geochemical effects of magmatic solitary waves ii some analysis summary as a complement to the numerical solutions of watson spiegelman 1994 hereafter known as paper i we present some analytic solutions and proofs to illustrate more generally how a single magmatic solitary wave affects the transport of trace elements in the absence of any diffusion or dispersion the equations for trace element transport can be solved by the method of characteristics this analysis shows that without diffusion a solitary wave can transport chemical signals but cannot permanently change the shape of an initial trace element distribution the solitary waves however can locally steepen trace element gradients by a distortion factor s which depends only on the ratio of initial and local transport velocities these results are quite general and are expected for any solitary wave of permanent form and constant velocity because these waves always provide a frame of reference where the melt and solid velocities are steady state

what controls submarine channel development and the morphology of deltas entering deep water fjords river deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet these systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords and why some locations have well developed submarine channels whilst others do not deltas and associated turbidity current systems are analysed initially in five fjord systems from british columbia in canada and then more widely this provides the basis for a general classification of delta and turbidity current system types where rivers enter relatively deep 200 m water fjord delta area is found to be strongly bimodal avalanching of coarse grained bedload delivered by steep mountainous rivers produces small gilbert type fan deltas whose steep gradient 11 25 approaches the sediment s angle of repose bigger fjord head deltas are associated with much larger and finer grained rivers these deltas have much lower gradients 1 5 10 that decrease offshore in a near exponential fashion the lengths of turbidity current channels are highly variable even in settings fed by rivers with similar discharges this may be due to resetting of channel systems by delta top channel avulsions or major offshore landslides as well as the amount and rate of sediment supplied to the delta front by rivers

particle size related mineral magnetic source sediment linkages in the rhode river catchment maryland usa characterization of both the source materials and the bottom sediments of an estuary by magnetic characters on a particle size basis allows the source of the bottom sediments to be determined and indicates a shift in sediment sources corresponding to changing land use patterns the major source of the sediments in the lower part of cores from the middle of the estuary is eroding shorelines soil derived particulates from the uplands dominate the upper part of the cores this change in source occurred in the early part of the 19th century the magnetic character of the most recent sediments suggests a progressive depletion through time of the upland soils in the finest fractions

sediment sources and transport pathways in a rural catchment herefordshire uk the relative significance of primary and secondary suspended sediment sources has been identified in a small 15 km2 agricultural catchment through the application of a linear unmixing model using mineral magnetic geochemical and radionuclide signatures of fine sediments roads appear to be important as a secondary source of suspended sediment and in the conveyance of topsoils to the river channel roads also alter the source signatures of sediment in transport especially that derived from agricultural topsoils modelling suggests that c 30 of the suspended sediment collected from suspended sediment traps in the river leadon were derived from or transported via roads relative contributions from primary sources of c 43 and c 27 were estimated from a combined subsoil and channel bank subsurface source and a combined arable and grassland topsoil source respectively

large scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows large scale 20 m to 7 km wavelength bedforms are common on the seafloor yet there is a lack of consensus on how they form and thus what to call them we conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments the data form three distinct groups 1 small scale 20 300 m wavelength sediment waves with mixed relief made of medium sand to cobble sized sediment that form in confined settings which we call small sediment waves 2 large scale 300 7000 m wavelength sediment waves with mixed relief made of fine grained sediment that form in relatively unconfined settings which we call large sediment waves and 3 large scale fully enclosed depressions in the seafloor which we call scours there is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact this data gap probably results from the environments in which sediment waves form being either confined e g channel or canyon or unconfined e g open slope bedform migration direction is available for 36 of the data and includes small and large scale sediment waves of these examples all are shown to migrate up current up current migration is indicative of supercritical flow thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves therefore we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size the migration direction for scours remains unknown scours may form from similar processes to small and large sediment waves or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre existing defects in the seafloor this novel statistical analysis of a global database shows that up current migrating bedforms associated with supercritical flow are unusually widespread and are recognised at two distinct scales

a new method in mineral magnetism for the separation of weak antiferromagnetic signal from a strong ferrimagnetic background 1 this paper describes a new magnetic method for determining the contribution of weakly magnetic hematite against a strongly magnetic e g magnetite background the method involves measurements of mfr a residual or final remanence after hysteretic demagnetization of saturated isothermal remanent magnetization in terms of sensitivity and precision our proposed method is superior to the currently popular approach known as the hirm method compared to the latter mfr has no error amplification caused by subtraction of terms the method utilizes the available software for hysteretic demagnetization function on an automated vibrating sample magnetometer vsm this in turn automatically decreases some of the disturbing effects of the strong magnetic background e g magnetic viscosity initially non zero offset of hirm for partially oxidized magnetite etc

sediment sorting and beach erosion along a coastal foreland magnetic measurements in point pelee national park ontario canada abstract in situ and laboratory based magnetic measurements were made on sand samples from the beach on the eastern side of point pelee national park ppnp in northwestern lake erie in order to examine both the composition and spatial distribution of magnetic minerals magnetic remanence measurements on bulk and particle size fractions showed that the magnetic properties were dominated by multidomain ferrimagnetic grains probably magnetite and or maghemite particle size specific measurements on different grain size fractions showed that the

suspended sediment characterization and tracing using a magnetic fingerprinting technique bassenthwaite lake cumbria uk robust identification of catchment suspended sediment sources is a prerequisite both for under standing sediment delivery processes and targeting of effective mitigation measures fine sediment delivery can pose universal management problems especially with regard to nutrient run off and lake siltation here 19 sus pended sediment samplers were located within the three main tributary inflows to lake bassenthwaite a key but vulnerable site of special scientific interest with water quality problems linked to accelerated delivery of fine sediment magnetic properties of contemporary suspended sediments collected on a monthly basis were measured on a particle size specific basis and compared with the lake sediment core tops ferrimagnetic grain size and magnetic hardness vary significantly between the suspended sediments collected from the different tributaries with the 8 31 m and 31 63 m clastic grain fractions displaying greatest magnetic contrasts post depositional formation of bacterial magnetosomes is evident in the 2 8 m and 2 m fractions of the lake sediments thus for comparison with the potential source suspended sediments we use only the detrital clas tic fractions 8 31 m and 31 63 m the lake sediment magnetic properties show little spatial variation indi cating through lake transport of sediment no evidence was found of postdepositional diagenetic sulphide formation magnetic comparisons between the potential sources and the lake surface sediments indicate that newlands beck providing only 10 of the lake s hydraulic load is the main contributor of sediment to the deep basin of the lake sediments from the river derwent subcatchment contributing 80 of the hydraulic load are possibly stored either on the floodplain or in shallower areas of the lake

