Download Copyright (©) 2008, Elsevier Inc. All rights reserved. Chapter

Copyright (©) 2008, Elsevier Inc.
rights reserved.
All
between the older Pacific Ocean and the
younger Atlantic Ocean. The edges of the
Pacific are ringed with trenches, volcanoes
and earthquakes that signal the gradual
descent of the ocean bottom crustal “plates”
under the surrounding continental plates. In
contrast, the Atlantic is the site of dynamic
sea-floor spreading as material added in the
center of the mid-Atlantic ridge pushes the
plates apart, enlarging the Atlantic Ocean by
a few centimeters each year. In addition the
Atlantic has a very marked “S” shape while
the Pacific has a much more oval shape. In
order to appreciate the shapes of the oceans
and seas it is useful to examine them on a
globe, since map projections on flat paper
introduce distortions when large portions of
the earth are to be represented. From the
oceanographic point of view it is convenient
to distinguish the various regions in terms of
their
oceanographic
characteristics,
particularly their circulations.
Chapter 2. Ocean dimensions, shapes and
bottom materials
2.1 Dimensions
The oceans are basins in the surface of
the solid earth containing salt water. The
purpose of this chapter is to introduce some
of the nomenclature and to direct attention to
features of the basins which have a close
connection with the circulation and are of
importance to the physical oceanographer.
More detailed descriptions of the geology and
geophysics of the ocean basins are given in
Seibold and Berger (1982), The Sea Floor: an
Introduction to Marine Geology, and in
Kennett (1982), Marine Geology.
The shape, depth and geographic
location of an ocean affect the general
characteristics of its ocean's circulation. A
case in point is the dramatic difference
Fig. 2.1 Map of the world ocean from the
(http://www.ngdc.noaa.gov/mgg/global/global.html)
The major ocean areas are defined as
the Atlantic Ocean, the Pacific Ocean, the
Indian Ocean, the Arctic Ocean and the
Southern Ocean (Fig. 2.1). The first four are
National
Geophysical
Data
Center
clearly divided from each other by land
masses but the divisions between the
Southern Ocean and the others to its north are
determined only by the characteristics of the
28
ocean waters and by their circulations, as will
be described in Chapter 9. The geographical
peculiarities of each ocean are described in
Section 2.4.
Marginal seas are fairly large basins
of salt water that are connected to the open
ocean by one or more fairly narrow channels.
Those that are connected by very few
channels are sometimes called Mediterranean
seas, after the prototype, the (European)
Mediterranean Sea. As discussed later, the
Mediterranean provides an example of a
negative water balance in a sea, with less
inflow (river runoff and precipitation) and
greater evaporation, while an excellent
example of a positive water balance marginal
sea, with net precipitation, is found in the
Black Sea which connects with the
Mediterranean. Both of these seas will be
discussed further in Chapter 5.
Other
examples of marginal seas that are separated
from the open ocean by multiple straits or
island chains are the Caribbean Sea, the Sea
of Japan, the Bering Sea, the North Sea, the
Baltic Sea, etc. The term sea is also used for
a portion of an ocean that is not divided off
by land but has local distinguishing
oceanographic characteristics. Examples are
the Norwegian Sea, the Labrador Sea and the
Tasman Sea.
Looking at a globe again, it is evident
that more of the earth’s surface is covered by
sea than by land, about 71% in contrast to the
29% for land surface. Furthermore, the
proportion of water to land in the southern
hemisphere is much greater (4:1) than in the
northern hemisphere (1.5:1). In area, the
Pacific Ocean is about as large as the Atlantic
and Indian Oceans combined. If one includes
the neighboring sectors of the Southern
Ocean with the three main oceans north of it,
then the Pacific Ocean occupies about 46% of
the total world ocean area, the Atlantic Ocean
about 23%, the Indian Ocean about 20%, and
the rest combined about 11%.
The average depth of the oceans is
close to 4,000 m while the marginal seas are
generally about 1,200 m deep or less.
Relative to sea level, the oceans are much
deeper than the land is high. While only 11%
of the land surface of the earth is more than
2,000 m above sea level, 84% of the sea
bottom is more than 2,000 m deep. However,
the maxima are similar: the height of Mt.
Everest is about 8,840 m while the maximum
depth in the oceans is 11,034 m in the
Mariana Trench in the western North Pacific.
