Thesis advisor: Jeremy D. Shakun / The Arctic is one of the fastest warming regions on the planet. Currently, much of the Arctic is covered by permafrost, which contains approximately 1,700 gigatons of organic carbon. Permafrost thaw could release a substantial amount of this carbon as greenhouse gases into the atmosphere through microbial decomposition, potentially dramatically amplifying anthropogenic warming. However, the risk of permafrost thaw is uncertain, with models exhibiting a wide range of possibilities. Assessing the stability of permafrost during past interglacial periods enables evaluation of the sensitivity of permafrost to warming. Cave mineral deposits (speleothems) in areas currently covered with permafrost can act as a proxy for past permafrost thaw, as liquid water is one criterion for speleothem growth and thus speleothem deposition implies thawed ground conditions. Previous uranium-thorium (U-Th) dating of speleothems (n=73) from a wide range of latitudes and permafrost zones across the southern Canadian Rockies, Northwest Territories, and northern Yukon indicate that most of these formations exceed the U-Th dating limit of 500 ka. In this study, I apply uranium-lead (U-Pb) geochronology to several of these speleothems to extend the record of speleothem growth further back in time. Results include a U-Pb age of 412.8 ± 1.3 ka that replicates a previous U-Th age of 415.0 ± 11.5 ka, demonstrating the reliability of the U-Pb methodology. Additionally, U-Pb ages on six other speleothems are determined to be 876 ± 9 ka, 1501 ± 31 ka, 1570 ± 66 ka, 2046 ± 106 ka, 7636 ± 184 ka, and 7697 ± 185 ka. Further application of this technique could result in long records of past permafrost thaw and Arctic terrestrial climate extending back millions of years. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

No description available.

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaves 26-27. / M.S.

This thesis addresses various issues in U-Pb zircon geochronology, proposing new experimental protocols in conventional chemical abrasion-isotope dilution thermal ionization mass spectrometry or CA-(ID)-TIMS and developing a new method for Pb evaporation-condensation from zircon that allows high precision Pb-Pb age determination on Precambrian samples. Various experiments are also done on zircon to extract U-Pb information by in situ flux aided fusion methods and to optimize a better silica gel Pb-ionization activator. Radiation damage caused by U decay in zircon disrupts its ‘closed system’ behavior leading to the loss of daughter radiogenic Pb and resulting in inaccurate ages. A high temperature thermal annealing procedure has been proposed to prevent such Pb loss. Studies presented here have been carried out using Laser Raman Spectroscopy and Scanning Electron Microscopy to characterize radiation damage and effects of laboratory induced thermal annealing on such damage. Backscattered electron images reveal a variety of textures for ZrO2 overgrowths on zircon annealed at 1450oC. Highly damaged zircon produces finer polycrystalline aggregates (<5µm) than zircon with less damage. Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000oC for 2–3 or 20 days. Annealing at 1450oC for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon showing that when highly damaged zircon is recrystallized, it becomes a polycrystalline aggregate that pseudomorphs the original single crystal. The whole grain Pb evaporation-condensation method is based on 206Pb-207Pb age analyses where zircon grains are pre-treated at 1450oC to drive out all disturbed Pb and then they are kept at 1600oC for an hour or two during which Pb atoms are evaporated out of the grain and deposited directly into a clean Savillex teflon vial or a wide Re filament. This technique allows the use of a 202Pb-205Pb double spike for precise isotopic fractionation correction. Examples are shown in which application of this technique to zircon from Precambrian samples has successfully yielded sub-million year age precisions.

The late Archean Mount McRae Shale of the Hamersley Basin in Western Australia may record the presence of oxygen in the atmosphere before the Great Oxidation Event (2.4-2.3 Ga). Several prior studies (Anbar et al., 2007; Blum and Anbar, 2010; Duan et al., 2010; Kakegawa et al., 1998; McManus et al., 2006) have used isotopic systems to analyze the Mount McRae Shale and conclude that there was a presence of oxygen before the Great Oxidation Event. The purpose of this study is to determine if the U-Pb system can be used to see through later events to the initial conditions. The uranium-lead values of the Mt McRae Shale provide evidence of the mobilization of U and Pb gain. The geochemical disturbances have been linked to the tectonic activity (460 Ma) in the neighboring Canning basin, which could have possibly opened the geochemical system. In terms of the depositional environment the U-Pb data gathered here do not point to oxygenation of the atmosphere.

