reconstructing multidecadal and seasonal variations of the West African Monsoon system in the last two millenia, based on sclerochronological study of fossil shell middens in the Saloum Delta, Senegal.

Les variations multidécennales et saisonnières de la Mousson Ouest-Africaine (MOA) au cours des deux derniers millénaires dans la région sahélienne sont peu documentées en raison du manque d'archives paléoclimatiques. Pour cela, on se propose dans ce travail de thèse d'explorer une nouvelle archive paléoclimatique basée sur l'étude sclérochronologique des coquilles du mollusque bivalve Anadara senilis dans des amas coquilliers fossiles afin de reconstituer les variations hydrologiques multidécennales et saisonnières dans le Delta du Saloum au Sénégal de 460 à 1090 A.D. L'hydrologie de cet estuaire hypersalin est très sensible aux variations de la MOA. Les variations hydrologiques passées sont reconstituées à travers des analyses isotopiques (δ18O, δ13C) des coquilles modernes et des coquilles fossiles dans le delta. Le signal isotopique saisonnier de ces coquilles retrace fidèlement les variations hydrologiques liées au régime de la mousson. Nos résultats montrent que ces variations isotopiques, associées aux stries de croissance dont la périodicité est connue, permettent de reconstituer les durées des saisons avec une précision de 25 jours, une précision jamais atteinte dans les études paléoclimatiques antérieures dans la région sahélienne. Les variations hydrologiques multidécennales sont reconstituées à travers la composition isotopique des coquilles fossiles prélevées dans un amas coquillier massif (Dioron Boumak) dont le taux d'accumulation est très important. Les coquilles fossiles prélevées dans cet amas présentent des valeurs isotopiques moyennes en δ18O plus négatives de 1.4 ‰ par rapport à leurs analogues modernes. Ceci est une indication que les conditions hydrologiques étaient plus douces qu'aujourd'hui dans le Saloum qui n'était pas hypersalin à cette époque. Le bilan Précipitation-Evaporation était par conséquent plus positif en réponse à des pluies plus intenses et/ou plus étalées dans le temps de 460 à 1090 A.D. Il semblerait que les pluies hivernales et printanières, caractéristiques de la frange littorale sénégalo-mauritanienne, plutôt rares et insignifiantes de nos jours se produisaient plus fréquemment pendant cette période. La jonction entre ces pluies et les pluies de mousson aurait favorisé l'établissement de saisons des pluies beaucoup plus longues (~5 mois environ au lieu de 3 aujourd'hui) et une augmentation du bilan Précipitation-Evaporation. Cette étude met en lumière le potentiel considérable d'A. senilis comme archive paléoclimatique à haute résolution de la variabilité des précipitation dans la région sénégalaise. Elle montre également l'importance de la saisonnalité des précipitations dans les cycles hydrologiques passés dans cette région qui doit être prise en compte dans les études paléoclimatiques futures.The multidecadal and seasonal variations of the West African Monsoon (WAM) in the last two millennia remain poorly documented in the Sahel region because paleoclimate archives are lacking. For this, we propose in this PhD thesis a sclerochronological study of the mollusk bivalve Anadara senilis from massive shell middens to reconstruct multidecadal and seasonal variations of hydrological conditions in the Saloum Delta (Senegal) between AD 460 and 1090. Hydrological conditions in this hypersaline estuary are highly sensitive to the WAM variations.Past hydrological variations are reconstructed by using isotopic composition (δ18O, δ13C) of modern and fossil shells in this Delta. The shells' seasonal isotopic signals reflect faithfully hydrological variations, linked to monsoonal regime. Our results show that the variations of these seasonal isotopic signals, associated to shell growth patterns with known periodicities allow the reconstruction of season durations with a precision of 25 days, a precision that has never been achieved in paleoclimate studies in the Sahel region.Multidecadal variations of hydrological conditions are reconstructed by using isotopic composition of fossil shells collected in the massive shell middens (Dioron Boumak), characterized by high accumulation rate. The averaged δ18O value of fossil shells was more negative by 1.4‰ compared to modern shells' isotopic signature. This result indicates fresher mean conditions in the Saloum Delta that was likely not hypersaline as it is today. The precipitation-evaporation budget was thus more positive in response to a more intense and/or longer rainfall season during from AD 460 to 1090. We propose that winter and early spring rainfall events, which are observed very occasionally today, were occurring frequently during this period. These rains restricted to the western Sahelian coast and followed by the monsoon would have increased the total duration of the rainy season (~ 5 months instead of 3 months today) and substantially increased the annual precipitation-evaporation budget.This study shed light on the high potential of A. senilis as a high resolution paleoclimate archive of rainfall variability in the Sahel region. It shows also the importance of rainfall seasonality in past hydrological cycles that should be taken into account in the future paleoclimate studies