sediment provenance in a tropical fluvial and marine context by magnetic fingerprinting of transportable sand fractions the sources and fluxes of sediment to the great barrier reef lagoon from northeastern australian rivers have been the subject of much concern and study with the large catchments of the burdekin and fitzroy rivers thought to be the key sources at present here the utility of newly developed magnetic fingerprinting methods for identifying sediment provenance both onshore and offshore and in association with individual large flood events is investigated within the burdekin catchment sediments are mobilized from different subcatchments by runoff generated by intense localized rainfall events magnetic measurements were made on untreated and acid treated samples of river channel sediments within the burdekin river subcatchments and from the estuarine and inner shelf depocentres of burdekin river sediments the acid treatment removes all discrete magnetic particles and coatings and leaves magnetic inclusions protected within host silicate grains as the basis of the measured magnetic signature of a sample the magnetic properties of the acid treated samples display statistically distinct sediment provenance groupings sand samples from the upper burdekin river appear magnetically distinct from samples from tributaries of the burdekin e g hann creek fanning river and also from nearby coastal rivers including the haughton suspended sand samples from a burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999 comparisons of the terrestrial acid treated sand fractions with the same acid treated sand size fractions from transects taken offshore suggest that the surface sediments in upstart bay and bowling green bay have different sources some of these sources are as yet unidentified but may represent the unsampled lower discharge south western burdekin subcatchments and or along shore drift of sand from the south perhaps even from the fitzroy river over millennial timescales of cyclone pumping the magnetic inclusion method precludes any obfuscation or confounding of sediment source which might arise from hydraulic sorting and or post depositional magnetic diagenesis or authigenesis

bedforms on the submarine flanks of insular volcanoes new insights gained from high resolution seafloor surveys a comparative analysis of bedform fields along the submarine flanks of insular volcanoes characterized by different morpho structural settings volcanic and meteo marine regimes vanuatu kermadec bismark madeira and aeolian archipelagos is presented here to provide insights on the size distribution morpho dynamic and genesis of such bedforms two main types of bedforms are recognized according to their size location and preconditioning triggering processes small scale bedforms have wavelengths of tens to hundreds of metres and wave heights of metres because of their small size they are typically not recognizable at water depths greater than 400 m from vessel mounted bathymetric surveys however few examples of small scale bedforms are reported from upper volcanic flanks where steep gradients commonly hinder their formation their recognition is mostly limited to the thalweg of shallow and flat bottomed channels that carve the insular shelf on slope gradients 15 small scale bedforms are mostly related to erosional depositional processes due to sedimentary gravity flows that are often the result of a cascading effect between volcanic and non volcanic processes for example flood discharges and retrogressive landslides large scale bedforms occur at all water depths having wavelengths of hundreds thousands of metres and wave heights up to few hundreds of metres the origin of large bedforms is more difficult to ascertain especially if only bathymetric data are available some diagnostic criteria are presented to distinguish between bedforms associated with landslide deposits and those associated with density currents in this latter case relevant sediment sources and slope gradients 8 are key factors for bedform development erosional depositional bedforms are typically related to eruption fed density flows formed during large caldera collapses or to large turbidity flows bedforms generated by turbidity flows are often observed in the lower volcanic flanks where an abrupt decrease of gradients is present often matching a change from confined to unconfined settings in summary this study provides insights to interpret bedforms in modern and ancient marine volcaniclastic settings elsewhere

fingerprinting upland sediment sources particle size specific magnetic linkages between soils lake sediments and suspended sediments accelerated erosion of fine grained sediment is an environmental problem of international dimensions erosion control strategies and targeting of mitigation measures require robust and quantitative identification of sediment sources here we use magnetic fingerprinting to characterize soils and examine their affinity with and contribution to suspended sediments transported within two subcatchments feeding bassenthwaite lake northwest england a high resolution soil magnetic susceptibility survey was made using a field susceptometer zh instruments sm400 probe combining the spatial and vertical down profile soil magnetic data a subset of soil profiles was selected for detailed laboratory based magnetic remanence analyses the magnetic properties of the catchment soils are highly particle size dependent magnetic analyses were performed on the 31 63 m fraction for particle size specific comparison both with the suspended sediments and lake sediments fuzzy cluster analysis groups the soil magnetic data into six clusters apparently reflecting variations in parent material and horizon type with three magnetically hard soils as unclassified outliers examination of the cluster affinity of the soils suspended sediments and lake sediments indicates that topsoils of the upper newlands valley and subsoils around keskadale beck are a major source of the newlands beck suspended load and the recent post nineteenth century sediments in the deep lake basin older lake sediments show strong affinity with a small number of the derwent suspended sediments and one of the glenderamackin soils a large number of derwent suspended sediments show no affinity with any of the soils or lake sediments instead forming a coherent discrete and statistically unclassified group possibly resulting from mixing between the magnetically hard subsoils of the medium to high altitude glenderamackin and troutbeck areas and softer lower altitude glenderamackin soils the lack of any affinity of these suspended sediments with the lake sediments may indicate deposition along the derwent flood plain and or in the shallow delta of lake bassenthwaite particle size specific magnetic fingerprinting is thus shown to be both highly discriminatory and quantitatively robust even within the homogeneous geological units of this catchment area such a methodological approach has important implications for small large scale catchment management where sources of sediment arising from areas with uniform geology have been difficult to determine using other approaches such as geochemical or radionuclide analyses

