Identification of Mineralization in Geochemistry Along a Transect Based on the Spatial Curvature of Log-Ratios

Detecting subcropping mineralizations but also deeply buried mineralizations is one important goal in geochemical exploration. The identification of useful indicators for mineralization is a difficult task, as mineralization might be influenced by many factors, including location, investigated media and depth. Here, a statistical method is proposed which indicates chemical elements related to mineralization along a transect. Moreover, the method determines the potential area of the deposit along a transect. The identification is based on general additive models (GAMs) for the element concentrations across the spatial coordinate(s). The log-ratios of the GAM fits are taken to compute the curvature, where high and narrow curvature is supposed to indicate the mineralization area. By defining a measure for the quantification of high curvature, the log-ratios can be ranked, and elements can be identified that are indicative of the anomaly patterns.

     

Assessment of boundary-element method for modelling a free-floating sloped wave energy device. Part 1: Numerical modelling

The boundary-element method has been widely used as a design tool in the offshore and ship building industry for more than 30 years. Its application to wave energy conversion is, however, more recent. This paper deals with the numerical modelling of a free-floating sloped wave energy device. The power take-off mechanism of the device consists of an immersed tube with a piston sliding inside. The modelling is done using the boundary-element method package WAMIT. The model is first worked out for the case where the axis of the tube is vertical. It is then derived for the tube inclined and successfully verified against numerical benchmark data. A companion paper presents results of a detailed comparison with a physical model study.     

Wave diffraction and near-trapping by a multi-column gravity-based structure

This paper is concerned with the diffraction of water waves by offshore structures, with the ultimate aim of proposing tools for guiding airgap design. The diffraction of monochromatic waves by an array of four bottom mounted cylinders and a gravity-based structure is studied in detail using linear and second order theory. The phenomenon of near-trapping is investigated, allowing guidelines for airgap design to be established. When contemplating airgap design, however, it is crucially important that consideration is given to the largest waves in a sea state. Therefore, in this study a design wave, called NewWave, is proposed as a realistic model for large ocean waves and is used as the incident wave field in the wave-structure diffraction analysis.     

Trophic modeling of the Northern Humboldt Current Ecosystem, Part II: Elucidating ecosystem dynamics from 1995 to 2004 with a focus on the impact of ENSO

The Northern Humboldt Current Ecosystem is one of the most productive in the world in terms of fish production. Its location near to the equator permits strong upwelling under relatively low winds, thus creating optimal conditions for the development of plankton communities. These communities ultimately support abundant populations of grazing fish such as the Peruvian anchoveta, Engraulis ringens. The ecosystem is also subject to strong inter-annual environmental variability associated with the El Niño Southern Oscillation (ENSO), which has major effects on nutrient structure, primary production, and higher trophic levels. Here our objective is to model the contributions of several external drivers (i.e. reconstructed phytoplankton changes, fish immigration, and fishing rate) and internal control mechanisms (i.e. predator-prey) to ecosystem dynamics over an ENSO cycle. Steady-state models and time-series data from the Instituto del Mar del Perú (IMARPE) from 1995 to 2004 provide the base data for simulations conducted with the program Ecopath with Ecosim. In simulations all three external drivers contribute to ecosystem dynamics. Changes in phytoplankton quantity and composition (i.e. contribution of diatoms and dino- and silicoflagellates), as affected by upwelling intensity, were important in dynamics of the El Niño of 1997–98 and the subsequent 3 years. The expansion and immigration of mesopelagic fish populations during El Niño was important for dynamics in following years. Fishing rate changes were the most important of the three external drivers tested, helping to explain observed dynamics throughout the modeled period, and particularly during the post-El Niño period. Internal control settings show a mix of predator–prey control settings; however a “wasp-waist” control of the ecosystem by small pelagic fish is not supported.     

A probabilistic methodology to estimate future coastal flood risk due to sea level rise

In this paper we present a methodology to estimate the probability of future coastal flooding given uncertainty over possible sea level rise. We take as an example the range of sea level rise magnitudes for 2100 contained in the IPCC Third Assessment Report [Church, J.A., Gregory, J.M., Huybrechts, P., Kuhn, M., Lambeck, K., Nhuan, M.T., Qin, D., Woodworth, P.L., Anisimov, O.A., Bryan, F.O., Cazenave, A., Dixon, K.W., Fitzharris, B.B., Flato, G.M., Ganopolski, A., Gornitz, V., Lowe, J.A., Noda, A., Oberhuber, J.M., O'Farrell, S.P., Ohmura, A., Oppenheimer, M., Peltier, W.R., Raper, S.C.B., Ritz, C., Russell, G.L., Schlosser, E., Shum, C.K., Stocker, T.F., Stouffer, R.J., van de Wal, R.S.W., Voss, R., Wiebe, E.C., Wild, M., Wingham, D.J. and Zwally, H.J., 2001. Changes in sea level. In Houghton, J.T. et al. (eds), Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, 881pp.] and infer a plausible probability distribution for this range. We then use a Monte Carlo procedure to sample from this distribution and use the resulting values as an additional boundary forcing for a two-dimensional model of coastal inundation used to simulate a 1 in 200 year extreme water level event. This yields an ensemble of simulations for an event of this magnitude occurring in 2100, where each member represents a different possible scenario of sea level rise by this time. We then develop a methodology to approximate the probability of flooding in each model grid cell over the ensemble and by combining these hazards maps with maps of land use values (consequence) we are able to estimate spatial contributions to flood risk that can aid planning and investment decisions. The method is then applied to a 32 km section of the UK coast in Somerset, South-West England and used to estimate the monetary losses and risk due a 1 in 200 year recurrence interval event under: (a) current conditions; (b) with the IPCC's most plausible value for sea level rise by 2100 (0.48 m) and (c) using the above methodology to fully account for uncertainty over possible sea level rise. The analysis shows that undertaking a risk assessment using the most plausible sea level rise value may significantly underestimate monetary losses as it fails to account for the impact of low probability, high consequence events. The developed method provides an objective basis for decisions regarding future defence spending and can be easily extended to consider other sources of uncertainty such as changing event frequency–magnitude distribution, changing storm surge conditions or model structural uncertainty, either singly or in combination as joint probabilities.     

