The waters around Antarctica has a salinity of little about 34 parts per million (ppt), and it may be as low as 30 parts per million (ppt) in some spots. Thawing icebergs adds freshwater since icebergs generated from ice sheets developed over land do not contain salt, and freezing saltwater into ice floes removes even more salt. As a result, most of the world's oceans are becoming less salty due to this process.
Saltwater is composed of sodium chloride (NaCl), which is also known as salt. During ice ages, when sea levels are lower, more salt is dissolved in water and less fresh water is exposed to the atmosphere, resulting in more salt overall. As climate changes and sea levels rise, this process becomes increasingly difficult if not impossible to sustainably remove enough salt from the ocean to offset its accumulation.
As a result, most oceans are becoming more acidic due to this process. The acidity of seawater can be measured in terms of pH, which ranges from 1 to 14. Water with a pH below 7 is considered acidic, while water with a pH above 7 is considered alkaline. Earth's seas and oceans are very acidic today, with a pH between 2.5 and 3.5. This number will increase as fossil fuel pollution increases in addition to other factors such as volcanic activity. Some scientists believe that if current trends continue, our oceans will become so acidic that no shellfish will be able to create calcium carbonate shells which would lead to their extinction.
Ocean water is typically about 35 ppt, so this saltwater is very dilute.
The Dead Sea has a surface salinity of approximately 45 ppt, but its depth ranges from 367 to 421 feet (112-128 meters). The bottom sediment of the sea is made up of compacted rock fragments that are saturated with water; if you could dig down there, the soil would be saturated with water to a depth of at least 20 feet (6 m).
The Bay of Fundy in Nova Scotia is one of the world's largest tidal estuaries. At certain times of the year, the bay can have a surface area of more than 20,000 square miles (50,000 square km), but at other times it can shrink to an area less than 2,000 square miles (5,000 square km). The cause of this fluctuation is the difference between ocean tides and waves. The amplitude of the tide in the bay ranges from 13 feet (4 m) to 156 feet (48 m), while the height of the highest wave reaches 350 feet (107 m).
Water with a salinity of 17% freezes at around 30°F (-1°C), whereas water with a salinity of 35% freezes at about 28.5°F (-2°C). Nonetheless, despite the salinity of the ocean, sea ice contains relatively little salt, around a tenth of the salt found in sea water. This is due to the fact that ice does not integrate sea salt into its crystal structure. Rather, it absorbs it into its surface layer.
Ice floats on water because it is less dense than water. In fact, ice is two thirds water by weight, while salt is almost as heavy as gold. If there were no other way for ice to stay afloat, this would be enough to cause problems for marine animals and plants that rely on ice for protection from predators and to grow their food. But there are other ways for ice to remain buoyant, such as floating objects like seaweed or plastic bags that act as air bubbles trapped inside the ice. These help ice floes survive in harsh conditions where they might otherwise break up and drift away.
The reason why ice at the edge of the ocean is usually saltier than the center is because rivers carry salt from the land into the ocean. The most important river system in this respect is the Mississippi River, which flows into the Gulf of Mexico. When it rains on the Great Plains, the runoff picks up salt from the soil and carries it into the Mississippi River Basin, where it is deposited in areas near its banks. As well as being salty, the water becomes cloudy from all the suspended sediment.
Evaporation transports fresh water vapor from the ocean to the atmosphere, resulting in greater salinity. As one moves closer to the poles, fresh water from melting ice reduces surface salinity once more. Adding salt to water reduces the freezing point. This makes sense because it takes more energy to freeze salty water than fresh water.
The amount of evaporation from oceans varies by season and location. In dry regions, such as much of North America, less moisture evaporates because there are no clouds or rain to wash it away. In wet regions, such as Europe and Asia, more moisture evaporates because it has a larger area to be covered by water. Evaporation also varies with altitude; at high altitudes, where there is less atmospheric pressure, water molecules move faster and escape into space more easily.
At any given place on Earth, fresh water flows toward lower elevations until it meets up with saltwater. The ratio of fresh to saltwater increases as you go down through the water column because there's more fresh water flowing in. At the bottom of the ocean, where there is no net flow of water, this ratio is constant across all locations. But everywhere else, it increases upward.
Surface waters are usually fresher near their sources in large rivers or at low altitudes on islands. As distance from source increases, so does salt concentration.
Typically, the salinity drops from the surface ocean to deep waters is extremely minor, ranging from around 36 g/L (ppt) at the top to 35 g/L (ppt) in deep water, implying a relatively tiny density loss with depth assuming a constant temperature. The freezing point temperature of saltwater is also affected by its salinity. As you go down into deeper waters, the salt concentration decreases which reduces the ice point temperature of the water.
At depths greater than about 1000 m (3456 ft), the heat from radioactive decay is sufficient to maintain a continuous flow of water between the surface and the depth. This layer, called the thermocline, is quite narrow (about 100 m or 328 feet wide).
Below the thermocline, the pressure increases rapidly; at 10,000 m (33282 ft), it is almost one million times higher than at the sea level. Water molecules are more tightly packed at high pressures, so there is less room for them to move about compared to low pressures. This means that liquid water cannot exist at those pressures below about 5000 m (16472 ft) because the molecules would be forced too close together to allow any spaces between them at all. However, even at lower pressures, water can remain in a super-cooled state where ice crystals can form without nucleating particles coming out of solution.