new bedform phase diagrams and discriminant functions for formative conditions of bedforms in open channel flows understanding of the formative conditions of fluvial bedforms is significant for both river management and geological studies diagrams showing bedform stability conditions have been widely used for the analyses of sedimentary structures however the use of discriminants to determine the boundaries of different bedforms regimes has not yet been explored in this study we use discriminant functions to describe formative conditions for a range of fluvial bedforms in a 3d dimensionless parametric space we do this by means of discriminant analysis using the mahalanobis distance we analyzed 3793 available laboratory and field data and used these to produce new bedform phase diagrams these diagrams employ three dimensionless parameters representing properties of flow hydraulics and sediment particles as their axes the discriminant functions for bedform regimes proposed herein are quadratic functions of three dimensionless parameters and are expressed as curved surfaces in 3d space these empirical functions can be used to estimate paleoflow velocities from sedimentary structures as an example of the reconstruction of hydraulic conditions we calculated the paleoflow velocity of the 2011 tohoku oki tsunami backwash flow from the sedimentary structures of the tsunami deposit in so doing we successfully reconstructed reasonable values of the paleoflow velocities

bedforms and flow resistance of cohesive beds with and without biofilm coating abstractthe bedforms on a biofilm coated sediment biosediment bed are different from those on a noncohesive sediment bed under an identical flow condition and sediment particle size without biofi

mud rich density driven flows along an early ordovician storm dominated shoreline implications for shallow marine facies models abstract dynamic physical and biological seafloor processes shape the three dimensional stratal architecture of modern and ancient mud dominated continental shelves when evaluating a mudstone unit as a potential source reservoir or seal it is imperative to understand the physical biological and chemical seafloor processes that control mudstone depositional fabric and diagenesis this study presents sedimentological ichnological and geochemical data of exceptionally preserved mudstone originating from hyperpycnal and possible combined flow deposits in the lower ordovician tremadocian beach formation bell island group newfoundland seven mudstone facies are described based on textural compositional and ichnological characteristics mudstone deposits interpreted to originate from hyperpycnal flows are well cemented exhibit high chlorite illite ratios and contain sharp grain size changes commonly forming beds with tripartite subdivisions deposits of wave enhanced sediment gravity flows are in contrast poorly cemented illitic mudstone these mudstone facies exhibit decimeter thick wave or combined flow structures within siltstone and very fine sandstone beds as well as laterally discontinuous nonbioturbated mudstone layers beds of this facies have abundant mudstone on mudstone and mudstone on sandstone erosional contacts low organic carbon loading from the nonvegetated early paleozoic hinterland combined with a high reworking frequency on a shallow marine wave dominated shelf is inferred to have resulted in high remineralization efficiency and low preservation potential of reactive organic carbon burial efficiency and bioavailability of organic matter are considered to be the critical variables controlling infaunal colonization and bioturbation in mud dominated open coastline paleoenvironments this work demonstrates that ancient fine grained coastal systems are incompletely incorporated into sequence stratigraphic models owing to their atypical proximal to distal facies relationships it is proposed that the spatial organization of fine grained sediment and facies architecture in such systems are controlled by 1 frequency of sediment supply events 2 direction of mud transport 3 diagenetic reactivity of minerals and bioavailable organic carbon and 4 residence time of mineral grains and organic matter close to the sediment water interface

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

holocene raised beaches at terra nova bay victoria land antarctica abstract more than 40 14c dates for raised beaches at terra nova bay ranging from the present to 7505 230 yr b p supply minimum or maximum limiting ages that bracket a relative sea level curve for this part of victoria land most samples consist of guano and penguin remains collected from abandoned nesting sites on beach deposits up to four 14c dates in stratigraphic sequence have been obtained in two soil profiles other samples consist of marine shells collected both within and on the surface of raised beaches all 14c ages for organisms that lived or fed in circumantarctic waters were corrected for a large reservoir effect the calibrated ages delimit a first relative sea level curve for victoria land rates of uplift ranged from about 10 mm yr following deglaciation to about 2 mm yr in the last 3 millennia widespread presence of adelie penguin nesting sites suggests that terra nova bay was deglaciated before 7065 250 yr b p 7059 6439 cal yr b p when environmental conditions in the coastal area were similar to the present ones

