OS41D-0498 0800h
The Simulation Of Residual Tidal Phenomena In The White Sea
Residual tidal phenomena (residual tidal circulation and residual tidal level) in the White Sea play a significant role in energy contribution to the formation of the hydrological regime. Non-linear residual tidal currents and sea level due to energy exchanges from the basic harmonics to the high frequency part of the spectrum have been poorly investigated in the White Sea until now. But residual tidal phenomena play an important role in the general circulation, which affects the temperature distribution and the drift of ice, plankton, pollution and other conservative tracers. We carried out numerical experiments with a non-linear hydrodynamic model to estimate the contributions of (shallow, friction and convective) nonlinearities to the residual tidal phenomena of White Sea's, using a `consecutive realisation' technique. The model grid's spatio-temporal resolution is more detailed than before: the Earth's geoid is approximated by the Krasovskiy rotation ellipsoid. A residual tidal level is observed in the low frequency part of spectrum of model results. In the linear approximation, the spectral density has a dominant peak at the frequencies of the basic harmonics in all parts of the sea, and the residual level was absent; overtones were not expressed. The experiments revealed that the residual circulation was defined by the convective non-linearity. The Earth's rotation does not change the current structure: only the circulation intensity decreases. These results will enhance the knowledge of tidal phenomena in the White Sea, and can be used for navigation, ecology and fisheries research.
OS41D-0499 0800h
Are There Poincare Waves in The Chesapeake Bay?
Kelvin and Poincare waves are both linear waves caused by the Coriolis force when waves travel in a large straight channel. Although they both propagate along the channel, their characteristics are different. For Kevin waves, although the wave amplitude decreases across the channel, there is no lateral current. For Poincare waves, the amplitudes oscillate across the channel, and there are lateral currents. These lateral currents provide a great potential for lateral mixing, if existed. Using two numerical three-dimensional (3-D) hydrodynamic models (EFDC and UnTRIM), a study was carried out to check the characteristics and the existence of Poincare waves in the Chesapeake Bay. Because the numerical models include nonlinear terms, the development of nonlinear waves is evident. The model results demonstrated that the features of Kelvin waves and Poincare waves are both presented in the modification of the input waves as well as in the high order nonlinear waves. In other words, the frequencies of Poincare waves derived from the classical linear wave system changed because of the non-linear effects. The model results also confirmed the existence of Poincare waves in the Chesapeake Bay which is highly irregular in geometry and bathymetry. Although the amplitudes for Poincare waves are small, on the order of centimeters, they still provide a strong lateral mixing mechanism, because of their long wave periods, at the center of channel.
OS41D-0500 0800h
An Influence Of Boundary on Chaotic Advection In a Vortex Flow
The investigation of transport features of ocean vortex flows is a very important problem of geophysical hydrodynamics. Chaotic advection (Hasan Aref,1999) has fundamental importance in studies of mixing ocean waters. The influence lateral boundary on entrainment and release of water by ocean eddy structure is investigated with using mechanism of chaotic advection. In represented work, the topographical eddy situated into barotropic background flow is considered. The flow passes along lateral boundary. If the velocity of background flow is stationary, the exchange fluid particles between vortex region and flowing region is absence. When background flow has perturbation, for example tide-generating force, there is transport of passive pollutant between background flow and vortex region. The different scenarios of the influence of boundary on intensification of transport features in system were founded. It was showed, that presence of lateral boundary leads to increase exchange up to 1,5 time. New type behavior of trajectories of passive pollutant was founded, with using numerical methods. It is presented, by the example of transport of the fish eggs and larvae from the southern Japan spawning ground to the food abundant Kuroshio-Oyashio transition zone, oceanological interpretation of considered model of chaotic advection.
OS41D-0501 0800h
Oil Dispersion By Breaking Waves And Wind/Wave Induced Currents
Oil spilled at sea often entrained by breaking waves in stormy conditions and forms clouds of oil droplets that are dispersed by subsurface turbulence and shear currents. In this paper we consequently consider main mechanisms of oil dispersion - wave breaking, diffusion by turbulence generated by wind currents and Stokes drift, formation of oil droplet sizes specter by breaking waves and transport of dispersed oil in the surface layer. The joint action of wind stress, Stokes forcing and wave breaking with use of time-dependent 1D model with two-equation turbulence closure was simulated. The model equations are derived by horizontal averaging of Langmuir circulation model (McWilliams et al., 1997). The wave-breaking layer with thickness of half significant wave height was included into consideration. An injection of turbulence by penetrating breakers in this layer was parameterized by source terms in the turbulent kinetic energy and dissipation rate equations. The Monte-Carlo simulations of intermittent mixing support assumption that observed (Agarwal et al., 1992) lognormal distribution of dissipation rate is associated with breaking of waves in many scales. The calculations show the essential influence of Stokes drift and wind waves breaking on the mixing process. The results of simulations were applied to modeling of formation of oil droplet sizes spectra under the breaking waves. Almost all statistical models of break-up of an immiscible fluid immersed into a turbulent flow were not able to reproduce observed distribution of oil droplet size. Instead, the new model of the breakup based on Kolmogorov (1941) approach was proposed to reproduce observed lognormal distribution of oil droplet sizes. The calculations show that knowledge of mean characteristics of breaking wave field is not enough to simulate the breakup and entrainment of liquid and gas in the surface layer. The subsurface dispersion of entrained oil is governed both wind driven and wave driven currents. The detailed results of simulations of oil dispersion in the wave enhanced layer for stormy conditions by linked model of surface turbulent layer and 3D Lagrangian model of oil spill are presented.
