H2 Diagnostics of Magnetic Molecular Shocks in Bipolar Outflows

The shock waves associated with molecular outflows may be of continuous (C) type or jump (J) type, depending on conditions in the preshock gas, notably the magnetic field strength and the degree of ionisation. Intermediate situations also exist, in which a J-discontinuity terminates or is embedded in a C-type flow. We show that proper allowance for the departure of the chemistry from equilibrium (particularly the dissociation/reformation of H₂) and for the inertia of charged dust grains, is crucial for an accurate treatment of the C to J transition. We illustrate the use of H₂ population diagrams and H₂ line profiles, in conjunction with our detailed shock model, to constrain conditions in shocks propagating in molecular outflows. We show that H₂ pure rotational lines yield evidence for C-type precursors in bipolar outflows from young stars, with transverse magnetic field strengths B (μG) ? 1–10 × $\sqrt {n_{H/{\text{cm}}^{ - 3} } } $ similar to those inferred from Zeeman splitting and from the dispersion of dust polarization vectors in dense clouds.     

The spatial and temporal distribution of fossil-fuel dervied pollutants in the sediment record of Lake Baikal, east Siberia

Spatial and temporal patterns of spheroidal carbonaceous particles (SCP) extracted from lake sediments provide an unambiguous record of the distributions of fossil-fuel derived pollutants. When applied to sediment cores taken from Lake Baikal spatial patterns show good agreement with the distribution of industry, with the highest concentrations found in the southern basin nearest to Irkutsk. SCP were found to occur in all cores from all areas of the lake in contrast to metal results where anthropogenically enhanced deposition was only demonstrable in the southern basin. SCP distribution within the sediments of Lake Baikal is seen to be distinctly regional and therefore long distance transport is not thought to be an important pathway for these pollutants. Temporal patterns of SCP show trends that reflect the development of industry in the area since the 1940s. Settling rates in the 1600 m water column suggest that the SCP sediment record may be approximately an order of magnitude more sensitive to depositional changes than that of trace metals.     

Temporal evolution of MHD shocks in the interstellar medium

We undertake calculations of the time-dependent structure of shock waves propagating in dark and diffuse interstellar clouds. The results of the time-dependent model are compared with those obtained by means of an independent steady-state code and found to agree well at sufficiently late times. Discontinuities in the flow of the neutral fluid are handled by introducing a pseudo-viscosity. Special procedures are adopted to correct for the associated widening of the discontinuity, in order not to distort the role of inelastic collision processes. We find that, in dark clouds, C shocks will tend to predominate, but are unlikely to have attained steady state in the cloud lifetime. On the other hand, in diffuse clouds, steady state may be reached but the discontinuity in the flow of the neutral fluid remains. We find no evidence for the existence of C* shocks, in which the neutral fluid undergoes a continuous transition from supersonic to subsonic flow (in the reference frame of the shock wave). Attention is drawn to the possible importance of these results for the interpretation of H₂ rovibrational line intensities.     

Paleolimnological assessment of recent environmental change in Lake Baikal: sediment chronology

Sediment cores collected from six sites in Lake Baikal as part of an extensive investigation of sediment records of recent environmental change were dated radiometrically by ²¹⁰Pb. Although the results obtained were in some respects comparable to those obtained by previous ²¹⁰Pb studies, there were some significant differences. Most notably, a core from abyssal depths in the centre of the middle basin appeared to record at least wo major episodes of rapid sedimentation, possibly caused by turbidity currents. All cores, including those from the North Basin, contained significant records of weapons test fallout ¹³⁷Cs, though these were of little chronological value due to the long residence time of ¹³⁷Cs in the water column. Models and budgets for sediment and radionuclide transport through Baikal are presented and used to help validate the ²¹⁰Pb results.     

