Caught in the act: the 20 December 2001 gravity flow event in Monterey Canyon

A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time. The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than 10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit. This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh, greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth. The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow events of this magnitude do not require an exceptional triggering event.     

Evidence of sediment gravity flows induced by trawling in the Palamós (Fonera) submarine canyon (northwestern Mediterranean)

Three mooring arrays were deployed in the Palamós Canyon axis with sediment traps, current meters and turbidimeters installed near the bottom and in intermediate waters. Frequent sharp and fast turbidity peaks along with current speed increases were recorded, particularly at 1200 m depth in spring and summer. During these events, near-bottom water turbidity increased by up to more than one order of magnitude, current velocity by two to four times and horizontal sediment fluxes by one to three orders of magnitude. When these events occurred, 9–11 days integrated downward particle fluxes collected by the near-bottom sediment trap increased by two to three times. These events were identified as sediment gravity flows triggered by trawling activities along the northern canyon wall. Sediment eroded by the trawling nets at 400–750 m depth on this wall seems to be channeled through a gully and transported downslope towards the canyon axis, where the 1200 m mooring was located. The sediment gravity flows recorded at the 1200 m site were not detected at deeper instrumented sites along the canyon axis, suggesting that they affect local areas of the canyon without traveling long distances downcanyon. These observations indicate that trawling can generate frequent sediment gravity flows and increase sediment fluxes locally in submarine canyons. Furthermore, in addition to the various natural processes currently causing sediment gravity flows and other sediment transport events, human activities such as trawling must be taken into account in modern submarine canyon sediment dynamics studies.     

Momentum and gravity effects during the constant velocity water entry of wedge-shaped sections

Computational fluid dynamics analysis was used to investigate the added mass momentum, flow momentum and gravity effects during the constant velocity water entry of wedge-shaped sections with deadrise angles from 5° to 45°. It is shown that the added mass continues to increase for a time after chine immersion and that added mass can be estimated in terms of a constant added mass coefficient and an effective wetted width. A momentum theory is presented in which the water entry force is explained as the sum of the rate of change of added mass momentum, which becomes zero at immersion to chine depth ratios greater than about three, and the rate of change of flow momentum, which continues at deep immersions. The effect of gravity on the water entry force is given as the hydrostatic force together with the force necessary to create the potential energy in the water pile up. Hydrodynamic forces are not significantly changed by the effect of gravity on the flow fields.     

General patterns of circulation, sediment fluxes and ecology of the Palamós (La Fonera) submarine canyon, northwestern Mediterranean

Currents, particle fluxes and ecology were studied in the Palamós submarine canyon (also known as the Fonera canyon), located in the northwestern Mediterranean. Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope. Additionally, local and regional hydrographic cruises were carried out. Current data showed that mean near surface and mid-depth currents were oriented along the mean flow direction (NE–SW), although at 400 and 1200 m depth within the canyon current reversals were significant, indicating a more closed circulation inside the canyon. Mean near-bottom currents were constrained by the local bathymetry, especially at the canyon head. The most significant frequency at all levels was the inertial frequency. A second frequency of about three days, attributed to a topographic wave, was observed at all depths, suggesting that this wave was probably not trapped near the bottom. The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow.In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher up-canyon from about 1200 m depth than offshore, defining, along with the different hydrodynamics, two canyon domains: one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf-slope front. The higher near-bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows. During the deployment period, the increase in downward particle fluxes occurred by mid-November, when a severe storm took place. On the canyon walls at 1200 m depth, suspended sediment concentrations, downward particle fluxes, chlorophyll and particulate C and N were higher on the southern wall than on the northern wall inversely to the current’s energy. This could be caused by an upward water supply on the southern canyon wall and/or the mean flow interacting with the canyon bathymetry. In the swimmers collected by the sediment traps, the dominant species was an elasipod holothurian, which has not been recorded in other canyons or elsewhere in the Mediterranean, indicating particular speciation.     

