Detectives of Time
Time detectives using atomic tools are piecing together a new and dramatic picture of the past, reaching back as far as 25,000 years. N. J. BERRILL, who gives us the clues to this fascinating discovery, is a lifelong student of the sea, a marine biologist now serving as Professor of Zoology at McGill diversity. Atlantic readers will remember his book Journey into Wonder, an account of the voyages and explorations of the great naturalists, from which we published three chapters last year.
by N. J. BERRILL
THE Atomic Age has started and explosions make more sound and fury than other atomic ventures. Yet if knowledge leads to wisdom as well as power, then one of the quieter projects may in the end be more potent. It concerns the discovery of an atomic clock which is built into everything that has been alive. Scientists are putting natural atomic energy to work to unravel distant human history and what has been and may be happening to this continent. When was the icecap farthest south? Did men come before or after? What has happened since, and what is the shape of things to come? Time detectives using atomic tools are piecing together a dramatic past, and month by month the reports are coming in.
The new science, if that is the name for it, is radiocarbon dating. Here at last science and the humanities have a common goal — modern physics joins hands with history in its broadest sense. Atoms are studied in connection with fossil elephants, ancient Peruvian architecture and customs, pollen grains in the beds of old filled-up lakes, and not least the charred wood and burned bones buried deep in wayside caves. Willard F. Libby of the Institute for Nuclear Studies of the University of Chicago was the first to see, in 1945, that radioactive carbon might be used for dating the past. Now there are radiocarbon laboratories not only at Chicago but at Yale, Columbia, Michigan, Pennsylvania, Cambridge, Copenhagen, and in New Zealand.
The discovery of the carbon clock goes back in part to the late thirties. At that time the scientists who were studying cosmic rays pouring in from outer space began to analyze what goes on in the upper atmosphere. They found that the rays set free neutrons which in their turn convert atmospheric nitrogen into radioactive carbon. Libby took this information and worked out the proportion of radiocarbon to ordinary carbon normally present in the air.
Both kinds of carbon join with oxygen to make carbon dioxide; carbon dioxide, both radioactive and normal, is absorbed by plants and by the animals that eat them.
That is the starting point. What makes it a clock is this: radiocarbon, like all radioactive atoms, has a limited life as such and is continually disintegrating. After 5508 years, on an average, only half an original store of radiocarbon atoms will be left. After another 5568 years only half of those will be left, and so on. By the time 25,000 years have passed, only about one thirtieth of the original store will be left; and since the proportion is low to begin with, it is difficult to estimate what the ratio is in material older than that.
All animals and plants while they are alive contain the two kinds of carbon in the same proportion as they occur in the air. The moment the plant or animal dies, no more carbon is taken in and the carbon clock begins to run down, ticking away the years until hardly any radiocarbon is left. But as long as some wood or charred bone remains which is less than 30,000 years old, the radiocarbon can be counted in a Geiger tube and the age of the piece determined. This is the tool. What it can do depends upon human ingenuity.
Copyright 1953, by The Atlantic Monthly Company, Boston 10, Mass. All rights reserved.
A clock that runs down may not always tell the time correctly — a check is better than any amount of theory. Libby and his co-workers began with the age of a giant redwood tree. They took wood from the heart of a tree known as the Centennial Stump. According to the number of its annual growth rings, the tree was about 2900 years old when it was cut in 1874. Radiocarbon counting gave it an age of 2700 years — a little short, though close enough for most purposes. Tor another redwood trunk, now in the American Museum of Natural History, the atomic clock gave a radiocarbon age of 930 years for the ring grown in the year 10,57, and an age of 1430 years for the growth ring of the year 570 — a pretty close reckoning.
So far so good, but Chicago possesses ancient Egyptian relics as well as nuclear physics laboratories, and these have proved to be even better than the redwoods, since their age is greater and is just as well known by independent means. Radiocarbon gave the age of a funeral ship from an Egyptian tomb at 3750 years; the records put it at 3020. A cypress beam from an older tomb was 4575 years old by the record, 4802 by radiocarbon; wood from a coffin of the Ptolemaic period, of known age 2280 years, came out by radiocarbon at 2190. The carbon clock is good but is not perfect. It takes forty-eight man-hours to make a single count: a longer time would give greater accuracy but would cut down the number of items studied. The venture is new, the clock is crude, and we want to know everything at once. For the moment, it is good enough. And for younger material the check is closer: an Inca Temple known to have an age of about 444 years was given an age of 450 by radiocarbon.
