No description available.

As part of the collaborative Dorset Research Project investigating mercury and energy fluxes in fresh-water lakes, I measured mercury in the seston (Chapter 1) and studied the biomanipulation impacts on the phytoplankton (Chapter 2) of two Precambrian Shield lakes. Sestonic-mercury (HgT) was measured in the metalimnion and hypolimnion of each lake throughout the summer of 1995 to determine seasonal fluctuations and the relationship with algal productivity. In each lake, sestonic-HgT (pg Hg/L) did not significantly change in the metalimnion but significantly increased in the hypolimnion by season's end. Combined influences of external HgT inputs, seston sedimentation and increased methylmercury production in the hypolimnia over the season may have contributed to these trends. In comparison to other variables measured, algal productivity was highly correlated with sestonic mercury concentrations in both lakes at each limnetic depth. Although there were no significant differences between lakes with respect to average weight-specific HgT (pg HgT/mg D.W.), chlorophyll a exhibited the best correlations with HgT in MouseL. whereas algal biomass was more highly correlated with HgT in Ranger L. This disparity between lakes may be the result of apparent inter-lake differences in light availability and algal community structure. It was also apparent that changes in the proportions of large and small cells over the season affected the magnitude of sestonic mercury measured. With respect to the potential for trophic transfer of mercury, I suggest that small edible algal cells may bioconcentrate more mercury per unit weight than larger, inedible ones. The data also indicate that seston samples should be collected throughout the season at discrete depths if sestonic-mercury measurements are to be used in trophic transfer models. I also examined the effects of fish biomanipulation on the phytoplankton community of these study lakes. Prior to the biomanipulation, Ranger L. had a top-piscivore community whereas Mouse L. had a top-planktivore community. The biomanipulation involved the removal of top-piscivores from Ranger L. and adding top-piscivores to Mouse L. Trophic Cascade theory predicts that algal biomass in these lakes, with their similar morphometries and resource characteristics, should be ultimately controlled by top-consumer abundance. In addition, model predictions expect "edible" algal size-classes and groups in the community to experience the greatest changes in abundance. Therefore in Ranger L., it was expected that the removal of piscivores would result in higher algal biomass (particularly edible algae), whereas the addition of piscivores in Mouse L. would result in lower algal biomass (particularly edible algae). However, for those years following the biomanipulation, algal biomass significant increased in both lakes compared to pre-manipulation years. This suggests that variables other than direct trophic forces were controlling algal biomass from year to year, regardless of changes in the fish communities. When algal size-classes were tested, only edible cells varying from 10-30 μm increased in Mouse L., contrary to what was predicted. In Ranger L., large cells and colonies > 30 μm unexpectedly increased when all other size-classes did not significantly change. With respect to algal group composition, both Greens and Cryptomonads significantly increased in Mouse L. whereas only Greens significantly increased in Ranger L.. Both of these groups were considered to be edible and thus these results were not consistent with the model predictions. As such, I suggested that "bottom-up" influences were important in controlling both size-class and taxonomic abundances. However, when individual size-classes of representative algal genera were compared between pre-and post-manipulation years, there were some effects which may be attributed to the biomanipulation. In particular, large Green colonies became prevalent in Mouse L. during post-manipulation years as a probable response to increased grazing pressure. Conversely, "edible" Greens became prevalent in Ranger L. after the biomanipulation, supporting the prediction of reduced zooplankton grazing pressure. These results have revealed the necessity to test specific algal genera of varying size-classes in order to detect the effects of biomanipulation. They also showed that the majority of algal genera, regardless of size, were not affected by the biomanipulation. Limitations to my interpretation of the data are discussed and vary from time-scale issues to consumer and resource availability unknowns. Along with recommendations for further studies in this area, I hypothesized that the trophic transfer of sestonic-mercury to zooplankton could be intensified if small, edible algal genera (shown to be impacted by Top-Down forces), have relatively higher weight-specific mercury concentrations. However, considering that the phytoplankton community as a whole has shown resilience to herbivory, I also suggest that the majority of mercury measured in the seston is not available for trophic transfer to zooplankton consumers. / Thesis / Master of Science (MSc)

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1971. / Vita. / Includes bibliograpies. / Ph.D.

