dynamic relations for the deposition of sediment on floodplains and point bars of a freely meandering river abstract fluvial features such as floodplains and point bars are built by sediment deposition and sculpted by erosion long term measurements 38 yr of the cross section topography of active floodplains and point bars along the freely meandering powder river in southeastern montana usa mean daily discharge of 12 5 m3 s 1 were used to develop dynamic relations between annual sediment deposition and peak flood discharge five floodplain sections and five point bar sections were selected from 24 cross sections along a 90 km study reach at each cross section the sediment deposition volume per unit streamwise distance was computed as the difference between two topographic surveys made in consecutive years snowmelt floods were found to be the dominant annual process however other processes such as flash floods ice breakup floods and autumnal floods were important episodically the dynamic relations were linear for both fluvial features the snowmelt deposition discharge relations showed in general that point bars were about two times more efficient at trapping sediment than floodplains each relation had a discharge threshold that must be exceeded before sediment was deposited although these discharge thresholds for floodplains and point bars had essentially the same mean value 69 and 71 m3 s 1 respectively they represented different processes thresholds for other rivers will probably differ from those for powder river because of different channel geometry and sediment characteristics

the role of feedback mechanisms in historic channel changes of the lower rio grande in the big bend region abstract over the last century large scale water development of the upper rio grande in the u s and mexico and of the rio conchos in mexico has resulted in progressive channel narrowing of the lower rio grande in the big bend region we used methods operating at multiple spatial and temporal scales to analyze the rate magnitude and processes responsible for channel narrowing these methods included hydrologic analysis of historic stream gage data analysis of notes of measured discharges historic oblique and aerial photograph analysis and stratigraphic and dendrogeomorphic analysis of inset floodplain deposits our analyses indicate that frequent large floods between 1900 and the mid 1940s acted as a negative feedback mechanism and maintained a wide sandy multi threaded river declines in mean and peak flow in the mid 1940s resulted in progressive channel narrowing channel narrowing has been temporarily interrupted by occasional large floods that widened the channel however channel narrowing has always resumed after large floods in 1990 and 1991 the active channel width of the lower rio grande has narrowed by 36 52 narrowing has occurred by the vertical accretion of fine grained deposits on top of sand and gravel bars inset within natural levees channel narrowing by vertical accretion occurred simultaneously with a rapid invasion of non native riparian vegetation tamarix spp arundo donax which created a positive feedback and exacerbated the processes of channel narrowing and vertical accretion in two floodplain trenches we measured 2 75 and 3 5 m of vertical accretion between 1993 and 2008 in some localities nearly 90 of bare active channel bars were converted to vegetated floodplain during the same period upward shifts of stage discharge relations occurred resulting in over bank flooding at lower discharges and continued vertical accretion despite a progressive reduction in stream flow thus although the magnitude of the average annual flood was reduced between 40 and 50 over bank flooding continued these changes reflect a shift in the geomorphic nature of the rio grande from a wide laterally unstable multi thread river to a laterally stable single thread channel with cohesive vertical banks and few active in channel bars

valley fill alluviation during the little ice age ca a d 1400 1880 paria river basin and southern colorado plateau united states valley fill alluvium deposited from ca a d 1400 to 1880 is widespread in tributaries of the paria river and is largely coincident with the little ice age epoch of global climate variability previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern colorado plateau the alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca a d 1200 1400 and 1860 1910 respectively the fill forms a terrace in the axial valleys of major through flowing streams this terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of hillslopes of sparsely vegetated weakly consolidated bedrock suggesting that eroded bedrock was an important source of alluvium along with in channel and other sources paleoclimatic and high resolution paleoflood studies indicate that valley fill alluviation occurred during a long term decrease in the frequency of large destructive floods aggradation of the valleys ended about a d 1880 if not two decades earlier with the beginning of historic arroyo cutting this shift from deposition to valley entrenchment near the close of the little ice age generally coincided with the beginning of an episode of the largest floods in the preceding 400 500 yr which was probably caused by an increased recurrence and intensity of flood producing el nino events beginning at ca a d 1870

modeling width adjustment in meandering channels widening in sinuous channels occurs when the retreat of the outer bank exceeds the advance of the opposite bank an analytical model is presented to simulate width adjustment in meandering channels of non cohesive bank material resulting from bank erosion of two interactive processes basal erosion and bank collapse bank collapse refers to the avalanche of non cohesive material in the upper part of bank above water surface resulting from over steepening of the bank surface due to basal erosion the rate of basal erosion including lateral erosion and bed degradation is calculated as a function of the longitudinal gradient of sediment transport rate and strength of secondary flow the transverse bed slope is treated as a variable that increases as channel sinuosity increases until the transverse bed slope reaches its maximum value by simplifying the bank collapse process for non cohesive materials the present study shows that the rate of bank line retreat is determined by lateral erosion rate near bank bed degradation rate sediment grain size and difference between flow depth and bank height the time dependent widening processes of two meandering channels in the laboratory are selected to test applicability of the model the result shows that the simulated bank lines at individual time intervals closely match the experimental measurements whether the sinuosity of a meandering channel will increase or decrease is primarily determined by didtribution of the lngitudinal gradient of sediment transport rate along the channel

aeolian dust in europe african sources and european deposits abstract a conceptual model is presented for the provenance and dispersal patterns of small dust that falls on europe generally its sources are in north africa and it is distributed across all europe several key sources can be distinguished sahelian dust comes largely from the old lake chad region this is a clay rich unimodal material saharan dust comes from the great sand sheets it contains small monomineralic particles and may have a bimodal size range three simple deposition zones can be recognised a d1a zone where sufficient dust is deposited to form a discrete soil layer not well classified as a rendoll in the extreme south of europe a d1b zone where the airborne dust simply provided a silty admixture to soil systems across middle europe and a northern zone d1c where the dust is a fugitive cloud but very occasionally forms noticeable deposits two particle formation methods can be noted particle control in sahelian dust is via the sedimentation in the original lake this gives an open structure which can be modelled using a simple monte carlo approach the open structure ensures that only small particles are produced size control is via particle packing a chipping mechanism can produce fine quartz particles from sandy deserts the aeolian energy is by and large not sufficient to cause major impact fracturing but small mineral chips can be produced in the small dust size fine and very fine silt which go into high level suspension and travel to europe and beyond the saharan material can have a wider more variable size distribution than the sahelian material the canary islands loess is largely sahelian material the cape verde islands deposits from the nearby sandy regions are saharan deposits large dust has fallen on europe and produced widespread loess deposits large dust is essentially an in continent deposit small dust comes from outside from africa

