A porção norte da região Nordeste do Brasil é uma das áreas mais interessantes para estudo dos mecanismos e processos relacionados à variação de pluviosidade dos trópicos, pois a maior parte da precipitação anual está associada à migração meridional da Zona de Convergência Intertropical do Atlântico (ZCIT). O clima no Nordeste sofreu alterações durante o Holoceno, no entanto, as interpretações paleoclimáticas são ainda muito controversas devido a pequena quantidade de estudos. De forma a contribuir para a discussão sobre mudanças paleoclimáticas e paleoambientais do Nordeste brasileiro, este estudo realizou análises geoquímicas, micropaleontológicas, biogeoquímica e de isótopos de deuterium e carbono em sedimentos lacustres e guano, e análises de isótopos de oxigênio e carbono em espeleotemas do Holoceno Médio e Tardio. Os estudos da Lagoa do Boqueirão sugeriram que sua formação ocorreu devido a dinâmica fluvio-eólica, que resultou em barramentos dos canais fluviais durante o período úmido do Holoceno. A transição entre o sistema fluvial e lacustre, que marca o barramento, foi definido em torno de 4.500 anos BP pela substituição de espículas de esponjas, tipicamente fluviais, pelas diatomáceas lacustres, predominantemente Mastogloia smithii var. lacustres. Foi demonstrado que a formação e as flutuações da profundidade da Lagoa do Boqueirão não estavam associadas a precipitação regional. As variações paleoclimáticas da Lagoa do Boqueirão e da Caverna do Trapiá puderam ser acessadas através dos dados de \'delta\'D em ácidos n-alcanóicos de 28 carbonos, produzidos por vegetação terrestre e macrófitas aquáticas, e dados de \'\'delta\'POT.18\'O dos espeleotemas. Valores mais positivos (negativos) desses isótopos indicaram que durante o MCA (LIA) a região apresentava condições de seca (umidade), devido ao posicionamento mais ao Norte (Sul) da ZCIT, que migrou em direção ao Hemisfério mais aquecido de acordo com a correlação observada com a Oscilação Multidecadal do Atlântico (OMA). A porção norte do NEB apresentou paleoclima diferente em relação ao da porção sul do NEB durante o LIA, que estava seco devido ao deslocamento da Zona de Convergência do Atlântico Sul (ZCAS) mais para sul, assim como a ZCIT, que não afetou a porção sul do NEB. Mudanças paleoambientais foram também definidas com base na idade de sequências sedimentares clásticas datadas por LOI e pelos depósitos de guano em condutos de cavernas em Felipe Guerra-RN. A deposição de sedimentos terrígenos na caverna Urubu entre 7 to 4 ky B.P. cave foi cronologicamente associada a clima mais úmido, o que é apoiado pelo baixos valores de \'\'delta\' POT.18\'O values de espeleotemas. Esse período também foi marcado pelo aumento da contribuição de de carbono orgânico do solo como indicado por valores mais baixos de \'\'delta\'POT.13\'C dos mesmos espeleotemas. Uma transição abrupta para clima mais seco pode está associada ao final da sedimentação clástica na caverna por volta de 4.2 ky devido a interrupção do fluxo do rio subterrâneo e completa erosão do solo ao redor da caverna. A erosão do solo é ressaltada por altos valores de \'\'delta\'POT.13\'C dos espeleotemas próximos aos da rocha encaixante (~0 %0). Estes resultados demonstraram uma relação entre preenchimento sedimentar da caverna Urubu com a formação e erosão de solo na sua volta. Esse aumento de aridez, particularmente nos últimos três mil anos na região, foi também importante para preservação de depósitos de guano sobre os sedimentos clásticos, visto que esse material é bastante solúvel. As idades das camadas de guano foram utilizadas para definir períodos de alta ocupação das cavernas por morcegos entre 1730 and 677 cal. anos A.P. e 200 cal anos A.P. até o recente. Durante o evento MCA houve redução na acumulação de guano que levou a um hiato deposicional por conta que o clima seco desfavoreceu a sobrevivência dos morcegos na região. A