"It really is the premier scientific method for dating structures made of wood."
Tree Rings Tell Time
The key relates to the way trees grow: Each year they lay down a new ring of wood around their trunk. In good years—when trees grow well—the rings are wide, and in bad years the rings are narrow.
Most of the time scientists use this information to figure out how the climate has changed over time in a particular area.
But it also gives archaeologists a valuable tool they can use to date wooden artifacts by cross-referencing tree-ring sequences in different samples.
In the case of the World Trade Center discovery, the ship was made of wood from hickory, spruce, pine, and oak, but Cook and his colleagues based their findings on tree rings contained in planks of white oak, which were used to build the ship's frame.
White oaks grow throughout eastern North America, but the scientists were able to narrow in on a more precise location because they already had information on tree-ring sequences in white oaks from North Carolina to Massachusetts. All that was required was a comparison between those sequences and the tree-ring sequences contained in the boat's timber.
As it turns out, "the same pattern of growth variability in the World Trade Center boat was found in timbers in southeastern Pennsylvania," Cook said. "There is no indication that timbers came from a more remote area."
The oldest ring that they identified dated back to the late 1400s, during the time of Columbus's maiden voyage to the New World. And the youngest ring—laid down in the year the timber was harvested—dated back to 1773, the same year that Bostonians held their famous Tea Party.
The later date not only tells us about when the wood was harvested, but also provides a window into the likely construction period.
"You can make an educated leap of faith that the timber was used as soon as it was cut down, and that there wasn't any seasoning involved," said Martin Bridge, an expert on dating wooden artifacts at the University College London in the United Kingdom.
"With shipbuilding you usually use [the wood] within a year or two [of it being harvested] because it's easier to work with," so it seems like a reasonable assumption that the ship was built in 1773 or soon after, noted Bridge, who was not involved in the study.
Short Life
Cook and his colleagues were able to surmise that the ship was probably built in a small shipyard in Philadelphia because all of the timber had the same tree-ring sequence. Large shipyards would have received wood from a variety of locations, each with different growth conditions, so experts wouldn't expect their tree-ring sequences to match up as nicely.
Once built, the ship "would have been taken down the Delaware River, used for various purposes, and then taken to Manhattan where it was scrapped," Cook said.
The area in Lower Manhattan where it was found was filled in in the 1790s, so—for reasons that aren't entirely clear—the ship's working life probably didn't extend beyond a decade or two.
