By reading the rings of pines as old as the earliest pharaohs, Henry Michael has changed the way we look at the past.
This article appeared in Archaeology, Volume 54 Number 2, March/April 2001 in a slightly different form.
On a rounded ridge in the Inyo National Forest, east of the Sierra Nevadas and west of Nevada's parched deserts, are high-altitude slopes where the thin air makes your head spin and the trees seem to live forever. This is the realm of the ancients, the home of the oldest bristlecone pine trees of the White Mountains in east-central California.
“Ancients” is not a word one hears often in this throwaway age, but these trees have earned it; they are the oldest living trees on the planet. Before Giza and Palmyra, before the ziggurat of Ur, before the invention of trumpets or leavened bread, the trees in this grove of Pinus longaeva were stubborn, thriving seedlings. Here, where the growing season is 45 days long and it rains barely ten inches a year, there are 200 trees more than 3,000 years old, and two dozen trees more than 4,000 years old. Here reigns Methuselah, the world's oldest living tree, which sprouted around 2700 B.C.
Bristlecone pines rarely reach 40 feet. Each one is unique and strangely beautiful, more evocative of a ghost or a spirit than of any tree you've ever seen. Their wood is mostly dead. They are twisted and bent, blackened in fire, carved and sculpted by wind-whipped sand and snow. But their hard, dense, resinous trunks resist bugs, disease, and rot, and their roots snake along the ground in search of water.
Discreetly out of sight, behind branches and low on fallen trunks, are shiny metal tags with numbers and dates, foil strips tacked to the trees by the scientists who study them. Most numerous, it seems, are those with an “H.” These are the tags of Dr. Henry Michael of the University of Pennsylvania's Museum Applied Science Center for Archaeology (MASCA).
Almost every summer for more than 30 years, in a square-mile swath of forest, Michael dug out long-buried logs and took cores from standing trunks, taking home samples thousands of years old. With these bits of aged wood he helped build a tree-ring chronicle of Earth's ever-changing environment and a yardstick by which radiocarbon dating, then in its infancy, could be calibrated, with results that forced archaeologists to rethink their theories about the spread of civilization.
“Quietly, year in and year out, he was out getting this stuff,” says Stuart Fleming, MASCA's scientific director. “He would bring back snags of wood, and lo and behold an older date would emerge. By its sheer ring-by-ring precision [the work] dramatically altered our perspective [on the past].”
I talked with Michael in his office, deep in the basement beneath the University Museum's turn-of-the-century galleries, where he worked for almost 50 years. It is a narrow little room, busy with maps, charts, slices of pines, Russian books, pictures of trees and his grand- and great-grandchildren, and a photo of Alfred E. Douglass, the father of tree-ring research.
Michael enrolled at the University of Pennsylvania more than 62 years ago, and has taught geography, physical anthropology, and the eastern Europeam archaeology at both Temple University and Penn. He no longer prospects for wood. After all, he is 87, and the last time he was on the mountain top, when he was only 81, his pulse jumped to 120 and he had to rest every 100 yards.
A self-effacing man with a dry sense of humor, Michael has a high forehead, bright blue eyes, and a wide, square face. Although his step has slowed and he has a slight stoop, there's little else in his appearance to suggest he's nearing 90. He has the sturdy build of someone who has spent decades hiking in the mountains and hefting logs, and when he speaks it's with the methodical pace of a man who makes meticulous measurements. True, he wears broad glasses and sometimes a hearing aid, and if names of old colleagues occasionally elude him, he retains the tenacity of spirit that helped rewrite the history of ancient Europe.
Since 1996 he has been busy translating from Russian into English volumes of legends and fairy tales of Siberian Eskimos and continuing 50 years of study probing the Paleolithic cultures of Alaska, eastern Europe, and Siberia. But it is his work with bristlecone pines that has had the biggest impact. Through his research, Michael helped unite two sciences: radiocarbon dating and dendrochronology, the science of dating past events by studying the annual growth rings of trees.
