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Brief Geologic History and Zonation of the Ocean
Objectives:
1.
Be able to provide a historical framework for
major events that happened in the ocean.
2.
Be able to label all oceans and major seas on a
map of the world.
3.
Be able to draw a cross-sectional image of an
ocean basin, and label and define all seafloor features.
4.
Be able to describe two different ways volcanic
islands are formed.
5.
Be able to list and describe all benthic zones.
6.
Be able to list and describe all oceanic zones.
Introduction
The ocean is the largest life-supporting
habitat on the planet. It covers 70.9%
of the Earth’s surface, has an average depth of 3700 meters, and contains over 1.3
billion km3 of living space.
In addition, the ocean is home to at least half of all known species,
yet over 95% of it still remains unexplored.
Geologic History of the Ocean
The Earth is 4.5 billion years old. It was so hot at first that water existed
only as water vapor. By 4 bya (billion
years ago) the atmosphere and planet cooled enough that water vapor condensed and
filled ocean basins for the first time.
This deluge also produced continental runoff that brought sediment and
salts to the ocean. Ocean salinity
increased until 1.5 bya when it stabilized at concentrations we observe today. We do not completely understand the processes
that maintain ocean salinity at the current stable level. We do know that salt is lost from water by
physical and chemical processes and that salt is added to water by an influx of
material from the continents.
Life evolved in the sea. The first evidence of life appeared about 3.5
billion years ago. The first organisms
were prokaryotes, including the photosynthetic cyanobacteria. It took
cyanobacteria nearly a billion years of ongoing photosynthesis to produce
enough oxygen so that it could start accumulating in the atmosphere and surface
waters of the ocean. Early oxidation of
the atmosphere and ocean occurred 2.5 to 1.8 bya in what we call the great oxidation event (Fig.
2-1). Little evidence of further
increases in ocean or atmospheric oxygen occurred between 1.8 – 0.8 bya and is
called the boring billion. The first eukaryotes appeared 1.5 bya, however,
making that billion years not totally boring.
Between 0.85-0.54 bya enough oxygen accumulated in the atmosphere and
ocean that even the deep sea became oxygenated.
The Earth also experienced alternating hothouse and ice age conditions during
that time. Oxygen levels continued to
increase, and about 530 million years ago the ocean experienced a massive
proliferation of anatomically complex animal life. This is called the Cambrian Explosion. The oldest
fossils of most modern animal body plans were produced at this time. Oxygen concentrations in the atmosphere and
ocean stabilized to modern levels a few hundred million years ago and have been
the same ever since.
The size and shape of the ocean is
in slow but constant flux. Changes occur
as tectonic forces create new
oceanic crust in some places and drive subduction
in others. These forces push and pull continental
plates around the Earth’s surface at the a few to several cm yr-1. That’s about the same rate your fingernails
grow. Sometimes tectonic forces move the
continents together creating a supercontinent
like Pangea (Fig. 2-2). When a
supercontinent exists the rest of the Earth is covered by one massive
ocean. At other times, like now, the
continents are dispersed around the planet surface, and the ocean is divided into
several smaller basins (Fig. 2-3).
Figure 2-1. High
and low ranges of oxygen accumulation in the atmosphere (top graph), ocean
surface waters (middle graph), and ocean deep waters (bottom graph). (Image
modified from Holland, 2006)
Figure 2-2. Tectonic
movement of contents between 250 mya and today. Pangea is the most recent
supercontinent. (Image: USGS)
Figure 2-3. Boundaries of oceans and major seas of the
modern world. (Image: NOAA)
Seafloor Topography
The seafloor extends from sea level
at the margins of all continents and islands down to ocean trenches more than
10,000 meters below the surface. Figure
2-4 shows a generalized profile of the seafloor and major features associated
with it. These features include the
continental shelf, shelf break, continental slope, continental rise, abyssal
plain, volcanic islands, trenches, seamounts, and mid-oceanic ridges.
Figure 2-4.