Sources, dispersal pathways and mass budget of sedimentary polycyclic aromatic hydrocarbons (PAH) in the NW Mediterranean margin, Gulf of Lions

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Linking Ocean Variabilty to Phytoplankton Changes Over the Last 2000 Years off N. Iceland

International audienceUnderstanding ocean variability at decadal to century time-scale is a key issue of climate research, yet it remains poorly described partly because high resolution marine records are lacking. Recent observations also suggested that climate-driven oceanic changes may also affect marine productivity reflecting adjustments of the pelagic ecosystem. To explore these issues we investigated sediment cores (a Calypso and a box core) off North Iceland at unprecedented high temporal resolution (2 to 5 years). The area lies in a sensitive boundary region with strong climatic gradients between the Arctic and the North Atlantic realms. Surface hydrology is affected by warm, high salinity Atlantic water (Irminger current-IC) and cold, low salinity water of the East Icelandic Current (EIC), as well as by sea ice and drifting ice exported from the Arctic Ocean and East Greenland. Using biomarker proxies we reconstructed, over the past two millennia, high frequency surface ocean properties, such as Sea Surface Temperature (SST) and patterns of major phytoplankton groups. Records from a box core covering the last 80 years and comparison with instrumental data allowed to improve constrain of the proxies used. The SST curve reveals 4 major oscillations of about 20-25 years, depicting also the Great Salinity Anomaly (GSA). It reflects ocean circulation changes with varying impact of Polar and Atlantic waters (EIC, IC). Similarities with the NAO (North Atlantic Oscillation) index suggest links between STT and westerlies intensity. Phytoplankton biomarker profiles (specific phytosterols and alkenones) are consistent with biological data acquired during annual surveys. They show significant increase in periods when Atlantic waters and subsequent mixing processes prevail during positive NAO phase, compared to low levels when Polar waters and stratified conditions occur. Over the last two millennia the SST curve depicts a broad cooling trend towards present, steepening over the last 500 years. Prolonged warm or cool centennial intervals are recognizable among which the Medieval Warm Period (MWP), the Little Ice Age (LIA), the Roman Warm Period (RWP) and cooler Dark Ages. The SST signal also shows intermittent 20-25 year oscillations that are likely reflecting the ocean response to wind forcing, presumably the NAO. Phytoplankton biomarkers show important high frequency oscillations. Diatom sterols infer a broad decrease of siliceous productivity since 2000 yr BP with minimum values during the LIA, and a sharp rise after 200 yr BP. Such a trend is not observed for coccolithophorid biomarkers (alkenones), which show low centennial scale variations since 1200 yrs BP, along with high abundance between 1600 and 1200 yrs BP. Similar trends are observed for dinoflagellate biomarkers (dinosterol). Overall, planktonic biomarkers show multidecadal to centennial scale fluctuations inferring modifications of phytoplankton community structure. These could be linked at least partly to hydrological changes indicated in the SST curve, namely modifications of the relative importance of IC and EIC, which likely reflect the ocean response to NAO forcing

Precipitation variability in the Sahel during the last 2000 years revealed by archaeological shell middens in Senegal

