Hot spring


A hot spring, thermal spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma or by circulation through faults to hot rock deep in the Earth's crust.
Hot spring water often contains large amounts of dissolved minerals. The chemistry of hot springs ranges from acid sulfate springs with a pH as low as 0.8, to alkaline chloride springs saturated with silica, to bicarbonate springs saturated with carbon dioxide and carbonate minerals. Some springs also contain abundant dissolved iron. The minerals brought to the surface in hot springs often feed communities of extremophiles, microorganisms adapted to extreme conditions, and it is possible that life on Earth had its origin in hot springs.
Humans have made use of hot springs for bathing, relaxation, or medical therapy for thousands of years. However, some are hot enough that immersion can be harmful, leading to scalding and potentially death.

Definitions

There is no universally accepted definition of a hot spring. For example, one can find the phrase hot spring defined as
  • any spring heated by geothermal activity
  • a spring with water temperatures above its surroundings
  • a natural spring with water temperature above human body temperature
  • a natural spring of water whose temperature is greater than
  • a type of thermal spring whose water temperature is usually or more above mean air temperature.
  • a spring with water temperatures above
The related term "warm spring" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al. suggest that the phrase "warm spring" is not useful and should be avoided. In 1923, Menzier proposed that a warm spring be defined as a thermal spring where the water is below that of the human body, but above that of the mean air temperature around the spring, though this definition is contested.

Chemistry

Because heated water can hold more dissolved solids than cold water, the water that issues from hot springs often has a very high mineral content, containing everything from calcium to lithium and even radium. The overall chemistry of hot springs varies from alkaline chloride to acid sulfate to bicarbonate to iron-rich, each of which defines an end member of a range of possible hot spring chemistries.
Alkaline chloride hot springs are fed by hydrothermal fluids that form when groundwater containing dissolved chloride salts reacts with silicate rocks at high temperature. These springs have nearly neutral pH but are saturated with silica. The solubility of silica depends strongly upon temperature, so upon cooling, the silica is deposited as geyserite, a form of opal. This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening.
Acid sulfate hot springs are fed by hydrothermal fluids rich in hydrogen sulfide, which is oxidized to form sulfuric acid,. The pH of the fluids is thereby lowered to values as low as 0.8. The acid reacts with rock to alter it to clay minerals, oxide minerals, and a residue of silica.
Bicarbonate hot springs are fed by hydrothermal fluids that form when carbon dioxide and groundwater react with carbonate rocks. When the fluids reach the surface, is rapidly lost and carbonate minerals precipitate as travertine, so that bicarbonate hot springs tend to form high-relief structures around their openings.
Iron-rich springs are characterized by the presence of microbial communities that produce clumps of oxidized iron from iron in the hydrothermal fluids feeding the spring.
Some hot springs produce fluids that are intermediate in chemistry between these extremes. For example, mixed acid-sulfate-chloride hot springs are intermediate between acid sulfate and alkaline chloride springs and may form by mixing of acid sulfate and alkaline chloride fluids. They deposit geyserite, but in smaller quantities than alkaline chloride springs.

Flow rates

Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a geyser, or fountain.

High-flow hot springs

There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Springs with high flow rates include:
  • The Dalhousie Springs complex in Australia had a peak total flow of more than 23,000 liters/second in 1915, giving the average spring in the complex an output of more than 325 liters/second. This has been reduced now to a peak total flow of 17,370 liters/second so the average spring has a peak output of about 250 liters/second.
  • The 2,850 hot springs of Beppu in Japan are the highest flow hot spring complex in Japan. Together the Beppu hot springs produce about 1,592 liters/second, or corresponding to an average hot spring flow of 0.56 liters/second.
  • The 303 hot springs of Kokonoe in Japan produce 1,028 liters/second, which gives the average hot spring a flow of 3.39 liters/second.
  • Ōita Prefecture has 4,762 hot springs, with a total flow of 4,437 liters/second, so the average hot spring flow is 0.93 liters/second.
  • The highest flow rate hot spring in Japan is the Tamagawa Hot Spring in Akita Prefecture, which has a flow rate of 150 liters/second. The Tamagawa Hot Spring feeds a wide stream with a temperature of.
  • The most famous hot springs of Brazil's Caldas Novas are tapped by 86 wells, from which 333 liters/second are pumped for 14 hours per day. This corresponds to a peak average flow rate of 3.89 liters/second per well.
  • In Florida, there are 33 recognized "magnitude one springs". Silver Springs, Florida has a flow of more than.
  • The Excelsior Geyser Crater in Yellowstone National Park yields about.
  • Evans Plunge in Hot Springs, South Dakota has a flow rate of of spring water. The Plunge, built in 1890, is the world's largest natural warm water indoor swimming pool.
  • The hot spring of Saturnia, Italy with around 500 liters a second
  • Lava Hot Springs in Idaho has a flow of 130 liters/second.
  • Glenwood Springs in Colorado has a flow of 143 liters/second.
  • Elizabeth Springs in western Queensland, Australia might have had a flow of 158 liters/second in the late 19th century, but now has a flow of about 5 liters/second.
  • Deildartunguhver in Iceland has a flow of 180 liters/second.
  • There are at least three hot springs in the Nage region south west of Bajawa in Indonesia that collectively produce more than 453.6 liters/second.
  • There are another three large hot springs north east of Bajawa, Indonesia that together produce more than 450 liters/second of hot water.

