• NEANIAS Underwater Research Community

[1] Tomographic images constrain the existence of a subducted seamount beneath the Gulf of Nicoya, Costa Rica. The subducted seamount is found at a depth of 30 km within the rupture area of the March 25, 1990, Mw = 7.0 Gulf of Nicoya earthquake. The Gulf of Nicoya earthquake was a typical thrust-type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting Cocos plate and the overriding plate. Precise relocation of the mainshock and its aftershocks in a 3-D P-wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount. Most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate. We interpret the subducted seamount as an asperity whose rupture caused the 1990 Gulf of Nicoya earthquake.

Complex principal component analysis (CPCA) is applied to examine medium-term geomorphological behavior of an exposed sandy beach adjacent to a gigantic breakwater of the Port of Sendai located near a river mouth in the Sendai Coast, Japan, using a 12-year series of bathymetry survey data. The results of CPCA and conventional real-PCA demonstrate that erosion and subsequent accretion of the submerged terrace formed in front of the river mouth appear in the first mode of CPCA and have the most significant influence on the medium-term geomorphology of the study area. The first mode is mostly caused by northward alongshore sediment transport driven by wave energy flux, explained from the observed wave data. The second mode of CPCA demonstrates that the sediments previously discharged from the river return again into the nearshore region, and furthermore, the topography changes due to cross-shore sediment transport emerge in the third mode of CPCA.

International audience; A new conodont species, Siphonodella leiosa, is described from the lower Carboniferous pelagic limestones of the Montagne Noire (France), deposited in North Gondwana on a outer platform environment. Specimens were obtained from one level dated to the Siphonodella jii conodont Zone. The major difference from other siphonodellid conodonts known in this area is that the elements of this new species have a practically entirely smooth and unornamented platform, apart from the development of one or two low rostral ridge-like nodes. Similar morphologies were generally observed in shallow marine deposits of the same time frame from China, Russia and East and Central European areas. The new discovery reinforces the idea that ornamentation of siphonodellids is not only related to bathymetry, but that temperature could play an important role in the diversification and radiation of unornamented species during the Siphonodella jii conodont Zone.

The proposed work intends to make use of Geomorphometry – science that quantifies the forms of relief – in the characterization and distinction of passive continental margins. The main objective is to provide means to distinguish continental slope from continental rise in a non-subjective and morphologically coherent way. Geomorphologically smooth continental margins, that reach abyssal depths in a gradual way, with no sharp breaks in gradient, yields a specific geomorphometric approach which is proposed by the present work and applied to digital grids generated from the ETOPO2 bathymetric database and profiles extracted from multibeam bathymetric data. No geological or geophysical arguments are used to corroborate the methodology.

A quantitative sampling was made on the non-cultivated molluscs of the soft bottom macrofauna in the bay of Marennes-Oleron. A stratified random sampling, including an optimal allocation strategy. provided adequate data to estimate abundance and biomass (total weight. dry weight and ash-free-dry weight) of each molluscan species. The main stratification factors were respectively the bathymetric levels. the different types of sediment and the differences between natural, oyster-free mud flats and cultivated areas. A Smith Mc-Intyre grab was used to sample the subtidal, shallow areas and the tidal flats as well. The total number of sampling stations was 370. The final estimation of the total weight of benthic molluscs was 14,400 ± 3,000 tons (at 95 % confidence limits). The main trophic competitors for oysters were : Cerastoderma edule (4,500 ± 2,200 tons), Mytilus edulis (2,600 ±. 2,000 tons), Crepidula fornicata (1,800 ±. 900 tons), Solen marginatus (800 ±. 600 tons) and Macoma balthica (800 ± 400 tons). La stratégie d'échantillonnage retenue pour l'estimation des stocks de mollusques benthiques non cultivés du bassin de Marennes-Oléron consiste en un type d'échantillonnage aléatoire stratifié, après allocation optimale de l'effort d'échantillonnage au sein des strates. L'engin de prélèvement, une benne Smith Mc-Intyre, a été utilisée sur l'ensemble de la zone soit 180 km. Les principaux stratificateurs sont la bathymétrie, les types biosédimentaires et la présence ou l'absence de culture d'huîtres. Un ensemble de 370 stations a été effectué. Le stock global de mollusques non cultivés est évalué en biomasse fraîche à 14 400 ± 3 000 tonnes (au seuil d'erreur de 5 %). Les principales espèces compétitrices sur le plan trophique des huîtres sont : Cerastoderma edule (4 500 ± 2 000 tonnes), Mytilus edulis (2 600 ±. 2 000 tonnes), Crepidula fornicata (1 800 ± 900 tonnes), Solen marginatus (800 ± 600 tonnes) et Macoma balthica (800 ±. 400 tonnes).

We conclude this book by considering three challenges and opportunities that are shared by all fields of submarine geomorphological research: (i) big data at multiple spatio-temporal scales, (ii) direct observation, and (iii) interaction with subaerial geomorphologists.

Abstract We present an empirical model of the gravitational field generated by the oceanic lithosphere computed over the world’s oceans with a spectral resolution complete to a spherical harmonic degree of 180. This gravity model is compiled based on applying methods for a spherical harmonic analysis and synthesis of the global gravity and crustal structure models. The in situ seawater densities and the density samples from ocean-floor drilling sites are utilized in the gravimetric forward modeling of bathymetry and marine sediments. The gravitational signal attributed to the oceanic lithosphere density structure is described empirically in terms of the ocean-floor age and depth. The former is explained by the increasing density with age due to conductive cooling of the oceanic lithosphere. The latter describes the gravitational signature of thermal lithospheric contraction, which is isostatically compensated by ocean deepening. The long-wavelength gravity spectrum reflects mainly the compositional and thermal structures within the sub-lithospheric mantle. We demonstrate that this empirical gravity model reproduces realistically most of the long-to-medium wavelength features of the actual gravity field, except for some systematic discrepancies, especially along continental slopes and large sedimentary accumulations, which cannot be described accurately by applied empirical models.

The global flux of tidal energy to mixing processes via topographically generated internal waves is estimated utilizing gridded databases for bathymetry, vertical density stratification and barotropic tides together with a simple, local model for the generation of progressive internal tides at vertical steps in the ocean floor. Both the horizontal distribution of the energy flux to internal tides and its ocean mean are discussed. The computed oceanic mean value is 44 × 10−4W m−2, a factor of about 2–3 greater than previous estimates (Munk, 1966, Deep-Sea Research, 13, 707–730; Bell, 1975, Journal of Geophysical Research, 80, 320–327). The global distribution of vertical diffusivity in the abyss is computed by assuming that topographically generated baroclinic motions dissipate locally and that the dissipation is distributed vertically according to an empirical law. Our results are linearly dependent on the flux Richardson number Rf. From the computed vertical diffusivities and the known vertical stratification we finally compute the global distribution of vertical velocities. Choosing a value of Rf≈0.05 we obtain an upward vertical transport in the interior of the ocean, at the 1000 m level, of about 15 × 106m3s−1, which agrees with Warren's (1981, in: Evolution of physical oceanography, B. A. Warren and C. Wunsch, editors, 6–41) estimated rate of sinking from surface waters at high latitudes. Below the 1000 m level the upward vertical transport increases and a maximum value of about 25 × 106m3s−1 is found at the 2000 m level, after which the transport decreases to about 8 × 106m3s−1 at the 4000 m level. This may be explained by the action of bottom currents. These currents entrain ambient water whereby the upward interior vertical transports tend to increase with depth. However, because of the entrainment of lighter ambient fluid the dense currents become less dense and only the most dense flows penetrate to the greatest depths.