Flat Seafloor Research Articles

Assemblage of glacial and related landforms in the fjords of southern Chile

At 42–55° S (Fig. 1a), the fjords of southern Chile are one of the lowest-latitude and mildest regions in which glaciers reach the sea today (Dowdeswell & Vasquez 2013). The Andean ice caps and glaciers of the Darwin Cordillera in Tierra del Fuego are of high mass throughput with precipitation often metres per year and, to balance this, very high rates of surface melting. These ice masses either reach the sea at fjord heads or feed braided-river systems that flow into the fjords. Large volumes of meltwater and glacial sediment are therefore delivered to Chilean fjords either directly from subglacial streams beneath tidewater glaciers or indirectly by glacifluvial processes. At the Last Glacial Maximum (LGM) the Patagonian Ice Cap expanded to fill these fjords, which were re-occupied by ocean waters as deglaciation proceeded. Fig. 1. Sun-illuminated multibeam-bathymetric images of submarine landforms in the fjords of Chilean Patagonia. Acquisition system Atlas Fansweep. Frequency 200 kHz. Grid-cell size 20 m. ( a ) Location of study area (red box; map from GEBCO_08). ( b ) Terminal moraine ridge and other transverse ridges produced by Tempano Glacier, inner Iceberg Fjord. S, streamlined lineations. ( c ) Hummocky but otherwise largely flat seafloor beyond Pio XI Glacier in Eyre Fjord. H, hummocky glacial debris. ( d ) Swath-bathymetric image of 100 m high recessional moraine in Europa Fjord. ( e ) Glacifluvial delta in Iceberg Fjord fed from an associated braided river. ( f ) Glacifluvial delta in Bernardo Fjord, showing a braided river and the adjacent Bernardo Glacier. ( g ) Small fluvial delta in Eyre Fjord. Images modified after Dowdeswell & Vasquez (2013). We thank the Servicio Hidrografico y Oceanografico de la Armada de Chile (SHOA) for allowing use of their swath-bathymetric data. Many Chilean fjords have experienced tidewater-glacier advance during the cool Little Ice Age (approximately sixteenth–nineteenth centuries), followed by retreat over the past century …

Can small islands protect nearby coasts from tsunamis? An active experimental design approach

Small islands in the vicinity of the mainland are widely believed to offer protection from wind and waves and thus coastal communities have been developed in mainland areas behind small islands. However, whether they offer protection from tsunamis is unclear. Do islands act as natural barriers? Recent post-tsunami survey data, supported by numerical simulations, reveal that the run-up on coastal areas behind small islands was significantly higher than on neighbouring locations not affected by the presence of the islands. To study the conditions of this run-up amplification, we solve numerically the nonlinear shallow water equations. We use the simplified geometry of a conical island sitting on a flat seafloor in front of a uniform sloping beach. By doing so, the experimental set-up is defined by five physical parameters, namely the island slope, the beach slope, the water depth, the distance between the island and the plane beach and the incoming wavelength, while the wave height was kept fixed. The objective is to find the maximum run-up amplification with the least number of simulations. To achieve this goal, we build an emulator based on Gaussian Processes to guide the selection of the query points in the parameter space. We thus reduce substantially the computations required to identify the run-up amplification. Our results show that the island acts as a focusing lens for energy and amplifies the run-up along the coastline behind its lee side, instead of protecting it, as popular beliefs suggest.

동해 후포퇴 왕돌초 주변의 정밀해저지형 및 해저면 특성 분석

동해의 주요 어장이고 레저 관광과 동해 해양환경 연구 등에서 매우 중요한 역할을 하고 있는 후포퇴 왕돌초 주변해역에 대한 정밀해저지형을 분석하고, 해저면 영상 자료 및 퇴적물 입도 결과를 이용하여 해저면 특성을 분류하였다. 후포퇴는 동해의 해안선과 평행하게 나타나며 가장 수심이 얕은 지역은 후포항 동쪽에 위치하고 있는 왕돌초 지역이다. 이 왕돌초는 셋잠(북쪽), 중간잠(중간), 맞잠(남쪽)이라는 3개의 봉우리로 구성되어 있다. 세 개의 봉우리 중 가장 수심이 낮은 지역은 중간잠으로 약 6 m의 가장 얕은 수심을 보이고, 북쪽의 셋잠은 약 8 m, 남쪽의 맞잠은 약 9 m의 수심에서 가장 얕은 지역이 나타난다. 왕돌초의 세 봉우리 주변으로는 얕은 해저지형을 보이며 복잡한 지형 기복과 많은 암반들이 산재한다. 그 암반들 사이 및 북동쪽의 평탄면과 급경사면의 서쪽 주변부로는 모래질자갈/자갈질모래 등의 비교적 굵은 조립질 퇴적물이 관측된다. 왕돌초 서쪽으로는 불규칙한 지형보다는 지형 기복이 감소하고 급격한 경사면이 존재하며 후포분지와 연결되는데 이 후포분지 지역은 니질/실트질 모래류의 퇴적물이 나타난다. 왕돌초 주변의 정밀해저지형과 해저면 특성 분석자료는 향후 왕돌초 해역의 해양환경연구 및 생태계의 지속적 이용을 위한 관리에 기여할 것으로 판단된다. The Wangdol-cho area, in the Hupo Bank, plays a very important role in main fishing grounds, leisure tourism and marine environmental researches of the East Sea. We analyzed the detailed bathymetry and classified the seabed characteristics of the Wangdol-cho area, based on seafloor backscattering images and sediment grain size. The Hupo Bank is developed in parallel with the eastern coastal line of Korean peninsula, and the shallowest area (Wangdol-cho) of the Hupo Bank is located along the eastern part of Hupo Port. The Wangdol-cho comprises three summits; north summit, middle summit, and south summit. The middle summit area among the three summits has the most shallow water depth with minimum about 6 m. The north summit shows about 8 m minimum depth and the south summit about 9 m. The bathymetry data around three summits represent undulating seabeds with many scattered underwater reefs and shallow water depth. The area between the underwater reefs, the flat seafloor in the northeastern part of the survey site, and the western steep slope area have relatively coarse sediments such as sandy gravel and gravelly sand. The bathymetry in the western side of the Wangdol-cho shows steep slope seabed, extending to the Hupo Basin. Fine sediments including mud and silty sand occur in the Hupo Basin area of the survey site. The submarine detailed topography and the analysis of the seafloor characteristics of the survey area are expected to contribute to management for marine environmental researches and sustainable use of ecosystems in the Wangdol-cho.

On how thin submarine flows transported large volumes of sand for hundreds of kilometres across a flat basin plain without eroding the sea floor

AbstractSubmarine gravity currents, especially long run‐out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north‐west Africa. This is the only data set where bed shape, internal distribution of lithofacies, changes in grain size and sea floor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km3) for large distances across very flat sea floor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0·02° to 0·05°) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying sea floor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and that deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field data set is a challenge for future experimental and numerical models.

FURONGIAN (LATE CAMBRIAN) SPONGE-MICROBIAL MAZE-LIKE REEFS IN THE NORTH CHINA PLATFORM

ABSTRACT During the Furongian (late Cambrian) and Early Ordovician, maze-like (maceriate) microbialites flourished in both Laurentia and Gondwana. The maze-like microbialites are characterized by centimeter- to decimeter-scale branching, complex structures. However, organisms responsible for the formation of maze-like structures are poorly known. In order to understand formational processes of maze-like microbialites, this study focuses on the Furongian microbialites of the North China Platform in which microbial components and siliceous sponges co-occur. The maze-like structures consist of microbial components such as microstromatolites, Girvanella, and Renalcis-like forms, as well as sponge spicule networks, whereas lime mud and bioclasts occupy the space between the structures. The maze-like structures developed on a relatively flat seafloor, forming low synoptic relief (<1 cm) above the sediment surface. Continuous growth of maze-like structures with balanced deposition of sediments led to meter-scale...