Atomic scientists contributed the clock and pointed out a few things it could be used for. But all a clock can do, whether it runs by a spring or by radiocarbon atoms, is to tell the time; how long ago this or that was a part of a living organism.
For a long time oceanographers have wanted to know the rate of turnover in the oceans. It is slow and difficult to measure, but all marine life, large and small, depends upon it: it brings certain salts in short supply continually to the surface. The water sinks in polar regions and moves in the depths toward the equator, where it may rise again. Once the water sinks, it is as cut off from fresh supplies of radiocarbon as though it were a plant or animal that died, and radiocarbon counts can give the age of oceanic carbon just as readily as the age of a piece of ancient charcoal. Guesses gave the travel time from polar to equatorial regions all the way from ten to thousands of years.
Radiocarbon has yielded the answer, but it has been hard to get. The difficulty is the small amount of carbon the water contains: 200 gallons of sea water arc required for a single carbon count. Yet Laurence Kulp of the Lamont Geological Observatory of Columbia University and Maurice Ewing of the Oceanographic Institute at Woods Hole combined their experience and effort and have succeeded in solving many supposedly impossible problems concerning the ocean.
They designed an open tank which could be lowered to a depth of 3 or 4 miles. At the right moment large doors are closed and 200 gallons of ocean water of a particular depth and location are sealed within. The sample is brought up to the deck of the ship and the carbon extracted and eventually sent to the radiocarbon laboratory. The first samples of water came from the two sides of the mid-Atlantic ridge at the latitude of Newfoundland from depths of over a mile. And radiocarbon counts have given the ages at 1600 years and older. The water left the surface in Arctic regions at the time the Romans abandoned Britain, and even now is only halfway to the equator. The flow is remarkably slow — so slow that it tells something of the crust of the earth that lies beneath the ocean floor.
The rate of oceanic circulation is but one of the recent candidates for radiocarbon analysis. Most of the materials so far analyzed have been chosen with care from widely separated parts of the earth in the hope of sketching in the major events of the last 10,000 years or so.
It is an extensive undertaking which has really only just begun. Radiocarbon laboratories will multiply in the future. Many organizations and institutions concerned with the finances and with the finding of suitable materials from near and far are coöperating in the venture. The list includes the American Museum of Natural History, the Geological Society of America, the U.S. Geological Survey, the Guggenheim Foundation, the American Anthropological Association, the WennerGrenn Foundation of Anthropological Research — not to mention several great universities involved.
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Two points catch the eye at once. At Two Creeks, Wisconsin, the trees of an ancient spruce forest lie shattered, all with splintered ends and all pointing in a southwestern direction. The evidence shows that the forest went down beneath the creeping edge of a glacier of the last advance of the continental icecap, and that the ice advanced only another 20 or 30 miles. It was the turning point of the last ice age, and according to radiocarbon the forest fell about 11,400 years ago — only half as long ago as earlier estimations had placed the start of the ice withdrawal. And only twice as far back in time as recorded human history.
The other point is this. Burned bison bone has been found in Texas, of a kind of bison now extinct, associated with man-made spear tips. The age according to radiocarbon is 9883 years. Several pairs of woven sandals found buried in Fort Rock Cave near Crater Lake in Oregon are 9053 years old. The exact figures may be larger or smaller, but these ages show that men were living and were hunting bison in North America when half the continent was still covered by ice. From here on the questions come faster than the answers.
One fact stands out. Fragments of skin and other tissue of an extinct superbison found in frozen muck near Fairbanks, Alaska, are older than 28,000 years according to the radiocarbon count. The permafrost must have been continuous in that region ever since the last major phase of the last ice age. Did men reach North America from Asia by traveling over the dry and frozen Bering Strait before the days of stone lamps and warm skin clothing? Did they get across in a warm interglacial period many thousand years before, or did they come in boats along the coastal waters?
