Louis Martin

The last 50,000 years in the Neotropics (Southern Brazil): evolution of vegetation and climate

Abstract In the “Lagoa Campestre” (Lake) of Salitre (19°S, 46°46′W, 970 m elev.), there are plant taxa belonging to many ecological groups that are encountered nowhere else at this latitude. Frequent incursions of polar advections causing cooling and humidity, a cool and foggy climate in the middle of the depression and warmer temperatures on the surrounding slopes help to maintain all these groups within a fairly restricted area. Late Pleistocene-Holocene climatic change has had a considerable impact on the flora and vegetation of Salitre. The pollen record of the 6 m deep core LC3 shows how cold forest trees such as Araucaria angustifolia and Drimys brasiliensis, semi-deciduous forest, halophytic plants and peat bog started to develop on this site. The initial period, between c. 50,000 and 40,000 yr B.P., was an arid phase not recorded in any other neotropical lowland site. It was followed by a period of high moisture levels (40,000 to 27,000 yr B.P.) with a maximum estimated at c. 35,000 yr B.P. The Late Glacial maximum is missing because of a gap in sedimentation. Humidity gradually increased during the Late Pleistocene, between 16,000 and 11,000 yr B.P. The early Holocene, 9500 to 5000 yr B.P., is characterized by a more marked seasonal pattern and higher temperatures, reaching a maximum c. 5000 yr B.P. The spread of semi-deciduous forest between 4000 and 3000 yr B.P. attests to a return of humidity. Comparison with the Serra Negra section (19°S, 46°45′–46′W, 1170 m elev.) not far from Salitre confirms the high moisture rates recorded at c. 35–40,000 yr B.P. (although temperatures were cooler at the altitude of Serra Negra, as is attested by the presence of Araucaria forest) and also confirms the strong impact of polar advections on the climate of Southeastern Brazil.

Variations of the Amazonian rainforest environment: a sedimentological record covering 30,000 years

Abstract Two cores (CSS 2 and CSS 10) were collected from two lakes situated in the Carajas area (6°35′S, 49°30′W) that show different stages of silting up. The results of geochemical and petrographic organic analyses, backed up by 14 C radiometric dating, lead to a reconstruction of the variations in the hydrological regime of these lakes over the last 30,000 years. Evidence is revealed for a period of drying up, characterized by a sedimentary hiatus between 22,000 and 13,000 14 C years BP, followed by a period of refilling between 13,000 and 11,000 14 C years BP. (15,400–12,900 cal years BP). The latter interval is primarily characterized by a mainly clastic sedimentary input rich in quartz, kaolinite and iron hydroxides, leading on to the authigenic formation of siderite. This type of sedimentation implies the erosion of the small catchment basin during periods of transition between arid and humid climatic regimes. From 8000 to 4000 14 C years BP (8900–4500 cal years BP), as well as from 2700 to 1500 14 C years BP (2780–1360 cal years BP), special hydrological conditions prevailed. They are characterized in the sedimentary record by the presence of micro-laminae containing wood charcoal interlayered with sponge spicule material. This interlamination reflects the rapid alternation of dry and humid periods. A comparison with the results obtained by pollen analysis shows that sedimentological indicators, once filtered for strictly local effects, can provide a full record of the regional-scale variations in the environment.

Late Quaternary vegetational and climatic changes in cerrado and palm swamp from Central Brazil

Abstract Geochemical and palynological analyses of core CR1 from Crominia, Goias, provided information on the paleoenvironmental history of central Brazil. The records show that prior to 32,400 yr B.P. the vegetation of the Crominia region was similar to the present one characterized locally by a complex of cerrado, gallery forest and palm swamp, indicating a semi-humid tropical climate. A palm swamp occupied the coring site. From 32,400 to about 20,000 yr B.P. a treeless grassland replaced the arboreal cerrado and the gallery forest suggesting that humidity increased but temperature probably decreased. The site was a shallow lake between ca. 27,000 and 20,000 yr B.P. Humidity started to decrease at ca. 18,500 yr B.P. and the period from ca. 18,500 to ca. 11,500 yr B.P. was very dry. A sparse vegetation was growing in the region during that time. The dry climate continued until 6500 yr B.P. and the core site probably dried out several times. At ca. 5000 yr B.P. humidity increased again, the palm swamp vegetation returned to the site and cerrado vegetation and gallery forest started to grow in the region. The abundant charcoal particles prior to 20,000 yr B.P. and from 10,500 to ca. 3500 yr B.P. document a long history of fires in the region. Results are compared with those from other sites in Central Brazil and with the climate sequence of the last glacial maximum and postglacial time in the tropical Andes.

