Palynomorph and Diatom Evidence - BioOne

Journal of Coastal Research

25

1

224–233

West Palm Beach, Florida

January 2009

Holocene Marine Transgression in the Coastal Plain of Rio Grande do Sul, Brazil: Palynomorph and Diatom Evidence Svetlana Medeanic†, Lezilda Carvalho Torgan‡, Luiz Carlos Pinheiro Clerot†, and Cristiane Bahi dos Santos‡ Instituto de Geocie`ncias Universidade Federal do Rio Grande do Sul Avenida Bento Gonçalves 9500, CEP 91509-900 Porto Alegre, RS, Brazil [email protected]

†

Museu de Cie`ncias Naturais Fundaçaõ Zoobotaˆnica do Rio Grande do Sul Rua Dr. Salvador França 1427, CEP 90690–000 Porto Alegre, RS, Brazil

‡

ABSTRACT MEDEANIC, S.; TORGAN, L.C.; CLEROT, L.C.P., and SANTOS, C.B., 2009. Holocene marine transgression in the coastal plain of Rio Grande do Sul, Brazil: palynomorph and diatom evidence. Journal of Coastal Research, 25(1), 224– 233. West Palm Beach (Florida), ISSN 0749-0208. Based on sedimentology, geochronology, palynology, and diatom analyses from core silt sediments in Cassino Beach (32⬚11⬘06⬙ S and 52⬚09⬘45⬙ W), southern Brazil, the Holocene marine transgressive stage was established. The absolute age of one sample is about 4940 ⫾ 80 years BP. The palynomorphs (pollen and spores of vascular plants, zygospores and colonies of Chlorophyceae, cysts of dinoflagellates and acritarchs, fungal spores, and microforaminifera), silicoflagellates, and diatoms indicate the presence of an inlet bay in the southern part of the coastal plain during the marine transgression. The changes in the taxonomic composition, abundance, and frequency of palynomorphs and diatoms from the samples corresponding to transgression show an oscillatory character of the sea level. The posterior marine regression resulted in sand deposition and dune formation. The results demonstrate the importance of palynomorph and diatom application for the palaeoenvironmental reconstructions in coastal plains. ADDITIONAL INDEX WORDS: Palaeoenvironmental reconstructions, southern Brazil.

INTRODUCTION The present coastal environments in the state of Rio Grande do Sul include dunes, wetlands, and brackish and salt marshes and lagoons. Their development has a long history connected with sea level oscillations and drastic climatic changes during the Quaternary. The Quaternary sea level oscillations resulted in transgressions and regressions and were the principal factors forming the present ‘‘face’’ of the Brazilian coastal plain (TOMAZELLI and VILLWOCK, 2000; VILLWOCK et al., 1986). Considerable sea level advances and retreats occurred on the Brazilian coast, too. Three stages of sea level rise during the Holocene were established. The most considerable sea level rise of 4–5 m amplitude occurred at about 5100 years BP (ANGULO and LESSA, 1997; ANGULO et al., 1999; LESSA et al., 2000; MARTIN, DOMINGUEZ, and BITTENCOURT, 2003). The multidisciplinary study of Quaternary sediments based on sedimentology, paleontology, palynology, diatom analysis, and absolute age dating methods makes it possible to reconstruct past environments, climatic changes, and sea level oscillations that occurred in the Holocene. The pollen and spores of terrestrial and aquatic plants are usually used for palaeoenvironmental and palaeoclimatic reconstructions (TRAVERSE, 1988). However, they appear to be insufficient DOI: 10.2112/07-0935.1 received 21 August 2007; accepted in revision 10 January 2008.

