Multi-proxy characterization of Saharan dust deposited on the West African margin at present : links with source regions and transport patterns, and implications for the interpretation of the Northeastern Atlantic Tropical sedimentary archives

L’étude des dépôts du matériel terrigène éolien dans les archives sédimentaires de l’Atlantique Tropical Nord-Est (ATNE) contribue à documenter les changements paléoclimatiques dans cette région. Toutefois, l’interprétation de ces enregistrements est rendue difficile puisque les outils employés dans les sédiments marins pour retracer les régions sources et les systèmes de transport atmosphérique associés sont encore mal contraints. Une étude des dépôts éoliens dans le contexte climatique actuel a été réalisée sur la marge Ouest africaine dans l’objectif d’améliorer notre compréhension des traceurs. Dans ce cadre, un capteur de dépôts de particules éoliennes a été mis en place en 2006 sur la côte sénégalaise permettant d’obtenir une série temporelle de dépôts éoliens continue dont le pas d’échantillonnage est inférieur ou égal à 7 jours. La majeure partie de cette étude consiste en une analyse multi-proxy - incluant granulométrie, minéralogie des argiles et rapports isotopiques du strontium et du néodyme - des dépôts éoliens collectés entre 2006 et 2009, et à l’élaboration d’une calibration de ces traceurs en terme de provenance et de système de transport dans le contexte climatique actuel. Cette calibration a ensuite été (1) testée sur les sédiments de surface de l’ATNE puis (2) utilisée pour déchiffrer le signal terrigène éolien enregistré au cours des derniers 26 000 ans au niveau du site MD03-2705 (18°05N ; 21°09W). Ce travail de thèse contribue à évaluer dans quelle mesure cette première calibration des traceurs terrigènes éoliens à l’actuel permet d’améliorer notre compréhension du signal terrigène éolien en ATNE.The study of the aeolian material in the sedimentary archives of the NEATO contributes to document the paleoclimatic changes in this region. However, the interpretation of these records is complicated by the fact that proxies used in the deep sea sediments to reconstruct the provenance regions and the associated atmospheric transport patterns remain poorly constrained. A study of the aeolian deposits in the present-day climatic context was carried out on the West African margin in order to improve our understanding of the proxies. In this frame, a mineral dust deposits trap was installed in 2006 on the Senegalese coast, providing a unique aeolian deposits time series with a temporal resolution better than a week. The first part of this study consists in a multi-proxy analysis of the aeolian deposits collected from 2006 to 2009 (including grain-size distribution, clay mineralogy and strontium and neodymium isotopic ratios) and the subsequent development of a calibration of these tracers in terms of provenance regions and transport patterns in the present-day climatic context. In a second part, this calibration was (1) tested against sea surface sediments of the NEATO and then (2) used to decipher the aeolian terrigenous signal recorded during the last 26 000 years in the MD03-2705 core (18°05N; 21°09W). This PhD work contributes to document in which ways this initial present-day calibration of the aeolian proxies can contribute to improve our understanding of the terrigenous signal in NEATO

Geochemical provenance of sediments from the northern East China Sea document a gradual migration of the Asian Monsoon belt over the past 400,000 years

The reconstruction of the long-term evolution of the East Asian Monsoon remains controversial. In this study, we aim to give a new outlook on this evolution by studying a 400 kyr long sediment record (U1429) from the northern East China Sea recovered during IODP Expedition 346. Neodymium isotopic ratios and rare earth element concentrations of different grain-size fractions reveal significant provenance changes of the sediments in the East China Sea between East Asian continental sources (mainly Yellow River) and sediment contributions from the Japanese Archipelago. These provenance changes are interpreted as the direct impact of sea level changes, due to the reorganization of East Asian river mouth locations and ocean circulation on the East China Sea shelf, and latitudinal shifts of the intertropical convergence zone (ITCZ) from the interior of Asia to the western North Pacific Ocean. Our data reveal the dominance of winter and summer monsoons during glacial and interglacial periods, respectively, except for glacial MIS 6d (∼150–180 ka) during which unexpected summer monsoon dominated conditions prevailed. Finally, our data suggests a possible strengthening of the interglacial summer monsoon rainfalls over the East Asian continent and Japan throughout the past 400 kyr, and between MIS 11 and MIS 5 in particular. This could result from a gradual northward migration of the ITCZ

