Variation of stable silicon isotopes: Analytical developments and applications in Precambrian geochemistry

Die vorliegende Dissertation behandelt die Gesamtgesteinsanalyse stabiler Siliziumisotope mit Hilfe einer „Multi Collector-ICP-MS“. Die Analysen fanden in Kooperation mit dem „Royal Museum for Central Africa“ in Belgien statt. Einer der Schwerpunkte des ersten Kapitels ist die erstmalige Analyse des δ30Si –Wertes an einem konventionellen Nu PlasmaTM „Multi-Collector ICP-MS“ Instrument, durch die Eliminierung der den 30Si “peak” überlagernden 14N16O Interferenz. Die Analyse von δ30Si wurde durch technische Modifikationen der Anlage erreicht, welche eine höherer Massenauflösung ermöglichten. Die sorgsame Charakterisierung eines adäquaten Referenzmaterials ist unabdingbar für die Abschätzung der Genauigkeit einer Messung. Die Bestimmung der „U.S. Geological Survey“ Referenzmaterialien bildet den zweiten Schwerpunkt dieses Kapitales. Die Analyse zweier hawaiianischer Standards (BHVO-1 and BHVO-2), belegt die präzise und genaue δ30Si Bestimmung und bietet Vergleichsdaten als Qualitätskontrolle für andere Labore. Das zweite Kapitel befasst sich mit kombinierter Silizium-/Sauerstoffisotope zur Untersuchung der Entstehung der Silizifizierung vulkanischer Gesteine des „Barberton Greenstone Belt“, Südafrika. Im Gegensatz zu heute, war die Silizifizierung der Oberflächennahen Schichten, einschließlich der „Chert“ Bildung, weitverbreitete Prozesse am präkambrischen Ozeanboden. Diese Horizonte sind Zeugen einer extremen Siliziummobilisierung in der Frühzeit der Erde. Dieses Kapitel behandelt die Analyse von Silizium- und Sauerstoffisotopen an drei unterschiedlichen Gesteinsprofilen mit unterschiedlich stark silizifizierten Basalten und überlagernden geschichteten „Cherts“ der 3.54, 3.45 und 3.33 Mill. Jr. alten Theespruit, Kromberg und Hooggenoeg Formationen. Siliziumisotope, Sauerstoffisotope und die SiO2-Gehalte demonstrieren in allen drei Gesteinsprofilen eine positive Korrelation mit dem Silizifizierungsgrad, jedoch mit unterschiedlichen Steigungen der δ30Si-δ18O-Verhältnisse. Meerwasser wird als Quelle des Siliziums für den Silizifizierungsprozess betrachtet. Berechnungen haben gezeigt, dass eine klassische Wasser-Gestein Wechselwirkung die Siliziumisotopenvariation nicht beeinflussen kann, da die Konzentration von Si im Meerwasser zu gering ist (49 ppm). Die Daten stimmen mit einer Zwei-Endglieder-Komponentenmischung überein, mit Basalt und „Chert“ als jeweilige Endglieder. Unsere gegenwärtigen Daten an den „Cherts“ bestätigen einen Anstieg der Isotopenzusammensetzung über der Zeit. Mögliche Faktoren, die für unterschiedliche Steigungen der δ30Si-δ18O Verhältnisse verantwortlich sein könnten sind Veränderungen in der Meerwasserisotopie, der Wassertemperatur oder sekundäre Alterationseffekte. Das letzte Kapitel beinhaltet potentielle Variationen in der Quellregion archaischer Granitoide: die Si-Isotopen Perspektive. Natriumhaltige Tonalit-Trondhjemit-Granodiorit (TTG) Intrusiva repräsentieren große Anteile der archaischen Kruste. Im Gegensatz dazu ist die heutige Kruste kaliumhaltiger (GMS-Gruppe: Granit-Monzonite-Syenite). Prozesse, die zu dem Wechsel von natriumhaltiger zu kaliumhaltiger Kruste führten sind die Thematik diesen Kapitels. Siliziumisotopenmessungen wurden hier kombiniert mit Haupt- und Spurenelementanalysen an unterschiedlichen Generationen der 3.55 bis 3.10 Mill. Yr. alten TTG und GMS Intrusiva aus dem Arbeitsgebiet. Die δ30Si-Werte in den unterschiedlichen Plutonit Generationen zeigen einen leichten Anstieg der Isotopie mit der Zeit, wobei natriumhaltige Intrusiva die niedrigste Si-Isotopenzusammensetzung aufweisen. Der leichte Anstieg in der Siliziumisotopenzusammensetzung über die Zeit könnte auf unterschiedliche Temperaturbedingungen in der Quellregion der Granitoide hinweisen. Die Entstehung von Na-reichen, leichten d30Si Granitoiden würde demnach bei höheren Temperaturen erfolgen. Die Ähnlichkeit der δ30Si-Werte in archaischen K-reichen Plutoniten und phanerozoischen K-reichen Plutoniten wird ebenfalls deutlich.

