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Among other things, they hope to learn whether the gigantic volcanic eruptions that have punctuated geological history have occurred as clusters in relatively short periods. In Irish and German oaks, longer-term effects of volcanoes are discernible: after major eruptions, the rings for several successive years are thinner than normal because of slow growth in protracted periods of cold.
Climatologists are also intrigued by fresh evidence from tree rings that huge ancient eruptions may have had far more lasting effects on climate than had been supposed. Large eruptions have stamped sharply defined benchmarks on tree rings and ice, and these markers have permitted scientists to reconcile many dating techniques and improve the accuracy of all.
Similar chronologies have been developed recently in two regions of West Germany.
It seems, therefore, that the Dutch masters imported their wood from Poland or Lithuania. Baillie said, ''all these techniques are based on circumstantial evidence.A major landmark in dendrochronology was reported last month in the British journal Nature by Dr. The Belfast scientist reported that traces of at least five major eruptions in prehistoric or ancient times are clearly visible in the rings of Irish oaks that grew over the last 7,272 years. Moreover, scientists in the United States and Ireland have discovered how to use tree-ring chronologies to greatly enhance the precision of carbon dating in determining the ages of organic objects by their radioactivity. Gordon Pearson of Queen's College, Belfast, and Dr.Minsi Stuiver of the University of Washington, Seattle, have independently discovered an important relationship between the true age of a wooden object and its apparent age as determined by carbon dating.In Europe, a lot of the material we use in this stage of the chronology comes from beams and planks in historic buildings, and that takes us back through the Middle Ages.''You just keep looking for overlapping ring patterns in wood farther and farther back, but once you get to the time of Christ, you begin to run out of man-made wooden objects.