mud deposition and diagenesis within an early palaeozoic clinothem power steps formation newfoundland canada the early ordovician 385 ma power steps formation newfoundland canada exposes a well preserved mudstone dominated clinothem that serves as an excellent archive for understanding how mud has been produced transported and converted into mudstone prior to the evolution of globally widespread deep soil horizons sedimentological analysis of four sandstone and five mudstone facies along the ochre cove clinothem reveal that mud and sand were delivered by unidirectional currents and experienced episodic reworking by storm waves petrographic examination and x ray diffraction from described mudstone facies reveal significant variability in the distribution of illite versus chlorite between the lower and upper part of the ochre cove clinothem this research highlights that in the present day clay mineral fraction illite is often detrital whereas chlorite originated via the alteration of silt sized highly unstable mafic volcanoclastic grains throughout all sedimentologic facies albeit in different proportions these mafic lithic grains were diagenetically altered via in situ weathering before significant compaction occurred resulting in the precipitation of significant volumes of pore bridging silica and iron rich chlorite cement compositional diagenetic and textural attributes across the ochre cove mud clinothem vary as a function of starting composition hydrodynamic sorting and grain density given that a significant proportion of clay minerals has been generated via in situ transformation of a mafic non stable precursor assemblage we recommend future studies to incorporate detailed petrographic description along with x ray diffraction analyses when aiming to employ trends in whole rock clay mineral data as a proxy in provenance and palaeoclimate studies of very old pre devonian mudstones and sandstones

insights into the holocene environmental setting of terra nova bay region ross sea antarctica from oxygen isotope geochemistry of adelie penguin eggshells an oxygen isotope record of adelie penguin pygoscelis adeliae eggshells from terra nova bay victoria land antarctica has been reconstructed in order to provide insights into the palaeoenviroment of this region during the last 8000 years the 18o values measured in eggshell carbonate show a great variability over the course of the studied period with maximum values consistently around 27 5 adopting a metabolic enrichment relative to water source of 28 5 eggshell 18o values can be interpreted in terms of marine water uptake by the penguins the spread in 18o of eggshell carbonate at any given time toward more negative values reflects the existence of a low 18o water end member most plausibly snow or snowmelt samples younger than 2000 years are characterized by a general lack of very negative 18o values as also attested by other proxies this shift toward more positive isotopic values is likely to be related both to a decreasing contribution of snow or meltwater in the seawaters and con

the melting history of the late pleistocene antarctic ice sheet spatial and temporal variations in the sea levels of the past 20 000 years around the globe place constraints on the melting history of the major late pleistocene ice sheets the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9 000 6 000 years ago when it may have lagged behind minor melting of the antarctic ice sheet continued throughout the holocene

mixed sand mud bedforms produced by transient turbulent flows in the fringe of submarine fans indicators of flow transformation the fringe of fine grained deep marine systems often exhibits complex sedimentary facies and facies associations because the presence of clay promotes the development of transient turbulent flows with complex depositional properties relatively little is known about the variation of current induced sedimentary structures found within these facies this study provides the first comprehensive description and interpretation of mixed sandstone mudstone bedforms observed in the fringe of the mud rich submarine fan that makes up the aberystwyth grits group and borth mudstone formation wales united kingdom using textural and structural descriptions 158 bedforms in sediment gravity flow deposits were characterized into three main types classic sandy current ripples large current ripples and low amplitude bed waves the sandy current ripples comprise clean sandstone with average heights and lengths of 11 mm and 141 mm respectively the large current ripples are composed of mixed sandstone mudstone and possess greater dimensions than the sandy current ripples with an average height of 19 mm and an average length of 274 mm the low amplitude bed waves are long thin bedforms composed commonly of mixed sandstone mudstone with an average height and length of 10 mm and 354 mm respectively the large current ripples and low amplitude bed waves are strikingly similar to experimental bedforms produced under decelerating mixed sand mud flows and are interpreted to form beneath transitional flows with enhanced and attenuated near bed turbulence respectively from the fringe to the distal fringe of the fan the dominant bedform type changed from sandy current ripples via large current ripples to low amplitude bed waves suggesting that the flows changed from turbulent to increasingly turbulence modulated it is proposed that the flow reynolds number reduced reflecting this flow transformation from a combination of constant or decreasing flow height flow deceleration from sediment deposition and increasing flow viscosity due to the shear thinning nature of clay rich suspensions large current ripples and low amplitude bed waves are likely to be common in the fringe of other submarine fans the presence and spatial trends in mixed sand mud bedform types may be an important tool in interpreting fan fringe environments

late pleistocene holocene marine conditions in the ross sea antarctica evidence from the diatom record statistical analyses of diatom assemblages from radiocarbon dated sediment cores were used to reconstruct changing palaeoceanographic conditions in the western and west central ross sea antarctica from c 12 14c kyr bp to the present data from three kasten cores support a north to south time transgressive glacial interglacial transition assemblages at the base of each core suggest that glacial processes affecting frustule preservation were significant during the late pleistocene increasing biogenic silic and decreasing per centages of reworked species suggest that upper water column productivity instead of preservational processes increasingly dominates diatom assemblages during grounding line recession c 12to c 6 14c kyr bp a warm ing during the middle to late holocene c 6to c 3 14c kyr bp may have resulted in a spring sea ice melting prior to the annual inception of the ross sea polynya in the west central ross sea and b increased plateletice delivery due to increased melting beneath t

