subsurface gas offshore of northern california and its link to submarine geomorphology abstract the northern california continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology analyses of seismic reflection sidescan sonar and high resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope subsurface gas inferred from enhanced reflectors and other geophysical indicators is spatially variable and related more to total depth and stratigraphy than to underlying structure with the exception of one band of gas that follows the regional structural trend shallow depressions on the seafloor pockmarks are used to infer expulsion sites the largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m where expulsion features are rare the upper slope water depths 400 600 m has a high concentration of pockmarks diameter 10 20 m in contrast to a near absence of pockmarks at water depths shallower than 400 m of nearly 4000 pockmarks observed on sidescan sonar records more than 95 are located in water depths deeper than 400 m bottom simulating reflectors bsrs on some seismic reflection profiles indicate the possible presence of gas hydrate we find that gas and pore fluid migration in the offshore eel river basin is 1 correlated to surface morphology 2 a contributor to seabed roughness 3 a significant mode of sediment redistribution on the upper slope and 4 potentially a factor in large slope failures

three dimensional quantitative modeling of clinoform development abstract clinoform geometry pervades stratigraphic sequences across a variety of scales it is a composite surface which records the complex interplay between accommodation sediment supply physiography and hydrodynamics much of the sequence stratigraphic technique employs clinoform geometry to discriminate between the various systems tracts that constitute depositional sequences however many sequence stratigraphic models treat the generation of accommodation and the input of sediment supply as a two dimensional problem by assuming a line source of sediment entering a system impacted by relative sea level fluctuations one common attribute of clinoforms regardless of size and or shape is that their formation is inherently three dimensional therefore the clinoform geometry varies spatially and temporally throughout a region depending on the proximity to the sediment source and the character of the hydrodynamic regime we present a three dimensional stratigraphic model that combines both diffusion and advection to simulate across and along margin sediment transport and deposition this model allows us to examine how the morphology and internal clinoform geometry of deltas vary away from the sediment source in this application we simulate the development of a delta in a low energy environment to understand the importance of along shelf diffusion on clinoform geometry and the morphology of the delta we predict the three dimensional geometry and how it varies throughout a relative sea level cycle our model indicates that when the clinoform rollover is spatially coincident with the shoreline position the clinoform geometry changes dramatically along strike away from the sediment source region throughout a relative sea level cycle for example oblique clinoforms develop near the source region and evolve into an onlapping sequence along strike on the basis of stratal geometry and the technique of sequence stratigraphy this deposit would be interpreted as two different sequences with very different connotations for their formation we surmise that when the clinoform rollover and the shoreline position are separated e g a subaqueous delta the along strike variability of the clinoform geometry diminishes because of the hydrodynamic forces operative across the shelf consequently it is important to determine how clinoforms develop in three dimensions and to ensure that our window of observation is sufficient to discern between local and regional variability

deposition and modification of a flood layer on the northern california shelf lessons from and about the fate of terrestrial particulate organic carbon the 1995 flood of the eel river in northern california provided an opportunity to follow the short term history of riverine particulate matter on a continental shelf particulate organic carbon including both vascular plant debris and soil carbon was utilized as a tracer of the distribution and modification of the shelf flood deposit these components were sorted during initial emplacement of the deposit both in the river plume and in the benthic boundary layer during the 8 months following the flood lateral transport shallow burial and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit study of the deposit suggests that this and previous flood layers may be preserved in the shelf and upper slope stratigraphic record as clay rich beds with relatively high carbon to nitrogen ratios and more negative 13c values than background non flood sediment concentrations of vascular plant debris are locally characteristic of such layers and may serve as distinctive stratigraphic markers investigation of organic carbon in the 1995 flood layer on the eel river shelf provides insight into the fate of terrestrial particulate carbon on continental margins in general flooding of the eel river and of similar river systems draining mountainous terrains introduces large quantities of terrestrial carbon to the marine environment and may favor the preservation of both terrestrial and marine carbon the abundant woody vascular plant debris discharged during floods is especially likely to survive oxidation and sulfate reduction in the shallower zones of the seabed this study reveals the dominant role that physical and biological processes may play in attenuating the terrestrial organic carbon signal on shelves over monthly to yearly time scales

across shelf variations in bed roughness bed stress and sediment suspension on the northern california shelf abstract field measurements of bed micromorphology roughness benthic flow bed stress and suspended sediment flux were made on the northern california continental shelf in connection with the strataform program a sediment profiling camera and side scan sonar were used to observe bed features in december 1995 biogenic roughness with 2 cm relief prevailed at both sites bottom boundary layer tripods were deployed on the 60 m and 70 m isobaths over the period 5 january to 7 march 1996 during which time two high energy events occurred skin friction shear stresses were subcritical under average conditions but appreciably exceeded the threshold for sediment suspension during storms for those high energy events near bed suspended sediment concentrations reached 2 g l at 60 m 15 cm above bed and 1g l at 70 m 27 cm above bed and suspended sediment induced stratification significantly affected bed stress estimates at those times increases in current shear were accompanied by increases in suspended sediment concentration gradients causing the gradient richardson number within the log layer to remain near the critical value of 1 4 this suggests suppression of turbulence by sediment induced stratification the abundance of under consolidated fine sediment on the shelf to the north of the eel river mouth presumably allows increases in stress to be accompanied by progressive increases in suspended sediment concentration within the log layer to maintain the critical richardson number this contrasts to situations where bed armoring limits the total amount of fine sediment in suspension applications of a wave current boundary layer model with stratification effects included reduces estimates of current friction velocities by u c about 24 relative to results from fitting the von karman prantdl equation without stratification the model suggests that the total wave current friction velocity u cw reached 3 0 cm s at 70 m and 3 5 cm s at 60 m depth integrated across shelf suspended sediment fluxes were seaward at the 60 m isobath and near zero to weakly landward at the 70 m isobath during high energy periods implying flux convergence this is consistent with the conclusions of other strataform investigators that rapid long term accumulation of mud is occurring on the mid shelf

the shallow porosity structure of the eel shelf northern california results of a towed electromagnetic survey a towed electromagnetic survey mapping the electrical resistivity of the seafloor was conducted over an area of the eel shelf off humboldt bay california continuous resistivity profiles to 20 m below the seafloor were measured along 120 km of track line from water depths of 100 m to around 30 m the shallow structure along the shelf is highly variable and we identify three distinct environments based on the recorded resistivities and the porosities inferred from them the first region is a mid shelf depocenter characterized by a thin 2 m moderately high porosity 45 60 surface layer which overlies a less porous 35 45 and homogeneous substrate uniform both laterally and vertically this region is found to the northwest of the humboldt bay entrance from water depths of about 65 m to at least 100 m and is roughly coincident with recent flood deposits the second region is located closer to shore and contains extremely high resistivities for a shallow sedimentary environment it reveals a high degree of spatial variability on length scales of several hundred meters several possibilities exist to explain such high resistivities and these include upwelling fresh water channeled to the seafloor through local fault and anticline systems a significant volume of natural gas within the sediments or a continuous process of carbonate precipitation through oxidation of methane near the seafloor which over time builds a substantial thickness of lithified material none of the above explanations are mutually exclusive and all could act in concert to increase resistivities the third region roughly coincides with the eel river delta and features a buried layer of moderately low porosity 30 35 at a depth of about 5 m and with a thickness of between 5 and 10 m this layer extends from near the entrance to humboldt bay in a southwesterly direction across the shelf 1999 elsevier science b v all rights reserved

measurement of size and settling velocity of suspended aggregates on the northern california continental shelf abstract a high resolution video camera attached to a miniature sediment trap was used to make time lapse images of suspended aggregates settling through the water column the video trap system was placed 2 m above the seabed in the strataform study area off eureka california and was programmed to take a 7 second time sequence of settling particles every 6 h over a 4 month period the camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 m data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size shape and settling velocity results of the first deployment of this system september 1995 show that particle sizes elliptical nominal diameter determined from the video images ranged from 130 to 740 m with a modal size of 300 m the average form factor ratio of short to long semi axis of the aggregates was 0 71 measured settling velocities varied from 0 09 to 8 13 mm s in terms of weight percent or mass the median size of the aggregate distribution is 600 m analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1 0 to 63 m with a modal size of 26 m and a median size of 11 m additionally an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity and these results were compared to observations using the equations for settling velocity of spheres and prolate spheroids and substituting published relationships for size versus excess density into these equations the settling velocity for combinations of shape and excess density has been calculated results show that differences between sphere and prolate spheroid with major axis both parallel and perpendicular to flow approximations caused less than a 13 difference in computed settling velocity in contrast the various size excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values in all cases however the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution

effects of triple junction migration and glacioeustatic cyclicity on evolution of upper slope morphologies offshore eel river basin northern california abstract the upper continental slope of the eel river basin is affected both by uplift and seismicity associated with the northward migrating mendocino triple junction mtj and by glacioeustatic fluctuations as a result seismically imaged slope sequences vary dramatically along strike the southern slope is dominated by the humboldt slide a stack of nine deformed sequences older undeformed sequences thicken landward while slide sequences maintain a constant travel time thickness onset of deformation was abrupt within the slide reflector amplitudes alternate high amplitude reflectors are wavy to shingled and suggest increasing deformation with depth low amplitude reflectors are sub parallel to wavy throughout high and low amplitude sequence couplets suggest repetitive deposition of contrasting lithologies a response we ascribe to late pleistocene glacioeustatic cyclicity if each couplet represents a 100 kyr late pleistocene sea level cycle then humboldt slide deformation began 450 ka this is approximately coeval with onset of deformation on the adjacent shelf 500 ka previously attributed to uplift and seismicity associated with northward encroachment of the mtj we believe migration of the mtj triggered the slide and is the cause for continuing deformation today by sediment creep along internal glide planes north of the humboldt slide we interpret an upper slope anticline as a seaward extension of the little salmon fault zone lsfz previously identified landward both faulting and folding extend to the seafloor indicating that deformation continues an adjacent buried anticline suggests that deformation has also jumped northward within the lsfz on some parts of the upper slope north of the lsfz the slope is dominated by downslope trending channels and gullies these features are v shaped vertically stacked and increase in depth downslope they are mostly infilled their physiographic expression is generally subdued by sediment draping channel fills generally consist of basal high amplitude reflectors overlain by reflectors of lower amplitudes several large channels on older surfaces are concentrated near the northern end of the seismic coverage smaller channels are more evenly distributed along the margin on younger surfaces channels also extend increasingly landward on progressively younger surfaces indicating migration in that direction with time these incisions are clearly inactive during highstands as at present slope deposition today is dominated by draping of hemipelagic sediments channels form and migrate landward through headward erosion during base level changes when both shoreface and fluvial sediment sources are more proximal to the upper slope the increasing lateral distribution of channels through time supports an increase in shoreface erosion north of the lsfz a possible explanation is a known decrease in fluvial sediment input from the north combined with mtj related uplift in the south along strike contrasts in upper slope sequence morphology in the southern eel river basin are clearly a function of both regional tectonism and proximity to sediment sources influenced by base level changes humboldt slide sequences while deformed are continuously influenced by systematic lithological variations occurring as a result of glacioeustatic cycles effects of base level changes similarly control the distribution and fill of incisions on the northern slope all of these observations reinforce previous seismic investigations of the offshore eel river basin shelf and confirm that the competing effects of tectonism and glacioeustacy on the preservation of continental margin stratigraphy can be differentiated

modeling sediment entrainment and transport processes limited by bed armoring abstract a sediment transport model including a bed armoring algorithm is used to ascertain the potential importance of bed armoring processes the parameters that control the bed armoring effects on the suspension of granular sediments in the marine environment are examined using the numerical model and data from the eel shelf results for the shelf indicate that bed armoring controls the total suspended sediment load and affects the across shelf gradients of suspended material the armoring effects are too significant to be neglected in most analyses of shelf and nearshore sediment transport and this is especially relevant for models that are developed to represent large scale and long term sedimentary regimes a principal controlling parameter is the thickness of the active layer and the dynamics of this layer are not well known

time integrated 3d approach of late quaternary sediment depocenter migration in the tagus depositional system from river valley to abyssal plain quantification of sediment volumes in continental to deep ocean basins is key to understanding processes of sediment distribution in source to sink depositional systems using our own and published data we present the first quantification of sediment volume changes in basins along the course of a major southwest european river during the deglaciation the salient points of this quantitative record in the tagus and equivalent north atlantic basins show crucial roles for sea level climate and land use in the distribution of sediments the bypass of sediments starved the tagus basins and subsequently sedimentation mainly occurred on the tagus abyssal plain during the sea level lowstand of the last glacial maximum the main sediment depocenter rapidly shifted via the continental shelf to the lower tagus valley during sea level rise in the deglaciation period finally the main sediment depocenter shifted further landward into the lower tagus valley during sea level high stand in the holocene during the high stand phase last 7 ky sediment flux increased up to 2 5 times due to climate and land use changes the average catchment denudation rate during the last 12 ky 0 04 0 1 mm y is in agreement with those of other european catchments our study clearly demonstrates the added value of detailed knowledge of 3d depocenter distribution size and chronology this allowed us to identify an increased sediment flux during the last 7 ky which was not identified using local observations from boreholes alone the uniqueness of the tagus depositional system lies in the combination of a large accommodation space in the bedrock confined lower tagus valley the steep lowstand surface gradient and the narrow continental shelf with canyons indenting the shelf break

landslide dammed paleolake perturbs marine sedimentation and drives genetic change in anadromous fish large bedrock landslides have been shown to modulate rates and processes of river activity by forming dams forcing upstream aggradation of water and sediment and generating catastrophic outburst floods less apparent is the effect of large landslide dams on river ecosystems and marine sedimentation combining analyses of 1 m resolution topographic data acquired via airborne laser mapping and field investigation we present evidence for a large landslide dammed paleolake along the eel river ca the landslide mass initiated from a high relief resistant outcrop which failed catastrophically blocking the eel river with an approximately 130 m tall dam support for the resulting 55 km long 1 3 km3 lake includes subtle shorelines cut into bounding terrain deltas and lacustrine sediments radiocarbon dated to 22 5 ka the landslide provides an explanation for the recent genetic divergence of local anadromous ocean run steelhead trout oncorhynchus mykiss by blocking their migration route and causing gene flow between summer run and winter run reproductive ecotypes further the dam arrested the prodigious flux of sediment down the eel river this cessation is recorded in marine sedimentary deposits as a 10 fold reduction in deposition rates of eel derived sediment and constitutes a rare example of a terrestrial event transmitted through the dispersal system and recorded offshore

strataform overview of its design and synthesis of its results abstract the strata formed on continental margins record the history of both marine and terrestrial events occurring on earth the strataform program was created to develop a better understanding of relationships between these events and margin stratigraphy in order to obtain the maximum information from strata they are being examined simultaneously on various temporal and spatial scales and their formation is being studied where events and accumulation are known to be active the general objectives of strataform are to add geological perspective to studies of sedimentary processes to constrain stratigraphic interpretations better and to develop numerical models relating processes and stratigraphy this research is being undertaken on margins off northern california and new jersey which provide unique opportunities for investigation of short term and long term stratigraphy respectively this volume and paper focus upon studies undertaken off northern california north of cape mendocino this area is tectonically active with a coastal mountain range narrow shelf and input of sediment primarily from the eel river recent estimates indicate that 1 to 3 10 7 t yr of mud silt and clay are supplied to the continental shelf sand accumulates on the inner shelf 70 reaches the continental slope or deeper during the previous lowstand of sea level sediment discharge was 25 greater and probably led to hyperpycnal debris and other flows transporting sediment down the slope on the eel shelf flood and storm events combine to create normally stratified fine grained layers with basal sand and much terrestrial organic debris these layers are modified through time but some can be buried and preserved subsurface flood layers seem to impact observations of acoustic backscatter on the middle shelf and subsurface fluids gas freshwater affect electromagnetic measurements on the inner shelf budgets of eel river mud accumulation for the past century indicate 20 trapped on the adjacent shelf and 20 on the upper slope 150 600 m the remaining 60 is dispersed to more distal locations over holocene time scales eel river sediment has built a small sandy subaqueous delta and a subtle bulge of muddy sediment on the middle shelf north of the river gullies have been eroded on the open slope during lowstands of sea level and have accreted upward during highstands a massive failure has formed the humboldt slide due to earthquake activity and unstable substrates possibly from excess pore pressures and gas the tectonic setting has impacted margin stratigraphy from small scales fluid expulsion pockmarks to large scales spatial variation of late cenozoic sedimentation the broad dispersal of eel sediment demonstrates the three dimensional character of the resulting stratigraphy including the large scale clinoform structure developed on the shelf upper slope the rollover point topset foreset boundary for this feature is the shelf break i e below sea level which is important information for stratigraphic interpretations of sea level change

rapid and widespread dispersal of flood sediment on the northern california margin the dispersal of flood sediment from small river systems is a poorly studied yet potentially important aspect of active continental margin sedimentation in january 1995 during a flood with a 30 yr return period the eel river northern california delivered an estimated 25 3 106 t metric tons of fine grained 62 m sediment to the ocean the flood formed a distinct layer on the sea bed that was centered on the 70 m isobath extended for 30 km along shelf and 8 km across shelf and was as thick as 8 5 cm but contained only 6 106 t of sediment thus 75 of the flood derived sediment did not form a recognizable deposit but was instead rapidly and widely dispersed over the continental margin stratigraphic models of and compilations of sediment flux to active continental margins need to take the dispersive nature of small river systems into account

tectonic and petrogenetic implications of major and rare earth element chemistry of troodos glasses cyprus chilled pillow margins dike margins and interpillow hyaloclastites from the extrusive sequences of the troodos ophiolite cyprus reveal two compositional groups of glasses 1 andesites to dacitic andesites containing plagioclase ca rich pyroxene and ca poor pyroxene as phenocrysts occur in a stratigraphically low position and 2 basaltic andesites containing olivine and ca rich pyroxene phenocrysts occur in an upper position in the lava sequence the glasses contain between 52 and 62 wt sio2 and have mg numbers between 65 and 27 the troodos glasses are very similar to basaltic andesite or boninitic glasses from the mariana trench and together these two groups define a trend intermediate between typical calcalkaline and tholeiite evolutions the troodos glasses are inferred by difference to contain 3 4 5 wt h2o which is taken to be of primary origin and to reflect the ocean depth of extrusion and quenching based on normative projections in the basalt phase diagram there are indications that elevated water pressures during formation of the phenocryst assemblages may have shifted the isobaric pseudo quaternary peritectic towards silica quantitative crystal fractionation models fail to explain the observed chemical variation it is suggested that the chemical variation may result from high pressure melts equilibrated to lower pressure 2 and 3 phase cotectic relations some mariana and the troodos glasses have similar ree patterns with relatively low abundances depletion in lree and flattening in the hree region the close similarities between the troodos and the mariana glasses point to an identical tectonic and petrogenetic control on their origin