for reconstructing aquatic environment parameters, such as temperature, depth, salinity, pH, and nutrients. The algal palynomorphs of coccoidal green algae are important for recognizing past freshwater environments (JANKOVSKA´ and KOMAˆREK, 2000; KOMAˆREK and JANKOVSKA´, 2001; VAN GEEL, 1976; VAN GEEL, BONCKE, and DEE, 1980/81; VAN GEEL and VAN DER HAMMEN, 1978) and coastal aquatic environments with high salinity (MEDEANIC, JANKOVSKA´, and DILLENBURG, 2003). Dinoflagellate and acritarch cysts are useful for reconstructing environments influenced by sea waters (DALE, 1976, 1978; DOMINGUEZ, 1987; GRILL and QUATROCCIO, 1996; SARJEANT, 1970; TRAVERSE and GINSBURG, 1967). All mentioned palynomorphs are resistant to destruction due to their sporopollenin-like layer in their cell walls. Fungal spores and microforaminifera composed by a pseudochitina, resistant to destruction, may also contribute to palaeoenvironmental reconstructions (ELSIK, 1971; THUNELL and WILLIAMS, 1983; TRAVERSE, 1988). In the southern coastal plain of Rio Grande do Sul, the palynomorphs from the Holocene sediments were registered by palynologists (CORDEIRO and LORSHEITTER, 1994; LORSCHEITTER, 1983; MEDEANIC, DILLENBURG, and TOLDO, 2001). Data based on diatoms that is about palaeoenvironments subjected to sea level influence during Holocene marine transgression are rare (ABREU et al., 1987; CALLEGARO and LOBO, 1990). This study represents the first study based on palynomorphs and diatoms that gives data on palaeoenvironments

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Figure 1. The study area and location of the core FS-20.

in the continental part of the extreme south of the coastal plain of Rio Grande do Sul during Holocene marine transgression; the study demonstrates the importance of these microfossils for coastal aquatic environmental reconstructions. There are a few papers in the world dedicated to the study of palynomorphs and diatoms from Quaternary samples from coastal plains. In the coastal plain of Rio Grande do Sul, the first records of palynomorphs and diatoms from Holocene sediments were reported by MEDEANIC, TOIGO-MARQUES, and ASHRAF (2000), MEDEANIC and DILLENBURG (2001), and MEDEANIC, JANKOVSKA, and DILLENBURG (2003). Use of palynomorphs and diatoms together for interpretations helps to easily distinguish transgressive and regressive stages in coastal plains, characterizing both aquatic and terrestrial adjacent environments. Implications of using palynomorphs and diatoms for reconstructions of climatic changes during the last two millennia in Patagonia show their importance (HABEZETTE et al., 2005). A multiproxy study of radiocarbondated lake sediments, including diatoms and palynomorphs, formed during the last millennia in Patagonia helps in the recognition of nature development influenced by human settlements (MAYR et al., 2005). Using diatoms and palynomorphs, sedimentology, and 14C-dated samples, GARCIA-RODRIGUEZ et al. (2004) established the Holocene trophic state changes in Lake Blanca, Uruguay, in relation to sea level variations during the Holocene. As seen from the above cited works, use of diatoms and palynomorphs together for palaeoreconstructions has not yet been well elaborated.

STUDY AREA The study area is situated in the continent, around 20 km from the Atlantic Ocean, near Cassino Beach (32⬚11⬘06⬙ S and 52⬚09⬘45⬙ W), Rio Grande do Sul State (Figure 1). The climate in this region is warm-temperate, due to the influence of the warm Brazilian and cold Falkland currents (VIEIRA and RANGEL, 1988). The mean annual temperature is around 18⬚C, and average monthly temperatures are 24.6⬚C

in January and 13.1⬚C in July. The average annual atmospheric precipitation is 1200 mm. At present, the geomorphology of the area is characterized by wide lowland with several connected lagoons, formed during the Holocene transgression and posterior regression, that covers an area of 33,000 km2, bordered at the east with highlands. The vegetation of the dunes and marshes, adjacent to the lagoons, are represented predominantly by halophilous and xerophilous herbs (CORDAZZO and SEELIGER, 1995; COSTA et al., 1997; SEELIGER, 1992). In the coastal lagoons, the Chlorophyceae are diverse and widely spread in the freshwater environments, and the diatoms are abundant both in fresh and brackish waters (TORGAN, BARREDA, and FORTES, 2001; TORGAN, BECKER, and PRATES, 1999; TORGAN, PILLAR, and NIENCHESKI, 2004; TORGAN, TUNDISI, and NIENCHESKI, 2002).

