4/12/2015
Composition of seawater
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. new tab.)
In order to understand the sea, some of its chemical
properties are important. This page details the
chemical composition of sea water, salinity, density,
its dissolved gases, carbon dioxide and pH as
limiting factor. Chemical elements in sea water do
not exist on their own but are attracted to
preferential ions of opposite charge: sulphur will
occur mainly as sulphate, sodium as sodium
chloride, and so on.
Detailed composition: abundance of the elements in seawater
Salinity: the main salt ions making the sea salty
Density: the density of sea water depends on temperature and salinity
Dissolved gases: the two important gases to life, oxygen and carbondioxide. Limiting hydrogen ions
and ocean pH.
Bicarbonate: the life of dissolved carbon dioxide in the sea.
Related chapters:
global climate: learn about global climate step by step, from a very wide perspective. Is
global warming real or fraudulent? (140p) Must­read!
acid oceans: are oceans becoming more acidic? How does it work? Threat or fraud?
(60p) Must­read!
abundance of the elements of life in the universe, earth, sea and organisms.
table of units & measures: units, measures, conversion constants, world dimensions, and
much more.
periodic table: the periodic table of elements, complete with elementary chemistry and
interesting facts.
soil/ecology: the main biomes of the land and their carbon sinks. How does soil work?
Sustainability? What to do against erosion? (large)
the Dark Decay Assay: new discoveries of the plankton ecosystem. pH as most
important limiting factor.
. ► Seawater
http://www.seafriends.org.nz/oceano/seawater.htm
► Composition
► Sea Water Fish
► PH in Water
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Composition of seawater
­­ Seafriends home ­­ oceanography ­­ sitemap ­­ Rev 20000714,20060825,20070515,20070718,20100608, Detailed composition of seawater Element Hydrogen H2O Oxygen H2O Sodium NaCl Chlorine NaCl Magnesium Mg Sulfur S Potassium K Calcium Ca Bromine Br
Helium He Lithium Li Beryllium Be Boron B Carbon C Nitrogen ion Fluorine F Neon Ne Aluminium Al Silicon Si Phosphorus P Argon Ar Scandium Sc Titanium Ti Vanadium V Chromium Cr Manganese Mn Ferrum (Iron) Fe Cobalt Co Nickel Ni
at 3.5% salinity
At.weight ppm Element 1.00797 110,000 Molybdenum Mo 15.9994 883,000 Ruthenium Ru 22.9898 10,800 Rhodium Rh 35.453 19,400 Palladium Pd 24.312 1,290 Argentum (silver) Ag 32.064 904 Cadmium Cd 39.102 392 Indium In 40.08 411 Stannum (tin) Sn 79.909
67.3
Antimony Sb
4.0026 0.0000072 Tellurium Te 6.939 0.170 Iodine I 9.0133 0.0000006 Xenon Xe 10.811 4.450 Cesium Cs 12.011 28.0 Barium Ba 14.007 15.5 Lanthanum La 18.998 13 Cerium Ce 20.183 0.00012 Praesodymium Pr 26.982 0.001 Neodymium Nd 28.086 2.9 Samarium Sm 30.974 0.088 Europium Eu 39.948 0.450 Gadolinium Gd 44.956 <0.000004 Terbium Tb 47.90 0.001 Dysprosium Dy 50.942 0.0019 Holmium Ho 51.996 0.0002 Erbium Er 54.938 0.0004 Thulium Tm 55.847 0.0034 Ytterbium Yb 58.933 0.00039 Lutetium Lu 58.71
0.0066
Hafnium Hf
Copper Cu Zinc Zn Gallium Ga Germanium Ge Arsenic As Selenium Se Krypton Kr Rubidium Rb Strontium Sr Yttrium Y Zirconium Zr Niobium Nb
63.54 65.37 69.72 72.59 74.922 78.96 83.80 85.47 87.62 88.905 91.22 92.906
0.0009 0.005 0.00003 0.00006 0.0026 0.0009 0.00021 0.120 8.1 0.000013 0.000026 0.000015
Tantalum Ta Tungsten W Rhenium Re Osmium Os Iridium Ir Platinum Pt Aurum (gold) Au Mercury Hg Thallium Tl Lead Pb Bismuth Bi Thorium Th Uranium U Plutonimu Pu
At.weight 0.09594 101.07 102.905 106.4 107.870 112.4 114.82 118.69 121.75
127.6 166.904 131.30 132.905 137.34 138.91 140.12 140.907 144.24 150.35 151.96 157.25 158.924 162.50 164.930 167.26 168.934 173.04 174.97 178.49
ppm 0.01 0.0000007 . . 0.00028 0.00011 . 0.00081 0.00033
. 0.064 0.000047 0.0003 0.021 0.0000029 0.0000012 0.00000064 0.0000028 0.00000045 0.0000013 0.0000007 0.00000014 0.00000091 0.00000022 0.00000087 0.00000017 0.00000082 0.00000015 <0.000008