evidence of saharan dust in upper pleistocene reworked palaeosols of north west sardinia italy palaeoenvironmental implications abstract a multi disciplinary approach was followed to investigate two thick palaeosolstrata that alternate with wind blown dominated deposits developed along thealghero coast north west sardinia italy optically stimulated luminescenceages reveal that both palaeosols were developed during cooler drier periods the rst one at around 70 ka marine isotope stage 4 and the latter around 50 ka marine isotope stage 3 in contrast the pedological features indicate that thepalaeosols underwent heavy weathering processes under warm humid to sub humid conditions characteristic of the sardinian climate during the lastinterglacial stage marine isotope stage 5e to reconcile this apparent datadiscrepancy a range of sedimentological and pedological analyses wereconducted these analyses reveal that the palaeosols possess a complexhistory with accumulation and weathering occurring during marine isotopestage 5e and erosion colluviation and nal deposition taking place during thefollowing cold stages thus even if these reddish palaeosols were last formedduring the glacial period the sediments building up these strata probablyrecord the climate of the last interglacial stage marine isotope stage 5e traceelement and x ray diffraction analyses together with scanning electronmicroscope images reveal the presence of saharan dust in the parentmaterial of the palaeosols however no evidence of any far travelled africandust has been observed in the marine isotope stage 4 3 aeolian deposits it ispossible to conclude that in the west mediterranean islands saharan dustinput even if of modest magnitude is preserved preferentially in soilsaccumulated and weathered during interglacial stages keywords marine isotopic stage mediterranean osl dating trace elementgeochemistry xrd analysis sedimentology 2012 59 917 938

late holocene environmental change and the anthropization of the highlands of santo antao island cabo verde abstract cabo verde was the first tropical archipelago colonized by europeans historians have suggested that the first colonizers initiated archipelago wide ecosystem degradation loss of vegetation cover and erosion however the human environment interactions that led to the archipelago s current environmental status remain poorly understood here we report the first palaeoecological study of past vegetation change and disturbance regimes for cabo verde we present a 2130 yr old sediment sequence from a volcanic caldera cova de paul located at 1200 m asl on santo antao island for which we analyzed fossil pollen non pollen palynomorphs npps charcoal silica bodies and grain size distribution our analyses do not show evidence of the presence of temperate tropical or subtropical forests growing on the summits of santo antao in pre human times the pollen record shows that scrubland and grasslands dominated the highlands and underwent compositional changes ca 1850 and 1300 cal yr bp these shifts overlap with erosion phases and are linked to intensified seasonality steady rates of sedimentation marked the period 1230 to ca 350 cal yr bp but an increase in charcoal concentrations indicate a drying phase increases in regional and local fire peaks of coprophilous fungi and the presence of new world crop pollen zea mays are interpreted as the onset of portuguese settlement of the highlands after 450 bp sustained erosion between ca 350 and 100 cal yr bp indicate soil degradation and the pollen record shows the increase in introduced herbaceous taxa rosaceae centaurea verbenaceae and exotic tree taxa e g pinus while shifting abundances of coprophilous fungi indicate changes in land use the record shows stabilization of soils in the last century due to recent afforestation of the highlands further palaeoecological studies have the potential to provide further detail of the long term dynamics of cabo verde ecosystems and to inform conservation initiatives

tracing dust sources and transport patterns using sr nd and pb isotopes continental dust transported through the atmosphere can be a tracer of atmospheric circulation and transport as well as being a source of information about climate in the dust s source area this information about the provenance of dust transported from desert areas and deposited elsewhere on the oceans and continents can be evaluated using several observational techniques satellite imagery has been used to this end in recent decades and air mass back trajectories have been calculated to trace dust deposits back to their source area another method consists of analysis of the concentration and composition of certain tracers in the dust itself and comparing them to soils and sediments from possible source areas this method has the advantage of being useable for paleo atmospheric circulation and paleo dust transport studies whereas satellite imagery and air mass back trajectories are restricted to relatively recent times we review here different kinds of isotopic tracers that have been used to trace dust provenance but special attention is focused on the use of sr nd and pb isotopes we evaluate their use in studies of both spatially as well as temporally varying continental dust source and transport

relationship between a syntectonic granitic intrusion and a shear zone in the southern carpathian balkan area almăj mountains romania implications for late variscan kinematics and cherbelezu granitoid emplacement the carboniferous cherbelezu batholith almmountains romania is a well preserved but poorly studied intrusion belonging to the upper danubian alpine nappe this pluton crops out along a pre existing major verticalized formation the corbu mylonitic zone cmz our study investigates the role of the cmz on the deformation recorded during the mush emplacement and cooling a detailed microstructural study of this granitic body coupled with investigations on both anisotropy of magnetic susceptibility ams and shape preferred orientation spo of biotite subfabric has been performed the surrounding rocks preserve evidence that the cmz has been reactivated as a sinistral strike slip fault before the pluton emplacement microstructural investigations of the granitic facies indicate that the pluton has undergone superimposed deformations during its cooling from submagmatic to lt condi tions foliation and lineation patterns obtained by ams and spo e both methods giving similar results e reflect either magmatic submagmatic or solid state flows magmatic flow preserved in the western and southern parts of the pluton is characterized by concentric foliation pattern with both divergent and parallel lineations the latter pointing to an early transcurrent regime subsequently a solid state deformation recorded during the pluton cooling and restricted to its eastern and northern parts argues for the concomitant cmz activity under a sinistral transpressive regime this is supported by the