Figure 2.2 shows the distributions of land
elevations and of sea depths relative to sea
level in 1,000-m intervals as the percentage of
the total area of the earth’s surface. The land
data are from Kossina (1921) and the ocean
data from Menard and Smith (1966).
Fig. 2.2 Areas of earth’s surface above and
below sea level as percentage of total area of
earth (in 1,000 m intervals).
Although the average depth of the
oceans of 4 km, is a considerable distance, it
is small compared with the horizontal
dimensions of the oceans, which are of the
order of 5,000 to 15,000 km. An idea of the
relative dimensions of the Pacific Ocean may
be obtained by stating that they are much the
same as a sheet of paper. Relative to the
major dimensions of the earth the oceans are
a thin skin, but there is a great deal of detail
29
and structure in this layer between the sea
surface and the bottom of the ocean.
2.2 Sea-floor dimensions
2.2.1 Scales
Very often we wish to present some of
the characteristics of the ocean by drawing a
vertical cross-section of a part of the oceans.
A drawing to true scale would have the
relative dimensions of the edge of a sheet of
paper and would be either too thin to show
details or too long to be convenient.
Therefore we usually distort our cross-section
by making the vertical scale much larger than
the horizontal one. For instance, we might
use a scale of 1 cm on the paper to represent
100 km horizontally in the sea while depths
might be on a scale of 1 cm to represent 100
m, i.e. 0.1 km. In this case the vertical
dimensions on our drawing would be
magnified by 1,000 times compared with the
horizontal ones (a vertical exaggeration of
1,000:1). This gives us room to show the
detail but exaggerates the slope of the sea
bottom or of contours of constant water
properties (isopleths) drawn on the crosssection, such as in Fig. 2.3. Such slopes are
in reality far less than they appear on the
cross-section drawings. For instance, a line
of constant temperature (isotherm) with a real
slope of 1 in 10,000 would appear as a slope
of 1 in 10 on the plot.
The continents form the major lateral
boundaries to the oceans, and the detailed
features of the shoreline and of the sea bottom
are important in their effects on circulation.
Starting from the land, the main divisions
recognized are the shore, the continental
shelf, the continental slope and rise, and the
deep-sea bottom, part of which is the abyssal
plain (shown schematically in Fig. 2.3).
Fig. 2.3 Schematic section through ocean
floor to show principal features.
Bottom topography often plays an
important role in the distribution of water
masses and the location of currents. For
instance, bottom water coming from the
Weddell Sea (Antarctica) is unable to fill the
eastern part of the Atlantic basin directly due
to the height of the Walvis Ridge (South
Atlantic Ocean). Instead the bottom water
travels to the north along the western
boundary of the South Atlantic, finds a deep
passage in the mid-Atlantic ridge and then
flows south to fill the eastern side of the
ridge.
At shallower depths the sills
(shallowest part of a channel) defining the
marginal seas strongly influence both the
mid-level currents and the distribution of
water masses associated with the sea. Near
the shore, of course, it is bottom topography
that dictates the breaking of surface gravity
waves, while bottom topography also directly
influences the local tidal expressions. Tidal
mixing is also strongly influenced by bottom
relief.
Coastal upwelling is a direct
consequence of the shape of the coast and its
related bottom topography.
Alongshore
currents are often determined by the coastal
bottom topography while the instabilities in
this system are also often a function of the
horizontal scales of the bottom topography.
2.2.2 Shore
The shore is defined here as a part of
the landmass, close to the sea, which has been
modified by the action of the sea. There is
ample evidence to indicate that sea level in
the past has varied over a range of about 120
30
m as land glaciers were smaller or larger than
they are now. The beach is the seaward limit
of the shore and extends roughly from the
highest to the lowest tide levels. Sandy
beaches are often in a state of dynamic
equilibrium. In other words, they may be
composed of sand all the time but it may not
always be the same sand. This sand may be
continually moving along the shore under the
influence of waves and nearshore currents
which are established to balance the
shoreward moving of water with wave action
and tidal currents. Evidence for this process
can be seen in the way in which sand
accumulates against new structures built on
the shore, or by the way in which it is
removed from a beach when a breakwater is
built in such a way as to cut off the supply of
sand beyond it. On some beaches, the sand
may be removed by currents associated with
high waves at one season of the year and
replaced by different currents associated with
lower waves at another season.
