Computer simulations of microstructures related to the olivine spinel transition

Tiefherd-Beben, die im oberen Erdmantel in einer Tiefe von ca. 400 km auftreten, werden gewöhnlich mit dem in gleicher Tiefe auftretenden druckabhängigen, polymorphen Phasenübergang von Olivine (α-Phase) zu Spinel (β-Phase) in Verbindung gebracht. Es ist jedoch nach wie vor unklar, wie der Phasenübergang mit dem mechanischen Versagen des Mantelmaterials zusammenhängt. Zur Zeit werden im Wesentlichen zwei Modelle diskutiert, die entweder Mikrostrukturen, die durch den Phasenübergang entstehen, oder aber die rheologischen Veränderungen des Mantelgesteins durch den Phasenübergang dafür verantwortlich machen. Dabei sind Untersuchungen der Olivin→Spinel Umwandlung durch die Unzugänglichkeit des natürlichen Materials vollständig auf theoretische Überlegungen sowie Hochdruck-Experimente und Numerische Simulationen beschränkt. Das zentrale Thema dieser Dissertation war es, ein funktionierendes Computermodell zur Simulation der Mikrostrukturen zu entwickeln, die durch den Phasenübergang entstehen. Des Weiteren wurde das Computer Modell angewandt um die mikrostrukturelle Entwicklung von Spinelkörnern und die Kontrollparameter zu untersuchen. Die Arbeit ist daher in zwei Teile unterteilt: Der erste Teil (Kap. 2 und 3) behandelt die physikalischen Gesetzmäßigkeiten und die prinzipielle Funktionsweise des Computer Modells, das auf der Kombination von Gleichungen zur Errechnung der kinetischen Reaktionsgeschwindigkeit mit Gesetzen der Nichtgleichgewichtsthermodynamik unter nicht-hydostatischen Bedingungen beruht. Das Computermodell erweitert ein Federnetzwerk der Software latte aus dem Programmpaket elle. Der wichtigste Parameter ist dabei die Normalspannung auf der Kornoberfläche von Spinel. Darüber hinaus berücksichtigt das Programm die Latenzwärme der Reaktion, die Oberflächenenergie und die geringe Viskosität von Mantelmaterial als weitere wesentliche Parameter in der Berechnung der Reaktionskinetic. Das Wachstumsverhalten und die fraktale Dimension von errechneten Spinelkörnern ist dabei in guter Übereinstimmung mit Spinelstrukturen aus Hochdruckexperimenten. Im zweiten Teil der Arbeit wird das Computermodell angewandt, um die Entwicklung der Oberflächenstruktur von Spinelkörnern unter verschiedenen Bedigungen zu eruieren. Die sogenannte ’anticrack theory of faulting’, die den katastrophalen Verlauf der Olivine→Spinel Umwandlung in olivinhaltigem Material unter differentieller Spannung durch Spannungskonzentrationen erklärt, wurde anhand des Computermodells untersucht. Der entsprechende Mechanismus konnte dabei nicht bestätigt werden. Stattdessen können Oberflächenstrukturen, die Ähnlichkeiten zu Anticracks aufweisen, durch Unreinheiten des Materials erklärt werden (Kap. 4). Eine Reihe von Simulationen wurde der Herleitung der wichtigsten Kontrollparameter der Reaktion in monomineralischem Olivin gewidmet (Kap. 5 and Kap. 6). Als wichtigste Einflüsse auf die Kornform von Spinel stellten sich dabei die Hauptnormalspannungen auf dem System sowie Heterogenitäten im Wirtsminerals und die Viskosität heraus. Im weiteren Verlauf wurden die Nukleierung und das Wachstum von Spinel in polymineralischen Mineralparagenesen untersucht (Kap. 7). Die Reaktionsgeschwindigkeit der Olivine→Spinel Umwandlung und die Entwicklung von Spinelnetzwerken und Clustern wird durch die Gegenwart nicht-reaktiver Minerale wie Granat oder Pyroxen erheblich beschleunigt. Die Bildung von Spinelnetzwerken hat das Potential, die mechanischen Eigenschaften von Mantelgestein erheblich zu beeinflussen, sei es durch die Bildung potentieller Scherzonen oder durch Gerüstbildung. Dieser Lokalisierungprozess des Spinelwachstums in Mantelgesteinen kann daher ein neues Erklärungsmuster für Tiefbeben darstellen.

Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene-Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and Ar-40/Ar-39 ages of lavas indicate that basin development and volcanism commenced by 12.5-11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5-9.3 Ma) than western sections (9.6-8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

Impact of suborbital climate changes in the North Atlantic on ice sheet dynamics at the Mid-Pleistocene Transition

[1] Early and Mid-Pleistocene climate, ocean hydrography and ice sheet dynamics have been reconstructed using a high-resolution data set (planktonic and benthicδ18O time series, faunal-based sea surface temperature (SST) reconstructions and ice-rafted debris (IRD)) record from a high-deposition-rate sedimentary succession recovered at the Gardar Drift formation in the subpolar North Atlantic (Integrated Ocean Drilling Program Leg 306, Site U1314). Our sedimentary record spans from late in Marine Isotope Stage (MIS) 31 to MIS 19 (1069–779 ka). Different trends of the benthic and planktonic oxygen isotopes, SST and IRD records before and after MIS 25 (∼940 ka) evidence the large increase in Northern Hemisphere ice-volume, linked to the cyclicity change from the 41-kyr to the 100-kyr that occurred during the Mid-Pleistocene Transition (MPT). Beside longer glacial-interglacial (G-IG) variability, millennial-scale fluctuations were a pervasive feature across our study. Negative excursions in the benthicδ18O time series observed at the times of IRD events may be related to glacio-eustatic changes due to ice sheets retreats and/or to changes in deep hydrography. Time series analysis on surface water proxies (IRD, SST and planktonicδ18O) of the interval between MIS 31 to MIS 26 shows that the timing of these millennial-scale climate changes are related to half-precessional (10 kyr) components of the insolation forcing, which are interpreted as cross-equatorial heat transport toward high latitudes during both equinox insolation maxima at the equator.

Regulation of the heat shock response by thiol-reactive compounds in the yeast Saccharomyces cerevisiae

Cells govern their activities and modulate their interactions with the environment to achieve homeostasis. The heat shock response (HSR) is one of the most well studied fundamental cellular responses to environmental and physiological challenges, resulting in rapid synthesis of heat shock proteins (HSPs), which serve to protect cellular constituents from the deleterious effects of stress. In addition to its role in cytoprotection, the HSR also influences lifespan and is associated with a variety of human diseases including cancer, aging and neurodegenerative disorders. In most eukaryotes, the HSR is primarily mediated by the highly conserved transcription factor HSF1, which recognizes target hsp genes by binding to heat shock elements (HSEs) in their promoters. In recent years, significant efforts have been made to identify small molecules as potential pharmacological activators of HSF1 that could be used for therapeutic benefit in the treatment of human diseases relevant to protein conformation. However, the detailed mechanisms through which these molecules drive HSR activation remain unclear. In this work, I utilized the baker's yeast Saccharomyces cerevisiae as a model system to identify a group of thiol-reactive molecules including oxidants, transition metals and metalloids, and electrophiles, as potent activators of yeast Hsf1. Using an artificial HSE-lacZ reporter and the glucocorticoid receptor system (GR), these diverse thiol-reactive compounds are shown to activate Hsf1 and inhibit Hsp90 chaperone complex activity in a reciprocal, dose-dependent manner. To further understand whether cells sense these reactive compounds through accumulation of unfolded proteins, the proline analog azetidine-2-carboxylic acid (AZC) and protein cross-linker dithiobis(succinimidyl propionate) (DSP) were used to force misfolding of nascent polypeptides and existing cytosolic proteins, respectively. Both unfolding reagents display kinetic HSP induction profiles dissimilar to those generated by thiol-reactive compounds. Moreover, AZC treatment leads to significant cytotoxicity, which is not observed in the presence of the thiol-reactive compounds at the concentrations sufficient to induce Hsf1. Additionally, DSP treatment has little to no effect on Hsp90 functions. Together with the ultracentrifugation analysis of cell lysates that detected no insoluble protein aggregates, my data suggest that at concentrations sufficient to induce Hsf1, thiol-reactive compounds do not induce the HSR via a mechanism based on accumulation of unfolded cytosolic proteins. Another possibility is that thiol-reactive compounds may influence aspects of the protein quality control system such as the ubiquitin-proteasome system (UPS). To address this hypothesis, β-galactosidase reporter fusions were used as model substrates to demonstrate that thiol-reactive compounds do not inhibit ubiquitin activating enzymes (E1) or proteasome activity. Therefore, thiol-reactive compounds do not activate the HSR by inhibiting UPS-dependent protein degradation. I therefore hypothesized that these molecules may directly inactivate protein chaperones, known as repressors of Hsf1. To address this possibility, a thiol-reactive biotin probe was used to demonstrate in vitro that the yeast cytosolic Hsp70 Ssa1, which partners with Hsp90 to repress Hsf1, is specifically modified. Strikingly, mutation of conserved cysteine residues in Ssa1 renders cells insensitive to Hsf1 activation by cadmium and celastrol but not by heat shock. Conversely, substitution with the sulfinic acid and steric bulk mimic aspartic acid led to constitutive activation of Hsf1. Cysteine 303, located in the nucleotide-binding/ATPase domain of Ssa1, was shown to be modified in vivo by a model organic electrophile using Click chemistry technology, verifying that Ssa1 is a direct target for thiol-reactive compounds through adduct formation. Consistently, cadmium pretreatment promoted cells thermotolerance, which is abolished in cells carrying SSA1 cysteine mutant alleles. Taken together, these findings demonstrate that Hsp70 acts as a sensor to induce the cytoprotective heat shock response in response to environmental or endogenously produced thiol-reactive molecules and can discriminate between two distinct environmental stressors.

