Kenitiro Suguio

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.

Relative sea-level changes in the last 5500 years in southern Brazil (Laguna–Imbituba region, Santa Catarina State) based on vermetid 14C ages

Abstract Twenty-six new radiocarbon dates from vermetid shells collected in the southernmost sector of the Brazilian rocky coast presented dates ranging from 5410±80 to 190±65 years B.P., with associated paleosea levels varying from +2.10 m to +0.20 m above present sea level. The overall suggested trend of the relative sea level (RSL), declining until at least 190 years B.P., is somewhat contradictory to a proposed RSL rise in the last 1000 years in southern Brazil. The data also seem to undermine a more widely accepted RSL trend that suggests that at least two negative RSL oscillations occurred between 4100 and 3800 years B.P. and between 3000 and 2700 years B.P. The maximum elevation of the RSL in the Holocene in southern Brazil was possibly lower than that observed in most of the Brazilian eastern coast. Discrepancies between ancient sea levels of similar ages are attributed to coincidental methodological problems, to imprecisions in determining past relative sea levels and to possible changes in the geomorphology and wave climate close to shore during the last 5000 years. A general trend of increasing δ 18 O with a reduction in age in the studied samples may suggest a gradual reduction of water temperature in the region during the same period.

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.

Holocene sea-level history on the Rio Grande do Norte State coast, Brazil

Abstract The Rio Grande do Norte State coast, northeastern Brazil, lies in an intraplate region which displays elevated Holocene shorelines, abandoned tidal flats and other intertidal deposits. A field and chronological investigation, which used 48 radiocarbon dates on unaltered mollusk shells, peats, coral reefs, and vermetids in living position and death assemblages encrusted on beachrock, tidal flat and peat deposits, was carried out along two littoral zones, one trending east–west and the other north–south. The sea-level envelope curve for the region indicates at least one sea-level oscillation after the maximum Holocene highstand, which occurred at ∼5000 cal. yr BP. This curve shows notable deviations from the mean sea-level curve for the Central Brazilian coast in the early and mid Holocene. However, in general, this envelope curve fits a glacio-isostatic prediction for the area. A significant deviation from this prediction, related to a small sea-level oscillation, occurred from ∼2100 to ∼1100 cal. yr BP. The comparison between the glacio-isostatic prediction and the sea-level observation enables us to conclude that local events may have disturbed the sea-level record. Minor Holocene sea-level oscillations, which were mainly associated with climate changes or tectonics and superimposed on a major smooth pattern, were observed in South Africa and Australia. But in northeastern Brazil, neotectonic controlled crustal movements or variation in marine and wind currents could provide the answer. The data indicate that local events may have an important role in the history of coastal progradation and retreat.

Late Pleistocene marine terrace deposits in northeastern Brazil: sea-level change and tectonic implications

Abstract Coastal areas of passive margins are among the best places to examine the reality of sea-level changes. Elevation of marine terraces is central to interpretation of their approximate age in this kind of tectonic setting. The recognition of Pleistocene shorelines on the South American eastern continental margin is becoming increasingly common, but data are commonly imprecise in both time and space, and the resulting chronology is based mainly on geomorphological attributes. Nine thermoluminescence dates and four optically stimulated luminescence dates of 220–206 ka and 117–110 ka were obtained for two marine terrace deposits along 340 km of coast in Rio Grande do Norte State, northeastern Brazil. They are correlated with the highstands of marine oxygen-isotope substages 7c and 5c sea level, respectively. The older deposit occurs mainly on the N–S-trending coast and ranges in elevation between 7.5 and 1.3 m. The latter is found along the E–W-trending coast at altitudes that vary between 1 m and 20 m. It indicates relative downfaulting of the 220–206-ka marine terrace and uplift of the 117–110-ka marine deposit, which is locally about 12 m higher than deposits of similar age described 1000 km to the south. It follows that elevation alone is an unreliable tool for establishing a Quaternary sea-level change chronology also on passive margin coastal areas.

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.

Paleovegetation and paleoclimate of "Vereda de Águas Emendadas", central Brazil.

Abstract The palynological analysis from an extensive palm swamp on top of a plateau, close to the city of Brasilia (15°34′S and 47°35′W), has shown a sequence of vegetational and climatic changes in the last 30,500 years. The area is the headwater of tributaries of the main Brazilian hydrographic basins: Amazonian (north), Parana (south), and Sao Francisco (east). At the base of the analysed core there is a clay deposit devoid of palynomorphs. At 30,480±100 yr BP, peat started to form in the area. Between 25,790±70 and ca. 24,200 yr BP, pollen grains and algal remains are abundant, indicating a marsh surrounded by open cerrado and patches of gallery forest on top of the plateau. Between ca. 24,000 and 21,450±100 yr BP, vegetation was denser than at present, with abundant marsh plants and the occurrence of some trees of a cooler climate, suggesting a climate more humid than at present and probably cooler. The absence of Mauritia pollen during that time indicates the palm swamp (“vereda”) characteristic of the present-day area did not occur. The drastic decrease in concentration and diversity of palynomorphs from ca. 21,000 yr BP until 7,220±50 yr BP suggests a dry climatic phase that resulted in drier, more seasonal climate and the desertification on top of the plateau. Shortly after 7,000 yr BP, the abundance of pollen grains and spores increased and Mauritia pollen occurred in the sediments, suggesting increased humidity and the beginning of the palm swamp; from ca. 5,600 yr BP onwards, the present types of vegetation grew on top of the plateau: arboreal cerrado, gallery forest, and palm swamp. The climate of this phase was similar to that of the present. Charcoal particles are found throughout the record but they increase mainly after 2,600 yr BP, suggesting at least three extensive episodes when the local vegetation was burned. The sequence of events found here follows, in general, the same pattern described for other areas in central Brazil. However, the dry climatic phase is more strongly marked in Vereda de Aguas Emendadas, probably due to its location on top of a plateau.

Tropical Spodosols in northeastern Amazonas State, Brazil

Abstract The white sand formation found in northeastern Amazonas State in Brazil showed three horizons—E, B, and C. The superficial horizon (E) was formed by sandy friable material of grayish to whitish colour with accumulations of organic matter in the form of wavy bands. The B horizon remained essentially sandy but showed a pale yellowish to orangish colour. The C horizon was friable with pink to creamy clayey sandy materials. In the contact zone between horizons B and C occurred an enrichment of organic matter, forming ortsteins. The profile structure, the upward increasing of quartz grains corrosion, and the predominance of quartz in the E horizon, and thus of iO 2 , instead of kaolinite, Al 2 O 3 , Fe 2 O 3 , and TiO 2 that were more abundant in the C horizon, suggest that the white sands are the product of podzolization. Soil horizonation and accumulation of organic matter are governed by the active decomposition of the forest litter. These data also allow us to relate the Spodosols with the mottled and the saprolitic horizons of truncated lateritic profiles of the Alter do Chao Formation. This process of podzolization, which continues up to the present moment, is very aggressive since the studied profiles have developed in less than 3000 years and beneath the modern forest.

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?).