Download Oceans

Oceans: The Last
Frontier
Dr. Masdouq Al-Taj
2017
The vast world ocean
Geography of the Oceans
• Earth is often referred to as the blue planet.
• 71 % (360 M km2) of Earth’s surface is represented by
oceans and marginal seas; seas around the ocean’s
margin like Mediterranean and Caribbean sea.
• Continents and islands comprise the remaining 29 %;
(150 M km2 )
• The Northern Hemisphere is called the land
hemisphere (61% water and 39% is land), and the
Southern Hemisphere, the water hemisphere
(81%water and 19% is land).
Views of the
Northern and
Southern
Hemispheres
• Oceanography
Is an interdisciplinary science that
draws on the methods and
knowledge of biology, chemistry,
physics, and geology to study all
aspects of the world ocean.
The vast world ocean
• Four main ocean basins
• Pacific Ocean—The largest and has the
greatest depth covering over half oceans area,
world’s deepest ~3940m, fit continent areas.
• Atlantic Ocean—About half the size of the
Pacific and not quite as deep and bounded
• Indian Ocean—Slightly smaller than the
Atlantic, largely a Southern Hemisphere body
• Arctic Ocean—About 7 percent the size of the
Pacific and quarter as deep as the rest of the
oceans.
The oceans of Earth
Composition of seawater
3.5 % by weight of sea water is dissolved minerals termed as
“Salts”
• Salinity
• Salinity is the total amount of solid
material dissolved in water
• Typically expressed as a percent
• Dissolved substances in seawater are small
numbers and therefore expressed in parts
per thousand (35‰)
• Most of the salt in seawater is sodium
chloride (table salt)
Dissolved components
in seawater
9 elements are very important
in maintaining necessary chemical
Environment for life in the sea
Composition of seawater
• Sources of sea salts
1. Chemical weathering of rocks on continents; 2.5
billion tons of dissolved delivered by streams.
2. Earth’s interior through volcanic eruptions, through
Process called outgassing ~ Major source of water in
ocean and atmosphere Cl, Br, S, B
• Composition of seawater has been relatively stable for
millions of years:
• Material is removed just as rapidly as it is added~
withdraw by plants, animals, sediments hydrothermal
activity (oceanic ridges).
Composition of seawater
• Processes affecting seawater salinity
• Primarily due to changes in the water
content of the solution. These include:
1. The addition of fresh water due to
precipitation, runoff, icebergs melting, and
sea-ice melting.
2. The removal of fresh water by evaporation
and the formation of sea ice also affect
salinity.
* This graph shows the variations in ocean surface
temperature (top curve) and surface salinity (lower
curve).
* Interpreting Diagrams. At which latitudes is sea surface
temperature highest? Why?
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25-35 N and S Evaporation is high
35-60 N and S Precipitation is high
Near equator lower salinities prevail.
Open ocean, salinity varies between 33‰ - 38‰
Some marginal seas (Red sea and Persian gulf), salinity
may exceeds 42‰
• Baltic sea in northern Europe’s , salinity is often below 10‰
• Temperature and salinity change with depth in the oceans founded
by oceanographers: A three-layered structure exists in the open
ocean
• 1. surface mixed zone: 2%, the warmest –solar energy-wave
turbulence varied according to latitude and season of the year
• 2. Transition zone:18%, falls abruptly with depth. A layer of rapid
temperature change below the zone of mixing is known as the
thermocline (200-1000 m).
• 3. Deep zone:80%: below 1000m ocean-water temperatures are
consistently below 4° C.
The ocean’s layered
structure
• Salinity variations with depth, correspond
to the general three-layered structure
described for temperature
• A zone of rapidly changing salinity, called
the halocline, corresponds to the
thermocline
Halocline
Density Variation with Depth
By sampling ocean waters, oceanographers have learned that
temperature and salinity—and the water’s resulting density—vary
with depth. Figure below shows two graphs of density versus depth.
One graph shows the density for low-latitude regions and the other
for high-latitude regions. Compare the density curves to the
temperature curves. The temperature is the most important factor
affecting seawater density. It also shows that temperature is
inversely proportional to density.
* When two quantities are inversely proportional,
they can be multiplied together to equal a
constant. Therefore, if the value of one quantity
increases, the value of the other quantity
decreases proportionately.
* When water temperature increases, its
density decreases.
The pycnocline (pycno = density, cline = slope) is the layer of
ocean water between about 300 meters and 1000 meters where
there is a rapid change of density with depth.
