Brief History of Oceanography Extent of the Ocean Composition of Seawater Ocean Layered Structure Resources of the Seawater

How deep is the ocean? How much of Earth is covered by the global sea? What does the seafloor look-like?

And what about the carpet of sediment that covers much of the seafloor? Where did it come from, and what can be learned by examining it?

Do you believe that the Earth is a “water planet” ?

Although the global comprises a much greater percentage of Earth’s surface than the continents, it has only been in the relatively recent past that the ocean became an important focus of the study. Oceanography is an interdisciplinary science that draws on the methods and knowledge of geology, chemistry, physics, and biology to study all aspects of the world ocean.

The area of the Earth is about 510 million square kilometers. Of this total, approximately 360 million square kilometers, or 71%, is represented by oceans and marginal seas. Continents and Islands comprise the remaining 29%, or 150 million square kilometers. In the Northern Hemisphere, for instance, nearly 61% of the surface is water, whereas about 31% is land. In the Southern Hemisphere, on the other hand, almost 81% of the surface is water, and only 19% is land.

The world ocean can be divided into four main basins: Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean.

The Pacific Ocean, which is the largest ocean (and the largest single geographic feature on the planet), covering over half of the ocean surface area on the earth. The Atlantic Ocean, which is relatively narrow ocean as compared to the Pacific and is bounded by almost parallel continental margins. The Indian Ocean, which is slightly smaller than the Atlantic Ocean but has about the same average depth. The Arctic Ocean, which is about 7% the size of the Pacific Ocean and is only a little more than one-quarter as deep as the rest of the oceans.

Salinity (salinus = salt) is the total amount of solid material dissolved in water. More specifically, it is the ratio of the mass of dissolved substances to the mass of the water sample. Many common quantities are expressed in percent (%), which is really parts per hundred. Because the proportion of dissolved substances in seawater is such a small number, oceanographers typically express salinity in parts per thousand. Thus the average salinity of seawater is 3.5% or 35% o

If one wanted to make artificial seawater, it could be approximated by the following recipe: chlorine, sodium, sulfate, magnesium, calcium, potassium, strontium, bromine, and carbon. The most evident salt in seawater is sodium chloride---common table salt.

One source of sea salts is the chemical weathering of rocks in the continents. The second major source of elements found in seawater is from Earth’s interior. Through volcanic eruptions, large quantities of water vapor and other gases have been emitted during much of geologic time. This process is called outgassing, is the principal source of water in the oceans. Certain elements----notably chlorine, bromine, sulfur, and boron--- were outgassed along with water and exist in the ocean in much greater abundance than could be explained by weathering of rocks alone.

Why doesn’t the sea get saltier ? The answer is because material is being removed just as rapidly as it is added.

Processes that add large amounts of fresh water to seawater—and thereby decrease salinity--- include precipitation, runoff from land, icebergs melting, and seawater melting. Processes that removed amounts of fresh water from seawater --- and thereby increase seawater salinity – include evaporation and the formation of sea ice.

When seawater freezes in winter, sea salts do not become part of the ice. Therefore, the salinity of the remaining seawater increases. In summer when the sea ice melts, the addition of the relatively fresh water dilutes the solution and salinity decreases.

Oil and Natural Gas. The ancient remains of microscopic organisms, buried within marine sediments before they could decompose, are the source of today’s deposits of oil and natural gas. The percentage of world oil produced from offshore regions has increased from traced amounts in the 1930s to over 30% today.

Gas Hydrate. Are unusually compact chemical structures made of water and natural gas. The common type of natural gas is methane, which produces methane hydrate. Most oceanic gas hydrates are created when bacteria break down organic matter trapped in seafloor sediments, producing methane gas with minor amounts of ethane and propane. These gases combine with water in deep ocean sediments in such a way that the gas is trapped inside a lattice like cage of water molecules.

Sand and Gravel. The offshore sand-and-gravel industry is second in economic value only to the petroleum industry. Sand and gravel, which includes rock fragments that are washed out to sea and shells of marine organisms, is mined by offshore barges using suction dredges.

Evaporative Salts. When seawater evaporates, the salts increase in concentration until they can no longer remain dissolved, so they precipitate out of solution and from salt deposits, which can then be harvested. The most economically important salt is halite (common table salt).

Manganese Nodules. It contains significant concentrations of manganese, iron, and smaller concentrations of copper, nickel, and cobalt, for example, is deemed “strategic” because it is required to produce dense, strong alloys with other metals and is used in high-speed cutting tools, powerful permanent magnets, and jet engine parts.

Nodules are widely distributed, but not all regions have the same potential for mining. Good locations have abundant nodules that contain the economically optimum mix of copper, nickel, and cobalt. Sites meeting these criteria, however, are relatively limited. Additionally, there are political problems of establishing mining rights far from land and environmental concerns about disturbing large portions of the deep-ocean floor.

Because solar energy is perceived at the ocean surface, it is here that water temperatures are warmest. The mixing of these waters by waves as well as the turbulence from currents and tides creates a rapid vertical heat transfer. Hence, this surface mixed zone has nearly uniform temperatures. The thickness and temperature of this layer vary, depending on latitude and season.

Below the Sun-warmed zone of mixing, the temperature falls abruptly with depth. Here, a distinct layer called the transition zone exists between the warm, surface layer above and the deep zone of cold water below.

Below the transition zone is the deep zone, where sunlight never reaches and water temperatures are just a few degrees above freezing.

In high latitudes, the three layer structure of ocean layering does not exist. This is because the water column is isothermal, which indicates that there is no rapid change in temperature with depth. Consequently, good vertical mixing between surface, sinks, and initiates deep ocean currents.