R. Boulet

The carbon isotope record in soils along a forest-cerrado ecosystem transect: implications for vegetation changes in the Rondonia state, southwestern Brazilian Amazon region

This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savannah (cerrado), a tropical semideciduous forest (cerradão), a forest tran sition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. 14C data of total soil organic matter and charcoal indicate that the organic matter in these soils is at least Holocene in age. The forest and forest transition sites are characterized by δ13C soil depth profiles gener ated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradão have experienced significant vegetation changes during the Holocene. The d13C data (-30‰ to -27‰) obtained in the deepest part of the profile at the cerradão site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C4 plant influence is reflected in the 13C-enriched 13C record shows a clear expansion of C3 vegetation, particularly at the cerradão site. The regression/expansion of the forest and savannah vegetation documented at the cerradão and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Ama zon region.

Origin and dynamics of soil organic matter and vegetation changes during the Holocene in a forest-savanna transition zone, Brazilian Amazon region:

Carbon isotope data on soil organic matter (SOM) were collected along an ecosystem transect 90 m in length that includes a tropical forest on the plateau, a transitional forest-savanna and savanna in a depression. Total organic carbon data show a significant increase in carbon content from sites representing forest ecosystem to sites representing savanna ecosystem. It was hypothesized that carbon accumulation in the depression is controlled by flooding conditions that slow down carbon decomposition and in part by carbon transport from the upper part of the transect (the savanna and the transition forest-savanna areas) into the depressions by water during the rainy season. The origin of the carbon was confirmed by using soil 13 C analysis. The savanna sites located in the depression showed δ 13C values between –19.5‰ and –22.5‰ indicating a mixture of C3 and C4 plants. The vegetation cover in the depression is predominantly C3 grasses with d 13C values of about –27‰ and –26‰. In the site under savanna located at an elevation slightly higher, the d 13C value was more enriched (–16‰) showing the predominance of C4 plants (d 13C of –13.6‰). At the forest-savanna transition and in the forest ecosystem the d 13C values were characteristic of C3 plants (–25‰ and –28.1‰). 14C and 13C data indicate that the organic matter of mixed origin has been deposited for at least the last 7000 years in the savanna depressions. The 13C pattern observed in the soil organic matter profiles indicate a predominance of C3 plants in the early part of the Holocene. About 7000 to 4000 years ago, the data show the influence of C4 plants, indicating forest regression associated with a drier climate than at present. The more recent 13 C records suggest forest expansion, and the return to a climate similar to the present.

Carbon isotopes in charcoal and soils in studies of paleovegetation and climate changes during the late Pleistocene and the Holocene in the southeast and centerwest regions of Brazil

This paper attempts to reconstruct vegetation changes and to infer climate changes during the late Pleistocene and the Holocene in the southeast (Botucatu, Anhembi and Jaguariuna, Sao Paulo State) and centerwest of Brazil (Pontes e Lacerda, Mato Grosso State). The research approach included the use of carbon isotopes ( 13 C and 14 C) in soil organic matter (SOM) and the evaluation of charcoal distribution and its identification at the species level. Soils sampled in this study were located under natural vegetation, along the slopes of small hills. Charcoal was found predominantly between 150 and 50-cm depth, indicating a period of greater frequency of fires in the study areas, between 6000 and 3000 years BP. For the Botucatu, Anhembi and Pontes e Lacerda sites, the d 13 C profiles suggest the predominance of C3 plants during the entire Holocene. The 13 C patterns obtained at the Jaguariuna site that show a more significant presence of C4 plants compared to the other regions, suggest that this region has been drier than the others during the Holocene. These patterns also indicate the presence of a drier climate compared with present-day conditions at the Jaguariuna region during late Pleistocene until the middle Holocene. This study shows the complexity of vegetation dynamics in the southeast of Brazil during the Holocene. It also shows that the analyses of multiple soil cores representative of the main vegetation communities are necessary for paleovegetation studies. D 2002 Published by Elsevier Science B.V.