Stable isotope investigations of atmospheric nitrous oxide

ZusammenfassungDie Analyse von Isotopenverhältnissen ist von wachsender Bedeutung bei der Untersuchung von Quellen, Senken und chemischen Reaktionswegen atmosphärischer Spurengase. Distickstoffoxid (N2O) hat vier isotopisch einfach substituierte Spezies: 14N15N16O, 15N14N16O, 14N217O und 14N218O. In der vorliegenden Arbeit wurden massenspektrometrische Methoden entwickelt, die eine komplette Charakterisierung der Variationen im Vorkommen dieser Spezies ermöglichen. Es wird die bisher umfassendste Darstellung dieser Variationen in Troposphäre und Stratosphäre gegeben und mit Bezug auf eine Reihe von Laborexperimenten detailliert interpretiert.Die Laborexperimente machen einen großen Anteil dieser Doktorarbeit aus und konzentrieren sich auf die Isotopenfraktionierung in den stratosphärischen N2O-Senken, d. h. Photolyse und Reaktion mit elektronisch angeregten Sauerstoffatomen, O(1D). Diese Prozesse sind von dominantem Einfluß auf die Isotopenzusammensetzung von atmosphärischem N2O. Potentiell wichtige Parameter wie Temperatur- und Druckvariationen, aber auch Veränderungen der Wellenlänge im Fall der Photolyse wurden berücksichtigt. Photolyse bei stratosphärisch relevanten Wellenlängen > 190 nm zeigte immer Anreicherungen von 15N in beiden Stickstoffatomen des verbleibenden N2O wie auch in 17O und 18O. Die Anreicherungen waren am mittelständigen N-Atom signifikant höher als am endständigen N (mit mittleren Werten für 18O) und stiegen zu größeren Wellenlängen und niedrigeren Temperaturen hin an. Erstmalig wurden für 18O und 15N am endständigen N-Atom Isotopenabreicherungen bei 185 nm-Photolyse festgestellt. Im Gegensatz zur Photolyse waren die Isotopenanreicherungen bei der zweiten wichtigen N2O-Senke, Reaktion mit O(1D) vergleichsweise gering. Jedoch war das positionsabhängige Fraktionierungsmuster dem der Photolyse direkt entgegengesetzt und zeigte größere Anreicherungen am endständigen N-Atom. Demgemäß führen beiden Senkenprozesse zu charakteristischen Isotopensignaturen in stratosphärischem N2O. Weitere N2O-Photolyseexperimente zeigten, daß 15N216O in der Atmosphäre höchstwahrscheinlich mit der statistisch zu erwartenden Häufigkeit vorkommt.Kleine stratosphärische Proben erforderten die Anpassung der massenspektrometrischen Methoden an Permanentflußtechniken, die auch für Messungen an Firnluftproben von zwei antarktischen Stationen verwendet wurden. Das 'Firnluftarchiv' erlaubte es, den gegenwärtigen Trend und die präindustriellen Werte der troposphärischen N2O-Isotopensignatur zu bestimmen. Ein daraus konstruiertes globales N2O-Isotopenbudget ist im Einklang mit den besten Schätzungen der Gesamt-N2O-Emissionen aus Böden und Ozeanen.17O-Messungen bestätigten die Sauerstoffisotopenanomalie in atmosphärischem N2O, zeigten aber auch, daß N2O-Photolyse die Sauerstoffisotope gemäß einem massenabhängigen Fraktionierungsgesetz anreichert. Eine troposphärische Ursache für einen Teil des Exzeß-17O wurde vorgeschlagen, basierend auf der Reaktion von NH2 mit NO2, wodurch die Sauerstoffisotopenanomalie von O3 über NO2 an N2O übertragen wird.