Generalized cross section of an ocean basin (not to scale). 1 - Continental shelf; 2 - Shelf break; 3 –
Continental slope; 4 – Continental rise; 5 – Abyssal plain; 6 – Volcanic
island; 7 – Trench; 8 – Seamount; 9 – Mid-oceanic ridge. (Image: ARH)
The continental shelf is the submerged edge of a continent. Continental shelves extend a few kilometers
to over 1000 kilometers in width, and the outer edge of the shelf is a few
hundred to several hundred meters deep.
The continental shelf break marks
the outer edge of the continental shelf.
This is where the shallow grade of the continental slope gives way to
the steep grade of the continental slope.
The continental slope plunges
down a few thousand meters before it reaches the continental rise. The continental
rise is the transitional area that shifts gradually from the steep grade of
the continental slope to a flat abyssal plain. The abyssal plain may be 3000-6000 meters deep and is a vast, muddy
expanse covering most of the seafloor, though volcanic islands, trenches,
seamounts, and mid-oceanic ridges interrupt it.
Volcanic islands form along trenches
where oceanic crust subducts under another tectonic plate (Fig. 2-5). Islands that form along trenches typically
form an island arc. A couple of examples
of island arcs include the Aleutian Islands and the Marianas Islands (Fig 2-6). Volcanoes form as subduction pushes crust
material and water trapped in the sediment downward. Heat from the mantle superheats the subducted
rock and water, but since that material is now under extreme pressure the water
remains liquid and facilitates the further heating of rock around it. Lower density rock in the subducted crust
becomes semi-pliable and gradually rises toward the surface. When this superheated rock material gets close
enough to the surface, pressure is reduced and the rock can transition into
magma that is released during a volcanic eruption. Ongoing or repeated magma release adds to the
height of underwater volcanoes until they sometimes break the ocean surface as
volcanic islands. You may not have known
this, but the tallest mountain in the world from base to peak is not Mt.
Everest, it’s Moana Kea on the big island of Hawaii. It is 10,200 m tall from base to peak; the
peak of Mt. Everest is only 8,848 m above sea level.
By the way, the deepest part of the
deepest trench in the ocean, the Challenger Deep of the Mariana Trench is
10,916 m deep. That is so deep that if
you put Mt. Everest (8,848 m tall) in it, its peak would still be over 2 km
below the ocean surface!
Figure 2-5. Subduction
of an oceanic plate and formation of an island arc volcano. (Image: ARH)
Figure 2-6. The
Marianas Trench and Aleutian Trench and associated island arcs. Island arcs include islands and seamounts.
(Images: modified from Google Earth)
Volcanic
islands can also occur where a tectonic plate slides over a hot spot in the
mantle where a plume of mantle material pushes through the crust toward the
seafloor. This is how the Hawaiian
Island chain was formed (Fig. 2-7). By
the way, mantle plumes/hot spots can occur on land. A mantle plume is what fuels the geysers and
thermal activity in Yellowstone National Park.
Figure 2-7. The
Hawaiian Island chain. The closest trench
to the Hawaiian Islands is the Aleutian Trench, over 3500 km away. (Image:
Google Earth)
When a
volcanic island does not reach the surface it is a seamount. Actually, a
seamount is any underwater rise that does not reach the surface. Scientists and fishermen discovered that
seamounts are often islands of high biomass and biodiversity surrounded by low
biomass habitats.
The last topographic feature
addressed in this section is the mid-oceanic
ridge. The mid-oceanic ridge is an
undersea mountain range that exists along divergent boundaries where seafloor
spreading occurs (Fig. 2-8). The peak of
mid-oceanic ridges usually rises a few thousand meters above the abyssal plain
on either side of it. The interconnected
mid-oceanic ridge system constitutes the longest continuous mountain range on
the planet.
Zonation of the Ocean
The ocean is divided into the
benthic and pelagic zones (Fig. 2-9).
The benthic zone includes the seabed, and the pelagic zone includes the
water column.