On the total geostrophic circulation of the Indian Ocean: flow patterns, tracers, and transports

The large-scale circulation of the Indian Ocean has several major components. There is a cyclonic gyre in the far southwest with its axis along about 60°S. It extends to the bottom. North of this the Circumpolar Current flows eastward south of 40°S to more than 3000 m. The axis of the great anticyclonic gyre lies along 35°S to 40°S down to about 2000 m. Below there the western end shifts northward and the axis lies along the central and southeast Indian ridges, with southward flow west of the ridges and northward flow on the east side.There is a westward flow along 10°S to 15°S, which includes water from the Pacific, through the Banda Sea. The flow near the equator is eastward down to the depth of the ridge near 73°E. Flow within both the Arabian Sea and Bay of Bengal is cyclonic down to great depth.There is a southward flow along the coast of Africa in the upper 2000 m joining the Circumpolar Current, and a southward flow along the coast of Australia that does not reach the Circumpolar Current.Below 2500 m there is a northward flow from the Circumpolar Current along the east coast of Madagascar and on into the Somali and Arabian basins.     

Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.     

Bedforms produced by the Kuroshio Current passing over the northern Izu Ridge

Current-generated bedforms were found on sandy seafloor at water depths of 200–400 m on the northern Izu Ridge, where the Kuroshio Current encounters and passes over the ridge. The observed bedforms include large dunes and sand ribbons and are interpreted to be products of present-day oceanographic conditions and to indicate intensive flow activity controlled by local topography. A comparison between the surface flow velocity estimated from empirical relationships for dune formation and the observed flow velocity suggests that the dunes are generated when the main axis of the meandering Kuroshio Current passes through this area, and that subsequent current velocities are sufficiently high to maintain the dunes up to the next event.     

台风作用下的港湾型潮滩沉积过程 以2008年“凤凰”台风对福建省罗源湾的影响为例

通过对"凤凰"台风的现场观测和沉积物样品的分析,结果表明,在台风影响下潮水淹没时间增长,增水达1.1 m;台风期间互花米草盐沼内流速变化较复杂,而且盐沼内部流速大于盐沼边缘的;台风期间盐沼边缘潮周期平均悬沙含量是台风前2 d的7倍;台风期间十分之一波高最大为1.54 m。滩面重复测量结果显示,台风登陆期间整个光滩滩面都发生了侵蚀,盐沼内部有部分地区发生侵蚀,侵蚀深度为4.5~5.5 cm,盐沼边缘的侵蚀深度仅为0.7 cm,侵蚀原因主要是植被在风浪作用下从根部折断,从而带走了滩面的沉积物;随着台风强度的减弱,整个滩面均接受悬沙沉降;台风带来的强降雨是影响滩面沉积物活动的重要因素。     

CFC Indicating Renewal of the Japan Sea Deep Water in Winter 2000–2001

The distributions of CFC (chlorofluorocarbon) in the water column was determined twice in 2000 and 2001 in the northwestern Japan Sea. In 2000 the CFC-11 concentration decreased almost exponentially with depth from 6 pmol/kg at a few hundred m deep to 0.3 pmol/kg or less at the bottom of about 3400 m depth at three stations (40–41°N, 132–133°E) about 300 km off Vladivostok. In 2001 the CFC-11 concentration increased sharply up to 2 pmol/kg in the bottom water, while it did not increase at a station (42.0°N, 136.5°E) about 450 km away to the northeast. This is due to the renewal of the bottom water which is replaced by the surface water flowing down along the continental slope, as suggested by Tsunogai et al. (1999), who proposed the continental shelf pump. Furthermore, an increase in the CFC-11 concentration was observed throughout the entire water column above 3000 m depth, although the proportion of the increase was about 20%, which was one order of magnitude smaller than that in the bottom water. The increase in inventory is almost four times larger than that in the bottom water below 3000 m depth which is equivalent to about 1/6 of the total inventory found in 2000. The increase also means that 3% of the deep water was replaced by the recent surface water, or, if the turnover occurs every year, that the turnover time of the deep water to be about 30 years. This revised version was published online in July 2006 with corrections to the Cover Date.     

