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The Chemistry of Sea Water
OCEA 101
• Why should you care?- the chemical composition of sea water is important for all marine organisms
- nutrients for phytoplankton and bacteria- trace metals control utilization of nutrients- salts influence fish metabolism- ocean acidity controls shell, skeleton and test formation
Overview
• Salts and salt balance• Gases in sea water• pH of sea water• Nutrient cycles
Salts
• Salinity (S) is a measure of salt conc.• S varies considerably with latitude.• Coastal S strongly controlled by river inflow.• Mid-ocean S strongly controlled by
evaporation and precipitation.• In polar regions S strongly controlled by
sea-ice (brine rejection).
Seawater Salinity
Global Average: 35 ‰
Ionic Bonds
• Atoms in salts are held together by ionic bonds. E.g. Sodium Chloride (NaCl)
• Ionic bonds are easily broken by polar water molecules.
Major Constituents of Seawater
•Conservative constituents (Cl-,Na+,SO42-,Mg2+,Ca2+,K+) – ratios do not change;
not removed or added by marine organisms.
Trace Elements in Seawater
•Some are non-conservative due to importance in biological reactions withinmarine organisms (e.g. Fe).
Salt Balance
• Geological evidence indicates that oceanic salt composition has not changed for ~1.5 billion years.
• This implies an equilibrium:
Rate of salt addition = Rate of salt removal
Sources and Sinks of Salt Ions
Concentration of ions inexcrement, shells, skeletons
Ions adhere to particlessuch as clays (from dust) andfecal pellets.
Loss to sediments
Spreading centresand vents are sourcesand sinks.
Residence Time
Inventory (total amount in ocean)
Total Fluxes Out
Fluxes are in units of amount per time (e.g. grams/year)
Inventory is total amount (e.g. grams)
Residence time = Inventory / Flux In
The average amount of time one atom of constituent spends in ocean
Approx. the amount of time it takes for the concentration of a constituent to significantly change
Total Fluxes In
Conservative
Non conservative
Gases in Sea Water
• Gas exchange occurs at the air-sea bndry.• Biological processes influence gas conc.
below surface.• Max amount of gas that can be held in
solution = saturation concentration (SC)• SC depends on T, S and p.
SC as T S p
•Photosynthesis and respiration controlO2 and CO2 in sea water.
•Euphotic zone: photosynthesis yields abundant O2 and depletion of CO2
•Below euphotic zone: respiration by marineorganisms and bacteria consumes O2 andgenerates CO2
•Compensation depth: rate of photosynthesis= rate of respiration ~100m
•Deep ocean O2 increase associated withfewer organisms and supply by deep ocean circulation
Carbon Dioxide and the Biological Pump
• At present, the ocean is a net sink of CO2
• The rate of uptake controlled by:- the solubility pump (a combination of temperature and ocean chemistry)- the biological pump (a combination of biology and ocean circulation).
The Biological and Solubility Pumps
The pH of Sea Water
• Water molecules can dissociate into ions:H2O H+ + OH-
• In pure water @ 25C, 1 molecule in every 107 dissociates (i.e. 10-7 as a fraction).
• In impure solutions, concentrations of H+
and OH- vary in inverse proportion.
Hydrogencation
Hydroxideanion
CO2 as a buffer
• pH=-log10[H+]• For pure water [H+]=[OH-]=10-7, so pH=7.• As [H+] increases, pH decreases: more acidic• As [H+] decreases, pH increases: more alkaline• Sea water is slightly alkaline: average pH~7.8• pH of seawater remains relatively constant:
CO2+H2O H2CO3 HCO3- + H+
or CO32- + 2H+
Carbonic acid Carbonate
BicarbonateEquilibrium reactions:
CO2 as a buffer
• If seawater becomes more acidic, reactions proceed to the right releasing more H+ ions.
• If seawater becomes more alkaline, reactions proceed to the left removing H+ ions.
CO2+H2O H2CO3 HCO3- + H+
or CO32- + 2H+
CO2(f) + H2O H2CO3 HCO3- + H+
CO32- + H+
K0 K1
K2
Equilibria in SolutionFree CO2
K0, K1, and K2 equilibrium/dissociation constants:
K0 ={H2CO3}/({H20}{CO2(f)})
K1 ={H+}{HCO3-}/{H2CO3}
K2 ={H+}{CO32-}/{HCO3
-}
Equilibria in Solution
CO2(f) + H2O H2CO3 HCO3- + H+
CO32- + H+
K0 K1
K2
K0, K1, and K2 equilibrium/dissociation constants:
K0 ={H2CO3}/({H20}{CO2(f)})
K1 ={H+}{HCO3-}/{H2CO3}
K2 ={H+}{CO32-}/{HCO3
-}
SLOW EQUILIBRATION TIME
Half-time to equilibrationfor CO2 in sea water
Note the relatively long Times at ocean pH.
This can cause problemsfor ocean respiration viagills or lungs, so a specialenzyme, carbonic anhydraseis present to speed up reactionsand aid respiration.
