Lake size is a strong control on lake function and on how lakes interact with the environment. For example, lake size is related to carbon burial rates in lake sediments. Lake size distribution (the number of small, medium, and large lakes per unit area) can be used to extrapolate lake function to landscapes at local, regional and global scales. This research examined the utility of using radar satellite imagery (ALOS PALSAR) and existing spatial data (CanVec) for the construction of a new Canadian lake database, which was then used to estimate carbon accumulation in Canadian boreal lake sediments. The capability of ALOS PALSAR images for classifying lakes from eight pilot regions across Canada was assessed by direct comparison to existing CanVec data. The PALSAR lake classification differed between -1.8% to 18.0% for overall lake area and -56.0% to 196.0% for overall lake count compared to CanVec. The wide range in difference can be explained by limitations in resolution, classification method, and how a lake was defined. While the temporal resolution of PALSAR was superior, it did not provide better spatial resolution and accuracy than existing datasets. PALSAR’s utility therefore is in short term change determination. Consequently, CanVec was used to construct the final database describing lake distribution in Canada, resulting in over 13.2 million features with a total area of almost 1.2 million km2. Lake database results suggest that the scaling rules used in previous studies to estimate the number of very small lakes regionally and globally have limits. The use of real lake data allowed for a better understanding of regional differences in lake distribution across Canada that was not possible with scaling rule approaches. Estimates of carbon accumulation in boreal Canada lake sediments based on the new CanVec lake distribution and literature-based accumulation rates ranged from 1.65 and 2.34 Mt C yr-1, or roughly equal to the carbon emissions of 300,000-450,000 cars per year. Similarly, it would require only 36 years for Canada’s total annual emissions to account for all the carbon accumulation in Canadian boreal lakes over the Holocene (last 10,000 years). Thematic products derived from the lake database suggest that number of lakes is more important than the distribution of small, medium and large lakes when estimating carbon accumulation in the lake sediments of boreal Canada. / Graduate

Pollen assemblages, peat humification and carbon:nitrogen stratigraphy were examined at high resolution in a core from a fen peatland in the Hudson Bay Lowlands, Northern Ontario, to interpret the factors that drive long-term peatland dynamics. Subtle changes in the vegetation community are evident over the record, suggesting both allogenic and autogenic influences, but a fen community appears to have been resilient to external perturbations including isostatic rebound and hydroclimatic changes between 6400 and 100 years BP. Paleoclimatic reconstructions from the fossil pollen assemblages indicate that precipitation increased 3000 years BP at the end of the Holocene Thermal Maximum, and that carbon accumulation in the fen was controlled more by effective surface moisture (precipitation) than by temperature. The pollen record suggests changes over the past century, including increases in shrub Betula, Alnus, Ambrosia, and Cyperaceae and a decrease in Sphagnum spores, consistent with the observed Pan-Arctic shrub increase.

Pollen assemblages, peat humification and carbon:nitrogen stratigraphy were examined at high resolution in a core from a fen peatland in the Hudson Bay Lowlands, Northern Ontario, to interpret the factors that drive long-term peatland dynamics. Subtle changes in the vegetation community are evident over the record, suggesting both allogenic and autogenic influences, but a fen community appears to have been resilient to external perturbations including isostatic rebound and hydroclimatic changes between 6400 and 100 years BP. Paleoclimatic reconstructions from the fossil pollen assemblages indicate that precipitation increased 3000 years BP at the end of the Holocene Thermal Maximum, and that carbon accumulation in the fen was controlled more by effective surface moisture (precipitation) than by temperature. The pollen record suggests changes over the past century, including increases in shrub Betula, Alnus, Ambrosia, and Cyperaceae and a decrease in Sphagnum spores, consistent with the observed Pan-Arctic shrub increase.

