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Late Holocene development of a mangrove ecosystem in southeastern Brazil (Itanhaém, state of São Paulo)
Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 608 – 620 www.elsevier.com/locate/palaeo
Late Holocene development of a mangrove ecosystem in southeastern Brazil (Itanhaém, state of São Paulo) Paula Garcia Carvalho do Amaral a,⁎, Marie-Pierre Ledru a,b , Fresia Ricardi Branco c , Paulo César Fonseca Giannini a a
c
Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São Paulo, rua do Lago 562, CEP 05508-080, São Paulo, SP, Brazil b Institut de Recherche pour le Développement (IRD), UR 055, France Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, rua João Pandiá Calógeras 51, CEP 13083-970, Caixa Postal 6152, Campinas, SP, Brazil Received 6 July 2005; received in revised form 29 March 2006; accepted 6 April 2006
Abstract Most palynological studies of mangroves have been carried out in the Indo-Pacific region, but few have investigated these ecosystems along the southern Atlantic coast. This paper provides information on the palynology of a mangrove at Itanhaém, state of São Paulo, on the southeastern Brazilian coast. This mangrove occurs on microtidal flats adjacent to a fluvial-tidal channel mouth (Itanhaém River), part of an estuarine lagoon system established in the mid-Holocene. Pollen samples were collected from the surface and from bromeliad water in order to define the composition of modern pollen rain in the study area. The results show a strong presence of rain forest taxa in the mangrove pollen spectra, which differ from those obtained from northern Brazilian mangroves, which are characterized by higher percentages of the mangrove taxa Rhizophora and Avicennia. Based on palynological and 14C chronological analysis of a shallow core measuring 135 cm in depth, the Itanhaém mangrove already existed by 1300 years BP and began expanding about 1000 years BP, reaching its present extent sometime after 300 years BP. Its evolution is related to the sedimentary filling of the estuarine–lagoonal depositional system. Two factors may have controlled the development of this mangrove: the relative increase of intertidal areas due to this sedimentary filling and the decrease in wave action and increase in pelitic deposition in the intertidal zone related to the partial closing of the estuarine–lagoonal inlets. These processes were favored by the advance of bayhead deltas and by the development of sand spits as a part of the regression of the Holocene strandplain. © 2006 Elsevier B.V. All rights reserved. Keywords: Holocene; Palaeoecology; Pollen record; Mangrove; Restinga; Tropical rain forest; Estuary; Brazil
1. Introduction
⁎ Corresponding author. Tel.: +55 11 3091 4138; fax: +55 11 3091 4207. E-mail address: [email protected] (P.G.C. do Amaral). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.04.010
A mangrove is a coastal ecosystem with constant influence of tides, which is typical of tropical and subtropical regions (Lamparelli, 1999); it is defined by the existence of flora consisting of an association of
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specific trees and bushes (e.g. Rhizophora mangle, Avicennia sp.) adapted to variations in salinity and low levels of free oxygen in the substrate (Suguio, 1998). This ecosystem can occupy a variety of substrates (e.g. coral rubble, sand) but is more common and most diverse on fine-grained sediments (Grindrod, 1988). Mangroves are exposed to many natural processes, such as sea level variation and coastal erosion that can modify their development and geographic distribution. Along the Brazilian coast, mangrove ecosystems are found from the extreme northern coast on the Oiapoque River (04°20′N) in the state of Amapá to Laguna (28°30′S) on the southern coast in the state of Santa Catarina. These mangroves become narrower and lower in stature from the north to the south, where both tidal range and temperatures are lower, and the coastal plain is restricted by the Serra do Mar (a Cenozoic fault scarp). On the southern coast, mangroves are restricted to microtidal bays and lagoonal or estuarine inlets (Schaeffer-Novelli, 1991). The widespread occurrence of this ecosystem along the Brazilian coast provides a large range of scenarios for the characterization of mangrove environments and reconstruction of corresponding paleoenvironments using pollen records. Studies of these records in mangroves have demonstrated that palynological analysis can provide important information about the history of the vegetation in the ecosystem as well as data for the reconstruction of sea level changes and the evolution of coastal sediments (Suguio, 1999; Grindrod et al., 2002). However, research with such a focus is rare and has been restricted to a few specific locations. Our objective is to reconstruct the development of a mangrove in southern Brazil and the history of sedimentary dynamics in this region, including relative sea level (RSL) and coastline variations, based on the palynological analysis of a shallow core (135 cm) extracted from an area currently occupied by this kind of ecosystem. 2. Study area: depositional systems and mangrove distribution Formed by the confluence of the Branco and Preto Rivers, the Itanhaém River enters the sea near the city of Itanhaém (24°16′59″S, 46°47′20″W; Fig. 1) on the coastal plain of the southern Brazilian coast in the state of São Paulo. The Itanhaém River and the lower courses of its affluents show cyclic flow reversion, thus functioning as tidal channels. In terms of Quaternary depositional systems (sensu Fisher and McGowen, 1967; Fisher, 1983), the Peruíbe–Itanhaém coastal
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plain can be described as a juxtaposition of strandplain and estuarine–lagoonal systems, although it has been modified by recent and restricted fluvial and tidal plain systems. The strandplain system is formed by sandy deposits, characterized in aerial photos by successively aligned foredune ridges and beach ridges. The evolution of this strandplain system is basically related to the progradations which followed two separate transgressive events (Suguio and Martin, 1978). The first, in the Late Pleistocene, is related to the high RSL (+ 8 ± 2 m) of the last interglacial near 120,000 years BP. The second, during the Holocene, is linked to the maximum RSL ca. 5100 years BP (around +4.5 m). The Pleistocene strandplain, up to 12 km wide, has its inland limit at the foot of the Serra do Mar scarp, whereas the Holocene strandplain is a narrow zone (less than 2 km wide) between the Pleistocene strandplain and the ocean. The areas currently drained by the Branco, Preto and Itanhaém Rivers contain sandy mud deposits of estuarine–lagoonal origin. According to the model developed by Giannini and Santos (1996) for the Peruíbe River southwest of Itanhaém, this estuarine– lagoonal system was formed during the Holocene transgression and it was probably developed by drowning and mud filling of fluvial valleys incised during the lowstand of the Würm glacial period. Modern tidal-fluvial channels and tidal flats are superimposed on the filled estuarine–lagoonal systems, and mangroves occur in association with the muddy intertidal facies of the tidal flat. The main factors determining the distribution and relative extension of supratidal, intertidal and infratidal facies in the Itanhaém region are tidal range, physiography of the tidal plain (width and dip) and the dynamics of coastal depositional systems. This third factor was the most variable during the Late Holocene and may have included closure and opening of inlets and filling and erosion of bays in the estuarine–lagoonal system. It is strongly influenced by the balance between sedimentary supply and RSL changes, which determine the position of the coastline and locations for the development of mangroves. On a regional scale, the tidal flat domain, with its supra-and intertidal facies, probably increased during the Holocene regression, leading to corresponding variations in the distribution of mangroves during the last millennia. 3. Climate and vegetation The annual average temperature in the region is greater than 20 °C and annual precipitation varies from 2000 to 2500 mm. Highest precipitation falls during the
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Fig. 1. Geological map of Itanhaém (modified by Suguio and Martin, 1978) showing the location of the modern samples.