Alleghanian-aged deformation and grain size reduction of quartzofeldspathic and mafic rocks of the Late Precambrian Ponaganset Gneiss proceeded at amphibolite grade conditions ( 600°C, 4-6 kb), and were accompanied by considerable mobility of Si, Na, K, Ca, Fe and Mg. Fluids present during and after ductile deformation facilitated hydration reactions within a narrow ( > 10 cm), foliation-parallel mafic layer composed largely of clinopyroxene and albitic plagioclase (An₃). The final product of these reactions was a K-feldspar/plagioclase (An₉)-rich amphibolite. Mass balance relations show that the amphibolite-forming reactions required an influx of Fe²⁺ and Mg²⁺ cations, and expulsion of Na⁺ into the surrounding gneisses. The source for Fe²⁺ and Mg²⁺ was probably the adjacent, actively-deforming quartzofeldspathic mylonite which, with reference to the protolith gneisses less than 30 cm from the gneiss-amphibolite contact, is reduced by 60% of its original concentration of these elements. Where no amphibolite layer is present, quartzofeldspathic mylonites display a similar depletion in Fe and Mg, but also carry textural and geochemical evidence for fluid flow on a millimeter to centimeter scale (e.g. foliation-parallel quartz veins walled by monomineralic layers of plagioclase or K-feldspar, and pronounced cm scale gradients in whole rock K, Na and Si concentrations). Despite the evidence in favor of fluid flow, reaction progress calculations in the amphibolite layer suggest that minimum volumetric fluid-rock ratios never exceeded 0.05 liters H₂O/liter of rock. The local presence of strained clinopyroxene microporphyroclasts and laminae of clinopyroxene-albite mylonite in the amphibolite layer suggest that hydration reactions did not go to completion. The presence of biotite in the amphibolite layer elsewhere at the outcrop suggests that fluid-rock ratios were not uniform during, or shortly after, deformation. Post deformational metamorphic re- actions in the amphibolite resulted in the formation of actinolitic rims on amphibole grains which continued to grow at the expense of adjacent, strained clinopyroxene microporphyroclasts. Textural relationships indicate that the actinolitic rims grew via diffusive processes in the presence of an intergranular fluid film. The ability of the zircon U-Pb system to yield useful geochronologic information on the timing of deformational events is a complex function of the degree of metamictization of the zircons, the chemistry and mineralogy of the enclosing rock, the P-T conditions of metamorphism and the presence and composition of metamorphic fluids. The possible effects of dynamic metamorphism on the isotopic ages obtained from zircons range from no apparent Pb loss to complete resetting of the isotopic "clock". The two examples presented here represent the two extremes: the mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone, and the PennsylvanianPermian amphibolite-grade Hope Valley Shear Zone. In the mylonites of the Rockfish Valley Fault Zone, zircons underwent brittle failure and were comminuted in the ductilely deforming matrix. Comparative U-Pb isotopic studies of zircons from the mylonites and from the Grenville-aged charnockitic protolith demonstrate that no appreciable Pb loss occurred as the result of intense mylonitization, despite the presence of aqueous fluids during deformation which affected the hydration of the mylonitic mineral assemblages. A detailed U-Pb isotopic study of a single outcrop of the late Precambrian Ponaganset gneiss was also undertaken. Amphibolite-grade mylonitization associated with movement along the Hope Valley Shear Zone caused some grain size reduction of the zircons in the mylonite zones. However, the primary cause of isotopic disturbance in these zircons is the extensive growth of U-rich metamorphic zircon during, or shortly after, metamorphism. Corrosion of pre-existing zircon also occurred during metamorphism/deformation, but it is difficult to quantify its effects on U-Pb systematics. The lower intercept ages of 289± 24 m.y. and 265± 62 m.y. from the pink gneiss and gray gneiss, respectively, both fall within the range of other published age estimates (e.g. Zartman et al. 1988) for Alleghanian deformation and metamorphism in southeastern New England. / Ph. D.

At present the Karoo Basin covers approximately 20 000 km2. It is a large intracratonic basin which, from Carboniferous to Jurassic times, was infilled with a succession of sediments ranging from glacial deposits to those deposited in warm, equable conditions. The Beaufort Group forms part of this succession, and was deposited in a terrestrial, river dominated environment. The dominant lithologies exposed in the Estcourt region in the KwaZulu-Natal Midlands belong to the lower and middle Beaufort divided by the PermoTriassic boundary. The Permo-Triassic palaeoenvironment in this region is reconstructed using sedimentary profiles combined with the study of the fossil remains discovered in the area, including plant, body, and trace fossils. The lower Beaufort sediments in this region belong to the Estcourt Formation, and the Middle Beaufort sediments to the Belmont Formation. The Estcourt Formation is dominated by a succession of alternating sandstones, siltstones and mudstones, which are interpreted as representing sediments deposited in a fluvial-floodplain environment, which can be divided into two sub-environments. The first is dominated by sediments that were deposited by meandering rivers on a semi-arid floodplain, and the second sub-environment is represented by those sediments deposited in lacustrine environments. Both of these subenvironments are closely linked and alternate in the rock record indicating many episodes of transgressive-regressive lacustrine episodes. The Estcourt Formation can be closely correlated with the lower Beaufort sediments mapped in other regions of the Karoo Basin, indicating similar climatic and environmental controls throughout the Karoo Basin of southern Africa. The Estcourt Formation also contains a wide variety of body and trace fossils. The PermoTriassic boundary can be traced along the western border of Estcourt by using the distribution pattern of the two mammal-like reptiles Dicynodon and Lystrosaurus. There is evidence of an overlap in the distribution between these to mammal-like reptiles, which together with palaeoflora evidence, indicates that Lystrosaurus evolved during the Late Permian and not Early Triassic as previously thought. The first Triassic sediments are represented in the Estcourt region by a series of maroon shales which can be correlated with the Palingkloof Member. / Thesis (M.Sc.)-University of Natal, 1997.

Call number: LD2668 .T4 1978 K623 / Master of Science

No description available.

In Alabama, the Paleozoic Appalachian thrust belt plunges southwest beneath the Mesozoic-Cenozoic Gulf Coastal Plain. In Arkansas, the Paleozoic Ouachita thrust belt plunges southeast beneath the Coastal Plain. The strikes of the exposed thrust belts suggest an intersection beneath the Coastal Plain. Well data and seismic reflection profiles confirm the strike and intersection of the thrust belts, and provide information to determine the structure and general stratigraphy of each thrust belt. In east-central Mississippi, the Appalachian thrust belt curves from the regional northeast trace to westward at the intersection with the southeastern terminus of the Ouachita thrust belt, to northwest where Ouachita thrust sheets are in the Appalachian footwall, and farther west, to a west-southwest orientation. At the intersection, the frontal Appalachian fault truncates the Appalachian thrust sheets. The Appalachian thrust sheets are detached in Lower Cambrian strata and contain a distinctive Cambrian-Ordovician passive-margin carbonate succession. The Ouachita thrust sheets are detached above the carbonate succession and contain a thick Carboniferous clastic succession. The Appalachian thrust sheets east of the intersection rest on an autochthonous footwall with a thin Lower Cambrian sedimentary cover above Precambrian crystalline basement. To the west, the Appalachian thrust sheets rest on an allochthonous footwall of thick Ouachita thrust sheets. The top of Precambrian crystalline basement rocks dips southwestward beneath the Ouachita thrust belt; large-magnitude down-to-southwest basement faults enhance the deepening. Appalachian thrust sheets on the northeast are detached above relatively shallow basement, but to the west, are detached above thick Ouachita thrust sheets, which overlie deeper basement. The structure of the basement reflects the Iapetan rifted margin, where the northwest-striking Alabama-Oklahoma transform bounds the southwest side of the Alabama promontory. The trends of basement structures and subsidence toward the Ouachita thrust belt parallel the Alabama-Oklahoma transform. Shallower basement and synrift basement grabens underlie the northeast-striking Appalachian thrust belt. The curves in strike and along-strike change in footwall structure of the Appalachian thrust belt reflect controls by basement structure and by the structure of the Ouachita thrust belt.

