1

Learning goals

• Understand the processes controlling the

concentrations and distributions of:

– Major solutes

– Dissolved gases

– Nutrients

– Trace elements

Evaporated seawater in bottom of five-gallon bucket

Why is the ocean salty?

Major components of seawater:

Salinity: quantity (by mass) of dissolved solids in seawater -

after carbonate is converted to oxide, bromide and iodide are converted

to chloride, and dissolved organic matter is oxidized

Usually expressed as g/kg, or o/oo, or psu (practical salinity units)

Average ocean salinity: 35 o/oo

Average salinity of deep water in Puget Sound: ~30 o/oo

surface water: ~22 to ~30 o/oo

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Major components of seawater and constancy of composition

Cl-, Na+, SO42-, Mg2+, Ca2+, K+, HCO3

-,

These constituents, plus hydrogen and oxygen (in water molecules),

make up 99.99% of the mass of seawater.

Relative concentrations of these components are invariant

throughout most of the ocean.

Implication: Measure one, calculate the others, and determine salinity

Measuring salinity

1. Chlorinity (salinity = 1.80655 x chlorinity (%o)

2. Conductivity

3. Refractive index (refractometer)

CTD photo from Sea-bird electronics

CTD:

Conductivity

Temperature

Depth

Oceanographer’s main

sampling device

Could be called:

CTDLTrDOChlA…

Water-sampling rosette or carousel

Average Average

Ion River (mM) Seawater (mM)

HCO3- 0.86 2.38

Ca2+ 0.33 10.2

Na+ 0.23 468

Cl- 0.16 545

Mg2+ 0.15 53.2

SO42- 0.069 28.2

K+ 0.03 10.2

Comparison of river and seawater composition

Seven major solutes in seawater make up its salinity.

Their relative concentrations do not match river water.

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Effects of salinity on seawater

1: Freezing point depression, boiling point elevation

(disrupts H-bonding, lowers vapor pressure)

Freezing point of seawater (35%o) ~ -1.88 deg C

2: Changes density

pure water density: 1 kg/l

seawater density (35%o, 20 deg C): ~1.024 kg/l

common density units: 24 st

3. Causes stratification (along with temperature)

4: Eliminates density-temperature anomaly

(24.7%o, freezing temp = -1.33 deg C)

Fresh water999.8

999.84

999.88

999.92

999.96

1000

0 2 4 6 8

Temperature (oC)

Den

sit

y (

g c

m-3

)

15 psu

1011.6

1011.68

1011.76

1011.84

1011.92

1012

1012.08

-1 1 3 5 7

Temperature (oC)

Den

sit

y (

g c

m-3

)

25 psu

1019.36

1019.52

1019.68

1019.84

1020

1020.16

-1 1 3 5 7

Temperature (oC)

Variation in water density

with changes in temperature

and salinity (calculated using

equation of state of seawater

at standard pressure)

Max density

Max density

0

0

Average Average

Ion River (mM) Seawater (mM)

HCO3- 0.86 2.38

Ca2+ 0.33 10.2

Na+ 0.23 468

Cl- 0.16 545

Mg2+ 0.15 53.2

SO42- 0.069 28.2

K+ 0.03 10.2

Comparison of river and seawater composition

Approx. residence Time

(Million years)

0.08

1

200

> 200

20

10

10

Residence time = Quantity of a solute in the ocean

Rate of supply or removal

Mg and sulfate removed at hydrothermal vents, but K? (reverse weathering)

4KAlSi3O8 + 4H+ + 2H2O → Al4Si4O10(OH)8 + 4K+ + 8SiO24KAlSi3O8 + 4H+ + 2H2O ← Al4Si4O10(OH)8 + 4K+ + 8SiO2

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Removal mechanisms

Evaporite formation, precipitation, coprecipitation, uptake by organisms,

sorption onto particle surfaces followed by burial in sediments, scavenging

in hydrothermal vent systems (Mg+, SO4-), reverse weathering (K)

What else is dissolved in seawater?