This thesis addresses various issues in U-Pb zircon geochronology, proposing new experimental protocols in conventional chemical abrasion-isotope dilution thermal ionization mass spectrometry or CA-(ID)-TIMS and developing a new method for Pb evaporation-condensation from zircon that allows high precision Pb-Pb age determination on Precambrian samples. Various experiments are also done on zircon to extract U-Pb information by in situ flux aided fusion methods and to optimize a better silica gel Pb-ionization activator. Radiation damage caused by U decay in zircon disrupts its ‘closed system’ behavior leading to the loss of daughter radiogenic Pb and resulting in inaccurate ages. A high temperature thermal annealing procedure has been proposed to prevent such Pb loss. Studies presented here have been carried out using Laser Raman Spectroscopy and Scanning Electron Microscopy to characterize radiation damage and effects of laboratory induced thermal annealing on such damage. Backscattered electron images reveal a variety of textures for ZrO2 overgrowths on zircon annealed at 1450oC. Highly damaged zircon produces finer polycrystalline aggregates (<5µm) than zircon with less damage. Raman spectroscopy indicates that crystals with different levels of radiation damage are only partially restored by annealing at 1000oC for 2–3 or 20 days. Annealing at 1450oC for 1 h results in partial breakdown of zircon but restores Raman peak widths and wave numbers. Raman spectra are much less sensitive to polarization angle for annealed highly damaged grains than for weakly damaged zircon showing that when highly damaged zircon is recrystallized, it becomes a polycrystalline aggregate that pseudomorphs the original single crystal. The whole grain Pb evaporation-condensation method is based on 206Pb-207Pb age analyses where zircon grains are pre-treated at 1450oC to drive out all disturbed Pb and then they are kept at 1600oC for an hour or two during which Pb atoms are evaporated out of the grain and deposited directly into a clean Savillex teflon vial or a wide Re filament. This technique allows the use of a 202Pb-205Pb double spike for precise isotopic fractionation correction. Examples are shown in which application of this technique to zircon from Precambrian samples has successfully yielded sub-million year age precisions.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Bibliography : leaves 248-259. / Ph.D.

As part of the wider European GTS Next project, I propose new constraints on the ages of the Late Cretaceous, derived from a multitude of geochronological techniques, and successful stratigraphic interpretations from Canada and Japan. In the Western Canada Sedimentary Basin, we propose a new constraint on the age of the K/Pg boundary in the Red Deer River section (Alberta, Canada). We were able to cyclostratigraphically tune sediments in a non-marine, fluvial environment utilising high-resolution proxy records suggesting a 11-12 precession related cyclicity. Assuming the 40Ar/39Ar method is inter-calibrated with the cyclostratigraphy, the apparent age for C29r suggests that the K/Pg boundary falls between eccentricity maxima and minima, yielding an age of the C29r between 65.89 ± 0.08 and 66.30 ± 0.08 Ma. Assuming that the bundle containing the coal horizon represents a precession cycle, the K/Pg boundary is within the analytical uncertainty of the youngest zircon population achieving a revised age for the K/Pg boundary as 65.75 ± 0.06 Ma. The Campanian - Maastrichtian boundary is preserved in the sedimentary succession of the Horseshoe Canyon Formation and has been placed ~8 m below Coal nr. 10. Cyclostratigraphic studies show that the formation of these depositional sequences (alternations) of all scales are influenced directly by sea-level changes due to precession but more dominated by eccentricity cycles proved in the cyclostratigraphic framework and is mainly controlled by sand horizons, which have been related by autocyclicity in a dynamic sedimentary setting. Our work shows that the Campanian - Maastrichtian boundary in the Western Canada Sedimentary Basin coincides with ~2.5 eccentricity cycles above the youngest zircon age population at the bottom of the section and ~4.9 Myr before the Cretaceous - Palaeogene boundary (K/Pg), and thus corresponds to an absolute age of 70.65 ± 0.09 Ma producing an ~1.4 Myr younger age than recent published ages. Finally, using advances with terrestrial carbon isotope and planktonic foraminifera records within central Hokkaido, Northwest Pacific, sections from the Cretaceous Yezo group were correlated to that of European and North American counterparts. Datable ash layers throughout the Kotanbetsu and Shumarinai section were analysed using both 40Ar/39Ar and U-Pb methods. We successfully dated two ash tuff layers falling either side of the Turonian - Coniacian boundary, yielding an age range for the boundary between 89.31 ± 0.11 Ma and 89.57 ± 0.11 Ma or a boundary age of 89.44 ± 0.24 Ma. Combining these U-Pb ages with recent published ages we are able to reduce the age limit once more and propose an age for the Turonian - Coniacian boundary as 89.62 ± 0.04 Ma.