a holocene record of ocean productivity and upwelling from the northern california continental slope abstract the holocene upwelling history of the northern california continental slope is examined using the high resolution record of tn062 o550 40 9 n 124 6 w 550 m water depth this 7 m long marine sediment core spans the last 7500 years and we use it to test the hypothesis that marine productivity in the california current system ccs driven by coastal upwelling has co varied with holocene millennial scale warm intervals a combination of biogenic sediment concentrations opal total organic c and total n stable isotopes organic matter 13c and bulk sedimentary 15n and key microfossil indicators of upwelling were used to test this hypothesis the record of biogenic accumulation in tn062 o550 shows considerable holocene variability despite being located within 50 km of the mouth of the eel river which is one of the largest sources of terrigenous sediment to the northeast pacific ocean margin a key time interval beginning at 2900 calibrated years before present cal yr bp indicates the onset of modern upwelling in the ccs and this period also corresponds to the most intense period of upwelling in the last 7500 years when these results are placed into a regional ccs context during the holocene it was found that the timing of upwelling intensification at tn062 o550 corresponds closely to that seen at nearby odp site 1019 as well as in the santa barbara basin of southern california other ccs records with less refined age control show similar results which suggest late holocene upwelling intensification may be synchronous throughout the ccs based on the strong correspondence between the alkenone sea surface temperature record at odp site 1019 and the onset of late holocene upwelling in northern california we suggest that ccs warming may be conducive to upwelling intensification though future changes are unclear as the mechanisms forcing sst variability may differ

temporal and spatial variability of the sediment grain size distribution on the eel shelf the flood layer of 1995 abstract sediment grain size characteristics observed on the eel shelf have been analyzed using a wet sieving technique that minimizes breakage of aggregates at several sites on the 70 m isobath north of the river where a 1995 flood layer attained a maximum thickness of about 9 cm replicate box cores were collected on seven cruises during february 1995 to january 1997 these samples provide a unique opportunity to follow the evolution of a flood layer over a two year period as it was modified and gradually buried one month after the flood a layer of tan colored high porosity sediment with up to 96 of its particles in the size range of 0 20 m had accumulated on the central part of the shelf 7 30 km north of the river and principally between the 50 m and 90 m isobaths substantial coarsening of this layer occurred between february 1995 and may 1995 particularly along the southern and the landward edge of the deposit in water depths of

high resolution swath sonar investigation of sand ridge dune and ribbon morphology in the offshore environment of the new jersey margin abstract sand ridges on the northeast us atlantic shelf form in the near shore environment most likely in response to storm driven flows as the holocene transgression has continued the ridges have been transferred to an offshore hydrodynamic regime where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly here we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller scale features in an effort to place constraints on bedform development in these deeper waters a recent high resolution swath sonar survey mapped a portion of the new jersey shelf from 20 m water depth to the shelf break 120 m imaging both sand ridges and smaller scaled dunes and ribbons in far greater detail than has been previously possible using a robust statistical analysis we find that the gross morphology of ridges height width length does not change with depth beyond 20 m water depth and changes in ridge orientation generally mirror changes in regional contour orientation hence ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime however on the inner shelf 20 m water depth to the mid shelf shore we do find evidence in the ridge shape which has an asymmetry opposite to that seen near shore and in the complex backscatter response that some important modifications to ridges are taking place at these water depths in contrast on the mid shelf from the mid shelf shore to the franklin shore ridges tend to have higher backscatter at the crests implying that these are largely winnowed relict features lineated smaller scale 100 500 m wavelength

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

reconstruction of tertiary progradation and clinoform development on the new jersey passive margin by 2 d backstripping we have reconstructed the oligocene to middle miocene paleobathymetry and stratigraphy of the new jersey margin using a modified backstripping technique by analyzing the geometry of the margin through time we investigate its response to fluctuating sea level changing climate and variable sediment supply during the tertiary the reconstructions reveal a change in the margin morphology from a more steeply dipping 1 300 to 1 500 carbonate ramp in the eocene to a flatter shelf with a sharp shelf edge at present this was accomplished by an increase in the terrigenous sediment supply that filled available accommodation and caused progradation across the margin we link the increase in sediment flux with climatic cooling rather than tectonic processes the progradation is evidenced by a series of clinoforms whose formation was modulated by sea level and which extend over 100 km across the shelf the height and dip of the clinoforms increased as they extended onto the deeper parts of the earlier ramp the miocene clinoform rollovers at the new jersey margin had water depths of60 130 m and are interpreted as the edge of a new continental shelf built over the older ramp sea level fall was probably insufficient to drive the miocene shorelines past the shelf breaks thus measurements of sea level amplitude based upon coastal onlap over the clinoforms are not reliable 1999 elsevier science b v all rights reserved

modeling studies of the coastal circulation off northern california statistics and patterns of wintertime flow we conduct modeling studies of the coastal circulation off northern california in the vicinity of the eel river 40 6 n the site of the 1995 2000 strata formation on margins strataform marine geology observational program using a series of nested hydrostatic primitive equation models to aid in the understanding of the shelf flow field that receives and transports sediments during the winter floods the basic objectives of our numerical studies are to model the continental shelf and slope flow and to understand the dominant dynamical processes we simulate the shelf and slope flow surrounding cape mendocino during 100 days in winter 1996 1997 using a 3 km resolution model embedded in a 9 km resolution regional model of the north pacific ocean the naval research laboratory s pacific west coast pwc model the validity of the model simulations is assessed by comparison with current measurements from a strataform shelf tripod outfitted with an acoustic doppler current profiler and with satellite altimetry good agreement is found between the amplitude and time variability of the 3 km resolution modeled and observed alongshore currents on the shelf the 3 km resolution model outperforms the 9 km resolution pwc model using statistical maps of flow variables strong alongshore variability in wintertime flow is documented the role of the forcing supplied by wind and river runoff in establishing this flow asymmetry between regions north and south of cape mendocino is examined using empirical orthogonal functions the evolution of a robust anticyclonic eddy over the shelf and slope adjacent to cape mendocino is described the eddy forms when strong poleward winds weaken and reverse direction during winter storms the important role of alongshore pressure gradients in the shelf circulation is documented

provenance of sand on the poverty bay shelf the link between source and sink sectors of the waipaoa river sedimentary system abstract the poverty shelf north island new zealand is a segment of the waipaoa sedimentary system wss a margins source to sink focus site our petrographic analysis of sand from shelf core samples indicates that the sand fraction is mainly derived from intra and potentially extrabasinal sources but surprisingly the major fluvial system that drains into poverty bay the waipaoa river mean qfl xqyfzl is not the dominant source only one vibracore at the mouth of poverty bay contained sand qfl xqyfzl potentially derived from the waipaoa river the shelf sand mean qfl xqyfzl more strongly resembles beach sand mean qfl xqyfzl collected along the coast which is likely sourced from pervasive local cliff erosion of miocene pliocene sedimentary units that exhibit similar sandstone detrital modes mean qfl xqyfzl texturally coarser more poorly sorted and more angular sand is located along the outer shelf while finer well sorted sand characterizes the mid shelf these findings suggest a shorter transport history for the material near the outer shelf bathymetric high areas and this observation along with the composition data suggests that they were sourced by erosion of locally exposed miocene pliocene units a potential extrabasinal source of shelf sediment is indicated by anomalous prolate and equant shaped greywacke torlesse and minor red chert pebbles collected in two outer shelf box cores on the lachlan anticline these are not lithologies found within the terrestrial segment of the wss or strata comprising the outer shelf highs the clast shapes are also distinctly different from the oblate shaped pebble sized greywacke gravel clasts on beaches in hawke bay rather these sediments are more similar to torlesse stream gravel seismic and multibeam data support the possibility that during the most recent sea level lowstand the hawke bay fluvial system flowed into poverty canyon bringing these unique gravels onto what is now the poverty outer shelf the presence of the torlesse gravel is significant in that it highlights how the wss was not a closed sedimentary system in the past just as recent sedimentary research has shown in the case today

