Luiz Carlos Ruiz Pessenda

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.

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.

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.

Development of accurate and reliable 14C chronologies for loess deposits: Application to the loess sequence of Nussloch (Rhine Valley, Germany)

Due to very high accumulation rates, loess sequences are best suited archives for the continental paleoclimate of glacial periods. Accurate chronologies cannot be easily established by radiocarbon-dating, because of the lack of organic macrorests, the only material for reliable 14 C dating so far. A chemical protocol is reported to extract the organic matter of loess (organic carbon content lower than 0.1% by weight) for accelerator mass spectrometry (AMS) 14 C dating. Sediments were taken from the loess sequence of Nussloch, for which a large dataset of luminescence ages (TL, IRSL/OSL) is available. The 14 C chronology of the organic matter extracted from loess is in good agreement with the corresponding luminescence ages. It allows high resolution correlations with climatic proxy signals (magnetic susceptibility, malacological assemblages, δ 13 C on organic matter, etc.) derived from the loess sequence and global environmental proxy records.

Natural radiocarbon measurements in Brazilian soils developed on basic rocks.

This paper presents 14 C, 13 C and chemical data of soil organic matter (SOM) in three soil profiles under native forests from Brazil: Londrina (southern), Piracicaba (southeastern) and Altamira (northern). The main objective is to use carbon isotopes in tropical and subtropical soils of Brazil to provide information about vegetation changes that occurred in relation to climate changes during the Holocene. 14 C data from SOM indicate that the organic matter in the soils studied is of at least Holocene age. 13 C data indicate that C 4 plants probably provided the dominant vegetation in Londrina and Piracicaba during the early and mid-Holocene and that C 3 plants provided the dominant vegetation in the Altamira region during the Holocene.

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.