the importance of bioturbation to continental slope sediment structure and benthic processes off cape hatteras north carolina abstract even though the continental slope off cape hatteras has sediment accumulation rates on the order of 1 cm year large areas of soft sediment are intensively reworked by infaunal organisms primary sedimentary structures have been completely replaced with biogenic structures surface sedimentary structures are dominated by the bioturbational activities of a deep burrowing infauna to at least 30 cm the layer actively mixed by the benthos as evidenced by sediment profile and x ray images is estimated to range from 5 to 20 cm with the residence time for particles within the surface mixed layer ranging from about 4 5 to 18 years the biological mixing parameter g ranges from 0 4 to 5 5 which indicates moderate to strong biological mixing relative to accumulation and strata formation bioturbation contributes to the dynamic forces affecting the surface sediments by decreasing compaction of sediment layers and dilating sediment fabrics by sediment mixing and introducing large water filled burrows and voids to subsurface sediments the sediment profile images captured numerous subsurface feeding voids and worms in the process of making deep burrows many of which extended below the 5 to 10 cm average depth of the apparent color redoxpotential discontinuity layer high rates of accumulation of organic rich sediment lead to high standing stocks of benthos and intensive feeding burrowing activity that result in organic rich stratagraphic sequences that are thoroughly mixed cape hatteras is a apparent focusing point for the transport of shallow water sediments to the deep sea sediments across other areas of the continental slope just 100 s of kilometers south of cape hatteras are not as thoroughly mixed or biologically active

observations regarding coarse sediment classification based on multi beam echo sounder s backscatter strength and depth residuals in dutch rivers this contribution investigates the behavior of two important riverbed sediment classifiers derived from multi beam echo sounder mbes operating at 300 khz data in very coarse sediment environments these are the backscatter strength and the depth residuals four mbes data sets collected at different parts of rivers in the netherlands are employed from previous research the backscatter strength was found to increase for increasing mean grain sizes depth residuals however are often found to have lower values for coarser sediments investigation of the four data sets indicates that these statements are valid only for moderately coarse sediment such as sand for very coarse sediments e g coarse gravel the backscatter strength is found to decrease and the depth residuals increase for increasing mean grain sizes this is observed when the sediment mean grain size becomes significantly larger than the acoustic wavelength of the mbes 5 mm knowledge regarding this behavior is of high importance when using backscatter strength and depth residuals for sediment classification purposes as the reverse in behavior can induce ambiguity in the classification

paleoclimatic implications of holocene plant remains from the sierra bacha sonora mexico abstract a total of 93 plant taxa were identified from 11 packrat neotoma sp midden samples from the sierra bacha on the coast of the gulf of california near puerto libertad sonora mexico nine indurated samples have radiocarbon dates ranging from 9970 to 320 yr b p sonoran desertscrub was present on rocky slopes throughout the holocene early holocene assemblages dominated by fouquieria columnaris boojum tree reflect vegetation and climate more like modern baja california with greater winter rainfall and cooler summers middle holocene vegetation was essentially modern with modest indications of greater monsoonal rainfall even though cold water upwelling locally inhibits summer precipitation the results are similar to all previous midden reconstructions of early and middle holocene climates in the sonoran desert but contradict general atmospheric circulation model simulations

estimations of heat transfer from grotto s north tower a neptune observatory case study abstract the overall heat transfer through an active hydrothermal sulfide edifice in particular the north tower of grotto has been estimated at 80 mw or greater based on the following in situ measurements and assumptions 1 the heat transfer by diffuse flow is estimated at 33 380 mw based on extrapolating the acoustically mapped area to all sides of the north tower visible area 30 m 2 extrapolated area 100 m 2 and using the range of available spot measurements of temperature 6 23 c and vertical velocity 0 07 0 28 m s the lower number 33 mw is more likely but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain 2 the heat transfer by focused flow is estimated at 30 70 mw based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges 3 conductive heat transfer out of the mound is unknown but is likely to be much less than the advective heat transfer additionally the plume transport of heat is estimated at 20 40 mw based on the direct measurement of temperature within the plume at 5 25 m above the top of the edifice despite uncertainties the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow furthermore not all plumes from individual smokers may merge even for so small an area as the north tower of grotto

simple equations for the speed of sound in neptunian waters two simple equations for the speed of sound in sea waters down to depths of 4 km have been obtained as approximations to a more precise equation developed by lovett the simpler of the two is valid for all neptunian waters except the red sea mediterranean sea and persian gulf and agrees with lovett s equation to within about 0 1 m s the other includes these water masses and is in agreement with lovett to within about 0 03 m s the two equations are well adapted to programmable handheld calculators

development of vegetation in the central mojave desert of california during the late quaternary vegetation analysis of 47 neotoma packrat middens from the core of the central mojave desert of california reveal changes in desert plant community composition over the last ca 24 000 years bp one of the lowest and most arid locations in north america habitat currently dominated by mojave desert larrea tridentata creosote bush was occupied prior to ca 11 500 years bp by pinus monophylla pinon juniperus osteosperma utah juniper purshia mexicana bitterbush cercocarpus ledifolius mountain mahogany and prunus fasciculata desert almond woodland above 1000 m p monophylla juniperus woodland was widespread over the southern and central mojave desert however less than 150 km north of the central mojave j osteosperma steppe dominated the landscape paleoecologic records spanning latitude 358n ca 378n document a biogeographic boundary between milder moister environments to the south and the colder drier environments to the north at elevations b1000 m desert scrub species of lycium cooperi wolfberry salvia mojavensis mojave sage and the mesophytic prosopis juliflora mesquite occurred near the shore of pluvial lake mojave from ca 8810 to 8430 years bp larrea is not recorded on the xeric carbonate substrates 605 m until ca 6990 years bp then took over 2000 years to dominate the regional landscape modern vegetation associates such as thamnosma montana desert rue and atriplex confertifolia shadscale were established by ca 4500 years bp late holocene vegetation changes document neoglacial cooling and wetter conditions than today between ca 3490 and 2640 years bp paralleling the regional pluvial lake highstands and marsh deposit records decreases of mesophytic mojave desert associates in the granite mountains suggest climates similar to today s aridity between ca 1970 and 1440 years bp mesophytic species increase in middens

epibenthic megacrustaceans from the continental margin slope and abyssal plain of the southwestern gulf of mexico factors responsible for variability in species composition and diversity abstract the community structure of megacrustaceans orders lophogastrida isopoda and decapoda collected in trawls on the continental margin upper slope and abyssal plain of the southern gulf of mexico was studied to determine to what extent broad scale variation in community composition and diversity was influenced by geographic regions environmental variability and depth trawls were collected in the mexican ridges the campeche bank and the sigsbee abyssal plain there was variability in species composition density and diversity among geographic regions and along the depth gradient a total of 106 species were identified and grouped in three orders five infraorders 40 families and 70 genera this study extends the known geographic ranges of the species homolodromia monstrosa and ephyrina benedicti the largest number of species was recorded in the mexican ridges and on the upper continental shelf lower values were found on the continental margin and in the abyssal plain the largest densities were recorded on the continental margin in the mexican ridges megacrustaceans show in general low frequencies and low abundances in trawls characterizing them as rare components of benthic assemblages contrary to an accepted paradigm about deep sea biodiversity the highest h diversity values were recorded in the sigsbee abyssal plain followed by values from the upper continental slope diversity values were correlated with evenness canonical redundancy analysis results showed a significant affinity to regions for 18 crustacean species 33 species showed a significant affinity to both regions and depth zones within regions

acoustic sensing of particles suspended by wave bottom interactions abstract the inability to measure the temporal and spatial distributions of particles suspended by the action of combined waves and currents is a limiting factor in obtaining quantitative data to test the various theoretical or empirical models which have been hypothesized to explain the process of particle suspension in a beach estuary or continental shelf environment an experiment has been conducted in a wave tank to examine the feasibility of using a 1 mhz acoustic system to study the temporal dependence and spatial distribution of sand grains raised from the tank bottom during the passage of waves of known period and amplitude data showing the acoustic detection of fine to medium sized sand grains suspended by surface gravity waves are presented to illustrate the suitability of using acoustic systems to study the particle suspension process acoustic systems used in conjunction with instruments such as current meters and reynolds stress measuring devices will allow a more complete understanding of wave current bottom interactions and the associated suspension of sediments

