Fe, Mn, and S in marine sediments Fe and Mn : first estimates of use as e acceptors Sulfate reduction Redox cycling within the sediment column Mn / O 2 S / Mn Fe and Mn : revised estimates of use as e acceptors?

Electron acceptor review: order of use -600 -500 -400 -300 -200 -100 0 O2 NO3 Mn(IV) Fe(III) SO4 Electron acceptor

EA order of use: addendum Thamdrup (2000) Adv Micr. Ecol 16, 41-83 MnIV,III Fe III , ligands FeIII

1. Solutes: SO 4 2 Electron Acceptors: Availability Thamdrup, 2000 >> O 2 >NO 3 2. Particles: Mn and Fe oxides Poorly Crystalline: Most reactive Almost always: Fe >> Mn Deep-sea: limited availability Margins: larger terrigenous input, abundant Fe(III) Riverine particles

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 C org oxidation  Reactions: Mn: 

CH

2

O

 106 

NH

3  16 

H

3

PO

4   212

MnO

2  438

H

  212

Mn

2   106

CO

2  16

NH

4  

HPO

4 2   318

H

2

O

Fe: 

CH

2

O

 106 

NH

3  16 

H

3

PO

4   212

Fe

2

O

3  756

H

  424

Fe

2   106

HCO

3   16

NH

4  

HPO

4 2   424

H

2

O

 x mn C org ox by MnO 2 = 0.5

  

x mn P Mn

2 

dx

 0.5

  

D Mn dMn

2 

dx

 

x



x mn

x fe  C org ox by Fe 2 O 3 = 0.25

  

x fe P Fe

2 

dx

 0.25

  

D Mn dFe

2 

dx

 

x



x fe



Fe and Mn: continental margins Reimers et al., 1994 (N.E. Pac) Lohse et al., 1998 (N.E. Atl.)

site

MANOP H MANOP C E. Eq. Atlantic Summary: Profile-based estimates Martin and Sayles, 2003

Table 3: Electron Acceptors in Pelagic Sediments (1) % of organic C oxidation by different electron acceptors region

E. Eq. Pacific

C org ox. rate (µmol/cm 2 /y)

12.0

O 2

99.2

NO 3 -

0.8

Mn(IV)

0.4

FE(III) SO 4 2-

Central Eq. Pac 0-3°N, 6-16°W (1) Summarized from Bender and Heggie, 1984 20.4

12.4

98.1

93.8

1.6

4.4

0.4

0.1

1.8

Table 4: Electron Acceptors in Continental Margin Sediments Location

N.E. Atlantic (1) N.W. Atlantic (2) N.E. Pac: O2< 50 µM (3) N.E. Pac: O2 =73-145 (3)

Water depths

208-4500 260-2510 780-1440 1900-4070

Total Corg ox (µmol/cm2/y)

36-158 36-52 66-75 36-74

O2 % of organic C oxidation by different electron acceptors

67-97 74-90 5.0-46 69-75 (1) Lohse et al., 1998; (2) Martin and Sayles, submitted; (3) Reimers et al., 1992

NO3-

1-8.5

1.8-6.0

41-69 11-18

Mn

0-2.1

0.1

0.1-6.9

Fe

0-1.7

8-20 0.7-1.3

0.3-0.7

SO4

1-20 5.7-25 5.6-18

SO 4 2 as an electron acceptor It’s the most abundant… It undergoes the larges change in oxidation state (+VI to -II) BUT: it’s low on the ∆G ladder  The reaction: 

CH

2

O

 106   16 

H

3

PO

4   53SO 4 2   106HCO  3  16NH 4  

HPO

 4  53HS   39H  BUT… HS is not the only reduced S product… Also: FeS, S 0 , S n 2 , and FeS 2

So: measuring SO4 reduction rates (A) From pore water profiles / benthic fluxes SO 4 2 , NH 4 + , HS (a) examples of SO 4 2 profiles (b) Canfield 1989, DSR 36, 121-138:

SR

 

D sed dC dx

    

x

 0   

x



x max



Incubation techniques for measuring SO4 reduction

Whole-core Incubations: Howarth and Jorgensen (1984) GCA 48, 1807-1818

SO 4 2 as an electron acceptor Canfield, 1989

The study of SO4 reduction in (coastal) sediments But sulfate reducers can only oxidize a limited suite of simple organic substrates. They typically function as part of a community that includes fermenters, acetogens, and methanogens, as well as sulfate reducers.

Redox cycling within the sediment column 15 10 5 O 2 /H 2 O NO 3 / N 2 Mn(IV)/Mn(II) NO 3 / NH + 4 ( org ) 0 -5 -10 Fe(III)/Fe(II) SO 2 4 / HS HCO 3 / CH 2 O (org ) Redox couple

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

•Fe and S closely

connected •Recycling is important!

 

aq



H

2

S



S

2 

n

 

FeS

2 

FeS



S n

 

FeS

2 

S

2 

n

 1

Bioturbation is key: With: 1.7 - 15 % preserved Without: 28-77% pres buried

Sedimentary Fe and S cycles: Long-term importance

Coastal Sediments… and contaminants

An alternative to profile-based methods? 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 35 SO 4 2 technique Data:

Interpretation of Incubation results Difference between Total Cox and Cox by SO4 reduction ==> Important role for Fe(III) reduction

Electron acceptor use in margin sediments: Incubation study results Thamdrup, 2000