Les conditions redox des océans ont considérablement évolué au cours du Précambrien. Ceci est principalement dû à l’arrivée de l’oxygène engendrant dans un premier temps des oasis d’oxygène puis un Grand Évènement d’Oxydation (GEO) vers 2.45 Ga. Néanmoins, les connaissances concernant la datation des premiers signes d’oxygénation et les mécanismes de leur enregistrement sédimentaire, propres à cette période, restent encore à approfondir. Depuis plusieurs années, la composition isotopique en molybdène (δ98Mo) dans les sédiments est utilisée comme indicateur des conditions redox locales et globales des océans. Une étude élémentaire et isotopique en Mo des différentes phases constituant les carbonates a été réalisée suite à des attaques séquentielles sur des carbonates d’âge et d’environnement de dépôts différents. Les résultats montrent que le Mo se loge dans la phase organique mais également au sein des carbonates authigènes. Ces derniers influencent la valeur isotopique étudiée lors des attaques pseudo totales par 6N HCl et utilisée dans toutes les études du proxy redox des carbonates. Une étude minéralogique et isotopique du Mo des argiles et des carbonates riches en Mn (II) des Formations Boolgeeda et Kazput (craton de Pilbara, Australie) s’intéresse à la capacité du δ98Mo d’enregistrer les conditions redox locales afin de comprendre les mécanismes d’enrichissement sédimentaire en manganèse sur la période du GEO. Couplées à d’autre proxy redox et comparées à un analogue moderne (le bassin réduit de Landsort, mer Baltique), les données argumentent pour une réduction des oxydes de Mn(IV) au sein de la colonne d’eau et à l’interface des sédiments. Ces observations invitent à la discussion concernant les enrichissements en Mn du Précambrien dont le processus de réduction des oxydes est automatiquement considéré comme intrasédimentaire. Finalement, une étude compare des valeurs maximales en δ98Mo de carbonate analysées et compilées aux valeurs isotopiques des black shales et des formations de fer compilées au cours des temps géologiques en exploitant la composante globale de ce proxy redox. Les données suggèrent l’absence de condition euxinique permettant l’enregistrement des valeurs de l’eau de mer par les black shales. L’étude permet également d’observer la présence d’un cycle oxydatif du molybdène dès le Méso-Archéen. / The redox conditions of the oceans evolved considerably during the Precambrian. This is mainly due to the arrival of the oxygen, at first generating oxygen oases, and then accumulating in the atmosphere during the Great Oxidation Event (GOE) ca. 2.45 Ga. However, the earliest traces of free oxygen during these periods, and the mechanisms of their sedimentary expression, remain poorly understood. For over a decade, the isotopic composition of molybdenum (δ98Mo) in sediments has been used as a global marine redox proxy.An elemental and isotopic study of the different phases constituting carbonate rocks was carried out using sequential digestions performed on carbonates of different ages and depositional environments. The results show that Mo is primarily hosted in the organic phase but also within authigenic carbonate phases. These latter influence the isotopic value obtained during the classic pseudo-total digestion by 6N HCl that is most frequently employed for Mo isotope proxy studies in carbonates.Mineralogical and Mo isotopic studies on Mn(II)–rich shales and carbonates in the Boolgeeda and KazputFormations (Pilbara craton, Australia) leveraged the ability of δ98Mo to record local redox conditions in order to understand mechanisms responsible for sedimentary Mn enrichments occurring around the GOE. Coupled with other redox proxies and compared to modern analogues (e.g., the Landsort basin, Baltic Sea), the data argues for a reduction of Mn(IV) oxides within the water column and at the sediment-water interface. These observations bring new perspective on Precambrian Mn enrichments, which have been traditionally considered as the result of Mn oxide reduction occurring at depth in the sedimentary pile.Finally, a study of the maximum values of δ98Mo of analyzed and compiled carbonates compared to compiled isotopic values of black shales and iron formations deposited through geological time make use of the global component of this redox proxy. Data suggest an absence of euxinic conditions required for the recording of seawater values by black shales. This study also highlight the presence of an oxidative cycle of molybdenum since at least the Mesoarchean.

