SciAm frequent contributor Charles Q. Choi writes from the Yukon on an expedition with researchers from the American Museum of Natural History in New York City. Their goal: to recover intact DNA from mammoths, which once roamed the tundra but went extinct some 11,000 years ago.DAY 1: June 13, 2008
Today I begin a dig in the Yukon! Ross MacPhee, the American Museum of Natural History's curator of mammals, and his colleagues are personally interested in finding mammoth DNA, either in bones or in frozen earth.
For our first day, we go to Paradise Hill, a site for gold mining for the past century or so, about 20 kilometers east of Dawson City. We drive right up to the site"”a real luxury in field work"”on curving, bumpy gravel roads, over wooden bridges, past abandoned rusting mining equipment, surrounded on all sides by spruce forests mixed with paper birch and aspen trees, explained archivist Clare Flemming, an associate at the American Museum of Natural History and Ross's wife.
The excavation site is a wasteland of orange and brown gravel dotted with pinkish white quartz chunks. Geologist Duane Froese at the University of Alberta points out a bed of tephra, or volcanic ejecta, about 50 feet high up the walls of the site, which likely came from the Wrangell Mountains hundreds of miles away in southeastern Alaska. Duane has worked here in the Klondike for the past 15 or so years, aiming to put together a full picture of the last few million years of history in the region, back when it was part of Beringia, the grassland steppe ranging from North America to Asia that nowadays lies submerged under the icy waters of the Bering Strait.
A view of the excavation site at Paradise Hill.
Underneath the tephra bed is what the miners call "muck""”silt loaded with decomposing organic material from the Pleistocene, which reeks of an odor Froese charitably compares with a horse barn. Loaded in this muck may be the DNA that Ross is looking for, as well as that of the plants, bacteria and other life once found here thousands to millions of years ago.
The miners here are very supportive of the work, and two days ago they used excavating equipment to help scrape off the overburden, the material on top of the frozen earth. That is another incredible luxury, given all the hours of work that can go into clearing away tons of overburden. Working with miners makes this research cost-effective"”"you can get way more done for relatively modest amounts of money," Duane explains. Ross agrees: "You'd be lucky to get one site done in Siberia in a week."
We quickly get to work as swallows and butterflies flutter nearby. Paleoentomologist Svetlana Kuzmina at the University of Alberta climbs up the walls of the excavation site about 40 feet high to fill a half-dozen bags with about 30 lbs. of dirt each, which I help lug to the cars. She later screens this sediment for insects, unloading the soil into a sieve and dunking it in water the miners pumped up from nearby Hunker Creek to help sift gold. She rocks the sieve back and forth to remove the excess silt and then dries what's left out in the sun. She is looking for beetles that are good indicators of what the environments were like back then"”whether they were wet tundra, dry tundra, forest or steppe. Some beetles found in the sediments are now only found in Colorado or in relic steppe tundra communities on the south-facing sedge- and grass-covered slopes of hills near Whitehorse, hundreds of miles away.
Meanwhile, Ross and Lee Arnold at the University of Wollongong in Australia get to work extracting samples. Lee digs holes in the walls and sticks in a gamma spectrometer probe, which measures extraordinarily minute amounts of radiation given off by naturally occurring radioisotopes of uranium, thorium and potassium. This is a key step in dating substances with optically stimulated luminescence (OSL), Lee's specialty. The sediments here may be up to 100,000 years old, past what radiocarbon dating can reliably measure, but OSL dating can go back to at least 150,000 years by analyzing quartz grains and 250,000 years with feldspar grains. Some results in China suggest the method can date up to 740,000 to one million years.
OSL dating works on the principle that buried sediments are constantly exposed to ionizing radiation from naturally occurring radioisotopes. This leads to a buildup of charge in the sediments directly proportional to the amount of time the sediment remains buried. This charge gets released when exposed to light. By exposing the 90- to-250-micron-wide quartz or feldspar grains in such sediments to infrared or blue light in the lab, scientists can measure how much charge the sediments accumulated and thus date the sample. The gamma spectrometer that Lee used helps detect how much radiation a given locale emits, to determine the rate of exposure the sediments experienced.
Ross hopes to find woolly mammoth DNA from 80,000 to 100,000 years, which could shed light on as-yet uncertain aspects of their evolution. To get such old samples, Ross aims for the frozen material, scraping away material that has thawed over the last two days with an ice axe and then using a gas-powered handheld coring machine, the kind used to punch holes in concrete to put in pipes. Ross jokes we're actually digging for gold: "How do you think we fund these expeditions?"
Using the corer is difficult work"”the frozen material is rock-hard. But hopefully it will have protected the DNA over the millennia from too much degradation. Cores are then put in PVC pipes or Ziploc baggies stuffed with ripped up magazine pages or toilet paper for cushioning and put in the freezer.Lee gets cores of frozen and thawed material as well for OSL dating"”the key there is to protect the cores from sunlight.
As samples are collected, Clare assigns each of them arcane codes such as RM2008-Y1-02-OSL-02A. In this case, RM stands for Ross MacPhee, Y1 stands for Yukon site 1, the first 02 stands for the second sample area at that site, OSL stands for the purpose for which the sample was extracted, and 02A is the specific tag given that sample.
At the end of the day, we collect 25 or so cores, and head out for a dinner of Arctic blackened char, salmon burgers and barbecued ribs.
VIDEO: Sifting for fossils