From subduction to transform motion: a seabeam survey of the Hellenic trench system

Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.     

Cloud processes associated with past and future climate changes

 To investigate the cloud response during cold and warm periods, we have performed simulations of the Last Glacial Maximum (LGM-21ky BP) and of double CO₂ concentration using the LMD AGCM model. We observe that the thermal characteristics of these two climates are opposite, but the cloud response is more complex and does not display the same symmetry When doubling the CO₂, the warming of the troposphere and the cooling of the stratosphere are clearly linked with a reduction in low-level clouds and an increase of high-level clouds associated with relative humidity changes. For the LGM, the cloud response is more complex. In the inter tropical region, we show that the Hadley cell is reinforced during LGM (+20%) whereas it is reduced (−10%) for the double CO₂ experiments. The most important feature is that we observe an enlarged Hadley cell for LGM climate which strongly modifies the atmospheric dynamics and water transport. For LGM conditions, the cloud response is then mostly driven by these dynamical changes at low latitudes though at high latitudes the thermal changes explain a large part of the cloud response. Two different versions of the model, using different parametrizations for the precipitation show that cloud feedbacks may act differently for cold and warm climates; and that the cloud response may be more complex that previously expected, but also indicate that the details of these effects are model dependent.     

The conditions for biodegradation of petroleum hydrocarbons at sea

The hydrocarbon degradation potential of seawater appears higher in zones chronically polluted by these materials; they are characterized by an abundant bacterial development. The salinity of the effluent from a refinery and also its hydrocarbon content are factors affecting the speed of degradation.     

The fit of the continents around the North Atlantic Ocean

A new reassembly of the continents around the North Atlantic Ocean is presented. The first criterion used for this reassembly is the identification of the structural framework related to the opening which consists of marginal fracture zones generated by offsets of the Rift. The Africa—North America, Eurasia—Greenland, Greenland—North America and Eurasia—North America adjustments are successively discussed. It is argued that the adjustments are best made at the 3000-meters isobath between Africa and North America and at the 2000-meters isobath for the younger rifts. The difference is attributed to subsidence and modification of continental margins with time. The importance of the Late Paleozoic tectonic phase in determining the subsequent pattern of Mesozoic rifting is emphasized.     

First evidence for large earthquakes on the Deshir Fault,Central Iran Plateau

Although sliced by several strike slip faults, a large part of Central Iran remained aseismic during the period of time covered by the instrumental and historical seismic records. Stating the existence of earthquakes in the Holocene is therefore important for the assessment of the regional seismic hazard. A palaeoseismic study of the Deshir fault demonstrates that Central Iran hosted large earthquakes during latest Pleistocene and Holocene. The last event corresponds to 1 m‐deep fissures, which sandy infilling yielded an optically stimulated luminescence (OSL) age of 2.8 ± 1.4 ka. At least two previous events, outlined by older fissures and/or colluvial wedges, have been recorded over the last 10–30 ka. The magnitudes are difficult to assess because the actual slips per event are unknown. The size of the fissures and the significant vertical displacement associated with a colluvial wedge are nevertheless compatible with M ≈ 7 events along a primary strike‐slip surface break.     

Bifurcation and second‐order work in geomaterials

In this paper, the ability of a material rate‐independent system to evolve toward another mechanical state from an equilibrium configuration, with no change in the control parameters, is investigated. From a mechanical point of view, this means that the system can spontaneously develop kinetic energy with no external disturbance from an equilibrium state, which corresponds to a particular case of bifurcation. The existence of both conjugate incremental strain and stress such that the second‐order work vanishes is established as a necessary and sufficient condition for the appearance of this bifurcation phenomenon. It is proved that this fundamental result is independent of the constitutive relation of the rate‐independent material considered. Then the case of homogeneous loading paths is investigated, and, as an illustration, the subsequent results are applied to interpret the well‐known liquefaction observed under isochoric triaxial loading conditions with loose granular materials. Copyright © 2006 John Wiley & Sons, Ltd.     

Accumulation,solid partitioning and remobilisation of 99Tc in subtidal and intertidal sediments in the Irish Sea

Recent studies have shown that accumulation of ⁹⁹Tc in seabed sediments labelled by authorised radioactive liquid discharges into the NE Irish Sea from the Sellafield reprocessing complex is greater than previously thought. In this paper, new data on ⁹⁹Tc concentration profiles in subtidal and intertidal sediments from the eastern and western Irish Sea are provided with a view to elucidating the processes responsible for the incorporation and retention of ⁹⁹Tc in the seabed. The data show that substantial amounts of ⁹⁹Tc have accumulated in the fine-grained subtidal sediments off the Cumbrian coast, particularly after increased releases from Sellafield following the commissioning of the Enhanced Actinide Removal Plant (EARP) in 1994. In all the cores taken in this area, ⁹⁹Tc has been found to be present to depths in excess of 30 cm. Analysis of ¹³⁷Cs and ²⁴¹Am profiles, together with other supporting geochemical data, show a high degree of homogenisation of the sediments down to these depths as a result of physical and biological processes, and confirm that incorporation of ⁹⁹Tc into the sediment compartment is actually the result of mixing and reworking, rather than active sediment accumulation. In contrast, active deposition of material transported from this mixed pool of sediment appears to be the dominant mechanism controlling ⁹⁹Tc profiles in intertidal areas close to the Sellafield discharge outfall.Data obtained from the analysis of subtidal sediment cores from the western Irish Sea mud basin suggest that similar mixing processes to those occurring in the subtidal sediments of the eastern Irish Sea are also active in this area. Time-series data on ⁹⁹Tc concentrations in surficial sediments from this basin, gathered in the period 1988–2004, inclusive, show a clear increase in concentrations, by a factor of ～2, between samples collected pre-EARP and post-EARP. The constancy of ⁹⁹Tc concentrations in surface sediments throughout the 1980s and the early-1990s suggests that little redissolution and export of ⁹⁹Tc occurred over this extended period. A similar observation applies to the post-EARP period, when concentrations remained relatively constant despite the reported steady decrease in ⁹⁹Tc concentrations in the overlying waters.This apparent lack of remobilisation is consistent with data from sequential extraction analyses, which indicate that the bulk of the ⁹⁹Tc is strongly bound to non-labile geochemical phases, with only a small proportion associated with exchangeable and acido-soluble phases. Further, these analyses show that ⁹⁹Tc is not associated with oxygen-sensitive and highly-reactive acid-volatile sulphides (AVS) to any significant extent.     

Variation of Alfven fluctuations across a slow shock near the sun

We consider how the Alfven fluctuation varies as it passes through a slow MHD shock, assuming the fluctuation to originate on the Sun and such slow shocks are present between 2 and 6 solar radii. The results are 1) low latitude region in the vicinity of the neutral sheet is where the fluctuation can propagate with no or little distortion; 2) high latitude region far from the neutral sheet is a region of small distortion only for low Alfven-Mach numbers (about 0.3 or lower); 3) the observation of the Alfven fluctuations in space is consistent with, and lends support to their originating in the Sun and the existence of slow shock around the Sun.