observations and geologic significance of marine snow in a shallow water partially enclosed marine embayment because of their fragile nature marine snow sized aggregates aggregates 0 5 mm in longest dimension have been generally overlooked in shallow water environments we report in situ observations and measurements in cape lookout bight north carolina that indicate that marine snow is probably always present and usually abundant underwater photographs show that over a period of hours marine snow varies in size from 8 to 71 10 5 ml and in abundance from 291 to 489 l 1 but that such variations in size and abundance are not obviously related to tide total suspended sediment concentration or current speeds in the 10 to 20 cm s 1 range photographs and concurrent transmissometer records suggest that variations in size shape and abundance of marine snow may be sufficiently great that particulates do not remain optically constant and thus affect beam transmission irrespective of total suspended sediment concentration levels marine snow is significant geologically in that it has high settling rates 50 to 200 m per day and may be the package in which much of the vertical mass flux occurs if so it offers an explanation for the high accumulation rates observed in coastal and estuarine fluid mud deposits

inferring domain state from magnetic hysteresis in high coercivity dolerites bearing magnetite with ilmenite lamellae abstract hysteresis loops were measured for 16 dolerite samples from separate precambrian dykes whose remanence is probably primary remanence is carried by grains of magnetite subdivided into fine particles usually by ilmenite lamellae as seems common in mafic igneous rocks of high coercivity coercive force h c ranges from 9 to 40 10 3 a m 110 500 oe the ratio j rs j s of saturation remanence to saturation magnetization ranges from 0 10 to 0 45 j rs j s varies in approximate proportion to h c with a constant of proportionality appropriate for pseudo single domain magnetite with one or two domain walls per particle a cooling cycle to 77k causes large 40 on average demagnetization of j rs in most samples supporting the dominance of pseudo single domain magnetite most samples also show a large 29 on average decrease in h c on cooling to 140k suggesting magnetostrictive control of h c perhaps through internal stresses opposing domain wall motion only the three dolerites with h c greater than 30 10 3 a m 380 oe show little change in h c on cooling suggesting dominance by single domain particles with shape anisotropy in magnetite grains subdivided into single domain particles by ilmenite lamellae magnetic interaction may lower j rs j s from the expected 0 5 some theories 1 2 suggests that this is due to a self demagnetizing field from magnetic poles on the surface of each grain of magnetite subdivided by ilmenite and should be corrected for another theory 3 suggests that this correction is not necessary because the self demagnetizing field is already approximately cancelled due to magnetic poles inside each subdivided grain at the ends of magnetite particles the latter suggestion is favoured by our three highest h c dolerites whose j rs j s averages 0 42 0 04 before and 0 66 0 06 after correction for the self demagnetizing field from poles on the surface of magnetite ilmenite grains applying this correction to our other dolerites usually raises j rs j s significantly above 0 5 which is unreasonable for pseudo single domain magnetite hence correcting for self demagnetizing fields from poles on the surface of magnetite ilmenite grains in rocks is not recommended whatever the domain state

influence of oxygen on heterotrophic reworking of sedimentary lipids at the pakistan margin abstract oceanic oxygen minimum zones omzs are thought to be regions of enhanced organic carbon burial with high organic matter om quality preservation resulting from low oxygen concentrations and faunal biomass both in the water column and at the seafloor here we use the pakistan continental margin to assess the controls on om deposition and quality along a transect through the omz from water depths of 140 1850 m we sampled five sites spanning the omz during the inter and late monsoon seasons to address the role of a oxygen b the benthic fauna and c seasonality on the composition and quality of om using lipid biomarkers organic carbon concentrations were highest within the omz and lipid biomarkers revealed that om at the pakistan margin was primarily of marine origin oxygen concentrations in bottom waters played an indirect role in controlling om quality by influencing faunal community composition and biomass both in the water column and at the seafloor benthic fauna appear to influence om quality via molecular filtration of certain compounds lipid biomarkers for example foraminifera that peak in the omz core 300 m 122 individuals 10 cm 2 0 11 ml l 1 4 91 m efficiently removed polyunsaturated fatty acids from the sediments whilst at the lower omz boundary suspension feeding pennatulids 1200 m 108 individuals hm 2 0 35 ml l 1 15 62 m stripped labile lipids from the water column particulates leading to the deposition of lipid poor om in the surficial sediments low infaunal and epifaunal biomass at the fully oxygenated deep site led to the persistence of phytodetritus and subsequently labile lipids in the sediments seasonal changes in om quality and composition were noted at 140 300 and 1850 m and were attributed to both particulate fluxes and faunal feeding patterns finally lipid biomarker indices demonstrated that the om at the pakistan margin was of low quality when compared to other similar environments

magnetic hysteresis as a function of low temperature for deep sea basalts containing large titanomagnetite grains inference of domain state and controls on coercivity hysteresis loops to 1200 oe 95 ka m 1 are measured between 295 and 105 k for two deep sea basalts dsdp legs 34 and 37 containing large 200 m unexsolved titanomagnetite grains the curie points electron microprobe analyses and saturation magnetizations of the magnetic grains are the same as for synthetic titanomagnetite xfe2tio4 1 x fe3o4 with x 0 6 as temperature is lowered from 295 to 190 k coercive force hc slowly rises from 40 to 95 oe 3 2 to 7 6 ka m 1 approximately in proportion to the rise in the magnetostriction constant presumably hc is controlled by through internal stresses impeding domain wall motion as expected of multidomain grains the ratio of saturation remanence to saturation magnetization in 1200 oe 95 ka m 1 cycles jr js rises approximately in proportion to hc with a constant of proportionality consistent with titanomagnetite x 0 6 as temperature is lowered from 190 to 120 k hc rises rapidly to 400 oe 32 ka m 1 as a roughly linear function of the

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

settling velocities of fine suspended particles in the changjiang estuary china abstract point sampled suspended sediment concentration profiles grain size largely c of sediment in suspension the best fit equation for the field settling velocity of fine particles in the changjiang estuary can be expressed by the power law ws 2 37 c 0 84 r2

in situ estimations of the density and porosity of flocs of varying sizes in a submarine canyon abstract this paper introduces a new method to estimate in situ floc density and porosity of three particle size classes 0 45 10 10 63 and 63 250 m lisst 100 and catnet a novel on board filtration system of nested filters were used to measure suspended particle volume and mass concentrations of 32 and 3 size classes respectively two field experiments were conducted in 2004 and 2006 in the head region of gaoping submarine canyon gpsc to test this method in each experiment ctd and lisst 100 profiling and simultaneous water sampling were carried out on board r v ocean research iii in 2004 the water samples were filtered in the laboratory after the cruise using traditional methods catnet was used in 2006 and the seawater filtration was done on board assuming the observed suspended particles were all flocs composed of primary particles with assumed density of 2 65 g cm 3 and interstitial water with assumed density of 1 025 g cm 3 we estimated the volume of primary particles the volume difference between flocs measured from lisst 100 and primary particles is the interstitial pore space in flocs subsequently the porosity of a floc particle is the ratio of the volume of the interstitial pore space to that of the floc particle results for 2004 show erroneous values of estimated floc porosities which do not appear in 2006 the major difference is that the seawater was filtered immediately on board using catnet in 2006 after removing erroneous data related to after cruise filtering the averaged floc porosity and density in the three grain size classes were 47 0 64 0 85 9 and 1 89 1 61 1 25 g cm 3 respectively there is a trend that larger particles have lower density which is supported by literature the temporal variations of floc density and porosity of all three particle size groups were larger in the surface than near the bottom of the submarine canyon this vertical variability resulted from flocculation and biogenic processes in the surface of the coastal ocean and the terrestrial source of the gaoping river floc density is an important physical parameter to evaluate the settling flux of suspended sediments in estuarine and marine environments this approach provides direct in situ floc bulk densities of different grain size classes which has advantages over conventional methods

recent advances in fine particle magnetism implications for paleomagnetism it is becoming increasingly obvious in paleomagnetism that the older practice of using large numbers of samples to improve statistical measures of accuracy is often not adequate to reveal the signal of greatest geological importance blanket demagnetization treatments are now largely outdated except for the most ideally behaved samples a rapidly vanishing phenomenon in many locales variations in characteristic remanent directions between neighboring and supposedly contemporaneous but lithogically distinct rock units can no longer be simply treated as noise we now recognize that such differences may actually reflect rock magnetic effects rather than such factors as the short term behavior of the geomagnetic field or local variations in structure multiple overprints often the result of moderate reheating and or chemical alteration can result in an uncertain relationship between the timing of emplacement and the acquisition of remanence in the past rocks which presented such problems could often be avoided in favor of more straightforward units much of that geological cream has been skimmed however further progress in many areas particularly the determination of the apw path and reversal stratigraphy in the paleozoic will require a better understanding of magnetization processes in order to resolve the above mentioned problems and produce accurate and reliable results

the role of the benthic biota in sedimentary metabolism and sediment water exchange processes in the goban spur area ne atlantic we provide an overview of the role of biological processes in the benthic boundary layer bbl and in sediments on the cycling of particulate organic material in the goban spur area northeast atlantic the benthic fauna sediment and bbl characteristics were studied along a transect ranging from 208 to 4460 m water depth in different seasons over 3 years near bottom flow velocities are high at the upper part of the slope 1000 1500 m and high numbers of filter feeding taxa are found there such that organic carbon normally passing this area during high flow conditions is probably trapped accumulated and or remineralised by the fauna overall metabolism in shelf and upper slope sediments is dominated by the macrofauna more than half of the organic matter flux is respired by macrofauna with a lower contribution of metazoan meiofauna 4 and anoxic and suboxic bacterial mineralisation 21 the remainder 23 being channelled through nanobiota and oxic bacteria by its feeding activity and movement the macrofauna intensely reworks the sediments on the shelf and upper slope mixing intensity of bulk sediment and of organic matter are of comparable magnitude the benthos of the lower slope and abyssal depth is dominated by the microbiota both in terms of total biomass 90 and carbon respiration about 80 the macrofauna 16 meiofauna 4 and megafauna 0 5 only marginally contribute to total carbon respiration at depths below 1400 m because large animals have a lower share in total metabolism mixing of organic matter within the sediments is reduced by a factor of 5 whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf the food quality of organic matter in the sediments in the shallowest part of the goban spur transect is significantly higher than in sediments in the deeper parts the residence time of mineralisable carbon is about 120 d on the shelf and compares well with the residence time of the biota in the deepest station the mean residence time of mineralisable carbon is more than 3000 d an order of magnitude higher than that of biotic biomass

in situ observations of floc settling velocities in glacier bay alaska abstract in situ floc settling velocities and diameters of particles ranging in size from 0 63 to 5 05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom tripod mounted floc camera assembly fca in tarr inlet glacier bay alaska these observations were used to estimate floc effective densities three results emerge from this work first fits of settling velocity and effective density to diameter are consistent with expressions published for other environments suggesting that common controls on floc size and settling velocity operate across diverse marine environments second the raw data show considerable scatter with upper and lower 95 prediction intervals on settling velocity and excess density differing by about a factor of 7 analysis of sources of error suggests that the variability is caused by differences in component grain composition among flocs and turbulent stirring within the stilling box third bin averaged effective densities and settling velocities are highly correlated with diameter thus while it is not possible based on diameter to predict accurately the settling velocity of a single floc it is possible to estimate the mean settling velocity of a population of like sized flocs

amino acid biogeochemistry and organic matter degradation state across the pakistan margin oxygen minimum zone to assess whether the oxygen minimum zone omz across the pakistan margin causes differences in the lability of sedimentary organic matter sediments were collected in the core of the omz in the upper and lower transition zones and below the omz sediment samples were analysed for total nitrogen tn and organic carbon oc contents mineral surface area sa and total hydrolysable amino acids thaa and enzymatically hydrolysable amino acids ehaa oc contents and organic carbon per unit of mineral surface area oc sa values were clearly elevated in the core and lower omz transition zone these sediments also contained more labile sedimentary organic matter as discerned by higher concentrations of thaa and the contribution of n in thaa to tn a protein amino acid based degradation index revealed that all sedimentary organic matter has undergone significant degradation but sediments in the upper omz transition zone and below the omz are more degraded than inside the omz changes in amino acid composition during diagenesis are attributed to a combination of factors 1 selective preservation in which amino acids in cell walls are better preserved than amino acids in cell plasma 2 formation and accumulation of bacterially derived organic matter there were relatively more living bacteria in the core of the omz and an accumulation of peptidoglycan derived amino acids in degraded sediments in the upper omz transition zone and below the omz and 3 bacterial transformation as the molar percentages of bacterial transformation products alanine bala amino butyric acid gaba and ornithine orn increased with increasing degradation

a simple method for calculating in situ floc settling velocities based on effective density functions abstract we present a simple method to obtain settling velocity estimates of fine grained suspended sediment using the following 1 in situ floc volume fraction measurements 2 calculations of in situ floc mass 3 measurements of in situ floc size 4 laboratory measurements of primary particle size and 5 laboratory measurements of organic matter obtained through loss on ignition this method is based on the mass distribution over the particle size spectra floc properties are incorporated by using information concerning the primary particles and the ratio between the volume concentration and the mass concentration of the in situ measurements in this manner the changes in effective densities caused by flocculation and biological mediation are approximated this method relies solely on measurements conducted with a particle sizing instrument and on an analysis of water samples the method is applied to measurements obtained with a lisst and data from three different estuaries in different climatic zones are used the settling velocity estimates found using this method are compared to settling tube measurements and to mass settling method estimates as well as to calculations using fractal dimension methods from the literature settling velocities calculated using the method presented in this study lie within the ranges of the three other methods demonstrating that this simple method produces results within the ranges of settling tube velocities and those predicted by the more comprehensive multiparameter fractal dimension models the presented method allows for easy intercomparisons between studies by utilising instruments such as the lisst and it is believed that this method would also produce reliable settling velocities using other instruments that yield a volume distribution as well as a proxy for total volume and mass concentration e g some camera systems

the short term fate of fresh algal carbon in continental slope sediments emplacement of a tracer mixture containing 13c labeled green algae on the sea floor of the continental slope offshore of cape hatteras north carolina elicited a rapid response over 1 5 d from the dense benthic community certain deposit feeding annelids e g scalibregma injlatum and aricidea quadrilobata became heavily labeled with 13c as a result of ingestion of the algae 13c labeled organic matter was transported to a depth of at least 4 5 cm into the seabed during the 1 5 d period presumably as a consequence of a feedingassociated activity nonlocal transport produced subsurface peaks in organic 13c at 2 3 cm dissolved inorganic 13c produced by the oxidation of the labeled algae penetrated to lo cm depth the transport of highly reactive organic matter from the sediment surface at initial velocities 1 3 cm d l is expected to be an important control of subsurface benthic processes in slope environments characterized by abundant macrofaunal populations anaerobic processes which are enhanced on the cape hatteras slope relative to adjacent areas may be promoted by the rapid injection of reactive material into subsurface sediments the transport in turn is a consequence of the dense infaunal populations that are supported by the rapid deposition of organic carbon in this region

oxygen and organic matter thresholds for benthic faunal activity on the pakistan margin oxygen minimum zone 700 1100 m abstract a transition from fully laminated to highly bioturbated sediments on continental margins is thought to derive from increased animal activity associated with increasing bottom water oxygen concentration we examined faunal community responses to oxygen and organic matter gradients across the lower oxygen minimum zone omz on the bathyal pakistan margin where sediments grade from fully laminated sediment at 700 m 0 12 ml l 1 o 2 5 m to highly bioturbated sediment at 1100 m 0 23 ml l 1 o 2 10 m high resolution sampling of the seafloor every 50 m water depth was conducted along a single transect during inter and post monsoon periods in 2003 to address a the existence of oxygen thresholds regulating macrofaunal abundance composition diversity and lifestyles b the interactive effects of organic matter quantity and quality c associated community effects on sediment structure and d potential seasonality in these processes macrofaunal biomass and bioturbation depth were positively correlated with organic matter availability which peaked at 850 950 m 3 39 3 53 org c in contrast macrofaunal diversity h dominance r1d and burrow number exhibited threshold responses at oxygen concentrations of 0 12 0 20 ml l 1 5 9 m with few animals and highly laminated sediments present below this concentration and most taxa present in fully bioturbated sediments above it the highly mobile burrowing amphinomid polychaete linopherus sp exhibited almost complete dominance and high density at 750 850 m 0 12 0 14 ml l 1 o 2 5 6 m but despite its activity sediment laminae remained faintly visible formation of permanent burrows and detritivory were dominant macrofaunal lifestyles within the omz allowing laminae to persist at surprisingly high animal density and biomass results reflect a shift from organic matter to oxygen regulation of body size and biogenic structures following the monsoon this study suggests that for assemblages evolving under permanent severe hypoxia food availability remains a significant determinant of animal abundance and biogenic structure depth oxygen influences patterns of diversity and dominance and interacts with organic matter to generate abrupt faunal transitions on the pakistan margin

rost and beast devices for in situ measurement of particle settling velocity abstract the principles and design of two devices are described that measure the particle settling velocity in situ the remote optical settling tube rost designed by a group at osu has been successfully deployed at the hebble site the benthic autonomous settling tube beast has recently been operated at the same site

adaptation of focused beam reflectance measurement to in situ particle sizing in estuaries and coastal waters abstract recent advances in focused beam reflectance technology have resulted in new instruments with the potential for particle size determination in natural waters a commercially manufactured instrument based upon this principle has been adapted to allow the size distribution of suspended particulate matter in estuaries and coastal waters to be determined in situ after calibration the standard instrument has been shown to be capable of providing accurate undisturbed particle size distribution data over a wide range of particle concentrations 10 mg l 1 50 g l 1 irrespective of current speed and to a large extent particle composition and density data obtained with the in situ instrument in the humber estuary have revealed large temporal and spatial variations in the size distribution of suspended particles over a tidal cycle particle sizes were largest near the bed during periods of slack water where median diameters exceeded 500 m this was attributed to the sedimentation of large aggregates rapid reductions in median particle size coincided with periods of strong tidal current acceleration after low water slack current at maximum current velocity on the flood tide mobilisation of sediment from the bed gave rise to larger particles in the water column multiple regression analysis indicated that a combination of current velocity and particle concentration provides the best predictor of particle size

investigation of flocculation dynamics under changing hydrodynamic forcing on an intertidal mudflat abstract in situ floc size and turbulent shear stress were measured together with suspended sediment concentration to investigate the floc properties under changing hydrodynamic forcing over the intertidal mudflat a tripod system was established in the field for a period of approximately one month including 6 days of stormy conditions in the middle of the investigation period mean floc size exhibited strong temporal variations within a tidal cycle and inverse relationship was found between mean floc size and shear stress suspended sediment concentration ssc can modulate the flocculation dynamics when shear stress decreases down to enhancing flocculation asymmetrical behaviors of floc sizes between flood and ebb phases were identified with overall larger floc sizes in flood than in ebb tide under the same shear stresses floc structure showed different properties under calm and stormy conditions and the variable fractal dimension and variable primary particle size were more convincing in simulating the variation of floc effective density with mean floc size during the storm period which was inferred to be related to the resuspension of bed sediment as well as organic matter a total of 110 mm bed erosion was measured during the storm and erosion events occurred only around low water due to the high current wave combined bed shear stress and off shore current after the storm 40 of the erosion recovered within one week and the fast settling of large flocs around high water plays significant role in the deposition process leading to 60 of the recovery

organic content and preservational controls in surficial shelf and slope sediments from the arabian sea pakistan margin abstract in the arabian sea organic rich deposits occur on the continental slope at depths roughly coincident with the impingement of an intense water column oxygen minimum zone omz this relationship has often been taken as causal and such deposits are often proposed as modern analogues of organic rich facies from the palaeorecord however there are numerous other potential preservational controls on continental margins and their relative importance remains unclear and much debated we report the results of organic inorganic and stable c isotope analyses on surficial sediments from three transects of the pakistan margin ranging in depth from well above to well below the omz organic carbon c org and total nitrogen n total contents show expected mid slope maxima although these do not mirror bottom water dissolved o 2 profiles maximal values are observed near the lower boundary of the omz or even in homogenous sediments below it together lignin yields and 13 c org values indicate a general paucity of terrigenous organic matter om this information combined with c org n total relationships suggests that the om contents of all sediments are dictated by differing accumulation of a predominant marine fraction while there are no obvious redox related differences in c org n total or om hydrogen richness across the margin there are clear minima in 13 c org within the omz in apparent contradiction with maximal accumulation of marine om the latter phenomenon is ascribed either to local source effects such as variable algal isotopic signatures or to differences in om preservation state possibly linked to bottom water redox conditions this remains to be clarified but along with the rough correspondence between mid slope sedimentary c org maxima and bottom water o 2 minima may indicate that o 2 availability is an important factor in determining sedimentary om content on this margin however evidence from redox sensitive indicators i c org mn al for persistent accumulation of om rich deposits under oxidizing conditions in homogeneous sediments below the omz clearly indicates that o 2 availability is not the sole or over riding control on om distributions

magnetic properties of synthetic analogues of the altered olivines of igneous rocks summary a laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions three compositions in the forsterite fayalite solid solution series were synthesized by a three firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite fe o as a contaminant 0 005 per cent the prepared olivines were oxidized between 850 and 1280 c at oxygen fugacities of between lo and lop5 atmospheres the composition and concentration of the multiphase oxidation products were studied by magnetic optical electron optical and x ray methods the magnetic constituent of the intergrowth was magnetite below about 1150 c the microstructure is similar to the dendritic intergrowths found for example in the altered olivine of the central layered intrusion of the isle of rhum above about 1150 c the form may be similar to symplectite intergrowths the magnetization process parameters coercive force etc vary systematically with temperature of oxidation to which evidently controls the microstructure of the magnetite the magnetite produced is magnetically hard samples altered at 900 c and 950 c having higher coercive force hc than any magnetite reported in the literature the room temperature properties appear to bridge the published trends found in grown crystal magnetites and crushed grains inferred effective grain sizes of the complex intergrowths range from 0 1 to 3 pm as to increases from 900 to 1200 c the temperature dependence of magnetization process parameters principally h measured between room temperature and the curie point are modelled in terms of i monodomain grains subject to thermal fluctuations ii monodomain grains not subject to thermal fluctuations and iii as multidomain grains model i yields plausible values for effective grain dimensions and length diameter ratios higher to producing larger more equidimensional particles model ii suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure dependent uniaxial particle particle interaction term model iii implies that if domain walls are pinned by both surface and volume defects magnetostriction plays a greater role in volume pinning than in surface pinning plausible values of demagnetizing factors appropriate to grains with few domains are produced by model iii

characteristics of suspended particles at an 11 hour anchor station in san francisco bay california hourly measurements of suspended sediment in situ floc size distributions constituent grain size distributions total concentrations and average particle densities are presented for five depths from an 11 hour anchor station in san francisco bay the flocs formed well sorted distributions with modal sizes between 100 and 500 m whereas the disaggregated sediment was poorly sorted with about the same volume of material in all size classes and relatively little material coarser than 100 m for both types of size spectra concentrations at the coarse end of the size distribution fall off rapidly at a size range which varied with bottom shear stress the settling rates corresponding to the modal floc diameters were generally related to the maximum disaggregated diameters by a square relationship but turbulence appeared to limit floc size in the coarsest samples both the floc and disaggregated grain size vary with total concentration suggesting that flocs are relatively stable entities which do not change during alternating settling and resuspension

a new instrument for measuring settling velocities in situ abstract an in situ settling cylinder for determining particle settling velocities has been deployed during a field experiment in san pablo bay california the instrument is a spring loaded cylinder which traps approximately 31 of water and suspended sediment between two end plates mounted between the end plates is a vertical array of five miniature nephelometers positioned 4 20 cm below the top end plate which continuously monitor 5 hz sampling rate suspended sediment concentration at each level as particles settle the resulting settling curve can be divided into components with characteristic settling velocities and a settling velocity distribution obtained for the suspension the results are conservative estimates of settling velocities for the suspension in situ estimates of particle settling velocity are higher than stokes settling velocities calculated for the disaggregated grain size distribution of a suspended sediment sample indicating that the sediment in suspension is in the form of aggregates the aggregates have settling velocities as high as those of sand size quartz spheres

inssev an instrument to measure the size and settling velocity of flocs in situ abstract an instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments inssev in situ settling velocity instrument is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column the settling flocs are viewed using a miniature video system subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 m from this information floc density can be estimated the main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary with as little disturbance to their hydrodynamic environment as possible preliminary field testing in the tamar estuary has given useful results in flow velocities up to 0 4 m s 1 and in concentrations up to 200 mg 1 1 the video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres

in situ particle size spectra and density of particle aggregates in a dredging plume abstract an in situ laser particle sizer the lisst 100 was used to describe the spatial variation of beam attenuation coefficient in situ particle size spectra and aggregate densities in a dredging plume in the sound oresund between denmark and sweden the results proved that the above mentioned parameters varied significantly within the investigated length of the plume approximately 2 km it is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution and at the same time how the mean density of particles aggregates decreases significantly this shift in the state in which the particles exist in the water effectively changes the optical response of the mass of particles suspended in the water it is shown that adequate correlations between mass concentrations and beam attenuation coefficients can only be obtained if parameters describing the in situ quality and state of the mass of particles e g standard deviation of the size spectra and volume concentration is included in the regression from the spatial variation in mean density and in situ particle size it was possible to calculate the spatial difference in settling velocity it was found that the difference in settling velocity was only about a factor of 1 7 because increasing in situ particle size was counter balanced by decreasing mean density furthermore the time scale of flocculation within the plume was found to be in the order of 50 min

macrofaunal communities and sediment structure across the pakistan margin oxygen minimum zone north east arabian sea abstract benthic macrofauna and sediment column features were sampled at five stations along a bathymetric transect depths 140 300 940 1200 1850 m through the pakistan margin oxygen minimum zone omz during the 2003 intermonsoon march may and late post monsoon august october periods objectives were to compare patterns with those described from other omzs particularly the oman margin of the arabian sea in order to assess the relative influence of bottom water oxygenation and sediment organic content on macrofaunal standing stock and community structure macrofaunal density was highest at the 140 m station subject to monsoon driven shoaling of the omz but there was no elevation of density at the lower omz boundary 1200 m numbers was extremely low in the omz core 300 m and were not readily explicable from the environmental data there was no consistent depth related trend in macrofaunal biomass macrofaunal densities were consistently lower than found off oman but there was less contrast in biomass a significant post monsoon decline in macrofaunal density at 140 m was driven by selective loss of polychaete taxa polychaeta was the most abundant major taxon at all stations but did not dominate the macrofaunal community to the extent reported from oman cirratulidae and spionidae were major components of the polychaete fauna at most stations but acrocirridae ampharetidae amphinomidae and cossuridae were more important at 940 m polychaete assemblages at each station were almost completely distinct at the species level polychaete species richness was positively correlated with bottom water dissolved oxygen and negatively correlated with sediment toc c n ratio and total phytopigments community dominance showed the opposite pattern the strongly inverse correlation between oxygen and measures of sediment organic content made it difficult to distinguish their relative effects the strongly laminated sediments in the omz core contrasted with the homogeneous heavily bioturbated sediments above and below this zone but were associated with minimal macrofaunal biomass rather than distinctive functional group composition in general data from the oman margin were weak predictors of patterns seen off pakistan and results suggest the importance of local factors superimposed on the broader trends of macrofaunal community composition in omzs

high temperature magnetic fabric development from plastically deformed magnetite in experimental shear zones summary magnetic fabric development has been studied in synthetic magnetite silicate aggregates in a series of high temperature simple shear deformation experiments samples composed of magnetite grains with a nominal size of 20 40 m dispersed at 3 wt in a matrix of plagioclase were deformed between 1000 and 1200 c with a confining pressure of 300 mpa shear stresses in the range 10 130 mpa and shear strains up to 3 we determined that both magnetite and plagioclase were deformed plastically at these conditions an oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of ams rapidly increases with strain up to a value of 2 5 theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains the shape preferred orientation of magnetite grains after deformation determined from reflected light image analysis was used to calculate independent magnetite strain estimates these results were then compared with strains estimated from theoretical magnetic anisotropy development both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions

can flocs and water stable soil aggregates be differentiated within fluvial systems abstract this paper uses a number of particle structural characterizations and behavioral assessments in an attempt to differentiate flocs formed in the water column from water stable soil aggregates formed in the terrestrial environment but transported to a river by overland flow using discrete populations of flocs and soil aggregates the study successfully illustrated differences in floc structure as defined by fractal dimension sphericity measures density and porosity and behavior as defined by settling velocity flocs were generally found to be porous and irregular in shape with high water content low density and low settling velocity whereas soil aggregates were found to be rounder more compact dense with lower porosity and a higher settling velocity the techniques were however inappropriate for differentiating the two particle forms from river storm samples which are collected at the time when both particle forms are believed to be in suspension results suggest that aggregates once within the river system undergo a quick transformation into hybrid particles aggregate floc combinations due to flocculation processes occurring on the river bed following quick settling or during suspended sediment transport it is also possible that the aggregates undergo flocculation processes during overland flow forming hybrid particles prior to entering the river system or never reach the river altogether ending up in long term depositional areas in the terrestrial environment

estuarine flocs their size settling velocity and density the size and settling velocity of flocs of suspended sediments from the chesapeake bay were measured in laboratory experiments and in the field the curves between floc diameter and settling velocity showed a non stoke s relationship that was best described by the following settling velocity cm s 1 73 floc diameter cm 0 78 furthermore the relationship between floc diameter and settling velocity indicated that the floe density decreased as the floes increased in size in addition to changing density a cylinder shape was found to be the best shape factor

sediment resuspension flocculation and settling in a macrotidal estuary national natural science foundation of china 40876043 papd fund china scholarship council 2010832427 mel young scientist visiting fellowship melrs0938

the short term fate of organic carbon in marine sediments comparing the pakistan margin to other regions abstract pulse chase experiments with isotopically labelled phytodetritus conducted across the pakistan margin reveal that the impact of biological activities on benthic c cycling varies markedly among sites exhibiting different seafloor conditions in this study patterns of biological c processing across the pakistan margin oxygen minimum zone omz are compared with those observed in previous tracer studies variations in site environmental conditions are proposed to explain the considerable variations in c processing patterns among this and previous studies three categories of c processing pattern are identified 1 respiration dominated where respiration accounts for 75 of biological c processing and uptake by metazoan macrofauna foraminifera and bacteria are relatively minor processes these sites tend to show several although not necessarily all of the properties of being cold and deep and having low inputs of organic carbon to the sediment and relatively low biomass metazoan macrofaunal communities 2 active faunal uptake where respiration accounts for 2 h 1 and seem to be dominantly controlled by seafloor temperature rates of metazoan macrofaunal uptake of organic matter om on the pakistan margin are larger than those in most other studies and this is attributed to the large and active metazoan macrofaunal communities in the lower omz characteristic of omz boundaries finally biological mixing of pakistan margin sediments was reduced compared to that observed in comparable tracer studies on other margins this probably reflects faunal feeding and burrowing strategies consistent with low oxygen concentrations and a relatively abundant supply of sedimentary om

deformation and magnetic fabrics in ductile shear zones a review abstract the anisotropy of magnetic susceptibility ams is a well established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings the magnetic susceptibility of a rock k depends primarily on the nature and abundance of magnetic minerals the physical arrangement and lattice preferred orientation of these magnetic minerals give rise to magnetic anisotropy the ams scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy p 1 a proxy for fabric intensity and the shape factor 1 t 1 an indicator of the magnetic fabric symmetry prolate vs oblate a number of studies have shown that a positive correlation generally exists between p and strain thus the ams shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones however a number of caveats exist i the increase of p with strain cannot be solely attributed to deformation because p also increases with k regardless of deformation ii strain across shear zones is typically heterogeneous and is often localized in units of different lithology thus making the separation of the lithological and strain controls on ams difficult also deformation is commonly accompanied by mineral segregation or fluid rock interaction that induces changes in magnetic mineralogy iii even if the undeformed lithology was uniform across a shear zone variations in strain rate or temperature may result in different deformation mechanisms hence the relationship between p and strain depends strongly on both the mineral carriers of ams and on deformation mechanisms and iv the ams is unable to resolve composite fabrics such as those resulting from s c structures where minerals on the c and s planes respectively contribute to ams

in situ settling behavior of marine snow1 the settling velocities of undisturbed macroscopic aggregates known as marine snow were measured with scuba in surface waters off southern california and analyzed as a function of aggregate size mass and density the mean settling velocity was 74f39 m d l for aggregates ranging from 2 4 to 75 mm in maximum length sinking rates in the field varied exponentially with aggregate size and dry weight and were consistently up to four times slower than rates measured in the laboratory the excess densities of the 80 aggregates examined were calculated from volume and dry weight and ranged over four orders of magnitude with a median of 1 4 x 1o 4 g cm 3 aggregates of marine snow sank more slowly than predicted for either solid or porous spheres of equivalent volume and density although their velocities were within the range expected for equivalent sinking prolate ellipsoids no relationships between settling velocity and either excess density or particle shape were found drag coefficients of marine snow were also higher than predicted by theory for spheres of equivalent volume and density these deviations from theoretical expectations may be partially explained by errors in the estimation of the excess densities of aggregates variability in the densities of the heterogeneous primary particles comprising marine snow fecal pellets claymineral particlcs phytoplankton molts etc and the potential for buoyancy regulation by individual phytoplankton cells inhabiting aggregates make determination of excess density especially

carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the pakistan margin of the arabian sea abstract the factors controlling sedimentary organic matter distributions are the subject of ongoing debate we report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m located across the indus margin of the arabian sea the sites spanned the intense midwater oxygen minimum zone omz and showed dramatic differences in bottom water redox conditions benthic communities and degrees of bioturbation five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 high resolution sampling was also conducted across the upper 140 275 m and lower omz 600 1200 m boundaries the sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition the extent of organic matter cycling within the sediments under contrasting depositional conditions and the fate of organic matter deposited during the southwest monsoon as previously noted on this and other omz margins there was a roughly inverse spatial relationship between bottom water oxygen and sedimentary organic matter concentrations however variability in organic carbon content within the omz that was unrelated to oxygen and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower omz boundary rather than at the omz core indicate that factors other than oxygen contribute significantly as controls on carbon distributions despite a 3 fold range in organic carbon content carbon to nitrogen ratios showed no systematic trends across the omz with values reflecting uniform predominantly marine organic matter inputs nitrogen isotopic signatures 15n values in the range of 6 5 8 5 also did not show clear cross omz trends and are attributed to a 14n depleted nitrate pool created by pelagic denitrification there was little downcore change in organic matter content at any site suggesting either that organic matter decay occurs largely in the water column or that decay within the sediments at oxygenated sites is masked by bioturbation seasonal variability in organic matter content and composition was restricted to the most surficial sediments and changes were ephemeral suggesting rapid turnover of organic matter deposited during the southwest monsoon on timescales of weeks stable c isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the omz core this is attributed to a chemosynthetic bacterial imprint

sedimentary oxygen consumption and microdistribution at sites across the arabian sea oxygen minimum zone pakistan margin abstract biogeochemical processes and fluxes occurring across the sediment water interface on continental margins impacted by oxygen minimum zones omzs are important to bioelement cycles ocean inventories and productivity the nature and magnitude of these processes depend heavily on spatial and temporal variability in dissolved o 2 concentrations in bottom waters and porewaters in 2003 four research cruises to the indus margin of the arabian sea pakistan were undertaken to survey the benthic biogeochemical processes in the resident omz before and after the southwest monsoon sediment o 2 microdistribution and consumption rates were measured at five stations along a depth transect 140 1850 m across the omz during the spring intermonsoon and the late to post southwest monsoon periods using in situ benthic research platforms landers lander o 2 electrode data show that the intermonsoon and late to postmonsoon bottom water o 2 levels had little to no variation 300 m no change 940 m 1 7 2 8 m 1200 m 10 2 12 6 m and 1850 m 82 80 m in contrast at the shallowest station 140 m a large fluctuation occurred between the intermonsoon o 2 44 5 m and the late to postmonsoon o 2 1 m due to monsoon forced shoaling of the upper omz boundary oxygen did not penetrate into the sediments at the 300 m site during either sampling season during the intermonsoon season at the 140 m site o 2 penetrated to a depth of 3 mm but no measurable o 2 penetration occurred after the monsoon at the 940 and 1200 m sites o 2 penetration into the sediments was small ca 1 2 mm at both sites and did not measurably change between the two sampling seasons in contrast at the 1850 m site o 2 penetration decreased after the monsoon 18 12 mm calculated late to postmonsoon o 2 consumption rates were generally similar to or lower than intermonsoon values 0 vs 2 22 mmol m 2 d 1 at 140 m 0 37 vs 0 31 mmol m 2 d 1 at 1200 m and 0 73 vs 1 01 mmol m 2 d 1 at 1850 m the relatively small seasonal signal suggests that organic matter delivered during the monsoon period may have already been largely remineralized by the late to postmonsoon sampling period modelling of porewater o 2 profiles indicates that subsurface o 2 consumption associated oxidation of reduced inorganic species makes a significant contribution to total o 2 consumption at some sites similarly differences in o 2 consumption rates determined by porewater profile modelling and whole core incubations at some sites indicate significant contributions associated with bioturbation and bioirrigation

in situ measurements of particle settling velocity in the deep sea two instruments the remote optical settling tube and the benthic autonomous settling tube have been deployed on the nova scotian continental rise at a depth of 4820 m the few results obtained suggest that optical and gravimetric methods give comparable results assumptions about particle density little affect the settling velocity distribution but do scale the total magnitude by using a distribution of declining density with increasing size indicating increasing water content the total concentration inferred from the light attenuation by using mie theory is quite close to the total found from gravimetric data the coarser end of the size distribution changes more with overall change in concentration than the fine end the settling velocity distribution shows the presence of several modes the two principal ones can be matched with modal sizes of 3 3 m and 60 m in coulter counter data allowing inference of stokes density contrasts of 1 5 and 0 061 g cm 3 respectively at concentrations of about 200 mg m 3 more than 50 of the material has a settling velocity less than 2 10 4 cm s 1 probably 3 m in size but this percentage appears to decrease with an increase in total concentration

evidence for magnetostrictive control of intrinsic susceptibility and coercive force of multidomain magnetite in rocks abstract it is shown that intrinsic susceptibility xi susceptibility in the absence of self demagnetizing fields as well as coercive force hc of multidomain magnetite in rocks are likely to be commonly magnetostrictively controlled presumably through internal stresses opposing domain wall motion we use observations of the thermal variation of observed susceptibility x per cm3 of magnetite in the sample and coercive force hc on cooling to 130 k for seven basaltic rocks and for 0 25 0 21 mm magnetite grains dispersed in cement as expected theoretically with magnetostrictively controlled xi a plot of 1 x versus j s 2 for each sample where is the polycrystalline magnetostriction coefficient and js is saturation magnetization gives a reasonably straight line whose 1 x intercept agrees with n average self demagnetizing factor of magnetite grains in the sample given by h c j r n h c j r h c j ri where jr is saturation remanence and ri is jr when n 0 as expected theoretically with magnetostrictively controlled hc a plot of hc versus j s for each sample reveals an approximate proportionality this also implies magnetostrictive control of xi because a plot of 1 x versus h c j s for each sample yields a straight line of 1 x intercept agreeing with n from h c j r n h c j r h c j ri as expected if hc and xi are both magnetorestrictively controlled

benthic biological and biogeochemical patterns and processes across an oxygen minimum zone pakistan margin ne arabian sea abstract oxygen minimum zones omzs impinging on continental margins present sharp gradients ideal for testing environmental factors thought to influence c cycling and other benthic processes and for identifying the roles that biota play in these processes here we introduce the objectives and initial results of a multinational research program designed to address the influences of water depth the omz 150 1300 m and organic matter om availability on benthic communities and processes across the pakistan margin of the arabian sea hydrologic sediment and faunal characterizations were combined with in situ and shipboard experiments to quantify and compare biogeochemical processes and fluxes om burial efficiency and the contributions of benthic communities across the omz in this introductory paper we briefly review previous related work in the arabian sea building the rationale for integrative biogeochemical and ecological process studies this is followed by a summary of individual volume contributions and a brief synthesis of results five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 taken together the contributed papers demonstrate distinct cross margin gradients not only in oxygenation and sediment om content but in benthic community structure and function including microbial processes the extent of bioturbation and faunal roles in c cycling hydrographic studies demonstrated changes in the intensity and extent of the omz during the sw monsoon with a shoaling of the upper omz boundary that engulfed the previously oxygenated 140 m site oxygen profiling and microbial process rate determinations demonstrated dramatic differences in oxygen penetration and consumption across the margin and in the relative importance of anaerobic processes but surprisingly little seasonal change a broad maximum in sediment om content occurred on the upper slope roughly coincident with the omz but the otherwise poor correlation with bottom water oxygen concentrations indicated that other factors are important in determining sediment om distributions downcore profiles generally showed little clear evidence of in situ om alteration and there was little sign of om enrichment resulting from the southwest monsoon in sediments collected in the late to postmonsoon sampling this is interpreted to be due to rapid cycling of labile om organic geochemical studies confirmed that sediment om is overwhelmingly of marine origin across the margin but also that it is heavily altered with only small changes in degradation state across the omz more negative stable c isotopic compositions in surficial sediments at hypoxic sites within the omz core are attributed to a chemosynthetic bacterial imprint dramatic changes in benthic community structure occurred across the lower omz transition apparently related to om availability and quality as well as to do concentrations high resolution sampling biomarkers and isotope tracer studies revealed that oxygen availability appears to exert threshold type controls on benthic community structure and early faunal c processing biomarker studies also provided evidence of faunal influence on sediment om composition together the results offer strong evidence that benthic fauna at sites across the margin play important roles in the early cycling of sediment om through differential feeding and bioturbation activities

the effects of suspended sediment on turbulence within an estuarine turbidity maximum predictions of cohesive sediment transport are hampered by poor understanding of the effects of sediment suspensions upon the nature of near bed turbulence in this study current velocities associated turbulence and flow characteristics were measured within an estuarine turbidity maximum during two spring ebb tides measurements at two heights both within and above a near bed high concentration layer show the importance of intermittent internal waves the ratio of vertical to horizontal turbulence intensities exceeded 1 0 when waves were present the waves were more significant above the layer than within it comparison between the turbulent kinetic energy and the reynolds stresses showed inactive turbulence occurring at stresses less than about 0 2 n m 2 during these periods negative suspended sediment fluxes occurred indicating that the turbulence could enhance settling of the sediment the effect of the intermittent internal waves on the turbulent bursting structure and their interaction with the concentration profile appears to increase the variability of the reynolds fluxes of suspended sediment and cause downwards reynolds fluxes especially beneath the lutocline

canyon conditions impact carbon flows in food webs of three sections of the nazare canyon submarine canyons transport large amounts of sediment and organic matter om from the continental shelf to the abyssal plain three carbon based food web models were constructed for the upper 300 750 m water depth middle 2700 3500 m and lower section 4000 5000 m of the nazare canyon eastern atlantic ocean using linear inverse modeling to examine how the food web is influenced by the characteristics of the respective canyon section the models were based on an empirical dataset consisting of biomass and carbon processing data and general physiological data constraints from the literature environmental conditions most notably organic matter om input and hydrodynamic activity differed between the canyon sections and strongly affected the benthic food web structure despite the large difference in depth the om inputs into the food webs of the upper and middle sections were of similar magnitude 7 98 0 84 and 9 30 0 71 mmol c m 2 d 1 respectively om input to the lower section was however almost 6 7 times lower 1 26 0 03 mmol c m 2 d 1 carbon processing in the upper section was dominated by prokaryotes 70 of total respiration though there was a significant meiofaunal 21 and smaller macrofaunal 9 contribution the high total faunal contribution to carbon processing resembles that found in shallower continental shelves and upper slopes although the meiofaunal contribution is surprisingly high and suggest that high current speeds and sediment resuspension in the upper canyon favor the role of the meiofauna the high om input and conditions in the accreting sediments of the middle canyon section were more beneficial for megafauna holothurians than for the other food web compartments the high megafaunal biomass 516 mmol c m 2 their large contribution to respiration 56 of total respiration and secondary production 0 08 mmol c m 2 d 1 shows that these accreting sediments in canyons are megafaunal hotspots in the deep sea conversely carbon cycling in the lower canyon section was strongly dominated by prokaryotes 86 of respiration and the food web structure therefore resembled that of lower slope and abyssal plain sediments this study shows that elevated om input in canyons may favor the faunal contribution to carbon processing and create hotspots of faunal biomass and carbon processing along the continental shelf

variations in bioturbation across the oxygen minimum zone in the northwest arabian sea abstract oxygen minimum zones are expected to alter substantially the nature rates and depths of bioturbation along continental margins yet these effects remain poorly studied using excess 210 pb profiles sediment x radiography and box core samples for macrofauna we examined bioturbation processes at six stations 400 700 850 1000 1250 and 3400 m deep along a transect across the oxygen minimum zone omz on the oman margin bottom water oxygen concentrations ranged from 0 13 ml l 1 at 400 m to 2 99 ml l 1 at 3400 m 210 pb mixed layer depth and bioturbation intensity db exhibited high within station variance and means did not differ significantly among stations however the mean mixed layer depth 4 6 cm for pooled omz stations 400 1000 m depths 0 13 0 27 ml l 1 bottom water oxygen was half that for stations from similar water depths along well oxygenated atlantic and pacific slopes 11 1 cm suggesting that oxygen stress reduced 210 pb mixing depth on the oman margin modal burrow diameter and the diversity of burrow types at a station were highly correlated with bottom water oxygen concentration from the edge to the core of the oman omz spearman s rho 0 89 p 0 02 suggesting that these parameters are useful proxies for bottom water oxygen concentrations under dysaerobic conditions in contrast neither the maximum diameter and nor the maximum penetration depth of open burrows exhibited oxygen related patterns along the transect reduced 210 pb mixing depth within the oman margin omz appeared to result from a predominance of surface deposit feeders and tube builders within this zone rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass the number of burrow types per station was highly correlated with macrofaunal species diversity suggesting that burrow diversity may be a good proxy for species diversity in paleo dysaerobic assemblages we conclude that bottom water oxygen concentrations of 0 13 0 27 ml l 1 substantially alter a number of bioturbation parameters of importance to diagenetic and biofacies models for continental margins

variability in cohesive sediment settling fluxes observations under different estuarine tidal conditions the mass settling flux which is defined as the product of the concentration and the settling velocity is of prime importance with respect to both stratified and well mixed estuarine conditions the determination of these fluxes for applied modelling purposes in high energy tidal estuarine environments is very problematic this is because the muddy sediments which dominate in estuaries flocculate producing a variety of sizes and settling velocities and this flocculation process is not understood well enough to be fully described theoretically by drawing on examples of floc spectra acquired in situ using the inssev system this study explains how mass settling fluxes in the near bed region can vary by three or four orders of magnitude in meso and macro tidal estuaries throughout a single tidal cycle a floc population representative of dilute suspension conditions on a neap tide indicated only 35 of the floc mass was macroflocs 160 m however the macrofloc settling velocity 2 4 mm s 1 three times faster than the microflocs which meant the former fraction contributed 57 of the 205 mg m 2s 1 settling flux both highly concentrated 4 6 g l 1 and very turbulent spring tide conditions 1 6 n m 2 produced a bi modal distribution in terms of the floc size and dry mass with the former 54 of the mass was contained within the 240 480 m size fraction with a further 25 of the dry floc mass in the flocs over 480 m in diameter these large flocs had settling velocities between 4 8 mm s 1 which meant 99 5 of the settling flux 33 5 g m 2s 1 was accredited to the macroflocs the high turbulence environment saw the dry floc mass distribution shift 60 40 in favour of the microflocs the microfloc settling velocity was 1 45 mm s 1 0 35 mm s 1 faster than the larger macrofloc fraction in terms of the total mass settling flux 0 9 g m 2 s 1 this translates into the microflocs contributing 70 during high turbulence at slack water the flux only reached 12 mg m 2 s 1 and macrofloc growth was mainly attributed to differential settling continuous floc observations made over a complete tidal cycle revealed that the asymmetrical distribution of the tidal energy generated throughout the spring conditions in the tamar estuary demonstrated a distinct control on the flocculation process the less turbulent ebb produced 86 of the total tidal cycle mass settling flux of which only 8 of the settling flux was outside the turbidity maximum an attempt to simulate these large settling fluxes by using a constant settling rate of 0 5 mm s 1 under estimated the tidal cycle settling flux by 78 with less than 15 of the total flux being estimated during the advection of the turbidity maximum on the ebb in contrast using a faster constant settling velocity parameter of 5 mm s 1 representative of the macrofloc fraction resulted in a mass flux over estimate of 116 for the tidal cycle duration

determination of self demagnetizing factor n for multidomain magnetite grains in rock abstract the magnetic poles on a multidomain grain with a magnetization j produce a field nj tending to demagnetize the grain strongly affecting many magnetic properties of rocks bearing multidomain magnetite the average self demagnetizing factor n of multidomain grains in a rock is thus an important parameter which is shown to be simply measured from a high field hysteresis loop using h c j r n h c j r h c j r i where h c is the coercive force j r the saturation remanence and j r i an estimate of j r when n 0 measuring magnetic susceptibility x and coercive force h c as a function of low temperature and then plotting 1 x versus h c j s where j s is saturation magnetization gives a straight line whose intercept on the 1 x axis also estimates n these two independent estimates of n are shown to agree for the seven magnetite bearing basaltic rocks studied yielding an average n 1 7 e m u and for the 0 2 mm natural magnetite grains yielding n 2 6 0 2 e m u in contrast estimating n from h c j r 1 x i where x i is x when n 0 is shown to be invalid and even use of n 1 x h c 45 is not recommended measured n is much below the 4 e m u expected of large equidimensional magnetite grains which is understandable for the rocks of higher coercive force but makes further study of rocks of low coercive force desirable

faunal responses to oxygen gradients on the pakistan margin a comparison of foraminiferans macrofauna and megafauna the pakistan margin is characterised by a strong mid water oxygen minimum zone omz that intercepts the seabed at bathyal depths 150 1300 m we investigated whether faunal abundance and diversity trends were similar among protists foraminiferans and gromiids metazoan macrofauna and megafauna along a transect 140 1850 m water depth across the omz during the 2003 intermonsoon march may and late post monsoon august october seasons all groups exhibited some drop in abundance in the omz core 250 500 m water depth o2 0 10 0 13 ml l 1 4 46 5 80 m but to differing degrees densities of foraminiferans 63 m were slightly depressed at 300 m peaked at 738 m and were much lower at deeper stations foraminiferans 300 m were the overwhelmingly dominant macrofaunal organisms in the omz core macrofaunal metazoans reached maximum densities at 140 m depth with additional peaks at 850 940 and 1850 m where foraminiferans were less abundant the polychaete linopherus sp was responsible for a macrofaunal biomass peak at 950 m apart from large swimming animals fish and natant decapods metazoan megafauna were absent between 300 and 900 m o2 0 2 ml l 1 8 92 m the progressively deeper abundance peaks for foraminiferans 63 m linopherus sp and ophiuroids probably represent lower omz boundary edge effects and suggest a link between body size and tolerance of hypoxia macro and megafaunal organisms collected between 800 and 1100 m were dominated by a succession of different taxa indicating that the lower part of the omz is also a region of rapid faunal change species diversity was depressed in all groups in the omz core but this was much more pronounced for macrofauna and megafauna than for foraminiferans oxygen levels strongly influenced the taxonomic composition of all faunal groups calcareous foraminiferans dominated the seasonally and permanently hypoxic sites 136 300 m agglutinated foraminiferans were relatively more abundant at deeper stations where oxygen concentrations were 0 13 ml l 1 5 80 m polychaetes were the main macrofaunal taxon within the omz calcareous macrofauna and megafauna molluscs and echinoderms were rare or absent where oxygen levels were lowest the rarity of larger animals between 300 and 700 m on the pakistan margin compared with the abundant macrofauna in the omz core off oman is the most notable contrast between the two sides of the arabian sea this difference probably reflects the slightly higher oxygen levels and better food quality on the western side

properties of suspended sediment over the hebble area on the nova scotian rise abstract twenty samples from 20 40 250 and 1000 m above bottom were analysed by coulter counter for size using three overlapping tubes giving a size range of 1 59 256 m the principal features are a fine mode in the region 2 8 m and a coarser one between 20 and 60 m aperture shear was probably responsible for lack of particles counted over 100 m log log cumulative particle number distributions are mainly wavy or curved but have segments particularly 1 6 10 m with a slope approaching 3 signifying equal particle volumes in logarithmic size grades if waviness and coarse end truncation are ignored there is a rough overall fit to 3 in many cases the samples were also filtered to determine weight concentration and examined by scanning electron microscope aggregates were common in most samples particularly with associated mucus blobs and strings of mucus were common with and without particles apparent density of particles a increases with overall concentration but b decreases with increasing concentration when only larger particles are included this probably reflects a aggregation during ageing of a suspension and b presence of more mucus in larger coarse mode populations

flocculation a key process in the sediment flux of a large glacier fed lake this study investigates the consequences of flocculation for sediment flux in glacier fed lillooet lake british columbia based on density fractal dimension in situ profiles of sediment concentration and size distribution and settling velocity equations presented in the literature sediment flux attributed to macroflocs during the late spring and summer accounts for a significant portion of sediment flux in the lake equivalent to at least one quarter of the average annual sediment flux fine sediment is reaching the lake floor faster in flocs than occurs if settling as individual grains this flux varies both spatially and temporally over the observation period suggesting a link between deposition via flocculation and the properties of bottom sediments macrofloc flux increased through june reached a peak during july and then declined into august macrofloc flux was greatest in the distal end of the first basin approximately 10 km from the point of inflow relatively high excess densities 0 1 g cm 3 at 500 m for flocs in situ are consistent with a composition dominated by inorganic primary particles microlaminations within lillooet lake varves have been linked by earlier workers to discharge events and the action of turbidity currents emanating from the lillooet river while turbidity currents undoubtedly occur in lillooet lake these results demonstrate flocculation as an adjunct process linking discharge lake level macrofloc flux bulk density and microlaminations in situ measurements of sediment settling velocity in glacier fed lakes are required to better constrain flux rates and permit comparison between flocculation in lacustrine environments with existing studies of estuarine marine and fluvial flocculation copyright 2009 john wiley sons ltd

uplift estimated from remanent magnetization munro area of superior province since 2150 ma ago a method for determining depth of burial from remanent magnetization was applied to three igneous contacts of two different ages in munro township of the superior province of the canadian shield sampling profiles perpendicular to the contact of a 2150 ma diabase dike one collected in an older 2690 ma diabase dike and a second in gabbro country rock yielded ambient temperatures for the present erosion surface at the time of dike emplacement of 219 23 and 181 7 c respectively a third sampling profile perpendicular to a 2690 ma dike failed to provide a usable magnetic contact zone a secondary magnetic component with shallow inclination and easterly declination was detected in individual samples collected from both dikes as well as from the surrounding country rock converging remagnetization circles indicate that this component acquired some time after emplacement of the younger 2150 ma dike has the direction d 70 i 39 and corresponding paleopole 39 e 4 s

a laboratory examination of floc characteristics with regard to turbulent shearing abstract turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters and their respective aggregate break up this study examines the inter relationships between floc characteristics over increasing turbidity 80 200 mg l 1 and turbulent shear 0 0 6 n m 2 environments by the use of a laboratory flume within which a suspension can be sheared at a controlled rate and with an unintrusive macro lens miniature video camera mounted in a viewing port on the flume channel wall the camera enables the direct simultaneous measurement of both floc size and settling velocity from which accurate estimates of floc effective density and porosity can be made measurements were made 120 s after the induced turbulence has ceased the instrument has an upper viewing turbidity limit of 210 mg l 1 and a lower resolution of 20 m the sediment was collected from the inter tidal mudflats at weir quay on the tamar estuary in devon southwest england the results indicated that increasing turbidity at low shear levels encouraged floc growth but the effect of the increasing turbulent shear 0 35 n m 2 together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process at shears up to 0 35 n m 2 the largest size and settling velocity flocs were produced at high concentrations whereas above 0 35 n m 2 disruption caused smaller flocs at higher concentrations the use of algorithms which were based either on a single floc characteristic i e size or settling velocity or a mean fractal dimension were seen not to accurately approximate the experimental data a multiple regression analysis of the experimental data produced the following formula based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions referred to as max20size mean values settling velocity 0 301 0 00337 rms of the gradient in turbulent velocity fluctuations 0 000606 spm 0 00335 floc size which proved to be the most accurate representation with an r 2 value of 0 95 a similar formula was determined for the average value of the four fastest settling flocs within each sample group max4w s this highlights the importance of modelling algorithms that are developed from data that take into account effective density variations i e simultaneous size and settling velocity measurement