Svetlana Medeanic

Holocene marine transgression in the coastal plain of Rio Grande do Sul, Brazil: palynomorph and diatom evidence.

Abstract 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.

Holocene Paleoinlet of the Bojuru Region, Lagoa dos Patos, Southern Brazil

Abstract The Holocene environmental evolution of the Bojuru region, Rio Grande do Sul state, southern Brazil, was inferred from seismic, geochronologic, sedimentologic, and paleontologic data. The absolute ages of two samples, determined by the 14C method, are 9400 ± 140 Cal BP and 7370 ±150 Cal BP. On the basis of lithology and palynomorph and diatom data, the six stages of paleoenvironmental development of this region were chosen. Layers of muddy sediments characterized by a significant incidence of marine palynomorphs and marine and estuarine diatoms were deposited during the Early and Middle Holocene. Seismic records (3.5 kHz) revealed a prominent buried paleochannel that can be correlated to the present Barra Falsa channel. The main period of the channel incision was related to the last regressive event of the Late Pleistocene. During the last Holocene transgression, this paleochannel was filled up by fluvial, estuarine, and marine sediments. The integrated analysis of data reveals that the Barra Falsa feature resulted from a former channel connecting the Patos lagoon and the Atlantic Ocean. The closure of this inlet channel is attributed to shifts in the sedimentation milieu related to the transgressive maximum and subsequent regression during the Late Holocene.

Algal Palynomorphs Response to Environmental Changes in the Tramandai Lagoon, Southern Brazil, and Climatic Oscillations in the 20th Century

Abstract This article presents an evaluation of the importance of algal palynomorphs for palaeoenvironmental and climatic reconstructions. The palynomorphs were identified in sediments formed during the 20th century in the Tramandaí Lagoon (southern Brazil). The sediments sample corresponds to the 1905–97 period as dated by 210Pb. Five algal palynomorph zones (IA–VA) were defined based on Botryococcus ∶ and Pediastrum frequency of occurrence ratio as well as on the presence of other freshwater algal palynomorphs. The sensitiveness of algal Chlorophyta palynomorphs to environmental variables, such as temperature and salinity, reveals that these palynomorph zones are related to five major stages of the Tramandaí Lagoons evolution, reflecting variations in the regional hydrological cycle. The last identified stage (VA) is characterized by a notable increase in marine algae and a decrease in freshwater algal palynomorphs. This marine algal palynomorph enrichment was caused by an increase in sea water influx as a result of the construction of an artificial channel connecting the lagoon with the Atlantic Ocean. The obtained results on algal palynomorph frequencies of occurrence were compared with available records of water-level measurements from the coastal Mirim Lagoon, situated relatively close to the Tramandaí Lagoon, and to the Southern Oscillation Index of the Australian Bureau of Meteorology. Wetter/drier periods of the past century as revealed by algal palynomorph frequencies generally coincide with low-frequency water-level oscillations in the Mirim Lagoon and are probably related to interannual variations of rainfall in the region, driven by the variability of the El Niño–Southern Oscillation phenomena.

Palynomorph Records from the Core of Rio Grande Cone, Brazil: Approaches for Palaeoenvironmental and Palaeoclimatic Reconstructions in the Middle Holocene

ABSTRACT Diniz, D. and Medeanic, S., 2013. Palynomorph records from the core of Rio Grande Cone, Brazil: approaches for palaeoenvironmental and palaeoclimatic reconstructions in the Middle Holocene. Palynomorphs are an important tool for understanding palaeoclimatic and palaeoenvironmental changes in the past. There are no studies in Brazil based in palynomorphs in deep sea oceans. Based on core REG-973, the objective of this study was to determine changes in palaeoenvironments during the Middle Holocene in the Rio Grande Cone, southern Brazil. A total of 39 samples were collected from the core, with intervals of 4 cm. Samples were treated using methodology for palynological analysis. There was 14C dating at three samples (20, 100, and 200 cm depth) with absolute ages of 2290, 4080 and 5050 ± 40 yr BP, respectively. The analysis of palynomorphs revealed three distinct origins: marine, terrestrial, and freshwater. Much of this material was allochthonous, transported by aquatic currents of palaeorivers and palaeochannels, which were present on the continental shelf of Rio Grande do Sul (RS) in periods when the mean sea level (MSL) was lower than today. The analysis of the taxonomic variety of palynomorphs showed five Palynomorph Zones. These zones were admitted as corresponding to five stages of palaeoclimatic and palaeoenvironmental development. The constant presence of marine indicators pointed out the formation of such sediment in a marine environment but with influences of coastal surroundings. It was concluded that sediment was generated in a marine environment but always influenced by terrestrial material. The preservation of palynomorphs reflected the MSL oscillations. When the source area was more distant, the preservation level was worse. The generation of sediments was found to be very complex, influenced by factors affecting both the terrestrial and marine environments.