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[Tubercular Ox]

This is more an outline than an explanation, that's the problem with hard scifi.  You can never really explain.

Imagine a world without magic but with adamantine spires.  They’re tall, air-filled columns.  Air convects heat well, meaning it’ll move around the column until the temperature is the same everywhere.  If it succeeds, the inside of the column will be the average temperature along the outside surface of the column.  How close it gets to this goal depends on the insulating value of the adamantine spire itself, but even if adamantine is only a wonky insulator, there will be a thermocline - a temperature difference - at the tip of the spire.

If you removed HFS when I said imagine a world without magic, the rule is deeper is hotter and the inside of the spire at the top will be hotter than the outside, because of heat convected from below.  If you assumed HFS uses a kind of magic that’s exempt from removing, then the inside of the spire is cooler than the outside.  Either way, you could paste peltiers all over the inside of the spire tip and use the generated voltage to electrolyze water.  When chlorophyll does it, cracking water like this is the first step of photosynthesis.  Obviously I’m going for adamantine: underground chlorophyll, but if that doesn’t work I can fall back on adamantine: structural support for underground chlorophyll.  My goal is to have this underground chlorophyll be a primary producer for the cavern layers, but it can’t be releasing oxygen directly because it would have to oxidize the entire magma layer before any useful oxygen could reach the cavern layer, which would change the predominant minerals worldwide to things that simply do not exist in the real world.  That's no good.

Luckily Toady included magmatic life -- fire snakes, magma crabs, etc. My copy of Life in the Universe, https://books.google.com/books?id=bsZ49I84twEC, suggests the most appropriate life for magma is based on silicate polymers.  It also calls it unlikely, but the crabs are there so obviously it's wrong on that part.

The basic building block of life (CH₂O) wants to be carbon dioxide and water, but living things spend their life avoiding that conclusion.  If you’ll trust me, silicate (SiO₄) wants to be silica (SiO₂), which magmatic life will likewise avoid, but when it happens there’s no choice but for the reaction to release a fair amount of oxygen.  Just like a human corpse doesn’t spontaneously explode into a cloud of CO₂ and water vapor, magmatic corpses are not going to explode into clouds of quartz and oxygen, but that’s the direction they’re headed in and they’re going to give off both in some quantity. 

If the magmatic ecosystem is full of a rich microbial life that spreads by diffusion, similar to surface life, then it could diffuse up the magma pipes and meet inevitable death at the hands of the hostile environment.  This would give off at least a little quartz, which under the temperature and pressure might accumulate on the walls of the pipe, explaining the obsidian shell surrounding each one.  It would also give off at least a little oxygen, but realistically this couldn’t be free O₂ because everything is just too reduced down there.  I think the best you can hope for are oxides useful for (hydrocarbon based) anaerobic respiration: https://en.wikipedia.org/wiki/Anaerobic_respiration

I’ll admit this is a weak point in my argument.  Chemistry is hard and knowing that oxygen is released doesn’t mean it’s released in a way useful to life.  I have found a few minerals that would decay in a way to provide some redox profit, but just as many with a tiny redox deficit.  Therefore I have to invoke a quirk of making silicate polymers: If you want to make the chain longer, you free up oxygen, as noted, all the way until quartz, which is a perfect lattice, everything connected to everything else.  Likewise, if you want to break a chain, you need an oxygen to cap the ends or you're in trouble.  These are also known as anabolism and catabolism.  Because statistics are a bitch, every once in a while you're going to have to break down a polymer and there's not going to be an oxygen handy from a nearby build up.  Either you're stuck waiting for anabolism to proceed somewhere, to free up an oxygen, or you keep a reserve of oxygen on hand (stored in some way) for these temporary emergencies.  So it's this reserve of oxygen, more than the decay of the silicate itself, which goes on to oxidize the cavern layers, but it still depends on the nature of the SiO₄ to SiO₂ decay for its existence.

So there are anaerobes, and the caves clearly have oxygen, which means the anaerobes are probably sequestered in poorly aerated soil, i.e wet and muddy, which is where they hide on the surface, as well.  This could be to the ecosystem’s benefit because it also implements an oxycline.  Using sulfur as the example again, sulfur oxidizing and sulfur reducing bacteria can toss the same bit of sulfur back and forth across an oxycline indefinitely, giving the sulfur more mobility than without the bucket chaining.  IOW, the oxycline could be why there’s an ubiquitous underground ecosystem and not just little rings around the magma pipes where the combination of mine gases and magmatic detritus is just right for anaerobic life.  The increased surface area would in turn allow for the better extraction of energy from said mine gases, resulting in a larger biomass overall.  As a side effect, absorbing mine gases makes cavern air far more breathable than analogous air in the real world: The life keeps it clean.