OS41D-0502 0800h
Strong nonlinear regimes of internal waves and currents in closed basin
The particularities of strong-nonlinear dynamics of the degeneration of basin-scale waves in a rectangular closed basin have been investigated using 3D non-hydrostatic model (Kanarska Maderich, 2003). The classification of the possible flow regimes depending on the amplitude of the initial basin-scale wave and the depth of undisturbed position of the interface (Horn et al. 2001) was extended with the investigation of gravitational current regimes. Such regimes occur when the interface crosses the upper and/or lower boundary of the basin including the case of lock exchange flow when the interface has vertical position with maximum available potential energy. These regimes can not be described in the frame of weakly-nonlinear models of internal waves, because of inverse impact of small scale nonlinear disturbances on large scale motions. The model reproduces variety of strong-nonlinear flows with the formation of Kelvin-Helmholtz instabilities and undular bores which are results of joint impact of nonlinearity, dispersion and dissipation effects. Such flows are important mechanisms for transferring energy and mixing processes in real lakes induced by strong wind forcing.
OS41D-0503 0800h
An improved QuikSCAT weekly wind speed data set for coastal and high latitude applications
Accurate satellite-derived measurements of marine wind speed are necessary for a variety of oceanographic applications. The SeaWinds instrument on the QuikSCAT satellite has measured global marine winds since its launch in 1999. However, microwave backscatter from land and ice limits the coverage and utility of QuikSCAT wind speed measurements near the coast and at high latitudes. A new QuikSCAT weekly wind speed data set has been developed with several improvements that address these and other limitations of the data. The improved QuikSCAT data set includes new land and sea ice mask information adapted from the AVHRR Pathfinder sea surface temperature (SST) v5.0 and the Optimally Interpolated SST version 2 data sets, respectively. Additionally, near shore pixels originally contaminated by backscatter from land have been filled by several methods, including simple median filling, an inverse-distance weighted mean, and an objective analysis technique that considers the spatial structure of the wind speed data. The gap filling methods and QuikSCAT wind speed data were then evaluated by comparing with in situ wind speed measurements taken from near shore and open ocean buoys and marine weather stations. The improvements in the new QuikSCAT weekly wind speed data set and statistical comparison with the in situ wind stations are discussed here.
OS41D-0504 0800h
Currents Along the Mesoamerican Barrier Reef, Western Caribbean
To characterize currents and the extent to which they are influenced by winds, Interocean S4 electromagnetic current meter data were analyzed from three locations between Lighthouse Reef (17.4441° N) and Sapodilla Cays (16.1509° N). A better understanding of these currents is important to local fishing efforts, ecotourists and SCUBA divers through its value to conservation efforts with respect to connectivity and repopulation of the reef. The currents were related to regional COAMPS\_CENTAM modeled wind data to determine the extent to which winds drive the near reef currents. Harmonic analysis of the currents was also conducted to determine the influence of tidal cycles. This study will test the extent to which:(i) currents seaward of the Mesoamerican Barrier Reef extending from Lighthouse Reef to Sapodilla Cays are wind driven, (ii) tidally influenced and (iii) coherent.
OS41D-0505 0800h
Tidal Convergence Fronts in Cook Inlet, Alaska
Cook Inlet, Alaska is an extremely dynamic system, with tidal range up to 9 m, maximum current up to 12 knots, and an average maximum surface current of 3 knots. The water depth is mostly between 20 and 40 m in the upper inlet. Since Cook Inlet is elongated, the strong current system tends to generate axial convergence fronts. Synthetic Aperture Radar images from satellite RADARSAT-1 have shown frontal lines along the axial direction of Cook Inlet. These frontal lines can be more than 100 km in length. It also appears that the fronts vary in position from time to time. We hypothesize that the convergence fronts are a result of strong tidal current under the influence of strong bottom friction. The cross channel variation in bottom friction caused by significant depth variations tends to generate differences in both magnitude and phase of tidal flow. These cross channel differences in flow field can generate convergence along the channel. This hypothesis is tested with some preliminary studies using a diagnostic 2-D tidal model using realistic bathymetry. The model is driven by tidal data along Cook Inlet from NOAA stations. The model results have shown that the strong tidal current does produce significant convergence across the channel with significant depth variations. The maximum convergence calculated from the model is consistent with that from SAR images.
OS41D-0506 0800h
Occurrence of Langmuir Supercells at LEO-15
Extended deployment of a "turbulence" VADCP has revealed that the major sediment resuspension events at the LEO-15 coastal observatory are associated with the occurrence of Langmuir supercells (LSC), Langmuir circulations that grow to fill the entire water column. We will describe the three-dimensional flow field associated with LSC and explore conditions leading to these events.