depositional processes on an ancient and modern muddy shelf northern california the modern eel river shelf and analogous pleistocene rio dell formation in northern california provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence and thereby enables further understanding of muddy shelf processes the modern shelf is the site of accumulation of a thick deposit of holocene mud both large scale sediment distribution patterns and small scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation even on a high energy storm dominated coast the major factors in preservation of this flood signature are the cohesive behaviour of fine grained sediments and episodically rapid rates of sediment input the rio dell formation includes approximately 400 m of mostly fine grained shelf deposits that accumulated offshore from a palaeo eel river mouth the shelf sediments comprise four depositional sequences sequence 1 records progradation from outer to inner shelf depths facies trends closely resemble across shelf trends on the modern shelf suggesting that processes were similar detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf muddy facies of the rio dell sequence are characterized by bioturbated clayey silts interbedded with event layers of several types clay rich silt layers are interpreted as flood deposits and physically stratified coarse silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms sediment transport calculations and consideration of grain size distributions of bioturbated sediments which form the bulk of the rio dell sequence suggest that these sediments are the result of biological homogenization of the fine grained flood deposits and of the coarser grained storm deposits the results of this study in general indicate that fine grained shelf deposits do preserve a distinguishable if subtle record of depositional processes

river plume sedimentation modeling for sequence stratigraphy application to the eel margin northern california abstract sediment deposition in the coastal zone from riverine input is of key importance in understanding the construction of sediment sequences through time a turbulent jet model is used to simulate sediment input to the continental margin from a riverine source turbulent jet models track the momentum which is the first order force carrying most of the sediment flowing into the ocean the froude number at the river mouth remains supercritical during floods of the eel river justifying the use of a jet model for the initial dispersion of the sediment laden plume sediment is deposited from the plume prior to the centerline froude number decreasing to its critical value empirically derived first order removal rate constants are used for each grain size in an advection diffusion equation to predict the areal extent of sediment deposition one strength of using a jet model is its ability to simulate deposits on a daily time step over intermediate time scales thousands of years using a limited number of input parameters a number of simulations of the eel river plume are shown for conditions of the present day and 18 000 yr bp during the last glacial maximum the model results for modern conditions show that a majority of the deposition occurs within 20 km north of the river mouth and is typically confined to within 10 km of the coastline corresponding well with the shelf sands on the eel margin however the initial mud deposit from a 1995 flood was farther offshore and northward additional advection of the mud fraction must occur after it leaves the surficial plume and prior to its initial deposition the model runs of 18 000 yr bp with 100 m lower sea level wetter and colder climate and the river mouth assumed to be at the canyon show that the plume is able to transport sand silt and clay out of the canyon and onto the continental slope

paleoenvironmental evidence for latest pleistocene sea level fluctuations on the new jersey outer continental shelf combining high resolution sequence stratigraphy and foraminiferal analysis foraminiferal assemblages in vibracored sediments within 5 m of the seafloor document high resolution sea level fluctuations and confirm the complex quaternary depositional history of the new jersey margin biostratigraphic data have been integrated with ongoing 2 d 3 d high resolution seismic analyses to interpret paleoenvironments of surficial stratigraphy beneath the modern middle to outer shelf cluster analysis of foraminiferal assemblages sampled every 20 cm average define four sample groups a d dominated by benthic species group a characterized by cibicides lobatulus is a middle to outer shelf assemblage 60 100 m water depths group b is composed of middle shelf species 30 60 m water depths and is characterized by low abundances and a fairly uniform distribution of foraminifera which suggests reworking and or a high rate of sediment accumulation group c dominated by elphidium excavatum is an inner to middle shelf assemblage 10 40 m water depths group d present in only one core and dominated by ammonia beccarii represents marginal marine environments 0 10 m water depths with fluctuations in salinity in general the modern sedimentary veneer i e the upper 20 cm is characterized by the deeper water fauna of group a sediments below these modern deposits are characterized by groups b and c reflecting shallower water depths and variable sediment accumulation rates on the inner and middle shelf taken together the group succession in the cores suggests an overall se to nw landward transgression in these latest pleistocene holocene deposits seismic analysis indicates that one core 27 penetrates the flank of a buried channel including its fill and the pre channel section within the channel three alternations of groups c and d suggest fluctuations between marginal marine and middle shelf paleoenvironments high frequency perhaps very small 10 m changes in base level are indicated using previously published ams 14c dates these fluctuations can be constrained as having occurred between 45 000 yr b p channel incision and 12 500 yr b p channel filling suggesting the non uniform nature of the last wisconsinan deglaciation

spatial variability in sedimentary processes on the eel continental slope the northern california continental margin receives episodically large quantities of sediment from the eel river which seasonally discharges to the shelf regional circulation broadcasts this sediment widely not only affecting sedimentation on the shelf but on the slope as well a suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the eel continental slope in waters 800 m deep spatial surveys of accumulation rate 0 2 1 3 g cm 2 year 1 and surficial sediment grain size 4 8 5 demonstrate that sediment is accumulating throughout the eel margin reflecting a rapid and widespread redistribution of the fluvial discharge downcore grain size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment delivery processes may be important for the slope in areas proximal to the river in more distal areas downcore profiles reveal relatively homogeneous sediments delivered dominantly by hemipelagic processes a budget for fine grained sediment shows that this upper portion of the slope contains at a maximum 20 of the river s annual discharge a combined budget for the shelf and upper slope suggests that at least 60 of the annual sediment load is not accounted for in these areas the eel continental slope because of the large terrigenous sediment input the narrow width of the adjacent shelf and its energetic environment is a good analogue for slopes during transitional stands of sea level and for modern tectonically active settings processes typically associated with these geological settings i e supply of sediment to the slope and downslope mass movement are actively occurring 1999 elsevier science b v all rights reserved

sediment dynamics during wet and dry storm events on the tet inner shelf sw gulf of lions abstract the importance of short term processes such as floods and storms on sediment delivery and reworking on the tet inner shelf was investigated the tet inner shelf is a small event dominated system located in the south western part of the gulf of lions the expected sedimentary scenario in this environment is that fluvial sediment should be deposited on the prodelta and later dispersed around the shelf and slope by wave and current reworking during storms this paper investigates differences in inner shelf sediment dynamics between storm events occurring during usual river discharges and those occurring during river floods waves current velocities near bottom water turbidity bottom sediment grain size and sea floor erosion accretion were concurrently measured at 28 m water depth on the tet inner shelf from november 2003 to march 2004 two major storms took place on 4 december 2003 and 21 february 2004 and two moderate storms occurred on 8 december 2003 and 14 march 2004 the two major storms displayed similar wave characteristics maximum significant wave height h s 7 m peak period t p 12 s and wave direction around 90 the main environmental differences during the two major storm events correspond to the amount of sediment discharged from the tet river about 2 10 4 t of sediment was delivered by the river during the 4 december flood wet storm and less than 5 10 2 t during the 21 february storm dry storm sediment dynamics were quite similar during the storm events resuspension caused by waves and sediment advection towards the southeast due to near bottom shelf currents were the dominant sedimentary processes the result was a bottom sediment erosion of several centimetres at the study site during both the wet and dry events the main differences between the wet and dry events arose after the storm immediately after the peak of the wet storm sediment supplied by the tet river and probably from other rivers was deposited around the river mouth a few days later during a moderate storm this unconsolidated sediment was resuspended transported offshore and deposited on the inner shelf in contrast a moderate storm which occurred some days after the dry february storm caused bottom erosion on the inner shelf because no fresh sediment was available on the shallower area the results from this study indicate that the tet inner shelf at 28 m water depth is mainly a bypass zone for sediment that is transported s se towards the middle outer shelf and slope although ephemeral sediment deposits may be favoured by the sediment supplied from flood events sediment transport across the shelf from the river to the slope follows a complex multi step pattern that needs to be addressed using a multi event approach

modern accumulation rates and a sediment budget for the eel shelf a flood dominated depositional environment the northern california continental margin is periodically impacted by geologically significant storms which have a marked influence on terrigenous sediment supply flood deposition and long term accumulation of fine grained sediment on the eel shelf accumulation of eel river muds on the adjacent shelf was investigated using 210pb and 137cs geochronologies in order to understand the fate of sediment discharged by the eel river and to relate patterns of net sediment accumulation 100 yr time scale to sediment dynamics 210pb data demonstrate that modern accumulation of river mud occurs from the 50 m isobath seaward across shelf accumulation rates decrease from maximum mid shelf values of 0 6 1 7 g cm 2 yr 1 to values of 0 2 0 4 g cm 2 yr 1 at the shelf break with a spatially weighted mean of 0 5 g cm 2 yr 1 0 4 cm yr for the entire shelf 210pbxs sediment depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity which are produced by flood deposition in some cores particular 210pbxs activity intervals may be associated with major eel river floods of 1955 1964 and 1974 it is postulated that because of the coincidence of high river flow events and southerly winds during cyclonic winter storms net northward transport allows for preferential deposition of fine grained sediment north of the river mouth over the past 100 years fluvial sediment input combined with marine dispersal processes have produced a mid shelf depocenter evident by both the spatial distribution of 210pb accumulation rates and by clay rich flood layers partially preserved in shelf deposits a fine grained sediment budget for the dispersal system based on hydrological data and 210pb geochronologies demonstrates that a maximum of 20 3 109 kg yr of the mean annual supply of fluvial mud 14 109 kg yr is trapped on the shelf the results of this study demonstrate that 1 short term sedimentation processes associated with floods can influence sediment accumulation on longer time scales and 2 a major fraction of fine grained sediment supplied to tectonically active margins by flood prone mountainous rivers bypasses narrow continental shelves