acumulação de guano voltou a ocorrer a 200 anos atrás, provavelmente porque a população de morcegos foi reestabelecida, devido a clima mais úmido. / The northern sector of Nordeste of Brazil (NEB) is one of the most interesting regions to study mechanisms and processes related to fluctuations tropical rainfall, as the majority of annual precipitation is associated to the meridional migration of Intertropical Convergence Zone (ITCZ). Climate in NEB changed during the Holocene, although paleoclimatic interpretations are still controversial due to small number of studies. This study contribute to the discussion about paleoclimatic and paleoenvironmental changes of NEB by using geochemistry, micropaleontological, biogeochemistry and deuterium and carbon isotope analyses in lacustrine sediments and bat guano, and also speleothem isotope records from middle to late Holocene. Studies from Boqueirão Lake suggested that its formation occurred due to the fluvio-eolian dynamic, which resulted in blockage of fluvial channels during the holocenic humid period. Transition between fluvial and lacustrine system recorded the barrage formation and was defined around 4,500 years BP and suggested by the substitution of fluvial sponge spicules to lacustrine diatoms, mainly Mastogloia smithii var. lacustres, preserved in sediments. These data demonstrated that depth fluctuations of Boqueirão Lake were not associated to local precipitation accumulation. Paleoclimatic changes were reconstructed from Boqueirão Lake and Trapiá Cave based on \'delta\'D of n-alkanoic acids of 28 carbons, which are produced by terrestrial vegetation and macrophytes, and from \'\'delta\'POT.18\'O of speleothems. More positive (negative) isotope ratios indicate persistent dry (wet) climatic conditions during the MCA (LIA) there, due to a north (south) displacement of ITCZ. This climate system migrated toward the warmest ocean according to the correlation with Atlantic Multidecal Oscillation (AMO). The northern portion of the NEB (nNEB) presented different paleoclimate conditions when compared to the southern NEB portion (sNEB) during the LIA. LIA in sNEB was dry due to the displacement of the South Atlantic Convergence Zone (SACZ) to a southernmost position. Paleoenvironmental changes were attested by determining the age of clastic sequences filling cave conduits dated by OSL method and also by radiocarbon in bat guano. The deposition of terrigenous sediments in Urubu cave from 7 to 4 ky B.P. cave was chronologically associated with more humid climate indicated by low \'\'delta\'POT.8\'O values in speleothems. In addition, this period was marked by increased contribution of organic carbon from soil indicated by negative values of \'\'delta\'POT.13\'C of the same speleothems. An abrupt transition to dry climate was indicated by the end of clastic sedimentation at about 4.2 ky because interruption of underground river flow and complete erosion of soil surrounding the cave as indicated by abrupt enrichment of \'\'delta\'POT.13\'C values of speleothems, reaching values similar to the carbonate bedrock (~0 %o). These data demonstrated the relationship between the sedimentary filling of the Urubu Cave and the formation and soil erosion in the region. These predominant dry conditions during last three millenniums in the region were also important for preservation ofbat Guano deposits above terrigenous sediments. The chronological data of guano was also used to indicate two periods of high accumulation associated with intense occupation of caves by bat colonies between 1,730 and 677 years cal BP and 200 cal years BP until recently. During the MCA occurred a reduction in guano accumulation until a complete hiatus, because dry conditions did not favor bats survival. Guano deposit accumulated again only 200 years ago, when the population of bats restored with stabilization of relatively humid conditions.

