Detection of deep water benthic macroalgae using image-based classification techniques on multibeam backscatter at Cashes Ledge,Gulf of Maine,USA

Benthic macroalgae form an important part of temperate marine ecosystems, exhibiting a complex three-dimensional character which represents a vital foraging and spawning ground for many juvenile fish species. In this research, image-based techniques for classification of multibeam backscatter are explored for the detection of benthic macroalgae at Cashes Ledge in the Gulf of Maine, USA. Two classifications were performed using QTC-Multiview, differentiated by application of a threshold filter, and macroalgal signatures were independently extracted from the raw sonar datagrams in Matlab. All classifications were validated by comparison with video ground-truth data. The unfiltered classification shows a high degree of complexity in the shallowest areas within the study site; the filtered demonstrates markedly less variation by depth. The unfiltered classification shows a positive agreement with the video ground-truth data; 82.6% of observations recording Laminaria sp., 39.1% of Agarum cribrosum and 100.0% (n = 3) of mixed macroalgae occur within the same acoustically distinct group of classes. These are discrete from the 8.1% recorded agreement with absences and nulls (>40 m) of macrophytes (n = 32) from a total of 86 ground-truth locations. The results of the water column data extraction (WCDE) show similar success, accurately predicting 78.3% of Laminaria sp. and 30.4% of A. cribrosum observations.     

Onshore and offshore seismic and lithostratigraphic analysis of a deeply incised Quaternary buried valley system in the Northern Netherlands

High-resolution seismic data (onshore and offshore), geophysical borehole data as well as detailed lithofacies from airlift boreholes were acquired in northern Netherlands on and around the island of Ameland. Marine and land seismic data combined with information from land boreholes have been explored with the objective of providing a sedimentary model. Qualitative seismic facies analysis of the valley fill commonly shows a thin unit with high amplitude reflectors at the base. Thick units of variable seismic facies (transparent to high amplitude) occur higher up in the sequence. Onlap is common at mid–upper levels within the sandy valley fill (with clay in mm layering), and a transparent seismic facies, corresponding to firm clays, is common at the top. Almost all lithological unit boundaries recognised within core parameters correspond with seismic unconformities within error margins. Subunits contain multiple cyclical trends in gamma ray and grain size. Cyclical trends show lower order fluctuations in gamma radiation on a scale of less than 1 m. Gamma-ray pattern variability between units, e.g. in general coarsening-up or fining-up units, suggests migration of subaqueous outwash fans or ice margin fluctuations. Seismic results could support a headward excavation and backfilling process suggested by Praeg [Morphology, stratigraphy and genesis of buried Elsterian tunnel valleys in the southern North Sea basin [PhD thesis]: University of Edinburgh, 207 pp.; Journal of Applied Geophysics, (this volume)] as being responsible for the formation of buried valleys. On a lithological scale, a more complicated, detailed and cyclical pattern arises. Catastrophic processes are considered unlikely as being responsible for the infill sequence because of the observed small-scale facies variability and because of the presence of diamicton layers. Diamicton layers at the base of basal unconformities as well as higher in sequence could suggest subglacial deformation by grounded ice before and during the valley-fill process.     

Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes

Brines in Cambrian sandstones and Ordovician dolostones of the St-Lawrence Lowlands at Bécancour, Québec, Canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path, in addition to quantifying their residence time. Major ion chemistry indicates that the brines are of Na-Ca-Cl type, possibly derived from halite dissolution. ⁸⁷Sr/⁸⁶Sr ratios and Ca excess indicate prolonged interactions with silicate rocks of the Proterozoic Grenville basement or the Cambrian Potsdam sandstone. The brines constrain a 2-3% contribution of mantle ³He and large amounts of nucleogenic ²¹Ne^∗ and ³⁸Ar^∗ and radiogenic ⁴He and ⁴⁰Ar^∗. ⁴He/⁴⁰Ar^∗ and ²¹Ne^∗/⁴⁰Ar^∗ ratios, corrected for mass fractionation during incomplete brine degassing, are identical to their production ratios in rocks. The source of salinity (halite dissolution), plus the occurrence of large amounts of ⁴⁰Ar^∗ in brines constrain the residence time of Bécancour brines as being older than the Cretaceous. Evaporites in the St-Lawrence Lowlands likely existed only during Devonian-Silurian time. Brines might result from infiltration of Devonian water leaching halite, penetrating into or below the deeper Cambrian-Ordovician aquifers. During the Devonian, the basin reached temperatures higher than 250 °C, allowing for thermal maturation of local gas-prone source rocks (Utica shales) and possibly facilitating the release of radiogenic ⁴⁰Ar^∗ into the brines. The last thermal event that could have facilitated the liberation of ⁴⁰Ar^∗ into fluids and contributed to mantle ³He is the Cretaceous Monteregian Hills magmatic episode. For residence times younger than the Cretaceous, it is difficult to find an appropriate source of salinity and of nucleogenic/radiogenic gases to the Bécancour brines.     

