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9.8 Oceanography: 1. The oceans have evolved

Syllabus reference: (October 2002 version)
1. The oceans have evolved over the history of the Earth	*Students learn to:* describe the modern oceans in terms of: average temperature mean depth average salinity average density identify the area of the Earth covered by oceans and explain how this influences conditions on the Earth’s surface identify the probable origins of the oceanic waters compare the evolution of the oceanic waters with the evolution of the atmosphere and explain how and why the two are linked	Students: process and present secondary information to produce a flow chart illustrating the movement of water, carbon and oxygen between the oceans and the atmosphere

Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edit: 7 Aug 08]

Prior learning: Preliminary module 8.2 Planet Earth and its Environment (subsections 4 and 5).
Preliminary module 8.4 Water Issues (subsection1)

Background: Viewed from space the oceans dominate the earth, composing 71% of the surface. They evolved relatively early in the evolution of the Earth.

process and present secondary information to produce a flow chart illustrating the movement of water, carbon and oxygen between the oceans and the atmosphere

Process information from your teacher or that you have gathered on the Internet or in text books, to illustrate trends or patterns of the movement of water, carbon and oxygen between the oceans and the atmosphere.
- Carbon would be in the form of carbon dioxide in the atmosphere and dissolved in the oceans as well as plants and animals (mostly in the form of bicarbonate) in the water that have taken the carbon and incorporated it into their bodies. Carbon dioxide is converted to carbohydrate during photosynthesis and then to other organic molecules.
- Oxygen is in the form of oxygen gas in the atmosphere and dissolved in water, especially where the water is flowing or waves are crashing against rocks and beaches. This oxygen is then taken in by organisms in the water (plants and animals) and combined with glucose during respiration to produce energy.
- Water evaporates from the oceans when it is heated by the sun and condenses into clouds when it is cooled by the cooler air higher in the atmosphere. The condensed water then falls back to the land and water as rain, mist, snow or hail. Water is also split during photosynthesis and oxygen gas (O₂) is released to the atmosphere or into the ocean. More information on the water cycle can be accessed, The Water Cycle NASA Oceanography, USA
When this information has been put in an ordered format, possibly using diagrams and arrows, arrange the information in the sequential order of a flow chart.
A website that explains The carbon cycle: a simple explanation The Hadley Centre, The Met Office, UK(Website last accessed 7 Aug 2008)

describe the modern oceans in terms of:

average temperature
mean depth
average salinity
average density

Background information

Generally the ocean structure at middle and low latitudes can be regarded as a 3 layered system:

an upper, well mixed surface layer (up to 500m thick)
a layer called the thermocline (500 to 1000m thick)
and a deep water layer.

The surface layer has a relatively high and constant temperature, salinity increases with depth and dissolved oxygen is relatively high.

The thermocline has the temperature fall fairly rapidly and is only about 4 degrees at the base. The salinity remains fairly constant in the Antarctic but in low latitude areas the salinity falls rapidly to about 34.5 0/00. An oxygen minimum laye also lies within this layer.

The deep water layer is characterised low temperatures, a gradually increasing salinity towards 35 0/00 and a relatively constant dissolved oxygen concentration.

Density depends on both temperature and salinity - temperature being the most important factor. Dense sea water predominantly forms near the poles (water being densest at close to freezing (-2 degrees C). At the equator evaporation also increases density and water will sink to a lower level. Density is important because it causes water to sink at the poles carrying oxygen to the deep sea floor.

Temperature is important to understand because it affects the volume of the oceans and the solubility of carbon dioxide.

Ocean temperatures vary according to depth and latitude but the average temperature is about 3.8°C.
The average or mean depth of the ocean is 3800 metres. The average ocean depth is 4.4 times greater than average land elevation.
The average salinity of oceans is 34.5‰ parts per thousand or psu (practical salinity units).

Useful information

The primary elements responsible for the ocean’s salinity are sodium and chlorine. The chloride ion (Cl^- ) is the most abundant constituent contributing to the oceans salinity and is the easiest to measure.

The salinity of ocean waters close to coastal areas shows greater variability due to a range of physical factors including mixing with coastal runoff of freshwater.

