The Oceans Have Evolved Over the History of the Earth

Oceanography

The oceans have evolved over the history of the earth

Describe modern oceans in terms of average temperature, mean depth, average salinity and average density

Feature / Description / Changes with depth
Average temperature / -Approx 3.8˚C (75% of ocean is at depth therefore low average)
-Vary according to depth and latitude
-Vary less than that of land because of the high heat capacity (able to take in/lose heat without changing temp too much compared to earth i.e. dessert boiling in day freezing at night)
-Mixing of surface water and deep water allows for heat to be distributed / -Sun can effectively heat just the top layer or photic zone (as ocean is transparent – this property called transmissibility)
-Distinct layering observed
-Surface/mixed layer most influenced by sun/climate – range of 15 - 18˚C
-Thermocline (500 – 1000m) – drops with depth and temp ranges from 4 - 15˚C
-Deep water layer – ranges from 4 – -1˚C (pressure and salinity allows for -˚)
Average depth / -Approx 3.75km (3800m) / -Enormous range with a max of 11km at Mariana Trench
Average salinity / -Average of 35%0
-Total amount of dissolved solids (grams) in 1000 grams (1kg) of water
-Mainly made up of sodium and chlorine (table salt)
-Salinity and temp closely related (i.e. evaporation which causes cooling and increased salinity) / -Increases with depth
-Varies greatly due to evaporation, ice melting, mixing with freshwater
-High salinity occurs in secluded areas and the subtropics (Mediterranean Seas)
-Low salinity occurs in the north (Baltic sea) and around continents (coastal areas)
-Middle layer = Halocline
Average density / -1.027 g/cm3 (grams per cubic centimetre) / -increases with depth but depends on other factors i.e. increased salinity and pressure and decreased temps (all present at depth) = increased density
-middle layer = Pycnocline
Pressure / -Humans can travel to about 3-4 atms (atmospheres) / -Increases with depth (due to weight of water above)

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

Oceans are a key part of the hydrological cycle with the oceans absorbing huge amounts of solar energy thus increasing water temp and promoting evaporation
Evaporated sea water is the main source of atmospheric water vapour which then supports life
Also a huge exchange of materials such as water, oxygen and carbon.
Carbon dioxide in the atmosphere is dissolved in water taken up by marine organisms and used in photosynthesis
Oxygen in the atmosphere is dissolved in water and taken up by marine organisms and used in respiration
Water evaporates from oceans when it is heated and then falls back on land and runs back into the oceans. Water is also spilt during photosynthesis and oxygen gas is released into the atmosphere or the ocean

Identify the area covered by oceans and explain how this influences conditions on the earth’s surface

71% of the worlds surface is covered by oceans (361 million square kilometres)
The biggest oceans are the Pacific, Indian, Atlantic and Southern
Water plays a significant role in the climate of the planet as ocean moderates atmospheric temps
Oceans absorb half the suns energy reaching earth and the ocean is able to transport heat by currents which distribute heat away from equatorial, tropical regions towards polar regions and by carrying cold water from polar regions towards the tropics (Cold ocean = cold atmosphere and vice versa)
Australia’s climate variability is strongly influenced by the Pacific Ocean
S hemisphere has a milder climate because there is a larger surface area and volume of ocean water
Influences of currents are referred to as El Nino (dry) and La Nina (wet)
Always remove lots of CO2 from atmosphere - this is used in photosynthesis of marine organisms

Identify the probable origins of the oceanic waters

Oceans formed a few thousands years after the earth and atmosphere but the composition has not changed in a significant way as the composition of the atmosphere has – physical and chemical properties of the ocean have stayed very stable for 1.5 billion years
Two main theories for the origin of the oceans:

Degassing/Out gassing – atmosphere and oceans formed gradually. Many volcanic eruptions released CO2 and water vapour into the atmosphere and when surface temp of earth cooled before boiling point of water (4 bya), 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. Formation of oceans helps the earth’s temp (due to its high heat capacity) Gravity prevents water leaving the planet and because volcanism continued through history of earth, volume of oceans increase. Most likely to be correct.

Comets – recent theories suggest large ice comets frequently bombarded the earth's atmosphere and vaporized above the earth's surface. Such comet rain supplied the earth's initial water mass as well as many of the basic compounds necessary for the origin of life. Given the current rate of collisions, 4 billion years represents a sufficient time span to enable oceans to reach their present day volume. Not as likely to be correct (as comets contain high levels of deuterium however earth's water do not contain high levels of deuterium)

Regardless of the origin, as the earth cooled, water vapour from the atmosphere cooled and condensed and formed rain. As this rain fell on the fresh solid surface of the earth, it dissolved some of the soluble minerals from the rocks – eventually this slightly saline water ran into large depressions on the earth's surface to form the ocean basins
Earth's oldest rocks include sedimentary strata (NB sedimentary rocks always form in water therefore) which indicates that as far back as 4 billion years ago, there was liquid water on the surface

