21 December 2010
San Francisco will be rainy through this week, but I am learning about weather 55 million years ago, as recorded in the rings of ancient trees found in a coal seam on Ellesmere Island, above the Arctic Circle.
The trees caught the interest of University of Hawaii geologists Hope Jahren and Brian Schubert and their collaborators. “The wood is of spectacular preservation,“ Jahren said during her Thursday afternoon talk given in session B44B. “This is mummified wood.”
Because the wood avoided petrification (mineralization), the scientists were able to examine the rise and fall of carbon-13 within the annual rings the trees grew. Carbon-13 is a heavier form of carbon, and its use in plants depends depends on climate–if nourishment is scarce, such as during times of drought, carbon-13 is more heavily incorporated into plants. When climate is right to cause the plant to thrive, less carbon-13 can be found in plants.
Because both temperature and precipitation affect carbon-13 levels on a daily basis in plant tissue, the rings in the fossil trees found on Ellesmere Island record detailed information about weather and climate 40 to 50 million years before human beings walked the Earth.
Unlike traditional tree ring studies, Schubert and Jahren’s methods don’t count the rings or measure their thickness one-by-one. Instead, they examine the carbon-13 levels at several dozen points within a single ring to see fine-grained changes in growth that occurred over a single year. The scientists take dozens of samples, separated by as little as 40 microns, within each of the annual rings the trees grew, and this provides a detailed picture of the ancient environment that ring-by-ring analysis cannot provide.
“When you look at these high resolutions, you get information that you utterly miss if you do one analysis per ring,” Jahren said. “I think that’s the future of where ring analysis is going.”
For example, the rise and fall of carbon-13 show different seasonal patterns of carbon usage by the trees, clearly differentiating between evergreen and deciduous species. The Ellesmere Island trees have all proven to be evergreen, yet most previous studies looking at this region’s ancient environment conclude the Arctic forests of this age were deciduous-dominant.
Furthermore, the high-resolution measurements of carbon peaks and troughs provide a means of identifying the size of the associated fluctuations in rainfall and temperature. Schubert’s studies of modern evergreens (reported in session B51F) produced a mathematical model that can predict the size of the annual peak in carbon with near certainty, based on precipitation and temperature data. Applying this model to the carbon-13 curves from the mummified Ellesmere Island wood, the scientists conclude that 50 million years ago over 90 cm of rain fell annually and at least 75 percent of this fell during the summer months.
Other scientists independently conclude that this ancient ecosystem saw such wet years, but the distribution of the rain within the year may force a revision of their understanding. Previous research usually compares this ancient Arctic forest ecosystem to the temperate rain forests of modern Washington and British Columbia. But the high-resolution tree ring studies show that Ellesmere Island saw dry winters and wet summers, opposite the pattern seen in the Pacific Northwest.
Because the months of heaviest precipitation coincided with the endless Arctic summer days– when sun remains above the horizon nearly constantly–Jahren concludes that the trees must have thrived. “(Water) is being delivered when light is available. Growth rates are certainly high,” she said.
If future research determines that other regions of the ancient Arctic shared the the wet summer pattern, scientists would have a good explanation for why thick organic deposits similar in age to the Ellesmere Island coal can be found all across the now treeless modern Arctic.
–Keith Rozendal is a science communication graduate student at UC Santa Cruz