Bruno Turcq

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.

Tropical forest changes during the late quaternary in African and South American lowlands

Abstract Arboreal pollen and montane elements of Late Quaternary pollen assemblages from three lacustrine cores (West Cameroon, southeastern Amazonia and central Brazil) are correlated, by the radiocarbon chronology, with other palaeoenvironmental records in Africa and South America. We observe in both continents a well-developed dense forest at 30,000 and 9000 yr B.P. The succession of vegetation types during the Late Quaternary appeared strongly related to the regional conditions: (1) the dense forest was more or less degraded depending on the regions during the last full glacial period (20,000–15,000 yr B.P.); (2) a slow increase of tree elements is evidenced in some areas during the Late Glacial (15,000–10,000 yr B.P.), whereas short-term fluctuations occurred in central Brazil during the same time; (3) a strong regression of the forest during the middle Holocene (6000–5000 yr B.P.), in the southern tropical zone of South America, was in opposition to a full forest development in Africa. In both continents two main features characterize the tropical forest evolution: (1) Montane elements developed in the lowlands during the last glacial period and in some southern or northern regions during the early Holocene; and (2) the climate seasonality was enhanced in several regions since 8500–7500 yr B.P. For a tentative explanation, we relate the cold or cool climate, inferred by palaeoecological evidences in the glacial period and glacial-interglacial transition, to polar air-masses reaching more frequently the tropical zone. This interpretation explains the apparent contradiction between the markedly low temperature of the continental lowlands opposed: (1) at 18,000 yr B.P., to the 1–2°C lower Sea Surface Temperature of tropical oceans and (2) to the global warming during the late glacial. During the middle and Late Holocene, climate evolution was mainly influenced by the latitudinal shift of the ITCZ positions in July and January and, in South America, by short-term changes of the zonal atmospheric circulation.

A 21 000 cal years paleoclimatic record from Caçó Lake, northern Brazil: evidence from sedimentary and pollen analyses

Abstract Sedimentological studies including seismic profiles, mineralogy and organic geochemistry on two cores from the center and margin of Caco Lake, Maranhao State, northern Brazil, revealed variable climatic and environmental conditions during the last 21 cal kyr BP. Between 21 and 17 cal kyr BP, during the Late Glacial Maximum, regional climate was predominantly dry, interrupted by short humid phases, as reflected by a succession of very thin layers of sand and organic matter. The Late Pleistocene climate was relatively wet as is suggested by rapid lake-level rise and forest expansion. The Late Pleistocene humid climate differed significantly from present conditions. We suggest that Late Glacial humid conditions were the consequence of intensification of the Inter-Tropical Convergence Zone or shifts of its position, resulting in Antarctic cold-front occurrences. The abrupt climatic changes during this period were marked by siderite deposition into Caco Lake, which appears to be related to regional hydrologic changes linked to global/Northern Hemisphere events. The Holocene was characterized by lower moisture availability and a distinct dry period until 7 cal kyr BP, in response to South American insolation conditions.

Accumulation of organic carbon in five Brazilian lakes during the Holocene

Abstract A comparative study was made of organic carbon accumulation in five lacustrine environments in Brazil (large open water lake, ponds with floating meadows and marshes). The most representative cores were used to calculate the accumulation of total organic carbon (TOC) in the sediment over the past 12,000 cal years BP. Carbon accumulation rates were determined using the TOC concentration, sediment «in situ» density, 14 C calibrated ages and the interpolated sedimentation rate (cm year −1 ) or sediment accumulation rate (g m −2 year −1 ). The C/N ratio and microscopic observations were used to characterise the sedimentary organic matter. Recent TOC accumulation in the lakes was estimated at 7–41 g m −2 year −1 by averaging the last 500 years. This variation roughly correlated with the present-day ratio of watershed area vs. lake area, indicating that nutrient input is important for the modern accumulation of organic matter. The accumulation rates were greatest in lakes with floating macrophytes. The largest lake (Dom Helvecio), with a strong thermal stratification, low primary production and low carbon accumulation, had relatively well preserved organic matter when its low watershed area/lake area ratio was considered. The general increase in carbon accumulation during the Holocene followed the expansion of lakes and the development of a wetter climate, due to a growing penetration of the Atlantic and Amazonian wet air mass over Brazil driven by insolation changes. This change in carbon accumulation was not controlled by the higher water and nutrient inputs alone, more organic matter was decomposed when lake levels were low at the beginning of the Holocene. Some lakes were also influenced by higher inputs of allochthonous organic matter during the early and middle Holocene. These inputs included high concentrations of charcoal fragments. Lake morphology and its changes as the lake level rose, as well as the nature of primary producers, strongly influenced the carbon accumulation rates. Our understanding of how climate changes are linked to changes in the lacustrine environment should not be based on measuring organic matter alone, but requires a good description of the organic matter and other limnological parameters.

Tropical climates in the game of two hemispheres revealed by abrupt climatic change

The climatically sensitive equatorial regions provide important information for evaluation of the phasing between high- and low-latitude climate variability. A high-resolution pollen record from northern Brazil demonstrates a significant abrupt and rapid environmental change associated with the Northern Hemisphere Younger Dryas temperature reversal. This finding is consistent with the model in which the Younger Dryas had a stronger influence on temperature in the Northern Hemisphere than south of the equator because of the larger temperature gradient between pole and equator in the Northern Hemisphere than in the Southern Hemisphere. One consequence of the Younger Dryas changes would be the location of the Intertropical Convergence Zone in a southern position, so that even tropical regions would have been under Arctic influence.

