Download Oceanography Lecture 15

“The highest form of human intelligence is
the ability to observe without judging”
Krishnamurti
“The intuitive mind is a sacred gift and the
rational mind is a faithful servant. We have
created a society that honors the servant
and has forgotten the gift”
Albert Einstein
“The mind is everything,
Oceanography
Lecture 15
a. Seawater Chemistry:
Sources of Sea salts – Constancy rule
c. Residence time: Steady-state
d. Dissolved gases
e. The carbonate system and Carbon cycle
what you think you become”
Buddha
Vertical Temperature & Salinity
Salinity vs Depth
Temperature vs Depth
Salinity (‰)
Temperature (°C)
0
5
10
Density Structure of the Oceans: Depth
34
15
20
34.5
35
35.5
36
0
0
2000
3000
4000
5000
1000
Thermocline
Halocline
2000
Depth (m)
Depth (m)
1000
3000
Halocline
4000
5000
The Ocean can be divided into three structures:
Surface layer; Pycnocline; Deep-layer.
Pycnocline: Amounts to ~18% of the Oceans volume)
Salinity
So, What is salinity?
• By weight, 96.5% of the Ocean are water, whereas
the remainder 3.5% are composed of dissolved
substances (salts).
• Oceans: 1,370.103 km3.
• Total weight of salts:
1,370.106 km3 ! 1.1015 cm3/km3 ! 1.03 g/cm3 ! 0.035
4.94! 1022 g of salts
~50 000 Trillion Tons!!!!!
! Could cover the entire planet with an even layer > 50
m in thickness!
Salinity: Composition
Major Elements
Only 7 elements belong to this
class > 100 ppm
Sodium
Sulfate
Magnesium
Common
form
ClNa+
SO4Mg2+
Concentration
(ppm)
18,980
10,556
2,649
1,272
Percent
by weight
50.08%
30.63%
7.69%
3.69%
Calcium
Potassium
Carbon
Ca2+
K+
HCO3-
400
380
140
1.16%
1.10%
0.41%
34.377
99.76%
Constituent
Chloride
Only 12 elements make up 99.9% of the dissolved
constituents of seawater!
Total
Minor Elements
Trace Elements
Only 5 elements belong to this
class 1 ppm > x > 100 ppm
Constituent
Common
form
Concentration
(ppm)
Percent
by weight
Bromide
Br-
65
0.19%
Strontium
Sr2-
8
0.02%
Boron
B3+
4.4
0.01%
Silicon
Si(OH)4
4.9
0.01%
Fluorine
F-
1.4
0.004%
83.7
0.234%
Total
Important trace elements:
Concentration < 1 ppm
Nitrogen
Lithium
Iodine
Phosphorus
Common
form
N
Li
I
P
Concentration
(ppb)
280
125
60
30
Zinc
Iron
Aluminum
Manganese
Zn
Fe
Al
Mn
10
6
2
2
Constituent
Etc…
…
Salinity distribution
Uniformity?
Uniformity?
Sea water chemistry
Percent
by weight
<0.001
Uniformity of Ocean Water
75% of the total Ocean volume
have a
• Temperature: 0-5°C
• Salinity: 34-35‰
Oceans’ depths are filled with
cold water (colder than the
~17.5°C average T of the
Oceans’ surface waters).
! Most of this water must have
originated in polar latitudes,
where it was chilled by losing
heat to the frigid air.
! Uniformity of T and salinity
of subsurface seawater from
Ocean to Ocean suggests that
Ocean basins are open systems!
Departure:
Departure:
Ø Utilization of carbon, calcium and magnesium by
biological activities (hard skeletons)
Ø Enclosed seas: estuaries and other regions can
receive substantial inflow of river water which
may contain less salts in different proportions
Ø Basins, Fjords, etc., where bottom circulation is
severely restricted (i.e. Black Sea). Complete
utilization of O2 followed by SO4-.
Principle of constant proportions
• In 1865, the chemist George Forchammer noted that,
although the total amount of dissolved solids (salinity)
might vary among samples, the ratio of major salts in
samples of seawater from many locations was
constant!
Na/Cl = 0.56; S/Cl = 0.14; Mg/Cl = 0.07
Regardless of changes in salinity (total amount of salts)!
The way salinity changes throughout the Oceans
depends almost entirely on:
- evaporation/precipitation balance, and
- extent of mixing between surface and deep waters.
Departure:
Departure:
Ø Extensive areas of warm, shallow waters (i.e.
Bahamas), characterized by very active chemical
and/or biological precipitation of calcium
carbonate leading to changes in Ca2+ and HCO3- to
total salinity.
Ø Areas of sea-floor where interstitial (pore)
waters in sediments react with sediments !
leading to large changes in ionic ratios.
Ø Regions of sea-floor spreading and active
submarine volcanism where heated seawater
circulates through cracks of the oceanic crust.
Ionic ratio in hydrothermal solutions are very
different from those of normal seawater
Sources of Sea Salts?
Example: Water on Earth
! Water is a great solvent: a great many substances
can dissolve within its matrix.
! Rain and river dissolve continental crustal rocks due
to the formation of acids (H2CO3; H2SO4, HCl) and the
solvent properties of water.
! H2O + CO2 " H2CO3 (acid)
! SO2 + OH + H2O # # H2SO4 (acid)
!These dissolved species then reach the Oceans
through surface flow in rivers (rivers provide most of
the dissolved elements to the Oceans!).
Sources of Sea Salts?
!So, does that mean that Oceans:
• Have a diluted but similar composition as crustal rocks?