the formation of shadow dunes abstract an investigation of the modification of airflow by a discrete semi circular roughness element in this case vegetation the resulting flow structure and the formation of pyramidal shaped shadow dunes is presented two erect pioneer sand dune grasses festuca littoralis and ammophila arenaria were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities 4 20 m sec 1 the three dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake a vertical separation envelope is also formed over the vegetation pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs shadow dune height is dependent on element basal width and repose angle of the sand shadow dune length is dependent on element basal width and wind velocity the results are extrapolated to include all semi porous to non porous semicircular discrete roughness elements the role of shadow dune formation in the development of foredunes and related environments is examined

climate and ephemeral stream processes twentieth century geomorphology and alluvial stratigraphy of the little colorado river arizona during the first 40 years of the twentieth century erosion was the dominant geomorphic process affecting the morphology of the little colorado river channel the discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation in the 1940s and early 1950s average annual precipitation declined reducing annual discharge to about 57 of that of the preceding period as well as reducing the frequency of large floods the channel adjusted to the new hydrologic regimen by reducing its width parts of the channel were frequently dry and riparian vegetation primarily nonnative salt cedar became established on the higher channel surfaces precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process as indicated by accretion of 2 to 5 m of flood plain alluvium between 1952 and 1978 events of 1980 however suggest that the flood plain has ceased to accrete although climate has not fluctuated the flood plain has probably reached a critical height above the channel beyond which further accretion is unlikely under the existing discharge regimen the recent history of the little colorado broadly suggests that flood plain development was initiated by climatically induced hydrologic fluctuations flood plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions

particle dynamics the continuum of bedrock to alluvial river segments abstract particle dynamics refers to production erosion transport and storage of particulate material including mineral sediment and organic matter particle dynamics differ significantly between the end members of bedrock and alluvial river segments and between alluvial river segments with different grain size distributions bedrock segments are supply limited and resistant to change with relatively slow linear adjustments and predominantly erosion and transport particle dynamics in alluvial segments in contrast are transport limited and dominated by storage of mineral sediment and production of organic matter alluvial segments are resilient to change with relatively rapid multidirectional adjustments and stronger internal influences because of feedbacks between particles and biota bedrock segments are the governors of erosion within a river network whereas alluvial segments are the biogeochemical reactors fundamental research questions for both types of river segments center on particle dynamics which limit network scale incision in response to base level fall bedrock segments and habitat biogeochemical reactions and biomass production alluvial segments these characterizations illuminate how the spatial arrangement of bedrock and alluvial segments within a river network influence network scale resistance and resilience to external changes in relative base level climate and human activities

multicomponent analysis of ftir spectra quantification of amorphous and crystallized mineral phases in synthetic and natural sediments abstract the study of weathering and erosion processes requires the establishment of a mass balance of minerals from the soils towards the sedimentation area this calls for the development of quantitative tools that can be applied to mixtures of finely divided and sometimes poorly crystallized minerals found in soils and sediments we present here the use of fourier transform infrared ftir absorbance spectroscopy in the mid infrared as a method to calculate mineral modes for such materials samples were prepared using the kbr disc method this ensures that lambert beer s law is valid a quantitative determination of the mineral content from various blends was performed by making a multicomponent analysis of the experimental spectrum using the spectra of each component in the mixture to check the validity of the procedure the multicomponent analysis was performed on spectra from synthetic mixtures of standard minerals quartz kaolinite gibbsite amorphous silica good agreement between actual and computed wt was obtained in the range 1315 315 cm 1 this application of ftir spectroscopy was used to quantify the mineral components of a lacustrine sediment cored at salitre minas gerais brazil the major phases are organic matter kaolinite gibbsite quartz anatase and amorphous silica phytoliths and sponge spicules fifty three samples were analyzed quantitative results from ftir spectroscopy were compared to chemical analyses to assess the validity of the method for the natural sediments phases with no sharp diagnostic spectral features such as anatase or amorphous silica were quantified using the multicomponent analysis the variations of the main mineral phases along the core reflect the mechanisms of transfer from the surrounding soils to the sediment area and the role of the vegetal cover over the drainage area

atmospheric contribution to cations cycling in highly weathered catchment guadeloupe lesser antilles abstract the important fertilizing role of atmospheric dust and particularly african dust in tropical rainforests is increasingly recognized but still poorly quantified to better evaluate dust input into the caribbean basin we sampled critical zone compartments of a small forested volcanic catchment in guadeloupe soils parent rock atmospheric dust plants soil solutions stream and rain waters the aims of this study are to track sources of cation nutrients ca mg k sr developed on highly weathered soil in the rainforest of guadeloupe to quantify plant recycling of these nutrients and to identify constraints on regolith development and its associated nutrient pool in the quiock creek catchment a large isotopic range of 87sr 86sr and end values was observed despite the small scale of observation sr isotopic composition of the dissolved load varied from 0 7084 in rainfall to 0 7110 in soil solution whereas it ranges between 0 7068 and 0 7153 for soil samples and between 0 7096 and 0 7102 for plants the nd isotopic composition varied between 8 39 in near surface soil samples to 2 71 in deeper soil all samples had an intermediate signature between that of the bedrock endmember 87sr 86sr 0 7038 end 4 8 and the atmospheric endmember sea salt 87sr 86sr 0 7092 and saharan dust 87sr 86sr 0 7187 end 11 5 the regolith was built on pyroclastic deposits but because of extreme leaching the regolith has lost its original bedrock signature and inherited an exogenous atmospheric signature our results show that only the chemical weathering of the fresh near surface minerals can provide nutrients to the ecosystem first 30 cm however this dust weathering is too low to sustain the tropical forest ecosystem on its own the cationic mass balance at the catchment scale as well as the sr isotopic signature show that cation and sr fluxes are of atmospheric origin only and that original bedrock no longer participates in nutrient cycles the vegetation reflects the 87sr 86sr of the dissolved pool of atmospheric sr at the soil plant scale the cation nutrient fluxes provided by vegetation litter fall leaf excretion are major compared to input and output fluxes the annual ca k sr and mg fluxes within the vegetation are respectively 31 28 20 and 3 times greater than the exported fluxes at the outlet of the basin the residence time of nutrients in the vegetation is 16 years for k and close to 45 years for sr ca and mg these results emphasize the highly efficient vegetative turnover that dominates the nutrient cycle in the quiock creek catchment this first characterization of biogeochemical cycles in the guadeloupean rainforest suggests that the forest community of quiock creek is sustained by a small near surface nutrient pool disconnected from the deep volcanic bedrock we also demonstrated that even with efficient nutrient recycling saharan dust plays a significant role in maintaining ecosystem productivity in guadeloupe over long time scales