Foraminiferal zonations as base lines for quaternary sea-level fluctuations in south-southeast Brazilian mangroves and marshes

Small vertical variations in occurrences of mangrove foraminifera and arcellaceans (thecamoebians) provide a zonation scheme that is applicable to the study of short-term sea-level change. Sampling along transects was carried out at three sites in southern Brazil and four sites along the Rio de Janeiro lagoon system. The Sai-Guacu fluvial-estuarine point bar that divides the states of Santa Catarina and Parana, and the Capinzal and Veiga islands at the ancient flood tidal delta of the Guaratuba Bay, were sampled during two summers for a total of 42 samples. In the state of Rio de Janeiro, the seaward borders of the Urussanga, Boqueirao and Fora lagoons, which comprise the incipient brackish mangroves of the Saquarema Lagoon system, were sampled in both summer and winter for a total of 17 samples. Three samples were collected during the summer at the freshwater marsh of Jacarepia Lake. These coastal sedimentary environments differ in latitude, climate, physio-chemical conditions, vegetation, and sediment type. Altitude, rate of sediment transport and salinity influence the distribution of three faunal zones: higher, middle, and lower. Statistics on the dead foraminiferal distribution and ecological indications given by live occurrences showed that Haplophragmoides sp. is the dominant species at the uppermost stations (higher high water—HHW, with a 6-cm vertical range) in both Guaratuba and Rio de Janeiro transects, whereas Trochammina inflata dominates the southernmost Sai-Guacu site. The Haplophragmoides - Trochammina association was also the most consistent in both dead and live HHW assemblages. Trochammina inflata and two forms of T. macrescens were found to be salinity-sensitive HHW assemblages, as were Polysaccammina hyperhalina and Miliammina fusca in the Guaratuba intertidal zone. In Saquarema, the intertidal zone is marked by T. macrescens, and Centropyxis aculeata predominates in Jacarepia. The subtidal zone in Parana contains Arenoparrella mexicana, Tiphotrocha comprimata, Trochammina macrescens f. macrescens, and Difflugia oblonga, whereas in Rio de Janeiro, T. macrescens f. polystoma predominates. Similar HHW assemblages are also observed in Oregon, British Columbia, Maritime Canada, and New England. Thus, the assemblages found in Brazil are potentially applicable to the study of sea-level variation as recorded in Quaternary deposits, like their counterparts in temperate marshes. These transects are the first well-constrained vertical transects of mangrove faunas in South America.

Mid-Holocene Climate of Tropical South America: A Model-Data Approach

Most of the Early and mid-Holocene paleoclimate studies in tropical South America indicate a drier climate in Amazon and Southeast Brazil and a wetter climate in Venezuela. This pattern has been interpreted as a northward migration of the Intertropical Convergence Zone (ITCZ) due to insolation changes explained by Milancovitch cycles. We show how model simulations and model-data comparisons can help to investigate further the reason of these changes by considering the mid-Holocene period (6 ka). The insolation effect and the vegetation interaction on the seasonal cycle are explored with emphasis on the regional impact on precipitation and on the atmospheric circulation. A major feature of the mean mid-Holocene simulated climate is indeed the decrease of the rainfall in the South Atlantic Convergence Zone (SACZ) region compared to present day, which is confirmed by the proxy data. However, the ITCZ migrates southward during the Southern Hemisphere summer thus enhancing the precipitation in Northeast Brazil. The SACZ and ITCZ displacements are enhanced by the vegetation feedback. The analysis of the transient meridional heat transport and of the baroclinicity of the model climate suggests more intense winter and early spring cold outbreaks in the central region of South America, which seems in agreement with paleoclimate proxies.

Carbon storage in Amazonia during the Last Glacial Maximum: secondary data and uncertainties

The Amazonian forest is, due to its great size, carbon storage capacity and present-day variability in carbon uptake and release, an important component of the global carbon cycle. Paleo-environmental reconstruction is difficult for Amazonia due to the scarcity of primary palynological data and the mis-interpretation of some secondary data. Studies of lacustrine sediment records have shown that Amazonia has known periods in which the climate was drier than it is today. However, not all geomorphological features such as dunes, and slope erosion, which are thought to indicate rainforest regression, date from the time of the Late Glacial Maximum (LGM) and these features do not necessarily correspond to episodes of forest regression. There is also uncertainty concerning LGM carbon storage due to rainforest soils and biomass estimates. Soil carbon content may decrease moderately during the LGM, whereas rainforest biomass may change considerably in response to changes in the global environment. Biomass per unit area in Amazonia has probably been reduced by the cumulative effects of low CO2 concentration, a drier climate and lower temperatures. As few paleo-vegetation data are available, there is considerable uncertainty concerning the amount of carbon stored in Amazonia during the LGM, which may have corresponded to 44-94% of the carbon currently stored in biomass and soils.

Palynological evidence of the replacement of the hygrophilous forest by field vegetation during the last 7,000 years B.P. in the northern coast of Rio de Janeiro, Brazil

Historians claim that European colonizers of the northern coast of Rio de Janeiro State found vast herbaceous fields when arrived in this region. Hypotheses about the origin of these fields include forest burning by the Goitacás indians and periodical floods by the Paraíba do Sul River and the lagoon system. The palynologycal analysis of two lake cores obtained in the municipality of Campos dos Goytacazes revealed opening episodes of hygrophilous forest and the establishment of field vegetation, recorded at ca. 6,500 and ca. 4,000 (14)C yr BP. The partial replacement of forest by field vegetation in the first episode was probably caused by floods of the lower areas during the development of the Holocene lagoon phase. During the second episode, successions of vegetational patterns occurred due to lowering of the sea level. Drying and enlarging of the coastal plain have allowed its colonization by herbs and heliophyte plants. The palynological analysis does not provide any evidence that sustains the theories of use of fire and agricultural activities by indigenous groups during these periods.