Electron impact excitation of Mg VIII: Collision strengths, transition probabilities and theoretical EUV and soft X-ray line intensities for Mg VIII

Context: Mg VIII emission lines are observed in a range of astronomical objects such as the Sun, other cool stars and in the coronal line region of Seyfert galaxies. Under coronal conditions Mg VIII emits strongly in the extreme ultraviolet (EUV) and soft X-ray spectral regions which makes it an ideal ion for plasma diagnostics.

Aims. Two theoretical atomic models, consisting of 125 fine structure levels, are developed for the Mg VIII ion. The 125 levels arise from the 2s(2)2p, 2s(2)p2, 2p(3), 2s(2)3s, 2s(2)3p, 2s(2)3d, 2s2p3s, 2s2p3p, 2s2p3d, 2p(2)3s, 2p(2)3p and 2p(2)3d configurations. Electron impact excitation collision strengths and radiative transition probabilities are calculated for both Mg VIII models, compared with existing data, and the best model selected to generate a set of theoretical emission line intensities. The EUV lines, covering 312-790 angstrom, are compared with existing solar spectra (SERTS-89 and SUMER), while the soft X-ray transitions (69-97 angstrom) are examined for potential density diagnostic line ratios and also compared with the limited available solar and stellar observational data.

Methods. The R-matrix codes Breit-Pauli RMATRXI and RMATRXII are utilised, along with the PSTGF code, to calculate the collision strengths for two Mg VIII models. Collision strengths are averaged over a Maxwellian distribution to produce the corresponding effective collision strengths for use in astrophysical applications. Transition probabilities are also calculated using the CIV3 atomic structure code. The best data are then incorporated into the modelling code CLOUDY and line intensities generated for a range of electron temperatures and densities appropriate to solar and stellar coronal plasmas.

Results. The present effective collision strengths are compared with two previous calculations. Good levels of agreement are found with the most recent, but there are large differences with the other for forbidden transitions. The resulting line intensities compare favourably with the observed values from the SERTS-89 and SUMER spectra. Theoretical soft X-ray emission lines are presented and several density diagnostic line ratios examined, which are in reasonable agreement with the limited observational data available.

Three new distal tephras in sediments spanning the Last Glacial-Interglacial Transition in Scotland

Three new microtephras are reported from a number of lake sites from the Inner Hebrides and Scottish mainland. One occurs stratigrapically in the middle of Greenland Interstadial 1 (GI-1) and has been named the Penifiler Tephra. It is rhyolitic and possesses a geochemical signature that is very similar to that of the Borrobol Tephra, which also occurs in three of the sequences reported here, but which lies close to the lower boundary of GI-1. The second occurs stratigraphically in the early Holocene below the Saksunarvatn Ash and is named the Ashik Tephra. This tephra is geochemically bimodal, with a rhyolitic component comparable to the An Druim Tephra that occurs later in the Holocene, and a basaltic component which is similar to the Saksunarvatn Ash. A third tephra occurs stratigraphically above the Saksunarvatn Ash and is provisionally named the Breakish Tephra. The consistent inter-site correlation demonstrated for these new tephras at several sites enhances the regional tephrostratigraphic framework, and increases the potential for correlating palaeoenvironmental events during GI-1 and the early Holocene. However, the occurrence of multiple tephras with similar geochemistry in close stratigraphic and temporal proximity has implications for the rigour with which tephrostratigraphic investigations must be performed.

