suspended sediment transport in the surf zone response to cross shore infragravity motion a field experiment to investigate the magnitude and temporal and spatial scales of suspended sediment transport in the inner surf zone was conducted at san marine oregon on october 7 13 1984 instrumentation consisted of five electromagnetic current meters three strain gauge type pressure transducers and 25 optical backscatter sensors to measure the concentrations of suspended sediment the instruments were deployed approximately 70 m x seaward of the mean high water line mean depth h 1 1 1 3 m in a closely spaced cross shaped pattern data were collected for 68 min periods spanning high tide during the 7 day experiment during the experiment waves varied in angle of approach from shore normal to southwesterly offshore significant swell heights were 3 5 m with periods of 12 14 s local sea conditions with wind waves of approximately 8 s also occurred a visual estimation of the width of the surf zone indicated it to be approximately 500 m and the breaker pattern suggested that there were two longshore bars at approximately 200 and 500 m offshore the data resulting from the experiment encompass a broad range of wave and current conditions which included periods of extreme infragravity motion 30 t 300 s and weak longshore currents and periods of low infragravity motion and strong longshore currents periods of incident wave dominance also occurred the field study section of this paper qualitatively describes the sediment response to strong 240 cm s 1 low frequency 0 01 hz cross shore fluid motions which dominated the inner surf zone during two of the data runs the infragravity band accounted for 85 of the cross shore velocity variance the peak 0 01 hz of which was associated with shore normal standing waves concentration gradients of suspended sediment appear to be associated with diffusive processes and develop and decay on time scales of the infragravity motions rather than those of incident waves suspension events during times of infragravity dominance often persisted for periods of 30 45 s and reached peak concentrations of 20 40 g l 1 at elevations in excess of 26 cm mean suspended load was 3 4 times larger than that associated with incident wave motions the modeling section of the paper attempts to quantitatively describe the observed time dependent vertical suspended sediment distribution with a one dimensional turbulent diffusion model formulations of eddy diffusion and erosion rate coefficients associated with peak cross shore flows are found to agree with or fall within the range of values from previous laboratory and field studies and are used in the model the model is evaluated by its ability to reproduce the time dependent vertical distribution of suspended sediment and to estimate the observed mean suspended load and the correct direction and magnitude of the mean longshore and cross shore flux level above the bed z 31 cm of suspended sediment

the effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites the wtt cupa roccamonfina volcano italy abstract the distribution of lithic clasts within two trachytic small volume pumiceous ignimbrites are described from the quaternary white trachytic tuff cupa formation of roccamonfina volcano italy the ignimbrites show a downslope grading of lithics with a maximum size where there is a major break in the volcano s slope rather than at proximal locations this is also the location where ignimbrites are thickest and most massive the break in slope is interpreted to have reduced flow capacity and velocity increasing the sedimentation rate so that massive ignimbrite formed by hindered settling sedimentation ignimbrite cc exhibits no vertical grading of lithics though it does show downslope grading with maximum size at the major break in slope and a rapid decrease further downslope ignimbrite cc thins away from the break in slope and shows an upward fining of the grain size within the topmost few decimeters of the unit the ignimbrite is stratified proximally and grades to massive facies at the break in slope and distally to stratified facies with numerous inverse graded beds the simplest mechanism accounting for these downslope variations is progressive aggradation from a quasi steady nonuniform pyroclastic density current the changes in deposit thickness and facies are interpreted to record downcurrent changes in sedimentation rate the upward fining reflects waning flow inversely graded bedded depositional facies in distal areas is interpreted to reflect flow unsteadiness and a decrease in suspended sediment load ignimbrite cd shows vertical as well as downslope grading of lithics this characteristic coupled with the widespread massive facies of the deposit and the tabular unit geometry are features that can be reconciled with both the debris flow plug analogy for pyroclastic flows sparks 1976 and the progressive aggradation model branney and kokelaar 1992 however none of them appears to satisfy completely the field evidences implying that when dealing with massive ignimbrites other evidence than lithic grading needs to be presented to better understand the related transport and depositional processes

a model for cross shore sediment transport a model for cross shore sediment transport due to random waves is described which adopts a vertically integrated transport description for sheetflow situations the formulation of the transport as a function of the instantaneous velocity field is based on the approach of bailard 1981 this approach assumes in essence that the instantaneous transport is proportional to some power of the instantaneous near bottom velocity implementation of this transport description in a time dependent model requires a formulation of the time mean and some low order moments of the near bottom velocity field an initial formulation based on a monochromatic second order stokes wave representation is presented the model is checked on the basis of both field and laboratory data some consequences for further study are indicated

geochemistry and origin of archaean quartz cordierite gneisses from the godthabsfjord region west greenland we report new data on the major minor and trace element compositions of metasedimentary quartzcordierite gneisses qcg an important member of the archaean malene supracrustal suite found throughout the godthabsfjord region of west greenland the analyzed qcg contain assemblages of quartz cordierite biotite garnet anthophyllite gedrite staurolite sillimanite plagioclase with abundant accessory zircon and minor rutile monazite and allanite and broadly resemble cordierite orthoamphibole rocks found in a great number of other metamorphic terrains chemically the qcg are characterized by 1 high but variable sio2 59 87 wt relative enrichments in mgo feo and al2o3 mg 0 35 0 85 and depletions in na2o k2o and especially cao 2 low concentrations of sc cr co ni and sr 3 high concentrations of y nb zr hf ta th u and ree rare earth elements with prominent negative eu anomalies in each case 5 high concentrations of ga 18 55 ppm with variable ga al ratios that are significantly higher than average crustal material low cr and ni together with enriched and fractionated ree displaying negative eu anomalies distinguish the malene qcg from published accounts of most other archaean sedimentary rocks furthermore all of the above mentioned trace element characteristics distinguish the qcg from ordinary malene clastic metasediments quartzites psammites and pelites suggesting a separate origin for the qcg these data point towards chemically evolved felsic igneous rocks being the source of the qcg consequently we propose that the malene qcg represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism which resulted in gain of mg and fe loss of alkalis and lime and possibly eu and sr the overall trace element characteristics of the qcg elevated ga zr nb ree etc are features shared by a type granites and their volcanic equivalents such igneous rocks may represent the ultimate source material for the qcg protolith