Northern peatlands are an important component of the global carbon (C) cycle and have been a net sink of atmospheric C during the Holocene. Under current climate warming conditions, the future sink-source balance of these peatlands is uncertain. In particular, peatlands near the southern limit of permafrost are likely to be sensitive to changes in topography as well as climate. In order to predict how the sink-source balance may change, this thesis focuses on determining the generality of observed patterns of C accumulation in Northeastern Canada. The methodological approach in this thesis is unique. A total of 30 cores were taken from 9 peatlands located in 3 ecoclimatic regions along the North Shore of the Gulf of St Lawrence. This replication of records allows for climate-scale (allogenic) signals to be separated from the internal or local factors (autogenic), and for statistical testing of differences between regions and within sites over time. Trends in carbon accumulation rates (CAR) were analysed on three levels: (1) within individual sites along a hydrological or microtopography gradient, (2) between overall regions located along a climatic or permafrost gradient, and (3) over time on a multi-centennial scale. Lead-210 (210Pb) dating was used throughout the analysis to increase temporal resolution for the last 150-200 years of C accumulation. The method was thoroughly tested from preparation to analysis and found to produce reliable results, comparable with other dating methods. These dates were then used to develop combined age-depth models for longer-term context. Replicated records of 210Pb inventories and fallout rates were also used to address questions of deposition patterns and post-depositional mobility in peat profiles. Total inventories decreased with water table depth, with lichen hummocks having significantly higher inventories. One site also received significantly higher 210Pb deposition than the other two, as it is more sheltered from the Gulf influence. Recent carbon accumulation rates for the 150-year period for all microforms across all regions was 62.1 ± 4.4 g C m-2 a-1, and were highest for Sphagnum hummocks (79.9 ± 8.9 g C m-2 a-1) and lowest for dry lichen hummocks (42.7 ± 6.2 g C m-2 a-1). Patterns and trends at this scale were mainly driven by autogenic processes, including incomplete decomposition in the acrotelm peat. Models of peat accumulation related to acrotelm thickness were found to be overly simplistic, as carbon accumulation for intermediate microforms showed large natural variability driven by changing ecohydrological feedbacks, in part due to permafrost degradation at one of the sites. Over a multi-centennial scale, carbon accumulation rates were driven by a combination of climatic changes and ecohydrological feedbacks due to shifts in the microform configuration in response to permafrost degradation. Changes in carbon accumulation rates were detected and coincided with Little Ice Age temperature/solar minima (including the Spörer, Maunder and Dalton Minima), permafrost degradation since the 1950s, and recent climatic changes in the mid-1990s. Snow cover and exposure of sites and microforms were found to play an important role, rather than solely climatic variables. Rapid Sphagnum re-establishment in post-permafrost degraded features and increasing temperatures meant that carbon accumulation was highest for the northernmost site in the transect. Age-depth models using a combination of lead-210 and radiocarbon dates allowed for the calculation of carbon accumulation rates at a decadal resolution. While peat carbon sequestration is projected to increase in northern regions, the fate of peatland C near the southern limit of permafrost is complex. Future studies seeking to interpret recent changes should include multiple cores and consider both regional climatic and local ecohydrological drivers.