crustal regime of deformation and ascent of granitic magma abstract structural studies and gravity data demonstrate the magma flow pattern of granitic plutons and their morphology at depth the deepest zones calculated from gravity data provided they show vertical lineations in outcrop are interpreted as the feeder channels which fed the pluton their morphology at depth as well as the position and number of root zones vary according to the type of deformation suggesting that regional deformation determines the emplacement of granitic plutons therefore deformation must also influence the segregation and the ascent of acid magmas a thermal model is constructed which demonstates the structure and heat generation of the hercynian crust according to that model the thermal gradient changes the conditions for the deformation of the crust obviously a large amount of heat must be supplied to induce melting of the lower crust as a result magma segregation and ascent occur entirely within the ductile field when upwelling magma reaches the brittle ductile transition zone the hydrodynamic force buoyancy is insufficient to fracture the upper brittle crust parameters which control diapirism wavelength viscosity thickness do not match those related to the volume of erupted magma as a consequence diapiric upwelling appears to be limited under favourable conditions to the lower ductile crust and does not apply to intrusions in a brittle crust shearing helps to segregate magma and to feed conduits toward the surface since it creates the required zones of local extension for the magma to infiltrate

granite magma migration and emplacement along thrusts this paper investigates the influence exerted by brittle tectonic structures in the emplacement of granite plutons in contractional settings we address both cases where contractional tectonics and magma intrusion are 1 coeval to study how active contractional tectonics controls the transport of magma and 2 diachronous to study the role of pre existing structures on the transport of magma in light of new experimental models we show that magma can rise along thrusts ramps and flats this phenomenon occurs for both low viscosity magma basalts to andesite and high viscosity magma dry granite the experimental results also allow the evaluation of the role played by magma viscosity in determining pluton geometries in addition a review of literature demonstrates a spatial and causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the pluton final shape the abundance of subhorizontal and tabular granitic intrusions indicates that the location of inflating granitic sills along thrust flats can be common we argue that active and pre existing flats and ramps thrusts provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway

sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed luquillo mountains puerto rico abstract sr isotope data from soils water and atmospheric inputs in a small tropical granitoid watershed in the luquillo mountains of puerto rico constrain soil mineral development weathering fluxes and atmospheric deposition this study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone 87sr 86sr data reveal that saharan mineral aerosol dust contributes significantly to atmospheric inputs watershed scale sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 700 mg cm 2 ka 1 nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of saharan dust in the regolith watershed scale sr isotope mass balance calculations are used to calculate the overall short term chemical denudation velocity for the watershed which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides the dissolved streamwater sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone a steep gradient in regolith porewater 87sr 86sr ratio with depth from 0 70635 to as high as 0 71395 reflects the transition from primary mineral derived sr to a combination of residual biotite derived sr and atmospherically derived sr near the surface and allows multiple origins of kaolinite to be identified

bulk composition of northern african dust and its source sediments a compilation abstract this paper presents a review of bulk compositional data of northern african dust and its potential source sediments and includes elemental isotope and mineralogical data northern african dust represents about one half of the total global atmospheric mineral dust burden and its uplift transport and deposition have strong impacts on climate and various terrestrial and marine ecosystems the chemical data set shows that an average northern african dust exhibits comparable si fe and mn contents with respect to the average composition of the upper continental crust is slightly depleted in the alkali metals k and na and enriched in ti and p however the complete data set yields clear evidence that northern african dust and its source sediments are compositionally heterogeneous on a regional scale and that this heterogeneity can be used to differentiate between major potential source areas on the basis of so called source markers an evaluation of these compositional fingerprints shows that the following parameters and especially their combination are effective in the discrimination of the most active source areas in northern africa ratio of ca mg fe wt calcite or carbonate content palygorskite occurrence and abundance illite kaolinite ratio nd 0 value and 87sr 86sr ratio for example the data set corroborates previous ideas which assign carbonate illite and palygorskite rich mineral dusts to north west ern source regions because most of the above listed source markers do not change substantially during transport even far traveled dusts may be assigned to specific potential source areas in northern africa some limitations of the presented data set are also discussed our compilation reveals some substantial gaps in the knowledge of the composition of source sediments and mineral dusts from important potential source areas that should be filled in the future the here compiled data set can be used as a reference frame when incorporating the composition of source sediments e g mineralogy into global or regional dust transport models and can be compared with source analysis by remote sensing or back trajectory analysis however source apportionment studies supported by our data set will not only be useful for actual dust samples but will also be helpful for the understanding of paleo wind directions and hence paleo climatological conditions through the investigation of quaternary eolian sediments deposited in and around northern africa

evolution of deepwater mixing and weathering inputs in the central atlantic ocean over the past 33 myr the isotopic composition of nd in present day deep waters of the central and northeastern atlantic ocean is thought to fingerprint mixing of north atlantic deep water with antarctic bottom water the central atlantic romanche and vema fracture zones are considered the most important pathways for deep water exchange between the western and eastern atlantic basins today we present new nd isotope records of the deepwater evolution in the fracture zones obtained from ferromanganese crusts which are inconsistent with simple water mass mixing alone prior to 3 ma and require additional inputs from other sources the new pb isotope time series from the fracture zones are inexplicable by simple mixing of north atlantic deep water and antarctic bottom water for the entire past 33 myr the distinct and relatively invariable nd and pb isotope records of deep waters in the fracture zones appear instead to have been controlled to a large extent by contributions from saharan dust and the orinoco amazon rivers thus the previously observed similarity of nd and pb isotope time series from the western and eastern north atlantic basins is better explainable by direct supply of labrador seawater to the eastern basin via a northern pathway rather than by advection of north atlantic deep water via the romanche and vema fracture zones