MATERIALS AND METHODS Sampling and Dating A core FS-20 of 26 m was taken in the beach near the small city of Cassino (32⬚11⬘06⬙ S and 52⬚09⬘45⬙ W; see Figure 1) with the aid of SPT (Standard Penetration Test) using piston corer (Raymond/Terzachi, Geotek Corporations) with an inner diameter of 13⁄8 in (35 mm) and an outer diameter of 2 in (50.8 mm). The core material includes four lithological layers, represented by silt, silty clay, silty sand, and sand (Figure 2). The granulometric compositions of the samples from the core are given for five samples, according to S HEPARD and YOUNG (1961), and are shown in Figure 3. The granulometric data corresponded to five horizons which were deposited in different conditions, connected with sea level oscillations and climatic changes between others. Ten samples were collected from the lowest layer, represented by silty clay and silt ranging from the depth of 16.10 to 25.4 m. Only this layer was represented by sediments favorable for palynomorph and diatom study. In addition, good preserved mollusk shell of Olivancillaria in living position (in situ) was found at the depth

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Figure 2. Lithology of the core FS-20.

of 23.0 m. This shell was dated by Beta Analytic, Inc. (Miami, Florida, United States) using the 14C method.

Chemical Treatment for Palynomorphs and Diatoms The samples were first desiccated in a furnace at a temperature of 60⬚C; then 50 g was treated with HCl (10%) and KOH (5%) and boiled for 10 minutes according to FAEGRI and IVERSEN (1989). The chemical treatment of HF was avoided, preserving the siliceous diatom valves and silicoflagellate skeletons. Inorganic substances were separated from the organic matter by ‘‘dense liquid,’’ an aquatic solution of ZnCl2

Figure 3. Granulometry of studied samples of the core FS-20. CGS ⫽ coarse-grained sand, MCS ⫽ mid–coarse sand, FS ⫽ fine sand, VFS ⫽ very fine sand, S ⫹C ⫽ silt ⫹ clay, S ⫽ silt, C ⫽ clay. Roman numerals show the depth of the samples: (I) 0.10 m, (II) 3.0 m, (III) 7.0 m, (IV) 15.0 m, (V) 23.0 m. Adopted from Clerot (2004).



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at 2.2 g/cm3 density. The residual material was mounted on glycerol jelly to make a permanent slide. In order to extract the diatoms, the same residues were processed again by a new portion of ZnCl2 solution (2.3–2.4 g/cm3 density). Next, an aliquot of 10 ml for every sample was mounted on permanent slides in Naphrax for identification and counting.

Palynomorph and Diatom Study

Figure 4. Palynomorph percentage diagram of the samples from the core FS-20. AP ⫽ arboreal pollen, NAP ⫽ nonarboreal pollen, S ⫽ spores of Bryophyta and Pteridophyta.

The taxonomic definitions of pollen and spores were based on a palynoteca of actual native plants, spread in the coastal plain of Rio Grande do Sul. In order to avoid the invalid definitions, pollen and spores were identified sensu lato (to the family or genus level). The freshwater coccoidal palynomorph identifications were based on VAN GEEL (1976), VAN GEEL and VAN DER HAMMEN (1978), and VAN GEEL, BONCKE, and DEE (1980–81). Cysts of dinoflagellates and acritarchs were recognized according to TOMAS (1997). The palynomorph slides are preserved in the Centro de Estudos de Geologia Costeira e Oceaˆnica, at the Instituto de Geocie`ncias, Universidade Federal do Rio Grande do Sul. For diatom analyses, the efficiency was up to 83% using the quantitative method for determining a representative algal sample count according to PAPAS and STOERMER (1996). Species were identified according to H USTED (1927–30), HENDEY (1964), ROSA (1982), BUSELATO-TONIOLI (1986), and MORENO, LICEA, and SANTOYO (1996). The diatom slides are

Figure 5. Diatom percentage diagram of samples from the core FS-20.


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Figure 6. Palynomorphs: (5) Cymatiosphaera; (6, 7) Micrhystridium; (8, 9) Dinoflagellate cysts indeterminate (indet.); (10–12) Dictyocha; (13) Operculodinium; (14) Spiniferites; (15–17) Fungal spores; (18) Debarya; (19, 20) Spirogyra; (21, 22) Botryococcus colonies; (23) Microforaminifera. Scale bar ⫽ 40 ␮m.

deposited at the Herbarium ‘‘Prof. Dr. Alarich Shultz’’ (HAS) in the Museu de Cie`ncias Naturais. The obtained data were plotted on diagrams (Figures 4 and 5) using Tilia software designed by GRIMM (1987). The observations and photomicrographs (Figures 6–9) were made using a Zeiss Axioplan microscope with 400⫻–1250⫻ magnification.

