Tree-rings have been used to date archaeological sites from the American Southwest to the Mediterranean, but are generally not considered practical outside temperate regions. Now growth rings found in well-preserved wood samples from Chankillo, a prehistoric ceremonial site in Casma Valley, Peru, stand to usher in a new era of more precise archaeological dating in the region.
The findings were presented in April by Ron Winters, Dr. Ron Towner (Laboratory of Tree-Ring Research, University of Arizona), and Ivan Ghezzi (Yale University) at the Society for American Archaeology meeting in Milwaukee, Wisconsin. The wood samples were collected last summer by Earthwatch team members working with Winters, Towner, and Ghezzi, a Peruvian archaeologist and principal investigator of the Earthwatch-supported Pre-Inca Civilization project.
"Chankillo yielded the first large collection of prehistoric tree-ring samples from Peru," said Ghezzi. The Chankillo site is the legacy of the Chavìn civilization, considered the source of later Andean cultures like the Inca, which occupied the Casma Valley from 800 to 200 b.c. "The benefits for Peruvian archaeology will be enormous, possibly bringing the field up to the level of chronological precision found in other areas."
Tree-ring dating of archaeological samples, or dendroarchaeology, provides precise "calender" dates for prehistoric sites by matching samples to "master tree-ring sequences" from the same region and time period. Radiocarbon dating, by comparison, has an estimated error range that is often as large as 100 years. For some periods, such as when Chankillo was built, they can be in error by several hundred years, giving only an approximate time frame.
It has generally been believed that tree-ring dating is only relevant to archaeological sites in temperate zones, and it has been widely used in North America and Europe to date charcoal from hearths, wooden tools, or timbers used in buildings. Tropical areas, with their less marked seasonality, were not expected to produce the distinct growth rings necessary for the technique.
Earthwatch teams helped Ghezzi and Towner sample wood from more than 100 algarrobo (Prosopis sp.) wood lintels from the Chankillo site, many still perfectly-preserved and socketed in their original positions after 2,500 years. The wealth of this large sample is a rare find in an area where prehistoric wood was often recycled by later populations.
Towner and Ghezzi are still in the process of building the "master tree-ring sequence" from the sample, after which it will be invaluable for reconstructing the chronological development of Chankillo and dating wood samples from other sites. It will also tell archaeologists about the prehistoric climate.
"Much has been written on the proposed influenced of El Niño induced rains, floods, or droughts in shaping Andean civilizations," said Ghezzi. "But obviously this can only be tested with a solid chronological record, such as that stored in the tree-rings of Chankillo."
Earthwatch Institute is an international nonprofit organization which supports scientific field research worldwide by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to engage people worldwide in scientific field research and education to promote the understanding and action necessary for a sustainable environment.