Ocean Waters and the Ocean Floor

Ocean Waters and the Ocean Floor introduces oceanography with a brief discussion of the extent and distribution of the world ocean. After examining the composition of seawater, the ocean’s layered temperature and salinity structures are presented. The ocean floor section of the chapter begins with an examination of continental margins and continues with submarine canyons and associated turbidity currents, ocean basin features, and mid-ocean ridges. The origin of coral reefs and atolls is also presented. The chapter ends with a discussion of seafloor sediments and how they are used to study climatic changes.

Learning Objectives

After reading, studying, and discussing this chapter, you should be able to:

•Describe the extent and boundaries of the world ocean.

•Discuss the chemical composition of ocean water.

•Explain the oceans layered temperature and salinity structures.

•Describe the relation between submarine canyons, turbidity currents, and turbidites.

•Describe the major features of the continental margin, ocean basin floor, and mid-ocean ridges.

•List the types of seafloor sediments.

•Describe how ocean floor sediments relate to climatic changes.

Chapter Summary

•Oceanography is a composite science that draws on the methods and knowledge of biology, chemistry, physics, and geology to study all aspects of the world ocean.

•Earth is a planet dominated by oceans. Seventy-one percent of Earths area consists of oceans and marginal seas. In the Southern Hemisphere, often called the water hemisphere, about 81 percent of the surface is water. Of the three major oceans, Pacific, Atlantic, and Indian, the Pacific Ocean is the largest, contains slightly more than half of the water in the world ocean, and has the greatest average depth–5940 meters (12,900 feet).

•Salinity is the proportion of dissolved salts to pure water, usually expressed in parts per thousand (‰). The average salinity in the open ocean ranges from 55‰ to 57‰. The principal elements that contribute to the ocean’s salinity are chlorine (55 percent) and sodium (51 percent). The primary sources for the salts in the ocean are chemical weathering of rocks on the continents and outgassing through volcanism. Outgassing is also considered to be the principal source of water in the oceans as well as in the atmosphere.

•Commercial products obtained from seawater include common salt (sodium chloride), the lightweight metal magnesium, and bromine, a gasoline additive also used in the manufacture of fireproofing material. Fresh water is derived from seawater by desalination, the removal of salts and other chemicals.

•In most regions, open oceans exhibit a three-layered temperature and salinity structure. Ocean water temperatures are warmest at the surface because of solar energy. The mixing of waves as well as the turbulence from currents can distribute this heat to a depth of about 450 meters or more. Beneath the sun-warmed zone of mixing, a layer of rapid temperature change, called the thermocline, occurs. Below the thermocline, in the deep zone, temperatures fall only a few more degrees. The changes in salinity with increasing depth correspond to the general three-layered temperature structure. In the low and middle latitudes, a surface zone of higher salinity is underlaid by a layer of rapidly decreasing salinity, called the halocline. Below the halocline, salinity changes are small.

•Ocean depths are determined using an echo sounder, a device carried by a ship that bounces sound off the ocean floor. The time it takes for the sound waves to make the round trip to the bottom and back to the ship is directly related to the depth. Continuous monitoring of the echoes are plotted to produce a profile of the ocean floor.

•The zones that collectively make up the continental margin include the continental shelf a gently sloping, submerged surface extending from the shoreline toward the deep-ocean basin), continental slope (the true edge of the continent, which has a steep slope that leads from the continental shelf into deep water), and in regions where trenches do not exist, the steep continental slope merges into a gradual incline known as the continental rise. The continental rise consists of sediments that have moved downslope from the continental shelf to the deep-ocean floor.

•Submarine canyons are deep, steep-sided valleys that originate on the continental slope and may extend to depths of three kilometers. Some of these canyons appear to be the seaward extensions of river valleys. However, most information seems to favor the view that many submarine canyons have been excavated by turbidity currents (downslope movements of dense, sediment-laden water). Turbidites, sediments deposited by turbidity currents, arc characterized by a decrease in sediment grain size from bottom to top, a phenomenon known as graded bedding.

•The ocean basin floor lies between the continental margin and the mid-oceanic ridge system. The features of the ocean basin floor include deep-ocean trenches (the deepest parts of the ocean, where moving crustal plates descend into the mantle), abyssal plains (the most level places on Earth, consisting of thick accumulations of sediments that were deposited atop the low, rough portions of the ocean floor by turbidity currents), and seamounts, (isolated volcanic peaks on the ocean floor that originate near oceanic ridges or in association with volcanic hot spots)

•Mid-ocean ridges, the sites of seafloor spreading, are found in all major oceans and represent more than 20 percent of Earth’s surface. These broad features are characterized by an elevated position, extensive faulting, and volcanic structures that have developed on newly formed oceanic crust. Most of the geologic activity associated with ridges occurs along a narrow region on the ridge crest, called the rift zone, where magma from the asthenosphere moves upward to create new slivers of oceanic crust.

•Coral reefs, which are confined largely to the warm, sunlit waters of the Pacific and Indian Oceans, are constructed over thousands of years primarily from the skeletal remains and secretions of corals and certain algae. Coral islands, called atolls, consist of a continuous or broken ring of coral reef surrounding a central lagoon. Atolls form from corals that grow on the flanks of sinking volcanic islands, where the corals continue to build the reef complex upward as the island sinks.

•There are three broad categories of seafloor sediments. Terrigenous sediment consists primarily of mineral grains that were weathered from continental rocks and transported to the ocean. Biogenous sediment consists of shells and skeletons of marine animals and plants. Hydrogenous sediment includes minerals that crystallize directly from seawater through various chemical reactions.

•Seafloor sediments are helpful when studying worldwide climatic changes because they often contain the remains of organisms that once lived near the sea surface. The numbers and types of these organisms change as the climate changes and their remains in the sediments record these changes.