Dans le cadre du projet Européen GTS Next, nous avons obtenu des nouvelles contraintes sur l’âge des étages du Crétacé Supérieur à partir de plusieurs techniques de géochronologie et d’interprétations stratigraphiques au Canada et au Japon. Dans le bassin sédimentaire du Western Interior Canada, nous proposons une nouvelle détermination de l’âge de la limite Crétacé - Tertiaire (K/Pg) enregistrée dans la coupe de Red Deer River (Alberta). Il a été possible de calibrer par cyclostratigraphie haute-résolution cette série sédimentaire fluviatile non-marine et d’identifier 11-12 cycles associés à la précession orbitale de la Terre. En considérant la technique 40Ar/39Ar intercalibrée avec la cyclostratigraphie, l’âge apparent de la base du chron magnétique C29r suggère que la limite K/Pg se trouve entre un minimum et un maximum de l’excentricité, avec une durée pour C29r de 66.30 ± 0.08 à 65.89 ± 0.08 Ma. En supposant que le cycle contenant le niveau de charbon soit associé à un cycle de précession, l’âge révisé de la limite Crétacé - Tertiaire est donné par la plus jeune des populations de zircon datée par U-Pb à 65.75 ± 0.06 Ma.La limite Campanien – Maastrichtien est également enregistrée dans ce même bassin canadien, et se trouve à environ 8 m sous le niveau de charbon No. 10 dans la formation de Horseshoe Canyon. L’étude cyclostratigraphique montre que le dépôt de cette séquence sédimentaire est directement influencé par les changements du niveau marin dû à la précession et dominés par l’excentricité Notre travail montre que la position de la limite Campanien – Maastrichtien dans ce bassin sédimentaire du Western Canada est placée à environ 2.5 cycles d’excentricité au dessus d’un niveau de téphra de la base de la coupe dont l’âge U-Pb est donné par la plus jeune population des zircons, et ~4.9 Myr avant la limite Crétacé - Tertiaire. Nous en déduisons un âge absolu de 70.65 ± 0.09 Ma pour la limite Campanien – Maastrichtien, ce qui est ~1.4 Myr plus jeune que les études récemment publiées.Enfin, à partir des isotopes du carbone et des foraminifères planctoniques enregistrés au centre d’Hokkaido (Pacifique Nord-Ouest), les coupes Crétacé du groupe Yezo ont été corrélée avec les séries européennes et nord-américaines. Plusieurs niveaux de téphra prélevés au sein des coupes de Kotanbetsu et Shumarinai ont été datés par les méthodes 40Ar/39Ar and U-Pb. Deux d’entre eux, placés de part et d’autre de la limite Turonien – Coniacien, ont donné des âges de 89.31 ± 0.11 et 89.57 ± 0.11 Ma, ce qui suggère un âge de 89.44 ± 0.24 Ma pour cette limite. En combinant notre résultat avec les âges récemment publiés, nous pouvons proposer un âge de 89.62 ± 0.04 Ma pour la limite Turonien – Coniacien. / As part of the wider European GTS Next project, I propose new constraints on the ages of the Late Cretaceous, derived from a multitude of geochronological techniques, and successful stratigraphic interpretations from Canada and Japan. In the Western Canada Sedimentary Basin, we propose a new constraint on the age of the K/Pg boundary in the Red Deer River section (Alberta, Canada). We were able to cyclostratigraphically tune sediments in a non-marine, fluvial environment utilising high-resolution proxy records suggesting a 11-12 precession related cyclicity. Assuming the 40Ar/39Ar method is inter-calibrated with the cyclostratigraphy, the apparent age for C29r suggests that the K/Pg boundary falls between eccentricity maxima and minima, yielding an age of the C29r between 65.89 ± 0.08 and 66.30 ± 0.08 Ma. Assuming that the bundle containing the coal horizon represents a precession cycle, the K/Pg boundary is within the analytical uncertainty of the youngest zircon population achieving a revised age for the K/Pg boundary as 65.75 ± 0.06 Ma. The Campanian - Maastrichtian boundary is preserved in the sedimentary succession of the Horseshoe Canyon Formation and has been placed ~8 m below Coal nr. 10. Cyclostratigraphic studies show that the formation of these depositional sequences (alternations) of all scales are influenced directly by sea-level changes due to precession but more dominated by eccentricity cycles proved in the cyclostratigraphic framework and is mainly controlled by sand horizons, which have been related by autocyclicity in a dynamic sedimentary setting. Our work shows that the Campanian - Maastrichtian boundary in the Western Canada Sedimentary Basin coincides with ~2.5 eccentricity cycles above the youngest zircon age population at the bottom of the section and ~4.9 Myr before the Cretaceous - Palaeogene boundary (K/Pg), and thus corresponds to an absolute age of 70.65 ± 0.09 Ma producing an ~1.4 Myr younger age than recent published ages. Finally, using advances with terrestrial carbon isotope and planktonic foraminifera records within central Hokkaido, Northwest Pacific, sections from the Cretaceous Yezo group were correlated to that of European and North American counterparts. Datable ash layers throughout the Kotanbetsu and Shumarinai section were analysed using both 40Ar/39Ar and U-Pb methods. We successfully dated two ash tuff layers falling either side of the Turonian - Coniacian boundary, yielding an age range for the boundary between 89.31 ± 0.11 Ma and 89.57 ± 0.11 Ma or a boundary age of 89.44 ± 0.24 Ma. Combining these U-Pb ages with recent published ages we are able to reduce the age limit once more and propose an age for the Turonian - Coniacian boundary as 89.62 ± 0.04 Ma.