Geological and Paleontological Sites of Brazil - 041
LAGOA SALGADA (RIO DE JANEIRO)
© Srivastava,N.K. 1999. Lagoa Salgada (Rio de Janeiro) - Recent stromatolites. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 28/09/1999 on Internet at the address http://www.unb.br/ig/sigep/sitio041/sitio041english.htm [Actually http://sigep.cprm.gov.br/sitio041/sitio041english.htm]
(The above bibliographic reference of author copy rights is required for any use of this article in any media, being forbidden the use for any commercial purpose)
The Lagoa Salgada (a highly saline lagoon), occupies an area of about 16 square kilometers, near the coastal town of Cape of São Tomé of the municipality of Campos, on the north coast of the state of Rio de Janeiro (Brasil), and contains the unique ocurrence of Recent columnar carbonate stromatolites of whole of Brazil and probably of the South America. The geological history of the lagoon is intimately associated with the fluctuations of the sea level during the Late Quaternary and the formation of the delta of Paraiba do Sul river. The stromatolites occur as discontinuous pathches, principally at the southwest and northwest borders of the lake , overlying the marine sands, and are normally covered by soil or water during high - tides. The thickness of stromatole bioherms and biostromes varies all along the western border. The discrete columnar stromatolites show great variations vertically. The phytological investigations of water samples, microbial mats and the poorly ithified horizonatally stratified stromatolites revealed nineteen species of cianobacterias belonging to the following families : Chroccocaceae, Oscillatoriaceae, Dermocapaceae, Entophysalidae, Hydrococcaceae, Mycrocystacae, Pleurocapsaceae, Rivulariacae and Xenocococcasae. Besides these some clorofite and chrysophyte algae also occur in the lake. All these cyanobacterian communities, along with other invertebrades ( pelecypodes, microgastropdes, ostracodes ) participate, directly or indirectly, in the formation of Recent stromatolites of the lagoon. The isotope date of the formation of lagoon is estimated to be 3780 ± 170 years B.P.
The site houses the only occurrence of Recent carbonate stromatolites in Brazil and, probably, in South America. Their geological and paleontological importance can be compared with the other few similar occurrences in Shark Bay (Australia), Pink or Spencer Lake (Australia), Solar Lake (Israel), Han-Nan Island (China), Persian Gulf, Salt Lake (USA), Green Lake (USA), Yellowstone National Park (USA), Florida (USA), Bahamas, Bermuda and Gulf of Mexico, among others.
Lagoa Salgada (21°54'10''S and 41°00'30''W) is situated on the north coast of the State of Rio de Janeiro, in the coastal municipal district of Campos, close to Cape of São Tomé, and is part of the Deltaic Complex of Paraíba do Sul river (total area of about 2500 square kilometers and occupying part of the onshore portion of the Campos Basin). The lake is located in a sandy plain formed by holocenic beach ridges south of the Mouth of the river Paraíba do Sul (Figure 1). The marine origin of these sands is confirmed by foraminiferal analysis.
Starting from Rio de Janeiro, Lagoa Salgada is reached by the national highway BR - 101 (Rio de JaneiroCampos280 km) and later by highway RJ - 216 (CamposCape of São Tomé - 50 km).From here to Lagoa Salgada, 20 km away, access is made by secondary dirt roads within the sugarcane plantations, passing through small villages of Santa Rosa and Quixaba towards Barra do Açu.