    Ecosystems

Hot springs often host communities of microorganisms adapted to life in hot, mineral-laden water. These include thermophiles, which are a type of extremophile that thrives at high temperatures, between. Further from the vent, where the water has had time to cool and precipitate part of its mineral load, conditions favor organisms adapted to less extreme conditions. This produces a succession of microbial communities as one moves away from the vent, which in some respects resembles the successive stages in the evolution of early life.
For example, in a bicarbonate hot spring, the community of organisms immediately around the vent is dominated by filamentous thermophilic bacteria, such as Aquifex and other Aquificales, that oxidize sulfide and hydrogen to obtain energy for their life processes. Further from the vent, where water temperatures have dropped below, the surface is covered with microbial mats thick that are dominated by cyanobacteria, such as Spirulina, Oscillatoria, and Synechococcus, and green sulfur bacteria such as Chloroflexus. These organisms are all capable of photosynthesis, though green sulfur bacteria produce sulfur rather than oxygen during photosynthesis. Still further from the vent, where temperatures drop below, conditions are favorable for a complex community of microorganisms that includes Spirulina, Calothrix, diatoms and other single-celled eukaryotes, and grazing insects and protozoans. As temperatures drop close to those of the surroundings, higher plants appear.
Alkali chloride hot springs show a similar succession of communities of organisms, with various thermophilic bacteria and archaea in the hottest parts of the vent. Acid sulfate hot springs show a somewhat different succession of microorganisms, dominated by acid-tolerant algae, fungi, and diatoms. Iron-rich hot springs contain communities of photosynthetic organisms that oxidize reduced iron to oxidized iron.
Hot springs are a dependable source of water that provides a rich chemical environment. This includes reduced chemical species that microorganisms can oxidize as a source of energy.

Significance to abiogenesis

Hot spring hypothesis

In contrast with "black smokers", hot springs similar to terrestrial hydrothermal fields at Kamchatka produce fluids having suitable pH and temperature for early cells and biochemical reactions. Dissolved organic compounds were found in hot springs at Kamchatka. Metal sulfides and silica minerals in these environments would act as photocatalysts. They experience cycles of wetting and drying which promote the formation of biopolymers which are then encapsulated in vesicles after rehydration. Solar UV exposure to the environment promotes synthesis to monomeric biomolecules. The ionic composition and concentration of hot springs are identical to the cytoplasm of modern cells and possibly to those of the LUCA or early cellular life according to phylogenomic analysis. For these reasons, it has been hypothesized that hot springs may be the place of origin of life on Earth. The evolutionary implications of the hypothesis imply a direct evolutionary pathway to land plants. Where continuous exposure to sunlight leads to the development of photosynthetic properties and later colonize on land and life at hydrothermal vents is suggested to be a later adaptation.
Recent experimental studies at hot springs support this hypothesis. They show that fatty acids self-assemble into membranous structures and encapsulate synthesized biomolecules during exposure to UV light and multiple wet-dry cycles at slightly alkaline or acidic hot springs, which would not happen at saltwater conditions as the high concentrations of ionic solutes there would inhibit the formation of membranous structures. David Deamer and Bruce Damer note that these hypothesized prebiotic environments resemble Charles Darwin's imagined "warm little pond". If life did not emerge at deep sea hydrothermal vents, rather at terrestrial pools, extraterrestrial quinones transported to the environment would generate redox reactions conducive to proton gradients. Without continuous wet-dry cycling to maintain stability of primitive proteins for membrane transport and other biological macromolecules, they would go through hydrolysis in an aquatic environment. Scientists discovered a 3.48 billion year old geyserite that seemingly preserved fossilized microbial life, stromatolites, and biosignatures. Researchers propose pyrophosphite to have been used by early cellular life for energy storage and it might have been a precursor to pyrophosphate. Phosphites, which are present at hot springs, would have bonded together into pyrophosphite within hot springs through wet-dry cycling. Like alkaline hydrothermal vents, the Hakuba Happo hot spring goes through serpentinization, suggesting methanogenic microbial life possibly originated in similar habitats.