Holocene fire and vegetation changes in southeastern Brazil as deduced from fossil charcoal and soil carbon isotopes

Abstract Carbon isotopes of soil organic matter (SOM) and radiocarbon dating on charcoal from nine soil profiles collected under native forest vegetation in Londrina, state of Parana, Anhembi, Botucatu and Jaguariuna, Sao Paulo and Salitre, Minas Gerais, southeastern Brazil, were used to evaluate the vegetation changes and to establish the chronology. 13 C and 14 C data in SOM and charcoal, respectively, indicate that C 4 plants were the dominant vegetation in Londrina and Jaguariuna during the Late Pleistocene until Middle Holocene, probably associated with the presence of a drier climate. In Anhembi and Botucatu, C 3 plants dominate the landscape during the period. A probable mixture of C 3 and C 4 plants occurred in Salitre during the Holocene. This study is part of a main research program related to palaeoenvironmental reconstruction of vegetation and climate in distinct regions of Brazil during the last 20,000 years.

14C Dating and Stable Carbon Isotopes of Soil Organic Matter in Forest–Savanna Boundary Areas in the Southern Brazilian Amazon Region

This study, which was carried out in the southern Brazilian Amazon region (Rondonia state and Humaita, Southern Amazon state), presents and discusses the significance of carbon isotope data measured in soil profiles collected across natural boundaries of forest to savanna vegetation. The main objective of this study was to evaluate the expansion-regression dynamics of these vegetation units in relation to climate changes during the Holocene. (super 14) C data from charcoal, soil organic matter (SOM) and its component humin fraction indicate that the organic matter in the studied soils is essentially Holocene in origin. (super 13) C data indicate that C (sub 3) type plants were the dominant vegetation at all study areas in the early Holocene, and during the entire Holocene, in the forest sites of Central Rondonia state and in the forest site 50 km from the city of Humaita. (super 13) C data also indicate that C4 plants have influenced significantly the vegetation at the transitional forest and the Cerrado (wooded savanna) sites of Southern Rondonia state and the forest ecosystem located 20 km from the Humaita city. These typical C (sub 4) type isotopic signatures probably reflect a drier climate during the mid-Holocene. The (super 13) C records representing probably the last 3000 yr show an expansion of the forest, due to a climatic improvement, in areas previously occupied by savanna vegetation. These results and other published data for the Amazon region indicate that the areas representing todays forest-savanna boundaries have been determined by significant vegetation changes during the Holocene. The boundary between forest and savanna vegetation seems to be quite sensitive to climatic change and should be the focus of more extensive research to correlate climate and past vegetation dynamics in the Amazon region.

Soils as an Important Sink for Mercury in the Amazon

This work focuses on the behaviour of mercury in lateritic soil profiles found in the Serra do Navio and Tartarugalzinhoareas of the State of Amapá in Northern Brazil. The Hg contents are high in the upper horizons of the soil profiles(100–300 μg kg-1), and decrease to less than 100 μg kg-1 at depths of 200 or 300 cm. The higher levels of Hg are associated with higher Fe concentrations, particularly in the ferruginous accumulations as mottles andnodules. For each horizon of the soil profile, balance calculations were used to distinguish the amount of mercury naturally accumulated from rocks through lateritic pedogenesis (lithogenic mercury) from the anthropogenic mercury introduced in the profile through atmospheric contamination. The results show that the anthropogenic contribution is significant in the upper horizons (up to 95%% of the total Hg), and decreases downward in the soil profile. Mercury burdens were calculated for soil profiles in both upslope (272 880 and 217 440 μg m-2 for the first 70 cm) and downslope positions (118 800 and 182 160 %μg m-2 for the first 70 cm). The loss of Hg in downslope profiles seems to be related to the natural evolution of iron duricrust into latossols, which has been brought about by climatic changes toward increasing humidityin the Amazon since the Tertiary.