the role of biophysical cohesion on subaqueous bed form size biologically active fine grained sediment forms abundant sedimentary deposits on earth s surface and mixed mud sand dominates many coasts deltas and estuaries our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt sand and gravel this approach underpins many paleoenvironmental reconstructions of sedimentary successions which rely on analysis of cross stratification and bounding surfaces produced by migrating bed forms here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology the results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships the findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record

topographic controls on the development of contemporaneous but contrasting basin floor depositional architectures abstract sediment laden gravity driven flow deposits on the basin floor are typically considered to form either discrete lobes that stack compensationally or packages of laterally extensive beds commonly termed sheets these end member stacking patterns are documented in several basinfills however whether they can coexist in a single basin or there are intermediate or transitional stacking patterns is poorly understood an analysis of depositional architecture and stacking patterns along a 70 km dip oriented transect in the upper broto turbidite system jaca basin south central pyrenees spain which displays disparate stacking patterns in contemporaneous strata is presented proximal and medial deposits are characterized by discrete packages of clean sandstones with sharp bed tops which exhibit predictable lateral and longitudinal facies changes and are interpreted as lobes distal deposits comprise both relatively clean sandstones and hybrid beds that do not stack to form lobes instead localized relatively thick hybrid beds are inferred to have inhibited the development of lobes hybrid beds developed under flows which were deflected and entrained carbonate mud substrate off a carbonate slope that bounded the basin to the south evidence for this interpretation includes 1 divergent paleoflow indicators and hummock like features in individual beds 2 a decrease in hybrid bed thickness and abundance away from the lateral confining slope 3 a carbonate rich upper division not seen in more proximal turbidites the study demonstrates the co occurrence of different styles of basin floor stacking patterns in the same stratigraphic interval and suggests that characterization of deep water systems as either lobes or sheets is a false dichotomy

bottomset architecture formed in the troughs of dunes and unit bars abstract bottomsets are formed in the troughs of migrating bedforms the flow and sediment dynamics downstream of bedforms differ and consequently the thickness grain size and internal structure of bottomsets vary greatly a classification of the controls on bottomset formation is proposed related to two and three dimensional processes flow and sediment unsteadiness and pre existing deposits structures and deposits created by flow and sediment unsteadiness as well as pre existing deposits are also considered in relation to changes they can induce in subsequent trough processes the classification is illustrated by observations from laboratory flumes the modern burdekin river australia and the rock record the roaches grit england and hawkesbury sandstone australia in bottomset examples described in this paper internal variability both vertically and laterally was much greater than that within the dominant foreset component complex compound structures were created by the interaction and predominance of different bottomset controls this combined with their high preservation potential makes them a highly useful paleoenvironmental indicator bottomset variation e g changes in the abundance of mud laterally will influence the permeability heterogeneity of fluvial reservoir rocks bottomsets formed under relatively steady conditions are likely to be laterally extensive and have similar characteristics over their entire length they may act as significant barriers to vertical flow conversely bottomsets generated in unsteady regimes tend to be highly variable potentially creating conduits between cross bed sets aiding inter set permeability

bedform development in mixed sand mud the contrasting role of cohesive forces in flow and bed abstract the majority of subaqueous sediment on earth consists of mixtures of cohesive clay and cohesionless sand and silt but the role of cohesion on the development and stability of sedimentary bedforms is poorly understood the results of new laboratory flume experiments on bedform development in cohesive mixed sand mud beds are compared with the results of previous experiments in which cohesive forces in high concentration clay flows dominated bedform development even though both series of mixed sand mud experiments were conducted at similar flow velocities the textural and structural properties of the bedforms were sufficiently different to permit the designation of key criteria for identifying bedform generation under cohesive flows against bedform generation on cohesive substrates these criteria are essential for improving bedform size predictions in sediment transport modelling in modern sedimentary environments and for the reconstruction of depositional processes in the geological record the current ripples developing on the cohesive mixed sand mud beds with bed mud fractions of up to 18 were significantly smaller than equivalent bedforms in noncohesive sand moreover the bedform height showed a stronger inversely proportional relationship with initial bed mud fraction than the bedform wavelength this is in contrast with the bedforms developing under the cohesive clay flows which tend to increase in size with increasing suspended clay concentration until the flow turbulence is fully suppressed selective removal of clay from the mixed beds i e clay winnowing was found to be an important process with 82 100 clay entrained into suspension after 2 h of bedform development this winnowing process led to the development of a sand rich armouring layer this armouring layer is inferred to have protected the underlying mixed sand mud from prolonged erosion and in conjunction with strong cohesive forces in the bed may have caused the smaller size of the bedforms winnowing was less efficient for the bedforms developing under the cohesive clay flows where bedforms consisting of muddy sand were more characteristic the winnowed sand was also found to heal irregularly scoured topography thus reestablishing classic quasitriangular bedform shapes in cohesive flows the bedforms had more variable shapes and the healing process was confined to lower transitional plug flows in which strong turbulence is only present close to the sediment bed furthermore the bedforms on the cohesive beds tended to form angle of repose cross lamination whereas low angle cross lamination was more common in bedforms under cohesive flows in general terms erosional bedforms prevail when cohesive forces in the bed dominate bedform dynamics whereas depositional bedforms prevail when cohesive forces in the flow dominate bedform dynamics empirical relationships between the proportion of cohesive mud in the mixed sand mud bed and the development rate and size of the bedforms are defined for future use in field and laboratory studies