Petrogenesis of dunite, wehrlite and websterite xenoliths from Kimberley, South Africa: Origin as mantle peridotites of cumulates

Dunite, wehrlite and websterite xenoliths occur amongst a large abundance of mantle xenoliths in kimberlites of the Kimberley cluster in South Africa. Up to know they have mostly been neglected. On the basis of texture, major and trace elements, oxygen isotopes as well as Re-Os isotope characteristics, they can be subdivided into two groups. A coarse-grained mantle peridotite group, comprising dunite, wehrlite and websterite xenoliths, that are similar to fertile peridotites and represent upper mantle assemblages that are differently influenced by mantle metasomatism. And a cumulate group, containing fine-grained Fe-rich dunite xenoliths that represent cumulates of flood basalt magmatism related to ~183 Ma Karoo and ~2.7 Ga Ventersdorp events in southern Africa. Dunite, wehrlite and websterite xenoliths have preserved a complex history of melt depletion and metasomatic re-enrichment events, which gives information about the different re-enrichment stages of the subcratonic lithospheric mantle and the spatial differences within the Kaapvaal craton upper mantle. Websterite xenoliths comprise orthopyroxene (40-85 Vol. %), clinopyroxene (5-42 Vol. %), garnet (4-10 Vol. %) and subordinately olivine, while dunite and wehrlite xenoliths contain predominantly olivine (65-100 Vol %) and subordinately orthopyroxene, clinopyroxene and garnet. High melt depletion and a dunitic to harzburgitic protolith composition are reflected by high forsterite (Fo90-92) and high olivine NiO contents (2800-5000 ppm) and high orthopyroxene Mg# (Mg/(Mg+Fe)) of 0.91-0.93. Re-depletion ages of predominantly 2.9 Ga reflect a minimum age of melt depletion. Melt depletion ceased in conjunction with collision of the Kimberley block with the Witwatersrand block ~2.9 Ga ago. Subduction related re-fertilisation of the previously depleted mantle xenoliths is documented by i) amoeboid textured orthopyroxene, clinopyroxene and garnet, which crystallized in schlieren along olivine grain boundaries, ii) high whole-rock SiO2, Al2O3, CaO, TiO2, FeO contents, iii) low oxygen isotope ratios in clinopyroxene and garnet of 4.8-5.4 ‰ and 4.7-5.3 ‰, respectively and iv) trace element compositions of wehrlitic clinopyroxene and garnet in equilibrium with high-pressure partial melts of eclogite. Trace element disequilibrium of orthopyroxene with clinopyroxene and garnet indicates a separate origin for orthopyroxene, on one side as primary mantle orthopyroxene in dunite and wehrlite xenoliths and on the other side as reaction product with Si-rich melts produced by partial melting of eclogite. This reaction triggered replacement of olivine by orthopyroxene in the surrounding mantle and produced the typical Si-rich composition of Kaapvaal mantle peridotites. Partial melting of eclogite at higher temperatures produced a second metasomatic melt with lower SiO2, but higher Al2O3, CaO, FeO, Ti, Zr, Hf and a low oxygen isotope ratio. This melt triggered clinopyroxene and locally garnet and rutile crystallization in percolation veins, replacing olivine and orthopyroxene in the Kaapvaal upper mantle. Additionally, websterite xenoliths have experienced late stage cryptic metasomatism by the host kimberlite melt, changing the trace element composition of clinopyroxene, orthopyroxene and garnet to different extent. Hence websterite and most fertile lherzolite xenoliths have experienced three metasomatic events: i) reaction with high-Si melt, ii) percolation of subduction related silica melt with lower SiO2 content and iii) cryptic metasomatism by kimberlite. In contrast, dunite and wehrlite xenoliths have only experienced the second metasomatic event. They represent mantle lithologies further away from metasomatising agents. The Fe-rich dunites comprise olivine neoblasts with subordinate olivine porphyroclasts and parallel-orientated needles of ilmenite, which may enclose spinel. The lower forsterite and NiO contents of olivine in Fe-rich dunites compared to mantle peridotite xenoliths (Fo87-89 vs. Fo93-95 and 1300-2800ppm vs. 2200-3900 ppm, respectively), rules out a restitic origin. Cr-rich spinels are remnants of the original cumulate mineralogy that survived a late stage metasomatic overprint related to the production of the host kimberlite, producing ilmenite and phlogopite in some samples. Olivine porphyroclasts and neoblasts have different trace element compositions, the latter having high Ti, V, Cr and Ni and low Zn, Zr and Nb contents, indicating contrasting origins for neoblasts and porphyroclasts. The dunites have high 187Os/188Os ratios (0.11-0.15) indicating young (Phanerozoic) model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyroclastic material. Most Fe-rich dunite xenoliths can be interpreted as cumulates of fractional crystallization of Karoo magmatism, whereas the porphyroclasts are interpreted to be remnants from the much earlier Archaean Ventersdorp magmatism.

