OS54A-01 INVITED
Axial Channel Morphology Fill and Movement Within Submarine Canyons off California
Axial channels of seven submarine canyons off California have recently been investigated with ultrahigh resolution multibeam surveys. Vibracores collected from remotely operated vehicles (ROVs) provide ground truth data on the materials within the axis of six of these canyons. Acoustic beacons were used to track movement of the seafloor within the axis of Monterey Canyon. Multibeam bathymetry (0.15 m vertical precision and 1.0 m horizontal resolution at 50 m survey altitude) and chirp seismic-reflection profiles (0.11 m vertical resolution) were collected from an autonomous underwater vehicle (AUV). Inertial navigation combined with Doppler velocity sonar allows the AUV to navigate the sinuous canyons at 3 kt on a pre-programmed route ~50 m above the seafloor. Submarine canyons that head near the shoreline (Monterey, Hueneme, Mugu, Redondo and La Jolla) contain similar wave-like bedforms with 20-100 m wavelengths and amplitudes up to 2.5 m oriented roughly perpendicular to the channel axis. These bedforms are asymmetric in profile with a steep down-canyon face and crescent-shaped, concave down-canyon. They are observed throughout the surveys (80-800 m water depth) and appear to form genetically similar packages (0.1 to 3 km long) that terminate upslope at a somewhat higher topographic step. Generally, bedform groups are found within the axial channel, but occasionally extend up the canyon walls. ROV-collected vibracores show that the wave-like bedforms comprise recent, coarse-grained gravity flow deposits, suggesting these canyons are active. In contrast, two submarine canyons with heads on the outer shelf (Soquel and Santa Monica) lack these wave-like bedforms and have relatively smooth axial channel floors. Vibracores show these canyons are filled with generally uniform fine-grained sediments, suggesting they are inactive. Apparently wave-like bedforms are common features of active submarine canyons. To assess whether sediment within a canyon floor is moved by traction currents or mass transport, three acoustic beacons were deployed in Monterey Canyon (~290 m water depth) in February 2007. The beacons were placed in 50-cm-high ~45 kg concrete monuments. These boulder-sized monuments were buried leaving only the top of the beacon extending ~6 cm above the seafloor. The position of these beacons was re-surveyed 8 times between February 2007 and July 2008. While the beacons did not move between most surveys, two down-canyon shifts of ~150 m and >1,200 m were measured. The episodic movement of the buried monuments suggests that the seafloor was remobilized to more than 50-cm- depth during sediment transport events. Four repeat AUV multibeam surveys were also conducted during the first half of 2007. These surveys show that the wave-shaped bedforms occur in the canyon axis down to at least 1,100 m water depth. Because the buried monuments have moved more than two wavelengths down- canyon during an individual event, identification and tracking of individual bedforms between events has proven to be difficult. Apparently, these wave-shaped bedforms are produced during brief, discrete mass transport events.
OS54A-02
Storm-triggered mass failure and sediment liquefaction in the Swatch of No Ground canyon, offshore Bangladesh
In November 2007 the eye of a major tropical storm, Cyclone Sidr, tracked over the offshore canyon associated with the Ganges-Brahmaputra river delta. Known as the Swatch of No Ground (SoNG), this large canyon incises within 30 km of the deltaic coast and is accreting rapidly (10 to >50 cm/yr) with fluvially derived sediments. Historically numerous tropical storms have affected the northern Bay of Bengal each year, but more recently cyclogenesis has been weak and Sidr is the first major cyclone to impact the Bengal shelf since 1991. As part of an ongoing study, our research team had just completed a high-resolution sub- bottom sonar (chirp) survey of the SoNG canyon head six months prior to Cyclone Sidr. Following the storm, we organized a second sub-bottom survey to assess the storm's impact on canyon morphology and sedimentation patterns. A comparison of our pre- and post-storm surveys reveal dramatic mass failures around the canyon wall, but also vast areas that were largely unaffected. Among the failures, many had dimensions 10s of meters thick and >1 km wide. These large-scale failures appear to have been preferentially located where there is pre-storm evidence for fluid escape, perhaps via submarine groundwater discharge through onshore sandy delta complex. Fluid chimneys, apparently gas charged, are also associated with many of the large failures. However, many locations with prominent shallow gas show no disruption after the storm, suggesting that zones prone to major failure are associated fluid flow. Other areas of the canyon show storm-induced liquefaction of surface sediments (<5 m below seabed) and resulting mudflows. Runout distances are only a few hundred meters, though, suggesting rapid consolidation of the flows even on sloping surfaces (1-3°). As interesting as these storm-related failures are the actively accreting, steep-walled (>5°) gullies that show no apparent effect of the storm. Overall it appears that areas of the canyon most susceptible to failure are not necessarily the steepest or most rapidly accreting, but rather those associated with subsurface fluid flow and planes of weakness along escape structures.