OS41D-0507 0800h
Instabilities Along a Coastal Downwelling Front
This research studies the instabilities that develop along stratified continental shelves under downwelling conditions. The generation and evolution of the instability field is investigated for idealized situations using the ROMS hydrodynamic model. The model domain has a straight coast and constant shelf slope. The model is initialized with constant stratification and forced with a downwelling-favorable wind impulse. This wind impulse is spatially uniform except for small perturbations introduced to excite instabilities. Three types of instabilities develop initially: baroclinic instabilities arise near the surface in the downwelling jet, shear instabilities grow along the shoreward edge of the jet, and baroclinic instabilities form within the frictional bottom boundary layer. The boundary layer instabilities begin as symmetric instabilities and are sensitive to bottom slope and friction. As the instability field matures, the dominant wavelength increases from O(10 km) to O(100 km) and shear instabilities become most important. Instabilities directly extract a small percentage of energy from the mean flow. The instabilities also lead to increased dissipation that slows the downwelling jet and decreases isopycnal slopes.
OS41D-0508 0800h
EXPERIMENTS ON SUBMARINE KNICKPOINTS
The results of a laboratory study on the upstream migration of submarine knickpoints are presented. The main goal of this research was to obtain the formation of these features at laboratory scale. In order to achieve the objective, eight experiments using different materials were conducted in a flume at the Saint Anthony Falls Laboratory, University of Minnesota. The sediment used in these experiments was: kaolin clay, silica flour and fine sand. Silica flour and fine sand were used as single sediment in four experiments. The other four experiments consisted of a combination of the previous materials. The total sediment concentration in the experiments was 5% (one experiment) and 10% (seven experiments) by volume. The discharge ranged from 0.011 to 0.055 l/s. The experiment started with an initial slope in the channel bed of 15%. The bed deposit inside the flume was built up as the experiment progressed. Knickpoints were observed in six of the eight experiments. They were formed when one material or a combination of them was used in the tests. Initial calculations on the upstream migration velocity indicate values in the vicinity of 0.5 mm/s.
OS41D-0509 0800h
Simulating Submarine Channels in Flume Experiments: Aspects of the Channel Incision Dynamic
We report the results of thirty laboratory experiments reproducing submarine straight canyons as well as meandering ones. Our experimental setup consists in 2 m X 0.5 m flume filled with fresh water. The flume bottom simulates a sub-marine ramp draped by a sediment blanket. A sustained density flow is simulated by a continuous brine stream injected at the top of the ramp. We control the three main parameters of the experiment: the slope of the plane, the input flow rate, and the brine density. It allowed us to test the reproducibility of the phenomenon and to roughly delineate a phase diagram specifying the channel incision conditions. An optical acquisition technique enables us to measure instantaneously the topography of the sediment surface at successive times, during the canyon formation and frontal lobe deposition. In this way we can define and then measure physical quantities representative of different channel types in order to visualize their evolution. Here we present a few aspects of the channel incision dynamic. After a development phase in which the bottom density current spreads over the bed, the channel inception phase suddenly begins, followed by a positive feedback mechanism facilitating further erosion. Then a phase of regressive erosion appears, and in some cases a steady state can be finally obtained. By computing the difference between successive maps, we can elaborate time-varying maps of sedimentation and erosion rates in the system. Stacking of these maps produce a 3D cube of sedimentation rates showing autocyclic phases of incision, by-pass and sedimentation. Cross-sections through this 3D cube show morphologies very similar to those observed on seismic acquisitions performed in the Recent Orenoque channel system.
OS41D-0510 0800h
In-Situ Measurements of Turbidity Currents in Monterey Submarine Canyon
Turbidity currents are thought to be the main mechanism to move ~500,000 m${^3}$ of sediments annually from the head of the Monterey submarine canyon to the deep-sea fan. Indirect evidence has shown frequent occurrences of such turbidity currents in the canyon, but the dynamic properties of the turbidity currents such as maximum speed, duration, and dimensions are still unknown. Here we present the first-ever in-situ measurements of velocity profiles and sediment transport of four distinct turbidity currents observed during a year-long deployment of three deep-sea moorings December 2002 - November 2003. Each turbidity current started abruptly and gradually dissipated within 5 - 8 hours. The body of the turbidity currents, whose maximum along-canyon velocity reached 190 cm/s, were mostly confined to the bottom 50 m of water, but the turbid cloud that followed occluded instruments as much as 170 m above the canyon floor. Triggering mechanisms of these turbidity currents will also be discussed.
OS41D-0511 0800h
Experimental Investigation on Submarine Channel Formation
Submarine mega fans and their associated canyons have long attracted the attention of a large community in the Earth sciences because they record the history of past erosional processes. As such their morphology and sedimentary infill may be of use to unravel past climatic and tectonic evolutions. Yet despite an important corpus of research on their structural characteristics we lack a well accepted quantitative description for the dynamics of the formation and growth of these sedimentary systems. One of the main reasons for this lies in our inability to successfully understand, reproduce and predict the dynamics of the channel systems, tens to hundreds of meters deep, several kilometres in width, and hundreds of kilometres in length, that build submarine fans. Here we report on a series of small-scale laboratory experiments on the formation of sub aqueous channels and lobes. Our experiments show that steady flow of a dense current on a bed of light particles can induce both spontaneous formation of channels longer than a few channel widths and spontaneous meandering.