Controls on the location,morphology and evolution of complex esker systems at decadal timescales,Breiðamerkurjökull,southeast Iceland

This paper uses detailed mapping of eskers to address three questions which are important for reconstructing meltwater behaviour beneath contemporary and ancient ice masses: ‘What controls the morphology of simple and complex esker systems?’, ‘How do esker systems evolve through time?’ and ‘Are esker patterns compatible with groundwater controlled hydraulic spacing of esker tunnels?’. Esker crestlines and widths are mapped on the Breiðamerkurjökull foreland for eight time slices between 1945 and 2007, from high resolution (~50 cm) aerial photography, permitting their long‐term morphological evolution to be analysed in a high level of detail. We find that complex eskers develop where meltwater and sediment is abundant, such that sediment clogs channels, forming distributary eskers. Isolated eskers are simpler and smaller and reflect less abundant meltwater and sediment, which is unable to clog channels. Eskers may take several decades to emerge from outwash deposits containing buried ice and can increase or decrease in size when ice surrounding and underlying them melts out. It has been suggested that groundwater–channel coupling dictates the spacing between eskers at Breiðamerkurjökull. Our results do not dispute this, but suggest that the routing of sediment and meltwater through medial moraines is an additional important control on esker location and spacing. These results may be used to better understand the processes surrounding esker formation in a variety of geographical settings, enabling a more detailed understanding of the operation of meltwater drainage systems in sub‐marginal zones beneath glaciers and ice sheets. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel <101> direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 μm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles. Received: 16 November 1998 / Accepted: 17 May 1999     

Papua New Guinea Highlands: palaeomagnetic constraints on terrane tectonics

Oblique convergence since the Early Cenozoic between the northward-moving Australian plate, westward-moving Pacific plate and almost stationary Eurasian plate has created a world-ranking tectonic zone in the eastern Indonesia–New Guinea–Southwest Pacific region (Tonga–Sulawesi megashear) that is notorious for its complex mix of tectonic styles and terrane juxtapositions. Unlike an ancient analog—the Mesozoic–Cenozoic Cordillera of North America—palaeomagnetic constraints on terrane motions in the zone are few. To improve the framework of quantitative control on such motions and therefore our understanding of the development of the zone, results of a palaeomagnetic study in the Highlands region of Papua New Guinea (PNG), in the southern part of the New Guinea Orogen, are reported. The study yields new insights into terrane tectonics along the Australian craton's active northern margin and confirms the complexity of block rotations to be expected at the local scale in tectonically intricate zones. The study is based on more than 500 samples (21 localities) collected from an interior and an exterior zone of New Guinea's central cordillera. The two zones are separated by the Tahin and Stolle–Lagaip–Kaugel Fault zones and collectively represent the para-autochthonous northern margin of the Australian craton. Samples from the interior zone, which in the study area comprises a cratonic spur of uncertain—probably displaced—origin, come from Triassic to Miocene sediments and subordinate volcanics of the Kubor Anticline, Jimi Terrane, and Yaveufa Syncline (16 localities) in the central and eastern Highlands. Samples from the exterior zone, which represent a basement-involved, Pliocene foreland fold-and-thrust belt, come from Middle Eocene to Middle Miocene carbonates and clastics (five localities) in the southern Highlands of the Papuan Fold Belt. Results permit us to constrain the tectonic evolution of the two zones palaeomagnetically. Using mainly thermal demagnetization techniques, three main magnetic components have been identified in the collection: (1) a recent field overprint of both normal and reverse polarity; (2) a pervasive overprint of mainly normal polarity that originated during extensive Middle to Late Miocene intrusive activity in the central cordillera; and (3) a primary component which has been identified in only 7 of the 21 localities (5 of 11 stratigraphic units represented in the collection). All components show patterns of rotation that are consistent within the zones, but differ between them. In the interior zone (central and eastern Highlands), large-scale counterclockwise rotations of between 30°+ and 100°+ have been established throughout the Kubor Anticline and Jimi Terrane, with some clockwise rotation present in the southern part of the Yaveufa Syncline. In contrast, in the Mendi area of the exterior zone (southern Highlands), clockwise rotations of between 30°+ and 50°+ can be recognized. These contrasting rotation patterns across the Tahin and Stolle–Lagaip–Kaugel Fault zones indicate decoupling of the two tectonic zones, probably along basement-involved faults. The clockwise rotations in the southern Highlands of the Papuan Fold Belt are to be expected from its structural grain, and are probably governed by regional basement faults and transverse lineaments. In contrast, the pattern of counterclockwise rotations in the Kubor Anticline–Jimi Terrane cratonic spur of the central and eastern Highlands was unexpected. The pattern is interpreted to result from non-rigid rotation of continental terranes as they were transported westward across the northeastern margin of the Australian craton. This margin became reorganised after the Middle Miocene, when the steadily northward-advancing Australian craton impinged into the westward-moving Pacific plate/buffer-plate system. Transpressional reorganisation under the influence of the sinistral Tonga–Sulawesi megashear became enhanced with Mio-Pliocene docking, and subsequent southward overthrusting, of the Finisterre Terrane onto the northeastern margin of the Australian craton.