subaqueous barchans and plane beds from deposition of quartz silt abstractthe suspension flow of quartz silt geometric mean grain size of 4 15 m in unfavorable conditions for deposition is the motivation for the development of different bed morphologies particles deposit over a plane nonerodible surface and develop well defined barchan ripples barchanoids and plane beds in flume experiments bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport the final bed morphology is controlled by the suspended sediment concentration and running time of the experiment the average dimensions of the bedforms after 1 day experiments are 4 93 cm wide 10 33 cm long and 0 45 cm high cohesive plane beds appear after 2 day and 3 day experiments with very high sediment concentrations 22 5 kg m3 viscous effects are deemed relevant for the formation of the beds

abrasion of detrital grain coating clays during sediment transport implications for diagenetic clay coats abstract detrital grain coating clay minerals have been identified as precursors of diagenetic grain coating clay minerals that inhibit quartz cementation and preserve good reservoir quality properties in deeply buried sandstone reservoirs between initial and final deposition detrital grain coats are likely to be affected by sediment transport processes to date it has not been demonstrated that detrital grain coating clays are able to survive sediment transport flume experiments simulating open channel flow conditions were performed using sediments from the ravenglass estuary cumbria uk where the distribution and characteristics of modern detrital grain coating clays is well established scanning electron microscopy and automated mineralogy techniques were employed to study the sediment before and after transport to determine the stability of the detrital grain coating clays the experimental results show that detrital grain coating clays are still present after sediment transport in turbulent flows with velocities up to 0 5 m s 1 the stability of the detrital grain coating clays was enhanced by 1 high initial clay coat coverage 2 turbulence characteristics of the flow and 3 entrapment of the detrital grain coating clays in bedforms this work documents the first quantifiable evidence that estuary derived detrital clay coats are routinely preserved after sediment transport clay coat rich estuarine sediments can therefore act as the source of clay coated grains found in cleaner estuarine sandstones and may even survive transport out into open marine settings

helium and carbon isotopic compositions of hot spring gases in the tibetan plateau abstract the tibetan plateau is one of the most active geothermal areas in the himalayan geothermal belt however the heat source of the geothermal manifestation is unknown because of a lack of quaternary volcanoes in this area except in western yunnan and northern tibet in order to unravel the heat source and the origin of volatile components in hot spring gases in the southern tibetan plateau we have measured helium and carbon isotope ratios of hot spring gases from yangbajain district and some northern sites the observed helium compositions in the hot spring gases are explained by the mixing of an air component and underground components whose mean end member 3 he 4 he ratio is 0 12 ratm for yangbajain and 0 22 ratm for the northern sites respectively despite the lack of quaternary volcanism such 3 he 4 he ratios are an order of magnitude higher than that of stable continental crust 0 02 ratm these results may imply the existence of intrusive magmas that contain mantle helium in the thick tibetan crust the mantle helium with high 3 he 4 he ratio may have been diluted to the observed values by radiogenic helium produced in the crust the observed high co2 3 he ratios of the samples 1010 1012 relative to that of morb 2 109 indicate that most carbon dioxide in the samples is derived from crustal materials such as organic matter and marine carbonate minerals regional differences are observed not only in the mean end member 3 he 4 he ratios of the underground components but also in the 13c co2 values of the hot spring gases between yangbajain 9 1 to 6 3 and the northern sites 4 3 to 1 0 these features suggest heterogeneity among basement rocks the lower 3 he 4 he ratio in yangbajain may be due to a larger addition of radiogenic 4 he from basement rocks that are enriched with uranium and thorium the higher 13 c co2 value of the hot spring gases from the northern sites may be due to the addition of carbon dioxide from underlying sedimentary rocks containing more marine carbonate minerals

deposits of flows transitional between turbidity current and debris flow the relationship between submarine sediment gravity flows and the character of their deposits is poorly understood annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment laden flows decelerating fast 3 m s flows with fixed sand content 10 vol and variable mud content 0 17 vol resulted in only four deposit types clean sand with a mud cap that resembled a turbidity current deposit turbidite formed if the flow was turbulent when deposition began or if the muddy fluid had insufficient strength to suspend the sand the clean sand could contain structures if mud content was low 300 s ungraded muddy sand with a mud cap that resembled a debris flow deposit debrite formed if the flow became laminar before sand could deposit clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late stage settling of sand from a muddy suspension these deposits resemble enigmatic submarine flow deposits called linked debrite turbidites the experiments provide a basis for inferring flow type from deposit character for submarine sediment laden flows