180.948 183.85 186.2 190.2 192.2 195.09 196.967 200.59 204.37 207.19 208.980 232.04 238.03 (244)
<0.0000025 <0.000001 0.0000084 . . . 0.000011 0.00015 . 0.00003 0.00002 0.0000004 0.0033 .
Note! ppm= parts per million = mg/litre = 0.001g/kg.
source: Karl K Turekian: Oceans. 1968. Prentice­Hall
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Composition of seawater
Salinity and the main salt ions The salinity of sea water (usually 3.5%) is made up by all the dissolved salts shown in the above table.
Interestingly, their proportions are always the same, which can be understood if salinity differences are caused by
either evaporating fresh water or adding fresh water from rivers. Freezing and thawing also matter.
Salinity affects marine organisms because the process of osmosis transports water towards a higher concentration
through cell walls. A fish with a cellular salinity of 1.8% will swell in fresh water and dehydrate in salt water. So,
saltwater fish drink water copiously while excreting excess salts through their gills. Freshwater fish do the opposite
by not drinking but excreting copious amounts of urine while losing little of their body salts.
Marine plants (seaweeds) and many lower organisms have no mechanism to control osmosis, which makes them
very sensitive to the salinity of the water in which they live.
The main nutrients for plant growth are nitrogen (N as in nitrate NO3­, nitrite NO2­, ammonia NH4+ ), phosporus (P
as phosphate PO43­) and potassium (K) followed by Sulfur (S), Magnesium (Mg) and Calcium (Ca). Iron (Fe) is an
essential component of enzymes and is copiously available in soil, but not in sea water (0.0034ppm). This makes
iron an essential nutrient for plankton growth. Plankton organisms (like diatoms) that make shells of silicon
compounds furthermore need dissolved silicon salts (SiO2) which at 3ppm can be rather limiting.
The main salt ions that make up 99.9% are the following: Chloride Cl
concentration part of molecular mmol/ ppm, mg/kg salinity %
weight
kg
­1
19345
55.03
35.453
546
Sodium Na
Sulfate SO4
Magnesium Mg
+1
­2
+2
10752
2701
1295
30.59
7.68
3.68
22.990
96.062
24.305
468
28.1
53.3
Calcium Ca
Potassium K
Bicarbonate HCO3
+2
+1
­1
416
390
145
1.18
1.11
0.41
40.078
39.098
61.016
10.4
9.97
2.34
Bromide Br
Borate BO3
­1
­3
66
27
0.19
0.08
79.904
58.808
0.83
0.46
Strontium Sr
Fluoride F
+2
­1
13
1
0.04
0.003
87.620
18.998
0.091
0.068
chemical ion
valence
By adding the µmol in last column up, multiplied by respective valences, like: ­546 +468 ­56.2 +106.6 + .... one
ends up with almost 0, suggesting that the above values are about right. During the Challenger Expedition of the
1870s, it was discovered that the ratios between elements is nearly constant although salinity (the amount of H2O)
may vary. Note that the figures above differ slightly in differing publications. Also landlocked seas like the Black
Sea and the Baltic Sea, have differing concentrations. http://www.seafriends.org.nz/oceano/seawater.htm
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This world map shows how the salinity of the
oceans changes slightly from around 32ppt (3.2%)
to 40ppt (4.0%). Low salinity is found in cold seas,
particularly during the summer season when ice
melts. High salinity is found in the ocean 'deserts' in
a band coinciding with the continental deserts. Due
to cool dry air descending and warming up, these
desert zones have very little rainfall, and high
evaporation. The Red Sea located in the desert
region but almost completely closed, shows the
highest salinity of all (40ppt) but the Mediterranean
Sea follows as a close second (38ppt). Lowest
salinity is found in the upper reaches of the Baltic
Sea (0.5%). The Dead Sea is 24% saline, containing
mainly magnesium chloride MgCl2. Shallow coastal
areas are 2.6­3.0% saline and estuaries 0­3%.
