Geological activity of oceans and seas
Features of the relief of the ocean floor
Destructive and accumulative activities of the sea
Sedimentation in seas and oceans
General information about the World Ocean
Ocean- a continuous water shell of the Earth surrounding continents and islands and having a common salt composition. The World Ocean makes up 94% of the hydrosphere and occupies 70.8% of the earth's surface. It is a giant depression of the earth's surface, containing the bulk of the hydrosphere - about 1.35 km 3. Parts of the World Ocean separated by land or elevated underwater relief and differing from the open part of the ocean in hydrological, meteorological and climatic regime are called seas. Conventionally, some open parts of the oceans (Sargasso Sea) and large lakes (Caspian Sea) are also called seas. From a geological point of view, modern seas are young formations: all of them took shape close to modern ones in Paleogene-Neogene times, and were finally formed in the Anthropocene. The formation of deep seas is associated with tectonic processes; shallow seas usually arose when the marginal parts of continents (shelf seas) were flooded by the waters of the World Ocean. The flooding of these areas could be due to two reasons: 1) a rise in the level of the World Ocean (due to the melting of Quaternary glaciers) or 2) subsidence of the earth's crust.
Salinity and composition of sea waters. The average salinity of the waters of the World Ocean is about 35 g/kg (or 35 ‰ - 35 ppm). However, this value is different in different parts of the World Ocean and depends on the degree of connection with the open ocean, climate, proximity to the mouths of large rivers, melting ice, etc.: in the Red Sea salinity reaches 42‰, while in the Baltic it exceeds 3 -6‰. Maximum salinity is observed in lagoons and bays separated from the sea, located in arid regions. Another reason for abnormally high salinity may be the supply of salts with hot aqueous solutions, which is observed in areas with an active tectonic regime; in some bottom areas of the Red Sea, where thermal brines emerge, salinity reaches 310‰. Minimum salinity is typical for seas that have a difficult connection with the ocean and receive a significant amount of river water (salinity of the Black Sea is 17-18‰), and water areas near the mouths of large rivers.
Sea water is a solution containing more than 40 chemical elements. The sources of salts are river runoff and salts entering during the process of volcanism and hydrothermal activity, as well as during underwater weathering of rocks - halmyrolysis. The total mass of salts is about 49.2 * 10 15 tons, this mass is enough for the evaporation of all ocean waters to cover the surface of the planet with a layer 150 m thick. The most common anions and cations in waters are the following (in descending order): among the anions Cl -, SO 4 2-, HCO 3 -, among the anions Na +, Mg 2+, Ca 2+. Accordingly, in terms of layers, the largest amount is NaCl (about 78%), MgCl 2, MgSO 4, CaSO 4. The salt composition of sea water is dominated by chlorides (while river water contains more carbonates). It is noteworthy that the chemical composition of sea water is very similar to the salt composition of human blood. The salty taste of water depends on the sodium chloride content in it; the bitter taste is determined by magnesium chloride, sodium and magnesium sulfates. The slightly alkaline reaction of sea water (pH 8.38-8.40) is determined by the predominant role of alkaline and alkaline earth elements - sodium, calcium, magnesium, potassium.
A significant amount of gases are also dissolved in the waters of the seas and oceans. These are mainly nitrogen, oxygen and CO 2 . At the same time, the gas composition of sea waters is somewhat different from the atmospheric one - sea water, for example, contains hydrogen sulfide and methane.
Most of all, nitrogen is dissolved in sea water (10-15 ml/l), which, due to its chemical inertness, does not participate and does not significantly influence sedimentation processes and biological processes. It is assimilated only by nitrogen-fixing bacteria that are capable of converting free nitrogen into its compounds. Therefore, compared to other gases, the content of dissolved nitrogen (as well as argon, neon and helium) changes little with depth and is always close to saturation.
Oxygen entering waters during gas exchange with the atmosphere and during photosynthesis. It is a very mobile and chemically active component of sea waters, therefore its content is very different - from significant to negligible; in the surface layers of the ocean its concentration usually ranges from 5 to 9 ml/l. The supply of oxygen to the deep ocean layers depends on the rate of its consumption (oxidation of organic components, respiration, etc.), on the mixing of waters and their transfer by currents. The solubility of oxygen in water depends on temperature and salinity; in general, it decreases with increasing temperature, which explains its low content in the equatorial zone and higher content in the cold waters of high latitudes. With increasing depth, the oxygen content decreases, reaching values of 3.0-0.5 ml/l in the oxygen minimum layer.
