A palynological study of the upper quaternary sequence in the El Abra corridor and rock shelters (Colombia)

Abstract

This palynological study is a contribution to the reconstruction of the vegetational, climatological and stratigraphical sequence in the El Abra corridor principally during the Last Glacial and the Holocene. It seems likely that the sequence reaches into the Last Interglacial and even further down into the last part of the penultimate Glacial. The El Abra corridor forms part of the Sabana de Bogotá located in the Colombian Eastern Cordillera (altitude of site 2,570 m). The sequence starts with a succession of several warmer and colder intervals (El Abra zones I and II) during which the El Abra valley lay mainly in the Andean forest belt with a shift to the subpáramo vegetation belt during the relatively coldest periods. Weinmannia was well represented among the trees. A series of peat, peaty clay and clay was deposited. We have named the warmer intervals, El Abra zones Ib, Id, IIb, IId, respectively, the Rocas de Sevilla I, II, IIIa, IIIb intervals. At the transition of the El Abra subzones Ild to IIe, a 14C-dating yields an age of approximately 50,000 years. Thus the El Abra subzones IIa-d probably correspond with Early Glacial interstadials and stadials, subzone IIe with the Lower Pleniglacial and subzone Id with the Last Interglacial (Europe: Eemian). However, these correlations are very tentative. During the next series of warmer and colder intervals (El Abra zone III), conditions changed in the valley. During the first two warmer intervals, named Rocas de Sevilla IVa and b (El Abra subzones IIIa 1 and IIIa 3), the valley was inundated due to a rise in the level of the Sabana Lake caused by an increase of the effective precipitation. From the end of the third warmer interval, named Rocas de Sevilla V interval (El Abra subzone IIIc), there is a 14C-dating of ca. 28,000 years B.P., which correlates this interval with the Santuario interval and approximately with the European Denekamp interstadial. El Abra zone III likely corresponds with the Middle Pleniglacial and the intervals RIVa and RIVb probably with the European Moershoofd and Hengelo interstadials. Lake clay was deposited during inundation periods and is followed by a series of “soils”. Alnus rather than Weinmannia becomes dominant among the trees; characteristic elements of the Andean forest disappear. The vegetation belts shift from the Andean forest/subpáramo zones to the subpáramo/páramo zones. El Abra zone IV forms the coldest part of the Last Glacial (Upper Pleniglacial). During the Fuquene stadial (actually the coldest part), conditions were dry and the effective precipitation was probably lower than it is today. A series of soils was deposited (or formed), with a high silt content (“loessic”), during the coldest phase. At the end of zone IV the humidity increased again, which resulted in a renewed inundation of the El Abra corridor during part of the following Late Glacial (El Abra zone V). This renewed rising of the Sabana Lake took place in the Guantiva interstadial, corresponding approximately with the European Bølling and Allerød interstadials. The Andean forest again invaded the valley. During the following El Abra stadial (type section: the succession in II-B3), conditions became colder and drier again. The Late Glacial sedimentary sequence consists of “soily” material, lake clay and peaty clay. At the beginning of the Holocene (El Abra zones VI, VII, VIII), the more definite amelioration of the climate took place. The El Abra corridor formed part of the Andean forest belt. Several species, representative of the Andean forest, reappear after a long absence or occur for the first time in the diagram. The sediment consists of a partly homogenized “soily” material. During the upper part of the Holocene, human influence on the vegetation is apparent. Apart from the pollen analysis, opaline silica bodies were studied; these bodies (= phytoliths) are remnants of plants. The main contributors are grasses. Each genus may have its own characteristic types. It appeared that the opaline silica bodies may provide very useful information in two ways. A quantitative analysis offers method for correlation. A qualitative analysis offers the possibility to identify various taxa within the Gramineae, which is otherwise impossible on the basis of pollen grains.