Water chemistry and heat budget of the Churchill River estuary region

A conceptual scheme for the transition from winter to spring is developed for a small Arctic estuary (Churchill River, Hudson Bay) using hydrological, meteorological and oceanographic data together with models of the landfast ice. Observations within the Churchill River estuary and away from the direct influence of the river plume (Button Bay), between March and May 2005, show that both sea ice (production and melt) and river water influence the region's freshwater budget. In Button Bay, ice production in the flaw lead or polynya of NW Hudson Bay result in salinization through winter until the end of March, followed by a gradual freshening of the water column through April-May. In the Churchill Estuary, conditions varied abruptly throughout winter-spring depending on the physical interaction among river discharge, the seasonal landfast ice, and the rubble zone along the seaward margin of the landfast ice. Until late May, the rubble zone partially impounded river discharge, influencing the surface salinity, stratification, flushing time, and distribution and abundance of nutrients in the estuary. The river discharge, in turn, advanced and enhanced sea ice ablation in the estuary by delivering sensible heat. Weak stratification, the supply of riverine nitrogen and silicate, and a relatively long flushing time (~6 days) in the period preceding melt may have briefly favoured phytoplankton production in the estuary when conditions were still poor in the surrounding coastal environment. However, in late May, the peak flow and breakdown of the ice-rubble zone around the estuary brought abrupt changes, including increased stratification and turbidity, reduced marine and freshwater nutrient supply, a shorter flushing time, and the release of the freshwater pool into the interior ocean. These conditions suppressed phytoplankton productivity while enhancing the inventory of particulate organic matter delivered by the river. The physical and biological changes observed in this study highlight the variability and instability of small frozen estuaries during winter-spring transition, which implies sensitivity to climate change.

Heat flow ratios of stations from the Iberian abyssal plain (Table 1)

New heat flow observations have been made in the Iberia abyssal plain off the Galicia margin along the transeat of Ocean Drilling Program Leg 149 drill sites. in order to investigate the nature of this unusually wide and deep continet-ocean transition region. Our results indicate the presence of three separate zones. Average values of 47.5 +/- 3 mW/m in the westernmost zone III agree with predictions of standard oceanic lithospheric models for its estimated age of 126 Ma. In contrast, the heat flow within zone II is 5-15 mW/m higher than predicted. assuming that the mantle heat flow remains constant across the basin. This region of high values is coincident with the location of a major intra-crustal "S"-type reflector east of ODP Site 900. and the anomaly is consistent with the presence of 2-3 km of primarily upper continental crust above the reflector, with concentrations of radiogenic components similar to those from granodiorite samplles dredged off Galicia Bank. It is not, however, consistent with the low values of heat production measured on gabbroic sanhples from its western end at ODP Site 900. In zone I, detailed measurements across the tilted fault block south of ODP Site 901 show consistent variations which closely match predictions due to the effects of basement structure and sediment deposition. There is no evidence for variations due to vertical convective transport along the dipping basement fault block. Once corrected for these variations. measurements in zone I yield average values that agree quite well with previous measurements across Calicia Bank. indicating no systematic landward increase in heat flow with decreasing amounts of continental, extension.

Transition from isentropic to isothermal expansion in laser produced plasma

The transition that the expansion flow of laser-produced plasmas experiences when one moves from long, low intensity pulses (temperature vanishing at the isentropic plasma-vacuum front,lying at finite distance) to short, intense ones (non-zero, uniform temperature at the plasma-vacuum front, lying at infinity) is studied. For plznar geometry and lqge ion number Z, the transition occurs for dq5/dt=0.14(27/8)k712Z’1zn$/m4f, 12nK,,; mi, and K are laser intensity, critical density,ion mass, and Spitzer’s heat conduction coefficient. This result remains valid for finite Zit,h ough the numerical factor in d$/dt is different. Shorter wavelength lasers and higher 4 plasmas allow faster rising pulses below transition.