These
currents are influenced by seasonal and
interannual wind variations. In addition to
their effects on sand transport it is important
to have knowledge of these near-shore
currents particularly in areas where people are
engaged in water sports. Locally intense and
narrow currents can carry swimmers very
rapidly out to sea.
The evolution of coastal regions is
influenced by changes in sea level that have
occurred over geologic time.
The
sedimentary record shows a series of marine
intrusions and retreats corresponding to layers
that reflect periods when the surface was
above and below sea level. The ability of the
coast to resist the erosionary forces of the
ocean depends directly on the type of material
the coast is made of. Sands are easily
redistributed by the ocean currents while
granitic coasts are slow to erode. Often sea
level changes are combined with the
hydrologic forces of an estuary, which
dramatically
changes
the
dynamical
relationship between the ocean and the solid
surface.
Coasts are relatively difficult to
classify due to the many types of variability.
One early method was to classify a coast as
rising or falling in a net, long-term sense
(Johnson, 1919). The causes of uplift or
subsidence were not clearly understood at that
time. More recently the emergence of our
understanding of plate tectonics has led to
new classifications of coasts, which can be
considered to be either active or passive.
Located at the front of a moving plate, active
coasts are fundamentally different from the
passive coasts located at the trailing edge of a
plate. However, coastal changes due to
extremely slow plate movements are mostly
obscured by shorter-term variations due to
changes in mean sea level (decadal to
millennial time scales) and due to waves and
tides (much shorter time scales). Coasts can
also be referred to as erosional or
depositional, depending on whether materials
are being removed or added.
Three basic factors contribute to longterm sea level change: (1) the total amount of
water in the ocean, (2) changes in the
containment volume of the world’s ocean,
and (3) changes in the temperature/salinity
characteristics of the ocean that alter its
density and hence cause the water to expand
or contract.
Changes in total amount of
water are due primarily to changes in the
volume of landfast ice, contained in ice sheets
and glaciers. (Since sea ice floats in water,
changes in sea ice volume, such as that in the
Arctic or Antarctic, do not affect sea level.)
Changes in containment volume are due to
tectonics, and include the slow rebound of
continents, continuing into the present, after
the melt of landfast ice after the last
deglaciation, and rebound due to continuing
melt of glacial ice. Changes in heat content
cause seawater to expand (heating) or
contract (cooling).
31
Sea level rose 20 cm from 1870 to
2003, including 3 cm in just the last 10 years
(1993-2003).
Because good global
observations are available for that last ten
years, it is possible to ascribe 1.6 cm to
thermal expansion, 0.4 cm to Greenland and
Antarctic ice sheet melt, and 0.8 cm to other
glacial melt, with a residual of 0.3 cm. Sea
level is projected to rise 30 + 10 cm in the
next 100 years, mainly due to warming of the
oceans, which absorb most of the
anthropogenic heat increase in the earth's
climate system. (See the summary in the 4th
assessment report of the Intergovernmental
Panel on Climate Change, Bindoff et al.,
2007.)
Erosional coasts are attacked by both
waves and currents, which carry fine material
that abrades the coast. Some coastal elements
are dissolved into the sea water while most of
the coastal material is carried out by local
currents that are forced by tides and by the
onshore buildup of water from wind-driven
surface waves.
This latter build-up is
generally compensated by a strong narrow
current, often referred to as a “rip current”,
that carries water and sediment out to sea
from the coast (Chapter 15.3).
Marine erosion is greatest on “highenergy coasts” which are frequently exposed
to large waves. These include the coasts of
Maine, British Columbia and the southern tip
of South Africa. Low energy coasts are
generally not exposed to strong wave fields;
areas such as the Gulf of Mexico are therefore
stable until dramatic events such as a
hurricane visit the region with strong and
destructive waves. Tidal levels modify the
elevation that the waves influence so a coast
with little tidal variation erodes more quickly
than one with a significant tidal range.
Coasts protected by offshore island arcs erode
more slowly than those directly exposed to
the wind wave environment.
Erosive forces carve characteristic
features on coastlines such as sea cliffs and
sea caves. A sea cliff is formed when the
ocean undercuts the land surface and then the
over-hanging portion falls into the sea. Sea
caves are subsurface features, formed at
points where the coastal composition material
is weak. These can lead to the creation of a
blowhole if the weakness extends to the
surface.
Initially the effect of marine erosion
on newly-exposed coastlines is to create
irregularities where weaker material is eroded
more quickly than the harder components.