Carbon dioxide is contained in seawater in small concentrations (no more than 0.5 ml/l), but the total content of carbon dioxide is approximately 60 times higher than its amount in the atmosphere. At the same time, it plays a crucial role in biological processes (being a source of carbon during the construction of a living cell), influences global climatic processes (participating in gas exchange with the atmosphere), and determines the characteristics of carbonate sedimentation. In seawater, carbon oxides are common in free form (CO 2), in the form of carbonic acid and in the form of the HCO 3– anion. In general, the content of CO 2, as well as oxygen, decreases with increasing temperature, so its maximum content is observed in cold waters of high latitudes and in deep zones of the water column. With depth, the concentration of CO 2 increases, since its consumption decreases in the absence of photosynthesis and the supply of carbon monoxide increases during the decomposition of organic residues, especially in the oxygen minimum layer.
Hydrogen sulfide in seawater is found in significant quantities in water bodies with difficult water exchange (a well-known example of “hydrogen sulfide contamination” is the Black Sea). Hydrothermal waters coming from the depths to the ocean floor, the reduction of sulfates by sulfate-reducing bacteria during the decomposition of dead organic matter, and the release of sulfur-containing organic residues during decay can serve as sources of hydrogen sulfide. Oxygen reacts quite quickly with hydrogen sulfide and sulfides, ultimately oxidizing them to sulfates.
The solubility of carbonates in seawater is important for ocean sedimentation processes. Calcium in sea water contains an average of 400 mg/l, but a huge amount of it is bound in the skeletons of marine organisms, which dissolve when the latter die. Surface waters are typically saturated with calcium carbonate, so it does not dissolve in the upper part of the water column immediately after organisms die. With depth, waters become increasingly undersaturated with calcium carbonate, and eventually, at a certain depth, the rate of dissolution of carbonate matter is equal to the rate of its supply. This level is named depth of carbonate compensation. The depth of carbonate compensation varies depending on the chemical composition and temperature of sea water, averaging 4500 m. Below this level, carbonates cannot accumulate, which determines the replacement of essentially carbonate sediments with non-carbonate ones. The depth where the concentration of carbonates is equal to 10% of the dry matter of the sediment is called the critical depth of carbonate accumulation ( carbonate compensation depth).
Features of the relief of the ocean floor
Shelf(or mainland shoal) is a slightly inclined, leveled part of the underwater margin of continents, adjacent to the shores of land and characterized by a common geological structure. The shelf depth is usually up to 100-200 m; The shelf width ranges from 1-3 km to 1500 km (Barents Sea shelf). The outer boundary of the shelf is delineated by an inflection of the bottom topography - the edge of the shelf.
Modern shelves were mainly formed as a result of the flooding of the margins of continents when the level of the World Ocean rose due to the melting of glaciers, as well as due to the subsidence of sections of the earth's surface associated with recent tectonic movements. The shelf existed in all geological periods, in some of them it grew sharply in size (for example, in the Jurassic and Cretaceous times), in others, occupying small areas (Permian). The modern geological era is characterized by moderate development of shelf seas.
continental slope is the next of the main elements of the underwater continental margins; it is located between the shelf and the continental foot. It is characterized by steeper surface slopes compared to the shelf and ocean bed (on average 3-5 0, sometimes up to 40 0) and significant dissected relief. Typical forms of relief are steps parallel to the edge and base of the slope, as well as underwater canyons, usually originating on the shelf and extending to the continental foot. Seismic studies, dredging and deep-sea drilling have established that, in terms of geological structure, the continental slope, like the shelf, is a direct continuation of the structures developed in adjacent areas of the continents.
Mainland foot is a plume of accumulative sediments that arose at the foot of the continental slope due to the movement of material down the slope (through turbidity currents, underwater landslides and landslides) and the deposition of suspended matter. The depth of the continental foot reaches 3.5 km or more. Geomorphologically, it is a sloping hilly plain. Accumulative deposits that form the continental foot are usually superimposed on the ocean floor, represented by oceanic crust, or are located partly on continental and partly on oceanic crust.