Figure 1 - Location Map of Lagoa Salgada ( Rio de Janeiro ), Brazil
Lamego (1946; 1955) suggested the first evolutionary model for the area, and considered that the formation processes of the deltaic complex of Paraiba do Sul river gave rise to several small lagoons, amongst them Lagoa Salgada itself. Dias (1981) described calcareous stromatolites growing on the lithified biodetrical substrate and suggested hypersaline conditions for their formation. Dias & Gorini (1980) accomplished a detailed morphologic study of the coastal environment of the Deltaic Complex of Paraiba do Sul river, including the area of Lagoa Salgada, and discriminated and characterized several progradational and erosional zones. Rodrigues et al. (1981) studied the foraminifers in the sediment cores of the lake and suggested a gradual process of colmatation, passing from a marine environment to a continental one. Martin et al. (1984,1993) suggested some modifications to the Lamego Model and proposed, on the basis of isotope dating of molluscan shells collected from the the basal sediments of Lagoa Salgada, an age of around 3850 years B.P., which could be considered as the age of the formation of this lagoon. This age corresponds to the elevation of the sea level between 3.900 and 3.000 years B.P. Lacerda et al. (1984) realised physico-chemical and biological investigations of water of Lagoa Salgada and determined the concentrations of several chemical elements and nutrients. The results obtained pointed to a high chlorophyll concentration in comparison with other Brazilian ecosystems indicating a high growth rate of primary production. In 1995, Lemos investigated depositional fácies and stromatolíte structures of Lagoa Salgada and observed that the stromatlites occur at the western border of the lake and their thickness varies locally. According to her, the vertical variation in internal structure of the stromatolites is directly linked to changing environmental conditions. Silva and Silva (1999) and Silva and Silva et al. (1999) identified the following 19 cyanophyte species (cyanobacterias) in water samples, microbial mats and poorly lithified stratiform stromatolites:three belonging to the family Chroococcaceae, one to Dermocarpaceae, two to Hydrococcaceae, three to Mycrocystaceae, two to Oscillatoriaceae, one to Entophysalidacae, one to Pleurocapsaceae, two to Rivulariacae and one of the family Xenocococcaceae, most of these species are identical to those of cyanobacterial communitity of the supratidal zone of Cabo Frio (State of Rio de Janeiro). Some chlorophyte and crysophyte algaes are also found in lake water and microbial mats. The cynophyte species (cyanobacterias) identified are: Phormidium komarovii, P.hypolimneticum, P. fine, Oscillatoria terebriformis, Lyngbya aestuarii, Calotrix confervicola, C. crustaceous, Chroococus turgidus, Aphanocapsa litoralis, Aphanothece saxicola and A . halophyica.
DESCRIPTION OF THE SITE
The lake with the main axis NWSE
has a maximum length of 8.6 km and a width of 1.9 km. According to aerial pictures of
1976, the water in its central portion was about 4.5 km in length and 1.2 km in width. The
maximum water depth rarely exceeds one meter or little over that and during dry seasons
the lake could become naked dry. The mean values of measured physico-chemical parameters
of the lake water measured are as follows : temperature: 28 to 31° C; pH: 8,7 to 9,7;
electric conductivity: 52 000 to 86 200 (s/cm; alkalinity: 7,0 m Eq/l; salinity:
6,35%; CO2 (total): 233,6 mg/l; O2 (dissolved): 3,2 to 3,6 mg/l; SO4: 733,3 ppm; Cl:
13 720,2 ppm; Ca: 71,8 ppm; Faith: 0,5 ppm; K: 543 ppm; In the: 8 846 ppm; Mg: 664 ppm;
Mn: traces; SiO2-Si: 1,3 mg/l; P (total dissolved ): 143,0 g/l. The lake is situated
in an area characterized by tropical climate with strong northeast winds, although the
winds in the southwest direction are also frequent. During the months of February to
April, low rates of rainfall prevail, provoking the diminution or even absence of water.
However during the rainy season (August and September ) the water level rises, flooding
the area. The lake has a connection with the sea through the river Açu (salinity: 4,23%;
pH: 8, 2). According to the residents, the lake used to dry up during the period of
drought, fifteen years ago, when there was no connection with the river Açu, which is
actually a tidal channel.
According to Martin et al. (1993) , the formation of Lagoa Salgada took place during an erosional phase of the costal plains of river Paraiba do Sul and elevation of the sea level between 3900 and 3600 years B.P. during which the sandy barriers moved towards the continent controlled by some specific hydrodynamic conditions generated by the sea waves, south of Cape of São Tomé. These events provoked the formation of barriers and displacement of elongated lagoons away from the coast. The isotopic dates of molluscan shells, collected at the base of Lagaoa Salgada furnished an age of 3850 ± 200 years B.P., which is now considered to be the age of the formation of Lagoa Salgada.
