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Keywords: matéria-orgânica;  Paleolimnologia;  Paleoambientes;  Paleoclimatologia;  organic matter;  Paleolimnology;  Paleoenvironmental;  Paleoclimatology
Issue Date: 26-Jun-1998
Abstract: During the end of the Pleistocene a lacustrine system was formed in the studied region by damming of the tributaries of the Doce River. The studies carried out in this area point out great importance of this region for the comprehension of the Quaternary paleoenvironmental evolution in southern-center Brazil. The major aim of the present study is to extend the contribution for the understanding of the Quaternary paleoenvironmental in the middle Doce River Valley, using a spatial-time scale approach. Five long and six short cores were collected along a bathymetric transect withins the Dom Helvecio Lake, one of the largest lakes among those in the lacustrine regional system. For the reconstruction of the climatic regional evolution together with the paleolimnology of the Dom Helvecio Lake were used multiple sedimentary organic matter tracers, including C:N, δ13C, δ15N, deposicional organic matter petrography, among others. The collected sediments were identified as representing three sedimentary facies: (1) facies of the paleo-Doce River alluvium, whose deposits were dated in ca. of 9.000 years BP, representing the damming of the paleo-Doce River and their tributaries, which was responsible for the initial formation of the Dom Helvecio Lake; (2) lacustrine-shore facies characterized by sediments deposited from about 8.500 years BP, probably, in a water column ranging from 0 to 3 meters; and (3) lacustrine facies, representing deposition that effectively occurred in a lake environmental since about 8.200 years BP. Using the time-spatial dynamics of the lacustrine-shore facies, it was possible to propose a variation curve fir the water level of Dom Helvecio, which evidence the neotectonic action as the main controller of the lake level until ca. 8.200 years BP, whilst the climatic factor should have prevailed since then. It was possible to evidenced that the lake water-column-level increased throughout their history, however, not necessarily continuously. These results and the great abundance of rests of shoreline animals nowadays than throughout the evolution of the lake, suggest that the morfometric changes due to the water level variation have leaded to a continuous increase in the shoreline deveploment. The centrales diatoms were represented 16 by Aulacoseira. A pattern of thermic stratification evolution of the lake was brought about by the compariosion between the bathymetric reconstruction of the Dom Helvecio Lake and the abundance of Aulacoseira. It was then proposed a frequent circulation pattern of the water column ( maybe polymitic) until ca. 4.200 years BP, and also hypothesized that until ca 4.200 year BP the nutrients were evenly distributed through the water column. As a consequence, a high productivity should have occurred due mainly to diatoms. Since 4.200 years BP the maximal depth of the water column surpassed the limit for occurrence of frequent circulation in the Dom Helvecio Lake, i.e., between 12 to 15 meters, reaching a more infrequent pattern. For that reason, scarcity of nutrients probably limited the primary productivity. This trend might have occurred until 2.000 years BP, when the lake apparently reached the stratification pattern observed nowadays. This phase was then classified as oligotrophic-stratified. The input of microscopic fragments from authoctonous and alloctonous sources to the sediments showed three periods of a proportionally higher income of alloctonous material. Therefore, three distrophic crises were characterized, which correlated to regional climatic changes. It was proposed an establishment of the climatic dynamics throughout the Holocene by counting the microscopic fragments from alloctonous sources. The analysis of micro charcoals size frequency showed that almost 90% of charcoals obtained from the lacustrine sediments were finer than 4.5 μm. These results suggest the atmosphere as main path of transport, charactering a regional source. In the alluvium sediments were observed a greater contribution of lager particles, suggesting a water transport into the lake. It was evidenced the occurrence of four drier phases: (1) before 8.500 years BP, which represents the drier climatic phase of the end of the Pleistocene, when the agradation of the paleo-Doce River occurred, resulting in the damming of their tributaries and the formation of the Dom Helvecio Lake; (2) 7.000-5.500 BP, represents the driest phase of the onset of the Holocene; (3) 4.000-3.000 years BP and (4) 2.000-1.500 years BP. These identified drier phases were totally or partially coincident with other ones identified at different localities in Brazil.
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