In short, adamantine could be magmatic chlorophyll, supporting the magmatic ecosystem.  Decaying microbial magmatic life, leaving behind obsidian detritus, could combine with mine gases to sustain anaerobic life in the caverns.  This anaerobic life, hiding in wet and muddy soil, plays tag with aerobic life in order to maximize biomass.  No muddy soil, no oxycline, no trees.  Since "maximizing biomass" means "drawing down mine gases", as a side effect, cavern air is made more agreeable to surface life than our experience in the real world might suggest.

Of course, if you look up how much thermal energy is available it seems like this kind of life is impossible: Earth has 47 terawatts of heat escaping and 173,000 TW of incident sunlight, but hydrothermal vents (https://en.wikipedia.org/wiki/Hydrothermal_vent) survive on similar flows, so something is possible.

[Greiger]

Ok, that was a really interesting read, I really like to dabble in explanations of complicated systems, but I sadly learned of that enjoyment too late to get any real proper education in any scientific fields.

Just to make sure I'm understanding a bit better I assume since there is a lack of oxygen there would be a lack of CO₂ as well since that includes oxygen? I always assumed the plants down there just produced the O₂ but from your explanation it seems they would not have any carbon dioxide to work with.  From the link I don't see any producers of O₂, but would anaerobic respiration theoretically be able to produce any kind of oxygen? 

Or alternately, could adamantine itself potentially have oxygen as a component and still have it's known properties?  That seems unlikely since I would think that would mean it would decay quickly and not be nearly as hard as it is, but just random thoughts.

[Tubercular Ox]

When I say there is a lack of oxygen, I mean there is a lack of free oxygen (O₂), and the reason why it's lacking is because all available oxygen is bound up in hard-to-use molecules like CO₂, SiO₂, and H₂O, all of which are available in abundance in the magma layer.  CO₂ and H₂O bubble out of the magma once the pressure/temperature get soft enough, which is why they're the two most common volcanic (or magmatic) gases.  They're also common as mine gases.

However, I don't think adamantine (or underground chlorophyll) would do the traditional H₂O + CO₂ --> CH₂O + O₂ conversion because that'd be useless to silicate life.  I picture instead 2H₂O + SiO₂ --> H₄SiO₄, turning silica quartz into silicic acid.  Cracking water would still be the first step for that, and the silicic acid is analogous to methane (CH₄), which has proven to be a kickstarter for hydrocarbon life: https://en.wikipedia.org/wiki/Methanotroph, so maybe silicic acid could be a kickstarter for silicate life.

Oxygen isn't really freed in that conversion, but magmatic life that starts from silicic acid has a number of excuses to walk around with oxygen stored in accessible molecules like sulfate (SO₄), and the things like sulfate still in their body when they die in the cavern layer is what keeps the anaerobic microbes breathing, so to speak.

Anaerobic microbes would find it almost impossible to generate free oxygen on their own.  It's climbing the wrong way on a cotton candy ladder.  I have no idea where the actual free oxygen in the cave comes from. >_>  I was hoping this would be it but by the time I was done I realized oxidizers like sulfate were my best hope for the adamantine process.  The soil is at least fertile in the model so far, and at least some oxygen will come in by diffusion from the surface, so at least tower caps are possible.  It's figuring out how to relay and/or reinforce the surface oxygen that's got me paused.

[GoblinCookie]

I’ll admit this is a weak point in my argument.  Chemistry is hard and knowing that oxygen is released doesn’t mean it’s released in a way useful to life.  I have found a few minerals that would decay in a way to provide some redox profit, but just as many with a tiny redox deficit.  Therefore I have to invoke a quirk of making silicate polymers: If you want to make the chain longer, you free up oxygen, as noted, all the way until quartz, which is a perfect lattice, everything connected to everything else.  Likewise, if you want to break a chain, you need an oxygen to cap the ends or you're in trouble.  These are also known as anabolism and catabolism.  Because statistics are a bitch, every once in a while you're going to have to break down a polymer and there's not going to be an oxygen handy from a nearby build up.  Either you're stuck waiting for anabolism to proceed somewhere, to free up an oxygen, or you keep a reserve of oxygen on hand (stored in some way) for these temporary emergencies.  So it's this reserve of oxygen, more than the decay of the silicate itself, which goes on to oxidize the cavern layers, but it still depends on the nature of the SiO₄ to SiO₂ decay for its existence.

Quite the chemist you are. 

Can you not get oxygen from the surface in the normal way because it is transported into the cavern dissolved into the water.  If there a shortage of oxygen in the cavern, would not the dissolved oxygen be released into the cavern?

