I'm working on a story set roughly 2 million years in the future involving future archeologists discovering evidence of a nuclear war and ensuing winter wiping out humanity. What sort of clues would survive for that long?
Ice cores from Antarctica (if the ice is not melted for global warming) or sediments would not only contain traces of the mentioned substances, but indicate exceptionally high levels of ash in the air only to be found in supervolcano eruptions and temperature proxies would indicate very cold climate.
It depends on the technology level of your futuristic society. If they already know about radiation and can detect it, then some radioactive isotopes will still remain. If they do not have advanced technology, then they will have to rely on digging up layers of soot/ash from the nuclear winter, or discovering vitrified remains.
Nuclear fission creates many radioactive elements. Each has it's own lifespan (measured in half-lifes it is constantly radiating ionized parts of itself off (that's why they are called "radioactive") and becoming other elements, some still radioactive and some stable. Only a few of them would still remain after 2 million years:
Generally speaking, after 20 half-lives, radioactive isotopes have only negligible activity.
So, depending on the technology level of your futuristic society, they might be able to detect some of these remaining radioactive isotopes, even tiny trace amounts.
Nuclear winter is produced by large having so much soot in the atmosphere that sunlight cannot warm the earth enough and winter-like conditions set in. Over time, this soot will eventually precipitate out of the atmosphere and end up as layers in the geological record.
However, some of the most obvious clues would be vitrified materials. These could be detected by even a stone age society. However, without knowledge of nuclear heat, they might not understand the significance of vitrification.
Nuclear weapons produce extreme heat in ways not found in any other natural phenomena normally on earth. One such process is vitrification of stone and other materials. There is a current example today--in Scotland, some 60 stone forts supposedly have vitrified walls:
For over 250 years, archaeologists studying ancient Scottish ruins have reported a type of construction said to defy explanation. About sixty of these rough stone wall enclosures have been found throughout Scotland, and even a few scattered across mainland Europe. Most are prehistoric. Called vitrified forts, they're notable for a unique and surprising feature. The rocks that make up the walls were originally stacked dry, with no mortar; but have been fused together into a solid surface through a process called vitrification, the transformation into glass. How can rock be melted into glass using prehistoric technology? Some say that it can't because the temperatures required to do it are far too high, and that the only plausible explanation is an ancient atomic blast.
Regardless of what vitrified these forts, they still offer clues as to what vitrified remains might be like. So, vitrified structures, particularly ones still obviously man-made, would be very significant clues.
The first thing is that archaeologists in the future will know we were here, and that we had some pretty sophisticated tech, because we'll get fossil plastic imprints. Fossil shells are commonplace throughout the geological record. We have enough plastic junk around that some of it will be lost in mud pits. Over years that plastic bottle or whatever will degrade, but it will leave its imprint in the same way as the shell of an ammonite. They probably won't know exactly what it was or what it was made of, but they'll know it wasn't simply metal or wood (because that would corrode too fast).
Evidence of a nuclear war - that's a lot harder. The fallout from a nuclear war isn't likely to last longer than a few decades, and whilst that's probably enough to kill off humanity, it's barely a flicker geologically. The meteorite which killed the dinosaurs (hypothetically; it's still only the best theory so far) may have been the largest planetary disaster so far, but the only evidence we have is a hole in the ground and a thin layer of iridium in the strata. There's ash in the strata too, but there's good evidence for much of that coming from the Deccan Traps. We have absolutely zero direct evidence of any global ecological effects from the meteorite, even though best guess from sophisticated models of impact effects and climate is that it caused a few years of nuclear winter, but that's not direct evidence. All we know is that dinosaurs were alive at some point on one side of these events and dead at some point on the other side, within a margin of tens of thousand years either way. The simple fact is that evidence of global events doesn't survive.
If there's no good evidence on Earth, how's about elsewhere? Satellites and space junk generally are all going to have either fallen to Earth or drifted off, so that's out.
That leaves one good place for evidence - the Moon. We can reasonably assume that our archaeologists are a curious species, and if their tech is hot enough to be looking for evidence of previous civilisations and nuclear war, they've probably made their own first steps into space. In that case it simply isn't possible that they haven't seen the Apollo remnants on the Moon, which will still be there for millions of years.
The Apollo missions won't say anything about a nuclear war, of course. But since this is your world, you could suppose that someone sent a one-way rocket to the Moon containing a decent library and some kind of self-learning language course so they can read it, and carved a big sign on the surface to say "look at this!" This library would naturally include details of how we killed ourselves, partly just for their information, and partly as an example of what not to do as a civilisation. I could see Richard Branson and Elon Musk setting this up together, and a few dozen astronauts heading off on a one-way mission as heroes whose names will be legendary for some future civilisation.
There are millions of bits of dinosaurs, and around 10,000 "full" dinosaur fossils.
Dinosaurs lasted for about 160 million years, and died off about 65 million years ago.
In terms of biomass, I'll assume humans are about as dominant as dinosaurs (all of them together) are. Dinosaurs left 1 artifact every 160 years on the planet (and one "full" specimin every 16000 years).