Equilibria in Solution
CO2(f) + H2O H2CO3 HCO3- + H+
CO32- + H+
K0 K1
K2
K0, K1, and K2 equilibrium/dissociation constants:
K0 ={H2CO3}/({H20}{CO2(f)})
K1 ={H+}{HCO3-}/{H2CO3}
K2 ={H+}{CO32-}/{HCO3
-}
K2 << K1
Fast Slow
Also K = (rate to left)/(rate to right)
Calcium Carbonate
CaCO3 Ca2+ + CO32-
Ksp = {Ca2+}{CO32-} = solubility product const.
K
If K < Ksp calcium carbonate unsaturated and dissolves
If K > Ksp calcium carbonate saturated and precipitates
Ion concentrations at saturation
Increasing CO2 Concentration
Increasing CO2 conc. increases [H+] more acidic
CO32- + H+ HCO3
- + H+
K= {Ca2+}{CO32-} < Ksp
Calcium carbonate unsaturated so more dissolves
(Note: This also occurs in the deep ocean where CO2 levels are highdue to cold temps and high respiration rates, which explains why CaCO3is unsaturated in the deep ocean, and the presence of the CCD)
Fast
Increasing Ocean Acidity
• Because pH is inversely proportional to [CO2], surface waters are more alkaline than deep waters.(i.e. less CO2 means less H+ and higher pH)
• Increases in anthropogenic CO2 in atmosincrease in surface CO2
decrease in pHincrease in acidityCCD will rise towards the surface
Figure 5.9IPCC, 2007WG1 ReportChapter 5
0.1 pH~30% changein [H+]
29N, 15W
23N, 158W
31N, 64W
Nutrients
• Nutrients are ions required for phytoplankton growth:
Nitrate, NO3-
Phosphate, PO43-
Silicate, SiO44-
Major Biogeochemical Cycles
• Carbon• Phosphorus• Nitrogen• Iron
Carbon
• Actually driven by carbonate chemistry and the balance between the oceans and atmosphere, and by the effects of biology on the inorganic carbon
• “Solubility Pump” is simply the “pumping” of carbon from the surface to depth by the change in solubility as pressure increases and temperature decreases
• “Biological Pump” is the packaging of inorganic carbon as organic material, which is rapidly exported to depth as organic matter (often fecal pellets). Sometimes referred to as the “fecal express”
• Carbonate Compensation Depth (CCD) is the point at which carbonate is soluble in seawater. Below that depth, carbonate will dissolve if exposed to seawater
Contributions of different processes to the overall global transfer ofCO2 into the ocean
Carbon Cycle
*
Feeding
Phosphorus• Phosphorous is considered the single most
limiting element in the ocean for biology because it is an essential constituent of RNA and DNA, and energy mechanisms
• Primary source of phosphorous is from rocks• Phosphorous is very particle reactive, so it is not
present in very high concentrations• In the oceans, phosphorous is primarily
regenerated at depth, and the major source is upwelling of deep, nutrient rich water, but it is also regenerated quickly, so it spends a lot of time being recycled in the surface waters
Phosphorous comes mostly from rocks
*
Secretions, mucus,undigested food
(DOM)
Fluids from punctured cells,fecal pellets leaching organic matter (DOM)
Bacteria return organic P to foodweb
(Return of DOM to the foodweb)
Nitrogen
• Geologists consider it to be non-limiting (biologically) because there is always more available from the atmosphere, via nitrogen fixation.
• Biologists argue that on shorter time scales, nitrogen is very limiting in the oceans (more than phosphorous)
• Very complicated biogeochemical cycling, with many sources and sinks, and several forms of nitrogen (nitrate, nitrite, ammonium, urea, amino acids, nitrogen gas, etc)
• Nitrogen is not regenerated as quickly as phosphorous in the surface ocean, but there is a lot of recycling relative to other elements (such as silicon)
The Nitrogen Cycle
Many sources and sinks
Controlled largely by biological processes
*
(Return of DOM to the foodweb)
Silicon• Only used by diatoms and radiolarians in the
modern ocean • Limits production of diatoms in some regions• ALWAYS undersaturated in the ocean (always
dissolves), so there is no equivalent of the CCD• Regeneration is limited in the surface ocean, so
silicon is exported from the surface to depth much more efficiently than either C, N, or P.
• Primary sources of Si are upwelled deep waters and river water (dissolved terrestrial rocks)
The Silica Cycle
Rapid export to depth
Major external source is river water
* *
Dissolves:undersaturated
Iron• Very scarce in the oceans (it is a “trace metal”)• In large regions of the ocean, it may limit
biological productivity• Primary sources are atmospheric, terrestrial
(rivers), and benthic (sediments), so it is most limiting in deep waters with no upwelling far from land
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Mixed Layer
Nutricline
Subsurface Max
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)– NO3, PO4, Fe
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)– NO3, PO4, Fe
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)– Na, Cl, K, Mg
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)– Na, Cl, K, Mg
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Mixed Layer
Nutricline
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)– Cd, C, Ca
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)– Cd, C, Ca
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Vertical Profiles in the Ocean
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged– Pb, Th
• Bio-Limiting (Nutrient Type)
• Bio-Unlimiting(Conservative)
• Bio-Intermediate (Mixed)
• Scavenged– Pb, Th
[element] (µM)
Dep
th (m
)
0
1000
2000
3000
4000
Chemical Profiles
Differences betweenbasins due to circulation and availability of iron.