Geochemical provenance of sediments from the northern East China Sea document a gradual migration of the Asian Monsoon belt over the past 400,000 years

The reconstruction of the long-term evolution of the East Asian Monsoon remains controversial. In this study, we aim to give a new outlook on this evolution by studying a 400 kyr long sediment record (U1429) from the northern East China Sea recovered during IODP Expedition 346. Neodymium isotopic ratios and rare earth element concentrations of different grain-size fractions reveal significant provenance changes of the sediments in the East China Sea between East Asian continental sources (mainly Yellow River) and sediment contributions from the Japanese Archipelago. These provenance changes are interpreted as the direct impact of sea level changes, due to the reorganization of East Asian river mouth locations and ocean circulation on the East China Sea shelf, and latitudinal shifts of the intertropical convergence zone (ITCZ) from the interior of Asia to the western North Pacific Ocean. Our data reveal the dominance of winter and summer monsoons during glacial and interglacial periods, respectively, except for glacial MIS 6d (∼150–180 ka) during which unexpected summer monsoon dominated conditions prevailed. Finally, our data suggests a possible strengthening of the interglacial summer monsoon rainfalls over the East Asian continent and Japan throughout the past 400 kyr, and between MIS 11 and MIS 5 in particular. This could result from a gradual northward migration of the ITCZ

Reconstruction of Ocean Circulation Based on Neodymium Isotopic Composition: Potential Limitations and Application to the Mid-Pleistocene Transition

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Environmental Hf–Nd isotopic decoupling in World river clays

The hafnium and neodymium radiogenic isotope systems behave differently during Earth surface processes, causing a wide dispersion of Hf and Nd isotopic compositions in sediments and other sedimentary rocks. The decoupling between Hf and Nd isotopes in sediments is generally attributed to a combination of preferential sorting of zircon during sediment transport and incongruent weathering processes on continents. In this study, we analysed size-fractions of sediment samples collected near the mouth of 53 rivers worldwide to better understand the factors controlling the distribution of Hf and Nd isotopes in sediments. Our results for rivers draining old cratonic areas and volcanic provinces demonstrate that both granite and basalt weathering can lead to significant grain-size dependent Hf isotopic variability. While silt-size fractions mainly plot along the Terrestrial Array, World river clays are systematically shifted towards more radiogenic Hf isotopic compositions, defining together with published data a new Clay Array (εHf=0.78×εNd+5.23εHf=0.78×εNd+5.23). The Hf–Nd isotope decoupling observed in volcanogenic sediments is best explained by selective alteration of Lu-rich mineral phases (e.g. olivine) and preferential enrichment of resistant unradiogenic minerals, such as spinel and ilmenite, in silt fractions. We also show that the extent to which World river clays deviate from the Clay Array (ΔεHf clayΔεHf clay) is not linked to the presence of zircons. Instead, it correlates positively with weathering indices and climatic parameters (temperature, rainfall) of the corresponding drainage basins. Overall, these findings demonstrate that the distribution of Hf–Nd isotopes in clay-size sediments is related to a large extent to weathering conditions on continents, although the precise mechanisms controlling this relationship remain unclear. We finally propose that the Hf–Nd isotope pair proxy could be used in palaeoenvironmental studies to provide semi-quantitative information on past climates

Eastern Atlantic deep-water circulation inferred from neodymium and carbon isotopic compositions over the past 1200 thousand years

