ALBERT

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1975

Beschreibung: The Mid-Atlantic Ridge is one of the most well known and yet poorly understood spreading centers in the world. A detailed investigation of the Mid-Atlantic Ridge crest near 37°N (FAMOUS) was conducted using a deeply towed instrument package. The objective was to study the detailed structure and spreading history of the Mid-Atlantic Ridge median valley, to explore the roles of volcanism and faulting in the evolution of oceanic crust, and to study the morphologic expression and structural history of the zone of crustal accretion. In addition, microearthquake surveys were conducted using arrays of free-floating hydrophones. The most recent expression of the accreting plate boundary in the Famous Rift is an alternating series of linear central volcanoes and depressions 1.5 km wide which lie within the inner floor. This lineament is marked by a sharp maximum in crustal magnetization only 2-3 km wide. Magnetic studies indicate that over 90% of the extrusive volcanism occurs within the rift inner floor, a zone 1 to 12 km wide, while volcanism is extremely rare in the rift mountains. Volcanoes created in the inner floor are transported out on, block faults, becoming a lasting part of the topography. Magnetic anomaly transition widths vary from 1 km to 8 km with time and appear to reflect a bi-stable median valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths); or it has a narrow inner floor and wide terraces, the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present the. Famous Rift has a narrow inner floor and volcanic zone (1-3 km) while the south Famous Rift is at the opposite end of the cycle with a wide inner floor and volcanic zone (10-12 km). Over 95% of the large scale (〉2 km) relief of the median valley is accounted for by normal faults dipping toward the valley axis. Normal faulting along fault planes dipping away from the valley begins just past the outer walls of the valley. Outward facing normal faulting accounts for most of the decay of median valley relief in the rift mountains while crustal tilting accounts for less than 20%. The pattern of normal faulting creates a broad, undulating horst and graben relief. Volcanic features contribute little to the large scale relief, but contribute to the short wavelength (〈2km) roughness of the topography. Spreading in the Famous area is highly asymmetric with rates twice as high to the east as to the west. At 1.7 m.y.b.p. the sense of asymmetry reverses in direction with spreading faster to the west, resulting in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Structural studies indicate that the asymmetric spreading is accomplished through asymmetric crustal extension as well as asymmetric crustal accretion. Spreading in the Famous area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate boundary system. Spreading has been stably oblique for at least 6 m.y., even through a change in spreading direction. Magnetic studies reveal that the deep DSDP hole at site 332 was drilled into a magnetic polarity transition, and may have sampled rocks which recorded the earth i s field behavior during a reversal. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been determined, and may be due to old crust left behind or recording of a geomagnetic field event. Crustal magnetization decays to lie of its initial value in less than 0.6 m.y. The rapid decay may be facillitated by very intense crustal fracturing observed in the inner floor. Microearthquake, magnetic and structural studies indicate that both the spreading and transform plate boundaries are very narrow (1-2 km) and well-defined for short periods, but migrate over zones 10-20 km wide through time.

Beschreibung: MapTool is an interactive computer program for the display of common marine geophysical data. At present, the program displays isolines, color-filled contours, navigation tracklines, and navigated scalar values in a variety of styles. A variety of map projections are supported. This document describes the basic requirements for running the MapTool program, for creating various displays, and generating hard copy output. The supported data file formats are described. All of the options, displays, menus, and windows are documented.

Beschreibung: Funding was provided by the Office of Naval Research under Grant N00014-90-J-1621.

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1971

Beschreibung: An experimental investigation of the different flow regimes in a rotating source-sink annulus is described. Both the steady and transient velocities are measured over a large range of Ekman Reynolds number and Rossby number. Differing probe configurations are used to investigate the corresponding motions in spatially separated regions of the annulus. The steady interior circulation field exhibits a strong dependence on the imposed flux values. The non-dimensional circulation increases with radius over a certain radial range for higher system Rossby number. The observed profile changes are related to the existence of an unstable Ekman layer at some inner radial position. The thickness of the observed Ekman layers is typically 85% of the theoretical scale height. For higher local Reynolds number (ReL), the thickness is generally much smaller. The width of the sidewall boundary layer adjacent to the sink increases with larger system Rossby number. Adjacent to the source, the radial boundary layer is wider than that at the sink wall. Observed oscillations are separable into three types. For ReL 〉 50, instability waves are observed in the Ekman layer flow. In the same Re range, inertial oscillations are detected in the interior region of the annulus. The observed inertial wave frequency at differing radial positions is explained by incorporating Doppler shift corrections and taking account of the steady circulation profiles. The radial wavelength of the inertial waves corresponds to the length of the Class A Ekman layer instabi1ity. For small values of Re and local Rossby number, an axisymmetric disturbance, with a characteristic frequency slightly greater than therotation rate, is observed at the outer radial positions.