a study of the chemistry of pore fluids and authigenic carbonates in methane seep environments kodiak trench hydrate ridge monterey bay and eel river basin analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the kodiak trench hydrate ridge monterey bay clam flats and the eel river basin coupled with the studies of the chemistry of the pore fluids have shown that these carbonates have grown within the sediment column geochemical profiles of pore fluids show that in deep water seeps kodiak trench 4450 m monterey bay 1000 m hydrate ridge 650 m 13c dic values are low isotopically light whereas in the eel river area 350 500 m 13c dic values are much higher isotopically heavier in all cases the 13c values indicate that processes of methane oxidation associated with sulfate reduction are dominant in the shallow sediments data on the isotopic composition of authigenic carbonates found at sites in kodiak trench eel river basin south and eel river basin north indicate a variable composition and origin in different geochemical environments some of the authigenic carbonates from the study sites show a trend in their 13c values similar to those of the pore fluids obtained in their vicinity suggesting formation at relatively shallow depths but others indicate formation at greater sediment depths the latter usually consist of high magnesium calcite or dolomite which from their high values of 13c up to 23 and 18o up to 7 5 suggest formation in the deeper horizons of the sediments in the zone of methanogenesis these observations are in agreement with observations by other workers at hydrate ridge in monterey bay and in the eel river basin

hydrodynamic and suspended sediment transport controls on river mouth morphology 1 river mouths building into standing bodies of water have strikingly varied growth habits this presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands here we present a generalized 2 5 dimensional potential vorticity pv theory that explains sedimentation patterns of a sediment laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields we explore the physical meaning of this new sediment pv definition and its impact on outflow depositional patterns by analyzing data from a shallow wall bounded plane jet experiment and by discussing new theoretical insights a key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition the new pv theory constitutes a more generalized mathematical framework that expands the rouse theory for the equilibrium of suspended sediment

the persistence of memory the fate of ancient sedimentary organic carbon in a modern sedimentary system the cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales the burial of organic carbon occurs predominantly at sea in association with clay sized particles derived from the erosion of uplifted continental rocks it follows that the history of the fine grained particles on land may bear on the nature of the organic carbon buried in this study the evolution of clay associated organic matter was followed from bedrock source to the seabed in the eel river sedimentary system of northern california using natural abundance 13c and 14c tracers approximately half of the fine grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted mesozoic tertiary franciscan complex of the watershed the short residence time of friable soils on steep hill slopes coupled with rapid sediment accumulation rates on the shelf slope act to preserve the ancient organic carbon a comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope the bimodal mixture of ancient and modern c in soils and sediments may be characteristic of many short mountainous rivers if the eel river chemistry is typical of such rivers more than 40 tg of ancient organic c may be delivered to the world s oceans each year a flux of that magnitude would have a significant influence on marine and global c cycles

inversion of sonobuoy data from shallow water sites with simulated annealing an enhanced simulated annealing algorithm is used to invert sparsely sampled seismic data collected with sonobuoys to obtain seafloor geoacoustic properties at two littoral marine environments as well as for a synthetic data set inversion of field data from a 750 m water depth site using a water gun sound source found a good solution which included a pronounced subbottom reflector after 6483 iterations over seven variables field data from a 250 m water depth site using an air gun source required 35 421 iterations for a good inversion solution because 30 variables had to be solved for including the shot to receiver offsets the sonobuoy derived compressional wave velocity depth vp z models compare favorably with vp z models derived from nearby high quality multichannel seismic data there are however substantial differences between seafloor reflection coefficients calculated from field models and seafloor reflection coefficients based on commonly used vp regression curves gradients reflection l

late holocene sediment accumulation on the northern california shelf oceanic fluvial and anthropogenic influences the late holocene sedimentary record of the northern california continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux and provides perspective on the cumulative impacts of major floods and land use change in the drainage basin during historical time piston cores collected on the shelf 50 150 m water depths off the eel and klamath rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k y and dated using 137cs and 14c methods to resolve spatial patterns in sediment accumulation rates sediment accumulation rates averaged over the past several millennia display along and across shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography the highest rates 3 6 mm yr 0 4 0 8 g cm2 yr are associated with structural lows on the middle shelf whereas the lowest rates 0 2 0 6 mm yr 0 03 0 05 g cm2 yr occur at structural highs and at the shelf edge this accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf and with depocenters that form on shorter time scales in response to oceanic flood events the sediment column exhibits an upward fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the holocene transgression sedimentary event beds indicative of major river floods including two probable hyperpycnal discharges of the eel river within the past 1 k y are preserved stratigraphic evidence of accelerated sediment delivery to the shelf during historical time in the form of increased burial of fluvial mud relative to fine grained sand is superimposed on this natural record this change is concordant with the regional history of forest clearing and related land use practices in the watershed after the early 1800s and appears to record the combined impacts of timber harvesting and several extreme floods after ca a d 1950

geological geochemical and microbiological heterogeneity of the seafloor around methane vents in the eel river basin offshore california marine methane vents and cold seeps are common features along continental margins worldwide serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities the eel river basin erb located on the continental slope off northern california contains active methane vents and seep associated chemosynthetic biological communities cbc on the crests of anticlines in 520 m water depth seep related features on the seafloor have a patchy distribution and include active bubbling vents chemosynthetic clam beds and sulfide oxidizing bacterial mats methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane aom to develop a comprehensive understanding of the complex biological geochemical and physical processes associated with and influencing seafloor methane seepage a multidisciplinary approach is required here we present an integrative multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the eel river basin and the complex interactions defining the geochemistry mineralogy and microbiology within this environment

mass wasting on the mississippi river subaqueous delta abstract the mississippi river delta front mrdf is an excellent natural laboratory for studying seafloor instability and evaluating the preconditioning factors and triggering mechanisms that lead to seafloor failures the occurrence of seafloor landslides on the mrdf has been recognized since the 1950s but the bulk of research on the topic was conducted in the 1970s 80s after seafloor landslides triggered by hurricane camille damaged offshore infrastructure through these efforts it was recognized that seafloor instability features are present across the entire delta front that rapid sediment delivery from the mississippi river is a primary preconditioning factor for failures and that cyclic loading from hurricane waves is the main triggering mechanism for deep seated catastrophic events however mounting evidence shows that mrdf seafloor motion occurs even during periods of no hurricane activity which could be attributed to creep like motion of underconsolidated sediment or smaller episodic flows triggered by winter storms or mississippi river floods given the extensive spatial distribution of instabilities on the mrdf the multiple mechanisms of failure and variety of resulting morphology and the range of potential triggering mechanisms with short recurrence the mrdf is an exceptional location to assess the complex problem of seafloor landslides an improved understanding of seafloor landslides is crucial for understanding global sediment budgets and associated biogeochemical cycling as they can evacuate large volumes of sediment into deep water over short time periods additionally seafloor landslides pose a hazard to offshore infrastructure and to coastal communities through possible tsunami inundation and advancing research on preconditioning factors and triggering mechanisms would improve hazard assessments this paper presents a review and synthesis of geological and geotechnical research on the mrdf particularly with respect to seafloor landslides their occurrence geological and geotechnical controls triggering mechanisms and knowledge gaps related thereto this synthesis is conducted at times of rapidly developing technologies for studying seafloor instability and as the mrdf enters a stage of degradation characterized by reduced sediment delivery and accumulation

observations of event scale sedimentary dynamics with an instrumented bottom boundary layer tripod abstract a new approach to examine event scale sedimentation is described that establishes a direct connection between water column signals and changes in the seabed the po river italy shelf is a good case study due to the dominant impact of winter storm events on sediment transport and the availability of coincident measurements necessary for the analysis repeated deployments of an instrumented bottom boundary layer tripod at this site have produced a nearly continuous record of sediment transport over 2 5 y sediment cores were collected every 3 mo during this time and seabed inventories of 7 be half life 53 3 d were used to determine net sediment deposition or erosion during each deployment comparison of these values with the net elevation change obtained from the tripod altimeter shows good agreement varying from 2 cm deposition to 1 cm erosion the altimeter s use is extended to quantify the net seabed change that results from individual storm induced sediment resuspension events lasting from days to weeks within each event analysis of the altimeter record has proved challenging likely due to changes in sediment porosity that occur during a storm event however net erosion and deposition that occurs over an event can be evaluated and was found to be

measuring the shallow porosity structure of sediments on the continental shelf a comparison of an electromagnetic approach with cores and acoustic backscatter we validate estimates of shallow sedimentary porosities made on the continental shelf by electromagnetic methods through a comparison with those measured directly in cores specifically we show that an electromagnetic surveying system we have used to measure sediment resistivity in a number of seafloor locations returns porosity depth profiles that are related to those measured in cores and that our data have good sensitivity to the structure within the uppermost few tens of centimeters of seafloor we then consider inferences of sediment type made by observations of acoustic backscatter in the light of our porosity measurements our results suggest that electromagnetic methods provide complementary information that can be used to improve grain type estimates at and just below the seafloor especially when combined with acoustic backscatter data because the system is pulled along the seafloor data can be quickly collected from a large area of seafloor and can be used to interpolate facies conditions between core locations

