The Chemistry of Sea Water - University of California ...ammoore/migrated/ocea101/... · The...

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

• 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+

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

CO32- + H+

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

CO32- + H+

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.

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)

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

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

[element] (µM)

Mixed Layer

Nutricline

Subsurface Max

• Bio-Limiting (Nutrient Type)– NO3, PO4, Fe

• Scavenged

• Bio-Limiting (Nutrient Type)– NO3, PO4, Fe

• Scavenged

[element] (µM)

• Scavenged

[element] (µM)

• Bio-Unlimiting(Conservative)– Na, Cl, K, Mg

• Scavenged

• Bio-Unlimiting(Conservative)– Na, Cl, K, Mg

• Scavenged

[element] (µM)

• Scavenged

[element] (µM)

Mixed Layer

Nutricline

• Bio-Intermediate (Mixed)– Cd, C, Ca

• Scavenged

• Bio-Intermediate (Mixed)– Cd, C, Ca

• Scavenged

[element] (µM)

• Scavenged

[element] (µM)

• Scavenged– Pb, Th

[element] (µM)

Chemical Profiles

Differences betweenbasins due to circulation and availability of iron.

The Chemistry of Sea Water - University of California ...ammoore/migrated/ocea101/... · The...

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