No description available.

In immature Jurassic shales of differing depositional environment from Great Britain (Ro<0.5) the C₃₂ hopane and C₂₉ sterane isomerisation provide an accurate indication of relative maturities. For mature Viking Graben samples (Ro>0.7) a reliable indication of relative maturity is gained if Tmax, Ts(Ts+Tm), C₂₉ββ/(ββ+αα) steranes and the Triaromatic Index are considered together. The most reliable parameters for the determination of depositional redox are pristane/phytane and the C₃₅/C₃₄ homohopane ratio, together with the concentrations of V, Cu, Ni and Mo. In the brackish lagoon of the Brora Coal Formation (BCF) low sulphate availability does not appear to have limited the pyritisation of iron relative to the open marine facies of the Brora Argillaceous Formation (BAF). Despite this δ³⁴Spyrite for BCF samples indicates that sulphate reduction took place in a closed system due to the rapid depletion of available sulphate. Only low concentrations of excess H₂S were available for incorporation within the organic matter, organic sulphur abundances in the BCF being much lower than for the BAF. In open-marine sequences an increase in TOC is associated with an increase in anoxia, the preservation of hydrogen-rich organic matter and the abundance of organic sulphur. A generally correlated increase in pyrite sulphur suggests both these modes of sulphur are associated with increasing concentrations of diagenetic sulphides (H₂S and its partial oxidation products) generated through bacterial sulphate reduction. The greatest abundances of organic sulphur are formed when sulphide concentrations are high whilst sub-oxic surficial sediments are maintained so that polysulphides are generated within the sediment (by partial oxidation of H₂S) for rapid sulphur incorporation. Improved preservation of labile organic matter leads to a increase in the abundance of organic compounds (e.g. sterenes and phytol derivatives) in which suitable functionalities exist for sulphur incorporation.

The evolution of the Metazoa is among the greatest success stories in Earth history. From modest origins, probably in the Cryogenian (~720 - 635 Ma), metazoans had acquired hard parts, and a vast range of life strategies and body plans by the middle Cambrian (around 520 Ma). This leaves a long delay between the origin of the Metazoa and their rise to ecological dominance. A popular explanatory hypothesis for this delay is that atmospheric oxygen levels, low in the Proterozoic (< 0.001 % PAL), began to rise towards modern levels towards the end of the Neoproterozoic. Among the earliest known putative metazoans are Namacalathus, Namapoikia and Cloudina, calcified marine invertebrates abundant in the latest Ediacaran (~ 548-541 Ma) Nama Group, Namibia. Although they were pioneers of metazoan biomineralisation, little is known of their affinities or palaeocology. The Nama Group, a well-characterised, relatively undeformed mixed carbonate and siliciclastic succession, provides a rare opportunity to investigate the palaeoecology of these important organisms in their environmental context. New geochemical data from the Nama Group confirm the heterogeneity of Ediacaran redox conditions. These contextualise in situ fossil assemblages which reveal diverse ecological strategies among the calcified metazoans of the Nama Group, and offer constraints on their affinities. Based on its large size (< 1 m), modular body plan and internal structure of interlinked tubules, Namapoikia was a long-lived specialist and possible Poriferan. I show that Namapoikia colonised both lithified and living microbial substrates in oxic, mid-ramp reef crypts. By contrast, size and occurrence data show that Namacalathus was an environmental generalist, forming large, thick aggregations in persistently oxic, mid-ramp reef environments but opportunistically exploiting the transiently oxic, inner ramp setting. Bilaterally symmetrical, asexual budding and a microlamellar skeletal ultrastructure suggest that Namacalathus may have been an early lophophorate, and had flexible growth depending on environmental setting, showing a cup diameter of 2 – 35 mm, and size distributions varying with substrate type, redox and water depth. In oxic mid-ramp reefs, Cloudina constructed large (> 20 m) reefs showing mutual attachment and consistent orientation in life position, making it the earliest known reef-building metazoan and suggesting that it was a passive suspension feeder. I further present food webs based on fossil assemblages from Ediacaran to Cambrian Stage 4 carbonate successions and evaluate their usefulness in tracking metazoan trophic diversification in the early Cambrian. Ediacaran redox conditions were a major control on the ecologies of the earliest metazoans. A requirement for oxygen made persistently oxic conditions a prerequisite for complex and long-lived ecologies, while highly flexible life strategies were used to exploit changeable environments. Ediacaran metazoans represent a phylogenetic and ecological foreshadowing of the complexity of the Phanerozoic, but it was not until much later that the Metazoa would attain their evolutionary potential.