Thesis submitted in fulfillment of the requirements for the Degree of Doctor of Philosophy University of the Witwatersrand South Africa 2015 / A multidisciplinary sedimentological, stratigraphic, mineralogical and geochronological analysis of a small, fossil-bearing, Holocene hillslope deposit, flanking a mesa, has enabled a reconstruction of the palaeo-environmental history of the region. The hillslope deposit, located on the farm Heelbo in the eastern Free State Povince, South Africa, overlies Jurassic mudrock and sandstone of the Elliot Formation, Karoo Supergroup. The deposit is located on a steep (~10°) slope and covers an area of ~7 km2 in two separate sections. It extends ~475 m downslope and reaches a maximum thickness of at least 6-8 m towards the base. Mineralogy indicates the deposit is sourced from the mesa but its fine grain size and location on a steep slope position Heelbo outside the scope of traditionally described alluvial fans or colluvial deposits. The hillslope deposit is described as an alluvial slope based on the morphology of the deposit and the grain size distribution against the slope gradient. The deposit comprises fine-sand to silt- grain size, but is found on a steep (~10°) slope. The source of the sediment is shown to be the sediment of the mesa, rather than input from an aeolian source. The grains are described as sub-angular and poorly sorted which also suggests a local provenance for the sediment. Microscopic and XRF analysis confirm the derivation of the sediment from the Elliot Formation, with the lithic fragments derived from subarkoses to arkoses of the underlying bedrock of the Upper Elliot Formation (UEF) in the Karoo Supergroup. Secondary calcite was visible in only two of the thin sections, thus it is likely due to a diagenetic overprint that is constrained by depth from the surface or time and not to specific layers. The deposit is cut by several mature and continuous gully networks with V-shaped profiles in the proximal slope, and combined V- and U-shaped profiles in the medial and distal sections. Gully formation is linked to both the sodium adsorption ratio (SARs), and high soil clay content, which facilitates swelling and shrinking. The Heelbo deposit comprises two palaeosols (BT1 and BT2) and four sediment (B1, B2, RB and TS) horizons. Through luminescence dating, the ages were found to be approximately 6390 ± 740 years BP for the oldest Brown (B1) horizon and 250 ± 170 years BP for the Red Brown (RB) horizon. The radiocarbon ages of the sediment were inconclusive, but the 14C ages of the fossil bones were in agreement with the luminescence ages. The multiple palaeosol horizons identified suggest two cycles of deposition, pedogenesis and erosion of the alluvial succession. The palaeosols and the presence of calcareous nodules and rhizocretions, and smectite and mixed-layer clay minerals, together with the total absence of illite and kaolinite, suggest protracted, dry periods with intermittent short periods of high rainfall. This is a similar climate regime to what the region experiences currently. Main findings: The Heelbo alluvial slope comprises locally derived sediment, rather than an aeolian source. Heelbo suggests that the hillslope deposits classification system needs to be re-evaluated and opens opportunities for wider study of Pleistocene-Holocene hillslope deposits in central / northern South Africa. This study also contributes to climate change debates in the Holocene.

Carbon isotope studies are commonly used to provide a proxy for past vegetation communities and for evaluating environmental change. Original studies suggested carbon isotope ratios of soil organic matter (SOM) faithfully preserved the isotopic composition of standing vegetation with little or no modification in the pedogenic and shallow burial environment. Recent studies of modern soils and laboratory experiments suggest that this may not necessarily be the case and that degradation of SOM in the burial environment may alter the original C-isotope ratio of bulk SOM. A first step in addressing the issue is to begin to understand the transformations of SOM in the burial environment; of particular interest in this study are transformations involving microbial residues. Sedimentary sequences with stacked buried soils afford the opportunity to study the changes that may occur through time and are especially useful if numerical ages and other environmental proxies are present. The objective of this study is to thoroughly investigate the composition and quantity of organic matter that has been preserved in the surface and buried soils at the Claussen site, using Fourier Transform Infrared Spectroscopy (FTIR), which provides an estimate for the abundance of organic matter components preserved in each paleosol's SOM. We can trace the fate of bioavailable OM and determine the magnitude of preferential decay of SOM with time by first comparing the composition of bulk SOM to the composition of physically protected carbon, located in soil microaggregates (Christensen, 1992) of the stacked buried soils. The results of this project suggest differences in the composition of paleosol and surface soil SOM that could impact paleovegetation interpretations derived from δ13C values.

