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Earth\u2019s marine bodies are home to innumerable species of living organisms adapted to a variety of conditions at sea. The properties of ocean habitat are responsible for the wide variety of marine life and the characteristics each of them exhibits. Earth\u2019s marine habitat is enormous and attempting to understand it from a narrow perspective may not yield desired results. It is essential to broaden our perspective to include greatly different time and distance scales to understand the marine environment and how it along with its inhabitants evolved into their present forms.\u00a0<\/span><\/p> <\/p> The Changing Marine Environment<\/span><\/strong>\u00a0<\/span><\/p> The evolution of complex organisms that developed efficient energy utilization methods resulted in the explosion of marine life forms. Around 500 million years ago, most of the main categories of marine life forms made their appearance. These marine species included worms, sponges, corals, and aquatic plants. Although abundant ancestors of terrestrial plants and animals existed at this time, there was no life outside the marine environment as the sea water formed a shield that protected the life underneath from intense solar radiation present during that time.\u00a0<\/span><\/p> <\/p> A Different View of the Ocean Floor<\/span><\/strong>\u00a0<\/span><\/p> In the beginning of the 20th\u00a0century, Alfred Wegener suggested that the oceans were changing in many ways. He worked out a detailed hypothesis of\u00a0the\u00a0continental drift to explain many global geologic features. He came up with the idea that our present continental formations had drifted apart after the breaking of a single supercontinent: Pangaea. However, most scientists at that time remained sceptical about this hypothesis. One reason for this scepticism was that his evidence was ambiguous. It was in the second half of the century that new evidence led to the proposal of concepts of seafloor spreading and plate tectonics. These closely related concepts were put forth by two geophysicists independently and almost simultaneously. Looking backwards, these concepts appear to be easily apparent: that the earth\u2019s crust is divided into large irregular plates. These rigid crustal plates float on the denser and relatively more plastic mantle where each plate is defined by oceanic trench or rigid systems and a few plates include both oceanic as well as continental crusts. Along the\u00a0axes of oceanic ridges and rises, new oceanic crustal material gets continuously formed. And as these crustal plates develop on either side of the ridge, they tend to move away from the ridge axis in opposite directions,\u00a0thereby carrying bottom sediments and attached continental masses along with them.<\/span><\/p> <\/p> <\/p> The Ocean World<\/span><\/strong>\u00a0<\/span><\/p> The continent of Antarctica is surrounded by the so-called Southern Ocean which has three giant\u00a0embayments\u00a0stretching in the northern direction. These three oceanic extensions are the three major oceans the Atlantic, Pacific and the Indian. They are, however, partially separated by continental barriers. The smaller Arctic Ocean and seas like the Mediterranean extend from the margins of the larger ocean basins.\u00a0<\/span><\/p> <\/p> Properties of Seawater<\/span><\/strong>\u00a0<\/span><\/p> Many common properties of seawater are important to the survival and well-being of the marine life forms. Most marine organisms consist of 80-90% water in volume. It gives them buoyancy and body support and helps them in swimming. It also reduces the need for heavy skeletal structures in marine organisms,\u00a0especially the ones that float. The life processes of marine species subsequently alter many physical and chemical properties of seawater at a fundamental level. This may include the transparency and chemical makeup of seawater. This reason makes organisms in the\u00a0sea\u00a0an integral part of the entire marine environment.\u00a0\u00a0<\/span><\/p> <\/p> Pure Water<\/span><\/strong>\u00a0<\/span><\/p> The ubiquitous nature of water does not make it any less remarkable. It is the only substance on our planet that is plentiful,\u00a0as a liquid with considerable quantities,\u00a0left over as a gas in the atmosphere and as a solid in the form of ice and snow.\u00a0<\/span><\/p> <\/p> Seawater<\/span><\/strong>\u00a0<\/span><\/p> Seawater is the collected result of billions of years of water condensing as rain from the atmosphere and eroding rocks and soil and washing it all to the sea. The seawater is composed of 96.5% pure water and the rest dissolved compounds from aforementioned sources. All naturally occurring substances are found in the oceans at least in traces. These substances can be categorized into inorganic and organic substances.\u202f Inorganic substances are generally called salts and these include nutrients essential for plant growth and dissolved gases like N2, O2, and CO2.\u00a0Organic compounds are those compounds derived from life forms. These include fats, oils, carbohydrates, vitamins, amino acids, proteins and other substances.\u00a0<\/span><\/p> The survival and well-being of living organisms demand relatively unchanging internal environmental conditions,\u00a0including a nearly constant balance of salt and water.\u00a0<\/span><\/p> <\/p> <\/p> Physical and Chemical Characteristics of Seawater<\/span><\/strong>\u00a0<\/span><\/p> Seawater is a mixture of various salts and water. Most of the water in the ocean basins is believed to originate from the condensation of water found in the early atmosphere as the Earth cooled after its formation. This water was released from the lithosphere as the Earth\u2019s crust solidified. Additional water has also been added to the oceans over geologic time from periodic volcanic action. Some scientists have recently speculated that comets entering the Earth\u2019s atmosphere may be another important source of water for the oceans.