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Geomorphology and Sea Level
Figure 1
The shape of the ocean basins has much to do with the way the ocean moves, the way the wind
blows, and the distributions of water temperature and salinity, among other physical phenomena.
This chapter discusses the geomorphology of the ocean basins in the context of physical
oceanography. This chapter also discusses the recent trends in sea level.
The Ocean Basins
The four ocean basins (Pacific, Atlantic, Arctic, Indian) and the major marginal seas comprise the
global ocean. Features that are important to the physics of the ocean include:
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The placement of the continents is asymmetrical (larger area in northern hemisphere).
The Southern Ocean is continuous zonally (along co-latitude lines).
The large marginal seas, some of which are evaporative basins are found mostly in the
Northern Hemisphere
The South Pole is land and the North Pole is ocean
River input to the Northern Hemisphere is higher than that to the Southern Hemisphere
The Pacific, Atlantic and Indian are connected only at very high southern latitudes.
The Arctic basin is only connected to the other basins by relatively shallow sills thus might
be considered a marginal sea.
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The global system of rises and rifts stands several thousand meters above the abyssal ocean floor.
Such relief is sufficient to affect the flow of deep and bottom waters. This is most noticeable in the
Atlantic basin where the Mid-Atlantic Ridge isolates the northward flow of Antarctic Bottom Water.
Some of the gaps in the ridge system such as the Vema Channel and the Iceland-Faroe gap exemplify
the effect of topography on water mass distribution.
A secondary effect of the distribution of land and water is related to the advection of cold air masses
from continents to the ocean. Clearly because the major continental land masses lie in the Northern
Hemisphere the major air sea exchanges will occur in the northern Hemisphere. The exception is
the cold air flowing off the Antarctic continent that modifies the Southern Ocean.
The typical ocean basin has several characteristic that are shown schematically in Figure 2.
Note the following:
The broad abyssal plain interrupted by the mid ocean ridge system
The presence of sea mounts
Deep trenches adjacent to island arc systems
The continental rise sloping up from the abyssal plain to the continental slope
The shallow shelf break forming the boundary between the relatively steep slope and the
gently sloping continental shelf.
The slope of the shelf break is like approaching the Blue Ridge or Rockies while the shelf itself
would resemble a coastal plain.
The hypsometric curve (Fig. 3) shows the frequency distribution of depth in the ocean basins.
The following should be noted about the depths of the ocean:
Average depth of ocean is ~4000 m in contrast to mean land height of 1000 m.
Most of the ocean is deep
Shelves and slopes occupy a small percentage of the ocean
Continental Margin
Continental margins are composed of the Continental Rise, Continental Slope, Continental Shelf
and Shelf Break (Fig. 4). Some regions have large shelf areas others have steep shelves Example
of California continental margin (Fig. 5).
Shelf Break
As you will learn the shelf break depth is very important. This is because wind-driven process
obviously originate at the surface and penetrate downward into the ocean. Over most of the ocean
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the downward penetration never reaches the bottom. However as we approach the coast the surface
processes on the ocean eventually reach the bottom. Thus the depth of the shelf break is important
in defining how the waters over the continental shelf interact with the deeper waters of the adjacent
ocean.
Figure 6 shows a plot of the shelf break depth along the east coast of the U.S. Note how shallow the
shelf break depth is off Florida compared to farther north. Off Virginia it is about 100 m and
deepens to 300 m off Newfoundland.
Figure 7 shows the width of various bathymetric features. Note that the widest feature is the abyssal
plain. Continental shelf and rise are also typically wide. Seamounts, trenches and canyons are
typically very narrow. These dimensions have important implications for the productivity of the
region as they may determine the transport and exchange of nutrients between different regions of
the ocean. Continental shelves are among the most productive regions of the ocean.
On the continental shelves, as we approach the coast we usually see one of two types of coasts: steep
or shallow (Fig. 8). The shallow coast slowly shoals to the beach. Often barrier islands lie offshore
creating shallow lagoons. These lagoons, because they are shallow, respond quickly to atmospheric
variability. The steep coast, as we see off the west coast features steep shore, narrow shelf, and
often, rocky headland. Here the currents of the open ocean often reach to within several kilometers
of the coast.
Estuaries such as the Chesapeake Bay (Fig. 9) can get extremely complicated just in their bathymetry
let alone the dynamics and biology. Here we see the complex system of channels, deeps, and shoals
that make up the Chesapeake. Note how the deep channels are not connected.
Submarine Canyons
Submarine canyons (Fig. 10) are important because they cut through the slope and often the shelf
providing a route for shelf and deeper slope waters to interact. A more obvious indicator of their
importance is the way fisherman congregate around them because of the concentrations of fish.
Canyons incise much of the U. S. coastline (Fig. 11) but some areas are noticeably lacking in them
(Gulf of Mexico, Southeast U.S., part of Oregon). When studying shelf areas you must be aware of
the proximity of canyons as they will affect the flow field significantly. Many studies seek to avoid
them.
The canyon system draining Baffin Bay (Fig. 12) is important in the flow of North Atlantic Deep
Water (NADW) that is sinking in this area.