The investigations of the lake sediments based on core samples (Rodrigues et al.,1981; Lemos, 1995) revealed that the basal sediments of the lake are marine, medium, dark green sands with abundant foraminifers. Overlying these sand are calcareous stromatolites, which are overlain by or interdigitated with lagoonal sediments represented by gray muds and marls, rich in organic matter, inercalated lightly with calcareous microbial mats, plant remains, microgastropods, diatoms, bivalves, ostracods and bioclastos of vertebrates.
Still in growth conditions, are found on the shores of the lake (Figures 2, 3, 4, 8) and are usually covered by a thin veneer of soil or vegetation or are submerged during rainy season or floodings. The lithified stromatolites form small, domical bioherms or thinly crusted biostromes, of varied thickness, but rarely more than a meter. The following types of microbial structures (microbiolites) exist at Lagoa Salgada: (a) Microbial Mats, (b) Columnar Stromatolites; (c) Oncolites; and (d) Thrombolites.
(a) Microbial Mats: gelatinous, dark, composed of a succession of very thin (0.5 - 1 mm thick), crenulated, dark (rich in organic matter) and light grey, calcareous layers (rich in magnesian calcite, calcite and aragonite), occasionally dispersed with microgastropods, ostracods and palinomorphs. They are normally exposed at the shores in two forms: when covered by a thin cover of water the mats are flat on the surface; at the emerged parts these microbian mats tend to break in small polygons, with a whitish covering of calcium carbonate (Figure 7).
Figure 2 Recent stromatolites on the western border of Lagoa Salgada.
(b) Columnar Stromatolites: This type of calcareous microbiolite structures forms small domed and subspherical (10 - 20 cm in diameter) bioherms and irregular biostromes, consisting of discreet subcylindrical columns, which are frequently linked laterally (Figure 5). The exteriors of the bioherms or the biostromes are highly lithified due to cementation, while the interior is usually porous or friable, and occasionally filled with microgastropods, worm- tubes, bioclasts of bivalves, organic material, etc. Magnesian calcite, calcite and aragonite are the common minerals of lithified columnar stromatolites.
(c) Oncolites: small, oval, ellipsoidal to irregular, up to 5 cm in size, still in growing position and associated with microbial mats, are common on the west shore of the lake ( Figures 6, 9). They are normally present in a slightly deeper and agitated aquatic environment and show thin concentric laminations.
(d) Thrombolites: microbiolites very similar in form and size to stromatolites, but devoid of internal lamination due to parasitic or symbiotic activities of invertebrates and other organisms and the accumulation of detrital material. Thrombolites are very common in this lake.
Figure 3 Small carbonate bioherms on the border of the lake.
Figure 4 General view showing microbial mats and bioherms.
Figure 5 - Polished section of a lithified calcareous bioherm
Figure 6 - Microbial oncolites in growth position on the east shore of the lake.
Figure 7 - Microbial mat with desiccation cracks and invertebrates
Figure 8 Lateral association of microbian mats and lithified bioherms.
Figure 9 - Association of microbial mat and oncolites.
The Recent stromatolites are found in a highly polluted area, principally because of the
presence of a large number of agricultural plots and areas of leisure around the lake.
This situation results in the fact that small farmers, normally employing noxious products
for the cultivation of agricultural products, directly or indirectly influence the
hydrochemistry of the lake. Although no concrete data concerning the environmental impact
of the agricultural activities on the stromatolite growth are available, enough
indications exist for such deductions, because the small agriculture producers break and
remove the stromatolite bioherms from the ground to give place to tomato, sweet potato
and corn plantations. These activities have already been practiced there for many years,
destroying a great part of the stromatolite outcrops. This destruction can be seen in
loco since blocks of biohermas are being accumulated in an area away from agricultural
plantations. Frequently, the small farmers make a trench cutting the bioherms and
biostromes to drain the polluted water into the ecosystem of the lake, provoking serious
damage to the hydrochemistry and, consequently, to the stromatolite growth. Further, the
stromatolitic limestone is used in building constructions (foundations of houses and other
constructions) by the population.