Mediterranean-type climate in the South Aegean (Eastern Mediterranean) during the Late Miocene: Evidence from isotope and element proxies

The present-day climate in the Mediterranean region is characterized by mild, wet winters and hot, dry summers. There is contradictory evidence as to whether the present-day conditions (“Mediterranean climate”) already existed in the Late Miocene. This thesis presents seasonally-resolved isotope and element proxy data obtained from Late Miocene reef corals from Crete (Southern Aegean, Eastern Mediterranean) in order to illustrate climate conditions in the Mediterranean region during this time. There was a transition from greenhouse to icehouse conditions without a Greenland ice sheet during the Late Miocene. Since the Greenland ice sheet is predicted to melt fully within the next millennia, Late Miocene climate mechanisms can be considered as useful analogues in evaluating models of Northern Hemispheric climate conditions in the future. So far, high resolution chemical proxy data on Late Miocene environments are limited. In order to enlarge the proxy database for this time span, coral genus Tarbellastraea was evaluated as a new proxy archive, and proved reliable based on consistent oxygen isotope records of Tarbellastraea and the established paleoenvironmental archive of coral genus Porites. In combination with lithostratigraphic data, global 87Sr/86Sr seawater chronostratigraphy was used to constrain the numerical age of the coral sites, assuming the Mediterranean Sea to be equilibrated with global open ocean water. 87Sr/86Sr ratios of Tarbellastraea and Porites from eight stratigraphically different sampling sites were measured by thermal ionization mass spectrometry. The ratios range from 0.708900 to 0.708958 corresponding to ages of 10 to 7 Ma (Tortonian to Early Messinian). Spectral analyses of multi-decadal time-series yield interannual δ18O variability with periods of ~2 and ~5 years, similar to that of modern records, indicating that pressure field systems comparable to those controlling the seasonality of present-day Mediterranean climate existed, at least intermittently, already during the Late Miocene. In addition to sea surface temperature (SST), δ18O composition of coral aragonite is controlled by other parameters such as local seawater composition which as a result of precipitation and evaporation, influences sea surface salinity (SSS). The Sr/Ca ratio is considered to be independent of salinity, and was used, therefore, as an additional proxy to estimate seasonality in SST. Major and trace element concentrations in coral aragonite determined by laser ablation inductively coupled plasma mass spectrometry yield significant variations along a transect perpendicular to coral growth increments, and record varying environmental conditions. The comparison between the average SST seasonality of 7°C and 9°C, derived from average annual δ18O (1.1‰) and Sr/Ca (0.579 mmol/mol) amplitudes, respectively, indicates that the δ18O-derived SST seasonality is biased by seawater composition, reducing the δ18O amplitude by 0.3‰. This value is equivalent to a seasonal SSS variation of 1‰, as observed under present-day Aegean Sea conditions. Concentration patterns of non-lattice bound major and trace elements, related to trapped particles within the coral skeleton, reflect seasonal input of suspended load into the reef environment. δ18O, Sr/Ca and non-lattice bound element proxy records, as well as geochemical compositions of the trapped particles, provide evidence for intense precipitation in the Eastern Mediterranean during winters. Winter rain caused freshwater discharge and transport of weathering products from the hinterland into the reef environment. There is a trend in coral δ18O data to more positive mean δ18O values (–2.7‰ to –1.7‰) coupled with decreased seasonal δ18O amplitudes (1.1‰ to 0.7‰) from 10 to 7 Ma. This relationship is most easily explained in terms of more positive summer δ18O. Since coral diversity and annual growth rates indicate more or less constant average SST for the Mediterranean from the Tortonian to the Early Messinian, more positive mean and summer δ18O indicate increasing aridity during the Late Miocene, and more pronounced during summers. The analytical results implicate that winter rainfall and summer drought, the main characteristics of the present-day Mediterranean climate, were already present in the Mediterranean region during the Late Miocene. Some models have argued that the Mediterranean climate did not exist in this region prior to the Pliocene. However, the data presented here show that conditions comparable to those of the present-day existed either intermittently or permanently since at least about 10 Ma.

