I want my novel's protagonist to leave a diary on the dying planet Earth for some far-in-the-future being to read.
He wants to leave some kind of marker/clue (or perhaps timepiece) for this future being to figure out when he, the last Earthling, left the planet.
Can someone suggest something suitable to leave? Carbon dating will not work as this future being might not visit the Earth for another 50,000 years.
Location, location, location
This is fairly easy you need a giant stone building in a dry tectonically inactive area, which carved and possibly inlaid writing in stone on the inside. Out of all humans structures these last the second longest, the longest lasting are carved directly into the stone, but those are also hard to find especially from orbit or aircraft. Clay is good, stone is better, inlaid stone is best, granite inlaid with gold is about as indestructible as you can get on earth. It is also fairly easy to make; carve stone, pour on liquid gold, rinse, repeat. writing no matter what it is needs to be protected, even paint lasts tens of thousands of years in cave, inlaid stone can easily last what you need, you just need to build an artificial cave. Note another good question would be what kind of "rosetta stone" to leave behind so they will understand what you wrote.
A massive block of stacked stone is the best protection against erosion. A dry climate will also keep erosion to a minimum. the pyramids are not bad but their location is not great, it is a little to erosion/burial prone, a colder climate may be better. If the outer cover of the pyramids had not been stripped off by people they would be far more resistant, and would not be as bad a shape as they are in But the location is the most important part, some place with minimal erosion and burial, non-sandy deserts for preference, egypt has too much shifting sand and deposition. the Nasca desert would be a great pick, although if you can find a less geologically active place even better. Honestly a question just about location might not be bad.
Dating it is really easy he just needs to leave several blocks (like inch cubes) of different radioactive material with the original ratios of parent and daughter material written on them or their stone container. carbon 14, potassium, and uranium will be good choices, plutonium is even better. If you can have a few blocks of uranium glass in a different area of the structure that will allow fission track dating as well. the nice thing about dating methods are the more you have the smaller your error bars get.
The hardest part is getting spotted, first it need to be an unnatural shape, like a triangle or square (another reason pyramids are good). if you can have consecutive shapes even better. three structures laid out in a triangle are even better they get noticed. Alternatively dig a giant triangular hole and fill it with concrete, that will stick out like a sore thumb on regular and gravity maps. Put your structure right on top. keep in mind structures don't have to be that big to be seen from space, many large buildings are visible with only minor magnification. If you can put it in geologically weird location even better, like right in the middle of the painted desert, or ayers rock, or large flat expanse so it stands out.
At present, humanity has the technology to store data for up to 13bn years using lasers to etch data into glass. So, with moderate technology, the protagonist could leave a significant amount of data in a relatively small space. Currently the discs are about the size of your thumbnail but this could be stored 3-dimensionally in cubes, spheres or other shapes. So that covers the storage medium.
One problem, as other answers have mentioned, is ensuring that the data is readable by whomsoever decodes it, so the protagonist would likely need to leave behind a 'Rosetta stone' / dictionary as part of the cache to help whomsoever discovers the data decode it.
In terms of dating the diary. Radioisotopes are a good shout but including the position of the stars, planets, moons, processions of earth etc might be an alternative over long periods of time and give the data reader a more deliberate method of tracking time.
Lastly, Earth is a highly volatile place, especially over longer periods and epochs. Even well away from tectonically active areas there's still the risk of glaciations, massive storms, flooding, erosion, interference by lifeforms, deposition etc.
The safest place (IMO) to store the data would probably not be on Earth itself but perhaps on the moon as he / she leaves Earth. Getting the data noticed out there would be difficult (radio beacons would eventually fail / lose power) but it would be more likely to survive for a longer period of time if shielded from any potential meteor impacts.
Your protagonist could maybe leave a radioisotope "trail of breadcrumbs" or even an etching on the moons surface, erosion is far less of an issue up there...
