Where did the oceans come from?

When the earth first formed it was much hotter than at present and there were no oceans. As the planet began to cool down and segregate into the core mantle and crust, volatile elements (gases) were released. The most abundant volatile was H2O, but also present were N2, CO2 and HCL. It is thought that the oceans could have began forming as soon as the earth was cool enough for liquid water to exist, about 4 billion years ago. At this time the released gases would have condensed and fallen as rain. Over millions of years sufficient water was generated to form the oceans. Additional volatiles are also released during volcanic eruptions and volcanoes much more abundant during the early very hot period of the earth’s history.

The bulk of the salt in the oceans comes from elements leached from rocks as well as those elements released during the early out gassing (hydrochloric acid and hydrogen sulphide) and from subsequent volcanism. It is thought that the concentration of salt in the ocean has been stable for the last million years, due to the equilibrium developed between input (mostly from rivers) and removal (mostly in sediments).

If the salt was extracted from the ocean today and dried it would form a 45m thick layer over the whole planet.

Ocean salinity

96.5% of the ocean is water and the other 3.5% is salt. Salinity is the total concentration of dissolved solids and is recorded as parts per thousand ie grams of dissolved solids per kilogram of seawater. While coastal waters can exhibit a wide range of salinity as a result of differences in freshwater runoff, most of the world’s ocean lies in the narrow salinity range of 33.8 to 36.8 0/00. The salt is composed of a number of components, the most abundant of which is chlorine (almost all the chlorine is thought to have come from volatiles mentioned above), followed by sodium, sulfate, magnesium, calcium, and potassium. The ratio of these components is extremely constant even in areas of differing salinity.

Differences in sea surface salinity can be due to a number of factors. Both evaporation and the formation of sea ice increase salinity. As water evaporates the salt it contains is left behind, thereby raising the salt concentration of the water. The structure of ice cannot accommodate very much salt, therefore as the water freezes the surrounding water becomes more saline. Alternatively, salinity is decreased by the input of fresh water from rivers, rain, snow and melting ice.

Map of the surface salinity of the oceans (from the Bigelow Laboratory for Ocean Sciences http://www.bigelow.org/)

Measuring salinity

Salinity can also be measured from space. The European Space Agency has embarked on a program that will make the first ocean salinity measurements from space. Due to be launched around 2004, the Soil Moisture and Ocean Salinity (SMOS) mission will also measure soil moisture over land. The sea surface salinity will be measured using microwave sensors. The technique relies on the fact that earth emits microwave signals that vary according to temperature and salinity. SMOS will use a frequency of 1.4GHz to make measurements. At this frequency the amount of microwave radiation emitted is particularly sensitive to changes in salinity.

A large antenna, several metres in diameter, is required to measure the microwave radiation. Until now the cost of launching such large equipment into space has been prohibitive. New technology has been developed that mimics a large antenna. Over 70 small antennae are deployed along three arms held in a Y-shape, which can be folded up to fit inside the launch vehicle. A measurement of sea surface salinity can be reconstructed from this large number of small antennae measurements using the principles of interferometry. A satellite image can then be put together of a 600km wide swath with a pixel size of 30 to 60km (from Dr Meric Srokosz
http://www.soton.ac.uk/~pubaffrs/oczone/spr2000/story13.htm).

NASA is also pursuing sea surface salinity (SSS) research. Their brief is to better understand the role of salinity at the ocean's surface in climate change http://oceans.nasa.gov/missions/.

Why Care About Salinity?

In the open ocean sea, surface salinity is fairly stable and most of the changes are due to precipitation. Therefore being able to monitor SSS gives us information about the global hydrological cycle, global warming, changes in ocean circulation and sea surface/air interaction. In contrast to the open ocean, salinity in the coastal zone is highly variable, affected by seasonal changes in precipitation and runoff. There are currently not enough long-term salinity measurements of the coastal zone to reliably discern changes. However, changes in land use patterns around the world, many of which result in greater runoff, suggest that coastal salinity would be affected. Many regions in the world's oceans have recently experienced a decline in salinity.

