A Comparison of Glacial Chronologies Between the Eastern and Western Cordilleras, Bolivia
Author | : Colby A. Smith |
Publisher | : |
Total Pages | : 119 |
Release | : 2008 |
ISBN-10 | : OCLC:246653535 |
ISBN-13 | : |
Rating | : 4/5 (35 Downloads) |
Book excerpt: The timing and forcing of glacial advance in the tropical Andes remains uncertain with regard to both higher latitude and intra-tropical glacial fluctuations over the last glacial cycle and through the Holocene. Surface exposure age dating of glacial deposits in both the Eastern and Western Cordilleras of Bolivia coupled with glacial mass balance modeling provides a step towards understanding these relationships. 36Cl Surface exposure age dating at Nevado Sajama in the Western Cordillera, Bolivia suggests that glaciers retreated from moraines at ~16.9 ka -10.2 ka, 7.0 - 4.4 ka, and 4.7 - 3.3 ka. In the Eastern Cordillera of Bolivia at Nevado Illimani glaciers retreated from moraines at 15 ka -13.0 ka, 10.5 ka - 8.5 ka, and 3.0 ka -1.5 ka based on 10Be surface exposure ages. The new data from the Eastern Cordillera agree well with other Late Glacial and late Holocene aged moraines in the Eastern Cordillera while the data from the Western Cordillera agree well with the one previous study. However, this study provides the first dates on Holocene moraines in the Western Cordillera. The major chronological contribution from the Western Cordillera is that the Late Glacial aged advance appears to be the most extensive of the last glacial cycle and approximately synchronous with the Late Glacial advance in the Eastern Cordillera. If indeed the two advances occurred at the same time, reconstruction of past equilibrium line altitudes (ELA) suggests that that the regional ELA was nearly flat between the Eastern (4870 m a.s.l.) and Western Cordilleras (>4690 m a.s.l.) as opposed to today where it rises 360 m from the Eastern to the Western Cordillera. It is hypothesized that the change in the slope of the regional ELA results form increased winter precipitation from the west. A positive degree-day glacier mass balance model was applied to the paleo-ELAs associated with the moraines dated at Nevado Illimani. Unique solutions were obtained for periods where independent estimates of either precipitation or temperature are available. According to the model only a ~2.3 oC cooling is required to advance glaciers to their Late Glacial extents if precipitation was 50-75% higher. The model was also applied to the Western Cordillera. However, it did not produce the correct modern ELA. This suggests that PDD models are not applicable to regions where sublimation is the dominant ablation mechanism.