Mrs Libby is a physicist.
(laughter and applause).
That explains my presence.
I want to speak about the most recent developments in radiocarbon dating,
emphasising our hopes for the future perfection of this method of determining the lapse of time
in archaeological and historical sites.
The method of radiocarbon dating derives from the production of radiocarbon in the atmosphere by cosmic rays.
As Professor Hofstadter showed you, these high energy particles viciously disintegrate the atmosphere producing neutrons
which are nearly quantitatively converted to radiocarbon by the nitrogen of the atmosphere.
So on the average we have produced about two carbon-14 atoms per second for each square centimetre over the earth’s surface.
And as time passes, this accumulates and the radiocarbon so produced burns in the air
to form carbondioxide, radioactive carbondioxide.
Which of course is mixed with the carbondioxide of the normal type and therefore finds its way into the life cycle,
since all living matter is derived either directly or indirectly from atmospheric carbondioxide.
That is the plants grow by the process of photosynthesis from the atmosphere.
So we have built into our substance, and all living things have built into their substance a natural clock,
set to be triggered at the moment of death.
For throughout the lifetime we constitute a small but I hope very significant part of a giant system
which is continually mixed by the processes of synthesis, photosynthesis and decay after death,
but mainly by the winds and ocean currents.
So that we have a fixed concentration of radiocarbon.
With the decaying atoms being replaced by the radiocarbon contained in the food.
So at the instance of death, when the supply of replacement, that is the eating of food stops,
the supply of radiocarbon proceeds to decrease according to the law of radioactive decay with a half period of 5,730 years.
So we do radiocarbon dating by taking long dead organic matter which once was part of a living system,
such as hair from a body, such as charcoal from an ancient camp fire, such as material in woven fabrics,
anything which was once part of a living system can be used to determine the lapse of time since death,
that is called the radiocarbon age.
Now, the principle, simple as it is, rests on several basic assumptions.
The first of these is that all living matter has always had the same concentration of radiocarbon.
For we do assume, when we take a piece of linen, say from a first dynasty tomb in Egypt,
and find that it has about 60% of the radiocarbon that modern tissue, modern living matter has,
we calculate its age to be say 4,800 years.
We calculate it on the assumption that when that flax was grown, when that linen was young,
it would have had the same concentration of radiocarbon that modern living matter has.
In other words, a basic assumption is that life has always had the same concentration of radiocarbon.
That rests on two further assumptions, namely that the cosmic ray intensity has not changed
and that the volume of carbon in mixing equilibrium with the atmospheric CO2 has not changed.
Of course the concentration is determined by the total amount of radiocarbon and the amount of carbon with which it is mixed.
The concentration which living matter has corresponds to about 15 disintegrations per minute, per gram of carbon in the lifecycle.
Or more importantly in the sea salts because in total amount the carbon contained dissolved in the ocean,
in non living form is about 95 or 96% of the total diluting reservoir.
So we must first say, ask the question, have the oceans changed in volume?
Since the ocean is the main part of the diluting reservoir and assuming that the compositions of the ocean has not changed
in the lifetime of radiocarbon or in a few lifetimes of radiocarbon,
our first question is whether the volume of the ocean has changed.
Now, we know that it hasn’t, and even if it had changed we would probably not appreciably affect the carbon in the reservoir
but merely concentrate the salts to some degree.
However, since the ocean is close to being saturated with respect to calcium carbonate formation,
we must study whether the volume of the ocean has changed.
Now, we have a way of doing this, if I may have the first slide, I’ll show you the evidence.
Dr. Dansgaard from Copenhagen has carefully studied ice cores from Greenland, going back as far as an estimated 87,000 years.
And he has measured the O18 concentration of these samples of ice.
Now, to a first approximation, as ice is formed, the water which of course came by evaporating from the ocean,
is depleted in the heavy oxygen isotope.
So beginning over here at the present,
at this concentration we have a constant O18 concentration back to about 10,000 or 12,000 years.
Then we have a drop off which corresponds to the formation of the last ice sheets.
And this drop off gives us a measure of the extent to which the volume of the ocean has changed.
We see incidentally that the ice lasted approximately 15,000 years
and there was a considerable period before it went back to the present normal state.
So from this we know that the volume of the ocean has been constant over the last 40,000, 50,000 years to about 1% or 1½%.
So our basic assumption that the reservoir has been constant is probably sound.
However the proof of the matter lies in the testing of the accuracy of the dates.
About 10 years after we began systematically measuring radiocarbon dates,
we began to observe that the dates about 3,000 years before the present were looking a little young.
And when we got back to the first dynasty’s of Egypt,
it seemed that either the Egyptian historians were wrong or radiocarbon was giving an erroneously young date.
The difference being some several centuries.