close spatial relationship between plutons and shear zones comment and reply comment weinberg et al 2004 1 present an intriguing model to explain why plutons might be emplaced in low stress sites next to faults in a geology paper it is difficult to fully present all aspects of such models and i remain puzzled by some of the authors interpretations thus in the hope that

a 184 year record of river meander migration from tree rings aerial imagery and cross sections abstract channel migration is the primary mechanism of floodplain turnover in meandering rivers and is essential to the persistence of riparian ecosystems channel migration is driven by river flows but short term records cannot disentangle the effects of land use flow diversion past floods and climate change we used three data sets to quantify nearly two centuries of channel migration on the powder river in montana the most precise data set came from channel cross sections measured an average of 21 times from 1975 to 2014 we then extended spatial and temporal scales of analysis using aerial photographs 1939 2013 and by aging plains cottonwoods along transects 1830 2014 migration rates calculated from overlapping periods across data sets mostly revealed cross method consistency data set integration revealed that migration rates have declined since peaking at 5 m year in the two decades after the extreme 1923 flood 3000 m 3 s averaged over the duration of each data set cross section channel migration occurred at 0 81 m year compared to 1 52 m year for the medium length air photo record and 1 62 m year for the lengthy cottonwood record powder river peak annual flows decreased by 48 201 vs 104 m 3 s after the largest flood of the post 1930 gaged record 930 m 3 s in 1978 declining peak discharges led to a 53 reduction in channel width and a 29 increase in sinuosity over the 1939 2013 air photo record changes in planform geometry and reductions in channel migration make calculations of floodplain turnover rates dependent on the period of analysis we found that the intensively studied last four decades do not represent the past two centuries

the magnetism of a glacial aeolianite sequence from lanzarote canary islands coupling between luvic calcisol formation and saharan dust trapping processes during wet deposition events off northwestern sahara summary in order to better document the climatic origin of pedogenized loess deposits west of sahara rock magnetic measurements were performed on a last glacial coastal sand sequence from la mala lm quarry lanzarote canary islands containing six interbedded loess palaeosol units hysteresis and susceptibility data point to a coarse grained magnetic enhancement in the coastal sand which contrasts with the superparamagnetic sp to pseudo single domain psd behaviour of the saharan loess and palaeosol high and low temperature experiments show that oxidized titanomagnetite contributions dominate in the sand while fine grained sp to psd iron oxidation products such as maghemite and goethite are evidenced in the saharan loess palaeosol units at room temperature the detrital psd multidomain titanomagnetite contribution of local origin is tentatively estimated from the ac field dependence of magnetic susceptibility surface oxidation of detrital spinel grains and authigenesis of fine grained iron oxides including sp goethite are proposed to explain the magnetic properties of the primarily fine saharan dust bearing material the dry local climate of the present day and late holocene means that loess deposits are not preserved the six pedogenized loess units however point to dust trapping under semi arid wetter conditions probably illustrating periods of reduced latitudinal temperature gradients and climate variability of the north atlantic climate respectively these findings suggest that both the canary palaeosol and its content of ultra fine iron oxide might be constrained by wet deposition and trapping of fine saharan dust

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

deciphering human climate interactions in an ombrotrophic peat record ree nd and pb isotope signatures of dust supplies over the last 2500 years misten bog belgium abstract a high resolution peat record from eastern belgium reveals the chronology of dust deposition for the last 2500 years ree and lithogenic elements in addition to nd and pb isotopes were measured in a 173 cm age dated peat profile and provide a continuous chronology of dust source and intensity calculated dust flux show pronounced increases c 300 bc 600 ad 1000 ad 1200 ad and from 1700 ad corresponding to local and regional human activities combined with climate change the industrial revolution samples 1700 1950 ad are characterised by a significant enrichment in sc normalised ree abundance sum ree sc 25 due to intensive coal combustion for the pre industrial revolution samples the sc normalised ree abundance 10

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

absence of saharan dust influence on the strontium isotope ratios on modern trees from the bahamas and turks and caicos islands we report on strontium 87 sr 86 sr isotope results from 91 modern trees growing on the bahamas and turks and caicos islands the average 87 sr 86 sr ratio of 0 709169 0 000010 is consistent with the late quaternary limestone of the islands and with the modern ocean value the absence of any detectable influence of 87 sr enriched saharan dust is notable given the known contribution of this material to both past and recent soils of the caribbean our results indicate that the impact of saharan dust to the modern biosphere of the bahamian archipelago is at least an order of magnitude less than modeled in currently available strontium isoscapes for the circum caribbean we suggest that the bioavailability of sr in saharan dust may be considerably less than previously thought nevertheless further work could usefully be carried out in the bahamian archipelago on plants with different rooting depths growing on different soil types and on limestone of different ages our results have particular relevance for the refinement of existing strontium isoscapes and the archaeological provenience of artifacts animals and people in the circum caribbean

geochemical and mineralogical evidence for sahara and sahel dust additions to quaternary soils on lanzarote eastern canary islands spain africa is the most important source of dust in the world today and dust storms are frequent on the nearby canary islands previous workers have inferred that the sahara is the most important source of dust to canary islands soils with little contribution from the sahel region soils overlying a late quaternary basalt flow on lanzarote canary islands contain in addition to volcanic minerals quartz and mica exotic to the island s bedrock kaolinite in the soils also likely has an exotic origin trace element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and african dust major element geochemistry clay mineralogy and interpretation of satellite imagery suggest that dust additions to the canary islands come not only from the sahara desert but also from the sahel region

