An Introduction to the Marine Environment

• ~71% of the Earth is covered by salt water• The average depth of the ocean is 3,800

meters (12,500 feet) and at its deepest depth is 36,163 feet (7,000 feet deeper than Mt. Everest is high)

• That’s a total volume of 1,349,929,000 km3!• The oceans constitute the largest repository of

organisms on the planet

The Geography of the Ocean Basins

• The oceans (Atlantic, Pacific, Indian, Arctic, and Southern) are not distributed evenly– Two-thirds of land found in Northern Hemisphere– 80% of Southern Hemisphere is ocean

• A number of shallow seas are connected to the main ocean basins

• All ocean basins are inter-connected, and together constitute 1 world ocean

One World Ocean

http://www.gebco.net/data_and_products/gebco_world_map/images/gda_world_map_large.jpg

Pacific OceanAtlantic Ocean

Indian Ocean

Arctic Ocean

Southern Ocean

The Ocean’s Origins• The Earth is believed to be ~4.6 billion years old• Originated from a tremendous debris cloud,

the result of the “Big Bang”, a massive cosmic explosion which occurred 9 billion years prior

• Tremendous amounts of heat were released from decaying radioactive elements deep within the newly-assembled planet, and from gravitational compression and nearly constant asteroid impacts

The Ocean’s Origins• So much heat was generated as the early

Earth formed that the planet was partially molten

• Gravity pulled most of the heavy metals such as iron and nickel towards the hot center

• At the same time, lighter elements such as aluminum and silicon rose towards the surface, cooling into a thin crust

The Ocean’s Origins• The cool, rigid

crust rests upon a hot molten layer of mantle that is denser still than the crust above it

The Ocean’s Origins

• Due to physical and chemical differences in the rock that makes up the crust, it is thinner and thicker in some places

• Thinner = oceans• Thicker = continents• Ocean floor (oceanic crust)

is geologically distinct from continental crust

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Geologic differences between oceans and continents

• Oceanic crust consists of a thin, dense, dark mineral called basalt– Rich in iron and magnesium

• Continental crust consists of a thick, light material known as granite– Rich in sodium, potassium, calcium and aluminum

• Dense oceanic crust sinks deeper into the molten mantle, and sits even lower still since it is so thin

e.g., light continental crust

e.g., heavy oceanic crust

• Eventually, the young Earth cooled enough such that water vapor (perhaps originating from impacts by icy comets) condensed and collected in the deep ocean basins

• As water collected, it dissolved minerals and salts within the rocks forming sea water

• After only 20 million years of rain, the oceans were born!

The Ocean’s Origins

The Ocean’s Origins

• Oceanic and continental crust also differ in age

• The oldest oceanic rocks are less than 200 million years old, quite young by geological standards

• Conversely, continental rocks can be as much 3.8 billion years old

• Why???

The Origin and Structure of the Ocean Basins

• Although the Earth appears stagnant in time, evidence for geologic change is all around us– From catastrophic earthquakes and volcanic

eruptions to the slow erosion of river valleys and oceanic cliffs

– Even a glimpse at a world map provides clues for the ocean’s origins…

From the time accurate charts became available in the 1700’s, observers noticed the

remarkable coincidence of shape of the Atlantic coasts of South America and Africa

Early Evidence of Continental drift

• In 1912, Alfred Wegener proposed the theory of continental drift, the idea that all Earth’s land was in fact once joined into a single supercontinent, called Pangaea

• Discovery of coal (the fossilized remains of tropical plants) in Antarctica, and similarities in fossils across separate continents supported his theory

Continental Drift

Pangaea Supercontinent

190 million years ago

Future Pacific Ocean

Future Indian Ocean

Future Arctic Ocean

Continental Drift

In fact, the Atlantic is expanding, while the Pacific is shrinking!

Atlantic Ocean formed first

How did our current oceans form?

Say What?!!?

• By the 1950’s and 1960’s, scientists were able to piece all of the evidence together and proposed a mechanism for continental drift, known as plate tectonics

• Plate tectonics describes the large-scale movements of the Earth’s upper layer, known as the lithosphere (includes oceanic and continental crust) over the molten, plastic layer below, the aesthenosphere– The lithosphere is broken up into pieces, or plates

Plate Tectonics

The lithosphere is broken into plates: some major, some minor

Seafloor Spreading

• The theory of plate tectonics is supported by a process known as seafloor spreading– Helps explain large-scale movement of continents

• Seafloor spreading occurs as new oceanic crust rises to the surface from the aesthenosphere below

• This new crust is very hot and so less dense than older oceanic (basaltic) crust; as a result it rises high as a ridge system in the center of our oceans

The Mid-Atlantic Ridge

Plate Tectonics/Seafloor spreading

• As seafloor spreading continues, new oceanic crust is laid down and the ocean steadily grows

• The ridge is highest in its center as the new, (warm) less-dense oceanic crust rises higher than the (cool) more-dense, older crust that surrounds it

• Takes millions of years to cool and sink; the center of the oceans are often the shallowest!