[Urist McScoopbeard]

A couple of notes:

-- the underground ecosystem is not self-contained, it is, at many points, connected through openings both large and small to the surface (caves in tangible terms.) The largest of which are more than enough for monolithic forgotten beasts and titans to freely transverse.

-- Thus, I would imagine, on the problem of oxygen, that though natural fires, chokepoints, and pockets of extremely dense life might lead to somewhat extreme cycles of death and rebirth, the atmosphere of the DF world(s) would eventually flood in to the caverns at relatively normal temperatures and pressures.

[Tubercular Ox]

Can you not get oxygen from the surface in the normal way because it is transported into the cavern dissolved into the water.  If there a shortage of oxygen in the cavern, would not the dissolved oxygen be released into the cavern?

Only if the concentration of oxygen were higher in the water.  The example of Movile Cave shows us that water can be deoxygenated within inches if food is present. 
https://microbiomejournal.biomedcentral.com/track/pdf/10.1186/s40168-017-0383-2

Considering the Gulf Sea Dead Zone and the limnic eruptions of Lake Nyos, I don’t think we can rely on water for gas transport at all.

But you’re right, I’m seriously underestimating the power of oxygen to get there the normal way: diffusion.  I’m thinking the cave openings we see, plus unmodeled cracks and fissures too small for dwarves to crawl through, may actually be able to support the whole load, but I’ve gone back and forth on it with myself all day so I’ve nothing coherent to say yet.  Let’s babble!

If you look at a different spot on that Movile Cave link, you’ll see that an air bell fed its oxygen by water has 10% oxygen in it.  I used to think this was an equilibrium point, but now I think it’s a saturation point, because 10% is also how much oxygen you need to keep a human being alive.  Every time the oxygen falls below 10%, things die (and stop consuming oxygen) until it rises above 10% again.  The circle of life.

The same paper mentions microaerophilic life, but apparently it only lives in water.  If some air-breathing microaerophile couldn’t come and displace the life in Movile Cave after 5 million years, I feel sort of safe that it’s not on the way.

To me this means that oxygen will be at least 10% everywhere in the caverns, with numerous situational exceptions, but they’re just that: situational.  The new 10% is as useful as the old 0%, so I don’t know what’s really changed, but I’m more comfortable with the idea of oxygen sinking all the way down without being poached.

I keep trying to figure out how much air is in a 5’x5’ tunnel, like we see at the bottoms of caves formed in world generation.  A 25 sqft window is legal ventilation for a 500 sqft room, whose maximum occupancy is limited by the size of the exits and nothing to do with ventilation.  FWIW, a single 36” door permits a maximum occupancy of 180, or 2.7 square feet per person.  Pretty tight packing, I’d say, may as well stop there.

Reverse it? 180 people need 3600 cfm of air, a 25 sqft window needs the air moving at 144 feet per minute, or 1.6mph, which is a breeze so soft you can’t even feel it on your face.  Okay so far.

Now multipliers.  Ventilation codes want air above 18% so everyone feels happy and comfortable, I’m willing to drop it to 15%, so my 180 people is now 360.  If I drop it all the way to 10% we’re at 720. 

But these are surface people with surface metabolisms.  The underground is blessed with constant temperature and humidity, so since I now believe oxygen will sink as far as necessary, if we let it sink to where the ambient temperature is 98.6 degrees F, then a creature can have a human-sized activity level while living on the dietary needs of a crocodile (one raccoon a week).  I found exact calories once and it suggested a 10:1 factor, so my 720 low-oxygen-adapted people balloons to 7200 low-oxygen-and-ideal-temperature-adapted people, at a depth of somewhere around half a mile or a full kilometer.  If you’re near the poles, it’s deeper.  If you’re next to a volcano, it’s shallower.

Or we say it’s 720 low-oxygen-and-ideal-temperature-adapted people, with an ecosystem ten times their size to support them.

Not bad at all for a single tile passage.  If we assume the innumerable cracks and fissures too small for dwarves to explore are being modeled as solid stone but are still there to support the ecosystem, we may even reach enough air to support the underground, but first I would have to trust this line of thought.

A couple of notes:

-- the underground ecosystem is not self-contained, it is, at many points, connected through openings both large and small to the surface (caves in tangible terms.) The largest of which are more than enough for monolithic forgotten beasts and titans to freely transverse.

-- Thus, I would imagine, on the problem of oxygen, that though natural fires, chokepoints, and pockets of extremely dense life might lead to somewhat extreme cycles of death and rebirth, the atmosphere of the DF world(s) would eventually flood in to the caverns at relatively normal temperatures and pressures.

Um, yes.  I stand corrected.  I did assume the underground was self contained.

