Please Explain This

[Scylla]

You said there are rounded stones in the matrix? That is consistent with a sedimentary origin. The radioisotope dating of anything can be confounded by various sources of error. I think you have found a petrified log in a sedimentary deposit. Congrats! As for the age, you cannot carbon date something 40 million years old since the radioactive carbon has all decayed away making measurement of a ratio crazily difficult, but if a porous segment of rock was infiltrated by carbon rich groundwater, that carbon might give an erroneous result closer to the time of the groundwater gaining the carbon.

[Duppa]

Yes that is right. OK wow, thanks for that info, much appreciated. It just made me a bit skeptical of dating techniques, but also understand the further you try and go back, the more unreliable the results will be. 

[Rockwood]

23 minutes ago, Duppa said:

Do shale clasts sound like wood when you tap it? I don't even know what they are. 

It would likely sound slightly more like modern wood than petrified wood would.

[Rockwood]

34 minutes ago, Duppa said:

I don't even know what they are. 

On a geologic scale these people are making puny number of shale clasts.

[FossilDAWG]

Different radioactive elements decay at different rates.  The half-life for C14 (the time for half the original C14 to decay into nitrogen-14) is only 5,730 years.  Using very sensitive mass spectrometers helps, but after ~10 half-lives there is so little C14 left that the accuracy of measuring it is poor, so the effective limit to C14 dating is ~50,000 years. 

 

Other elements/isotopes have much longer half lives and so they are useful on different time scales.  Uranium-238 decays to uranium-234 at a rate that makes it useful to date samples between 100,000 and 1.2 million years.  Uranium-238 will decay all the way to lead (Pb-206) with a half-life of 4.47 billion years.  A different isotope of uranium, U-235, decays to Pb-207 with a half life of 710 million years.  These time scales make uranium decay useful for measuring geological events, including the oldest rocks found on the Earth or the moon, and so allow us to determine the minimum age of the Earth, the moon, etc.  The best material for dating a geological formation is zircon crystals, which can be found in lots of volcanic deposits.  As zircon are crystals forming, they strongly exclude lead but not uranium, which means at the time they form they can contain U-238 and U-235 but no Pb (lead).  As the uranium decays to lead, the lead remains trapped in the already-formed crystal.  This means one can use the ratio of U-238 to Pb-206 to determine how many half-lives of U-238 have passed since the crystal formed, which gives of the age of the volcanic event that formed the zircon crystal.  At the same time, the ratio of U-235 to Pb-207 gives a second independent measurement.  Measuring multiple radioactive decay series  form the same sample is a form of replication that allows us to estimate the age of a sample such as a zircon crystal with remarkable precision.

 

Of course, the nature of the sample is critical to consider.  Zircon crystals (we are usually talking about very small crystals here) can be eroded from a layer of volcanic ash or lava and redeposited in a much younger layer.  The age of the zircon will reflect the original volcanic event that created the zircon crystals, not the age of the rock they are re-deposited in.  In the case of a conglomerate, the clasts (the chunks of rock in the conglomerate) will always be older than the conglomerate itself.  That is why, when dating a rock formation, geologists will look for layers of volcanic ash (such as bentonite) or lava both below and above the layer of interest.  That way you can say your layer is older x (the ash layer above) but younger that y (the ash layer below). 

 

Don

[Duppa]

Awesome stuff Don, thanks for taking the time to share this. One question I have is: can we be sure the decay is at a consistent rate over millions of years and is not influenced by environmental factors?

I'm only trying to get my head around it using my normal thought process (I question most things that are put to me haha) 

Thanks again

[Duppa]

Rockwood. Yes it does sound like modern wood, that is what had me so puzzled. Cool picture, I've learned heaps already and it's cool I can be on here nightshift when people overseas are awake

[Arlie]

I’m not an expert on carbon dating but from my understanding there are lots of things that can effect carbon dating readings. For one it works best on living things that get there carbon from the air and not secondary means. Animals that live in the ocean get most of there carbon from other animals and such so you will get reading in the thousands of years from newly deceased things. Seals, penguins, mollusk, fish and so on will all have funky dates since the carbon 14 has already started to decay when get there carbon. 
Also things like being close to radioactive materials will through off readings. Like I said I’m not an expert but believe I’m recalling info I have read correctly please someone correct me if I’m misremembering 

[Duppa]

OK cool thanks. I read the article on the nearby discovery but must admit I only understand part of it as I have never studied anything like that. 

