Radiocarbon Dating and Calibration with Tree Rings and Lake Sediments
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Lindau Nobel Laureate Meetings74 / 340
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Transcript: English(auto-generated)
00:12
Mrs. Libby is a physicist, and that explains my presence.
00:23
I want to speak about the most recent developments in radiocarbon dating, emphasizing our hopes for the future perfection of this method of determining the lapse of time in archaeological
00:41
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
01:00
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
01:21
centimeter over the Earth's surface. And as time passes, this accumulates, and the radiocarbon so produced burns in the air to form carbon dioxide, radioactive carbon dioxide, which, of course, is mixed with the carbon dioxide of the normal type and therefore finds its way into the life
01:47
cycle since all living matter is derived, either directly or indirectly, from atmospheric carbon dioxide. That is, the plants grow by the process of photosynthesis from the atmosphere.
02:04
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
02:25
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
02:41
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
03:05
decay, with a half period of 5730 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 campfire,
03:29
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.
03:42
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.
04:02
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.
04:23
We calculated 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
04:42
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
05:01
changed. Well, 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 life cycle, or more importantly, in the sea salts
05:26
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.
05:41
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,
06:02
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.
06:21
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.
06:41
Dr. Dansgaard of Copenhagen has carefully studied ice cores from Greenland. Going back as far as an estimated 87,000 years. And he has measured the O-18 concentration of these samples of ice.
07:04
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 O-18
07:26
concentration back to about 10 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
07:47
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.
08:01
So from this we know that the volume of the ocean has been constant over the last 40, 50,000 years to about one percent or one and a half percent. So our basic assumption that the reservoir has been constant is probably sound.
08:24
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
08:49
a little young. And when we got back to the first dynasties of Egypt, it seemed that either the Egyptian historians were wrong or radiocarbon was giving an erroneously young date, the difference
09:06
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.
09:23
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.
09:42
So we turned to the use of another method of checking, and that is the tree ring technique. There's 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
10:06
after it's once ceased to be part of the living system of the tree. The outer several rings may have some contact with the life-bearing sap, but as far as
10:21
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
10:43
or detecting on 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, I think of what, you know, the first measurements were made on about a hundred
11:02
year slice because our error of counting and measuring the radiocarbon was at least a hundred 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
11:26
in another assumption we had to make in the 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
11:44
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
12:00
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
12:27
three or four thousand 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
12:43
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 there were perfect agreement, the data would fall right on this horizontal line. Back here at about 1000 B.C. they
13:12
largely, to the work of Professor Hans Seuss at La Jolla, show the results from the most
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ancient living tree, the bristlecone pine tree. These wonderful trees grow in only one place on earth, namely the Southwestern United States. They live to be about four or five thousand years old. They're a scraggly, stunted looking little tree with great ability
13:44
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
14:00
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. Seuss measured
14:24
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. Seuss believes are beyond experimental error. And I believe it's true also, for we have used these wiggles as we
14:45
call them, of Dr. Seuss'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
15:06
corrected with the Seuss wiggles. These wiggles continue throughout the whole record, but at about two thousand years ago, there's a general drift towards radiocarbon dates
15:23
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 five thousand years ago, by the way, this is the four thousand eight hundred year point, and if
15:46
we use this correction to our forty-three hundred year date for radiocarbon, we get the answer the classical Egyptian historians had given us the whole while. In other words, they
16:02
are right. They've been, I must say, pleased to learn that. But these wiggles are superimposed on a massive general trend which apparently continues back to at least six thousand years. Now we have some additional methods of checking in the radiocarbon
16:27
content of lake sediments. Next slide please. We have here the data of Dr. Mintz-Steiver of the University of Washington, who studied a sedimentary lake, the sediment in a lake,
16:45
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
17:00
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 Mintz-Steiver would say, and does say, that at about ten thousand
17:22
or eleven thousand years ago, we're back on the beam. And therefore, apparently, there was a period of time beginning at about three thousand years ago and backward in time, or I should say, the other way around, beginning about ten thousand years ago, and up until about three thousand years ago, the cosmic rays were extra intense.
17:50
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 twelve thousand years ago. That's all
18:02
for the slide, please. We have some thirty thousand radiocarbon dates. 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,
18:27
namely the historians, there is a deep interest and concern in the accuracy that we feel that it is a very important task that we have to press the calibration curve.
