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

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

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

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

More complex reaction better reflection of photosynthesis More complex reaction better reflection of photosynthesis 106CO NO HPO H H + + trace elements = C 106 H 263 O 110 N 16 P O 2

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Example IRL

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

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

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

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

Lakes Vertical stratification Vertical stratification Epilimnion – warm low density water, well mixed from wind Epilimnion – warm low density water, well mixed from wind Metalimnion (thermocline) – rapid decrease in T with depth Metalimnion (thermocline) – rapid decrease in T with depth Hypolimnion – uniformly cold water at base of lake Hypolimnion – uniformly cold water at base of lake Stable – little mixing between hypolimnion and epilimnion 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 Amount of nutrient in lake determines type Oligotrophic – low supply of nutrients, water oxygenated at all depth Oligotrophic – low supply of nutrients, water oxygenated at all depth Eutrophic – high supply of nutrients, hypolimnion can be anaerobic Eutrophic – high supply of nutrients, hypolimnion can be anaerobic

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

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

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

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

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

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

Broecker’s Conveyor Belt

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

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

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

Ground Water Difficult to generalize about controls on redox reactions Difficult to generalize about controls on redox reactions

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

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

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

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