Sedimentação de Longo e Curto Período na Lagoa dos Patos, Sul do Brasil

Abstract

Lagoa dos Patos in southern Brazil is part of the largest lagoonal system in South America. The Holocene lagoonal sediments of the Lagoa dos Patos, mostly muds, have an average thickness of about 6 m as determined by 297 km of 7.0 kHz echograms. Holocene muddy sedimentation developed over a probable Upper Pleistocene coastal plain, whose surface has a subbottom reflector that could not be penetrated by the energy of a 7.0 kHz seismic wave. The characteristics of this reflecting surface suggest indurated Pleistocene muds and/or sediments that are coarser than the overlying muddy deposits of Holocene lagoon. Based on stratigraphic correlation and the local sea level curve, we estimate that Holocene sedimentation started about 8.0 ka ago. This yields an average deposition rate of 0.75 mm/yr. A broadly comparable average rate of 0.52 ± mm/yr was obtained for cored intervals between 14C samples from the upper part of these muddy Holocene lagoon deposits. These long-term sedimentation rates are much slower than rates based on two determinations of 210Pb for surface muds deposits in the last 150 years, which yielded values of 3.5 and 8.3 mm/yr. Quite possibly the high short-term rates may be the result of more rapid lagoonal sedimentation related to deforestation of the watershed of the lagoon and other impact types related with human activities during the 150 years of European colonization in the Rio Grande do Sul state. Also, the aim of this study is to identify present and possible future environmental problems related with high lagoonal sedimentation rates such as the water quality, port dredge and the presence of mud deposits on the oceanic beach.

Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

“…The Patos Lagoon is oriented NE-SW. It is about 240 km long, with average width of 40 km, and the surface area is ~ 10,000 km 2 (Kjerfve 1986, Toldo Jr. et al 2006b). The single permanent connection with the ocean is the Rio Grande channel, and the tidal range average in the lagoon is about Figure 1.…”

“…The segmentation process, as proposed by Zenkovitch (1959), is believed to be incomplete because of currents associated with freshwater influx (Toldo Jr. 1991). The submerged relief of each lagoonal cell comprises two sections: the margins (< 5 m water depth) are mainly sandy deposits, over which grow sandy spits in water depths of circa 1 m depth; the central portion of the cells have an average depth about 6 m, and the deposits are mainly muddy sediments (Toldo Jr. 1991, Toldo Jr. et al 2006b.…”

“…The Patos Lagoon, in southern Brazil, is the biggest barrier lagoon in the world (~ 10,000 km 2 ), with a single permanent tidal inlet in Rio Grande (Kjerfve 1994, Toldo Jr. et al 2006b. Initial studies using high-resolution seismic data investigated Late Quaternary paleoenvironments in the Patos Lagoon (Weschenfelder et al 2006(Weschenfelder et al , 2010a(Weschenfelder et al , 2014, whose results established a general and regional geological framework.…”

Sedimentação da Lagoa Itapeva, RS, Brasil

“…A inclinação registrada a leste é comum a outras lagoas costeiras do estado, segundo trabalhos realizados por Tabajara (1994), Toldo Jr. et al (2000 e Meyer et al (2006). Esta característica é condicionada pela declividade da superfície pleistocênica de sub-fundosegundo estudos de geofísica realizados na Lagoa dos Patos por Toldo Jr. et al (2006) -e pela influência de sedimentação eólica relativa à presença de dunas transgressivas nas margens leste lacustres. Devido a este processo de colmatação, muitos destes corpos costeiros já sofreram grandes modificações nas suas morfologias iniciais, com expressiva redução de seus tamanhos originais (Tomazelli & Villwock, 1991).…”

“…Comparativamente os valores de CSS na Lagoa Itapeva são próximos ao encontrado no estuário da Chesapeake Bay -USA, de 10 a 150 mg.l -1 , mas considerado elevado em relação aos valores no estuário da Gironde -França, de até 3 mg.l -1 (Nichols & Biggs 1985). Por outro lado, a concentração média é semelhante a outros corpos aquosos do estado, como no sistema lagunar Patos, que apresenta uma CSS entre 50 e 150 mg.l -1 (Toldo Jr. et al, 2006) e ao Guaíba, com média de 35 mg.l -1 , e concentração máxima de 150 mg.l -1 (Andrade Neto et al, 2012).…”