International audienceMid-Pleistocene transition (MPT; 1200 to 800 thousand years, ky) is marked by an intensification and a shift of glacial-interglacial cycle from 41 ky to 100 ky that is not directly supported by solar insolation. These changes can be explained by combined effects of the stabilization of ice sheets associated with regolith removal withatmospheric CO2 reduction caused by active biological pumping in the Southern Ocean. Significant changes in ocean circulation was also suggested.To improve the spatiotemporal coverage of past ocean circulation records, we analysed Nd isotopic ratios recorded in foraminiferal authigenic fractions and epibenthic foraminiferal stable isotopes on two cores: MD03-2705 (18◦06’N, 21◦09’W, 3085 m) in the eastern tropical Atlantic where the reconstruction based on Nd isotopes is absent for this timescale and ODP1085 (29◦22’S, 13◦59’E, 1713 m) that is located at the present boundary between a northern source water (NSW, North Atlantic Deep Water) and a southern source water (SSW, Antarctic Bottom Water).Sequential cleaning and SEM observation of foraminiferal tests of MD03-2705 indicate that Nd isotopic ratios of authigenic phases reflect seawater values despite the fact that the core is located under Saharan dust plume. Reconstructed seawater Nd values of MD03-2705 ranges between -12.7 and -10.3 with more radiogenic values during glacial periods, suggesting an increase in SSW proportion. The glacial-interglacial eNd amplitude is smaller for the MPT than for the post-MPT. Epibenthic d13C values of ODP1085 varies from -0.6 to 0.9h with a clear positive shift at MIS 13 that is not observed for core MD03-2705. These results suggest the reorganisation of Atlantic meridional overturning circulation since the MPT. The analysis of eNd for core ODP1085 is in progress.We will discuss variability in relative proportion between SSW and NSW in relation to carbon cycle by combining reconstructed seawater eNd and d13C values from the two cores with available record

Eastern Atlantic deep-water circulation inferred from neodymium and carbon isotopic compositions over the past 1200 thousand years

International audienceMid-Pleistocene transition (MPT; 1200 to 800 thousand years, ky) is marked by an intensification and a shift of glacial-interglacial cycle from 41 ky to 100 ky that is not directly supported by solar insolation. These changes can be explained by combined effects of the stabilization of ice sheets associated with regolith removal withatmospheric CO2 reduction caused by active biological pumping in the Southern Ocean. Significant changes in ocean circulation was also suggested.To improve the spatiotemporal coverage of past ocean circulation records, we analysed Nd isotopic ratios recorded in foraminiferal authigenic fractions and epibenthic foraminiferal stable isotopes on two cores: MD03-2705 (18◦06’N, 21◦09’W, 3085 m) in the eastern tropical Atlantic where the reconstruction based on Nd isotopes is absent for this timescale and ODP1085 (29◦22’S, 13◦59’E, 1713 m) that is located at the present boundary between a northern source water (NSW, North Atlantic Deep Water) and a southern source water (SSW, Antarctic Bottom Water).Sequential cleaning and SEM observation of foraminiferal tests of MD03-2705 indicate that Nd isotopic ratios of authigenic phases reflect seawater values despite the fact that the core is located under Saharan dust plume. Reconstructed seawater Nd values of MD03-2705 ranges between -12.7 and -10.3 with more radiogenic values during glacial periods, suggesting an increase in SSW proportion. The glacial-interglacial eNd amplitude is smaller for the MPT than for the post-MPT. Epibenthic d13C values of ODP1085 varies from -0.6 to 0.9h with a clear positive shift at MIS 13 that is not observed for core MD03-2705. These results suggest the reorganisation of Atlantic meridional overturning circulation since the MPT. The analysis of eNd for core ODP1085 is in progress.We will discuss variability in relative proportion between SSW and NSW in relation to carbon cycle by combining reconstructed seawater eNd and d13C values from the two cores with available record

Rapid termination of the African Humid Period triggered by northern high-latitude cooling

International audienceThe rapidity and synchrony of the African Humid Period (AHP) termination at around 5.5 ka are debated, and it is unclear what caused a rapid hydroclimate response. Here we analysed the hydrogen isotopic composition of sedimentary leaf-waxes (δD wax) from the Gulf of Guinea, a proxy for regional precipitation in Cameroon and the central Sahel-Sahara. Our record indicates high precipitation during the AHP followed by a rapid decrease at 5.8-4.8 ka. The similarity with a δD wax record from northern East Africa suggests a large-scale atmospheric mechanism. We show that northern high-and mid-latitude cooling weakened the Tropical Easterly Jet and, through feedbacks, strengthened the African Easterly Jet. The associated decrease in precipitation triggered the AHP termination and combined with bio-geophysical feedbacks to result in aridification. Our findings suggest that extratropical temperature changes, albeit smaller than during the glacial and deglacial, were important in triggering rapid African aridification during the Holocene