No description available.

Speleothems (cave carbonate deposits) have been recognized as a multi-proxy paleoclimate archive. Variations in carbon and oxygen isotopes in speleothems can record past climate changes (e.g., temperature, rainfall and vegetation) under isotopic equilibrium conditions. However, non-climatic noises caused by in-cave processes may affect these stable isotope records under non-equilibrium isotopic conditions. The identification of equilibrium and non-equilibrium isotopic conditions in speleothems is still disputed in the speleothem research community; however, this is a prerequisite for the interpretation of carbon and oxygen isotope records in speleothems as paleoclimate proxies. In this Ph.D. thesis, a series of laboratory experiments under cave-analogue conditions were performed to simulate the formation of speleothems in natural caves. The results of these experiments demonstrate that stable isotope equilibrium in speleothems is achieved under slow carbonate precipitation in pool-like settings (pool carbonates). On the basis of these pool carbonates, equilibrium carbon and oxygen isotope fractionation factors between calcite and water (or DIC for carbon) were determined. Our experiments show larger carbon and oxygen isotope non-equilibrium fractionations between calcite and water (or DIC for carbon) in stalagmite-like settings (fast carbonate precipitate) than those determined in pool-like settings. The flow rate of drip water above the surface of stalagmite appears to control the magnitude of these non-equilibrium isotope effects which increase with decreasing the flow rate. Furthermore, a natural speleothem sample was examined as a paleoclimate archive in this thesis. The growth of a double stalagmite (WS-5d) in Wadi Sannur Cave from the Northeastern Sahara was used to infer the greening of the Sahara (intensive rainfall and vegetation). The U/Th dating in the WS-5d stalagmite suggests that greening conditions extended widely in the Sahara during the interglacial Marine Isotope Stages MIS 5.5, MIS 7.3, and the early MIS 9. Based upon oxygen isotope compositions from the WS-5d, we attributed the source of these greening periods to long-traveling rains from the Atlantic Ocean that were delivered via the West African monsoon system. Our study suggests that the two youngest greening periods were concurrent with the arrival of Homo sapiens in the Levant and an earlier possible change in human population at 244 ka, indicating a key role of the Sahara route in early human dispersal out of Africa. Finally, clumped isotope measurements (Δ47) on carbonate-derived CO2 have been shown to reflect the formation temperature of the carbonate minerals. The absolute Δ47 values of these isotopic measurements seem to be sensitive to the standardization methods (heated CO2 gases and water-equilibrated CO2 gases) that are used to normalize the raw Δ47 measurements. Neither the hypothetical base for the heated CO2 gas standardization method nor the theoretical base for the water-equilibrated CO2 gas standardization method has been experimentally tested. A series of CO2 gases were heated in pre-dehumidified quartz tubes to obtain equilibrium Δ47 values of these CO2 gases at temperature range of 50 – 1100 °C. Consequently, the first experimentally derived Δ47 – T calibration in a CO2 gas phase was proposed. This experimental calibration provides a validated base for the standardization of the raw Δ47 data. Moreover, heating CO2 in a pre-humidified quartz tube enables us to easily prepare a CO2 standard gas of a similar Δ47 value to the CO2 sample (i.e., similar Δ47 correction matrix). This will lead to an improvement in the correction scheme of the carbonate clumped isotope thermometry and reliably adjust the absolute Δ47 scale. / Thesis / Doctor of Philosophy (PhD)

Abstract Amorphous Calcium Carbonate (ACC) is a naturally occurring amorphous form of the widely distributed mineral calcium carbonate (CaCO3). ACC has been found increasingly as a precursor phase, calcium storage site, or strengthening structural phase in a wide array of different biomineralizing organisms. An accurate understanding of the widely used classic carbonate-water paleothermometry relies on formation of CaCO3 minerals and associated oxygen isotope effects. Moreover, ACC has oft been pointed to as a possible reason for non-equilibrium isotope effects, also called vital effects, in biogenic carbonates. It is, therefore, vital to understand whether ACC can reach equilibrium with its surrounding solution, as well as the role of ACC precursors in the isotopic composition and evolution of the final crystalline phase they transform into. This study is designed to answer these questions through the precipitation of stable ACC by two methods, the alkaline method (AM) which utilizes high pH to precipitate ACC, and the silica method (SM) which envelopes precipitating ACC particles in silica vesicles to prevent crystallization. These differently precipitated ACCs are then subjected to several different experimental treatments. This is achieved by monitoring the crystallization by X-ray Diffraction (XRD), and isotopic evolution of the ACC precipitates by Isotope Ratio Mass Spectrometry (IRMS) as they age and concurrently crystallize in parent solution, or in 18O enriched re-equilibration solution. This research indicated a marked difference in the crystallization behaviour, isotopic composition, and isotopic evolution of ACC produced by these two precipitation methods. With the AM method, ACC precipitates (AM-ACC) crystallized more predictably to calcite and maintained δ18O signatures that were slightly lower than the equilibrium CO32- and resisted further isotopic exchange with surrounding solution. We propose that the former is mostly due to an incomplete DIC-water oxygen isotope equilibrium prior to the AM-ACC precipitation and the latter is a result of the high pH of the precipitating solution decreasing the solubility of the precipitated ACC phase, disallowing isotope exchange, and favouring crystallization by solid-state transformation. Conversely, while ACC precipitated using the SM (SM-ACC) yielded much more variable results, both in terms of mineralogical identity upon crystallization, and δ18O values. Isotopic results were much closer to the expected equilibrium δ18O value for calcite, hinting at an expedited oxygen isotope exchange between SM-ACC and parent solution. Furthermore, SM-ACC was capable of isotopic exchange with the 18O enriched re-equilibration solution, a feat corresponding AM-ACC was incapable of. Overall, our experimental results gleaned here that precipitation method or precipitation environment play a critical role in the isotopic evolution of precursor ACC to crystalline CaCO3, suggesting ACC as an important source of the vital effect. / Thesis / Master of Science (MSc)

No description available.

