安第斯埃达克岩 :三种成因模式

在安第斯埃达克岩是常见的。由这种岩石的痕量元素特征揭示的高压组成矿物 ,表明其有三种可能的成因。这三种成因按相对重要性依次为 :安第斯地壳的构造加厚 ;弧前壳的俯冲 -侵蚀 ;年轻洋壳的俯冲。在过去 30 Ma,每种成因模式的埃达克岩在智利 -阿根廷安第斯地区均有喷发。它们的产状与智利 -阿根廷边缘快速变化的构造背景中特定构造条件和事件相吻合。埃达克岩在过去安第斯边缘的产状可望成为矿化的优异构造标志 ,也可用于指导找矿     

Issues in sediment toxicity and ecological risk assessment

This paper is based on a facilitated Workshop and Roundtable Discussion of key issues in sediment toxicology and ecological risk assessment (ERA) as applied to sediments that was held at the Conference on Dredged Material Management: Options and Environmental Considerations. The issues addressed included how toxicity is defined and perceived, how it is measured, and how it should be used within the context of ERA to support management decisions. The following conclusions were reached regarding scientific considerations of these issues. Toxicity is a measure of hazard and not a risk per se. Thus, toxicity testing is a means but not the end to understand risks of sediments. Toxicity testing cannot presently be replaced by chemical analyses to define hazard. Toxicity test organisms need to be appropriate to the problem being addressed, and the results put into context relative to both reference and baseline comparisons to understand hazard. Use of toxicity tests in sediment ERAs requires appropriate endpoints and risk hypotheses, considering ecological not just statistical significance, and recognizing that hazard does not equate to risk. Toxicity should be linked to population and community response to support decision-making, assessing possible genotypic adaptations that can influence risk estimates, and addressing uncertainty. Additionally, several key scientific issues were identified to improve future sediment ERAs, including the need to improve basic understanding of ecological mechanisms and processes, recognition of variability in the assessment process, and an improved focus and ability to assess risks to populations and communities.     

Influence Diagrams for Representing Uncertainty in Climate-Related Propositions

In order to respond to policy questions about the potential impacts of climate change it is usually necessary to assemble large quantities of evidence from a variety of sources. Influence diagrams provide a formal mechanism for structuring this evidence and representing its relationship with the climate-related question of interest. When populated with probabilistic measures of belief an influence diagram provides a graphical representation of uncertainty, which can help to synthesize complex and contentious arguments into a relatively simple, yet evidence-based, graphical output.Following unusually damaging floods in October–November 2000 the UK government commissioned research with a view to establishing the extent to which the floods were a manifestation of hydrological climate change. By way of example application, influence diagrams have been used to represent the evidential reasoning and uncertainties in responding to this question. Three alternative approaches to the mathematization of uncertainty in influence diagrams are demonstrated and compared. In situations of information scarcity and imprecise expert judgements, methods based on interval probabilities have proved to be attractive. Interval probabilities can, it is argued, represent ambiguity and ignorance in a more satisfactory manner than the conventional Bayesian alternative. The analysis provides a quantified commentary on the uncertainties in the conclusion that the events of October–November 2000 were extreme, but cannot in themselves be attributed to climate change.     

Integrated Interpolation Methods for Geophysical Data: Applications to Mineral Exploration

Any mine planning requires careful prediction of both the head grade andtonnage ofmineralization. There are various methods of interpolation that attempt to provide reasonable estimatesat unsampled locations. All of these give realizations that are unduly smooth and extremevalues that occur in reality are not reflected in these estimates. Such methods, therefore,provide limited scope for accurate risk assessment. An alternative approach that is rapidlygaining popularity is the method of conditional simulation. This approach attempts to reproduceboth the grade distributions of the sample data as well as its spatial variability. In this paper,a case study is presented on a platinum mineralization to demonstrate and compare sequentialGaussian and sequential conditional simulation techniques and to quantify and discuss therelevant sensitivities.     

Lattice-preferred orientation and microstructure of peridotites from ODP Hole 1274A (15°39′N), Mid-Atlantic Ridge: Testing models of mantle upwelling and tectonic exhumation

Eleven harzburgites and one dunite from Ocean Drilling Program Leg 209 Hole 1274A preserve high-temperature mantle textures. Electron backscatter diffraction (EBSD) analysis shows moderately developed crystal lattice preferred orientations (LPOs) in olivine and orthopyroxene (M-indices ≈ 0.1) indicative of crystal-plastic deformation at ~ 1250 °C. These rocks preserve a protogranular texture with a weak olivine foliation, a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene LPO, and minor interstitial clinopyroxene and spinel. Olivine grain size distributions, along with melt-related microstructures in orthopyroxene, clinopyroxene and spinel suggest that high-temperature deformation textures have been overprinted by pervasive post-deformation melt-rock interaction. Paleomagnetic data constrain the olivine [100] axes to be subhorizontal and oriented at low angle (≤ 28.6° ± 10.6°) to the ridge axis at the onset of serpentinization. This orientation is consistent with either complex 3-D mantle upwelling or 2-D mantle upwelling coupled with complex 3-D tectonic emplacement to the seafloor.     

Near-surface electromagnetic, rock magnetic, and geochemical fingerprinting of submarine freshwater seepage at Eckernf?rde Bay (SW Baltic Sea)

Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernf?rde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations.     

Sea-ice retreat in the Sea of Okhotsk and the ice–ocean albedo feedback effect on it

Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat flux from the atmosphere at the water surface is about 77 W m⁻² on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m⁻² because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input into the ice–upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased) open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive feedback is regarded as the ice–ocean albedo feedback, and explains in part the large interannual variability of the ice cover in the ice melt season.