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.     

Search for Phobos and Deimos gas/dust tori using in situ observations from Mars Global Surveyor MAG/ER

More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons.     

Oxygen consumption in the epilimnia and hypolimnia of two eutrophic,warm‐monomictic lakes

Oxygen consumption was estimated for two eutrophic New Zealand lakes using a simple 2‐layer model, from estimates of net changes in oxygen, phytoplankton photosynthesis, inflow and outflow, diffusion, eddy diffusion, and changes in the depth of the thermocline. Of the total oxygen consumption, 75–85% occurred in the epilimnia. Epilimetic oxygen consumption per m³ and per m² in Lake Johnson were higher than in Lake Hayes, reflecting known differences in the trophic status of the lakes, but estimates for the hypolimnia of the two lakes were similar. Daily hypolimnetic areal deficits were sensitive to both the depths and time intervals chosen.

In the epilimnia, monthly oxygen production by photosynthesis averaged 1.1 and 1.8 times the initial oxygen content in Lakes Hayes and Johnson respectively. Diffusion appeared to be of similar magnitude to photosynthesis in the isothermal period, but was less important during stratification. Losses to the hypolimnia by eddy diffusion were less than 10% of photosynthesis. Photosynthesis in the hypolimnia contributed 30–40% of the oxygen consumed there and eddy diffusion supplied a further 20%.

In Lake Johnson a 3–5 m stratum of water in the metalimnion became anoxic for 2–3 months each summer while there were still up to 5.2 g.rer³ of oxygen remaining in the hypolimnion. Metalimnetic anoxia is attributed largely to oxygen consumption in situ, resulting from the highly eutrophic state of the lake and intense thermal stratification. Zooplankton respiration accounted for less than 10% of the maximum net decline in metalimnetic oxygen in 1970–71, but was more important in the other 2 years.

Changes in the oxygen content of the hypolimnion indicate that Lake Hayes has become more eutrophic since 1954–55. In Lake Johnson low oxygen concentrations at the autumn overturn (<5 g.m^‐3) represent a threat to the trout population.     

The freshwater lens of Benjamín Aceval,Chaco, Paraguay: a terrestrial analogue of an oceanic island lens

The occurrence of a freshwater lens in the Paraguayan Chaco, 900 km away from the ocean, is reported. It is located underneath sandstone hills, surrounded by lowlands with predominantly saline groundwater. Its geometry was delineated using geoelectrical and electromagnetic investigations. The unusual height of the fresh groundwater level can be attributed to the presence of a confining layer at depth. The lens receives its recharge exclusively from rainfall during the hot and humid summer months. It predominantly contains water predating the atmospheric atomic bomb tests, some of it probably up to a thousand or more years old. The water balance shows that extraction currently does not exceed recharge in normal years. However, the available volume of groundwater leaves little room for a further increase of extraction in the future. Recharge is augmented by return flow from thousands of latrines and cess pits, and this has lead to widespread contamination of the groundwater by faecal bacteria.     

Zircon evidence for a ~200 k.y. supereruption-related thermal flare-up in the Miocene southern Black Mountains,western Arizona,USA

The Silver Creek caldera (southern Black Mountains, western Arizona) is the source of the 18.8 Ma, >700 km³ Peach Spring Tuff (PST) supereruption, the largest eruption generated in the Colorado River Extensional Corridor (CREC) of the southwestern United States. Within and immediately surrounding the caldera is a sequence of volcanics and intrusions ranging in age from ~19 to 17 Ma. These units offer a record of magmatic processes prior to, during, and immediately following the PST eruption. To investigate the thermal evolution of the magmatic center that produced the PST, we applied a combination of Ti-in-zircon thermometry, zircon saturation thermometry, and high-precision U–Pb CA–TIMS zircon dating to representative pre- and post-supereruption volcanic and intrusive units from the caldera and its environs. Similar to intracaldera PST zircons, zircons from a pre-PST trachytic lava (19 Ma) and a post-PST caldera intrusion (18.8 Ma) yield exceptionally high-Ti concentrations (most >20 ppm, some up to nearly 60 ppm), corresponding to calculated temperatures that exceed 900 °C. In these units, Ti-in-zircon temperatures typically surpass zircon saturation temperatures (ZSTs), suggesting the entrainment of zircon that had grown in hotter environments within the magmatic system. Titanium concentrations in younger volcanic and intrusive units (~18.7–17.5 Ma) decline through time, corresponding to an average cooling rate of 10^?3.5 °C/year. The ~200 k.y. thermal peak evident at Silver Creek caldera is spatially limited: elsewhere in the Miocene record of the northern CREC, Ti-in-zircon concentrations and ZSTs are much lower, suggesting that felsic magmas were generally substantially cooler.     