Modelling population retention in Australia’s Northern Territory – how do current forms of migration contribute to population turnover and retention?

ABSTRACT

In the past, population growth in Australia's Northern Territory, as in other peripheral parts of high-income countries, has been driven by internal labour migration and migration from outside of Australia. These have been contributing to the high population turnover experienced in peripheral areas. Since 2010, the Northern Territory has experienced low (and even negative) population growth, and public policy is currently focused on migration as a lever to reverse this trend. However, the extent to which the characteristics of migrants influence the potential for longer-term population growth is poorly understood. This paper uses a new method to analyse the contributions of various types of migrants to both population turnover and retention. Two major sets of findings emerge: First, the significance of separating newer in-migrants from longer-term residents when analysing migration patterns; and secondly, the contribution of age, gender, Indigenous status, international origin, wages and industry of employment to the Northern Territory's population turnover. The research suggests that current forms of migration favour people who are likely to stay for only short periods, and have high wage demands. The main policy inference is that long-term population growth will likely not eventuate unless new forms of migration can be stimulated.     

Time to talk knowledge

As part of the secondary school qualification repackaging, the Ministry of Education has requested the geography education community define what constitutes their “critical body of knowledge.” In this article, I discuss three knowledge‐oriented themes focused on: (a) recent assessment trends in geography; (b) the predictive value of NCEA Level 3 geography with 100‐level success and (c) the nature of knowledge as expressed by the New Zealand Curriculum and assessment. Taken together, these themes suggest a need for a serious conversation about knowledge in school geography, as a pre‐cursor to the re‐design of the geography standards matrix.     

Salt thickness and composition influence rift structural style,northern North Sea,offshore Norway

“Salt” giants are typically halite‐dominated, although they invariably contain other evaporite (e.g. anhydrite, bittern salts) and non‐evaporite (e.g. carbonate, clastic) rocks. Rheological differences between these rocks mean they impact or respond to rift‐related, upper crustal deformation in different ways. Our understanding of basin‐scale lithology variations in ancient salt giants, what controls this and how this impacts later rift‐related deformation, is poor, principally due to a lack of subsurface datasets of sufficiently regional extent. Here we use 2D seismic reflection and borehole data from offshore Norway to map compositional variations within the Zechstein Supergroup (ZSG) (Lopingian), relating this to the structural styles developed during Middle Jurassic‐to‐Early Cretaceous rifting. Based on the proportion of halite, we identify and map four intrasalt depositional zones (sensu Clark et al., Journal of the Geological Society, 1998, 155, 663) offshore Norway. We show that, at the basin margins, the ZSG is carbonate‐dominated, whereas towards the basin centre, it becomes increasingly halite‐dominated, a trend observed in the UK sector of the North Sea Basin and in other ancient salt giants. However, we also document abrupt, large magnitude compositional and thickness variations adjacent to large, intra‐basin normal faults; for example, thin, carbonate‐dominated successions occur on fault‐bounded footwall highs, whereas thick, halite‐dominated successions occur only a few kilometres away in adjacent depocentres. It is presently unclear if this variability reflects variations in syn‐depositional relief related to flooding of an underfilled presalt (Early Permian) rift or syn‐depositional (Lopingian) rift‐related faulting. Irrespective of the underlying controls, variations in salt composition and thickness influenced the Middle Jurassic‐to‐Early Cretaceous rift structural style, with diapirism characterising hangingwall basins where autochthonous salt was thick and halite‐rich and salt‐detached normal faulting occurring on the basin margins and on intra‐basin structural highs where the salt was too thin and/or halite‐poor to undergo diapirism. This variability is currently not captured by existing tectono‐stratigraphic models largely based on observations from salt‐free rifts and, we argue, mapping of suprasalt structural styles may provide insights into salt composition and thickness in areas where boreholes are lacking or seismic imaging is poor.     

Strain analysis of a seismically imaged mass‐transport complex,offshore Uruguay

Strain style, magnitude and distribution within mass‐transport complexes (MTCs) are important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally driven passive margins have been shown to approximately balance between updip extensional and downdip contractional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1,500 mbsf) and well‐imaged MTC, offshore Uruguay using a high‐resolution (12.5 m vertical and 15 × 12.5 m horizontal resolution) three‐dimensional seismic‐reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic‐attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub‐orthogonal shear zones, and fold‐thrust systems within the basal shear zone beneath rafted‐blocks. These features suggest multiple transport directions and phases of flow during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and contractional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra‐MTC rafted‐blocks due to their kinematic interactions with the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is included, a strain difference between extension (1.6–1.9 km) and contraction (6.7–7.3 km) is calculated. We attribute this to a combination of distributed, sub‐seismic, ‘cryptic’ strain, likely related to de‐watering, grain‐scale deformation and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits.