CHRONOLOGIE ET INTERPRETATION PALEOCLIMATIQUE DES FLUCTUATIONS DES GLACIERS DANS LES ANDES DE BOLIVIE (16°S) DEPUIS LE MAXIMUM DU PETIT AGE GLACIAIRE (17EME SIECLE)

Abstract : The evolution of about fifteen glaciers of the Bolivian Eastern Cordillera (16-17°S; 68°W), since their Little Ice Age (LIA) maximum till the late 20th century, has been reconstructed on the basis on ten main moraines observed on the glacier forelands and six aerial photographs. A correspondence between the moraines from a glacier margin to another has been established using geomorphological criteria. These moraines have been dated by lichenometry (Rhizocarpon geographicum s.l.) using a new statistical method based on the extreme values theory. Glacier maximal extent has been dated from the second half of the 17th century. Glaciers kept, until the first half of the 18th century, a quite similar position. From the 1730s – 40s, they began to retreat nearly continuously. The most important moraines attest to periods of standstill or small advances which were never strong enough to remove former moraines. Glacier retreat accelerated during the late 19th and early 20th centuries. Over the 20th century retreat remains fast. Over the whole studied period, glaciers lost about 60% of their maximal extent and the ELA rose by about 135 m. Moraines' dating as well as Bolivian glaciers' evolution since the late 19th century are in good agreement with data available in other places of the tropical Andes such as in Peru and Equator. A synchronism in glacier evolution at a regional scale due to a common climatic forcing can be stressed. We suppose that the Bolivian glaciers' maximal extent was forced by enhanced precipitation (20 to 30% higher than current mean) and reduced temperature (0.4 to 0.6°C lower than current mean). In addition, we can note the concomitance between periods of low solar activity and periods of glacier advances. Retreat from the first half of the 18th century could be mainly due to a decrease in precipitation. These hypothesis agree with the results obtained from the scarce climate proxies available in this part of the Andes such as the ice core retrieved in the southern Peru Quelccaya ice cap. During the late 19 – early 20th century, numerous and intense El Niño events, pointed out by several indicators, could be responsible of the accelerated glacier recession which precipitated the LIA ending in the tropical Andes. Unlike the former periods during which glacier evolution appears to have been mainly controlled by precipitation, their retreat during the 20th century seems to have been the result from a combined increase in temperature and humidity.
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Antoine Rabatel. CHRONOLOGIE ET INTERPRETATION PALEOCLIMATIQUE DES FLUCTUATIONS DES GLACIERS DANS LES ANDES DE BOLIVIE (16°S) DEPUIS LE MAXIMUM DU PETIT AGE GLACIAIRE (17EME SIECLE). Autre. Université Joseph-Fourier - Grenoble I, 2005. Français. ⟨tel-00012124⟩

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