The hydrologic cycle of the ocean (ie pattern of evaporation and precipitation) is one of the least understood elements of the climate system. However, it is now considered one of the most important, especially for ocean circulation changes on decadal to millennial time-scales. The hydrologic cycle has a direct impact on the thermohaline circulation of the ocean (that part of the circulation driven by heat and salt differences), which is recognized as an important element in short term climate variability. Therefore data that record changes in sea surface salinity may be useful in predicting droughts, floods and other climatic variables (see section on temperature below).

Salt and What Else?

We now know that water is continually cycled through the crust and upper mantle, changing its composition at it goes (it has been estimated that it takes about 10 million years for all the water in the oceans to move through this cycle). In addition terrestrial erosion introduces material and an exchange of gas and particles occurs with the atmosphere. Therefore seawater contains a range of substances – major constituents, trace elements, nutrients, gases and organic compounds. Particles can be either in suspension or dissolved (the later defined as particles less than 4 microns).

As we have mentioned above, dissolved salts are the major constituents of seawater. The most abundant solutes are sodium (Na) and chlorine (Cl), followed by magnesium (Mg), sulphate (SO4–2), calcium (Ca), potassium (K), bromine (Br), bicarbonate (HCO3-), boric acid (H3BO4), strontium (St0 and fluorine (F). These elements are known as major constituents and occur in concentrations greater than 1 ppm (part per million). Major constituents are rapidly cycled by chemical and biological processes, however their concentration in seawater remains very constant. Also because mixing in seawater occurs rapidly the ratio of major constituents remains constant. This is termed the principle of constant proportions and is exploited when measuring salinity.

Trace elements are inorganic constituents that occur in very low concentrations, generally less than 1ppm. Trace elements are important in many chemical and biological reactions and concentrations can vary in both space and time. Trace elements include arsenic, iodine, manganese, copper, cadmium, mercury, tin, zinc, lead, radium, phosphorus and many others.

Selected major and minor constituents play a role as plant nutrients, essential for growth, eg iron, nitrogen and phosphorus. Gases dissolved in seawater include CO2 and O2 and vary according to rates of photosynthesis and respiration.

Ocean Temperature

The ocean and atmosphere are together responsible for distributing heat around the earth. Water has approximately 30 times the heat capacity of the atmosphere, so that small changes in circulation and sea surface temperature can have a marked affect on the world’s climate. For example the Southern Oscillation Index is used to predict short term climate variability around the globe. It has been found that the cyclic warming and cooling of the eastern and central Pacific can be recognised in changes in sea level pressure. In particular, when the pressure measured at Darwin is compared with that measured at Tahiti, the difference between the two can be used to generate an "index" number. When there is a positive number, we have a La-Niña (or ocean cooling), but when the number is negative we have an El-Niño (or ocean warming).

In normal, non- El-Niño conditions the trade winds blow towards the west across the Pacific. These winds push the surface water west, so that the sea surface is about 1/2 meter higher at Darwin than at Ecuador. This pile up of warm surface water results in a sea surface temperature about 8 degrees C higher in the west. The fact that the warm water is moving west produces upwelling of cold water off the coast of South America. Rainfall occurs due to rising air over the warmer water, resulting in the summer monsoon in the west and dryer conditions in the east.

During El Niño the trade winds relax in the central and western Pacific leading to cooler water in the western Pacific and relatively warmer water in the east. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in distant locations.

At other times, the injection of cold water becomes more intense than usual, causing the surface of the eastern Pacific to cool - this is a La-Niña event. This results in droughts in south America and heavy rainfall, even floods, in eastern Australia. In this way, Australia experiences it's characteristic cycle of droughts and floods

El Nino was first recognised by fisherman off the coast of South America as unusually warm water occurring near the beginning of the year. El Nino means little boy of Christ child in Spanish and was termed because the phenomena happened around Christmas time. Al Nina means the girl.

Science has as yet not fully explained what drives the El Nino/Al Nina cycles, however they appear to be related to the solar activity.