Controls on Quaternary coastal evolution of the East-Northeastern coast of Brazil : roles of sea-level history, trade winds and climate

Abstract East-northeastern Brazil has a wave-dominated, micro- to meso-tidal coast, lying entirely within the southern Atlantic trade wind belt. Integration of geologic mapping, radiocarbon dating and vibracoring data shows that the Quaternary coastal evolution of this area was controlled by three major factors: (1) sea-level history; (2) trade winds; and (3) climate change. Sea-level history. Along the east-northeastern coast of Brazil, relative sea level has fallen approximately 5 m during the last 5000 y. Correlation of this sea-level history with the evolution of beach-ridge, lagoonal and coastal plain deposits shows that: (1) sea-level rise favours the formation of barrier island—lagoonal systems and the construction of intralagoonal deltas; (2) sea-level lowering is not conductive to barrier island formation. Rather, lagoons and bays become emergent and beach-ridge plains rapidly prograde. Trade winds. Sediment dispersal systems along the coastal zone of east-northeastern Brazil have been highly persistent since Pleistocene time, as deduced from beach-ridge orientation. This persistence results from the fact that sediment dispersal in wave-dominated settings is ultimately controlled by atmospheric circulation which, for the east-northeastern coast of Brazil is associated with the South Atlantic high-pressure cell. The remarkable stability of this cell through time, has allowed the accumulation of extensive beach-ridge plains at the longshore drift sinks located along the coast. Climate change. Effects of Quaternary climate changes on coastal sedimentation are twofold. Climate changes may affect rainfall patterns, thus exerting an important control on coastal dune development. Along the coast of northeastern Brazil, active coastal dunes are only present in those areas in which at least four consecutive dry months occur during the year. Mapping of these areas has shown that dune development during the Holocene has been episodic, these episodes being probably controlled by variations in rainfall patterns associated with climate changes. Secondly, despite its overall stability, the position of the high-pressure cell has experienced small shifts in position during the Holocene in response to climate changes. Changes in wind direction associated with these shifts have induced modifications in the coastal dispersion system, which are recorded in the strandplains as small truncations in the beach-ridge alignments. These results have important implications in understanding accumulation of ancient sandstone shoreline sequences.

Variation of coastal dynamics during the last 7000 years recorded in beach-ridge plains associated with river mouths: example from the central Brazilian coast

Abstract The reconstruction of relative sea-level changes, during the last 7000 years, has shown that the central Brazilian coast was been subjected to submergence until approximately 5100 yr. B.P. However, the emergence after 5100 yr. B.P. was interrupted by two important fluctuations with an amplitude of approximately 2–3 m and a duration of 200–300 yr. This evolutionary history, which is quite different from that of several other regions in the world, played an essential role in the development of the central Brazilian coastal plains, whether or not they are associated with important river mouths. The periods of submergence, characterized by erosional phases, introduced noticeable changes in the geometry of the coastal deposits. The periods of emergence gave rise to sandy terraces covered by beach-ridges whose orientation is determined by the longshore drift. Elsewhere on a low sandy coast, the direction of longshore sand transport depends on the orientation of the swell. In a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction. Such swells are not necessarily the most prevalent. For instance, along the central Brazilian coast the southern sector waves, in spite of their infrequent occurrences, are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north. When fossil beach-ridges are present, as is the case on the central Brazilian coast, their geometry reflects the past directions of longshore sand transport. This makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns. A detailed study of the beach-ridge geometry of some sections of the central Brazilian coast, showed a sequence of reversals of the longshore drift during the last 5100 yr, with a duration ranging from 10 to 100 yr. These reversals represent changes in the direction of effective waves, which determine the longshore transportation of sediments, and, consequently, indicate changes in the wind pattern.

