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
Comparison of river and seawater composition
Why is the ocean salty?
Ion
HCO3Ca2+
Na+
ClMg2+
SO42K+
Average
River (mM)
0.86
0.33
0.23
0.16
0.15
0.069
0.03
Average
Seawater (mM)
2.38
10.2
468
545
53.2
28.2
10.2
Seven major solutes in seawater make up its salinity.
Their relative concentrations do not match river water.
1
Major components of seawater and constancy of composition
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
Measuring salinity
1. Chlorinity (salinity = 1.80655 x chlorinity (%o)
2. Conductivity
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
CTD:
Conductivity
Temperature
Depth
3. Refractive index (refractometer)
Oceanographer’s main
sampling device
Could be called:
CTDLTrDOChlA…
Water-sampling
rosette or carousel
CTD photo from Sea-bird electronics
2
Effects of salinity on seawater
-3
Density (g cm )
1000
1: Freezing point depression, boiling point elevation
(disrupts H-bonding, lowers vapor pressure)
Freezing point of seawater (35%o) ~ -1.88 deg C
1019.84
Max density
999.88
999.84
1019.68
1019.52
Fresh water
0
2
4
6
8
25 psu
1019.36
-1
1
0
o
Temperature ( C)
3
5
7
o
Temperature ( C)
1012.08
-3
Density (g cm )
4: Eliminates density-temperature anomaly
(24.7%o, freezing temp = -1.33 deg C)
1020
999.92
999.8
2: Changes density
pure water density: 1 kg/l
seawater density (35%o, 20 deg C): ~1.024 kg/l
common density units: 24 t
3. Causes stratification (along with temperature)
1020.16
999.96
Variation in water density
with changes in temperature
and salinity (calculated using
equation of state of seawater
at standard pressure)
1012
1011.92
1011.84
Max density
1011.76
1011.68
15 psu
1011.6
-1
5: Salt is hygroscopic (moisture attracting)
0
1
3
5
7
o
Temperature ( C)
Fig. 5.7
Comparison of river and seawater composition
Ion
HCO3Ca2+
Na+
ClMg2+
SO42K+
Average
River (mM)
0.86
0.33
0.23
0.16
0.15
0.069
0.03
Average
Seawater (mM)
2.38
10.2
468
545
53.2
28.2
10.2
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+ + 8SiO2
3
Removal mechanisms
Evaporite formation, precipitation, coprecipitation, uptake by organisms,
sorption onto particle surfaces, scavenging in hydrothermal vent systems
(Mg+, SO4-) followed by burial in sediments
Gas concentrations in air and seawater
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
0.04
1.4
Carbon Dioxide CO2
Neon
Ne
0.0018
0.00097
Helium
He
0.00052
0.00023
Methane
CH4
0.0002
0.00038
Krypton
Kr
What else is dissolved in 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: very soluble:
Carbonate system (Ocean’s buffering system):
0.00011
0.00038
Carbon Monoxide CO
0.000015
0.000017
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
Nitrous Oxide
N2O
0.00005
0.0015
Xenon
Xe
0.0000087
0.000054
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
4
CO2 pumps: solubility and biological
• Solubility pump
Forms CaCO3 shells
Forms organic matter – both can sink
– 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.
Biological Pump
Respiration Photosynthesis
Solubility Pump
High latitude
Low latitude
Spatial CO2 air-sea exchange
Flux into the ocean
Solubility pump accounts for ~ 90% of carbon stored in ocean
Figure from wikipedia.com
Flux out of the ocean
From Takahashi et al. 2002 Deep-Sea Research
5
Parts of Hood Canal become dead zone
Spatial variation in biological pump
CO2 is taken out of the atmosphere by biological productivity
From Takahashi et al. 2002 Deep-Sea Research
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
CO2 and O2 profiles in seawater
?
This DO profile
makes little sense
Southern Bering Sea
Why would most surface waters in the ocean be supersaturated with O2?
This is what a DO profile
actually looks like.
6
AOU = O2 content at saturation minus measured O2 content
Water depth = 4000 m
Chlorofluorocarbons as tracers of large-scale circulation
Concentrations and
ratios of CFCs in
the northern
hemisphere
(Fine 2011)
Units:
pmol per Kg
Figure from wikipedia.com
Distribution of dissolved CFC-12 in the North Atlantic Ocean (Bullister, 1989)
7
Seawater age in North Atlantic
Temperature, Salinity, Chlorophyll and nutrients
(nitrate and ammonium) in the southern Bering Sea
(Data from C. Mordy and E. Cokelet, NOAA)
years
Fine
(2011)
Distributions of “reactive” solutes: Nutrients
Why are N or P thought to be limiting for algal populations?
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)
N: Amino acids, nucleic acids, chlorophyll, etc.
How are limiting nutrients affected by biological processes?
P: membranes (phospholipids), nucleic acids, etc.
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- + HPO42- +122 H20 + 18 H+ ↔
C106H263O106N16P1 + 138O2
8
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 +122 H20 + 18
C106H263O106N16P1 + 138O2
-
2-
H+
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)
↔
Slope ≈ 1/16
Elemental ratios can tell
you about the nutritional
quality of POM – C:N, N:P
From Broeker and Peng 1982
Dissolved phosphate versus nitrate
Trace metals in the sea – Is the ocean becoming cleaner?
Vertical profiles of nutrients:
Depleted in surface waters due to biological uptake
Regenerated at depth due to organic matter decomposition
Nitrate
( µM (µM)
)
Phosphate
( µM )
202
00
500
500
2.5
30
403
3.5
50
Nutricline
Depth (m)
1000
1000
1500
1500
2000
2000
2500
2500
3000
3000
3500
3500
Southern Bering Sea (Spring 2007)
9
Processes affecting the distributions of “reactive” trace elements
Zn and Cd profiles
look like nutrient
profiles, with removal
in euphotic zone.
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
Cu profile indicates
removal at intermediate
depths in the water
column.
Regeneration: Breakdown of particles can release trace elements back
into solution
Data from Bruland 1980. Figure from Broeker and Peng 1982
Types of trace element profiles and residence times ()
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.
Accumulated:  > 106 years
Recycled: : 103 – 105 years
Scavenged:  < 103 years
Data from Bruland 1980. Figure from Broeker and Peng 1982
10
Online version of periodic table
of elements in seawater
Figure by Y. Nozaki
http://www.mbari.org/chemsensor/pteo.htm
Sampling and measuring trace
elements
Seawater sampling:
CTD + Rosette
CTD photo from Sea-bird electronics
CTD = conductivity, temperature, depth
24 12-L bottles
Sensors:
CTD
Dissolved O2
Fluorometer
Light meter
Transmissometer
Altimeter
Nitrate sensor?
Others?
11
SOIREE
2 tons Fe
SERIES
SOFEx
Evaporated seawater in bottom of five-gallon bucket
12