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

The original atmosphere formed at the same time as the earth due to residual gasses left from its formation from a solar nebula – these gases were largely hydrogen and helium
Soon after the atmosphere began to change as a result of degassing and a dense atmosphere emerged from vapour and gases that were expelled during degassing – these gases may have consisted of hydrogen, water vapour, methane, carbon monoxide and carbon dioxide
Degassing contributed to changing atmosphere as the volcanic activities produced new gases and it is thought that a strong solar wind blew them away in addition to plants using CO2 to produce oxygen as well as bacteria releasing nitrogen (more a decrease of other gases than an increase of nitrogen)

Atmosphere / First Atmosphere / Volcanic Atmosphere / Present Atmosphere
Origin / Residual gasses from formation / Degassing / Life processes (photosynthesis)
Gases present / Hydrogen
Helium / Hydrogen
Methane
Carbon Monoxide
Carbon Dioxide
Water vapour / Oxygen
Nitrogen

The early water vapour present in the volcanic atmosphere would not have condensed as it was still too hot however dense clouds would have formed.
The most important feature of the ancient environment was an absence of free oxygen
About 3 billion years ago cyanobacteria began to photosynthesis creating oxygen and as this oxygen increased, the carbon dioxide decreased
In the upper atmosphere, some oxygen molecules absorbed energy from UV rays and split to form single oxygen atoms. These atoms combined with remaining oxygen molecules to form ozone and consequently an ozone layer which was very efficient at absorbing UV rays
As a result of this evolution of cyanobacteria, the earth acquired an atmosphere and an ocean
Salt in the oceans has accumulated over 3 billion years however as rivers flow into it carrying dissolved salts from soil erosion – oceans should be far saltier than they are i.e. simultaneous with the salt entering the sea, there must be an equal amount of salt leaving the sea.
The mechanism is not known but salt may have been somehow locked in seafloor sediments and then recycled through subduction, melted, and redistributed into other minerals.

The shape, distribution and age of the current oceans has been determined by plate tectonics

Identify the region of the crust where new ocean basins are forming and where ocean floors are subducting

Oceanic crust, due to its composition, is more dense and so continental crust floats higher
The only entirely oceanic plate is the Pacific Plate
Ocean ridges are where new oceanic lithosphere is created by upwelling convection currents which partially melt the mantle and result in basaltic magmas which intrude and erupt at the oceanic ridges to create new oceanic crust
As new oceanic crust is created it is pushed aside in two directions thus the age becomes progressively greater in both directions away from the ridge in a mirror image
Because oceanic crust may be subducted, the age of ocean basis is relatively young and the oldest ocean crust occurs furthest away from a ridge
Sediment thickness also increases in both directions away from the ridge and is thickest where the crust is oldest with the bottom layer of sediment being the oldest
When oceanic crust collides with another plate it subducts, thus losing old crust

Outline the types of evidence used to date ocean floors

Relative dating of ocean floors indicates that the further from the ridge, the older the rocks
Fossils can be taken from sediment and dated using radiometrics and relative dating
There are two other methods:

Using magnetism of the crust (Magnetic Timescale

-The magnetic zebra striping that can be seen on the sedimentary rocks in the ocean are from changes in the magnetism of the earth

-A reconstruction of the history of the magnetic reversals for the last 4 million years was made using radiometric dating and measuring the magnetic orientation of rocks

-Radiometrics not good for ocean crust because it never stays very old (subduction)

-However the magnetic timescale was already established therefore scientists measured the magnetism of the rocks on the crust and then compared it to the magnetic time scale

Looking at microfossils in seafloor sediments

-Biological processes dominate sediment formation in areas that receive little terrigenous material (land derived)

-Inorganic debris from dead organisms turns into oozes (calcium carbonate or silica)

-The microfossils in sediment immediately overlying the basalt sea floor gives an estimate of the age of the sea floor at that locality as the basalt is considered to be the same age as this bottom most sediment

-Microfossils already have known eras and if a certain microfossil is found the rock can be dated to the era the organism lived in

Assess the reliability of information used to estimate the age of ocean beds

The magnetic timescale is reliable because calculations are done and the observed data was matched to the theorised data
Magnetic timescales can be used to narrow down a time range
Microfossils are also very reliable because age and distribution of microfossils is very well studied and already well know therefore they are easy to date however direct samples of fossils and ocean beds are needed every time and still they only give a time period
Processes should be used in conjunction with each other to achieve enhanced accuracy of results

Outline the reasons why the oldest sea floor present on the earth today is generally less than 250 mya