Holocene palaeoenvironmental evolution in the São Paulo State (Brazil), based on anthracology and soil δ13C analysis

This paper presents a reconstruction of the Holocene palaeoenvironmental evolution in the central São Paulo State (Brazil) based on anthracological analyses, in association with soil isotopic composition (δ13C) and radiocarbon dating from four sites. Anatomical identification of charcoal particles allows the reconstitution of past plant associations, and consequently of the vegetation and climate history. Rather precise interpretations may be achieved when associating anthracology and soil δ13C analysis. In the early Holocene, climate was dry and an open cerrado vegetation (savanna) covered most of this area. A cerradão (forested savanna) or a semideciduous forest existed in the more humid localities. After 3500/3000 14C yr BP the climate was more humid, similar to the present, leading to the establishment of forested vegetation in all the studied sites. Com parison of these results with various palaeoenvironmental studies carried out in the Brazilian phytogeographical zone of cerrado and semideciduous forest suggests that at least its greater part presented a similar trend in the climatic evolution during the Holocene. This phytogeographical zone presented a dry climate during the early Holocene, then a more humid climate during the late Holocene. Climatic conditions similar to the present appeared from 5000 to 1000 yr BP, depending on the site.

Hydrogeochemistry of a small catchment in Northeastern Amazonia: a comparison between natural with deforested parts of the catchment (Serra do Navio, Amapá State, Brazil).

To undertake a comprehensive study of the disturbanceeffects due to mining activities (manganese oreexploitation) on the chemical composition of drainagewaters, a 164 ha catchment in North-eastern Amazoniatropical forest was sampled weekly for a year period(June 1993 to June 1994) to determine the contrasts inrainwater, throughfall and stream water chemistrybetween the upper undisturbed area and the lower partaffected by deforestation and mining works. During the3 dry months season only about 15% of the totalannual input-output of chemical species to and fromthe catchment occurs. In the three wettest months,about 30% of the flux occurs, except 60% of thepotassium and nitrate release from the catchmentoccurs in these three months. The rain waters areslightly acidic (pH ≈ 5.2) with lowalkalinity, while the stream water is nearly neutral(pH ≈ 6.9): alkalinity is generated within thesoil and soil water system. The rainfall-throughfallrelationship of the chemical species in the naturalareas exhibits strong internal recycling and littleexport from the catchment. In the area changed bydeforestation and mining works almost all chemicalspecies show a net export in surface waters: these aremainly derived from the deforested area of the basin,where most probably due to enhanced weatheringprocesses because the vegetation cover has tore-establish itself.

Iron bands, fragipans and duripans in the northeastern plateaus of Brazil — properties and genesis

Iron bands, fragipans and duripans are common in yellow low-activity clay soils developed from the Barreiras Group in coastal plateaus of northeastern Brazil. Such indurated horizons are found in depressions of the plateaus where sugar cane growth is greatly reduced. Little research has being done on the nature of their bonding agents and their genesis. Research carried out in arid zones has frequently attributed duripan consistency to amorphous silica; however, duripans in northeast Brazil occur under higher rainfall (1500–2000 mm). The objective of this work was to study the nature and distribution of soil features in a plateau to better understand the processes associated with soil hardening. Two transects, across a small and a large depression, were investigated using field soil characterisation, micromorphological techniques, and mineralogical and geochemical analyses.The formation of the indurated horizons studied is due to two sequential processes: development of aquic conditions and incipient podz...

Evolution and opening of closed depressions developed in a quartz-kaolinitic sedimentary substratum at Taubaté basin (São Paulo, Brazil), and analogy to the slope evolution

Abstract The occurrence of dated peat (17,000–12,500 yr B.P.) in the soil in a closed depression in quartz-kaolinitic rocks of the Taubate basin, Brazil, allowed (1) reconstruction of the evolution of this depression, and (2) estimation of the rate of sinking. The analogy with the soil cover of a slope in the same area has shown that this valley developed by the same mechanisms as those of the depression. Chemical erosion is the main agent for the generation of landforms in this region.