OS54A-03 INVITED
Contemporary sedimentation processes in the Gulf of Lions submarine canyons: the role of dense shelf water cascading
Cascading of dense shelf waters is a global oceanographic phenomenon whose effects on sedimentation processes on continental margins have been poorly studied and largely underestimated. The north-western Mediterranean is one of the regions of the world where massive dense water formation occurs because of cooling and evaporation of surface waters during winter-time. Recent observation within the frame of successive research initiatives found that, concurrent with the well known open-sea convection process, coastal surface waters over the wide shelf of the Gulf of Lions also become denser than the underlying waters and cascade downslope during sustained periods of time until reaching their equilibrium depth. Through this climate-driven phenomenon, dense shelf waters carrying large quantities of particles in suspension and as bed load are rapidly advected hundreds of meters deep, mainly through submarine canyons, acting as an efficient sediment transport and sea floor shaping process. Several evidences of sediment erosion and deposition within canyons attributed to this phenomenon have been identified (e.g. sedimentary furrows, sand-filled axial channels, sediments waves), providing an alternative explanation to the classical view that turbidity currents and associated processes are the main sediment transport mechanisms from shallow to deep-sea regions. At least, during highstands of sea level, dense shelf water cascading in the Gulf of Lions appears to effectively contribute to cross-margin sediment transport. Our findings allow anticipating that under present conditions dense shelf water cascading sites identified worldwide can constitute preferential regions for active contemporary sediment transport processes through submarine canyons, from the coastal ocean to the deep basins.
OS54A-04
Dense-water cascading and the resulting sedimentation patterns in Lacaze-Duthiers and Cap Creus Canyons, Gulf of Lions
Modern sediment dispersal in the Gulf of Lions is controlled by the interaction of strong marine storms and dense-water formation on the shelf. Regional cyclonic circulation and the narrowing of the southwest shelf lead to off-shelf sediment export via Lacaze-Duthiers and Cap Creus Canyons. Canyon heads are areas of sediment bypassing due to intense current interaction and the frequent occurrence of dense-water cascading. In winter 2004-2005, off-shelf export was dominated by dense-water cascading with additional contributions from internal-wave resuspension, hemipelagic settling and sediment gravity flows. Sedimentation in the canyon heads is asymmetrical with erosion (and coarse sediment) dominating the western flank and main entrant, and pockets of fine-grained sediment on the eastern flank where the Northern Current drapes over the canyon edge. Sedimentation in the lower canyon depths (>400 m) shows evidence of both hemipelagic settling and intermittent sediment gravity flows. Modern fine-grained sediment accumulation in the southwest canyons accounts for <10% of the annual sediment discharge from the Rhône River. This is likely a conservative estimate as the area used to calculate the budget was based on channel-floor areas only. Areas of increased sediment focus may occur within deeper portions of canyons, which were not resolved in this study. Lacaze-Duthiers and Cap Creus Canyon are modern conduits for sediment transport from the shelf to the deep sea with increased deposition likely occurring in years when dense-water cascading occurs in conjunction with winter-storm resuspension events.
