How can we know how old fossils are?

[Anomalocaris]

The I think organic materials attributed to the original animal have also been found in dinosaur egg shell fossil, and in impressions of primitive feathers on dinosaurs.  These have provided direct indications of  the colors of dinosaur egg shells and plumage.

As extraction, preservation and probing techniques improve, and as experience have prompted paleontologist to look carefully for these things, I suspect more and more original organic material from the original organism preserved along with mineralized fossils will be found, and these will provide vastly more complete picture of paleobiology, paleoecology and biochemical aspect of evolution than would have been deemed even theoretically attainable just 10-15 years ago.

[Rhondazvous]

Rifht. It's used for dating the fossils (organic marerial) in rock? The next question is what is meant by decay? I'd think this is different from the physical wear and tear of water on rocks. I think I'll leave t hat question to those who know more than I do.

Decay, in this context, means that the nuclei of some of the atoms are unstable and will spontaneously change into another type of atom while emitting an alpha particle (two neutrons and two protons) or an electron.

This spontaneous change happens at a regular rate that is determined by the characteristics of the nucleus.

But all nuclei are surrounded by a cloud of electrons that, at ordinary temperatures (say, below that of the surface of the sun) shield the nucleus from outside influences. This is helped by the fact that the nucleus is *very* small. If an atom were as big across as a football field, the nucleus would be smaller than a tenth of an inch across. So those electrons are a pretty good shield.

Physical wear and tear removes whole atoms but leaves the nuclei the same. Similarly, chemical reactions affect the electrons but not the nucleus.

That is why radioactive dating can be done: to influence the rate of decay of the nucleus takes very high temperatures of other conditions that will kill off any living thing. Such conditions haven't existed on the Earth since it was formed (yes, they would have left traces if they had occurred).

So, to date a rock requires knowing the radioactive nuclei that were in the rock originally (this can be determined by the crystal structure) and what sort of atoms the change is into (the daughter nuclei). it is then, basically a count of how many daughter nuclei there are and how many 'parent' nuclei have decayed. Since the rate of decay is known (it can be measured today), the age is then straightforward to determine.

Are there technical issues to resolve? Of course. For example, carbon-14 has a relatively fast rate of decay, so it can only be used for fairly recent things (the last 50,000 years or so---which can be extended to 100,000 years with a loss of accuracy). So no dinosaur bones will be dated by C14 dating: all you will pick up is the background amount of C14 and not that from the dinos. Also, things that live in the deep sea tend to have already decayed carbon around, so cannot be dated by this method.

As another example, in Potassium-Argon dating, the daughter nucleus is a gas (argon). If the rock is too porous, this gas can leak out and give an incorrectly *young* age for the rock.

I was beginning to understand until you mentioned daughter nuclei. That sounds like mitosis. but how do non biotic atoms have daughter nuclei?

[Anomalocaris]

Daughter nuclei are just any fragments of a larger nuclei that broke apart as a result of spontaneous radioactive decay, and happen to contain at least one proton, so it would qualify as a full fledged atomic nuclei itself.

[Peebothuhlu]

So... when two nuclei fission themselves in a very special way...





Not at work.

[The Valkyrie]

Old people?

Cut them in half and count the growth rings.

Oh, not that kind of fossil?

[Anomalocaris]

Old people?

Cut them in half and count the growth rings.

Oh, not that kind of fossil?

What if they stopped putting on new rings years ago?

[The Valkyrie]

Old people?

Cut them in half and count the growth rings.

Oh, not that kind of fossil?

What if they stopped putting on new rings years ago?

Then they're dead and don't matter.

[Gawdzilla Sama]

Old people?

Cut them in half and count the growth rings.

Oh, not that kind of fossil?

[polymath257]

Decay, in this context, means that the nuclei of some of the atoms are unstable and will spontaneously change into another type of atom while emitting an alpha particle (two neutrons and two protons) or an electron.

This spontaneous change happens at a regular rate that is determined by the characteristics of the nucleus.

But all nuclei are surrounded by a cloud of electrons that, at ordinary temperatures (say, below that of the surface of the sun) shield the nucleus from outside influences. This is helped by the fact that the nucleus is *very* small. If an atom were as big across as a football field, the nucleus would be smaller than a tenth of an inch across. So those electrons are a pretty good shield.

Physical wear and tear removes whole atoms but leaves the nuclei the same. Similarly, chemical reactions affect the electrons but not the nucleus.