That is we were finding for the first dynasty of Egypt about 4,300 years,
whereas the historians expected something more like 4,800 years.
We read a lot of Egyptian history then and when you do go into the evidence the historians have used,
it does not inspire you with great confidence.
So we were not at all certain that we had evidence of cosmic ray variation in this discrepancy.
So we turned to the use of another method of checking and that is the tree ring technique.
It is a remarkable fact that the wood in a large tree is dead, chemically dead, and is unable to interact with the sap,
even though it’s continuously in contact with it, after it’s once ceased to be part of the living system of the tree.
The other several rings may have some contact with the life bearing sap,
but as far as we can tell from the radiocarbon datings of the inner parts of large trees,
they are dead within a few years of being themselves part of the outer ring.
Thus when we take a large redwood tree, among the first tests we ever used for calibrating the radiocarbon dating technique,
was a very large redwood tree which had nearly 3,000 rings in it.
And we took borings through the tree and using about 10 years, no,
the first measurements were made on about 100 year slice,
because our error of counting and measuring the radiocarbon was at least 100 years.
We found an agreement between the radiocarbon age and the number of tree rings.
Now, this has never ceased to amaze me, as a chemist,
but it gave us great confidence in another assumption we had to make in developing the radiocarbon dating method,
namely that we could do our laundry well, we could clean up.
We could wash and purify, say charcoal that had been lying in the ground for 10,000 years,
and we could remove the contaminating humic acids and other carbonaceous dirt that might have accumulated.
But returning to the tree ring, this early result had left us with the impression that if worse came to worse
we could use tree ring samples themselves for checking the radiocarbon dating method.
So mainly due to the efforts of Dr. Paul Damon of the University of Arizona,
early measurements were made on trees which had been dated by the tree ring method.
And these dates, even though they stopped at something like 3,000 or 4,000 years themselves,
did show a tendency for the older dates to be a few centuries too young.
May I have the next slide, please?
I’m afraid this is not visible, maybe you can see a little of it.
On the one side here is the radiocarbon date and on the other side is the tree ring date
and if they were in perfect agreement, the data would fall right on this horizontal line.
Back here at about 1,000 BC, they begin to move up.
Next slide, please.
These data due largely to the work of Professor Hans Suess at La Jolla,
show the results from the most ancient living tree, the bristlecone pine tree.
These wonderful trees grow in only one place on earth, namely the south western United States.
They live to be about 4,000 or 5,000 years old.
They’re a scraggly stunted looking little tree with great ability to survive.
These materials are invaluable for our problem of testing the method of radiocarbon dating.
For by using various pieces of wood, which are found lying on the forest bed,
it has been possible to constitute a dendrochronological scale which goes back to something like 8,000 years.
This remarkable piece of work was done by Wes Ferguson of the Douglas tree ring laboratory at the university of Arizona
and Dr. Suess measured the radiocarbon content.
Now you will see, beginning at the present time and going backward in time,
we have some small wiggles and deviations which Dr. Suess believes are beyond experimental air, and I believe it’s true also,
for we have used these wiggles, as we call them, of Dr. Suess’s, for checking some very accurately known materials,
namely some of the English manor houses and French chateau
and we do find that the historical records agree far better with the radiocarbon date
after the radiocarbon dates have been corrected with the Suess wiggles.
These wiggles continue throughout the whole record but at about 2,000 years ago
there’s a general drift towards radiocarbon dates being younger,
the radiocarbon dates are given above and the tree ring dates below.
Next slide, please.
This deviation continues, the deviation amounting, say at 5,000 years ago,
by the way this is the 4,800 year point and if we use this correction to our 4,300 year date for radiocarbon,
we get the answer, the classical Egyptian historians had given us the whole while.
In other words, they are right.
They’ve been, I must say, pleased to learn that.
(laughter).
But these wiggles are superimposed on a massive general trend which apparently continues back to at least 6,000 years.
Now, we have some additional methods of checking.
In the radiocarbon content of lake sediments.
The next slide, please.
We have here the data of Dr. Minze Stuiver of the University of Washington who studied a sedimentary lake,
the sediment in a lake which he’s measured in terms of ash content, pollen grains,
total organic matter and then the radiocarbon ages.
Now, the radiocarbon ages are given here and what he calls the true ages, in terms of the thickness of sediment,
as derived from the younger layers, would be here.
And as you see, the radiocarbon ages are too young again.
But the interesting part about this is that Minze Stuiver would say and does say
that at about 10,000 or 11,000 years ago we’re back on the beam.
And therefore apparently there was a period of time beginning at about 3,000 years ago and backward in time,
or I should say the other way around, beginning about 10,000 years ago and up until about 3,000 years ago,
the cosmic rays were extra intense.
