MUDANÇAS CLIMÁTICAS NA REGIÃO DE TAQUARUÇU (MS) DURANTE O HOLOCENO

Resumo

A Unidade Geomorfológica Taquaruçu e suas lagoas associadas (e do Mato Grosso do Sul), representam um importante registro de variações paleoclimáticas e paleoambientais do Holoceno, no alto curso do rio Paraná, correlacionáveis com o Centro sul do Brasil e NE da Argentina. Por meio do estudo de susceptibilidade magnética, datação por termoluminescência (TL) e de análise micromorfológica e sedimentar dos depósitos, foi possível confirmar que a região possui uma história evolutiva fortemente influenciada pelas flutuações climáticas do Quaternário. Uma discordância erosiva na coluna sedimentar, caracterizada pela susceptibilidade magnética e pela datação por TL, sugere uma intensa atividade eólica em 3,3 ka BP ocorrida em um período de maior aridez. Por outro lado, a ocorrência de zonas de ferruginação no perfil sedimentar e o reconhecimento de paleoníveis mais altos das lagoas são indicativos de um período úmido anterior a 3,5 ka BP. Os dados paleoclimáticos, definidos neste trabalho, encontram-se dentro do contexto da evolução paleoclimática já definida para todo centro-sul brasileiro e NE argentino.

CLIMATIC CHANGES IN THE TAQUARUÇU REGION (MS, BRAZIL) DURING THE HOLOCENO

Abstract

The purpose of this work was to establish the paleoclimatic reconstruction of the Taquaruçu region (MS) during the Holocene and to discuss it in the regional paleoclimatic context defined by. The studied area is located near the right margin of Paraná river, in Taquaruçu, southeast of Mato Grosso do Sul state, center-east of Brazil (lat. 22° 30' and 22° 45’S and long. 53° 15’and 53° 30’W) (figure 1). The area belongs to the Taquaruçu Geomorphologic Unity and it is dominated by a flatten reliev defined between 240 and 290 m of altitude and 40 m above average water-level of the Paraná river. The best characteristic of this unity is the presence of hundreds of pans (some of them dry) with diameter of 300 and 6000 m (figure 2). The sedimentar covering is constituted by massive sand to muddy sand with a thickness of 20 m layed over a 1 m bank of limonite-cemented gravel. The geologic basement is constituted by sandstones of the Caiuá Formation. The paleoclimatic and chronological data were obtained through magnetic susceptibility, thermoluminescence dating, micromorphologic analysis, detailed topographic profiles and granulometric compositive analysis. It has been processed the granulomteric analysis of 53 samples gotten from auger and trenches. Material is constituted of 60 to 70% of fine to medium massive, quartzose sand and 30 to 40% of mud (figure 5). The highest concentration of clay in the samples is probably related to eluviation. Analysis of 14 thin sections from samples collected in the trenches revealed that material has a quartzose composition in large part with corroded and fregmented grains. The cavities formed between the grains are fulfilled with ferruginous mud due to the oscillation of the ground water, in a period it as above the present level (figure 8 b).The magnetic susceptibility values, measured in sucessive sampling 0.30 m, presented two intervals with different values (superior with 45 x 10 -8 m 3 . kg -l ; inferior 5 x.10 -8 m 3 . Kg -l ) limited in the depth of 2.28 m. It has been realized 5 thermoluminescence determinations in sediments using the Aditional Doses and Total Regeneration methods. Ages varied between 2,200 and 26,000 years BP.The age/depth ratio curve (figure 1) shows an expressive inflection between the depths of 1.5m (3,390 ± 300 BP) and 2.8m (12,480 ± 400 BP), suggesting a temporal hiatus of 9,090 years in sedimentary column. The corroboration of the above mentioned data allows important inferences about the paleoclimatic evolution of the area. An expressive erosive discordance in the depth of 2.28 m was identified by the temporal hiatus in TL datations and by the abrupt change in the magnetic susceptibility values (figure 8 a). The relief quite plain permits to suppose this discordance has been provoked by intense eolic remobilization about 3,000 years BP Palinologyc data in correlated deposits of the pans suggest a semiarid period between 3.5 and 2.5 ka BP for the region. The detailed topographic survey possibilited the recognition of a former level of the lakes about 1.8 to 2.0 m above the present one (figure 8 b). This fact is reinforced by the study of micromorphology, where it has been identified evidences of processes related to oscillation of the ground-water positioned 2.0 to 2.5 m above the present level. As such evidences are found below the discordance mentioned before, it is probable that the ground water has been standing in this position in a period posterior to12.48 ka BP (age of the material) and inferior to 3.39 ka BP (age of the discordance). In this way, the existence of a humid period in this interval becomes quite suggestive for the region. Such fact matches the scheme of climatic changes of the Holocene established for the area, where a period of great humidity was defined between 8.5 to 3.5 ka BP. The deposits of Taquaruçu Geomorphologic Unity record two importan moments in the climatic history of the Holocene in the southeast region of Mato Grosso do Sul which are coherent with the pre-established evolution for the region. Due to sandy and homogeneous characteristics of the material studied, the quality and characteristics of the proxy data are quite limited. However the continuity of the studies in the pans correlate deposits of the region permit to foresee an important source of paleoclimatic and paleoambiental information.