A new and less destructive laboratory procedure for the physical separation of distal glass tephra shards from sediments

Tephrochronology, a key tool in the correlation of Quaternary sequences, relies on the extraction of tephra shards from sediments for visual identification and high-precision geochemical comparison. A prerequisite for the reliable correlation of tephra layers is that the geochemical composition of glass shards remains unaltered by natural processes (e.g. chemical exchange in the sedimentary environment) and/or by laboratory analytical procedures. However, natural glasses, particularly when in the form of small shards with a high surface to volume ratio, are prone to chemical alteration in both acidic and basic environments. Current techniques for the extraction of distal tephra from sediments involve the ‘cleaning’ of samples in precisely such environments and at elevated temperatures. The acid phase of the ‘cleaning’ process risks alteration of the geochemical signature of the shards, while the basic phase leads to considerable sample loss through dissolution of the silica network. Here, we illustrate the degree of alteration and loss to which distal tephras may be prone, and introduce a less destructive procedure for their extraction. This method is based on stepped heavy liquid flotation and which results in samples of sufficient quality for analysis while preserving their geochemical integrity. In trials, this method out-performed chemical extraction procedures in terms of the number of shards recovered and has resulted in the detection of new tephra layers with low shard concentrations. The implications of this study are highly significant because (i) the current database of distal tephra records and their corresponding geochemical signatures may require refinement and (ii) the record of distal tephras may be incomplete due to sample loss induced by corrosive laboratory procedures. It is therefore vital that less corrosive laboratory procedures are developed to make the detection and classification of distal glass tephra more secure.

Two-dimensional images of dissolved sulfide and metals in anoxic sediments by a novel diffusive gradients in thin film probe and optical scanning techniques

A novel diffusive gradients in thin film probe developed comprises diffusive gel layer of silver iodide (AgI) and a back-up Microchelex resin gel layer. 2D high-resolution images of sulfide and trace metals were determined respectively on the AgI gel by densitometric analysis and on the Microchelex resin layer with laser-ablation-inductively-coupled plasma mass spectrometry (LA-ICP-MS).We investigated the validity of the analytical procedures used for the determination of sulfide and trace metals. We found low relative standard deviations on replicate measurements, linear trace-metal calibration curves between the LA-ICP-MS signal and the true trace-metal concentration in the resin gel, and a good agreement of the sulfide results obtained with the AgI resin gel and with other analytical methods. The method was applied on anoxic sediment pore waters in an estuarine and marine system. Simultaneous remobilization of sulfide and trace metals was observed in the marine sediment.

Dataflow toolset for soft-core processors on FPGA for image processing applications

With security and surveillance, there is an increasing need to be able to process image data efficiently and effectively either at source or in a large data networks. Whilst Field Programmable Gate Arrays have been seen as a key technology for enabling this, they typically use high level and/or hardware description language synthesis approaches; this provides a major disadvantage in terms of the time needed to design or program them and to verify correct operation; it considerably reduces the programmability capability of any technique based on this technology. The work here proposes a different approach of using optimised soft-core processors which can be programmed in software. In particular, the paper proposes a design tool chain for programming such processors that uses the CAL Actor Language as a starting point for describing an image processing algorithm and targets its implementation to these custom designed, soft-core processors on FPGA. The main purpose is to exploit the task and data parallelism in order to achieve the same parallelism as a previous HDL implementation but avoiding the design time, verification and debugging steps associated with such approaches.

Contaminants in surface water and sediments around the Tynagh Mine, Galway, Ireland.

A former silver mine in Tynagh, Co. Galway, Ireland is one of the most contaminated mine sites in Europe with maximum concentrations of Zn, As, Pb, Mn, Ni, Cu, and Cd far exceeding guideline values for water and sediment. The aims of this research were to 1) further assess the contamination, particularly metals, in surface water and sediment around the site, and 2) determine if the contamination has increased 10 years after the Environmental Protection Agency Ireland (EPAI) identified off-site contamination. Site pH is alkaline to neutral because CaCO3-rich sediment and rock material buffer the exposed acid generating sulphide-rich ore. When this study was compared to the previous EPAI study conducted 10 years earlier, it appeared that further weathering of exposed surface sediment had increased concentrations of As and other potentially toxic elements. Water samples from the tailings ponds and adjacent Barnacullia Stream had concentrations of Al, Cd, Mn, Zn and Pb above guideline values. Lead and Zn concentrations from the tailings pond sediment were 16 and 5 times higher, respectively, than concentrations reported 10 years earlier. Pb and Zn levels in most sediment samples exceeded the Expert Group (EGS) guidelines of 1000 and 5000 mg/kg, respectively. Arsenic concentrations were as high as 6238 mg/kg in the tailings ponds sediment, which is 62 and 862 times greater than the EGS and Canadian Soil Quality Guidelines (CSQG), respectively. Cadmium, Cu, Fe, Mn, Pb and Zn concentrations in water and sediment were above guideline values downstream of the site. Additionally, Fe, Mn and organic matter (OM) were strongly correlated and correlated to Zn, Pb, As, Cd, Cu and Ni in stream sediment. Therefore, the nearby Barnacullia Stream is also a significant pathway for contaminant transport to downstream areas. Further rehabilitation of the site may decrease the contamination around the area.

