The objective of this research is to interpret high-resolution palaeo-proxy data sets to understand the Asian summer monsoon variability in the past. This was done by synthesizing published palaeo-records from the Asian monsoon region, model simulation comparisons, and analysing new lake sedimentary records from northeast Thailand. Palaeo-records and climate modeling indicate a strengthened summer monsoon over Mainland Southeast Asia during the Last Glacial Maximum (LGM), compared to dry conditions in other parts of the Asian monsoon region. This can be explained by the LGM sea level low stand, which exposed Sundaland and created a large land-sea thermal contrast. Sea level rise ~19 600 years before present (BP), reorganized the atmospheric circulation in the Pacific Ocean and weakened the summer monsoon between 20 000 and 19 000 years BP. Both the Mainland Southeast Asia and the East Asian monsoon hydroclimatic records point to an earlier Holocene onset of strengthened summer monsoon, compared to the Indian Ocean monsoon. The asynchronous evolution of the summer monsoon and a time lag of 1500 years between the East Asian and the Indian Ocean monsoon can be explained by the palaeogeography of Mainland Southeast Asia, which acted as a land bridge for the movement of the Intertropical Convergence Zone. The palaeo-proxy records from Lake Kumphawapi compare well to the other data sets and suggest a strengthened summer monsoon between 10 000 and 7000 years BP and a weakening of the summer monsoon thereafter. The data from Lake Pa Kho provides a picture of summer monsoon variability over 2000 years. A strengthened summer monsoon prevailed between BC 170-AD 370, AD 800-960 and since AD 1450, and was weaker about AD 370-800 and AD 1300-1450. The movement of the mean position of the Intertropical Convergence Zone explains shifts in summer monsoon intensity, but weakening of the summer monsoon between 960 and 1450 AD could be affected by changes in the Walker circulation. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript..</p>

There is little consensus on what forced the climate of southern Africa to change during the Last Glacial Maximum (LGM). Because of southern Africa's latitudinal position, changes in seasonal precipitation can help resolve the influence of internal climate factors such as groundwater and external climate forcers such as large scale atmospheric circulation patterns. This paper presents a simple model of groundwater discharge based on permeability and topography in comparison with general circulation model precipitation results and paleoenvironmental proxy records. Results show that during the LGM the Intertropical Convergence Zone (ITCZ) likely weakened and moved slightly further south while the westerlies likely expanded slightly northward, with no significant change in strength. The climate and groundwater results were compared to the distribution of LGM and pre-LGM archaeological sites. Results show that the Later Stone Age peoples of southern Africa were likely inhabiting a relatively wet environment rather than an arid one.

There is little consensus on what forced the climate of southern Africa to change during the Last Glacial Maximum (LGM). Because of southern Africa's latitudinal position, changes in seasonal precipitation can help resolve the influence of internal climate factors such as groundwater and external climate forcers such as large scale atmospheric circulation patterns. This paper presents a simple model of groundwater discharge based on permeability and topography in comparison with general circulation model precipitation results and paleoenvironmental proxy records. Results show that during the LGM the Intertropical Convergence Zone (ITCZ) likely weakened and moved slightly further south while the westerlies likely expanded slightly northward, with no significant change in strength. The climate and groundwater results were compared to the distribution of LGM and pre-LGM archaeological sites. Results show that the Later Stone Age peoples of southern Africa were likely inhabiting a relatively wet environment rather than an arid one.

