So a few lakes are salty because rivers carried salts to the lakes, the water in the lakes evaporated and the salts were left behind. After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.
Because of the huge volume of the oceans, hundreds of millions of years of river input were required for the salt content to build to its present level. Rivers are not the only source of dissolved salts.
About twenty years ago, features on the crest of oceanic ridges were discovered that modified our view on how the sea became salty. These features, known as hydrothermal vents, represent places on the ocean floor where sea water that has seeped into the rocks of the oceanic crust, has become hotter, and has dissolved some of the minerals from the crust, now flows back into the ocean. Stringfield writes of this situation as follows:.
On the other hand, if the head of water is sufficiently great, a hydraulic gradient will be established in the aquifer, the saltwater will be pushed back to the submarine outcrop, and freshwater will escape into the sea.
Under the first condition, there is no discharge of freshwater into the sea. Hence, there is no hydraulic gradient, the head of the water in the aquifer is the same at all points, and the piezometric surface becomes an even surface at some height above sea level… If the head is relatively high or the submarine out-crop is relatively near sea level, active conditions may prevail.
Under these conditions, there is a hydraulic gradient; water is moving through the aquifer, and the soluble salts have generally been removed if there has been sufficient time and if the artesian water is relatively free of mineral matter. Conversely, if the head is low or the submarine outcrop is at great depth, static conditions may prevail. The aquifer is plugged by pressure of salt-water, and there is no leaking out of the soluble salts, except as artesian water may escape through the overlying confining beds or through nearby submarine outcrops where the aquifer is near enough to sea level so that the freshwater head in the aquifer at the outcrop exceeds the back pressure from the saltwater column in the ocean.
This has been observed in the Miami, Florida, area where the gradient of the water table is low, but the aquifer is very permeable and fresh water is constantly discharged into Biscayne Bay. Dark water in the photo is fresh water, while the lighter shade is salt water. Photo courtesy NASA. One way minerals and salts are deposited into the oceans is from outflow from rivers, which drain the landscape, thus causing the oceans to be salty.
You may know that the oceans cover about 70 percent of the of Earth's surface, and that about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on our planet. By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth's land surface it would form a layer more than feet meters thick, about the height of a story office building Source: NOAA.
But, where did all this salt come from? If you get into folk stories and mythology you will see that almost every culture has a story explaining how the oceans became salty. The answer is really very simple. Salt in the ocean comes from rocks on land. Here's how it works The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid.
The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions.
The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time. The two ions that are present most often in seawater are chloride and sodium. By the way, the concentration of salt in seawater salinity is about 35 parts per thousand.
In other words, about 35 of 1, 3. And, just so you don't think seawater is worthless, a cubic mile of it also can contain up to 25 pounds of gold at a concentration of 0. For a steady-state ocean, it is possible to find out how long a particular element stays in the ocean i. The ocean is at steady state for a particular element if that element is added and removed at the same rate. For example, Na sodium is added to the ocean at the rate of about 7.
The concept of residence time is informative in several ways. Elements with long residence times in the ocean tend to be very soluble in sea water and to be evenly mixed throughout the ocean. Thus, Na, Cl, and other elements Table 2 have long residence times and are known as conservative elements, occurring in the same ratio to one another throughout the ocean regardless of the total salinity. Elements with short residence times such as iron and aluminum are relatively reactive, or insoluble in sea water; they are easily removed and are unevenly distributed throughout the ocean.
This makes sense in comparison with the mixing time for the whole ocean, which is about 1, years. An element that remains dissolved for millions of years will have been mixed through the ocean many times over, and hence should be evenly distributed.
In contrast, an element with a residence time of only years will not be able to make it around once without being removed. Rivers add huge amounts of dissolved materials to the ocean each year as well as many tons of soil and rock particles.
At the boundary between the land and the sea are estuaries, or bodies of water that are. As salinity and pH increase seaward, some dissolved substances, such as Fe iron , may precipitate to form solids and then remove other dissolved elements, such as Mn manganese , onto their surfaces.
Other elements that arrive in the estuary adsorbed bound to river particle surfaces may be desorbed unbound by the influence of the higher salt content they encounter in the ocean. The dissolved substances enter the ocean, but as much as 90 percent of river-borne particles are trapped in the estuary and on the continental shelf. Particles carried through the air are known as aerosols. They come from a variety of sources.
Natural aerosols include sea-spray residues, windblown soil particles, volcanic particles, smoke from forest fires, and particles condensed from natural gases. Anthropogenic human-derived aerosols, often considered pollutants, include direct emissions such as from smokestacks and particles from conversion of anthropogenic gases.
Because most of the river-derived sediment load is trapped in estuaries and on the continental shelf, a large fraction of particles reaching the ocean from land consists of aerosols. The amounts in air vary over time and tend to be concentrated in latitude zones. Sea water continually reacts with its "container"—the basalt rocks that underlie the ocean.
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