The Transport Time pdf for a Simple Turbulent Flow

In a turbulent fluid, the time a particle needs to travel from a point source to the observation point, can be considered as a random variable. It is shown that the probability density function (pdf) for this random variable is determined by the Lagrangian particle position pdf. The characteristics of the transport time pdf are discussed for the simple case of a turbulent fluid moving with a constant mean velocity.     

Temperature assimilation into a coastal ocean-biogeochemical model: assessment of weakly and strongly coupled data assimilation

Ocean Dynamics - Satellite data of both physical properties as well as ocean colour can be assimilated into coupled ocean-biogeochemical models with the aim to improve the model state. The physical...     

Southern Hemisphere Westerly Winds have modulated the formation of laminations in sediments in Lago Fagnano (Tierra del Fuego,Argentina) over the past 6.3 ka

Tierra del Fuego in Argentina is a unique location to examine past Holocene wind variability since it intersects the core of the Southern Hemisphere Westerly Winds (SHWW). The SHWW are the most powerful prevailing winds on Earth. Their variation plays a role in regulating atmospheric CO₂ levels and rainfall amounts and distribution, both today and in the past. We obtained a piston core (LF06-PC8) from Bahía Grande, a protected sub-basin at the southern margin of Lago Fagnano, the largest lake in Tierra del Fuego. This article focuses on the uppermost 185 cm of this core, corresponding to laminated sediment from the last ~6.3 ka. Laminations consist of millimetre-scale paired dark and light layers. Previous studies and new geochemical analysis show that the dark and light layers are characterized by differing concentrations of Mn and Fe. We attribute the distribution of Mn and Fe to episodic hypolimnic oxic–anoxic variations. The age model suggests an approximately bidecadal timescale for the formation of each layer pair. We propose a new model of these redox changes with the SHWW variations. The most likely phenomenon to produce complete water-column mixing is thermobaric instability, which occurs in colder winters with low-intensity SHWW (El Niño-like conditions). In contrast, windier winters are characterized by higher temperatures and reduced mixing in the water column, facilitating a decline in oxygen concentration. Laminations, and the inferred presence of periodic hypolimnion redox changes, are common features of the past ~6.3 ka. Geochemical proxy variability is compatible with an intensification of El Niño/Southern Oscillation activity during the past ~2 ka.     

The rotations opening the Central and Northern Atlantic Ocean: compilation, drift lines, and flow lines

We provide an up-to-date compilation of Euler rotations that model the evolution of the Central and Northern Atlantic Ocean (Table 1). The data basis forms seafloor spreading magnetic anomalies of the Atlantic. We checked the published rotations and selected those that form a consistent model. The increments of the Euler rotations going back in time from magnetic anomaly to magnetic anomaly can be illustrated by chains of points on “drift lines” that are paths of motions from continent to continent. Along these paths, the continents bordering the Atlantic Ocean can be moved back to their Mesozoic position within Pangea. Other figures exhibit the early rifting of the North Atlantic, the drift of Iberia, and the evolution of the Greenland-Ellesmere region. The points on the drift lines do not correspond directly to the lines of magnetic anomalies or their “picks” displayed today symmetrically in the Atlantic Ocean. To acquire correspondence, symmetric “flow lines” are constructed analogous to the spreading procedure. But points on the flow lines constructed by half of the increments partially also deviate from the expected symmetric position and in this way quantify displacements or jumps of the axis of rifting or spreading. Most of the selected rotations are from the excellent analyses of previous work. Essential deviations from published rotations are the M 0 rotations of Eurasia and of the Porcupine unit with respect to North America (EUR-NAM and POR-NAM). They lead to a better coincidence between the back-rotated M 0 magnetic anomalies in the Bay of Biscay on the one side and a change of the first transform motions between Greenland and Svalbard on the other side. Through this explanation, an overlap in Pangea SW of Svalbard is avoided and transform motions instead of strong extension are predicted. Some additional data are needed to complete the model: the earliest part of the path of Iberia to North America (IBA-NAM) up to M 4 is calculated assuming that Iberia moved parallel to the African plate, though with slower spreading rates. The evolution of the Central and North Atlantic Ocean system is described in short. This model of the Central and North Atlantic was produced with the primary intention of clearing and fixing the positions of Africa, Iberia, and Eurasia as a framework for an improved reconstruction of the Western Tethys evolution.     