La haute montagne est un terrain particulièrement sensible aux variations climatiques. La hausse de température depuis plusieurs décennies a un fort impact sur les parois du massif du Mont Blanc : la dégradation du permafrost s’y traduit par une activité gravitaire majeure. Une augmentation du nombre d'écroulements (>100 m3) liés à des périodes chaudes a en effet été mis en évidence à plusieurs échelles de temps, lors des étés particulièrement chauds de 2003 et 2015 comme au cours des trois dernières décennies. La fréquence des écroulements dans le massif devrait continuer à s’accroitre avec l’augmentation de la température au 21e siècle.En revanche, la fréquence des écroulements dans le massif antérieurement à la fin du Petit Âge Glaciaire (PAG) est très largement inconnue. Pendant l'Holocène voire le Tardiglaciaire, les écroulements dans le massif du Mont Blanc ont-ils également été favorisés par les hausses de température ? Pour répondre à cette question, cette thèse poursuit quatre objectifs :i. Dater un grand nombre d'écroulements dans la partie centrale du massif pour comprendre leur distribution pendant l'Holocène et le Tardiglaciaire. L'âge des niches d’arrachement est obtenu par datation cosmogénique.ii. Vérifier les possibles corrélations entre périodes à forte occurrence d’écroulements et périodes climatiques post-glaciaires.iii. Quantifier le volume des écroulements par reconstruction 3D de la forme des blocs écroulés, et étudier la relation entre volumes écroulés et périodes climatiques.iv. Etudier la relation entre âge d'exposition et couleur des niches d’arrachement quantifiée avec la spectroscopie de réflectance.Un total de 70 surfaces a été échantillonné dans les parois du massif au cours de trois campagnes de terrain en 2006, 2011, et 2015-2016. Les âges d'exposition de 63 surfaces ont été obtenus, compris entre 30 ± 20 ans et 100.50 ± 8.50 ka. Trois groupes d’âges peuvent être corrélés aux périodes climatiques chaudes que sont : les Périodes Chaudes de l'Holocène moyen (7.50 – 5.70 ka), l'Optimum de l'âge de Bronze (3.35 – 2.80 ka) et le Période Chaude Romaine (2.35 – 1.75 ka) ; un quatrième groupe d'âges est daté entre 4.91 et 4.32 ka. Le groupe d'âges le plus nombreux, entre 1.09 ka et l'Actuel, aux volumes généralement réduits, est interprété comme représentatif de l'activité gravitaire annuelle du massif avec le climat actuel.Les données spectrales des échantillons datés ont permis de développer un index de la couleur du granite (GRIGRI) par combinaison des valeurs de réflectivité de deux longueurs d'onde différentes. Cet index est corrélé avec l'âge d'exposition (R=0.861) ; il a permis de proposer la datation de 10 échantillons d'âge inconnu à partir de leurs caractéristiques spectrales. / High mountain is particularly sensitive to climate variations. The raising temperature that is currently taking place due to climate change has a strong impact on the Mont Blanc massif rock walls: a higher rockfall (>100 m3) occurrence has been noticed, caused by permafrost thawing. The raising in number of rockfalls has been successfully correlated to warm periods at different timescales, e.g., during extreme warm episodes like the 2003 and 2015 heat waves, and during the last 30 years. According to the expected raising temperatures, during the 21st century rockfall occurrence should continue to rise.Rockfall frequency in the Mont Blanc massif before the Little Ice Age is still largely unknown. During Lateglacial and Holocene, high occurrence has been related to warm periods as well? In order to answer this question, this PhD thesis has four aims:i. To date several rockfalls having taken place in the central part of the Mont Blanc massif, in order to understand their frequency during Lateglacial and Holocene. Exposure age of rockfall scars is obtained using Terrestrial Cosmogenic Nuclide dating.ii. To verify possible relationships between high rockfall occurrence periods and post-glacial climate periods.iii. To quantify rockfall volumes by means of 3D reconstruction of the rockfall shapes, to explore the possible relationship between cumulate volumes and climate periods.iv. To study the relationship between exposure ages and colours of rock surfaces. Colours are quantified by reflectance spectroscopy.A total of 70 rock surfaces have been sampled during three field campaigns that took place in 2006, 2011 and 2015-2016. 63 exposure ages were obtained, ranging 30 ± 20 a to 100.50 ± 8.50 ka. Three age clusters can be correlated to warm periods, corresponding to: two Holocene Warm Periods (7.50 – 5.70 ka), the Bronze Age Optimum (3.35 – 2.80 ka) and the Roman Warm Period (2.35 – 1.75 ka). A fourth age cluster has been detected with ages ranging 4.91 – 4.32 ka. The biggest cluster, ranging 1.09 ka – recent, shows rather small volumes. This is interpreted as the normal erosion activity corresponding to the current climate.The samples reflectance spectra allowed to develop a granite colour index (GRIGRI) by combining the values of two different wavelengths. This index is correlated to the samples exposure age (R = 0.861), and has been used to date the exposure age of 10 samples where Terrestrial Cosmogenic Nuclide dating failed