Gases

Nutrients

Metals

Organic matter

Gas Chemical symbol % in air % in water

Nitrogen N2 78.08 62.6

Oxygen O2 20.95 34.3

Argon Ar 0.934 1.6

Carbon Dioxide CO2 0.04 1.4

Neon Ne 0.0018 0.00097

Helium He 0.00052 0.00023

Methane CH4 0.0002 0.00038

Krypton Kr 0.00011 0.00038

Carbon MonoxideCO 0.000015 0.000017

Nitrous Oxide N2O 0.00005 0.0015

Xenon Xe 0.0000087 0.000054

Gas concentrations in air and seawater Dissolved gases in seawater:

Major gases: N2, O2, CO2, Ar, etc…

Factors influencing dissolved concentrations in surface ocean

(1) Concentration in atmosphere

(2) Solubility

Water temperature (cold temperature increases solubility)

Salinity (high salinity reduces solubility)

(Air-sea exchange leads toward saturation: conc=f[atm conc, solubility(T,S,P)] )

CO2: high solubility

Carbonate system (Ocean’s buffering system):

CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3

2-

Carbonate content of the ocean 60x the content of the atmosphere.

(Ocean strongly affects atmospheric CO2 concentration)

Below the air-water interface: (3) In situ production or consumption

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Forms CaCO3 shells Forms organic matter – both can sink

Seawater carbonate system CO2 pumps: solubility and biological

• Solubility pump– Higher CO2 solubility + sinking of cold water means high latitude

seawater takes up CO2

– Lower solubility in warm water + upwelling of deep water means equatorial seas release CO2 into the atmosphere

• Biological pump– CO2 (as bicarbonate) is taken up by phytoplankton to form

particulate organic matter (POM) and CaCO3 during photosynthesis

– Some of this POM and carbonate will sink to deep water

– Fate of POM: -Most is oxidized to CO2 (and carbonate) which can reach the surface again via upwelling.

-Some POM is converted to dissolved OM

-A small fraction is buried in sediments.

Figure from wikipedia.comSolubility pump accounts for ~ 90% of carbon stored in ocean

Biological Pump Solubility PumpRespiration Photosynthesis High latitude Low latitude

Spatial CO2 air-sea exchange

From Takahashi et al. 2002 Deep-Sea Research

Flux into the ocean Flux out of the ocean

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Spatial variation in biological pump

From Takahashi et al. 2002 Deep-Sea Research

CO2 is taken out of the atmosphere by biological productivity

Parts of Hood Canal become dead zone

Lack of dissolved oxygen leaves sea life gasping

JOHN DODGE THE OLYMPIAN

Portions of Hood Canal have turned into a dead zone

for sea life this fall, according to area residents and state

officials. Hundreds of shrimp, crab, small fish, rockfish

and striped perch have washed ashore in the past week

from the Potlatch area north to Hoodsport. A lack of

dissolved oxygen in the water -- a chronic late summer

and fall problem that appears to be worsening in the

60-mile-long fjord -- is to blame, according to the state

Department of Ecology.

The life-and death-struggle for marine life in the canal is a

telling example of how human-caused pollution, an unusually

dry summer and ocean conditions can upset an ecosystem

already handicapped by poor water circulation even in the best

of times.

In most years, the dissolved-oxygen problem is worse at depths

of 30 feet and lower, which allows most fish and sea life to find

some breathing room near the surface. But this year, even the

surface waters are starved for oxygen, Ecology oceanographer

Jan Newton said.

"The fish and other marine organisms just can't escape it,"

Newton said. Recent storms with winds from the south might

have pushed the oxygen-rich surface water out of the southern

end of the canal, she said.

Hoodsport residents Bob and Donna Sund said scuba divers in

front of their waterfront home have reported all sorts of dead

sea life, including octopus and rockfish, since late summer.

"We didn't see the dead fish last year like this year," said Bob

Why would most surface waters in the ocean be supersaturated with O2?

CO2 and O2 profiles in seawater

Southern Bering Sea

Dissolved oxygen in the

southern Bering SeaProfiles from MBARI.org

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AOU = O2 content at saturation minus measured O2 content

Water depth = 4000 m

Figure from wikipedia.com

Chlorofluorocarbons as tracers of large-scale circulation

Distribution of dissolved CFC-12 in the North Atlantic Ocean (Bullister, 1989)

Units:

pmol per Kg

Concentrations and

ratios of CFCs in

the northern

hemisphere

(Fine 2011)

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Seawater age in North Atlantic

Fine

(2011)

ye

ars

Gas summary

• At saturation, gas concentration controlled by

atmospheric concentration and solubility

• Solubility varies strongly with temperature

• Photosynthesis and respiration affect CO2 and

O2 concentrations, particularly at depth

• CO2 enters the deep ocean via the solubility and

biological CO2 pumps

• Gas concentrations vary in the deep ocean

along the deep ocean conveyor (O2, CO2, CFC)

Distributions of “reactive” solutes: Nutrients

Liebig’s law of minimums: Maximum population size, or production,

or growth rate is controlled by one limiting factor (e.g., in the case

of marine algae, a single nutrient or light)

How are limiting nutrients affected by biological processes?