A paleoecological study was carried out on a 4.7 m peat core from Anderson Fen on central Vancouver Island, using a multi-proxy approach. Pollen, non-pollen palynomorphs, and physicochemical analyses were used to document past vegetation, peatland developmental history, and carbon and nitrogen accumulation rates over the last 14,000 years. Lake sediment and aquatic plant remains at the base of the core indicate a shallow pond was present at the site after deglaciation. By ~11,700 calendar years before present (cal yr BP), the shallow pond became a herb-dominated wetland (marsh) through terrestrialization. Bog formation started around 10,500 cal yr BP with decreasing water levels, as indicated by high C:N, Sphagnum and fungal remains, and testate amoebae such as Archerella flavum and Heleopera. A fen developed by ~9400 cal yr BP with fluctuating water levels through the rest of Holocene. Carbon accumulation rates were highest towards the surface and during the early Holocene warm period, with an overall mean rate of 12.9 g/m2/cal yr, which is low compared to continental and northern peatlands. Pollen analysis reveals that non-arboreal communities dominated by Salix prevailed soon after deglaciation before the expansion of Pinus forests 13,200 cal yr BP. Pseudotsuga menziesii dominated forests between ~10,700 and 8400 cal yr BP under warm and dry conditions. Tsuga heterophylla rainforest was established by ~7000 cal yr BP under increasingly cool and wet conditions. Overall, Anderson fen and the surrounding area experienced major and rapid changes in environmental conditions and vegetation in response to climate change during the late glacial and early Holocene, while mid- to late Holocene changes have been more subtle and relatively gradual. / Graduate / 2020-10-25

Les tourbières boréales jouent un rôle important dans le cycle global du carbone en tant que puits de CO2 à long terme et en tant que l’une des plus grandes sources de méthane naturel (CH4). Ces importants réservoirs de carbone seront exposés à l’avenir au réchauffement et aux conditions plus humides caractérisant le changement climatique dans les hautes latitudes et, en raison de la grande quantité de carbone stockée dans les tourbières boréales, comprendre leurs dynamiques est important. Dans cette thèse, j'ai intégré une représentation du cycle de l'eau et du carbone dans les tourbières dans le modèle de surface terrestre ORCHIDEE-MICT (LSM), dans le but d'améliorer la compréhension du C des tourbes et de sa dynamique depuis l'Holocène, afin d'explorer les effets du changement climatique.Tout d'abord (chapitre 2), J'ai implémenté les tourbières en tant qu'unité hydrologique de sol (HSU) sous-réseau indépendante qui reçoit les eaux de ruissellement provenant des HSU non tourbeuses environnantes dans chaque cellule du réseau et ne possède pas de drainage, conformément la representation propose par Largeron et al. (2018). Pour modéliser les flux d’eau verticaux des sols tourbeux et non tourbeux, j’ai représenté les paramètres hydrologiques spécifiques à la tourbe pour l’HSU des tourbières, tandis que dans d’autres HSU, les paramètres hydrologiques sont déterminés par la texture dominante du sol de la cellule de la grille. j'ai choisi un modèle diplotelmique pour simuler la décomposition et l'accumulation de tourbe de C. Ce modèle à deux couches comprend une couche supérieure (acrotelm) inondée de manière variable et une couche inférieure (catotelm) inondée en permanence. Ce modèle a montré de bonnes performances dans la simulation de l'hydrologie des tourbières, du C et des flux d'énergie dans 30 tourbières boréales sur des échelles de temps quotidiennes à annuelles. Mais la simplification excessive de la dynamique du carbone pourrait limiter sa capacité à prévoir la réponse des tourbières boréales aux futurs changements climatiques.Deuxièmement (chapitre 3), j'ai remplacé le modèle carbone de tourbe diplotelmique par un modèle multicouche afin de prendre en compte les hétérogénéités verticales de la température et de l'humidité le long du profil de la tourbe. J'ai ensuite adapté TOPMODEL et les critères d'établissement des tourbières de Stocker et al. (2014) pour simuler la dynamique de la zone des tourbières dans une unité de la grille. Ici, la zone inondée donnée par TOPMODEL est traversée avec des conditions de croissance de tourbe appropriées pour définir la zone occupée par une HSU de tourbe. Ce modèle a été testé sur plusieurs sites de tourbières du nord et pour des simulations en 2D sur l'hémisphère nord (> 30 ° N). La superficie totale simulée de tourbières et le stock de carbone en 2010 est de 3,9 million de km2 et 463 PgC, conformément aux observations (3,4 à 4,0 million de km2 et 270 à 540 PgC).Enfin (chapitre 4), avec le modèle multicouche, j’ai réalisé des simulations factorielles à l’aide de données climatiques passées et futures