Call number: LD2668 .T4 1961 L36

Γνωρίζουμε ως δεδομένο ότι το κεντρικό Αιγαίο έχει υποβληθεί σε ηπειρωτική διαστολή κατά το Ολιγόκαινο. Κατά το στάδιο της διαστολής είχαμε διείσδυση ενός γρανιτικής σύστασης πλουτωνίτη οποίος σχετίζεται με το σύστημα αποκόλλησης της Μυκόνου. Η Μύκονος κυριαρχείται από έναν μονζογρανίτη, Ι-τύπου, η ηλικία του οποίου είναι περίπου 10Μa όπως υποδεικνύεται από τα θερμοχρονολογικά δεδομένα, ο λακκόλιθος της Μυκόνου είχε εκταφεί πολύ γρήγορα. Ο λακκόλιθος της Μυκόνου διείσδυσε στην κατώτερη ενότητα και έχει μονζογρανιτική έως γρανοδιοριτική σύσταση και υπέρκειται από ένα σύστημα μικρής κλίσεως ρηγμάτων. Η αποκόλληση της Λιβάδας η οποία ήταν ενεργή ταυτόχρονα με την εκταφή του γρανίτη με αποτέλεσμα τη δημιουργία μίας μικρής κλίσεως εύπλαστης διατμητικής ζώνης η οποία επαναενεργοποίησε την επαφή διείσδυσης του γρανίτη μεταξύ του μεταβασίτη και της ανώτερης ενότητας. Η κύρια απόθεση του βαρύτη έγινε σε περιοχές στις οποίες παρατηρείται μεγάλη παρουσία ρηγμάτων. Από τα δείγματα τα οποία συλλέχθηκαν από την περιοχή του Πανόρμου απομονώθηκαν ρευστά εγκλείσματα μέσα από διαφανή ορυκτά όπως ο βαρύτης και ο χαλαζίας και χρησιμοποιήθηκαν για την ισοτοπική ανάλυση. / We know as a fact that the central Aegean domain has undergone continental extension during the Oligocene. The extensional stage was accompanied by the intrusion of granitic plutons and associated detachments, such as the Mykonos and Livada detachments. Mykonos Island is dominated by an I-type monzogranite. The age of the monzogranite is about 10Ma and as indicated by the thermochronological data, Mykonos laccolith was exhumed very fast. Mykonos laccolith intruded into the Lower Unit and has a monzogranitic to granodioritic composition and are topped by a low angle normal fault system. Livada detachment, which was active along with the exhumation of the granite, corresponds to a low-angle ductile shear zone that reworked the intrusive contact of the granite within the metabasites of the upper plate and has undergone through intensive mylonitization and brecciation. Where the faults are most highly concentrated, they are mineralized with barite and iron sulphides with local concentrations of argentiferous galena and sphalerite. The barite vein system which is deposited in the area of Panormos Bay is hosted in the monzogranite and consists of a set of ten major NW- to NNW- crustified tension gashes filled with barite and other minerals. Sample material that was collected from Panormos Bay was used for the study of fluid inclusions that were trapped in coarse-grained transparent minerals such as barite and quartz. Looking at fluid inclusions trapped within minerals can provide very useful information about as part of a target selection process and the conditions existing during the formation of barite as well as the origin of the mineralization on Mykonos Island.

published_or_final_version / Radio Chemistry / Master / Master of Science

Despite the suspected biological imprint on the terrestrial silicon (Si) cycle, plant contribution to the Si continental reservoir is poorly quantified. Within the soil-plant system, aqueous silicon (H4SiO40) can be retrieved from soil solution by plant uptake, clay formation and adsorption onto secondary oxides, or leached out and transferred to stream waters. Two approaches are very promising to trace Si within the soil-plant cycle: the Si stable isotopes and Ge/Si ratio. This thesis aims at quantifying the Si isotopic fractionation induced by plants and soil processes in both controlled (in vitro) and natural conditions (in situ). The model used is a tropical soil-plant system involving a Si-accumulating plant (banana, Musa acuminata Colla, cv Grande Naine) cropped on soils derived from basaltic ash but differing in weathering stage (Cameroon, West Africa). Si isotopic compositions in the different compartments of the soil-plant system were measured by MC-ICP-MS in dry plasma mode with external Mg doping. The analytical method required specific developments, and was validated by an inter-laboratory comparison of reference materials. Plant root uptake of Si, and Si transport within the plant induce an isotopic fractionation quantified in vitro and measured in situ. The plant Si isotopic signature is influenced by soil weathering stage, precisely by soil contents of clay and iron oxide. Soil clay-sized fractions sequestrate light Si isotopes following abiotic fractionating processes of mineral weathering and clay formation, leaving a residual solution enriched in heavy Si isotopes. Biogenic Si, enriched in light Si isotopes, constitutes a Si source for clay formation. Clay-sized Fe-oxides concentrate with increasing weathering and soil development. They selectively adsorb light Si isotopes by surface complexation of monomeric H4SiO40. Silicon transport within the plant produces a Ge/Si fractionation involving Ge sequestration in roots whereas weathering and clay formation induce a selective Ge sequestration in clay minerals, producing an increased Ge/Si ratio with increasing weathering, in excellent agreement with Si isotopic data. In the soil-plant system studied, Si stable isotopes thus trace three major continental processes: plant uptake and phytolith formation, sequestration of Si in clay minerals, and adsorption of Si by Fe-oxides. These biotic and abiotic processes all lead to a progressive enrichment of pore waters in heavy Si isotopes.

No description available.

No description available.