Brief Geologic History and Zonation of the Ocean
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.
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)
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.