Produção Nacional
Revisado por pares
Human occupation and paleoenvironment during the Holocene of southern Brazil: Sangão and Garivaldino sites
2012; Elsevier BV; Volume: 279-280; Linguagem: Inglês
10.1016/j.quaint.2012.08.290
ISSN1873-4553
Autores Tópico(s)Geology and Paleoclimatology Research
ResumoA 586 m long core, Funza II, was recovered from the lake sediments of the high plain of Bogotá (Eastern Cordillera, Colombia, 2550 m alt.), which represent the bottom of a drained lake. Results of the palynological analysis of the interval 2–158 m with sample distances of 1 m are presented. Time control of the Funza II core is based on zircon fission-track datings of intercalated volcanic ash horizons. In addition, pollen r cord Funza II could be correlated in detail with pollen record Funza I, located at 1 km distance. The Funza I pollen record has been graphically correlated with the oxygen isotope record of ODP Site 677 (East Pacific). Oxygen isotope stages 19.1 to 3.0 have been tentatively recognized in the pollen record, indicating that the interval 158-2 m represents approximately the period of 735-28 ka. The average calculated time resolution is 4550 yr with considerable fluctuations depending changes in accumulation rate.The interval 158-131 m core depth (estimated age 735-569 ka) shows warm climatic conditions most of the time and is tentatively correlated with the oxygen isotope stages 19.1 to 15.1. The pollen spectra have no direct modern analogues because of the absence of Quercus and related conditions. The upper forest line oscillated mainly from 2100 to 2700 m during most of the interval. The corresponding average annual temperature on the high plain is 6.5–11°C. The high plain was situated in the Andean forest belt most of the time. The upper limit of the subandean forest belt (Acalypha, Alchornea) was situated some hundreds of meters below the modern elevation. Podocarpus was most important in the lower part of the Andean forest belt. Weinmannia forest, the precursor of the modern Weinmannietum, included a substantial contribution of Hedyosmum as well as lower frequencies of Myrsine (= Rapanea) and Eugenia. A type of Vallea-Miconia forest, including low frequencies of Ilex and Myrsine (= Rapanea), could have occurred on the drier parts of the high plain. The lake was shallow most of the time, with local marsh vegetation of cyperaceous reed swamp and Hydrocotyle. Myrica thickets M. parvifolia) a nd Alnus carr (swamp forest) covered the wet flats around the lake. Myrica (M. pubescens) and Alnus possibly contributed also with low frequency to the zonal Andean forest belt. Dwarf forest of Polylepis, Myrica and Compositae scrub occurred at the upper forest line.The interval 131-100 m core depth (estimated age 569-350 ka) shows cold climatic conditions most of the time and is tentatively correlated with oxygen isotope stages 14.4 to 11.1. The upper forest line oscillated mainly from 1800 to 2500 m. The corresponding average temperature on the high plain is ca. 5–9.5°C. The high plain was situated in the grassparamo belt most of the time. Apart from Gramineae (e.g. Calamagrostis, Chusquea) and woody stem rosettes of Espeletia (Compositae) a variety of paramo herbs (Valeriana, Caryophyllaceae, Geranium, Aragoa, Lycopodium) were present with substantial frequencies, and abundant cushion bogs of Plantago were present. The water level in the lake was high and marsh vegetation limited. Polylepis dwarf forest occurred in the subparamo belt, along with shrub of Compositae, Hypericum and Ericaceae. In the Andean forest belt Vallea-Miconia forest and Weinmannia-Hedyosmum forest were most important.The interval 100-57 m core depth (estimated age 350-186 ka) shows warm climatic conditions most of the time and is tentatively correlated with the oxygen isotope stages 10.2 to 7.1. The upper forest line oscillated from 2000 to 2600 m in the first part and from 2600 to 2900 m in the last of this interval. The corresponding average annual temperature is 6–10° and 10–12°C, respectively. The high plain was in the first part of this interval most of the time situated in the paramo and in the last part of this interval in the Andean forest belt. During this interval Quercus migrated into the area of the high plain. Quercus forests occurred in a wide altitudinal range (1000–2800 m) and constituted at first local patches of forest, but at the end of this interval zonal Quercus forests were a major part of the Andean forest belt. Acalypha and Alchornea reached higher elevations in the Quercus forests, and the upper limit of subandean forest rose to modern elevations. Weinmannia dominated in the Weinmannia-Hedyosnum forest type. At the end of this interval the contribution of Vellea-Miconia forest increased markedly and replaced Weinmannia forest. Podocarpus-rich forest occurred in the lower part of the Andean forest belt. Alnus carr and Myrica thickets were abundant around the lake, which was of a shallow type. Algae (Botryococcus) became very abundant from the beginning of this interval to the top of the record.The interval 57-2 m core depth (estimated age 186-24 ka) shows for the first time in the record abundant presence of zonal Quercus forests. The composition of the Andean forest belt had changed dramatically. Based on arboreal percentages, climatic conditions seem warm most of the time, but the high frequency of Quercus, a wind-pollinated tree that produces large amounts of pollen, exaggerates real conditions. This is tentatively correlated with the oxygen isotope stages 6 to 3.0. The forest line oscillated from 2000 to 3000 m most of the time. The corresponding average annual temperature is 6–12.5°C. Quercus forests, resembling the modern Saurauia-Quercus humboldtii forest, and Weinmannia-Hedyosmum forest, resembling the modern Weinmannietum, dominated in the Andean forest belt. Vallea-Miconia forest probably resembled the modern Xylosma-Duranta-Vallea forest, but the latter is palynologically difficult to recognize. Eugenia, Ilex and Myrsine (= Rapanea) contributed substantially to this rather dry forest type of low stature. Polylepis dwarf forest was frequent at the forest line and possibly also in the paramo belt up to 4000 m. Alnus carr dominated completely the flat parts of the high plain. Myrica thickets and marsh vegetation diminished in the last part of this period. Sediment accumulation was very rapid (up to 60 cm per 1000 yr). Supposedly, erosion of the Tequendama Falls in the Bogotá River, the only outlet of the high plain, led to the final draining of the lake between ca. 28 and 22 ka.The Andean biozones VI and VII are represented in this part of the record. Biozone VI (158-94 m core interval; estimated age 735-350 ka) is characterized by the presence of Alnus and absence of Quercus. Biozone VII (94-0 m core interval; estimated age 350-0 ka) is characterized by the presence of Alnus and Quercus, both from northern hemisphere origin.
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