Chapter 3 Geological History of Coastal Lagoons

This paper discusses the geological processes controlling the origin and evolution of coastal lagoons. The study of a lagoon is intimately connected with the barrier island enclosing it-one cannot exist without the other. Three main factors control the origin and maintenance of sandy barriers and thus determine the ultimate fate of coastal lagoons: (1) sea-level history; (2) shoreface dynamics; and (3) tidal range. The history of infilling of coastal lagoons is controlled primarily by rates of sediment supply relative to rates of sea-level rise. In areas of high relative sediment supply, such as on mesotidal coasts or where rivers empty directly into lagoons, the lagoon floor is rapidly brought to the intertidal level. Thus, sediments deposited in the intertidal zone will dominate the sedimentary record of the lagoons. In areas of low sediment supply, lagoons will be characterized by unobstructed water bodies and the lagoonal sedimentary record will be dominated by sediments deposited in sub-tidal environments. The effects of sea-level history and rates of sediment supply on the evolution and infilling of ‘fossil’ coastal lagoons along the eastern coast of Brazil are documented and discussed for three major scenarios: (1) lagoons formed on a low-relief coastal plain during a sea-level rise; (2) lagoons formed on a low-relief coastal plain during a sea-level drop; and (3) lagoons formed on a high-relief coastal plain during a sea-level rise.

Hauts niveaux marins pléistocènes du littoral brésilien

Abstract The high sea-level, corresponding to the last Pleistocene interglacial stade, left the most important records represented by extensive beach-ridge plains along the central and southern coasts of Brazil. During the maximum level, which was dated at 123,000 yr B.P. by coral samples, the relative sea-level was situated 8 ± 2 m above present level. Four generations of Quaternary marine sandy deposits have been described in southern Brazil (State of Rio Grande do Sul), where they are known as Barrier I, II, III and IV. The Barrier IV, about 4 m high, corresponds to the last great transgressive-regressive phase, whose maximum occurred about 5100 yr B.P. The Barrier III, with a height of about 8–10 m, corresponds to the penultimate transgressive-regressive period, whose maximum was produced about 123,000 yr B.P. The Barrier II, about 13–15 m high, and the Barrier I, about 20–25 m high, situated at inner positions in relation to the previous ones, have been formed during high sea-levels older than 123,000 years. Along the coastal plains of the States of Sergipe and Bahia, the high sea-level older than 123,000 years is represented only by one phase through cliffs carved within the Barreiras Formation. On the other side, along the coastalp plains of Sao Paulo, Parana and Santa Catarina, there are marine terraces with a minimum height of 13 m above present sea-level, which are certainly older than the Barrier III, and are probably recording the transgressive-regressive period of the Barrier II (middle Pleistocene?).

Quaternary evolution of the Caravelas strandplain - Southern Bahia State - Brazil

An evolutionary model is proposed for the Caravelas strandplain. The model encompasses integration of: (i) mapping of Quaternary deposits, (ii) cartography of beach-ridge alignments and their truncations, (iii) relative sea-level history, (iv) development history of the Abrolhos coral reefs, (v) vibra-coring and (vi) C 14 dating of Quaternary deposits. Seven major evolutionary stages were identified. These stages show that the strandplain has had its Quaternary evolution strongly controlled by relative sea-level changes. In addition, the development of the Abrolhos coral reefs has also played an important role in dispersion and accumulation of sediments along the coastline, causing localized inversion in longshore sediment transport.

Vibrotestemunhador leve: construção, utilização e potencialidades

Undisturbed or minimally disturbed cores are very hardly obtainable from soft sedimentary deposits, mostly of Quatemary age. On the other hand, details of primary structures must be preserved making possible to reconstruct, as perfect as possible, their sedimentary environments. A very light and cheap vibracorer, suitably used for coring clayey-sandy organic sediments, like bay-bottom, lacustrine or lagoonal deposits, is here described.

Significance of quaternary sea-level fluctuations for delta construction along the Brazilian coast

Many important areas of Quaternary deltaic sedimentation along the Brazilian coast are practically unknown in the geologic literature, especially outside Brazil. Our studies show that these areas, previously considered as highly destructive wave-dominated deltas of Holocene age, were affected by a wave-dominated phase during the Pleistocene, succeeded by highly constructive, intralagoonal stage in the Holocene, in turn followed by wave-dominated deltaic sedimentation that continues until today. The geologic evolution of these coastal plains is exemplified here by the Doce River mouth area, State of Espírito Santo, where relative sea-level fluctuations during the Quaternary played an important role in the construction of the deltaic complex.