Making sea salt Sea salt is made by evaporating sea water, but this is not straight­forward. Between 100% and 50% first the calcium
carbonate (CaCO3= limestone) precipitates out, which is chalk and not desirable. Between 50% and 20%, gypsum
precipitates out (CaSO4.2H2O), which also tastes like chalk. Between 20% and 1% sea salt precipitates (NaCl) but going
further, the bitter potassium and magnesium chlorides and sulfates precipitate, which is to be avoided, unless for health
reasons. In commercial salt production, the water is led through various evaporation ponds, to achieve the desired result. Note that this process has also happened where large lakes dried out, laying down the above salts in the above sequence.
Note that normal sea water is undersaturated with respect to all its salts, except for calcium carbonate which may occur in
saturated or near­saturated state in surface waters. An artificial salt solution of 3.5% (35ppt) is made by weighing 35g of salt in a beaker and topping it up with fresh water to
1000g.
Density The density of fresh water is 1.00 (gram/ml or kg/litre) but added salts can increase this. The saltier the water, the
higher its density. When water warms, it expands and becomes less dense. The colder the water, the denser it
becomes. So it is possible that warm salty water remains on top of cold, less salty water. The density of 35ppt
saline seawater at 15ºC is about 1.0255, or s (sigma)= 25.5. Another word for density is specific gravity.
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Composition of seawater
The relationship between temperature, salinity and
density is shown by the blue isopycnal (of same
density) curves in this diagram. In red, green and
blue the waters of the major oceans of the planet is
shown for depths below ­200 metre. The Pacific has
most of the lightest water with densities below 26.0,
whereas the Atlantic has most of the densest water
between 27.5 and 28.0. Antarctic bottom water is
indeed densest for Pacific and Indian oceans but not
for the Atlantic which has a lot of similarly dense
water.
Dissolved gases in seawater The gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations
depend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amount of
gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperature
increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount
of gas dissolved.
Inert gases like nitrogen and argon do not take part in the processes of life and are thus not affected by plant and
animal life. But non­conservative gases like oxygen and carbondioxide are influenced by sea life. Plants reduce the
concentration of carbondioxide in the presence of sunlight, whereas animals do the opposite in either light or
darkness. gas molecule
Nitrogen N2
Oxygen O2
% in atmosphere
78%
21%
% in surface seawater
47.5%
36.0%
ml/litre sea water
10
5
mg/kg (ppm) in sea water
12.5
7
Carbondioxide CO2 0.03%
15.1%
40
90 *
Argon
1%
1.4%
.