OS41D-0512 0800h
High Frequency Time-series of the Dynamic Sedimentation Processes on the Western Shelf of the Mississippi River Delta
Rivers annually transport billions of tons of organic and inorganic sediment to coastal environments, making them an extremely important part of global biogeochemical cycles. However, the majority of the freshwater and suspended materials are delivered to the coastal ocean by only a few rivers. In these river-dominated ocean margins (RiOMar), sediments are deposited and re-suspended repeatedly before stable deposition. This sediment cycling is poorly understood and is critical to understanding how deltas and continental shelves, considered to be major repositories of organic carbon in marine sediments, manipulate the global carbon cycle and biogeochemical processes affecting coastal environments. During six cruises in the fall of 2003 (October, November, and December) and spring of 2004 (March, April, and May), on the shelf west of the Mississippi River Delta, sediment samples collected from cores were analyzed for particle reactive radionuclides (210Pb, 137Cs, and 234Th) to create a quantitative high frequency time-series of sediment deposition and erosion processes and evaluate the transport and fate of material on the shelf. Based on previous work completed by Corbett et al. (2004), seasonal variations in short-lived tracers could be explained by river flow and weather conditions. Inventories of the tracers collected during the fall cruises suggest increased deposition during the late summer months and that most sediment reworking and export occurs during the winter months, typically a period of low/increasing river discharge and increased weather forcing.
OS41D-0513 0800h
Sedimentation Response to Holocene Landscape Disturbance on the Poverty Bay Continental Margin, East Coast New Zealand
Since human settlement, dramatic landscape changes have occurred on the Raukumara Peninsula, East Coast North Island of New Zealand. In particular, European destruction of native forests for pasture caused accelerated erosion of the mudstone and sandstone dominated hinterland. Sediment eroded from the Raukumara Ranges is primarily carried by three small-catchment river systems, which collectively deliver approximately 70 Mt/y of suspended sediment, representing about 0.3% of total global input to the ocean. Today, the Waipaoa River delivers 15 Mt/y of mud to coastal Poverty Bay, accumulating in an actively subsiding mid-shelf basin and outer shelf lobe. The shelf is bordered along its seaward edge by two emergent ridges, but a significant component of hemipelagic sediment leaks through the 13 km-wide Poverty Gap between the ridges, and is deposited on the slope in a large structural indentation that is heavily incised by the Poverty submarine canyon system. Using Holocene tephrochronology, and accepting near-full capture of Holocene riverine sediment on the shelf and slope, accumulation rates indicate that the modern (post-colonisation) sediment input from the Waipaoa River is probably an order of magnitude higher than the average for the Holocene. Previous studies suggest that a five-times increase in accumulation rates by the early 1900's on the shelf is contemporaneous with deforestation. Modern sediment mass accumulation rates determined from excess $^{210}$Pb profiles suggest that shelf sedimentation increases seaward, reaching a maximum of 0.9 cm/y on the outer shelf, with no net accumulation apparent on the inner-middle shelf. In general, accumulation rates are an order of magnitude lower on the slope, around 0.1 cm/y, decreasing slightly down-slope. Palynological data show a succession of destruction of native forests by burning, extensive land clearance for pasture, and the establishment of exotic forests. These markers date the arrival of Polynesian settlers and suggest that the sediment accumulation rate on the Poverty slope doubled upon European deforestation.
OS41D-0514 0800h
Sedimentary Particle Transport During an Upwelling Event - A Case Study from the Namibian Passive Margin
Upwelling events are accompanied by a complex interplay of erosion, transport, and deposition of sedimentary particles. The characteristics of individual transport paths are controlled by the physical properties of the respective particle (grain size, shape, and density), the location and time of its release and the water velocity field, which is, in turn, dependent on the seafloor topography and the oceanographic boundary conditions. In this study, we combine velocity data provided by ROMS circulation models with lagrangian particle tracing techniques to detect key parameters controlling the respective transport patterns. The model geometry mimics a two-dimensional bathymetric profile from the passive continental margin offshore Namibia. Each model run describes a five day upwelling event driven by increased surface wind stress and the subsequent period of relaxation under mean wind stress conditions. Background circulation is roughly re-established 10 days after the drop of driving wind forces. Based on the calculated velocity fields transport paths of a large number of lagrangian tracers are calculated, which are released at variable times and positions relative to the shoreline either at the sea surface or bottom. Since water currents are directed offshore at the surface and landwards near the bottom, particles inserted a the surface typically traverse different currents during their gravitational settling. Thereby, rapidly sinking or early released tracers experience a net landward transport, whereas slowly sinking or lately relased particles undergo an overall offshore transport. In addition, there are prominent lateral variations in the shape of particle paths. Surface source tracers from the shelf area tend to show an overall landward offset, while those emitted outside the shelf break are uniformly transported offshore. Near the coast, clay and fine silt particles mobilized from the bottom sources are transported onshore and to some extent backwards into the open ocean driven by the surface current. Modified model runs simulating the situation at the LGM with a significantly diminished shelf indicate transport of particles from the continental slope landwards onto the shelf.