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

explosive felsic volcanism on el hierro canary islands the canary islands consist of seven basaltic shield volcanoes whose submerged portion is much more voluminous than the subaerial part of each island like so many other volcanic oceanic islands the indicative deposits of explosive felsic volcanism are not a common feature on the canary archipelago hitherto they have only been documented from the central islands of gran canaria and tenerife which are the largest volcanic complexes of the islands on the other canary islands the presence of felsic rocks is mostly restricted to intrusions and a few lava flows generally within the succession in the oldest parts of individual islands in this paper we present a detailed stratigraphic lithological and sedimentological study of a significant felsic pumice deposit on the island of el hierro referred here as the malpaso member which represents the only explosive episode of felsic volcanism found on the canary islands outside of gran canaria and tenerife the products of the eruption indicate a single eruptive event and cover an area of about 15 km2 this work provides a detailed stratigraphic and chronological framework for el hierro and four subunits are identified within the member on the basis of lithological and granulometric characteristics the results of this study demonstrate the importance of an explosive eruption in a setting where the activity is typified by effusive basaltic events given the style and the spatial distribution of the malpaso eruption and its products a future event with similar characteristics could have a serious impact on the population infrastructure and economy of the island of el hierro

formation mechanism of gongchangling high grade magnetite deposit hosted in archean bif anshan benxi area northeastern china abstract banded iron formations are main resources of global iron ore in which high grade ore is mainly composed of martite goethite and hematite they are also the major resource of iron ore in china mainly distributing in liaoning and hebei province in china the iron ore with fe greater than 50 is classified as high grade iron ore the high grade iron ore mainly consists of magnetite and displays its unique characteristics gongchangling iron deposit is one typical bif iron deposit which contains 150 mt of high grade iron ore in china the high grade magnetite ore bodies mainly occur around magnetite quartzite faults and the cores of folds and show positive relation to the development of the altered rocks in this deposit this research shows that high grade magnetite comes from magnetite quartzite and they are both formed with little or no addition of aluminum containing detrital material by marine chemical deposition in reduced environment and they are closely related to seafloor hydrothermal activity muddy silty rocks are original rocks of altered rocks of which the primitive mantle normalized ree pattern except eu is consistent with that of iron ore reflecting that their formation is related to the formation of high grade magnetite ore therefore the formation mechanism of high grade iron ore is proposed as following the regional metamorphism provides storage space for the formation of high grade magnetite ore and required temperature and pressure conditions for the mineral transformation the regional metamorphic hydrothermal fluid leaches feo out of magnetite quartzite when it passes by and the feo that leached out moves near faults or cores of folds together with the metamorphic hydrothermal fluid and aluminum containing rocks of which the original rocks are muddy silty in the formation of high grade iron ore aluminum containing rock appears in the intervals of sedimentation of iron containing rock series and consumes the silicon leached out of magnetite quartzite and forms garnet chlorite and biotite

dynamics of pyroclastic density currents conditions that promote substrate erosion and self channelization mount st helens washington usa abstract the may 18th 1980 eruption of mount st helens msh produced multiple pyroclastic density currents pdcs burying the area north of the volcano under 10s of meters of deposits detailed measurements of recently exposed strata from these pdcs provide substantial insight into the dynamics of concentrated currents including inferences on particle particle interactions current mobility due to sedimentation fluidization and internal pore pressure particle support mechanisms the influence of surface roughness and the conditions that promote substrate erosion and self channelization four primary flow units are identified along the extensive drainage system north of the volcano each flow unit has intricate vertical and lateral facies changes and complex cross cutting relationships away from source each flow unit is an accumulation from an unsteady but locally sustained pdc or an amalgamation of several pdc pulses the pdcs associated with units i and ii likely occurred during the pre climactic waxing phase of the eruption these currents flowed around and filled in the hummocky topography leaving the massive to diffusely stratified deposits of units i and ii the deposits of both units i and ii are generally more massive in low lying areas and more stratified in areas of high surface roughness suggesting that surface roughness enhanced basal shear stress within the flow boundary units iii and iv are associated with the climactic phase of the eruption which produced the most voluminous and wide spread pdcs both flow units are characteristically massive and enriched in vent derived lithic blocks these currents flowed over and around the debris avalanche deposits as evidenced by the erosion of blocks from the hummocks unit iii is massive poorly sorted and shows little to no evidence of elutriation or segregation of lithics and pumice suggesting a highly concentrated current where size density segregation was suppressed unit iv shows similar depositional features but typically has a basal lithic rich region is variably fines depleted and contains pumice lobes suggesting density segregation in a less concentrated current relative to unit iii deep erosive channels cut by the unit iii current and thick lithic levee deposits within unit iv occur in an area where debris avalanche relief is limited suggesting self channelization developed as a function of internal flow dynamics an increase in the proportion and size of lithic blocks is found 1 downstream of debris avalanche hummocks suggesting the pdcs were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream and 2 within large channels cut by later pdcs into earlier pdc deposits suggesting self channelization of the flows increased the carrying capacity of the subsequent channelized currents finally the combination of thick massive deposits with a high percentage of fine ash within unit iii and in the medial distal depositional regions of units ii iv suggests that the pdcs developed and maintained a high internal pore pressure during transport and deposition the most important results include our ability to understand the role of internal pore pressure on current mobility the influence of self channelization on carrying capacity of the currents and the influence of surface roughness on substrate erosion these observations have critical consequences for understanding the flow dynamics and hazard potential of pdcs

internal flow variations and diachronous sedimentation within extensive sustained density stratified pyroclastic density currents flowing down gentle slopes as revealed by the internal architectures of ignimbrites on tenerife during a protracted explosive eruption at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of las canadas volcano tenerife each pyroclastic current developed marked local and regional spatial variations in response to the incised gently concave substrate topography the locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography this complex shifting response of the density currents to minor topographic variations has been reconstructed in high resolution over a wide area 500 km2 using the internal architecture of cryptic time surfaces entrachrons marked by compositional zoning in the deposit including variations in clast types valley side field relations reveal that the currents were density stratified yet at a single instant in time the lower levels of each current comprised a granular fluid at some locations but were fully dilute and turbulent at others moreover the locations of these variations shifted geographically as the topography changed during the eruption the variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies both laterally and in the downcurrent direction individual currents were regionally widespread and travelled 15 km but deposited only in longitudinally restricted localised zones that spanned several small valleys and interfluves the currents bypassed slopes up and downcurrent of the restricted depositional zones without depositing the locations of deposition then gradually shifted with time such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow units are markedly diachronous deposition of a flow unit commenced and ceased at different times in different places this study suggests that in hazard assessments a models of density currents that incorporate only pre existing topography e g from dems may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement altering flow paths b frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under estimated because currents commonly bypass without leaving a deposit record and c even where preservation appears to be complete an ignimbrite at a single exposure commonly will not record the current s entire flow history at that site

morphologic properties derived from a simple cross shore sediment transport model this paper builds on the now classical discussions by bowen 1980 and bailard 1981 on the applicability and implications of bagnold s 1963 sediment transport model to nearshore profile modeling we focus on the morphologic implications of both the strengths and weaknesses of bagnold s model isolating the transport terms that are well predicted i e mean flow terms from those that are not well predicted i e transport due to correlations between flow and sediment load we factor bagnold s model into a dimensional transport magnitude and a nondimensional term the nondimensional term describes the relative importance of transport due to undertow gravity and correlations between flow and sediment load the transport magnitude largely determines the response time of nearshore profiles for typical nearshore environments this response time was estimated to vary as a function of incident rms wave height hrms from 500 years hrms 0 5 m to 2 years hrms 3 m the relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height defined as the ratio of the rms wave height to the local depth simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles several differences from previously computed equilibrium profiles were noted first because the relative wave height saturates in natural surf zones equilibrium profiles converge to a relatively flat profile near the shoreline second under some situations a seaward sloping equilibrium profile may not exist third the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature

bars formed by horizontal diffusion of suspended sediment abstract the mechanism responsible for the ubiquitous presence of convex beach profiles and shoreward migration of linear bars is examined using numerical circulation and sediment transport models the models are validated against laboratory measurements and observed natural beach cross sections while not discounting the importance of infragravity and advective horizontal circulation or bed return flow mechanisms a robust diffusive process explains the convex profile shape and bar formation in the presence of concentration gradients across the surf zone a diffusive sediment flux from high to low concentration results in the transfer of sediment outwards from the breakpoint both onshore and offshore and the subsequent formation of a diffusion bar and diffusion profile the profiles are characterised by single and double convex dome like shapes developing during shoreward migration of the bars by the diffusion mechanism the mechanism explains several phenomena observed on natural beaches including i convex beach profiles ii shoreward migration of the bar with concomitant beach accretion under narrow band swell iii reduced propensity for bar formation on low gradient fine sand beaches or under wide band wave spectra even though multiple bars are common on some low gradient beaches and iv offshore migration of the bar during periods of increasing wave height the diffusion mechanism can be dependent on orbital motion alone and as such requires no frequency selection or strong correlation between multiple processes for bar formation

high resolution field measurements and numerical modelling of intra wave sediment suspension on plane beds under shoaling waves abstract intra wave sediment suspension is examined using high resolution field measurements and numerical hydrodynamic and sediment models within 120 mm of a plane seabed under natural asymmetric waves the detailed measurements of suspended sediment concentration at 5 mm vertical resolution and at 4 hz showed two or three entrainment bursts around peak flow under the wave crest and another at flow reversal during the decelerating phase at flow reversal the mixing length was found to be approximately double the value attained at peak flow under the crest to examine the cause of multiple suspension peaks and increased diffusion at flow reversal a numerical side view hydrodynamic model was developed to reproduce near bed wave induced orbital currents predicted currents at the bed and above the wave boundary layer were oppositely directed around flow reversal and this effect became more pronounced with increasing wave asymmetry when the predicted orbital currents and an enhanced eddy diffusivity during periods of oppositely directed flows were applied in a lagrangian numerical sediment transport model unprecedented and extremely close predictions of the measured instantaneous concentrations were obtained the numerical models were simplified to incorporate only the essential parameters and by simulating at short time scales empirical time averaged parameterisations were not required key factors in the sediment model were fall velocities of the full grain size distribution diffusion separation of entrainment from settlement and non constant but vertically uniform eddy diffusivity over the plane bed sediment convection by wave orbital vertical currents was found to have no significant influence on the results

the effect of sea breeze on beach morphology surf zone hydrodynamics and sediment resuspension abstract beach morphodynamic processes were investigated over part of a sea breeze cycle on a microtidal low wave energy sandy beach in southwestern australia prior to the onset of the sea breeze offshore winds with speeds less than 5 m s prevailed during the sea breeze alongshore winds with speeds higher than 10 m s were experienced the sea breeze induced pronounced changes to the nearshore morphodynamics which were similar to that of a storm event 1 root mean square wave height increased from 0 3 to 0 5 m 2 zero upcrossing wave period decreased from 8 to 4 s 3 mean cross shore flows reached velocities of 0 2 m s directed offshore and 4 the longshore current increased in strength from 0 05 to 1 0 m s before the sea breeze sediment resuspension typically occurred during isolated high wave events associated with the passage of wave groups flux coupling between the wave oscillatory currents and the suspended sediment jaffe b e sternberg r w sallenger a h 1984 the role of suspended sediment in shore normal beach profile changes proc 19th int conf coastal engineering asce pp 1983 1996 induced net onshore suspended sediment transport resulting in beachface accretion and a steepening of the foreshore profile in contrast during the sea breeze sediment resuspension was almost continuous the suspended sediment load increased six fold and as a consequence the longshore suspended sediment transport rate increased by a factor of 100 during the sea breeze cross shore transport was directed offshore and primarily associated with mean offshore flows this resulted in erosion of the beachface and deposition in the surf zone and consequently a flattening of the beach profile the suspended sediment load was strongly related to the shields parameter defined for the combined action of waves and currents investigation of the suspended sediment profiles indicated that the degree of vertical mixing parameterised by the sediment diffusion coefficient increased with distance from the bed in addition the sediment diffusion coefficient exhibited a large temporal variation ranging between 0 005 and 0 018 m2 s the variability in the sediment diffusion coefficient is possibly due to changes in the sea bed morphology

sediment transport near the break point associated with cross shore gradients in vertical eddy diffusivity sediment clouds entrained and mixed vertically into the water column at the breakpoint were observed to advect offshore with the return current following wave breaking at a natural beach an advection diffusion model is applied to this phenomenon providing for cross shore gradients in vertical eddy diffusivity within and beyond the surf zone which result in differential entrainment and settlement near the break point results indicate that the horizontal gradient in turbulent vertical sediment mixing can provide a significant mechanism for bar generation even if the surface gravity and infra gravity waves remain uncoupled quantitative estimates of the rate of bar accretion are obtained after calibrating the sediment transport model using field measurements of currents and suspended sediment loads in the absence of other processes accretion rates of order 20 mm hour associated with the horizontal gradient in vertical diffusivity are predicted on the offshore bar

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

suspended sediment load during an asymmetric wave cycle over a plane bed abstract suspended sediment measurements made by ribberink and al salem 1992 between 5 mm and 46 mm from a plane bed exhibited apparent changes to the boundary layer thickness and eddy diffusivity during the wave cycle bed ripples were absent but the data still showed entrainment near the time of flow reversal these laboratory measurements were examined using a numerical model to study sediment suspension at short time scales to assess relevance the data were considered in conjunction with concentration time series of similar character from a natural beach each of three distinct concentration peaks during one wave cycle evident in the laboratory measurements had to be treated differently however all peaks exhibited a high correlation to steady state logarithmic concentration profiles with uniform but unequal vertical eddy diffusivities the first peak under the wave crest could be predicted with standard empirical relationships derived from long term averaged data the remaining peaks could be simulated only when unexpected entrainment was permitted for a very short time 0 1 s near flow reversal and separately during the accelerating phase of the wave trough the paper provides the basis for further work but at present the results cannot be used for predictive purposes