I just don't understand how their dates could be out by a factor of 1000? Human error has been suggested and that could well be the answer. 

[Rockwood]

48 minutes ago, Duppa said:

Rockwood. Yes it does sound like modern wood

Up close the shale I pictured would be a poor example, but some shale has an almost page like nature due to many of it's minerals crystals being plate shaped. As the substance weathers it would tend to sort of fray leaving a surface that would deaden the sound of a tap.

[Mahnmut]

concerning the influences on radioactive decay:

As best as modern physics knows, radioactive decay is not influenced significantly by the environment. (if we are not talking about nuclear reactors and chain reactions)

What we don´t know for sure is how much radioactive carbon there was to begin with (I think that is less of a problem with other isotopic dating methods, because for example uranium doesn´t come from the air as carbon does when it gets incorporatedin plant material.)

So with C14 there is a lot of ongoing calibration and recalibration using samples whos age is known through other methods. that can lead to an artifact dated at 1100 years being 1200 years old after all, but it doesn´t make the difference between 40.000 and 40.000.000.

Best Regards,

J

 

 

[FossilDAWG]

It's important to be precise in how one answers things.  Radioactive decay involves one of the fundamental forces, the "weak" force that helps to hold atomic nuclei together.  (The other fundamental forces are the "strong" force binding sub-atomic particles, electromagnetism, and gravity).   Many many experiments to test the rate of radioactive decay has failed to show any effect of temperature or pressure.  So no, environmental conditions have no effect on the half-life of any radioactive isotope, and therefore no effect on the age estimate.  Of course in a nuclear reactor (or bomb) the uranium (or plutonium) is highly purified, which means that highly energetic particles released from one decaying nucleus can collide with and "split" adjacent nuclei in a fission reaction, which is very different from spontaneous radioactive decay.  In nature the concentration of radioactive nuclei is far too dilute for fission to contribute significantly to the decay, and the daughter products are different which would be seen/measured if it were to happen.

 

The amount of a radioisotope left after a period of time will be dependent on how much was there to start with, and how much time (how many half-lives) have elapsed.  In the case of uranium series decay, especially in zircon crystals, we can know how much uranium (U) was there to begin with as the decayed uranium is still there, as lead (Pb).  So U-238 + Pb-206 = starting amount of U-238, and U-235 + Pb-207 = starting U-235.  The amount of each element/isotope is determined by ultra-sensitive mass spectrometry which can count even small numbers of atoms.  It is not determined by measuring radioactivity with a geiger counter or scintillation counter, which would be far too imprecise and insensitive.

 

C-14 is a bit of a different animal, that requires more care to interpret.  C-14 is produced in the atmosphere by cosmic rays impacting nitrogen atoms.  So, the amount of C-14 in air can vary within smallish limits, depending on the frequency of cosmic ray generating events such as solar storms, and the strength of the Earth's magnetic field.  More recently open-air testing of nuclear weapons added some C-14 to the atmosphere.  Plants will absorb CO2, and so some amount of C-14, and incorporate the C into their leaves, wood, etc.  Herbivores that eat the plants will pick up this C-14 and incorporate it in their tissues, and similarly carnivores will pick up C-14 from their herbivorous prey.  In plants, the concentration of C-14 will be a direct reflection of the concentration of C-14 in atmospheric CO2, and this will carry through to herbivores and carnivores.  This accumulation of C-14 occurs throughout the lifetime of the plant/animal, but after death C-14 will decay but no new C-14 is added. 