holocene deglaciation sea level change and the emergence of the windmill islands budd coast antarctica a holocene deglaciation sequence for the windmill islands was determined from the 14c age of raised marine shorelines lakebottom sediments and adelie penguin remains found in abandoned rookeries a north south gradient in the elevation of the upper marine limit was observed with the highest marine limit 31 32 m observed on browning peninsula and hull island at the southern edge of the islands correspondingly the southern islands were found to have been deglaciated by 8000 corr yr b p while the northern islands were deglaciated by 5500 corr yr b p isostatic uplift rates were calculated as 0 5 to 0 6 m 100 yr with an estimated total uplift of around 53 m which indicates late pleistocene ice sheet thicknesses of 200 and 400 m over the islands and adjacent petersen bank respectively the margin of the late pleistocene grounded ice sheet extended an estimated 8 15 km offshore which coincides with the location of the 200 m isobath

adelie penguin dietary remains reveal holocene environmental changes in the western ross sea antarctica carbon and nitrogen stable isotope analyses were performed on modern and holocene adelie penguin guano samples collected from ornithogenic soils along the scott coast southern victoria land antarctica from cape irizar to dunlop island and at cape bird ross island guano samples also were sieved and sorted under stereomicroscope in order to select penguin dietary remains such as fish bones and otoliths carbon and nitrogen stable isotope composition coupled with the taxonomic identification of fish otoliths from scott coast holocene samples indicated a mainly fish based diet for this area with pleuragramma antarcticum as the most eaten prey throughout the investigated period from 390 cal bp to ca 7300 cal bp the isotopic values of ross island samples from modern to 3850 cal bp showed a krill consumption increase in the samples younger than 2000 cal bp with the maximum in modern samples scott coast and ross island holocene samples showed 13c and 15n trends similar to those previously published from terra nova bay northern victoria land whereas modern samples from ross island have similar 15n composition but different 13c values this 13c divergence started at ca 2000 bp and follows the abandonment of the scott coast colonies the 13c trend observed in ross island and terra nova bay samples and the abandonment of the scott coast colonies could suggest the stability and the persistence of the previous oceanographic conditions i e polynya for the terra nova bay area and the establishment of new conditions for water circulation in the southern ross sea since 2000 bp when persistent sea ice sealed the scott coast

the influence of clay on the threshold of movement of fine sandy beds the erosion threshold i e surface shear strength of mixed sediment deposits has been tested under the action of unidirectional currents mean critical speeds 4 mm above the bed ranging from 12 4 cm s 1 to 18 2 cm s 1 or simulated waves mean critical periods ranging from 2 0 s to 6 7 s for near bed wave amplitudes of 0 28 0 57 m using an oscillating tray in a rectangular recirculating flume the sediment mixtures investigated consisted of angular fine grained quartz sands d50 152 5 and 215 m and cohesive estuarine mud the investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits the rate of increase is smaller for mud contents 30 clay mineral content 11 14 by dry weight and is attributed to an increase of the internal angle of friction when the clay mineral content exceeds 11 14 by dry weight then the sand particles are not in contact with each other consequently the clay component controls the erodibility of the sediment mixture finally a prolonged and intense stress history can increase considerably by a factor of 1 3 1 7 the original erosion threshold value of these sediments

antarctic ice volume and contribution to sea level fall at 20 000 yr bp from raised beaches the contribution of the antarctic ice sheets to global sea level fall at the last glacial maximum lgm depends largely on how the extent and thickness of peripheral ice changed model studies1 3 suggest that there was widespread thickening from 500 m to more than 1 000 m of the ice sheet margins sufficient to induce a drop in sea level of at least 25 m geological evidence4 5 on the other hand indicates only limited ice expansion and a sea level fall of just 8 m here we use recent data on the altitudes and ages of raised beaches from the ross embayment and east antarctica to investigate the timing and extent of antarctic deglaciation these indicate that the ice margin during the lgm was thinner and less extensive than has been formerly thought and that its contribution to the drop in sea level was only 0 5 2 5 m deglaciation was well advanced by 10 kyr bp and was complete by 6 kyr bp these findings imply either that sea level during the lgm fell less than present estimates suggest or that an ice volume considerably greater than currently accepted must have been present in the northern hemisphere

the antarctic ice sheet during the last glacial maximum and its subsequent retreat history a review abstract an emerging body of evidence from studies of the last glacial interglacial cycle suggests that the east and west antarctic ice sheets have not advanced and retreated in concert the west antarctic ice sheet advanced to the outer shelf in most regions during the last glacial maximum lgm the retreat from the shelf commenced shortly after the lgm and continued into the late holocene the west antarctic ice sheet in ross and amundsen seas slid across a deforming bed at least during the final phases of extended glaciation this implies that at this time the ice sheet had a low profile differences in the number and locations of grounding zone wedges and smaller grounding zone features from trough to trough imply that individual west antarctic ice sheet ice streams retreated episodically details concerning the expansion retreat and behavior of the east antarctic ice sheet eais are more sparse the picture emerging is that the eais did not expand to the continental shelf edge during the lgm rather it achieved a maximum extent of a mid shelf position in some locations while in other areas the ice terminus was situated near its present location the timing of retreat along sectors within the eais appears diachronous and in places occurred prior to the lgm the antarctic peninsula shelf contained considerably more ice during the lgm than previously proposed the results presented in this paper support more recent published ice sheet models that call for greater contributions of melting ice from west antarctica including the antarctic peninsula to the post glacial rise in sea level

