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Analysis of biological proxies in lake sediment and geochemical analysis of soil profiles reveal natural vegetation dynamics, with climate inferences, since the late Pleistocene in a fragment of the pristine lowland Atlantic Forest of southeastern Brazil. Carbon isotopes from soil organic matter and 14C ages from the humin fraction indicate the dominance of C3 plants since ~17,000 cal BP. Palynological analysis of a sediment core indicates the presence of Atlantic Forest vegetation since 7700 cal BP. Changes in the relative abundance of tree ferns and palms suggest the predominance of a humid period from ~7000-4000 cal BP and establishment of the modern seasonal climate at ~4000 cal BP. Data indicate maintenance of the regional forest coverage since the late Pleistocene, corroborating previous suggestions that this region was a forest refuge during less humid periods of the late Pleistocene and Holocene. Some plant taxa with currently divided distributions between Amazonia and the Atlantic Forest colonized the region since at least 7500 cal BP, indicating an earlier connection between Amazonia and Atlantic Forest. [ABSTRACT FROM AUTHOR]

A sediment core was studied to characterize the influences of Holocene sea-level variations in the Barra Seca River valley, in the Atlantic rainforest, Linhares, Espírito Santo, southeastern Brazil. Biological proxies (pollen, spores, and sponge spicules), 14C dating, granulometry, d13C, d15N, C/N and major chemical elements revealed the establishment and the evolution of a paleo-estuary during the interval from ~7700-585 cal BP. During the interval ~7700-7000 cal BP, the study site was occupied by a bay-head delta, the inner portion of the paleo-estuary, presenting the most dense mangrove coverage of the entire record. In the interval ~7000-3200 cal BP, the site was occupied by the central basin, possibly a consequence of the landward migration of the paleo-estuary. This interval presents reduced mangrove coverage, probably due to the permanent flooding of the valley. From ~3200 cal BP, the marine influence at the site decreased probably as result of the seaward migration of the coast line. From ~600 cal BP, the modern floodplain and freshwater lake were established. This interpretation is in agreement with the sea-level curves for the southeastern Brazilian coast, except for the fact that evidence of sea levels lower than the present at ~4000 and ~2500 cal BP as suggested by some authors were not found. [ABSTRACT FROM AUTHOR]

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Abstract: The lack of paleoecological records from the montane Atlantic Rainforest of coastal Brazil, a hotspot of biological diversity, has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle. We present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the Serra do Mar rainforest in São Paulo, Brazil. From ca. 28,000 to ∼22,000 14C yr BP, a subtropical forest with conifer trees is indicative of cool and humid conditions. In agreement carbon isotopic data on soil organic matter suggest the presence of C3 plants and perhaps C4 plants from ∼28,000 to ∼19,000 14C yr BP. The significant increase in the sedimentation rate and algal spores from ∼19,450 to ∼19,000 14C yr BP indicates increasing humidity, associated to an erosion process between ∼19,000 and ∼15,600 14C yr BP. From ∼15,600 14C yr BP to present there is a substantial increase in arboreal elements and herbs, indicating more humid and warmer climate. From ∼19,000 to ∼1000 14C yr BP, δ13C values indicated the predominance of C3 plants. These results are in agreement with studies in speleothems of caves, which suggest humid conditions during the last glacial maximum. [Copyright &y& Elsevier]

Abstract: The present work integrates sedimentary facies, 14C dating, δ 13C, δ 15N, and C/N with geologic and geomorphologic data available from literature. The aim was to characterize the depositional settings of a late Quaternary estuary in northeastern Marajó Island and analyze its evolution within the context of relative sea level fluctuations. The data derive from four continuous cores along a proximal-to-distal transect of a paleoestuary, previously recognized using remote sensing information. Fifteen sediment samples recorded ages ranging from 42,580±1430 to 3184±37 14C yr B.P. Facies analysis indicated fine- to coarse-grained sands with parallel lamination or cross stratification, massive or laminated muds and heterolithic deposits. δ 13C (−28.1‰ to −19.7‰, mean=−23.0‰), δ 15N (+14.8‰ to +4.7‰, mean=+9.2‰) and C/N (14.5 to 1.5, mean=7.9) indicate mostly marine and freshwater phytoplankton sources for the organic matter. The results confirm a large late Quaternary paleoestuary in northeastern Marajó Island. The distribution of δ 13C, δ 15N, and C/N, together with facies associations, led to identify depositional settings related to fluvial channel, floodplain, tidal channel/tidal flat, central basin, tidal delta, and tidal inlet/sand barrier. These deposits are consistent with a wave-dominated estuary. Variations in stratigraphy and geochemistry are controlled by changes in relative sea level, revealing a main transgression from an undetermined time around 42,000 14C yr B.P. and 29,340 (±200) 14C yr B.P., which is synchronous to the overall drop in sea level after the last interglacial. Following this period, and probably until 9110±37 14C yr B.P., i.e., during a time interval encompassing two glacial episodes including the Last Glacial and the Younger Dryas, there was a pronounced drop in sea level, recorded by the development of a major erosional discontinuity due to valley re-incision. Sea level rose again until 5464±40 14C yr B.P, just before the main worldwide mid-Holocene transgressive peak. Mid to late Holocene coastal progradation ended the Marajó paleoestuarine history, and promoted the establishment of continental conditions throughout the island. The divergence comparing the Marajó sea level behavior with the eustatic curve allows hypothesizing that post-rifting tectonics along the Brazilian Equatorial margin influenced the sedimentary evolution of the studied paleoestuary. Considering that sedimentary facies in estuarine settings are highly variable both laterally and vertically, the present integration of facies with isotope and elemental analyses was crucial to provide a more precise interpretation of the Late Pleistocene and Holocene Marajó paleoestuary, and analyze its sea level history within the eustatic and tectonic context. [Copyright &y& Elsevier]

Due to the many factors controlling δ13C values in stalagmites, complicating their paleoclimatic and paleoenvironmental interpretation, most studies do not present δ13C values, but instead focus mainly on δ18O values. This is also the case for most cave studies from tropical South America, where many new δ18O stalagmite records covering the last millennia were recently published. Here, we review the δ13C values in stalagmites, investigating the influence on this proxy of local hydroclimate, altitude, temperature and vegetation types, by employing a new dataset composed of published and unpublished carbon isotope records from various sites in tropical South America. The main factors influencing δ13C values are associated with the local hydroclimate, followed by minor effects from temperature. Most of the isotopic records show a significant correlation between the δ13C and δ18O values, indicating a close relationship between local hydroclimate and atmospheric convective processes related to the South American Monsoon System. • δ13C values in stalagmites from tropics are related with local hydrology. • Prior calcite precipitation is the main driver for δ13C values in stalagmites. • The mean conditions of local hydroclimate are associated with South American monsoon. • Dataset of δ13C values in stalagmites is useful as paleoclimate record. [ABSTRACT FROM AUTHOR]