0.4
One kg of fresh water contains 55.6 mol H2O * also reported as 80 mg/kg molecular weight
28.014
31.998
mmol/ kg
0.446
0.219
42.009
39.948
2.142
0.01
Please note that these figures may be incorrect as too many different values have been published
In the above table, the conservative gases nitrogen and argon do not contribute to life processes, even though
nitrogen gas can be converted by some bacteria into fertilising nitrogen compounds (NO3, NH4). Surprisingly the
world under water is very much different from that above in the availability of the most important gases for life:
oxygen and carbondioxide. Whereas in air about one in five molecules is oxygen, in sea water this is only about 4
in every thousand million water molecules. Whereas air contains about one carbondioxide molecule in 3000 air
molecules, in sea water this ratio becomes 4 in every 100 million water molecules, which makes carbondioxide
much more common (available) in sea water than oxygen. Note that even though their concentrations in solution
differ due to differences in solubility (ability to dissolve), their partial pressures remain as in air, according to
Henry's law, except where life changes this. Plants increase oxygen content while decreasing carbondioxide and
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animals do the reverse. Bacteria are even capable of using up all oxygen.
All gases are less soluble as temperature increases, particularly nitrogen, oxygen and carbondioxide which become
about 40­50% less soluble with an increase of 25ºC. When water is warmed, it becomes more saturated, eventually
resulting in bubbles leaving the liquid. Fish like sunbathing or resting near the warm surface or in warm water
outfalls because oxygen levels there are higher. The elevated temperature also enhances their metabolism, resulting
in faster growth, and perhaps a sense of wellbeing. Likewise if the whole ocean were to warm up, the equilibrium with the atmosphere would change towards more
carbondioxide (and oxygen) being released to the atmosphere, thereby exacerbating global warming.
Since the volume of all oceans is 1.35E21 kg (see table of units & measures) and CO2 concentration is 9E­5 kg/kg
(90ppm), it follows that the total amount of CO2 in all oceans is 12.2E16 kg = 121,000 Pg (Mt) and the partial
carbon amount (12/42) = 34,700 Pg (600Pg in surface waters + 7000Pg in mid waters + 30,000Pg in deep ocean = 37,600Pg [1]).
Compare this with the amount of carbon in soil and vegetation (1301 + 664 = 1965 Pg, see soil/ecology) and the
carbon in the atmosphere, about 1 kg per square metre over 510E6 km2 = 510E12 kg = 510 Pg (700Pg [1]). It
follows that the ocean is a very large reservoir of carbondioxide, also called Dissolved Inorganic Carbon (DIC). In
addition to this, it contains Dissolved Organic Carbon (DOC) of unknown quantity. The difference between DIC
and DOC is an arbitrary particle size of 0.45µm which passes DIC through filtration paper. This definition does not
distinguish our newly discovered slush (incompletely decomposed biomolecules) as DOC. See our DDA section. What is dissolved, particulate, inorganic and organic carbon? Carbon is a miraculous element located in the middle of the Periodic Table, next to nitrogen, which is also a surprising
element. Elements to the left are basic with positive valence (attracting free electrons) and those to the right are acidic
with negative valence (owning loose electrons). Carbon with a valence of 4 can bind with both sides of the table and with
itself. When combined with hydrogen, it forms long chains of organic molecules like CH3.CH2.CH2......X where the end
group X gives it the character of an alkane (CH3), alcohol (OH), acid (COOH), aldehyde (COH), amino (NH2), and so on.
The organic carbon chains can form loops and bonds with other elements, all being organic compounds. Only few
inorganic carbon compounds are known, of which carbondioxide (CO2) is by far the most common. Natural gas or
methane (CH4) is either the last inorganic molecule or the first organic molecule. So it is safe to say that dissolved
inorganic carbon is CO2, particularly since it dissolves so readily in water.
All biomolecules that make up the structure of an organism are organic (except for salts in body liquids), and when these
are decomposed, the leftover molecules are also organic, except for inorganic nutrients and CO2, for the whole purpose of
decomposition is to turn organic molecules into inorganic nutrients and CO2 for plants. All biomolecules can be
transported by being dissolved in water. When an organism dies and decomposes, most of its organic molecules end up in
solution as dissolved organic carbon (DOC), molecules that are very much smaller than the smallest of organisms
(viruses).