RESULTS The obtained absolute age by 14C dating of this layer at the depth of 23.0 m was 4940 ⫾ 80 years BP. The studied samples are characterized by differences in the frequency of palynomorphs and diatoms. In some samples, palynomorphs are predominant and diatoms are rare; in others, palynomorphs are absent or rare and diatoms are predominant; and in some samples, both are common. This pattern difference between palynomorph and diatom frequency may be caused by different sedimentological and taphonomic conditions that may be favorable or adverse for the preservation of organic matter in palynomorphs and siliceous diatoms. More frequent palynomorphs, especially pollen and spores of terrestrial plants, may be connected with proximity of lands, and abundance of aquatic palynomorphs and diatoms may be evidence of favorable ecological conditions of aquatic basins.

Figure 7. Palynomorphs: (25) Phaeoceros; (26) Anthoceros; (27) Blechnum; (28) Microgramma; (29) Azolla filiculoides; (30) Palmae; (31, 32) Alchornea; (33) Asteraceae; (34, 35) Chenopodiaceae; (36) Cyperaceae; (37) Juncaginaceae (Triglochin type); (38) Fabaceae; (39) Fabaceae (Cotula type); (40) Lamiaceae; (41) Myrtaceae; (42) Poaceae; (43) Polygonaceae (Rumex type); (43) Verbenaceae. Scale bar ⫽ 30 ␮m.

Palynomorphs The palynomorph frequencies in the samples are relatively low. In the diagram (Figure 4) and in Table 1, only the results of four samples where the total sum of palynomorphs constitutes more than 200 examples (specimens) are included. The other six samples, containing a few palynomorph grains, are not represented in the palynodiagram and in Table 1. In the lower part of the layer (23.30–25.0 m), the dinoflagellate and acritarch cysts make up 32.3–33.6%, represented by Operculodinium, Spiniferites, Micrhystridium, and Cymatiosphaera. Microforaminifera compounds constitute 1.5– 2.3%. The Chlorophyceae palynomorphs (3.7–4.4%) are represented by Botryococcus, Spirogyra, and Pseudoschizaea. Arboreal pollen (3.9–7.4%) is relatively frequent and diverse. Nonarboreal pollen (31.4–49.8%) is characterized by a variety of taxa and abundances (Table 1). Poaceae and Chenopodiaceae species are the most frequent. Pteridophyta and Bryophyta spores (12.3–18.7%) are common. Fungal palynomorphs of Tetraploa are registered in one sample. Other unidentified algal palynomorphs make up 2.3–2.6%. At 19.0–19.45 m, the dinoflagellate cysts increase. Microforaminifera compounds constitute 2.3%. The Chlorophyceae palynomorphs represented by Botryococcus and Spirogyra in-



Figure 8. Diatoms: (44) Actinoptychus vulgaris; (45) A. senarius; (46) A. splendens; (47) Coscinodiscus obscurus; (48) Thalassiosira eccentrica; (49) C. radiatus. Scale bar ⫽ 10 ␮m.

crease. Botryococcus predominates at 22.4%. Arboreal and nonarboreal pollen notably decrease. Bryophyta spores of Phaeoceros increase and Pteridophyta spores decrease. Fungal palynomorphs are extremely rare. Dictyocha skeletons are encountered. At 17.30–17.45 m, the dinoflagellate and acritarch cysts and microforaminifera notably decrease. The Chlorophyceae palynomorphs of Botryococcus and Spirogyra decrease, too. Pollen Chenopodiaceae increase to 28.1%, and pollen Poaceae increase to 8.5%. Bryophyta spores, represented by Phaeoceros, significantly increase (Table 1). The most frequent palynomorphs are shown in Figures 5 and 6.

Diatoms The diatoms are encountered in five core samples. The assemblage is composed of 26 taxa (17 centric and 9 pinnate forms) presently distributed in marine and estuarine environments. The taxonomic variety and abundance is shown in Table 2 and in Figures 8 and 9.