So a full human skeleton is unlikely to be found by this future civilization. Bits of human skeleton are likely to be found.
I suspect industrial civilization artifacts far outnumber us in volume and count. Buildings are huge, our garbage is everywhere. So I suspect they'll get fossil evidence of the strange thing we build (buildings, cars, plastic bottles).
Both of these would show up around about the same time as our civilization discovered and understood dinosaurs.
Geological Evidence of War
There are two kinds of geological evidence. The first is the direct blast effects of the nukes. Spread all over the world at the same strata will be nuclear-fuzed glass, both from direct blast and from erosion/ejecta.
These will be similar to comet/asteroid impacts that we have geological evidence of.
This layer will also have a large number of strange isotopes. A concentration of moderately unstable isotopes of medium-long length will be spread over the planet, and concentrated in this layer. These won't match anything else in nature really; unlike the dinosaur-killer, whose isotope mix matches other asteroids, it will be a mystery for a while where this mix comes from.
Only when they themselves discover radioactive decay, nuclear transmutation, and nuclear chain reactions/bombs will those isotope mixes start to make more and more sense.
Some larger bombs may even leave craters large enough to last millions of years, depending on what kind of bombs are in fashion at the time. Such lakes would have large amounts of the nuclear glass in them. Connecting said nuclear glass to an ancient intelligent civilization will be tricky.
Geological Evidence of Civilization
Beyond actual artifacts, humans move a lot of dirt. We have reshaped the world. Over 2 million years an average of 100 feet of material is stripped off (and replaced) over the world; humans have, on average, done about 1 to 10 feet over the time we have been on this planet (usually moving it somewhere else). While it may not be visible everywhere, it may be visible some places. A project we do (roadworks, canals, pyramids, etc) that reshapes the landscape may be buried by erosion from elsewhere, and unearthed either by natural processes or by digging by the later civilization.
Recognizing it as being caused by a particular civilization may be challenging, but the layer of ash, dead plant material, and glaciation evidence directly above that strange layer may draw attention to it.
A nuclear war is likely to wipe out cities. Any cities spared by such a war would be a decent concentration of metals, as we have spent centuries mining metals and concentrating them in our cities. Some may be ground to dust by glaciers, with the dust deposits being mined; but somewhat intact cities could draw economic activity, and induce curiosity.
There could also be geologic evidence in the form of shatter cones (https://en.wikipedia.org/wiki/Shatter_cone). They are typically indicators of meteorite impacts but they can also be formed by a nuclear explosion.
Most of the other answers focused on the Archaeology of physical artifacts, but what about the archaeology of humans and society? So for example, your archaeologists themselves came from somewhere. We trace our own human timeline back hundreds of thousands of years and can say things about early humans.
Do people in your future society wonder what their own origins are? What kinds of traditions and mythology do they have? Do they have any "Great Flood/Great Fire/Long Winter" myths, especially ones that have striking similarities in very different cultural contexts? What about regional languages? Are there any commonalities in language that are difficult to explain other than there might have been a global civilization long ago? Obviously there were survivors of the war from eons ago. What kind of stories and rituals did they pass down from generation to generation that have lost their obvious meaning over time but still retain potent symbolism that might point to a great disaster?
None of this would be direct evidence for a nuclear war per se, but combined with some physical artifacts might lead a canny investigator down the right path.
Nuclear fallout will be around for an long, long time
Nuclear fallout (note: "nuclear fallout" usually refers to radioactive fallout. I will be using a slightly different meaning due to the context of the question). will be detectable until earth is destroyed - if geology allows. I am no expert in formation of rock sediments or the spread of nuclear fallout, so I will assume that nuclear fallout will be preserved in relatively clear strata in e.g. sandstone.
If so, any civilization with a relatively basic understanding of nuclear physics will be able to see that there was a big release of fissile material (uranium-235 and plutonium-239), along with products from fission processes.
An atomic bomb works through a chain reaction in which a lot of either uranium-235 or plutonium-239 fusions (there are a few other choices of isotopes which might be technically possible to use, but they seldom are) . However, not all of the bomb material will be spent; a significant fraction will be released as fallout. This means that a bomb will spread isotopes from three different groups:
- Unspent fissile material
- Decay products from fission processes
- Activated material from surroundings (e.g. bomb casing, air, soil)
We can ignore the last category for our purposes here: the other two will provide clearer evidence.
Unspent fissile material
The unspent fissile material will be either uranium-235 or plutonium-239.
Uranium-235 has a half-life of over 700 million years. Thus, virtually all of it will still be around after the two million years in the question. This will be an excellent clue for the archaeologist: not only will it show that someone spread a lot of expensive, highly enriched uranium over the planet, it will also give them a good estimate of when, as you can analyse the amount of decay products from the decay chain to see how long since this took place.
Plutonium-239 has a much shorter half-life, only 24000 years. This means that after two million years, it will be gone. However, it will not have disappeared entirely, but have also turned into uranium-235. This will mess with the time-span analysis of above somewhat, and introduce an uncertainty of about 100 000 years.