There are few facts to go on; yet in a cave near the southern tip of South America the burned bones of the giant sloth, horses, and the South American camel have been found together with human bones and artifacts. Their radiocarbon age is about 9000 years, and as long ago as that men were cooking the flesh of animals 7000 miles to the south of the ice.
We are accustomed to thinking of Stone Age man as hunting mammoths and other extinct elephants in Europe. The superb paintings on the walls of Laseaux Cave in France prove it beyond all question. Charcoal found in the same cave has a radiocarbon age of a little more than 11,000 years, although there is nothing to show that the fires were lit by the same men that did the drawing. It. is more startling to find evidence that men were hunting elephants in America as well, and at a time long before the ice began to melt.
Both at Tepexpan, Mexico, and at Clovis, New Mexico, burned bison bone, of a kind of bison now extinct, has been found with spear points of a certain kind. The radiocarbon age is about 10,000 years, and the period is that of Folsom man, the name given to the hunters who made their spear points in this particular way. But at each place the Folsom layer lies over a deeper one of a very different kind and greater age. This deeper layer contains no bison bones but does contain the bones of elephants — not of the existing kinds, but of mastodons and mammoths — and the remnants of man-made weapons. At Tepexpan in 1952, Mexican scientists found weapon points actually embedded in the ribs, with knives and scrapers lying around. The radiocarbon age of pine pollen in the surrounding muck is about 9000 years — pollen in the bone cavities is mixed pine and spruce and is probably older. It looks as if this particular hunting party were disturbed after the kill had been made: perhaps saber-toothed tigers, which also hunted elephants at that time and place, interfered with the process of dismemberment.
Human history on this continent begins somewhere in the age of ice or during an earlier interglacial period. Men hunted elephants, camels, horses, giant sloths, and armadillos as large as an ox, superbison, and giant beavers. They had dire wolves and saber-toothed tigers for unwanted company. All the animals were here before the ice age began more than half a million years ago, and all surv ived the alternating periods of hot and cold well into the present postglacial times. Remains of the giant sloth with a radiocarbon age of 10,000 to 11,000 years have been found in caves as far apart as the tip of Patagonia and Las Vegas, Nevada.
This is the beginning and the end of a story, for all of these mammals disappeared, except the llama — one of the smaller South American camels — and the horse, which was reintroduced by the Spaniards. Their going coincided with the presence of man, but it flatters the skill of these primitive hunters too much to suppose that they were the cause. There is a mystery here. Sometime after the continental ice began to withdraw, a rich fauna disappeared. Men witnessed the extinction, which makes it part of human experience and something more than just a. matter of curiosity.
Giant sloths in Nevada are out of place. They were great beasts that stood 12 to 14 feet high; they plodded along on bearlike hind legs, feeding on the foliage of trees or grubbing for roots in the soft earth. Nevada territory in their day must have been very different from what it is now — more humid, with thick forests and semitropical undergrowth. Then something happened, and men may have been victims as well as the beasts
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THE 9000-year-old sandals from the Oregon cave were worn by men long before Mount Mazuma blew up and covered the region with ashes and brimstone, leaving a hole where the mountain stood that is now Crater Lake. Radiocarbon dates the charcoal of a tree that was killed and buried by the eruption at 6453 years, or about 2500 years after the making of the sandals. The Newberry Crater, which took the place of the exploded mountain, went on erupting until 2000 years ago — again according to countings made of the charcoal recovered from the crater. Then the hole filled up and became the lake.
This is not the answer to the vanishing animals, but it does suggest climatic changes and some increase in aridity. And at this point pollen grains unite with radiocarbon dating to give a clearer picture.
Pollen grains survive in a recognizable form for thousands of years in the mud of ancient bogs and lake beds. After treatment they can be identified as spruce or pine, birch, oak, or whatever they happen to be. And either the pollen grains or associated wood can be dated by radiocarbon. When the information is pul together we see the forest creeping northward in the wake of the retreating ice. More than that, we see the changing nature of the forest and the climate it belongs to.