a flume study on the development and equilibrium morphology of current ripples in very fine sand an empirical model is constructed for the development and equilibrium dimensions of small scale unidirectional bedforms in sand with a median grain size of 0 095 mm based on a series of steady flow experiments in a flume current ripples always attain a linguoid plan morphology with constant average height 13 1 mm and wavelength 115 7 mm provided that sufficient time is allowed for their formation the development pattern of these ripples on a flat bed is independent of flow velocity and involves four stages 1 incipient ripples 2 straight and sinuous ripples 3 non equilibrium linguoid ripples and 4 equilibrium linguoid ripples straight and sinuous ripples are non equilibrium bedforms at all flow velocities the time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours at flow velocities where washed ripples are stable the equilibrium wavelength is similar to that of equilibrium linguoid ripples but the equilibrium height rapidly decreases from 13 1 mm to zero towards upper stage plane bed conditions the results of the flume experiments correspond reasonably well with those of previous studies provided that various complicating factors such as different experimental methods different sediment characteristics shallow flow depths and non equilibrium runs are accounted for

despiking ultrasonic doppler velocity profiling data abstract a practical method for detecting and removing anomalous velocity spikes in ultrasonic doppler velocity profiling udvp data is presented udvp applies the pulsed ultrasonic doppler effect to measure instantaneous flow velocities using the rate of movement of suspended particles within the flow as a proxy this technique has become popular in laboratory studies of sediment erosion transport and deposition because of its ability to acquire high resolution velocimetric data at a wide range of suspended sediment concentrations in for example hyperconcentrated river flows transient turbulent mudflows and turbidity currents velocity data recorded by udvp transducers that are not part of the genuine velocity signal are collectively known as velocity spikes the new technique to detect velocity spikes proposed here is based on a simple combined acceleration and magnitude threshold method with a piecewise hermite cubic interpolation of four adjacent velocity measurements used to remove the detected velocity spikes 110 datasets covering different combinations of velocity and suspended sediment concentration were used to test the matlab coded despiking algorithm and to make recommendations for best practice in future udvp based studies

late pleistocene interactions of east and west antarctic ice flow regimes evidence from the mcmurdo ice shelf we present new interpretations of deglaciation in mcmurdo sound and the western ross sea with observationally based reconstructions of interactions between east and west antarctic ice at the last glacial maximum lgm 16000 12 000 8000 and 4000 bp at the lgm east antarctic ice from mulock glacier split one branch turned westward south of ross island but the other branch rounded ross island before flowing southwest into mcmurdo sound this flow regime constrained by an ice saddle north of ross island is consistent with the reconstruction of stuiver and others 1981a after the lgm grounding line retreat was most rapid in areas with greatest water depth especially along the victoria land coast by 12 000 bp the ice flow regime in mcmurdo sound changed to through flowing mulock glacier ice with lesser contributions from koettlitz blue and ferrar glaciers because the former ice saddle north of ross island was replaced by a dome the modern flow regime was established 4000 bp ice derived from high elevations on the polar plateau but now stranded on the mcmurdo ice shelf and the pattern of the transantarctic mountains erratics support our reconstructions of mulock glacier ice rounding minna bluff but with all ice from skelton glacier ablating south of the bluff they are inconsistent with drewry s 1979 lgm reconstruction that includes skelton glacier ice in the mcmurdo sound through flow drewry s 1979 model closely approximates our results for 12 000 4000 bp ice sheet modeling holds promise for determining whether deglaciation proceeded by grounding line retreat of an ice sheet that was largely stagnant because it never approached equilibrium flowline profiles after the ross ice shelf grounded or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present day outlet glaciers after the ross ice shelf grounded

wave ripple development on mixed clay sand substrates effects of clay winnowing and armoring based on bed form experiments in a large scale flume we demonstrate that the rate of development of wave ripples on a mixed sand clay bed under regular waves is significantly lower than on a pure sand bed even at clay fractions as low as 4 2 and that this rate of development decreases exponentially from 4 2 to 7 4 clay these experiments also showed that despite the slow growth of the bed forms in the mixed sand clay the equilibrium length and height of the wave ripples were independent of the initial bed clay fraction given that the ripple crests were composed of pure sand at the end of all the experiments that started with well mixed sand clay it is inferred that the clay was removed from the bed during the development of the wave ripples through winnowing into the water column and possibly also by sieving into the subsurface where the final clay fractions were found to be higher than the initial clay fractions these clay removal processes are interpreted to have facilitated the wave ripples to reach equilibrium lengths and heights that are similar to those in pure sand clay carrying pore flow initiated by pressure gradients between the wave ripple troughs and crests might also have contributed to the accumulation of clay in the sediment below the wave ripples the formation of the clay enriched armoring layer in the substrate is likely to further reduce erosion rates and could influence the dispersion of nutrients and pollutants in coastal seas