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summer (January to March) and the lowest in winter (July to August), although there is no well-defined dry season. Relative humidity is high, averaging around 80% (Lamparelli, 1999). The strandplain in the vicinity of the Itanhaem River system is occupied by an association of trees and shrubs known as restinga, with subordinate presence of secondary forests and cultivated areas. Atlantic Coastal Rain Forest covers the Serra do Mar. The mangrove under study covers an area of 3.75 km2 and is located in the intertidal zone of the Itanhaém River and its confluents. The vegetation and ecology of the Itanhaém mangrove were studied by Lamberti (1969), who found the dominant species in this ecosystem included Avicennia schaueriana (Avicenniaceae), Laguncularia racemosa (Combretaceae) and R. mangle (Rhizophoraceae). In the outlying zones only occasionally reached by spring tides, one also finds Hibiscus tiliaceus (Malvaceae), Crinun attenuatum (Amarylidaceae), Spartina brasiliensis (Poaceae), Fimbristylis glomerata (Cyperaceae) and Acrostischum aureum (Polypodiaceae). The vegetation of the neighboring restinga growing in the sandy soil typical of a strandplain depositional system is characterized by a wide variety of palm trees, as well as the orchids and bromeliads that grow on trunks and branches of larger trees (Branco, 1984). The restinga in this area is related to the Atlantic Coastal Rain Forest, although the forest is shorter and less diverse than at sites on the slope of the Serra do Mar. Garcia (2003) identified 373 species from 215 genera and 88 families in the tropical rain forest in Itanhaém (23°59′06″S, 46°44′36″W). According to this author, species-rich genera include: Mikania (Asteraceae), Leandra (Melastomataceae), Vrisea (Bromeliaceae), Paspalum (Poaceae), Baccharis (Asteraceae), Ocotea (Lauraceae), Panicum (Poaceae), Psychotria (Rubiaceae), Miconia (Melastomataceae), Eugenia (Myrtaceae), Piper (Piperaceae), Tibouchina (Melastomataceae), Xyris (Xyridaceae), Myrcia (Myrtaceae) and Habenaria (Orchidaceae). 4. Materials and methods Nine samples of modern pollen rain were collected along a transect cutting through the following vegetation types present in the Itanhaém drainage basin: mangrove, restinga, Atlantic Coastal Rain Forest and secondary forest stands. At the beginning of the rainy season in November 2000, sediment samples at the water/sediment interface and surface soils were collected in an area of approximately 100 m2. In addition, bromeliad-stored
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water was sampled from pollen content. Bromeliads are omni-present in the region and these plants are natural collectors of pollen rain; where possible, paired samples of sediment and bromeliad water were collected for a comparative analysis. The core location was selected to avoid bioturbation and as far as possible (about 50 m) from the nearest channel. However, since the channel of the Itanhaém River meanders extensively in the narrow area occupied by the mangrove, it is difficult to find a point, which is not close to the river and tidal creeks. Manual sampling was used to extract a core of 135 cm depth. Once the core was opened, color changes, grain size, sedimentary structures, roots and other fragments of vegetation were recorded. Special care was taken to identify possible signs of bioturbation in the sediments of the core. Samples for palynological extraction using the classical method described by Faegri and Iversen (1989) were also taken every 4 cm. Four levels were selected for radiocarbon dating based on lithological changes and the presence of organic material. The sample residues were mounted in glycerine. The identification and counting of pollen grains involved a study of the slides using a biological microscope. For each sample, at least 250 pollen grains were counted and identified by comparison with our reference pollen collection and various pollen keys (Absy, 1975; Markgraf and D'Antoni, 1978; Thanikaimoni, 1987; Roubick and Moreno, 1991; Colinvaux et al., 1999). Due to the similarity of their pollen grains, the Meliaceae/Sapotaceae and Melastomataceae/Combretaceae families were not separated here. The results are presented in two types of diagrams (Figs. 2 and 3). Fig. 2 provides detailed pollen data expressed as percentages of each taxon in relation to the sum of arboreal (AP), non-arboreal (NAP) and undetermined terrestrial pollen, with aquatic pollen and spores excluded from the total. Relative frequencies of spores and aquatic pollen were calculated in relation to the total AP and NAP pollen sum. Fig. 3 provides percent values for summarised pollen groups, based on plant form and habitat type. The concentration values (pollen grains per gram of wet sediment) were calculated using the volumetric method of Cour (1974). 5. Results and interpretation 5.1. Modern pollen rain The results obtained from both surface and bromeliad-water samples, as well as those from the top of the
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Fig. 2. Summary pollen percentage diagram of core IT-01.
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Fig. 3. Summarised pollen spectra of vegetation groups. Mangrove = Rhizophora and Avicennia; Trees and shrubs (restinga and Atlantic rain forest) = Alchornea, Arecaceae, Arrabidea, Cecropia, Ilex, Melastomataceae/Combretaceae, Myrsine, Myrtaceae, Podocarpus, Proteaceae, Weinmania; herbs = Asteraceae, Cyperaceae, Poaceae; spores = Cyatheaceae, Polypodiaceae. Concentration diagram is also presented.
core, are presented in Fig. 4. Pollen from Rhizophora and Avicennia are recorded only inside the mangrove, Rhizophora being more frequent than Avicennia (1–2% and 0.4%, respectively). This difference seems to be related to the pollen dispersal strategies of these genera as observed in palynological studies in other mangroves (Muller, 1959; Behling and Costa, 1994, Behling et al., 2001). The pollen collected in the restinga is represented by samples 3, 4 and 5, which show high frequencies of Alchornea (2–8%), Cecropia (3–4%), Myrtaceae (10– 18%), Myrsine (1–4%), Asteraceae tubuliflorae (3–8%) and Amaranthaceae/Chenopodiaceae (0.3–2%). The high frequency of Piper in sample 9 (21%) coincides with the transition between the restinga and the Atlantic rain forest itself. High frequencies of Piper, indicating a spatial change in vegetation, have also been observed in Carajás (Amazonas state) by Absy et al. (1991) and in Lago Aquiri (Maranhão state) by Behling and Costa (1994). A high frequency of pteridophyte spores (63%) is recorded in sample 8, while other samples reveal much lower levels (from 7% to 32%). It is possible that oxidation and degradation of fragile pollen types on the surface soil may have led to an increase in the relative frequency of more resistant spore and pollen types. Field observations, however, tend to show a large number of
pteridophytes in more humid areas of the Atlantic Coastal Rain Forest. Paired samples were collected at two locations, one from surface sediments and the other from the water stored in Bromeliaceae (samples 3, 3a, 5 and 5a). Similar frequencies of pollen were observed in the samples from the two sources for Cecropia (3% in both pairs), Podocarpus (1% in both pairs) and Weinmannia (1% in one pair, but 0% in the other). Higher frequencies of some of them were observed in the water from Bromeliaceae: Alchornea (7–8% vs. 2–3% on the surface); Myrsine (4% vs. 1% on the surface) and Arrabidea (2% vs. 1% on the surface). Aquatic taxa were present in greater concentrations in the samples from the surface sediments (6% and 15% vs. 2% and 4% in the bromeliad water) (Fig. 4). Hence, the use of the Bromeliaceae as natural collectors of pollen rain seems to be a useful option for present-day rain forests, although for the sake comparison, surface sediment samples should also be collected at the same sites. The separation of the pollen spectra from restinga forest and those from the Atlantic Coastal Rain Forest was problematic, as the two areas contain many of the same species, although the rain forest exhibits greater diversity. A more precise definition of the pollen spectrum from the vegetation of these two areas
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Fig. 4. Summary pollen percentage of the surface and bromeliad samples, with top of core (It-01) also represented.
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would require exhaustive sampling along the mountain range. Another important finding is that forest pollen taxa are extensively deposited in the mangrove. In fact, the mangrove pollen signal is masked by the high quantity of allochthonous pollen types. This may reflect fluvial transport of pollen from upland sources. Similar results were reported by Crowley et al. (1994) and Moss et al. (2005). Another explanation may be the proximity of the forest to the mangrove site, as the mountain range covered by rain forest is only 15 km from Itanhaém. This proximity could have facilitated wind dispersal of pollen. The strong influence of allochthonous pollen in the Itanhaém mangrove contrasts with mangrove pollen assemblages from northern Brazil, where mangrove taxa account for around 90% of the pollen assemblages (Behling and Costa, 1994; Behling et al., 2001). Previous studies have demonstrated the significant differences in the palynological records in the mangroves of the Brazilian southeast–south and north–northeast. The most noticeable difference between these regions seems to be the low frequencies of Rhizophora and Avicennia (typi-
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cal mangrove association) in the pollen record of mangroves in the southeast–south region (Absy and Suguio, 1975; Slompo, 1997; Coelho et al., 2001) and over-representation of pollen of Rhizophora in the sedimentary record of mangroves in the north–northeast (Behling and Costa, 1994; Behling et al., 2001). The results of modern pollen rain analysis of the Itanhaém coastal plain have led to a slight modification of the model of Grindrod (1988) to account for the presence of the Atlantic coastal forest on the slopes of the nearby mountain (Fig. 5). The presence of the local taxa (Rhizophora and Avicennia) is obvious, while longdistance dispersal is attested by the presence of Alnus pollen; the allochthonous regional taxa from the restinga and the Atlantic Coastal Rain Forest include Alchornea, Arrabidea, Cecropia, Ilex, Myrtaceae, Myrsine, Podocarpus and Weinmannia, whereas the transition area is defined by the presence of such pollen as Piper. 5.2. Core results Sedimentological variations in the core are shown in Table 1. The core is made up of muddy sediments rich in
Fig. 5. Model of pollen rain dispersion along coastal plain of the Itanhaém River. The distance between the Itanhaém mangrove and the “Serra do Mar” slope is nearly 15 km (modified by Grindrod, 1988).
