As water is vital for life on Earth, the Blue Planet in our Solar System, a very short summary of its main physical and chemical properties will be giv-en here. Water is a unique molecule and behaves
differently from most other chemical compounds.
The a-symmetrical atomic arrangement in a mol-ecule of pure water (H2O) is responsible for its special physical characteristics and a critical fac-tor for climate. Its oxygen atom (O) and the two hydrogen atoms (H) have an angle of 104.5°. This produces a dipole, a molecule with one negatively and one positively charged end. Because the water dipoles tend to hold together like small magnets, water reacts sluggishly to warming or cooling. For a given amount of heat absorbed, water has a lower rise in temperature than other substances; it has, in other words, a high heat capacity. In fact, water has the highest heat capacity of all liquid and solid substances with the exception of ammonia (World Ocean Review, 2010). That is why water can absorb and release large amounts of heat with very little change in temperature, thus causing the inertia heat reservoir of the ocean. But only 37 % of the sun’s radiation penetrates the water column to 1 m depth, 16 % to 10 m depth and only 0.5 % to 100 m depth (if the water is very clear) (Fig. 1.17). Global sea surface temperatures, show pronounced east-west temperature belts approximately paralleling the equator. During August, the warmest waters exceed 28°C and occur in a belt between 30°N and 10°S latitude where the solar radiation is at its max-imum. During winter in the Northern Hemisphere, this zone shifts south together with the belt of warm water until it is largely below the equator.
Fig. 1.15
a diameter of several centimetres and are composed primarily of manganese and iron. (Photo credit: D. Kelletat).
Manganese nodules from a Tertiary deep ocean floor, now exposed on land in the Negev Desert of Israel. These nodules have
ocean floors, over a very long time (millions of
the flanks of undersea mountain ranges (between
of climate processes influenced by the gigantic
Distribution of mineral resources in the world’s oceans
distribution of cobalt crusts occurences of maganese nodules continental plate margins occurences of black smokers
© 2011 Hans van der Baan / Ingeborg Scheffers
Fig. 1.16 Distribution of mineral resources in the world’s oceans.
Fig. 1.17 Global sea surface temperatures in August. This sea surface temperature map was produced using MODIS data acquired daily over the whole globe. The red pixels show warmer surface temperatures, whereas yellows and greens are intermediate values, and blue pixels indicate cold water. (Photo credit: NASA’s Visible Earth, http://visibleearth.nasa.gov/).
Oceans play a central role in the climate system.
Beside their heat capacity, the physical proper
ties of seawater vary with depth and drive ocean circulation. The water column of the oceans is vertically layered as a result of variations in the density of seawater which is a function of salin
ity and temperature. Cold, salty water is heavy and sinks to great depths until it reaches a level where the surrounding water has the same den
sity thus causing the stratification of water in the ocean. This powerful phenomenon, which primarily occurs in a few polar regions of the ocean, is called convection and we come back to it later. Oceanographers recognize three major depth zones in the oceans: A relative warm sur
face zone or “mixed layer”, extending to a depth of 100 – 500 m, where wind, waves and tempera
ture changes cause a constant mixing of this layer warmed by the radiation of the sun. Below lies a zone in which temperature, salinity and den
sity undergo significant changes with increas
ing depth. This zone is called the thermocline, a zone in which temperature decreases with depth.
Under this lies the deep zone, which contains about 80 % of the ocean’s water volume.
The most important chemical constituent of ocean water is salt with a mean concentration of 35 ‰ (varying between 33 and 37 ‰). However, salinity is closely related to latitude and is con
trolled by: precipitation as rain and snow which adds freshwater making the seawater less salty;
evaporation, which removes freshwater and makes the ocean water more salty; inflow of rivers car
rying freshwater into the sea; and the freezing of sea ice because during the salt minerals are excluded from the ice leaving the unfrozen sea
water more salty. Even if the salt content in gen
eral differs, the chemical substances are stable in their percentage: chlorides of sodium and mag
nesium comprise 88.7 % of the total salt content, sulphates (of magnesium, calcium and potassium) 10.8 %, and carbonates and bromides the remain
ing 0.5 %. Nearly all chemical elements can be found dissolved in ocean water, including gases like oxygen and nitrogen and rare metals. If all the salt were precipitated, it would form a layer of 53 m over the entire seafloor. But where does the salt in the sea come from? Each year rivers and streams carry an estimated 2.5 billion tons of dissolved substances to the sea. This includes cations such as sodium and potassium which are
leached out by weathering processes of rocks and become part of the dissolved load of rivers and streams flowing into the sea. Volcanic eruptions release gases such as water vapour and carbon dioxide, but also two important anions, chloride and sulphate, which dissolve in atmospheric water and return to the surface as precipitation, much of which falls directly into the ocean. Volcanic gas with its anions is also released directly into the ocean by submarine eruptions along the mid ocean ridges. Here, interactions between the heated rock and sea water play an important role in the composition of sea water as calcium, iron, and manganese together with trace elements are removed from the rock and added to the seawater.
Other important sources of salts in the oceans are dust particles eroded from the desert regions and blown out to the sea. Altogether, the quantity of dissolved ions added by these processes over the billions of years of Earth’s history exceeds the amount now dissolved in our today’s oceans. But over time the composition of seawater remains virtually unchanged! The reason is that the sub
stances are removed at the same time as they are added: Aquatic plants and animals are withdraw
ing elements such as silicon or calcium to build their skeletons; other elements like potassium or sodium are absorbed or removed by clay particles as they settle slowly on the sea floor, still others are precipitated to form new minerals in the oce
anic sediments. Overall, the processes of extrac
tion are equal to the combined inputs and the salinity of the sea remains unchanged over time.
An open question is if the oceans always have been salty? Best evidence of past saltiness are evaporates of marine origin, which are common in young sedimentary basins, but are not present in rocks older than about one billion years. Most geologists agree that there is evidence in many ancient strata that evaporates once were present.
Salt concentration in the oceans has also a major influence on marine organisms. The main
tenance of cell fluid composition is crucial to sur
vival of marine organisms (as it is to fresh water life). Their body cells must have a means by which to adapt to changing salt concentrations in their environments. This balance is met through the processes of osmosis, the passive movement of water particles from a region of higher concentra
tion to a lower concentration across a semi perme
able membrane. Osmose regulation is the active
regulation of particles within a cell en abling the on their cell walls.