(IV) very small ** Re-oxidation of reduced species is important part of O2 consumption ?Do incubations reflect in situ processes accurately?

Mn cycling within the sediment column: Deep-sea sediments Froelich et al., 1979 Removal: re-oxidation of upward-diffusing Mn by O2 Dissolved Mn production (by Corg oxidation)

Data: Internal cycles of Mn in deep-sea sediments

Mn(IV) <--> Mn(II) cycling in nearshore sediments: A larger role for Mn? Aller, 1994, Journal of Marine Research 52, 259-295 At times of: high bottom water [O2] ; moderate Corg flux to seafloor; rapid bioturbation, Mn(IV) and Corg are mixed into anoxic zone; Mn is reduced by Corg and S; Upward-diffusing Mn(II) is trapped by re-oxidation

Summary of Results: Mn recycling and O2 consumption in Long Island Sound

A look at Fe in coastal sediments Note: Rapid net dissolved Fe production; and rapid removal by oxidation by O2 at sediment-water interface Very insoluble sulfides, FeS, which convert to pyrite (FeS2) by reaction of FeS with S(-II) or S0

Coastal Fe: benthic fluxes The 2 chambers Have similar Corg Oxidation rates Nonzero Mn flux When O2 present; Increases at low O2 Fe trapped when O2 Present; released at Low O2

Are benthic dissolved Fe fluxes from continental shelf sediments an important source of Fe to the ocean? Elrod et al., 2004 Geophys Res Lett 31, L12307 Dissolvable Fe in surface waters: Higher near California coast

Compilation of benthic dissolved Fe fluxes Fe fluxes and Corg ox rates vs water Depth Triangles: Monterey Bay Open: from O2 minimum Assert correlation between Fe flux and Corg ox. Rate Mean Cox flux on cont. shelves ~ 12 mmol C/m2/day…. Using correlation ==> benthic dissolved Fe flux ~ input of aerosol Fe to the ocean

Another interpretation? Cox rate at the Boston Harbor site was > 30 mmol/m2/day Benthic [O2] ~ 300 µmol/l Very low or no Fe flux… unless overlying [O2] was allowed to drop Data from O2 minimum MB Fe flux vs Cox as shown in Elrod et al. Elrod et al. data replotted As Fe flux vs. bw [O2]