calculation of littoral sand transport rate abstract an energy related littoral sand transport rate formula is presented based on extensive laboratory tests and on a broad set of known field data an example of application of the new expression to an actual measured field situation is also given

distal ash hurricane pyroclastic density current deposits from a ca 2000 yr b p plinian style eruption of mount pelee martinique distribution grain size characteristics and implications for future hazard plinian style activity has not occurred on any volcano in the lesser antilles since european settlement however pumice rich deposits from this eruptive style are found throughout the lesser antilles and such eruptions were witnessed by pre columbian native american populations one of these eruptive events from mount pelee on martinique occurring between 1800 and 2200 yr b p produced pumice and ash flow deposits that followed the main drainage channels on the volcano ash hurricane deposits that both mantle the volcano s flanks slopes up to 25 and extended at least 20 km from the crater we examined the latter in the context of modern developments in understanding transport and depositional mechanics of pyroclastic density currents based on grain size analysis they are subdivided into proximal and distal deposits the distal deposits become progressively more depleted in fines and enriched in crystals with distance from the crater we conclude these are deposits of dilute or inertia dominated pyroclastic overcurrents which decoupled from their underflows as they surmounted the topographic barrier formed by the older pitons du carbet volcanic center a combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff and resulting extreme crystal enrichment their areal distribution suggests much of the northern half of martinique must have been severely affected and the zone of devastation extended at least to the northern outskirts of the capital fort de france although their final runout distance may have extended significantly further south major coignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation

sources of iron formations in the archean isua and malene supracrustals west greenland evidence from la ce and sm nd isotopic data and ree abundances abstract isotopic ratios of ce and nd and abundances of rare earth elements rees and ba are reported for the iron formations of the archean isua and malene supracrustals west greenland it is suggested that the sm nd system of the isua iron formations was disturbed by metamorphisms at 1680 ma oxide facies iron formation and 2740 ma carbonate facies iron formation however the malene iron formation samples show similar nd isotopic ratios and ree patterns indicating that they suffered little disturbance after deposition at 2800 ma initial ce and nd values estimated at 2800 ma for the malene iron formation are close to chur chondritic uniform reservoir of 3 samples two show negative ce and positive nd and the other gives chondritic ce and negative nd these initial values suggest that the sources for the malene iron formation had a chiefly chondritic or rather mantle like depleted nature without a notable long term negative ce anomaly similar ree features are indicated for the sources of the isua iron formation by the estimated initial ce and nd values at 3760 ma of these samples

facies characteristics and magma water interaction of the white trachytic tuffs roccamonfina volcano southern italy the quaternary white trachytic tuffs formation from roccamonfina volcano southern italy comprises four non welded trachytic pyroclastic sequences bounded by paleosols each of which corresponds to small to intermediate volume explosive eruptions from central vents from oldest to youngest they are white trachytic tuff wtt cupa wtt aulpi wtt s clemente and wtt galluccio the wtt galluccio eruption was the largest and emplaced 4 km3 of magma the internal stratigraphy of all four wtt eruptive units is a complex association of fallout surge and pyroclastic flow deposits each eruptive unit is organized into two facies associations facies association a below facies association b the emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking and erosion facies association a consists of interbedded fallout deposits surge deposits and subordinate ignimbrites this facies association involved the eruption of the most evolved trachytic magma and pumice clasts are white and well vesiculated the grain size coarsens upward in facies association a with upward increases of dune bedform wavelengths and a decrease in the proportion of fine ash these trends could reflect an increase in eruption column height from the onset of the eruption and possibly also in mass eruption rate facies association b comprises massive ignimbrites that are progressively richer in lithic clast content this association involved the eruption of more mafic magma and pumice clasts are gray and poorly vesiculated facies association b is interpreted to record the climax of the eruption phreatomagmatic deposits occur at different stratigraphic levels in the four wtt and have different facies characteristics the deposits reflect the style and degree of magma water interaction and the local hydrogeology very fine grained lithic poor phreatomagmatic surge deposits found at the base of wtt cupa and wtt galluccio could record the interaction of the erupting magma with a lake that occupied the roccamonfina summit depression renewed magma water interaction later in the wtt galluccio eruption is indicated by fine grained lithic bearing phreatomagmatic fall and surge deposits occurring at the top of facies association a they could be interpreted to reflect shifts of the magma fragmentation level to highly transmissive regional aquifers located beneath the roccamonfina edifice possibly heralding a caldera collapse event

large volume phreatomagmatic ignimbrites from the colli albani volcano middle pleistocene italy abstract in this paper we describe four large volume ash and accretionary lapilli rich phreatomagmatic compound ignimbrite units mafic in composition from colli albani volcano south of rome the four units that form the pisolitic tuffs succession are separated by paleosols and represent the earliest explosive large volume eruptive episodes from the quaternary alban hills volcano the occurrence of large volume phreatomagmatic phreatoplinian eruptions implies the availability of large quantities of water interacting with the rising magma the paleogeography of the area below the volcano has been reconstructed by the analysis of stratigraphic data from more than a thousand bore holes distributed around the volcano that allowed to identify nw trending and ne trending paleotopographic lows that underlie the central area of the volcano and interpreted as extensional tectonic basins these lows are filled with lower to middle pleistocene pre volcanic lacustrine and fluvial deposits and suggest that at least the central part of the colli albani volcanic area hosted a large lake or lagoon the absence of sedimentary xenoliths in the pisolitic tuffs and the low vesicularity of scoria and shards suggest that water interacted with a poorly fragmented magma at very shallow level triggering the large explosivity of the eruptions we suggest that water interacting with magma was mostly surficial water related to the presence of the lagoon or lake in this environment it is likely that vents were subaqueous allowing a continuous access of water to the conduit considering that the minimum calculated volume of products for the pisolitic tuffs succession is 37 km3 and each eruption unit averages approximately around minimum volume of 10 km3 we suggest that each eruption was related to a caldera collapse which would have allowed the persistence of a taupo like lake in the central area and of phreatoplinian activity after the last phreatomagmatic eruption however the fragmentation style of large volume ignimbrites from colli albani became magmatic suggesting the extinction of the lake after that date each of the four units show a basal phreatoplinian fallout level overlain by a complex association of low aspect ratio surge deposits and ignimbrites phreatomagmatic pyroclastic flow deposits are found at distances of more than 40 km from the central area of the volcano and show important facies variation according to the paleotopography to the west of the volcano pyroclastic flows reached the tyrrhenian coast and emplaced mostly stratified facies on a flat topography interpreted to reflect both the spreading of pyroclastic flows on an unconfined topography and their interaction with lacustrine and lagoon areas by contrast to the east of the volcano where pyroclastic flows were confined within paleovalleys the main facies is thick and massive

