I'm writing a story in which a world has been terraformed into an Earth-like planet and settled by a group of colonists. However, a cataclysm forces society to regress to the Stone/Bronze Age, at which point they basically have to restart their development from 0.
My question is, without prior eons of life forming fossil fuel deposits, would there just be no fuel to power industrialization or other societal advancement? What alternatives might they use which could be comparable?
Taking a lifeless planet and making it long-term suitable for human life is such a complex and difficult task that I am absolutely sure that the Ancient Gods would have included in their project plan a sub-sub-subproject of making sure that they included some nice coal deposits as risk mitigation against improbable but potentially devastating threats.
The thing is, the Ancient Gods could not have taken a planet like pre-biotic Earth and terraformed it into something resembling modern Earth suitable for sustaining pre-modern human life.
Or maybe they could do it, using absolutely unimaginably advanced technology; but in this case they would have surely spent 0.1% of their effort to produce nicely placed coal and maybe also petroleum deposits. You see, making sure to add some coal and petroleum to the terraformed planet is such a minor task compared to everything else they had to do that they would have certainly done it, if only as insurance against really horrible unforseen circumstances.
Pre-biotic Earth had enormous amounts of carbon dioxide in the atmosphere.
Once upon a time, all that coal and all that petroleum we enjoy were carbon dioxide in the atmosphere. It took life hundreds of millions of years to convert the carbon dioxide in the air into coal and petroleum.
Pre-biotic Earth had no soil for plants to grow in.
Soil is entirely a product of life. Without life there is only barren sand at best. Modern Earth has maybe 2 trillion tonnes of coal: compare with about 700 trillion tonnes of soil. I cannot believe that the Ancient Gods expended the effort to make 700 trillion tonnes of soil and did not bother to expend 0.3% of that effort to make some coal to be available in case of dire necessity.
And by soil I mean everything that's in it, including earthworms and most importantly the nitrogen-fixing bacteria and fungi without which plants cannot survive.
Pre-biotic Earth had no oxygen for the humans and other animals to breathe.
On Earth, photosynthetic life spent billions of years making oxygen from water and carbon dioxide, only for it to be promptly consumed to oxidize everything that could be oxidized. Notably, all those nice iron ore deposits we use were once upon a time elemental iron... Only after spending billions of years making oxygen to oxidize everything that could be oxidized could photosynthetic life even begin to accumulate oxygen in the air and in the ocean.
The point is that it was not nearly enough for the Ancient Gods to make the 1.2 trillion tonnes of oxygen we currently have in the air, because it would have been gone in a very short time. Oxygen is a very reactive gas, and it will promptly find something to oxidize. They must have first made enormous amounts of oxygen for it to oxidize everything that could be oxidized on the planet, and only after that could they begin making the oxygen in the air.
Oh, and in order for the planet to maintain free oxygen in the air the planet must be teeming with photosynthetic life. Oxygen is really a very reactive gas; without a constant supply it just won't stay as free oxygen.
Long story short, in order for them to convert a lifeless planet into a planet able to sustain human life long-term, the Ancient Gods must have used their unimaginably advanced technology to create the intricately complicated ecosystems which span the world, and the environmental conditions into which those ecosystems fit. Compared to this, the task of creating some well-placed coal deposits as insurance against cataclysms which might force society into the Bronze Age is so trivial that I'm sure their project manager included it as a cheap risk-mitigation measure.
Smelting metal ores, be they tin and copper ores for the Bronze Age, or iron ores for the Iron Age, needs carbon. Metal ores are metal oxides; you heat up the metal oxide in a carbon-rich environment, the carbon is more reactive than the metal and combines with the oxygen in the ore, and the elemental metal is released.
In the ancient world they used charcoal made from wood to smelt their ores into metals. This is not sustainable. Without access to coal deposits, the world would not have been able to reach the technological level of the Early Modern period before running out of forests. The Industrial Revolution is not even on the horizon.
Allow me to introduce you to the Diesel Tree, a tropical plant that naturally produces some petroleum-like precursors (turpenes). It's not exactly high yield, but it is a fuel source that literally grows on trees.
Public Domain - By Mauroguanandi
Seeing as there's no "hard science" tag, and we're talking about people who can terraform whole planets, I'd recommend a genetically modified version that has both a higher oil yield and a wider climactic range (seeing as it can't be "the tropics" everywhere). If you need a reason to have the trees pre-apocalypse, well even today, there's a lot of non-fuel things made from petrochemicals, so it'd be useful to have a low-maintenance self-sustaining source (especially on a frontier world)
N.B. if you want a purely organic solution, you're going to have to come up with a work-around for the fact that the world of today is chewing through our fossil fuels many, many, orders of magnitude faster than they were laid down to begin with. You'll probably have to accept a limited, albeit renewable, supply of fuel until civilization is able to build some solar panels and electrify everything.
