The Seafloor

Lesson Objectives

Describe the methods scientists have for studying the seafloor.
Describe the features of the seafloor.
List the living and non-living resources that people use from the seafloor.

Vocabulary

bottom trawling
manganese nodule

Introduction

Oceanographers like to say that we know more about the dark side of the Moon than we do about the oceans. That statement is doubly true of the seafloor. Although modern technology has allowed us to learn more about the seafloor, vast regions remain unexplored.

Studying the Seafloor

Scuba divers can only dive to about 40 meters and they cannot stay down there for very long. Although this is good for researching the organisms and ecosystems very near a coast, most oceanic research requires accessing greater depths.

Mapping

In the Plate Tectonics chapter you learned that echo sounders designed to locate enemy submarines allowed scientists to create bathymetric maps of the seafloor (Figure below). Prior to this advance, explorers mapped a small amount of the seafloor by painstakingly dropping a line over the side of a ship to measure the depth at one tiny spot at a time.

Map of San Francisco Bay, oblique view.

KQED: Sea 3-D: Charting the Ocean Floor

Using sound and laser technology, researchers have begun to reveal the secrets of the ocean floor from the Sonoma Coast to Monterey Bay. By creating complex 3-D maps, they’re hoping to learn more about waves and achieve ambitious conservation goals. Learn more at: http://science.kqed.org/quest/video/sea-3-d-charting-the-ocean-floor/.

Sampling Remotely

Samples of seawater from different depths in the water column are needed to understand ocean chemistry. To do this bottles are placed along a cable at regular depths and closed as a weight is dropped down the cable. The water trapped in the bottle can be analyzed later in a laboratory (Figure below).

A Niskin bottle being deployed off the side of a research ship.

Scientists are also interested in collecting rock and sediment samples from the seafloor. A dredge is a giant rectangular bucket that is dragged along behind a ship to collect loose rocks. Gravity corers are metal tubes that fall to the seafloor and slice into the sediments to collect a sample. The research vessel, the Joides Resolution, drills deep into the seafloor to collect samples of the sediment and ocean crust. Scientists analyze the samples for chemistry and paleomagnetism.

Videos of the drill bit and core samples taken by the Joides Resolution are seen here: http://www.youtube.com/watch?v=4c9HlHBCSCY; http://www.youtube.com/watch?v=MXqlRUbqHz0; http://www.youtube.com/watch?v=50VeS0TxCvU.

Submersibles

Samples of seawater and rocks can be collected directly by scientists in a submersible. These subs can take scientists down to make observations and the subs have arms for collecting samples. The submersible Alvin is an HOV, a human operated vehicle. Alvin can dive up to 4,500 m beneath the ocean surface and has made more than 4000 dives since 1964 (Figure below). Other submersibles can dive deeper.

Alvin allows two people and a pilot to make a nine hour dive.

View a slide show of DSV Alvin and its history from the Woods Hole Oceanographic Institution: http://www.whoi.edu/page.do?pid=8422&tid=201&cid=14616&ct=362#.

Remotely Operated Vehicles

To avoid the expense, dangers, and limitations of human missions under the sea, remotely operated vehicles or ROV’s, allow scientists to study the ocean’s depths by using small vehicles carrying cameras and scientific instruments. ROVs were used to study the Titanic, which would have been far too dangerous for a manned sub to enter. Scientists control ROVs electronically with sophisticated operating systems (Figure below).

Footage of the NOAA Titanic Expedition of 2004 is visible in this video: http://www.youtube.com/watch?v=6Z7REEnwKOQ.

Remotely operated vehicles such as this one allow scientists to study the seafloor.

A video of the ROV Nereus from the Woods Hole Oceanographic Institution is shown here: http://www.youtube.com/watch?v=wwdF_2wMRfU.

Ocean Resources

The ocean provides important living and non-living resources. To be maintained for future use, these resources must be managed sustainably.

Living Resources

Most fish are caught by lines or nets as they swim in the open waters of the ocean. Some species of fish are being overharvested, which means their rate of reproduction cannot keep up with the rate at which people consume them.

Bottom trawling is a method of fishing that involves towing a weighted net across the seafloor to harvest fish. In many areas where bottom trawling is done, ecosystems are severely disturbed by the large nets. For this reason, in a few areas in the world, laws limit bottom trawling to waters not more than 1,000 m deep or waters far from protected and sensitive areas. Still these actions protect some of the seafloor. Besides food, ocean organisms have other uses. Some provide us with medications.

Non-living Resources

Oil and natural gas are the most valuable non-living resources taken from the ocean. Extracting these resources requires drilling into the seafloor. Oil platforms have dozens of oil wells that are drilled in places where the ocean is sometimes 2,000 m deep (Figure below). A description of the Deepwater Horizon oil spill affecting the Gulf of Mexico is located in the Human Actions and Earth’s Waters chapter.

Oil platforms can be fixed or they can float.

The seafloor has some valuable minerals. Manganese nodules containing manganese, iron, copper, nickel, phosphate, and cobalt (Figure below) may be as small as a pea or as large as a basketball. Estimates are that there may be as much as 500 billion tons of nodules on the seafloor. The minerals in manganese nodules have many uses in the industrial world, but currently they are not being mined. Think back to the discussion of ore deposits in the Earth’s Minerals chapter. Why do you think these seafloor resources are not being mined?

Manganese nodules from the seafloor are often rich in metals such as manganese, iron, nickel, copper, and cobalt.

Lesson Summary

Scuba divers can only explore near the surface so most oceanographic research is done from satellites, ships, and submersibles.
The oceans are divided into zones by water depth, distance from shore, and the slope of the seafloor.
The oceans provide us with both living and non-living resources.
Living oceanic resources include fish and invertebrates used for food.
The most valuable non-living resources from the oceans are oil and natural gas.

Review Questions

1. Scientists sometimes say that we know less about the oceans than the far side of the Moon. Why is it so difficult to learn about the oceans?

2. The atmospheric pressure is about 1 kilogram per centimeter squared (14.7 pounds per square inch or 1 atmosphere) at sea level. About what is the pressure if you are 100 meters deep in the ocean?

3. Where on the ocean floor will you find the greatest amount of living organisms?

4. Compare and contrast the continental shelf and the abyssal plain.

5. Why do you think mapping the seafloor is important to the Navy? What part of the seafloor is the navy most interested in?

6. Why is bottom trawling damaging to the seafloor?

7. As world population grows and the ocean is called on to provide more and more resources, what can people do to be sure the resources are used sustainably?

8. What is a manganese nodule?

Points to Consider

If the seafloor is not well known, how much do you think is known about marine life?
What methods are needed to learn about organisms at the ocean surface, the mid-depths, or on the seafloor?
Are techniques that are used for understanding ocean physics, chemistry, and geology also useful for studying marine life?

Earth’s Oceans