OS41D-0515 0800h
Preservation and Alteration of the 1995 Eel River Flood Deposit
The Eel River shelf of northern California is episodically subject to seasonal flood events and associated sediment deposition. Flooding of the Eel River in the winter and spring of 1995, and the winter of 1997, resulted in a recognizable sequence of layers deposited across much of the adjacent continental shelf. Event-response sampling allowed the short term fate of the deposits to be documented (Wheatcroft, 2000). Within a few years after the 1997 event, the flood deposit was mixed or remobilized such that the original deposit was not recognized in core x-radiographs anywhere on the shelf. However, remnants of the 1995 flood deposit were still identifiable as of October 2001 (Wheatcroft {\it et al}., in progress). In May 2004, a total of 31 box cores were collected at 23 different stations along transects established by previous studies. Flood layers were still present and recognizable in x-radiographs at several stations. Many of the layers identified in x-radiographs were, although preserved, visibly disrupted. In locations where x-radiographs were inconclusive, the unusually fine-grained nature of the deposit allowed them to be detected using standard sediment size analyses. The mean percent of sediment $<$20 $\mu$m ranged from 68-84% in the 1995 flood deposit. Core subsamples were analyzed from the upper 16 cm in order to determine biological abundance. Polychaete worms were identified as the dominant taxonomic group, constituting approximately 80% of the identified organisms, which is consistent with previous research (Wheatcroft {\it et al}., in progress; Bentley and Nittrouer, 2003). The dominance of polychaetes is notable in light of the fact that these subsurface deposit feeders are likely to mix sediment to substantial depths (Wheatcroft {\it et al}., in progress). Despite this biological activity, the flood layer is still recognizable a decade after the initial event. As a result, the 1995 flood deposit may ultimately become a permanent piece of the stratigraphic record.
OS41D-0516 0800h
A Lipid Molecular Marker Assessment of Sediments from the Northern Gulf of Mexico before and after the Passage of Hurricane Lili
A lipid molecular marker (hydrocarbons, sterols and fatty acid) characterization was undertaken to identify the sources of organic matter to surface sediments from the Louisiana Shelf, west of the Atchafalaya River before and after the passage of Hurricane Lili. The data obtained point to relatively small changes in source with no large pulse of terrestrial materials following the passage of this category 2 hurricane. During both sampling periods, the terrestrial/aquatic ratios (TAR) estimated from the hydrocarbon compositions of surface sediments were consistently high (increased terrestrial component) at the near-shore sites while smaller TAR values (decreased terrestrial component) were observed at the offshore sites. All samples displayed unresolved complex mixtures (UCM), which ranged in concentration from 54.9 Yg/g OC to 426.5 Yg/g OC. Generally; the UCM concentrations were highest at the near-shore sites and decreased by an order of magnitude at the offshore sites. Sterols, linear, mono-and poly-unsaturated fatty acids from multiple organic matter sources were also detected at each site. Overall, these compounds are consistent with similar sources (higher plant, phytoplankton, zooplankton, diatom and bacterial) of organic matter albeit small differences between the pre- and post-hurricane samples were observed. Principle component analysis of the lipid concentration data revealed subtle differences in the distribution of organic matter sources among the samples. The pre-hurricane samples were characterized by lipid contributions from a mixture of higher plant, zooplankton, diatom and dinoflagellate sources while the post-hurricane samples were characterized by lipid contributions from higher plant, zooplankton and dinoflagellate sources, with little or no contributions from diatoms. The apparent difference between sampling cruises was not detected in the simple autochthonous/allochthonous molecular marker ratios or in other less specific molecular markers.
OS41D-0517 0800h
A One-Dimensional Physical-Ecosystem Model Study of Plankton, Nutrient and Dimethylsulfide Dynamics in the Southeastern Bering Sea middle shelf domain
As a high-latitude sea exhibiting great variability in sea ice extent and high productivity, the Bering Sea is an opportune location for studies on how geophysical phenomena (such as climate variability) influence plankton dynamics and thus biogeochemical cycling in the Arctic. With this in mind, we have developed a one-dimensional physical model coupled to a one-dimensional ecosystem model and applied it to the time series data from the NOAA-PMEL biophysical mooring M2 in the middle shelf domain of the Southeastern Bering Sea (Hunt and Stabeno, 2002, Progress in Oceanography, 55, 5-22). The ecosystem submodel is a depth-resolved, N-based model of the lower trophic level ecosystem. Two types of phytoplankton (diatoms and flagellates), three nutrients (nitrate + nitrite, ammonium, and silicon), three size-classes of zooplankton, detritus and dimethylsulfide cycling are included in the model. The model is driven by meteorological data: wind velocity, air temperature, sea surface temperature, specific humidity, cloud cover and light. The model realistically simulates the spring bloom, seasonal cycles of temperature and chlorophyll, and large seasonal variations of nutrients. Simulations for the southeastern Bering Sea middle-shelf domain in relatively high and low sea surface temperature years, 2000 and 1999, respectively, suggest that increased water column mixing in spring delays phytoplankton bloom onset and that zooplankton grazing pressure is an important factor in the maintenance of the coccolithopore bloom in the Bering Shelf. Model sensitivity studies point out several important factors, such as, that the timing of the spring phytoplankton bloom is most sensitive to cloud cover (light) and phytoplankton growth rates, the latter also affecting bloom strength. Maximum rate of grazing on flagellates appears to control the timing of the end of the spring bloom and the strength of the autumn diatom bloom. Work in progress and recent model developments are described, moving towards a three-dimensional version of the model, including sea ice and having applications elsewhere in the Arctic.