issus des scenarios de trajectoire de concentration représentative (RCP) à partir de deux modèles de circulation générale (GCM) afin d’explorer les réactions des tourbières boréales au changement climatique. Les impacts des tourbières sur le futur bilan en carbone de l'hémisphère nord ont été examinés, notamment la réaction directe du bilan en carbone de la tourbière existante (simulée) et les effets indirects des tourbières sur le bilan de carbone terrestre lorsque les tourbières se modifient à l'avenir.Les travaux futurs se concentreront sur l’inclusion des influences du changement d’affectation des sols et des incendies sur les tourbières dans le modèle, étant donné que des pertes importantes de C pourraient survenir en raison de ces perturbations. Pour avoir une image complète du bilan C des tourbières, il faut prendre en compte les pertes de CH4 et de C organique dissous (DOC). / Northern peatlands play an important role in the global carbon (C) cycle as a long-term CO2 sink and the one of the largest natural methane (CH4) sources. Meanwhile, these substantial carbon stores will be exposed in the future to large warming and wetter conditions that characterize climate change in the high latitudes and, because of the large amount of C stored in northern peatlands, their fate is of concern. In this thesis, I integrated a representation of peatlands water and carbon cycling into the ORCHIDEE-MICT land surface model (LSM), with the aim to improve the understanding of peatland C and area dynamics since the Holocene, to explore effects of projected climate change to northern peatlands, and to quantify the role of northern peatlands in the global C cycle.Firstly (Chapter 2), I implemented peatland as an independent sub-grid hydrological soil unit (HSU) which receives runoff from surrounding non-peatland HSUs in each grid cell and has no bottom drainage, following the concept of Largeron et al. (2018). To model vertical water fluxes of peatland and non-peatland soils, I represented peat-specific hydrological parameters for the peatland HSU while in other HSUs the hydrological parameters are determined by the dominant soil texture of the grid cell. I chose a diplotelmic model to simulate peat C decomposition and accumulation. This two-layered model includes an upper layer (acrotelm) that is variably inundated and a lower layer (catotelm) that is permanently inundated. This model showed good performance in simulating peatland hydrology, C and energy fluxes at 30 northern peatland sites on daily to annual time scales. But the over simplification of the C dynamics may limit its capacity to predict northern peatland response to future climate change.Secondly (Chapter 3), I replaced the diplotelmic peat carbon model with a multi-layered model to account for vertical heterogeneities in temperature and moisture along the peat profile. I then adapted the cost-efficient version of TOPMODEL and peatland establishment criteria from Stocker et al. (2014) to simulate the dynamics of peatland area within a grid cell. Here the flooded area given by TOPMODEL is crossed with suitable peat growing conditions to set the area that is occupied by a peat HSU. This model was tested across a range of northern peatland sites and for gridded simulations over the Northern Hemisphere (>30 °N). Simulated total northern peatlands area and C stock by 2010 is 3.9 million km2 and 463 PgC, fall well within observation-based reported range of northern peatlands area (3.4 – 4.0 million km2) and C stock (270 – 540 PgC).Lastly (Chapter 4), with the multi-layered model, I conducted factorial simulations using representative concentration pathway (RCP)-driven bias-corrected past and future climate data from two general circulation models (GCMs) to explore responses of northern peatlands to climate change. The impacts of peatlands on future C balance of the Northern Hemisphere were discussed, including the direct response of the C balance of the (simulated) extant peatland area, and indirect effects of peatlands on the terrestrial C balance when peatlands area change in the future.Future work will focus on including influences of land use change and fires on peatland into the model, given that substantial losses of C could occur due to these disturbances. To have a complete picture of peatland C balance, CH4 and dissolved organic C (DOC) losses must be considered.

