Pollen analysis of Holocene sediments from the Poço das Antas National Biological Reserve , Silva Jardim , Rio de Janeiro , Brazil

The Poço das Antas National Biological Reserve is located in Rio de Janeiro State, southeast Brazil. This paper presents information on past environmental characteristics of the area through pollen analysis. Two sedimentary columns were collected and five samples were selected for radiocarbon dating. The following ages of the columns from bottom to top were detected: column 1 – 1.20-1.16 m: 6080 ± 40 years BP, 0.775-0.735 m: 4090 ± 40 years BP, 0.385-0.345 m: 1880 ± 80 years BP; column 2 – 1.22-1.18 m: 3520 ± 40 years BP, 0.23-0.19 m: 1810 ± 40 years BP. Three samples from column 1 and two samples from column 2 were selected for pollen analysis: 1.20 m, 0.77 m and 0.37 m of column 1 and 1.22 m and 0.21 m of column 2. Chemical treatment followed standard methodology. The palynological analysis shows that around 6080 years BP the study area was dominated by a rain forest and from around 4090 years BP the vegetation changed to a fragmented forest, restricted to low hills and surrounded by an open area of grassland and pioneer plants, swamps and peat areas. The pollen assemblage of the samples 3520, 1880 and 1810 years BP suggest the permanence of this kind of vegetation between 4000 years BP and the actual.


INTRODUCTION
The Poço das Antas National Biological Reserve (RE-BIO) was established in 1974 to preserve the natural habitat of the golden lion tamarin (Leontopithecus rosalia rosalia), an endangered species, native to this area.Located between 22 • 30 and 22 • 35 S and 42 • 15 and 42 • 19 W (Fig. 1) on a broad plain of Tertiary and Quaternary sediments, the REBIO occupies an area of 5000 ha in Silva Jardim municipality, Rio de Janeiro State, Southeast Brazil (IBDF/FBCN 1981).
The rivers that cut through the Reserve or originate inside it drain into the São João River (IBDF/FBCN 1981).In the 1970s, the "Departamento Nacional de Obras e Saneamento" (DNOS) channeled part of the São João River and its affluents, opened new channels and constructed a dam near Juturnaíba lagoon, increasing the region´s irrigation capacity for farming activities, and also improving water quality (FEEMA 1991, IBAMA 1999).Due to dam construction, the swampy area was subjected to desiccation (IBDF/FBCN 1981), resulting in annual fires in the REBIO.
The vegetation of the REBIO is ombrophilous and seasonal (Lima 2000).Correia J.M.F., unpublished data, based on data obtained from the Rio de Janeiro Botanical Garden´s Mata Atlântica Programe, defined six vegetation units in the REBIO: submontane forest, alluvial forest, pioneer formation with fluvial influence, submontane "capoeira", alluvial "capoeira" and manmade grassland.
Besides an extensive continuous forest, hills isolated within the eastern border in swampy and peaty areas are also covered by forest (IBDF/FBCN 1981).Fernandez et al. (1998) and Castro and Fernandez (2004) described the forest fragments in the REBIO as "Barbados Islands", surrounded by an open area of peaty soil dominated by grasses, ferns, and pioneer trees, maintained in initial successional stages by frequent fires.
This research provides information on the age and past vegetation of the REBIO through pollen analysis of five 14 C-dated samples, collected in the open area on muddy soils surrounding the forest fragments.

