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I am thinking of a world where bubbles of hydrogen emerge from the seafloor, and are trapped by people for use as fuel.

But what is the source of the hydrogen? In other words, what geological or maybe biological/bacterial processes would cause masses of H₂ to bubble up through the sea in a reliable way?

The people will have technology like 20th-century Terra with which to trap/harvest it.

Thanks

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Seafloor flatulence

A little known fact is that while many cases of flatulence are ignitable, only some people produce methane. Many others release hydrogen in significant quantities.

The hydrogen results when something needs an electron acceptor for some reason, and O2 isn't available somewhere deep where the sun doesn't shine. (Hydrobacteria grow in multiple such environments, some beneath the sea and some much closer to home) If you have electrons to spare in any aqueous environment, you can combine 2H+ + 2e- = H2. (Of course, in air that would be a horrible waste of fuel, but the usefulness of fuel depends on the local atmosphere!)

Now all this depends on some manner of undigested food that needs some sort of what can loosely be categorized as fermentation. Perhaps your sea was on the legendary ancient Bean Trail. Or perhaps more likely, toxic herbivores are dying and scavengers can't touch them.

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  • $\begingroup$ This is why I love SE. I knew about hydrogen farts, but would never apply it to this. $\endgroup$
    – Murphy L.
    Commented May 2, 2022 at 17:57
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Outgassing of primordial hydrogen trapped in the core and mantle.

There is a lot of hydrogen in the universe. When the planet was formed, hydrogen was trapped down deep. Little by little it escapes.

Degassing of primordial hydrogen and helium as the major energy source for internal terrestrial processes

In our former article (Gilat and Vol, 2005) we hypothesized that during Earth’s accretion primordial helium and hydrogen were trapped and stored in the planet’s interior as He- and H- solutions and compounds, stable only under ultrahigh pT-conditions; some of them were discovered in laboratory experiments under similar conditions, some were deduced from their atomic structure and composition of natural He-rich gases. Since stabilization of the planet the energy spent on trapping H and He is quasi-constantly released by dominantly exothermal reactions of the Earth’s degassing.

The authors lay out their theory in some depth. To me it works better for a fictional worldbuilding scenario than on our real earth. I have to think that this scenario would lead to a lot of hydrogen coming out of the ground, mixed with natural gas if nothing else. As far as I know that is not the case.

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  • $\begingroup$ In fact it is the case. Hydrogen is coming out of the ground and the sea all the time, stays in the atmosphere for a while until it either reacts or escapes into space. But it is not "primordial". $\endgroup$
    – Avun Jahei
    Commented Apr 28, 2022 at 11:14
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On Earth - and certainly on other bodies in the solar system - molecular hydrogen is formed by quite a few pathways. In fact hydrogen is ubiquitous in the earth crust - although in low concentration - providing an energy source for bacteria thousands of meters underground. Sometimes that hydrogen can be trapped in geological formations (from which it may escape into the ocean as in your world).

Few people are aware of it, but wells and seapages of molecular hydrogen do exist in the earth crust. So far only one is used commercially (in Mali), many are known to exist and many more are still undiscovered because not many people search for them.