Beschreibung: An important question in geophysics is whether earthquakes (EQs) can be anticipated prior to their occurrence. Pre-seismic electromagnetic (EM) emissions provide a promising window through which the dynamics of EQ preparation can be investigated. However, the existence of precursory features in pre-seismic EM emissions is still debatable: in principle, it is difficult to prove associations between events separated in time, such as EQs and their EM precursors. The scope of this paper is the investigation of the pre-seismic EM activity in terms of complexity. A basic reason for our interest in complexity is the striking similarity in behavior close to irreversible phase transitions among systems that are otherwise quite different in nature. Interestingly, theoretical studies (Hopfield, 1994; Herz and Hopfield 1995; Rundle et al., 1995; Corral et al., 1997) suggest that the EQ dynamics at the final stage and neural seizure dynamics should have many similar features and can be analyzed within similar mathematical frameworks. Motivated by this hypothesis, we evaluate the capability of linear and non-linear techniques to extract common features from brain electrical activities and pre-seismic EM emissions predictive of epileptic seizures and EQs respectively. The results suggest that a unified theory may exist for the ways in which firing neurons and opening cracks organize themselves to produce a large crisis, while the preparation of an epileptic shock or a large EQ can be studied in terms of “Intermittent Criticality”.

Beschreibung: Numerous factors control the topographic evolution of mountain belts. Crustal thickening, rock uplift rate, and denudational forcing doubtlessly interact, but the feedback mechanisms amongst these are disputed, because they operate over entirely different time scales. Cosmogenically-derived denudation rates cover time-scales of 10(3) to 10(5) years, providing a denudational tool that allows us to shed light on interactions between long-term topography-forming processes and short-term factors destroying topography. Prerequisites for the application of this method in presently uplifting and fast-eroding mountain belts like the Central Alps are an investigation of appropriate watershed sizes for systematic sampling. Denudation rates in Maggia tributaries of various sizes reveal that the trunk stream yields statistically the same denudation rate (0.85 ± 0.14 mm/yr) as the tributaries (0.74 ± 0.14 mm/yr). Therefore, sampling of large watersheds is a feasible approach. Denudation rates of watersheds from the Central Alps are amongst the highest ever measured in similar complex settings, ranging in mean from 0.27 ± 0.05 mm/yr for the Alpine foreland to 1.42 ± 0.4 mm/yr for the high crystalline Central Alps. The measured cosmogenic denudation rates are in good agreement with post-LGM lake infill rates; they are significantly higher than recent denudation rates from river loads. We attribute this discrepancy to differences in methodology and integration time scale. We will show that denudation is high in areas of high altitude and high relief. Furthermore, our data shows that denudation rates are low in areas of low rock uplift, and are high in areas of high rock uplift, respectively. It appears that rock uplift and denudation are intimately linked. It follows that either crustal thickening is generating rock uplift; the mountain belt reacts with erosional unloading. Alternatively, high precipitation and glaciers, most pronounced at high altitude, result in high denudation rates at these sites. Topography then would respond by increasing rock uplift.

