RESUMO

Holocene variations of tropical moisture balance have been ascribed to orbitally forced changes in solar insolation. If this model is correct, millennial-scale climate evolution should be antiphased between the northern and southern hemispheres, producing humid intervals in one hemisphere matched to aridity in the other. Here we show that Holocene climate trends were largely synchronous and in the same direction in the northern and southern hemisphere outer-tropical Andes, providing little support for the dominant role of insolation forcing in these regions. Today, sea-surface temperatures in the equatorial Pacific Ocean modulate rainfall variability in the outer tropical Andes of both hemispheres, and we suggest that this mechanism was pervasive throughout the Holocene. Our findings imply that oceanic forcing plays a larger role in regional South American climate than previously suspected, and that Pacific sea-surface temperatures have the capacity to induce abrupt and sustained shifts in Andean climate.

Assuntos

RESUMO

Para el valle de la quebrada Saisay en Los Andes centrales venezolanos, fueron analizados los procesos y factores que intervinieron en su evolución glacial y postglacial. La metodología utilizada consistió en la realización de trabajos de campo, fotointerpretación, representación cartográfica, descripción y análisis de las facies sedimentarias para interpretar los procesos geológicos y geomorfológicos que afectaron el área. Los resultados indican principalmente la presencia de formas de erosión glacial (circos, aristas, valles en forma de U, escalones rocosos, estrías glaciales) y depósitos morrénicos neoglaciales (facies IA- 1-b). Igualmente, en el área se localizan sedimentos postglaciales de origen fluvioglacial (facies II-B-1-b) glaciodeltáicos (facies II-B-2-a y facies II-B-2-b) y depósitos glaciolacustrinos (facies II-B-2-c). El modelo de sedimentación general propuesto para esta secuencia sedimentaria se relaciona con un sistema de depositación asociado a un delta bifurcado (braid delta). Una edad mínima de 8.000 ± 500 años (TL) A.P., indica que este proceso de sedimentación pudo haberse iniciado a principios del Holoceno. En conclusión, la región estudiada se caracteriza por la presencia de formas típicas de erosión glacial, depósitos de origen glacial alpino y sedimentos postglaciales.

For the valley of the Saisay creek in the central Venezuelan Andes, the processes and factors that intervened in it´sglacial and postglacial evolution were analyzed. The methodology used consisted in photo-interpretation and detailed field works to draw the geomorphological map, and description and analysis of the sedimentary facies to explain the geologic and geomorphologic processes that affected the area. The results indicate mostly the presence of glacial erosion forms (circus, edges, U shaped valleys, rocky steps, glacial grooves) and neoglacial morrenic deposits (facies I-A-1-b). As well, there are postglacial sediments of fluvial-glacial origin (facies II-B-1-b), glacial-deltaic (facies II-B-2-a and facies II-B-2-b) and glacial-lacustrine (facies II-B-2-c). The model of general sedimentation proposed for this sedimentary sequence is related to a system of deposit associated to a bifurcated delta or braid delta. A minimal age of 8,000 ± 500 years (TL) A.P., indicate that this sedimentation process could have started at the beginning of the Holocene. In conclusion, the studied region is characterized by the presence of typical shapes of glacial erosion. Deposits of glacial alpine origin and postglacial sediments.

Assuntos