Evidence from speleothem for abrupt climatic changes during the Holocene and their impact on human settlements in the Eastern Mediterranean region: Dating methods and stable isotope systematics

摘要

Evidence from speleothem for abrupt climatic changes during the Holocene and their impact on human settlements in the Eastern Mediterranean (EM) region. Climate change is thought to drive ho-minid evolution and especially the "out of Africa" dispersal of hominids. One of the main routes of human dispersal out of Africa was the eastern route along northeast Africa and the Levant. Tracing climate variability in this region is crucial in understanding the environmental settings that accompanied the human settlements. Here we examine the changes in the rainfall amount, based on accurate U-Th dating of cave deposits (speleothems) in the EM region (central Israel) during the Holocene, and their oxygen and carbon isotopic composition, and ask if these changes are evident in the archeological records of the area. We estimate the annual paleorainfall amounts, based on the present-day relationship between rainfall amount and its isotopic composition, temperatures and the isotopic composition of the speleothems. The time period with the highest rainfall occurred during the earlier part of the Holocene. A general trend towards aridity associated with a drop of ~ 40% in the rainfall amount, is observed from about 7.5 kyr. Superimposed on this general trend there are several sharp climatic events associated with decrease in rainfall. These events almost always coincide with major archeological events. The transition into the Chalcolitic period immediately followed a sharp drop in the rainfall amount at ~ 5.2 kyr, which was the most severe drop during the Holocene, but was very short-lived, lasting less than 200 years. The highest rainfall amount during the later part of the Holocene occurred from 4.8 to 4.6 kyr and was followed by 600 years of gradual decrease in rainfall. At about 4.0 kyr the rainfall had dropped by 30-50%. This sharp drop coincided with a relative increase in the proportion of C4 type vegetation in the region. These aridity events overlap in time with periods of destruction of human civilization in the Middle East and North Africa. Since about 3.5 kyr, the amplitude of changes in the rainfall amount became smaller, but still large enough to affect human populations. This paper is divided into two topics: l) A short summary of the principles of radioactive C and U-Th dating methods; 2) The use of karstic cave deposits (speleothems) from the EM region as paleocli-mate indicators, and the impact on human settlements.