Air pollutant contamination and acidification of surface waters in the North York Moors, UK: Multi-proxy evidence from the sediments of a moorland pool

Despite the extensive geographical range of palaeolimnological studies designed to assess the extent of surface water acidification in the United Kingdom during the 1980s, little attention was paid to the status of surface waters in the North York Moors (NYM). In this paper, we present sediment core data from a moorland pool in the NYM that provide a record of air pollution contamination and surface water acidification. The 41-cm-long core was divided into three lithostratigraphic units. The lower two comprise peaty soils and peats, respectively, that date to between approximately 8080 and 6740 cal. BP. The uppermost unit comprises peaty lake muds dating from between approximately ad 1790 and the present day (ad 2006). The lower two units contain pollen dominated by forest taxa, whereas the uppermost unit contains pollen indicative of open landscape conditions similar to those of the present. Heavy metal, spheroidal carbonaceous particle, mineral magnetics and stable isotope analysis of the upper sediments show clear evidence of contamination by air pollutants derived from fossil-fuel combustion over the last c. 150years, and diatom analysis indicates that the naturally acidic pool became more acidic during the 20th century. We conclude that the exceptionally acidic surface waters of the pool at present (pH=c. 4.1) are the result of a long history of air pollution and not because of naturally acidic local conditions. We argue that the highly acidic surface waters elsewhere in the NYM are similarly acidified and that the lack of evidence of significant recovery from acidification, despite major reductions in the emissions of acidic gases that have taken place over the last c. 30years, indicates the continuing influence of pollutant sulphur stored in catchment peats, a legacy of over 150years of acid deposition.

In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake

Understanding the labile status of phosphorus (P) in sediments is crucial for managing a eutrophic lake, but it is hindered by lacking in situ data particularly on a catchment scale. In this study, we for the first time characterized in situ labile P in sediments with the Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique at a two-dimensional (2D), submillimeter resolution in a large eutrophic lake (Lake Taihu, China, with an area of 2338km²). The concentration of DGT-labile P in the sediment profiles showed strong variation mostly ranging from 0.01 to 0.35mgL^-1 with a considerable number of hotspots. The horizontal heterogeneity index of labile P varied from 0.04 to 4.5. High values appeared at the depths of 0-30mm, likely reflecting an active layer of labile P under the sediment-water interface (SWI). Concentration gradients of labile P were observed from the high-resolution 1D DGT profiles in both the sediment and overlying water layers close to the SWI. The apparent diffusion flux of P across the SWI was calculated between -21 and 65ngcm^-2d^-1, which showed that the sediments tended to be a source and sink of overlying water P in the algal- and macrophyte-dominated regions, respectively. The DGT-labile P in the 0-30mm active layer showed a better correlation with overlying water P than the labile P measured by ex situ chemical extraction methods. It implies that in situ, high-resolution profiling of labile P with DGT is a more reliable approach and will significantly extend our ability in in situ monitoring of the labile status of P in sediments in the field.

