Description: Highlights • The proposed method automatically assesses the abundance of poly-metallic nodules on the seafloor. • No manually created feature reference set is required. • Large collections of benthic images from a range of acquisition gear can be analysed efficiently. Abstract Underwater image analysis is a new field for computational pattern recognition. In academia as well as in the industry, it is more and more common to use camera-equipped stationary landers, autonomous underwater vehicles, ocean floor observatory systems or remotely operated vehicles for image based monitoring and exploration. The resulting image collections create a bottleneck for manual data interpretation owing to their size. In this paper, the problem of measuring size and abundance of poly-metallic nodules in benthic images is considered. A foreground/background separation (i.e. separating the nodules from the surrounding sediment) is required to determine the targeted quantities. Poly-metallic nodules are compact (convex), but vary in size and appear as composites with different visual features (color, texture, etc.). Methods for automating nodule segmentation have so far relied on manual training data. However, a hand-drawn, ground-truthed segmentation of nodules and sediment is difficult (or even impossible) to achieve for a sufficient number of images. The new ES4C algorithm (Evolutionary tuned Segmentation using Cluster Co-occurrence and a Convexity Criterion) is presented that can be applied to a segmentation task without a reference ground truth. First, a learning vector quantization groups the visual features in the images into clusters. Secondly, a segmentation function is constructed by assigning the clusters to classes automatically according to defined heuristics. Using evolutionary algorithms, a quality criterion is maximized to assign cluster prototypes to classes. This criterion integrates the morphological compactness of the nodules as well as feature similarity in different parts of nodules. To assess its applicability, the ES4C algorithm is tested with two real-world data sets. For one of these data sets, a reference gold standard is available and we report a sensitivity of 0.88 and a specificity of 0.65. Our results show that the applied heuristics, which combine patterns in the feature domain with patterns in the spatial domain, lead to good segmentation results and allow full automation of the resource-abundance assessment for benthic poly-metallic nodules.

Description: The Logatchev hydrothermal field (14 degrees 45'N on the MAR) is one of a few submarine hydrothermal systems associated with ultramafic rocks. It is situated on the eastern inner flank of the rift valley wall, 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation. Geological mapping during various ROV dives, geological sampling, and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks. The mixed mafic/ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks. Massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor. The Logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems. One of these are so-called,smoking craters", seafloor depressions that are several meters wide, characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material. At these smoking craters hydrothermal venting occurs directly from holes within the craters and from small, cm to dm high, Cu-rich chimneys occurring at the crater rim. Based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic-hosted, off-axis location with abundant landslides, as well as off-axis gabbroic intrusions providing the heat for the hydrothermal convection cell. (C) 2009 Elsevier B.V. All rights reserved.

Description: The sediment-hosted Grimsey hydrothermal field is situated in the Tjörnes fracture zone (TFZ) which represents the transition from northern Iceland to the southern Kolbeinsey Ridge. The TFZ is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area. Hydrothermal activity occurs in the Grimsey field in a 300 m×1000 m large area at a water depth of 400 m. Active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field. Clear, metal-depleted, up to 250 °C hydrothermal fluids are venting from the active chimneys. Anhydrite samples collected from the Grimsey field average 21.6 wt.% Ca, 1475 ppm Sr and 3.47 wt.% Mg. The average molar Sr/Ca ratio is 3.3×10−3. Sulfur isotopes of anhydrite have typical seawater values of 22±0.7‰ δ34S, indicating a seawater source for SO42−. Strontium isotopic ratios average 0.70662±0.00033, suggesting the precipitation of anhydrite from a hydrothermal fluid–seawater mixture. The endmember of the venting hydrothermal fluids calculated on a Mg-zero basis contains 59.8 μmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of 0.70634. The average Sr/Ca partition coefficient between the hydrothermal fluids and anhydrite of about 0.67 implies precipitation from a non-evolved fluid. A model for fluid evolution in the Grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater. Before and during mixing, seawater is heated to 200–250 °C which causes anhydrite precipitation and probably the formation of an anhydrite-rich zone beneath the seafloor.

Description: Osbourn Trough is a key piece in an outstanding problem: do the Ontong Java, Manihiki and Hikurangi large igneous provinces represent a single ~100 million km3 magmatic pulse? Bathymetric mapping of a 145-km-wide swath across the ∼900-km-long Osbourn Trough revealed three segments offset by 23–35-km-long basins that strike perpendicular to the trough axis. Each segment comprises a 10–15-km-wide axial valley bounded by 300–500-m-high ridge mountains, has inside corner highs at its NW and SE margins that rise 1000–1200 m above the axial valley, and has a flanking set of subparallel abyssal hills. Dredging on steep escarpments successfully penetrated thick sediments and recovered Fe–Mn oxyhydroxide-encrusted volcaniclastic breccias. Lava clasts within the breccias have undergone variable degrees of marine weathering, leading to strong enrichment in most alkali elements and the light REE (except Ce). Nevertheless, their immobile element concentrations are consistently MORB-like and they plot within the MORB fields of tectonic discrimination diagrams. Isotope analyses indicate an affinity with Pacific MORB-source mantle. Both the morphology of Osbourn Trough and geochemistry of its lavas establish that it represents an extinct spreading ridge system. The trough is nearly equidistant (1750 km vs. 1550 km) from the Manihiki and Hikurangi Plateaus, which we interpret as remnants of a formerly contiguous Ontong Java–Manihiki–Hikurangi large igneous province. Inception of the Osbourn spreading ridge was coincident with reorganization of the former Pacific–Phoenix–Farallon spreading system and mega-plateau fragmentation at ∼118 Ma. Spreading across Osbourn Trough ceased when the Hikurangi Plateau collided with and blocked a southward-dipping subduction system developed along the Chatham Rise (eastern New Zealand) sector of the Gondwana margin at ∼86 Ma.

Description: Mineralogical and geochemical investigation of altered host rock samples from the Logatchev hydrothermal field reveal a large variety of alteration styles at this site. Serpentinization is most intense in former harzburgites and dunites varying between 90-95%, whereas gabbros are mostly rather fresh. A combination of serpentinization, interaction with hot hydrothermal fluids, melt/rock interaction, and low-temperature seafloor weathering lead to significant gains and losses of major and trace elements. Serpentinization within the Logatchev hydrothermal field proceeds mainly isochemical for the major elements, except for a loss of TiO2 and CaO. However, the concentration of the trace elements Cu, Nb, Ba, La, Sm, Eu, Th or U increases significantly in the serpentinites. Gabbroic intrusions act as a sink for MgO during alteration due to the formation of chlorite and serpentine after clinopyroxene. Interaction between gabbros and hydrothermal fluids leads to significant redistribution of SiO2, TiO2, CaO, and Na2O as well as numerous trace elements. The different styles of alteration and their associated element changes reveal that samples from the entire Logatchev field have been influenced by hydrothermal fluids to some degree. Therefore, the hydrothermal fluid-dominated alteration of the ultramafic oceanic crust is a sink for many trace elements which were provided by mafic intrusions and mobilized by hydrothermal fluids and melt-rock interaction, whereas the gabbros accumulate high amounts of Mg from the seawater. Summarized the alteration processes at Logatchev are a combination of serpentinization, melt/rock interaction of serpentinites and mafic intrusions, and low-temperature seafloor weathering. (C) 2008 Elsevier B.V. All rights reserved.