Ancient Stalactites Have A Tale To Tell


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The stalactites and stalagmites of Israel's Soreq Cave - a well-known local tourist attraction - now have another claim to fame: they provide the only continuous record known of eastern Mediterranean rainfall and vegetation over the past 58,000 years.

A study conducted by Dr. Aaron Kaufman of the Weizmann Institute's Environmental Sciences and Energy Research Department together with Drs. Miryam Bar-Matthews and Avner Ayalon of the of the Geological Survey of Israel demonstrates that these ancient cave formations are a unique repository of information about climatic fluctuations.

"Past changes in climate are reflected in the way stalactites and stalagmites grow and develop," says Kaufman. "By carrying out isotopic analyses on more than a hundred samples from each individual structure, we were able to piece together an ongoing picture of diverse climatic changes."

The study showed that throughout most of the period between 20,000 and 58,000 years ago, the climate in the region was dry and cool. Afterwards, the annual rainfall fluctuated significantly, and about 6,000 years ago the general climatic conditions became similar to those of today. These changes were reflected in the types of local vegetation.

Stalactites are icicle-shaped formations hanging down from cave roofs, while stalagmites look similar but point upward from the cave floor. Both are created when water picks up calcium carbonate from the soil and rocks surrounding the cave, and then trickles through cracks in the cave roof, leaving some of the mineral behind as it drips down onto the floor. Every hundred years or so a distinctive layer, or ring, is formed, so that the cross section of a stalactite or stalagmite resembles that of a tree trunk.

The scientists performed three types of isotopic analyses on the samples. For purposes of dating, they took advantage of the fact that when stalactites and stalagmites are first formed, they contain a known quantity of one particular radioactive isotope - uranium 234 - but none of a different isotope, thorium 230. Uranium 234 decays at a uniform rate, during which some of it is converted into thorium 230; the older a sample is, therefore, the higher its thorium-to-uranium ratio.

To obtain data about the climate from these same samples, the researchers analyzed the ratios of stable isotopes that are known to be a function of specific temperature and precipitation conditions: the ratios of carbon 13 to carbon 12, and of oxygen 18 to oxygen 16.

The knowledge of past climatic changes provided by this study may lead to a better understanding of today's climate and may also help make climatic projections for the future. These findings were partly reported in Quaternary Research (1997, Vol. 47, no. 2), and will be further described in the Proceedings of the International Symposium on Isotope Technique in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.