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Table 1 Sedimentological variation in the core Depth (cm)
Sedimentological description
− 26 to 0 cm
Uncompacted black muddy sediments, rich in organic matter, with plants remains present Compacted grey muddy sediments, with plants remains present Compacted dark grey muddy sediments Grey compacted muddy sediments, with plants remains present Muddy sediments with little quantity of sand Muddy sediments with medium quantity of sand Muddy sediments with large quantity of sand Light gray fine sand laminated, with organic matter
− 41 to − 26 cm − 46 to − 41cm − 64 to − 46 cm − 84 to − 64 cm − 106 to − 84 cm − 125 to − 106 cm − 135 to 125 cm
organic matter, except for the lowest 10 cm, which are composed of fine sand, with horizontal plane-parallel lamination. The amount of sand decreases upwards. No bioturbation was observed. Radiocarbon dating shows that the base of the core (−120 to −125 cm) is at least 1300 years old (Table 2). Results for −37 to−42 cm (590± 130 years BP) and −70 to −75 cm (450 ± 40 years BP), overlap at one standard deviation. Vertical sediment mixing in the mangrove environment may explain these results. The use of different dating methods, AMS and conventional methods, could also be the source of this discrepancy. For the purposes of this study, an average age of 500 years BP is assumed for this interval (−37 to −75 cm). 5.3. Description of the diagrams The results of pollen analysis of the core samples are presented in Figs. 2 and 3. Two zones may be distinguished on the basis of pollen frequency in the mangrove as registered in the core. Zone I is characterized by low frequency of mangrove index taxa (0.3%) and Zone II by the progressive upward increase in the pollen of these taxa. 5.3.1. Zone I (− 135 cm to − 98 cm) This zone is composed basically of sandy sediments with little organic matter and includes the base of the core (Fig. 2). In general terms, this pollen zone is characterized by low pollen concentrations, with only 140–470 pollen grains per gram of sediment (Fig. 3), and high frequency of spores. Arboreal pollen (AP) include mainly Myrtaceae (13–29%), but also Melastomataceae/Combretaceae (2– 11%), Alchornea (5–9%), Cecropia (2–7%), Arecaceae (3–5%) and Myrsine (2–3%). Non-arboreal pollen (NAP) is dominated by Poaceae (12–18%). Percentage values for
the mangrove taxa, Rhizophora and Avicennia, are very low (never exceeding 1%), with Rhizophora pollen recorded in only two samples. When compared to modern pollen rain, the composition of the pollen spectra observed in this zone suggests the presence of a restinga coastal forest, which is consistent with the sandy nature of this part of the core. 5.3.2. Zone II (−98 to 0 cm) This pollen zone is characterized by progessive increase in pollen concentration and frequency of mangrove taxa. The zone may be divided into two sub-zones (IIa and IIb) on the basis of pollen concentration and presence of the mangrove taxa. 5.3.3. Sub-zone IIa (− 98 to − 33 cm) This sub-zone is characterized by high pollen concentrations (both AP and NAP) and by a decrease in spore frequency. Although values for many tree taxa remain similar to those recorded in Zone I, with Myrtaceae still accounting for the largest percentage of tree pollen (11–21%); other abundant pollen types include Alchornea (5–10%), Arecaceae (3–6%) and Myrsine (0.4–3%). Some other taxa which were present in relatively high concentrations in Zone 1, such as Melastomataceae/Combretaceae and Cecropia, decrease in this sub-zone (from a maximum of 11% to 8% and from 7% to 3%, respectively). The frequency of others increases: Arrabidea (from 1% to 4%), Ilex (from 2% to 4%), Podocarpus (from 0.3% to 3%) and Weinmannia (from 2% to 5%). Among the NAP, the pollen of Poaceae continues to be abundant (11–20%) and that of Asteraceae tubuliflorae (now 2–6%) has increased. A progressive increase in the frequency of the pollen of Rhizophora is observed, whereas Avicennia is recorded in a single sample (< 1% at − 38 cm). This sub-zone attests to the development of the restinga and the mangrove. The increase in the percentage of finergrained sediments and the large concentration of organic matter indicate a shift to a lower energy and more sheltered depositional environment. This change would have allowed the deposition of more pelitic material, thereby Table 2 C ages obtained for the samples of the core
14
Sample (no. of lab)
Dating method
Beta-177002 AMS PaBO 29 Conventional method Beta-177003 AMS Beta-177004 AMS
Depth (cm)
13
C/12C
− 10 to −15 − 37 to −42
− 28.4‰ Modern − 31.72‰ 590 ± 130
− 70 to −75 − 28.4‰ − 120 to − 125 − 28.4‰
14
C age (years BP)
450 ± 40 1250 ± 50
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favoring the establishment of a mangrove and preservation of its pollen. At the top of Sub-zone IIa (−33 cm), an abrupt decrease in pollen concentration (from 451 to 175 grains per gram of sediment; Fig. 3) characterizes the transition from Sub-zone IIa to Sub-zone IIb. 5.3.4. Sub-zone IIb (− 33 cm to 0 cm) This sub-zone is characterized by the upward increase in mangrove taxa Rhizophora and Avicennia
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(from 1% to 6% and from 0% to 1%, respectively), the consequent decrease in some Arboreal Pollen, and the disappearance of Cecropia and Proteaceae. Other tree taxa show minor increases towards the top of this zone, with Podocarpus increasing from 0% at the transition to 3% at the top of the core, and Ilex increasing from 1% to 3% at the top. Among the herbs, the same pattern emerges, with Poaceae decreasing to 11% at the transition between the two sub-zones, but increasing to
Fig. 6. Model showing the evolution of the Itanhaém mangrove: I: formation of the estuary during tansgressive phase; II and III: beginning of gradual sedimentary filling of the estuary by the progradation of bayhead deltas during a regression phase, with the partial closing of the estuary enhancing the deposition of pelitic sediments in the inter-tidal zone and favoring the expansion of the mangrove; IV: current situation of the estuary.
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21% close to the top of the core and Cyperaceae decreasing from 2% to 1% in the transition zone, but increasing to 7% near the top. With the increase in pollen of index taxa for the mangrove ecosystem, this sub-zone marks the consolidation and expansion of mangrove vegetation in the area of the core site. 5.4. Core interpretation Pollen analysis and radiocarbon dating indicate that the Itanhaém mangrove already existed by 1300 years BP, although the limited frequency of pollen of Rhizophora at the base of the core suggests that the mangrove at that time was restricted to small areas or occurred at some distance from the core site. Around 1000 years BP, the mangrove began expanding and probably reached the core site around 330 years BP (basal portion of Sub-zone IIb). The evolutionary model of the Itanhaém mangrove during the Late Holocene presented here uses both palynological and sedimentological data from the core and is based on previous knowledge of behavior of the RSL in its regional context and its implications for the sedimentary dynamics of an estuarine–lagoonal system (Fig. 6). The estuarine–lagoonal system of the Itanhaém region was created during the last transgression, brought about by the rise in RSL following the final glacial maximum around 18,000 years BP (Correa, 1996). Along many stretches of the Brazilian coast, including that of the area of study, this transgression led to the drowning of fluvial valleys and the consequent formation of estuarine–lagoonal bays. With a decrease in the rate of rise of the RSL about 6000 years BP, the transgression ceased, and a regressive phase was initiated, characterized by the advance of the strandplain depositional system and the closure and/or filling of the lagoons and estuarine bays mainly with muddy sediments. The present southern Brazilian coastal depositional systems continue in this phase (Suguio et al., 1985; Angulo and Lessa, 1996; Angulo and Giannini, 1996; Ybert et al., 2003). Within this context, the Itanhaém estuarine–lagoonal system would have been more open during the transgressive phase, with its tidal flats more exposed to wave action. These tidal flats would have been sandy, with muddy intertidal facies occupying only a narrow strip of the estuary. Considering that mangroves tend to develop mostly on muddy substrates, in zones protected from wave action, this phase would not have been favorable for extensive development of mangrove vegetation, although such conditions could have existed in localized areas with minor embayments.