acoustic backscatter of the 1995 flood deposit on the eel shelf abstract acoustic swath mapping and sediment box coring conducted on the continental shelf near the mouth of the eel river revealed regional variations in acoustic backscatter that can be related to the shelf sedimentology the acoustic backscatter variations observed on the shelf were unusually narrow compared to the response of similar sediment types documented in other areas however the acoustic data revealed four principal bottom types on the shelf that can be related to sedimentologic differences observed in cores the four areas are 1 low acoustic backscatter associated with the nearshore sand facies and the prodelta terraces of the eel and mad rivers composed of fine sands and coarse silts with low porosity 2 high acoustic backscatter associated with fine silts characterized by high porosity and deposited by the 1995 flood of the eel river 3 intermediate acoustic backscatter in the outer shelf muds where clayey silts are accumulating and the 1995 flood apparently had limited direct effect and 4 intermediate acoustic backscatter near the fringes of the 1995 flood deposits and in areas where the flood sediments were more disrupted by post depositional processes the highest acoustic backscatter was identified in areas where the 1995 flood sediments remained relatively intact and near the shelf surface into the summer of 1995 cores collected from these areas contained wavy or lenticular bedding the rapid deposition of the high porosity muddy layers results in better preservation of incorporated ripple forms than in areas less directly impacted by the flood deposit the high porosity muddy layers allow acoustic penetration into the sediments and result in greater acoustic backscatter from incorporated roughness elements

post lgm sedimentation on the outer shelf upper slope of the northernmost part of the sao paulo bight southeastern brazil abstract sedimentary and isotope data as well as 14 c ams dating on 15 box cores were used to identify the main post last glacial maximum lgm processes in the outer shelf and upper slope of the northernmost part of the sao paulo bight southeastern brazil results show slight but significant variations in calcium carbonate total sulphur and nitrogen contents as well as carbon and oxygen isotope ratios sedimentation rates varying from 5 to 184 mm kyr 1 are controlled by the shelf and upper slope morphology the brazil current meander dynamics and the coastal water offshore motion

tectonics sedimentation and erosion in northern california submarine geomorphology and sediment preservation potential as a result of three competing processes abstract on the continental margin of northern california north of the mendocino triple junction the geomorphic grain varies dramatically between the coast and the toe of the accretionary wedge this grain represents the composite signature of three competing processes tectonics sedimentation and erosion all three are active across the region but their rates and orientations vary spatially and temporally affecting the resultant grain and preservation potential in this rapid sedimentation environment at the toe of the accretionary wedge the relative rate of tectonic deformation exceeds the rate of sediment accumulation and erosion is largely limited to areally restricted but at times large mass wasting features in this region isobaths representing the geomorphic grain are parallel to structural highs and lows as in most forearcs and sediment accumulation is restricted to small basins located between anticlinal ridges the slope between the shelf break and the accretionary complex shows the competing effects of sediment accumulation erosion and tectonism slope perpendicular gullies are the prominent erosional features in this area especially in the region surrounding the trinity submarine canyon anticlinal ridges crop out on the seafloor although the ridge crests decrease in bathymetric expression both up and down slope on the shelf the rate of sediment accumulation 6 12 mm yr exceeds the relative rate of tectonic deformation with 1 3 mm yr of active uplift on land in this region the geomorphic grain is controlled by sediment accumulation with compressed but continuous stratigraphic sections above anticlinal highs and expanded sections in actively subsiding regions

sub decadal turbidite frequency during the early holocene eel fan offshore northern california remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an 100 m thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on eel submarine fan offshore northern california usa these outcrops provide a rare opportunity to connect young deep sea turbidites with their feeder system 14 c measurements reveal that from 12 8 ka to 7 9 ka one turbidite was being emplaced on average every 7 yr this emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the pacific margin of north america the turbidites contain abundant wood and shallow dwelling foraminifera demonstrating an efficient connection between the eel river source and the eel fan sink tur bidite recurrence intervals diminish fivefold to 36 yr from 7 9 ka onward reflecting sea level rise and re routing of eel river sediments

early muddy deposits along the gulf of lions shoreline a key for a better understanding of land to sea transfer of sediments and associated pollutant fluxes large quantities of sediment introduced by rivers especially during flood events lead to the formation of prodeltaic structures characterized by fine grained deposits in the proximal area of the shelf according to their mean grain size it is largely admitted in the literature that these deposits mainly stand below the storm wave base in the gulf of lions nw mediterranean however our data show that these muddy sediments also exist above the storm wave limit in the uppermost unit of the holocene prism the purpose of this work is therefore to draw a detailed sedimentological and geochemical portrait of these units and to discuss the processes involved in the sedimentation and remobilization of these sediments our results show that the fine particles accumulate earlier in the inner shelf in front of the nearby rivers in association with the chemical elements which are commonly associated with this grain size fraction e g organic carbon radionuclides and heavy metals this accumulation probably mainly occurs during storm and flood events the early deposits are then exposed to resuspension events and can actively participate to the distribution of fine grained sediment in the entire margin by advective transfers via the benthic nepheloid layer these fine grained nearshore prodeltas are a common feature of the mediterranean deltaic systems and mark the frontal processes that occur at the continent sea interface of non tidal seas our results especially point out the potential use of heavy metals as markers of the tracking of riverine sediments which is naturally an important condition when evaluating the ultimate fate of land derived pollutants in the marine realm but also more fundamental studies on the understanding of sedimentological processes and reconstruction of margin stratigraphies should consider these deposits as functional units for the distribution of platform sediments

detailed investigation of continental shelf morphology using a high resolution swath sonar survey the eel margin northern california abstract the eel shelf northern california lies within an active compressional tectonic margin subject to abundant terrigenous sediment input from the eel river a recent high resolution swath sonar survey provides us with the opportunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region our analysis of the statistical character of bathymetry demonstrates a clear separation into large and small scale morphologies at a 3 10 km scale with smaller scale morphology heavily damped relative to large scale morphology the eel shelf bathymetry is subtle but several structures can be readily discerned in a residual bathymetry formed by removal of the downslope gradient some shelf structures are evidently related to depositional processes as evidenced by correlation with 100 yr sediment accumulation rates whereas others appear related to tectonic processes as evidenced by correlation with subsurface synclines and anticlines the sidescan structure of the shelf is dominated by the low backscatter over the eel and mad river subaqueous deltas evidently associated with the sand to mud transition however contrary to usual correlations between backscatter and grain size in this situation higher backscatter is associated with the muddy sediments in addition we observe a series of shore perpendicular striations or ribbons spaced 0 2 1 0 km apart which extend northward from the eel river subaqueous delta and lie at or near the sand to mud transition some aspects of ribbon morphology suggest that they may be associated with down slope flows

shelf record of climatic changes in flood magnitude and frequency north coastal california continental shelf deposits off the eel river north coastal california document a recent increase in the magnitude and frequency of major hydrologic floods 10 yr recurrence interval the shelf record reveals a sudden three fold increase in sedimentation rate ca 1954 and a concomitant increase in the frequency of preserved flood beds comparison of sedimentary and river discharge records reveals that major floods after ca 1950 had a more pronounced effect on coastal sediment delivery and accumulation offshore than previous recorded events of similar magnitude and that stratigraphic preservation of flood events is highly dependent on flood frequency and net sedimentation rate we contend that this change in marine sedimentation is a response to documented climatic phenomena that have increased the frequency of major floods throughout the western united states during the past half century together with intrabasinal impacts of extreme floods in 1955 and 1964 anthropogenic increase in watershed sediment production is a probable secondary factor

two dimensional numerical modeling of storm deposition on the northern california shelf abstract a two dimensional across shelf sediment transport model has been developed to simulate storm deposition on continental shelves this paper describes the structure of the model and related algorithms in the model the time dependent sediment resuspension transportation and deposition and across shelf transport gradients are simulated as responses to storm waves on the eel shelf northern california the simulations show that the pattern of storm deposition on the sandy inner shelf is different from that on the muddy middle and outer shelf storm bed thickness decreases seaward across the inner shelf sandy zone as far as the 50 m isobath farther seaward storm bed thickness increases as the mud component in the sea floor becomes abundant then decreases again as the weakening of bottom wave motion with greater depth becomes the dominant control storm bed thickness in the mud zone thus has a nearly symmetrical cross shelf pattern with the maximum thickness at a water depth of 70 m the pattern suggests that while the sequence of new beds found in february 1995 on the eel shelf may have had a flood source their geometry reflects storm resuspension and transport the new muds found in these beds may have been reworked transported and redeposited several times by storm waves and other oceanic forces numerical simulations illustrate that the critical bottom shear stress for mud erosion e and the coefficient for sand resuspension 0 are more important than other parameters in determining storm depositional patterns on the eel shelf

the use of 7be to identify event and seasonal sedimentation near the po river delta adriatic sea in october 2000 a major flood event of the po river occurred the resulting seabed deposit was initially sampled in december 2000 and most sites were reoccupied on subsequent cruises over a 3 yr period using the maximum penetration depth of the short lived radioisotope 7be half life 53 3 d the flood deposit was found to be up to 15 cm thick individual depocenters of thick strata were located immediately adjacent to the distributaries at the river mouth in relatively shallow water 30 m these flood deposit thicknesses are minimum estimates because the depth of 234th half life 24 1 d exceeds that of 7be in physically stratified flood sediment indicating that the first sediment deposited during the flood event likely originated from the river channel 30 55 of the estimated sediment load delivered to the shelf during the flood event can be accounted for using 7be penetration depths seasonal deposition also can be quantified using 7be after removing the effects of biological mixing seasonal deposition rates are a maximum of 6 cm yr 1 near the pila distributary decreasing to 2 cm yr 1 in the southern portions of the dispersal system