Eventually, however, continued erosion
smoothes the entire coast, with the loose,
eroded material deposited into relatively calm
bays located between the headlands that are
exposed to the strong waves focused upon
them by the local topography.
The rip currents referred to above are
fed by alongshore currents that move water
from the incoming waves to feed the offshore
rip currents.
The directions of these
alongshore currents are generally determined
by the propagation direction of the incoming
waves which generally are not precisely
aligned to the coast. Frequently the load of
sediment carried by the alongshore currents is
joined by river sediment discharge, which can
form a “river delta” which fans out the
sediment from the river.
The most familiar feature of a
depositional coast is a beach, which is a zone
of unconsolidated particles that covers part or
all of a shore. Beaches result when sediment,
usually sand, is transported to places suitable
for continued deposition. Again these are
often the quiet bays between headlands and
other areas of low surf activity. Often a
beach is in equilibrium, with new sand being
deposited to replace sand that is being
scoured away. The landward limit of the
beach might be vegetation, permanent sand
dunes or human construction. The seaward
limit of a beach, where the sediment
movement on and offshore ceases, is at a
depth of about 10 m at low tide.
32
Beaches are made of material that can
range from boulders, cobbles, pebbles, and
gravel to very fine silt. Some beaches consist
of shell debris or fragments of coral. In
general, the flatter the beach the finer the
material from which it is made.
This
relationship between “sand” particle size and
beach slope depends on the wave energy
incident on beach, the particle shape and the
porosity of the material.
features of the shelf and slope are the
submarine canyons, which are of worldwide
occurrence. They are valleys in the slope,
either V-shaped or with vertical sides, and are
usually found off coasts with rivers. Some,
usually in hard granitic rock, were originally
carved as rivers and then submerged examples are around the Mediterranean and
southern Baja California. Others, commonly
in softer sedimentary rock, are formed by
turbidity currents, described in the next
paragraph. The lower part of the slope, where
it grades into the deep-sea bottom, is referred
to as the continental rise.
Turbidity currents are common on
continental slopes. These are not water
currents like those we have been referring to
in terms of the circulation. Instead these are
episodic events that transport large volumes
of sediment and bottom material down the
slope into the deep ocean (Fig. 2.4). In these
events material builds up on the slope until it
is no longer stable and the force of gravity
wins out with the material all crashing down
the slope at once, at speeds up to 100 km/hr.
These events have often snapped underwater
cables that they cross. The precise conditions
that dictate when a turbidity current occurs
vary with the slope of the valley and the
nature of the material in the valley. Turbidity
currents carve many of the submarine
canyons found on the slopes. Some giant
rivers like the Congo carry such a dense load
of suspended material that they form
continuous density flows of turbid water
down their canyons.
2.2.3 Continental shelf
The continental shelf extends seaward
from the shore with an average gradient of 1
in 500. Its outer limit (the “shelf-break”) is
set where the gradient increases to about 1 in
20 on average to form the continental slope
down to the deep sea bottom. The shelf has an
average width of 65 km. In places it is much
narrower than this, while in others, as in the
northeastern Bering Sea or the Arctic shelf
off Siberia, it is as much as ten times this
width. The bottom material is dominantly
sand, with rock or mud being less common.
The shelf break is usually clearly evident in a
vertical cross-section of the sea bottom from
the shore outward. The average depth at the
shelf break is about 130 m. Most of the
world’s fisheries are located on the
continental shelves for a multitude of reasons
including proximity of estuaries, depth of
penetration of sunlight compared with bottom
depth and upwelling of nutrient-rich waters
onto some shelves, particularly those off
western coasts.
2.2.4 Continental slope and rise
The continental slope averages about
4,000 m vertically from the shelf to the deep
sea bottom, but in places extends as much as
9,000 m vertically in a relatively short
horizontal distance.
In general, the
continental slope is considerably steeper than
the slopes from lowland to highland on land.
The material of the slope is predominantly
mud, with some rock outcrops. Very typical
33
valleys and plains on the ocean bottom just as
on land.
Movement of the earth’s tectonic
plates shapes the sea floor.
Sea floor
spreading creates new sea floor as the earth's
plates spread apart. This creates the midocean ridge system. The ocean plates spread
apart at a rate of about 2 cm/year (Atlantic) to
16 cm/year (Pacific), causing extrusion of
magma into the surface at the centers of the
ridges. Over geologic time the orientation of
the Earth’s magnetic field has reversed
causing the ferromagnetic components in the
molten new surface material to reverse.