A Soft Sensor for Viscosity Control of Polymer Extrusion

Viscosity represents a key indicator of product quality in polymer extrusion but has traditionally been difficult to measure in-process in real-time. An innovative, yet simple, solution to this problem is proposed by a Prediction-Feedback observer mechanism. A `Prediction' model based on the operating conditions generates an open-loop estimate of the melt viscosity; this estimate is used as an input to a second, `Feedback' model to predict the pressure of the system. The pressure value is compared to the actual measured melt pressure and the error used to correct the viscosity estimate. The Prediction model captures the relationship between the operating conditions and the resulting melt viscosity and as such describes the specific material behavior. The Feedback model on the other hand describes the fundamental physical relationship between viscosity and extruder pressure and is a function of the machine geometry. The resulting system yields viscosity estimates within 1% error, shows excellent disturbance rejection properties and can be directly applied to model-based control. This is of major significance to achieving higher quality and reducing waste and set-up times in the polymer extrusion industry.

Distal tephrochronology in volcanic regions: Challenges and insights from Kamchatkan lake sediments

Kamchatka is one of the world’s most active volcanic regions and has hosted many explosive eruptions during the Holocene. These eruptions had the potential to disperse tephra over wide areas, forming time-synchronous markers wherever those tephras are found. Recent research in Kamchatka has begun to focus on the geochemical analysis of individual glass shards in order to characterise tephra layers. We have applied this approach to the study of visible tephras from three lakes – one in central and two in northern Kamchatka – with the aim of identifying key tephras and potential issues in the application of distal (>100 km from an active volcano) tephra in volcanically complex regions. In total, 23 tephras from 22 tephra beds have been geochemically analysed, representing products from at least four volcanic systems in Kamchatka. We demonstrate that distal lake sediments in the region can yield reliable tephrostratigraphies, capturing tephra from eruptions that have the greatest potential to disperse volcanic ash beyond the region. We draw attention to issues relating to correlating and distinguishing key marker horizons from the highly active Shiveluch Volcano, namely the need to ensure inter-lab comparability of geochemical data and good chronological control of the proximal and distal tephras. Importantly, we have also extended the known distribution of two key tephra isochrons from the Ksudach volcano. Our work contributes valuable glass geochemical on data several key marker beds that will facilitate future tephra and palaeoenvironmental research within and beyond Kamchatka.

Holocene climate and environmental change in north-eastern Kamchatka (Russian Far East), inferred from a multi-proxy study of lake sediments

A sediment record from a small lake in the north-eastern part of the Kamchatka Peninsula has been investigated in a multi-proxy study to gain knowledge of Holocene climatic and environmental change. Pollen, diatoms, chironomids and selected geochemical parameters were analysed and the sediment record was dated with radiocarbon. The study shows Holocene changes in the terrestrial vegetation as well as responses of the lake ecosystem to catchment maturity and multiple stressors, such as climate change and volcanic eruptions. Climate change is the major driving force resulting in the recorded environmental changes in the lake, although recurrent tephra deposition events also contributed. The sediment record has an age at the base of about 10,000 cal yrs BP, and during the first 400 years the climate was cold and the lake exhibited extensive ice-cover during winter and relatively low primary production. Soils in the catchment were poor with shrub alder and birches dominating the vegetation surrounding the lake. At about 9600–8900 cal yrs BP the climate was cold and moist, and strong seasonal wind stress resulted in reduced ice-cover and increased primary production. After ca. 8900 cal yrs BP the forest density increased around the lake, runoff decreased in a generally drier climate resulting in decreased primary production in the lake until ca. 7000 cal yrs BP. This generally dry climate was interrupted by a brief climatic perturbation, possibly attributed to the 8.2 ka event, indicating increasingly windy conditions with thick snow cover, reduced ice-cover and slightly elevated primary production in the lake. The diatom record shows maximum thermal stratification at ca. 6300–5800 cal yrs BP and indicates together with the geochemical proxies a dry and slightly warmer climate resulting in a high productive lake. The most remarkably change in the catchment vegetation occurred at ca. 4200 cal yrs BP in the form of a conspicuous increase in Siberian dwarf pine (Pinus pumila), indicating a shift to a cooler climate with a thicker and more long-lasting snow cover. This vegetational change was accompanied by marked shifts in the diatom and chironomid stratigraphies, which are also indicative of colder climate and more extensive ice-cover.