Dias,G.T.M. 1981. Complexo deltaico do rio Paraíba do Sul. In: Simpósio sobre o Quaternário do Brasil. 4, Rio de Janeiro, 1981, CTQC/SBG. Publicação Especial, 2: 58-79.
Dias,G.T.M.; Gorini,M.A. 1980. A Baixa Campista - Estudo morfológico dos ambientes litorâneos. In: Congresso Brasileiro de Geologia, 31, Camboriú/SC,1980, SBG. Anais, 1: 588 - 602.
Esteves,F.A.; Ishii,I.H.; Camargo,A.F.M. 1984. Pesquisas limnológicas em 14 lagoas do litoral do Estado do Rio de Janeiro. In: Simpósio sobre as Restingas Brasileiras, Niterói/RJ, 1984, CEUFF/UFF. Anais: 443-454.
Lamego,A.R. 1946. O Homem e a Restinga, Brasil. Rio de Janeiro. Conselho Nacional de Geografia.
Lacerda,L.D.; Araújo, D.S.D.; Cerqueira, R.; Turcq,B. 1984. Restingas - Origem, Estrutura e Processos. In: Simpósio sobre as Restingas Brasileiras, Niterói/RJ, 1984, UFF/CEUFF. Anais: 443-454.
Lamego,A.R. 1955. Geologia das Quadrículas de Campos , São Tomé , Lagoa Feia e Xexé. Rio de Janeiro, DNPM/ DGM, Boletim 154: 1-60.
Lemos,R.M.T. 1995. Estudo das fácies deposicionais e das estruturas estromatolíticas da Lagoa Salgada - Rio de Janeiro. Programa de Pós- Graduação em Geologia e Geofísica Marinha da Universidade Federal Fluminense, Niterói, Tese de Mestrado, 122p.
Lemos,R.M.T.; Silva,C.G.; Spadini,A.R. 1994. Estratigrafia e estromatólitos recentes da Lagoa Salgada - RJ. In: Congresso Brasileiro de Geologia, 38, Camboriú/SC,1994, SBG. Anais,3: 258- 260.
Martin,L.; Suguio,K.; Dominguez,J.M.L.; Flexor,J.M. ;Azevedo,A.E.G. 1984. Evolução da planície costeira do Rio Paraíba do Sul (RJ) durante o Quaternário : Influência do nível do mar. In: Congresso Brasileiro de Geologia, 33, Rio do Janeiro, 1984, SBG. Anais, 1: 84 - 97.
Martin,L.; Suguio,K.; Flexor,J.M. 1993. As flutuações de nível do mar durante o Quaternário Superior e a evolução geológica de "deltas "brasileiros". Boletim IG - USP, Publicação Especial 15, 186 pp.
Rodrigues,M.A.; Carvalho,M.G.P.; Dias,G.T.M. 1981. Interpretação paleoambiental das Lagoas Salgadas e das Ostras (RJ), Brasil. In: Congresso Latino-Americano de Paleontologia,II, Porto Alegre/RS. Anais: 893 - 906.
Silva e Silva,L.H. 1999. Estromatólitos e Cianofíceas na Lagoa Salgada, Rio de Janeiro, R.J. Levantamento Preliminar. In: Congresso Brasileiro de Paleontologia,16, Crato/CE, SBP. Boletim de Resumos: 106 - 107.
Silva e Silva,L.H.; Senra, M.C.E.; Srivastava,N.K. 1999. Composição Ficológica dos Estromatólitos Estratiformis e Esteiras Algais com Biodetritos Associados na Lagoa Salgada, Rio de Janeiro, Brasil. In: Congresso da Associação Brasileira de Estudos Quaternários, Porto Seguro/BA, 1999, ABEQUA. (no prelo).