The gradual sedimentary filling of the estuary during the regressive phase, and possible advance of bayhead deltas, increased the intertidal areas, leaving the subtidal area restricted to tidal channels. Moreover, the partial closing of the estuary related to its sedimentary filling may have increased the deposition of pelitic sediments in the intertidal zone due to the decreased action of waves. This scenario would favor the development and expansion of mangrove, which thrives on pelitic substrates. 6. Conclusions 1. Palynological analysis of modern pollen rain and of a core collected in the Itanhaém mangrove reveal similarities to those previously reported for mangroves along the southern and southeastern coast of Brazil, in which allochthonous pollen types exhibit accentuated representation. Concordant with this, the pollen spectra of the Itanhaém mangrove show a greater influence of pollen of wider regional origin than that of a more local origin. 2. The differences in the pollen records of mangroves from different stretches of the Brazilian coast can be explained by variations in the physiography (width and dip) of the coastal plain, which determine the distance between the mangroves and the neighboring rain forest, as well as tidal range, which tends to be higher in the northern of Brazil. These two variables define the areal extent of potential mangrove occupation. 3. Utilization of the Bromeliaceae as natural collectors of pollen rain seems to be a useful method for the study of pollen rain in tropical rain forests. 4. Assuming that the mangrove develops preferentially on wave-sheltered coasts and muddy substrates, the transition from sandy to muddy sediments in the core together with increased frequency of mangrove pollen were interpreted as reflecting gradual estuary closure. 5. Pollen analysis of the core extracted from the Itanhaém mangrove made it possible to suggest an evolutionary scheme to this ecosystem. The mangrove was already present in the region by 1300 years BP, although it was probably restricted to small areas within an estuarine–lagoonal system. Around 1000 years BP, the mangrove expanded in the general area around the core location, occupying the core site itself around 330 years BP. 6. The development of the Itanhaém mangrove was associated with the dynamics of sedimentation in paleoestuarine–lagoonal system. Conditions for full development of mangrove appeared only after the paleoestuariny–lagoon became filled with sediment
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as bayhead deltas advanced to the sea, increasing the size of the intertidal areas. Partial closure of the inlets or river mouths could have led to increased deposition of pelitic sediments in the intertidal zone, thus favoring the development of mangrove. 7. Mangroves develop in intertidal zones which are protected from direct wave action, such as in estuarine and/or lagoonal depositional systems. Therefore, the historical reconstruction of a mangrove must be based on interpretations of the pollen record in relation to sedimentary dynamics of estuarine–lagoonal systems. These dynamics are controlled not only by the behavior of the RSL, but also by sediments supply, as well as by interaction with other coastal systems such as strandplain and fluvial systems. The appearance or disappearance of a mangrove can be caused by changes in these sedimentary dynamics and may even occur without any variation in RSL. The vast majority of papers about mangrove palynology, however, have used pollen records to make direct interpretations of RSL changes. However, any such interpretation of the palynological record of a mangrove to explain RSL changes must necessarily take into account all available knowledge regarding the sedimentary evolution of the coastal depositional system in which the mangrove occurs. Acknowledgements The authors would like to thank Fabio Branco, Samuel Branco (in memoriam) and Jorg Bogumil for their help in the field. They would also like to thank André O. Sawakuchi, Paulo E. de Oliveira, Chahrazed L. Morenghi and Thomas R. Fairchild for their helpful discussions. Support for this research was provided by the BIOTA Program (01/09881-2) and a grant from the Brazilian National Research Council (CNPq-131212/ 02-8). M.-P. Ledru is funded by a convention between the CNPq (Brazil) and the French Research Development Institute (IRD-France). We would like to thank the reviewers for the suggestions which were of great importance for the development of this work. References Absy, M.L., 1975. Polem e Esporos do Quaternário de Santos (Brasil). Hoehnea 5, 1–26. Absy, M.L., Suguio, K., 1975. Palynological content and significance of the drilled sediment samples from the Baixada Santista, Brazil. Anais da Academia Brasileira de Ciências 47, 287–290 (supl.). Absy, M.L., Cleef, A., Fournier, M., Martin, L., Servant, M., Sifeddine, A., Ferreira da Silva, M., Soubies, F., Suguio, K.,
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Turq, B., Van der Hammen, T., 1991. Mise en evidence du quatre phases d'overture de la forêt dense dans le sud-est de l'Amazonie au cours des 60 000 dernières années: première comparaison avec d'autres régions tropicales. Compte Rendu de l'Académie des Sciences de Paris 312 (II), 673–678. Angulo, R.J., Giannini, P.C.F., 1996. Variação do nível relativo do mar nos últimos dois mil anos na região sul do Brasil: uma discussão. Boletim Paranaense de Geociências 44, 67–75. Angulo, R.J., Lessa, G., 1996. The Brazilian sea level curves: a critical review with emphasis on the curves from Paranaguá and Cananéia regions. XXXIX Congresso Brasileiro de Geologia. Anais, Salvador, pp. 285–288. Behling, H., Costa, M.L.D., 1994. Studies on Holocene tropical vegetation mangrove and coast environments in the state of Maranhão, NE Brazil. In: Rabassa, J., Salemme, M. (Eds.), Quaternary of South America and Antarctic Peninsula, 10. A.A. Balkema, Netherlands, pp. 93–118. Behling, H., Cohen, M.C.L., Lara, R.J., 2001. Studies on Holocene mangrove ecosystem dynamics of the Bragança Peninsula in northeastern Pará, Brazil. Paleogeography, Paleoclimatology, Paleoecology 167, 225–242. Branco, S.M., 1984. O fenômeno Cubatão. CETESB, São Paulo. Coelho, L.G., Barth, O.M., Chaves, H.A.F., 2001. Palynological records of environmental changes in Guaratiba mangrove area, southeast Brazil, in the last 6000 years B.P. VIII Congresso da Assoc. Bras. de Estudos do Quaternário, Boletim de resumos, Imbé 403–404. Colinvaux, P., de Oliveira, P.E., Patiño, J.E.M., 1999. Amazon Pollen Manual and Atlas. Harwood Academic Publishers, Netherlands. Correa, I.C.S., 1996. Les variations du niveau de la mer durant les derniers 17.500 ans P.B.: l'exemple de la plate-forme continentale du Rio Grande do Sul-Brésil. Marine Geology 130, 163–178. Cour, P., 1974. Nouvelle techniques de détection des flux et des retombées polliniques: étude de la sedimentation des pollens et des spores a la surface du sol. Pollen et Spores XVI, 103–141. Crowley, G.M., Grindrod, J., Kershaw, A.P., 1994. Modern pollen deposition in the tropical lowlands of northeast Queensland, Australia. Review of Paleobotany and Palynology 83, 299–327. Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis. John Wiley and Sons, LTD, Chichester. Fisher, W.L., 1983. Facies Analysis in Reservoir Geology. Ouro Preto, Univ. Federal de Ouro Preto. Short course. 66 p. Fisher, W.L., McGowen, J.H., 1967. Depositional systems in Wilcox Group (Eocene) of Texas and their relation to occurrence of oil and gas. Bulletin of the American Association of Petroleum Geologists 53 (1), 30–54. Garcia, R.J.F., 2003. Estudo florístico dos campos alto-montanos e matas nebulares do Parque Estadual da Serra do Mar, Núcleo Curucutu, São Paulo, SP, Brasil. Unpublished doctoral thesis, Instituto de Biociências, Universidade de São Paulo, São Paulo, 356 pp. Giannini, P.C.F., Santos, E.R., 1996. Caracterização sedimentológica da lama negra de Peruíbe (SP). III Congresso Brasileiro de Termalismo. Anais, Santa Catarina, pp. 1–20. Grindrod, J., 1988. The palynology of Holocene mangrove and saltmarsh sediments, particularly in northern Australia. Review of Palaeobotany and Palynology 55, 229–245. Grindrod, J., Moss, P., Kaars, S., 2002. Late Quaternary mangrove pollen records from continental shelf and ocean cores in the north Australian–Indonesian region. In: Kershaw, P., David, B., Tapper, N., Penny, D., Brown, J. (Eds.), Bridging Wallace's Line: The Environmental and Cultural History and Dynamics of the SEAsian–Australian Region. Catena Verlag GMBH, Reiskirchen.
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Lamberti, A., 1969. Contribuição ao conhecimento da ecologia das plantas do manguezal de Itanhaém. Boletim da Fac. Fil. Ciên., 23. Letras da Universidade de São Paulo, pp. 7–217. Lamparelli, C.C., 1999. Mapeamento dos Ecossistemas Costeiros do Estado de São Paulo. Secr. Meio Ambiente. CETESB, São Paulo. coord. Markgraf, V., D'Antoni, H.L., 1978. Pollen Flora of Argentina— Modern Spore and Pollen Types of Pteridophyta, Gymnospermae, and Angiospermae. The University of Arizona Press, Tucson. Moss, P.T., Kershaw, A.P., Grindrod, J., 2005. Pollen transport and deposition in riverine and marine environments within the humid tropics of northeastern Australia. Review of Paleobotany and Palynology 134, 55–69. Muller, J., 1959. Palynology of recent Orinoco delta and shelf sediments. Micropaleontology 5, 1–32. Roubick, D.W., Moreno, J.E., 1991. Pollen and Spores of Barro Colorado Island. Missouri Botanical Garden, Missouri. Schaeffer-Novelli, Y., 1991. Manguezais brasileiros. Unpublished thesis, Instituto Oceanográfico, Universidade de São Paulo, São Paulo. 42 pp. Slompo, C.T.J., 1997. Estudo palinológico dos sedimentos inconsolidados do mangue de Itacorumbi, Ilha de Santa Catarina, SC, Brasil. Boletim Paranaense de Geociências 45, 67–79.
Suguio, K., 1998. Dicionário de Geologia Sedimentar e Áreas Afins. Bertrand, Rio de Janeiro, Brasil. Suguio, K., 1999. Geologia do Quaternário e mudanças ambientais (passado + presente = futuro?). Paulo's Comunicação e Artes Gráficas, São Paulo. Suguio, K., Martin, L., 1978. Formações quaternárias marinhas do litoral paulista e sul fluminense. Intern. Symp. on Coastal in the Quaternary, vol. 1, pp. 1–55. São Paulo. Suguio, K., Martin, L., Bittencourt, A.C.S.P., Dominguez, J.M.L., Flexor, J.-M., Azevedo, A.E.G., 1985. Flutuações do nível relativo do mar durante o Quaternário superior ao longo do litoral brasileiro e suas implicações na sedimentação costeira. Revista Brasileira de Geociências 56, 273–286. Thanikaimoni, G., 1987. Mangrove Palynology. UNDP/UNESCOInstitut Français de Pondichery Tome, Paris. Ybert, J.P., Bissa, W.M., Catharino, E.L.M., Kutner, M., 2003. Environmental and sea-level variations on the southeastern Brazilian coast during the Late Holocene with comments on prehistoric human occupation. Paleogeography, Paleoclimatology, Paleoecology 189, 11–24.
Late Holocene development of a mangrove ecosystem in southeastern Brazil (Itanhaém, state of São Paulo) Paula Garcia Carvalho do Amaral a,⁎, Marie-Pierre Ledru a,b , Fresia Ricardi Branco c , Paulo César Fonseca Giannini a a
c
Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São Paulo, rua do Lago 562, CEP 05508-080, São Paulo, SP, Brazil b Institut de Recherche pour le Développement (IRD), UR 055, France Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, rua João Pandiá Calógeras 51, CEP 13083-970, Caixa Postal 6152, Campinas, SP, Brazil Received 6 July 2005; received in revised form 29 March 2006; accepted 6 April 2006
Abstract Most palynological studies of mangroves have been carried out in the Indo-Pacific region, but few have investigated these ecosystems along the southern Atlantic coast. This paper provides information on the palynology of a mangrove at Itanhaém, state of São Paulo, on the southeastern Brazilian coast. This mangrove occurs on microtidal flats adjacent to a fluvial-tidal channel mouth (Itanhaém River), part of an estuarine lagoon system established in the mid-Holocene. Pollen samples were collected from the surface and from bromeliad water in order to define the composition of modern pollen rain in the study area. The results show a strong presence of rain forest taxa in the mangrove pollen spectra, which differ from those obtained from northern Brazilian mangroves, which are characterized by higher percentages of the mangrove taxa Rhizophora and Avicennia. Based on palynological and 14C chronological analysis of a shallow core measuring 135 cm in depth, the Itanhaém mangrove already existed by 1300 years BP and began expanding about 1000 years BP, reaching its present extent sometime after 300 years BP. Its evolution is related to the sedimentary filling of the estuarine–lagoonal depositional system. Two factors may have controlled the development of this mangrove: the relative increase of intertidal areas due to this sedimentary filling and the decrease in wave action and increase in pelitic deposition in the intertidal zone related to the partial closing of the estuarine–lagoonal inlets. These processes were favored by the advance of bayhead deltas and by the development of sand spits as a part of the regression of the Holocene strandplain. © 2006 Elsevier B.V. All rights reserved. Keywords: Holocene; Palaeoecology; Pollen record; Mangrove; Restinga; Tropical rain forest; Estuary; Brazil
1. Introduction
⁎ Corresponding author. Tel.: +55 11 3091 4138; fax: +55 11 3091 4207. E-mail address: [email protected] (P.G.C. do Amaral). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.04.010
A mangrove is a coastal ecosystem with constant influence of tides, which is typical of tropical and subtropical regions (Lamparelli, 1999); it is defined by the existence of flora consisting of an association of
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specific trees and bushes (e.g. Rhizophora mangle, Avicennia sp.) adapted to variations in salinity and low levels of free oxygen in the substrate (Suguio, 1998). This ecosystem can occupy a variety of substrates (e.g. coral rubble, sand) but is more common and most diverse on fine-grained sediments (Grindrod, 1988). Mangroves are exposed to many natural processes, such as sea level variation and coastal erosion that can modify their development and geographic distribution. Along the Brazilian coast, mangrove ecosystems are found from the extreme northern coast on the Oiapoque River (04°20′N) in the state of Amapá to Laguna (28°30′S) on the southern coast in the state of Santa Catarina. These mangroves become narrower and lower in stature from the north to the south, where both tidal range and temperatures are lower, and the coastal plain is restricted by the Serra do Mar (a Cenozoic fault scarp). On the southern coast, mangroves are restricted to microtidal bays and lagoonal or estuarine inlets (Schaeffer-Novelli, 1991). The widespread occurrence of this ecosystem along the Brazilian coast provides a large range of scenarios for the characterization of mangrove environments and reconstruction of corresponding paleoenvironments using pollen records. Studies of these records in mangroves have demonstrated that palynological analysis can provide important information about the history of the vegetation in the ecosystem as well as data for the reconstruction of sea level changes and the evolution of coastal sediments (Suguio, 1999; Grindrod et al., 2002). However, research with such a focus is rare and has been restricted to a few specific locations. Our objective is to reconstruct the development of a mangrove in southern Brazil and the history of sedimentary dynamics in this region, including relative sea level (RSL) and coastline variations, based on the palynological analysis of a shallow core (135 cm) extracted from an area currently occupied by this kind of ecosystem. 2. Study area: depositional systems and mangrove distribution Formed by the confluence of the Branco and Preto Rivers, the Itanhaém River enters the sea near the city of Itanhaém (24°16′59″S, 46°47′20″W; Fig. 1) on the coastal plain of the southern Brazilian coast in the state of São Paulo. The Itanhaém River and the lower courses of its affluents show cyclic flow reversion, thus functioning as tidal channels. In terms of Quaternary depositional systems (sensu Fisher and McGowen, 1967; Fisher, 1983), the Peruíbe–Itanhaém coastal
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plain can be described as a juxtaposition of strandplain and estuarine–lagoonal systems, although it has been modified by recent and restricted fluvial and tidal plain systems. The strandplain system is formed by sandy deposits, characterized in aerial photos by successively aligned foredune ridges and beach ridges. The evolution of this strandplain system is basically related to the progradations which followed two separate transgressive events (Suguio and Martin, 1978). The first, in the Late Pleistocene, is related to the high RSL (+ 8 ± 2 m) of the last interglacial near 120,000 years BP. The second, during the Holocene, is linked to the maximum RSL ca. 5100 years BP (around +4.5 m). The Pleistocene strandplain, up to 12 km wide, has its inland limit at the foot of the Serra do Mar scarp, whereas the Holocene strandplain is a narrow zone (less than 2 km wide) between the Pleistocene strandplain and the ocean. The areas currently drained by the Branco, Preto and Itanhaém Rivers contain sandy mud deposits of estuarine–lagoonal origin. According to the model developed by Giannini and Santos (1996) for the Peruíbe River southwest of Itanhaém, this estuarine– lagoonal system was formed during the Holocene transgression and it was probably developed by drowning and mud filling of fluvial valleys incised during the lowstand of the Würm glacial period. Modern tidal-fluvial channels and tidal flats are superimposed on the filled estuarine–lagoonal systems, and mangroves occur in association with the muddy intertidal facies of the tidal flat. The main factors determining the distribution and relative extension of supratidal, intertidal and infratidal facies in the Itanhaém region are tidal range, physiography of the tidal plain (width and dip) and the dynamics of coastal depositional systems. This third factor was the most variable during the Late Holocene and may have included closure and opening of inlets and filling and erosion of bays in the estuarine–lagoonal system. It is strongly influenced by the balance between sedimentary supply and RSL changes, which determine the position of the coastline and locations for the development of mangroves. On a regional scale, the tidal flat domain, with its supra-and intertidal facies, probably increased during the Holocene regression, leading to corresponding variations in the distribution of mangroves during the last millennia. 3. Climate and vegetation The annual average temperature in the region is greater than 20 °C and annual precipitation varies from 2000 to 2500 mm. Highest precipitation falls during the
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Fig. 1. Geological map of Itanhaém (modified by Suguio and Martin, 1978) showing the location of the modern samples.