Thus, spreading at the mid-ocean ridge was
proven by observing the reversals in the
magnetic orientations in the surface material
(Figure 2.5). The recurrence interval for
magnetic reversals is of the order of 500,000
to 1,000,000 years.
Fig. 2.4 Turbidity current schematic
2.2.5 Plate tectonics and deep-sea
topography
From the bottom of the continental
slope, the bathymetric gradient decreases
down the continental rise to the deep-sea
bottom, the last and most extensive area.
Depths of 3,000 to 6,000 m are found over
74% of the ocean basins with 1% being
deeper.
Perhaps the most characteristic
aspect of the deep-sea bottom is the variety of
its topography. Before any significant deep
ocean soundings were available the sea
bottom was regarded as uniformly smooth.
When detailed sounding started in connection
with cable laying, it became clear that this
was not the case and there was a swing to
regarding the sea bottom as predominantly
rugged. Neither view is exclusively correct,
for we know now that there are mountains,
Fig. 2.5 Schematic of ridge with series of
magnetic reversals
34
Fig. 2.6 Schematic reconstruction of continents prior to and after sea-floor spreading
The earlier geographic layout of the
Earth can be reconstructed from the magnetic
pattern left by sea-floor spreading. Here in
Fig. 2.6 we see a suggested pattern for fitting
the continents together in the way they might
have been millions of year ago. Extrapolating
this activity forward in time it is clear that the
redistribution of land and ocean basins will
continue to take place
The 14,000 km long Mid-Ocean
Ridge is a tectonic spreading center. The
ridge is the most extensive feature of the
earth’s topography. Starting in the Arctic
Ocean, it extends through Iceland, down the
middle of the Atlantic, wrapping around the
tip of Africa and then through the Indian and
Pacific Oceans, ending in the Gulf of
California. In the Atlantic the ridge separates
the bottom waters, as can be seen from their
very different properties east and west of the
ridge. However, there are narrow gaps in this
ridge, called fracture zones, created by lateral
jogs in the spreading center. The fracture
zones
are
roughly
vertical
planes,
perpendicular to the ridge, on either side of
which the crust has moved in opposite
directions perpendicular to the ridge. This
leaves a gap in the ridge through which water
below the ridge top may leak from one side of
the ridge to the other. One example is the
Romanche Fracture Zone through the MidAtlantic Ridge close to the equator.
Individual mountains (seamounts) are
widely distributed in the oceans. Some
project above the surface to form islands,
while the tops of others are below the surface.
Virtually all are volcanic in origin. Some of
these seamounts have reached the surface
where they have been worn flat before
29
sinking again below the surface to form a
feature know as a “guyot.”
In some of the large basins the sea
floor is very smooth, possibly more so than
the plains areas on land. Stretches of the
abyssal plain in the western North Atlantic
have been measured to be smooth within 2 m
over distances of 100 km.
The deepest parts of the oceans are in
the trenches (Fig. 2.7).
Trenches are the
active locations where the oceanic plates are
sinking (subducting) beneath other plates
causing earthquakes and volcanoes. In many
cases oceanic plates sink beneath the lighter
granitic continental plates.
Along the
landward side of trenches are volcanoes
caused by the introduction of water into the
hot interior of the earth. The majority of the
deep trenches are in the Pacific, including the
Aleutian, Kurile, Tonga, Philippine and
Mariana Trenches, with a few in other oceans
such as the Puerto Rico and the South
Sandwich Trenches in the Atlantic and the
Sunda Trench in the Indian Ocean. In
dimensions these trenches are narrow relative
to their length and have depths to 11,000 m.
They are often shaped like an arc of a circle
in plan form with an island arc on one side.
Examples of island arcs are the Aleutian
Islands (Pacific), the Lesser Antilles
(Atlantic) and the Sunda Arc (Indian). The
landward side of a trench extends as much as
10,000 m from the trench bottom to the sea
surface while the other side is only half as
high, terminating at the ocean depth of about
5,000 m.
