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. 350
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. 302