1,721,006 research outputs found
In situ modification of modern submarine hyaloclastic / pyroclastic deposits by oceanic currents: an example from the Southern Kermadec arc (SW Pacific)
No full textSubmarine volcaniclastic deposits, both modern and ancient, pose a conundrum in distinguishing between syn- and post-eruptive processes. High-standing, submarine volcanic edifices of the late Quaternary southern Kermadec arc (SW Pacific) are point sources of pyroclastic/hyaloclastic deposits that are bathed and modified by a complex current system of the South Pacific gyre flowing southeast along the northern margin of New Zealand, which in part comprises the anticyclonic flow of the warm-cored East Cape Eddy (ECE). Flow of the ECE across the southern Kermadec arc provides a present-day case of extensive and in situ, post-eruptive, textural modification of modern pyroclastic/hyaloclastic deposits on the crests and upper flanks of submarine stratovolcanoes. Photographic observations (and limited textural data) from seven Kermadec volcanoes reveal pervasive evidence of sediment winnowing (including crag and tail structures, scour and moating around volcanic blocks, coarse sand-granule lag deposits, epifaunal deflection, lineated mud streaking, and moulded bioturbation mounds) and asymmetric current-ripple bedforms at water-depths of at least 1500 m. All bedforms indicate increasing current speed at progressively higher elevations (decreasing water-depth) for each volcano. Current-ripples mostly have discontinuous, asymmetric, shorted-crested, linguoid–lunate forms below 1000 m water-depth, progressing to semi-continuous, asymmetric, shorted-crested, linear-sinuous forms above 500 m. Current elutriation of the Kermadec deposits progressively removes fines with decreasing water-depth resulting in relatively fines-depleted, volcaniclastic sands and granules. This post-eruptive process overprints syn-eruptive processes that notionally generate more comminuted fine-grained clasts with decreasing water-depth as phreatomagmatic explosive eruptions become more vigorous. Current-elutriation also modifies volcaniclastic detritus prior to subsequent removal by episodic, mass-gravity flow. In addition the sand-granule traction load, driven by current-flow, moves sediment nearly continuously to gully and rill heads for removal down-slope, independently of syn-eruptive sediment flux. The underlying observation is that volcaniclastic deposits rarely reflect just syn-eruptive processes, and that significant in situ current-elutriation of at the least surficial pyroclastic/hyaloclastic eruption products can occur on submarine volcanoes.Threshold current velocities, derived assuming unidirectional flow over cohesionless sand-lapilli grainsizes, and accounting for bed friction, yield current velocities (at 100 cm above the bed) of ?15 cm s?1 for water-depths >1500 m through to 70 cm s?1 for depths <500 m at the crests of Rumble III and V volcanoes. Estimated velocities are consistent with short-term current velocities of 30–40 cm s?1, measured directly from either acoustic doppler current profile data or relative geostrophic flow, since the latter do not account for seafloor topographic intensification. The variable hydrographic climatology of the ECE, known from sea-surface dynamic heights and repeat CTD surveys, is possibly recorded by seafloor substrates as evinced by worm-trails post-dating ripple formation and differing orientations of winnowed structures and ripples.<br/
Seamount gravity anomaly modelling with variably thick sediment cover
No full textInversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple 'Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple 'flood' geometry. Complete burial, with a typical sediment density of 2300 kg m–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m
RRS James Clark Ross Cruise 253, 26 Jul -25 Aug 2011. Arctic methane hydrates
Full text linkThe cruise built on the successful geophysical and geochemical mapping, undertaken during the 2008 IPY voyage JCR211 (Westbrook et al., 2009) that made the first comprehensive survey of methane bubble plume venting along the western Svalbard margin. The main achievements of JCR253 included the recovery of the ESONET demonstration mission AOEM - MASOX seafloor lander (with recovery of 10 months of physical and biogeochemical parameters from a vigorous bubble plume site) and its deployment for a further 12 months at the same site (for recovery in August 2012), completion of 23 HyBIS ROV dives, totaling 35 hr. Seafloor video and photographs, were completed along transects in both 420 – 380 m and 80-90 m water-depths, but additionally HyBIS was used to sample bubble plume fluids at seafloor “vents” for geochemical analysis, and bubble imaging to measure bubble sizes and ascent rates. A suite of 14 piston / gravity cores were acquired along three transects perpendicular to both the interpreted position of the hydrate stability zone outcropping at the seafloor and general linear band of bubble plumes emitting from the seafloor around ~ 390 m. A comprehensive suite of 28 CTD stations were completed for physical / chemical sensing and water-sampling along the three transects (co-located with sediment and box cores) and the shallow-water sites. Additionally, the active acoustic bubble BOB imaging system was deployed to record active methane bubble release at a representative bubble stream at 390 m for an 18-day deployment. A major “discovery” of the cruise is the observation of active methane bubble release in shallow- ater (80-90 m water-depth) landward of the previously described edge of the hydrate stability zone outcropping near the seafloor at water-depths of 420 – 380 m
