The Oldest Wood in the World
On a bright summer day in 1987, researcher Jane E. Francis was busy with her wood saws on Axel Heiberg Island, high in the Canadian Arctic. The wood she cut was not driftwood destined for the campfire, though it did burn perfectly well. Instead, it was to be saved for laboratories and museums. Which was entirely as it should be, for the wood was 45 million years old. Dr. Francis is an expert on fossil trees, and she was sampling the remains of ancient forests.
Fossil forests are not particularly rare, even in what are now the extreme cold ends of the Earth. That Canada's far north once harbored stands of trees has been known at least since 1883, when a member of the Greely expedition found petrified wood on Ellesmere Island. But petrified wood is no longer truly woody---its cell structure has been penetrated and replaced by dissolved minerals. What remains may have the approximate appearance of wood, down to growth rings and bark scales, but it is stone, usually quartz or calcite. What Jane Francis investigated was something else entirely---true wood that had been mummified, not petrified.
Francis recounts the story of the mummified forest of Axel Heiberg Island in last January's issue of Natural History magazine (it takes a while for me to get through all the wonderful things there are to read). For someone like her, accustomed to working with wood long since turned to stone or reduced to a carbon smear between layers of sedimentary rock, investigating the ancient wood must have been a joyful experience. She even had quantities of perfectly preserved dry leaves and feathery conifer needles to examine.
The extraordinary state of preservation of trunks and stumps, leaves and litter came about because of floods and chance. When they lived, the trees formed a mostly swampy forest in a broad plain cut by several rivers. Every so often, the rivers overflowed in a massive flood---perhaps akin to the hundred-year floods that hydrologists today use to assess flood plain hazards. The rivers carried heavy sediment loads, the eroded products of a mountain range to the west of the forest site. When the rivers flooded, the sediments they dumped on the plain sealed the forest floor, smothering stumps and fallen logs in the process.
The sediment particles were very fine, so the cap they formed sealed out both rot-causing bacteria and entry of petrifying mineral solutions. That was good luck for future scientists, but still not quite enough. If sediments build up thick enough for long enough---and 45 million years is plenty long---then increasing temperatures and pressures can turn buried organic matter into coal. Yet even though the process of flooding and forest regrowth was repeated many times over thousands of years, the sediments on the old flood plain only built up to a depth of a few hundred feet. That was enough to compress the wood and squash some delicate bits of forest residue, but not enough to change the organic materials into anything else.
The final element of favorable chance was changed climate. The gradual chilling of the high latitudes killed the forests but preserved their remains. As erosion brought the mummified forest close to the surface, the refrigeration effect of the ice age held decay at bay.
The different forest layers document the slow coming of the cold. A subtropical swamp like the Florida Everglades, complete with bald cypress trees and alligators, was replaced over thousands of years by mixed woods including birch and alder, then by stands of pine, fir, and spruce.
Today, the only woody plant found on Axel Heiberg Island is the arctic willow. Its sprouts may reach a few inches high, even in the protection of stumps that once bore trees reaching a hundred feet high.