Next are structures formed on oceanic-type crust. The largest elements of the relief of the oceans (and the Earth as a whole) are the ocean floor and mid-ocean ridges. The ocean floor is divided by ridges, swells and hills into basins, the bottom of which is occupied by abyssal plains. These areas are characterized by a stable tectonic regime, low seismic activity and flat topography, which allows them to be considered as oceanic plates - Thalassocratons. Geomorphologically, these areas are represented by abyssal (deep-sea) accumulative and hilly plains. Accumulative plains have a leveled, slightly inclined surface and are developed primarily along the periphery of the oceans in areas of significant influx of sedimentary material from the continents. Their formation is associated with the supply and accumulation of material by suspension flows, which determines their inherent features: a depression of the surface from the continental foot toward the ocean, the presence of underwater valleys, gradational layering of sediments, and leveled relief. The latter feature is determined by the fact that, moving deeper into the ocean basins, sediments bury the primary dissected tectonic and volcanic relief. The hilly abyssal plains are characterized by dissected topography and low sediment thickness. These plains are typical of the inner parts of basins, distant from the shores. An important element of the relief of these plains are volcanic uplifts and individual volcanic structures.
Another element of the megarelief is mid-ocean ridges, which are a powerful mountain system stretching across all oceans. The total length of mid-ocean ridges (MORs) is more than 60,000 km, width 200-1200 km, height 1-3 km. In some areas, the peaks of the MOR form volcanic islands (Iceland). The relief is dissected, the relief forms are oriented mainly parallel to the extent of the ridge. The sedimentary cover is thin, represented by carbonate biogenic silts and volcanogenic formations. The age of sedimentary strata becomes older with distance from the axial parts of the ridge; in the axial zones the sedimentary cover is absent or represented by modern deposits. The MOR regions are characterized by intense endogenous activity: seismicity, volcanism, and high heat flow.
MOR zones are confined to the boundaries of the separation of lithospheric plates; here the process of formation of new oceanic crust occurs due to incoming mantle melts.
The zones of transition from continental to oceanic crust—the margins of continents—deserve special attention. There are two types of continental margins: tectonically active and tectonically passive.
Passive outskirts They represent a direct continuation of continental blocks, flooded by the waters of the seas and oceans. They include the shelf, continental slope and continental foot and are characterized by the absence of manifestations of endogenous activity. Active ocarinas are confined to the boundaries of lithospheric plates, along which oceanic plates move under continental plates. These ocarinas are characterized by active endogenous activity; areas of seismic activity and modern volcanism are confined to them. Among active ocarinas, two main types are distinguished by structure: Western Pacific (island arc) and Eastern Pacific (Andean). The main elements of Western Pacific type margins are deep-sea trenches, volcanic island arcs, and marginal (or inter-arc) marine basins. The region of the deep-sea trench corresponds to the boundary at which the subduction of a plate with oceanic crust occurs. The melting of part of the subducting plate and the above-lying rocks of the lithosphere (associated with the influx of water into the subducting plate, which sharply lowers the melting temperature of the rocks) leads to the formation of magma chambers, from which melts flow to the surface. Due to active volcanism, volcanic islands are formed, stretching parallel to the plate subduction boundary. The margins of the East Pacific type are distinguished by the absence of volcanic arcs (volcanism occurs directly on the edge of the land) and marginal basins. The deep-sea trench gives way to a steep continental slope and a narrow shelf.
Destructive and accumulative activity of the sea
Abrasion (from lat. “abrasion” – scraping, shaving) – the process of destruction of rocks by waves and currents. Abrasion occurs most intensely near the shore under the influence of the surf.
The destruction of coastal rocks consists of the following factors:
· wave impact (the force of which reaches 30-40 t/m2 during storms);
· abrasive effect of debris brought by the wave;
· dissolution of rocks;
· compression of air in the pores and cavities of the rock during the impact of waves, which leads to cracking of rocks under the influence of high pressure;
· thermal abrasion, manifested in the thawing of frozen rocks and ice shores, and other types of impact on the shores.
The impact of the abrasion process manifests itself to a depth of several tens of meters, and in the oceans up to 100 m or more.
The impact of abrasion on the shores leads to the formation of clastic deposits and certain forms of relief. The abrasion process proceeds as follows. Hitting the shore, the wave gradually creates a depression at its base - wave-breaking niche, over which the cornice hangs. As the wave-breaking niche deepens under the influence of gravity, the cornice collapses, the debris ends up at the foot of the shore and, under the influence of the waves, turns into sand and pebbles.
The cliff or steep ledge formed as a result of abrasion is called cliff. At the site of the retreating cliff, a abrasion terrace, or bench (English "bench"), consisting of bedrock. The cliff may border directly on the bench or be separated from the latter by a beach. The transverse profile of the abrasion terrace has the form of a convex curve with small slopes near the shore and large slopes at the base of the terrace. The resulting debris material is carried away from the shore, forming underwater accumulative terraces.
As abrasion and accumulative terraces develop, the waves end up in shallow water, become rough and lose energy before reaching the bedrock shore, and because of this, the abrasion process stops.
Depending on the nature of the ongoing processes, shores can be divided into abrasive and accumulative.
A, B, C - various stages of retreat of the coastal cliff, destroyed by abrasion; A 1, B 2, C 3 - various stages of development of an underwater accumulative terrace.
Waves carry out not only destructive work, but also work on the movement and accumulation of debris. The oncoming wave carries out pebbles and sand, which remain on the shore when the wave recedes, thus forming beaches. Beach(from French "plage" - sloping seashore) called a strip of sediment on the sea coast in the zone of action of the surf flow. Morphologically, there are beaches of full profile, which have the appearance of a gently sloping ridge, and beaches of incomplete profile, which are an accumulation of sediment inclined towards the sea, adjacent to the foot of the coastal cliff with its back side. Beaches with a full profile are typical for accumulative shores, while beaches with an incomplete profile are typical for abrasive shores.
When waves pile up at depths of the first meters, the material deposited under water (sand, gravel or shell) forms an underwater sand bank. Sometimes an underwater accumulative shaft, growing, protrudes above the surface of the water, stretching parallel to the shore. Such shafts are called bars(from French "barre" - obstacle, shallow).
The formation of a bar can lead to the separation of the coastal part of the sea basin from the main water area - lagoons are formed. Lagoon (from lat. "lacus" - lake) is a shallow natural water basin, separated from the sea by a bar or connected to the sea by a narrow strait (or straits). The main feature of lagoons is the difference in water salinity and biological communities.
Sedimentation in seas and oceans
Various sediments accumulate in the seas and oceans, which, based on their origin, can be divided into the following groups:
· terrigenous, formed due to the accumulation of products of mechanical destruction of rocks;
· biogenic, formed due to the vital activity and death of organisms;
· chemogenic, associated with precipitation from sea water;
· volcanogenic, accumulating as a result of underwater eruptions and due to eruption products brought from land;
· polygenic, i.e. mixed sediments formed by materials of different origins.
In general, the material composition of bottom sediments is determined by the following factors:
· depth of the sedimentation area and bottom topography;
· hydrodynamic conditions (presence of currents, influence of wave activity);
· the nature of the supplied sedimentary material (determined by climatic zonation and distance from the continents);
· biological productivity (marine organisms extract minerals from water and supply them to the bottom after dying (in the form of shells, coral structures, etc.));
· volcanism and hydrothermal activity.
One of the determining factors is depth, which makes it possible to distinguish several zones that differ in sedimentation characteristics. Littoral(from lat. "litoralis"- coastal) - a border strip between land and sea, regularly flooded at high tide and drained at low tide. The littoral zone is the area of the seabed located between the levels of the highest high tide and the lowest low tide. Nerite zone corresponds to the depths of the shelf (from Greek. "erites"- sea mollusk). Bathyal zone(from the Greek “deep”) roughly corresponds to the area of the continental slope and foot and depths of 200 – 2500 m. This zone is characterized by the following environmental conditions: significant pressure, almost complete absence of light, minor seasonal fluctuations in temperature and water density; The organic world is dominated by representatives of zoobenthos and fish; the plant world is very poor due to the lack of light. Abyssal zone(from the Greek “bottomless”) corresponds to sea depths of more than 2500 m, which corresponds to deep-sea basins. The waters of this zone are characterized by relatively weak mobility, constantly low temperature (1-2 0 C, in the polar regions below 0 0 C), constant salinity; Here there is a complete absence of sunlight and enormous pressures are reached, which determine the originality and poverty of the organic world. Areas with a depth of more than 6000 m are usually identified as ultra-abyssal zones, corresponding to the deepest parts of basins and deep-sea trenches.
A huge number of chemical elements are dissolved in the waters of the World Ocean. There are enough of them to cover the entire land surface of our planet with a layer of 240 m. Sea water by mass consists of 95% pure water and more than 4% of salts, gases and suspended particles dissolved in it. Therefore, sea water differs from fresh water in a number of features: bitter-salty taste, specific gravity, transparency, color, and a more aggressive effect on building materials.
All this is explained by the content in sea water of a significant amount of dissolved solids and gases, as well as suspended particles of organic and inorganic origin.
The amount of dissolved mineral solids (salts) expressed in grams per kilogram (liter) of seawater is called its salinity.
The average salinity of the World Ocean is 35 ‰. In certain areas of the World Ocean, salinity can deviate widely from the average value depending on hydrological and climatic conditions.
There are many different substances dissolved in seawater, but they are not represented equally. Some substances are contained in it in relatively large quantities (in grams per 1 kg (liter) of water), others - in quantities calculated only in thousandths of a gram per ton of water. These substances are trace elements common in sea water.
For the first time, the composition of sea water was determined by Ditmar based on a study of 77 samples collected at various points in the World Ocean. The entire mass of ocean water is a liquid "ore body". It contains almost all the elements of the periodic table.
Theoretically, sea water contains all known chemical elements, but their weight content is different. There are two groups of elements contained in sea water. The first group includes 11 main elements, which, in fact, determine the properties of sea water, the most important of which we have already named; The second group includes all other elements - they are often called microelements, the total content of which does not exceed 3 mg/kg. For example, 1 kg of sea water contains 3x10-7 g of silver, 5x10-7 of gold, and elements such as cobalt, nickel, tin are found only in the blood of marine animals that capture them from the water.
The main elements are found in seawater usually in the form of compounds (salts), the main of which are:
1) chlorides (NaCl and MgCl), constituting 88.7% by weight of all solids dissolved in seawater;
2) sulfates (MgSO4, CaBO4, K2804), components
3) carbonates (CaCO3) - making up 0.3%.
Changes in the salinity of surface waters of the World Ocean by latitude. Salinity on the surface of the ocean in its open parts depends mainly on the relationship between the amount of precipitation and the amount of evaporation. The greater the difference in temperature between water and air and wind speed, the greater the amount of evaporation.
Precipitation reduces surface salinity. In addition, the mixing of ocean and sea waters has a significant impact on changes in salinity. In polar regions, salinity changes as ice melts and forms. Near river mouths, salinity depends on freshwater flow.
All of these factors make it possible to judge the change in salinity by latitude.
Variations in salinity across latitudes are approximately the same for all oceans. Salinity increases from the poles to the tropics, reaches a maximum value around 20-25 northern and southern latitudes and decreases again at the equator. This pattern is associated with the regime of precipitation and evaporation.
In the trade wind circulation zone, clear, sunny weather without precipitation remains for most of the year, constantly blowing strong winds at fairly high air temperatures, which causes intense evaporation, reaching 3 m per year, as a result of which the salinity of surface waters in the tropical latitudes of the oceans is constantly the highest.
In the equatorial zone, where winds are very rare, despite the high air temperature, and precipitation is abundant, there is a slight decrease in salinity.
In the temperate zone, precipitation prevails over evaporation and salinity therefore decreases.
The uniform change in surface salinity is disrupted due to the presence of oceanic and coastal currents, as well as as a result of the removal of fresh water by large rivers (Congo, Amazon, Mississippi, Brahmaputra, Mekong, Yellow River, Tigris, Euphrates, etc.).
The area of the highest salinity in the World Ocean (S = 37.9%), not counting some seas, lies to the west of the Azores. The salinity of the seas differs more from the salinity of the ocean, the less the seas communicate with the ocean, and depends on their geographical location. The seas have a higher salinity than ocean waters: Mediterranean - in the west 37-38%, in the east 38-39%; Red - in the south 37%, in the north 41%; Persian Gulf - in the north 40%, in the eastern part 41%. Salinity on the surface of the seas of Eurasia varies widely. In the Sea of Azov in its middle part it is 10-12%, and off the coast it is 9.5%; in the Black Sea - in the middle part 18.5%, and in the northwestern part 17%; in the Baltic Sea with eastern winds it is 10%, with western and southwestern winds it is 20-22%, and in the Gulf of Finland, in some rainy years, with eastern winds the salinity decreases to 2-3%. The salinity of the polar seas in areas remote from the coast is 29-35% and may vary slightly depending on the influx of water from other areas of the ocean.
The closed seas (Caspian and Aral) have an average salinity of 12.8% and 10%, respectively.
Change in salinity with depth. In depth, noticeable fluctuations in salinity occur only up to 1500 m, and below this horizon salinity changes insignificantly. In a number of places, salinity levels stabilize starting from shallower depths.
In the polar regions, when ice melts, salinity increases with depth, and when ice forms, it decreases.
In temperate latitudes, salinity varies little with depth.
In the subtropical zone, salinity quickly decreases to a depth of 1000-1500 m.
In the tropical zone, salinity increases to a depth of 100 m, then decreases to a depth of 500 m, after which it increases slightly to a depth of 1500 m and below remains unchanged.
The distribution of salinity in depth, just as on the surface, is affected by horizontal movements and vertical circulation of water masses.
The distribution of salinity on the surface of the World Ocean on maps is shown using lines called isohalines - that is, lines of equal salinity.
At different times of the year, salinity also has its own fluctuations. To analyze changes in salinity over time, a graph is constructed - a halinic isopleth, on which the salinity value is written along the vertical axis, and the observation time along the horizontal axis. The horizontal distribution of salts at different depths differs significantly from its distribution over the surface. This is due to a number of reasons. One of them is that the distribution of water in the ocean across layers is determined by its density, and since the temperature of water usually decreases with depth, stable equilibrium does not require an increase in salinity as depth increases. Salinity can decrease with depth (anahaline), increase (catagaline) or remain unchanged (homogeneity).
For example, in high latitudes, heavy precipitation desalinates the surface water, making it less dense, which causes greater stability of the waters and prevents mixing. Therefore, in areas of minimum surface salinity, it is not necessary to expect a similar salinity position at depth. Deep currents play a major role in disrupting the consistency in the horizontal distribution of salinity on the surface and at depths. Thus, in the horizon of 75-150 m near the equator in the Pacific and Atlantic oceans, the secondary salinity minimum characteristic of surface horizons is no longer visible. Here, the surface waters are underlain by a horizon of highly saline water (36%o) and deep equatorial countercurrents of the Cromwell and Lomonosov.
Origin of salts in the World Ocean. Scientists have not yet given a definite answer to the question of the origin of salts in the World Ocean. Until recently, there were two assumptions about this. According to the first, the water of the World Ocean has been salty since its inception. According to the second, the ocean became saltier gradually, due to the removal of salts into the ocean by rivers and due to volcanic activity.
To confirm the correctness of the first assumption, analyzes of the composition of the most ancient deposits of potassium salt, formed in distant eras of the Earth’s existence, are provided. These deposits arose as a result of the drying out of sea basins with salt water. The remains of ancient marine organisms preserved in the mentioned sediments suggest that they existed in salty waters. In addition, water is an excellent solvent, and it is impossible to assume that the waters of the primary ocean were fresh.
The second assumption about the variability of salinity and salt composition under the influence of river runoff and degassing processes in the Earth’s Mantle is obvious. And this statement is especially true for the period preceding the advent of the biological regulator of salt composition.
In recent years, another hypothesis has been put forward regarding the origin of the salinity of the World Ocean, which is, as it were, a synthesis of various aspects of the assumptions just considered. According to this hypothesis:
1. The waters of the primordial ocean were salty from the moment of its origin, but their salinity and salt composition were certainly different than they are now.
2. The salinity of the World Ocean and the composition of its salts in their genesis are the result of complex and long-term processes associated with the history of the development of the Earth. The role of river runoff alone, although it can explain the accumulation of the entire mass of salts in quantity, is not sufficient to explain the current composition. The entry of the main cations into the ocean waters is indeed due to the processes of weathering of rocks and river runoff, but most of them probably came from the bowels of the earth.
3. Salinity changed throughout the entire period of the existence of the World Ocean, both upward and downward, and not unilaterally, as follows according to the second assumption. By the end of the Paleozoic, judging by the composition of the salts of the seas that existed then and subsequently dried up, the chemical composition of the ocean was already close to modern.
4. The salinity and composition of water are still changing, but this process is so slow that due to the insufficient sensitivity of chemical analysis methods, people cannot notice these changes. The change of geological periods, sharply different in the nature of mountain-building, volcanic activity, as well as climatic conditions, the appearance of life in the ocean are milestones that mark the direction of the process of variability in the salt composition and salinity of the World Ocean.
Let's remember: How are the planet's waters divided by salinity? Why do travelers and sailors take fresh water on sea voyages?
Key words:sea water, salinity, water temperature, ppm.
1. Water salinity. In all seas and oceans, water has a bitter-salty taste. It is impossible to drink such water. Therefore, sailors setting sail on ships take with them a supply of fresh water. Salt water can be desalinated in special installations that are available on sea vessels.
Mostly table salt is dissolved in sea water, which we eat, but there are other salts (Fig. 92).
* Magnesium salts give water a bitter taste. Aluminum, copper, silver, and gold were found in ocean water, but in very small quantities. For example, 2000 tons of water contains 1 g of gold.
Why are ocean waters salty? Some scientists believe that the primary ocean was fresh, because it was formed by river waters and rains that fell abundantly on the Earth millions of years ago. Rivers brought and continue to bring salt to the ocean. They accumulate and lead to salinity in ocean water.
Other scientists suggest that the ocean immediately became salty upon its formation, because it was replenished with salty waters from the bowels of the Earth. Future research may answer this question.
Rice. 92. The amount of substances dissolved in ocean water.
** The amount of salts dissolved in ocean water is enough to cover the surface of the land with a layer 240 m thick.
It is assumed that all naturally occurring substances are dissolved in seawater. Most of them are found in water in very small quantities: thousandths of a gram per ton of water. Other substances are contained in relatively large quantities - in grams per kilogram of sea water. They determine its salinity .
SALINITY sea water is the amount of salts dissolved in water.
Rice. 93. Salinity of surface waters of the World Ocean
Salinity is expressed in p r o m i l l y e, i.e. in thousandths of a number, and is denoted -°/oo. The average salinity of the waters of the World Ocean is 35°/oo. This means that every kilogram of sea water contains 35 grams of salts (Fig. 92). The salinity of fresh river or lake waters is less than 1°/oo.
The Atlantic Ocean has the most saline surface waters, the Arctic Ocean has the least saline (see Table 2 in Appendix 1).
The salinity of the oceans is not the same everywhere. In the open part of the oceans, salinity reaches its highest values in tropical latitudes (up to 37 - 38 °/oo), and in the polar regions the salinity of surface ocean waters decreases to 32 °/oo (Fig. 93).
The salinity of water in marginal seas usually differs little from the salinity of adjacent parts of the ocean. The water of inland seas differs from the water of the open part of the oceans in salinity: it increases in the seas of the hot zone with a dry climate. For example, the salinity of water in the Red Sea is almost 42°/oo. This is the saltiest sea in the World Ocean.
In the temperate seas, which receive a large amount of river water, salinity is below average, for example in the Black Sea - from 17°/oo to 22°/oo, in the Azov Sea - from 10°/oo to 12°/oo.
* The salinity of sea water depends on precipitation and evaporation, as well as currents, the influx of river water, the formation of ice and its melting. When seawater evaporates, salinity increases, and when precipitation falls, it decreases. Warm currents usually carry saltier water than cold ones. In the coastal strip, sea waters are desalinated by rivers. When seawater freezes, salinity increases; when seawater melts, on the contrary, it decreases.
The salinity of sea water varies from the equator to the poles, from the open part of the ocean to the shores, with increasing depth. Changes in salinity cover only the upper water column (down to a depth of 1500 - 2000 m). Deeper salinity remains constant and is approximately equal to the average ocean level.
2. Water temperature. The temperature of ocean water at the surface depends on the input of solar heat. Those parts of the World Ocean that are located in tropical latitudes have a temperature of + 28 0 C – +25 0 C, and in some seas, for example in the Red Sea, the temperature sometimes reaches +35 0 C. This is the warmest sea in the World Ocean. In the polar regions, the temperature drops to - 1.8 0 C (Fig. 94). At a temperature of 0 0 C, fresh water in rivers and lakes turns into ice. Sea water does not freeze. Its freezing is prevented by dissolved substances. And the higher the salinity of sea water, the lower its freezing point.
Fig.94. Temperature of surface waters of the World Ocean
With strong cooling, sea water, like fresh water, freezes. Sea ice forms. They constantly cover most of the Arctic Ocean, surround Antarctica, and appear in shallow seas at temperate latitudes in winter, where they melt in summer.
*Up to a depth of 200 m, the water temperature varies depending on the time of year: in summer the water is warmer, in winter it becomes colder. Below 200 m, the temperature changes due to the influx of warmer or colder waters by currents, and in the near-bottom layers it can increase due to the influx of hot water from faults in the oceanic crust. In one of these sources at the bottom of the Pacific Ocean, the temperature reaches 400 0 C.
The temperature of ocean waters also changes with depth. On average, for every 1,000 m of depth, the temperature drops by 2 0 C. At the bottom of deep-sea depressions the temperature is about 0 0 C.
1. What is called the salinity of sea water, how is it expressed? 2. What determines the salinity of sea water and how is it distributed in the World Ocean? What explains this distribution? 3. How does the temperature of the waters of the World Ocean change with latitude and depth? 4*. Why does salinity in tropical areas reach the highest values for the open part of the ocean (up to 37 - 38°/oo), while in equatorial latitudes salinity is much lower?
Practical work.
Determine salinity if 25 g of salts are dissolved in 1 liter of sea water.
2*. Calculate how much salt can be obtained from 1 ton of Red Sea water.
Competition of experts . There is a sea on earth in which a person can stand on the surface of the water like a float (Fig. 95). What is the name of this sea and where is it located? Why does the water in this sea have such properties?
Rice. 95 “The sea” in which non-swimmers can swim.
Among the properties of the waters of the World Ocean, temperature and salinity are distinguished.
Water temperature The world's oceans change in the vertical direction (decreases with depth, since the sun's rays do not penetrate to great depths) and horizontally (the temperature of surface waters decreases from the equator to the poles from +25 ° C to - 2 ° C due to the difference in the amount of water received solar heat).
Surface water temperature. Ocean water is heated by the influx of solar heat onto its surface. The temperature of surface waters depends on the latitude of the place. In some areas of the ocean, this distribution is disrupted by the uneven distribution of land, ocean currents, constant winds, and water runoff from the continents. Temperature naturally changes with depth. Moreover, at first the temperature drops very quickly, and then quite slowly. The average annual temperature of the surface waters of the World Ocean is +17.5 °C. At a depth of 3-4 thousand m it usually ranges from +2 to 0 °C.
Salinity of the water of the World Ocean.
Ocean water concentrates different salt: sodium chloride (gives water a salty taste) - 78% of the total amount of salts, magnesium chloride (gives water a bitter taste) - 11%, other substances. The salinity of sea water is calculated in ppm (the ratio of a certain amount of substance to 1000 weight units), denoted ‰. The salinity of the ocean varies, it varies from 32‰ to 38‰.
The degree of salinity depends on the amount of precipitation, evaporation, and desalination of rivers flowing into the sea. Salinity also changes with depth. To a depth of 1500 m, salinity decreases slightly compared to the surface. Deeper down, changes in water salinity are insignificant; it is 35‰ almost everywhere. The minimum salinity is 5‰ in the Baltic Sea, the maximum is up to 41‰ in the Red Sea.
Thus, water salinity depends : 1) on the ratio of precipitation and evaporation, which varies depending on the geographic latitude (since temperature and pressure change); Salinity may be lower where the amount of precipitation exceeds evaporation, where the influx of river water is large, where the ice is melting; 2) from depth.
Table “Properties of ocean waters”
the total amount of all solid minerals in grams dissolved in 1 kg of sea water. Expressed in thousandths - ppm, denoted o/oo. Determined by hydrochemical analysis of water samples or by the electrical conductivity of sea water. The salinity of the surface layer of the ocean depends on the relationship between the process of evaporation of sea water and the amount of precipitation: evaporation increases, and precipitation decreases the salt content. In coastal regions, salinity is greatly influenced by river runoff, and in polar regions - by the processes of ice formation and melting. When water freezes and sea ice grows, some of the salts flow into the water and salinity increases; when sea ice and icebergs melt, it decreases. Mixing of waters (diffusion) and advection of salts by currents also participate in the formation of the salinity field. The salinity of deep and bottom waters is determined exclusively by these 2 processes, since there are no internal sources and sinks of salts at depths and near the ocean bottom. The influence of biochemical processes on salinity is negligible. In the oceans far from the coast, salinity varies from 29 to 38o/oo. High salinity is observed in surface waters of tropical latitudes, where evaporation greatly exceeds precipitation. Water with the highest salinity (up to 37.9°/oo) is formed in the Atlantic Ocean in the zone of the Azores anticyclone. In the equatorial zone of the oceans, where heavy rainfall is frequent, salinity is low (34-35°/oo). In temperate latitudes it is relatively equal to 34°/oo. The lowest salinity of ocean waters - up to 29 °/oo - is observed in the summer among melting ice in the Arctic Ocean. The salinity of deep and bottom waters in the oceans is approximately 34.5 - 34.9 °/oo, and its distribution is determined by the circulation of the waters of the World Ocean. The average salinity of the World Ocean is 34.71°/oo (Atlantic - 35.3, Pacific - 34.85, Indian - 34.87°/oo). In coastal areas of the oceans with significant river flow (Rio de La Plata, estuaries of the Amazon, St. Lawrence, Niger, Ob, Yenisei, etc.), salinity can be significantly less than the average salinity and equal only 15-20 °/oo. The salinity of waters in the Mediterranean seas can be either less or greater than the salinity of ocean waters. Thus, the salinity of surface waters in the Black Sea is 16-18°/oo, in the Azov Sea 10-12°/oo, and in the Baltic Sea 5-8°/oo. In the Mediterranean and Red Seas, where evaporation significantly exceeds precipitation, salinity reaches 39 and 42°/oo, respectively. Salinity, together with temperature, determines the density of sea water, which determines the draft of the ship, the propagation of sound in water and many other physical characteristics of water. Salinity in some cases determines the characteristics of the technical use of sea water (feeding steam boilers, desalination plants, etc.). Salinity affects the development of life in the sea. In some areas of the World Ocean, the behavior of fish, and therefore their catches, depends on changes in water salinity.