The fundamental fuel of industrialization is getting something to spin. Once you have that, you can use factories to automate production, generate electrical power, and take off. Fortunately, there's a lot of ways to get something to spin. The oldest industrial factories used water wheels along rivers, which are a great stable source of power. Dams can be built to increase your energy output and reliability. Wind also works, but you can't generate as much torque.
You can also get spin through heat, via steam turbines and sterling engines. This gives your society access to locomotives, steam ships, and more electricity. Although coal and oil are certainly the best easily portable fuels for this, any dry cabron-based biomass will burn and generate heat. Additionally, non-biotic heat sources exist. Stationary heat plants can harvest geothermal power, and reflective arrays or large lenses can be used to concentrate solar energy to heat water or salt.
Without much good transportable fuel, power production will be fairly centralized until your civilization can develop a sufficient electrical grid and batteries. A successfully industrialized civilization would form cities around these centers of power and production. Cities in turn fuel many of the circumstances for other societal advancements- art, research, medicine, government. Case in point, the first water-powered textile factory in the US was also the site of the country's first factory strike, by women who were sick of working long hours in unsafe conditions for little money.
Finally, there's engines that can take almost any plant life and turn it into speed and power. They're very portable. They have been used for thousands of years. They let you ride them and will plow your fields. I hope your colonists brought some horses and oxen.
A society which can terraform worlds isn't suddenly going to go back to the bronze age - many of the artifacts which were used to terraform the world and maintain it's climate, atmosphere and hydrosphere should still be available, and more importantly, running, otherwise you might suddenly discover your planetary ecology is suffering system and cascade failures as it reverts to the more stable equilibrium of pre terraforming times.
Orbiting mirrors. These would be redirecting sumlight to or away from the planet
But if we hand wave this - perhaps reverting to a more stable equilibrium will require thousands of years, then there are still several potential work arounds.
Abiogenic hydrocarbon formation. While according to this theory you still need an active core and mantle for hydrocarbons to form and accumulate, making Mars a poor candidate, if you are willing to use this idea in your story, there will be deposits of hydrocarbons that can be accessed by drilling equipment.
Agricultural fuel sources. Plants can be harvested for oils, which can be used directly as fuel and chemical feed stock, or the oils can be converted via chemical processes into biodiesel and similar fuels. Alcohols can also be fermented from plant materials, and of course dry biomass can also be used directly as fuel - look up wood gasification and it's use in powering cars during WWII.
Genetic engineering. Some plants already produce oils which can be harvested for industrial use. Genetic engineers can modify these plants for higher outputs (although generally at the cost of some other attribute), or plants can be cross bred in the traditional way to produce more oils.
Of course other sources of energy exist as well, like wind or geothermal, but as the writer, you can decide which idea or combination of ideas would work best.
Spanish windmills from the middle ages
If there is enough organic matter to live from, there should be enough organic matter to turn into some kind of bio-fuel. Skip parts based on available tools/materials.
Mine some ores with stone tools. Make copper/bronze ones using wood to melt those. Turn wood to charcoal for forging steel to work towards steam engines. At which point you can start machining better parts for better engines.
Once you have precise engines you won't have a need for other fossil fuels. Machine the parts to make a still, and make alcohols. They burn well, and engines run very smooth on it. It just isn't commercially viable, and about 1/3 less energy dense than the average gasoline but that doesn't matter when you only require enough to further your tools-to-built-the-tools chain.
When you get to the point where your tools you can build solar panels, batteries, and electric motors you only require fuels for plastics and chemistry. Or use a biological source for plastics, which can be made from the things like maize or olives.
Bio-fuels quickly become a dead-end when you need multiple times the amount of land you would need to feed a group of people who could do more work. Once you get to solar, you could scale up or move towards fusion/fission energy sources. At which point you'll have enough power to serve until you reach your space-age tech.
Assuming the terraforming is young, and the colony is not populated by amish and not mor ethan a few million people, then the cataclysm destroying the space age technology the humans relied on to survive will have resulted surely in total collapse and extinction. These humans are most likely totally inept to be farmers, they relied on automated harvesters and computers to tell them when to send the plow and seeding machine out. They worked in maintenance or high skill machining or engineering and design. Almost all will work secondary (industry) or tertiary (service) jobs. Almost none of them was working the primary sector: food generation.
Now, the cataclysm hits. It destroys all machinery. Food generation and manufacture breaks down. Nobody can fix the machinery. Millions are killed in the cataclysm and the following societal uproar as food grows sparse. Maybe small pockets of dozens of people survive for a little.
Small pockets that have to relearn farming. Many will starve in the years till they figured that out. The survivors of those dispersed groups will be too few to keep the genetics intact, and too far apart to effectively form one community. They are at best a few hundred, widely strewn over the planet in tiny settlements of one to three families and possibly unable to ever get into contact with one another. But it takes at least a local minimum viable population to keep a society alive for a few generations. You need one stable community of about 500 to 1000 individuals within a somewhat concise area. But your demanded cataclysm throwing them back to the stone age precludes that many survivors in one area, if even allowing so many in total. Your edge condition says, everybody will be dead before they even get to mining.