The Meers Fault (Oklahoma) is one of few seismogenic structures with Holocene surface expression in the stable continental region of North America. Only the ~37 km-long southeastern section of the ~55 km long Meers Fault is interpreted to be Holocene-active. The ~17 km-long northwestern section is considered to be Quaternary-active (pre-Holocene); however, its low-relief geomorphic expression and anthropogenic alteration have presented difficulties in evaluating the fault length and style of Holocene deformation. We reevaluate surface expression and earthquake timing of the northwestern portion of the Meers Fault to improve fault characterization, earthquake rupture models, and seismic hazard evaluations based on fault length. We use a combination of airborne lidar (0.5--2 m-resolution), historical aerial photos, and new balloon-based photogrammetric (Structure from Motion) topography (0.25--0.5 m-resolution) collected in this study to analyze and characterize the fault scarp and local fault zone geomorphology. In the northwest, complex surface deformation includes fault splays, a left step, subtle monoclinal warping, and a minor change in fault strike. The fault is evident in the landscape as linear escarpments, incised channels on the up-thrown side of the scarp, and closed depressions on the downthrown side. I use topographic profiles, measured perpendicular to the fault scarp to show that the northwest scarp is characterized by decimeter surface offsets. Where the fault traverses the Post Oak Conglomerate the fault zone width rarely exceeds 25 m, in the Hennessey Shale I document an increase in fault zone width with deformation occurring over 20 m to 115 m. I further examined the northwest section of the fault in a paleoseismic excavation where weathered Permian Hennessey Shale and a ~1--2 m-thick veneer of Holocene alluvial deposits have been folded and warped during three surface-folding earthquakes. In an adjacent stream exposure these units are also faulted near the ground surface. Paleoearthquake age modeling (Oxcal) constrained by accelerated mass spectrometry (AMS) dating of detrital charcoal and optically stimulated luminescence (OSL) dating of sandy alluvial beds indicates two earthquakes occurred since ~6152-5550 cal. years BP and one possibly older event along the erosional unconformity along the Hennessey Shale bedrock. This analysis lengthens the Holocene extent of the Meers Fault by ~6 km, to ~43 km, and extends the paleoseismic record of the Meers Fault to ~9598 cal. years BP. These data will improve fault-rupture and earthquake recurrence models used for seismic hazard analysis of the Meers Fault.

The Columbia River is the largest fluvially dominated estuary in the Pacific Northwest, yet the Holocene transgressive fill of this system has not previously been studied. Nearly 1500 industry borehole and water well records in the lower Columbia River basin (LCRB) were analyzed. These records document the sedimentary infilling of the lower 120 miles of the drowned river valley that occurred during the Holocene marine transgression (10-0 ka) . Of particular importance is a key stratigraphic marker horizon of volcanic tephra that has been identified throughout the LCRB. INAA was used to determine the geochemical composition of the target tephra layer. The tephra geochemistry was then compared to geochemical data from potential Cascade source volcanos to determine whether the tephra layers are geochemically related, and the possible age and source of the tephra. The geochemical comparisons indicate that the suspect tephra horizon was derived from the climax eruption of Mount Mazama approximately 6845 years ago. Cross-sections have been constructed that record the lateral and longitudinal depositional development of the river basin. Sediment grain size distribution data have also been compiled and shows that grain size distribution does not change with respect to subsurface elevation within the LCRB. The results indicate that the LCRB has been dominated by fine sand deposition throughout the Holocene period, and silt and clay sized fractions were bypassed through the system to be deposited offshore. A total volume of 74.6 km3 of sediment has accumulated in the basin since the time of the catastrophic floods 12,700 years ago. Sediment volume analysis was used to predict past fluvial sediment supply rates and sediment retention. The volume of sediment deposition from early to late Holocene time has decreased by a factor of 2.4. Sedimentation rates in the basin are estimated from a basin isopach of the Holocene fill and from an extrapolated sediment sea-level curve. Basin sedimentation rates ranged from 12. 6 mm/yr-1 for the early Holocene to 2. 5 mm/yr-1 for the late Holocene period. This factor of 5 decrease in the sedimentation rate also indicates that the Columbia River bypassed much of its fine grained fraction through to the marine environment during the mid-late Holocene.

