Despite the recognized importance of understanding late Holocene climatic changes in southern Africa, a dearth of available evidence has resulted in a frag- mented view of the recent past. South Africa has been identified as a key focus region for palaeoclimatic studies, as it reflects the dynamics of both tropical and temperate climate regimes. This study aims to create a catchment-integrated view of palaeoenvironmental conditions in the country's Winter Rainfall Zone (WRZ) and Southern Benguela region during the late Holocene, in order to establish the linkages between oceanic and terrestrial climate components. A multiproxy analysis was conducted on a west coast mudbelt sediment core, where continuous deposition over the last 2,250 years has aided the production of an uninterrupted age model. Alkenone and isotopic analyses were conducted at a multi-decadal resolution to assist in the reconstruction of sea-surface tem- perature (SST) in the St. Helena Bay region, and hydrological variation in the WRZ. Changes in moisture availability were inferred via the application of in- organic proxies, including grain size variation (promoted through the use of an end-member proxy algorithm) and the Fe/K ratio, a proxy interpreted to be rep- resentative of changes in chemical weathering. Furthermore, a newly-developed proxy, TEX 86 , was used as an alternative palaeothermometer. TEX 86 produced an additional record of SST, independent of alkenones, which elicited a com- parative study between SST TEX86 and SST UK'37 . The comparison facilitated a review of the effectiveness of TEX 86 within the Benguela Upwelling System, and assisted in quantifying reasons for the observed differences between the two methods. The results of the multiproxy analysis sheds new light on southwest African late Holocene climatic dynamics. This study documents a decrease in SST accompanied by increasing WRZ rainfall, which is hypothesized to be a result of large-scale changes in the position and/or intensity of the austral westerly wind belt. A northerly migration/increase in intensity of the winds acts to produce cooler SSTs and wetter west coast continental conditions, both of which were most acutely experienced during the so-called Little Ice Age (LIA) (1300 - 1850 CE). Zonal symmetry across the Southern Hemisphere is hypothesized to be a consequence of the large geographical extent of the westerly winds, as cooler and wetter conditions have been experienced in coastal, winter-rainfall areas of Chile and Australia.

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It is widely accepted that the late Pliocene spans a time with globally warmer conditions compared to today. Regional specifics in temperature patterns from this period, however, remain poorly known. In this study, we reconstruct quantitatively late Pliocene climates for eight sites at the southeastern border of the Qinghai-Tibet Plateau (SBTP), based on palaeobotanical data compiled from published sources using the Coexistence Approach (CoA), and analyze anomalies with respect to modern climates. The reconstructed temperatures indicate that in the late Pliocene, the northwestern part of the study area was cooler than its southern part. This spatial differentiation in temperature was largely due to differences in altitude: the northwest of the SBTP probably had higher altitudes than the south at that time. Mean annual temperatures (MATs) were around 1. °C higher than today, suggesting a cooling trend since the late Pliocene. Our data show that summer temperatures have declined significantly since the late Pliocene while winter temperatures have remained similar to those of the present, different from observations in other territories. The unexpected summer and winter temperature changes can be explained by the regional orogenic uplift plus the global cooling. The eastward extrusion of the Qinghai-Tibet Plateau might have blocked the southward cold high pressure of the winter monsoon and forced it to circumvent the eastern flank of the plateau, weakening its impact on the SBTP. The post-Pliocene mountain uplift increased the overall altitude of the region, which caused the temperature decline for both summer and winter. The reconstructed summer precipitation was lower while the winter precipitation was higher than today, suggesting a weaker monsoon climate during the late Pliocene.

Cette thèse étudie la dynamique de la végétation Holocène pour Burmarrad dans le NW Malte et fournit une reconstruction paléoclimatique quantitative à base de pollen pour cet archipel méditerranéen situé au centre. Le record de pollen donne un nouvel aperçu l'échange végétation 7280-1730 cal BP qui correspondent bien avec other régional dossiers. La reconstruction du climat fournit également forte corrélation avec les sites du sud (en dessous de 40oN) de la Méditerranée. L'interprétation suggère un paysage initialement ouvert au début du néolithique, se développer en un dense Pistacia brousse ca. 6700 calBP. Depuis environ 4450 calBP le paysage devient de nouveau ouvert, coïncidant avec le début de l'âge du bronze sur l'archipel. Cette période coïncide avec une instabilité accrue du climat (entre 4500 et 3700 calBP) qui est suivie par une diminution progressive de la disponibilité de l'humidité de l'été à la fin de l'Holocène. Durant la période romaine début de l'occupation (1972-1730 calBP) le paysage reste généralement ouvert avec une augmentation modérée de Olea. Cette augmentation correspond à des preuves archéologiques pour la production de l'huile d'olive dans la région, avec l'augmentation des taxons cultivés des cultures et des espèces rudérales associés, ainsi que d'une hausse des cas d'incendies. Cette thèse propose également une synthèse des résultats d'un autre noyau (BM1) provenant de la même zone de chalandise, ainsi que les résultats d'une étude préliminaire de la pluie de pollen de surface moderne. L'archipel fournit des indications sur la végétation, les impacts humains et les changements climatiques dans un contexte de l'île au cours de l'Holocène. / This thesis investigates the Holocene vegetation dynamics for Burmarrad in north-west Malta and provides a pollen-based quantitative palaeoclimatic reconstruction for this centrally located Mediterranean archipelago. The pollen record from this site provides new insight into the vegetation changes from 7280 to 1730 cal BP which correspond well with other regional records. The climate reconstruction for the area also provides strong correlation with southern (below 40oN) Mediterranean sites. The interpretation suggests an initially open landscape during the early Neolithic, surrounding a large palaeobay, developing into a dense Pistacia scrubland ca. 6700 cal BP. From about 4450 cal BP the landscape once again becomes open, coinciding with the start of the Bronze Age on the archipelago. This period is concurrent with increased climatic instability (between 4500 and 3700 cal BP) which is followed by a gradual decrease in summer moisture availability in the late Holocene. During the early Roman occupation period (1972 to 1730 cal BP) the landscape remains generally open with a moderate increase in Olea. This increase corresponds to archaeological evidence for olive oil production in the area, along with increases in cultivated crop taxa and associated ruderal species, as well as a rise in fire events. This thesis also provides a synthesis with the results from another core (BM1) taken from the same catchment area, as well as results of a preliminary modern surface pollen rain study. The Maltese archipelago provides important insight into vegetation, human impacts and climatic changes in an island context during the Holocene.

L’étage Valanginien est caractérisé dans le registre sédimentaire par une excursion positive des isotopes du carbone (amplitude 1,5-2‰), appelée « Episode Weissert ». Cet événement coïncide avec des changements paléoenvironnementaux et paléoclimatiques majeurs. La formation de la province volcanique ignée du Paraná-Etendeka a été proposée comme principal facteur forçant contrôlant ces changements. Cependant, de récentes études démontrent que l’Episode Weissert précède systématiquement l’activité volcanique. Ainsi, les facteurs déclenchant qui contrôlent ces changements sont encore source de débat. Dans cette étude des analyses cyclostratigraphique, biostratigraphique, minéralogique et géochimique ont été réalisés sur des sites de moyennes et hautes latitudes (coupes d’Orpierre et de La Charce/Arnayon dans le Bassin Vocontien, site 765C dans la plaine abyssale Argo) dans le but (i) d’explorer la dynamique des changements paléoenvironnementaux à l’échelle locale versus globale et (ii) de discuter des possibles facteurs forçant contrôlant l’épisode Weissert. Les processus diagénétiques et sédimentologiques locaux, qui pouvaient modifier le signal paléoclimatique, ont été discuté avant des interprétations paléoenvironnementales. La calibration astronomique de l’épisode Weissert, réalisée sur la coupe d’Orpierre, a permis pour la première fois le calcul de flux détritiques et de nutriments dans le Bassin Vocontien. Ces résultats indiquent de rapides changements climatiques aux moyennes latitudes, marqués par des conditions plus humides pendant l’Episode Weissert et des conditions plus sèches au Valanginien supérieur. En parallèle de nouvelles données géochimiques et minéralogiques, réalisés sur le site ODP 765C, ont été intégrées à une compilation globale de données géochimique et minéralogique sur 23 coupes répartie dans cinq grands environnements. Trois ceintures climatiques latitudinales majeures ont été identifié durant le Valanginien : (i) une ceinture aride dans l’océan proto Nord Atlantique (~15-17°N), (ii) une ceinture climatique subtropical dans la marge NW téthysienne (~25-30°N), et (iii) une ceinture climatique chaude et tempéré aux sites de hautes latitudes (~53°S et ~40 à ~70°N). Il apparaît que durant cette période, des facteurs orbitaux et tectoniques peuvent avoir contrôlé la distribution de ces ceintures et les principaux changements durant l’Episode Weissert. / The Valanginian stage witnesses a positive carbon isotope excursion (amplitude 1.5-2‰) in the sedimentary records, the so-called « Weissert Episode ». This event coincides with major palaeoenvironmental and palaeoclimatic changes. The formation of the Paraná-Etendeka large igneous province has widely been proposed as the major driving force behind these changes. However recent investigations demonstrate that the Weissert Episode precedes the volcanic activity. Thus the driving force(s) behind these changes are still under debate. In the herein study, cyclostratigraphic, biostratigraphic, mineralogical and geochemical analyses have been performed on mid- and high latitudes sites (the Orpierre and Arnayon/La Charce sections in the Vocontian Basin, and the ODP hole 765C in the Argo abyssal plain) in order (i) to explore the dynamic of the local versus global processes in the palaeoenvironmental changes and (ii) to discuss the possible forcing factors behind the Weissert Episode. Local diagenetic and sedimentological processes that might bias the palaeoclimatic signals are highlighted and discussed before any palaeoenvironmental interpretation. The astronomical calibration of the Weissert Episode performed at Orpierre, allowed for the first time, the calculation of detrital and nutrient influxes into the Vocontian basin. They highlight rapid climate changes at mid latitudes, characterized by pronounced humid conditions during the Weissert Event (Early–Late Valanginian transition) and drier conditions in the Late Valanginian. In parallel, new geochemical and mineralogical analyses performed on the ODP hole 765C have been integrated to a global compilation of published geochemical and mineralogical data from 23 sections, located in five depositional environments. Three latitudinal belts have been recognized during the Valanginian : (i) an arid palaeolatitudinal belt in the Proto North Atlantic Ocean (~15–17°N), (ii) a subtropical climatic belt in the Northwestern Tethyan margin (~25–30°N) ; and (iii) a warm temperate palaeoclimatic belt in the high latitudes site (~53°S and ~40 to 70°N). It appears that during this period, both tectonic and orbital factors might have controlled the distribution of the palaeoclimatic belts and the major palaeoenvironmental changes during the Weissert Episode.