DXL: a sounding rocket mission for the study of solar wind charge exchange and local hot bubble X-ray emission

The Diffuse X-rays from the Local galaxy (DXL) mission is an approved sounding rocket project with a first launch scheduled around December 2012. Its goal is to identify and separate the X-ray emission generated by solar wind charge exchange from that of the local hot bubble to improve our understanding of both. With 1,000 cm² proportional counters and grasp of about 10 cm² sr both in the 1/4 and 3/4 keV bands, DXL will achieve in a 5-min flight what cannot be achieved by current and future X-ray satellites.     

Hydrogen-line profiles of six λ Bootis stars

Based on high resolution spectroscopic observations hydrogen-line profiles of six Bootis stars: HD 31295, HD 101108, HD 105058, HD 106223, HD 142703 and HD 183324 are presented. Peculiar line profiles with weak cores and broad but shallow wings are found for four of the stars. Influence of metallicity parameter [Z/H] and effective temperature on the appearance of the observed hydrogen lines is discussed.     

A near infrared speckle imaging study of T Tauri Stars

The results of a speckle imaging survey of T Tauri stars suggest that most, if not all, young low mass stars have companions. Repeated observations of these young binary stars have revealed orbital motion in the closest pairs (0.3), proving that these systems are indeed gravitationally bound and providing the basis for mass estimates in the upcoming years. These mass estimates are necessary to distinguish between the various binary star formation mechanisms that have been proposed to date.     

The earthquake of 250 a.d. in Augusta Raurica, A real event with a 3D site-effect?

The Roman city Augusta Raurica is located East of Basel, Switzerland. One important topic of the city’s history concerns the hypothesis of an earthquake striking the city in the middle of the third century a.d. This idea had been formulated according to archaeological features and findings, but had not been tested so far. A selection of the archaeological features were reviewed and dated in order to test the hypothesis of a single event. However, archaeological investigations do not draw a conclusive picture; it could not be proven that all features of possible destruction date to the same event. Detailed seismological investigations were performed. These included geological and geotechnical mapping of the unconsolidated sediments. Important parameters such as the thickness and composition of the unconsolidated sediments, the terrain topography and the topography of the bedrock surface were mapped. Ambient vibration H/V measurements provided the fundamental frequency of resonance for the unconsolidated sediments. The velocity of shear waves traveling through sediments is the controlling parameter for amplification of seismic waves. This material property is estimated using the relation between the ellipticity of the fundamental mode Rayleigh wave and the H/V curve. From all information we compiled a three-dimensional model of the surface geology. This model is used to simulate earthquake ground motion and amplification effects in the city, and to map the variability of the amplification. In the part of the city where possible earthquake damage was recognized, amplification occurs in the frequency band of building resonance (2–8 Hz). In the other part of the city amplification occurs much above the building’s resonance. From 1D modelling we estimate a difference in spectral amplification of about a factor of 2.5 to 3 between the two parts of the city. This corresponds approximately to a difference in macroseismic intensity of one unit. 3D modelling showed a large variability of ground motion within very close distance in the part of the city where possible earthquake damage was recognized. The maximum amplification reaches values up to a factor of nine, which is due to 3D effects and the choice of using vertically incident waves. Finally, all paleoseismological findings in the area of Basel were reviewed in order to find indications of a large event in the time-period of interest. Paleoseismological findings provide no hints to a large earthquake in the third century. If we assume that an earthquake caused at least part of the identified damage in Augusta Raurica, we have to assign to this event a magnitude Mw of about 6.0 or even lower, that is much smaller than the value of 6.9 that is actually in the Swiss earthquake catalogue. The earthquake source of this event must then be very close to the site of Augusta Raurica and a strong site-effect occurred in one part of the city.