Peatlands in the sub-humid Boreal Plains of Alberta exist at the limit of their climatic tolerance and are vulnerable to wildfire. This is especially true at the interface between the peatland and forestland (margins) due to water table fluctuation resulting in high peat bulk density and low moisture content during dry periods in some peatland systems. Deep burning at the margins may reduce a peatland's ability to recover to its previous state, leading to a reduction in area and/or collapse following fire, and bringing into question the long-term stability of Boreal Plains peatlands on the landscape under current and future climate predictions. Previous research has identified small peatlands located at a mid-topographic position on coarse sediments as hotspots for deep burning, as these peatlands are not regularly connected to regional groundwater flow. The ability of these peatland systems to recover lost carbon from both the interior and margin within the fire return interval, however, has not yet been investigated. This thesis further examines the relationship between surficial sediment assemblages and the impact of wildfire on overlying peatlands through assessment of organic soil carbon accumulation following wildfire across the Boreal Plains landscape. Peatland organic soil recovery along a chronosequence was assessed in the interior and margin of 26 ombrotrophic bogs located at various positions on the post-glaciation landscape of Northern Alberta using estimates of organic soil carbon accumulation calculated through loss on ignition of peat above the uppermost charcoal layer in peat cores from each site, as well as characterization of peat properties along a transect from the adjacent forestland into the peatland interior. Soil organic carbon accumulation with time since fire was greater in studied peatland interiors than margins. Underlying sediments were found to have little effect on total soil organic carbon accumulation in the interior and margins of the studied peatlands, indicating that organic soil carbon accumulation rates following wildfire estimated in this study can be extended to ombrotrophic bogs across the Boreal Plains landscape. Though total soil organic carbon accumulation following wildfire does not appear to be influenced by hydrogeological setting, the ability of a peatland to recover the quantity of carbon lost within the fire return interval will be dependent on the amount of carbon which was released through smouldering, which is influenced by hydrogeological setting for peatland margins. Based on published measurements of organic soil carbon loss during wildfire and organic soil carbon accumulation rates estimated in this thesis, peatlands located at topographic lows on coarse grained glaciofluvial outwash sediments or on low-relief, fine grained sediment deposits from glaciolacustrine or subglacial paleoenvironments are predicted to be resilient to wildfire on the Boreal Plains landscape. Peatlands which experience severe smouldering at the margins, such as ephemerally perched systems on glaciofluvial outwash sediments, will likely undergo permanent loss of legacy carbon stores. The resilience of peatlands which are perched above regional groundwater on glaciofluvial outwash or stagnant ice moraine deposits is unknown at this time; further investigation into water table dynamics, margin peat properties, and smouldering depths in these systems is required. Identification of peatland systems which are at risk of permanent carbon loss at the margins and those which are most resilient to wildfire in this thesis can be applied to wildfire management strategies and the design of peatland systems for reclamation of oil sands leases. The stability of natural and created peatlands through time on a landscape where wildfire is frequent is an important consideration in terms of both lasting ecosystem services and the potential risk to fire suppression and community safety that vulnerable systems pose. / Thesis / Master of Science (MSc)

Submitted by Biblioteca de Pós-Graduação em Geoquímica BGQ (bgq@ndc.uff.br) on 2016-04-26T17:27:20Z No. of bitstreams: 1 TeseKAniceto.pdf: 7215824 bytes, checksum: b930fcc52874c9e437f1d692f113bde8 (MD5) / Made available in DSpace on 2016-04-26T17:27:20Z (GMT). No. of bitstreams: 1 TeseKAniceto.pdf: 7215824 bytes, checksum: b930fcc52874c9e437f1d692f113bde8 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Universidade Federal Fluminense. Instituto de Química. Programa de Pós-Graduação em Geociências-Geoquímica. Niterói, RJ / As variações paleohidrológicas pelas quais passaram os lagos Hubos e Quistococha ao longo do Holoceno Médio e Superior, foram reconstruídas através de análises semi-quantitativas e qualitativas dos argilo-minerais e dos elementos Al, Si, S, K, Ca, Mn Ti, Fe, Rb, Sr e Zr. A fim de avaliar como o Rio Amazonas influencia os processos sedimentares destes ecossistemas, determinou-se a concentração elementar de carbono (carbono orgânico total-COT) e nitrogênio (nitrogênio total-NT), composição isotópica do carbono e nitrogênio (δ13C e δ15N). O Lago Hubos, está situado próximo à confluência dos rios Ucayali e Marañon, faz parte da planície de inundação do Rio Ucayali e sua conexão com o canal principal se dá de forma indireta e através do pequeno rio Yarapá e sua distância do canal rio é de aproximadamente 2 km. A sedimentação do lago Hubos nos últimos 4130 anos cal AP, se deu sob a influência da forte dinâmica do Ucayali, que resultou em datações 14C invertidas ou com idades muito próximas em trechos extensos do testemunho. Entre 4130 e 2870 anos cal AP a concentração de COT era baixa (<1%), os valores de δ13C e δ15N do lago apresentaram uma matéria orgânica que é mistura de fitoplancton e com matéria orgânica típica de solos e plantas C3 e a presença de laminações, o que sugere um sistema de baixa energia hidrodinâmica. A assembléia mineral com assinatura típica do Ucayali, apresentou elevados percentuais de esmectita (~61%), caulinita (~15%), clorita (~8%) e ilita (18%) e a mineralogia por FRX sugere variabilidade da maior parte dos dados, sugerindo pulsos de inundação neste período. Uma lacuna na sedimentação foi observada entre 2870 e 700 anos cal AP (unidade III). Este sedimento foi caracterizado por concentrações mais elevadas de COT (4,3%), matéria orgânica originada de plantas C3, elevados percentuais de esmectita (67%), caulinita (~12%), clorita (~6%) e ilita (15%) e predominância de material intemperizado e grosso no sedimento. Após esse período, o material sedimentado era pós-moderno e bioturbado, as concentrações de COT de 2,1% e esmectita (~52%), caulinita (~29%), clorita (~4%) e ilita (15%), esta matéria orgânica é característica de solos e plantas C3, com redução de frações mais grossas no sedimento e aumento de material inalterado. O material mais superficial é antigo, provavelmente retrabalhado, com concentração de COT 1,29% e esmectita (~46%) caulinita (~12%), clorita (~26%) e ilita (18%). A FRX, sugere material de granulometria mais grossa e mais alterado quimicamente. O lago Quistococha faz parte da Reserva Quistococha, e está localizado à 10 km do centro da cidade de Iquitos e aproximadamente 10 Km do rio Amazonas. Entre cerca de 6.100 e 4.900 anos cal AP, o lago estava sob forte influência do rio Amazonas. O que induziu a maiores taxas de sedimentação (~0,5 cm ano-1), a deposição predominante foi de partículas relativamente grossas (silte grosso) e laminadas, em detrimento do material orgânico (~5%). A mineralogia da fração argila determinou percentuais de esmectita (~41%), clorita (~9%), ilita (~20%), e caulinita (~29%). A razão FRX, sugere presença de material alterado quimicamente e laminações de 0,7 cm de espessura. A diminuição da granulometria e da espessura das laminações a partir de 5800 anos cal AP, até seu completo desaparecimento indicam a perda gradual da influência do Amazonas. Um hiato foi observado a partir de 4.900 anos calAP, cujo motivo provável ainda precisa ser investigado. Condições mais seca do Holoceno Médio impediram a volta da sedimentação, que só aconteceu apos 2600 anos cal AP, com o retorno das condições mais úmidas estabelecidas durante o Holoceno Tardio. A perda da influência do rio Amazonas induziu alterações significativas na sedimentação lacustre, determinando sua atual condição de isolamento. Caracterizado por taxas de sedimentação extremamente baixas (0,02 cm ano-1), sedimentos mais finos, ricos em material orgânico (20-80%) e fração argila composta de esmectita (~35%); clorita (~5%); ilita (~16%); e caulinita (~44%) / The palaeo-hydrological variations through which passed Hubos and Quistococha lakes along the Middle and Upper Holocene were reconstructed using a semi-quantitative and qualitative analyzes of clay minerals and the elementsAl, Si, S, K, Ca, Mn Ti, Fe, Rb, Sr e Zr. In order to assess how the Amazon River influences the sedimentary processes of these ecosystems, was determined the elementary concentration of carbon (total organic carbon-TOC) and nitrogen (total nitrogen-NT), isotopic composition of carbon and nitrogen (δ13C and δ15N). The Hubos Lake, is situated near the Ucayali and Marañon rivers confluence, part of the floodplain of the Ucayali river and its connection with the main channel occurs indirectly and through small river Yarapá and its distance from the river channel is aproximately2 Km. The sedimentation of the lake Hubos in the last 4130 years cal BP, occurred under the influence of the strong dynamics of the Ucayali, which resulted in 14C datings reversed or very close in age in extensive excerpts of testimony . Between 4130 and 2870 years cal AP TOC concentration was low (<1%), the values of δ13C and δ15N of lake showed an organic matter which is mixed with phytoplankton and typical organic matter from soils and plants C3 and the presence of laminations, which suggests a system of low hydrodynamic energy. The mineral meeting with the typical signature of Ucayali showed high percentages of smectite (~ 61%), kaolinite (~ 15%), chlorite (~ 8%) and illite (18%) and mineralogy by XRF suggests variability of most data, suggesting flood pulses during this period. A gap in sedimentation was observed between 2870 and 700 cal years BP (Unit III). This sediment was characterized by higher concentrations of TOC (4.3%), organic matter originating from plants C3 smectite high percentage (67%), kaolinite (~ 12%), chlorite (~ 6%) and illite (15 %) and predominance of the weathered and coarse sediment. After this period, the settled material was postmodern and bioturbed, TOC concentrations of 2.1% and smectite (~ 52%), kaolinite (~ 29%), chlorite (~ 4%) and illite (15%) this organic matter is characteristic of soils and C3 plants, with reducing coarser fractions in the sediment and unaltered material increase. The most superficial material is old, probably reworked, with 1.29% TOC concentration and smectite (~ 46%) kaolinite (~ 12%), chlorite (~ 26%) and illite (18%). The XRF suggests a thicker granulometry and chemically altered material. The Quistococha lake is part of Quistococha Reserve, and is located 10 km from the center of Iquitos city and approximately 10 km of the Amazon River. Between about 6100 and 4900 years cal BP, the lake was under strong influence of the Amazon River. What led to higher sedimentation rates (~ 0.5 cm-1 year), was the predominant deposition of relatively coarse particles (coarse silt) and laminated, to the detriment of the organic material (~ 5%). The the clay fraction mineralogy determined percentage of smectite (~ 41%), chlorite (~ 9%), illite (~ 20%), and kaolinite (~ 29%). The XRF reason, suggests the presence of chemically altered material and laminations of 0.7 cm thick. The decrease of granulometry and the laminations thickness from 5800 cal years BP, until its complete disappearance indicate the gradual loss of the Amazon influence. A gap was observed from 4900 cal years BP, whose probable reason still needs to be investigated. Drier conditions in the Middle Holocene prevented the sedimentation return, which only happened after 2600 cal years BP, with the return of most humid conditions established during the late Holocene. The loss of influence of the Amazon river induce significant changes in lacustrine sedimentation, determining its current condition of isolation. Characterized by extremely low sedimentation rates (0.02 cm year-1), fine sediments, rich in organic material (20-80%) and clay fraction composed of smectite (~ 35%); chlorite (~ 5%); illite (~ 16%); and kaolinite (~ 44%)

Dans l’éventualité où l’important réservoir de carbone que représentent les tourbières venait à être déstabilisé, les impacts sur le climat planétaire pourraient être majeurs. Pourtant, les facteurs influençant les taux d’accumulation de carbone à l’échelle décennale et centennale dans les tourbières n’ont pas clairement été déterminés et les dynamiques passées et contemporaines en matière d’accumulation de carbone ont rarement été comparées pour le même site d’étude. Ce projet visait à (1) reconstruire à l’échelle fine (cm) les taux d’accumulation de carbone pour la tourbière Degerö Stormyr (Suède), (2) identifier les principaux facteurs influençant ces taux d’accumulation et (3) comparer les taux reconstruits aux flux de carbone contemporains mesurés sur le site. Deux approches méthodologiques complémentaires ont été employées : les reconstructions paléoécologiques et la modélisation. Les principales conclusions de ce mémoire sont les suivantes : (1) les taux apparents d’accumulation de carbone sont principalement liés aux conditions hydroclimatiques dans l’acrotelme, mais sont influencés par la composition végétale de la tourbe dans le catotelme. (2) Les taux réels d’accumulation de carbone pour la période récente (≈ 130 dernières années) sont semblables au bilan écosystémique net de carbone mesuré sur le site. (3) La comparaison entre les dynamiques récentes et à plus long terme suggère que Degerö Stormyr a séquestré davantage de carbone au cours des dernières décennies. Les résultats de ce mémoire soulignent l’importance d’étudier les facteurs influençant les taux d’accumulation de carbone pour différentes échelles temporelles et suggèrent que l’accent doit être mis sur l’accumulation de carbone à l’échelle décennale et centennale. / In the event that the important carbon reservoir sequestered in peatlands were to be destabilized, the impacts on global climate could be major. However, the factors influencing carbon accumulation rates at decadal and centennial scale in peatlands have not yet been clearly identified and past and contemporary carbon accumulation dynamics of the same study site have rarely been compared. This project aimed to (1) reconstruct fine scale (cm) carbon accumulation rates for the Degerö Stormyr peatland (Sweden), (2) identify the key factors influencing accumulation rates and (3) compare the reconstructed rates to modern carbon fluxes measured at the site. Two complementary methodological approaches were used: paleoecological reconstructions and modelling. The main conclusions of this thesis are that: (1) Apparent rates of carbon accumulation are mainly related to hydroclimatic conditions in the acrotelm, but are influenced by the vegetation composition of the peat in the catotelm. (2) True rates of carbon accumulation for the recent period (≈ last 130 years) are similar to the net ecosystem carbon balance measured on site. (3) The comparison between recent and long-term carbon accumulation dynamics suggests that Degerö Stormyr has sequestered more carbon over the past decades. The results of this thesis highlight the importance of studying factors influencing carbon accumulation rates at different time scales and suggest that emphasis should be placed on decadal and centennial scales.

Ce travail s'inscrit dans le cadre du projet SIMED fédérant les activités de modélisation à l'échelle méditerranéenne, et plus globalement dans le programme MERMEX qui vise à étudier les cycles biogéochimiques en mer Méditerranée et leurs évolutions futures. L'étape préliminaire a été de coupler la plateforme de modélisation hydrodynamique (NEMO) à celle de modélisation biogéochimique mécaniste (Eco3M), afin de réaliser une simulation (2000-2012) utilisant les sorties hydrodynamiques de la configuration NEMO-MED12 pour forcer le modèle biogéochimique Eco3M-MED. Les nombreuses comparaisons menées dans cette thèse (chlorophylle, sels nutritifs, production primaire, etc.) ont aidé à s'assurer de la capacité du modèle à reproduire les principales caractéristiques biogéochimiques de la Méditerranée. Ce travail a permis de généraliser le rôle majeur joué par le carbone organique dissous dans la pompe biologique à l'échelle de la mer Méditerranée. Les résultats montrent que la production de carbone organique particulaire est restreinte aux régions de forte dynamique physique, tandis que l'accumulation de carbone organique dissous dans les eaux de surface est commune à la plupart des régions du bassin. Ce dernier processus s'est avéré dépendant des contenus cellulaires du phytoplancton et des bactéries hétérotrophes. Finalement d'après le modèle, la fraction dissoute du carbone organique contribuerait à hauteur d'environ 64 % à l'exportation dans le bassin Ouest, et de 90 % dans le bassin Est. Le bassin Est -en dépit de sa plus forte oligotrophie- s'avère participer à près de 60 % à l'exportation de carbone organique en mer Méditerranée. / This work is part of the SIMED project which is dedicated to basin-scale modeling of the Mediterranean Sea. It also belongs to the MERMEX program which aims at studying biogeochemical cycles in the Mediterranean Sea and their evolution. The first step of this work was to couple the hydrodynamic modeling platform (NEMO) to the mechanistic biogeochemical modeling platform (Eco3M). We ran a simulation (2000-2012) using the hydrodynamic outputs from NEMO-MED12 configuration to force the biogeochamical model Eco3M-MED. The model evaluation was conducted using numerous field measurements (chlorophyll, nutrients, primary production, etc.). The simulation strengthens and extends to the whole basin the prominent role of dissolved organic carbon in the biological carbon pump in the whole Mediterranean Sea. A comprehensive analysis of organic carbon (particulate and dissolved) production processes production was performed. Results reveal that particulate organic carbon production is restricted to the highly dynamic areas, whereas dissolved organic carbon accumulation in the surface layers is a common process in much areas of the basin. This latter process appeared to dependant on the cellular contents of phytoplancton and heterotrophic bacteria, themselved being controled by low phosphate availability. Finally, the dissolved organic carbon contribution to carbon export is around 64 % in the Western basin, and up to 90 % in the Eastern basin. When taking into account the dissolved fraction, total organic carbon export in the Eastern basin -despite its higher oligotrophy- exceeds the one in the Western basin (60% against 40 %).