On the wall outside of Michael's office hang two important chunks of tree, “captured,” as Michael says, four decades ago. One is a slice from the stump of a giant sequoia, the second an irregular slab from a California bristlecone pine. The sequoia's contemporaries included Hannibal, Caesar, Charlemagne, and Lincoln. The bristlecone pine is much older. It spans 3050 to 2700 B.C., from Horus' Aha, the first pharaoh of Egypt's first dynasty, to Horus Qahedhet, the last pharaoh of the third dynasty.
Bristlecone pine rings are paper-thin and so closely packed they meld into a homogeneous brown, but with a microscope the trained eye can distinguish their differences - the rings are thick in warm or wet years and thin when stressed by frost or drought. Certain sequences of years have distinct patterns of thick and thin rings. This “signature” is shared by all the trees living at the same time in the same climatic region. By matching these signatures in trees with overlapping ages, dendrochronologists have compiled charts, some of which are almost 50 feet long, showing growth ring patterns going back millennia.
By matching the pattern of rings on wood from an archaeological site to such master charts, the record is extended farther into the past.
Dendrochronologists have built sequences for a number of tree species, including German, Irish and Polish oaks, Patagonian cypresses, Lebanese cedars, pine, yew, spruce, and chestnut. They've plotted rings from shipwreck timbers and roof beams, and wood from the Mediterranean, Russia, and China. The bristlecone pine series, the longest built from a single species in a single location, stretches back to 7040 B.C. It is based on thousands of rings from 81 living trees and 118 dead trees, and Michael found more than three-quarters of the samples. He also recovered one older piece of wood, carbon dated to more than 10,000 years ago, whose rings cannot yet be linked to the the sequence.
A typical bristlecone pine is a bulky slab of mostly dead wood with a thin ribbon of living bark snaking up its side. The bark leads to branches thick with tufts of green needles and bristle-tipped cones. From time to time a branch dies and a new one grows, or the central trunk dies and side branches take over. In essence, these trees grow sideways and wider, not taller, adding a partial ring each year only beneath the living bark.
In 41 years of work, Michael has developed a practiced eye. “The weathering is the clue,” he says. “You can judge by the abrasion of the wood whether it has been there tens of years, hundreds of years, or thousands of years. I can tell with an error of about twenty per cent. But you always take a chance. Some days I'd pick up a piece of wood, and it would turn out to be 1,000 to 2,000 years old. Other times I'd pick one up and it would be 4,000 years old.”
Much has changed during Michael's years at MASCA. As he led me from the dark Victorian-era halls into the modern sunlit lunch room, we navigated a new passcard security system with winking red diodes and an elevator full of students, budding archaeologists of whom none seemed to have the slightest idea who he was. We talked of his stubborn pursuit of the past over light helpings of salad and soup.
Michael's wood-hunting career has its roots in the World War II Manhattan Project. During the war chemist Willard F. Libby devised radiocarbon dating, a method that can measure the age of organic material. It remained classified, Michael says, until 1947, when the government allowed Libby to announce it. (He received the Nobel prize for his discovery in 1960).
Radiocarbon dating takes advantage of the fact that cosmic rays from outer space bombard the earth's atmosphere and convert nitrogen in the air into carbon-14, a radioactive isotope of common carbon-12. Like carbon-12, it combines with oxygen to form carbon dioxide, is absorbed by plants as they respire and is incorporated into the tissues of herbivores, and then carnivores, throughout the food chain.
Like all radioactive materials, carbon-14 disintegrates at a steady rate, but it remains at a constant level in living plants and animals because they absorb more from the air and their food. The ratio of carbon -14 to carbon-12 in their tissues is the same as in the atmosphere. When an organism dies, however, absorption stops, and the carbon -14 in its tissues steadily decays back into nitrogen. The rate of decay, its half-life (the time it takes for half of it to vanish), is 5,730 years. To obtain the age of a specimen, scientists compare the amount of carbon-14 in it to the amount of C-14 in a present day sample of similar material. By using this ratio and C-14's half-life, they can determine the date the organism died.