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

modelling magnetization data using simplex two new approaches to modelling magnetization data are described the first approach models the magnetic properties of natural materials in terms of mixtures of possible source materials the second approach models the magnetic properties of natural materials in terms of mixtures of magnetite and haematite crystals of varying concentrations and grain sizes in the second approach the magnetic characteristics of magnetite and haematite and their variation with grain size are described by bicubic spline functions the methods are applied to remanence and susceptibility measurements on a suite of lake sediment particle size fractions and on a set of samples from a soil profile

use of composite fingerprints to determine the provenance of the contemporary suspended sediment load transported by rivers sediment fingerprinting appears to offer a valuable alternative to direct monitoring for elucidating the provenance of suspended sediment and the relative importance of spatial zones or subcatchments comprising larger 500 km 2 drainage basins against this background a quantitative composite fingerprinting technique incorporating both statistically verified multicomponent signatures and a multivariate sediment mixing model has been employed to determine the spatial origin of contemporary suspended sediment transported from the upper and middle reaches of the river exe 601km 2 and river severn 4325 km 2 basins uk spatial origin is addressed in terms of the relative contribution from three distinct geological subareas constituting each study basin the consistency of the composite fingerprinting approach is examined using the estimates for mean and seasonal variations in source area contributions and also a comparison between the results obtained for individual flood events and alternative lines of evidence provided by flood travel times and the spatial distribution of precipitation it is argued that fingerprinting estimates for sediment provenance are consistent with existing information on suspended sediment yields from different subcatchments within the study basins although in the severn the role of storage and remobilization in producing signature averaging may complicate comparison of the fingerprinting data with typical floodwater routing times validation represents the greatest problem for the cost benefit of fingerprinting and scope still exists for further refinement of the procedures involved 1998 john wiley sons ltd

pyroclastic dune bedforms macroscale structures and lateral variations examples from the 2006 pyroclastic currents at tungurahua ecuador pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions for much of their dynamics they behave as particulate density currents and share similarities with turbidity currents they occasionally deposit dune bedforms with peculiar lamination patterns from what is thought to represent the dilute low concentration and fluid turbulence supported end member of the pyroclastic currents here we present a high resolution dataset of sediment plates lacquer peels with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the august 2006 eruption of tungurahua ecuador most of the sedimentary features contain backset bedding and preferential stoss face deposition from the ripple scale few cm to the largest dune bedform scale several m length similar patterns of erosive based backset beds are evidenced recurrent trains of sub vertical truncations on the stoss side of structures reshape and steepen the bedforms in contrast sporadic coarse grained lenses and lensoidal layers flatten bedforms by filling troughs the coarsest clasts up to 10 cm least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend the stratal architecture exhibits strong lateral variations within tens of centimeters with very local truncations both in flow perpendicular and flow parallel direction we infer that the bedforms sedimentary patterns result from four formation mechanisms differential draping slope influenced saltation truncative bursts and granular based events whereas most of the literature makes a straightforward link between backset bedding and froude supercritical flows we reconsider this interpretation indeed features that would be diagnostic of subcritical dunes antidunes and chute and pools can be found on the same horizon and in a single bedform only laterally separated by short distances 10s of cm our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as gortler vortices backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre existing morphology quantification of near bed flow velocities are made via comparison with wind tunnel experiments we estimate that shear velocities of ca 0 30 m s 1 equivalent to pure wind velocity of 6 to 8 m s 1 at 10 cm above the bed could emplace the constructive bedsets whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 40 m s 1

the late holocene history of gormire lake ne england and its catchment a multiproxy reconstruction of past human impact sedimentary evidence spanning the last 3500 years from cores taken from the centre of gormire lake cleveland northeast england points to two periods of deforestation and catchment erosion the first mainly late iron age romano british phase began c 200 bc and ended c ad 600 the second began c ad 1200 the time of early monastic activity and continued into the nineteenth century the chronology for the core rests on 137cs 210pb and 14c measurements and on the history of lead accumulation in the sediments the majority of the ams 14c dates are shown to be too old to varying degrees depending on the source of the organic matter in the sediments the record of vegetation change is derived from pollen analysis evidence for changes in sediment composition and source and for catchment erosion comes from magnetic measurements element analyses using xrf and organic biogeochemical analyses there is a high degree of coherence between all these lines of evidence interpretation of the magnetic record

on the architecture of intra formational mass transport deposits insights from the carbonate slopes of great bahama bank and the apulian carbonate platform abstract large sub tropical carbonate platforms shed huge volumes of sediment onto their adjacent slopes resulting carbonate successions are comprised of gravity flow beds turbidites debrites which are occasionally reworked into mass transport deposits mtds by the failure of inconsistently lithified portions of slope strata despite their significance in the depositional record the internal architecture of mtds along relatively tectonically stable and sediment producing carbonate platforms is poorly understood in comparison with counterparts in active compressional tectonic settings this study compares carbonate slope derived mtds imaged in seismic reflection data with those observed in outcrop the internal architecture of mtds from two case studies is presented the seismically imaged mtds in miocene recent carbonate slope deposits from the bahamas and mtds off the cretaceous apulian carbonate margin exposed in outcrops of the ionian basin albania understanding the large scale kilometers to tens of kilometers internal architecture of mtds in seismic is crucial for identification of these structures in outcrop in turn only outcrop studies allow the detailed analysis of the reworking style within these massive sediment bodies at a smaller scale meters to kilometers the internal architecture of the studied carbonate mtds testifies to the existence of an along slope continuum of deformation structures characterized by vertical partitioning into distinct mass flows during single mass transport events mass wasting processes are inferred from strain partitioning structures recording along slope and slope parallel architectural heterogeneities three deformation domains are recognized extensional headwall domain translational body domain and contractional toe domain confined mtds are limited by steep and deep lateral margins and frontal ramps that mark the boundaries between failed deposits and undisturbed strata vertical strain partitioning is pronounced in the toe domain upward intensification of deformation within mtds is common and corresponds to a top down reorganization of failed strata during downslope transport intra formational mtds along carbonate slopes that are lacking exotics such as the ones studied here differ from extra formational mtds occurring in compressional tectonic settings that form sedimentary melanges ductile layer deformation structures are predominant in carbonate mtds as opposed to block in matrix fabrics in sedimentary melanges the development of shear surfaces rather than ductile shear zones in the body and toe domains presents predictive in modern environments and diagnostic criteria in fossil examples of mass wasting events along carbonate slopes