Fig. 2.7 Perspective map of a trench
Many seamounts and islands were
created by volcanic hot spots that are
relatively stationary in contrast to the tectonic
plates. As the plates move across them,
chains of seamounts are formed. Examples
include the Hawaiian Islands/Emperor
Seamounts chain, Polynesian island chains,
the Walvis Ridge, and the Ninetyeast Ridge
in the Indian Ocean.
2.2.6 Methods for mapping bottom
topography
Our present knowledge of the shape of
the ocean floor results from an accumulation
of sounding measurements, most of which
have been made within the last 60 years, and
very recently using the gravity field measured
by satellites. The early measurements were
made by lowering a weight on a measured
line until the weight touched bottom, as
discussed in chapter 1. This method was
slow; in deep water it was uncertain because
it was difficult to tell when the weight
touched the bottom, and to be certain that the
line was vertical. Since 1920 most depth
measurements have been made with echo
sounders, which measure the time taken for a
pulse of sound to travel from the ship to the
bottom and be reflected back to the ship. One
half of this time is multiplied by the average
speed of sound in the seawater under the ship
to give the depth.
With present-day
29
equipment, the time can be measured very
accurately and the main uncertainty over a
flat bottom is in the value used for the speed
of sound. This varies with water temperature
and salinity (see Section 3.7) and if these are
not measured at the time of sounding an
average value must be used. This introduces
a possibility of error. Over trenches or places
where rapid changes with depth occur there
may also be some uncertainty about whether
the echo comes from directly under the ship
(to give the true depth) or from one side (to
give too small a value).
Great detail has been added to our
knowledge of the sea floor topography by
satellite measurements.
These satellites
measure the earth's gravity field, which
depends on the local mass of material. These
measurements allow mapping of many
hitherto unknown features such as fracture
zones and seamounts in regions remote from
intensive echo sounder measurements (Fig.
2.1), and provide much more information
about these features even where they had
been mapped. Echo sounder measurements
are still needed to verify the gravity-based
measurements since the material on the ocean
bottom is not uniform. For instance a bottom
shaped by extensive sediment cover might not
be detected from the gravity field.
On the continental shelf and slope,
most of the bottom material comes directly
from the land, either brought down by rivers
or blown by the wind. The material of the
deep-sea bottom is often more fine-grained
than that on the shelf or slope. Much of it is
pelagic in character, i.e. it has been formed in
the open ocean itself. The two major deep
ocean sediments are “red” clay and the
biogenic “oozes”. The former has less than
30% of biogenic material and is mainly
mineral in content. It consists of fine material
from the land (which may have traveled great
distances through the air before finally
settling into the ocean), volcanic material, and
meteoric remains. The oozes are over 30%
biogenic and originate from the remains of
living organisms (plankton). The calcareous
oozes have a high percentage of calcium
carbonate from the shells of animal plankton,
while the siliceous oozes have a high
proportion of silica from the shells of silicasecreting planktonic plants and animals. The
siliceous oozes are found mainly in the
Southern Ocean and in the equatorial Pacific.
The relative distribution of calcareous and
siliceous oozes is clearly related to the
nutrient content of the surface waters, with
calcareous oozes common in low nutrient
regions and siliceous oozes in high nutrient
regions.
Except when turbidity currents deposit
their loads on the ocean bed, the average rate
of deposition of the sediments is from 0.1 to
10 mm per 1,000 years, and much
information on the past history of the oceans
is stored up in them. Samples of bottom
material are obtained with a “corer” which is
a steel pipe 2 to 30 m long, which is lowered
vertically and forced to penetrate into the
sediments by the heavy weight at its upper
end. The “core” of sediment retained in the
pipe may represent material deposited over a
period from 1,000 to 10 million years per
meter of length. Sometimes the material is
layered, indicating stages of sedimentation of
2.2.7 Sills
One other term used frequently in
reference to bottom topography is sill. This
refers to a ridge, above the average bottom
level in a region, which separates one basin
from another or, in the case of a fjord
(Chapter ?), separates the landward basin
from the sea outside. The sill depth is the
depth from the sea surface to the deepest part
of the ridge, i.e. the maximum depth at which
direct flow across the sill is possible.
2.3 Bottom Material
30
different materials. In some places, layers of
volcanic ash can be related to historical
records of eruptions; in others, organisms
characteristic of cold or of warm waters are
found in different layers and suggest changes
in temperature of the overlying water during
the period represented by the core. In some
places gradations from coarse to fine
sediments in the upward direction suggest the
occurrence of turbidity currents bringing
material to the region with the coarser
material settling out first and the finer later.
Large sediment depositions from
rivers create a sloping, smooth ocean bottom
for thousands of miles from the mouths of the
rivers. This is called a deep-sea sediment fan.
The largest, the Bengal Fan, is in the
northeastern Indian Ocean and is created by
the outflow from many rivers including the
Ganges and Brahmaputra. Other examples of
fans are at the mouths of the Yangtze,
Amazon and Columbia Rivers.
The physical oceanographer looks at
the sediments for the information that they
give on the movement of the water at the
ocean floor. Photographs of the deep-sea
bottom have been obtained in recent years
and some of them show ripples such as one
sees on a sand beach after the tide has gone
out. Such ripples are only found on the beach
where the water speed is high, such as in the
backwash from waves. We conclude from
the ripples on the deep-ocean bottom that
currents of similar speed occur there. This
discovery helped to dispel the earlier notion
that all deep-sea currents are very slow.
Sediments can affect the properties of
sea water in contact with them. For instance,
silicate and carbonate are dissolved from
sediments into the overlying seawater.
Organic carbon mainly from fecal pellets is
biologically decomposed (remineralized) into
inorganic carbon dioxide in the sediments,
with oxygen consumed in the process. The
carbon dioxide-rich, oxygen-poor pore waters
in the sediments are released back into the sea
water, affecting its composition. . Organic
nitrogen
and
phosphorus
are
also
remineralized in the sediments, providing an
important source of inorganic nutrients for
sea water. In regions where all oxygen is
consumed, methane forms from bacterial
action. This methane is often stored in solid
form, called a methane hydrate.
Vast
quantities (about 1019 grams) of methane
hydrate have accumulated in marine
sediments over the earth's history. They can
spontaneously turn from solid to gaseous
form, causing submarine landslides and
releasing methane into the water, affecting its
chemistry.
2.4 Ocean Basins
The Pacific Ocean (Fig. 2.8) is the
world’s largest ocean basin. To the north
there is a physical boundary broken only by
the Bering Strait, which is quite shallow
(about 50 m) and 82 km wide. There is a
small net northward flow from the Pacific to
the Arctic through this strait. At the equator,
the Pacific is very wide so that tropical
phenomena that propagate east west take
much longer to cross the Pacific than across
the other oceans. The Pacific is rimmed in
the west and north with trenches and ridges.
The associated volcanoes have led to this
being called the “ring of fire". The EastPacific Rise, a major topographic feature of
the South-Pacific, is a spreading ridge that
separates the deep waters of the southeast
from the rest of the Pacific. Fracture zones
allow some communication. Where the ridge
encounters the major eastward current of the
Southern Ocean, the Antarctic Circumpolar
Current (Chapter 8), the current is deflected
and becomes unstable.
31
Fig. 2.8 Map of the Pacific Ocean with topography from NGDC
The Pacific has more islands than any
other ocean. Most of them are located in the
western tropical regions.
The Hawaiian
Islands and their extension northwestward
into the Emperor Seamounts were created by
motion of the Pacific oceanic plate across the
hot spot that is now located just east of the
big island of Hawaii.
The Pacific Ocean has numerous
marginal seas, mostly along its western side.
Some for the North Pacific are the Bering,
Okhotsk, Japan, Yellow, East China and
South China Seas in the west and the Gulf of
California in the east. In the South Pacific are
found the Solomon, Coral and Tasman Seas.
In the southern South Pacific is found the
Ross Sea, which contributes to the bottom
waters of the world ocean.
The Atlantic Ocean has an “S” shape
(Fig 2.9).
The mid-Atlantic ridge, a
spreading ridge down the center of the ocean,
dominates its topography. Deep trenches are
found just east of the Lesser Antilles in the
eastern Caribbean and east of the South
Sandwich Islands. The Atlantic is open both
at the north and the south connecting to the
Arctic and Southern Oceans. The northern
North Atlantic is the formation location of
much of the world's deep water. One of the
Atlantic's marginal seas, the Mediterranean, is
evaporative and contributes high salinity,
warm water to the mid-depth ocean. At the
29
southern boundary, the Weddell Sea is the
formation location for most of the bottom
water found in the oceans. Other marginal
seas connecting to the Atlantic are the
Norwegian, Greenland and Iceland Seas
(sometimes known collectively as the Nordic
Seas), the North Sea, the Baltic Sea, the
Black Sea, and the Caribbean The Irminger
Sea is the region east of southern Greenland,
the Labrador Sea is the region between
Labrador and Greenland and the Sargasso Sea
is the open ocean region surrounding
Bermuda. Fresh outflow from the Amazon
and Congo Rivers form marked low-salinity
tongues at the sea surface.
eastern boundary of the Indian Ocean is
porous and connected to the Pacific Ocean
through
the
Indonesian
archipelago.