Observations on the crystal structures of lueshite

Laboratory powder XRD patterns of the perovskite-group mineral lueshite from the type locality (Lueshe, Kivu, DRC) and pure NaNbO₃ demonstrate that lueshite does not adopt the same space group (Pbma; #57) as the synthetic compound. The crystal structures of lueshite (2 samples) from Lueshe, Mont Saint-Hilaire (Quebec, Canada) and Sallanlatvi (Kola, Russia) have been determined by single-crystal CCD X-ray diffraction. These room temperature X-ray data for all single-crystal samples can be satisfactorily refined in the orthorhombic space group Pbnm (#62). Cell dimensions, atomic coordinates of the atoms, bond lengths and octahedron tilt angles are given for four crystals. Conventional neutron diffraction patterns for Lueshe lueshite recorded over the temperature range 11–1,000 K confirm that lueshite does not adopt space group Pbma within these temperatures. Neutron diffraction indicates no phase changes on cooling from room temperature to 11 K. None of these neutron diffraction data give satisfactorily refinements but suggest that this is the space group Pbnm. Time-of-flight neutron diffraction patterns for Lueshe lueshite recorded from room temperature to 700 °C demonstrate phase transitions above 550 °C from Cmcm through P4/mbm to \(Pm\overline{3} m\) above 650 °C. Cell dimensions and atomic coordinates of the atoms are given for the three high-temperature phases. The room temperature to 400 °C structures cannot be satisfactorily resolved, and it is suggested that the lueshite at room temperature consists of domains of pinned metastable phases with orthorhombic and/or monoclinic structures. However, the sequence of high-temperature phase transitions observed is similar to those determined for synthetic NaTaO₃, suggesting that the equilibrated room temperature structure of lueshite is orthorhombic Pbnm.     

A near‐annual palaeohydrological study based on testate amoebae from a sub‐alpine mire: surface wetness and the role of climate during the instrumental period

We present the first testate amoeba‐based palaeohydrological reconstruction from the Swiss Alps, and the first depth to the water table (DWT) calibration dataset for this region. Compared to existing models, our new calibration dataset performs well (RMSEP = 4.88), despite the length of the water table gradient covered (53 cm). The present‐day topography and vegetation of the study mire Mauntschas suggest that it is partly ombrotrophic (large Sphagnum fuscum hummocks, one of which was the coring site) but mostly under the minerotrophic influence of springs in the mire and runoff from the surrounding area. Ombrotrophic Sphagnum fuscum hummocks developed at the sampling site only during the last 50 years, when testate amoebae indicate a shift towards dry and/or acid conditions. Prior to AD 1950 the water table was much higher, suggesting that the influence of the mineral‐rich water prevented the development of ombrotrophic hummocks. The reconstructed DWT correlated with Pinus cembra pollen accumulation rates, suggesting that testate amoebae living on the mire and P. cembra growing outside of it partly respond to the same factor(s). Finally, temperature trends from the nearby meteorological station paralleled trends in reconstructed DWT. However, contrary to other studies made on raised bogs of northwestern Europe, the highest correlation was observed for winter temperature, despite the fact that testate amoebae would more logically respond to moisture conditions during the growing season. The observed correlation with winter temperature might reflect a control of winter severity on surface moisture during at least the first part of the growing season, through snow melt and soil frost phenomena influencing run‐off. More ecohydrological work on sub‐alpine mires is needed to understand the relationships between climate, testate amoebae and peatland development. Copyright © 2009 John Wiley & Sons, Ltd.