A pycnocline presents a significant barrier to mixing between lowdensity water above and high-density water below. A pycnocline is
not present in high latitudes; instead, the water column is about the
same density throughout.
Mapping the ocean floor
If all water were removed from the ocean basin, a great variety of
features would be seen: Chains of volcano, deep canyons, rift
valleys and large submarine plateaus
Until the 1920s, ocean depths were determined laboriously, by
lowering rope until it touched bottom.
• Bathymetry — measurement of
ocean depths and the charting of the
shape or topography of the ocean
floor.
• Echo sounder (also called sonar,
sound navigation and ranging, first
device)
• Invented in the 1920s
• Primary instrument for measuring depth
• Depends on reflects sound from ocean
floor
Echo sounder
Mapping the ocean floor
• Sidescan sonar :following
WWII by U.S
navy for mines.
• Multibeam sonar
• Employs and array of sound sources
and listening devices
• Obtains a profile of a narrow strip of
seafloor.
Multibeam sonar
Fan of sound
received by narrowly
receivers at different
angles
“Mowing the lawn”: regularly
back-and-forth pattern: 10 km
wide with <1m resolution
Torpedo-shaped instrument
no bathymetric data
Both Sidescan and Multibeam sonars obtain an image of narrow swath
)‫(رقعة‬of sea floor every few seconds,
• Measuring the shape of the ocean
surface from space
• Employs satellites of microwave pulses
and equipped with radar altimeters. After
compensating for waves, tides, currents,
and atmospheric effects: the ocean
surface is not perfectly flat, this because
gravity attracts water toward regions were
massive sea floor feature occur.
Mapping the ocean floor
• Three major topographic units of the
ocean floor:
A. Continental margins (Passive and
active)
B. Deep-Ocean basin
C. Oceanic (mid-ocean) ridge
Major topographic divisions of
the North Atlantic Ocean
Active and Passive margin
Features of a passive
continental margin
A.Continental margins
• Passive continental margins
• Not associated with plate boundaries
• Experience little volcanism
• Few earthquakes
• Found along most coastal area that
surround the Atlantic Ocean
Continental margins
•
Passive continental margins
•
Features comprising a passive continental margin
I. Continental shelf
1.
Flooded extension of the continent and represent
7.5% of ocean area.
2. Varies greatly in width~ 1500km, 130m depth, gently
sloping about 0.1º.
3. Contains oil and important mineral deposits, source
of fishing.
4. Some areas are mantled by extensive glacial deposits
during Ice age: Pleistocene.
5. Most consist of thick accumulations of shallow-water
sediments (several km thick), particularly during
earlier period of coral reef building.
Coastal plain
A.Continental margins
II. Continental slope
1. Narrow feature and extends to about 20km.
2. Marks the seaward edge of the continental shelf
3. Relatively steep structure: ~5º but may exceed 25º
4. Consists of a thick accumulations of sediments that
moved downslope from the continental shelf
5. At the base of the continental slope turbidity currents that
follow submarine canyons deposit sediments that forms
deep-sea fans
6. Boundary between continental crust and oceanic crust
Continental margins
• Submarine canyons and turbidity currents
• Submarine canyons
• Deep, steep-sided valleys cut into the
continental slope
• Some are seaward extensions of river valleys
• Most appear to have been eroded by turbidity
currents
Continental margins
III. Continental rise
1. Found in regions where trenches are absent: 20 km
width and may extend to hundred km into the deepocean basin.
2. Continental slope merges into a more gradual
incline—The continental rise: slope drops to about
0.3º
Continental margins
• Turbidity currents: originate along the continental
slope and continue across the continental rise
• Downslope movements of dense, sedimentladen water
• Created when sand and mud are dislodged
and thrown into suspension
• Because mud-chocked water is denser than
normal seawater , it flows downslope as a
mass, eroding and accumulating more
sediments as it goes
• Deposits are called turbidites: coarse sand
sediment dropped at the base first, then
followed by successively finer silt and clay
Continental margins
• Active continental margins
• Continental slope descends abruptly into a deep-ocean
trench, here the continental margin is very narrow , the
trench may lie 50km offshore.
• Located primarily around the Pacific Ocean
• Accumulations of deformed sediment and scraps of
ocean crust form accretionary wedges.
• Some subduction zones have little or no accumulation
of sediments : ocean sediments are being carried into
the mantle with subducting plate.