OS41D-0518 0800h
Surficial Geology of the Sea Floor in West-Central Long Island Sound as Shown by Sidescan Sonar Imagery
The United States Geological Survey is working cooperatively with the National Oceanic and Atmospheric Administration (NOAA) and Connecticut Department of Environmental Protection to conduct detailed studies of the surficial geology in Long Island Sound (LIS). Sidescan sonar imagery, processed from data collected during NOAA survey H11044, details almost 300 km$^{2}$ of the sea floor in west-central LIS and is used in conjunction with bathymetry, sediment, bottom video, and seismic data to interpret the area's surficial geology. The distributions of sediments and sedimentary environments interpreted from these data sets are products of the Quaternary geology, regional bathymetry, and effects of modern tidal and wave-driven currents. Most of the study area sea floor slopes gently to the southeast or south, though more complex bathymetry consisting of shoals, lesser bathymetric highs, and troughs is also present. Four distinct sedimentary environments consisting of 1) erosion or nondeposition, 2) coarse-grained bedload transport, 3) sorting and reworking, and 4) fine-grained deposition were identified and mapped. High-energy conditions characterized by erosion or nondeposition occur on bathymetric highs where gravel and gravelly sediments are present. Sedimentary environments characterized by coarse-grained bedload transport, distinguished by sandy sediments with current-derived bedforms, are located on an unnamed shoal in the northwestern part of the study area and on the northern flanks of Stratford Shoal. Fine sand, silty sand, and sand-silt-clay are the dominant sediment types within sedimentary environments characterized by sorting and reworking. This environment, which marks the transition from relatively higher energy conditions to lower energy conditions, occurs on the sides of basins and flanks of bathymetric highs. Elongate sinuous features of low backscatter also occur within sorting and reworking environments and are interpreted to be down-slope density-flow pathways through which fine-grained sediments are remobilized and transported deeper into the basins. Relatively low-energy environments prevail where deposition of clayey silts occurs in deeper water throughout the central part of the study area and in the protected areas of the far northeastern corner. Thus, bathymetric highs tend to have high-energy environments with coarser grained sediments, bathymetric lows tend to have low-energy environments with finer grained sediments, and transitional environments and conditions occur between the two.
OS41D-0519 0800h
Geomorphology of the Southern Gulf of California Seafloor
A Spring 2004 multibeam sonar survey defined the seafloor geomorphology of the southern part of Gulf of California and the intersection of the East Pacific Rise with the North American continent. Survey goals included mapping structural patterns formed during the rifting that opened the Gulf and identifying the spatial transition from continental rifting to seafloor spreading. Multibeam sonar imagery, augmented with archival data and a subaerial DEM of Mexico, illuminates the principal features of this boundary zone between obliquely diverging plates: (i) active and inactive oceanic risecrests within young oceanic basins that are rich in evidence for off-axis magmatic eruption and intrusion; (ii) transforms with pull-apart basins and transpressive ridges along shearing continental margins and within oceanic crust; (iii) orphaned blocks of continental crust detached from sheared and rifted continental margins; and (iv) young, still-extending continental margins dissected by submarine canyons that in many cases are deeply drowned river valleys. Many of the canyons are conduits for turbidity currents that feed deep-sea fans on oceanic and subsided continental crust, and channel sediment to spreading axes, thereby modifying the crustal accretion process. We present a series of detailed bathymetric and seafloor reflectivity maps of this MARGINS Rupturing Continental Lithosphere focus site illustrating geomorphologic features of the southern part of the Gulf, from Guaymas Basin to the Maria Magdalena Rise.
OS41D-0520 0800h
Seismic facies analysis of shallowly buried channels, New Jersey continental shelf: understanding late Quaternary paleoenvironments during the last transgression
We are investigating the late Quaternary sedimentary record of the New Jersey mid-outer continental shelf using deep-towed chirp sonar (1-4 kHz and 1-15 kHz) profiles, coupled with lithologic and chronostratigraphic control from long sediment cores collected using the DOSECC AHC-800 drilling system. We have seismically mapped extensive, shallowly buried, dendritic drainage systems. Observed seismic facies distributions suggest the complex nature of channel fills, and synthetic seismograms derived from MST logs enable us to correlate the chirp data to changes in lithology and physical properties of the cored samples, including channel fills, confirming that fine-grained material is transparent seismically, while interbedded sand and mud produce laminated reflections. We suggest that these channels probably formed during shelfal exposure coincident with the last glacial lowstand along this margin. Observed seismic facies superposition within valley fills is in part consistent with a tripartite zonation derived from wave-dominated estuary models. We have mapped four main facies within these dendritic incised valleys: (1) The lower facies, SF1, consists of a high-amplitude chaotic configuration. We interpret this facies as lowstand fluvial fill; (2) Overlying facies SF2 is generally a thin layer (<1-2m) of stratified, high amplitude reflectors in valley axes. This facies is characterized by small wedges along channel flanks, with a generally transparent acoustic response, but occasionally also by internal clinoforms. This facies could have been deposited as transgression began, by backfilling of valleys (bayhead delta? aggradational alluvial deposits?); (3) SF3 is generally transparent; subtle horizontal and parallel reflectors onlap channel flanks. We interpret this facies as representing central basin/bay deposits, a low-energy zones during the transgression, perhaps related to turbidity maxima; (4) SF4 is observed only in the seaward end of the valley. This facies is more variable in amplitude and configuration, and includes a laminated acoustic response, small erosional surfaces, and some wavy reflections. We think the complexity of this facies likely reflects deposition of an estuary mouth complex in a dynamic environment, including frequent lateral variations in sedimentary facies from tidal inlets, washovers, tidal-deltas and barriers. A seismic transition upward from chaotic to flat-lying reflections and a more transparent acoustic response indicates less depositional energy, suggesting replacement of fluvial systems by tidal/estuarine environments. This has been confirmed by vibra-coring of one channel. Our paleo-flow reconstructions also yield velocities in the range of 0.5-1.5 m/s, which are reasonable estimates for flows in estuarine environments.