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Figure 9. Diatoms: (50) Thalassiosira sp.; (51) Paralia sulcata; (52) Cyclotella striata; (53) Triceratium favus; (54) Odontella rhombus; (55) Cocconeis disculus; (56) Cymatosira belgica; (57) Diploneis bombus; (58) Delphineis surirella. Scale bar ⫽ 10 ␮m.

At the bottom, at 25.00–25.40 m, marine planktonic species (Actinoptychus senarius, Thalassiosira eccentrica, and Thalassiosira spp.) and marine benthic species (Cocconeis disculoides, Diploneis bombus, and Grammatophora marina) are present, and the tichoplanktonic, brackish-marine diatom P. sulcata is the most abundant (80.15%). At 19.00–19.25 m, an increase in the species diversity and a significant decrease in Paralia sulcata are observed. The species Cymatosira belgica, Delphineis surirella, Nitzschia punctata, and Odontella rhombus are common, besides the other diatom species mentioned above. The highest diversity of diatoms is registered at 17.30–17.35 m deep, where 20 taxa are identified. Some diatom species, like Actinocyclus octonarius, Actinoptychus splendens, Coscinodiscus curvatus, C. obscurus, C. radiatus, and Raphoneis surirella, are encountered only in this depth, and P. sulcata is the most abundant (28.85%). At 17.20–17.30 m, a decrease in diatom diversity is observed: only 11 species are identified. At the upper part of the layer (16.10–16.20 m deep), a significant decrease in di-


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Table 1. Palynomorph taxa and relative frequency (%) in the samples of the core FS-20.

Table 1. Continued. Depth (m)

Depth (m) Palynomorph Taxa

25.00– 24.40

23.20– 23.30

19.00– 19.45

17.30– 17.45

Arboreal pollen (AP) PINOPHYTA Ephedra Podocarpus MAGNOLIOPHYTA Alchornea Anacardiaceae Boraginaceae Euphorbiaceae Mimosaceae Palmae Rapanea Trema Ulmaceae

DICTYOCHOPHYCEAE Dictyocha MICROFORAMINIFERA

— 0.4

— —

— —

1.5 1.2

— 5.2 — — 0.9 — 0.9 — —

0.3 1.8 0.3 0.3 — 0.3 0.3 0.3 0.3

— 0.3 — — — 0.9 0.3 — —

— — — — — — — — —

— 0.9 4.8 — 9.2 5.2 1.7 — 0.4 — — 8.7 — — — — 0.4

— 0.3 4.1 0.3 16.4 3.1 0.3 0.5 — 1.0 — 14.6 — 0.3 0.5 3.1 0.5

0.3 0.3 3.2 0.3 6.4 4.4 — — — — — 6.7 0.3 0.3 0.3 0.3 0.3

— 2.3 1.2 — 28.1 3.8 — 1.2 — — 2 8.5 — 0.4 — 1.5 0.8

BRYOPHYTA Phaeoceros Sphagnum

10.0 0.4

2.6 0.3

19.5 0.6

22.3 0.8

PTERIDOPHYTA Alsophyla Anemia Azolla filiculoides Blechnum Dicranoglossum Dicranopteris Equisetum Lycopodiella Microgramma Ophioglossum Polypodiaceae

— — 4.4 0.9 1.3 — — — 0.4 — 1.3

0.8 0.5 0.3 0.3 2.0 — 0.5 1.3 1.3 0.3 2.3

— 0.3 2.7 — — 0.3 0.3 0.3 0.6 0.3 0.3

— — 0.4 0.4 0.4 — — 1.0 — — —

CHLOROPHYTA Botryococcus Debarya Mougeotia Spirogyra Pseudoschizaea

3.1 — — 0.9 0.4

2.6 0.3 0.3 0.5 —

22.4 — — 0.6 —

8.0 — — 2.3 —

ACRITARCHA Cymatiosphaera Micrhystridium

0.4 31.9

0.5 29.7

2.3 12.2

— 4.6

DYNOPHYCEAE Operculodinium Spiniferites Dinoflagellate cysts indet.