International audienceIn the second half of the 20th century, a strong multidecadal drought occurred in the Sahel that had catastrophic social and economic consequences. This event made it urgent to better understand the forcing driving precipitation changes in the Sahel and potential impact of anthropogenic global warming. Here, we reconstruct past changes of the precipitation regime in the Saloum Delta, Senegal, from stable isotope analyses in fossil shells of Senilia senilis. The shells were collected in large radiocarbon dated shell middens built by ancient fishermen during the last two millennia. Oxygen isotopic composition in shells is related to salinity conditions in this hypersaline estuary that is very sensitive to changes in precipitation and evaporation. Our dataset shows that salinity is today at its highest level in the past 2000 years. These values were reached abruptly in the 20th century and lie outside the range of natural climatic variability in this site. Monthly resolved isotopic profiles in fossil shells combined with analyses of shell growth lines allowed us to explore the role of rainfall seasonality in the variability of mean conditions

Mid-Holocene mean climate in the south eastern Pacific and its influence on South America

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The peopling of mangroves in Senegal in the late Holocene: relationship with climate and political changes in West Africa

International audienceWe present a reconstruction of human demography and shell fishing activity in the Sine-Saloum mangrove Delta (Senegal) in the Late Holocene using the summed probability density (SPD) of radiocarbon dates in archaeological shell middens. We explore how this local history relates to the climatic and political history of West Africa. We find that traces of human presence were scarce from 6000 to 2000 yr BP, partly because the geomorphology of the estuary was less favourable to human settlements at that time. A specialized shell fishing population migrated massively to the Sine-Saloum around 2000 yr BP, at the end of the aridification trend that followed the African humid period. This population, likely coming from the northern coast in search of land and resources, fleeing the Saharan desertification and the subsequent warfare, found refuge in the coastal mangroves and reached a maximum activity at about 1700 yr BP. This period corresponds to the beginning of trans-Saharan trade, and to a political complexification that would give rise to the Ghana empire. The incoming migration may have occurred in two waves as suggested by two peaks in the SPD curve at 200-400 CE and 600-800 CE and by cultural differences within the Delta. Most sites in the Sine-Saloum islands were abandoned in the 14th century, before the arrival of Europeans, possibly because intensive shell fishing was not sustainable anymore, or because of the regional political destabilization associated to the fall of the Ghana empire and the beginning of the Mali empire. Shortly after, in agreement with oral traditions, a new population lead by the Manding Guelwars, moved to the Sine Saloum after a military defeat and founded the modern towns. They had a reduced shellfishing activity compared to previous inhabitants, possibly because activities were more oriented to the new trade with Europeans or to a prosperous agriculture in more humid climatic conditions that prevailed from 1500 to 1800 CE

The peopling of mangroves in Senegal in the late Holocene: relationship with climate and political changes in West Africa

International audienceWe present a reconstruction of human demography and shell fishing activity in the Sine-Saloum mangrove Delta (Senegal) in the Late Holocene using the summed probability density (SPD) of radiocarbon dates in archaeological shell middens. We explore how this local history relates to the climatic and political history of West Africa. We find that traces of human presence were scarce from 6000 to 2000 yr BP, partly because the geomorphology of the estuary was less favourable to human settlements at that time. A specialized shell fishing population migrated massively to the Sine-Saloum around 2000 yr BP, at the end of the aridification trend that followed the African humid period. This population, likely coming from the northern coast in search of land and resources, fleeing the Saharan desertification and the subsequent warfare, found refuge in the coastal mangroves and reached a maximum activity at about 1700 yr BP. This period corresponds to the beginning of trans-Saharan trade, and to a political complexification that would give rise to the Ghana empire. The incoming migration may have occurred in two waves as suggested by two peaks in the SPD curve at 200-400 CE and 600-800 CE and by cultural differences within the Delta. Most sites in the Sine-Saloum islands were abandoned in the 14th century, before the arrival of Europeans, possibly because intensive shell fishing was not sustainable anymore, or because of the regional political destabilization associated to the fall of the Ghana empire and the beginning of the Mali empire. Shortly after, in agreement with oral traditions, a new population lead by the Manding Guelwars, moved to the Sine Saloum after a military defeat and founded the modern towns. They had a reduced shellfishing activity compared to previous inhabitants, possibly because activities were more oriented to the new trade with Europeans or to a prosperous agriculture in more humid climatic conditions that prevailed from 1500 to 1800 CE

The peopling of mangroves in Senegal in the late Holocene: relationship with climate and political changes in West Africa

International audienceWe present a reconstruction of human demography and shell fishing activity in the Sine-Saloum mangrove Delta (Senegal) in the Late Holocene using the summed probability density (SPD) of radiocarbon dates in archaeological shell middens. We explore how this local history relates to the climatic and political history of West Africa. We find that traces of human presence were scarce from 6000 to 2000 yr BP, partly because the geomorphology of the estuary was less favourable to human settlements at that time. A specialized shell fishing population migrated massively to the Sine-Saloum around 2000 yr BP, at the end of the aridification trend that followed the African humid period. This population, likely coming from the northern coast in search of land and resources, fleeing the Saharan desertification and the subsequent warfare, found refuge in the coastal mangroves and reached a maximum activity at about 1700 yr BP. This period corresponds to the beginning of trans-Saharan trade, and to a political complexification that would give rise to the Ghana empire. The incoming migration may have occurred in two waves as suggested by two peaks in the SPD curve at 200-400 CE and 600-800 CE and by cultural differences within the Delta. Most sites in the Sine-Saloum islands were abandoned in the 14th century, before the arrival of Europeans, possibly because intensive shell fishing was not sustainable anymore, or because of the regional political destabilization associated to the fall of the Ghana empire and the beginning of the Mali empire. Shortly after, in agreement with oral traditions, a new population lead by the Manding Guelwars, moved to the Sine Saloum after a military defeat and founded the modern towns. They had a reduced shellfishing activity compared to previous inhabitants, possibly because activities were more oriented to the new trade with Europeans or to a prosperous agriculture in more humid climatic conditions that prevailed from 1500 to 1800 CE

The peopling of mangroves in Senegal in the late Holocene: relationship with climate and political changes in West Africa

International audienceWe present a reconstruction of human demography and shell fishing activity in the Sine-Saloum mangrove Delta (Senegal) in the Late Holocene using the summed probability density (SPD) of radiocarbon dates in archaeological shell middens. We explore how this local history relates to the climatic and political history of West Africa. We find that traces of human presence were scarce from 6000 to 2000 yr BP, partly because the geomorphology of the estuary was less favourable to human settlements at that time. A specialized shell fishing population migrated massively to the Sine-Saloum around 2000 yr BP, at the end of the aridification trend that followed the African humid period. This population, likely coming from the northern coast in search of land and resources, fleeing the Saharan desertification and the subsequent warfare, found refuge in the coastal mangroves and reached a maximum activity at about 1700 yr BP. This period corresponds to the beginning of trans-Saharan trade, and to a political complexification that would give rise to the Ghana empire. The incoming migration may have occurred in two waves as suggested by two peaks in the SPD curve at 200-400 CE and 600-800 CE and by cultural differences within the Delta. Most sites in the Sine-Saloum islands were abandoned in the 14th century, before the arrival of Europeans, possibly because intensive shell fishing was not sustainable anymore, or because of the regional political destabilization associated to the fall of the Ghana empire and the beginning of the Mali empire. Shortly after, in agreement with oral traditions, a new population lead by the Manding Guelwars, moved to the Sine Saloum after a military defeat and founded the modern towns. They had a reduced shellfishing activity compared to previous inhabitants, possibly because activities were more oriented to the new trade with Europeans or to a prosperous agriculture in more humid climatic conditions that prevailed from 1500 to 1800 CE