Patterns of event and chronic hydrothermal venting following a magmatic intrusion: new perspectives from the 1996 Gorda Ridge eruption

The remote detection of a seismic swarm on the northern Gorda Ridge on 28 February 1996 prompted a three-cruise response effort to investigate event and chronic hydrothermal discharge associated with a dike intrusion. The GR1 cruise reached the northern Gorda only 10 days after seismicity began and discovered a 15 km-diameter event plume, EP96A, centered between depths 1800 and 2800 m above the shallowest portion of the axial valley axis (3100 m). One month later, GR2 returned and found only a weak, near-bottom chronic plume at the EP96A site. A few kilometers to the south, however, GR2 mapped a distinctly different chronic plume (2500–2900 m depth) as well as the edge of a second event plume, EP96B (1800–2400-m depth), above the western wall of the axial valley. EP96B was seeded with a neutrally buoyant float, which traveled 10 net km to the northwest before surfacing on 10 June at the start of GR3. Mapping around the float location fully revealed EP96B, a 10 km diameter plume with a heat content 25% that of EP96A. Extensive observations within the axial valley determined that chronic venting was effectively exhausted within three months. Models seeking to explain the perturbation of hydrothermal venting by a dike intrusion and eruption must satisfy several criteria generalized from this and previous events: (1) venting begins (or increases) with the intrusion/eruption and declines exponentially afterwards; (2) the time scale of the post-intrusion decline varies within and among sites; (3) the discharge of multiple event plumes is common; (4) an existing high-temperature vent field may not be necessary or even conducive to event plume formation; and (5) the ratio of total chronic to event discharge varies among intrusion events.     

Coastal Bathymetric Studies from Space Imagery

Abstract

Studies of coastal bathymetry are important where littoral drift has implications on the planning of fishing and dredging operations. Also, there is a possibility of finding hitherto unknown bottom features in relatively less explored regions of the shallow seas around the globe. High resolution satellite imagery over oceans provides us with quantitative methods for estimating depth in shallow parts of the seas. One of the methods is the analysis of the refraction of coastal gravity waves observed on satellite imagery. A panchromatic image acquired by SPOT with 10 m resolution on March 22, 1986, over Bay of Bengal near Madras Coast, was used for this analysis. The image was enhanced to clearly bring out the wave structure seen on the sea surface. The image was then superimposed with a 1 km × 1 km grid. For each grid cell, 64 × 64 pixels at the center were considered for getting a Fast Fourier Transform to determine the wave spectrum and the dominant wavelength present there. The classical theory of gravity waves was used to relate the shallow water wavelengths obtained as above with the corresponding wavelengths in the deep water. The deep‐water wavelength was estimated to be 110 m using the known chart depths at a set of control points. The resulting depth estimates, when compared with standard bathymetric charts, were found, in general, to be well in agreement up to a depth of 30 m in the sea, with an r.m.s. error of 2.6 meters. The method seems to be very useful for remotely sensed bathymetric work. However, further research is required to reduce the error margin and operationalize the method.     

Gas and fluid venting at the Makran accretionary wedge off Pakistan

The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane and H₂S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively destabilized and dissociated into hydrate water, methane and H₂S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth, upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed at water depths of 350 to 800 m. Received: 14 June 1999 / Revision accepted: 6 February 2000     

Live (stained) benthic foraminifera in the Whittard Canyon, Celtic margin (NE Atlantic)

Living (Rose Bengal stained) benthic foraminifera were investigated at 18 deep-sea stations sampled in the Whittard Canyon area (NE Atlantic). The stations were positioned along 4 bathymetric transects ranging from 300 to 3000 m depth: two along the main canyon axes (Western and Eastern branches) and two along adjacent open slopes (Western and Eastern slopes). The aim of this study was to assess changes of foraminiferal standing stock, composition and microhabitat in relation to the physico-chemical conditions prevailing at and below the sediment-water interface in various canyon and open-slope environments. Minimal oxygen penetration depths and maximal diffusive oxygen uptakes were recorded at upper canyon stations, suggesting a high mineralisation rate. This is confirmed by the high phytopigment concentrations measured in the sediment of the upper canyon axes. Foraminiferal abundance was positively correlated with diffusive oxygen uptake and phytopigment concentration in the sediment. This suggests a control of organic matter fluxes on the foraminiferal communities. Foraminiferal abundance was generally higher along the canyon axis compared to open-slope sites at comparable water depths. The species composition varied with water depth along all four transects, but was also different between canyon branches and adjacent slopes. The silty/sandy intercalations at many of the deeper canyon stations may have been rapidly deposited by fairly recent gravity flows. At station 51WB (3002 m), the faunal characteristics (strong dominance, shallow infaunal microhabitats) suggest that the foraminiferal community is in an early state of ecosystem colonisation after these recent sedimentation events, which would have supplied the important amounts of phytopigments.     