high field remanence properties of synthetic and natural submicrometre haematites and goethites significance for environmental contexts haematite and goethite are significant magnetic components both of marine and terrestrial sediments variable magnetic behaviour in haematite and goethite has been reported reflecting variations in grain size crystal defects nonstoichiometry and mode of formation here we provide new data first for a range of synthetic haematite and goethite powders of known grain size and second for a variety of haematite and goethite bearing natural samples including red bed samples and modern and fossil soils based on the synthetic data we identify two high field parameters which may be of value in identifying and characterizing the high coercivity components of natural environmental samples even when these are dominated magnetically by the presence of trace concentrations of ferrimagnets h indicates the proportion of the room temperature rt remanence acquired in fields from 2 tesla t up to 7 t and hcool indicates the increase or decrease in high field remanence upon cooling in zero field to 77 k lt used in tandem these parameters can differentiate between goethite dominated and haematite dominated samples and indicate differences in haematite grain size h values for the synthetic haematites vary between 27 and 38 with some apparent grain size dependence and for the goethites between 84 and 92 hcool values range from 130 to 157 for the goethites and 75 to 95 for those haematites of grain size 100 nm

how turbidity current frequency and character varies down a fjord delta system combining direct monitoring deposits and seismic data submarine turbidity currents are one of the most important processes for moving sediment across our planet they are hazardous to offshore infrastructure deposit petroleum reservoirs worldwide and may record tsunamigenic landslides however there are few studies that have monitored these submarine flows in action and even fewer studies that have combined direct monitoring with longer term records from core and seismic data of deposits this article provides one of the most complete studies yet of a turbidity current system the aim here is to understand what controls changes in flow frequency and character along the turbidite system the study area is a 12 km long delta fed fjord howe sound in british columbia canada over 100 often powerful up to 2 to 3 m sec 1 events occur each year in the highly active proximal channels which extend for 1 to 2 km from the delta lip about half of these events reach the lobes at the channel mouths however flow frequency decreases rapidly once these initially sand rich flows become unconfined and only one to five flows run out across the mid slope each year many of these sand rich channelized delta sourced flows therefore dissipated over a few hundred metres once unconfined rather than eroding and igniting upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes and also across the unconfined mid slope these supercritical flows deposited thick sand beds in proximal channels and lobes but thinner and finer beds on the unconfined mid slope the distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years this study shows how sand rich delta fed flows dissipate rapidly once they become unconfined that supercritical flows dominate in both confined and unconfined settings and how a second type of more hazardous and much less frequent event is linked to a different scale of margin failure

erosion and deposition by supercritical density flows during channel avulsion and backfilling field examples from coarse grained deepwater channel levee complexes sandino forearc basin southern central america abstract erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans field examples from coarse grained channel levee complexes from the sandino forearc basin southern central america show that cyclic step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion bypass levee construction and channel backfilling during channel avulsion large scale scour fill complexes 18 to 29 m deep 18 to 25 m wide 60 to 120 m long were incised by supercritical density flows the multi storey infill of the large scale scour fill complexes comprises amalgamated massive normally coarse tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones interpreted as deposits of the hydraulic jump zone of cyclic steps the large scale scour fill complexes can be distinguished from small scale channel fills based on the preservation of a steep upper margin and a coarse grained infill comprising mainly amalgamated hydraulic jump zone deposits channel fills include repeated successions deposited by cyclic steps with superimposed antidunes the deposits of the hydraulic jump zone of cyclic steps comprise regularly spaced scours 0 2 to 2 6 m deep 0 8 to 23 m long infilled by intraclast rich conglomerates or pebbly sandstones displaying normal coarse tail grading or backsets these deposits are laterally and vertically associated with subhorizontally stratified low angle cross stratified or sinusoidally stratified sandstones and pebbly sandstones which were deposited by antidunes on the stoss side of the cyclic steps during flow re acceleration the field examples indicate that so called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate grain size distribution and amalgamation the deposits of small scale cyclic steps with superimposed antidunes form fining upwards successions with decreasing antidune wavelengths indicating waning flows such cyclic step antidune successions form the characteristic basal infill of mid fan channels and are inferred to be related to successive supercritical high density turbidity flows triggered by retrogressive slope failures

using magnetic susceptibility to facilitate more rapid reproducible and precise delineation of hydric soils in the midwestern usa abstract standard field indicators currently used for hydric soil delineations usda nrcs 1998 field indicators of hydric soils in the united states version 4 0 in g w hurt et al ed united states department of agriculture nrcs fort worth tx are useful but in some cases they can be subjective difficult to recognize or time consuming to assess magnetic susceptibility ms measurements acquired rapidly in the field with a portable meter have great potential to help soil scientists delineate and map areas of hydric soils more precisely and objectively at five sites in illinois from 5 to 15 ha in area with contrasting soil types and glacial histories the ms values of surface soils were measured along transects and afterwards mapped and contoured the ms values were found to be consistently higher in well drained soils and lower in hydric soils reflecting anaerobic deterioration of both detrital magnetite and soil formed ferrimagnetics at each site volumetric ms values were statistically compared to field indicators to determine a critical ms value for hydric soil delineation such critical values range between 22 10 5 and 33 10 5 si in silty loessal or alluvial soils in illinois but are as high as 61 10 5 si at a site with fine sandy soil a higher magnetite content and slower dissolution rate in sandy soils may explain the difference among sites with silty parent material the lowest critical value 22 10 5 si occurs in soil with low ph 4 5 5 5 since acidic conditions are less favorable to ferrimagnetic mineral neoformation and enhance magnetite dissolution because of their sensitivity to parent material properties and soil ph critical ms values must be determined on a site specific basis the ms of studied soil samples 0 5 cm depth is mainly controlled by neoformed ultrafine ferrimagnetics and detrital magnetite concentrations with a minor contribution from anthropogenic fly ash neoformed ferrimagnetics are present in all samples but based on high fd 5 to 10 are most prevalent in high ph mollisols of northeastern illinois scanning electron microscope images display significantly more detrital magnetite alteration in hydric soils substantiating that reductive dissolution of magnetite aided by microorganisms is a primary cause for lower ms fly ash comprises 8 50 of the 5 m strongly magnetic particles and typically accounts for 5 15 of the total ms signal the proportion of fly ash in 5 m strongly magnetic fractions is greater in hydric soils because of lower natural magnetite contents possibly combined with historical topsoil accumulation in lower landscapes magnetic fly ash particles are also more altered in low ms soils implying that significant magnetite dissolution can occur in less than 150 years