The process of sea floor spreading is constantly forming new ocean crust
The new sea floor at MORs where magma comes to surface and forms new curst
The process of subduction destroys old ocean floor as it is subducted under continental plates
Ocean floor acts as a huge conveyor belt, transporting crust/sediment to subduction zones

Identify the role of plate tectonics in maintaining the equilibrium between the area of sea floor and area of continental land present on the earth

Seafloor is constantly being generated and destroyed but this ongoing process has not significantly altered the total area of the seafloor that exists over time i.e. subduction and rifting happens at a constant rate and stays in balance – some oceans are growing whilst others are shrinking
Erosion of continental crust is a constant process and as mountains are eroded away there is a significant change in the overlying weight of crustal material. In response to these changing forces the underlying crustal material will gradual rise to bring back balance – known as isostatic adjustment i.e. a piece of wood floating in water. If you cut it in half, it will still float in the middle

Discuss the reasons for, and impacts of, possible shifts in the equilibrium between the area of sea floor and area of continental land

The area of seafloor and continental land has remained fairly stable over the earth's history
What has varied dramatically is the distribution of land masses and oceans
200 million years ago the continents were combined into one large land mass called Pangaea and surrounded by one huge ocean called Panthalassa (Pan = all, Gaea = earth, Thalasaa = sea)
over time this single land mass spilt apart forming Laurasia and Gondwana and gradually reformed into the continents of today
This has resulted in the formation of new ocean basins through the formation/subduction and redistribution of ocean floor
On land, mountains are eroding away and this is carried down stream by rivers, some is then deposited in river deltas as the water slows. This can cause significant loading on areas of the crustal plate and result in the slow sinking of continental plate regions

Describe evidence for the closing of former ocean basins in terms of the presence of deep marine sedimentary rocks in present day continental mountain belts

One of the best examples of the presence of deep marine sedimentary rocks present in continental mountain belts are the Himalayas
The Himalayas formed as a result of the collision between the Eurasian and the Indo-Australian crustal plates and the resultant folding and faulting produced the mountain belt
India was an island but it moved north and squashed out the sea that was originally there – India moved at rates of up to 15cm a year and pushed out the Tethys Ocean. Today Himalayas are growing and eroding at a rate of 8cm/year (never getting bigger or smaller)
There are a number of visual clues to the origin of the crustal material making the Himalayas – these including the layering of sedimentary rocks which formed long ago as horizontal sediments on a sea bed and ammonite fossils (c.f. nautilus looking animal) have been found in large numbers. In addition, volcanoes that once fringed the edges of the ocean remain.

There are differences in physical, chemical and biological environments within and between past and present day oceans

Outline the origin of salinity in the earth’s seas and oceans

No single theory explains origins of the oceans but it is mostly agreed that it was due to degassing
Most of the oceans salts were derived from gradual processes such as the breaking up of the cooled igneous rocks of the earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams that transported their mineral washings to the sea
A portion is due to rain, groundwater and moving surface water dissolving minerals in rocks
Most of the salt accumulated over time due to weathering of continental rocks – as rain falls on the land it slowly dissolved parts of the rocks and soluble salts are washed into the sea
A simultaneous with salt entering the sea, it must be an equal amount leaving (salt never increased)
Volatiles from the mantle are released into oceans when molten material is rapidly cooled when it comes in contact with ocean water and in the process ions become dissolved from sediments deposited on the ocean floor have also contributed
Major contributing component of salinity is the Chloride Ion (Cl-1)
3.5% of the ocean is salt

Identify data sources, plan, choose equipment and perform a first hand investigation to compare the solubility of common salts in water of different temperatures

Investigating Solubility and Temperature:
Aim: to compare solubility of common salts in water at different temperatures
Method: using a measuring spoon add solute to 20ml of water

Salt / Amount Dissolved
Iced Water (3˚C) / Tap Water (21.5˚C) / Hot Water (51˚C)
Sodium Chloride / 6.3 grams / 6.8 grams / 8 grams
Sodium Sulphate / 0.1 grams / 0.7 grams / 1.2 grams
Potassium Nitrate / 2.1 grams / 4.4 grams / 5.3 grams

Results: Sodium Chloride has more salt dissolved in each water temperature and therefore this indicates that it has a higher solubility. Nitrate was next, with Sodium Sulphate having the least solubility. Generally the most salt dissolved in hot water and the least in cold water
Conclusion: Hot water does dissolve the most because it contains more energy to break up other particles. Hot water needed to be kept hot (which was hard) and cold water needed to stay cold (and not get heated) Results were based on an average as the experiment was repeated 4 times in the class (repetition) however this approach lacks consistency (everyone stirred their beakers differently, everyone judged when salt had dissolved differently, etc) there were also time constraints therefore was rushed.

Perform a first hand investigation to demonstrate the precipitation of salts from a cooling solution and solve problems to use this information to predict precipitation in naturally occurring bodies of water