Nuclear fission creates a range of different nuclides. In general, these are short-lived and unstable, but they will also generate daughter nuclides that are stable. The archaeologist will thus find a mixture of different isotopes, in proportions that does not occur naturally, in the same layers as the above uranium. This will clearly show that not only has someone spread a lot of uranium-235, but also a lot of fission products.
The theory that would best explain this is an all-out nuclear war:
- Large-scale nuclear disasters are unlikely. They would also release a lot of uranium-238, which would also be detectable. One could postulate a civilization which has reactors that only runs on highly enriched uranium, but that would be an extreme expense without any clear benefit.
- Large-scale deployment of dirty bombs is even less likely, for similar reasons: they would tend to leave a lot of fission products, but there is no reason for there to be a lot of uranium-235 and no uranium-238.
A smart archaeologist (or rather: a team of different specialists) would thus not only be able to figure out that a nuclear war had taken place, but also roughly when it had. This will even be possible as long as there is geological strata generated during the war around to analyse.
Layers of Sediment
These will tell the story. Huge layer of radioactive ash, followed by changes in temperature and a massive extinction event. This would be indicated by an increase in the amount of fossils discovered to come from the exact same period of time followed by no fossils... This includes plants which point to features of climate change when combined with other evidence.
I remember seeing a program where they were talking about evidence of a meteorite hitting the earth showing the same signs as a massive nuclear explosion however without any radioactive isotopes.
I would look up the archaeological evidence surrounding extinction events such as the meteorite from Cretaceous–Paleogene and also Permian–Triassic if you're talking about all life being wiped out.
FYI no expert but I think evidence from Ice would either not go back far enough or would be warped in some way to be nearly useless for >2 million years.
Anything that was already in the ground when it happened
This is a very broad question... but there will be a few very conspicuous things to find.
First there is the question about finding remains of the civilization. Will they do that? Yes, they will... because we have so many underground structures made of things like concrete. Since they are in the ground they will be protected from the initial destruction and have a good chance of withstanding erosion.
They will notice that the fall was very sudden, if nothing else from gravestones and tombs. They will find that the numbers on these burial sits gradually increase and that they become more and more frequent. And then all of a sudden: stop... nothing else. They will be able to pin the event by year and probably even month. It is another thing to match that year to any kind of calendar as they know it though.
They may find traces of the nuclear war but what they will find first is the traces of other ecological disasters. Oil spills, chemical contamination, heavy metal releases, nuclear plant meltdowns. If humans very suddenly go away, and the infrastructure is destroyed by weapons of mass destruction, then these things will happen and make patches of the planet quite inhospitable. That will leave very clear marks, and many chemicals, not to mention elements like heavy metals, have no half-life but exist forever.
As was stated elsewhere... the best place to find a clear time-line of events is most likely the Antarctic ice. There will be sudden layer of all kind of very "foreign" substances that do not exist elsewhere in the layers. Sooth, fission products from the weapon, chemicals of other sorts. And after that, there will be a steady decline as these things are washed out of the atmosphere by precipitation. And after that even the traces of civilization that existed before the calamity will cease to exist in the ice.
How about looking about some of our remains on the moon or mars? We can still make out footsteps in the dust after 50 years. While weather, bacteria and other movements will deteriorate most of what we have on earth, we have quite a few rovers and landers on the moon.
The moon doesn't have volcanoes or big earthquakes, it is also lacking vegetation. There is no water or oxygen to rust away the materials either.
All you have to worry about is small floating dust and radiation from space, and since I am neither a physicist nor an archaeologist I cannot comment on that.
How about the generation after million years is smart enough to design a spacecraft that travels multiple times faster than the light. and they go farther to capture today's light. They can come back at the speed they want and actually see everything that as if it is live. just a thought. If this light capturing sounds interesting you can build the story further on your own.
Take a look at the short story No Connection, by Asimov. Spoilers ahead.
The future Archaeologists just discovered isotope dating and are having a lot of progress. But the results on all artifacts from a few sites are wildly implausible. Off the charts implausible. These sites are also very, very rich in human fossils.
Towards the end, it's very heavily implied that these sites are our nuked cities, and that the dating is being garbled by the residual radiation from the bombs.
A lot of people have mentioned the half-life idea, but nobody has specifically mentioned Cesium 135. It has a half-life of around 2.3 million years and is produced in a fission reaction (I've called it "Humanities longest lasting gift to the Earth" in a previous post). A dust layer with higher concentrations of cesium 135 should be recognizable.
Although not directly related to nuclear war, given the difficulty in finding ANY evidence of a 2million year old civilization I just want to point out the one obvious remenant. Satellites in higher orbits, well clear of our atmosphere, will likely still be in orbit after 2 million years, even if solar wind and micro debris impacts have scoured their surfaces severely. Some of these satellites would be visible to the naked eye.
You would need radioactive isotopes that will last beyond 2,000,000 years. In the list of the half-lives of radioactive isotopes this would be the substances with half-lives around 10^12 seconds since 2 million years is about 6.3072x10^12 seconds. You probably need to find an isotope that's part of the decay chain of whatever radioactive material was used to set off the nuclear war in your story.