A regiäo de Monte Verde (Camanducaia, MG) está sìtuada a 1500 m de altitude, na porção sul da serra da Mantiqueira. Alvéolos do relevo serrano, como o entroncamento dos córregos do cadete e da Minhoca com o rio Jaguari, junto à sua área urbana, propìciaram o acúmulo de sucessões métricas de sedimentos argilo-arenosos em condiçöes favoráveis à formação de material turfoso e à preservação de palinomorfos. Nesta área, o presente estudo reconstitui, a partir da integração dos registros palinológico, sedimentológico (granulometria e minerais pesados) e geoquímico/isotópico (C e N) de três testemunhos rasos (até 230 cm), a evoluçäo e os possíveis controles sedimentares e paleoclimáticos do cenário paleoflorístico no Quaternário tardio, com especial atençäo para a Floresta de Araucária. O contexto geral registrado nos testemunhos é de planície de inundação fluvial, com cobertura arbórea próxima, e influência varìável de fluxos de encosta. Na parte montante do vale do cadete, obtiveram-se idades compreendidas entre 38695 - 40522 anos cal A.P., em 220 cm de profundidade, e 2060 - 1880 anos cal 4.P., em 5 cm. Na parte inferior da coluna, até cerca de 16000 anos A.P., há indícios de aumento de distalidade para cima. Entre cerca de 16000 e 3500 anos A.P., evidencia-se redução da influência da matéria orgânica de plantas terrestres, em detrimento de algas, possivelmente em momento de abeÍura relativa da cobertura florestal. Os últimos 3500 anos seriam de restabelecimento da cobeÍura florestal, possivelmente já nos moldes da existente hoje, com redução gradual do aporte de areia. Mais a jusante do mesmo vale, as idades ficaram compreendidas entre 26764-26023 anos cal 4.P., em 210 cm de profundidade, e 2350 - 2150 anos cal A.P., em 10 cm, A parte inferior do intervalo, mais antiga que 20830-20370 anos cal A.P., é dominada por influência de áreas fontes locais, graníticas. Em 20000 anos A.P., fontes distais, metamórficas, passam a atuar, com aumento da influência de algas. Nos últimos 5000 a 6000 anos A.P., tem-se o readensamento da Floresta de Araucária, com manutenção de brejos, sob condiçöes climáticas frias e úmidas. Os resultados sugerem que as mudanças climáticas nos últimos 40 mil anos em Monte Verde não foram acentuadas a ponto de exercer grande impacto florístico e que houve predomínio de Araucaria e elementos associados a essa floresta durante todo intervalo estudado, sob condições climáticas frias e úmidas. / he Monte Verde region (Camanducaia, MG) is located in the southern portion of the Mantiqueira Range, with 1500 m of elevation. Relief alveoli, as the junction of cadete and Minhoca creeks with Jaguari River, sited close to the urban area, propitiated the accumulation of metric sucessions of sandy-mud sediments, under cond itions favorable to the formation of peat material and preservation of paìynomorphs. ln this area, the present study integrates palynological, sedimentological (grain size and heavy minerals) and geochemical/isotopic (C and N) records of three shallow cores (up to 230 cm deep) to reconstruct the evolution and possible sedtmentary and paleoclimatic controls of the Laie Quaternary paleofloristic scenario, with special emphasìs in the Araucaria Forest. The general sedimentary context recorded in the cores is a river floodplain close to tree cover areas, and with variable influence of slope flows. On the upstream Cadete valley, were obtained ages of 38695-40522 cal years BP, at the deep of 220 cm, and 2060-1880 cal years BP, at 5 cm. From the bottom of the column until about 16000 BP, there is evidence of increased distality upwards The time interval between about 16000 and 3500 yr BP shows a reduction in the influence of organic matter derived from terrestrial plants, to the detriment of algae, possibly in a moment of relative opening of the forest cover. The last 3500 years would be characterized by the restoration of forest cover, possibly similar to the existing today, with gradual reduction of the sand supply ln the downstream of the same valley, the ages vary from 26,764 fo 26,023 cal years BP, at 210 cm deep, to 2350 to 2150 cal years BP, at 10 cm. The lower part of the sedimentary column, older than 20830-20370 AP\' is dominated by the influence of local granitìc sources areas. ln 20000 years BP, distal metamorphic sources begin to act paralel with a increased influence of algae. over the last 5000-6000 years, a increase of Araucaria Forest is recorded, with maintenance of weflands under cold and wet climatic conditions. This results suggest that climate changes during the last 40 k years in Monte Verde region were not sufficient to exert great floristic impact. Araucaria and associated forest elements predominated during the whole studied interval, under cold and wet weather conditions.

Despite being separated by millions of years of evolution, black bears (Ursus americanus) and brown bears (Ursus arctos) can be difficult to distinguish based on skeletal and dental material alone. Complicating matters, some Late Pleistocene U. americanus are significantly larger in size than their modern relatives, obscuring the identification of the two bears. In the past, fossil bears have been identified based on differences in dental morphology or size. This study used geometric morphometrics to look at overall differences in cranial shape and used step-wise discriminant analysis to identify specific characters that distinguish cranial morphology between black and brown bears. Such differences could prove important in identifying fossil bears when crania are present but teeth are missing. Furthermore, being able to properly identify U. arctos and U. americanus crania is important in understanding evolutionary and ecological distinctions among both fossil and modern bears. Principal components, discriminant, and thin plate spline analyses indicated a clear morphological separation between the crania of U. americanus and U. arctos and highlighted key identifying features including a more convex forehead and a narrower, more elongate rostrum in U. arctos than U. americanus.

Quaking aspen (Populus tremuloides) has the largest natural distribution of any tree native to North America, ranging from Alaska through the breadth of Canada and south to mid-Mexico. The Laurentide ice sheet occupied most of the current range of P. tremuloides until the late Pleistocene epoch, so this species has undergone a significant, geologically recent range expansion. Surprisingly, range-wide patterns of genetic variation in P. tremuloides have never been described. Using a sample set representing the full longitudinal and latitudinal extent of the species distribution, I have conducted a phylogeographic analysis for P. tremuloides. Preliminary results comparing both nuclear and chloroplast DNA sequences revealed surprisingly low levels of divergence across the range. Because of this remarkably shallow genetic divergence among aspen populations, I used a set of rapidly-evolving molecular markers (microsatellites) to describe patterns of gene flow and diversity and to correlate those patterns with landscape features and histories. I analyzed eight microsatellite loci in 794 individuals from 30 sampling sites. From this multilocus data set, I identified pronounced genetic structuring across the range. Strikingly, sampling sites representing the southwestern portion of the range, the western United States and Mexico, form a distinct cluster. Sites within this southwestern cluster display dramatically reduced within-site genetic diversity but elevated regional genetic diversity, which suggests that populations in the southwestern portion of the range make up a stable edge persisting through multiple climate oscillations. Based on the uniqueness of the southwestern cluster and the climatic differences between the southwest and northern portions of the range, I propose that the southwestern cluster may represent a distinct ecotype. I also identified hotspots of diversity that correspond with potential refugia during the last glacial maximum but additional work is needed to refine these patterns. Further, my findings provide a solid foundation for a range of future studies on adaptive genetic and trait variation in this species.

