[kevinkeller]

If PETM was due to large scale use of hydrocarbons, there should be evidence of depletion in the strata, right? For example, currently not all sources are uniformly exploited -- some coal seams and oilfields are all but depleted, others are currently being extracted, and others are yet to be found/exploited.

We should have seen signs of similar non-uniform usage in the strata from before that time period. I wonder if any research has been done on this.

[kergonath]

> If PETM was due to large scale use of hydrocarbons, there should be evidence of depletion in the strata, right?

And all the byproducts of this use. We are leaving abundant traces of our existence basically everywhere. We can see it in the ground, in the seas, in the ice caps. Our existence will be very obvious to anything caring to just dig for quite a long time.

[Terr_]

Just because traces are widespread doesn't mean they'll last for 60 million years.

For example, we don't have any ice core data older than ~5 million years. (Much lower, if you limit it to continuous spans.)

[kergonath]

The content of the atmosphere gets washed out by rainfall and does end up in the sedimentary record. Similarly, we can follow the oceans’ acidity from sediments. Large chemical changes would also show up in isotope ratios.

[jjk166]

The ice caps are only about 30 million years old, and the land and oceans are similarly transformed over long time scales. The only techno signature I'm aware of that would be clearly detectable 50 million years after our civilization is gone is reinforced concrete, which should produce a worldwide layer of a limestone-like mineral with iron inclusions that would be both unique in the geologic record and difficult to explain by any natural process. If a civilization did not use reinforced concrete nor some other such material that would leave a similar signature, it would be quite difficult to detect at that timescale.

[lovecg]

We’re talking about 50 million years. Literal continents move in that amount of time.

[kergonath]

We get bones and feathers from animals living 10 million years before that. We can find the traces from the Chicxulub impact across the whole world. Same for large volcanic eruptions. Burning enough stuff to cause this kind of global warming would leave plenty of chemical traces. It’s just not serious.

[jjk166]

I'm not sure if that's a safe assumption. Do we know what the sources we are exploiting right now are going to look like in tens of millions of years? Our models of the evolution of hydrocarbon reserves are based on those we've found, and we simply assume the differences between different deposits are natural in origin. It's easily possible that places where we didn't find hydrocarbons did in fact have them at a previous point in time, or that there was more in a given reserve in the past.