No description available.

PhD - Science / Analysis of charcoal from Middle Stone Age layers at Sibudu Cave, KwaZulu-Natal, shows evidence of environmental change during the Last Glacial. Layers analysed encompass the end of the cold stadial, Oxygen Isotope Stage (OIS) 4, and the warmer interstadial, OIS 3. Layers are divided – on the basis of lithic industry associations and taxonomic content – into the Howiesons Poort (HP) (dated to ca. 61 000 years ago), and the early, middle and late post-HP assemblages (ca. 60-55 000, 55-50 000 and 50-33 000 years ago respectively). This project aims to identify evidence for environmental change, characterise this change using qualitative and quantitative analyses, and compare the results to evidence from other proxies at Sibudu Cave and elsewhere in South Africa. HP layers (GS, GR, GR2) are dominated by evergreen forest taxa, including Podocarpus spp., Buxus sp., Brachylaena sp., Sapium/Spirostachys and Ptaeroxylon obliquum. Kirkia sp. suggests a warm, woodland savanna habitat grew beyond the forest vegetation. Early post-HP layers (Eb, SPCA, BSp) contain taxa from evergreen, riverine forest communities, including Erica spp., Leucosidea sericea, and Rapanea melanophloeos. Some of the taxa in these layers suggest a shift in vegetation, possibly related to the marine regression of the Last Glacial, bringing taxa currently found further inland towards the site. Fewer evergreen forest components, and more bushveld taxa, are present in the middle post-HP (RSp, OMOD, MOD) than in the previous layers. Some of the taxa are only found in northern South Africa in regions that are significantly drier than modern KwaZulu-Natal. These layers also contain more Acacia spp. and other Fabaceae taxa and fewer Erica spp. than the samples from the early post-HP. This may be a result of environmental change, a change in wood selection, charcoal fragmentation, or sampling bias. Layer Bu, within the late post-HP, contains evergreen and deciduous taxa many of which are found in KwaZulu-Natal today. Kirkia sp. again provides evidence for a dry habitat not iv currently found in the region. Many Types were defined in Bu, which may indicate a vegetation community for which there is currently no reference material. Temperature and moisture indices produced using the Factor Analysis suggest warm and moist conditions during the HP layers. During the early post-HP conditions became cooler with intermediate moisture levels. Subsequently, conditions were warm and dry (middle post-HP) and then warm with a little more moisture indicated (late post-HP). Good fuelwoods were routinely collected during the post-HP. In contrast, the HP layers are dominated by Podocarpus spp. and many of the good fuelwoods, such as Acacia spp. and Erica spp., are absent. This pattern may be a result of changes in the environment, sample bias or a development of awareness of wood properties between the HP and post-HP occupations. The charcoal results corroborate palaeoenvironmental interpretations provided by seeds and macrofauna from Sibudu Cave. When the Sibudu Cave data are combined with data from other sites it is apparent that, although conditions in the summer rainfall region during the Last Glacial were generally dry, there is evidence for localised variation in vegetation and climate.

Climate change during the last 2.5 million years is characterised by glacial-interglacial cycles of fluctuating sea level and temperature increasing in magnitude and duration towards the present day. The central North Sea preserves these glacial-interglacial cycles in an expanded sedimentary sequence creating a high resolution palaeo-climatic record. Basin-wide, low-resolution 3D seismic data, covering more than 80,000 km2 of the central North Sea, is combined with high-resolution, broadband 3D seismic, regional 2D seismic and local ultra-high resolution seismic from the Dogger Bank windfarm development zone in order to investigate in full the sedimentary sequence. The evolution of the basin is analysed along with the preserved geomorphological landforms in order to build a framework for the development of the North Sea and its changing palaeo-environments from the inception of the Quaternary (2.58 Ma) until the extensive glacial unconformity formed during the Elsterian (0.48 Ma).At the onset of the Quaternary the structure of the North Sea was that of an elongate marine basin, rapidly infilled from the south by continued progradation of the large clinoformal deposits of the southern North Sea deltaic system. The basin rapidly decreased in extent and depth however it was not until around 1.1 Ma that the broad, shallow shelf of the present day was fully established. A revision of the current seismic stratigraphy is proposed, identifying four new Members within the Aberdeen Ground Formation taking into account the development of the basin through time. Powerful downslope gravity currents dominated the basin during much of the early Quaternary, although a well-established, anti-clockwise tidal gyre acted to gently modify the gravity currents. Iceberg scouring was nearly continual from the onset of the Quaternary until grounded ice sheets began to penetrate into the basin from 1.7 Ma, more than half a million years before any previous estimates. Effects of confluence of the British and Fennoscandian ice sheets are observed from 1.3 Ma. The tunnel valleys of the Dogger Bank represent a continuation of the North Sea tunnel valley network, interacting with both older glaciotectonic thrusting and younger glaciotectonic folded deformation.

Sclerochronology affirms the well-established fact that banded growth increments in marine molluscs accurately record oscillations in climate and environment for the past millennia or more. This study considers how such records can enhance understanding of environmental shifts across the 18th to 20th centuries. Specifically, it investigates whether sclerochronological data are fundamentally associated with stable carbon and oxygen isotope values and if the climate phenomenon of the Little Ice Age impacts the marine radiocarbon reservoir effect. Furthermore, this study compares the North Sea and the Irish Sea to discover whether observed environmental changes are equivocal or not. Three shells of Arctica islandica were retrieved from the Dutch natural history museum "Naturalis" in Leiden, the Netherlands, to construct a chronology dating back to the 18th century from growth increments. Collection dates for the three shells vary between 1882 and 1954 in the North Sea and the Irish Sea. Shells were prepared, photographed and milled for calcium carbonate material. Digital images allowed counting of increments while the milled material was measured for stable carbon and oxygen isotopes and was radiocarbon dated. The ontogenetic bias was removed from sclerochronological data using MATLAB software to produce a purely environmental growth signal. The results agreed with the proposed hypothesis that there is a link between growth increments and the stable isotope data. However, further analysis is required to validate the presence of the Little Ice Age in the radiocarbon reservoir effect. These results indicate that the stable carbon and oxygen isotopes and growth increments are a good indicator of favourable growth conditions for Arctica islandica. Therefore, this study highlights that Arctica islandica is a suitable proxy for the North Sea and Irish Sea palaeoenvironment reconstructions. On this basis, future climate research can accurately depend on sclerochronological data to aid in understanding the patterns of anthropogenic climate change.

Idéalement située à la confluence entre climat tropical et climat tempéré, l'Afrique australe est une zone très sensible aux variations des conditions climatiques. Cette région fait cependant preuve d'un manque de données paléoclimatiques important, et les méthodes de reconstruction traditionnelles trouvent rapidement leurs limites face aux conditions particulières qui y sévissent. Les méthodes de reconstruction quantitatives communément employées se révèlent inutilisables face aux conditions d'aridité extrêmes qui ne permettent que très rarement l'accumulation et la préservation de pollen moderne et fossile et se confrontent également aux particularités de la végétation abritée par cette région. L'objectif de ces travaux de thèse vise à développer une méthode de reconstruction quantitative basée sur des données polliniques fossiles, à partir de la relation entre la distribution actuelle des taxons polliniques et le climat en Afrique australe. Nous avons développé une fonction de transfert utilisant les propriétés des fonctions de densité de probabilité (pdfs), permettant de transformer l'information contenue dans un assemblage pollinique en estimation quantitative du climat. En parallèle, cette étude a permis de développer une méthode permettant de sélectionner les espèces les plus probables à inclure dans chaque type pollinique qui compose un assemblage. Cette méthode de sélection des espèces (SSM) a permis de pallier la faible résolution taxonomique des données polliniques de cette région caractérisée par une biodiversité élevée et d'affiner la méthode des espèces indicatrices, afin de la rendre utilisable en Afrique australe. Cette méthodologie a été appliquée aux données polliniques du site de Wonderkrater (Afrique du Sud). Les résultats observés et leur comparaison avec ceux provenant de sites régionaux ont permis de déterminer que les températures estivales et hivernales étaient 6±2°C inférieure au cours du LGM et du Younger Dryas et que les précipitations au cours de la saison humide étaient 50% moins importantes qu'actuellement. Ces résultats montrent que les SST enregistrées dans le canal du Mozambique régissent les conditions hydrologiques du continent adjacent, en opposition avec la possible implication de l'ITCZ sous ces latitudes. Les résultats indiquent également que les deux tropiques montrent des tendances climatiques similaires au cours des derniers 20 000 ans. La méthode a ensuite été appliquée à un enregistrement pollinique provenant de la région du fynbos (Afrique du Sud). Les résultats ont montré les limites de la méthode au vu de la faible amplitude de reconstruction obtenue pour les températures entre le LGM et l'Holocène (~2°C). Les résultats ont néanmoins permis de mettre en évidence que les températures montraient un pattern similaire à celui observé en Antarctique. Nous avons également pu montrer que les périodes glaciaires coïncidaient avec les périodes les plus humides, en accord avec le modèle de Cockroft (1987). Ce modèle dérive du mécanisme de migration des « westerlies » vers l'équateur au cours des périodes glaciaires, en réponse au déplacement du vortex circcum polaire. Les travaux issus de cette thèse ont montré qu'il était possible d'utiliser la distribution actuelle des plantes pour estimer les variations quantitatives des changements climatiques passés, dans la plupart des configurations botaniques et climatiques rencontrées en Afrique australe. La méthode de sélection des espèces se révèle être un outil indispensable dans cette région de haute biodiversité. Ces travaux offrent des perspectives intéressantes dans cette zone actuellement dépourvue de reconstructions climatiques quantitatives. Cependant, les résultats émanant des deux cas d'études ont mis en évidence des faiblesses et des limites méthodologiques qui devront faire l'objet d'études supplémentaires afin d'en améliorer les performances. / Located at the interface between tropical and temperate climate systems, southern Africa is a particularly sensitive region in terms of long-term climate change. However, few reliable paleoclimatic records exist from the region – largely as a result of the arid climate with precludes the preservation of wetland sequences - , and virtually no quantitative reconstructions are available.The aim of this thesis is to develop quantitative palaeoclimate reconstruction method based the relation between modern plant distributions and climate in southern Africa. We develop botanical-climatological transfer functions derived from probability density functions (pdfs), allowing for quantitative estimates of the palaeoclimatic variables to be calculated from fossil pollen assemblages. In addition, a species-selection method (SSM) based on Bayesian statistics is outlined, which provides a parsimonious choice of most likely plant species from what are otherwise taxonomically broad pollen-types. This method addresses limitations imposed by the low taxonomic resolution of pollen identification, which is particularly problematic in areas of high biodiversity such as many regions of southern Africa.This methodology has been applied to pollen record from Wonderkrater (South Africa). Results indicate that temperatures during both the warm and cold season were 6±2°C colder during the Last Glacial Maximum and Younger Dryas, and that rainy season precipitation during the Last Glacial Maximum was ~50% of that during the mid-Holocene. Our results also imply that changes in precipitation at Wonderkrater generally track changes in Mozambique Channel sea-surface temperatures, with a steady increase following the Younger Dryas to a period of maximum water availability at Wonderkrater ~3-7 ka. These findings indicate that the northern and southern tropics experienced similar climatic trends during the last 20 kyr, and highlight the role of variations in sea-surface temperatures over the more popularly perceived role of a shifting Intertropical Convergence Zone in determining long-term environmental trends.This method has also been applied to a pollen record from Pakhuis Pass, in the Fynbos Biome (South Africa). Results show the limitations of quantitative methods, with only unrealistically low amplitude being reconstructed between the Last Glacial Maximum and Holocene (~2°C). However, results indicate that the reconstructed temperature trends, if not amplitudes, are similar to trends observed in Antarctic ice core records. Further, in reconstructing past humidity, we show that over the last 18 kyr, cooler conditions appear to be generally wetter at the site. These results are consistent with Cockcroft model (1987), derived from equatorward shift of the westerlies resulting from expansions of the circum-polar vortex.This study shows the potential of using modern plant distributions to estimates past climate parameters in southern Africa, and the species selection method proves to be a useful tool in region with high biodiversity. This work provides a novel perspective in the region, where no quantitative paleoclimatic reconstructions have been available. However, results from Pakhuis Pass highlight some of the limitations of this methodology, which will be subject of future work in this promising field of inquiry.

Ce travail de doctorat a pour objectif de comprendre les mécanismes et les réponses régionales de la variabilité climatique Holocène en ciblant plus spécifiquement la Rade de Brest et la Baie de Douarnenez. Dans les environnements côtiers, la vulnérabilité des sociétés humaines face aux aléas climatiques est particulièrement élevée et les pressions anthropiques s’exerçant sur les écosystèmes sont de plus en plus fortes. Ainsi, il s’agit de discuter la part respective des forçages naturels de ceux liés aux activités humaines dans les différents signaux paléoenvironnementaux holocènes reconstruits. Pour cela, plusieurs séquences sédimentaires nous ont permis de renseigner différentes périodes au cours des 10 000 dernières années. Des études multiproxies principalement basées sur des analyses palynologiques (pollen et kystes de dinoflagellés) ont été conduites permettant de discuter en parallèle des signaux continentaux et marins. Les études polliniques permettent de caractériser l’évolution des paysages terrestres tandis que les dinokystes, dénombrés à partir des mêmes lames palynologiques que les grains de pollen et spores, permettent de discuter les conditions paléohydrologiques de surface. De plus, dans le but d’affiner la compréhension de l’enregistrement des palynomorphes dans les sédiments marins de la Rade de Brest, des échantillons de surface délivrant un message palynologique actuel ont été analysés en préambule de l’étude paléoenvironnementale Holocène. Cette thèse a mis en évidence la robustesse des études palynologiques pour l’étude des variations paléoenvironnementales à l’échelle régionale en milieu côtier, et a démontré la pertinence de l’outil pour mettre en évidence l’imbrication des influences humaines et naturelles. Ce travail apporte ainsi une contribution régionale à l’étude de la variabilité environnementale Holocène en Bretagne, fournissant des enregistrements palynologiques haute résolution sur les derniers 150 ans (résolution de 1 à 4 ans) ainsi que sur les derniers milliers d’années caractérisés par l’influence grandissante des sociétés humaines en lien avec l’accroissement de la déforestation et des pratiques agro-pastorales sur les bassins versants (résolution moyenne de 30 ans). / This PhD study aims at understanding mechanisms and regional responses of the Holocene climatic variability, targeting the Bay of Brest and the Bay of Douarnenez. In coastal environments, the vulnerability of human societies to climate hazards is particularly high and anthropogenic pressures on ecosystems are increasing. In this context, the respective part of natural forcings from those related to human activities can be discussed thanks to paleoenvironmental Holocene reconstructions carried out on sediment archives.Several sedimentary sequences allowed us to focus on different key periods across the last 10,000 years BP.Multiproxy studies mainly based on palynological analyzes (pollen and dinoflagellate cysts) allowed discussing continental and marine signals in parallel, and thus to better understand forcing mechanisms.Pollen studies are used to characterize terrestrial landscape evolutions while dinocysts, counted from the same palynological slides as pollen grains and spores, make it possible to discuss surface palaeohydrological conditions. In addition, in order to complete the understanding of our Holocene palynomorph records conducted on Bay of Brest marine sediments, Bay of Brest surface sediments were investigated so as to deliver a current palynological message.This work has demonstrated the robustness of palynological studies conducted at a regional scale and in a macro-estuarian environment, and also the relevance of our tools to highlight the intertwining of human and natural influences. This work therefore represents a regional contribution to the study of Holocene environmental variability in Brittany, providing high resolution palynological records over the last 150 years (resolution of 1 to 4 years) as well as over the last 9,500 years BP (averaged resolution of 30 years) characterized by the growing influence of human societies in relation to increased deforestation and agropastoral practices on watersheds.