 

Because the concentration of C-14 in the atmosphere can vary a small amount, there can be a small discrepancy between the actual starting C-14 amount and the amount one would estimate based on average atmospheric C-14 concentrations, and this discrepancy could create a small amount of either extra or missing C-14 in the age calculation.  To correct this, scientists have measured the C-14 level in samples of precisely known age and compared that to the long-term average.  For example, dendrochronology is important in this regard.  Trees deposit rings in their trunk tissue (in temperate ecosystems at least), and everyone knows you can determine the age of a tree by counting rings.  However the rings are not all uniform, they will be thinner in drought years and thicker in wet years, they may record fires (a burned ring), insect outbreaks etc.  A single very old tree may provide a ring record going back a few hundred years, or some species may live for many hundreds of years.  Older wood, such as wooden beams, of known age, may be obtained from archeological sites.  The ring pattern (normal, thick, thin, or burnt) can be compared to the modern tree and if the patterns overlap the ring record can be extended further back in time.  In some areas this has been done to extend the tree ring record back ~ 10,000 years.  (A similar thing has been done with spelothems, stalactites and stalagmites in caves, going back over 40,000 years.)  Once you can determine by this method exactly what the age is of each ring on several wood samples, you can take a sample of the ring, determine the C-14 count, and compare the observed C-14 with what you would expect based on average C-14 and the age of the ring. This allows researchers to correct for the natural variation in atmospheric C-14, and so make a more precise estimate of the age of non-tree-ring samples. 

 

Every scientific measurement should include an estimate of possible error.  For example, a C-14 based age estimate could be presented as 2,350 +/- 85 years.  This means there is a 95% chance that the actual age falls between 2,265 and 2,435 years.  The error will include the sensitivity of the machine you are using to count C-14 atoms in your sample, possible variation in estimated starting C-14 concentration, and anything else you can think of that might affect the calculation of number of half-lives.  Some people will say "well if it could be 2,265 or 2,435 then you don't really know, you are just guessing, it could be 100 years or 50,000."  This is just garbage, when people say that they are revealing that they have no idea how science actually works. 

 

Don

[Duppa]

OK cool, man I'm leaning a lot already! I just had in mind volcanic eruptions (many in the area at some point), meteorite strikes, being submerged in seawater, changes in the earth's magnetic fields, changes in the atmosphere.. That type of thing. 

I'm not trying to push any sort of ideology or agenda, just thinking of all possible events that could have happened long before we were here

[Duppa]

So the whole starting point is a bit of an assumption then? If there is no real way of telling what the sample started with when it was first produced? 

[Duppa]

It's just a whole lot of new information to digest, forgive me if I don't understand straight away. I'll take your advice and do some more research. 

Thanks for your time

[FossilDAWG]

To clarify the C-14 story a bit:

Plants (and animals that directly or indirectly eat plants) take up C-14 from the air, so atmospheric C-14 concentrations give a good estimate of C-14 in plants/animals at the time that they died.  C-14 decays to nitrogen at a rate that has half the C-14 gone in 5,370 years, half of what remains in the next 5,370 years, half of what remains of that in the next 5,370 years, and so on.  We can express that in a table"

year from death       % C-14 remaining

0                              100

5,370                         50

10,740                       25

16,110                       12.5  and so on

 

We can use the concentration of C-14 in modern organisms to get an idea of what the time = 0 content would have been, since atmospheric C-14 levels don't vary too much.  If a sample has lost 75% of the C-14 it should have, it is ~ 10,700 years old.

 

If we want to be even more precise, we can analyze samples of precisely known age and make a graph of age against C-14 content.  Such samples might be wood from specific tree rings for example.  Then we can make a graph of C-14 content against age.  This will be very similar to the first graph, but individual points might be slightly higher or lower as the rate of C-14 formation (and so concentration in the atmosphere) could be a tiny bit higher or lower depending on solar storms and such.  Then we can measure C-14 in our unknown sample, compare the amount of C-14 to the graph, and interpolate the age.  This will be incrementally more accurate than the first approach.  The actual difference (in estimated age) will be greater the older the sample is, as year-to-year variation will accumulate over time.

 

In no case is the starting conditions a random guess, or totally unknown.  The starting C-14 concentration can be estimated with a high degree of precision.

 

When it comes to uranium series dating we can be even more precise because both the starting and the ending elements can be measured, so we can measure precisely how much uranium has decayed into lead.  Also, U-238 and U-235 decay at different rates and give different products (Pb-206 and Pb-207) so we have two different clocks running in the same sample at the same time.  Both should (and do) give the same estimate of age.  There are other radioactive isotopes as well that have different half-lives again, and these provide even more independent clocks.