Sediment accumulated in lakes stores valuable information about past environments and paleoclimatological conditions. Cores previously obtained from Saint Albans Bay, located in the Northeast Arm of Lake Champlain, VT record the transition from the Champlain Sea to Lake Champlain. Belrose (2015) documented the presence of a peat horizon separating the sediments of the Champlain Sea from those of Lake Champlain. Initially, this layer was thought to comprise the transition from the marine environment of the Champlain Sea to a freshwater wetland. However, based on the results from this study, the transition between marine and freshwater conditions is thought to be represented by an erosional unconformity, indicative of a lowstand at the end of the Champlain Sea period. For this study, five additional cores were collected from Saint Albans Bay along a transect following the long axis of the bay moving into progressively deeper water. These cores better constrain the spatial extent, thickness and age variability of the peat layer within the bay and allow us to better understand the environmental conditions that preceded the period of peat deposition. In each of the cores there is evidence of sediment reworking in the uppermost Champlain Sea sediments, indicated by the presence of coarse-grained sediment, which is suggestive of a lowstand at the end of the Champlain Sea period before the inception of Lake Champlain. This coarse-grained layer is immediately overlain by a thick peat horizon. The widespread occurrence of the peat layer points to a large wetland that occupied the entire inner portion of Saint Albans Bay, and lake level ~ 9 m lower than at present during the Early Holocene. Based on radiocarbon dating, this paleo-wetland existed in Saint Albans Bay from ~ 9,600-8,400 yr BP. The development of this wetland complex is time transgressive, reflecting rapidly increasing lake level during the Early Holocene. This hypothesis is supported by the basal peat radiocarbon dates, as well as by the composition of plant macrofossils recovered from the peat horizons. The shift from peat deposition to fine-grained, low organic content lacustrine sedimentation is believed to have occurred at ~8.6-8.4 ka and is likely the result of continued isostatically driven lake level rise coupled with a changing climate. Although it was not its primary focus, this study also seeks to address the variations in sediment composition in the Lake Champlain sections of the cores. Evidence from the Lake Champlain record in Saint Albans Bay indicates that there were notable fluctuations in sedimentation, which were likely linked to both climatic variations and a change in the morphology of the bay. The rebound in productivity from ~8-5 ka is likely the result of warmer conditions during the Hypsithermal period. An increase in terrigenous sedimentation during this same time suggests a change in the morphology of the bay in which the Mill River delta migrated towards the inner bay. Initially, the cooler conditions of the Neoglacial are reflected in Saint Albans Bay by a decrease in organic matter content from ~5-3 ka. During the latter part of the Neoglacial (~3-1 ka), increases in organic matter content and detrital input point to enhanced productivity in response to increased precipitation and runoff from the watershed. The most recently deposited sediments in Saint Albans Bay bear out the legacy of anthropogenic nutrient enrichment of the bay in the form of increased algal productivity.

The main outcome of this dissertation is a highly detailed RSL record constructed for the time interval 600 to 1600 AD to explore the eustatic response of sea level to the atmospheric warming that occurred during the Medieval Warm Period (∼900 to ∼1200 AD) and the transition into the Little Ice Age (∼1400 to ∼1900 AD). This new record was obtained in the microtidal Mississippi Delta and contains 28 sea-level index points that track ∼60 cm of relative sea-level rise. Basal peat was used as a tracer of sea-level, and age control was obtained by AMS 14C dating. All data were plotted as error boxes using 2 sigma confidence levels. A clustering of index points between 1000 and 1200 AD suggests an increase in the rate of sea-level rise. While this suggests a fluctuating eustatic sea level, any centennial-scale fluctuation could not have had an amplitude larger than ~30 cm. Rates of sea-level change calculated from the first derivative of a third-order polynomial function fitted through the central point of each error box (r2=0.96) indicate that the maximum rate of sea-level rise occurred around 1100 AD, which slightly postdates peak Medieval warmth according to most Northern Hemisphere paleotemperature records. The dominant control of the long-term rate (0.56 mm yr-1) of RSL rise obtained from the new reconstruction is