Plate Tectonics/Seafloor spreading

• Since the Earth is not continuously growing, the creation of new oceanic crust must be balanced by the destruction of crust elsewhere!

• In fact, while oceanic crust is being created in the center of an ocean, oceanic crust is being destroyed at the edges of the surrounding continents; a process known as subduction

Dense oceanic crust always sinks below the less-dense continental crust; explains why oceanic crust is so young, relative to the much older continental

crust which floats along on the asthenosphere

Plates come together at convergent boundaries

• What happens when two plates come together, or converge?

• Plates may come together in one of the following ways– Oceanic to continental plate: oceanic plate subducts

beneath continental– Oceanic to oceanic plate: the older (denser) oceanic

crust subducts beneath the newer (less dense) oceanic crust

– Continental to continental plate: the 2 plates share equal density (same age) & so they are compressed, folded & uplifted into each other forming mountain ranges

Oceanic to continental convergence

Oceanic to oceanic convergence

• The Marianas Trench – the world’s deepest ocean trench was formed by 2 oceanic plates converging

• The Marianas Trench is ~11,033 meters (36,163 feet)

Pacific plate

Mariana plate

Continental to continental convergence

Island Living

• Volcanic island chains such as Hawaii, are not the result of plate convergence, but rather the result of an oceanic plate moving over a stationary plume of mantle, called a hot spot

• Usually a hot spot is far from a plate boundary, but results from a rising mantle plume–As the plate moves over the plume, volcanic

islands are created

• The Hawaiian island chain was formed this way!

Functions as a giant assembly line; volcanoes become inactive as they move away from the hot spot

Seafloor spreading pushing plate westward

Geological Provinces of the Ocean• The structure of the ocean floor is determined

largely by geological processes• The sea floor is divided into 2 main regions– Continental margins (submerged continental crust)– Deep sea floor (oceanic crust)

Continental Margins

• Continental margins generally consist of a gently-sloping continental shelf, and a steeper continental slope, leading to the continental rise, the junction between continental and oceanic crust

• Continental shelves make up ~8% of the ocean’s surface area, but are the biologically richest part of the ocean

• Continental shelves is composed of continental crust that happens to be submerged underwater

http://oceanexplorer.noaa.gov/explorations/02hudson/background/mapping/mapping.html

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Continental margin (examples)

Continental Margins

• Continental margins are shallow water areas close to the continents; they literally are the submerged outer edges of a continent

(raised up because hot = buoyant)

Deep Ocean Basins

• Ocean basins are deep-water areas farther from land, beyond the continental margin

(raised up because hot = buoyant)

Deep Ocean Basins

• Deep sea floor is oceanic crust (basaltic) and so sinks deeply into the asthenosphere

• As a result, most of the deep sea floor lies at a depth of 3,000 to 5,000 meters (10,000 to 16,500 feet)

• Known as the abyssal plain, the deep sea floor rises at a very gentle slope (~1°) towards the mid-oceanic ridge

• Blanketed with up to 3 miles of sediments

The Abyss?

• Despite the misnomer, abyssal plains are not entirely flat, but are dotted with volcanic islands, submarine volcanoes (seamounts) and channels, rises, plateaus and other features

• In fact, the abyssal plain and seamounts are home to a tremendous variety of marine life

Mid-Oceanic Ridge Systems

• Mid-oceanic ridges form when material rising from the mantle pushes up new oceanic crust

• At the center of the ridge, however, 2 plates are pulling apart, leaving a gap, or depression, known as the central rift valley

• Seawater seeps through cracks and fissures along the sides of the valley, becomes super-heated by the very high temperatures of the hot mantle

Hydrothermal Vents

• The heated water then forces its way back up through the crust and emerges in hydrothermal vents

• As the hot water seeps through cracks, it dissolves a variety of minerals, e.g., sulfides

• When the super-heated water emerges at the vent and mixes with the surrounding water, it cools and causes the minerals to solidify, forming chimney-like structures around the vent

• The average temperature in the vicinity of hydrothermal vent activity is ~8-16°C (46-61°F), much warmer than usual for ocean-

bottom water (~3-4°C)

Hydrothermal Vents• Despite the temperatures, hydrothermal

vents host a unique assembly of organisms, fueled by the chemicals dissolved in the vent fluids– Chemosynthetic Archaea form the base

of the food chain (we return to this later)– Hydrothermal vents dot the mid-oceanic

ridge systems, but have also now been found near trench systems where 2 oceanic plates converge

Our present knowledge of vent systems