Examinando por Materia "Bacillariophyta"

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    Palynofacies analysis of the late Holocene San Nicolás terrace of the Cauca paleolake and paleohydrology of northern South America
    (ELSEVIER SCIENCE BV, 2011-01-01) Garcia, Yuri C.; Ignacio Martinez, J.; Velez, Maria I.; Yokoyama, Yusuke; Battarbee, Richard W.; Suter, Fiore D.; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    The detailed palynofacies analysis of the late Holocene laminated succession of the San Nicolás terrace in the middle Cauca Valley, northern Colombia, is interpreted as the product of sedimentation in a lacustrine to fluvial dominated setting. Radiocarbon analyses reveal that the succession was deposited between ~. 3500 and a few hundreds of years with sedimentary rates in excess of ~. 600. cm/ka. The milimetric laminae and the high frequency fluctuations in the content of the palynodebris of terrestrial origin, i.e., the high altered ligno-cellulosic debris/amorphous organic matter ratio, together with diatoms typical of fluvial conditions, such as Aulacoseira granulata do suggest an intermittent and continuous hydrological and biological connectivity with the Cauca River, akin to a varzea lacustrine environment. Drier to wetter conditions appear to occur, moving upward in the succession. This might reflect regional precipitation conditions all along the Cauca Valley and the southern migration of the intertropical convergence zone. © 2010 Elsevier B.V.
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    Pollen-and diatom based environmental history since the Last Glacial Maximum from the Andean core Fúquene-7, Colombia
    (JOHN WILEY & SONS LTD, 2003-01-01) Vélez, Maria Isabel; Hooghiemstra, Henry; Metcalfe, Sarah; Martínez, Ignacio; Mommersteeg, Herman; Universidad EAFIT. Departamento de Geología; Ciencias del Mar
    The late Pleistocene-Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene-7. Time control is provided by 11 accelerator mass spectrometry (AMS) 14C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake-levels were low and the area was surrounded by paramo and subparamo vegetation. Late-glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake-level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late-glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake-level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. © 2003 John Wiley and Sons, Ltd.