Plankton organisms are classified by size from femtoplankton (smaller than 0.2µm), picoplankton (0.2­2µm) to
megaplankton (0.2­2m). Note that the wavelength of visible light is 0.4­0.7µm, which means that organisms smaller than
1µm are not visible under a light microscope (all viruses and most bacteria). What all this means is that measuring the
biomass of plankton is almost impossible. For practical reasons, scientists decided that anything passing through fine
filtration paper (0.45µm) is dissolved and all that is retained is particulate. Unfortunately this marks a substantial amount
of particulate biomass as dissolved.
Phytoplankton consists of organisms from bacteria to diatoms and large dinoflagellates (like sea spark, Noctiluca
scintillans). Their biomass can be estimated by measuring their chlorophyl (green pigment) from light measurements.
However, other pigments (brown, red) are also common and the amount of chlorophyl is only a small part of biomass. So,
even quantifying the amount of phytoplankton is almost impossible.
The bottom line is that the boundaries between dissolved, particulate, inorganic and organic are rather vague. Also the
functional difference between producers (phytoplankton) and decomposers (most bacteria) is seldom acknowledged.
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Composition of seawater
Deep sea temperature, oxygen & nutrients In general the ratios between the various elements in
seawater is constant, except where modified by life.
In this diagram one can see how light penetrates no
deeper than 150m for photosynthesis. Indeed at
800m, the ocean is pitch dark. In the surface mixed
layer above the thermocline, water mixes
sufficiently to sustain life. Gas exchange with the
atmosphere is near­perfect such that the oxygen
concentration in the water is in equilibrium with the
atmosphere. But it rapidly decreases below 50­75m
as photosynthesis declines while animals use up
most oxygen. At around 800m oxygen levels reach a
minimum (as also carbondioxide levels reach a
maximum, not shown). Towards the deep and
bottom water, oxygen levels increase slightly due to
an influx of cold bottom water from the poles. Due to lack of oxygen, deep sea fish cannot be very active.
The coloured curves for phosphate and nitrate show how these nutrients are almost completely used near the
surface and how they gradually become available in the thermocline layer. Note how the Atlantic Ocean ends up
with less nutrients than the Pacific and Indian oceans. The temperature curve shows the general idea of staying relatively high and constant in the mixed layer, then
declining rapidly in the thermocline layer until reaching a near constant temperature of +3ºC in deep and bottom
water. The maximum surface temperature of course depends on many factors, like latitude and season.
Note that the concentration of CO2 in the atmosphere has increased from 280 ppm in 1850 to 360 ppm in 1998, and
is still rising. It is estimated that about 50% of anthropogenic CO2 has been absorbed by the oceans. Because the
upper atmosphere is bombarded by cosmic rays, some of the nitrogen atoms become radioactive isotopes C­14 with
a half life of 5730 years. Once incorporated into organisms, its radioactivity decays slowly, allowing scientists to
calculate the age of organic substances. Fossil fuels which have been underground for over 60 million years, have
lost nearly all their radioactive carbon isotopes, and in this manner CO2 from burning fossil fuels can be
distinguished from normal CO2 circulation. The diagrams below shows how fossil carbondioxide is absorbed by
the oceans. Radioactive Carbon­14 As cosmic rays bombard the outer atmosphere, they are slowed down by the thin gases there. With their energy of billions
of electron­Volt (eV) they produce fast neutrons that gradually slow down to that of thermal neutrons. At a height of about
9­15km, these neutrons collide with nitrogen­14 (normal nitrogen), producing radioactive carbon­14 (carbon with one
extra neutron). The total amount of C­14 produced each year is about 9.8kg for the whole Earth, or about 1 atom C­14 for
1 trillion (1E­12) normal C­12 atoms. Nuclear tests have almost doubled the quantity in the atmosphere in a peak (year
1964) that is gradually becoming normal again as the peak is absorbed by organisms and the ocean. Radioactive carbon