Beschreibung: The Aral Sea Basin is a critical area for studying the influence of climate and anthropogenic impact on the development of hydrographic conditions in an endorheic basin. We present organic-walled dinoflagellate cyst analyses with a sampling resolution of 15 to 20 years from a core retrieved at Chernyshov Bay in the NW Large Aral Sea (Kazakhstan). Cysts are present throughout, but species richness is low (seven taxa). The dominant morphotypes are Lingulodinium machaerophorum with varied process length and Impagidinium caspienense, a species recently described from the Caspian Sea. Subordinate species are Caspidinium rugosum, Romanodinium areolatum, Spiniferites cruciformis, cysts of Pentapharsodinium dalei, and round brownish protoperidiniacean cysts. The chlorococcalean algae Botryococcus and Pediastrum are taken to represent freshwater inflow into the Aral Sea. The data are used to reconstruct salinity as expressed in lake level changes during the past 2000 years. We quantify and date for the first time prominent salinity variations from the northern part of the Large Aral Sea. During high lake levels, I. caspienense, representing brackish conditions with salinities of about 10–15 g kg− 1 or less, prevails. Assemblages dominated by L. machaerophorum document lake lowstands during approximately 0–425 AD (or 100? BC–425 AD), 920–1230 AD, 1500 AD, 1600–1650 AD, 1800 AD and since the 1960s. Because salinity in the Aral Sea is mostly controlled by meltwater discharges from the Syr Darya and Amu Darya rivers, we interpret changes in salinity levels as a proxy for temperature fluctuations in the Tien Shan Mountains that control snow melt. Significant erosion of marine Palaeogene and Neogene deposits in the hinterland, evidenced between 1230 AD and 1400 AD, is regarded as sheet-wash from shore. This is controlled by the low pressure system that develops over the Eastern Mediterranean and brings moist air to the Middle East and Central Asia during late spring and summer. We propose that the recorded environmental changes are related primarily to climate, but perhaps to a lesser extent by human-controlled irrigation activities. Our results documenting climate change in western Central Asia are fairly consistent with reports elsewhere from Central Asia.

Beschreibung: The P and S seismic anomalies in the upper mantle beneath the Pamir–Hindukush collision zone and surrounding areas are investigated using more than 4 × 10 hoch 5 traveltimes from 5775 events located in the study region, recorded by the stations of the worldwide seismic network and reported in the ISC catalogue. All available epicentral distance ranges of seismic rays are considered. All sources are relocated using an algorithm, which includes the double-difference method. In addition, source parameters are corrected in the main inversion step, simultaneously with the velocity models. The high reliability of the resulting P-velocity images is confirmed by various tests. The S model provides much smoother patterns, which are, however, generally consistent with P anomalies. Our model shows a complex image of lithospheric plate under the study region. Under SW Hindukush, the northwards subducting Indian plate is imaged throughout the upper mantle and transition zone. Further to the NE, within a 300-km-wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at ~250 km depth. The tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern Tien Shan down to at least 250 km depth. To the north of Tarim, we observe a northward-dipping of the Tarim plate down to ~500 km depth. At the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults, which indicate complex crustal structure in this region.

Beschreibung: This paper presents results of geochemical investigations of lake sediments from the karst lake An Loch Mór, Aran Islands, including the first highly resolved record of atmospheric Roman Pb pollution for Ireland. The natural Pb influx into the lake is largely contributed by 3 Pb components, which differ in their isotopic composition: detrital influx of Pb from the siliciclastic input, dissolved influx of Pb released by weathering of the local limestone, and dissolved influx of seawater Pb. The balance between the 3 Pb components varies in concert with the hydrological evolution of the lake. The influx of Pb in dissolved form is estimated by geochemical mass balance assuming that the siliciclastic influx is characterised by the Pb/Al-ratio of the Late Glacial clastic sediments. It typically accounts for 50–80% of total Pb input in the Holocene sediments of An Loch Mór. The natural dissolved influxes of Pb, Sc, and Y reach a similar order of magnitude. Normalisation with Sc and Y is applied to quantify contributions from anthropogenic Pb. Based on continuous sampling of 1 cm sample slices, variations in the influx of Roman Pb could be reconstructed at a time resolution of c. 5 a. Combined geochemical and Pb isotope mass balance is used to characterise the isotopic composition of anthropogenic Pb. Distinctly enhanced influx of anthropogenic Pb occurs in the 1st and 2nd century AD and shows high variability on decadal scale. This is in contrast to central European Pb records, which document a gradual increase and decrease in ancient atmospheric pollution by Roman Pb. The reconstructed high variability in the influx of Roman Pb in An Loch Mór documents variations in the wind regime of western Europe, temporarily favouring the transport of atmospheric Pb to western Ireland.