 

There is a reason atomic clocks are used for extremely precise time measurements!

 

Don

[Duppa]

OK cheers Don, that was really well explained. I'm starting to get it haha 

[Scylla]

10 hours ago, Duppa said:

Yes that is right. OK wow, thanks for that info, much appreciated. It just made me a bit skeptical of dating techniques, but also understand the further you try and go back, the more unreliable the results will be. 

That is generally correct

[Scylla]

4 hours ago, FossilDAWG said:

 

 

There is a reason atomic clocks are used for extremely precise time measurements!

 

Don

I know you were joking about atomic clocks, but radioactive decay has nothing to do with why they are so accurate.

 

Other sources are used to calibrate carbon dating as well. For example, ice cores can show banding by season like tree rings so a very precise record of years can be traced back hundreds of thousands of years. Bubbles in the ice can be sampled to reflect atmospheric ratios from those times. Cave formations as mentioned above. Lake sediments can have yearly layers too so the carbon in the sediments can give a clue about variation in C14 level in the past. 

 

The only things I know of that can alter radioactive decay rates are things like traveling near the speed of light, approaching a mass like a black hole, or heavy neutron bombardment. None of those are likely to have happened anywhere on earth except inside reactors and nuclear bomb sites.

[Duppa]

I was actually thinking about ice core samples as a way of looking back in time. That makes a lot of sense, being able to analyse what the atmosphere was like year by year. I have heard scientific evidence has been found for increasing carbon levels from the beginning of the industrial revolution. It also makes sense to have a reference to compare dating techniques to. 

[grandpa]

Kudos to @FossilDAWG and @Scylla for giving us a carefully detailed and well-explained synopsis of how geologists determine geological time of strata and age of fossils.  I think I once knew most/all of that, but Time has dulled the edge of my detailed recall on this subject as it is not something that I use daily.  So thanks much for the refresh.  (Took a Historical Geology class in 1990 where I was exposed to this.  Wow, that's 30 years ago now.  Seems more like 10 yrs.  What does that say?)

 

Being the generalist that I am, the main point that I take away from this discussion is that Science approaches any question with an idea of what the answer might be (hypothesis, or even just a guess), tests that idea (experimentation), validates or revises the idea of the answer to the question based on the results of the tests and tests again until the question is fully understood and the answer is fully tested (ideally).  I find that a robust approach for separating fact from supposition and giving us answers to all, from our most basic to our most sublime, questions about "truths" that are "testable". 

 

[N.B. This might be a good topic for those in the know and most up to date on the subject to compile this info and make it a pinned document in the e.g., Documents section of TFF for future reference when this question comes up again, as I assume it may well, along with the already pinned Geologic Time Scale.  Just a thought]

[Duppa]

Yes I do remember the whole testing a hypothesis in highschool... That was a long long time ago haha. 

This is all good and well, but going back to the study mentioned at the start of the thread. How did they end up with such a large disparity for two objects that had obviously combined in the same event? 

Everything on earth would inherently be the same age, just re arranged during catastrophic events ect and break down into different elements over time. I know things are made up of microscopic "building blocks" that do not disappear, just change into different forms or elements. Would you say that its accurate? 

[grandpa]

So, what is not being said (I think) is the approach to the problem in one case (the pictured article) is (IMHO) based on a presumption of fact/truth before any evidence, such that all subsequent evidence must be forced into a pattern to support the presumption.  Science (in purity and as tested by peer review) is impartial.  It does not presume to know what is true before the testing and then twist the testing to support the predetermined "truth".  If the evidence does not support the hypothesis, the hypothesis must be changed, not the evidence contorted to support the hypothesis.

 

[doushantuo]

i very much recommend reading this:

 

 

or this: a a good review of detrital zircon geochronology methodological issues

 

 

 

 

[minnbuckeye]

This has been an interesting discussion, from which I learned much. But I have a SIMPLE question about the last picture. I notice that the white objects in the matrix are generally round or oval but the brown objects (lower right has quite a few) are sharply triangular. Does this help explain anything???

 

Mike