Depositional framework of the East Baltic Tremadocian black shale revisited

This article presents a centimetre- to micrometre-scale study of sedimentary fabrics from Lower Ordovician metalliferous black shale from the Baltic palaeobasin. Two sections of the Türisalu Fm. NW and NE Estonia were analysed with light microscopy and scanning electron microscopy. This rock unit is characterised by mostly thin bedding ( < 10 mm), common occurrence of minor erosional features, and a large variety of sedimentary fabrics, including graded, cross-laminated and massive fabrics. Based on this, we suggest that dynamic sedimentation events, rather than commonly assumed slow net sedimentation, may be the dominant mechanism behind the accumulation of these beds. The storm-related near-bottom flows and the bed-load transport of mud particles were likely common distribution agents of organic-rich mud. The mud (re)distribution, mainly via near-bottom flows and controlled by flat seafloor topography and general clastic starvation, might explain the present lateral distribution and diachronous character of the Türisalu Fm. Documented traces of microbial mat growth and siliceous sponges in the NW Estonia indicate that in more sheltered settings, biogenic factors played a vital role in developing primary mud characteristics. The geochemical palaeoredox proxies, and high trace metal and organic matter content suggest that mud sedimentation could occur under anoxic conditions. The observed sedimentary fabrics and traces of bioturbation, however, favour prevailing oscillating redox conditions in the lower water column. The recorded heterogeneity of microfabrics indicates that dynamic transport and intermittent deposition together with biogenic factors likely forced the development of an array of unique (bio)geochemical microenvironments for syngenetic trace element sequestration.

North Cascadia heat flux and fluid flow from gas hydrates: Modeling 3‐D topographic effects

AbstractThe bottom‐simulating reflector (BSR) of gas hydrate is well imaged from two perpendicular seismic grids in the region of a large carbonate mound, informally called Cucumber Ridge off Vancouver Island. We use a new method to calculate 3‐D heat flow map from the BSR depths, in which we incorporate 3‐D topographic corrections after calibrated by the drilling results from nearby (Integrated) Ocean Drilling Program Site 889 and Site U1327. We then estimate the associated fluid flow by relating it to the topographically corrected heat flux anomalies. In the midslope region, a heat flux anomaly of 1 mW/m2 can be associated with an approximate focused fluid flow rate of 0.09 mm/yr. Around Cucumber Ridge, high rates of focused fluid flow were observed at steep slopes with values more than double the average regional diffusive fluid discharge rate of 0.56 mm/yr. As well, in some areas of relatively flat seafloor, the focused fluid flow rates still exceeded 0.5 mm/yr. On the seismic lines the regions of focused fluid flow were commonly associated with seismic blanking zones above the BSR and sometimes with strong reflectors below the BSR, indicating that the faults/fractures provide high‐permeability pathways for fluids to carry methane from BSR depths to the seafloor. These high fluid flow regions cover mostly the western portion of our area with gas hydrate concentration estimations of ~6% based on empirical correlations from Hydrate Ridge in south off Oregon, significantly higher than previously recognized values of ~2.5% in the eastern portion determined from Site U1327.

Comparison Between Boussinesq and Shallow-Water Models in Predicting Solitary Wave Runup on Plane Beaches

The nonlinear shallow water equations and Boussinesq equations have been widely used for analyzing the solitary wave runup phenomenon. In order to quantitatively assess the merits and limitations of these two approaches, a shock-capturing scheme has been applied to numerically solve both sets of equations for predicting the runup processes over plane beaches. The analytical and experimental data available in the literature have been used as references in the assessment. In this study, the uniform sloping beach is preceded by a length of flat seafloor. When incident solitary waves are specified close to the slope, the two approaches are found to produce almost identical results for nonbreaking waves. For breaking waves, the Boussinesq equations give a better representation of the wave evolution prior to the breaking point, whereas they overestimate the short undulations accompanying the breaking process. If incoming solitary waves need to travel a long distance over a flat bed before reaching the slope, the shallow water equations cannot capture the correct waveform transformation, and the predicted runup depends heavily on the length of the flat-bed section. As this length approaches zero, however, the shallow water equations somehow give roughly the same maximum runup heights as those predicted by the Boussinesq model. The bed friction has little effect on the runup for small waves, but becomes important for large waves. The roughness coefficient needs to be calibrated to reproduce the measured runup heights of breaking waves.