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

supercritical flow structures on a late carboniferous delta front sedimentologic and paleoclimatic significance comment ventra et al 2015 propose a model of hyperpycnal supercritical flow regime deposition related to a megamonsoonal climate for a series of interpreted cyclic steps on a carboniferous delta front the new model has major implications for paleoenvironmental reconstructions of this well studied area in terms of climate and geographical setting we contend that fundamental sedimentological observations do not support their proposed model ventra et al interpret that each cross bedded set is the deposit of a single mega monsoonal flood event we propose that each set represents multiple events e g their set 3 is at least 30 discrete events event bed sandstones are represented by variable to normal grading crude cross stratification and plane parallel lamination and a finer grained top which is generally ripple cross laminated fig 1 these are capped by a very thin silty to fine grained sand layer with abundant organic material representing interflood deposition hampson 1997 this repeated motif is only interrupted where bed tops are eroded abundant loading dewatering and flame structures between beds and rip up clast horizons support the interpretation of discrete depositional events fig 1 a mega monsoonal origin cannot be invoked for 20 cm thick beds ventra et al state that there is a lack of ripple cross lamination or stratification aside from some upflow dipping crude laminae most bed tops are laminated to crosslaminated fig 1 these are interpreted as current ripples rather than antidunes or climbing ripple backsets as they have clearly developed tangential foresets these ripples suggest paleoflow broadly toward the east in contrast to the overall southwesterly paleoflow eastward paleocurrents typically occur on the updip side of the undulatory beds of mccabe 1977 and hampson 1997 suggesting that the exposed section is oblique to depositional dip rather than parallel as proposed we infer that there is a significant lateral as well as downstream accretion component to these three dimensional bedforms the lack of bioturbation between beds cannot be used as evidence for the short duration of events as there is in general a lack of bioturbational structures in the sandy marine sediments of the british carboniferous basins however the depositional environment as proposed by ventra marine delta front is at odds with the existing fluvial models which are based on detailed mapping and correlations mccabe 1977 hampson 1997 the large scale architecture shows a progressive decrease in depositional dip that we interpret to reflect healing of scour related topography the depth of the scour minimum of 13 m corresponds to that observed in major fluvial confluences and is aligned with hampson s 1997 interpretation the overlying section which ventra et al do not discuss is represented by many smaller scale trough cross beds interpreted to represent local shallowing in a fluvial setting associated with filling of the scour in this stratigraphic unit features of a similar geometry but at a smaller scale are convincingly shown to represent supercritical flow conditions bijkerk 2014 in summary we concur with the observations and interpretations of hampson 1997 that these beds represent the fill of a major scour potentially associated with a fluvial confluence which filled in a stepwise manner by gravity flow in the deepest parts with periods of minor erosion of the downslope sides of the bedforms e g ullah et al 2015 figure 1 a short sedimentological log from the base of ventra s 2015 set 2 showing the general character of event beds b character of event beds and finer interbeds i and amalgamation surfaces a c small dune with tangential foresets f d ripple cross lamination r with tangential foresets e loading l between beds

fingerprinting suspended sediment sources in the catchment of the river ouse yorkshire uk statistically verified composite fingerprints and a multivariate mixing model have been employed to establish the main sources of the suspended sediment transported through the lower non tidal reaches of the river ouse and one of its main tributaries the river wharfe during the period 1994 1997 in the case of the suspended sediment samples collected from the river ouse the load weighted mean contributions from uncultivated topsoil cultivated topsoil and channel bank sources were estimated to be c 25 38 and 37 respectively while for the river wharfe these sources contributed c 70 4 and 23 respectively c 4 was derived from woodland topsoil suspended sediment samples collected during higher flows evidenced a greater contribution from channel banks than samples collected during lower flows source materials were also differentiated according to the three main geological source areas carboniferous permian and triassic and jurassic and their load weighted mean contributions were estimated to be c 24 41 and 35 for the river ouse and c 91 9 and 0 there are no jurassic rocks in this catchment for the river wharfe respectively when suspended sediment samples from tributary streams were used to characterize each geological source area the equivalent results for the river ouse were c 30 46 and 24 considering the three main tributaries that contribute to the river ouse the load weighted mean contributions from the rivers swale ure and nidd were estimated to be 82 15 and 3 respectively these values have been compared with estimates of the relative magnitude of the annual suspended sediment loads of these three rivers for the years 1995 and 1996 derived from continuous monitoring of discharge and turbidity differences between the two sets of results are ascribed to the different periods of record involved and to the timing of suspended sediment sampling relative to the overall storm hydrograph and thus the degree to which the available samples are representative of the overall suspended sediment flux although a number of limitations must be recognized the fingerprinting approach to source ascription is seen as providing valuable information regarding suspended sediment sources in the study catchments copyright 1999 john wiley sons ltd

on mass transport in porosity waves abstract porosity waves arise naturally from the equations describing fluid migration in ductile rocks here we show that higher dimensional porosity waves can transport mass and therefore preserve geochemical signatures at least partially fluid focusing into these high porosity waves leads to recirculation in their center this recirculating fluid is separated from the background flow field by a circular dividing streamline and transported with the phase velocity of the porosity wave unlike models for one dimensional chromatography in geological porous media tracer transport in higher dimensional porosity waves does not produce chromatographic separations between relatively incompatible elements due to the circular flow pattern this may allow melt that originated from the partial melting of fertile heterogeneities or fluid produced during metamorphism to retain distinct geochemical signatures as they rise buoyantly towards the surface