decays back to nitrogen by emitting an electron (beta radiation) at the initial rate of 14 disintegrations per minute per
gram carbon. The C­13 carbon isotope which is not radioactive, occurs for about one in every 100 atoms C. The age of
organic remains can thus be measured by counting beta radiation from disintegrating atoms, but a much more sensitive
method is by counting all C14 atoms by mass spectrometry. Because of its slow decay rate of 50% in 5700 years, the total amount of C­14 in the atmosphere, biosphere and oceans is
much higher than 10kg. According to Libby (1955) who invented carbon dating, the distribution of carbon and carbon­14
is as follows: carbon reservoir
percentage
CO2 dissolved in oceans
87.5
Dissolved Organic Carbon (DOC) in oceans 7.1
Biosphere, all living organisms
4.0
Atmospheric CO2
1.4
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Composition of seawater
Note that at a pH of 7.0 (neutral water) only 0.1 µmol/kg (10­7 ) of water is dissociated into positive hydrogen ions
H+ and negative hydroxyl ions OH­ . In the ocean where a pH of around 8 is found, this becomes even less at 0.01
µmol/kg, which makes hydrogen ions twenty times scarcer than oxygen and 200 times scarcer than
carbondioxide. It explains how important the pH is to the productivity of aquatic ecosystems. Visit our latest
plankton discoveries in the Dark Decay Assay section where this limiting factor was quantified in freshwater lakes. This world map of ocean acidity shows that ocean pH varies from about 7.90 to 8.20 but along the coast one may
find much larger variations from 7.3 inside deep estuaries to 8.6 in productive coastal plankton blooms and 9.5 in
tide pools. The map shows that pH is lowest in the most productive regions where upwellings occur. It is thought
that the average acidity of the oceans decreased from 8.25 to 8.14 since the advent of fossil fuel (Jacobson M Z,
2005).
Carbondioxide as bicarbonate Carbondioxide binds loosely with water to form bicarbonate:
CO2 + H2O <=> H2CO3 <=> H+ + HCO3­ <=> H+ + H+ + CO32­
in the ratios CO2 & carbonic acid H2CO3 = 1%, bicarbonate HCO3­ = 93%, carbonate CO32­ =6%. These variants
of CO2 (species) add up to the total amount of Dissolved Inorganic Carbon (DIC), which also includes a smaller
amount of Dissolved Organic Carbon (DOC) that passes filtration techniques. The <=> symbol means 'in equilibrium with'.
These forms of carbon are always in close equilibrium with the atmosphere and with one another. When one talks
about dissolved carbondioxide, it is the slightly acidic bicarbonate. When the concentration of CO2 in the
atmosphere increases, presumably also the concentration in the ocean's surface increases, and this works itself
through to the right in above equation.
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Photosynthesis of organic matter is often simplified as: CO2 + H2O + sunlight => CH2O +O2, which happens
only in the sunlit depths to 150m and down to where the sea mixes.
The average composition of marine plants is: H:O:C:N:P:S = 212:106:106:16:2:1 which comes close to CH2O.
Respiration is often simplified as : CH2O => CO2 + H2O + energy, which can happen at all depths, depending on
the amount of food sinking down from above.
Therefore the concentrations of oxygen and carbondioxide vary with depth. The surface layers are rich in oxygen
which reduces quickly with depth, to reach a minimum between 200­800m depth. The deep ocean is richer in
oxygen because of cool and dense surface water descending from the poles into the deep ocean.
It is thought that the carbondioxide in the sea exists in equilibrium with that of exposed rock containing limestone
CaCO3. In other words, that the element calcium exists in equilibrium with CO3. But the concentration of Ca
(411ppm) is 10.4 mmol/l and that of all CO2 species (90ppm) 2.05 mmol/l, of which CO3 is about 6%, thus 0.12
mmol/l. Thus the sea has a vast oversupply of calcium. [1] Report of the Royal Society (June 2005): Ocean acidification due to increasing atmospheric carbon dioxide.
http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13539 (1MB) . ► PH in Water
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► Ocean
► Soil PH Levels
► PH
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