18:41
Another aspect to it, too, and that is the cause of 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 ten thousand years ago? What made them go back to their present lower level? The fluctuations I'm talking
19:06
about here amount to a few percent. It was by a few percent that the cosmic rays increased beginning about ten thousand years ago, though I must say the question of whether they were at the present level prior to that time is still not definitely answered.
19:24
In addition to the curves, the data I showed on the slides, there's work in Sweden with the lake sediments there, the beautiful work of the Baron de Geer on the so-called Swedish barbs, and we are able to correlate the barbed dates with the radiocarbon dates
19:44
though the barbs 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 Dr. Tauber of Copenhagen has done considerable work on the barbed dating correlating it with radiocarbon
20:04
and he as well as Dr. Deuver finds that the radiocarbon is back on the present curve at about ten thousand years ago. So we have some confidence in saying in the
20:21
deduction something made the radiocarbon, the cosmic rays, increase in intensity beginning about ten thousand years ago and that something ceased to exist about three thousand years ago plus or minus perhaps a thousand years. When we send an instrument such as the one
20:46
Professor Hofstadter spoke to us about up in a rocket, we see a fourfold increase in the cosmic ray intensity. Now half of that is due to just geometry, that is when
21:01
you're on the surface of the earth you have only two pi steradians from which you can receive cosmic rays. When you get away from the earth you have four pi and this factor of two. But the other factor of two is due to the earth's magnetic field. Cosmic rays are charged and as Professor Hofstadter said, you can bend charged particles and
21:25
that's the way they analyze, 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 two, that is if we turned off the earth's
21:44
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
22:04
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 a discrepancy between the radiocarbon ages and the historical
22:23
ages. Well we found such a discrepancy. In the direction that 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
22:40
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 affect
23:02
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
23:21
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 within a percent. So the mixing is very rapid. So it isn't enough to turn the north pole and south pole onto the
23:41
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
24:01
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
24:23
years ago, it must have been the earth's 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. However it's interesting that from the study of history we can say that the earth's field
24:44
weakened in these ancient times. And so we see a bringing together of the disciplines here in an interesting way. Now we look at Seuss's Wiggles. The general trend we'll call Elsasser, the Wiggles we'll call Seuss. Now the general trend we can easily imagine
25:05
is due to a gradual weakening and then a recovery of the earth's field. But the Wiggles presumably are due to something else. Seuss 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
25:26
gives off a plasma vapor which proceeds to flow outward from it. This plasma wind called a solar wind has in it, it's totally ionized matter, and in the clumps of plasma which
25:41
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 incoming cosmic rays. And we think the Wiggles are
26:05
due to variation in the intensity of the solar wind in past times. Seuss goes so far, and I think there's an 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.
26:24
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
26:42
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 greatest 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 Seuss theory is
27:02
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
27:22
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 one or two other Asia Minor societies. There are no written records in any of the Americas beyond the Mayans. And in Middle Europe and throughout all of
27:45
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 Seuss work back
28:05
in this one forest in the White Mountains of the southern Sierra Nevada, we got the whole thing. We have the whole thing if we can just carry them back in that one place, namely the Schumann Forest Grove in the southern Sierra Nevada. Now, let me
28:22
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 anyplace else. The requirements are that it live at an altitude of something over
28:42
3,000 meters, and then it'll 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's supposed to bring home a core. You know, they take a core and
29:02
go through, and then they bring that back and count the rings and saving the tree. But for one another reason, he misunderstood, and he saw this tree down and brought back a slab, this whole section. I have part of that on my desk. 4,200 years old when
29:21
that tree was cut down. But more importantly, lying on the ground in this forest are trees which died 5,000 years ago and which themselves were 4,000 years ago when they died. So the technique is to take these ancient pieces of what looks like driftwood
29:44
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,
30:03
one year will be thicker, the next year will be thinner. It will vary widely from year to year. That attractive feature of this marvelous 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
30:27
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
30:41
in this way, Ferguson and Seuss 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 two years ago when they were up there, said, there's a pretty piece, let's take it back. And we
31:01
radiocarbon dated that at 9,200 years, and it's 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
31:22
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.
31:44
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 Shulman Forest, but we have an unusually teddy set of forest rangers around there, and we have not put a fence and a padlock
32:03
on it. We had thought to have a big project where we go gather all of the pieces of driftwood and put them, or fallen wood, and put them in a 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
32:26
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 think of any other ways of calibrating
32:40
radiocarbon dates beyond 10,000 years or even back to 10,000 years, we would certainly appreciate knowing about them.