Bottom-up and top-down control of seagrass overgrazing by the sea urchin Tripneustes gratilla

The lack of top-down control on Tripneustes gratilla, a sea urchin commonly known to graze on seagrass, and the bottom-up control of its feeding preference, led to the overgrazing of seagrass meadows of the species Thalassodendron ciliatum in Changuu Island (Zanzibar Archipelago). The impact of overgrazing on seagrasses was assessed by mapping the presence of grazed versus non-grazed seagrass patches in the study site, while the top-down control on T. gratilla was assessed by measuring the abundance of its fish predators. The feeding preference and distribution of T. gratilla were characterized by calculating the electivity indexes for each seagrass species and measuring sea urchin density, respectively. Approximately half of the patches of T. ciliatum were overgrazed, while predatory fishes of T. gratilla were absent from the site. The Vanderploeg and Scavia's Relativized Electivity Index indicated that T. gratilla had a feeding preference for T. ciliatum, which was also supported by higher urchin densities within T. ciliatum dominated patches. Bottom-up control of grazing activity was observed by quantifying and analyzing morphological, nutritional, and the chemical defense traits of the seagrass in relation to feeding preference and urchin density. Feeding was positively correlated to the seagrass tissue C:P ratio (ρ = 0.9), whereas urchin density showed no correlations. The bottom-up control of the feeding preference and agglomeration of T. gratilla in T. ciliatum meadows, together with the lack of evidence of substantial top-down control and the long recovery time of T. ciliatum led to the overgrazing of this species at this site. Overgrazing, therefore, was shown to be the result of multiple factors ranging from the traits of the seagrass and feeding preference of T. gratilla, to the abundance of predators in this area.     

中国东部岩石圈地幔的演化——地幔岩捕虏体微量元素的证据

对中国东部赋存于新生代玄武岩中的地幔岩捕虏体的全岩和单斜辉石等作了主元素和微量元素分析，证实了二辉橄榄岩及其单斜辉石在主元素有连续变化的趋势，反映了具部分熔融后残留相的性质。方辉橄榄岩及其中的单斜辉石的主元素，Ｎｄ／Ｙｂ，Ｔｉ／Ｚｒ和Ｓｒ／Ｚｒ值与二辉橄榄岩的同类矿物是不连续过渡。     

Introduction to TETHYS—an interdisciplinary GIS database for studying continental collisions

The TETHYS GIS database is being developed as a way to integrate relevant geologic, geophysical, geochemical, geochronologic, and remote sensing data bearing on Tethyan continental plate collisions. The project is predicated on a need for actualistic model ‘templates’ for interpreting the Earth's geologic record. Because of their time-transgressive character, Tethyan collisions offer ‘actualistic’ models for features such as continental ‘escape’, collision-induced upper mantle flow magmatism, and marginal basin opening, associated with modern convergent plate margins. Large integrated geochemical and geophysical databases allow for such models to be tested against the geologic record, leading to a better understanding of continental accretion throughout Earth history. The TETHYS database combines digital topographic and geologic information, remote sensing images, sample-based geochemical, geochronologic, and isotopic data (for pre- and post-collision igneous activity), and data for seismic tomography, shear-wave splitting, space geodesy, and information for plate tectonic reconstructions. Here, we report progress on developing such a database and the tools for manipulating and visualizing integrated 2-, 3-, and 4-d data sets with examples of research applications in progress. Based on an Oracle database system, linked with ArcIMS via ArcSDE, the TETHYS project is an evolving resource for researchers, educators, and others interested in studying the role of plate collisions in the process of continental accretion, and will be accessible as a node of the national Geosciences Cyberinfrastructure Network—GEON via the World-Wide Web and ultra-high speed internet2. Interim partial access to the data and metadata is available at: http://geoinfo.geosc.uh.edu/Tethys/ and http://www.esrs.wmich.edu/tethys.htm. We demonstrate the utility of the TETHYS database in building a framework for lithospheric interactions in continental collision and accretion.     

A personal perspective on four decades of paleolimnology and environmental change research

A personal summary of paleolimnological research activities in the UK and elswehere since the mid 1970s.