EXPLORE: Earth's Oceans

1. Ocean Overview

Earth is nicknamed the Blue Planet because approximately 70% of its surface is covered with water. Most of that water is found in the oceans. There are 5 oceans - the Pacific, Atlantic, Indian, Arctic, and Southern Oceans. The Pacific Ocean is the largest of the world's oceans. The Southern Ocean surrounds Antarctica, and is composed of waters from the southern portions of the Atlantic, Pacific, and Indian Oceans. Both the Arctic and Southern Oceans are covered with vast expanses of sea ice during the winter. The ice melts in the summertime except in the coldest parts of these oceans, where the ice does not thaw in the summer.

The average depth of the ocean is 4, 267 meters (14,000 feet). The deepest part of the ocean is located in the western Pacific and is part of the Mariana Trench. This deep part of the ocean is called Challenger Deep. Here the ocean is 11,030 meters (36,200 feet) deep.

The following video provides an overview of the world's oceans. It explores the biodiversity of the world's oceans and seafloor topography. It introduces the concepts of ocean pollution and the overuse of ocean resources, and describes the impacts of human activities on ocean ecosystems.

2. Earth's Early Atmosphere

Gases found in the tail of comets became part of Earth's early atmosphere. Photo courtesy of CK-12. 

Earth’s first atmosphere was made of hydrogen and helium, the gases that were common in this region of the solar system as it was forming. Most of these gases were drawn into the center of the solar nebula to form the Sun. When Earth was new and very small, the solar wind blew off atmospheric gases that collected. If gases did collect, they were vaporized by impacts of comets and asteroids.

Eventually things started to settle down and gases began to collect. High heat in Earth’s early days meant that there were constant volcanic eruptions, which released gases from the mantle into the atmosphere. Just as today, volcanic outgassing was a source of water vapor, carbon dioxide, small amounts of nitrogen, and other gases to our atmosphere.

Scientists have calculated that the amount of gas that collected to form the early atmosphere could not have come entirely from volcanic eruptions. Frequent impacts by asteroids and comets brought in gases and ices, including water, carbon dioxide, methane, ammonia, nitrogen, and other volatiles from elsewhere in the solar system. Calculations also show that asteroids and comets cannot be responsible for all of the gases of the early atmosphere, so both impacts and outgassing were needed.

As outgassing and comet impacts continued, the atmosphere stayed with the planet. This atmosphere had lots of water vapor, carbon dioxide, nitrogen, and methane but almost no oxygen. Why was there so little oxygen? Plants produce oxygen when they photosynthesize but life had not yet begun or had not yet developed photosynthesis. In the early atmosphere, oxygen only appeared when sunlight split water molecules into hydrogen and oxygen and the oxygen accumulated in the atmosphere.

Without oxygen, life was restricted to tiny simple organisms. Why is oxygen essential for most life on Earth?

1. Oxygen is needed to make ozone, a molecule made of three oxygen ions, O₃. Ozone collects in the atmospheric ozone layer and blocks harmful ultraviolet radiation from the Sun. Without an ozone layer, life in the early Earth was almost impossible.

2. Animals need oxygen to breathe. No animals would have been able to breathe in Earth’s early atmosphere.

Source: Early Atmosphere and Ocean. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/11.15/ on September 1, 2013. 

3. Origin of the Oceans

Early Oceans

The early atmosphere was rich in water vapor from volcanic eruptions and comets. When Earth was cool enough, water vapor condensed and rain began to fall. The water cycle began. Over millions of years enough precipitation collected that the first oceans could have formed as early as 4.2 to 4.4 billion years ago. Dissolved minerals carried by stream runoff made the early oceans salty. What geological evidence could there be for the presence of an early ocean? Marine sedimentary rocks can be dated back about 4 billion years.

By the Archean, the planet was covered with oceans and the atmosphere was full of water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.

Scientists hypothesize that the water that makes up the global oceans came from meteorites, comets, and volcanism. The early atmosphere was rich in these gases. It is believed that both comets and meteorites released some of their water content to Earth when they've collided with our planet. Volcanoes release water vapor as a main constituent of their eruptions. It is hypothesized that our planet was once much hotter and underwent a period of intense volcanic activity, which released a huge quantity of water vapor into the atmosphere.

The ocean basins are continuously changing today. Geological processes associated with the movement of Earth's tectonic plates are responsible for changing the size and shape of the world's ocean basins over time. The process of seafloor spreading is creating new basaltic ocean crust at the Mid-Atlantic Ridge in the Atlantic Ocean basin. Therefore, the Atlantic Ocean is slowly getting bigger. When new lithosphere is created at one point, it is simultaneously being destroyed at another location. This is happening at subduction zones, located at deep ocean trenches in the Pacific Ocean. Therefore, the Pacific Ocean basin is slowly shrinking over time. 

Approximately 250 million years ago, during the time of Pangaea, there was one global ocean that surrounded the supercontinent. This ocean has been called the Panthalassic Ocean.  By 180 million years ago, Pangaea began to break up. When Pangaea broke into the continents of Laurasia and Gondwanaland, the Panthalassic Ocean basin also changed. It was split into two oceans, the Pacific Ocean in the west and the Tethys Ocean (composed of today's Atlantic and Indian Oceans) in the east. Even today, the waters of all the oceans are connected. 

Source: Eary Atmosphere and Oceans. Retrieved from http://www.ck12.org/book/CK-12-Earth-Science-Concepts-For-High-School/r16/section/11.15/ on September 1, 2013. and USOE OER Resources. Retrieved from http://www.slcschools.org/departments/curriculum/science/documents/4-Earth-Science.pdf on September 1, 2013. 

4. Oceans & Atmosphere

Oceans cover more than 70% of the Earth's surface and make up 97% of its surface water. It's no surprise they have a big influence on the planet. They affect the atmosphere, climate and living things.

Oceans are a major source of water vapor in the atmosphere. Sunlight heats water near the surface. As the water warms, some of it evaporates. The water vapor rises into the air. It may form clouds and precipitation. Precipiation provides fresh water needed by plants and other living things. 

Ocean water absorbes gases from the air too. It absorbs oxygen and carbon dioxide. Oxygen is needed by living things in the oceans. Much of the dissovled carbon dioxide sinks to the bottom of the water. Carbon dioxide is a major greenhouse gas; by absorbing carbon dioxide, the oceans help control global warming. 

The oceans and atmosphere exchange gases like carbon dioxide, oxygen and water vapor. Image courtesy of USOE. 

Compared with inland areas, coastal areas have a milder climate. They are warmer in the winter and cooler in the summer. That’s because land near an ocean is influenced by the temperature of the water. The temperature of ocean water is moderate and stable. Why?
There are two major reasons:

• Ocean water is much slower to warm up and cool down than land. As a result, it never gets as hot or cold as land.
• Water flows through all the world’s oceans. Therefore, warm water from the equator mixes with cold water from the poles. The warm and cold water tend to “cancel each other out.”

Even inland temperatures are milder because of oceans. Without oceans, there would be much bigger temperature swings all over Earth and life would not be able to exist as it does now.

Source: USOE OER Resources. Retrieved from http://www.slcschools.org/departments/curriculum/science/documents/4-Earth-Science.pdf on September 1, 2013. 

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