\u00a0<\/span><\/p> Most of the dissolved chemical constituents or salts found in seawater have a continental origin. It seems that these chemicals were released from continental rocks through weathering and then carried to the oceans by stream runoff. Over time, the concentration of these chemicals increased until an equilibrium was met. This equilibrium occurred when the ocean\u2019s water could not dissolve any more material in solution. Similarities between fossilized sea life and organisms living today indicate that the composition of seawater stopped changing drastically about 600 million years ago.\u00a0<\/span><\/p> About\u00a099% of sea salts\u00a0are comprised of only six elements and compounds: chlorine (Cl-), sodium (Na+), sulphur (SO4-2), magnesium (Mg+2), calcium (Ca+2), and potassium (K+) (Figure 8p-1). The relative abundance of the major salts in seawater are constant regardless of the ocean. Only the amount of water in the mixture varies because of differences between ocean basins because of regional differences in freshwater loss (evaporation) and gain (runoff and precipitation).\u00a0<\/span><\/p> The chlorine ion makes up 55% of the salt in seawater. Calculations of seawater salinity are made of the parts per 1000 of the chlorine\u00a0ion\u00a0present in one kilogram of seawater. Typically, seawater has a salinity of 35 parts per thousand.<\/span><\/p> <\/p> <\/p> Water is one of the few substances existing on the Earth\u2019s surface in all three forms of matter. At zero degrees Celsius liquid water turns into ice and has a density of approximately 917 kilograms per cubic meter. Liquid water at the same temperature has a density of nearly 1,000 kilograms per cubic meter. The density of seawater generally increases with decreasing temperature, increasing salinity, and increasing depth in the ocean. The density of seawater at the surface of the ocean varies from 1,020 to 1,029 kilograms per cubic meter. Highest densities are achieved with depth because of the overlying weight of water. In the deepest parts of the oceans, seawater densities can be as high as 1,050 kilograms per cubic meter.\u00a0<\/span><\/p> Seawater freezes at a temperature that is slightly colder than fresh water (0.0\u00b0 Celsius). The freezing temperature of seawater also varies with the concentration of salts. More salt the lower the initial freezing temperature. At a salinity of 35 parts per thousand, seawater freezes at a temperature of -1.9\u00b0 Celsius.\u00a0<\/span><\/p> Sea ice normally contains considerably less salt than seawater. Most of the salts found in liquid seawater are forced out it when freezing occurs. The reason for the exclusion is because the molecules of the various salts do not fit well in the highly orderly molecular structure of frozen water. Because of the density difference between ice and seawater, ice floats on the surface of the ocean.\u00a0<\/span><\/p> Seawater also contains small amounts of dissolved gases. Many of these gases are added to seawater from the atmosphere through the constant stirring of the sea surface by wind and waves. The concentration of gases that can be dissolved into seawater from the atmosphere is determined by temperature and salinity of the water. Increasing the temperature or salinity reduces the amount of gas that ocean water can dissolve. Some of the important atmospheric gases found in seawater include: nitrogen, oxygen, carbon dioxide (in the form of bicarbonate HCO3), argon, helium, and neon. Compared to the other atmospheric gases, the amount of carbon dioxide dissolved in saturated seawater is unusually large.\u00a0<\/span><\/p> Some gases found within seawater are also involved in oceanic organic and inorganic processes that are indirectly related to the atmosphere. For example, oxygen and carbon dioxide may be temporally generated or depleted by such processes to varying concentrations at specific locations within the ocean.\u00a0<\/span><\/p> Density of Ocean Water\u00a0<\/span><\/strong>\u00a0<\/span><\/p> The density of pure water is 1000 kg\/m3. Ocean water is denser because of the salt in it. Density of ocean water at the sea surface is about 1027 kg\/m3.\u00a0<\/span><\/p> There are two main factors that make ocean water more or less dense than about 1027 kg\/m3: the temperature of the water and the salinity of the water. Ocean water gets denser as temperature goes down. So, the colder the water, the denser it is. Increasing salinity also increases the density of sea water\u00a0<\/span><\/p> <\/p> Less dense water floats on top of more dense water. Given two layers of water with the same salinity, the warmer water will float on top of the colder water. There is one catch though! Temperature has a greater effect on the density of water than salinity does. So, a layer of water with higher salinity can actually\u00a0float on top of water with lower salinity if the layer with higher salinity is quite a bit warmer than the lower salinity layer.\u00a0<\/span><\/p> The temperature of the ocean decreases and decreases as you go to the bottom of the ocean. So, the density of ocean water increases and increases as you go to the bottom of the ocean. The deep ocean is layered with the densest water on bottom\u00a0and the lightest water on top. Circulation in the depths of the ocean is horizontal. That is, water moves along the layers with the same density.\u00a0<\/span><\/p> The density of ocean water is rarely measured directly. If you wanted to measure the density of ocean water, you would have to collect a sample of sea water and bring it back to the laboratory to be measured. Density is usually calculated using an equation. You just need to measure the salinity, temperature and pressure to be able to find density. These measurements are often made with a CTD instrument, where the instrument is placed in the ocean water from a ship or a platform.<\/span><\/p> <\/p>![\"5mLRTBcCrzesXsPEx2vVpw01Zu1uFdWxdgzpHvAxYHEypZxm.jpeg\"](\"https:\/\/images.teemill.com\/5mLRTBcCrzesXsPEx2vVpw01Zu1uFdWxdgzpHvAxYHEypZxm.jpeg\")
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