| The cold ice caps of the
highest peaks of the Andes represent natural archives of
past environmental conditions. During the past 15 years,
Thompson et al. have recovered several ice cores from
sites within the tropical circulation regime (Quelccaya,
Huascarán, and Sajama). Their analyses allow to
reconstruct the climatic history of the tropics since the
last glaciation [1]. In order to find a glacier
preserving the chemical and isotopic signal from a
precipitation regime affected by the Westerlies, we
performed an exploratory drilling on top of Cerro Tapado
(5550m, 30°08'S, 69°55'W) in Northern Chile in February
1998. Cerro Tapado is one of the northernmost glaciers south of the "South American Arid Diagonal" within the Westerlies circulation regime. At the drilling site, the glacier has a maximum thickness of 42 m. Two ice cores, 10 m each and 50 m apart could be recovered with a portable solar powered electromechanical 2-inch drill. From ice core stratigraphy, chemical analyses, isotope measurements as well as the evidence of various regional volcanic eruptions we conclude that these cores cover the period between 1980 and 1998 (Fig.1). Precipitation events in the area of Cerro Tapado occur primarily between May and September (i.e. winter). Because of their predominant Pacific moisture source and their regular annual deposition period, we assume the chemical concentrations of the measured species to be rather uniform in fresh precipitation. Hence, the observed strong variability of the chemical signal may be related to post-depositional effects such as sublimation (estimated to be between 3 and 6 mm water per day [2]) or dry deposition. We therefore use the structure of the chemical signal and particle concentration to distinguish between dry (high concentration) and humid conditions (low concentration) during the time period covered by the ice cores. Dating was supported by volcanic eruptions (Pinatubo - June 1991, and El Chichon - April 1982) identified by high concentrations of sulphate, and in addition, by several distinct d18O depletions, which may be caused by extreme precipitation events in connection with the phenomenon El Niño. By applying this time scale, it is possible to reconstruct 15 years of precipitation history of the Cerro Tapado ice cap. The results are in agreement with climatic data (precipitation and temperature) recorded at a meteorological station in Elqui Valley [3] with El Niño events established with Sea Surface Temperature Anomalies. |
 Fig. 1: Chemical concentration (mg/l) and isotopic ratio along Core 3, Cerro Tapado (Chile). During summer 1998, we developed a 3-inch light solar powered electromechanical 50-m-depth drill. In February 1999, we drilled to bedrock on the top of Cerro Tapado, installed a meteorological station and performed various experiments to study post-depositional effects. This 36 m long ice core should recover the 20th century and more and will allow to evaluate anthropogenic influences and to reconstruct the intensity of past El Niño- Southern Oscillation events. |