Implications of dredging induced changes in sediment particle size composition for the structure and function of marine benthic macrofaunal communities

A meta-analysis approach was used to assess the effect of dredging induced changes in sediment composition, under different conditions of natural physical disturbance, for the structure and function of marine benthic macrofaunal communities. Results showed the sensitivity of macrofaunal communities increased as both the proportion of gravel increased and the level of natural physical disturbance decreased. These findings may be explained by the close association of certain taxa with the gravel fraction, and the influence of natural physical disturbance which, as it increases, tends to restrict the colonisation by these species. We conclude that maintaining the gravel content of surface sediments after dredging and, where practicable, locating extraction sites in areas of higher natural disturbance will minimise the potential for long-term negative impacts on the macrofauna.     

From richer to poorer: zircon inheritance in Pomona Island Granite,New Zealand

Previous U–Pb zircon dating of the Pomona Island Granite (PIG) pluton (South Island, New Zealand) yielded either Permo-Carboniferous or Late Jurassic ages for five samples essentially indistinguishable in their field, petrographic, and geochemical characteristics. Detailed cathodoluminescence imaging and LA-ICP-MS dating of zircon in new and previously dated samples reveal that portions of the pluton contain either delicately oscillatory-zoned Late Jurassic zircon grains with rare Permo-Carboniferous cores, or Permo-Carboniferous grains with ubiquitous but thin Late Jurassic rims. Based on zircon dissolution-overgrowth textures, zircon rim and core trace element compositions, and the limited extent of sub-solidus rock recrystallisation textures, the bipartite age distribution is unlikely to reflect variable Pb-loss or metamorphic re-equilibration. Magmatic Zr-saturation temperatures were ≥851°C for samples dominated by Jurassic zircon and ≤809°C for samples with a predominance of Permo-Carboniferous zircon. Together, these data are consistent with PIG magmas having been derived from partial melting of a Permo-Carboniferous felsic igneous source at variable temperature wholly in the Late Jurassic (157 ± 3 Ma). The lowest temperature melts would have been incapable of dissolving significant amounts of pre-existing zircon and consequently generated inheritance-rich magmas, with the very thin rims on the pre-existing zircon grains the only evidence of the Late Jurassic magmatic age. As the partial melting temperature increased and nearly all pre-existing zircon grains dissolved into the magma, an inheritance-poor batch of melt was generated, which precipitated new zircon grains upon crystallisation. Concentrations of major and many trace elements in both magma batches may have been buffered by retention of residual quartz and feldspar in the source, which would explain the limited geochemical differences between inheritance-rich and inheritance-poor portions.     

Extended planar defects and the rapid incorporation of Ti4+ into olivine

The formation of extended planar defects in minerals such as olivine is related to high point defect concentration and can be driven by large gradients in chemical potential, where the energy of the system is lowered by the ordering of defects along specific planes in the crystal. The presence of extended defects has the potential to create the (apparently) anomalous ionic diffusion in olivine as reported recently (Spandler and O’Neill in Contrib Mineral Petrol 159(6):791–818, 2010). High-resolution transmission electron microscopy and energy-filtered imaging were done using experimental samples designed to examine the impact of a _TiO2 and f _O2 on the potential to form such defects in ferromagnesian olivine. Doped basalt (5 wt% TiO₂)–olivine reaction couple experiments were run at 1 atm and 1,310 and 1,410 °C for 50 h at various f _O2, ranging from 10² below to 10² above the quartz–fayalite–magnetite buffer. Our results show that extended planar defects in olivine, parallel to {101}_ol and occurring in ordered “clusters” with a prolate spheroid geometry ~5–25 nm across and extending up to 150 nm into the olivine, are present near the olivine–glass interfaces in all of our experimental high-TiO₂ basalt–olivine samples. Increased Ti content in the olivine is associated with the defects; ordering of Ti⁴⁺ and octahedral site vacancies leads to a two- or three-layer superstructure in the olivine. Defect nucleation and growth is driven by the large TiO₂ chemical potential gradient across the phase boundary at the start of the experiments, which provides access to microstructures not otherwise present.     

First evidence for Ordovician igneous activity in the dial range trough,Tasmania

A small andesitic intrusion, previously considered to be a Middle Cambrian lava, cuts fossiliferous upper Middle Cambrian sediments in the Leven Gorge section of the Dial Range Trough, northwestern Tasmania. Sixteen total‐rock samples of this intrusion produce a perfect‐fit, rubidium‐strontium isochron, which gives an Early Ordo‐vician age of 490 ± 18 m.y. for λ⁸⁷Rb = 1.39 × 10‐¹¹y‐¹, and NBS 70A feldspar = 522 ppm Rb and 65.3 ppm Sr respectively. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7112 ± 0.0003 suggests a continental type intrusion rather than an island‐arc andesite. Rather similar intermediate igneous rocks, at least some of which are intrusive, are common in the Dial Range Trough and could be largely post‐Cambrian in age. From the geological time‐scale point of view, the only definite information obtained in this work is that the Middle‐Late Cambrian boundary is older than the date given above.     