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summer (January to March) and the lowest in winter (July to August), although there is no well-defined dry season. Relative humidity is high, averaging around 80% (Lamparelli, 1999). The strandplain in the vicinity of the Itanhaem River system is occupied by an association of trees and shrubs known as restinga, with subordinate presence of secondary forests and cultivated areas. Atlantic Coastal Rain Forest covers the Serra do Mar. The mangrove under study covers an area of 3.75 km2 and is located in the intertidal zone of the Itanhaém River and its confluents. The vegetation and ecology of the Itanhaém mangrove were studied by Lamberti (1969), who found the dominant species in this ecosystem included Avicennia schaueriana (Avicenniaceae), Laguncularia racemosa (Combretaceae) and R. mangle (Rhizophoraceae). In the outlying zones only occasionally reached by spring tides, one also finds Hibiscus tiliaceus (Malvaceae), Crinun attenuatum (Amarylidaceae), Spartina brasiliensis (Poaceae), Fimbristylis glomerata (Cyperaceae) and Acrostischum aureum (Polypodiaceae). The vegetation of the neighboring restinga growing in the sandy soil typical of a strandplain depositional system is characterized by a wide variety of palm trees, as well as the orchids and bromeliads that grow on trunks and branches of larger trees (Branco, 1984). The restinga in this area is related to the Atlantic Coastal Rain Forest, although the forest is shorter and less diverse than at sites on the slope of the Serra do Mar. Garcia (2003) identified 373 species from 215 genera and 88 families in the tropical rain forest in Itanhaém (23°59′06″S, 46°44′36″W). According to this author, species-rich genera include: Mikania (Asteraceae), Leandra (Melastomataceae), Vrisea (Bromeliaceae), Paspalum (Poaceae), Baccharis (Asteraceae), Ocotea (Lauraceae), Panicum (Poaceae), Psychotria (Rubiaceae), Miconia (Melastomataceae), Eugenia (Myrtaceae), Piper (Piperaceae), Tibouchina (Melastomataceae), Xyris (Xyridaceae), Myrcia (Myrtaceae) and Habenaria (Orchidaceae). 4. Materials and methods Nine samples of modern pollen rain were collected along a transect cutting through the following vegetation types present in the Itanhaém drainage basin: mangrove, restinga, Atlantic Coastal Rain Forest and secondary forest stands. At the beginning of the rainy season in November 2000, sediment samples at the water/sediment interface and surface soils were collected in an area of approximately 100 m2. In addition, bromeliad-stored
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water was sampled from pollen content. Bromeliads are omni-present in the region and these plants are natural collectors of pollen rain; where possible, paired samples of sediment and bromeliad water were collected for a comparative analysis. The core location was selected to avoid bioturbation and as far as possible (about 50 m) from the nearest channel. However, since the channel of the Itanhaém River meanders extensively in the narrow area occupied by the mangrove, it is difficult to find a point, which is not close to the river and tidal creeks. Manual sampling was used to extract a core of 135 cm depth. Once the core was opened, color changes, grain size, sedimentary structures, roots and other fragments of vegetation were recorded. Special care was taken to identify possible signs of bioturbation in the sediments of the core. Samples for palynological extraction using the classical method described by Faegri and Iversen (1989) were also taken every 4 cm. Four levels were selected for radiocarbon dating based on lithological changes and the presence of organic material. The sample residues were mounted in glycerine. The identification and counting of pollen grains involved a study of the slides using a biological microscope. For each sample, at least 250 pollen grains were counted and identified by comparison with our reference pollen collection and various pollen keys (Absy, 1975; Markgraf and D'Antoni, 1978; Thanikaimoni, 1987; Roubick and Moreno, 1991; Colinvaux et al., 1999). Due to the similarity of their pollen grains, the Meliaceae/Sapotaceae and Melastomataceae/Combretaceae families were not separated here. The results are presented in two types of diagrams (Figs. 2 and 3). Fig. 2 provides detailed pollen data expressed as percentages of each taxon in relation to the sum of arboreal (AP), non-arboreal (NAP) and undetermined terrestrial pollen, with aquatic pollen and spores excluded from the total. Relative frequencies of spores and aquatic pollen were calculated in relation to the total AP and NAP pollen sum. Fig. 3 provides percent values for summarised pollen groups, based on plant form and habitat type. The concentration values (pollen grains per gram of wet sediment) were calculated using the volumetric method of Cour (1974). 5. Results and interpretation 5.1. Modern pollen rain The results obtained from both surface and bromeliad-water samples, as well as those from the top of the
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Fig. 2. Summary pollen percentage diagram of core IT-01.