OS54A-05
Examining sediment transport processes within two submarine canyons off coastal southern California using sediment trap arrays and naturally-occurring radionuclides
The transport of sediment was examined in two submarine canyons (Mugu and Hueneme) located in southern California, just south of Ventura. Two vertically-separated sediment traps were deployed in each canyon for approximately 6 months. The mooring in Mugu Canyon, which presumably receives most of its sediment load from the Calleguas Creek, was deployed in 180m of water and the two sediment traps were positioned 30m and 60m above the sea floor. Huemene Canyon lies just south of the Santa Clara River and in this system the sediment traps were also located at 30m and 60m above the sea floor on a mooring deployed in 190m of water. As has been shown previously, the offshore transport of sediment from land in this region occurs mostly during episodic winter storm events that can deliver sediment as distinct pulses that are usually correlated to heightened precipitation events. Naturally-occurring radionuclides were measured in discrete time-stamped intervals of the four extruded sediment trap columns to obtain time-series flux estimates that can provide insight into particle scavenging and removal processes. In general, 226Ra fluxes (1-31 dpm g-1 d-1) are highly correlated (R2 up to 95) to total mass fluxes (1-13 g m-2 d-1), while excess xs210Pb:total mass fluxes reveal substantial additional input/removal processes. An xs234Th-based geochronology (0.4cm d-1) of the Mugu Canyon upper sediment trap is consistent with the observed (80cm in 190 d) sedimentation rate. Our results suggest that in these two submarine canyon systems, the supply of sediment consists of two components (allochthonous and reworked/resuspended) that can be tracked using naturally-occurring radionuclides.
OS54A-06
Tidal Signatures of the Benthic Nepheloid Layer (BNL) in the Gaoping/Kaoping Submarine Canyon off Southwestern Taiwan
Temporal and spatial variations of benthic nepheloid layer (BNL) have been observed in the head region of the Gaoping/Kaoping Submarine Canyon (KPSC) along the canyon thalweg in different seasons between 1999 and 2000. The top of the BNL could be as high as 100 m above the canyon floor whose suspended sediment concentration (SSC) could be as high as 30 mg/l. In the BNL, silt comprises the largest size-class in the suspended sediment population. In 2000, 2002, and 2004 three taut-line moorings were deployed at different locations in the head region of the KPSC for one month. Time series measurements of along- canyon flow velocity, water temperature, and the volume concentration (VC) of clay, very-fine-to-medium silt, coarse silt and sand size-classes were obtained near the canyon floor from each mooring. Results show that the BNL is strongly modulated by the tides in the descending order at semidiurnal, diurnal, and spring-neap frequencies. In the course of a semidiurnal tidal cycle, the flood (up-canyon) current brings colder water from seaward part of the canyon and the SSC and thickness of the BNL increases. The SSC near the canyon floor also increases in response to the peaks of flood and ebb currents of the semidiurnal tide. The tidal-to-total energy ratio (ER) for the along-canon flow is between 70-80%, and between 50-80% among the suspended sediment of clay, very-fine-to-medium silt, coarse silt and sand size-classes. M2 is the most important constituent in the along-canyon flow, water temperature, and the VC of the four size-classes. The local phase difference between the forcing (velocity), and the responses (temperature and VC) at the M2 frequency suggest the a mixture of progressive and standing waves and that topographic effect caused the phase varied along the canyon. The VC tidal amplitude ratio between M4 and M2 constituents of the four size-classes indicates that the temporal fluctuations of the suspended sediment concentration in the BNL are highly nonlinear. The generation of nonlinearity could be through the flow-topography interaction and through the alternate entrainment and deposition of suspended sediment in the course of a semidiurnal tidal cycle. At this point, the relationship among barotropic tides, internal tides, and typhoon events and BNL is not clear. The role of the BNL in the sediment transport and sedimentation in submarine canyons worldwide is also not fully understood. Studies on these subjects in the KPSC are in progress.