[Rowanas]

A couple of notes:

-- the underground ecosystem is not self-contained, it is, at many points, connected through openings both large and small to the surface (caves in tangible terms.) The largest of which are more than enough for monolithic forgotten beasts and titans to freely transverse.

I think you mean traverse, unless you do indeed think that these forgotten beasts spread themselves across these openings and cavemouths like taut flesh bridges.

[Dorsidwarf]

I’m not sure how effective a few dozen 1-2 tile openings spread across the entire world would be at providing a notable amount of oxygen to even the first layer of the caverns, which are a colossal world spanning labyrinth of deep tunnels, let alone the second and third layers which are only reached by elusive downward passages. And how does this deal with the issue that CO2 is inevitably heavier than O2 and therefore tends to pool in deep delvings?

[Tubercular Ox]

I’m not sure how effective a few dozen 1-2 tile openings spread across the entire world would be at providing a notable amount of oxygen to even the first layer of the caverns, which are a colossal world spanning labyrinth of deep tunnels, let alone the second and third layers which are only reached by elusive downward passages. And how does this deal with the issue that CO2 is inevitably heavier than O2 and therefore tends to pool in deep delvings?

I waver.  When I agree with you, I think the adamantine idea could still be useful.  When I don't, I lean on the cracks-and-fissures idea to multiply air contact.

As for CO2, two tricks and some research.  First, sinking CO2 may be a myth: https://www.researchgate.net/publication/228475055_The_legend_of_carbon_dioxide_heaviness
Even though that's published in a respectable journal and cited a fair number of times, I wonder about its authenticity just because of the weight of tradition it's bucking.  If you believe it, carbon dioxide pools only when there's a low-lying local source, around which it pools.  When the source is distributed, like an entire ecosystem, diffusion keeps it at height.

Second: Caves breathe, like Wind Cave.  https://en.wikipedia.org/wiki/Wind_Cave_National_Park  All caves breathe, Wind Cave just does it particularly forcefully.  The volume of a cave's breath is related to the volume of the cave, which we know for the caverns is huge.  The force of a cave's breath is inversely related to the size of the openings.  The cracks-and-fissures idea is slightly supported if one assumes the known entrances to the caverns aren't howling windstorms.

Breathing will regularly refresh both oxygen and carbon dioxide levels, although I have no idea how to even check if a complete reset is possible for any part of the caves beyond the immediate neighborhood of the entrances.

Third, CO2 can be recycled biologically.  Synthesis, however it's done, will scrub CO2 from the air and replace it with organic carbon.  CO2 traps (which do exist, they're just not the result of heaviness) would be preferred by synthesizers because of the ease of pulling CO2 out of the air.  Animals will come to graze on the traps because of the abundance of food, then retreat to where there's better air, which is usually closer to the entrance.  So there's a slow conveyor belt of carbon towards the entrance, where finally cave breathing can whisk it out of the caves.

[Urist McScoopbeard]

Very interessante. A "few dozen 1-2 tile openings" is a lot me thinks. That's hundreds of meters worth of space--and. At the very least, on a grand scale, I imagine pressure would indeed dictate that oxygen makes its way into the cave systems in considerable quantities. On the flip hand, I think it's probably impossible for an average sized DF world to support an internal vacuum of the size of the cave systems without imploding.

*also size/scale is somewhat problematic. Tiles are TECHNICALLY about >2m^2, but also you can fit multiple sperm whales in a single tile so... I imagine that the entrances into the underground are at least large enough to fit two adult sperm whales. The average width of a sperm whale is 20.5 meters and since you can indeed fit more than one in a single tile space,  you COULD say that the openings into the caves are roughly 41m^2. So let's say there are 36 such openings at 2 tiles a pop in a world, that's almost 3000m^2 of capacity.

[Bumber]

Don't be confused by the lack of proper multi-tile creatures. A single tile is only 2m x 2m x 2.8m.

You wander into absurd physics territory if you start treating them as larger. (How big is a bed? How much force will a speeding minecart impart? How much water does dwarf drink?)

[Urist McScoopbeard]

Well let's logic it out then. A mainstay of DF is forgotten beasts, titans, et al. If those become multi-tile, especially the ones that come from underground, then let us suppose that cave entrances into the cavern layers might become bigger. The point still stands im--you've got very large openings for very large creatures to easily pass between the surface and the underground (and I doubt that will change).

[Bumber]

It's not certain they're supposed to leave their habitats (without provocation.) They wouldn't be "forgotten" beasts if they just popped up every now and then to attack a surface town.

[Urist McScoopbeard]

Correct me if I am wrong, but in legends mode, aren't some non-FB megabeasts listed as having been born in subterranean caverns only to terrorize the surface?

[Starver]

How much water does dwarf drink?

As little as possible. And the inverse for alcohol.