The Campbell ferromanganese nodule field in the southern part of New Zealand’s Exclusive Economic Zone
No full textNew multibeam mapping and geochemistry of the 30°–35° S sector, and overview, of southern Kermadec arc volcanism
No full textNew multibeam mapping and whole-rock geochemistry establish the first order definition of the modern submarine Kermadec arc between 30° and 35° S. Twenty-two volcanoes with basal diameters > 5 km are newly discovered or fully-mapped for the first time; Giggenbach, Macauley, Havre, Haungaroa, Kuiwai, Ngatoroirangi, Sonne, Kibblewhite and Yokosuka. For each large volcano, edifice morphology and structure, surficial deposits, lava fields, distribution of sector collapses, and lava compositions are determined. Macauley and Havre are large silicic intra-oceanic caldera complexes. For both, concentric ridges on the outer flanks are interpreted as recording mega-bedforms associated with pyroclastic density flows and edifice foundering. Other stratovolcanoes reveal complex histories, with repeated cycles of tectonically controlled construction and sector collapse, extensive basaltic flow fields, and the development of summit craters and/or small nested calderas.Combined with existing data for the southernmost arc segment, we provide an overview of the spatial distribution and magmatic heterogeneity along 780 km of the Kermadec arc at 30°–36°30? S. Coincident changes in arc elevation and lava composition define three volcano–tectonic segments. A central deeper segment at 32°20?–34°10? S has basement elevations of > 3200 m water-depth, and relatively simple stratovolcanoes dominated by low-K series, basalt–basaltic andesite. In contrast, the adjoining arc segments have higher basement elevations (typically < 2500 m water-depth), multi-vent volcanic centres including caldera complexes, and erupt sub-equal proportions of dacite and basalt–basaltic andesite. The association of silicic magmas with higher basement elevations (and hence thicker crust), coupled with significant inter- and intra-volcano heterogeneity of the silicic lavas, but not the mafic lavas, is interpreted as evidence for dehydration melting of the sub-arc crust. Conversely, the crust beneath the deeper arc segments is thinner, initially cooler, and has not yet reached the thermal requirements for anatexis. Silicic calderas with diameters > 3 km coincide with the shallower arc segments. The dominant mode of large caldera formation is interpreted as mass-discharge pyroclastic eruption with syn-eruptive collapse. Hence, the shallower arc segments are characterized by both the generation of volatile-enriched magmas from crustal melting and a reduced hydrostatic load, allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions. Proposed initiation parameters for submarine pyroclastic eruptions are water-depths < 1000 m, magmas with 5–6 wt.% water and > 70 wt.% SiO2, and a high discharge rate.<br/
Predicted seafloor topography of the New Zealand region: a nonlinear least-squares inversion of satellite altimetry data
No full textWe use a nonlinear least squares inversion to derive predicted seafloor topography (hereinafter referred to as RW99) for the New Zealand region (146°E–165°W, 60°S–25°S), combining altimetry data from ERS-1 and Geosat Geodetic Missions, as well as available shipborne gravity and echo sounding data. Currently, the lithospheric component of the model is principally applicable to thinly sedimented oceanic basins; however, we have attempted, though with only partial success, to compensate for regional crustal variations. The upper part of the oceanic lithosphere has an elastic behavior related to a half-space cooling model and flexing under seafloor relief load. Using least squares theory, the topographic solution is derived as a linear combination of altimetry and in situ measurements with adjusted coefficients. These coefficients are iteratively fitted using nonlinear operators between bathymetry and altimetry-derived gravity anomalies assuming their error distributions are Gaussian. The theory enables sparse in situ data to be included in the inversion, such as depth soundings and marine gravity profiles. In comparison with the global model of Smith and Sandwell [1997] (hereinafter referred to as SS97), the RW99 predicted topography is constrained by over threefold more shipborne soundings data and the inclusion of shipborne gravity data. Three strategies are used to validate the RW99 model. Compared to the root-mean-square (rms) error of 310 m of the SS97 model, final residual differences for the RW99 model are within the range of 104–250 m. These rms errors are the result of uncertainties of model parameters, especially the elastic thickness and the relief density, but also the complexity of seafloor topography. In addition, the model inversion does not presently consider gravitational contributions of marine sediments of variable thickness. <br/
Hydrothermal mineralization in arc-type submarine volcanoes (abstract of paper presented at: 16th Annual V.M. Goldschmidt Conference 2006, Melbourne, Australia, 27 Aug-1 Sept 2006)
No full textA refined geochemical dataset of volcanic rocks from the Kermadec intra-oceanic arc venting (abstract of paper presented at: 16th Annual V.M. Goldschmidt Conference 2006, Melbourne, Australia, 27 Aug-1 Sept 2006)
No full textAcoustic detection of seabed gas leaks, with application to carbon capture and storage (CCS), and leak prevention for the oil and gas industry: preliminary assessment of use of active and passive acoustic inversion for the quantification of underwater gas releases
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