
Redox largely controlled by quantity and quality (e.g. reactivity) of organic matter


Organic matter generated with photosynthesis
Organic matter decomposes ( remineralized ) during respiration


Reaction that converts CO matter and oxygen
2 plus nutrients
(N, P, other micronutrients) to organic
CO
2
+ N + P + other = C organic
+ O
2

This equation controls atmospheric oxygen

If not driven to right by primary production, all O
2 would be consumed



Photosynthesis occurs until essential nutrients are depleted
Various nutrients may be limiting:

N, P, Fe…


Organic matter is approximately constant composition
C
106
H
263
O
110
N
16
P
1

Redfield ratio is thus 106C:16N:1P (molar ratio)


More complex reaction better reflection of photosynthesis
106CO
2
+ 16NO
3
+ HPO trace elements = C
106
H
4
2-
263
O
+ 122H
2
110
N
16
P
1
0 + 18H + +
+ 138O
2


This reaction reflects the importance of P in the reaction:



106 moles C consumed/ mole of P
16 moles of N consumed / mole of P
138 moles of O
2 consumed / mole of P




Reverse reaction (remineralization: respiration/decay) equally important
Products include


Nitrate
Phosphate

CO
2
– decrease pH
Much respiration results from microbes
(bacteria, archea etc).


Oxidation of organic carbon also generates electrons:
C org
+ 2H
2
O = CO
2
+ 4H + + 4e -


Because no free electrons, a corresponding half reaction must consume them
Terminal electron acceptors – TEAs


For example – reduction of oxygen to water:
O
2
+ 4H + + 4e = 2H
2
O

Here oxygen is the terminal electron acceptor.


There are multiple terminal electron acceptors:
2NO
3
+ 12H + + 10e = N
2
+ 6H
2
O
FeOOH + 3H + + e = Fe 2+ + 2H
2
O
SO
4
2+ 10H + + 8e = H
2
S + 4H
2
O

Terminal electron acceptor controlled by microbes and by concentration of acceptor
MnO
2
/Mn 2+
Rare
FeOOH/Fe 2+
Decreasing amount of energy derived per mole of electrons transferred


Denitrification ( dissimilatory nitrate reduction)
7e -
5C organic
+ 4NO
3
+ 4H + = 2N
2
+ 5CO
2
+ 2H
2
0

Final product is molecular nitrogen

Conversion of nutrient to inert gas


Other nitrate reduction pathways

Reduction to nitrite:
2e -

C org
+ 2NO
3
= CO
Reduction to ammonia
2
+ 2NO
2
-
2C org
+ NO
3
+ H
2
O + H + = 2CO
2
+ NH
3
10e -



Ammonia also derived from decomposition of amino acids in proteins
Ammonia raises pH by formation of ammonium ion
NH
3
+ H
2
O = NH
4
+ + OH -
(now an acid-base reaction)

3


Haber Process (early 20 th century)



N
2 fixation to NH utilize CH
4
3 with Ni and Fe catalysts to generate needed H
2
NH
3 oxidized to NO
3 and NO
2
Prior to this fertilizers required

 mining fixed N (guano)
N fixing plants (legumes)



C org
+ 4Fe(OH)
3
+ 8H + = CO
2 e -
+ 4Fe 2+ + 10H
2
O
Common in groundwater where metal oxides concentrated. Rare in surface water
Fe 2+ commonly precipitates as carbonate or sulfide depending on solution chemistry




C org
+ SO
4
2+ 2H
2
O = H
2
S + 2HCO
3
-
8e -
Commonly driven by microbes
Products are H
2
HCO
3
-
S or HS and H
2
CO
3 depending on pH or
Microbes require simple carbon (e.g. < 20
C chains

Formate HCOO -


Acetate CH
3
COO -
Lactate C
3
H
5
O
3






Sulfate common seawater ion
Sulfide and bisulfide highly toxic
Used by oxidizing bacteria for chemosynthesis
Oxide to sulfides change sediment color
Metal chemistry



P and some metals adsorb to oxides
Other metals soluble in oxidizing solution (Cu,
Zn, Mo, Pb, Hg)
Other metals precipitate as sulfides




Breakdown of complex carbohydrates to simpler molecules
Products can be used by sulfate reducing bacteria
Don’t require terminal electron acceptors


Fermentation
CH
3
COOH = CH
4
+ CO
2

Oxidized and reduced C

Methanogenesis
CO
2
+ 4H
2
= CH
4
8e -
+ 2H
2
O

Oxidized to reduced C



Each terminal electron acceptor requires specific bacteria
Bacteria derive energy from reactions


Essentially catalyze breakdown of unstable to stable system
Reactions occur in approximate succession with depth in the sediment



The range of reactions are very common in marine sediments
Controls


Amount of organic matter
Sedimentation rate – controls diffusion

Depth variations depend on:
(1) Sedimentation rate
(2) Diffusion rate
(3) Amount of electron acceptor
(4) Amount of organic carbon
Sediment-water interface
Oxygen depleted
MnO
2
/Mn 2+
Nitrate depleted
N, P, CO
(alkalinity) increase
2
Mn 2+ increase
FeOOH/Fe 2+
SO
4
2decrease
Fe 2+ increase
Sulfide increase
Methane increase

Eastern equatorial
Atlantic:
Slow sed rate low OC content
Coastal salt marsh
High sed rate high OC content

 pe can be buffered just like pH


Depends on the electron receptor present
Example of surface water, contains oxygen and SO
4
2(no nitrate, metals etc).



With oxygen present, pe remains fairly constant at around 13
In oceans, once oxygen reduced, sulfate becomes terminal electron acceptor, pe = about -3

Oxygen consumed, pe rapidly decreases
Occurs in water with no NO or
Fe(III)
3
-


There could also be solid phases controlling redox conditions
Stepwise lowering of pe as various terminal electron acceptors are depleted



Vertical stratification



Epilimnion – warm low density water, well mixed from wind
Metalimnion (thermocline) – rapid decrease in
T with depth
Hypolimnion – uniformly cold water at base of lake
Stable – little mixing between hypolimnion and epilimnion

Generic Lake:
 May have multiple metalimnions
 Depends on depth of lake


Amount of nutrient in lake determines type


Oligotrophic – low supply of nutrients, water oxygenated at all depth
Eutrophic – high supply of nutrients, hypolimnion can be anaerobic



Cooling T in fall

Surface water reaches 4ºC – most dense
Causes breakdown of epilimnion – Fall turnover


Metalimnion breaks down
Wind mixes column





At T < 4º C, stably stratified

Ice forms
Warming in spring to 4º C is maximum density

Spring turnover
Monomictic – once a year turnover
Dimictic – twice a year turnover


Oxygen content (redox conditions) depends on turnover



Oxygen in hypolimnion decreases as organic matter falls from photic zone and is oxidized
The amount of oxygen used depends on production in photic zone
Production depends on nutrients, usually phosphate

O
2 more soluble in cold water
Oligotrophic
Eutrophic
High productivity, O
2 consumed






Pollution convert oligotrophic lakes to eutrophic ones (e.g. Lake Apopka, Florida)
Difficult to reverse process
Nutrients (P) buried in sediments because adsorbed to Fe-oxides
When buried Fe-oxides reduced and form
Fe 2+ and Fe-carbonates and sulfides
Released P returns to lake



Oceanic turnover


Continuous – Broecker’s “conveyer belt”
Nutrient distribution controlled by decay in water column and circulation/upwelling
Oxygen profiles controlled by settling organic matter from photic zone

Rate of input of organic matter controls oxygen minimum zone

Photic zone – OC production
Pycnocline = halocline + thermocline
High OC input upwelling system
Low OC input



Bottom configuration also important
Silled basins



Cariaco Basin – Venezuela
Sanich Inlet – B.C.
Santa Barbara Basin - California
Stratified – little mixing
NO
3
, Fe, Mn, SO reduction
4


Little deep water circulation



Oxygen rapidly depleted
May go to sulfate reduction in water column
Sediment affected

Black (sulfides)

Laminated (no bioturbation)


Difficult to generalize about controls on redox reactions


Oxygen content of recharge water


“Point recharge” – sinkholes, fractures well oxygenated
“diffuse recharge” – low oxygen, consumed by organic matter


Distribution of reactive C



Aquifers vary in amount of organic carbon
Quality of carbon variable – usually refractory
Refractory because

Old
 subject to heat


Distribution of redox buffers

Aquifers may have large amounts of Mn and
Fe oxides


Circulation of groundwater


Flow rates, transit times, residence times
Longer residence times generally mean lower pe