Calibration study of bivalve shells : implications for paleoenvironmental reconstruction

The accretionary shells of bivalve mollusks can provide environmental information, such as water temperature, precipitation, freshwater fluxes, primary productivity and anthropogenic activities in the form of variable growth rates and variable geochemical properties, such as stable oxygen and carbon isotopes. However, paleoenvironmental reconstructions are constrained by uncertainties about isotopic equilibrium fractionation during shell formation, which is generally acknowledged as a reasonable assumption for bivalves, but it has been disputed in several species. Furthermore, the variation in shell growth rates is accepted to rely on multiple environmental variables, such as temperature, food availability and salinity, but can differ from species to species. Therefore, it is necessary to perform species-specific calibration studies for both isotope proxies and shell growth rates before they can be used with confidence for environmental interpretations of the past. Accordingly, the principal objective of this Ph.D research is to examine the reliability of selected bivalve species, the long-lived Eurhomalea exalbida (Dillwyn), the short-lived and fast growing species Paphia undulata (Born 1778), and the freshwater mussel Margaritifera falcata (Gould 1850), as paleoenvironmental proxy archives.rnThe first part is focused on δ18Oshell and shell growth history of live-collected E. exalbida from the Falkland Islands. The most remarkable finding, however, is that E. exalbida formed its shell with an offset of -0.48‰ to -1.91‰ from the expected oxygen isotopic equilibrium with the ambient water. If this remained unnoticed, paleotemperature estimates would overestimate actual water temperatures by 2.1-8.3°C. With increasing ontogenetic age, the discrepancy between measured and reconstructed temperatures increased exponentially, irrespective of the seasonally varying shell growth rates. This study clearly demonstrates that, when the disequilibrium fractionation effect is taken into account, E. exalbida can serve as a high-resolution paleoclimate archive for the southern South America. The species therefore provides quantifiable temperature estimates, which yields new insights into long-term paleoclimate dynamics for mid to high latitudes on the southern hemisphere.rnThe stable carbon isotope of biogenic carbonates is generally considered to be useful for reconstruction of seawater dissolved inorganic carbon. The δ13Cshell composition of E. exalbida was therefore, investigated in the second part of this study. This chapter focuses on inter-annual and intra-annual variations in δ13Cshell. Environmental records in δ13Cshell are found to be strongly obscured by changes in shell growth rates, even if removing the ontogenetic decreasing trend. This suggests that δ13Cshell in E. exalbida may not be useful as an environmental proxy, but a potential tool for ecological investigations. rnIn addition to long-lived bivalve species, short-lived species that secrete their shells extremely fast, can also be useful for environmental reconstructions, especially as a high-resolution recorder. Therefore, P. undulata from Daya Bay, South China Sea was utilized in Chapter 4 to evaluate and establish a potential proxy archive for past variations of the East Asian monsoon on shorter time-scales. The δ18Oshell can provide qualitative estimates of the amount of monsoonal rain and terrestrial runoff and the δ13Cshell likely reflect the relative amount of isotopically light terrestrial carbon that reaches the ocean during the summer monsoon season. Therefore, shells of P. undulata can provide serviceable proxy archives to reconstruct the frequency of exceptional summer monsoons in the past. The relative strength of monsoon-related precipitation and associated changes in ocean salinity and the δ13C ratios of the dissolved inorganic carbon signature (δ13CDIC) can be estimated from the δ18Oshell and δ13Cshell values as well as shell growth patterns. rnIn the final part, the freshwater pearl shell M. falcata from four rivers in British Columbia, Canada was preliminarily studied concerning the lifespans and the shell growth rates. Two groups separated by the Georgia Strait can be clearly distinguished. Specimens from the western group exhibit a shorter lifespan, while the eastern group live longer. Moreover, the average lifespan seems to decrease from south to north. The computed growth equations from the eastern and western groups differ as well. The western group exhibits a lower growth rate, while bivalves from the eastern group grow faster. The land use history seems to be responsible for the differences in lifespans of the specimens from the two groups. Differences in growth rate may be induced by differences in water temperature or nutrient input also related to the land use activities.