1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. Mn and Fe for oxidizing Corg… results from solute profiles

8. 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:

9. 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.

10. 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?

11. 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)

12. Data: Internal cycles of Mn in deep-sea sediments

13. Mn(IV) <--> Mn(II) cycling in nearshore sediments: A larger role for Mn? Aller, 1994, Journal of Marine Research 52,
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

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

15. 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

16. 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

17. 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

18. 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

19. 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]