Upper Mantle Imaging with Array Recordings of Converted and Scattered Teleseismic Waves

This paper provides a review of array-based imaging techniques that use converted and scattered teleseismic waves. It addresses various aspects of the imaging process, from the preprocessing of the data to the application of the imaging algorithms. The reviewed techniques form a continuum with respect to the level of complexity adopted in the treatment of the scattering problem. On one end of the spectrum, images may be produced by simple stacking of normalized P-to-S conversion records (i.e., receiver functions), which are binned according to station or common conversion points (CCP) and mapped to depth. Finer resolution can be achieved through the stacking of singly scattered wavefields along diffraction hyperbolae to recover relative scattering intensity/potential at individual points through a 2-D or 3-D model space. Moving to higher levels of complexity, we find methods that involve inversion/backprojection of scattered teleseismic wavefield to recover estimates of localized material property perturbations with respect to an a priori background model.     

Recent Results from Studies of Electric Discharges in the Mesosphere

The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.     

Phytate as a novel phosphorus-specific paleo-indicator in aquatic sediments

A reliable geochemical paleo-indicator for phosphorus remains elusive, despite the importance of understanding historical changes in the nutrient status of aquatic ecosystems. We assessed the potential of phytate (salts of myo-inositol hexakisphosphate) as a novel phosphorus-specific paleo-indicator by measuring its concentrations in dated sediments from an embayment in Helsinki, Finland, with a known 200-year history of trophic changes. Phytate was extracted in a solution containing sodium hydroxide and EDTA and detected by solution ³¹P NMR spectroscopy with spectral deconvolution. Concentrations varied markedly with sediment depth and paralleled previously determined changes in diatom assemblages and geochemical indicators linked to trophic status. In contrast, total sediment phosphorus did not reflect phosphorus inputs to the embayment, presumably due to the mobilization of inorganic phosphate under anoxic conditions during periods of high pollutant loading. Importantly, phytate appeared to be stable in these brackish sediments, in contrast to other organic and inorganic phosphates which declined abruptly with depth. We therefore conclude that phytate represents a potentially important indicator of historical changes in phosphorus inputs to water bodies, although additional studies are required to confirm its stability under conditions likely to be encountered in lakes and coastal ecosystems.     

The role of iron and sulfur in the visual appearance of lake sediment varves

Easily discernible sediment varves (annual laminations) may be formed in temperate zone lakes, and reflect seasonal changes in the composition of the accumulating material derived from the lake and its catchment (minerogenic and organic material). The appearance of varves may also be influenced by chemical processes. We assessed the role of iron (Fe) and sulfur (S) in the appearance of varves in sediments from Lake Nylandssjön in northern Sweden. We surveyed Fe in the lake water and established whether there is internal transport of Fe within the sediment. We used a unique collection of seven stored freeze cores of varved sediment from the lake, collected from 1979 to 2004. This suite of cores made it possible to follow long-term changes in Fe and S in the sediment caused by processes that occur in the lake bottom when the sediment is ageing. We compared Fe and S concentrations using X-ray fluorescence spectroscopy (XRF) in specific years in the different cores. No diagenetic front was found in the sediment and the data do not suggest that there is substantial vertical transport of Fe and S in the sediment. We also modeled Fe and S based on thermodynamic, limnological, and sediment data from the lake. The model was limited to the five components H⁺, e^?, Fe³⁺, SO₄ ^2?, H₂CO₃ and included the formation of solid phases such as Fe(OH)₃ (amorphous), FeOOH (aged, microcrystalline), FeS and FeCO₃. Modeling showed that there are pe (redox) ranges within which either FeS or Fe(OH)₃/FeOOH is the only solid phase present and there are pe ranges within which the two solid phases co-exist, which supports the hypothesis that blackish and grey-brownish layers that occur in the varves were formed at the time of deposition. This creates new possibilities for deciphering high-temporal-resolution environmental information from varves.     

A note on the M w 6.3 earthquake in Iceland on 29 May 2008 at 15:45 UTC

This preliminary study aims to investigate a M _w 6.3 earthquake that occurred in South Iceland on Thursday 29 May 2008 at 15:45 UTC. The epicentre was in the Olfus District between the towns of Selfoss and Hveragerdi. This study examines the data recorded and the damage observed immediately after the event. Horizontal accelerations of up to 80%g were recorded in the epicentral region and there is visual evidence that the vertical acceleration exceeded 1 g. The PGA data is compared to a ground motion estimation model developed for the South Iceland earthquakes in June 2000. In general the basic properties of this event are found to be similar to the characteristics of the South Iceland earthquakes in June 2000. The duration of strong-motion is short and the intensity attenuates rapidly with increasing distance. The earthquake action resisted by buildings in the near fault area is inspected through evaluation of elastic as well as inelastic response spectra. The vast majority of structures seemed to withstand the strong-motion fairly competently and without significant visual damage due firstly to the low-rise, predominantly reinforced concrete or timber, style of buildings. Secondly, the short duration of strong-motion contributed to the endurance of structures.     

Functioning of intertidal flats inferred from temporal and spatial dynamics of O<Subscript>2</Subscript>, H<Subscript>2</Subscript>S and pH in their surface sediment

In this article, we describe the dynamics of pH, O₂ and H₂S in the top 5–10 cm of an intertidal flat consisting of permeable sand. These dynamics were measured at the low water line and higher up the flat and during several seasons. Together with pore water nutrient data, the dynamics confirm that two types of transport act as driving forces for the cycling of elements (Billerbeck et al. 2006b): Fast surface dynamics of pore water chemistry occur only during inundation. Thus, they must be driven by hydraulics (tidal and wave action) and are highly dependent on weather conditions. This was demonstrated clearly by quick variation in oxygen penetration depth: Seeps are active at low tide only, indicating that the pore water flow in them is driven by a pressure head developing at low tide. The seeps are fed by slow transport of pore water over long distances in the deeper sediment. In the seeps, high concentrations of degradation products such as nutrients and sulphide were found, showing them to be the outlets of deep-seated degradation processes. The degradation products appear toxic for bioturbating/bioirrigating organisms, as a consequence of which, these were absent in the wider seep areas. These two mechanisms driving advection determine oxygen dynamics in these flats, whereas bioirrigation plays a minor role. The deep circulation causes a characteristic distribution of strongly reduced pore water near the low water line and rather more oxidised sediments in the centre of the flats. The two combined transport phenomena determine the fluxes of solutes and gases from the sediment to the surface water and in this way create specific niches for various types of microorganisms.