Ancient world gets precise chronology.

Scholarly debate and uncertainty have dogged efforts to specify precisely the years when various ancient civilizations thrived in the lands bordering the eastern Mediterranean Sea. An ongoing analysis of tree-ring evidence, described in the June 27 Nature, promises to bring unprecedented exactitude to the calendar of ancient history.

New data from this project yield an exact chronology of eastern Mediterranean cultures from 2220 B.C. to 718 B.C., a time span that encompasses the rise and fall of early urban centers in Mesopotamia and Egypt, as well as the emergence of societies in Greece and Rome.

"Tree-ring dating now offers the route to a new, absolute chronology of the Old World that is independent of existing assumptions, gaps in evidence, and debates," asserts a scientific team headed by Peter Ian Kuniholm, an archaeologist at Cornell University.

Although this line of investigation will probably generate a reliable time line for archaeological sites in the eastern Mediterranean, doubts still remain about the dating sequence currently proposed by Kuniholm's group, writes Colin Renfrew of the McDonald Institute for Archaeological Research in Cambridge, England, in an accompanying comment.

Prior attempts to devise chronologies for early civilizations in the Near East and Egypt relied largely on recovered documents, such as clay tablets, which outline regional successions of kings and other royal figures. Three different chronologies have been proposed on the basis of such information.

Kuniholm and his colleagues aimed to calibrate a sequence of radiocarbon dates using tree rings from a variety of ancient timbers, most of which came from modern-day Turkey. They identified what they called a floating chronology of 1,503 years, a slice of time from around the second millennium B.C. that could not be pinned to exact years.

The scientists then obtained 18 high-precision radiocarbon dates from a juniper log at a Turkish archaeological site. A statistical comparison of these measurements to radiocarbon measurements from Europe and North America, all of which have established calendar dates, resulted in a chronological sequence for the eastern Mediterranean.

That estimate still contained a slight margin of error. Confirmation of the new chronology emerged with the observation at another Turkish site of exceptional growth in tree rings that correspond to comparably enlarged European and North American tree rings dated at 1628 B.C., the investigators contend.

Some kind of environmental disturbance produced a dramatically cooler, wetter climate throughout much of the world, resulting in these alterations in tree growth, they propose. The main candidate for this disturbance is a volcanic eruption that took place on the Aegean island of Thera (SN: 4/16/88, p. 251), the researchers hold.

That volcanic blast, or perhaps another one that has yet to be identified, spewed out a blanket of dust that drastically reduced the amount of sunlight reaching Earth's surface, they propose. As a result, soil remained unusually moist that year and contributed to expanded growth of annual tree rings.

Archaeologists have recently uncovered pumice, presumably derived from the Thera eruption, in sediment surrounding the remains of a palace from ancient Egypt, Renfrew notes. The palace belonged to a dynasty traditionally thought to have assumed power in 1550 B.C. If Kuniholm's group proves correct about the 1628 B.C. date of Thera's eruption, substantial changes may have to be made in the chronology of ancient Egypt, says Renfrew.

However, an "unassailable causal link" does not yet exist between the Thera eruption and the unusual spurt of tree growth in ancient Turkey or elsewhere, the British researcher contends.

"We're saying we have evidence for a global climatic event in late 1629 B.C. or early 1628 B.C.," asserts Cornell's Maryanne Newton, a tree-ring researcher and a member of Kuniholm's team. "It wasn't necessarily the Thera eruption, but the Turkish sites in our study were downwind from Thera."

Although critical questions remain, Kuniholm's project "offers the best hope we have for a really sound chronology for the later prehistory and history of the Near East and Egypt-and indeed the eastern Mediterranean in general," Renfrew concludes.