Element
Si
Al
Fe
Ca
Na
K
Mg
Ti
Mn
P
% by weight
28.2
8.2
5.6
4.2
2.4
2.4
2.0
0.6
0.1
0.1
Average percentages by weight of
the ten most abundant elements
(other than oxygen) in the Earth’s
crust
The three most abundant
elements in the crust (Si, Al, Fe)
do not appear at all in the
major-minor dissolved elements
of the Oceans. Why?
! Solubility and reactivity of
different elements.
! Si, Fe, Al (very common) not
very soluble. Remain as particles
! Na, Ca, K, are relatively
soluble and remain in solution
!So, does that mean that Oceans:
• Have a diluted but similar composition as crustal
rocks?
• Are concentrated versions of river water?
Sources of Sea Salts?
!So, does that mean that Oceans are:
• Concentrated versions of rivers?
Rain
River
Seawater
Sources of Sea Salts?
! So, what is it?
It’s a matter of how fast the “clock ticks”…
• As water moves through the hydrological cycle on
Continents, it picks up salts (dissolved constituents) !
Weathering.
• Salts can then be maintained in solution for a long time
(rates!).
Sources of Sea Salts?
Origin of Chloride:
Negligible proportions of chloride in river water comes
from weathering.
Chlorine comes from volcanism
(excess volatile)
So does sulfur!
But, if input is so small how can
the total mass be so high?
Simplified Examples:
H2O + CO2 " H2CO3 (acid)
•
CaCO3 (calcite - sed) + H2CO3 (acid) " Ca2+ + 2 HCO3(bicarbonate)
•
2NaAlSi3O8 (albite - metam/ign) + 5H2CO3 " Al2Si2O5(OH)4
(kaolinite) + 2Na+ + H2CO3 + 4 SiO2 (partly sol)
Ocean Salinity
• Paradox: Oceans contain, in high concentrations,
elements that are rare in Earth Crust; and, in low
concentrations, elements that are abundant in
the Earth’s crust.
Sources of Sea Salts?
Residence time!
Total mass of dissolved compounds in the Ocean
Rate of supply (or removal)
Chloride:
261 1014 t/2.54 108 t/yr
– Selective weathering
– Residence time
1.03 108 yrs
Iron:
103 Million Years! (Corrected !
)
1.4 109 t/0.22 108 t/yr
64 yrs!
Changes in residence time = Changes in reactivity
Residence time – reactivity – mixing rate
Steady State ! It seems so for several 100’s of My!
And how do these elements leave?
Ø Biological uptake/Chemical precipitation
Ø Precipitation/Adsorption
Ø Reverse weathering
Ø Hydrothermal fluids
Major elements
Minor elements
Biolimiting elements
Steady State ! It seems so for several 100’s of My!
And how do these elements leave?
•
Biological uptake/chemical precipitation:
Ca2+ + CO32- " CaCO3
•
Precipitation/Adsorption
Fe + particles ! Iron-rich Clay particles
Sediment diagenesis (Fe, Mn, S, metals)
•
Reverse Weathering
! 5Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2"
2KAl5Si7O20(OH)4 (Illite)+ 7H2O + 2 CO2
! Aluminosilicate + Na+ + HCO3- + SiO2 "
Na0.33Al2.33Si3.67O10(OH)2 (Montmorillonite)+ H2O + CO2
•
Hydrothermal fluids (1979)
Sink for Mg. But source of Ca, K, Si, Fe… (?)
Dissolved gases in water
Take a break, Grab a soda…
Let’s assume we have a clear bottle of soda…
You pop the lid ! gas escapes! (pressure)
You shake it ! more gas escapes! (mixing)
You warm it up ! even more gas escapes!
(temperature)
Does it stop?
!When final gas content of soda is at equilibrium
with temperature and pressure
Dissolved gases in water
Take a break, Grab a soda…
soda…
Density structure of the Ocean
The total amount of gas that can be dissolved
eventually reaches an equilibrium concentration
which is proportional to:
- Atmospheric concentration of gas in atmosphere
- Temperature,
- Pressure,
- Salinity of water.
The amount of gas usually $ as:
P$
T%
S%
Oxygen in the Ocean
CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas)
Photosynthesis possible only in photic zone (0-200 m)
Oxygen minimum (utilization) at pycnocline – O2 Minimum
Dissolved gases in the Ocean
CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas)
Photosynthesis only in possible in photic zone (0-200 m)
Oxygen limitation (utilization) at pycnocline – O2 Minimum
Dissolved gases in the Ocean
Temperature is the most important controlling factor!
The most abundant gases in the atmosphere:
- N2 (78%)
- O2 (21%)
- Argon (0.9%)
- CO2 (0.03%)
The most abundant gases in the Oceans:
- CO2 (94.3%). Much more soluble!
- N2 (3.4%)
- O2 (2%)
- Argon (0.3%)
Difference due to the reactivity of CO2 is seawater leading
to the various carbonate and bicarbonate equilibria
Dissolved gases in the Ocean
Argon and the other noble gases (He, Ne, Kr, and Ra) and
Nitrogen are essentially unreactive in the Oceans:
Conservative gases
In contrast, CO2 and O2 concentrations are altered by many
biological and chemical processes in the sea.
Non-conservative gases
Photosynthesis/Respiration:
CO2(gas) + H2O + Energy # H12C6O6 (sugar) + O2 (gas)
H12C6O6 (sugar) + O2 (gas) # CO2(gas) + H2O + Energy
CO2(gas) + H2O " H2CO3 " H+ + HCO3- " H+ + CO32-
Dissolved gases/nutrients in the Ocean
CO2(gas) + H2O + Nutrients (N,P) + Energy " OM + O2 (gas)
Dissolved gases in the Ocean
CO2(gas) + H2O + Energy " H12C6O6 (sugar) + O2 (gas)
Photosynthesis only possible in photic zone (0-200 m)
Oxygen limitation (utilization) at pycnocline – O2 Minimum