234th and 21pb evidence for rapid ingestion of settling particles by mobile epibenthic megafauna in the abyssal ne pacific particle scavenged 234 th and 210 pb can be used to trace the fate of particulate matter reaching the deep sea floor we used this technique to demonstrate rapid ingestion of particles arriving at the sea floor at a depth of 4 100 m by mobile epibenthic holothuroids abyssocucumis abyssorum and oneirophanta mutabilis excess 234 th and 210 pb activities of sediment trap material detrital aggregates from the sea floor and animal gut contents all were similar while activities of surface sediments top 0 5 mm were considerably lower a simple calculation using the excess 210 pb concentration of two potential food sources sediment trap material and surface sediments shows that 91 of gut content material of a abyssorum must have come from material similar to that found in the sediment trap cups by setting the sediment trap material as age 0 apparent ages of 100 d and 12 13 d were estimated for the aggregates surface sediments and a abyssorum guts respectively the population of a abyssorum at this site could potentially process 0 2 4 of the vertical mass flux m 2 d 1 during the high particle flux period between june and october 1994 the percentage of vertical flux processed could be substantially greater when all of the mobile epibenthic megafauna species are considered

classification of seafloor geology using multibeam sonar data from the scotian shelf abstract we describe a method for classifying multibeam sonar data and illustrate the method using data collected with a simrad em1000 sonar on the scotian shelf canada the method involves comparing various attributes of the bathymetry and backscatter with typical examples of specified seafloor types and computing their statistical degree of similarity sediment ponds are identified as areas of low echo amplitude and low topographic gradient and curvature ridges and troughs are identified by fitting a paraboloid to patches of the bathymetry once the sediments ridges and troughs are located we are able to use the database to extract orientations and other characteristics of these features for example the orientation of the topography can be computed from the paraboloid surface and rose diagrams of the ridge and trough orientations reflect the fabric of up turned cambro ordovician sedimentary beds outcropping in this area the unconsolidated sediments ponded within small basins have low topographic gradients with consistent tilts towards the southeast mean 133 which we interpret as due to offshore transport of holocene sediments possibly in response to storms

the path to covis a review of acoustic imaging of hydrothermal flow regimes abstract acoustic imaging of hydrothermal flow regimes started with the incidental recognition of a plume on a routine sonar scan for obstacles in the path of the human occupied submersible alvin developments in sonar engineering acoustic data processing and scientific visualization have been combined to develop technology which can effectively capture the behavior of focused and diffuse hydrothermal discharge this paper traces the development of these acoustic imaging techniques for hydrothermal flow regimes from their conception through to the development of the cabled observatory vent imaging sonar covis covis has monitored such flow eight times a day for several years successful acoustic techniques for estimating plume entrainment bending vertical rise volume flux and heat flux are presented as is the state of the art in diffuse flow detection

bioturbation in the abyssal arabian sea influence of fauna and food supply abstract in order to evaluate bioturbation in abyssal arabian sea sediments of the indus fan profiles of 210 pb half life 22 3 yr and 234 th half life 24 1 d were measured in cores collected during september and october 1995 and april 1997 respectively the density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during april 1997 mean eddy diffusive mixing coefficients according to the distribution of excess 210 pb 210 pb db were 0 072 0 028 0 068 0 055 0 373 0 119 0 037 0 009 and 0 079 0 119 cm2 yr 1 in the northern western central eastern and southern abyssal arabian sea respectively mean eddy diffusive mixing coefficients according to the distribution of excess 234 th 234 th db were 0 53 1 64 and 0 47 cm2 yr 1 in the northern western and central abyssal arabian sea respectively mobile epibenthic megafauna at the western northern central and southern study sites were dominated by ophiuroids holothurians ophiuroids and natant decapods the respective densities were 100 82 29 and 6 individuals 1000 m 2 the northern study site was characterized by a high abundance of spoke traces and fecal casts the central site showed spoke traces and many tracks the southern site displayed the highest abundance of spoke traces whereas at the western site hardly any lebensspuren were observed there is evidence for at least two functional endmember communities in the arabian sea in the northwestern arabian sea wast vertical particle displacement seems to be dominated by macrofauna and primarily eddy diffusive in the southern arabian sea sast non local and incidental mixing due to spoke trace producers might become more important and superimpose reduced eddy diffusive mixing with respect to biological data cast is an intermediate location given the biological data average 210 pb db is higher and decimeter scale variability of 210 pb db smaller at cast than expected these findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity as reflected by 210 pb db and reduces decimeter scale 210 pb db heterogeneity in comparison to the simple sum of the isolated effects of the endmembers for time scales 210 pb db and 234 th db bioturbation intensity should be controlled primarily by the composition of the benthic fauna its specific adaptation to the environmental setting and the abundance of each species of the benthic community food supply can have only an indirect influence on bioturbation intensity in certain parts of the ocean the a priori overall positive relationship between poc flux and biodiffusivity might include restricted intervals displaying no or even negative relations

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 remote sensing of benthic activity a statistical approach acoustic backscatter at 40 khz was recorded from a circular area of silt 75 m in radius every 0 1 d at a water depth of 91 m off the coast of northern california for 49 d in winter 1988 1989 data were resolved into pixels that extended 7 5 m radially and 5 azimuthally we analyzed the relatively small change in backscatter intensity and phase from a given pixel in a time series analysis this change in backscatter was decomposed into three components putatively due to large rare transient nekton and epibenthos more abundant benthopelagic organisms that caused volume reverberation and more sedentary benthos benthic change in backscatter was presumed due to movement of animals and other biogenic modifications of sound speed in the sediment water medium patterns of benthic activity and volume backscatter near the seabed changed abruptly twice with the middle interval showing highest levels during this middle interval winter storms disturbed surficial sediments offshore transport occurred near the seabed and the benthic component was dominated by activity perfectly in phase with and linearly proportional to median local downwelling irradiance we suspect that this circadian pattern was due to the burrowing urchin brisaster latifrons that is known from previous studies at this site to emerge on the sediment surface at night when food resources are available there and to reburrow during the lighted part of the day extensive correlative physical data collected at the site as part of interdisciplinary studies code and stress allowed us to exclude other physical forcings as likely explanations for diurnal change in backscatter in a spatial analysis the transient component was removed explicitly pixel by pixel and volume reverberation was removed implicitly to leave a benthic remainder that could be analyzed for spatial variation benthic activity showed distinct patchiness with coherence scales of roughly 5 10 m and negative spatial autocorrelation in activity levels near the largest spatial lag available 140 m patches did not move appreciably during the study

acoustic mapping of diffuse flow at a seafloor hydrothermal site monolith vent juan de fuca ridge diffuse flow of hydrothermal solutions commonly occurs in patchy areas up to tens of meters in diameter in seafloor hydrothermal fields it is recognized as a quantitatively significant component of thermal and chemical fluxes yet is elusive to map we report a new acoustic method to detect and map areas of diffuse flow using phase coherent correlation techniques the sonar system was modified to record phase information and mounted on dsv sea cliff the submersible occupied a stationary position on the seafloor and the transducer scanned the seafloor surrounding monolith vent a sulfide edifice venting black smokers at a nominal range of 17 m at a depth of 2249 m on the juan de fuca ridge patchy areas of uncorrelated returns clearly stood out from a background of returns that exhibited ping to ping correlation the areas of uncorrelated returns coincided with areas of diffuse flow as mapped by a video survey with the navy s advanced tethered vehicle atv correlated returns were backscattered from invariant seafloor uncorrelated returns were distorted by index of refraction inhomogeneities as they passed through diffuse flow between the seafloor and the transducer the acoustic method presented can synoptically map areas of diffuse flow when combined with standard in situ measurement and sampling methods the acoustic mapping will facilitate accurate determination of diffuse thermal and chemical fluxes in seafloor hydrothermal fields

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

the effects of fine scale surface roughness and grain size on 300 khz multibeam backscatter intensity in sandy marine sedimentary environments abstract high frequency acoustic backscatter intensity that is returned from sediment is affected by a number of seabed characteristics including sediment grain size distribution the nature and magnitude of seabed surface roughness and volume scattering by subsurface scatterers in order to develop quantitative remote sediment classification techniques it is important to determine the relative and combined effects of the dominant variables to which the acoustic signal is sensitive data used in this study were collected at two sandy sites off the south shore of long island new york and include 300 khz multibeam simrad em 3000 backscatter intensity data as well as grab samples and stereo bottom photographs multivariate statistical analyses were used to investigate the relationships between grain size properties seabed roughness and backscatter intensity at a number of sample locations at each study site encompassing a moderate range of backscatter intensities from 32 5 to 24 db the results of this study demonstrate that the relative importance of variables that affect backscatter intensity varies at different sites suggesting that backscatter intensity alone cannot be used to quantitatively predict seabed characteristics however our study indicates that in sandy sediments the median grain size the standardized magnitude of seabed roughness and the sorting of sediment can together be used to predict 300 khz backscatter intensity with high significance and that the relative importance of these variables varies from site to site we present an empirical model derived from a combined dataset that includes two sites of similar sedimentary and hydrodynamic conditions to predict the magnitude of seabed roughness based on backscatter intensity and sediment grain size characteristics the spatially coherent patterns in predicted seabed surface roughness revealed through the use of this kind of model greatly improve our understanding of variations in backscatter intensity in sandy marine sedimentary environments as a result the multivariate statistical approach that we employ improves our overall ability to remotely assess the details of the sedimentary environment that are significant with respect to benthic habitats and physical processes and are applicable to other geophysical datasets including sidescan sonar data

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

storm dominated bottom boundary layer dynamics on the northern california continental shelf measurements and predictions measurements of near bottom velocity profiles in 85 m water depth during a storm on the continental shelf off northern california using the geoprobe tripod in december 1979 provided estimates of shear velocities u and roughness lengths z0 when the near bottom velocity profiles were logarithmic these estimates agree within 90 confidence intervals with values computed from a simple near bottom combined wave current model that includes movable bed effects the reasonably good comparison between model and profile estimates of u suggests that such models can be used to predict bed shear stresses or shear velocities under combined flows of waves and currents typical of stormy conditions on continental shelves if stratification corrections to the velocity profile resulting from suspended sediment are small the repeated occurrences of storms of similar intensities and wind velocities during the winter months off northern california suggest that the resultant high bottom stresses due to the combined effects of waves and currents are major factors in controlling the distribution of surficial sediment on the central portion of the northern california shelf

late holocene vegetation changes in greenwater valley mojave desert california abstract small scale late holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the greenwater valley northern mojave desert california although the site is above the modern lower limit of coleogyne ramosissima black brush macrofossils of this shrub are only present in samples younger than 270 yr b p in order to measure changes more subtle than presence vs absence macrofossil concentrations were quantified and principal components and factor analyses were used to distinguish midden plant assemblages both the presence absence data and the statistical analyses suggest a downward shift of 50 to 100 m for coleogyne blackbrush communities between 1435 and 1795 a d

high frequency scattering from saturated sand sediments measurements of bottom scattering strengths were made with a multi frequency echo sounder mounted on a tower on a sandy bottom off west destin fl data were measured at five frequencies in the range 265 1850 khz at subcritical grazing angles ranging from 9 to 20 and at azimuths up to 90 around the tower scattering strength increased at about f1 4 up to 400 khz peaked between 700 and 1100 khz and decreased at higher frequencies

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

seafloor characterization and backscatter variability of the almeria margin alboran sea sw mediterranean based on high resolution acoustic data abstract an acoustic study of the almeria margin eastern alboran sea sw mediterranean was carried out by means of an integrated dataset comprising deep towed tobi sidescan sonar swath bathymetry topas high resolution seismics and sediment gravity cores the almeria margin is a complex system dominated by volcanic and tectonic features shaped by recent sedimentary processes in which a regional hemipelagic sedimentation is intercalated by sporadic gravitative events the aim of this work is twofold the first objective is to recognize the sedimentary and tectonic features that shape the almeria margin the main morpho sedimentary and morpho structural elements observed are the almeria canyon channel turbidite system the dalias tributary valley system landslides active faults and folds and volcanic banks the second aim is to explore and quantify the surface and subsurface seafloor parameters that characterize the backscatter of the study area sedimentary geomorphological and biological elements that play a role in the acoustic signature of the almeria margin were detected nevertheless it should be noted that seafloor acoustic backscatter is also affected by geometrical and instrumental factors despite the wide variability of the environments studied a low average acoustic backscatter reflects a diffused draping of hemipelagic sedimentation in the area higher backscatter values were observed at the top of some volcanic banks along the almeria canyon and in the shallower sector of the dalias tributary valley system the influence of subseafloor properties in the acoustic signature of the area was revealed by backscatter grain size correlations which were carried out for different depth intervals in sediment cores collected in the almeria turbidite system a poor relationship was found between backscatter and superficial silty sediments of the area whereas a higher correlation resulted in the upper 50 cm the presence of subsurface turbidites in the cores associated with the higher backscatter values suggests volume interface scattering of these sandy layers as a controlling factor of the acoustic signature obtained in the almeria margin on the other hand in rough settings such as the ones observed in the dalias tributary valley system topographic relief of up to 1 1 5 m strongly interacted with the tobi acoustic pulse this suggests that large scale roughness is an additional parameter that characterizes the acoustic strength of the area extremely high reflective patches distributed along some of the volcanic banks chella and pollux banks for depths ranging from 230 to 470 m coincide with areas where carbonate cold water coral mounds develop in the tobi images coral facies reach the maximum value of acoustic backscatter which is probably related to the rough morphology of coral ecosystems the position of coral mounds in the banks suggests that the occurrence of strong bottom currents and reduced sedimentary inputs are environmental factors that favour their settling and development in the almeria margin

isotopic and geochemical evidence of palaeoclimate changes in salton basin california during the past 20 kyr 1 δ18o and δ13c records in lake tufa deposits abstract investigation of 18 o and salt content of surface waters in the salton basin california shows that there is a strongly positive correlation between 18 o values and salt contents and that the 18 o of salton sea is much heavier than that of its input waters due to strong evaporation using a steady state 18 o model we have discussed the 18 o of lake cahuilla an ancient lake in the basin under through flowed closed and overflowed situations an ams 14 c dated stable isotope record of a 60 cm lake tufa slice lc 1 from the ancient shoreline of lake cahuilla indicates that the lake experienced closed and or overflowed conditions between 1 300 and 20 500 yr bp the variations in 18 o and 13 c values of lc 1 ranging from 8 52 to 2 50 pdb and from 0 8 to 3 76 respectively reflect changes in the relative humidity in salton basin and variations in the colorado river inflow and hence climatic conditions in the colorado river drainage basin the tufa record exhibits palaeoclimates of salton basin as follows 1 relatively dry climate between 20 and 18 kyr bp was followed by a wet climate between 18 and 17 kyr bp 2 decreasing effective moisture precipitation evaporation from 17 to 16 kyr bp corresponded to heinrich 1 event whereas effective moisture increased at ca 15 5 kyr bp during the trans us wet period contrasting patterns between the lc 1 record and the greenland ice core and chinese speleothem records during the bolling allerod interstadial from 15 2 to 11 7 kyr bp may be caused by age uncertainty of the lc 1 record further study of this interval is needed 3 wet warm early holocene corresponding to the maximum effective moisture period perhaps brought about by the strengthening of north american monsoon 4 toward 2 kyr bp the climate became dryer apparently resulting from weakening of the monsoons especially around 6 2 and 2 5 kyr bp 5 a wet regime occurred in the basin during the medieval warm period

acoustic imaging of structures in the upper 10 cm of sediments using a megahertz backscattering system preliminary results abstract a rapid in situ bottom survey technique is needed to provide information about sedimentary structures boundary roughness and macrofaunal distributions in estuarine continental shelf and deep sea sediments we have made preliminary studies using ultrasound imaging as a possible means of providing this kind of information our laboratory studies have shown that a high frequency acoustic backscattering system operated at 1 6 mhz can image the distribution of laminations and tubes in muddy sediments we compare the acoustic images with radiographs taken from the same sections of the test sediment to demonstrate the accuracy and resolution of the acoustic record this acoustic technique could be adapted to a towed sled vehicle to provide in situ high resolution 1 cm subbottom 0 10 cm profiles of sedimentary fabrics and possibly biological structures

moored wind temperature and current observations made during coastal ocean dynamics experiments 1 and 2 over the northern california continental shelf and upper slope moored observations of winds currents and temperature made over the northern california shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the coastal ocean dynamics experiment code are described in both field experiments code 1 and code 2 the onset of the upwelling season is triggered by the strengthening of the north pacific high off northern california and signalled by the presence of water colder than 9 c near the bottom over the shelf vertical temperature stratification over the shelf is reduced during active upwelling local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction and the mean wind stress is directed equatorward response of alongshelf currents to wind forcing is similar at all depths over the shelf the response of the cross shelf current is more complex in the surface layer offshelf flow is significantly and positively correlated with the upwelling favorable alongshelf wind stress beneath this layer a weak return flow exists the thickness of the upper mixed layer varies greatly reaching 60 m during upwelling events thus direct wind forcing affects a substantial fraction of the water over the shelf low frequency wind stress current and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure the most energetic mode typically accounts for 75 or more of the variance of wind stress and temperature over the middle and inner shelf the most energetic current mode accounts for 90 or more of the variance is dominated by he alongshelf component and veers to the left with depth the low frequency alongshelf flow fluctuations are quasi barotropic with some vertical shear but no reversal with depth vertical shear in alongshelf current and the cross shelf density gradient obey the thermal wind relation the simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross shelf transects in code 2 volume transport over the shelf is often nearly two dimensional but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope correlation of low frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf where a 15 cm 1 alongshelf flow lags a 1 dyn cm 2 alongshelf wind stress by the order of 10 hours currents over the upper slope are not significantly correlated with local wind alongshelf transport over the shelf is highly correlated with local wind stress explaining 80 of the variance in the low frequency transport during code 2 volume transport is best correlated with wind stress approximately 30 km to the south winds in the code area during spring and summer are characterized by strong and persistent upwelling favorable winds interrupted by shorter periods of weak winds regression analysis indicates that in the absence of wind a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the code area an upwelling favorable wind counters this jet over the shelf so that the alongshore flow is everywhere equator ward during strong wind events since the maximum response to wind forcing is found at midshelf and near surface the mean current over the upwelling season is strongly sheared in both the vertical and cross shelf directions mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward

detection of diffuse seafloor venting using structured light imaging 1 currently we lack a systematic and remote method for locating and quantifying diffuse seafloor venting using underwater robotic vehicles diffuse flow is characterized by both low temperature and low flux rates which cannot readily be distinguished using current remote visual acoustic or vehicle mounted environmental sensors the result is a poor understanding of the distribution contribution and context of diffuse flow sources an underwater structured light imaging system also used for high resolution seafloor bathymetric mapping has however shown promise in detecting diffuse flows while completing seafloor imaging surveys at a typical altitude of 3 m the system creates sequential bathymetric profiles by imaging a laser line projected on the seafloor in the presence of venting fluids the laser line exhibits a detectable level of distortion due to variations in the refractive index along the optical path by characterizing the degree of distortion it is possible to create maps indicating areas of potential venting with sub meter spatial resolution results from three distinct vent fields are presented and discussed analysis of these data sets indicates this system is capable of detecting both small point source vents and near bottom diffuse flow

shelf construction in a foreland basin storm beds shelf sandbodies and shelf slope depositional sequences in the upper cretaceous mesaverde group book cliffs utah the upper cretaceous campanian kenilworth member of the blackhawk formation mesaverde group is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of north america analysis of this section at a combination of small sedimentological and large stratigraphical scales reveals the dynamics of progradation of a shelf slope sequence into a subsiding foreland basin four major lithofacies are present in the upper mancos and kenilworth beds of the book cliffs a lag sandstone and channel fill shale lithofacies constitutes the thin basal transgressive sequence which rests on a marine erosion surface it was deposited in an outer shelf environment shale interbedded sandstone and shale and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave dominated delta and its flanking strand plains prograded seaward analysis of grain size and primary structures in kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies the fine to very fine grained graded strata of the interbedded facies are tempestites deposited out of suspension by alongshelf storm flows geostrophic flows there is no need to call on cross shelf turbidity currents density underflows to explain their presence very fine to fine grained hummocky strata are likewise suspension deposits created by waning storm flows but were deposited under conditions of more intense wave agitation on the middle shoreface cross strata sets in this region are bed load deposits that accumulated on the upper shore face in the surf zone lag strata are multi event bed load deposits that are the product of prolonged storm winnowing they occur on transgressive surfaces while the graded beds are tempestites in the strict sense all four classes of strata are storm deposits the distribution of strata types and their palaeocurrent orientations suggests a model of the kenilworth transport system driven by downwelling coastal storm flows and probably by a northeasterly alongshore pressure gradient the stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth but also because of progressive sorting which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments the kenilworth member and an isolated outlier the hatch mesa lentil constitute a delta prodelta shelf depositional system their rhythmically bedded lenticular sandstone and shale successions are a prodelta shelf facies and may be prodelta plume deposits major upper cretaceous sandstone tongues in the book cliffs are underlain by erosional surfaces like that beneath the blackhawk formation which extend for many tens of kilometres into the mancos shale these surfaces are the boundaries of upper cretaceous depositional sequences the sequences are large scale genetic stratigraphic units they result from the arranging of facies into depositional systems the depositional systems are in turn stacked in repeating arrays which constitute the depositional sequences the anatomy of these foreland basin sequences differs

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

rapid bioturbation in equatorial pacific sediments evidence from excess 234th measurements in order to evaluate bioturbation in equatorial pacific sediments profiles of excess 234th were measured in cores collected during november and december 1992 at six abyssal sites along the jgofs equatorial pacific 140 w transect from 5 s to 9 n excess 234th half life 24 1 days was detected at all sites mean inventories ranged from a low of 0 25 dpm cm 2 at 9 n to a high of 2 6 dpm cm 2 at 2 n eddy diffusive models of the excess 234th profiles suggest that on 100 day timescales particle mixing is slow at 5 s and 2 s eddy diffusion coefficient db 0 4 cm2 year 1 faster at 0 and 2 n db 0 34 4 8 cm2 year 1 and fastest at 5 n db 4 7 11 cm2 year 1 at 9 n 234th is found almost entirely in the upper 5 mm of the seabed and db values are 0 1 cm2 year 1 db values generally increase as poc flux increases although the linear relationship r2 0 334 p 0 365 displays considerable scatter however if the only sites included in the regression are those where 234th profiles were measured in at least two core sets i e 0 2 n 5 n and 9 n the linear regression improves substantially r2 0 754 p 0 0012 these results suggest that measurements on a single core may not fully characterize the sediment mixing regime in a particular region and highlight the need for multiple samples from an individual site thorium 234 db values at 5 s 2 s and 9 n are in rough agreement with published db values calculated from the profiles of intermediate half life radioisotopes such as 210pb and 137cs in open ocean abyssal sediments however at 0 2 n and 5 n 234th db values are as much as an order of magnitude greater than published 210pb db values this phenomenon has been observed in other marine environments and may result from age dependent mixing non steady state mixing or a combination of the two processes particle mixing in cores collected at 5 s and 5 n appears to be best described as a largely eddy diffusive process however in cores collected at 2 s 0 2 n and 9 n non local mixing processes appear to affect 15 30 of the excess 234th inventory at these sites non local mixing appears to transport recently deposited material at least 3 4 cm below the sediment water interface within 100 days of its arrival at the seafloor the combination of rapid biodiffusive mixing and non local transport allows recently deposited reactive biogenic debris to be rapidly mixed into the seabed this may affect a wide range of diagenetic processes including oxygen consumption dissolution of biogenic debris and organic carbon remineralization models of early diagenesis in deep sea sediments may need to incorporate both non diffusive transport terms and rapid biodiffusive mixing of recently deposited particles

process related classification of acoustic data from the hudson river estuary acoustic surveying provides valuable information on the distribution of sedimentary environments in subaqueous settings frequently differences in acoustic backscatter strength are used to distinguish sedimentary environments on the basis of their grain size composition here we are presenting acoustic backscatter data from the haverstraw bay section of the hudson river estuary that has been mapped using sidescan sub bottom profiling and multibeam bathymetry as part of the benthic mapping project of the hudson river estuary program funded by new york state in addition hundreds of gravity cores and grab samples provide ground truth for a classification of the river bottom into discrete substrate types analyses of sedimentary environments reveal patterns in backscatter strength beyond those that can be related to the sediment grain size distribution alone an integrated interpretation of sidescan sub bottom profiling and high resolution bathymetry data indicates that the backscatter pattern can be attributed to spatial variations in the modern depositional environments which cause differences in bottom roughness and sediment compaction based on an integrated interpretation of the acoustic and sample data sets we distinguished eight different sedimentary classes many of the classes can be linked to dynamic processes including contemporary deposition erosion and sediment migration in sand waves the results provide a better understanding of the dynamic processes of the hudson river estuary and improve the interpretation of the acoustic backscatter data from fine grained sedimentary environments

the effects of biological and hydrodynamic processes on physical and acoustic properties of sediments off the eel river california abstract the spatial trends in surficial sediment macro and microstructure and the resultant values of sediment physical and geoacoustic properties are controlled by the water depth dependent interplay of biological depositional and hydrodynamic processes along shore normal 25 100 m water depth and shore parallel 70 m contour transects north of the eel river northern california values of sediment compressional and shear wave speed porosity bulk density mean grain size and shear strength cluster within reasonably well defined surficial sediment facies inshore sands 21 42 m water depth offshore muds 90 100 m flood deposits 57 80 m and transition sediments between flood deposits and inshore sands 47 57 m statistical relationships among seafloor impedance calculated from in situ and laboratory measurements or measured remotely by acoustic methods sediment physical properties and geoacoustic properties are sufficiently robust to allow prediction with confidence the high local spatial variability in values of sediment physical and geoacoustic properties reflects the variability in macro and microstructure exhibited in x radiographs and ct imagery large scale distribution of sedimentary facies is controlled by flood deposition from the eel river and subsequent remobilization by hydrodynamic processes high local variability in sediment properties occurs where no single process is consistently dominant transition sediments the high variability of seafloor properties in flood deposits reflects the high structural heterogeneity associated with the interaction among numerous flood deposits resuspension and redeposition of flood deposits by surface gravity waves and mixing by bioturbation faunal reworking of sediments rapidly

rates of sediment reworking at the hebble site based on measurements of th 234 cs 137 and pb 210 abstract th 234 cs 137 and pb 210 measurements have been made on ten cores from the hebble site on the nova scotian continental rise th 234 mixing coefficients from hebble sediments show substantial lateral variability with values ranging from 1 to 33 cm 2 yr 1 these mixing coefficients are one to two orders of magnitude greater than values from typical continental rise and abyssal sediments because of high densities of benthic macrofauna th 234 data from hebble cores indicate that particles at the sediment water interface are mixed to depths of 1 5 cm on a 100 day time scale cs 137 and pb 210 data indicate that on time scales of 30 100 yrs surface sediments are reworked to depths ranging from 1 to 12 cm based on th 234 profiles from two hebble cores collected less than 200 m apart during consecutive years no temporal variability in mixing rate could be resolved in some hebble sediments there is good agreement between the th 234 mixing coefficient 100 day time scale and the pb 210 mixing coefficient 100 yr time scale in other hebble cores however th 234 data indicate that the pb 210 profile has not reached steady state if steady state solutions are used to model these non steady state profiles the calculated pb 210 mixing coefficient may underestimate the true mixing coefficient by as much as an order of magnitude

velocity structure in the izu bonin seismic zone and the depth of the olivine spinel phase transition in the slab in this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes the method is an extended arrival time difference atd scheme that incorporates a realistic model several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the atd technique the errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three dimensional ray tracing routine in general the comparisons suggest that these errors are usually much less than 0 1 s the inverse method is tested on the same set of hypothetical data the results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event the arrival time differences are relatively insensitive to the other parameters describing the slab strike dip half width and the position of the master event which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab convergence of the inverse routine is rapid in the tests usually less than five iterations are required before a stable solution is reached it appears that coupling between solutions for slab parameters and ray directions is insignificant unlike the standard atd approach solutions for hypocenters and velocities are not directly coupled in this scheme although well located events are required to insure that the hypocenters are not unduly biased by variations in structure once the velocity structure of the slab is determined that information can be used routinely by this scheme to relocate less well recorded events this method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the izu bonin subduction zone structure in the 200 300 km depth range could not be resolved and results in the 500 600 km depth range proved to be too sensitive to noise data from events between 27 n and 31 n show that p wave velocities within the slab from 400 to 500 km depth are 4 6 higher than those of the ambient mantle between 300 and 400 km depth the slab structure varies regionally north of about 32 5 n the p wave velocity within the slab is 3 4 higher than ambient from 180 to 375 km depth in contrast the velocities south of 32 5 n increase from 6 7 higher than ambient in the 375 410 km depth range to 8 10 higher in the 325 375 km depth range and appear to be 9 higher than ambient in the 270 325 km depth range the determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model above 325 km depth however the velocity determinations are somewhat dependent on the choice of starting model as a result of this dependence no upper bound on the depth of the high velocities could be resolved a plausible explanation for the exceptionally high p wave velocities within the slab south of 32 5 n is that the olivine spinel phase change boundary is elevated in this part of the slab while north of 32 5 n it is not significantly elevated it is not known if the phase change is elevated south of 31 n but the velocities between 400 and 500 km depth in this region imply that it is not depressed the high velocity zone south of 32 5 n lies in an area that marks the beginning of a seismic gap increasing in breadth to the south between deep and shallow seismicity the dip of the slab also becomes gradually steeper to the south of this region in contrast the lower velocities north of 32 5 n occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane if the seismic gap south of 32 5 n represents a region of high velocity then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine spinel phase boundary the decreasing width of the gap to the north suggests that the evolution of the izu bonin seismic zone has been governed by a propagating instability from the south

time series measurements of macrobenthos abundance and sediment bioturbation intensity on a flood dominated shelf abstract four 70 m stations on the continental shelf offshore from the eel river northern california were occupied at roughly four month intervals between february 1995 and march 1998 and in august 1999 at each of the stations profiles of excess 234 th were used to quantify sediment bioturbation intensity in addition at two of the stations macrofaunal abundance species composition and functional groupings were quantified during the study period the eel river displayed a range of hydrological conditions with historically significant floods in january 1995 and january 1997 return periods of 15 and 40 y respectively relatively low flows during the winters of 1995 1996 and 1998 1999 and an el nino year characterized by moderate but frequent discharges in 1997 1998 the january 1995 and 1997 floods deposited 3 7 cm of fine grained high porosity sediment with high c n ratios and a terrestrial organic carbon signature at the study sites the following general questions are addressed herein 1 how do macrofaunal abundance species composition and functional groupings vary over time 2 does the sediment deposition following the january 1997 flood constitute a major disturbance to the eel shelf macrobenthos 3 how does sediment bioturbation intensity vary in time space and what are the main factors controlling this variation the eel shelf macrofauna is strongly dominated by subsurface deposit feeding polychaetes with anomalously low abundances of surface deposit feeders and virtually no suspension feeders among the community dominants the abundance data revealed a clear seasonal pattern with peak density 4 5 10 4 m 2 in the fall and a factor of two lower density in the late winter spring 2 10 4 m 2 within this seasonal context there was little evidence for extraordinary mortality caused by the january 1997 flood in that overall wintertime mortality and the mortality of most community dominants during a year 1995 1996 when there was no flood deposition were comparable to the mortality observed following the january 1997 flood in contrast the depth distribution of the macrofauna revealed a distinctive post flood pattern whereby a majority 55 70 of individuals were temporarily found at depths 4 cm this pattern suggests an active response by the resident fauna to sediment deposition and supports the idea that the floods did not cause a widespread disturbance although there may not be clear evidence for short term flood effects the overall species composition and functional groupings do imply that the sedimentary environment high sediment accumulation rates and abundant terrestrial organic matter has had a long term influence on the eel shelf macrofauna model fits to 75 profiles of excess 234 th show that in general the data are consistent with a steady state biodiffusive model the resultant mixing intensities ranged from 3 to 325 cm 2 y 1 with averages standard deviation of 35 33 24 19 37 35 and22 9 cm 2 y 1 at stations c70 i70 l70 and o70 respectively the average biodiffusivity for all stations and times was 29 25 cm 2 y 1 n 62 due to the large amount of variability which is consistent with other continental margin studies it was not possible to detect significant spatial or temporal variability although there is a hint of higher mixing intensities during the late summer early fall the period of maximal carbon flux to the seabed correlations between total macrofaunal abundance and mixing intensity are notably poor whereas a slightly better correlation r 2 0 22 was obtained between the abundance of large animals and bioturbation intensity by explicitly considering organic carbon flux or some measure of seabed food resources and the abundance of larger organisms it may be possible to predict bioturbation intensity better in future studies although the pervasive small scale variability detected on the eel river shelf warrants in depth theoretical and experimental consideration

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

spatial heterogeneity of benthos associated with biogenic structures on the north carolina continental slope abstract the objective of this study was to determine if biogenic features such as mounds pits and tubes produce small scale 0 1 100 m spatial heterogeneity in macrofaunal community structure on the continental slope off north carolina at 850 m macrofaunal and microbial communities associated with sediment mounds pits and level areas were compared off cape lookout north carolina no significant differences were found in sediment microbial counts or total macrofaunal distributions one paraonid polychaete levensenia gracilis was more abundant in pits than in the other samples and infaunal anemones exhibited depressed densities in sediment mounds at a second site off cape hatteras north carolina infaunal heterogeneity associated with the tube building foraminiferanbathysiphon filiformis was examined by comparing an area with high tube densities 93 8 m 2 with an area 100 m away without tubes no significant differences were found in the distribution and abundances of bacteria between the two areas the only significant difference found in infaunal densities was the presence of high numbers of reproductive oligochaetes in the 5 10 cm fraction beneath tube beds oneterebellid polychaete species nicolea sp which lives exclusively onb filiformis tubes was absent in the non tube area with a few exceptions the biogenic structures examined at these two sites appeared to exert only minor influence on macrofaunal or microbial community structure within each site slope assemblages examined in this study appeared to be homogeneous on the small scales examined

high frequency bottom backscatter measurements in shallow water bottom backscatter measurements have been made at six shallow water sites over the frequency range 20 85 khz for bottom types including silt sand and gravel bottom samples were taken at all experimental sites and some sites are well characterized in terms of seabed acoustic parameters and roughness a slight frequency dependence was observed and scattering strength was generally found to decrease as grazing angle decreased backscatter measurements in four azimuthal directions showed no measurable anisotropy for grazing angles between 10 and 40 spatial variability of backscatter strength was measured along two tracks approximately 35 and 75 km long

benthic pelagic coupling in the greenland norwegian sea and its effect on the geological record the sedimentation pattern of organic material in the greenland norwegian sea is reflected in the surface sediments although less than 0 5 of the organic matter is buried in the sediment maximum fluxes and benthic responses are observed during june and or august september following the pattern of export production in the pelagial zone the annual remineralization rate on the voring plateau is 3 0 g c m 2 a 1 freshly settled phytodetritus as detected by chlorophyll measurements is rapidly mixed into the sediment and decomposed it stimulates the activity of benthic organisms especially foraminifera the mixing coefficient for this material is db 0 2 cm2 d 1 which is two to three orders of magnitude higher than that estimated from radiotracer methods the effect on the geological record however is likely to be small chlorophyll containing particles are at first very evenly distributed on the seafloor after partial decomposition and resuspension a secondary redistribution of particles occurs which can result in the formation of a high accumulation area with an up to 80 fold increase in the sedimentation rate by lateral advection this is mainly due to physical processes because biodeposition mediated by benthic animals increases sedimentation by only a factor of two or three

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

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

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

experimental tests for particle size dependent bioturbation in the deep ocean the potential for particle size dependent bioturbation rates was experimentally tested at 1 240 m in the santa catalina basin eastern pacific spherical glass bead tracers in five size classes 816 17 31 32 62 63 125 and 126 420 m were spread over the sediment surface and tube cored 997 d later downcore concentrations of glass beads were enumerated in each of the five size categories and page s l test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer in all cores the null hypothesis was rejected finer tracers penetrated deeper into the sediment in two of the three cores vertical biodiffusivities were computed from concentration profiles of downcore tracers these also showed size dependence with biodiffusivities ranging from 1 cm2 yr for the 8 1 6 pm fraction to 0 1 cm2 yr for the 1 25 420 pm size class these data demonstrate that vertical bioturbation rates are particle size dependent in santa catalina basin the likely cause is preferential ingestion and downward transport of fine particles by deposit feeders

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

photographs of deep sea lebensspuren a comparison of sedimentary provinces in the venezuela basin caribbean sea abstract twenty five types of lebensspuren were identified and quantified from sites representative of the three major sedimentary provinces in the venezuela basin caribbean sea only seven types were unique to any one site most of the lebensspuren have been found elsewhere in the deep sea and some have identities as ichnogenera densities of lebensspuren and megafauna were inversely correlated suggesting that the relative abundances of functional groups may be more important in determining densities of lebensspuren than overall megafaunal densities diversities of lebensspuren and megafauna were positively correlated lebensspuren attributable to sedentary and mobile animal activities were identified and compared among sites percent organic carbon content of sediment and the ratio of sedentary to mobile types were positively correlated suggesting that mobile deposit feeding activity predominates when organic carbon is low mean densities of total lebensspuren and the ratio of sedentary to mobile features were positively correlated suggesting that mobile deposit feeding activity tends to smooth out seafloor microrelief while sedentary deposit feeding helps maintain and accentuate it in the absence of hydrodynamic influences at the deep sea floor as the case with the provinces studied here densities of lebensspuren are maintained in a steady state by rates of destruction and production of these features by various sediment reworking activities of animals

numerical modeling of gravity driven sediment transport and deposition on an energetic continental shelf eel river northern california 1 a two dimensional numerical model was applied to predict large scale deposition by wave supported sediment gravity flows on the eel river continental shelf for four consecutive flood seasons using measured bathymetry waves and river forcing the model assumes that sediment induced stratification maintains the near bed richardson number at its critical value which determines the sediment carrying capacity of the wave boundary layer deposition is predicted when the gravity driven flux of sediment exceeds the carrying capacity the model predicted 26 of fine sediment discharged by the eel river to be deposited on the midshelf with a magnitude and distribution largely consistent with field observations greatest deposition on the midshelf was predicted well north of the river mouth despite greater sediment input nearest the river mouth model results indicate that when the river delivers sufficient sediment to critically stratify the wave boundary layer wave intensity and the bathymetry of the eel shelf are the dominant factors controlling the observed pattern of deposition large wave energy caused the majority of fine sediment 65 to escape the shelf as gravity driven flows the greatest amount of sediment was predicted to leave the shelf from the region off shelf of the river mouth including 11 into the eel canyon where inshore sediment input was high and the concave downward bathymetry associated with the eel river subaqueous delta prevents significant midshelf gravity driven deposition

impact of local morphology on sedimentation in a submarine canyon rov studies in eel canyon northern california u s a abstract despite existing paradigms about marine sedimentation some submarine canyons are receiving large amounts of sediment during the present highstand of sea level these modern examples can be used to unlock secrets of past canyon sedimentation however submarine canyons have complex morphology and consequently dramatic variations in sedimentary processes and deposits can occur over a range of spatial scales as small as meters to tens of meters operations from surface ships usually cannot place sampling devices on the canyon seabed with this level of accuracy the purpose of the present study was to investigate the variability of sedimentation over a range of scales in order to delineate accurate trends along and among channels at the head of a canyon sampling with the rov ventana facilitated detailed examination of microenvironments i e wall thalweg in eel submarine canyon the combination of intense sedimentation from nepheloid layers and gravity flows in a complicated morphologic system leads to clear distinctions between microenvironments as well as some recognizable and unifying trends in sedimentation inherent small scale variability due to canyon morphology is evident in narrow and steep channels at 1 m horizontal resolution cores exhibit consistent sediment fabric but laminae can differ between cores at the 10 m horizontal scale the fabric is not consistent broader more gently sloping channels reveal consistent sediment fabric at the 10 m scale in most cases physical stratification decreases along the thalwegs of channels as water depths increase in contrast channel walls generally exhibit elevated impacts of bioturbation and variable amounts of physical stratification radiochemical profiles 210pb 137cs and the predominance of physical stratification including discrete laminae with high sand content suggest that the northern entrants are receiving more sediment than their southern counterparts however 210pb profiles in this study demonstrate rapid accumulation of sediment everywhere in the head of eel canyon with the highest rates of accumulation 40 mm yr found in the channel thalwegs in thalwegs evidence from sedimentary structures e g erosional bases cross bedded sand layers demonstrates that gravity flows occur frequently many times each year and have been linked by other investigations to storm impacts on the adjacent continental shelf on decadal timescales these processes are temporarily depositing sediment in the head of the canyon which is removed over longer timescales probably as larger gravity flows triggered by earthquakes the radiochemical and sedimentological data collected at the base of entrant channels confirm that modern sediment is moving to deeper portions of the canyon

the july 1996 flood deposit in the saguenay fjord quebec canada implications for sources of spatial and temporal backscatter variations in july 1996 a major rainstorm and flood took place in the saguenay fjord backscatter strength measurements made with a simrad em1000 multibeam echosounder in 1993 1997 and 1999 have shown spatial and temporal variations which are interpreted in relation to the occurrence of the flood after empirical calibration of the different maps the data show an overall diminution of 5 db in backscatter strength in 1997 1 yr after the flood took place while the data sets obtained in 1993 and 1999 show comparable levels of backscatter strength the different data sets show a similar pattern of low and high backscatter patches which represent variations of the backscatter strength of a few decibels several grain size and water content measurements were also carried out on sediment box core and grab samples from the fjord bottom in 1997 and 1999 these have shown that the areas with higher acoustic backscatter correspond to the finer sediments while the low backscatter patches correspond to the coarser material the data show little relation between water content and backscatter strength thus indicating a poor dependence between the impedance terms bulk density and sound speed and the backscatter having taken this into account the major contribution to backscatter strength is assumed to result from differences in surface and volume roughness of the sediment since finer grain sizes offer higher backscatter the grain size of the sediment is not considered as being a major contributor to roughness and hence backscatter strength the major factor that seems to control roughness generation in the saguenay fjord is considered to be bioturbation the areas where geological processes most physically disturb the bottom which coincide with the areas where also most accumulation takes place and the coarser grain sizes are deposited are more sparsely colonized by organisms this results in a lower degree of bioturbation lower roughness and thus backscatter strength this hypothesis helps explain the variation in backscatter strength observed between the different measurement years the several million tons of sediment deposited in the fjord bottom after the 1996 flood buried the benthic fauna which was still recovering 1 yr later the less bioturbated sediment at this time would present lower roughness with respect to the 1993 and 1999 data resulting in a lower backscatter strength

multibeam backscatter as a tool for sea floor characterization and identification of oil spills in the galicia bank this study is focused on the potential of using the multibeam acoustic backscatter both for sea floor characterization of the galicia bank and for identification of sea bottom oil spills an extended multibeam data set was acquired during the rg 1 and rg 2 surveys aboard rv hesperides in 2003 for the geological and geophysical characterization of the galicia bank where the oil tanker prestige wreck is located 3 d images have been made by integrating bathymetry and backscatter data multibeam data have been additionally supported by gravity core information in order to correlate sea floor acoustic backscatter with the surficial sediments and their physical properties the major factors that seem to control the zonation of the backscatter intensity images are i the type of the sedimentary processes gravitational turbiditic hemipelagic which control at the same time the grain size distribution and ii the sea bottom morphologies such as basement outcrops however there are some backscatter halos that cannot be explained by these processes in the backscatter maps the bow and stern of the prestige wreck are clearly marked by a high backscatter intensity anomaly whose existence is discussed

early holocene vegetation record from the salton basin california abstract plant and vertebrate macrofossils in an early holocene fossil packrat neotoma sp midden with a radiocarbon age of 8640 100 14 c yr b p are reported from the chocolate mountains near the salton sea riverside county california an inventory of the midden has permitted a comparison of the modern flora and fauna of the site with that extant during the early holocene whereas the biota had assumed most aspects of its modern sonoran desert aspect by this date statistically significant evidence of differences is attributed to an increased flow of surface water in salt creek a high standing low salinity lake leconte and the late arrival of some characteristic sonoran desert plants these observations are consistent with models of significant fall winter precipitation in the sonoran desert although we cannot exclude alternative explanations

an anomalous low velocity region above the deep earthquakes in the japan subduction zone p wave travel time residuals for local intermediate and deep focus earthquakes are studied to infer a velocity model of the asthenosphere beneath the southern okhotsk sea and the western japan sea tectonically both regions belong to plate subduction zones large positive travel time anomalies are observed at stations above the deepest earthquakes consideration of source path and receiver effects leads to a conclusion that within the asthenosphere there is a lower velocity region below about 300 km depth with velocities about 5 10 below the jeffreys bullen velocity model

relaxation from upwelling in the coastal ocean dynamics experiment a relaxation of the upwelling system on the northern california shelf occurs when upwelling favorable winds decay or reverse observations from the coastal ocean dynamics experiment are used to investigate this process with emphasis on the near surface temperature evolution models of relaxation warming are tested for the surface layer 0 30 m over midshelf with the result that the surface heat flux and poleward alongshelf advection can account for the observed temperature behavior onshore advection of the upwelled isopycnal field is a relatively unimportant contribution the warming patterns that are observed and which provide the gradients for the advective process have wedgelike shapes warmest inshore and in the south of the experimental area these patterns appear to be related to a topographic influence of point reyes possibly through the generation of gradients in the wind field remote advection of warm water from as far south as san francisco occasionally occurs as well the nature of the relaxation mechanism has general implications a new warm saline nutrient rich water mass may be formed in the surface layer since the warming processes are not the reverse of the cooling an important asymmetry is present it causes time variable upwelling to generate conditions very different from steady upwelling a new upwelling front is created during each event with upwelled water seaward of it and reversed alongshelf flow contributes to the warming and exchange mechanisms the system cannot be described or understood without the irreversible processes during upwelling and relaxation or without a mixed layer

emergence of burrowing urchins from california continental shelf sediments a response to alongshore current reversals abstract two sequences of bottom photographs taken every two or four hours for two months during the coastal ocean dynamics experiment code off the russian river california reveal the dynamic nature of interations between the water column the sediments and benthic organisms in the mid shelf silt deposit time lapse photographs taken between late spring and early summer in 1981 and 1982 show that the subsurface dwelling urchin brisaster latifrons one of the largest invertebrates found in shelf depth fine sediment off the u s pacific coast occasionally emerged from the sediment plowed the sediment surface during the course of a few hours to several days then buried themselves again frame by frame study of the film sequences shows that the urchins typically emerged following relaxation of coastal upwelling periods characterized by current direction reversals and increases in bottom water turbidity among the possible causes of the emergence of urchins and the consequent bioturbation of the upper few cm of sediment a response to an enhanced food supply seems most plausible circumstantial evidence suggests the possibility that phytoplankton sedimentation during periods of upwelling relaxation could provide a new source of food at the sediment surface

evidence of fluid seepage in gronfjorden spitsbergen implications from an integrated acoustic study of seafloor morphology marine sediments and tectonics abstract this study integrates high resolution surface and sub surface geophysical data to investigate possible fluid gas and or liquids migration pathways to the seafloor in gronfjorden west spitsbergen gronfjorden is an n s aligned tributary fjord located in the western part of the isfjorden fjord system pockmarks were identified on the bathymetric data in the southern and northern parts of the fjord three types of shallow seep related acoustic features have been identified in the marine sediments i enhanced reflections ii acoustic turbid zones and iii acoustic blankings they occur beneath some of the pockmarks and circular patches of high backscatter on the seafloor in the central part of the fjord which are potential seep related seafloor features 6 8 km long nnw sse striking ridges have been identified in the southern part of the study area thrust faults were interpreted on 2d multichannel seismic data truncating the triassic jurassic and cretaceous successions of western isfjorden located north of gronfjorden a possible link has been found between these fault systems and the seafloor ridges which strike in a similar direction further a normal fault has been identified within the shallow marine sediments which drapes the bedrock of the fjord the fractured bedrock of the fjord and faults associated with the west spitsbergen fold and thrust belt could possibly represent major conduits for fluid migration from deeper stratigraphy to shallow marine sediments and the seafloor

three dimensional numerical modeling of earthquake migration along a northwestern pacific subduction slab three dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern pacific subduction slab on the assumption that the lithosphere is a linear visco elastic body but the asthenosphere and the mesosphere respond to the applied stress as non linear newtonian materials obeying the power law of creep on the basis of a three dimensional thermal structure of the northwestern pacific region the preliminarily computed stress regimes show that 1 to induce a stick slip type trench event prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required 2 the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep focus earthquake and 3 the co seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab

a shallow scattering layer high resolution acoustic analysis of nocturnal vertical migration from the seabed using bottom mounted high frequency inverted echo sounders the tracor acoustic profiling system to examine acoustic backscattering every 2 min in 50 cm range bins above the seabed we recorded strong remarkably regular nightly migrations of organisms from 0 5 m above the seabed to the upper pelagic layers over a period of 6 d in july august 1995 population migration speeds on ascent versus descent mean 6 1 sd were not significantly different when estimated from the mode or leading trailing edge of the population and were 0 59 6 0 24 cm s 21 in ascent 0 96 6 0 25 cm s 21 leading edge and 0 67 6 0 13 cm s 21 on descent 0 78 6 0 14 cm s 21 trailing edge the sd in start times of the ascent over the six nights was only 3 min and that in the times of the completion of the descent was 7 min emergence began after and reentry was completed before downwelling irradiance at photosynthetically active wavelengths 1 m above bottom reached 0 5 3 10 26 mmol quanta cm 22 s 21 migrators increased column scattering strength at 265 khz by a factor of 14 emergence trap samples and nighttime plankton tows suggested that the migrating population was dominated by mysids mostly neomysis kadiakensis our acoustic results suggest that emergence is much more prevalent and important in coastal and estuarine environments than trapping and net sampling can reveal and hence that this important component of benthic pelagic coupling requires additional attention both in measurements and in theoretical treatments

an acoustic backscatter thermometer for remotely mapping seafloor water temperature a bottom mounted circularly scanning sonar operating at 40 khz has been used to map changes in water sound speed over a circular region 150 m in diameter if it is assumed that the salinity remains constant the change in sound speed can be converted to a change in temperature for the present system the spatial resolution is 7 5 m and the temperature resolution is 0 05 c the technique is based on comparison of successive sonar scans by means of a correlation algorithm the algorithm is illustrated using data from the sediment transport events on slopes and shelves stress experiment

the benthic boundary layer and sediment transport the major problems facing scientists studying marine boundary layer fluid mechanics in 1979 concerned the specific mechanismic operating in the regions of the top and the bottom of the boundary layer at the top the mechanisms of fluid escapement from the boundary layer and mixing into the ocean interior could only be postulated approximations of diffusivity across the top of the layer were being debated the values of which were more articles of faith than physically validated estimates cf armi 1979 garrett 1979 at the bottom the intermittent generation of stress cf gordon 1975 heathershaw and simpson 1979 was held responsible for the wide variability observed in the drag coefficient and the thickness of the viscous sub layer was claimed to be greater than that traditionally found from laboratory studies caldwell and chriss 1979

silurian thalassinoides in an offshore carbonate community wisconsin usa abstract silicified boxwork of the ichnogenus thalassinoides in the silurian wenlockian racine formation milwaukee wisconsin usa represents open burrow systems that extended decimeters below the sediment surface in offshore carbonate muds of an intracratonic basin the most completely silicified burrows contain a core of organic rich spiculitic chert a surrounding zone of chert and an outer zone of mixed chert and dolomite burrows reach 3 cm in diameter and include larger turning chambers they were probably produced by an arthropod bioturbation affected the epibenthic fauna by extensive vertical mixing of skeletal material and possibly by construction of small mounds at the sediment surface the same skeletal fauna of siliceous sponge spicules silicified crinozoan ossicles and a diverse array of other groups occurs in both burrows and surrounding dolomite matrix brachiopods including dicoelosia and skenidioides indicate silurian benthic assemblage 5 which represents an outer shelf setting trilobite sclerites are more common within thalassioides burrows than in the surrounding matrix two enrolled calymenid individuals and one molted calymenid exoskeleton occur within thalassinoides suggesting that commensal trilobites occupied the burrow systems

14c as a tracer of labile organic matter in antarctic benthic food webs abstract 14c measurements were made on surface plankton particle trap material surface sediment benthic invertebrate gut contents and body tissue samples to assess the effectiveness of this radioisotope as a tracer of labile organic carbon in antarctic benthic food webs samples were collected on five cruises to the west antarctic peninsula wap shelf between november 1999 and march 2001 as part of the food for benthos on the antarctic continental shelf foodbancs project the 14c contents of the body tissues from a variety of deposit feeders 126 13 per mil were substantially enriched relative to the surface sediment 234 13 per mil and statistically similar to the organic matter collected in plankton tows 135 10 per mil indicating that recently produced marine plankton are the primary source of nutrition for these deposit feeders on the west antarctic shelf selective ingestion was the primary feeding strategy used by echiuran worms and certain holothurians i e peniagone vignoni for incorporating labile organic carbon into their tissues as demonstrated by the large differences 105 13 per mil between surface sediment and gut content 14c activities in contrast digestive and or assimilatory selection was the predominant strategy used by an irregular urchin amphipneustes lorioli and several other holothurians protelpidia murrayi bathyplotes fuscivinculum and the head down conveyor belt feeder molpadia musculus as demonstrated by large differences 42 7 per mil between the 14c activities of their foregut or whole gut organic contents and their body tissues despite large fluctuations in carbon export from the euphotic zone benthic feeding strategies remained essentially constant over the 15 month sampling period no seasonal variation was evident in either the 14c abundance of the deposit feeder body tissues or in the 14c abundance of their gut contents the mean 14c abundance in the body tissues of the two sub surface deposit feeders a lorioli and m musculus mean 136 2 8 5 per mil was distinct p 0 0008 from the mean 14c abundance in the body tissues of the four surface deposit feeders echiuran worm p vignoni p murrayi and b fusciviculum 122 6 12 3 per mil the mean 14c abundance of the gut contents from the sub surface deposit feeders 178 0 18 6 per mil also was significantly depleted p 0 0009 relative to that of the surface deposit feeders 149 5 26 6 per mil the 14c measurements proved to be a much more sensitive tracer for tracking labile organic carbon during ingestive and assimilatory processes than the stable isotopes of carbon or nitrogen

the late pleistocene of death valley a climatic reconstruction based on macrofossil data abstract wood rat neotoma macrofossil deposits from death valley california northern mohave dessert dating from the wisconsinan glacial epoch provide a basis for reconstruction of the glacial age climate at low elevations of the area elevational depression of the woodland zone 1200 1500 m relative to today presence of a woodland semidesert ecotone at approximately 450 m and species composition of the low elevation semidesert which included a yucca species yucca whipplei chaparral yucca now locally extinct point toward a pleistocene climate which was less arid and more equable than today precipitation is estimated to have been three to four times the present values and summers 8 14 c cooler the increase in precipitation is inferred to be part of a generally wetter regional in the northern mohave desert and the central great basin during the last glacial presence of warm desert species at 10 200 yr b p indicates that climatic conditions similar to those of the present were established at the lower elevations by this time

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

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

correlation of sidescan backscatter with grain size distribution of surficial seabed sediments the dependence of acoustic backscatter on sediment grain size distribution is examined using dual frequency 100 and 410 khz sidescan sonar and 22 sediment grab samples from the loch linnhe artificial reef site on the west coast of scotland the sidescan data were processed to remove an empirically estimated average grazing angle dependence on backscatter the processed data were analysed by forming histograms of pixels extracted from a 20 m 2 box around each ground truth site a positive correlation r 0 73 between mean backscatter intensity and mean grain size was obtained i e the coarsest samples had the brightest backscatter a positive correlation r 0 59 was also found between the standard deviations of the backscatter and grain size distributions i e poorly sorted sediments gave the most variable backscatter the performance of the sidescan data was compared to results from a co incident single beam echo sounder roxann survey the roxann roughness index e1 compared well with the sidescan whilst the roxann hardness index e2 did not this may be due to a physical link between the acoustic measures the comparison showed the sidescan to have delivered a significantly higher resolution image of the seabed for a similar amount of ship time imaging of the artificial reef modules themselves was found to be frequency dependent d 2004 elsevier b v all rights reserved

acoustic scattering from the seafloor modeling and data comparison an existing model of seafloor backscattering d r jackson d p winebrenner and a ishimaru j acoust soc am 79 1410 1422 1986 was implemented with the additions of volume scattering from a random inhomogeneous continuum and scattering from subbottom interfaces results of computer simulations with the extended model were compared with values of scattering strength obtained from processed gloria data from the monterey fan off the coast of california regions of well delineated high and low backscatter are seen in the gloria imagery the geoacoustic input parameters for the simulation runs were either taken directly from or estimated from core data obtained by ground truth sampling in the image area from the model simulation results it appears that the main factor causing the observed dichotomous character of the strength of returns in the fingers area is the presence or absence of a topmost inhomogeneous silt clay layer with a thickness of around 1 1 5 m in the high backscatter areas inh