apatite in early archean isua supracrustal rocks southern west greenland its origin association with graphite and potential as a biomarker abstract rare earth element ree abundances in individual apatite crystals in banded iron formations bifs metacherts metacarbonates and mafic dykes in the isua supracrustal belt isb have been determined by laser ablation inductively coupled plasma mass spectrometry the results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential proposed in earlier studies for use of the apatite graphite association as a biomarker the chondrite normalized distribution patterns of apatite in metasedimentary bifs and metacherts fall into three groups relatively flat profiles with distinct positive eu anomaly are interpreted as characterizing sedimentary diagenetic apatite that carry the ree signature of the archean ocean secondary apatite in isua metasdiments with either middle ree enriched profiles or with light ree depleted profiles is interpreted to have crystallized from percolating carbonate rich metasomatic fluids or from fluids derived from cross cutting mafic dykes respectively the occurrence together of these different genetic types of apatite with distinct ree signatures within cm scale samples shows the immobility of ree in preexisting apatite during metamorphic episodes apatite crystals in isua rocks of uncontested chemical sedimentary origin bif and metachert samples do not have graphite inclusions or coatings graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks in these rocks graphite is produced by thermal metamorphic reduction of carbonate ion derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor oxidizing to form magnetite in view of the non sedimentary metasomatic origin of isua metacarbonates and the abiogenic source of graphite the apatite graphite assemblage can not be considered as a biomarker and does not provide information on early archean life in the isb

on an sn w bearing iron formation in the archaean malene supracrustals west greenland abstract in the archaean malene supracrustal rocks west greenland small horizons of iron formation occur they are rarely more than a few metres wide and can be traced with intervals for several hundreds of metres along strike the iron formation is mainly silicate and sulphide facies with high contents of trace elements such as 1 1 zn 0 18 mo 0 12 sn and 170 ppm w the iron formation is associated with stratabound scheelite mineralization and tour malinites the iron formation is suggested to be of submarine exhalative origin

breaker turbulence and sediment suspension in the surf zone field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices related to wave breaking in suspended sediment dynamics the measurements were obtained from a barred intermediate type beach and the instrument rig was exposed to a range of wave types during the experiment including shoaling waves and both spilling and plunging breakers the time scale of the observed breaker vortices was 5 hz and thus in the frequency range of wave orbital motion coherent breaker vortices were separated from wave orbital motions and incoherent higher frequency turbulence using a threshold magnitude for the vertical velocity component plunging breakers displayed the largest root mean square rms and maximum vertical velocities the latter reaching approximately 1 m s 1 these measurements were made at an elevation of 0 15 m above the bed given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment on the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited rms vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers different phase relationships were observed between the horizontal fluid velocity and vortex induced sediment suspension for each of the wave types which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery respectively

three simple models of wave sediment transport abstract the process of sediment transport by nonbreaking waves over horizontal rippled beds is presented by three different models which are evaluated through comparison with wave flume data the conclusion from this testing is the simpler the better it is shown that the process in question can in fact be modelled adequately without quantitative consideration of the suspended sediment distribution only a reference average concentration at the bed is needed on the other hand classical diffusion models severely underpredict the transport of coarse sand in suspension because the process is much more organized than diffusion the possibility of adapting the models to conditions involving sheet flow and or breaking waves is discussed

origin of negative ce anomalies in barberton sedimentary rocks deduced from la ce and sm nd isotope systematics abstract the origin of sedimentary rocks with negative ce anomalies among several samples derived from the barberton greenstone belt 3250 ma south africa was investigated using la ce and sm nd isotopic systematics together with light rare earth element lree abundances negative ce anomalies were recognized in two sedimentary rocks a banded ferruginous chert and a shale their ce ce values are 0 40 and 0 81 respectively initial ce isotopic ratio calculated for the ferruginous chert 138 ce 142 ce 3250 ma 0 0225059 is far lower than chur value at 3250 ma and even the value at earth formation 138 ce 142 ce chur 4560 ma 0 0225409 the calculated initial ce isotopic ratio for the shale with a ce anomaly is also lower than that of other rocks without ce anomalies these results indicate that the negative ce anomalies were not inherited from an archean redox change but occurred following later episodes of alteration two possible formation models were examined to estimate a plausible occurrence age for the negative ce anomalies one is characterized by ce extraction and the other is by an addition of la nd and sm both models indicate that negative ce anomalies arose before 1100 ma this study suggests that la ce isotopic systematics provides a unique tool to constrain the occurrence age of negative ce anomalies observed in archean sedimentary rocks and gives some hints about the redox condition in the archean ocean and the evolution of the earth

facies associations of rain generated versus crater lake withdrawal lahar deposits from quaternary volcanoes central italy abstract two syneruption lahar deposit successions from quaternary italian volcanoes are presented displaying different facies associations interpreted to reflect different water sources the lahar deposits associated with the white trachytic tuff cupa wttc ignimbrites from the quaternary roccamonfina volcano located 150 km to the southeast of rome have been interpreted in terms of rain generated lahars the wttc ignimbrites are made of more than 1 km 3 of loose pumice and lava lithic debris emplaced along the hyperbolic slope of the volcano at ca 300 ka during an interglacial period characterised by mild and wet climate the lahar deposits are organised in a coarsening upward aggradational and back stepping succession of medium to thick bedded progressively juvenile poorer non cohesive debris flow to fluvial deposits box shaped channels cut the wttc ignimbrites along the steep upper slopes channels are filled with lava lithic rich fluvial to hyperconcentrated flow sand and conglomerate which are interpreted as lag deposits related to processes of bulking due to the removal of light pumice and ash debris from the upper slope along the lower slopes of the volcano and in the surrounding ring plains where the average slope inclination decreases to few degrees lahars emplaced an aggradational succession of bedded ash rich hyperconcentrated flow deposits entirely derived from wttc components the succession coarsens upward with increasing presence of lava rich conglomerate lenses fluvial in origin interpreted to record the progressive restoration through time of the drainage network the succession is cut by incised gullies filled with polygenetic fluvial deposits which indicate the restoration of intererruption condition by contrast the ca 23 ka small volume peperino albano phreatomagmatic eruption from colli albani volcano located 30 km to the southeast of rome emplaced a valley ponded block and ash ignimbrite which along the western slope of the volcano grades laterally into a single far reaching thick lahar deposit the lahar deposit coarsens upward from coarse ash hyperconcentrated flow deposit into a lithic block rich debris flow deposit this lahar deposit has been interpreted to be directly derived from a pyroclastic flow and particularly related to the entrance of the pyroclastic flow into a pre existing maar crater lake along the pyroclastic flow path the basal sand size hyperconcentrated flow deposit is interpreted to represent early deposition from the fast frontal flood wave whereas the coarse lithic rich debris flow deposit at the top may represent the rear of the lahar the separation of the two facies can be related to processes of hydraulic sieving operated by the lake water which couples with ash particles leaving behind the coarser fraction

rare earth elements in the early archaean isua iron formation west greenland abstract the rare earth element ree patterns in the 3 8 ga old isua iron formation are generally flat resembling those of some primitive basalts samples with positive negative or no europium anomaly were found it is shown that diagenesis and metamorphism did not significantly change the ree patterns the presence or absence of europium anomalies in iron formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the archaean and precambrian the ree contents cannot be used to distinguish algoma type from superior type iron formations there appears to be a striking similarity between the archaean submarine exhalations and modern submarine hydrothermal systems it is considered likely that archaean and early precambrian seawater had a chondritic ree pattern with a slight enrichment of light ree