A multitude of canyons concentrate in the Nova Scotia Shelf (Fig. 13). Also note the large channels
(Northeast and Laurentian) connecting the deeper ocean to basins (Gulf of Maine and Gulf of St.
Lawrence, respectively). Also note the basin system on this shelf partially isolated from the deep
ocean by shelf edge banks.
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The Hudson canyon (Fig. 14) reaches the coast at the entrance to the New York-New Jersey Harbor
estuary. Represents an important source of nutrients to the shelf but also may become anoxic in the
stratified summer. The Monterey and Indus canyons(Fig. 15) are other examples of canyons that
extend across the shelf close to the coast or river mouth. The Congo canyon (Fig. 16) is the extreme.
It extends into the river mouth. The Magdalena in Colombia is similar.
The Mississippi River, because of human control, has extended its delta completely across the shelf
and spills over the slope (Fig. 17). Thus Mississippi River waters do not enter the shelf area directly
and may be returned onshore by the slope-to-shore flow. Mississippi River waters can interact
directly with currents in the Gulf of Mexico.
Deep Ocean Basin - Submarine Trenches
In addition to the extensive abyssal plains, a relevant feature in the deep basins are the trenches and
the seamounts (Fig. 18). Trenches (Fig. 19) are the deepest parts of the ocean and represent areas
of crust subduction. The Mariana Trench ~11 km is the deepest point on Earth, Ocean currents in
trenches are very weak but it would be interesting to see what kind of vertical flows develop around
these features. Seamounts are very ubiquitous throughout the world’s oceans (Fig. 20). They tend
to disturb the typical flow pattern in a given region (Fig. 21). Such disturbances may enhance the
productivity of the area by retaining nutrients or early life stages of different organisms, thus
providing a strong base for a food chain.
Importance of bathymetric features
The importance of bathymetric features in coastal waters is illustrated in three figures. The first
(U.S. east coast, Fig. 22) shows surface color that is nearly equivalent to biological production. Note
the flow of low chlorophyll water into the Gulf of Maine through Northeast Channel. Note the high
values over shoals, and headlands. The second (Fig. 23) illustrates pigment concentration in the
Mediterranean and the Arabian Sea. Note the highest productivity around coastal areas. The third
is a sea surface temperature (SST) image for the U.S. west coast and the coast of Chile (Fig. 24).
Note the activity off some of the major headlands.
Sills, analogously to seamounts but over shallower water, also produce important disturbances to the
flow. Sills are rises in the bottom that isolate one deep water area from another. The sills at the Strait
of Gibraltar are classic examples. Figure 25 shows how dense waters formed in the Mediterranean
are isolated and flow over the sill as the basin fills. What other large ocean basins have sills? Look
at Figure 1.
One last look at sills. Fjords are by definition formed by glaciers and often the entrance to the fjord
is blocked by the terminal moraine of the glacier. Here is an example of such a sill and the effect.
First figure (Fig. 26a) shows the inlet with bathymetry. The bottom rises from 80 to 10 meters over
the sill. The water accelerates as it flows over the sill and causes disturbances at the pycnocline. This
process enhances mixing between bottom and surface waters and likely provides food to upper
trophic levels.
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Sea Level
The final part of this lecture deals with sea level variations. Obviously sea level changes will not
have an immediate effect on the open ocean. However you must be aware that sea level has changed
in the past and is now. This is because interpretation of historical data from coastal areas may
require this knowledge. The fact that sea level is now rising may be of little significance to people
who live on shorelines that have relief in regions close to the coast. But for those who live in
relatively flat regions, like the western Pacific, there are reasons to be very concerned. Within several
generations their territories may disappear.
Approximately 15,000 years ago the sea level was over 100 m lower (Fig. 27). The coastal line
during these low stands of sea level did not have a continental shelf (Fig. 28). But what is sea level
doing now? Where is it going?
Sea level is rising at a rate of about 2 mm per year or about 20 cm per century (Douglas, 1991).
What is the significance of this? There is probably little significance to ocean dynamics at less than
century time scales. The noticeable short term effect is on coastlines and thus indirectly on ocean
circulation. For a shallow slope coastline having a slope of 1:500, such a rise in sea level means a
horizontal transgression speed of 1 meter per year. Such transgression will have significant effects
on the coast by causing morphological changes that will no doubt affect coastal ocean flow.
The sea level changes produced by glaciation/deglaciation can be understood quantitatively through
the principle of Isostacy. This principle can also be used to explain ocean currents. It assumes that
the pressure at a certain depth underneath the earth’s crust is the same everywhere. Thus, if we add
mass to a continent in the form of ice, the continent will sink. Quantitatively, this is explored with
the relationship also know as the hydrostatic equation that relates pressure to the density of the
material, the acceleration due to gravity and the thickness of the material. This can be illustrated
with a couple of examples in class.
Summary
We have discussed the shape of the ocean and changes in sea level. While this will not be discussed
any further you must keep these ideas in mind as you think about the deep or shallow ocean. Size
and shape are important; they often are the most significant factors in ocean dynamics. Sea level has
changed and is presently rising. The effects on the coastal ocean may be significant, especially on
shallow slope coasts.
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