The main retreat of the icecap began about 8000 B.C. Some 500 years later the belt of pines extending along the southern edge of the Arctic tundra lay on a line passing through West Virginia. By 7000 B.C. it had shifted north to Connecticut. In 6000 B.C. the line passed through southern Minnesota, in 5000 B.C. through northern Minnesota, and in 4000 B.C. through northern Maine. With extensive tundra to the north of the pine, the continental ice must by this time have virtually melted away.
During the several thousand years that it took the ice to melt, and while the pine belt was shifting slowly to the north, oak dominated the southern forests. Altogether the tree pollens of this period indicate a temperature and humidity much greater than at present. This is plausible in any case. A lot of heat was needed to melt the icecap, and as long as it was melting and running off the land, the humidity would be high.
At that time of humid warmth, men were hunting bison of a kind that no longer lives. The plains supported camels and horses, while what is now the southwestern desert region had deep forests and rivers, with at least three times the present rainfall — the haunts of the giant sloth and the men who laid traps for them. The elephants may have already gone—perhaps there was too much water or too much sticky heat.
Around 4000 B.C. — at about the time Mount Mazuma blew its top — the climate changed; not suddenly, hut all too fast. From warm and moist it became warm and dry, and the reason is fairly clear. With no more ice to melt, the lakes and swamps and rivers dwindled, evaporation from shrinking water surfaces grew less, and the water table sank. The land grew dry, the hazy overcast gave way to bluer skies — and the vegetation changed. The details are not yet clear. But the forests thinned out and the plains became pit relied and hard—that much seems certain. During this period the long-familiar animals disappeared. They were there a little earlier and were gone a little later. They were too big, needed too much food, and couldn’t get enough of it.
The time was critical and important, and a lot of the mystery remains. Long, warm interglacial periods have occurred at intervals through the whole of the ice age. The mammals now vanished survived them all. Our own species of man came into being early in this age but remained a primitive hunter for several hundred thousand years. Why should the great extinction and the first beginnings of human civilizations coincide with the onset of the hot, dry millennia ?
Ice appears to be almost a thing of the past, but that may or may not be true. There is still a lot of it left in Greenland and Antarctica, and sooner or later it will grow again or melt entirely away. It spells discomfort either way. If it goes on melting until all is gone, the ocean level will rise by another 140 to 190 feet—high enough to drown New York and London and much of the best agricultural land in the world. Radiocarbon again has something to say. Cedar logs from an old drowned forest dredged with mud from the sea at Bermuda were alive in 9000 B.C., when the sea level was much lower. They were growing at the same time as the spruce in the Two Creeks forest in Wisconsin. Sediments of the same age dredged from the lower Mississippi show that the sea was 80 feet lower than at present; in 7000 B.C. it was 70 feet lower, in 5000 B.C. it was 50 feet, and in 1000 B.C. it was 25 feet lower than now. In the last twenty years it. has risen by nearly half a foot — a pace that seems to bring the end in sight.
Yet somewhere there appears to be a catch. All radiocarbon counts of the age of peat in the Florida Kverglades show that it started to form about 5000 years ago; earlier than that, the lower Florida peninsula was under water. This is the period known as the thermal maximum—the first thousand years of the warm dry spell. The sea level was higher than it has been since, implying that there was less ice in the icecap and more water in the ocean than there is at present. Perhaps the warm postglacial peak was reached between 6000 and 5000 years ago, and the ocean became lower as the ice began to grow again. The recent rise may well be only a minor fluctuation. For, in spite of the milder trend of the last few decades, the climate of the temperate earth has become comparatively cool and moist during the last two thousand years. The Norse colonies in Greenland were frozen out by expansion of the icecap, and many existing glaciers may be successors to and not descendants of the last ice age.
We may be two or three thousand years past the middle of a short interglacial period — and a further fall in average temperature of about 4.5 degrees Fahrenheit is all that is needed to bring the end. We seem to be caught between too much sea and too much ice. Atomic clocks may show which will be our fate.