suspended particulate matter along the coastal ocean dynamics experiment central line during upwelling and relaxation events profiles of light transmission temperature and salinity and analyses of suspended matter samples acquired along the coastal ocean dynamics experiment central line in june 1981 and april may 1982 suggest that the development of subsurface turbid plumes and temperature inversions may be associated with the relaxation of winds favorable for upwelling wind relaxations and reversals appear to trigger flow field adjustments characterized by sinking of previously upwelled water over the inner and middle shelf and the rapid growth of a barotropic poleward current over the inner shelf complexities in the cross shelf current structure during relaxation may be associated with the formation of the subsurface layers of plankton rich water and the small temperature anomalies renewed upwelling produces a relatively simple three layer distribution of suspended particles and the elimination of temperature inversions

in situ studies of megafaunal mounds indicate rapid sediment turnover and community response at the deep sea floor sediment mounds composed of faecal material from megafaunal deposit feeders1 2 occur in virtually all deep sea environments forming the dominant decimetre scale topography on the ocean floor3 4 mounds are often zones of deep convective bioturbation1 2 and their formation is thought to disturb infaunal benthos5 6 we are studying mounds in the deep sea to evaluate rates and patterns of megafaunal sediment mixing and to assess the disturbance effects of mound building on the infaunal community with the goal of using mounds to investigate natural processes of succession at the deep sea floor7 our results indicate that sediment turnover associated with mound building by echiuran worms can be very rapid and that bathyal infauna can respond quickly to this type of disturbance these findings suggest that spatially rare but dynamic elements in the sediment fabric such as mounds may be important in controlling microstratigraphy chemical diagenesis and community structure at the deep sea floor

effects of orientation on acoustic scattering from antarctic krill at 120 khz abstract backscattering measurements of 14 live individual antarctic krill euphausia superba were made at a frequency of 120 khz in a chilled insulated tank at the long marine laboratory in santa cruz ca individual animals were suspended in front of the transducers were only loosely constrained had substantial freedom to move and showed more or less random orientation one thousand echoes were collected per animal orientation data were recorded on video the acoustic data were analyzed and target strengths determined from each echo a method was developed for estimating the three dimensional orientation of the krill based on the video images and was applied to five of them giving their target strengths as functions of orientation scattering models based on a simplified distorted wave born approximation dwba method were developed for five animals and compared with the measurements both measured and modeled scattering patterns showed that 120 khz acoustic scattering levels are highly dependent on animal orientation use of these scattering patterns with orientation data from shipboard studies of e superba gave mean scattering levels approximately 12 db lower than peak levels these results underscore the need for better in situ behavioral data to properly interpret acoustic survey results a generic e superba dwba scattering model is proposed that is scalable by animal length with good orientation information this model could significantly improve the precision and accuracy of krill acoustic surveys

210pb in the sediments and water column of the northeast atlantic from 47 to 59 n along 20 w abstract over much of the deep north atlantic ocean the water column and atmospheric input deficit of the radionuclide 210 pb is not matched by the inventories found in the underlying sediments this implies that an unidentified extensive or intense sink or sinks for 210 pb must exist in the atlantic to counterbalance the large area of open ocean in deficit one likely sink area which has been suggested is the atlantic north of 50 n because of its moderately high surface productivity and the high inventories observed in the limited number of cores available this work tests the hypothesis by investigating the water column at 48 n 20 w and a set of cores from 47 to 59 n along 20 w the water column deficit is 21 dpm 210 pb cm 2 in a water column of 4500 m depth and a 226 ra content of 54 dpm cm 2 the important unmeasured parameter is the atmospheric input of 210 pb to the surface ocean but the 100 yr average fluxes for the majority of the cores eight out of eleven can be modelled to within 40 of their observed values with a value of 0 30 dpm cm 2 y 1 for the atmospheric input added to the flux implied by the water column deficit this leads to best estimate values of the characteristic water column parameters scavenging effectiveness and scav of 48 and 35 yrs respectively which are typical north atlantic values it is concluded that removal from the water column and steady state supply to the sediments are in approximate balance along this transect and that this part of the north atlantic is not a preferential sink for 210 pb two cores from a sediment drift area do show active enhanced 210 pb excess accumulation but this is ascribed to current driven fine sediment redistribution rather than to productivity driven enhanced removal from the water column

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

isotopic and geochemical evidence of palaeoclimate changes in salton basin california during the past 20 kyr 2 87sr 86sr ratio in lake tufa as an indicator of connection between colorado river and salton basin abstract as a part of the colorado river drainage system southern california s salton basin has two major tributaries colorado river and whitewater river interpretations on palaeoclimatic records from the basin strongly depend on the colorado river inflow to the basin in this study we have used the 87sr 86sr ratio in lacustrine deposits to identify the time and duration of the colorado river input our measurements of 87sr 86sr ratios made on water samples collected from salton basin indicate that the 87sr 86sr ratio 0 710169 of the colorado river input is identical to that of salton sea 0 710105 implying little change in the lake water ratio since 1905 when salton sea formed and began receiving water mostly from colorado river and the 87sr 86sr ratio of colorado river is much lower than that of whitewater river 0 715960 these results enable us to trace past changes in the colorado river input using 87sr 86sr in lake carbonates using a two end member mixing model we discuss the major factors that control the 87sr 86sr variation of lake cahuilla which was an ancient lake in the salton basin between 400 and 20 500 yr bp thirty measurements of 87sr 86sr were made on ams 14c dated tufa carbonates in two slabs from lake cahuilla the results show relatively constant 87sr 86sr ratios throughout the past 20 kyr ranging from 0 709944 to 0 710140 the average of these 87sr 86sr ratios 0 710060 0 000049 n 30 is very close to that of the colorado river input our results indicate that 1 the colorado river input was the dominant water source for lake cahuilla at least during 800 20 500 yr bp when the tufa grew in the basin and 2 the provenance of run off to the colorado river did not change significantly during 800 20 500 yr bp hence the palaeoclimatic proxies retrieved from the lake cahuilla tufas in the salton basin can decipher the discharge and flood history of the colorado river under the influence of climate variability in the colorado river drainage basin

rotary sidescan images of nearshore bedform evolution during a storm abstract high resolution acoustic images are presented of bedform development on the crest of a nearshore bar during a storm the images were obtained using a 2 25 mhz rotating sidescan sonar the sonar system provides a continuous record of the evolution of the bedform field through time within a 10 m diameter field of view the images document the occurrence of ripples cross ripples and megaripples during the waning stages of the storm transitions among ripple types occurred on time scales of 1 3 h beginning with flat bed and culminating in highly 3 d short crested ripples

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

habitat compression and expansion of sea urchins in response to changing climate conditions on the california continental shelf and slope 1994 2013 abstract echinoid sea urchins with distributions along the continental shelf and slope of the eastern pacific often dominate the megafauna community this occurs despite their exposure to naturally low dissolved oxygen do waters

the lower colorado river valley a pleistocene desert abstract a chronological sequence of plant macrofossil assemblages from twenty five pack rat middens provides a record of desert scrub vegetation for most of the last 13 380 yr b p from a hyperarid portion of the lower colorado river valley at the end of the late wisconsin and probably during much of the quaternary the picacho peak area imperial county california supported a typical mohave desert association of larrea divaricata creosote bush coleogyne ramosissima blackbrush yucca brevifolia joshua tree and y whipplei whipple yucca recent arrivals of sonoran desert plants such as olneya tesota ironwood and fouquieria splendens ocotillo suggest that the area supported relatively modern sonoran desert scrub species for relatively short periods during interglaciations

focal mechanisms of small earthquakes in the southeastern brazilian shield a test of stress models of the south american plate summary focal mechanisms of small earthquakes with magnitudes of about 3 in the se brazilian shield are calculated using s p amplitude ratios low attenuation q p from 400 to 800 in the shield upper crustal layers allowed sharp s arrivals to be recorded up to distances of 100 km besides p wave polarities sh wave first motions were also used to constrain the nodal plane orientations normal and reverse faulting mechanisms with strike slip components were found the inversion of four mechanisms to estimate the stress tensor indicated a strike slip stress regime with roughly e w orientated s 1 and n s s 3 both the orientations and the shape factor q 0 7 of the inverted stress are in excellent agreement with theoretical predictions for that part of brazil from the driving force model of coblentz richardson 1996 good agreement with the nature of the stress as well as its orientation was also found for the model of meijer 1995 both of these theoretical models include spreading stresses along the continent ocean lithospheric transition because the earthquakes are more than 300 km from the continental shelf they should not be avected by the local flexural forces caused by sediment load in the marginal basins the agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses

bistatic bottom scattering model experiments and model data comparison a model is presented for bistatic scattering from ocean sediments it treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment accordingly the scattered intensity is assumed to be a sum of two terms one proportional to the roughness scattering cross section and the other proportional to the volume scattering cross section the model is tested against data acquired as part of the coastal benthic boundary layer cbbl research program as part of that program an autonomous circularly scanning sonar system was deployed in well characterized regions this sonar operated at 40 khz had a 5 horizontal beam and acquired backscattering data over a 50 m radius during part of the deployment it operated in conjunction with a mobile receiving array so as to acquire bistatic data the experimental apparatus and procedures are presented and results are compared with model predictions