0.4 0.9 —

0.3 0.3 0.5

2.9 1.5 2.6

0.8 0.8 1.5

Nonarboreal pollen (NAP) Amaryllidaceae Apiaceae Asteraceae Brassicaceae Chenopodiaceae Cyperaceae Gunneraceae Fabaceae Juncaceae Juncaginaceae Myriophyllum Poaceae Polygonum hydropiperoides Primulaceae Scrophulariaceae Verbenaceae Vernonia

Palynomorph Taxa

Spores

FUNGI Tetraploa

25.00– 24.40

23.20– 23.30

—

—

2.3

1.5

—

Other indet. Total sum of palynomorphs

2.3 229

0.5 2.6 391

19.00– 19.45

17.30– 17.45

1.7

3.5

2.3

1.1

—

—

1.7 344

0.8 260

atom taxa diversity is registered. P. sulcata is the most abundant (89.2%).

DISCUSSION Marine palynomorphs (dinoflagellate and acritarch cysts), silicoflagellates, and predominant marine and estuarine diatoms were identified from a silty clay layer of the core FS20 taken from a site 20 km from the modern coastal line. This silty clay layer was formed during Holocene marine transgression. The correspondence of the silty clay layer to marine transgression was confirmed by one marine mollusk shell of Olivancilaria whose 14C age dating (4940 ⫾ 80 years BP) corresponded to marine transgression. The palynomorphs, especially marine algae cysts, microforaminifera, and diatoms identified from samples in this layer, indicate aquatic basin spread with elevated salinity in the Cassino region in the coastal plain of Rio Grande do Sul during Holocene marine transgression. We submit the spreading of an extensive bay whose outline, dimensions, and salinity were influenced by sea level rise, which, influenced by climatic oscillations, was changed from time to time during this marine transgression. The presence of zygospores of freshwater algae such as Spirogyra and Mougeotia may be the result of their transport by freshwater influxes into the bay inlet during pluvial periods from adjacent regions. Colonies of Botryococcus were common in this environment and also indicate a freshwater influence. Rare fungal palynomorphs (zoospores) whose transport capacity is very restricted may indicate distant lands away from the bay inlet. The variations in diatom diversity and P. sulcata abundance reveal changes in salinity and depth of the environment, occurring during transgression. The broad bays are a habitat available to tycoplanktonic diatoms, like P. sulcata (MCQUOID and HOBSON, 1998). During the beginning of the marine transgression, when the sea water advanced into the coastal plain, highly saline aquatic environments appeared where marine phytoplankton (acritarchs, dinoflagellates, and diatoms) were common. Micrhystridium and Cymatiosphaera acritarchs were relatively abundant. In the coastal plain of Rio Grande do Sul, they are registered in modern surface sediments of intertidal marshes and in the Patos Lagoon estuary (MEDEANIC, 2006). The brackish-marine diatom P. sulcata was abundant. Presence of this species is greatly increased in bays, a favorable habitat for P. sulcata development (living on the bottom, they may be



Table 2. Diatom taxa and abundance (%) in the samples of the core FS-20. Depth (m) Diatom Taxa

Achnanthes curvirostrum Achnanthes octonarius Actinoptychus senarius Actinoptychus splendens Actinoptychus vulgaris Cocconeis disculoides Cocconeis sp. Coscinodiscus curvatus Coscinodiscus obscurus Coscinodiscus radiatus Cyclotella striata Cyclostephanos sp. Cymatosira belgica Delphineis surirella Diploneis bombus Grammatophora marina Nitzschia punctata Odontella rhombus Paralia sulcata Rhaphoneis surirella Thalassiosira excentrica Thalassiosira oestrus Thalassiosira sp. 1 Thalassiosira sp. 2 Thalassiosira sp. Triceratium favus Number of species

25.40– 25.00

19.25– 19.00

17.35– 17.30

17.30– 17.20

16.20– 16.10

— — 0.74 — — 1.47 — — — — 3.68 — — — 0.74 0.74 — — 80.15 — 6.62 — 0.74 3.68 1.47 — 10

— — 3.85 — — 7.69 0.96 — — — 3.85 2.88 7.69 1.92 1.92 — 1.92 0.96 28.85 — 10.58 — 18.27 5.77 1.92 — 15

0.66 5.96 6.62 1.32 0.66 4.64 — 0.66 0.66 2.65 7.95 — 1.99 3.31 — — — 3.31 34.44 1.9 1.99 — 10.60 8.61 — 1.99 20

— — 5.47 — 0.78 0.78 — — — — 1.56 — — — 0.78 — — — 73.44 — 2.34 — 3.91 3.13 7.03 0.78 11

— — — — — — — — — — 1.54 — — — — — — — 89.23 — — 1.54 — 4.62 — 3.08 5

easily lifted into the plankton); it is frequent in estuaries and lagoons where salinity ranges between 5% and 25% (ZONG, 1977). Next, an increase is seen in open-marine influences, causing an increase in the salinity of the bay inlet confirmed by a higher frequency of marine palynomorphs (especially Operculodinium and Spiniferites cysts, among other identified dinoflagellate cysts), silicoflagellates (Dictyocha), and marine diatoms (Actinoptychus, Coscinodiscus, and Thalassiosira). Further, a decrease in P. sulcata and an increase in marine diatoms may be evidence of a sea level rise causing an increase in the salinity depth of the bay inlet. Low abundance of terrestrial palynomorphs (arboreal and nonarboreal pollen, and fern spores) in the sample may be evidence of relatively distant lands of foredunes and intertidal marshes, where halophilous and xerophilous Chenopodiaceae and Poaceae plants grew. Later, when the sea level began to fall, a decrease in the taxonomic diversity of marine palynomorphs and marine diatoms occurred. The portion of terrestrial pollen and spores and brackish-water diatoms, especially P. sulcata, increased. The bay inlet became shallow as a result of sea water retreats. Consequently, adjacent lands were also freed from the brackish waters where halophilous species of Chenopodiaceae and Poaceae grew. An abundance of terrestrial pollen and spores may be evidence of the sea level fall and may indicate the beginning of a marine regression. Salt marshes were prevailing on adjacent lands. Species of Cyperaceae, Asteraceae, Chenopodiaceae, Poaceae, Juncaceae, Juncaginaceae, Phae-

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oceros, Anthoceros, the aquatic fern Azolla filiculoides, Lycopodiella, and Equisetum were frequent. The rare arboreal pollen, such as Alchornea, Anacardiaceae, Boraginaceae, and others, may indicate their allochtonous origin and that they were transported from a distance. When the sea level fell, the deposition of the silty clay layer stopped. Latter marine regression resulted in sand dune deposition.

CONCLUSIONS The marine palynomorphs, diatoms, and silicoflagellates identified in this study indicated the Holocene transgressive stage, when the majority of the southern coastal plain was vastly covered by sea waters. During this transgression, diversity, abundance, and frequency of palynomorphs and diatoms were changed, showing the oscillating character of the sea level in the bay inlet. The end of the transgressive stage was registered when the abundance of marine palynomorphs and diatoms decreased, indicating the beginning of a regressive stage.

ACKNOWLEDGMENTS We thank the Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) for grants 300005/2007-5 and 302926/2004-6. We are grateful to Dr. S.R. Dillenburg, who provided the 14C dating, and to Haywood Dail Laughinghouse IV for reviewing the English in the manuscript. We are indebted to two anonymous reviewers for their criticism, suggestions, and corrections which improved this manuscript.

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䡺 RESUMO 䡺 Baseada nos dados de sedimentologia, geocronologia, palinologia e ana´lise de diatomaćeas nas amostras de um testemunho executado na praia do Cassino (32⬚11⬘06⬙ S e 52⬚09⬘45⬙ O), sul do Brasil, uma transgressaõ marinha foi descoberta. A idade absoluta de uma amostra e´ a cerca de 4.940 ⫾ 80 anos AP. Os palinomorfos (polens e esporos de plantas vasculares, os zigo´sporos e coloˆnias de clorofilas, cistos de dinoflagelados e acritarcas, esporos de fungos e microforaminı´feros), silicoflagelados, e diatomaćeas indicam sobre existe`ncia no passado de uma baıá na parte sul da Planıćie Costeira durante transgressaõ marinha. As mudanças da composiçaõ taxonoˆmica, abundaˆncia e frequë`ncia de palinomorfos e diatomaćeas descobertos nas amostras correspondentes a` transgressaõ marinha mostram oscilaço˜es do nı´vel do mar. A regressaõ marinha posterior resultou na deposiçaõ dos sedimentos de areia e formaçaõ de dunas. Os resultados mostram a importaˆncia de aplicaçaõ de palinomorfos e de diatomaćeas para reconstruço˜es paleoambientais nas planıćias costeiras.
