Susy Eli Marques Gouveia

Radiocarbon dating of total soil organic matter and humin fraction and its comparison with 14C ages of fossil charcoal

During the last decade radiocarbon dating has been used extensively in distinct regions of Brazil to provide information about soil chronology in paleoenvironmental studies. This paper presents (super 14) C dating of soil organic matter (SOM), humin fraction, and charcoal in several soil profiles under natural vegetation from different Brazil locations (north, central, and southeast regions). The main objective is to compare the obtained (super 14) C dating of total SOM with humin, the oldest fraction of SOM. In order to validate the humin ages these data are compared with the age of charcoal collected at similar depths. The (super 14) C ages obtained on charcoal were, in most of the cases, in agreement with the humin fraction considering the experimental errors, or 20% older in average. The dates obtained from total SOM showed significantly younger ages than the humin fraction indicating contamination by younger carbon. These results show the humin fraction is considered a reliable material for (super 14) C dating in soils. However, the humin fraction ages could be assumed as the minimum ages for carbon in soils.

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.

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.

Datação da fração humina da matéria orgânica do solo e sua comparação com idades 14C de carvões fósseis

The radiocarbon dating of the soil organic matter (SOM) is a polemic subject, due mainly to the complexity of the formation of the soils and to the variable contamination from several sources. Soil samples from 4 different Brazilian localities were submitted to physical and chemical pre-treatment for the extraction of humin fraction, which is the most stable organic compound and theoretically the oldest and representative of the age of the SOM. The radiocarbon dating obtained from the total SOM and their humin fractions are compared to the 14C ages from buried charcoals at similar depths. The radiocarbon ages obtained from such charcoals are, in most of the cases, concordant within the experimental errors of those obtained on humin fractions, or are in average 10% higher, with one exception. Thus, the ages on humin fractions could be assumed as the minimum ages for the associated soils, while the results obtained on total SOM, even at depths until 200 cm, exhibit pronounced contamination effect by modern carbon, rejuvenating their ages.

Last millennium environmental changes and climate inferences in the Southeastern Atlantic forest, Brazil

This study presents paleoenvironmental data based on pollen, elemental and isotopic compositions of organic matter (TOC, N, δ(13)C and δ(15)N) and (14)C dating of 170 cm lake sediment record. Samplings have been made in Lagoa Grande at Parque Estadual Turístico do Alto Ribeira - PETAR, Southern São Paulo State, Southeastern Brazil. The variations in relative frequencies (in percentage) of arboreal pollen along the core range between 40 and 80%. The δ(13)C values ranged from -23% to -30% and C/N of ~10 to 15, indicating the contribution of terrestrial C(3) plants and algae in the sediment organic matter. The δ(15)N results presented values from 3 to 4.5%, also suggesting a mixture of algae and terrestrial C(3) plants. The (14)C dating indicates modern age for the shallow horizons to ~1030 BP at the base of the core. A probable wetter climate in the period of ~370 BP to ~340 BP was inferred from the data set, which corresponds to a part of the period covered by the Little Ice Age (LIA).