Oceans For Dummies. Joseph Kraynak

Чтение книги онлайн.

СКАЧАТЬ scale) increased the concentration of salt in seawater to the level at which it is today — on average about 3.5 percent.

      Investigating variations in salt concentrations

      Because water evaporates faster from some parts of the ocean than others and rainfall and river discharge to the ocean isn’t uniform around the globe, certain parts of the ocean are saltier than others. For example, salinity in the Red Sea ranges from 36 ppt (parts per thousand) to 41 ppt (about 3.6 to 4.1 percent). In the Mediterranean Sea, surface waters average 38 ppt but can approach 40 ppt in the extreme western parts. Saltwater taffy, anyone?

      Salinity also fluctuates in certain areas of the ocean. For example, in polar regions, the salinity of water near the surface increases as ice forms, leaving much of the salt behind. As the ice melts, salinity decreases due to the influx of fresh water. Likewise, in coastal areas, salinity is lower in areas where freshwater from rivers and streams enters the ocean. When the salinity dips below about 30 ppt, the water is often called brackish.

      Realizing that sea water is more than just salty water

      The recipe for cookin’ up a batch of seawater might seem quite simple: Start with 1 gallon (about 4 liters) of water, add about 4 tablespoons (60 grams) salt, and shake till dissolved. However, seawater is more complex than mere salt and water — it contains a very large number of chemical elements and compounds, including dissolved organic materials, particulates, and dissolved gasses, including oxygen. (Marine mammals can’t breathe the gasses dissolved in seawater, but apparently the crew of the movie The Abyss can.)

      Don’t worry, we’re not going to deliver a lecture on the chemical composition of seawater, but knowing that seawater is more than just salty water is important for when we get into discussions about certain issues related to preserving the ocean’s health, such as climate change and ocean acidification (see Chapter 21).

      Checking out what’s at the bottom of the ocean (and below)

      At the bottom of the ocean is a landscape that’s as varied as what you see above the ocean, complete with plains, ridges (mountain ranges), seamounts (mountains), hills, canyons, trenches (valleys), cliffs, volcanoes, hydrothermal vents (underwater geysers), and even rivers and massive “waterfalls” of sand and sediment that you do indeed want to go chasing.

      The shape of the ocean floor (as well as all land) is largely determined by a process called plate tectonics — the bumping, grinding, and separating of tectonic plates (massive sections of rock that form Earth’s crust). These plates ride atop a semi-liquid layer of molten rock (imagine heating a rock till it melts — that’s HOT). Convection currents in the molten rock move the plates at three to five centimeters (one to two inches) per year. (See Chapter 6 for more about plate tectonics.)

      Plate tectonics are also responsible for creating most tsunamis. Whenever land shifts at the bottom of the ocean or an underwater volcano erupts, the resulting displacement of water can send a wave thousands of miles across the ocean in a matter of only a few hours. (See Chapter 16 for more about waves and tsunamis.)

      Riding waves, tides, and currents

      Ocean water is constantly in motion thanks to a variety of forces, including the rotation of Earth; the gravitational fields of the sun, Earth, and moon; wind; and variations in the temperature and salinity of ocean water.

      On a cosmic scale, the interaction of the Earth’s, moon’s, and sun’s gravitational forces cause tides that move water toward and away from coastlines once or, more often, twice daily.

      At the ocean’s surface, wind, along with other forces (including Earth’s rotation and variations in water temperature and salinity) drive the formation of large rotating waters called gyres that generally spin clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (And no, this has no effect on the direction water swirls down your bathtub drain.)

Below the surface, differences in water density drive currents. Near the poles, when water freezes at the surface, it leaves behind its salt. This colder, saltier water near the surface is denser than the warmer, fresher water below it, so it dives down, hits the seafloor, and heads toward the equator. As this cold water moves down and away from the poles, warmer surface water rushes in toward the poles to replace it, resulting in a continuous current that transports water, heat, and nutrients around the globe. This process is called thermohaline circulation, and the resulting system of currents is called the global ocean conveyor belt. It is one of the many reasons the poles are so important. They literally drive our entire ocean system, supporting marine life (and seafood) all around the globe.

      See Chapter 16 for more about waves, tides, currents, and gyres and how water, heat, and nutrients are circulated around the world via ocean currents.

      Recognizing the ocean’s role in climate control and weather

      The ocean plays a critical role in keeping our planet at a steady temperature as it transports heat (and energy) around the globe. Of course, it’s not that simple, and the ocean doesn’t do this all by itself; it works together with the atmosphere and land to create our climate and influence the ever-changing weather patterns:

       Climate: The prevailing weather over a long period of time (usually more than 30 years), such as tropical (warm and wet), desert (hot dry), polar (cold and dry), and temperate (neither extremely hot nor extremely cold).

       Weather: Atmospheric conditions over a short period of time in respect to temperature, sunshine, storms, wind, and precipitation (rain, snow, sleet, hail).

      The impact of the ocean on weather is most dramatic when the ocean releases some of its energy in the form of a tropical cyclone (a hurricane, cyclone, or typhoon). See Chapter 17 for more about how the ocean influences climate and weather.

Meeting the Ocean’s Inhabitants

      The ocean is teeming with life, from coastline to open ocean and from surface to seafloor. These living beings can be broken down into six groups (technically known as kingdoms) — Plants, Animals, Protists, Fungi, Archaebacteria, and Eubacteria. In this book, we break them into three groups — microbes (generally too small to see with the naked eye), plants (and other organisms that require sunlight for energy and growth), and animals. Each of these groups can be broken down further; for example, the animal kingdom includes simple invertebrates (such as sponges and jellies), mollusks (such as snails and clams), crustaceans (such as crabs and lobsters), fish, reptiles, birds, and mammals.

      In this section, we cover the bare bones of taxonomy — the classification system used to assign organisms to specific groups — and introduce you to the groups of organisms we cover in Part 3.

      Recognizing СКАЧАТЬ