Traditionally, two elements are considered to be limiting in coastal waters:

N, and P.

But, other elements, such as Si, Fe, Co, Zn and other trace metals

might limit phytoplankton growth from time to time as well.

Adding N and P to the photosynthesis equation:

106 CO2 + 16 NO3- + HPO4

2- +122 H20 + 18 H+ ↔

C106H263O106N16P1 + 138O2

Why are N or P thought to be limiting for algal populations?

N: Amino acids, nucleic acids, chlorophyll, etc.

P: membranes (phospholipids), nucleic acids, etc.

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Redfield Ratios of elements

Most particles in the open ocean are of biological origin.

The ratios of limiting elements in the ocean tend to

conform to the following ratio (by atoms) - C : N : P = 106:16:1

106 CO2 + 16 NO3- + HPO4

2- +122 H20 + 18 H+ ↔ C106H263O106N16P1 + 138O2

From Broeker and Peng 1982

Elemental ratios can tell

you about the nutritional

quality of POM – C:N, N:P

Dissolved phosphate versus nitrate

Slope ≈ 1/16

Dissolved nitrogen, phosphorus and silica species in the ocean

•N Forms: DIN: N2, NO3-, NH4

+, NO2-, N2,

Organic N: Particulate organic N, dissolved organic N

•P Forms: DIP: HPO42- (84%), H2PO4

-, PO43-

(DIP species depends upon pH: ↓ pH → ↑ H)

Organic P: Particulate and dissolved

•Si Dissolved forms: H4SiO4 (silicic acid)

Si(OH)4 (~ 97%), SiO(OH)31- (remainder)

Other dissolved forms: [SiOx(OH)4-xx-]

Particulate forms: SiO2.nH20 (opal)

Vertical profiles of nutrients:

Depleted in surface waters due to biological uptake

Regenerated at depth due to organic matter decomposition

Southern Bering Sea (Spring 2007)

0

500

1000

1500

2000

2500

3000

3500

20 30 40 50

Nitrate (µM )(µM )

0

500

1000

1500

2000

2500

3000

3500

2 2.5 3 3.5

Phosphate (µM)

De

pth

(m

)

Nutricline

Trace metals in the sea – Is the ocean becoming cleaner?

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Processes affecting the distributions of “reactive” trace elements

Adsorption: elements with low solubility often “stick” to the surfaces

of particulate material

Biological incorporation: Some metals are of nutritional importance

or mimic nutrients and are taken up by phytoplankton

(Together, these processes are referred to as “sorption”)

Precipitation: concentrations are low enough that this is not important

Coprecipitation: Due to low solubility, many metals will coprecipitate

Regeneration: Breakdown of particles can release trace elements back

into solution

Data from Bruland 1980. Figure from Broeker and Peng 1982

Zn and Cd profiles

look like nutrient

profiles, with removal

in euphotic zone.

Cu profile indicates

removal at intermediate

depths in the water

column.

Nearly perfect correlations

between Zn and Cd and

nutrients indicate complete

removal by phytoplankton

and identical recycling

processes at depth.

Correlations between Zn/Cd

and nutrients are better than

the correlation between

nitrate and phosphate.

Data from Bruland 1980. Figure from Broeker and Peng 1982

Types of trace element profiles and residence times (t)

Accumulated: t > 106 years

Recycled: t: 103 – 105 years

Scavenged: t < 103 years

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Figure by Y. Nozaki

Online version of periodic table

of elements in seawater

http://www.mbari.org/chemsensor/pteo.htm

Seawater sampling:

CTD + RosetteCTD = conductivity, temperature, depth

24 12-L bottles

Sensors:

CTD

Dissolved O2

Fluorometer

Light meter

Transmissometer

Altimeter

Nitrate sensor?

Others?

CTD photo from Sea-bird electronics

CTD = conductivity, temperature, depth

Sampling and measuring trace

elements

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SOIREE

SOFEx

SERIES

2 tons Fe

Evaporated seawater in bottom of five-gallon bucket