There was an augmentation of their intensity as compared to what it is today
and what it was at the time of the last ice sheet some 12,000 years ago.
That’s all for the slide, please.
We have some 30,000 radiocarbon dates and in most cases the archaeologists and historians
and the geologists are not so extremely concerned about the dates being translatable into accurate calendar dates.
But in enough cases, namely the historians, there is a deep interest and concern in the accuracy
that we feel it is a very important task that we have to press the calibration curve.
There’s another aspect to it, too,
and that is that the cause or the variation of the cosmic ray intensity has considerable scientific interest.
What is it that made the cosmic rays become, what was it that made the cosmic rays become more intense,
beginning about 10,000 years ago.
What made them go back to their present lower level.
The fluctuations I’m talking about here amount to a few percent.
It was by a few percent that the cosmic rays increased, beginning about 10,000 years ago.
Though I must say that the question of whether they were at the present level prior to that time is still not definitely answered.
In addition to the curves and data I showed on the slides, there is work in Sweden with the lake sediments up there,
the beautiful work of the Baron De Geer on the so called Swedish varves,
that we are able to correlate the varve dates with the radiocarbon dates,
though the varves themselves contain too little carbon for a dating.
We occasionally find material embedded in the lake sediments and we’re able to date those.
And doctor Tauber of Copenhagen has done considerable work on the varve dating, correlating it with radiocarbon and he,
as well as Dr. Stuiver, finds that the radiocarbon is back on the present curve at about 10,000 years ago.
So we have some confidence saying in the deduction something made the cosmic rays increase in intensity
beginning about 10,000 years ago.
And that something ceased to exist about 3,000 years ago, plus or minus perhaps 1,000 years.
When we send an instrument such as the one Professor Hofstadter spoke to us about, up in a rocket,
we see a 4-fold increase in the cosmic ray intensity.
Now, half of that is due to just geometry because when you’re on the surface of the earth,
you have only 2 pi steradians from which you can receive cosmic rays.
When you get away from the earth, you have 4 pi in this factor of 2.
But the other factor of 2 is due to the earth’s magnetic field, cosmic rays are charged, and as Professor Hofstadter said,
you can bend charged particles and that’s the way they determine the energy in the Stanford laboratory.
But the bending of the incoming cosmic rays causes about half of them to escape the earth and just go back out again.
So we have a factor of 2 that is, if we turned off the earth’s magnetic field right now,
the cosmic ray bombardment intensity and therefore the radiocarbon production rate would double.
So about 15 years ago Dr. Elsasser and collaborators suggested
that we should look carefully for deviations in the radiocarbon curve of just the sort that we have seen, has actually been found.
He didn’t predict which way it would be but he gave a formula for it
and suggested that if the earth’s magnetic field had varied
we would see it in the discrepancy between the radiocarbon ages and the historical ages.
Well, we found such a discrepancy.
In the directions we would have said, if the Elsasser theory is correct,
the earth’s magnetic field was weakened beginning about 10,000 years ago
and then it recovered about 3,000 years ago and we can even say how much, a few percent.
Now, we know from magnetic records that the earth’s field has changed in direction in historical times.
The navigational charts show this, the earth’s pole wanders.
Now, however, such a wandering in direction will not in any way effect the radiocarbon age,
because we know that even though the cosmic rays vary strongly with intensity over the earth’s surface,
the mixing is so rapid that a tree grown on the equator has exactly,
well, within our experimental area, namely a percent or so, the same radiocarbon concentration as a tree say in the Yukon.
Well, we made this measurement many times, going all over the earth
and all living matter has the same concentration to within a percent.
So the mixing is very rapid.
So it isn’t enough to turn the north pole and south pole on to the equator,
what you’ve got to do is change the total strength, the strength of the total magnetic field.
If you do that and you weaken that, then the cosmic rays will come in and your radiocarbon ages will get too young.
By the way, at the present time we are about twice the primeval level, due to the atomic explosions in the atmosphere.
Archaeologists who dig us up are going to be very surprised at the radiocarbon age we show.
But assuming such perturbations were not possible in past times, there were no hydrogen bombs 10,000 years ago,
it must have been the earth’s field, magnetic field weakened.
Now, of course we do not know much about the origin of the earth’s field and so perhaps anything is reasonable.
Or it’s interesting that from the study of history we can say that the earth’s field weakened in these ancient times.
And so we see a bringing together of the disciplines here in an interesting way.
Now, we’ll look at Suess’s wiggles, the general trend we’ll call Elsasser, the wiggles we’ll call Suess.
Now, the general trend we can easily imagine is due to a gradual weakening and then a recovery of the earth’s field.
But the wiggles presumably are due to something else.
Suess suggests and I agree, though I don’t think either of us would say it’s proven, that the wiggles are due to the sun.