In 1951, the University of Pennsylvania opened the world's second radiocarbon laboratory. Elizabeth Ralph was its chief scientist. “She was Libby's first student [at the University of Chicago],” Michael says. “After three months, he said `Go and establish a lab, I can't teach you any more.' I started working with her the second year the lab was up.”
Michael was already an expert on the ancient peoples of the eastern far north. Born in Pittsburgh, he was nine when his mother returned with him to her native country of Czechoslovakia after his father died. An interest in the region's prehistory eventually led him to studies further east; when he joined Ralph he was completing his thesis, “The Neolithic Age in Eastern Siberia.”
When the lab opened at Penn, it added a new dimension to his research. In the summer of 1952, MASCA director Froelich Rainey, an arctic archaeologist, sent Michael to Alaska to collect wood from ancient native huts. “I excavated over the summer and took radiocarbon samples, which nobody had done before,” he recalls.
But as Michael went to work as Ralph's assistant - cleaning and burning samples, extracting gaseous carbon and dating it in a counter - critics were questioning the accuracy of radiocarbon dating. Libby had assumed that the amount of carbon-14 in the atmosphere remained constant over time and that today's carbon-14 to carbon-12 ratio could be used as a benchmark to determine how much carbon-14 had decayed in the sample. Physicists countered that fluctuations in the earth's magnetic field would affect the amount of cosmic rays reaching the atmosphere, and thus alter the air's carbon-14 content. Soon their predictions proved true.
Before radiocarbon dating, archaeologists relied on written dynastic records, especially the royal annals of Egypt, to date ruins, artifacts, and the civilizations that created them. But the radiocarbon dates and the Egyptian dynastic dates clashed. The carbon-14 dates were too young. For example, artifacts known to be from 1400B.C. had radiocarbon dates of 1250 B.C.
By 1959, dozens of radiocarbon labs worldwide were dating thousands of samples. What good were the dates if they were wrong? Radiocarbon specialists needed a method to correct their dates, to establish how much the biosphere's carbon-14 changed over thousands of years. To do this, they turned to dendrochronology.
Although many observers, including Leonardo da Vinci, had examined the varying widths of tree-rings for clues to past climate, it was not until the early twentieth century that dendrochronology became a systematic science. In 1901, Alfred E. Douglass, an astronomer at the Lowell Observatory in Flagstaff, Arizona, began to study tree-rings in hopes of finding a correlation between ring width, sunspot activity, and Earth's weather. Douglass also saw the relevance of dendrochronology to archaeological dating. Over the next 35 years he established a 3,200-year chronology for giant sequoias, and, with a chronology he had developed for the Southwest, dated Indian pueblos and cliff dwellings by using the tree-rings of their roof beams. In 1937 he established the Laboratory of Tree-Ring Research at the University of Arizona.
In 1959, Ralph and Michael, in conjunction with radiocarbon labs at the University of Arizona and the University of California at San Diego, set out to restore radiocarbon dating's credibility. Their method was to obtain radiocarbon dates for thousands of years of tree-rings; since each annual ring offers a precisely dated snapshot of that year's atmospheric carbon-14, they could devise a year-by-year correction system which researchers could apply to radiocarbon-dated artifacts.
The first thing they needed was a supply of old wood, and that job fell to Michael. While visiting his daughter in California, he took a trip to the Sequoia National Forest. Rangers told him the fee to fell a sequoia was $5,000. But then he heard of a fallen redwood that was still propped against its stump and was threatening to crush a cabin. After forest rangers removed the tree trunk, Michael borrowed a chain saw and sliced a slab from the stump. It was 2,200 years old. Michael was hooked. “I went to the University of Arizona Laboratory of Tree Ring Research and picked up the essentials of dating trees.”
