期刊名称：Bulletin de l'Institut français d'études andines
印刷版ISSN：0303-7495
出版年度：1998
卷号：27
期号：3
语种：English
出版社：Institut Français d'Études Andines
其他摘要：Many of the models that are being developed to obtain more accurate predictions of the El Niño phenomenon require historical data on the behavior of climate. It is especially important for climatologists to obtain data of great antiquity. We find in sediment cores from lakes in the Galápagos Islands some of the oldest and most complete records of the frequency of the more intense El Niño events. The torrential rains that El Niño brings cause the lowering of salinity of the surface water of saline lagoons of the islands. It is possible to detect these changes in the sediments by mineralogical and geochemical analyses. Also the composition of species of microscopic algae, mainly diatoms changes as a result of a layer of fresh water that is formed in the surface of these lagoons during times of rain. The diatoms have cell walls of Silica that allow the preservation of the whole community in the sediments. A 4.17 m core obtained in the saline lagoon of the crater of Bainbridge to the south-east of Santiago Island provides us with a detailed history of these fluctuations in salinity. This core has been extensively radiocarbon dated (AMS) with a date at the bottom of 6 170± 55 years. We describe the complex stratigraphy of this core and the mineralogical and geochemical data. They suggest that the lagoon has stayed supersaturated with salts in all its history with several short periods of lower salinity that we attribute to increases in precipitation related to ENSO. Such periods are characterized by 83 thin bands (< 2 mm) each, rich in organic material and containing species of diatoms that indicate lower salinity. Most of these bands appear in the last 2 500 years and are more frequent in the last 800-1 000 years. Besides the record of Bainbridge we have other cores from Galápagos with a possible history of the ENSO phenomenon. A core of Genovesa Crater Lake (6 000 ± 50 years) shows fluctuations in the amount of phosphorus (coming from the guano of the red-footed boobies that nest around the crater) and of pollen of Bursera (a plant that produces flowers only when it rains). A core of El Junco Lake in island San Cristobal shows fluctuations in species of diatoms that indicate changes in lake level during the last 550 years (dated by means of Pb- 210). Cores of two coastal lagoons in Isabela and Santiago show, by the presence of marine diatoms and foraminifera, possible fluctuations in sea level.
