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I was intrigued to learn recently of the Paleocene–Eocene thermal maximum from this blog of John Baez. For those who haven't seen, it's the sharp spike labelled "PETM" in this graph (from Wikipedia) of global temperatures over the past 65 million years:

Global temps over time

It immediately struck me that this event is somewhat parallel to modern day climate change–an observation which is also mentioned in the Wikipedia article. Apparently, it is conjectured that the Paleocene–Eocene thermal maximum may have been caused by volcanic eruptions.

(In asking the following question, I am not saying that I believe this to be the case (indeed, I find it highly implausible). I ask simply out of scientific curiosity.)

Is there any way we could tell whether the Paleocene–Eocene thermal maximum was caused by an ancient race of intelligent life forms, who caused a period of global warming in similar fashion to what humans are doing currently, and subsequently went extinct?


The answer I expect is along the lines of "No, we can't exclude that for sure. But we would expect to see some fossil records from such a civilization, which we don't." But I would be happy to be surprised in either direction by someone more knowledgeable. Some food for thought:

  • This answer indicates that there is uncertainty about what actually caused the PETM.
  • I have heard it said that there are only two things humans have accomplished which will leave a permanent mark on the geological record. The first is climate change. The second is leaded gasoline, which will leave an identifiable stratum around the globe.
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    $\begingroup$ This is a really interesting question and reminds me of a similar one I discussed with colleagues in an Earth Science department at coffee once, without much solution. Inspired by a Star Trek episode we wondered: if a dinosaur species had developed civilisation (and space flight), would we still find archaeological signs and would we recognise them as such? We couldn't agree. $\endgroup$ Jan 23 at 19:11
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    $\begingroup$ Even if such a race had existed, they surely had no nuclear technology yet. It had left a trace in the form of mysteriously changed isotope ratios. The remains of the humanity will be traceable billions year long on this way. $\endgroup$
    – peterh
    Jan 23 at 23:38
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    $\begingroup$ Btw, even a fish or a living organism can leave a trace which we can find hundred millions year later. I see it very unlikely that they would have some industry, enough capable to boost the CO2 levels, but they had not left such traces. For example, a piece of glass, as far I know, will remain there until the eternity if you just dig it. $\endgroup$
    – peterh
    Jan 23 at 23:44
  • $\begingroup$ The Q is if it was from disturbing surface deposits or volcanism. So they either scarred their earth 10M times worse than us or they could make volcanoes erupt. - "thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates" - "This [study] leads us to identify volcanism associated with the North Atlantic Igneous Province, rather than carbon from a surface reservoir, as the main driver of the PETM." nature.com/articles/nature23646 $\endgroup$
    – Mazura
    Jan 24 at 15:14

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tl;dr:

No, we can't exclude that for sure. But we would expect to see some fossil records from such a civilization, which we don't.

Yes, pretty much this.


To elaborate, I would refer to your statement that:

I have heard it said that there are only two things humans have accomplished which will leave a permanent mark on the geological record. The first is climate change. The second is leaded gasoline, which will leave an identifiable stratum around the globe.

This is absolutely incorrect. There are so many signatures that our civilisation is leaving at the moment in the geological record, permanently. Some examples:

  1. Mining activities. Assuming your civilisation is extracting resources from the Earth to power their fossil fuel industry, then need to mine the stuff. We are talking about huge holes in the ground. There are no currently known cases of an ore deposit that looks like someone has mined out the valuable stuff millions of years ago, leaving behind a hole (whether currently filled or not) surrounded by lower grade material. What about tailings ponds? Large areas of shallow water (with sediments forming at the bottom), full of stuff that isn't usually found together. This would be very easy to find.
  2. Electricity. High voltage power lines. You just don't have very long and thin lines of copper or aluminium forming in nature. Even if they don't remain in their metallic form, copper oxide or carbonate, and aluminium oxide or hydroxide don't form horizontal veins that cut across pretty much everything.
  3. Agriculture. The huge amount of fertiliser dumped onto farms, and then washed away in rivers forms very large anomalies of nitrogen and phosphate. Again, something that would stick out like a sore thumb in the geological record.
  4. Rare earth elements (lanthanides) and gadolinium. These elements form very distinct concentration distributions in natural rocks and sediments. Humans are using individual elements out of the entire lanthanide group, and this ends up in rivers and then in the ocean. Once the concentration anomalies are out there, they are not going anywhere. We have very sensitive ways to detect even tiny differences of these elements from what they should be in natural systems.
  5. Landfills. A whole soup of elements that should not be together. No natural system can form this. They're buried in the ground, and they stay there.
  6. Megacities. Tokyo, New York, London, Sydney. Huge amounts of asphalt, concrete, glass, steel, aluminium. Overground and underground. Even if you collapse and erode whatever remains, you will have huge areas covered with former cities, forming rock formation that are otherwise not possible in natural geological processes.
  7. Nuclear waste. I reckon there's no need to add anything here - right? Obvious that these things will not form in nature.

The list goes on and on. Trust me, if there was a civilisation large enough and advanced enough to destroy itself, we would see that very well in the geological record.

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    $\begingroup$ It's a good answer, although I don't find all points equally convincing. My problem is that this was 55 Million years ago, a really long time. The global geochemical markers (your points 3 and 4) are quite convincing and I agree they would show up. Local sites (mining, landfill, cities) I find less convincing as they may be in places we don't look (even Antarctica) and cities are often in erosion areas; not sure how much would be left after 55ma and if we'd recognise it. And radioactive waste is only a small quantity, will have decayed by now and would've been buried deep. $\endgroup$ Jan 23 at 19:06
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    $\begingroup$ @StephanMatthiesen many cities are built along rivers and river deltas, which are environments with exceptional preservation in the geological record. I wouldn't expect anything in Switzerland to survive, but the enormous population centres in south east Asia are here to stay. "Places we don't look" - humans populated the entire Earth, not hard to find it. Radioactive waste will decay to a very distinct isotope composition. Should be easy to recognise it. $\endgroup$
    – Gimelist
    Jan 24 at 0:20
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    $\begingroup$ @LorenPechtel yes, and this odd mix of elements is what would raise the suspicion that it's not natural. $\endgroup$
    – Gimelist
    Jan 24 at 3:32
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    $\begingroup$ Rivers are both erosion and sedimentation areas. You're right there are many well-preserved sites, but there were also many that are gone or are now buried too deep. And concrete can be hard to distinguish from conglomerates. So it's not quite so easy. $\endgroup$ Jan 24 at 7:56
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    $\begingroup$ don't forget nuclear trace from nuclear detonations, this is actually used as a marker for geologic strata and can identify the age of materials like glass and steel. then you have artificial materials like glasses and bricks that can easily be preserved. $\endgroup$
    – John
    Jan 25 at 1:34
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The answer by @Gimelist is a good list of hints we would see, although not all of them are equally convincing or strong, we would expect to see at least some of the signs.

The problem with any archaeological sign on Earth is of course that Earth's surface has changed a fair bit in 55 million years. For example, some of the coastlines of that time (where you would expect large cities) are now folded up in the Alps, the Himalayas and other mountain ranges or lie under the Greenland and Antarctica ice sheet, and there's generally been much time for erosion and sedimentation. So we might not have been able to find them as they are buried deep, or they've been changed beyond recognition.

So I want to add something totally different: we would likely see some artefacts on the Moon. Of course this would assume that the civilisation developed near Earth space flight, but I don't think this is much of an assumption given that you're looking for an industrial civilisation.

As there's virtually no erosion on moon, even items from 55 million years ago would still be there.

Now we have maps of the lunar surface with a resolution of about 100 metres. This would by far not be enough to find the Apollo landers by chance, but our current civilisation is not far away from leaving larger structures on the moon, so it seems to me there is a big probability that previous industrial civilisations would've left something that sticks out like a sore thumb on the lunar maps that we already have.

There are plans to build the Lunar Gateway Station, a space station in orbit around the moon, by the end of this decade. While that's still not large enough to be found on current maps, you can easily expect a larger station only a few decades away. A station the size of the ISS from 55 million years ago would either still be orbiting the moon or (ore likely) have crashed into it by now, but in either case we should've found the debris. We will probably also build structures on the moon itself in just a few decades, whether research stations on the scale of the Antarctic stations or perhaps some exploratory mining for trace metals (whether that's ever economically viable is a different matter).

The point is that Earth's surface has changed so much that likely only very large structures (like megacities) can still be found, but may be deformed so much that we don't recognise them, while on the Moon even comparatively small structures of just a few hundred metres (or much less in some places) would still be very obvious.

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    $\begingroup$ Lunar orbits are notoriously unstable--I doubt the ISS would still be up there and if it came down it would be possible for an impact crater to come down on top later. $\endgroup$ Jan 24 at 2:48
  • $\begingroup$ @LorenPechtel It would be a very low velocity crash at shallow angle, so you don't get a crater but a trail of debris on the surface. $\endgroup$ Jan 24 at 6:12
  • $\begingroup$ Also I was thinking of structures actually build on the moon, even just a research station the size of Antarctic stations or some exploratory trace metal mining. The lunar gateway station was just meant as example of what we are actually planning; give our civilization a few more decades and we'll certainly leave more traces on moon even though I doubt there will ever be any big cities there. $\endgroup$ Jan 24 at 6:36
  • $\begingroup$ I thought of that--but you could also have a shallow-angle impact crater that buried the debris. $\endgroup$ Jan 24 at 21:34
  • $\begingroup$ As for big things on the moon--suppose you wrap a linear motor around the equator. It has two sets of tracks so something above orbital velocity can still use it. Max ejection velocity with human-tolerable acceleration will take you anyplace in the solar system. $\endgroup$ Jan 24 at 21:39
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I would add an item to Gimelist's answer, and that is roads. Our road network is extensive enough that it is implausible that geological processes will completely obliterate all terrain containing roads. And future processes which expose a geological cross-section, like the erosion of a canyon, can hardly avoid cutting roads. Crushed rock would presumably fuse or be cemented somewhat over 50M years, but the structure would still be distinctive and anomalous. So future geologists would see all these little beds of formerly crushed rock, perhaps with the remains of a layer of asphalt on top. I think that would be pretty hard to miss.

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    $\begingroup$ The road network is extensive, yes, but it's also fragile. 50 years is enough to remove the asphalt from a paved road; after a century, an unpaved road will need a detailed examination to distinguish it from a forest or field. Some exceptionally well-built ones like the Roman roads will survive a thousand years or more without maintenance, but we're looking at a timescale of millions of years -- even mountains don't survive that. $\endgroup$
    – Mark
    Jan 25 at 3:39
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I think you need to ask the inverse:

Could a species significantly change the climate without mining of hydrocarbons, or could they mine hydrocarbons on the scale needed without having a large technological footprint (roads, electrical grid, nuclear-power, etc) that should leave traces?

I would posit that you would be looking for something like a carnivorous civilization that did not need assistance to hunt (no weapons like flints needed to increase take-home calories from hunts), had little interest in increasing prey by farming/feeding them (no fertilizers/farming to increase feed for prey, no herds of fat cows driving out other wildlife), and did not need housing/goods to add protection / comfort - which are going to require resources to be re-distributed. Of course, once you posit a civilization like that - what would they need the power for? Mating displays / cultural reasons can only burn up so much gasoline. And, they have to have seen bad effects happening, and had a reason not to curb their excesses.

For avoiding extensive road networks, maybe they used airships/hot air ballons? I think you'd have to blame a cultural or biological tendency for that (former avian stock that really enjoyed being in the air?), as roads seem far easier to develop. Cultural imperative to maintain pristine land to grow prey in? (but that would conflict with burning massive amounts of carbon, I think)

Carbon:

You don't need an long-distance electrical-grid to burn carbon. So you might not have evidence from power-lines.

A reason they might've avoided/never developed nuclear power, is that they were burning carbon for power instead. We got into atom-smashing for war and scientific curiosity.

How much carbon was locked up in the geological cycle at that point?

What evidence would a pumped-out oil reservoir look like after 65mya? How big were coal-beds at that point?