Divisions of the
Benthic Zone
The marine
benthos extends from the high tide mark of the intertidal or littoral zone to
the bottom of the deepest trench. The littoral zone benthos includes all
seafloor that is covered and uncovered periodically by tidal exchange. Littoral benthic habitats include rock to mud
substrates. The sublittoral zone extends from the bottom of the littoral zone to
the continental shelf break. Depending
on water turbidity, latitude, and depth, light may reach the seafloor up to a
few hundred meters deep. This is where
we find marine benthic communities including kelp forests, kelp beds, seagrass
beds, turtle grass beds, and coral reefs.
The benthic zone of continental slopes is called the bathybenthic or bathyal zone. The abyssal benthic zone includes depths of
the continental rise and abyssal plains.
This is the largest benthic habitat in the ocean. The hadal
benthic zone is found only in trenches.
The deeper benthic zone is, the less we know about it.
Figure 2-8. Age
of oceanic crust. The newest crust
exists at divergent boundaries at mid-oceanic ridges, and the oldest material
is at trenches. (Image: NOAA)
Divisions of the Oceanic
Zone
The term water column refers to all water from the surface all the way to
the bottom in a particular location. The
water column of the ocean can therefore extend to more than 10,000 m in some
places. Scientists have divided the
water column into divisions by depth and other factors to reduce confusion when
referring to different regions of the ocean.
Keep in mind that there is no rigid line separating one division from
the next, and these divisions are used only as general guidelines in discussing
environments at different depths.
All water that is not part of the
intertidal or littoral zone is called the oceanic
zone. The oceanic zone is divided
into water that lies over continental shelves and deeper water. The division over the shelf is called the neritic zone. The rest of the ocean is called the pelagic zone.
The divisions of the open ocean,
starting at the shoreline and moving offshore are indicated in Figure 2-9. The uppermost horizontal layer of the oceanic
zone is the epipelagic zone. It usually extends to a depth of a few
hundred meters. This is also called the photic zone. This zone’s maximum depth is usually defined
as the depth where only 1% of surface incident solar radiation remains. Most pelagic ocean life lives in the
epipelagic zone because this is where photosynthesis can take place.
Figure 2-9.
Divisions of the ocean. (Image: Wikimedia Commons)
The mesopelagic zone spans extends from the bottom of the epipelagic
zone to 700-1000 m. Most solar radiation is absorbed or scattered in the
epipelagic zone, but the mesopelagic zone is not entirely dark. When you are in this zone and you look toward
the surface on a sunny day you can still discern a faint glow. There is too little light here for
phytoplankton to carry out enough photosynthesis to meet their basic energy needs. Because of this, the mesopelagic zone is also
called the disphotic zone. Many animals that live here bio luminesce and
migrate vertically into the epipelagic zone each night in order to feed.
The next layer in the water column
is the bathypelagic zone. The bathylpelagic
zone extends from about 1000 m to depths as deep as 4000 m. The upper bound of this layer is defined as
the depth where surface light is no longer discernible. The lower bound of this layer generally
corresponds with the lower end of the continental slope. Some bathypelagic animals may migrate up into
the mesopelagic zone to feed, but most organisms in the bathypelagic zone feed on
material that drifts down from above as well as on each other.
The abyssalpelagic zone exists below the bathypelagic zone. This layer extends from about 4000m to the abyssal
plain, usually 4000-6000 m deep. Organisms
in the bathypelagic zone make a living by consuming whatever drifts down from
above, by eating each other, and by feeding on benthic organisms.
The deepest pelagic layer is hadalpelagic zone. This zone exists only in trenches, in water
as much as 10,000 m deep. We know the
least about life in the hadalpelagic zone of any ocean depth, though fish were
observed in the Japan Trench via ROV in 2010 at depths approaching 8000 m.
Every division of the ocean has its
own set of challenges and opportunities for organisms that live there. One of the goals of marine biology is to
identify what those challenges and opportunities are, and then discover how
marine organisms exploit them.
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