Comparison of the Worlds Major Oceans

Ocean	Surface Area (million square kilometres)	Average Depth (metres)	Volume (million cubic kilometres)
Pacific	166	4188	696
Atlantic	84	3844	323
Indian	73	3872	284

The average density of seawater varies between 1.020 and 1.030 g.cm^-3 . Seawater density increases with increasing salinity and increasing pressure, due to depth and decreasing temperature.

identify the area of the Earth covered by oceans and explain how this influences conditions on the Earth’s surface

The main oceans of the Earth are: the Pacific, Indian, Atlantic and Southern Oceans. The oceans in total cover 71% of the Earth’s surface, which is 36,000,834,734 hectares or approximately 361 million square kilometres.
The ocean moderates atmospheric temperature and dramatically influences the Earth’s weather. Climate patterns and variations generated over the oceans are closely related to the distribution of solar energy and the wind belts of the world.
The ocean absorbs about half the energy reaching Earth from the Sun. The distribution of ocean and land surfaces creates a pattern of uneven heating over the Earth.
The ocean is able to transport heat via ocean currents by distributing heat away from the tropics towards the polar regions and by carrying cold water from the polar regions towards the tropics.
These ocean currents influence atmospheric heating. Warm currents from the south of the northern hemisphere result in high evaporation rates in this region bringing monsoons to Bangaladesh and India and typhoons to the Carribean and Central America, as well as to southern US states like Texas, Louisiana and Florida. Currents from the north in the southern hemisphere bring cyclones to northern Queensland, northern Western Australia and the Northern Territory .
Australia ’s climate variability is strongly influenced by the Pacific Ocean . Due to a much larger surface area and volume of ocean water than in the northern hemisphere, the southern hemisphere has a milder climate.
The influences of the currents are often referred to as El Nino or La Nina.

Extra information

The ocean also plays a major role in reducing the amount of carbon dioxide (a major greenhouse gas) in the atmosphere. This is due to the high solubility of carbon dioxide in water, especially at lower temperatures. The carbon dioxide absorbed by the oceans is often incorporated into organisms through photosynthesis and through the formation of carbonate shells.

identify the probable origins of the oceanic waters

There are several theories about the origin of the oceans, but no single theory explains all aspects of this puzzle. The t wo main theories for the origin of the Earth’s oceanic waters are listed below.
One hypothesis is that both the atmosphere and the oceans have accumulated gradually through geologic time from some process of "degassing" of the Earth's interior. The ocean had its origin from the prolonged escape of water vapour and other gases from the molten igneous rocks of the Earth to the clouds surrounding the cooling Earth.
About 4 billion years ago, after the Earth's surface had cooled to a temperature below the boiling point of water, rain began to fall and continued to fall for centuries. As this water drained into hollows in the Earth's surface, the primeval ocean came into existence. Gravity prevented water from leaving the planet. Since that time, water vapour from volcanic vents has continued to add to the ocean waters. Because volcanic activity has persisted throughout the Earth’s history, the volume of the world ocean is likely to have increased through time.
Planetary scientists have suggested that a large portion of the Earth’s oceans have originated from collisions with comet like structures which are predominately large balls of interstellar ice. Given the current rate of collisions with the Earth of these comet-like structures, four billion years represents a sufficient time span to enable the oceans to reach their present day level or volume.
Recent research suggests that this is not the most likely origin of the bulk of the Earth’s oceanic water as much of the ice based material which reaches the Earth from space contains large amounts of deuterium. Deuterium is not present in the higher concentrations in the Earth’s oceanic waters. This would indicate that this theory is not likely to be correct.

Further evidence

The Earth’s oldest rocks include sedimentary strata that were deposited by water and are similar to strata that we see deposited today. This evidence indicates that, as far back in geological history as 3.95 billion years, the Earth has had liquid water on its surface. Therefore, this indicates that ocean waters began to accumulate on the Earth’s surface sometime between 4.6 billion years ago, when the Earth formed, and 3.95 billion years ago, when the oldest known sedimentary rocks formed.

compare the evolution of the oceanic waters with the evolution of the atmosphere and explain how and why the two are linked

The early atmosphere would have contained water vapour. However the Earth’s surface would have been too hot to allow water vapour to condense into droplets.
Dense clouds would have formed.
As these dense clouds of water vapour began to cool some of the outgassed water vapour would have formed into droplets.
These droplets would have fallen to the surface, only to be vaporised by the hot rocky surface.
This cycle would have continued until the Earth’s surface had cooled sufficiently by heat radating into space from the atmosphere. This would allow water to remain on the surface in its liquid state and to collect in the basins formed by crustal development.
Both the atmosphere and the Earth’s oceans would have originated as gases expelled from the Earth’s interior.

Addition of Oxygen to the Atmosphere

Today, the atmosphere is approximately 21% free oxygen.

Oxygen Production

Photochemical dissociation – the break up of water molecules by ultraviolet radiation produced oxygen levels approximately 1-2% of current levels. At these levels ozone(O₃) can form to shield the Earth's surface from UV rays.
Photosynthesis - The rest of the oxygen (20% of today's atmosphere) was supplied initially by cyanobacteria, and eventually higher plants.
The equation for the reaction is: carbon dioxide + water + sunlight = glucose + oxygen.