The sun continually gives off a plasma vapour which proceeds to flow outward from it.
This plasma wind, called the solar wind, has in it totally ionised matter
and in the clumps of plasma which fly outward are entrained, rather substantial magnetic fields.
And these magnetic fields will scatter cosmic rays.
Now, the main part of the cosmic rays are coming from outside the solar system.
So as the solar wind blows outward from the sun, it’s continually pushing back and scattering back the incoming cosmic rays.
And we think the wiggles are due to variation in the intensity of the solar wind in past times.
Suess goes so far, and I think it's interesting speculation, to say that correlates with the weather,
when the solar wind is strong, the sun is hot or cold - I forget which, I think it’s hot –
and when the solar wind is weak, the opposite is true and he’s gone back looking at the historical weather records
and finds a fairly convincing correlation between his wiggles and the mean temperatures.
This is probably too speculative to pursue much further but in any case it offers an opportunity for future research.
Much more can be done to establish the history of the weather, there’s a Professor Lamb at Oxford
who is the world’s great authority on the history of the weather
and I’ve talked with him at some length about it and he thinks much more can be done.
And if the Suess theory is correct we’ll be able to get a figure for the variation of the solar constant in past time
or may be able to get from a study of the radiocarbon correction curve.
However the establishment of accurate historical records will reside entirely on our being able to correct the radiocarbon curve.
Most places on earth there are no written records beyond 2,000 years ago.
The only written records in the world are the Egyptians and 1 or 2 other Asia minor societies,
there are no written records in any of the Americas beyond the Mayan.
And in middle Europe and throughout all of the world, except for the middle east, there are no written records of any substance.
All we have in terms of past history relies on our doing something about these corrections.
Now, radiocarbon mixes rapidly, therefore, if we can carry the Ferguson-Suess work back
in this one forest in the white mountains of the southern Sierra Nevada, we’ve got the whole thing.
We have the whole thing if we can just carry them back in that one place,
namely the Schulman forest grove in the southern Sierra Nevada.
Now, let me tell you about this.
This little laboratory in the university of Arizona, the only place in the world, this bristlecone pine work is done.
The bristlecone pine doesn’t live any place else.
The requirements are that it lives at an altitude of something over 3,000 metres
and then it will live fine and it may live as long as 5,000 years.
We found one tree down there that was 4,200 years old when unfortunately a graduate student sawed it down.
He was supposed to bring home a core, you know, they take a core to go through
and then they bring that back and count the rings and saving the tree.
But for one or another reason he misunderstood and he sawed this tree down and brought back a slab, a whole section.
I have part of that on my desk, 4,200 years old when that tree was cut down.
But, more importantly, lying on the ground in this forest are trees which died 5,000 years ago,
which themselves were 4,000 years old when they died.
So the technique is to take these ancient pieces of what looks like driftwood lying on the forest floor,
take it to our radiocarbon dating lab, we make a date of it and that gives them the guideline as to where to start matching.
Then they look at the fine structure of the tree rings.
And they match the fine structure that is one year will be thicker, the next year will be thinner,
it will vary widely from year to year, that's attractive feature of this marvellous tree,
is that it has a great deal of variation from year to year in the thickness of the rings.
And so, from this single bed, a single forest stand of trees,
they build up a structure for the variation from year to year by overlapping the rings,
they can extend the chronology back beyond the time of the oldest living tree now, which is about 5,000 years.
And in this way, Ferguson and Suess have pushed the bristlecone pine back to 8,000 years.
We hope that we can push it back to 9,200 years, because Mrs. Ferguson about 2 years ago, when they were up there,
said there’s a pretty piece, let’s take it back and we radiocarbon dated that at 9,200 years in its oldest part.
Now, we have to find another log which will be 8,000 years old on the inner part and 9,000 years old
and so we can go back to 10,000 years hopefully in this way.
But all of quantitative human history depends on keeping the fact
that the bristlecone pine is the most valuable historical research object in the world, secret.
We must keep it secret, we must not let people come and use it for firewood or take it home and put it on their mantles.
We have decided however that the best way to keep it secret is to act as though it was unimportant.
So there is a new road into the Schulman forest, but we have an unusually attentive set of forest rangers around there.
and we have not put a fence and a padlock on it.
We had thought to have a big project where we go to gather all of the pieces of drift wood or fallen wood,
and put them in the warehouse, we’ve even decided against that
because it’s a pretty large job and seemed thoroughly beyond our budget.
I guess you know that radiocarbon dating is not an extremely richly financed field of research,
nothing like the field of space physics and high energy physics.
Well, I wanted to tell you this, if any of you can think of any other ways of calibrating radiocarbon dates
beyond 10,000 years or even back to 10,000 years, we would certainly appreciate knowing about them.
Thank you very much.
(Applause).