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Fig. 3. Summarised pollen spectra of vegetation groups. Mangrove = Rhizophora and Avicennia; Trees and shrubs (restinga and Atlantic rain forest) = Alchornea, Arecaceae, Arrabidea, Cecropia, Ilex, Melastomataceae/Combretaceae, Myrsine, Myrtaceae, Podocarpus, Proteaceae, Weinmania; herbs = Asteraceae, Cyperaceae, Poaceae; spores = Cyatheaceae, Polypodiaceae. Concentration diagram is also presented.
core, are presented in Fig. 4. Pollen from Rhizophora and Avicennia are recorded only inside the mangrove, Rhizophora being more frequent than Avicennia (1–2% and 0.4%, respectively). This difference seems to be related to the pollen dispersal strategies of these genera as observed in palynological studies in other mangroves (Muller, 1959; Behling and Costa, 1994, Behling et al., 2001). The pollen collected in the restinga is represented by samples 3, 4 and 5, which show high frequencies of Alchornea (2–8%), Cecropia (3–4%), Myrtaceae (10– 18%), Myrsine (1–4%), Asteraceae tubuliflorae (3–8%) and Amaranthaceae/Chenopodiaceae (0.3–2%). The high frequency of Piper in sample 9 (21%) coincides with the transition between the restinga and the Atlantic rain forest itself. High frequencies of Piper, indicating a spatial change in vegetation, have also been observed in Carajás (Amazonas state) by Absy et al. (1991) and in Lago Aquiri (Maranhão state) by Behling and Costa (1994). A high frequency of pteridophyte spores (63%) is recorded in sample 8, while other samples reveal much lower levels (from 7% to 32%). It is possible that oxidation and degradation of fragile pollen types on the surface soil may have led to an increase in the relative frequency of more resistant spore and pollen types. Field observations, however, tend to show a large number of
pteridophytes in more humid areas of the Atlantic Coastal Rain Forest. Paired samples were collected at two locations, one from surface sediments and the other from the water stored in Bromeliaceae (samples 3, 3a, 5 and 5a). Similar frequencies of pollen were observed in the samples from the two sources for Cecropia (3% in both pairs), Podocarpus (1% in both pairs) and Weinmannia (1% in one pair, but 0% in the other). Higher frequencies of some of them were observed in the water from Bromeliaceae: Alchornea (7–8% vs. 2–3% on the surface); Myrsine (4% vs. 1% on the surface) and Arrabidea (2% vs. 1% on the surface). Aquatic taxa were present in greater concentrations in the samples from the surface sediments (6% and 15% vs. 2% and 4% in the bromeliad water) (Fig. 4). Hence, the use of the Bromeliaceae as natural collectors of pollen rain seems to be a useful option for present-day rain forests, although for the sake comparison, surface sediment samples should also be collected at the same sites. The separation of the pollen spectra from restinga forest and those from the Atlantic Coastal Rain Forest was problematic, as the two areas contain many of the same species, although the rain forest exhibits greater diversity. A more precise definition of the pollen spectrum from the vegetation of these two areas
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Fig. 4. Summary pollen percentage of the surface and bromeliad samples, with top of core (It-01) also represented.
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would require exhaustive sampling along the mountain range. Another important finding is that forest pollen taxa are extensively deposited in the mangrove. In fact, the mangrove pollen signal is masked by the high quantity of allochthonous pollen types. This may reflect fluvial transport of pollen from upland sources. Similar results were reported by Crowley et al. (1994) and Moss et al. (2005). Another explanation may be the proximity of the forest to the mangrove site, as the mountain range covered by rain forest is only 15 km from Itanhaém. This proximity could have facilitated wind dispersal of pollen. The strong influence of allochthonous pollen in the Itanhaém mangrove contrasts with mangrove pollen assemblages from northern Brazil, where mangrove taxa account for around 90% of the pollen assemblages (Behling and Costa, 1994; Behling et al., 2001). Previous studies have demonstrated the significant differences in the palynological records in the mangroves of the Brazilian southeast–south and north–northeast. The most noticeable difference between these regions seems to be the low frequencies of Rhizophora and Avicennia (typi-
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cal mangrove association) in the pollen record of mangroves in the southeast–south region (Absy and Suguio, 1975; Slompo, 1997; Coelho et al., 2001) and over-representation of pollen of Rhizophora in the sedimentary record of mangroves in the north–northeast (Behling and Costa, 1994; Behling et al., 2001). The results of modern pollen rain analysis of the Itanhaém coastal plain have led to a slight modification of the model of Grindrod (1988) to account for the presence of the Atlantic coastal forest on the slopes of the nearby mountain (Fig. 5). The presence of the local taxa (Rhizophora and Avicennia) is obvious, while longdistance dispersal is attested by the presence of Alnus pollen; the allochthonous regional taxa from the restinga and the Atlantic Coastal Rain Forest include Alchornea, Arrabidea, Cecropia, Ilex, Myrtaceae, Myrsine, Podocarpus and Weinmannia, whereas the transition area is defined by the presence of such pollen as Piper. 5.2. Core results Sedimentological variations in the core are shown in Table 1. The core is made up of muddy sediments rich in
Fig. 5. Model of pollen rain dispersion along coastal plain of the Itanhaém River. The distance between the Itanhaém mangrove and the “Serra do Mar” slope is nearly 15 km (modified by Grindrod, 1988).
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Table 1 Sedimentological variation in the core Depth (cm)
Sedimentological description
− 26 to 0 cm
Uncompacted black muddy sediments, rich in organic matter, with plants remains present Compacted grey muddy sediments, with plants remains present Compacted dark grey muddy sediments Grey compacted muddy sediments, with plants remains present Muddy sediments with little quantity of sand Muddy sediments with medium quantity of sand Muddy sediments with large quantity of sand Light gray fine sand laminated, with organic matter
− 41 to − 26 cm − 46 to − 41cm − 64 to − 46 cm − 84 to − 64 cm − 106 to − 84 cm − 125 to − 106 cm − 135 to 125 cm
organic matter, except for the lowest 10 cm, which are composed of fine sand, with horizontal plane-parallel lamination. The amount of sand decreases upwards. No bioturbation was observed. Radiocarbon dating shows that the base of the core (−120 to −125 cm) is at least 1300 years old (Table 2). Results for −37 to−42 cm (590± 130 years BP) and −70 to −75 cm (450 ± 40 years BP), overlap at one standard deviation. Vertical sediment mixing in the mangrove environment may explain these results. The use of different dating methods, AMS and conventional methods, could also be the source of this discrepancy. For the purposes of this study, an average age of 500 years BP is assumed for this interval (−37 to −75 cm). 5.3. Description of the diagrams The results of pollen analysis of the core samples are presented in Figs. 2 and 3. Two zones may be distinguished on the basis of pollen frequency in the mangrove as registered in the core. Zone I is characterized by low frequency of mangrove index taxa (0.3%) and Zone II by the progressive upward increase in the pollen of these taxa. 5.3.1. Zone I (− 135 cm to − 98 cm) This zone is composed basically of sandy sediments with little organic matter and includes the base of the core (Fig. 2). In general terms, this pollen zone is characterized by low pollen concentrations, with only 140–470 pollen grains per gram of sediment (Fig. 3), and high frequency of spores. Arboreal pollen (AP) include mainly Myrtaceae (13–29%), but also Melastomataceae/Combretaceae (2– 11%), Alchornea (5–9%), Cecropia (2–7%), Arecaceae (3–5%) and Myrsine (2–3%). Non-arboreal pollen (NAP) is dominated by Poaceae (12–18%). Percentage values for
the mangrove taxa, Rhizophora and Avicennia, are very low (never exceeding 1%), with Rhizophora pollen recorded in only two samples. When compared to modern pollen rain, the composition of the pollen spectra observed in this zone suggests the presence of a restinga coastal forest, which is consistent with the sandy nature of this part of the core. 5.3.2. Zone II (−98 to 0 cm) This pollen zone is characterized by progessive increase in pollen concentration and frequency of mangrove taxa. The zone may be divided into two sub-zones (IIa and IIb) on the basis of pollen concentration and presence of the mangrove taxa. 5.3.3. Sub-zone IIa (− 98 to − 33 cm) This sub-zone is characterized by high pollen concentrations (both AP and NAP) and by a decrease in spore frequency. Although values for many tree taxa remain similar to those recorded in Zone I, with Myrtaceae still accounting for the largest percentage of tree pollen (11–21%); other abundant pollen types include Alchornea (5–10%), Arecaceae (3–6%) and Myrsine (0.4–3%). Some other taxa which were present in relatively high concentrations in Zone 1, such as Melastomataceae/Combretaceae and Cecropia, decrease in this sub-zone (from a maximum of 11% to 8% and from 7% to 3%, respectively). The frequency of others increases: Arrabidea (from 1% to 4%), Ilex (from 2% to 4%), Podocarpus (from 0.3% to 3%) and Weinmannia (from 2% to 5%). Among the NAP, the pollen of Poaceae continues to be abundant (11–20%) and that of Asteraceae tubuliflorae (now 2–6%) has increased. A progressive increase in the frequency of the pollen of Rhizophora is observed, whereas Avicennia is recorded in a single sample (< 1% at − 38 cm). This sub-zone attests to the development of the restinga and the mangrove. The increase in the percentage of finergrained sediments and the large concentration of organic matter indicate a shift to a lower energy and more sheltered depositional environment. This change would have allowed the deposition of more pelitic material, thereby Table 2 C ages obtained for the samples of the core
14
Sample (no. of lab)
Dating method
Beta-177002 AMS PaBO 29 Conventional method Beta-177003 AMS Beta-177004 AMS
Depth (cm)
13
C/12C
− 10 to −15 − 37 to −42
− 28.4‰ Modern − 31.72‰ 590 ± 130
− 70 to −75 − 28.4‰ − 120 to − 125 − 28.4‰
14
C age (years BP)
450 ± 40 1250 ± 50
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favoring the establishment of a mangrove and preservation of its pollen. At the top of Sub-zone IIa (−33 cm), an abrupt decrease in pollen concentration (from 451 to 175 grains per gram of sediment; Fig. 3) characterizes the transition from Sub-zone IIa to Sub-zone IIb. 5.3.4. Sub-zone IIb (− 33 cm to 0 cm) This sub-zone is characterized by the upward increase in mangrove taxa Rhizophora and Avicennia
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(from 1% to 6% and from 0% to 1%, respectively), the consequent decrease in some Arboreal Pollen, and the disappearance of Cecropia and Proteaceae. Other tree taxa show minor increases towards the top of this zone, with Podocarpus increasing from 0% at the transition to 3% at the top of the core, and Ilex increasing from 1% to 3% at the top. Among the herbs, the same pattern emerges, with Poaceae decreasing to 11% at the transition between the two sub-zones, but increasing to
Fig. 6. Model showing the evolution of the Itanhaém mangrove: I: formation of the estuary during tansgressive phase; II and III: beginning of gradual sedimentary filling of the estuary by the progradation of bayhead deltas during a regression phase, with the partial closing of the estuary enhancing the deposition of pelitic sediments in the inter-tidal zone and favoring the expansion of the mangrove; IV: current situation of the estuary.
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21% close to the top of the core and Cyperaceae decreasing from 2% to 1% in the transition zone, but increasing to 7% near the top. With the increase in pollen of index taxa for the mangrove ecosystem, this sub-zone marks the consolidation and expansion of mangrove vegetation in the area of the core site. 5.4. Core interpretation Pollen analysis and radiocarbon dating indicate that the Itanhaém mangrove already existed by 1300 years BP, although the limited frequency of pollen of Rhizophora at the base of the core suggests that the mangrove at that time was restricted to small areas or occurred at some distance from the core site. Around 1000 years BP, the mangrove began expanding and probably reached the core site around 330 years BP (basal portion of Sub-zone IIb). The evolutionary model of the Itanhaém mangrove during the Late Holocene presented here uses both palynological and sedimentological data from the core and is based on previous knowledge of behavior of the RSL in its regional context and its implications for the sedimentary dynamics of an estuarine–lagoonal system (Fig. 6). The estuarine–lagoonal system of the Itanhaém region was created during the last transgression, brought about by the rise in RSL following the final glacial maximum around 18,000 years BP (Correa, 1996). Along many stretches of the Brazilian coast, including that of the area of study, this transgression led to the drowning of fluvial valleys and the consequent formation of estuarine–lagoonal bays. With a decrease in the rate of rise of the RSL about 6000 years BP, the transgression ceased, and a regressive phase was initiated, characterized by the advance of the strandplain depositional system and the closure and/or filling of the lagoons and estuarine bays mainly with muddy sediments. The present southern Brazilian coastal depositional systems continue in this phase (Suguio et al., 1985; Angulo and Lessa, 1996; Angulo and Giannini, 1996; Ybert et al., 2003). Within this context, the Itanhaém estuarine–lagoonal system would have been more open during the transgressive phase, with its tidal flats more exposed to wave action. These tidal flats would have been sandy, with muddy intertidal facies occupying only a narrow strip of the estuary. Considering that mangroves tend to develop mostly on muddy substrates, in zones protected from wave action, this phase would not have been favorable for extensive development of mangrove vegetation, although such conditions could have existed in localized areas with minor embayments.
The gradual sedimentary filling of the estuary during the regressive phase, and possible advance of bayhead deltas, increased the intertidal areas, leaving the subtidal area restricted to tidal channels. Moreover, the partial closing of the estuary related to its sedimentary filling may have increased the deposition of pelitic sediments in the intertidal zone due to the decreased action of waves. This scenario would favor the development and expansion of mangrove, which thrives on pelitic substrates. 6. Conclusions 1. Palynological analysis of modern pollen rain and of a core collected in the Itanhaém mangrove reveal similarities to those previously reported for mangroves along the southern and southeastern coast of Brazil, in which allochthonous pollen types exhibit accentuated representation. Concordant with this, the pollen spectra of the Itanhaém mangrove show a greater influence of pollen of wider regional origin than that of a more local origin. 2. The differences in the pollen records of mangroves from different stretches of the Brazilian coast can be explained by variations in the physiography (width and dip) of the coastal plain, which determine the distance between the mangroves and the neighboring rain forest, as well as tidal range, which tends to be higher in the northern of Brazil. These two variables define the areal extent of potential mangrove occupation. 3. Utilization of the Bromeliaceae as natural collectors of pollen rain seems to be a useful method for the study of pollen rain in tropical rain forests. 4. Assuming that the mangrove develops preferentially on wave-sheltered coasts and muddy substrates, the transition from sandy to muddy sediments in the core together with increased frequency of mangrove pollen were interpreted as reflecting gradual estuary closure. 5. Pollen analysis of the core extracted from the Itanhaém mangrove made it possible to suggest an evolutionary scheme to this ecosystem. The mangrove was already present in the region by 1300 years BP, although it was probably restricted to small areas within an estuarine–lagoonal system. Around 1000 years BP, the mangrove expanded in the general area around the core location, occupying the core site itself around 330 years BP. 6. The development of the Itanhaém mangrove was associated with the dynamics of sedimentation in paleoestuarine–lagoonal system. Conditions for full development of mangrove appeared only after the paleoestuariny–lagoon became filled with sediment
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as bayhead deltas advanced to the sea, increasing the size of the intertidal areas. Partial closure of the inlets or river mouths could have led to increased deposition of pelitic sediments in the intertidal zone, thus favoring the development of mangrove. 7. Mangroves develop in intertidal zones which are protected from direct wave action, such as in estuarine and/or lagoonal depositional systems. Therefore, the historical reconstruction of a mangrove must be based on interpretations of the pollen record in relation to sedimentary dynamics of estuarine–lagoonal systems. These dynamics are controlled not only by the behavior of the RSL, but also by sediments supply, as well as by interaction with other coastal systems such as strandplain and fluvial systems. The appearance or disappearance of a mangrove can be caused by changes in these sedimentary dynamics and may even occur without any variation in RSL. The vast majority of papers about mangrove palynology, however, have used pollen records to make direct interpretations of RSL changes. However, any such interpretation of the palynological record of a mangrove to explain RSL changes must necessarily take into account all available knowledge regarding the sedimentary evolution of the coastal depositional system in which the mangrove occurs. Acknowledgements The authors would like to thank Fabio Branco, Samuel Branco (in memoriam) and Jorg Bogumil for their help in the field. They would also like to thank André O. Sawakuchi, Paulo E. de Oliveira, Chahrazed L. Morenghi and Thomas R. Fairchild for their helpful discussions. Support for this research was provided by the BIOTA Program (01/09881-2) and a grant from the Brazilian National Research Council (CNPq-131212/ 02-8). M.-P. Ledru is funded by a convention between the CNPq (Brazil) and the French Research Development Institute (IRD-France). We would like to thank the reviewers for the suggestions which were of great importance for the development of this work. References Absy, M.L., 1975. Polem e Esporos do Quaternário de Santos (Brasil). Hoehnea 5, 1–26. Absy, M.L., Suguio, K., 1975. Palynological content and significance of the drilled sediment samples from the Baixada Santista, Brazil. Anais da Academia Brasileira de Ciências 47, 287–290 (supl.). Absy, M.L., Cleef, A., Fournier, M., Martin, L., Servant, M., Sifeddine, A., Ferreira da Silva, M., Soubies, F., Suguio, K.,
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