Marginal seas for the Indian Ocean are the
Andaman Sea, the Red Sea and the Persian
Gulf. The open ocean region west of India is
called the Arabian Sea and the region east of
India is called the Bay of Bengal.
Fig. 2.10 Map of the Indian Ocean with
topography from Smith and Sandwell (1997)
The differential heating of land and
ocean in the tropics results in the creation of
the monsoon weather system. Monsoons
occur in many places, but the most dramatic
and best described monsoon is in the northern
Indian Ocean. The Northeast Monsoon from
October to May sends cool, dry winds from
the continental land masses in the northeast
over the Indian subcontinent to the ocean.
Starting in June and lasting until September,
the system shifts to the southwest monsoon,
which brings warm, wet rains from the
western tropical ocean to the Indian
subcontinent.
While these monsoon
conditions are best known in India they also
dominate the climate in the western tropical
and South Pacific.
Another interesting factor in the
geography of this region is that fact that most
Fig. 2.9 Map of the Atlantic Ocean (xx map is
inaccurate, replace xx)
The Indian Ocean (Fig. 2.10) is closed
off by land just north of the tropics. The
topography of the Indian Ocean is very rough
because of the ridges left behind as the Indian
plate moved northward into the Asian
continent, creating the Himalayas. The only
trench is the Sunda trench where the Indian
plate subducts beneath Indonesia.
The
29
of the Indian rivers outflow on the eastern
side of India. This causes the surface of the
Bay of Bengal east of India to be quite fresh.
The enormous amount of silt carried from the
eroding Himalayan Mountains into the Bay of
Bengal creates the Bengal Fan that slopes
smoothly downward for thousands of
kilometers. West of India the Arabian Sea,
Red Sea and Persian Gulf are very salty due
to the dry climate and subsequent high
evaporation. Much like the Mediterranean,
the saline Red Sea water is sufficiently dense
to sink to mid-depth in the Indian Ocean and
affects water properties over a large part of
the Arabian Sea.
The Arctic Ocean (Fig. 2.11) is
sometimes not regarded as an ocean, but
rather as a mediterranean sea connected to the
Atlantic Ocean. It is characterized by very
broad continental shelves surrounding a
deeper region, which is split down the center
by the Lomonosov Ridge. Areas around the
Arctic are named seas: the Beaufort, Chukchi,
East Sibersian, Laptev, Kara and Barents.
The Arctic is connected to the North Pacific
through the shallow Bering Strait. It is
connected to the Nordic Seas (Norwegian,
Greenland and Iceland Seas) through
passages on either side of Svalbard. The
Nordic Seas are separated from the Atlantic
Ocean by the submarine ridge between
Greenland, Iceland and the United Kingdom,
with a maximum sill depth of about 480 m
between Greenland and Iceland (Denmark
Strait). Dense water formed in the Nordic
Seas spills into the Atlantic over this ridge.
The central area of the Arctic is perennially
covered with sea ice.
30
Fig. 2.11 The Arctic Ocean (xxmap has obvious mistakes, replacexxx)
The Southern Ocean (Fig. 2.12) is not geographically distinct from the Atlantic, Indian and
Pacific Oceans, but is often considered separately since it is the only region outside the Arctic
where there is a path for eastward flow all the way around the globe. This occurs at the latitude of
Drake Passage between South America and Antarctica. This allows the three major oceans to be
connected. The South Sandwich Islands and trench east of Drake Passage partially block the open
circumpolar flow. The ocean around Antarctica includes permanent ice shelves as well as seasonal
sea ice. Unlike the Arctic there is no perennial long-term pack ice; except for some limited ice
shelves and all of the first year ice melts and forms each year. The densest bottom waters of the
world ocean are formed in the Southern Ocean, primarily in the Weddell and Ross Seas and also in
other areas distributed along Queen Adelie Land.
29
Fig. 2.12 The Southern Ocean around Antarctica
30