It's much more believable if the settlers belonged to a religious or societal group that hated technology in the first place and took the terraformed planet as their holy land, and went almost fully into manual farming but for a handful of tradesmen. The technological regression would have been manmade, but survivable: deliberate destruction of their own high-tech to return to their "roots" as subsistance farmers. Two, three, maybe four, generations later and a strict damnatio memoriae that they came from the stars in place, nobody will remember that they were colonists just a century ago.
The preferred light and heat source will be burning biomass or biomass product, such as wood, charcoal or alcohol, depending on how much tech the humans allowed themselves to keep.
Wood falls short for a full-on industrial revolution because the sheer amount of fuel required to reach basic industry is just too much for any forest, even continent-wide to provide.
The vast majority of that fuel went into smelting ores, producing steel. But your regressors don't need that. They already have the ore mined and processed into noticeably pure remnants of civilisation. Even if rusted, they need to remove the oxygen, not the 90+% of non-metal that ores often have. Scavenging wrecks and ruins is also far easier than digging it kilometres deep under mountains (in often abysmal purities no less).
And then you still have alternative power sources like water (which was even the most important at the beginning) which, combined with a greatly reduced need for metal processing heat, might even suffice for a slightly slowed industrial revolution.
In short: The kickoff will happen anyway (because it doesn't depend on fossil fuels) and then people will find a way because the advantages are just too great.
Here on Earth, nearly every molecule of water has been some animal's pee at least a couple of times and every complex hydrocarbon was some animal's corpse. But just like water does not need to go through an organism's body to exist, neither do complex hydrocarbons.
Here on Earth, we have identified several processes capable of forming hydrocarbons from simple molecules under the right conditions. While traditional petroleum geology focuses on biogenic origins, there is growing evidence that some may form through abiogenic processes occurring within the Earth’s crust, as well as spectrographic evidence that similar complex hydrocarbons are common naturally occuring substances on exoplanets without any other evidence of life.
In particular, hydrothermal systems within ultramafic rocks suggest that hydrocarbon formation can occur at relatively shallow depths on the order of 2 to 10 km, where temperatures range from approximately 150–400°C and pressures from 100–1,000 bar.
In these environments, water penetrates fractured rock containing olivine (peridotite) and undergoes a process known as serpentinization. During this reaction, the iron in olivine is oxidized to form magnetite (Fe₃O₄), while the water is reduced to produce hydrogen gas (H₂):
$$ 3FeO + H_2O \rightarrow Fe_3O_4 + H_2 $$
This process generates both a continuous hydrogen source and magnetite, which serves as a catalyst in Fischer–Tropsch-type (FTT) synthesis.
Carbon can be introduced into the system through any carbonate minerals in the rock or dissolved carbon compounds in the water. Under various expected reactions, these carbonates can contribute carbon in the form of CO₂ which is reduced to CO in the presence of the Hydrogen being released in the Serpentine process. As long as you are generating more H₂ than CO₂, your will end up with CO and H₂ which will be able to undergo FTT synthesis to form complex hydrocarbons at the pressure and temperature commonly experienced at depths of over 2km.
$$ nCO + (2n + 1)H_2 \rightarrow C_nH_{2n+2} + nH_2O $$
After millions of years, plate tectonics will cut off the influx of water and the process of Catagenesis can begin. Just like with organic petroleum the pressure will squeeze out the excess Hydrogen and and Water over the course of millions of additional years leaving you with a liquid of long chain hydrocarbons similar to crude oil.
Here on Earth, we have a relatively large amount of petroleum like fuels close to the surface where they are easy to mine. Your terraformed planet may have some near-surface reserves due to plate tectonics and volcanic activity, but the vast majority of its usable petroleum like fuels will require deep sea oil rigs to reach.
That said, you can offset this difficulty with a more carbon rich planet. Compared to other solar systems, the Sol System is relatively carbon scarce meaning that the average Earth-Like planet in another solar system will actually have much more Carbon than Earth. That means more natural hydrocarbons in the environment, meaning that you could have much larger abiogenic oil reserves than on Earth to offset the greater average depth.
OK so instead of fossil fuels, bio-fuels.
Both #1 and #2 were historically used to power steam boilers that powered other machines.
Windmills and water wheels
Both have historically been used to power all kinds of machines.
A water wheel placed next to a large river can easily power a large factory. Generally, a shaft attached to the wheel by gears would extend into the factory to power the machines in the factory. In some cases, this involved locating worker stations along the shaft and then having the workers attach to it via a belt to draw mechanical power.
Geothermal
If you can find naturally occurring hot-springs, you can power boilers from it in those locations. But that's going to be geographically limited.