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions abstract many coastal and estuarine environments are dominated by mixtures of non cohesive sand and cohesive mud the migration rate of bedforms such as ripples and dunes in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology however these models tend to ignore parameters describing the physical and biological cohesion resulting from clay and extracellular polymeric substances eps in natural mixed sediment largely because of a scarcity of relevant laboratory and field data to address this gap in knowledge data were collected on intertidal flats over a spring neap cycle to determine the bed material transport rates of bedforms in biologically active mixed sand mud bed cohesive composition changed from below 2 vol up to 5 4 vol cohesive clay as the tide progressed from spring towards neap the amount of eps in the bed sediment was found to vary linearly with the clay content using multiple linear regression the transport rate was found to depend on the shields stress parameter and the bed cohesive clay content the transport rates decreased with increasing cohesive clay and eps content when these contents were below 2 8 vol and 0 05 wt respectively above these limits bedform migration and bed material transport was not detectable by the instruments in the study area these limits are consistent with recently conducted sand clay and sand eps laboratory experiments on bedform development this work has important implications for the circumstances under which existing sand only bedform migration transport formulae may be applied in a mixed sand clay environment particularly as 2 8 vol cohesive clay is well within the commonly adopted definition of clean sand

ripples on stream bed some 30 years ago john f kennedy laid the foundations for the description of the geometry of sand waves on stream beds by the potential flow model since then the analysis has been extensively refined and extended but no rigorous description of the small features called ripples exists the present study attempts a stock taking of the data and the various behavioral concepts with the aim of encouraging researchers to improve our knowledge of the physics of features called ripples various trends are identified from the data it is deduced that ripples act as roughness elements of the boundary the ripple troughs are filled with the energy consuming lee vortices the main flow is over the ripple crests and lee vortices and does not interact with the bed geometry the roughness layer of ripples and vortices is within the constant shear layer and is affected by the flow depth only as far as it affects the velocity distribution and alters the shear stress on the bed

architecture and morphodynamics of subcritical sediment waves in an ancient channel lobe transition zone in modern systems submarine channel lobe transition zones show a well documented assemblage of depositional and erosional bedforms in contrast the stratigraphic record of channel lobe transition zones is poorly constrained because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings several locations from an exhumed fine grained base of slope system unit b laingsburg depocentre karoo basin south africa show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions the regional stratigraphic context lack of a basal confining surface wave like morphology in dip section size and facies characteristics support an interpretation of subcritical sediment waves within a channel lobe transition zone setting some sediment waves show steep 10 to 25 unevenly spaced 10 to 100 m internal truncation surfaces that are dominantly upstream facing which suggests significant spatio temporal fluctuations in flow character their architecture indicates that individual sediment wave beds accrete upstream in which each swell initiates individually lateral switching of the flow core is invoked to explain the sporadic upstream facing truncation surfaces and complex facies distributions vertically within each sediment wave variations in bedform character are related to the axial to marginal positions within a channel lobe transition zone the depositional processes documented do not correspond with known bedform development under supercritical conditions the proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture

bedform migration and bed load transport in some rivers and tidal environments reliable field data obtained by directly measuring bed load transport of fine to coarse grained bed material are extremely scarce mainly because of the difficulty of sampling accurately therefore the verification of bed load transport formulae is largely based on flume experiments which refer to unrealistic shallow water conditions in this study some bed load transport formulae were tested against data from natural environments as an alternative to ascertaining the bed load transport rate by sampling the bed load the transport rate was deduced from data on bedform height and bedform celerity for this purpose 43 sets of data from rivers representing a wide range of bed material bedform dimensions and hydraulic conditions were collected as were some sets of data from tidal settings two formulae were used for the prediction of the bed load transport the formula of van rijn 1981 and the kalinske 1947 formula as approximated by elzerman frijlink 1951 and in the present study slightly modified for application to tidal waters both the bed load function of van rijn and the modified formula of kalinske frijlink require data which are easily obtained and that can be measured accurately at those stages of the flow when bed load transport was high the van rijn function tended to overestimate that transport for flow stages when bed load transport was low the opposite was true the modified kalinske frijlink function gave consistently good results 86 of the transport rates predicted using the river data were within 0 5 2 0 times the values actually measured

