by Rachel E. Gross
Oct 21, 2012
The trees were quite dead.
But Jonathan Palmer, one of the world’s leading experts on tree rings, had expected them to be deader. Palmer has a strange calling. He chases the corpses of ancient trees throughout New Zealand bogs, and probes their annual growth rings for clues of climates past.
These trees were his specialty: New Zealand Kauri, a native plant that towers at more than 160 feet and lives up to two millennia. More than 25,000 years ago, Kauri would die and topple into peat bogs, where chemicals conspired to keep their bodies near-perfectly preserved. Today, people have been digging up the pickled time machines buried beneath the ground’s surface to make specialty furniture.
Whenever they do, Palmer gets a call. In 2006, he got one from a saw-miller he knew on the northern end of North Island. Cranes were lifting the trunks of four partially fossilized Kauri from a nearby swamp, and he’d better come quick if he wanted a sample. The New Zealander rushed to get a cookie—a cross-section up to 13 feet in diameter sliced through the trunk of the tree—to study. It wasn’t until lab results came back that Palmer and his colleague Chris Turney, a professor of climate change at the University of New South Wales, realized what they had on their hands.
These trees were 13,000 years old. Compared to the rest, positively youthful. More than that, the Kauri had occupied a pivotal chapter in earth’s climate history.
“It was unique and unusual,” Palmer says of the finding.
“Just brilliant,” is how Turney puts it.
As a dendrochronologist, Palmer reads the patterns in tree rings—layers of girth the tree lays down annually—to figure out what the earth was like at the time they formed. Looking at these rings, he could tell the Kauri had lived during a mysterious cold spell in the earth’s past known as the Younger Dryas.
Climate scientists regard the Younger Dryas as one of the most fascinating yet poorly understood periods in the earth’s past. A bout of cooling in the midst of an overall warming trend, it was a time when glaciers spread throughout Canada, encompassed the Great Lakes, and crept across the British Isles. The abrupt freeze is typically thought to have begun around 12,900 years ago and thawed 11,500 years ago.
But since the Younger Dryas happened mainly in the Northern Hemisphere, the Kauri down under had escaped the frost. To Palmer and Turney’s delight, the oldest Kauri tree in this grove had lived a ripe 1,350 years—covering most of the abrupt climate switch into this mystifying freeze. That made its rings a treasure trove.
As any climate scientist or historian will tell you, we need to understand the past to predict the future. Climate modelers rely on what we know about the world’s transition into and out of the Younger Dryas to see what’s in store for the planet today—a goal made more urgent by disappearing species, a melting Greenland ice sheet, and an upward-creeping global thermostat.
As Turney writes on his website, quoting the poet T.S. Eliot:
Time present and time past
Are both perhaps present in time future,
And time future contained in time past.
To understand this critical moment in earth’s past, we need to know “what happened when,” as Turney says. In other words, we need a good dating system. Palmer and Turney rely on a system researchers have developed using a powerful combination of old and new(ish) dating techniques: traditional tree ring-counting and radiocarbon dating.
Here’s how it works.
First, radiocarbon dating. We used it on the pyramids of Egypt. We used it on the Dead Sea Scrolls. Radiocarbon dating measures the amount of decay of a radioactive isotope of carbon stored in an organic material, which means we can use it on anything that was once alive.
But there’s a problem. If we go back far enough, radiocarbon years don’t match up to calendar years. That’s because radiocarbon levels in the atmosphere fluctuate based on the earth’s magnetic field, how much sunlight reached the planet, and how much radiocarbon got exchanged between the air and the sea.
So we also need a good calibration curve for radiocarbon, to translate radiocarbon years into calendar years.
That’s where tree rings come in. Their growth rings paint a clear picture of climate change year-by-year. Narrow rings mean years the tree survived harsh drought or frost, while fat rings reveal years of plenty. We can pair the calendar dates with the amount of radiocarbon the tree stored in its flesh during those years. “A two-pronged attack,” Turney calls it.
Researchers have cobbled together a tree calibration curve relying mostly on oaks and pines from central Europe. The project started in the 1960s, when researchers at the University of Arizona arranged a calibration sequence using the gnarled bodies of the Methuselian bristlecone pine going back 8,700 years. But the curve gets sketchy right around the start of the Younger Dryas.
At that time, most of the trees in the Northern Hemisphere were “runty little pines struggling to survive,” says John Southon, a radiocarbon dating researcher at the University of California at Irvine who ran the dates for the recent Kauri project. Their tree rings might as well have been written in chicken scratch. That’s why Southern Hemisphere Kauri are so prized: nowhere on earth but in the swamps of New Zealand can we find tree rings going back 45,000 years—or more.
Back in 2006, Palmer and Turney sent off one ancient Kauri sample for dating at the University of Waikato in New Zealand by one Alan Hogg. But they had to make sure what they were seeing was true. So they also shipped some billets to Southon, in Irvine.
“This was a big deal,” recalls Southon, a Kiwi himself. At the time, he was dating stalagmites from Hulu Cave in China spanning a stretch of 16,000 years. When the wood came in 2008—two years of funding challenges delayed its arrival—he took out his microscope and peered at it through thick black spectacles. Then he did a double-take.
“It was just like reading pages out of a book,” he says. “I’ve never seen anything that looked that great.”
The researcher took tiny chunks comprising ten rings each, burned them to form graphite, and measured the ratio of the different isotopes of carbon with a mass spectrometer in his California lab. His results matched Hogg’s. The new Kauri sequence would firm up 1,300 years of the shaky calibration curve, decisively bridging the transition into and out of the Younger Dryas.
In September, Southon presented his findings to a roomful of climate scientists at the annual Comer Conference on abrupt climate change in Wisconsin—but cautioned that the sequence was a work in progress.
There is still work to be done before researchers can say exactly what the new Kauri sequence means for the Younger Dryas and climate change today. Before they publish later this year, they have to make sure the sequence is matched correctly with the rest of the calibration curve. Then, Turney hopes to input the new data into climate models at the University of New South Wales.
Plus, they’d like to get more trees in on the action, a goal that has depended so far largely on luck.
In 2009, Palmer and Turney returned to New Zealand’s North Island on a hunch they might find more wood. The country was in the midst of a devastating drought, which have grown more common as the planet’s thermostat rises. But this drought was also fortuitous: it had squeezed moisture from peat bogs and desiccated the top layer of grass, so that brown scorch marks now outlined ancient Kauri just below the surface.
“It was like somebody had come along and stenciled on the surface a tree,” Turney says.
He and Palmer watched as tractors extracted from the earth first one, then a dozen, and finally 25 trees. For climate research, it was a windfall. Yet without global warming, it’s possible the trees may never have been found.
“Isn’t that deliciously perverse?” Turney says.
Palmer will soon join Turney at the University of New South Wales to continue probing the data. He spent this past year as a visiting scientist at The Kauri Museum in Northland, compiling an archive of Kauri samples for future researchers—as he has neither the time nor funds to analyze them all himself.
Palmer describes with reverence the first time he watched ancient Kauri being extracted: he says he was “in awe at the earth releasing such enormous logs from the ground.” Yet even after 30 years of searching, the tree ring expert still finds the Younger Dryas Kauri particularly exciting.
“Leaves you wondering what else might be still there,” he says.