Palynological analysis of a late Holocene core from Santo Antônio da Patrulha, Rio Grande do Sul, Southern Brazil

“main” — 2010/8/10 — 11:12 — page 731 — #1 Anais da Academia Brasileira de Ciências (2010) 82(3): 731-745 (Annals of the Brazilian Academy of Sciences) ISSN 0001-3765 www.scielo.br/aabc Palynological analysis of a late Holocene core from Santo Antônio da Patrulha, Rio Grande do Sul, Southern Brazil RENATO B. MACEDO1 , PAULO A. SOUZA1 , SORAIA G. BAUERMANN2 and SÉRGIO A.L. BORDIGNON2 1 Laboratório de Palinologia, Departamento de Paleontologia e Estratigrafia, Instituto de Geociências Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91540-900 Porto Alegre, RS, Brasil 2 Laboratório de Palinologia, Universidade Luterana do Brasil, Av. Farroupilha, 8001, 92425-900 Canoas, RS, Brasil Manuscript received on June 19, 2009; accepted for publication on December 8, 2009 ABSTRACT A sedimentar core collected at Santo Antônio da Patrulha, Rio Grande do Sul State, southmost Brazil, was submitted to pollen analysis to provide the vegetational history of this region, and the paleoecological and paleoclimatic changes. A total of 98 taxa of palynomorphs was identified from 35 subsamples. Three radiocarbonic datings were obtained along a section of 115 cm depth, including the basal age of 4730 ± 50 yr BP. Pollen diagrams and cluster analysis were performed based on palynomorphs frequencies, demonstrating five distinct phases (SAP-I to SAP-V), which reflected different paleoecological conditions. The predominance of plants associated with grasslands in the phase SAP-I suggests warm and dry climate conditions. A gradual increasing of humidity conditions was observed mainly from the beginning of the phase SAP-III, when the vegetation set a mosaic of grasslands and Atlantic rainforest. Furthermore, the presence of some forest taxa (Acacia-type, Daphnopsis racemosa, Erythrina-type and Parapiptadenia rigida-type), from the phase SAP-IV, is interpreted as an influence of the seasonal semideciduous forest in the study region. From the phase SAP-V (ca. 4000 yrs BP), the vegetation became similar to the modern one (extant Atlantic rainforest Biome), especially after 2000 yrs BP (calibrated age). Key words: Palynology, Paleoecology, Paleoclimatology, Atlantic rainforest, late Holocene, Rio Grande do Sul State. INTRODUCTION The vegetation of Rio Grande do Sul State (RS), Southern Brazil, is composed by grassland-forest mosaic as a result of paleoenvironmental changes, mainly characterized by climate variations during the Quaternary (Marchiori 2004). These grassland-forest mosaics were previously studied by pioneer naturalists at the end of the XIX century. Lindman (1906) observed that forests could expand over the grassland vegetation under dry paleoclimate conditions. Based on phytogeographic evidence, Rambo (1956, 1961) and Klein (1975) interpreted that the grasslands were the first plants constituents of the RS; therefore, advances of the forests is a recent answer to the humid climatic conditions. Correspondence to: Renato Backes Macedo E-mail: The palynology is an important tool for the understanding of vegetation dynamics and paleoclimate reconstructions. Several studies based on pollen analysis have contributed in solving questions on the vegetacional succession during the late Quaternary in Southern Brazil, as summarized by Behling (2002), Lorscheitter (2003), De Oliveira et al. (2005) and Leal and Lorscheitter (2007). Palynological results showed that, during the latest Pleistocene, the landscape in these regions was characterized by the dominance of grasslands due to the cold and dry climate associated with glacial times. During the early to middle Holocene, the vegetation related to these grasslands prevailed in lowlands and highlands of the Southern region due to warm and dry climatic condictions after the glacial period. Pollen analysis of the late Holocene in Rio Grande do Sul Coastal Plain (RSCP) and adjacent An Acad Bras Cienc (2010) 82 (3) “main” — 2010/8/10 — 11:12 — page 732 — #2 732 RENATO B. MACEDO, PAULO A. SOUZA, SORAIA G. BAUERMANN and SÉRGIO A.L. BORDIGNON areas showed that forests had expanded from 3500 yrs BP, especially after 2000 yrs BP, due to higher humidity. This paper presents interpretations about the vegetational succession of the Atlantic rainforest and seasonal semideciduous forest, as well as paleoclimates related to them, based on palynological data obtained from a core in Santo Antônio da Patrulha, RS, southmost Brazil. The results were compared with other palynological studies previously conducted in South Brazil. STUDY AREA The present study was made from a peat bog located in Santo Antônio da Patrulha municipality (coordinates 29◦ 440 4500 S, 50◦ 320 5600 W, high 37 m), far about 76 km from Porto Alegre and ca. 48 km from the Atlantic Ocean. The access to this site from Porto Alegre is through the highway RS-474, followed by a secondary road (Figs. 1a-b). According to Fortes (1959), this area is part of the physiographic region of the Lower Northeast Slopes of Serra Geral, RS. The peat bog sediments from Santo Antônio da Patrulha were deposited on sandstones of the Botucatu Formation, Jurassic/Cretaceous of Paraná Basin, which constitutes the basement of the analyzed core. The presence of a grassland-forest mosaic characterizes the phytophisiognomy of the landscape. The forest comprises a mixture of floristic elements of Atlantic rainforest and seasonal semideciduous forest (Rambo 1956, Teixeira et al. 1986, Reitz et al. 1988, Leite and Klein 1990). The climate of Southern Brazil is influenced by the South Atlantic Anticyclone, a semi-permanent high pressure system that transports moist tropical air masses over the continent from easterly and north-easterly directions during the whole year. Disturbances are related to polar cold fronts, when it meets the tropical air masses and produces strong rainfall in Southern Brazil (Nimer 1989). This region is characterized by subtropical humid (Cfa) in the Koppen classification (Moreno 1961), with regularly distributed rainfalls during the year and hot summers. The mean precipitation is around 1676.5 mm.a−1 , and the average annual temperature is 19.8◦ C; the average temperature of the hottest month is 24.4◦ C, while the coolest months are 15.4◦ C (data from Osório and Tramandaí meteorological station, IPAGRO 1979). An Acad Bras Cienc (2010) 82 (3) MATERIALS AND METHODS The core was taken from the deepest portion near the center of the peat bog using a Russian corer sampler, whose maximum depth reached 115 cm. Sections of 50 cm length were extruded in situ, wrapped in plastic film and aluminum foils. The section was transported to the Laboratory and stored in special conditions (ca. +4◦ C) before sampling. Three sediment subsamples of 3 cm thickness were taken from the core and dated by the Accelerator Mass Spectrometry (AMS) in the CAIS Laboratory of the University of Georgia (USA). The calibration of radiocarbon datings was carried out after CALPAL (Weninger et al. 2004). Ages (...truncated)