OS41D-0521 0800h
3D Seismic Data From the mid-Norwegian Margin Provide Evidence for Marine Based Ice Sheet Dynamics
3D- seismic exploration data and shallow cores have been used to study glacial deposits along the shelf break/upper continental slope off mid Norway. Glacial fed fans are ubiquitous phenomena on the Norwegian continental slope. Such fans were deposited along the ice front when it reached the shelf break, and are usually found distal to cross-shelf troughs, where glacial debris flows reached hundreds of kilometres down the slope (e.g. c. 500 km on the North Sea Fan). The study area is located in an interfan area, where an up to 250 m thick glacial package (reaching c. 100 km down slope) has previously only been imaged as a seismically transparent wedge, even on high-resolution 2D data. However, the use of seismic attribute analysis on the 3D data volume has made it possible to resolve the internal geometry of the wedge. The attribute analysis reveals a complex internal structure, showing that the wedge consists of coalescing debris flow lobes. The wedge has an average gradient of 0.7 deg, and the bulk of the lobes are from 0.5 to 2.5 km across. Continuous flow lines, which can be followed more than 15 kilometres delineate individual flow bodies. No scarps, blocks, transverse flow elements or indications of brittle behaviour of the sediment are observed. The flow lines show a winding, sinuous pattern and examples of convergence (bottlenecks), which may indicate that the movement of the debris flows was controlled by the underlying topography. These 3D data show that the glacigenic wedge in the interfan area has many of the same characteristics as the glacigenic debris flows on the large fans. However, the run-out distance is much smaller. The core investigations confirm that the material (diamicton) is very similar to that found on the large fans, also with regard to geotechnical properties. This suggests that the rate of sediment delivery to shelf edges is the key difference for explaining the various run-out distances for GDFs in an interfan and fan setting. This again is most likely related to different dynamics of the marine based ice sheet on the shelf in the two areas.
OS41D-0522 0800h
Transgressive Sequence Development on the Northern California Continental Shelf
It has long been recognized that stratigraphic patterns and associated facies on continental margins are controlled by three independent variables: eustatic sea level fluctuations, tectonic subsidence/uplift (rate and distribution), and sediment supply. In addition to this triad of processes, climate (e.g., transport and dispersal) and physiography of the margin also play an important role in controlling the resultant geometry of stratigraphic sequences. Numerous attempts have been made to understand how changes in these variables affect the stratigraphic record; that is, trying to define the link between process and product. Despite these efforts controversy remains regarding the importance of tectonics versus eustatic sea-level fluctuations in controlling sequence architecture. The Eel Basin in Northern California is an ideal location to tease out the influence of tectonics versus eustasy because the margin is undergoing shore-perpendicular tectonic deformation in response to the northward propagation of the Mendocino Triple Junction. This northeast-southwest oriented compression is inducing folds and high-angle reverse faults and thrusts with their fold/fault axis oriented northwest-southeast at a high angle to the shoreline. This configuration creates a tectonic signal with varying sign and amplitude parallel to the coastline and provides the ideal opportunity to examine how tectonic deformation affects stratigraphic architecture and facies assemblages. High resolution chirp data acquired in the region during multiple cruises (1998, 1999, 2000) indicate that the bathymetry and subsurface structure are not well correlated. For example, the thickness of the Holocene sediment varies along strike in the Eel Basin from less than 2 m across the crest of the Little Salmon Anticline to greater than 30 m toward the Eel River Syncline. In the Freshwater and Eel River synclines, which also control the location of Mad and Eel rivers, there is greater sediment input and the depositional events have a greater propensity for being preserved due to the rapid subsidence (up to 4 mm/yr in Eel River Syncline). The onlapping reflectors in the syncline diverge and exhibit increasing dip with depth implying that sedimentation is concomitant with tectonic deformation. Using modern tectonic uplift and subsidence rates determined from beach terraces and applying these rates for the Holocene reveals that the relief observed across the transgressive surface and differential Holocene sediment thickness can be explained predominantly by tectonic deformation. In addition to seismic and core data, we will present three-dimensional visualization to illustrate the contributions of tectonics versus eustasy for the Eel Basin.
OS41D-0523 0800h
Craters in the Seabed of the Gulf of California Caused by Volcanic Explosion? Volcanic Subsidence? Meteoroid Impact? Hydrocarbon Venting? or Dissolution?