Setting limits for acceptable change in sediment particle size composition: Testing a new approach to managing marine aggregate dredging

A baseline dataset from 2005 was used to identify the spatial distribution of macrofaunal assemblages across the eastern English Channel. The range of sediment composition found in association with each assemblage was used to define limits for acceptable change at ten licensed marine aggregate extraction areas. Sediment data acquired in 2010, 4 years after the onset of dredging, were used to assess whether conditions remained within the acceptable limits. Despite the observed changes in sediment composition, the composition of sediments in and around nine extraction areas remained within pre-defined acceptable limits. At the tenth site, some of the observed changes within the licence area were judged to have gone beyond the acceptable limits. Implications of the changes are discussed, and appropriate management measures identified. The approach taken in this study offers a simple, objective and cost-effective method for assessing the significance of change, and could simplify the existing monitoring regime.     

Infometric and statistical diagnostics to provide artificially-intelligent support for spatial analysis: the example of interpolation

The wider uptake of GIS tools by many application areas outside GIScience means that many newer users of GIS will have high-level knowledge of the wider task, and low-level knowledge of specific system commands as given in reference manuals. However, these newer users may not have the intermediate knowledge that experts in GI science have gained from working with GI systems over several years. Such intermediate knowledge includes an understanding of the assumptions implied by the use of certain functions, and an appreciation of how to combine functions appropriately to create a workflow that suits both the data and overall goals of the geographical analysis task.

Focusing on the common but non-trivial task of interpolating spatial data, this paper considers how to help users gain the necessary knowledge to complete their task and minimise the possibility of methodological error. We observe that both infometric (or cognitive) knowledge and statistical knowledge are usually required to find a solution that jointly and efficiently meets the requirements of a particular user and data set. Using the class of interpolation methods as an example, we outline an approach that combines knowledge from multiple sources and argue the case for designing a prototype ‘intelligent’ module that can sit between a user and a given GIS.

The knowledge needed to assist with the task of interpolation is constructed as a network of rules, structured as a binary decision tree, that assist the user in selecting an appropriate method according to task-related knowledge (or ‘purpose’) and the characteristics of the data sets. The decision tree triggers exploratory diagnostics that are run on the data sets when a rule requires to be evaluated. Following evaluation of the rules, the user is advised which interpolation method might be and should not be considered for the data set. Any parameters required to interpolate the particular data set (e.g. a distance decay parameter for Inverse Distance Weighting) are also supplied through subsequent optimisation and model selection routines. The rationale of the decision process may be examined, so the ‘intelligent interpolator’ also acts as a learning tool.     

Standard chemical‐based tephra extraction methods significantly alter the geochemistry of volcanic glass shards

The chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible chemical alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well‐characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to chemical alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K₂O, Na₂O, CaO, MgO) and SiO₂, and the most stable oxides are Al₂O₃ and FeO. Felsic glasses are considerably less susceptible to alteration by both acidic (HCl, HNO₃, H₂SO₄) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses. Copyright © 2019 John Wiley & Sons, Ltd.     

Calculation of photosynthetically available radiation using multispectral data in the Arctic

Photosynthetically Available Radiation(PAR) is an important bio-optical parameter related to marine primary production.PAR is usually measured by a broadband sensor and can also be calculated by multispectral data.When the PAR is calculated by multispectral data in polar region,four factors are possible error sources.PAR could be overestimated as the wavelengths of multispectral instrument are usually chosen to evade main absorption zones of atmosphere. However,both PARs calculated by hyperspectral and m...     

Examining the physical components of boundary shear stress for water‐worked gravel deposits

It is argued in this commentary that, in order to understand better the physical mechanisms that generate boundary shear stress over water‐worked gravel beds, flow velocity data should be re‐evaluated by spatial averaging the Reynolds equations to produce time‐ and space‐averaged (double‐averaged) momentum equations. A series of laboratory experiments were conducted in which the flow velocities were measured using a PIV system over two water‐worked gravel deposits. Combined with detailed data on the bed surface topography and vertical porosity, the physical components of shear stress were obtained. This enabled the various momentum transfer mechanisms present above, within and at the interface of a porous, fluvial deposit, to be quantified. This included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer. It is demonstrated that double‐averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment–water interface exchange mechanisms within the roughness layer in water‐worked gravel deposits. By considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel–sand mixtures and the transfer of solutes into porous, gravel deposits. This analysis reveals the importance of obtaining co‐located, high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress. Copyright © 2010 John Wiley & Sons, Ltd.