lithospheric thickness antarctic deglaciation history and ocean basin discretization effects in a global model of postglacial sea level change a summary of some sources of nonuniqueness the global model of postglacial relative sea level variations that has been developed over the past decade is employed to investigate the constraints that it may be invoked to place on the timing of the deglaciation of west antarctica the analyses presented here confirm the suggestion of p wu and w r peltier 1983 geophysical journal of the royal astronomical society74 377 450 that the model of this event employed inj a clark and c s lingle 1979quaternary research11 279 298 may be simply modified to rectify the misfits between theory and observation that are otherwise obtained at southern hemisphere sites a large number of southern hemisphere relative sea level data are shown to require that the retreat of antarctic ice substantially lagged the retreat of northern hemisphere ice if the deglaciation of antarctica was abrupt the time of onset of antarctic deglaciation is thereby shown to coincide with the time of most rapid northern hemisphere deglaciation sensitivity tests are performed which demonstrate that this result is relatively insensitive to the discretization employed to represent the ocean basins the only exception to this general rule obtains at some coastal sites at which a trade off is revealed between the delay of west antarctic melting and the thickness of the lithosphere required to reconcile the observed local variations of relative sea level at such sites which are all in the far field of the ice sheets some attention must be paid to the accuracy of the local finite element representation of the oceans and to the details of the antarctic deglaciation history

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

bed level changes on intertidal wetland in response to waves and tides a case study from the yangtze river delta short term bed level variability in tidal wetlands has important implication both for ecology and engineering in this study we combined in situ measurements with model simulations to quantify short term bed level changes on a meso macrotidal wetland in the yangtze river delta on the middle flat we observed erosion during neap to mean tides under onshore moderate to strong winds and bed recovery during subsequent spring tides when winds were both offshore and weaker suggesting that winds can overturn the neap spring cyclicity of bed level changes even on meso macrotidal mudflats the magnitude of bed level changes was smaller on both sides of the middle flat while the smallest changes occurred on the salt marsh observed bed level changes were reconstructed using a single point bed level change model which incorporates in situ measured parameters of hydrodynamics waves and currents suspended sediment concentrations and bed sediment properties we conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally due to changes in balance between wave and tidal energies and spatially because of changes in elevation and vegetation in the cross shore profile our study also reflects the advantage of combination of in situ measurement with simulation in detecting short term variability of tidal flats

late wisconsin and early holocene glacial history inner ross embayment antarctica abstract lateral drift sheets of outlet glaciers that pass through the transantarctic mountains constrain past changes of the huge ross ice drainage system of the antarctic ice sheet drift stratigraphy suggests correlation of reedy iii reedy glacier beardmore beardmore glacier britannia hatherton darwin glaciers ross sea mcmurdo sound and younger terra nova bay drifts radiocarbon dates place the outer limits of ross sea drift in late wisconsin time at 24 000 13 000 yr b p outlet glacier profiles from these drifts constrain late wisconsin ice sheet surface elevations within these constraints we give two extreme late wisconsin reconstructions of the ross ice drainage system both show little elevation change of the polar plateau coincident with extensive ice shelf grounding along the inner ross embayment however in the central ross embayment one reconstruction shows floating shelf ice whereas the other shows a grounded ice sheet massive late wisconsin holocene recession of grounded ice from the western ross embayment which was underway at 13 040 yr b p and completed by 6600 6020 yr b p was accompanied by little change in plateau ice levels inland of the transantarctic mountains sea level and basal melting probably controlled the extent of grounded ice in the ross embayment the interplay between the precipitation low late wisconsin and high holocene values and the influence of grounding on outlet glaciers late wisconsin thickening and late wisconsin holocene thinning with effects dying out inland probably controlled minor elevation changes of the polar plateau

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

history of the grounded ice sheet in the ross sea sector of antarctica during the last glacial maximum and the last termination knowledge of variations in the extent and thickness of the antarctic ice sheet is key for understanding the behaviour of southern hemisphere glaciers during the last ice age and for addressing issues involving global sea level ocean circulation and climate change insight into past ice sheet behaviour also will aid predictions of future ice sheet stability here we review ter restrial evidence for changes in ice geometry that occurred in the ross sea sector of antarctica at the last glacial maximum lgm and during subsequent deglaciation during the lgm a thick grounded ice sheet extended close to the continental shelf edge in the ross embayment this ice reached surface elevations of more than 1000 m along the coast of the central and southern trans antarctic mountains and marie byrd land the local lgm occurred by 18 ka on the coast but as late as 7 10 ka inland the first significant thinning took place at roughly 13 ka with most ice loss happening in the holocene this history makes it unlikely that the ross sea sector was a major contributor to meltwater pulse 1a mwp 1a resolution of a possible antarctic origin for mwp 1a awaits detailed reconstructions from all sectors of the ice sheet the antarctic ice sheet contains the world s larg est reservoir of fresh water and is a major control on southern hemisphere climate a history of changes in ice sheet extent and thickness is vital for answering questions involving the initiation of southern hemisphere glaciation and deglaciation