Curvelet Approach for Deep-sea Sonar Image Denoising, Contrast Enhancement and Fusion

Side-scan sonar acquires high quality imagery of the seafloor with very high spatial resolution but poor locational accuracy. However, multi-beam sonar obtains high precision position and underwater depth in seafloor points. In order to fully utilize all information of these two types of sonars, it is necessary to fuse the two kinds of sonar data. This paper gives curvelet transform for enhancing the signals or details in different scales separately. It also proposes a new intensity sonar image fusion method, which is based on curvelet transform. Considering the sonar image forming principle, for the low frequency curvelet coefficients, we use the maximum local energy method to calculate the energy of two sonar images. For the high frequency curvelet coefficients, we take absolute maximum method as a measurement. The main attribute of this paper is: Firstly, the multi-resolution analysis method is well adapted the cured-singularities and point-singularities. It is useful for sonar intensity image enhancement. Secondly, maximum local energy is well performing the intensity sonar images, which can achieve perfect fusion result. The experimental results show that the method can be used in the flat seafloor or the isotropic seabed. Compared with wavelet transform method, this method can get better performance.

Enhanced nutrient fluxes at the shelf sea seasonal thermocline caused by stratified flow over a bank

Patches of enhanced chlorophyll a (Chl) concentrations within the thermocline were observed over the slopes of several banks in the Celtic Sea. The turbulent mixing of nutrients from the bottom water into the thermocline was found to be greatly enhanced over the slope of a bank (up to 52mmolnitratem−2day−1), compared to over nearby flat seafloor (∼2mmolnitratem−2day−1). This increased nutrient supply, forced by locally generated lee waves and internal mixing, is greater than nitrate supplies to the productive tidal mixing fronts or to the shelf edge. We hypothesize this nutrient flux promotes an increase in phytoplankton growth in the thermocline over and downstream of shelf sea banks, contributing to the horizontal patchiness in the thermocline Chl signal. The persistence of the strong biological response to mixing at the bank, combined with the ubiquity of shelf sea banks, suggests these bathymetric features have wide importance for “new” primary production in shelf seas.

High‐resolution Remotely Operated Vehicle (ROV) mapping of a slow‐spreading ridge: Mid‐Atlantic Ridge 45°N

AbstractAxial volcanic ridges (AVRs) are found on most slow‐spreading mid‐ocean ridges and are thought to be the main locus of volcanism there. In this study we present high‐resolution mapping of a typical, well‐defined AVR on the Mid‐Atlantic Ridge at 45°N. The AVR is characterized by “hummocky terrain,” composed typically of hummocks with pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of “flat seafloor,” composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are of a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50 ka time frame as the flat seafloor.

Habitat engineering by decadal-scale bioturbation around shipwrecks on the Great Barrier Reef mid-shelf

Conspicuous, systematic patterns of holes in soft sediment around wreck sites on the Great Barrier Reef mid-shelf are documented. Multibeam bathymetry data show concentric 'halos of holes' extending hundreds of meters around the wrecks. These halos at 5 wreck sites between 16 and 111 yr old consist of hundreds of tightly spaced depressions with diameters of up to 10 m. Seafloor imagery clearly indicates a biogenic origin of the holes. The halos of the 2 youngest wrecks (<25 yr) have fewer holes than older wrecks, suggesting a relationship between wreck age and excavation activity. Conversely, the size of a wreck does not appear to control the sediment turnover activity. Holes are comparatively long-lived, and some of them are actively maintained by bioturbation activity. The spatial distribution of holes is consistent with resource depletion by a central-place forager, albeit on yearly to decadal time scales. Tow-camera data show that some holes support diverse sessile faunal assemblages of sponges, soft corals, and hard corals in an otherwise flat seafloor devoid of sessile fauna. The (to date unknown) bioturbator(s) are thus allogenic habitat engineers. The systematic occurrence of halos around wrecks suggests that 'the ecosystem wreck' extends beyond the spatial confines of the hull of a wreck to the seafloor, by a distance 10-fold greater than the size of a wreck itself. This has implications for the evaluation of ecological effects of artificial reefs such as wrecks and offshore wind farms on decadal time scales.

Identification of deep subaqueous co-seismic scarps through specific coeval sedimentation in Lesser Antilles: implication for seismic hazard

Abstract. During the GWADASEIS cruise (Lesser Antilles volcanic arc, February–March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent "ponding" of reworked sediments in the deepest areas, similar to the deposition of Mediterranean "homogenites". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimentological analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclastics). This layer can be up to 2 m thick, and overlies fine-grained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210Pb and 137Cs activities lack in the massive sands, while a normal profile of unsupported 210Pb decrease is observed in the hemipelagite below, together with a 137Cs peak corresponding to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (Mw = 6.3 and Mw = 7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogenites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cumulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210Pb decrease curve (0.5 mm yr−1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five &gt;M6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.

Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5 km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9 km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.

Paleoenvironmental implications of an extensive maceriate microbialite bed in the Furongian Chaomidian Formation, Shandong Province, China

This study focuses on the unique occurrence of an extensive microbialite bed (10–20 m thick) in the Chaomidian Formation (Furongian) in Shandong Province, China in order to understand its paleoenvironmental implications. The microbialite bed can be traced for over 6000 km 2 in area. The microbialites are characterized by centimeter- to decimeter-scale branching maze-like constituents (maceriae) of microbial and muddy sediments with chaotic texture, including tabular maceriate microbialite (type 1), columnar maceriate microbialite (type 2), and columnar chaotic microbialite (type 3). Within the bed, each microbialite unit is generally well correlated for tens of kilometers. The entire bed is bounded by limestone–marlstone alternation at the base, and an erosional surface at the top that is subsequently overlain by a grainstone bed of various thicknesses. The microbialite bed formed when the seafloor reached suitable water depth for the microbial growth during sea-level rise accompanied with carbonate production. The maze-like maceria structures formed to acquire a larger surface area under a relatively large input of lime mud. The tabular maceriate microbialites initially formed in relatively deep-water environments and flourished on broad and flat seafloor, whereas the columnar microbialites (types 2 and 3) developed under the influence of strong waves and currents. The microbialite bed was eroded and terminated by frequent storm events and buried under the reworked grainstone. Further rise in sea level and increased metazoan activities afterwards limited the resurgence of microbes in the late Furongian.

Acoustic-Based Techniques for Autonomous Underwater Vehicle Localization

This paper presents two acoustic-based techniques to solve the localization problem for an autonomous underwater vehicle (AUV). After a brief description of the Bayesian filtering framework, the paper analyses two of the most common underwater scenarios. Each scenario corresponds to one of the two localization techniques. The scenarios are as follows: map-based localization, when the environment is sufficiently distinctive; transponder-based localization for navigation in large and not distinctive environments. An environment is said to be distinctive when there are possible reference points. For example, a completely flat featureless sea floor is not considered as distinctive. The proposed techniques were validated in simulation. The map-based localization technique was also validated in trials with the autonomous underwater vehicles Ictineu and Nessie IV. The trials with Ictineu took place in an abandoned marina with Ictineu travelling along a trajectory that was more than 600 m long. The trials with Nessie IV took place in a rectangular pool and during an AUV competition.

서해 경기만에서의 고해상도 탄성파 탐사를 이용한 퇴적환경 및 퇴적층서 연구

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sedimentary sequence and distribution pattern of the late Holocene deposits in Gyunggi Bay, the Yellow Sea. The bay is located in the western part of Korea, east of the Yellow Sea. The sedimentary sequence divided into three units bounded by erosional bounding surface: (1) acoustically parallel to subparallel reflectors with cross bedding structures (Unit 1); (2) confused inner reflectors and top of unit exposed partially at the seafloor (Unit 2); and (3) approximately parallel reflections and regressive to transgressive incision-fills (Unit 3). On the basis of seafloor morphology, surface bedforms, and subbotom acoustic characters, echo types in the study area were identified following the schemes of Chough et al. (2002); (1) flat seafloor with sharp bottom echoes (echo types 1-1, 1-2 and 1-3; transgressive sediment sheets or relict sands), (2) mounded seafloor with either smooth surface or superposed bedforms (echo types 2-1 and 2-2; tidal ridges), and (3) various-scale eroded seafloor (echo types 3-1 and 3-2; channels). Suspect features of acoustic turbid zones which is related to gas charged sediment are reported.

The structures beneath submarine methane seeps: Seismic evidence from Opouawe Bank, Hikurangi Margin, New Zealand

The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. Within the New Vents project we investigate cold seeps and seep structures at the Hikurangi Margin, east of New Zealand. In the area of Opouawe Bank, offshore the southern tip of the North Island, numerous extremely active seeps have been discovered. High-resolution seismic sections show a variety of seep structures. We see seismic chimneys either characterised by high-amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways. The bathymetric expression of the seeps also varies: There are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds. The images of the 3.5 kHz Parasound system reveal the near-surface structure of the vent sites. While high-amplitude spots within the uppermost 50 m below the seafloor (bsf) are observed at the majority of the seep structures, indicating gas hydrate and/or authigenic carbonate formations with an accumulation of free gas underneath, a few seep structures are characterised by the complete absence of reflections, indicating a high gas content without the formation of a gas trap by hydrates or carbonates. The factors controlling seep formation have been analysed with respect to seep location, seep structure, water depth, seafloor morphology, faults and gas hydrate distribution. The results indicate that the prevailing structural control for seep formation at Opouawe Bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone.

Adaptive finite element modelling of two-dimensional magnetotelluric fields in general anisotropic media

An adaptive unstructured mesh finite element (FE) procedure is presented for improving the quality of numerical solutions to the magnetotelluric forward problem in a general 2-D anisotropic conductivity structure. We implement a self-adaptive, goal-oriented grid refinement algorithm in which a finite element analysis is performed on a sequence of refined meshes. The mesh refinement process is guided by a dual error estimate weighting to bias refinement towards elements that affect the solution at the EM receiver locations. We validate the finite element code against a layered 1-D model with a sea water layer. Further, we compare the FE results with those obtained by a finite-difference (FD) scheme for both a block seamountain and a sea bottom hill model. Both FE and FD schemes show very good agreement for the block seamountain model. For the sea bottom hill model, however, only on the flat seafloor segments both the FE and FD solutions fit very well, but on the seafloor slope, FD results are oscillating due to a simplistic staircase approximation of the bathymetric undulations. The FD scheme for 2-D anisotropic conductors, developed primarily for the modelling of magnetotelluric data on a flat Earth surface, is thus not an adequate tool for dealing with structures with sloping bathymetry and topography, whereas the FE method with adapting mesh can easily handle such structures at almost any level of complexity.

Onset of submarine debris flow deposition far from original giant landslide

Submarine landslides can generate sediment-laden flows whose scale is impressive. Individual flow deposits have been mapped that extend for 1,500 km offshore from northwest Africa. These are the longest run-out sediment density flow deposits yet documented on Earth. This contribution analyses one of these deposits, which contains ten times the mass of sediment transported annually by all of the world's rivers. Understanding how this type of submarine flow evolves is a significant problem, because they are extremely difficult to monitor directly. Previous work has shown how progressive disintegration of landslide blocks can generate debris flow, the deposit of which extends downslope from the original landslide. We provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide, encased within deposits of a more dilute flow type called turbidity current. Very little sediment was deposited across the intervening large expanse of sea floor, where the flow was locally very erosive. Sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea-floor gradient from 0.05 degrees to 0.01 degrees. This debris flow was probably generated by flow transformation from the decelerating turbidity current. The alternative is that non-channelized debris flow left almost no trace of its passage across one hundred kilometres of flat (0.2 degrees to 0.05 degrees) sea floor. Our work shows that initially well-mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean.