Apport des méthodes basées sur le déséquilibre radioactif (238U-234U-230Th-226Ra-210Pb) aux études des variations et changements climatiques

Dating methods based on radioactive disequilibrium in the 238U-series have been of great importance for understanding the climatic variability during the last climatic cycle. These methods allow the obtention of time constraints and sometimes the dating of recent formations from 500 ka to now according the radioisotopes that are used. For instance, for the most recent period (120 years to zero), 210Pb (T1/2 = 22.6 years) can yield information on the mixing processes or bioturbation and, in some cases, date the sediments accumulated recently in lakes, rivers or peat bogs. For the period documenting the Holocene period, 226Ra (T1/2 = 1600 years) is the most appropriate chronometer. Finally, uranium-series method based on the disequilibrium between 238U and its daughters 234U-230Th was particularly efficient for the reconstitution of variability and climatic changes during the last climatic cycle. The U-series results were used to validate the Milankovitch parameters by dating the high marine levels during the interglacial periods. Short events recorded in the continental carbonates (speleothems and travertines) can be also dated by U-series. The comparison between isotopic records (δ13C and δ18O from the carbonated phase) with the ages can document abrupt changes in climatic events. On the other hand, a calibration curve of 14C ages versus calendar ages was established by dating corals reefs with U-series and compared with their 14C ages. We present several examples in this paper.