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

vertical distribution of suspended sediment concentrations seawards of the breaking zone abstract on the basis of the current concepts on the wave flow structure and solid particle motion in turbulent flow an equation was derived which enables the suspended sediment concentration profile to be computed from known wave parameters the equation was verified by laboratory and full scale experiments which proved that it can be implemented for practical computations of the concentration profile seawards of the breaking zone the formula yields the best results when operating on wave characteristics corresponding to maximum spectral density

strata bound tourmaline in the archaean malene supracrustals west greenland thin tourmaline rich layers closely associated with strata bound tungsten mineralization occur in layered amphibolites and metasediments of the archaean malene supracrustals in west greenland locally tourmaline is intergrown with scheelite microprobe analyses showed that the tourmaline is of intermediate composition between schorl and dravite with small variations in composition in different layers the tourmaline is interpreted as being of submarine exhalative origin

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

holocene recurrent explosive activity at chimborazo volcano ecuador abstract ice capped chimborazo is one of the large composite ecuadorian volcanoes whose recent eruptive activity is poorly known this paper presents the characteristics and the ages of a newly discovered holocene sequence of pyroclastic deposits on the east and north sides of the cone lying upon a moraine of the late glacial period the most complete section of 4 5 m in thickness is located 5 km from the present summit crater it consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils based on field study most flow deposits were assessed as surge layers and six radiocarbon analyses obtained from charcoal fragments and paleosoils indicate that eruptions occurred at quite regular intervals between about 8000 and 1000 years ago the first two and most potent events generated thick lahars over the north and west flanks of the cone surface textures of volcanic clasts were analysed by scanning electron microscopy blocky and blocky fluidal vitric clasts indicate fragmentation during vulcanian explosions of a quite solidified shallow magma body in addition aggregates either cemented at a cooling stage with surface fluidal textures or consisting of fine particles moss looking aggregates form a large part of the surge deposits these characteristics indicate powerful explosions and intense fragmentation due to phreatic water reaching the conduit probably from the ice cap since the last eruption occurred between the early part of the 5th century ad 420 and the end of the 7th century these results highlight that chimborazo is a potentially active volcano given its dominating presence over the densely populated ambato and riobamba basins and owing its large ice cap chimborazo should be considered a dangerous volcano

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

suspended sediment concentrations under waves abstract a predictive formula is derived for calculation of the magnitude of time averaged suspended sediment concentrations under waves by taking into account the effect of flow contraction near ripple crests the formula applies equally well for rippled beds and sheet flow conditions the formula is in qualitative agreement with the bed load flux formula of madsen and grant 1976

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

bar formation due to wave groups and associated long waves abstract field observations reveal that waves propagating onto a beach from deeper water have a grouped structure associated with this pattern of alternating high and low waves is a long wave forced by radiation stress variations due to the changing short wave height as the wave groups propagate onshore the relationship between the variation in the short wave height and the long wave motion is complicated by breaking of the short waves reflection of the long wave and the generation of a free long wave by time variation of the breakpoint position nevertheless there may still be a fixed phase relation between the short wave envelope and the long wave motion at any particular location resulting in a net transport of sediment over the timescale of the short wave groups this transport arises from sediment grains mobilised by short waves and moved by the long wave motion model predictions of wave group propagation onto a beach and the associated long wave motions are presented which suggest that sediment transport due to short wave long wave coupling may result in the formation of a bar within the inner surf zone and the possibility of additional bars further offshore outside the surf zone this mechanism for bar formation may provide a new framework for understanding bar morphodynamics

suspended sediment transport in the swash zone of a dissipative beach abstract simultaneous high frequency field measurements of water depth flow velocity and suspended sediment concentration were made at three fixed locations across the high tide swash and inner surf zones of a dissipative beach the dominant period of the swash motion was 30 50 s and the results are representative of infragravity swash motion suspended sediment concentrations loads and transport rates in the swash zone were almost one order of magnitude greater than in the inner surf zone the vertical velocity gradient near the bed and the resulting bed shear stress at the start of the uprush was significantly larger than that at the end of the backwash despite similar flow velocities this suggests that the bed friction during the uprush was approximately twice that during the backwash the suspended sediment profile in the swash zone can be described reasonably well by an exponential shape with a mixing length scale of 0 02 0 03 m the suspended sediment transport flux measured in the swash zone was related to the bed shear stress through the shields parameter if the bed shear stress is derived from the vertical velocity gradient the proportionality coefficient between shear stress and sediment transport rate is similar for the uprush and the backwash if the bed shear stress is estimated using the free stream flow velocity and a constant friction factor the proportionality factor for the uprush is approximately twice that of the backwash it is suggested that the uprush is a more efficient transporter of sediment than the backwash because the larger friction factor during the uprush causes larger bed shear stresses for a given free stream velocity this increased transport competency of the uprush is necessary for maintaining the beach otherwise the comparable strength and greater duration of the backwash would progressively remove sediment from the beach

in situ iron isotope analysis of pyrites in 3 7ga sedimentary protoliths from the isua supracrustal belt southern west greenland abstract the timing of the emergence of life remains one of the principal unresolved questions in the earth sciences putative relicts of microorganisms in the eoarchean ca 3 6 3 85 ga high grade metamorphic terranes do not preserve morphological evidence for early life but some relicts can be identified by their geochemical signatures created by metabolic processes among the oldest rocks of sedimentary origin ca 3 8 ga occur in the isua supracrustal belt isb southern west greenland these have undergone metamorphism up to the amphibolite facies despite intense metamorphism the carbon isotope compositions of graphite clots from the isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process microbial dissimilatory iron reduction dir is considered to be an early form of metabolism the microbial dir produced fe 2 aq with a lower 56 fe value from a precursor fe 3 bearing iron mineral however 56 fe values lower than 1 are not found in sedimentary rocks prior to about 2 9 ga here we report in situ iron isotope analysis of pyrites in sedimentary rocks from the isb using a near infrared femtosecond laser ablation multicollector icp ms nir fs la mc icp ms a large variation of 56 fe values from 2 41 to 2 35 was documented from 190 points within pyrite grains from 11 rock specimens including those interpreted to be banded iron formations bifs chert amphibole rich chert quartz rich clastic sedimentary rocks mafic clastic sedimentary rocks carbonate rocks and conglomerates we found that the distribution of 56 fe values depends on the lithology whereas there is no correlation between their 56 fe values and the metamorphic grade the 56 fe values of pyrites in bifs range from 0 25 to 2 35 indicating partial oxidation in the deep ocean especially the high 56 fe values up to 2 35 suggest that the bif was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions pyrite grains in a conglomerate carbonate rocks mafic clastic sedimentary rocks and amphibole rich cherts show negative 56 fe values around 1 5 down to 2 41 pointing to microbial dir in the eoarchean shallow sea in addition the relatively low 56 fe values of pyrites in the shallow water sediments suggest anoxic anoxygenic photoautotrophic iron oxidation in the photic zone

bar trapped edge waves and longshore currents edge waves react to the presence of a mean longshore current as though the current was a change in bottom topography in this way the cross shore shape of the edge wave can be amplified over the current relative to the shoreline in the same way as the edge wave shape can be amplified over bars on a beach with both longshore currents and bars the degree of edge wave amplification or trapping depends on the strength and location of the longshore current shear relative to the steepness of the bottom slope and also on the direction of edge wave propagation relative to the longshore current weak longshore current shear over well developed bars does not significantly alter edge wave trapping over bars simply enhancing or reducing the existing bar trapping effect conversely strong current shears over gentle topography can govern the existence and location of edge wave trapping the extreme example being edge wave trapping on plane beaches where the amplification is entirely due to the existence of mean longshore currents in reality the possibilities are restricted since the shape of the longshore current profile is primarily determined by the pattern of incident wave breaking which itself depends on the beach profile the importance of realistic current shear relative to realistic bottom slope in determining edge wave shape is demonstrated using frequency wavenumber spectra taken during the duck experiment on low frequency and incident band longshore and across shore hydrodynamics delilah at duck north carolina these frequency wavenumber spectra show a general dominance of edge waves traveling with the longshore current in cases where the current shear is strong relative to the bottom slope the location of edge wave trapping changes as the longshore current profile changes with the tide when the bar was well defined the longshore current shear was rarely strong enough to influence the existence or location of edge wave trapping but could have a enhancing effect while past work i e schonfeldt 1995 has suggested that edge waves trapped and amplified on longshore currents may cause bars to form on plane beaches and bars to move on barred beaches these observations suggest that under most conditions edge wave trapping on longshore currents is a subtle effect modulated by sea level changes and is unlikely to produce significant morphological change unless the current is very strong

silicate facies iron formation and strata bound alteration tuffaceous exhalites derived by mixing evidence from mn garnet stilpnomelane rocks at redstone timmins ontario silicate facies iron formation composed of mn garnet stilpnomelane and ferrotschermakitic hornblende mark an exhalative sedimentary stratigraphic horizon located regionally at the break between two volcanic supercycles consisting of lowermost calc alkaline felsic pyroclastic rocks overlain by komatiitic ultramafic flows locally mn garnet stilpnomelane rock flanks and in places is footwall to a strata bound volcanic hosted nickel sulfide deposit at redstone timmins mn garnets have normal reverse and oscillatory zoning patterns with a coupled substitution of mn ca for fe mg stilpnomelane amphibole chlorite and biotite each have elevated mno contents but do not display any concentric zoning patterns the al ti and ti zr ratios of mn garnet stilpnomelane rock are similar to those of dacitic tuff at redstone however the former rock is enriched in fe mg and mn but depleted in na relative to the latter discordant alteration pipes and concordant strata bound altered rocks associated with massive base metal sulfide and archean lode au volcanic hosted deposits are predominated by fe mg mn al enriched alkali depleted rocks very similar to silicate facies iron formation at redstone mn garnet stilpnomelane rocks represent a mixture of pyroclastic material plus chemical sediment whereby all of the al ti zr v and p have a volcaniclastic origin and most of the fe mn mg ca co and rb are chemically precipitated whereas cr ni sr na nb ba and k have been leached from a volcanic precursor y is mobile perhaps due to a preferred partitioning into the mn garnet structure both silicate facies iron formation and semiconcordant alteration zones are enriched in elements commonly mobile in hydrothermal systems such as fe mg mn si relative to 100 percent tuff as well as immobile elements including al ti zr y nb rare earth elements relative to 100 percent exhalite the resultant group of aluminosilicate minerals within these altered rocks is a function of the bulk rock composition which in turn represents a mixture of exhaled chemical sediment and volcaniclastic debris strata bound fe mg mn al enriched tuffaceous exhalites typically occur in a mixed volcano sedimentary setting distal from a volcanic center but proximal to the hydrothermal vent often flanking or stratigraphically at or near a prospective gold or base metal ore horizon

petrogenesis of archaean malene supracrustal rocks nw buksefjorden region west greenland geochemical evidence for highly evolved archaean crust abstract we report new data for major minor and trace elements in forty nine samples of malene supracrustal rocks nw buksefjorden region west greenland the samples consist principally of quartz cordierite biotite garnet anthophyllite gedrite sillimanite with abundant accessory zircon and minor rutile allanite and monazite plagioclase is rare to absent while muscovite occurs in only a few cases thus the malene paragneisses closely resemble cordierite orthoamphibole rocks found in other metamorphic complexes the nw buksefjorden malene paragneisses have on average high sio2 moderate al2o3 fe2o3 t and mgo and low k2o na2o and especially cao distinctive trace element characteristics include 1 low sc co cr ni and sr 2 high y zr nb hf ta th u and ree with prominent negative eu anomalies in nearly every case and 3 high ga up to 65 ppm with ga al ratios significantly higher than average crustal material on the basis of afm assemblage type the paragneisses can be divided into distinct groups which preserve the field distinctions noted by previous workers however these groupings primarily reflect variations in mg total range from 0 26 0 89 with no meaningful distinctions appearing among trace elements except possibly cu the trace element characteristics of the nw buksefjorden malene paragneisses are identical to those of the malene quartz cordierite gneisses found throughout the greater godthabsfjord region these gneisses are different from the malene quartzite psammite pelite group of clastic metasediments the trace element data of the quartz cordierite gneisses are compatible with a protolith dominated by material derived from felsic igneous precursors an interpretation supported by immobile element discriminant plots and ternary la th sc and th co hf plots however negative eu anomalies and low sr and ca suggest that plagioclase was lost from the protolith or from the precursor lithology or both the high concentrations of ga indicate either that the source rocks for the paragneisses were unusually ga rich or that assembly of the protolith was accompanied by a major fractionation of al from ga we propose that the protolith for most of the nw buksefjorden malene paragneisses was a quartz chlorite sericite schist derived from felsic volcaniclastic sediment hydrothermally altered by seawater prior to metamorphism this alteration resulted in gain of mg and possibly fe and loss of lime and alkalis and possibly sr and eu this hypothesis contrasts with earlier proposals that the protolith included mg rich clay palygorskite that precipitated directly from seawater but it is more consistent with the new data there is a remarkably close correspondence between the element enrichments and depletions shown by the malene paragneisses and those found in a type granitic rocks and their volcanic equivalents consequently we suggest a comparable source for the paragneisses the petrogenesis of a type granites is associated typically with thickened stable continental crust this implies that the protolith for the malene paragneisses formed after or in conjunction with a major event of archaean crustal thickening

lithofacies and architecture of a basinwide tuff unit in the miocene eoil basin se korea modes of pyroclastic sedimentation changes in eruption style and implications for basin configuration the miocene eoil basin se korea is a small half graben basin that was rifted by both extensional and dextral strike slip deformations during backarc opening of the east sea sea of japan the basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits the paljori tuff is a 2 20 m thick basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation the tuff consists of two pyroclastic units units ii and iii which are underlain and overlain by two resedimented volcaniclastic units units i and iv unit i at the base is composed mainly of fine grained resedimented tephra unit ii is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin unit iii is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin margin relief and unit iv at the top is composed of coarse grained resedimented tephra the general coarsening up of these units the lack of nonvolcaniclastic sedimentary interbeds and the offset stacking of the units toward the northeast suggest an episodic short duration eruption from the southwest that waxed in mass eruption rate and changed eruption style from phreatomagmatic to magmatic these units show basin scale variations in geometry including 1 thick accumulations of unit i in the northeastern and southwestern extremities of the basin 2 ponding of unit ii in the southwestern part of the basin 3 truncation of unit iii in the same area and 4 thickening of unit iv toward the northeastern basin margin these provide important clues to the distribution of and temporal changes in accommodation space and hence the configuration and structural evolution of the eoil basin it is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin the southwest northeast trending axes of the basin plunge gently toward the northeast and the major intrabasinal faults of the basin were produced before the eruption of the paljori tuff the basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax the paljori tuff shows that important clues to and high resolution records of the large scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit

are there ancient soils in the 3 7 ga isua greenstone belt greenland abstract a lens of black schist within 3 7 ga quartzites of greenland may be earth s oldest known alluvial paleosol the suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite but it has a truncated top above dark gray grading down to gray color ptygmatically folded surface cracks filled with silt grains and large sand crystals unusual for sedimentary or metamorphic rocks the paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends and by chemical analysis showing molar weathering ratios and ree distribution like those of soils the schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the isukasia area the protolith can be reconstructed as a saponite clay with a salt rich horizon of kieserite like other acid sulfate paleosols of the early earth models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 510 ppm o2 and 820 201 ppm co2 and humid cool temperate paleoclimate the profile has organic 13cpdb consistently of 24 2 to 27 4 and modest raleigh distillation near the top similar consistent values and trends are produced by decay of organic matter in living soils but biotic carbon isotopic composition of sediments is erratic from bed to bed and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle thus life in this very ancient soil is not precluded by our analyses but ultrastructural and geochemical testing of carbon particles would further test this hypothesis

the origin of early archean banded iron formations and of continental crust isua southern west greenland abstract a long standing geological debate is whether the fe component of precambrian banded iron formations bifs was derived from continental sources or had its origin in hydrothermal alteration of the oceanic crust recent ree and nd and fe isotope studies of iron rich layers in bifs have provided strong supporting evidence for the hydrothermal origin of the fe component pillow basalts from the 3800 3700 ma isua greenstone belt west greenland underwent significant enrichment of fe up to 21 wt fe2o3 and some chalcophile elements e g pb 5 4 7 2 ppm prior to a late archean tectonothermal metamorphic event recorded in the region published and new pb pb isotopic data from the isua pillow basalts and the spatially associated bifs suggest that the enrichment of fe and pb and deposition of the bifs took place between 3800 and 3600 ma similar pb isotopic compositions in pillow basalts and bifs are consistent with a common source of pb for both supracrustal rocks iron and pb were probably scavenged from the oceanic crust and were re deposited on the ocean floor by hydrothermal fluids suggesting that high temperature hydrothermal alteration of early archean oceanic crust played an important role in the deposition of the isua bifs geochemical lithological and structural data from the isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting the isua bifs are spatially and temporally associated with boninitic and island arc picritic pillow basalts given the observations that cenozoic boninites and picrites tend to form in an intra oceanic arc forearc setting we suggest that the isua bif boninite and bif picrite associations were also deposited in a similar geodynamic setting the original pillow basalt bif stratigraphy was dismembered and transposed throughout the belt during early to late archean tectonothermal events the high temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of fe necessary to contribute to the deposition of the isua bifs the hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early archean arc forearc region the ridge subduction model can also explain the origin of the contemporaneous tonalite trondhjemite granodiorite ttg intrusions in the isua region partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced ttg melts the diapirically upwelling ttg melts formed the early archean continental crust and the complementary eclogitic residues contributed to the sub continental lithospheric mantle

erosion of archean continents the sm nd and lu hf isotopic record of barberton sedimentary rocks abstract knowing the composition nature and amount of crust at the surface of the early earth is crucial to understanding the early geodynamics of our planet yet our knowledge of the hadean archean crust is far from complete limited by the poor preservation of archean terranes and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants to address this problem and get a more comprehensive view of what an archean continent may have looked like we investigated the trace element and sm nd lu hf isotopic records of archean metasedimentary rocks from south africa we focused our study on sandstone and mudstone from drill core in the fig tree group 3 23 3 26 ga of the barberton granite greenstone belt but also analyzed the 3 4 ga buck reef cherts and still older 3 5 3 6 ga meta igneous rocks from the ancient gneiss complex swaziland based on principal component analysis of major and trace element data the fig tree metasedimentary rocks can be classified into three groups crustal detritus rich sediments si rich sediments and ca fe rich sediments the detritus rich sediments have preserved the sm nd and lu hf isotopic signatures of their continental sources and hence can be used to constrain the composition of crust eroded in the barberton area in the paleoarchean period based on sm nd ratios we estimate that this crust was more mafic than today with an average sio 2 content of 60 5 2 wt this composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic ultramafic and felsic rocks this implies that the archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth neodymium and hf crustal residence ages show that the eroded crust was on average 300 400 ma older than the deposition age of the sediments which highlights the importance of intracrustal reworking of older crust at 3 2 ga in the barberton area the si rich sediments have slightly positive e nd t 3 23ga but extremely radiogenic e hf t 3 23ga up to 11 based on analyses of 3 4 ga buck reef cherts we suggest that the radiogenic hf isotopic signature of the si rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks the latter have extremely high lu hf ratios such that their e hf values would increase dramatically by about 100 epsilon units every 100 ma in the ca fe rich sediments one sample contains carbonate that preserves the typical rare earth element features of seawater precipitates the initial nd isotopic composition of this sample e nd t 3 23ga 1 7 is within the range of previous estimates for archean anoxic seawater