Dynamics at an elongated,intermediate depth seamount in the North Atlantic (Sedlo Seamount, 40°20′N, 26°40′W)

Observations from a five-mooring array deployed in the vicinity of Sedlo Seamount over a 4-month period, together with supporting hydrographic and underway ADCP measurements, are described. Sedlo Seamount is an elongated, intermediate depth seamount with three separate peaks, rising from 2200 m water depth to summit peaks between 950 and 780 m depth, located at 40°20′N, 26°40W. Currents measured in depth range 750 and 820 m – the layer close to the summit depth of the shallowest southeast peak – showed a mean anti-cyclonic flow around the seamount, with residual current velocities of 2–5 cm s⁻¹. Significant mesoscale variability was present at this level, and this is attributed to the weak and variable background impinging flow. Stronger, more persistent currents were found at the summit mooring as a result of tidal rectification and some weak amplification. Below 1300 m, currents were extremely weak, even close to the seabed. Time series of relative vorticity for the depth layer 750–820 m showed persistent anti-cyclonic vorticity except for two periods of cyclonic vorticity. A mean relative vorticity of −0.06f (f=the local Coriolis frequency) was calculated from a triangle of current meters located at the flanks of the seamount. Modelling results confirmed that anti-cyclonic flow above the seamount was likely due to Taylor Cone generation driven by a combination of steady impinging and tidally rectified flow. The closed circulation pattern over the seamount was found to extend to ∼150 m above the summit level, consistent with simple idealised theory and the supporting hydrographic observations. At shallower depths (<500 m) model simulations predicted a predominantly cyclonic recirculation most likely controlled by topographic steering along the zonal axis of the seamount. There was some indication of flow reversal at these depths from Acoustic Doppler Current Profiler (ADCP) measurements carried out at one hydrographic survey. The model results were in good agreement with observations at the seamount summit, but were unable to reproduce the mesoscale variability patterns recorded in shallower layers. Kinetic energy patterns derived from the model revealed high variability in the oceanic far field downstream of the seamount summit probably as a result of complex flow interaction along the chain of seamount peaks. Possible impacts of the flow dynamics on the biological functioning at Sedlo Seamount and its surroundings are discussed.     

Morphology and architecture of the typical erosion-genesis submarine canyons on the East China Sea slope

The Yithi submarine canyons,composed of four canyons less than 60 km in length,are located on the narrowest part of the East China Sea(ECS) slope.They extend from the shelf break at 160 m down to water depth of 1 500 m with an average gradient(along the canyon axis) of 3°(<1 000 m) and 0.7°(>1000 m).The sinuosity of the canyons ranges form 1.02 to 1.14 and their pathways extend radially from the shelf break to the axis of the Okinawa Trough.Structural and evolution pattern of the Yithi canyons are mainly controlled by sediment mass-movements and turbidity current and similar with that of the canyons in Ebro continental slope.The whole canyon system consists of three parts:the canyon,the channel and the fan.Slumps and slides often develop in the upper part of canyon where the water depth is less than 1000 m,and the turbidities usually developed on the fan.The scale of turbidites becomes smaller and their inner structures become more regular towards the ends of the canyons.Canyon-fans are often associated with small angle progradational reflection.Most canyon-fans and levees were transversely cut by active normal faults with NEE-SWW trending that are coupled to the modern extension of the Okinawa Trough.According to the age of formation of canyon-fans and sediments incised by canyons,we can infer that the Yithi canyons were formed since the middle the Medio-Pleistocene.     

The evolution and depth structure of shelf and slope temperatures and velocities during the 1997–1998 El Niño near Point Conception, California

