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) Must-read!

acid oceans: are oceans becoming more acidic? How does it work? Threat or fraud? (60p) Must-read!

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.

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 PO4^3-) 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:
 

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.
 

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 2.6-3.0% saline and estuaries 0-3%.

 

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.

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 non-conservative 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.
 
 

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 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 40-50% 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 9E-5 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.
 
 

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 near-perfect such that the oxygen concentration in the water is in equilibrium with the atmosphere. But it rapidly decreases below 50-75m 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.

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 C-14 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.
 
 

Note that at a pH of 7.0 (neutral water) only 0.1 µmol/kg (10^-7 ) 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).

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.

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 200-800m 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.