OS54A-07
Tidal-dominated Sediment Transport in the Gaoping Submarine Canyon off Southwestern Taiwan: Radiochemical, Sediment Trap and Current Meter Observations
A sediment trap-CTD-current meter array was deployed ~17 m above the seafloor at 22°21.239'N, 120°16.419'E (water depth: 659 m) in the Gaoping submarine canyon (GPSC) from Jan. 10 to Mar. 20, 2008, a period with relatively quiescent weather conditions off southwestern Taiwan. It resulted in a "suspended core" 62 cm in length, which was sectioned at 1-cm intervals and analyzed for water content, particle size, excess 210Pb (210Pbex) and excess 234Th (234Thex), among others. Downcore profiles of both 210Pbex and 234Thex show a pronounced cyclic pattern, in phase with each other and with water content, reflecting the fact that fine-grained particles (with higher porosity and water content) are more effective in scavenging particle-reactive chemical substances from seawater. By normalizing the activity concentration of 234Thex against that of 210Pbex, the particle size effect is largely removed, resulting in a well-constrained exponential decrease with depth of the 234Thex/210Pbex activity ratio that can be simply explained by decay of radionuclides following burial (in the trap barrel) of the trapped material. The data points and the regression line suggest that, during the deployment period, the flux of sediment to the trap was fairly steady with a mean accumulation rate of 0.887 cm/d in the trap barrel, in excellent agreement with that expected from the thickness (62 cm) of the accumulated material and the length (70 d) of the deployment time. Thus, 234Thex is an ideal, built-in time element for tracing particles in this environment. Current meter and water temperature data recorded near the trap indicate that, the topographically-steered flow along the canyon was dominated by tidal motions at semidiurnal frequencies. Superimposed on the semidiurnal oscillation is a low-frequency oscillation at a periodicity of about two weeks, as manifested by changes in the amplitude of along-canyon velocity. The timing of maximum and minimum amplitudes corresponds to that of spring tide and neap tide, respectively. The temperature data also exhibit strong semidiurnal fluctuations, with flood tide bringing in deeper and colder offshore water up the canyon and ebb tide carrying shallower and warmer inshore water down the canyon. In general, daily mean temperature is lower during neap tides than in spring tides. During spring tides, wider tidal range leads to greater semidiurnal oscillation in along-canyon velocity, thus providing a more energetic condition for the suspension and dispersal of coarser grained particles (with lower 210Pbex and 234Thex). Conversely, finer particles prevail in the less energetic conditions during neap tides. We conclude from this observation that sediment dispersal in the GPSC is tidal-dominated during non-flood seasons.
OS54A-08
Sedimentary Characteristics of Cores Along the Gaoping Submarine Canyon and its Nearby Ocean Basin
To study their sedimentary facies and to interpret their possible processes of deposition, three cores, each approximately 3.5 to 4.6 meters long, along the Gaoping Submarine Canyon and one from the nearby South China Sea Basin were sampled. Three cores of the Gaoping Submarine Canyon were taken from the head, the mid-section, and the end of the canyon respectively. The other one core is taken from the nearby South China Sea Basin that has a similar water depth when compared to the one taken from the end of the canyon. Each core was sliced longitudinally to two halves and was x-ray radiographed, photographed, and visually described. Three main units of sedimentary sequences are recognized in these cores: (1) a fining-upward unit with sharp erosional base contact that varied from coarse silt or, in some cases, very fine sand to clay, (2) a fine parallel-laminated silt/clay alternation layer, and (3) a grey muddy deposit with scattered, brown organic matters. The core taken from the head of canyon is mainly characterized by the fining-upward units and the silt/clay alternation layers in between. The cores taken from the mid-section and the end of the canyon have similar characteristics in that they consist of mainly the silt/clay alternation layers. These layers appear as black silt laminae alternate with grey clay laminae. The silt-grade sediments consist mainly of the black slate fragments. The fining-upward units in the core from the head of canyon may be produced by hyperpycnal turbidity currents triggered during typhoon-related flash floods. The alternation of silt and clay laminae may be explained by the alternation of burst events in the basal boundary layer, as well as normal hemipelagic background sedimentation. The one long core taken from the nearby ocean basin is characterized by the grey clay deposit in the entire core, which may be a distinctive sequence representing the hemipelagic mud deposition. Key words: sedimentary facies, Gaoping Submarine Canyon, South China Sea Basin, hyperpycnal turbidity currents, hemipelagic mud