Salt (sodium chloride or halite), for details of the chemistry of Salt see https://en.m.wikipedia.org/wiki/Salt_(chemistry), has been the focus of human interest for thousands of years. It has been much sought after and traded since humans first realised its value. But there is much more to salt than simply an added taste at the dinner table. Salt is a common substance, chemically consisting mainly of sodium chloride ( NaCl) used extensively as a condiment and preservative while sulphate is (organic chemistry) any ester of sulphuric acid. 

The earliest known treatise on pharmacology was published in China. This was the Peng-Tzao-Kan-Mu published in China about 4,700 years ago and it revealed within it descriptions of two methods of extracting salt and transferring it into usable form, processes that are very similar to ones used today. A major portion of this writing is devoted to a discussion of more than 40 kinds of salt, including descriptions of two methods of salt extraction that are similar to processes used today. 

Humans have been aware of the precious value of salt for thousands of years.

We now know that salts exist in many different colors, which arise either from the anions or cations. For example

* sodium chromate is yellow by virtue of the chromate ion

* potassium dichromate is orange by virtue of the dichromate ion

* cobalt nitrate is red owing to the chromophore of hydrated cobalt(II) ([Co(H2O)6]2+)

* copper sulfate is blue because of the copper(II) chromophore

* potassium permanganate has the violet color of permanganate anion.

* nickel chloride is typically green of [NiCl2(H2O)4]

* sodium chloride, magnesium sulfate heptahydrate are colorless or white because the constituent cations and anions do not absorb in the visible part of the spectrum

Sulphates have played a major role in the process which led to oxygen forming on Earth, that in turn promoted the growth of photosynthetic life.

In an earlier blog I have mentioned tectonic plates. We know that when tectonic plates move, they can alter the amount of oxygen in the atmosphere. There is a relationship between the movements of tectonic plates and the consequential volcanic eruptions and the mineral and sulphate gases which spew out. The chemical reaction is part of the process of creating oxygen. For billions of years the Earth had no oxygen, then suddenly it came into being. Scientists have been trying to pinpoint when the Great Oxidation Event (GOE) occurred. Current thinking is that it was about 2.3-2.4 billion years ago.

Sulphates are described at https://en.m.wikipedia.org/wiki/Polyatomic_ion.

In a Nature Geoscience paper by scientists at Rice University they commented on what might happen when tectonic plates bump into each other, some sliding underneath another to depths where the temperature is high enough to melt it (subduction) and molten rock rises to form volcanoes at the surface. Those volcanoes can spew gasses into the atmosphere. The researchers suggest melting could have separated molecules of carbon and oxygen in the bodies of microbes long deceased (first life dates to at least 3.5 billion years ago) and settled to the ocean floor as sediments on the subducting plate. This separation could have sent the carbon even deeper into the Earth, for millions or billions of years, and expelled the oxygen out from volcanoes. 

Plate tectonics and biology are unique to Earth, as far as we know. Many speculate a symbioses between the two, geology and life working in tandem.

Geologists have discovered preserved salt lying 1.2 miles deep into the earth on the edge of Lake Onega, in western Russia near the border with Finland. This lake’s Geological history is of glacial-tectonic origin and is a small remnant of a larger body of water which existed in this area during an Ice Age. 
In geologic terms, the lake is rather young, formed – like almost all lakes in northern Europe – through the carving activity of the inland ice sheets in the latter part of the last Ice Age, about 12,000 years ago: In the Paleozoic Era (400–300 million years ago) the entire territory of the modern basin of the lake was covered with a shelf sea lying near the ancient, near-equatoric Baltic continent. Sediments at that time – sandstone, sand, clay and limestone – form a 200-metre-thick (660 ft) layer covering the Baltic Shield which consists of granite, gneiss and greenstone. The retreat of the Ice Age glaciers formed the Littorina Sea. Its level was first 7–9 m (23–30 ft) higher than at present, but it gradually lowered, thereby decreasing the sea area and forming several lakes in the Baltic region. (For more details see https://en.m.wikipedia.org/wiki/Lake_Onega)

The research teams established the buried salt was 2 billion years old, a first to find pristine salt of such a great age. Somehow this sample, which formed when an ancient sea evaporated, had remained unaltered by any geologic processes that occurred after burial.

By using computer models to recreate what the team of researchers found, they were able to identify details about the ancient ocean in which these samples formed, including just how oxidized it was.

The senior author based at Princeton University, Aivo Lepland, from the Geological Survey of Norway and Tallinn University of Technology, described their findings as “the strongest ever evidence” that the ancient seawater “had high sulphate concentrations reaching at least 30 percent of present-day oceanic sulphate as our estimations indicate.” 

Salt is indispensable to some living creatures, while also proving deadly for others. The last of the great mammoths died when they found themselves marooned on an island (Saint Paul Island, off the Alaskan coast) as sea levels rose and their fresh water sources became contaminated with salt water.

Salt has a myriad of important uses and was once even used as a form of currency in ancient Rome. The relationship between salt and water is perhaps one of the greatest balancing acts in all of nature, a partnership that has endured for millions of years.

Today, the Mineral Information Institute (MII) reports that about one-fifth of the world’s salt is produced in the United States, with other leading producers including China and Germany. But it is found globally in differing amounts.

According to the U.S. Office of Naval Research (ONR), the average ocean salinity is 35 ppt or parts per thousand, which means that for every 1,000 grams of water, there are 35 grams of salt. The ONR also reports that most of the salt in the ocean comes from rain, rivers and streams that wash sodium chloride into larger bodies of water. Other major sources of salt in the ocean include undersea volcanoes and hydrothermal vents. The term “brackish water” refers to bodies of water where freshwater and ocean water mix. In these areas, the average salinity ranges from 0.5 ppt to 17 ppt.

If salt water floods agricultural land the soil becomes contaminated by the salt and produce which was growing there, or grass for cattle, will die. Freshwater plants cannot thrive in salt water. The U.S. Department of Agriculture claims that soil salinity is responsible for reducing crop yields by as much as 25 percent in the United States. However, recent developments by the Agricultural Research Service have created new strains of wheatgrass that can withstand higher concentrations of salt by using genetic markers borrowed from saline-resistant plants.

Here is an extract about fish and how their bodies utilise and balance the salinity levels around them

Osmoregulation in fish (see https://en.m.wikipedia.org/wiki/Osmoregulation)

Osmoregulators tightly regulate their body osmolarity, maintaining constant internal conditions. They are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., Flounder. Flounder have been observed to inhabit two utterly disparate environments—marine and fresh water—and it is inherent to adapt to both by bringing in behavioral and physiological modifications.

But when the salmon move from the sea to freshwater rivers they adapt with brilliant abilities (see https://evolutionnews.org/2015/08/how_salmon_adju/)

Here is an extract from the above link:

Three main things must occur for the young salmon, called a smolt, to prepare for life in the salty ocean. First, it must start drinking a lot of water. Second, the kidneys have to drop their urine production dramatically. Third, and very important, molecular pumps in the cells of the gills have to shift into reverse, pumping sodium out instead of in. All these physiological changes have to change back when then the mature fish re-enters the freshwater river on its way to spawn. The fish will spend a few days in the intertidal zone as these changes are made automatically.

The observers of the natural world explain to us how these miracles of body evolution have led to so many species balancing the vital part salt plays in their survival, but also how some creatures cope when when otherwise salt would threaten their existence.

The next blog will delve into the threats and opportunities to human existence posed by the ever present sodium chloride in our environment.