An active continental margin
• Deep-ocean trenches
• Long, relatively narrow features
• Deepest parts of ocean:11022m the deepest at Mariana
trench
• Most are located in the Pacific Ocean
• Sites where moving lithospheric plates plunge into the
mantle
• Associated with volcanic activity
• Volcanic islands arcs
• Continental volcanic arcs
B. Deep-ocean basin
• Abyssal plains
• Likely the most level places on Earth
• Sites of thick accumulations of sediment: transported
sediments coming from turbidity current
• Found in all oceans: Atlantic ocean has fewer trenches, so it
has more extensive abyssal plains than does the pacific
• Seamounts and guyots
• Isolated volcanic peaks (conical steep sided and hundred of
meters high)
• Many form near oceanic ridges and the greatest number
found in the Pacific.
• May emerge as an island
• May sink and form flat-topped seamounts called guyots or
tablemounts: Some of the volcanoes are eroded to near sea
level by water wave action and over millions of years it sink
as moving plate carry them away where they originated
C. Mid-ocean ridge:
developed at divergent plate boundaries forming a broad
linear swell called oceanic ridge, or mid-ocean ridge
Characterized by:
•An elevated position
•Extensive faulting
•Numerous volcanic structures that have developed on
newly formed crust
• Interconnected ridge system is the longest topographic
feature on Earth’s surface: Over 70,000 kilometers in
length
•Representing 23 % of Earth’s surface
•Winds through all major oceans similar to the seam
on baseball and have a widths from 1000 to 4000 km
~half area of ocean floor.
•Along the axis of some segments are deep downfaulted
forming structures called rift valleys, this ridge is
segmented by large transform faults.
• Features associated with rift valleys:
•
active volcanoes, recent underwater lava flows, and
hydrothermal vent (black smokers).
• Consist of layer upon layer of basaltic rocks that have
been faulted and uplifted ~ 2-3 km high
• Mid-Atlantic Ridge has been studied more thoroughly
than any other ridge system ~ the submerged structures
standing 2500-3000 m
Oceanic ridge system
Seafloor sediments
• Ocean floor is mantled with sediment
• Sources
• Turbidity currents
• Sediment that slowly settles to the
bottom from above
• Thickness varies
• Thickest in trenches—Accumulations
may approach 10 kilometers
Seafloor sediments
• Thickness varies
• Pacific Ocean—About 600 meters or
less
• Atlantic Ocean—From 500 – 1000
meters thick
• Mud is the most common sediment
on the deep-ocean floor , Sand-sized
sediments are found also.
Seafloor sediments
• Types of seafloor sediments
They are of 3 types and categorized
according to their origin, but all are found as
mixtures.
1. Terrigenous sediment
• Material weathered from continental
rocks and transported to the ocean
• Virtually every part of the ocean
receives some terrigenous sediments,
the coarse sand and gravel settle near
shore, whereas fine sediments take
years to settle to ocean floor (1cm of
abyssal clay layer requires 50,000yrs).
• Fine particles remain suspended for a
long time and chemical reaction occurs
such as Oxidation often produces red
and brown colored sediments (rust)
Seafloor sediments
• Distribution
• Coarse terrigenous deposits dominate
continental margin areas
• Fine-grained terrigenous material is
common in deeper areas of the ocean
basin
Seafloor sediments
2. Biogenous sediment
Consists of Shells and skeletons of marine animals and plants,
algae.
*Most common are oozes; and they are of two types: - calcareous oozes :(CaCO3) produced from microscopic
organisms living near ocean water, the calcareous hard parts
begin to dissolve in cool deeper water (acidic water due to
carbon dioxide).
- siliceous oozes composed of skeletons of diatoms
(single–celled algae) and radiolarians (single–celled animals).
- Phosphate rich materials derived from the bones, teeth, and
scales of fish and other marine organisms.
Seafloor sediments
3. Hydrogenous Sediments
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Minerals that crystallize directly from seawater. Most
common types include:
Manganese nodules: rounded, hard lumps of Fe, Mn and
other, forming concentric layers around object, reaches
20 cm in diameter
Calcium carbonates: precipitate in warm climate
forming limestone
Metal sulfides: found as coating on rocks near black
smokers contain iron, nickel, copper, zinc, silver and
others
Evaporites: form where evaporation is high with
restricted open-ocean circulation, the evaporite
minerals called salts: halite NaCl, anhydrite CaSO4 and
gypsum CaSO4.2H2O
Seafloor sediments
• Distribution
• Hydrogenous sediment comprises only
a small portion of deposits in the ocean
• There are a few places where very little
sediment accumulates (mid-ocean
ridges)
End of Chapter 9