Moored, survey, and drifter observations are used to describe the evolution of temperature, sea level, velocity and salinity from 1997 to 1998 over the California shelf, between San Luis Obispo Bay and the eastern entrance to the Santa Barbara Channel. The dominant event during this time was the 1997–1998 El Niño. Its relation to background seasonal and interannual variability depended on which property is considered. Subsurface temperature and local sea level showed extreme anomalies between March 1997 and October 1998. Three distinct peaks occurred. The first two are associated with the regional response to El Niño, while the cause of the third remains unclear. The first peaked in June 1997, and decayed until August. The main El Niño peak reached maximum amplitude between November 1997 and January 1998. After the collapse of the regional El Niño anomaly in February 1998, a final peak occurred locally during the summer of 1998.The central result presented here concerns the spatial structure of temperature during these events. The initial peak was surface intensified and was barely detectable at 45 m. Its amplitude varied with position along the coast, decaying with distance north. The main peak showed a strong signal down to at least 200 m. The amplitude and timing of temperature anomalies during this event were depth dependent. The largest absolute amplitudes relative to seasonal cycles were in excess of 4 °C and occurred between 45 and 65 m depth. The anomalies reached their maximum values at later time with increasing depth, between October 1997 and January 1998. The amplitude of this main peak was comparable at all mooring sites. The final peak in August 1998 had a comparable amplitude at all mooring sites to a depth of 100 m. Temperature increases during the three events were accompanied by a corresponding rise in sea level.The El Niño signals in currents and salinity are more difficult to distinguish from background variability than those in temperatures and sea level. However, stronger than average poleward flow was observed at the eastern entrance to the Santa Barbara Channel between 5 and 100 m depth for most of 1997, and there are indications for greater than usual poleward flow over the Santa Maria Basin in fall 1997. Surface drifter evidence, although qualitative, also suggests greater than usual poleward displacement in November 1997 relative to other years. Along with increased temperature, survey observations of salinity suggest changes to the regional temperature–salinity relation during November 1997 with greater than usual salinity at temperatures below 12 °C.     

东北太平洋中国合同区的叶绿素a和初级生产力

根据东北太平洋中国合同区东区和西区从1994年至2003年6个航次(DY85-4,DY95-7,DY95-8,DY95-10,DY105-11和DY105-12.14)初级生产力调查资料编写.结果表明:次表层高值分布为叶绿素a分布的一般特征,从表层至水深50 m,数值为0.1 mg/m3左右,在水深50~100 m上下,数值约在0.2~0.4 mg/m3之间,在水深大于100 m,特别是150 m以深数值逐渐减小为零;东区初级生产力(C)测值在76.8~191.9 mg/(m2·d),平均为116.1 mg/(m2·d);西区的在73.1~222.5 mg/(m2·d),平均为127.1 mg/(m2·d);在水深0~125 m,0.2~2μm颗粒的微微型浮游植物的叶绿素a比例占叶绿素a总量的70%以上,而在水深125 m以深,占50%以下;反之,2~20μm和大于20μm两种粒级所占比例的和在0~125 m深度范围占30%以下,在水深125m以深时为50%以上.调查显示东、西两区属于初级生产力比较低的海域.     

Barotropic tide in the northeast South China Sea

A moored array deployed across the shelf break in the northeast South China Sea during April-May 2001 collected sufficient current and pressure data to allow estimation of the barotropic tidal currents and energy fluxes at five sites ranging in depth from 350 to 71 m. The tidal currents in this area were mixed, with the diurnal O1 and K1 currents dominant over the upper slope and the semidiurnal M2 current dominant over the shelf. The semidiurnal S2 current also increased onshelf (northward), but was always weaker than O1 and K1. The tidal currents were elliptical at all sites, with clockwise turning with time. The O1 and K1 transports decreased monotonically northward by a factor of 2 onto the shelf, with energy fluxes directed roughly westward over the slope and eastward over the shelf. The M2 and S2 current ellipses turned clockwise and increased in amplitude northward onto the shelf. The M2 and S2 transport ellipses also exhibited clockwise veering but little change in amplitude, suggesting roughly nondivergent flow in the direction of major axis orientation. The M2 energy flux was generally aligned with the transport major axis with little phase lag between high water and maximum transport. These barotropic energy fluxes are compared with the locally generated diurnal internal tide and high-frequency internal solitary-type waves generated by the M2 flow through the Luzon Strait.     

Cyclostrophic balance in surface gravity waves

The cyclostrophic balance (pressure forcevs. force centrifugal force) is shown to be satisfied for all fluid particles in surface gravity waves with sinusoidal form and circular particle orbits. Consequences of the cyclostrophic balance are 1) that the normal dispersion relation for deep water hold and 2) that the orbital radius decrease with increasing depth at the usual exponential rate, from which it follows that the wave pressure and particle speed also decrease with depth exponentially. In addition, the cyclostrophic and hydrostatic balances together predict wave breaking at the crests for amplitudes exceeding one divided by the wave number. In contrast to the traditional perturbation method, based on irrotational flow, the cyclostrophic method does not demand that the amplitude be much less than a wave length and does not require an infinite wave train.     

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay,Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.