# Have we looked for human-like signals in the geological record?

This is somewhat of a follow up to some interesting discussion here about using synthetic radioisotopes to define the start of the anthropocene. It is also sort of related to a question asked here, but with some additional thought put into it that I hope merits its own response. My question is simply:

Has there been any serious thought put into the search for ancient industrialised civilisations that would generate a signal comparable to human civilisation in the sediment record?

As a complete layman, it seems at least possible that paleontological (archaeological?) record may be incomplete enough that we have completely missed or misinterpreted evidence of an ancient civilisation. I would imagine that not a lot of human structures/artifacts would survive weathering over millions of years. It would stand to reason that there would not be a whole lot left from this hypothetical ancient civilisation.

Given the discussion in the question I linked above, what would last a long time would be evidence of our atmospheric nuclear explosions/accidents. One could imagine that isotopes that are not normally naturally present, such as long lived fission products like I-129, unused nuclear fuel like U-235, or their decay products such as U-236 from the alpha decay of Pu-240 could be present in a thin layer in the sediment record. In addition, one could also look for a spike of naturally present isotopes that would result from the incorporation of radioisotopes by living organisms (for example spikes in Zr-90 from the incorporation of Sr-90 replacing calcium in foraminifera). Given the types of isotopes present, one might even be able to guess at their origin (e.g. nuclear detonation, reactor accident, or breach of waste storage)

Moving away from the radioisotopes, this hypothetical civilisation might also have used fossil fuels, resulting in global warming similar to what we see today. Intriguingly, from my reading it seems that there are events that produced abrupt warming (e.g. Paleocene–Eocene Thermal Maximum) whose cause and/or trigger is poorly understood, though I won't speculate too much about this as it's not my area.

I think there are probably other signals that one could look for, for example evidence of industrial pollution or agriculture. However, I think these might be trickier to see than the radioisotope signal, as they are chemical and therefore might be difficult to distinguish from a sudden natural process. Nevertheless, it seems like the combination several of these types of signals with an abrupt onset and end in the sediment records would be pretty convincing evidence for an ancient civilisation. Perhaps more exciting, it seems like these could be tested with existing methods or even existing samples.

I don't want to come off as a total crackpot, but I hope that this has shown why this seem like an interesting and accessible line of inquiry to a non-geologist, even if I don't think the hypothesis of an ancient civilisation is particularly likely. What I will jokingly call SATI (Search for Ancient Terrestial Intelligence) seems to me equally significant to the much more well known SETI, and is much more likely to give some kind of result, whether positive or negative. Genuinely curious if this sort of thing has been seriously thought about and researched, or if there is something that I'm not understanding that would make this not as clear-cut as I'm making it out to be.

• This completely ignores earth history, evolution and any of the tags applied to the question. No serious geoscientist ponders about these things because they are right away devious. There's a whole discipline called paleontology that researches the evolution of life on earth, and it has painted a comprehensive picture, discussed, published in books for the genral public, reconstructed in paleo-environments, presented in museums all over the world. – user20217 Aug 6 '20 at 1:35
• You are right, i am not an expert in these things, which I is why I am asking. I guess I have always heard that the fossil record is very incomplete, perhaps I am assuming too much about the incompleteness. – phi1123 Aug 6 '20 at 1:42
• Definitely, sorry to be so blunt. Evolution doesn't jump, e.g. from amphibians to an worldwide civilization and back again. – user20217 Aug 6 '20 at 1:52
• It is highly unlikely, that a) this happened and they went extinct, or b) this happened and we found no trace. And Ockham's Razor says "the simplest explanation is most likely the right one" - so sorry, but nope. – Erik Aug 6 '20 at 6:47
• While it's hard to prove a negative, there is lots of evidence that ours is the first industrial society on Earth. Ever. Pre-Renaissance society extracted barely a dent in the resources needed by modern society. The early Industrial Age proceeded rapidly thanks to coal seams that extended to the surface of the Earth and mineral deposits that were very easily accessible. Those easily accessible resources would not have existed had some previous society (human or otherwise) achieved modern industrial status. – David Hammen Aug 6 '20 at 13:17

Any nuclear activity would be visible in the changes of the isotope ratios far longer than the existence of our Universe. Nothing was found until now, except one in Gabon. It was natural phenomenon: at the time, 2billion years ago, there was far more $${}^{235} \rm U$$ and the ground water could moderate uranium ore, if it was enough rich.