Several circular enclosed depressions were mapped at depths of 500 to 2500m during a multibeam sonar survey of the Gulf of California. Those on the summits and flanks of volcanic seamounts were caused by volcanic explosions or by subsidence following magma withdrawal, but the origins of the others, in both oceanic and continental crust, are more problematic. Multiple working hypotheses are being tested with geophysical data and rock-sample analysis. A fresh circular crater similar in size to Meteor Crater, Arizona but deeper (2km diameter, with a flat floor at 2185m below sea-level, 500m deeper than parts of its rim) indents young, faulted oceanic crust in rift mountains just 6km from the North Pescadero spreading axis. This axis, believed to spread a half-rate of 24km/Myr, accretes mainly a sediment-sill complex, with very localized eruptions of tholeiitic lava. Dredging of the steep (30 to $45\deg$) crater walls and rim recovered mudstone and angular boulders of basalt, and destroyed one dredge that grabbed a boulder too large to lift. The faulted rift walls 2km from the crater yielded mudstone and basalt fragments, plus rounded boulders (crater ejecta?) composed mainly of fresh tholeiitic glass. The most plausible, perhaps equally improbable, genetic hypotheses are (i) volcanic explosion when magma intruded off-axis into the sediment-sill complex and interacted with pore water, and (ii) meteoroid impact. Craters of a different type dissect the thick deformed sediment on the 1700 to 400m-deep crest of a transform ridge along the Guaymas Basin margin. Shallow pockmarks around discharges of hydrothermally generated hydrocarbons have been explored with submersibles on the deeper part of the ridge. The larger craters (0.5 to 2km diameter, 100 to 400m deep) that we found where the crest is 600 to 500m deep probably have the same origin (explosive release through a near-surface clathrate horizon of hydrocarbons that migrated up-dip from Guaymas Basin); a possible alternative is dissolution of evaporite strata, for which we have no direct evidence.
OS41D-0524 0800h
Identification of Late Pleistocene Ice-Rafted Debris (IRD) on the New Jersey Shelf
The purpose of this study is to evaluate the potential for ice-rafted debris (IRD) on the New Jersey shelf and develop procedures for IRD identification on a shelf environment using a variety of techniques to assess texture and age of the sediments. Pleistocene New Jersey shelf sedimentology is strongly defined by glacially driven sea level changes. IRD and its provenance may be identified on a shelf environment through analysis of grain size distribution, heavy mineral content (higher % suggests non-fluvial processes), mineralogical point counts (anomalous mineralogy indicates distal source), isotopic dating methods (age value determination to narrow down potential source rocks), and surface texture analysis (specific glacial transport features). IRD must be differentiated from sediment derived from the NJ bedrock. Likely sources for IRD include the bedrock of Maine and of the southeastern Canadian Shield. Duncan and Goff (2001) reported iceberg grounding along the NJ shelf. IRD is typically identified in the deep sea through anomalously large grain size within pelagic mud, but different methods are needed for the shelf, where regressive shoreline processes, subaerial exposure, fluvial downcutting, and deposition and reworking during transgression have influenced the sediment composition found today. We analyzed grab samples in or near the features believed to be iceberg scour marks and downcore samples from recent Geoclutter drilling in the same area. The coarse grain size fractions of shelf samples were separated by phi classes before heavy mineral separation methods were employed. Initial analyses show high percentages of heavy minerals in the 2 phi and 3 phi size fractions, consistent with past NJ shelf studies. Hornblende grains were hand-picked from select samples for K-Ar dating, providing age values of about 0.96 ± 0.03 Ga for three sites within iceberg scours. Mineral content of each size fraction is determined by point count. Qualitative assessment of surface textures of select grains as seen with a Hitachi S-2500 SEM is useful in interpretation of source and transport.
OS41D-0525 0800h
Post-Transgressive and Modern Erosion on the New Jersey Outer Shelf
Recent erosion is evident on the outer New Jersey shelf ($>$ 50 m water depth) based on analysis of multibeam bathymetry, backscatter, and chirp seismic reflection data, as well as grab samples and short cores. Truncation at the seafloor of the transgressive ravinement surface indicates that erosion occurred in the post-transgressive environment, i.e. after passage of the shoreline and development of the surficial sand sheet that caps the ravinement. Apparently moribund oblique sand ridges are also truncated by large, erosional swales oriented along the primary modern current direction, indicating that erosion likely post dates sand ridge evolution, which is known to persist to water depths of 40 m. Post-transgressive erosion has exposed a variety of strata at the seafloor, including: shallowly buried, fluvial channel systems, formed during or somewhat after the Last Glacial Maximum and filled during the transgression; the outer shelf wedge, likely deposited during falling sea level conditions; and a regional reflector, "R", that likely represents erosion during the last regression , i.e., exposing material $>$40 kyr. Depths of erosion range from a few meters to $>$10 m. A "ribbon" seafloor morphology marks much of the eroded regions. Ribbons are observed in the backscatter data as alternating bands of low and high backscatter elongated in the direction of bottom flow. Samples from the high backscatter regions are a mixture of shell hash, mud and sand; the latter exhibit populations of both abraded and unabraded grains. The shell hash is likely an erosional lag, perhaps remnants of the transgressive ravinement surface. The muds and unabraded grains are, because of negligible modern sediment input, evidence of newly eroded, previously undisturbed sediment. The lower-backscatter areas of the ribbon morphology consist of a well-sorted medium sand unit only a few tens of cm thick, overlying the shelly-muddy-sands. Well-rounded gravels and cobbles have been found in areas with very high backscatter; seismic data through one gravel mound indicates that it is likely derived from the base of an eroded fluvial/estuarine channel. Reworking of seafloor sediment in the post-transgressive regime appears to change from sand ridge evolution in inner to middle shelf depths to more predominantly erosional modification at outer shelf depths. We speculate that this change may be related to the reduction in the effectiveness of wave resuspension of sediment with increasing water depth.