<p>Tephrochronology, or the dating and correlation of stratigraphic sediment sequences by the use of tephra, is a dating method that has been much refined in the last decade. This refinement has enabled the detection of cryptotephra, which has expanded the known distribution of different tephras. At the site Lake Mulakullegöl in the southwest of Sweden, three distinct tephra horizons have been detected, of which two have been geochemically identified. The identified tephras are the Vedde Ash (ca 12,000 cal. yrs BP) and the Hässeldalen Tephra (ca 11,500 cal. yrs BP), both of Icelandic origin. These tephras were deposited during the middle of the Younger Dryas Stadial and the Younger Dryas/Preboreal transition, respectively. The Vedde Ash is one of the most widespread tephras of Icelandic provenance, while the Hässeldalen Tephra so far only has been found at two sites in southeastern Sweden, prior to the current study. A third, unidentified, tephra was also detected. Based on the stratigraphic position, this tephra might be the Borrobol Tephra, which has been dated to ca 14,400 cal. yrs BP. This might indicate that warming had not yet occurred in this part of Sweden. The extended known distribution of especially the Hässeldalen Tephra, but also the Vedde Ash, has enabled a higher potential for correlations between different records deposited during the Last Glacial/Interglacial transition.</p>

Tephrochronology, or the dating and correlation of stratigraphic sediment sequences by the use of tephra, is a dating method that has been much refined in the last decade. This refinement has enabled the detection of cryptotephra, which has expanded the known distribution of different tephras. At the site Lake Mulakullegöl in the southwest of Sweden, three distinct tephra horizons have been detected, of which two have been geochemically identified. The identified tephras are the Vedde Ash (ca 12,000 cal. yrs BP) and the Hässeldalen Tephra (ca 11,500 cal. yrs BP), both of Icelandic origin. These tephras were deposited during the middle of the Younger Dryas Stadial and the Younger Dryas/Preboreal transition, respectively. The Vedde Ash is one of the most widespread tephras of Icelandic provenance, while the Hässeldalen Tephra so far only has been found at two sites in southeastern Sweden, prior to the current study. A third, unidentified, tephra was also detected. Based on the stratigraphic position, this tephra might be the Borrobol Tephra, which has been dated to ca 14,400 cal. yrs BP. This might indicate that warming had not yet occurred in this part of Sweden. The extended known distribution of especially the Hässeldalen Tephra, but also the Vedde Ash, has enabled a higher potential for correlations between different records deposited during the Last Glacial/Interglacial transition.

Earth is characterized by episodes of glaciations and periods of minimal or no ice through geologic time. Using the linear energy balance model (EBM), nonlinear EBM with empirical ice sheet schemes, the general circulation model coupled with an ice sheet model, this study investigates the occurrence and stability of glaciations in geologic time. The simulations since the last glacial maximum (LGM) suggest that the summertime thawline of ice sheets conforms closely to the equatorward edge of the ice sheets and implies the relative stability toward deglaciation. CO2 levels are indispensable in controlling the initiation of ice sheet in the Cretaceous. At low CO2 levels, ice sheets exist in all periods no matter LGM or the last interglacial (LIG) orbital elements; however, at high CO2 levels ice sheets rarely exist. The simulations agree well with recent geological evidence of the hysteresis of glaciations in the Permo-Carboniferous. Gondwanaland reached its glacial maximum when CO2 level was roughly the same or slightly higher than the preindustrial value. With a further increase of CO2, deglaciation dominates and results in an ice free state. Again, if CO2 decreased to the present level, Gondwanaland would be glaciated once more and start a new cycle of glaciation and deglaciation. Simulations from five paleogeography maps in Gondwanaland with a suite of CO2 levels and different orbital elements reveal that paleogeography, CO2 levels and the Milankovitch cycles all contribute to the glaciations of Gondwanaland. This study shows that orbital elements alone are insufficient to account for the evolution of ice sheets. Net radiative forcing caused by greenhouse gases, such as CO2 and solar constant change are the primary drivers to glacial inception or demise. Continental geography, CO2 levels, solar constant change, and the Milankovitch cycles complicate the glacial history of Earth.