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

close spatial relationship between plutons and shear zones this paper explores the relationship between granitic plutons and transcurrent shear zones using examples from the borborema province northeastern brazil syntectonic plutons of this province generally have en cornue shapes and were emplaced preferentially within or at the margins of wide regional scale transcurrent shear zones particularly where shear zones intersect major lithological boundaries it is argued that the plutons shapes and their closeness to shear zones are indicative of a causal relationship between granite plutons and shear zones with the support of numerical models it is further argued that pluton emplacement in transcurrent terranes is controlled by low mean pressure magma traps at the shoulders of shear zones resulting from strain incompatibility caused by regional variations in rheological properties

anti atlas moroccan chain as the source of lithogenic derived micronutrient fluxes to the deep northeast atlantic ocean identifying the source of atmospheric dust is crucial to better understand the global marine biogeochemical cycle as phytoplankton growth relies on dissolved micronutrient elements available in the open ocean mineralogical geochemical and sr isotope analyses of a one year time series april 2003 april 2004 of deep ocean particle flux at the madeira abyssal plain in the subtropical northeast atlantic are presented the lithogenic fraction has a high occurrence of palygorskite and smectite and an absence of kaolinite together with sr isotopic compositions similar to the northeast atlantic aerosols this indicates the anti atlas moroccan chain of paleozoic age as the source region the lithogenic fraction supplies 4 times more fe during a dust event than during the spring summer aeolian input a continuous input of only 1 of the lithogenic iron made available over a year period could lead to an increase in primary productivity of 40 relative to today s value

simulating internal and external controls on fluvial terrace stratigraphy a qualitative comparison with the maas record palaeohydrological studies indicate that the use of sedimentary palaeoclimatic indicators in fluvial sediments still requires rigorous calibration of the climatic controls on the formation of those indicators an explorative attempt is made using a simulation experiment with the fluver2 model the simulation illustrates that climate control on the erosion sedimentation behaviour of a large fluvial system differs along the fluvial system while in the uplifting hinterland only glacial extremes are recorded as sediments with a relatively limited preservation chance as terraces due to the uplift the more downstream reach towards the hinge zone displays a sensitive fluvial record registering even stadial interstadial dynamics well preserved in an extensive glacial interglacial terrace staircase downstream in a subsiding basin the most complete sedimentary record is preserved which still contains temporal gaps of approximately 20 related to the spatial delay effects of the external control of climate and related base level change a systematic comparison of simulated climate changes and resulting fluvial dynamics illustrates that due to the internal control of fluvial systems a one to one correlation between recorded fluvial dynamics and climate change becomes less the further one moves downstream along the longitudinal profile a qualitative comparison of the simulation results with the maas record reveals an interesting match suggesting that the simulated system dynamics may well apply the maas system

the use of high resolution elevation histograms for mapping submerged terraces tests from the eastern tyrrhenian sea and the eastern atlantic ocean abstract histogram plots can be used as a cost and time efficient tool to detect horizontal or gently dipping geomorphic surfaces within a background regional topography one of the main classes of these surfaces is typified by terraces formed during marine still stands this contribution uses elevation histogram plots as morphometric indicators for detecting the possible existence of concealed terrace orders in digital elevation models dems derived by the processing of bathymetric data the approach is particularly useful when dealing with submerged terraces and platforms because of the obvious difficulties in mapping those surfaces the advantage of using this indicator consists in the rapid identification of dem recurrent bathymetric values this tool is most powerful when dealing with shortly developed terraces and or multiple terrace orders whose detection could have been missed by direct observation two case studies characterized by the presence of a single terracing event are tested the southern slope of the ischia island and the top of the palinuro seamount both located in the eastern tyrrhenian sea margin elevation histograms are used for terrace order computation in a more complex setting the gorringe bank located offshore sw portugal in the eastern atlantic ocean the results show that a joint use of elevation histogram computation and geomorphologic analysis can be particularly efficient in the study of planar surfaces

fire and alluvial chronology in yellowstone national park climatic and intrinsic controls on holocene geomorphic processes we employed a systemwide approach a large and robust set of radiocarbon ages and modern process analogs to interpret the holocene history of forest fire related sedimentation and overall alluvial activity in northeastern yellowstone national park debris flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire related sedimentation within valley side alluvial fans of soda butte creek fire related deposits make up approximately 30 of the late holocene fan alluvium fifty 14 c ages on fire related events cluster within the intervals of 3300 2900 2600 2400 2200 1800 and 1400 800 yr b p and suggest earlier episodes of large fires and fan aggradation around 7500 5500 and 4600 4000 yr b p a major pulse of fire related debris flow activity between 950 and 800 yr b p coincided with the height of the widely recognized medieval warm period ca a d 1050 1200 instrumental climate records over the last 100 yr in yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods enhancing the potential for severe short term drought major forest fires and storm generated fan deposition along lower soda butte creek fill cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca 8000 yr b p terrace level t1a 7000 5600 t1b 3100 2600 t2 2000 1300 t3 and post 800 yr b p t4 these periods coincide with overbank sedimentation on slough creek and the lamar river but alternate with intervals of fire related fan deposition implying a strong climatic control local paleoclimatic data suggest cooler effectively wetter conditions during terrace tread formation in warmer drier intervals reduced average runoff in axial streams results in meander belt narrowing concurrent channel incision may be caused by infrequent large floods greater resistance to downcutting however allowed fewer terraces to be formed along slough creek and the lamar river alluvial systems in northeastern yellowstone show a clear response to millennial scale climatic cycles wherein alluvial fans aggrade and prograde over flood plains in drier periods axial streams widen their flood plains and trim back the fans during wetter periods small scale climatic fluctuations of the holocene thus had substantial impact on postglacial landscapes in northeastern yellowstone

recent large magnitude floods and their impact on valley floor environments of northeastern yellowstone abstract the lamar river watershed of northeastern yellowstone contains some of the most diverse and important habitat in the national park broad glacial valley floors feature grassland winter range for ungulates riparian vegetation that provides food and cover for a variety of species and alluvial channels that are requisite habitat for native fish rapid neogene uplift and quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine riparian ecosystems uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock as a result steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley floor meadows tree ring dating identifies major floods in 1918 ca 1873 and possibly ca 1790 in 1996 and 1997 discharge during snowmelt runoff on soda butte creek approached the 100 year flood estimated by regional techniques with substantial local bank erosion and channel widening indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996 with similar duration and much greater flood plain impact nonetheless 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper yellowstone region the 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of soda butte creek channels may still be locally enlarged from flood erosion and net downcutting has occurred in some reaches leaving the pre 1790 flood plain abandoned as a terrace gravelly overbank deposits raise flood plain surfaces above levels of frequent inundation and are well drained therefore flood plain soils are drier noncohesive gravels also reduce bank stability and may have persistent effects on channel form overall floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley floor ecosystems in northeastern yellowstone

