So it's a record of the fraction of carbon-14 in some of those years.And you can go down to resolutions of as small as 10 years.Furthermore, different types of plants discriminate differently.This also has to be corrected for. Second, the ratio of C in the atmosphere at that time to be estimated, and so partial calibration of the “clock” is possible.So just to answer the question, it's actually probably in really, the last 50 years where the fossil fuel use has really exploded that we've really been changing the proportion of carbon-14 relative to the other isotopes of carbon.But anyway, hopefully that rests some of your worries about the assumption that I made in the last video about carbon-14 being relatively constant.
So I said maybe it's 5,730 years since this bone was part of a living animal, or it's roughly that old.
Obviously, this works only for things which were once living.
It cannot be used to date volcanic rocks, for example.
And you can look at their tree rings, and I think most of us are familiar with this idea that every year that a tree grows, it forms another layer of bark.
And so you can look back to that layer of bark just for the half life of carbon-14, and then figure out how much carbon-14 was there in the atmosphere at that period in time. Those are those speleothems that are kind of coming out of the bottom of the cave, or stalactites.