upstream migrating knickpoints and related sedimentary processes in a submarine canyon from a rare 20 year morphobathymetric time lapse capbreton submarine canyon bay of biscay france abstract the capbreton submarine canyon is a striking feature of the south east of the bay of biscay this canyon forms a deep incision through the continental shelf and slope and displays remarkable structures linked to its present day hydrosedimentary activity its head has been disconnected from the adour river since 1310 ce but remains close enough to the coast to be supplied with sediment by longshore drift gravity processes in the canyon body are abundantly described and documented but activity in the head and the fan of the canyon is poorly constrained furthermore many questions remain regarding the details of processes affecting the head the body and the fan of the capbreton canyon in this work we address the paucity of documentation concerning 1 the temporal evolution of sediment transfer between the head and the deep reaches of the canyon and 2 the interaction between gravity processes and the morphology of the canyon floor including both shaping and feedback mechanisms this study is based on the analysis and comparison of eight multibeam bathymetric surveys acquired in the upper part of the capbreton canyon between 1998 and 2018 in depths ranging 10 320 m below sea level this rare time series exposes the morphological evolution of this outstanding dynamic area over the last 20 years our work shows that much of the changes are located in the canyon floor and head following a period characterized by a unique flat floor thalweg the canyon was affected by an incision with low lateral terraces which resulted in a narrow axial thalweg the deepening of the narrow thalweg was induced by retrogressive erosion according to the presence of upstream migrating knickpoints while low elevation residual terraces formed as the canyon reached a local equilibrium profile the flat thalweg observed in 1998 is likely a result of a partial filling of the canyon thalweg by a substantial emptying of the canyon head and significant mass transfer to the proximal part of the canyon a flat floor thalweg was not observed again in the remaining of our time series since 2010 suggesting a possible quieter working mode of the canyon we also propose the first accurate volume quantification of sediment displacement on the canyon floor our findings underline the alternation of filling and erosive periods in the canyon axis and an unexpected continuous sediment deposition in the canyon head during the last 20 years

holocene sediment dynamics in an upland temperate lake catchment climatic and land use impacts in the english lake district accelerated erosion and transport of fine sediment from upland temperate catchments can reflect increased erosivity and or erodibility due in turn to climatic and or human forcing identification of sediment fluxes and sources over holocene timescales can both enable understanding of the relative impacts of these forcings and provide perspective on recent sediment fluxes here we present a 5500 yr record of sediment fluxes and sources from lake bassenthwaite in the english lake district utilizing magnetic measurements and fuzzy clustering coupled with independent pollen and archaeological records to identify the timing and impact of catchment disturbance this record shows that recent sediment flux increases ie within the last 150 years are unprecedented in scale throughout the mid late holocene and appear to be in response to specific human changes occurring within the catchment earlier episodes of human activity from the mid holocene onwards show no link with increased lake sediment fluxes

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

reconstruction of bedform dynamics controlled by supercritical flow in the channel lobe transition zone of a deep water delta sant llorenc del munt north east spain eocene

upslope migrating bedforms in a proglacial sandur delta cyclic steps from river derived underflows abstract backstepping cross strata on steep foresets of a gilbert type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical bedforms they provide insight into how sandur river flows transition into the marine realm these sedimentary structures are located on steep foresets up to 17 with corresponding top lying flat based topsets in an upper pleistocene delta on the north shore of the gulf of st lawrence quebec canada packages of backstepping cross strata of sand and gravel lying in the lower part of the delta front outcrop are organized in 10 20 m spaced pseudo foresets with a mean slope of 11 12 seawards backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo foresets and are interpreted as cyclic steps narrow deep and asymmetrical scours and upslope climbing cross beds are interpreted as chutes and pools and antidunes respectively very shallow 15 m depositional paleo bathymetry is inferred from the preservation of the delta brink the well organized stratal pattern in cyclic step to antidune deposits indicates relatively steady and uniform flow patterns there is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net depositional cyclic steps these deposits are sandier than the topsets beds and are thus not derived from them but rather correspond to topset erosional surfaces the development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes the resulting sediment laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets the cyclic steps have a high aspect ratio and represent an end member of coarse grained sediment deposited on steep slopes in contrast to low gradient low aspect ratio muddy deposits

mineral magnetic characterization of sediment sources from an analysis of lake and floodplain sediments in the catchments of the old mill reservoir and slapton ley south devon uk research on suspended sediment transport in the catchments of the old mill reservoir and slapton lower ley south devon has attempted to discriminate changing catchment sources on the basis of downcore variations in the mineral magnetic properties of lake reservoir and floodplain sediments here we examine these downcore variations and also explore the variability in catchment sources and the influence of topographic controls on mineral magnetic signatures of topsoils and subsoils particle size controls on the mineral magnetic signatures are explored by an analysis of a fractionated sediment sample whilst the possible impact of diagenesis is assessed by an examination of the mn profiles in the lake and reservoir sediments from this analysis it is evident that the mineral magnetic signatures of well sorted floodplain deposits are more likely to reflect the particle size composition of the transported material by contrast the mineral magnetic record in the sediment of slapton ley appears to be most strongly influenced by dissolution of magnetic minerals the sediment of the old mill reservoir provides the only suitable record for the application of a simple mixing model which is developed in order to quantify changes in the relative contribution of topsoil and subsoil through time the research has important implications for attempting to reconstruct sediment sources in highly eutrophic lakes and emphasizes the uncertainty in the application of simple mixing models 1998 john wiley sons ltd

how to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record insights from active submarine channels submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea modern observations show that submarine channels can be sculpted by supercritical turbidity currents seafloor sediment flows that can generate upstream migrating bedforms with a crescentic planform in order to accurately interpret supercritical flows and depositional environments in the geologic record it is important to be able to recognize the depositional signature of crescentic bedforms field geologists commonly link scour fills containing massive sands to crescentic bedforms whereas models of turbidity currents produce deposits dominated by back stepping beds here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations time lapse seabed mapping and sediment cores thus providing the link from flow process to depositional product these data were collected within the proximal part of a submarine channel on the squamish delta canada we demonstrate that bedform migration initially produces back stepping beds of sand however these back stepping beds are partially eroded by further bedform migration during subsequent flows resulting in scour fills containing massive sand as a result our observations better match the depositional architecture of upstream migrating bedforms produced by fluvial models despite the fact that they formed beneath turbidity currents

cyclic steps review and aggradation based classification abstract cyclic steps are a type of upper flow regime bedform consisting of trains of upstream and upslope migrating bed undulations the overriding flow is characterised by a series of hydraulic jumps occurring in the troughs of the undulations cyclic steps form in open channel flows such as rivers and comprise a common bedform in subaqueous density flows in oceans lakes and reservoirs cyclic steps are associated with alternating froude subcritical and froude supercritical flow on respectively the stoss side and lee side of individual bedforms the transition between these flow states is embodied by the hydraulic jump in the trough of the bedform leading to the permanent or quasi permanent morphology of cyclic steps over the past decade numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash fluvial delta and turbidite environments as reviewed here cyclic steps were previously discriminated as net depositional climbing transportational and net erosional falling in different parts of sedimentary systems the following cyclic step descriptors with distinct depositional signatures are proposed fully depositional partially depositional transportational partially erosional and fully erosional partially depositional cyclic steps are most common they are associated with backset bedded sets high aggradation rate and nested scours filled with massive to backset bedded deposits low aggradation rate the new classification can be used as a predictive tool in the reconstruction of modern and ancient sedimentary successions using repeat bathymetry seismic reflection or outcrop data it is applied in three turbidite case studies classical deep sea system small scale delta slope and line sourced carbonate slope

solitary waves in magma dynamics we investigate the stability of the one dimensional solitary waves solutions of the equations proposed by mckenzie to model the ascent of melts in the earth interior we show that for small porosity and two dimensional horizontal disturbances with long wavelength these solitary waves are unstable we also exhibit two and three dimensional solitary wave solutions of the mckenzie equations

lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows from cyclic steps to humpback dunes abstract the preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record although these bedforms are known from a variety of depositional environments this field based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps chutes and pools antidunes and humpback dunes from three dimensional outcrops the well exposed middle pleistocene successions from northern germany comprise glacilacustrine ice contact subaqueous fan and glacial lake outburst flood deposits the studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high energy depositional environments such as fluvial settings coarse grained deltas or turbidite systems deposits of cyclic steps occur within the glacial lake outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets cyclic steps formed due to acceleration and flow thinning when the glacial lake outburst flood spilled over a push moraine ridge these bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes and pools and antidunes the subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms which are interpreted as deposits of aggrading stationary antidunes which require quasi steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation humpback dunes are characterised by downflow divergent cross stratification displaying differentiation into topsets foresets and bottomsets and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common the absence of planar parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions the large scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows which was strongly affected by the occurrence of hydraulic jumps small scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge bed topography and pulsating unstable flows

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

modern sedimentary processes at the heads of martinez channel and steffen fjord chilean patagonia abstract chilean fjord sediments constitute high resolution archives of climate and environmental change in the southern andes to interpret such records accurately it is crucial to understand how sediment is transported and deposited within these basins this issue is of particular importance in glaciofluvial martinez channel and steffen fjord 48 s due to the increasing occurrence of glacial lake outburst floods glofs originating from outlet glaciers of the northern patagonian icefield hence the bathymetry of the head of martinez channel and steffen fjord was mapped at high resolution and the grain size and organic carbon content of grab sediment samples were examined results show that the subaquatic deltas of baker and huemules rivers at the head of martinez channel and steffen fjord respectively are deeply incised up to 36 m by sinuous channels the presence of sediment waves and coarser sediments within these channels imply recent activity and sediment transport by turbidity currents although several triggering mechanisms are possible we argue that elevated river discharge and the associated relatively high suspended sediment loads is the main cause of the occurrence of turbidity currents at the head of these fjords and are of key importance in shaping the fjord s subaquatic morphology this study shows that the heads of martinez channel and steffen fjord are dynamic sedimentary environments with rapidly migrating channels it highlights the importance of site selection and multi coring in any future project aimed at reconstructing environmental change using the sediments of either fjord

source storage and mobilisation of fine sediment in a chalk stream system concern for sediment problems in the upper river piddle dorset including accumulation of fine sediment on the channel bed and elevated turbidity levels during periods of stable flow provided the stimulus for a detailed field investigation of the sediment dynamics of this chalk stream system undertaken during the period november 1991 to april 1993 two monitoring stations were established at south farm piddletrenthide and lawrence mede piddlehinton and estimates of suspended sediment load were obtained for these two sites in addition suspended sediment concentrations and bed sediment loadings were regularly sampled at several other sites sediment traps were deployed at ten representative sites along the river channel field observations of sediment sources and pathways were made during storm events and a sediment source fingerprinting study was undertaken estimates of annual suspended sediment yield provided values of 9 12 t km 2 year 1 which must be seen as low by uk standards elevated suspended sediment concentrations were not restricted to storm events but occurred throughout the monitoring period in response to remobilisation of sediment stored in the channel the evidence provided by the sediment loads recorded at the two monitoring stations by the sediment traps and by the measurements of bed sediment loading indicated that substantial amounts of sediment accumulated in the channel of the upper and middle reaches of the river during the winter months and that this slug of sediment was slowly transmitted downstream during the following summer fingerprinting of the transported sediment and of potential sources indicated that the dominant source of suspended sediment was likely to be surface soil from cultivated areas field reconnaissance surveys undertaken during storm events indicated that substantial inputs of surface runoff and suspended sediment entered the stream much of this sediment is however stored within the channel system and slowly transmitted downstream copyright 1999 john wiley sons ltd

supercritical and subcritical turbidity currents and their deposits a synthesis common facies models of turbidite deposits are based on idealized sequences of turbidite units which are assumed to reflect the depositional processes of a decelerating turbidity current we show how suites of turbidite units i e distinct turbidite facies associations that are easily described from core and outcrop may characterize the entire range of large scale dynamics of turbidity currents enabling estimates of their densimetric froude number fr subcritical versus supercritical and suspension fall out rate stratified versus nonstratified flows the linking of facies associations with large scale flow dynamics resolves process facies links that were hitherto unresolved by the common turbidite facies models

retreat pattern of glaciers controls the occurrence of turbidity currents on high latitude fjord deltas eastern baffin island glacier and ice sheet mass loss as a result of climate change is driving important coastal changes in arctic fjords yet limited information exists for arctic coasts regarding the influence of glacial erosion and ice mass loss on the occurrence and character of turbidity currents in fjords which themselves affect delta dynamics here we show how glacial erosion and the production of meltwaters and sediments associated with the melting of retreating glaciers control the generation of turbidity currents in fjords of eastern baffin island canada the subaqueous parts of 31 river mouths along eastern baffin island were mapped by high resolution swath bathymetry in order to assess the presence or absence of sediment waves formed by turbidity currents on delta fronts by extracting glaciological and hydrological watershed characteristics of these river mouths we demonstrate that the presence and areal extent of glaciers is a key control for generating turbidity currents in fjords however lakes formed upstream during glacial retreat significantly alter the course of sediment routing to the deltas by forming temporary sinks leading to the cessation of turbidity currents in the fjords due to the different deglaciation stages of watersheds in eastern baffin island we put these results into a temporal framework of watershed deglaciation to demonstrate how the retreat pattern of glaciers through the formation and filling of proglacial lakes affects the turbidity current activity of deltas

backset lamination produced by supercritical backwash flows at the beachface shoreface transition of a storm dominated gravelly beach middle pleistocene central italy abstract bedforms and resulting sedimentary structures of interpreted upper flow regime origin are fairly common in various ancient and modern depositional settings yet outcrop examples of shallow water strata dominated by sedimentary structures arising from such flow conditions are rarely documented this outcrop based study presents the first detailed analysis of mid pleistocene beachface shoreface strata containing sedimentary structures interpreted to have been formed by storm generated backwash flows during the transition in flow regime from supercritical to subcritical here we propose that moving down the steep beachface backwash flows rapidly accelerated and became thinner experienced abrupt deceleration and flow thickening passing through an erosional hydraulic jump in the trough at the toe of the beachface and then waned to thicker and subcritical conditions just downflow of the hydraulic jump in this frame landward dipping backset laminae deposited on the downflow side of the trough evolved into thinner upper stage plane parallel laminae further downstream the transition from the proximal to the distal upper shoreface records a significant increase of preserved wave ripples and burrowed intervals in concert with a progressive decrease in thickness and grain size of individual sets of plane parallel laminae and increase of gravel filled gutter casts

supercritical flow structures on a late carboniferous delta front sedimentologic and paleoclimatic significance deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood prone regime alternating with lengthy periods of ineffective discharge distally linked deltaic successions should also feature distinctive attributes with great potential to preserve the stratigraphic evidence of exceptional discharge events we describe late carboniferous delta front valley confined sandstones from the pennine basin uk originally deposited at paleoequatorial latitudes during final assembly of the pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical flow conditions leading to aggradation of cyclic steps recently identified bedforms developing in high energy flows and of which this is the first complete outcrop example the lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors cyclic step formation in a deltaic setting suggests that pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow marine deposits

froude supercritical flow processes and sedimentary structures new insights from experiments with a wide range of grain sizes recognition of froude supercritical flow deposits in environments that range from rivers to the ocean floor has triggered a surge of interest in their flow processes bedforms and sedimentary structures interpreting these supercritical flow deposits is especially important because they often represent the most powerful flows in the geological record insights from experiments are key to reconstruct palaeo flow processes from the sedimentary record so far all experimentally produced supercritical flow deposits are of a narrow grain size range fine to medium sand while deposits in the rock record often consist of a much wider grain size distribution this paper presents results of supercritical flow experiments with a grain size distribution from clay to gravel these experiments show that cyclic step instabilities can produce more complex and a larger variety of sedimentary structures than the previously suggested backsets and scour and fill structures the sedimentary structures are composed of irregular lenses mounds and wedges with backsets and foresets as well as undulating planar to low angle upstream and downstream dipping laminae the experiments also demonstrate that the froude number is not the only control on the sedimentary structures formed by supercritical flow processes additional controls include the size and migration rate of the hydraulic jump and the substrate cohesion this study further demonstrates that froude supercritical flow promotes suspension transport of all grain sizes including gravels surprisingly it was observed that all grain sizes were rapidly deposited just downstream of hydraulic jumps including silt and clay these results expand the range of dynamic mud deposition into supercritical flow conditions where local transient shear stress reduction rather than overall flow waning conditions allow for deposition of fines comparison of the experimental deposits with outcrop datasets composed of conglomerates to mudstones shows significant similarities and highlights the role of hydraulic jumps rather than overall flow condition changes in producing lithologically and geometrically complex stratigraphy

large scale scours formed by supercritical turbidity currents along the full length of a submarine canyon northeast south china sea abstract high resolution multibeam bathymetric and seismic data enables a detailed morphological investigation of a submarine canyon west penghu canyon on the northeastern south china sea margin where twenty three 23 scours are observed along the canyon thalweg these scours form narrow topographic depressions in plan view and show asymmetrical morphologies in cross section the identified scours can be further divided into two groups types a and b based on their sizes and relative locations they are separated by a slope break at a water depth of 2850 m type a scours s1 s18 occur upslope from the slope break whereas type b scours s19 s23 lie downslope from this same break the scours are interpreted as net erosional cyclic steps associated with turbidity currents flowing through the west penghu canyon the currents that form type a scours reflect higher v q and el compared to the currents forming type b scours a change in slope gradient and loss of lateral confinement are proposed to control the change from type a to type b scours furthermore coriolis force influences the flow direction of turbidity currents leading to the preferential development and larger incision depths of scours towards the southwestern flank of the west penghu canyon our results contribute to a better understanding on the origin of scours in submarine canyons across the world

sediment source tracing using environmental magnetism a new approach with examples from australia environmental magnetism can be used for sediment source tracing in situations such as stream junctions where two catchments deliver magnetically distinguishable well combined average source mixes to a trunk stream consistent mineral magnetic parameter relationships show that spatially and temporally constant magnetic mineral assemblages occur along reaches of natural streams for most of the confluence mixing situations examined it is possible to distinguish between tributaries using magnetic parameter relationships using these relationships relative tributary contributions to the resultant downstream mix can be quantified the method can be used for tracing suspended and bedload sediments so that the principal source catchments can be determined by a sequence of measurements at stream junctions along a drainage network there is considerable potential for using this new and easily applied method for studies of sediment delivery processes and sediment movement in fluvial environments

preconditioning and triggering of offshore slope failures and turbidity currents revealed by most detailed monitoring yet at a fjord head delta rivers and turbidity currents are the two most important sediment transport processes by volume on earth various hypotheses have been proposed for triggering of turbidity currents offshore from river mouths including direct plunging of river discharge delta mouth bar flushing or slope failure caused by low tides and gas expansion earthquakes and rapid sedimentation during 2011 106 turbidity currents were monitored at squamish delta british columbia this enables statistical analysis of timing frequency and triggers the largest peaks in river discharge did not create hyperpycnal flows instead delayed delta lip failures occurred 8 11 h after flood peaks due to cumulative delta top sedimentation and tidally induced pore pressure changes elevated river discharge is thus a significant control on the timing and rate of turbidity currents but not directly due to plunging river water elevated river discharge and focusing of river discharge at low tides cause increased sediment transport across the delta lip which is the most significant of all controls on flow timing in this setting