glacial isostasy in the form of forebulge collapse. Subtracting the rate of eustatic sea level rise for the 20th century (1.7 mm yr-1) (Church and White, 2006) from the rate of sea level rise captured by the Pensacola (Florida) tide gauge (2.2 mm yr-1), an area that is considered tectonically stable, yields a residual of 0.5 mm yr-1. This figure is in agreement with the long-term rate obtained in this study. This value is also very close to what some geophysical models predict as the present-day rate of glacial isostatic adjustment for the north central Gulf Coast. Taking the ratio of the instrumental rate for the last 90 years captured by the Pensacola tide gauge (2.2 mm yr-1) to the long-term geologic rate (0.56 mm yr-1) indicates a four-fold increase in the rate of relative sea level rise during the 20th century compared to that of the previous millennium. Given that at time scales of one-thousand years processes like glacial isostasy and tectonic subsidence behave essentially linearly, and that both records are free of compaction, the increase can only result from an acceleration in the rate of global sea level rise. / acase@tulane.edu

Arroyos are steep-walled, entrenched, typically ephemeral streams commonly found in dryland river systems that form when streams incise into previously deposited alluvial fill. Arroyos in the southwestern U.S. have been studied extensively following the historic period of arroyo cutting in the late 1800s and early 1900s A.D. The upper Escalante River in south-central Utah similarly began incising in 1909, and records evidence for past cut and fill cycles in well-exposed walls along the now continuous arroyo. Establishing robust geochronologies of past arroyo cycles in these fluvial settings has been difficult. Recent improvements in accelerator mass spectrometry (AMS) radiocarbon (14C) and optically stimulated luminescence (OSL) dating provide an opportunity to link more highly resolved fluvial records to existing paleoclimate records. This allows hypotheses regarding the causes of arroyo cycles to be tested, and for the role of climate versus intrabasinal characteristics to be examined. One major objective of this research was to examine the applicability of OSL and AMS 14C dating in the upper Escalante, as both methods have proved problematic in similar settings. In total, 37 ages were obtained, 21 OSL and 16 14C ages. The Holocene fluvial history of the upper Escalante River was reconstructed using these age results and stratigraphic relationships. The chronostratigraphic record developed in this study suggests that at least six arroyo cycles have occurred in the upper Escalante since the middle Holocene, with incision occurring ~4.4 – 4.2 ka, ~2.6 – 2.4 ka, ~1.8 – 1.5 ka, ~1.0 – 0.9 ka, ~0.5 - 0.4 ka, and during the historic period of arroyo entrenchment. While semi-synchronous arroyo cutting (indicative of a climate signal) appears to have occurred in the Paria and Escalante drainages over the last 1 ka, correlations between proximal drainages are less clear prior to 1 ka, although this may be due in part to preservation effects. Overall, linkages to specific climate regimes and correlations to regional drainages are difficult to identify, suggesting that internal geomorphic thresholds are important in determining when arroyo entrenchment occurs in individual catchments.

O leste da região amazônica é uma região ainda pouco explorada pela palinologia, sendo que algumas questões sobre o paleoclima e o paleoambiente ainda necessitam de respostas. Além disso, o conhecimento da vegetação de planícies de inundação é fundamental para o entendimento da formação e evolução das Florestas de Várzea e Igapó. O trabalho foi desenvolvido na porção leste da Amazônia na região da Volta Grande do Rio Xingu, município de Altamira no Estado do Pará. Foram coletados dois testemunhos intitulados XC06 - Lago Irirí (3°48\' 56.09\" S - 52°40\' 29.60\" O) e XC01-2 - Ilha Arapujá (3°48\' 57.56\" S - 52°40\' 27.42\" O). Os objetivos da pesquisa foram: Verificar a evolução da vegetação de várzea e igapó durante o Holoceno; investigar o possível evento seco durante o Holoceno Médio no leste da Amazônia; avaliar o clima que prevaleceu ao longo do perfil sedimentar e buscar evidências de alteração na paisagem relacionadas a atividades antrópicas. A técnica empregada para o desenvolvimento da pesquisa foram a palinologia, partículas carbonizadas, granulometria, XRF e isótopos (C/N, NT, ? 15N e COT). Os resultados obtidos mostraram que a vegetação alcançou dois momentos de expansão e desenvolvimento sendo o primeiro em cerca de 8.700 anos cal. AP e o segundo cerca de 2.000 anos cal. AP. Oscilações na concentração de táxons polínicos, mostraram momentos de períodos de inundação prolongados que afetou diretamente a vegetação. Durante o Holoceno a região de estudo apresentou umidade sempre presente, o que revela a ausência de evento seco no Médio Holoceno em Altamira. Atividades antrópicas estão evidenciadas a partir de 1.900 anos cal. AP, pelo aumento de esporos, presença de vegetação característica de abertura de floresta e presença de vegetação de interesse humano como é o caso de Mauritia flexuosa (Buriti). / The eastern part of the Amazon region is a region not yet explored by palynology, and some questions about paleoclimate and paleoenvironment still require answers. In addition, the knowledge of floodplain vegetation is fundamental for understanding the formation and evolution of the Várzea and Igapó Forests. The work was carried out in the eastern portion of the Amazon region of the Volta Grande region of the Xingu River, in the municipality of Altamira in the State of Pará. Two cores entitled XC06 - Lago Irirí (3 ° 48 \'56.09 \"S - 52 ° 40\' 29.60\" W) and XC01-2 - Ilha Arapujá (3 ° 48 \'57.56 \"S - 52 ° 40\' 27.42\" W). The objectives of the research were: To verify the evolution of the vegetation of várzea and igapó during the Holocene; to investigate the possible dry event during the Middle Holocene in eastern Amazonia; to evaluate the climate that prevailed along the sedimentary profile and to seek evidence of alterations in the landscape related to anthropic activities. The technique used for the development of the research was palynology, carbonized particles, granulometry, XRF and isotopes (C/N, NT, ?15N and TOC). The results showed that the vegetation reached two moments of expansion and development, being the first in about 8,700 cal. years BP and the second about 2,000 cal. years BP. Oscillations in the concentration of pollen taxa showed moments of prolonged flood periods that directly affected the vegetation. During the Holocene, the study area presented constant moisture, which reveals the absence of a dry event in the Middle Holocene in Altamira. Anthropogenic activities are evidenced from 1,900 cal. years BP, by the increase of spores, presence of vegetation characteristic of opening of forest and presence of vegetation of human interest as is the case of Mauritia flexuosa (Buriti).

Alors que de nombreuses études paléoenvironnementales traitent de l'évolution du climat et de la végétation au cours de l'Holocene, il n'existe que très peu d'études portant sur l'évolution des sols au cours de cette même période. Cependant, les propriétés physiques et chimiques des sols ayant un rôle clef au sein d'un écosystème, la reconstitution de l'histoire des sols constitue un enjeu important pour comprendre les changements environnementaux passés. Un des facteurs limitant pour reconstituer l'histoire des sols réside dans l'absence d'enregistrement continu des processus pédologiques sur de longues échelles de temps (103 à 104 ans). Notre intérêt s'est donc porté sur l'utilisation d'archives sédimentaires lacustres dans le but de reconstruire l'histoire de la couverture pédologique en lien avec celle de l'écosystème. Cette étude a été menée sur deux petites dépressions lacustres de l'étage subalpin (Vallée de la Maurienne, Savoie) ayant a priori des histoires de l'occupation humaine différentes. <br /><br />Dans un premier temps, une approche géochimique a été appliquée sur des sols actuels afin de caractériser les pédosignatures des processus de podzolisation et d'altération chimique. Il en résulte que les terres rares (numéros atomiques de 57 à 71) constituent un indicateur du degré d'altération chimique tandis que les proportions des complexes organo-métalliques d'Al et Fe permettent de tracer l'intensité de la podzolisation. Dans un deuxième temps, ces mêmes traceurs ont été mesurés sur deux séquences sédimentaires couvrant 4500 ans Cal. BP (lac du Thyl) et 13500 ans (lac du Loup). La comparaison de ces pédosignatures avec des marqueurs indépendants (végétation, climat, feux) révèle des pédogenèses progressives et régressives après la déglaciation de la fin du Tardiglaciaire. <br /><br />Les sols et l'environnement du lac du Loup sont caractérisés par une évolution progressive et stable menant aux forets de résineux et aux podzols actuels. L'enregistrement du lac du Thyl est plus contrasté : le développement progressif d'une forêt de pin cembro associé au processus de podzolisation est suivi par des perturbations secondaires (principalement l'augmentation du régime des feux) qui pourraient expliquer la mise en place d'une végétation semi-ouverte associée à des processus de dépodzolisation. <br />Finalement, ce travail a permit d'ajouter un volet pédologique aux études paléoenvironnementales classiques. Il en résulte que les deux schémas d'évolution des sols sont très cohérents avec les marqueurs indépendants et qu'ils offrent une meilleure résolution temporelle que l'on ne pourrait l'obtenir avec les chronoséquences.