near bottom suspended sediment variability caused by storms and near inertial internal waves on the ebro mid continental shelf nw mediterranean abstract near bottom currents and suspended sediment concentrations ssc recorded on the ebro continental shelf at 60 m depth during the transition between fair and stormy weather have revealed some of the major processes controlling the sediment re suspension and transport at mid shelf depths in this nw mediterranean area storm waves were the main contributor to the transport of sediment although they only resuspended surface sediments during extreme wave conditions h s 4 5 m and t s 11 s during the recording period the net suspended sediment flux along the shelf was one order of magnitude higher than the net suspended sediment flux across the shelf and the resultant transport was towards the sw and seaward fluctuations of the near bottom ssc during fair weather showed a periodicity around 17 h and were related to the occurrence of near inertial internal waves caused by wind pulses during storm events the energy propagation of these internal waves reached critical angles at the mid and outer shelf which could favor the development and maintenance of the bottom nepheloid layer on the ebro continental shelf as well as on other mediterranean continental shelves

flood dispersal and deposition by near bed gravitational sediment flows and oceanographic transport a numerical modeling study of the eel river shelf northern california 1 a large flood of the eel river northern california created a thick sediment deposit between water depths of 50 and 70 m in january 1997 the freshwater plume however confined sediment delivery to water depths shallower than 30 m mechanisms proposed to explain the apparent cross shelf transport include dispersal by oceanographic currents resuspension by energetic waves and gravitationally forced transport of a thin layer of fluidized mud field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location size and timing of deposition approximately 30 of the sediment delivered by the eel river is accounted for in the midshelf mud bed and inner shelf but the fate of the remaining 70 is uncertain a three dimensional hydrodynamic model was used to examine potential mechanisms of sediment transport on the eel river shelf the model includes suspended sediment transport and was modified to account for a thin near bed layer of fluidized mud it was used to simulate flood dispersal on the eel river shelf to compare the relative importance of transport within the near bed fluid mud layer to suspended sediment transport and to evaluate sediment budgets for floods settling properties of fine grained sediment both within the flood plume and the fluid mud layer critically impact depositional patterns to a lesser degree wind driven ocean currents influence the volume of sediment that escapes the shelf and wave magnitude affects the cross shelf location of flood deposits though dilute suspension accounts for a large fraction of total flux cross shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume location and timing to those seen offshore of the eel river

seasonal interannual and mesoscale variability of the black sea upper layer circulation derived from altimeter data 1 topex poseidon and ers altimeter data comprising the period from may 1992 to may 1999 are assimilated into a shallow water model for providing a dynamically consistent interpretation of the sea surface height variations and estimation of the temporal and spatial characteristics of the upper layer circulation in the black sea these 7 year long observations offer a new capability for interpretation of major transient and quasi permanent features of the upper layer circulation the instantaneous flow fields involve a complex eddy dominated system with different types of structural organizations in which the eddies and the gyres of the interior cyclonic cell interact continuously among themselves and with meanders and filaments of the rim current the circulation possesses a distinct seasonal cycle whose major characteristic features repeat every year with some year to year variability an organized two gyre winter circulation system disintegrates gradually into a series of interconnecting eddies in the summer and autumn months which are also characterized by more pronounced and complex mesoscale activity in the peripheral flow system our analyses suggest a revised schematic circulation picture of the major quasi permanent and recurrent elements of the black sea

suspended sediment fluxes and transport processes in the gulf of lions submarine canyons the role of storms and dense water cascading contemporary suspended sediment transport was studied in seven submarine canyons of the gulf of lions gol current meters equipped with turbidity sensors were moored 4 m above bottom at 300 m depth in the canyon axis from november 2003 to may 2004 sediment transport events were monitored and studied in relation to forcing conditions there was a large flood in early december during which discharges from all of the coastal rivers increased by more than one order of magnitude a smaller flood of the rhone river occurred later in mid january followed by a persistent high river discharge that lasted until mid february there were also several e se storm events during the measurement period two of them causing large swell one in early december max hs 8 4 m coinciding with the major river flood andoneinlatefebruary maxhs 7m duringaperiodoflowerriverdischarge mostofthevariabilityindown canyoncurrentspeeds waslinkedtostrongdownwellinginducedbye sestormsandtocascadingofdenseshelfwaterinducedbynandnwwinds theintensity and timing of these processes strongly varied spatially eastern storms generated higher waves and induced stronger downwelling in the western than in the eastern sector of the gol shelf dense water cascading events were enhanced during eastern storms and they were also more intense in the western sector of the gol these events occurred frequently from january to may along the western canyons and from february to april along the eastern canyons from february to april they occurred simultaneously in all the canyons sediment transport was mainly down canyon and mostly concentrated during storm induced downwelling and dense water cascading events during the high river discharge season most of the newly supplied and resuspended sediment remained stored on theshelf however duringthelatefebruarystormandcascadingevent thestoredsedimentwasquicklyresuspendedandtransported mainly through the westernmost submarine canyon cap de creus following a flushing pattern later although minor storms and shelfdensewatercascadinghardlyincreasedsuspendedsedimentconcentrationswithinthecanyons theyinducedstrongnear bottom current velocities increasing down canyon sediment fluxes and generating small but significant sediment transport events most of the shelf canyon transfer took place during the late february storm and cascading event through the cap de creus canyon where the net suspended sediment flux was between one and two orders of magnitude higher than in the other canyons

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

sediment transport events on the northern california continental shelf abstract a long term monitoring tripod has been maintained in 60 m water depth at the northern end of the strataform study site on the northern california continental shelf as part of this ongoing study tripod data for 1 year beginning 24 september 1995 are used to provide a sediment transport analysis on an event by event basis the objective of this paper is to highlight the energetic nature of this shelf region in terms of the frequency duration and magnitude of sediment suspension events and the associated particle flux analyses are based on measurements from two current meters and two optical backscatterance sensors located at 30 and 100 cm above the bed data from these instruments and a pressure sensor were averaged over 7 5 min every hour during the year of record 41 distinct sediment suspension events occurred i e sustained suspended sediment concentrations greater than 20 mg l above background level at 100 cm above bed suspended sediment events were associated with significant wave activity tidal currents and river discharge the average duration of a sediment suspension event was 3 1 days and varied from 0 7 to 8 days during events mean suspended sediment concentrations of 110 mg l were observed with peak hourly observations exceeding 1000 mg l concentrations between events averaged 35 mg l the overall sediment flux for the period of record was directed seaward and southward the distribution and magnitude of sediment suspension events and particle flux showed a strong seasonality for example of the sediment transport during events 98 of the net along shelf and 73 of the net across shelf sediment transport occurred in the winter three major winter storms could account for 72 of the total along shelf transport but only 10 of the across shelf transport the across shelf flux was more evenly distributed over the year and occurred as a result of mean currents and very low frequency seaward flows associated with mesoscale circulation patterns

regional variability of slope stability application to the eel margin california abstract relative values of downslope driving forces and sediment resisting forces determine the locations of submarine slope failures both of these vary regionally and their impact can be addressed when the data are organized in a geographic information system gis the study area on the continental margin near the eel river provides an excellent opportunity to apply gis spatial analysis techniques for evaluation of slope stability in this area swath bathymetric mapping shows seafloor morphology and distribution of slope steepness in fine detail and sediment analysis of over 70 box cores delineates the variability of sediment density near the seafloor surface based on the results of ten geotechnical studies of submarine study areas we developed an algorithm that relates surface sediment density to the shear strength appropriate to the type of cyclic loading produced by an earthquake strength and stress normalization procedures provide results that are conceptually independent of subbottom depth results at depth are rigorously applicable if sediment lithology does not vary significantly and consolidation state can be estimated otherwise the method applies only to shallow seated slope failure regional density slope and level of anticipated seismic shaking information were combined in a gis framework to yield a map that illustrates the relative stability of slopes in the face of seismically induced failure when a measure of predicted relative slope stability is draped on an oblique view of swath bathymetry a variation in this slope stability is observed on an otherwise smooth slope along the mid slope region north of a plunging anticline the section of slope containing diffuse pockmarked gullies has a lower measure of stability than a separate section containing gullies that have sharper boundaries and somewhat steeper sides such an association suggests that our slope stability analysis relates to the stability of the gully sides the remainder of the study area shows few obvious indications of slope instability except for a feature that has become known as the humboldt slide but it is too deep seated to be amenable to the slope stability prediction techniques presented herein in general few slope failures have been mapped in the eel margin study area despite the high level of seismicity the relatively high rates of sediment accumulation and the extent of gas charging observed by others