MATERIALS AND METHODS
Two sedimentary columns (column 1 and column 2) were collected in the peaty area between forest fragments of the REBIO (Fig. 1).Column 1 (22 • 34 57,117 S;  42 • 15 03,166 W) was located in the middle of the open vegetation.Column 2 (22 • 34 58,807 S; 42 • 15 24,058 W) was located near of the base of one of the hills.Handexcavated holes in the ground facilitated the exposure of sediment profiles.Aluminum boxes, 40 cm long and 2 cm wide, were used to obtain soil samples.The holes were less than 1.30m deep, since the ground water welled up at this depth.The holes were refilled with the removed sediments and the remaining vegetation cover.
Material for dating was collected at the following levels starting from the bottom sediments: 1.20-1.16m, 0.775-0.735m and 0.385-0.345m of column 1, and 1.22-1.18m and 0.23-0.19m of column 2. Ages were established through radiocarbon methodology by Beta Analytic Inc., USA.
The sedimentological description of the columns was carried out in the Laboratory of Palynology, Botany Department, Institute of Biology, Federal University of Rio de Janeiro, in collaboration with geologist Dr. Marco André Malmann Medeiros.
Three samples from column 1 and two samples from column 2 were selected for palynologycal analysis: 0.37 m, 0.77 m and 1.20 m of column 1, and 0.21 m and 1.22 m of column 2. Chemical treatment followed Ybert et al. (1992).All samples were treated with 10% HCl, 40% HF for a minimum of 12 hours, 40% KOH, C and Zn 2 Cl with density 2. Exotic spores (Lycopodium clavatum) were added to the samples to calculate palynomorph concentration.Five microscope slides of each sample were prepared.The calculation of pollen grain percentages was based on total pollen that includes aquatic and hygrophyte taxa, herbs, shrubs and trees, but not algae and spores.Therefore, care was taken to stop counting only after obtaining a minimum of 300 pollen grains per sample, excluding aquatic and hygrophyte taxa and Poaceae.Pollen identification relied on the reference slide collection of the Laboratory of Palynology, as well as specialized literature.
TILIA software was used for statistical treatment of the palynological data.Palynomorphs were grouped in two different ways for plotting the pollen data through TILIAGRAPH software, based on the habit of the identified plants and on the floristic-unit classification (Correia J.M.F., unpublished data) for the REBIO.

RESULTS AND DISCUSSION
The analyzed sediments were mainly composed of mud.Samples 0.37 and 1.20 m, from column 1, were composed of brown mud with plant fragments, while sample 0.77 was composed of black mud with plant fragments.Sample 0.21 m, from column 2, revealed brown to grey mud with plant fragments and sample 1.22 a grey micaceous mud.
Pollen grain identification comprised 133 pollen types (Table II).In sample 1.20 m, base of column 1, tree pollen types of Cecropia (Moraceae), Ficus (Moraceae), Moraceae, Myrtaceae, Piper (Piperaceae) and Trema (Ulmaceae) prevailed, while non-arboreal pollen types  The pollen sum of each sample and pollen grain concentration/g of sediment are presented in Table III.
Total palynomorph concentration in the samples of column 1 was manifestly greater than that of column 2 samples.This may be attributed to the difference between sedimentation rates.Though the columns are practically the same depth, column 1 was nearly double the age of column 2, and therefore the sedimentation rate of column 2 was almost double that of column 1.The intense sediment accumulation rate in column 2 could be causing a reduction in total palynomorph concentration in these samples.
The palynological analysis showed a strong dominance of tree pollen types from the rain forest around 6080 years BP in the study area, which today is composed mainly of grassland and pioneer plants, besides forest elements, according to Fernandez et al. (1998).The forest receded around 4090 years BP, firstly grassland and "capoeira" areas became abundant and around 3520 years BP hygrophyte vegetation increased, as in the 1880-and 1810-years-BP samples, and like the vegetation described for the study area today (IBDF/FBCN 1981, Fernandez et al. 1998).
Palynological research in Quaternary sediments in Rio de Janeiro State provided results similar to the data of the present study.Coelho et al. (2002)  4650 years BP in the Guaratiba mangrove area, where tree pollen types from the Atlantic forest prevailed; between 4650 and 1350 years BP, a reduction of wet conditions was observed, associated with progressive forest reduction.Luz et al. (1999), studying sediments from Lagoa de Cima, in northern Rio de Janeiro State, observed a warm, humid phase around 6000 years BP; the swampy areas around the lake were well developed as was a pioneer forest, followed by a drier phase characterized by dry grassland vegetation in the vicinity of the lake.The period around 4000 years BP was the driest phase with high temperatures and the rain forest was confined to hillsides and valleys.Both studies indicate a reduction of rain forest area in periods similar to those observed in the present study.
In conclusion, palynological analysis indicates that the study area, today composed of forest fragments surrounded by grassland and "capoeira" vegetation, was dominated by rain forest around 6080 years BP.The dated samples of 4090, 3520, 1880 and 1810 years BP point to the installation of vegetation similar to that described recently for this fragmented area.