Meaning in rings. To pinpoint such precise dates, the researchers used an innovative statistical method that integrates two different types of data. Wood samples were first subjected to radiocarbon testing, a dating mechanism with a large margin of error. The archaeologists then analyzed tree-ring growth. Each year, trees in temperate zones add a layer of new wood under the bark, creating a ring that is visible in a trunk's cross section.
Trees growing at the same time, under the same conditions, have similar patterns.
Analysis of the Seahenge samples suggested several possible dates, which were then cross-referenced with the radiocarbon results. The two dating methods together allowed for only one date, says Alex Bayliss of England's Ancient Monuments Laboratory. The exact season in which the trees were felled was divined by analyzing the fragmented nature of the final rings, which were only partially formed. Since tree rings begin growing in the spring, Bayliss concluded that the central oak must have fallen before summer's onset. The surrounding stumps all featured an additional ring, indicating that they lived a year longer.