the influence of sea level and tectonics on late pleistocene through holocene sediment storage along the high sediment supply waipaoa continental shelf abstract we present geophysical and core evidence showing how subsidence caused by forearc shortening has accommodated late pleistocene and holocene sediments supplied to the tectonically active waipaoa shelf nz limiting off shelf export during the early sea level highstand the last glacioeustatic fall and subsequent rise exposed and then flooded a shelf segmented into subbasins separated by zones of uplift leaving key stratigraphic markers of shoreline regression and transgression that vary strongly in character across the shelf highstand sediment isopachs tied to piston cores dated using tephra correlation and a radiocarbon age model provide a sediment budget at 2000 yr intervals from the mid holocene 5500 cal yr bp to present sediment load estimates from our shelf budget are in agreement with published model estimates for suspended sediment discharge from the waipaoa river for the past 3000 yr but importantly do not show the 6 fold increase in the waipaoa s sediment output that began with human settlement 700 yr ago and accelerated with deforestation over the last century bypassing of waipaoa sediment to the slope may therefore be a recent phenomenon caused by unnaturally high sediment loads a conclusion supported by data reported elsewhere in this volume our study also reveals evidence for 1 a relatively thick mid shelf transgressive section deposited during the last eustatic rise that may correlate to estuarine sequences reported from numerous sites on the modern coastline of the north island 2 a slight decrease in total basin filling rates during the highstand and 3 variability in the partitioning of highstand sediments between individual subbasins that may reflect differing degrees of tectonic accommodation

estimates of suspended sediment flux and bedform activity on the inner portion of the eel continental shelf energetic waves strong bottom currents and relatively high rates of sediment discharge from the eel river combined to produce large amounts of suspended sediment transport on the inner continental shelf near the eel river during the winter of 1995 1996 bottom boundary layer bbl measurements at a depth of 50 m using the geoprobe tripod showed that the strongest near bottom flows combined wave and current speeds of over 1 m s and highest sediment concentrations exceeding 2 g l at 1 2 m above the bed occurred during two storms one in december 1995 and the other in february 1996 discharge from the eel river during these storms was estimated at between 2 and 4 103 m3 s suspended sediment flux ssf was measured 1 2 m above the bed and calculated throughout the bbl by applying the tripod data to a shelf sediment transport model these results showed initially northward along shelf ssf during the storms followed by abrupt and persistent southward reversals along shelf flux was more pronounced during and after the december storm than in february across shelf ssf over the entire measurement period was decidedly seaward this seaward transport could be responsible for surficial deposits of recent sediment on the outer shelf and upper continental slope in this region sediment ripples and larger bedforms were observed in the very fine to fine sand at 50 m depth using a sector scanning sonar mounted on the tripod ripple wavelengths estimated from the sonar images were about 9 cm which compared favorably with photographs of the bottom taken with a camera mounted on the tripod the ripple patterns were stable during periods of low combined wave current bottom stresses but changed significantly during high stress events such as the february storm two different sonic altimeters recorded changes in bed elevation of 10 to 20 cm during the periods of measurement these changes are thought to have been caused principally by the migration of low amplitude long wavelength sand waves into the measurement area

the high frequency backscattering angular response of gassy sediments model data comparison from the eel river margin california a model for the high frequency backscatter angular response of gassy sediments is proposed for the interface backscatter contribution we adopted the model developed by jackson et al j acoust soc am 79 1410 1422 1986 but added modifications to accommodate gas bubbles the model parameters that are affected by gas content are the density ratio the sound speed ratio and the loss parameter for the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment we treat the bubbles as individual discrete scatterers that sum to the total bubble contribution this total bubble contribution is then added to the volume contribution of other scatters the presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth the backscatter response of fine grained gassy sediments is dominated by the volume contribution while that of coarser grained gassy sediments is affected by both volume and interface contributions in deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected in shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased multibeam data acquired offshore northern california at 95 khz provides raw measurements for the backscatter as a function of grazing angle these raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model the analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that when compared to the model show general agreement

eel river margin source to sink sediment budgets revisited abstract the eel river coastal margin has been used as a representative source to sink sediment dispersal system owing to its steep high sediment yield river and the formation of sedimentary strata on its continental shelf one finding of previous studies is that the adjacent continental shelf retains only 25 of the eel river fine grained sediment less than 63 m discharged over time scales of both individual floods and the 20th century thus suggesting that the eel shelf trapping efficiency is uniquely lower than other similar systems here i provide data and analyses showing that sediment discharge relationships in the eel river have varied strongly with time and include substantial decreases in suspended sediment concentrations during the latter 20th century including these trends in margin wide sediment budgets i show that previous eel river sediment discharge rates were overestimated by a factor of two thus revised sediment budgets shown here reveal that the eel shelf retained 50 of the discharged river fine grained suspended sediment during intensively sampled events of 1995 97 and over the 20th century in light of this hypotheses about high rates of sediment export away from the primary shelf depocenter should be reevaluated

high resolution climate of the past 7300 years of coastal northernmost california results from diatoms silicoflagellates and pollen piston core tn062 o550 collected about 33 km offshore of eureka california contains a high resolution record of the climate and oceanography of coastal northernmost california during the past 7 34 kyr chronology established by nine ams ages on a combination of planktic foraminifers bivalve shell fragments and wood yields a mean sedimentation rate of 103 cm kyr 1 marine proxies diatoms and silicoflagellates and pollen transported by the nearby eel river reveal a stepwise development of both modern offshore surface water oceanography and coastal arboreal ecosystems beginning at 5 4 cal ka the relative abundance of coastal redwood pollen a proxy for coastal fog displays a two fold increase suggesting enhanced coastal upwelling a decline in the relative contribution of subtropical diatoms at 5 0 cal ka implies cooling of sea surface temperatures ssts at 3 6 cal ka an increase in the relative abundance of alder and oak at the expense of coastal redwood likely signals intensified riverine transport of pollen from inland environments cooler offshore ssts and increased precipitation characterize the interval between 3 6 and 2 8 cal ka a rapid stepwise change in coastal climatology and oceanography occurs between 2 8 and 2 6 cal ka that suggests an enhanced expression of modern pacific decadal oscillation like pdo cycles a three fold increase in the relative abundance of the subtropical diatom fragilariopsis doliolus at 2 8 cal ka appears to mark an abrupt warming of winter ssts soon afterwards at 2 6 cal ka a two fold increase in the relative abundance of coastal redwood pollen is suggestive of an abrupt intensification of spring upwelling after 2 8 cal ka a sequence of cool warm pdo like cycles occurs wherein cool cycles are characterized by relative abundance increases in coastal redwood pollen and decreased contributions of subtropical diatoms whereas opposite proxy trends distinguish warm cycles

origin and morphosedimentary evolution of fine grained modern continental shelf deposits the gironde mud fields bay of biscay france the gironde mud fields are thin blankets 4 m that extend from the inner to mid shelf over a restricted area 600 km 2 and overlie palimpsest sands and gravels seaward of the modern gironde estuary south western france the bulk of the mud has accumulated over the last two millennia and reflects the complex interactions of 1 an increasing estuarine supply of suspended particulate matter 2 resuspension of substrate material by storm events and 3 biological mixing the combination of these processes has produced various features within the deposits related to 1 the water depth and distance from source of the fine grained material which are manifested as a proximal distal cross shelf trend from an area of alternating mud sedimentation temporary after large river flows with resuspension by storm events to an area of mud accumulation in deeper water where bioturbation predominates 2 expansions of the mud depocentres over time such that they become more remote from sources of coarser shelf sediment resulting in an upward fining trend the lithofacies of the gironde shelf mud fields exhibit a number of similarities to those of larger scale modern muddy shelves and ancient shallow water mudrocks this work provides a detailed description of a facies model for fine grained shelf sediments deposited under relatively constant sea level

geochemical stability of fine grained silicic holocene tephra in iceland and scotland this paper tests two assumptions fundamental to the use of fine grained silicic tephras from iceland for creating chronological frameworks in northwest europe it is shown firstly that glass shards can retain their overall chemical integrity on at least a four millennial time scale in contrasting depositional environments in iceland and scotland and secondly that an acid digestion process the most practical method for extracting tephra from peat does not significantly bias the results of major element analysis by electron microprobe the implication is that there is great potential and an appropriate method for developing both the resolution and the spatial coverage of chronologies based on silicic icelandic tephras in northwest europe

estimating river sediment discharge to the ocean application to the eel margin northern california abstract hydrotrend a synthetic river discharge numerical model can simulate the natural variability daily annually and longer in the flux of water and sediment delivered to the coastal ocean the numerical model is designed to make discharge predictions based on climate even though field measurements of river flow are not available provided that appropriate assumptions are made regarding past climate the model can predict how a river may have behaved in the geological past sediment load is determined from a river s rating coefficients when applied to the flood dominated eel river northern california hydrotrend captures the intermonthly and interannual variability of discharge and loads over a century or more hydrotrend is applied to the ice age paleo eel river when sea level was lower and climate much cooler and wetter the paleo eel river at 18 000 year bp would see its hinterland runoff increase but not the magnitude of the peak discharges the post dicted sediment load of the paleo eel river was 25 greater than today s although basin precipitation was 45 greater and the hinterland area 4 greater through sea level lowering a method is provided to obtain sediment discharge rating coefficients for unmonitored modern rivers and paleo rivers the relationship q s g 1 2 h 3 2 a 1 2 provides for an estimate of a river s sediment load q s to within a factor of 3 over 7 orders of magnitude from knowledge of basin relief h and basin area a rating coefficients are determined from this estimate of q s and the power law rating curve q s aq b 1 where q is discharge