Characteristics of Oceans Help (page 2)
It has been estimated that if all of the oceans’ water were poured off, salt would cover the continents to a depth of 1.5 m. That is a lot of salt!
Salinity is the amount of salt found in 1 kg of water. Salinity, or salt content, is written in parts per thousand (ppt) because there are 1000 g in 1 kg.
The average ocean salinity is 35 ppt. This number varies as rainfall, evaporation, river runoff, and ice formation changes it slightly (32–37 ppt). For example, it is said that the Black Sea is so diluted by river runoff, its average salinity is commonly around 16 ppt.
Freshwater salinity is usually less than 0.5 ppt. Water between 0.5 ppt and 17 ppt is called brackish . In areas where fresh river water joins salty ocean water, like estuaries, the water becomes brackish.
When salt water gets to the polar regions, it cools and/or freezes, getting saltier and denser. Cold, salty water sinks. The level of ocean salinity increases by depth. It is divided into three vertical layers. The surface layer has a mixed salinity depending on rainfall or runoff from the land. The middle layer is called the halocline with a medium range of salinity. The deepest and coldest ocean water has the highest level of salinity.
Fish and animals that live in seawater have worked out ways to survive in a salty environment. Most marine creatures are able to maintain nearly the same concentration of salinity within their bodies as the surrounding environment. When they are moved to an area of much less salinity, they die. You can’t put a saltwater fish in a freshwater aquarium!
The ocean has a broad temperature range from warm (38°C) shallow coastal waters of the equator to the nearly freezing arctic waters.
The freezing point of seawater is about –2°C, instead of the 0°C freezing point of ordinary water. Salt lowers the freezing point of seawater. As seawater increases 5 ppt in salinity, the freezing point decreases by –17.5°C.
The ocean is also divided into three vertical temperature zones. The top layer is the surface layer or mixed layer . This warmest layer is affected by wind, rain, and solar heat. Have you ever been swimming in a deep lake? As you get farther away from the sun-heated surface, it gets colder. Your feet are colder than your upper body.
The second temperature layer is known as the thermocline layer. Here the water temperature drops as the depth increases, since the sun’s penetration drops too.
The third layer is the deep - water layer . Water temperature in this zone sinks slowly as depth increases. The deepest parts of the ocean are around 2°C in temperature, with inhabitants that either like very cold water, or have found specialized environments like a volcanic vent that heats the water dramatically. Figure 13-2 illustrates the thermocline’s location in the ocean’s temperature layers.
Fig. 13-2. The oceans have distinct temperature zones.
Temperature, salinity, and pressure come together to influence water density , which is the weight of water divided by its volume. Cold seawater is denser than warm, coastal water and will sink below the less dense layer.
temperature + salinity = density
The ocean waters are similarly divided into three density zones. Less dense waters form a surface layer. The temperature and salinity of this layer varies according to its contact with the air. For example, when water evaporates, the salinity goes up. If a cold north wind blows in, the temperature dips and that also affects density.
The middle layer is the pycnocline or transition zone . The density here does not change very much. This transition zone is a barrier between the surface zone and the bottom layer, allowing little to no water movement between the two zones.
The bottom layer is the deep zone , where the water stays cold and dense. The polar regions are the only places where deep waters are ever exposed to the atmosphere because the pycnocline is sometimes not present. Figure 13-3 shows the three-transition layers of density (pycnocline), salinity (halocline), and temperature (thermocline) according to depth.
Fig. 13-3. The ocean has transition zones for temperature, salinity, and density.
Where a river meets the ocean, fresh water flows into salty water. The colder river waters are often less dense than the warmer ocean waters. The density differences can create distinct layers in the relatively shallow waters. A submarine or diver operating in this environment and wanting to stay at the same horizontal level would have to alter buoyancy to adjust for density changes. The same thing happens when a river flows into the ocean. It drags along a portion of sediment into the ocean. This changes the shape of the ocean floor by sedimentation and silting and shifts the layering.
Although no one really thinks about it, air pushes against us at a constant pressure. At sea level, this pressure is 14.7 pounds per square inch (psi), or 1 kg/cm 2 . Our body handles this constant push, by pushing back with the same amount of force. On the top of a mountain, the pressure is less.
But water is a different story. Water is a lot heavier than air. The pushing force (pressure) goes up when you enter the water. In fact, at 10m in depth, one atmosphere (14.7 psi) pushes down on you.
It’s possible for humans to dive three or four atmospheres with the right scuba equipment, but to go any deeper, tough pressurized vehicles like research vessels and submarines are needed.
Ocean citizens like whales seem to be unaffected by pressure shifts. They chug along in the ocean and dive through rapid pressure changes, all the time, without even thinking about it. Although who really knows what a whale thinks about?
Sperm whales are known to dive to depths of 2250m and can stay down for over an hour. The pressure change from the surface is more than 220 atmospheres! Scientists are still trying to figure out how they do it.
Though Jacques Cousteau titled a book about the sea, The Silent World , the oceans are pretty noisy in places. Where currents smash up against rocky cliffs, the turbulence fills the ocean with sound. However, human ears aren’t sensitive enough to hear all the different frequencies.
Water is a great sound conductor. It doesn’t absorb sound, but allows it to travel great distances before it fades away. The speed of sound through the water is 1450 to 1570 m/s. The travel time increases as the water temperature increases.
Ocean animals are fairly loud. They chatter and call to each other while swimming, squeak when scared, brag when they find food, whistle to send out warnings, ping to inspect their surroundings, and sing to each other on Valentine’s Day. Well, they sing anyway.
Dolphins and whales use a method called echolocation . By emitting a series of clicks and whistles and then listening for the echoes of the sounds bouncing off objects, they can tell where things are. They do this to accurately locate fish, turtles, logs, boats, reefs, and whatever else is around. From the direction and strength of the echo, dolphins and whales get a mental image of their environment. By echolocation, they can tell the size, distance, and direction of objects in their path.
Echolocation is the method used by whales and dolphins to find out what is happening in the ocean around them.
Sonar is also used by oceanographers to study the ocean floor. It works a lot like echolocation. By sending out signals and picking up the echoes, scientists can get a picture of the features of the ocean floor. This is how shipwrecks like the Titanic were located. Fishermen also use sonar to find large schools of fish.
Oceanographers discovered one part of the ocean that has different and better acoustics than other parts of the ocean. It is known as the SOFAR channel , which is short for sonic fixing and ranging channel. In the SOFAR channel, low-frequency sounds can travel for hundreds of kilometers very well. If you rise or go deeper vertically the sound fades much faster. Scientists think that this is the main “long distance telephone line” that whales use to communicate to each other over the ocean’s expanse.