The Sulaiy Formation, which is the oldest unit in the Lower Cretaceous succession, is conformably overlain by the Yamama Formation and it is a challenge to identify the precise age of the two formations using foraminifera and other microfossil assemblages. In the eastern side of Saudi Arabia, the Sulaiy Formation and the base of Yamama Formation are poorly studied. The main objectives of this study is to enhance the understanding of the Sulaiy Formation sequence stratigraphical correlation, regional lateral variations and palaeoenvironmental investigation. Lithological and semi-quantitative micropalaeontological analysis of 1277 thin sections taken from core samples from nine cored wells providing a geographically representative distribution from the Saudi Arabian Gulf. These cores intersected the base of the Yamama Formation and the Sulaiy Formation in the total thickness of cored wells of 843.23 meters (2766.5 feet). On the evidence provided by the foraminifera, the Sulaiy Formation is considered to represent the Berriasian to the lowermost Valanginian. The investigation of the micropalaeontology has provided considerable insights into the biocomponents of Sulaiy and the base of Yamama formations in order to identify their biofacies. These microfossils include rotalid foraminifera, miliolid foraminifera, agglutinated foraminifera, calcareous algae, calcispheres, stromatoporoids, sponge spicules, problematica (e.g. Lithocodium aggregatum), molluscs, corals, echinoderms and ostracods. Systematics of planktic and benthic foraminifera is accomplished using the foraminiferal classification by Loeblich and Tappan (1988) as the main source. The assemblage contains foraminifera that recorded for the first time in the Sulaiy Formation. Other microfossils were identified and recorded to help in the identification of the sedimentary environments. The investigation of the micropalaeontology and the lithofacies analysis have provided evidence the identification of the various lithofacies. About twenty four microfacies were identified on the basis of their bio−component and non-skeletal grains. The lithofacies and the bio−component results have provided the evidence of the sedimentary palaeoenvironmental model namely the Arabian Rimmed Carbonate Platform. This palaeoenvironmental depositional model is characterised by two different platform regimes. They are the Platform Interior and the Platform Exterior each of which have unique sedimentary lithofacies zones that produce different types of lithofacies. Each lithofacies is characterised by special depositional conditions and palaeobathymetry that interact with sea level changes and the accommodation space. The important palaeoenvironments are intertidal, restricted lagoon (subtidal), open marine, deeper open marine, inner shoal, shoal and platform margin. Generating, and testing, a depositional model as a part of formulating a sequence stratigraphical interpretation of a region is a key to understanding its geological development and – ultimately – reservoir potential. The micropalaeontology and sedimentology of the Sulaiy Formation in the subsurface have indicated a succession of clearly defined shallowing−upwards depositional cycles. These typically commence with a deep marine biofacies with wackestones and packstones, capped with a mudstone-wackestone maximum flooding zone and an upper unit of packstone to grainstones containing shallow marine biofacies. The upper part of the Sulaiy Formation is highstand-dominated with common grainstones that host the Lower Ratawi reservoir which is capped by karst that defines the sequence boundary. This karst is identified by its abundant moldic porosity that enhanced the the reservoir quality by increasing its porosities into greater values. Integration of the sedimentology and micropalaeontology has yielded a succession of shoaling−upwards depositional cycles, considered to be 4th order sequences, that are superimposed on a large scale 3rd order system tract shallowing−upwards, highstand-associated sequence of the Sulaiy Formation. The Lower Ratawi Reservoir is located within the latest high-stand portion of a third-order Sulaiy Formation sequence. The reservoir consists of a succession of several sequences, each of which is sub-divided into a lower transgressive systems tract separated from the upper highstand systems tract by a maximum flooding surface (MFS/Z). The last of these depositional cycles terminates in beds of porous and permeable ooid, or ooidal-peloidal, grainstone. The reservoir is sealed by the finer-grained sediments of the Yamama Formation.