Beschreibung: A high-resolution image of the base of the lithosphere from S-to-P converted seismic waves revealed the collision architecture of the Indian and Asian continental plates beneath the Tibetan Plateau. The base of the Indian lithosphere dips northward from a depth of 160 km beneath the Himalayas to a depth of 220 km just south of the Bangong suture. The base of the Asian lithosphere is nearly horizontal at the depth of 160–180 km from central to northern Tibet. There is a vertical gap of about 50 km between Indian and Asian lithospheres. Our observation of a well-defined, thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere, which would predict a very thin Asian lithosphere. The hypothesized sequential southward subduction of Tibetan and Asian continental lithospheres leading to the growth of the Tibetan Plateau, if correct, cannot be occurring below ∼180 km depth. Our results, along with available geological and geophysical data, strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted Indian continental lithosphere in the south, which presently can be traced to the middle of the plateau, and a south dipping subducted Asian lithosphere in the north at a shallower depth.

Beschreibung: The Potsdam Magnetic Model of the Earth (POMME) is a geomagnetic field model providing an estimate of the Earth's core, crustal, magnetospheric, and induced magnetic fields. The internal field is represented to spherical harmonic (SH) degree 90, while the secular variation and acceleration are given to SH degree 16. Static and time-varying magnetospheric fields are parameterized in Geocentric Solar-Magnetospheric (GSM) and Solar-Magnetic (SM) coordinates and include Disturbance Storm-Time (DSt index) and Interplanetary Magnetic field (IFM-By) dependent contributions. The model was estimated from five years of CHAMP satellite magnetic data. All measurements were corrected from ocean tidal induction and night-side ionospheric F-region currents. The model is validated using an independent model from a combined data set of Oersted and SACC-C satellite measurements. For the core field to SH degree 13, the root mean square (RSM) vector difference between the two models at the center of the model period is smaller than 4nT at the Earth's surface. The RMS uncertainty increases to about 100 nT for the predicted fields in 2010, as inferred from the difference between the two models.

Beschreibung: The uplift and evolution of a noncollisional orogen developed along a subduction zone, such as the Andean system, is a direct consequence of the interrelation between plate tectonic stresses and erosion. Tectonic stresses are related to the convergence velocity and thermal state, among other causes. In this paper, a new model designed to investigate the evolution of the topography and the upper crustal deformation of noncollisional orogens in a subduction zone produced by the oceanic crust being subducted is presented. The mechanical behavior of the crust was modeled by means of finite elements methods to solve Stokes equations for a strain-rate-dependent viscoplastic rheology. The model takes into account erosion effects using interface-tracking methods to assign fictitious properties to nonmaterial elements.

Beschreibung: A fully flexible ionic interaction model for alumina, Al2O3, is developed. The model accounts for ionic polarizability and shape deformations up to the quadrupolar level. The parameters of the interaction potential are optimized by reference to ab initio density-functional theory (DFT)-based electronic structure calculations. Two potential models, derived from DFT using the local density approximation (LDA) and the generalized gradient approximation (GGA), respectively are compared. Both potentials reproduce accurately bulk and surface properties of the corundum phase, and transfer well over a wide range of pressures and temperatures. The sequence of high pressure polymorphic phases is predicted correctly. The potential derived from LDA-DFT also gives an excellent description of the melt whereas the potential fitted to GGA-DFT considerably overestimates the equilibrium molar volume of the melt. The accuracy of the potentials regarding, e.g., lattice parameters or elastic constants is comparable to the respective electronic structure method.

Beschreibung: Unsere Erde ist von einem Magnetfeld umgeben, welches durch Vorgänge im Erdkern erzeugt wird, die wir nun im Großen und Ganzen verstehen. Das Erdmagnetfeld ist sowohl in der Stärke als auch in seiner Ausrichtung zeitlich variabel. Ziel unserer Arbeit ist es, die Veränderungen des Magnetfelds zu analysieren und Prognosen für seine zukünftige Entwicklung möglich zu machen.