Hydrogen forming processes

  • Hydrogen is abundantly formed by the reaction of iron or magnesium containing minerals like olivin and pyroxene with water, producing serpentine rock and hydrogen (serpentinization). That happens frequently in deep sea hydrothermal vents. As I understand it, the hydrogen is produced in this process by a side reaction that decomposes iron silicate into magnetite, silica and hydrogen: 3 Fe2SiO4 + 2 H2O -> 2 Fe3O4 + 3 SiO2 + 2 H2. This side reaction only occurs under reducing conditions (= not much oxygen around)
  • Another important process in the Earth crust is the radiolysis of water in the presence of radioactive elements like uranium and thorium. They produce ionizing radiation which decomposes water into oxygen and hydrogen.
  • Industrially hydrogen is produced by reacting hydrocarbon (like methane) with water which forms hydrogen and carbon dioxide. That can happen in nature, too, you just need enough temperature and pressure.
  • Another industrial process which can happen naturally is the reaction of carbon or carbon monoxide with water. Both processes produce again carbon dioxide and hydrogen.
  • If you like it more exotic, hydrogen can be produced by many elements when they react with water, for example aluminium. Aluminium can occur in its native form on Earth, but only in extremely reducing conditions.
  • Simple heat can decompose hydrogen containing compounds like methane, ammonia and hydrogen sulfide (pyrolysis). That works with water, too, but it has to get very hot (above 2000°C or so).
  • It is believed by some scientists, that deep in the earth hydrogen is formed from the crystal water in hydrated minerals because of high pressure and heat. According to this model rocks deeper than 20,000 meter underground should be saturated with hydrogen which, like all gases, could find its way to the surface.
  • Biologically hydrogen is a side product of bacterial fermentation. Certain algae, like Chlamydomonas, can under certain conditions produce hydrogen, too.
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    $\begingroup$ +1 - for anyone who wants some more details, I found this article which talks some more about naturally-occurring hydrogen: geoscientist.online/sections/unearthed/… (I am currently working on a project doing research into this too 😆) $\endgroup$ Commented Apr 28, 2022 at 4:22
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You could create a fantastical type of hydrothermal vent. enter image description here

In reality, hydrothermal vents release copious amounts of sulfide minerals, usually particles, but you could make one that emits hydrogen gas. Have 'forests' of these vents scattered everywhere in your oceans, but since they tend to exist in the bathypelagic and abyssopelagic zone, make them more shallow-water benthic dwellers.

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    $\begingroup$ Real world hydrothermal vents frequently emit hydrogen gas. $\endgroup$
    – Avun Jahei
    Commented Apr 28, 2022 at 11:20
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Not exactly Hydrogen in its diatomic form, but Methane Hydrates or Methane Clathrate are a white ice like solid in appearance and are basically methane molecules surrounded by a hydrate cage. They are stable at high pressures and cold temperature on the sea floor. They contain a lot of energy, and if taken off the sea floor, or slightly under it can be held in you hand and burn. So perhaps you could have a form of undersea mining and meet your stories energy needs.

They are within the reach of 20th century technology, and are actively being considered today as an energy source by Japan and others, although there is concern that they could also release a lot of greenhouse gasses.

enter image description here

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    $\begingroup$ burning them is actually better for global warming than just letting them melt into the atmosphere (as is caused by global warming) $\endgroup$ Commented Apr 28, 2022 at 9:30
  • $\begingroup$ Combine with geothermal activity to increase temperature, or bubbles of any kind of gas at all from the seafloor to reduce pressure, and it will melt and bubble up. This could be steady if the deposit were right at the margin between freezing and not freezing and for example, sea current changes increased temperatures. $\endgroup$
    – ohwilleke
    Commented Apr 28, 2022 at 22:45
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The easiest way to have hydrogen bubbling up is have a source of hydrogen under/in the sea floor. But not too deep else it will dissolve and not per say bubble up. The diffusion rate of hydrogen means that even if there exists a source capturing reliably will be a challenge.

However is hydrogen is unlikely to form bubbles from a chemical properties standpoint.

  • Hydrogen is so small not many materials can contain it long term. Hydrogen tends to stay diffuse and rarely concentrates enough to form bubbles. Hydrogen out gassing from water will tend to dissipate to the atmosphere and escape without notice.
  • Hydrogen is reactive. The sea floor is at high pressure which will change equilibrium favoring hydrogen compounds. Also often including higher temperatures which will again tend to cause compounds to form.

If considering hydrogen in the form of compounds: it is very well documented the large number of hydrogen compounds being released into the seas/oceans from the sea floor.

  • Hydrogen sulfide.
  • Hydrogen dioxide.
  • Hydrocarbons. eg: Methyl hydrate formations.
  • Methane, swamp gas from swamps/muskeg etc. common enough to be its own category
  • etc.

If you really wanting such a system, I expect hand waving and or lamp shading will be required. Additionally such a system would be near surface perhaps less then 50m under water else the hydrogen will diffuse out to wide to be economical to collect.

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