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Title: Sedimentação orgânica recente da lagoa do Caçó- Maranhão (MA)
Keywords: Marcadores geoquímicos;  Petrografia da matéria orgânica;  Caracterização recente;  Paleoprofundidade;  Elemental compositions;  Isotopic compositions;  Palynofacies;  Organic matter;  Surficial sediments
Issue Date: 12-Jun-2001
Abstract: Elemental, isotopic compositions and palynofacies observations of organic matter in surficial sediments from a transect across Lagoa do Caçó (MA Brazil) have been analyzed to investigate the processes that participate in the production and deposition of sedimentary organic matter. Five cores were collected along a bathymetric profile with 1,5; 3; 4; 6 and 9 meters respectively. This transverse profile starts in a margin (0 to 0,5m), provides a clear biological zonation with the occurrence of emergent macrophytes, declines through a region with macrophytes in association with epiphytic algae (0,5 to 2 m), exceeds a region with submerged vegetation (2 to 5,5 m) up to a complete absence of vegetation (over 5,5 m). The variation of these parameters shows in the marginal zone a gradient between 0 to 4 meters decreasing for TOC, C/N, chlorophyll derivates, and d13Co/oo and increasing for d15N o/oo values. They start at 0,5 m with values around 22, 14, -25 o/oo and between 0 and 1 o/oo and reach 10, 8, -29 o/oo and 5 o/oo respectively at 4 meters water deep characterizing therefore this emergent macrophytes vegetation. These values remain stable between 4 and 10 meters to the phytoplanktonic production with weaker marginal influence. The palynofacies results show a same pattern marked by an increase of the terrestrial organic matter from the margin to the center. Concluding, it is strictly relevant to consider that the bulk and isotopic compositions mark the biological zonation of Lagoa do Caçó (MA- Brazil), which show that the high productivity occurs in the marginal area linked to macrophyte vegetation and tend to decrease until disappear at 4 meters. The variations of these elemental, isotopic and petrographyc parameters obtained from the five cores collected in those different zones of Caçó Lake have provided 5 paleohydrological phases marked by an increase of the lake level until the last unit when it starts to decrease. These lake level changes are probably linked to the regional hydrological balance whic h is associated to the annual rainfall distributions influenced, at the same time, by the Pacific and Atlantic variability.
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