modeling effects of bank friction and woody bank vegetation on channel flow and boundary shear stress in the rio puerco new mexico 1 we have applied a physically based model for steady horizontally uniform flow to calculate reach averaged velocity and boundary shear stress distributions in a natural stream with woody vegetation on the channel banks the model calculates explicitly the form drag on woody plant stems and includes the effects of vegetation on the boundary shear stress velocity and turbulence fields average channel shapes bed gradients and shrub characteristics were determined for four long internally similar reaches covering an 81 km segment of the lower rio puerco in north central new mexico hydraulic geometries for each of three flow events were determined from reach average channel shapes slopes and measured silt line heights above the bed model results show that friction on the lateral boundaries reduced the boundary shear stress in the center of the rio puerco channel by as much as 20 in reaches with moderate to dense bank shrubs perimeter averaged boundary shear stresses were reduced by almost 40 during near bankfull flows model calculated discharges for all three flow events indicate there was a loss of about 40 of the upstream discharge through the 81 km river segment infiltration rates estimated from discharge losses during the quasi steady peak flows and areas of the bed and banks suggest rates of flow loss into the silty sand of the upper banks are about 8 times greater than rates of loss into the clayey silt covered bed and lower banks

holocene saharan dust deposition on the cape verde islands sedimentological and nd sr isotopic evidence holocene aeolian silts deposited on the cape verde islands provide information about the origin of african palaeodusts that have fallen on the north eastern atlantic ocean over the last 10 000 years sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin their sr and nd isotopic composition identifies a saharan origin suggesting transport by harmattan winds we estimate that saharan dust comprises 75 95 of material in these holocene silts the rest coming from the weathering of local basaltic bedrock

flood disturbance recovery and inter flood incision on a large sand bed river abstract the lower 800 km of the missouri river has been incising for decades however the 2011 flood scoured more sediment than the previous 12 yr combined the river rebounded in the following two years re depositing over 70 of the sediment scoured during the 2011 flood historic evidence suggests that past missouri river floods may have followed similar scour rebound patterns this paper analyzes 16 channel surveys from 1987 to 2014 including 50 723 total cross sections to document the morphological history of the lower 800 km of the missouri river over the last thirty years including flood disturbance and long term trends the last thirty years on the missouri river included two basic morphologic regimes steady incision during low to moderate flow periods and rapid scour rebound responses to floods surveys collected during the 2011 flood demonstrate that the river scoured throughout the flood including the falling limb and did not begin a rebound phase until after the flood the analysis also identifies sediment sinks associated with each of these scour regimes flood scour and incision between floods that exceed the total volume eroded from the bed during those periods results suggest that floodplain deposition may induce the supply limitation that drives flood scour on this large sand bed river additionally aggregate mining removes substantially more sediment than the total river incision between floods and the reaches with maximum scour during the 1993 and 2011 floods correspond to the zones of aggregate mining

quantitative model of the growth of floodplains by vertical accretion a simple one dimensional model is developed to quantitatively predict the change in elevation over a period of decades for vertically accreting floodplains this unsteady model approximates the monotonic growth of a floodplain as an incremental but constant increase of net sediment deposition per flood for those floods of a partial duration series that exceed a threshold discharge corresponding to the elevation of the floodplain sediment deposition from each flood increases the elevation of the floodplain and consequently the magnitude of the threshold discharge resulting in a decrease in the number of floods and growth rate of the floodplain floodplain growth curves predicted by this model are compared to empirical growth curves based on dendrochronology and to direct field measurements at five floodplain sites the model was used to predict the value of net sediment deposition per flood which best fits in a least squares sense the empirical and field measurements these values fall within the range of independent estimates of the net sediment deposition per flood based on empirical equations these empirical equations permit the application of the model to estimate of floodplain growth for other floodplains throughout the world which do not have detailed data of sediment deposition during individual floods copyright 2000 john wiley sons ltd

marine sr ca input in quaternary volcanic rock weathering profiles from the mediterranean coast of morocco sr isotopic approach carbonate commonly accumulates in weathering profiles developed on noncalcareous parent material under arid and semiarid climatic conditions such weathering profile types are widespread in morocco making it critical to know the sources of the calcium in order to understand the genesis of carbonate rich soils in this study we use 87sr 86sr as a geochemical tracer of the ca sources in weathering profiles developed on volcanic rocks basalt tuff andesite and latite in the mediterranean coast of morocco the results show that the most of the sr and hence ca is of an external origin derived from i sea water for the profiles located near to the coast or ii sea water plus liassic carbonate outcropping in the catchment for the profiles more distant from the coastline the contribution of the host rock is small being less than 25 the ratio of the sr ca from the parent material to that from the external sources is controlled by the degree of porosity developed in the profile

variscan granitoids related to shear zones and faults examples from the central sudetes bohemian massif and the middle odra fault zone the granitoid intrusions of the central sudetes cs and of the middle odra fault zone mofz ne part of the bohemian massif are both spatially and temporally related to large scale shear zones and faults including possible terrane boundaries that provided effective channels for melt migration summarizing common features of the cs and mofz granitoids we have delineated a set of characteristics of the fault related and shear zone related granitoids 1 they are mainly generated by partial melting of crustal sources with variable contribution or no contribution of mantle materials 2 the sheet like steeply inclined narrow and rather small granitoid intrusions are emplaced within shear zones at mid crustal level c 20 km depth whereas the larger flat lying plutons intrude into the upper crust outside or above these shear zones 3 the magmatic foliation and lineation in granitoids of the deeper sheet like intrusions are concordant with those in the surrounding metamorphic rocks suggesting that the solidification of granitoids was coeval with the deformation in the shear zones instead the magmatic foliation in the shallower and larger dome like plutons reflects magma flow 4 ductile transcurrent movements along the shear zones postdate medium pressure regional metamorphism and are accompanied by an increase in the local thermal gradient as documented by the crystallization of cordierite andalusite and sillimanite 5 the increase in the thermal gradient precedes the emplacement of granitoids and their concomitant thermal influence on the country rocks the granitoids related to the final stages of tectonothermal activity of the shear zones are good time markers of their evolutionary path