Carbonist manifesto says we've evolved because the geological storage of carbon has gotten out of control (not being cycled thru biosphere), I'm assuming it wasn't such an issue back then.

If enough carbon was available in the biosphere, you don't need to mine it. If you're not mining, you don't need to dig out ore-bodies for metal to make tools/engines to mine.

We had extensive surface deposits of high quality ore (Michigan), how new/old are those?

Could you have burnt off massive amounts of flora, and/or put lots of methane in the air? Maybe it was a massive spike in breeding up cow-equivalents (methane producing, and eating all the flora), but over a very small time-span, and none of them got preserved in the fossil record? We have Lazarus taxa and ghost lineages, and (currently) disappearing breeds of farm animals. Fossil record is definitely spotty.


Also I specifically want to take issue with some of @Gimelist's answer:

If a civilization only developed near seashores or deltas - current humans might not find anything, as most of the former seashore is now under-water. (Has anyone found a fishing village / midden-heap in Doggerland? - AFAIK; no. We've found a couple of flints, a skull fragment, an antler point). We've done very little exploration in these spaces.

That said, there are some nice things in the suggestions - which a traceless (near-traceless?) civilization would have to have avoided.


Another thing we'd find with nuclear bombs/power:

Radionuclides are also deposited in bones, and can be used to date disasters / bodies born after such incidents (eg: FNPP 2011). Famous study found: 'that the teeth of children born in St. Louis in 1963 had 50 times the levels of strontium 90 as those of children born in 1950'.

I'm interested in whether a thorium-powered civilization would leave traces - it's not able to be weaponized, and can get all nuclear waste to a 300 year decay window. Of course if you have thorium, why would you burn hydrocarbons?

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    $\begingroup$ I doubt we'd see radionuclides. Sr-90 has a half life of 30 years so it's totally gone after 55ma. More generally, practically all the radioisotopes in the environment come from overground bomb testing in the 1950s, very small traces from reprocessing plants (mostly before the 80s) and almost nothing from power plants, so if the civilization didn't use many bombs or only test them underground we wouldn't see much, and by now only the stable decay products are left which are not so unusual. $\endgroup$ Jan 24 at 6:31
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    $\begingroup$ A civilisation that is only capable of building fishing villages will probably not be able to cause a spike in atmospheric CO2. Remember that we only started emitting major CO2 after the industrial revolution. The question was about a civilisation advanced enough too case a CO2 spike observable in the geological record. Not whether a small fishing society will be detected from a distance of several tens of million of years. $\endgroup$
    – Gimelist
    Jan 24 at 9:01
  • $\begingroup$ @gimelist If you read, I've already noted that. I'm presuming human civilization in Doggerland was (perhaps) fishing villages, based on current assumptions. 65mya potential climate-changing civilization that is based around coastal areas most likely would not be 'fishing villages'. Both could be 'not discovered' because underwater exploration is poor, ie: Doggerland is merely an example of: coastal == not found, cuz underwater. Absence of evidence is not evidence of absence - esp. if you're not looking. Not claiming such 65mya civ. exists. But being repeatedly part of shore/underwater? $\endgroup$
    – user3082
    Jan 24 at 21:55
  • $\begingroup$ @user3082 a climate-changing civilisation that is only located on coastal areas is sufficiently large and influential to affect the chemical signatures of sediments on a global scale. The huge complex of Guangzhou, Shenzen, Hong Kong is in a coastal area, and even if all humanity disappeared tomorrow, that area will remain in the geological record pretty much for eternity. $\endgroup$
    – Gimelist
    Jan 24 at 23:31
  • $\begingroup$ @Gimelist It wouldn't necessarily need to be a civilization like ours to produce the PETM. The Wiki says carbon emissions during the PETM are estimated at 0.24GT/year. Current CO2 emissions are 20 times that. (It's unclear whether the units are the same - mass of carbon only, vs. the mass of CO2.) $\endgroup$ Jan 25 at 21:55
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The answer is no, it could not have been. Period.

