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SIGEP
Geological and Paleontological
Sites of Brazil - 041
LAGOA SALGADA (RIO DE JANEIRO)
RECENT STROMATOLITES
Date:28/09/1999
Narendra K.
Srivastava
narendra@geologia.ufrn.br
Departamento de Geologia (UFRN)
Department of Geology (UFRN)
Natal ( RN) - Brasil
© 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
https://sigep.eco.br/sitio041/sitio041english.htm]
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ABSTRACT
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.
INTRODUCTION
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.
LOCATION
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 Janeiro–Campos–280 km) and later by highway RJ - 216 (Campos–Cape 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
HISTORICAL
NOTES
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 NW–SE
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.
STROMATOLITES
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.
PROTECTIVE MEASURES
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.
Considering these facts, it is essential that urgent and drastic
measures should be taken to protect this geological–paleontological site, before the
impressive occurrences of Recent stromatolites are destroyed by predative anthropogenic
activities. Similar occurrences, for instance, in Australia (Shark Bay), of the recent
stromatolites, are being preserved not only as a natural and scientific patrimony, but
also to make ecotourism. The necessary measures to protect this site would be to remove
the small land farmers from the surrounding areas and isolate the lake from degradation.
BIBLIOGRAPHIC REFERENCES
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Simpósio sobre o Quaternário do Brasil. 4, Rio de Janeiro, 1981, CTQC/SBG.
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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
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Salgada, Rio de Janeiro, R.J. Levantamento Preliminar. In: Congresso Brasileiro
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