That is why radioactive dating can be done: to influence the rate of decay of the nucleus takes very high temperatures of other conditions that will kill off any living thing. Such conditions haven't existed on the Earth since it was formed (yes, they would have left traces if they had occurred).

So, to date a rock requires knowing the radioactive nuclei that were in the rock originally (this can be determined by the crystal structure) and what sort of atoms the change is into (the daughter nuclei). it is then, basically a count of how many daughter nuclei there are and how many 'parent' nuclei have decayed. Since the rate of decay is known (it can be measured today), the age is then straightforward to determine.

Are there technical issues to resolve? Of course. For example, carbon-14 has a relatively fast rate of decay, so it can only be used for fairly recent things (the last 50,000 years or so---which can be extended to 100,000 years with a loss of accuracy). So no dinosaur bones will be dated by C14 dating: all you will pick up is the background amount of C14 and not that from the dinos. Also, things that live in the deep sea tend to have already decayed carbon around, so cannot be dated by this method.

As another example, in Potassium-Argon dating, the daughter nucleus is a gas (argon). If the rock is too porous, this gas can leak out and give an incorrectly *young* age for the rock.

I was beginning to understand until you mentioned daughter nuclei. That sounds like mitosis. but how do non biotic atoms have daughter nuclei?

When a nucleus is unstable and changes into another type of nucleus, we say the original one is the parent and the one produced is the daughter. It's just terminology that is common for the subject.

I can go into a bit more detail. Nuclei are made of two types of particles: protons and neutrons. Protons are positively charged, neutrons are neutral and electrons are negatively charged. In general the number of electrons for an atom is the same as the number of protons. The chemical properties of an atom are determined by the number of electrons and thereby the number of protons. So, for example, carbon atoms all have 6 protons and 6 electrons. Iron atoms all have 26 protons and 26 electrons. All atoms that have the same number of protons are said to be the same 'element'.

But the number of neutrons in the nucleus can be different from one atom, even for the same element. So, Carbon-12 has 6 neutrons in addition to the 6 protons in its nucleus. But carbon-14 has 8 neutrons in addition to the 6 protons. Similarly, Uranium-238 has 92 protons and 146 neutrons, while Uranium-235 has 92 protons and 143 neutrons.

Whether a nucleus is stable or not depends on how many protons and neutrons are in it. In essence, the protons, being all positively charged, repel each other bu the neutrons act as a type of glue. But the neutrons themselves can be unstable.

So, when a nucleus is unstable, that just means that it can change into another type of nucleus (the daughter nucleus) in some way. One way is to emit an alpha particle, which is just a helium-4 nucleus (2 protons and 2 neutrons). So, when the Uranium-238  nucleus does this, it goes from 92 protons and 146 neutrons down to 90 protons and 144 neutrons, thereby becoming a Thorium-234 nucleus.

Another way that nuclei can change (decay) is to have a neutron change into a proton while emitting an electron. This is what happens with Carbon-14. Here, the nucleus originally has 6 protons and 8 neutrons. When one of the neutrons changes into a proton, the result has 7 protons and 7 neutrons, which is a nucleus of Nitrogen-14.

And, of course, some nuclei are stable. Carbon-12 is stable as is Nitrogen-14.

In all of these cases, the *rate* at which the parent nucleus changes into the daughter nucleus can be measured. Furthermore, we know that such things as pressure, temperature, chemical environment, weather, etc do NOT affect this rate. It is different for each type of nucleus, but is not affected by things outside the nucleus, primarily because there is a cloud of electrons surrounding the nucleus that acts as a buffer. The energies required to affect the nucleus are *very* high, corresponding to those of nuclear bombs. This doesn't happen in ordinary or even extraordinary environments on Earth.

We can go into more specifics if you wish. The field is well studied and not too difficult to learn about.

[Anomalocaris]

What if they stopped putting on new rings years ago?

Then they're dead and don't matter.

Speaking rings, tree rings provide both a independent confirmation of the general accuracy of straight (as in unsophisticated) C14 dating, as well as an reliable bench mark for fine scale calibration for sophisticated C14 dating that corrects for the fact that atmospheric ratio of C14 and C12 has fluctuated slightly over time, so the age inferred from assumption that the initial C14 ratio in organisms over history had always been the same as they are now would actually drift slightly from their true age.

By matching rings in remains of successive generations of dead trees, it is possible to provide an exact calibration for C14 dating back to more than 12,000 years.