Just the one thing, fossil fuel usage, makes this so. Fossil fuels have not been replaced since their making over-to-around 450-400 million years ago. Yet when mankind began using them, even the "low-hanging fruit" was there, ready for the plucking. If a society in your time period had passed through a fossil fuel using stage, and they WOULD have, the low-hanging fruit would have been used, along with much, much more.

Yes, tectonic activity could have moved some not-then-low-hanging fruit to be so, but not on the scale we observed when plucking it ourselves. Nor would it replace the used materials.

No society would have gone from a few fireplaces straight to nuclear plants powering a world, nor geothermal ones or wind powered ones. And in further (logical) proof of that, how would those three sources have generated the warming on the required scale without massive use on a scale certainly greater than ours (certainly so if not also consuming fossil fuels)? Greater than ours because while ANY energy converted from storage to use then becomes part of the planet's heat load, even without greenhouse effects, though a rising temperature from electricity used from geothermal plants, say, has the effect of... rising temperature due to anything, though without the positive feedback element of adding greenhouse gases.

In fact, the use of non-replacing fossil fuels is the reason we need to jump up a level and pretty fast. Without doing so, we will reach a point at which no more are available (doesn't matter if that is the "20 years from now" I've heard since I was 10 for oil supplies, or 2600 AD I keep seeing for natural gas, or 3846 AD: the day cometh if their use is not replaced).

When that happens, a very harsh situation will develop, akin to hunter-gathers feasting on a positive feedback loop of grain and seeing their populations grow, then the grain is gone, largely, for a generation or six, and their populations fall drastically. Aided probably by fighting along the way, but also just by fewer children being supportable.

As we fall from modernity, our chance as a species will then be lost forever. Forever. Because probably no one will ever make the jump from burning wood and replacing it with wind power then developing nuclear technology (or even geothermal). So we won't "rise" again, but rather just fall away, though probably getting our 5-15 million more years before the species dies out.

If there had been such a civilization as posited in the question, we'd've run up against a wall (in Europe) hundreds of years ago when we began to actually see deforestation with no little story for ourselves that it was to increase farmland.

So WE wouldn't exist as we clearly actually do. Ergo, there was NO such civilization then, or 10,000 years BC, or 20,000 years BC, or whatever quack garbage is spewed forth by people on the fringe. When Edgar Cayce is your main scientist, you have problems.

So no, such a civilization did not exist. They did not fall from grace, nor did they reach to the stars and meet Admiral Janeway. If they had, we would not be here, and we are.

It is also a sobering thing to think about instead of worrying about our beachfront homes in Colorado and South Dakota being overrun by water someday. Decent chance the lack of alien contact (via radio, not starships) could be due to this degeneration occurring on other planets as well, if they even had a suitably long period of time between trees, bushes, plants, ferns, etc. and the evolution of bacteria that could break down their fibers. Planets with user a species that (like us) did not seem on a path to solving the energy source jump (fusion, really) before they started down the energy/civilization slope and reached a point at which they could never have another chance. Planets that (hopefully, unlike us!) that did not see the user species solve, or even seriously consider the problem itself, and tanked, and so never reached (via radio, not starships... it will NEVER be starships... we are in this system and alone, forever: well, as long as we last) any other star's planetary system's people. No more paradox, eh?

No, it could not have given the evidence of us and the ease with which incredible amounts of oil, coal, and gas were easy to obtain in the not so long ago past. Some amount, even moderate amounts could have been tectonic forces at work, but even today, some of all of them are still very easy to obtain. So no.

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    $\begingroup$ Fossil fuels are convenient, but hardly paramount for success. We were burning coal in steam engines long before the ICE took over, and even had electric cars in the first half of the 19th century. Renewable energy, in the form of trees, exists all around us. Hydrogen as a fuel isn't hard to produce to power ICEs, and fuel cells are becoming pretty good. The future without fossil fuels isn't automatically doomed. $\endgroup$
    – MichaelS
    Jan 26 at 0:15
  • $\begingroup$ Also, don't forget farmable resources like whale oil that could potentially be used in the place of diesel, like the Dishonored video games portray. $\endgroup$
    – MichaelS
    Jan 26 at 0:17

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