Bristlecone pines grow in eight western states. For six years, Michael sampled trees from Montana to Arizona, before confirming that the oldest grow in the Methuselah Grove in California's White Mountains. This was nearly virgin territory. Dendrochronologist Edwin Schulman had established the great age of bristlecones in 1953 and announced the discovery of the 4,700-year-old Methuselah in 1958.
For ten hours a day, starting at dawn, Michael would walk the slopes with maps, saws, a shovel, and an auger that cores pencil-thin cross sections without harming the tree. To find old wood, Michael says he became “a forest floor gazer,” discerning in the size, feel, and location of eroded pieces on the ground the hints of their age and value. The challenge and the mountains entranced him. “The setting is very dramatic,” he recalls. “I want to say beautiful, but it is more dramatic than beautiful.”
A 4,000 year old tree that dies is likely to remain standing for more than a millenium. Once it collapses, the wood resists rot thanks to its high resin content, the dry mountain air, and because it can lie frozen under a protective cover of snow for up to nine months a year. It does, however, break into large and small pieces that wash a foot or two downhill in the spring thaws. Over the centuries these bits of wood can slide over a mile from where they originally grew. Sometimes a piece of wood's location alerted Michael to its age. For example, a large, eroded slab of wood, lying between two very old trees, probably predates both of them, having come to rest before the living trees sprouted. Other times, it was the wood's quality; the oldest chunks are heavy and dense, with tight, narrow rings.
Michael would ship samples he suspected were ancient back to Philadelphia for a rough radiocarbon date. If tests confirmed his hunch, he would climb the mountain, following dried water courses, looking for additional pieces that might mate with his original sample. “Sometimes it works and sometimes it doesn't,” Michael recalls.
Often Michael worked side by side with the Tree-Ring Laboratory's Wesley Ferguson, the leader of the field. Ferguson was interested in climate as well as radiocarbon dating. “Ferguson figured that the oldest wood would be tiny little pieces, worn down and abraded. So he looked for those, and he found quite a few ancient ones. But it occurred to me to look at big hunks of wood.” Michael found one from a tree that lived from 6200 to 1900 B.C, and a 600-ring specimen that weighed 75 pounds. Small pieces were of value, too. In 1983 he found a thin sliver with 150 rings, the oldest of which dated to 7070 B.C. Michael's finds repeatedly pushed the tree-ring chronology further into the past. The continuous record now stretches back to 7400 B.C.
Finding old wood was just the first step. Next was the tedious radiocarbon dating process. A typical bristlecone ring is less than a hundredth of an inch across. Michael had to study the tree-rings under a microscope, slice off a tiny sample, clean it, burn it, capture its vapors, and seal the gas in a carbon-14 counter overnight.
Eventually, Ralph and Michael had obtained 900 dates in their own lab. They combined the results with 2,000 dates from Arizona and the University of California at San Diego. The resulting data, plotted on a graph called a calibration curve, tells researchers how many years to add to make radiocarbon dates conform with calendar dates. Presented in 1972 at a historic conference in New Zealand, the curve brought new precision to radiocarbon dating. This accuracy turned history on its head.
Before radiocarbon dating, archaeologists had used Egyptian dynastic records to establish the succession of stylistic changes in Egyptian statuary, building, and pottery. In turn, they established a chronology for Mycenaean Greece and Crete by dating Egyptian trade goods, found in Greek and Cretan excavations. They then linked Greece and Crete to cultures in other parts of Europe. From this followed theories that cultural advances were established sequentially. Knowledge diffused outward, the theories said, propelled on expanding waves of migration, travel, and trade, from the fertile crescent of the Tigris and Euphrates, into Egypt, to Greece, and then in a two-pronged path through the Iberian peninsula and north across the continent through the Balkans.
The corrected radiocarbon dates created a new chronology that upset a century of archaeological thinking. For example, some of the oldest stone monuments of prehistoric Europe - Stonehenge, the stone temples of Malta, the megalithic tombs of the Iberian Peninsula, northern Europe and Scandinavia - were now found to be up to 800 years older than the Egyptian and Greek monuments that had supposedly inspired them. And Balkan copper metallurgy, thought to have arrived through diffusion from the Mediterranean, was centuries ahead of comparable work in Greece.
“In Europe,” archaeologist Colin Renfrew wrote in Before Civilization (1973), “the conventional framework for our prehistoric past is collapsing about our ears.” Out of this wreckage came new theories and studies of cultural change, innovation, and social organization.
The radiocarbon calibration also raised questions about the history of the earth itself. The calibration curve reveals unexplained short and long-term oscillations in the air's carbon-14. Scientists cite a variety of suspects for the changes, including shifting ocean currents, supernovae explosions, and sunspot cycles. Some scientists think the fluctuating carbon-14 reflects a change of strength in the earth's magnetic field, a weakening that allowed more cosmic rays to enter the atmosphere as the magnetic poles began to reverse. However, this process takes tens of thousands of years, and Michael thinks it unlikely that the bristlecones will offer proof. Ice Age glaciers covered the peaks for centuries. “When the glaciers retreated, the trees started growing, and that happened 12,000 years ago. So the oldest trees wouldn't be older than 12,000 years.”
Michael's role in developing the calibration curve was not his only contribution. He confirmed the estimated ages of the Egyptian pyramids of Meidum and Dashur by coring the cedar wood tenons that secure their huge blocks. And in 1965, he and archaeologist Bryant Bannister from the Laboratory of Tree-ring Research cored the cedars of Lebanon, the supposedly 2,000-year old trees said to have shaded Jesus. Although the Lebanese government permitted the study, it did not want the results published. “We found that the oldest cedars of Lebanon were 800 or so years old,” says Michael. “They show off to the tourists an enormous tree, and actually it's sixteen trees connected by their bark.”
Simultaneously with this pioneering research, Michael also found time to pursue his original love of Siberian prehistory. He had learned Russian in 1948 for his doctorate, and since the 1960s he has supplied a steady stream of translated works on native Siberian cultures, shamanism, and nineteenth-century narratives of travel and exploration, such as the Alaskan expeditions of Russian lieutenant Lavrentii Zagoskin. “He was the first one to get into the interior,” says Michael, “and the way he described it was just fascinating.” Michael has visited Russia numerous times and worked to bridge the divide between Soviet and American researchers during the Cold War. He is now in the midst of a multivolume series of Siberian fairy tales. “Siberian natives today have been fairly Russianized,” he notes, but their stories show us “how differently they thought. They were always worried about food, because food is so difficult to obtain there. It is the wilderness, so there's always the danger of hunger.” These translations are the main focus of his energy today. “It's an art to translate accurately, yet readably,” says Michael. “If you translate word for word, readers get tired of it and uninterested.”
Michael retired from teaching in 1980. Elizabeth Ralph died in 1984, and MASCA closed the radiocarbon lab two years later because of its expense and the development of other dating techniques. In 1993, Michael made his last journey to the White Mountains, to show a new generation of tree-ring researchers where he found his treasures. “I got too old to climb the steep slopes. I'm now 87. And I feel it.” At home, he no longer does the maintenance and yardwork, and he travels less and less.
In April 2000, MASCA awarded Michael its Director's Award for the volunteer research, writing, and lectures he has undertaken since his 1980 “retirement.” “I believe that if you want to stay alive, you have to do something with both your body and brains,” he says. “I've come across people who just couldn't wait until they retired so they could sit down on a chair and be comfortable. And those people died within a few years.” Michael and his wife Ida could certainly stay busy keeping up with their daughter, two sons, four grandchildren, and two great-grandchildren, but he also credits the longevity that runs in his family - his uncles and aunt lived well into their 90s, so, as Michael quips, “I've got to catch up.”
Copyright © 2001 Tom Gidwitz