Estromatólitos foram estudados em nove localidades ao sul de Itapeva (SP), principalmente em metacalcários dolomíticos cinza-claro e, secundariamente, em metacalcários calcíticos cinza-escuro, do Grupo Itaiacoca, uma unidade vulcanossedimentar mesoproterozóica da Faixa Ribeira. Foram diferenciados em cinco formas colunares, a mais comum consistindo de colunas coniformes, não ramificadas, de diâmetros e alturas centimétricas a decimétricas, atribuídas a Conophyton. As outras quatro formas, com laminação convexa mas não coniforme, diferem em tamanho, silhueta e estilo/freqüência de ramificação. As diferenças na preservação dos estromatólitos são relacionadas ao comportamentos tectônicos distintos entre o metacalcário dolomítico (mais puro), com comportamento competente, e o calcítico (mais argiloso) que atuou de forma mais plástica. Nas melhores exposições desta área os estromatólitos estão agrupados em bioermas de Conophyton, sem indícios de exposição ou retrabalhamento sub-aéreo ou por ondas, evidenciando um ambiente calmo e relativamente profundo, provavelmente abaixo do nível de base de ondas, de talvez até algumas dezenas de metros de profundidade. Conophyton de Itapeva é semelhante a estromatólitos coniformes próximo a Abapã (PR), também no Grupo Itaiacoca, a cerca de 100 km de Itapeva, mas difere de outras formas, incluindo Conophyton cylindricum e C. metulum, de unidades proterozóicas associadas a margem oeste do Cráton do São Francisco. O Conophyton do Grupo Itaiacoca é semelhante a formas na ex-União Soviética que são geralmente encontrados no Mesoproterozóico ou Neoproterozóico inferior, que é consistente com as datações radiométricas disponíveis que colocam esta unidade próximo ao final do Mesoproterozóico. / Stromatolites were studied at nine localities south of Itapeva, São Paulo, Brazil, generally in light-gray metadolostones and secondarily in dark-gray metalimestones of the Itaiacoca Group, a Mesoproterozoic volcanosedimentary unit of the Ribeira Belt. Five columnar forms were distinguished, the most common consisting of unbranched, coniform columns, with centimetric to decimetric diameters and heights, attributed to Conophyton. The other four forms exhibit convex, but not coniform lamination and differ in size, silhouette and style/frequency of branching. Differences in stromatolite preservation are related to the differing tectonic behavious of the purer and more competent metadolostones and the more argillaceous metalimestones which behaved more plastically. In the best exposures in this area the stromatolites are grouped into Conophyton bioherms, without any evidence of subaerial exposure or reworking by waves, which suggests that they formed in a calm and relatively deep setting (perhaps up to several tens of meters in depth), probably below the base of fairweather water. Conophyton from Itapeva is similar to other coniform stromatolites in the Itaiacoca Group near Abapã (Paraná), about 100 km SW of Itapeva, but differs from other forms, including Conophyton cylindricum and C. metulum, from Proterozoic successions associated with the western margin of the São Francisco Craton. The Conophyton from the Itaiacoca Group is most similar to forms in the ex-Sovietic Union that are usually found in the Mesoproterozoic or lowest Neoproterozoic, which is consistent with available radiometric age dates that place this unit near the end of the Mesoproterozoic.