influence of flow variability on floodplain formation and destruction little missouri river north dakota resolving observations of channel change into separate planimetric measurements of floodplain formation and destruction reveals distinct relations between these processes and the flow regime we analyzed a time sequence of eight bottomland images from 1939 to 2003 along the little missouri river north dakota to relate geomorphic floodplain change to flow along this largely unregulated river at the decadal scale floodplain formation and destruction varied independently destruction was strongly positively correlated with the magnitude of infrequent high flows that recur every 5 10 yr whereas floodplain formation was negatively correlated with the magnitude of frequent low flows exceeded 80 of the time at the century scale however a climatically induced decrease in peak flows has reduced the destruction rate limiting the area made available for floodplain formation the rate of destruction was not uniform across the floodplain younger surfaces were consistently destroyed at a higher rate than older surfaces suggesting that throughput of contaminants would have occurred more rapidly than predicted by models that assume uniform residence time of sediment across the floodplain maps of floodplain ages produced by analysis of sequential floodplain images are similar to maps of forest ages produced through dendrochronology confirming the assumption of dendrogeomorphic studies that riparian tree establishment in this system is limited to recent channel locations

ontogeny of a flood plain the ontogeny of five flood plain segments is described for a period of 18 yr following a major flood in 1978 on the powder river in southeastern montana the flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood in cross section the flood plains resemble benches with well developed natural levees flood plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick resulting in some lateral but mostly vertical accretion annual and biannual measurements indicated that as the flood plain segments grew upward the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0 16 to 1 3 yr it is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood plain ontogeny these flood plains did not grow on migrating point bars and vertical accretion at least initially occurred within the channel rather than across the valley flat during extensive overbank flows sediments of these flood plains define narrow elongated stratigraphic units that border the active channel and onlap older flood plain deposits these characteristics are considerably different from those of many facies models for meandering river deposits facies similar to those described in this paper are likely to be preserved thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers

anatomy and dynamics of a floodplain powder river montana u s a abstract centimeter scale measurements on several powder river floodplains provide insights into the nature of overbank depositional processes that created the floodplains during a 20 year period after a major flood in 1978 rising stages initially entered across a sill at the downriver end of the floodplains later as stages continued to rise water entered the floodplains through distinct low saddles along natural levees the annual maximum depth of water over the levee crest averaged 0 19 m from 1983 through 1996 and the estimated flow velocities were approximately 0 15 m s 1 water ponded in the floodplain trough a topographic low between the natural levee and the pre flood riverbank and mud settled as thin layers of nearly constant thickness mud layers alternated with sand layers which were relatively thick near the channel together these beds created a distinctive natural levee in some locations individual flood deposits began as a thin mud layer that gradually coarsened upwards to medium grained sand coarsening upwards sequences form initially as mud because only the uppermost layers of water in the channel supply the first overbank flows which are rich in mud but starved of sand at successively higher stages fine sands and then medium sands increase in concentration in the floodwater and are deposited as fine and medium sand layers overlying the initial mud layer theoretical predictions from mathematical models of sediment transport by advection and diffusion indicate that these processes acting alone are unlikely to create the observed sand layers of nearly uniform thickness that extend across much of the floodplain we infer that other transport processes notably bedload transport must be important along powder river even with the centimeter scale measurements of floodplain deposits daily hydraulic data and precise annual surface topographic surveys we were unable to determine any clear correspondence between the gauged flow record of overbank floods and the depositional layers mapped in the floodplain these results provide a detailed example of floodplain deposits and depositional processes that should prove useful for interpreting natural levee deposits in a variety of geologic settings

channel adjustments to changing discharges powder river montana sixteen years of annual surveys reveal how powder river responds to varying discharges during 1978 the second largest recorded daily mean discharge occurred cutbank migration bed degradation net bank erosion and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62 during the ensuing years the channel area decreased as sediment was stored in low lying benches adjacent to the active bed of the channel the survey data indicate that the balance between bank erosion and deposition varies with discharge in years when the annual maximum daily mean discharge is 3 s a flow of 60 m 3 s has a recurrence interval of 1 1 yr bank erosion and deposition are approximately equal in years when the annual maximum daily mean discharge is between 60 and 150 m 3 s a discharge of 150 m 3 s has a recurrence interval of 2 7 yr bank deposition exceeds bank erosion and the channel contracts often by developing benches in years with higher discharges the channel expands through net bank erosion these results demonstrate that the channel of powder river is influenced by a wide variety of formative discharges powder river9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single channel forming discharge

terrace aggradation during the 1978 flood on powder river montana usa abstract flood processes no longer actively increase the planform area of terraces instead lateral erosion decreases the area however infrequent extreme floods continue episodic aggradation of terraces surfaces we quantify this type of evolution of terraces by an extreme flood in may 1978 on powder river in southeastern montana within an 89 km study reach of the river we 1 determine a sediment budget for each geomorphic feature 2 interpret the stratigraphy of the newly deposited sediment and 3 discuss the essential role of vegetation in the depositional processes peak flood discharge was about 930 m 3 s 1 which lasted about eight days during this time the flood transported 8 2 million tons of sediment into and 4 5 million tons out of the study reach the masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach the flood inundated the floodplain and some of the remnants of two terraces along the river lateral erosion decreased the planform area of the lower of the two terraces 2 7 m above the riverbed by 3 2 and that of the higher terrace 3 5 m above the riverbed by 4 1 however overbank aggradation on average raised the lower terrace by 0 16 m and the higher terrace by 0 063 m vegetation controlled the type thickness and stratigraphy of the aggradation on terrace surfaces two characteristic overbank deposits were common coarsening upward sequences and lee dunes grass caused the deposition of the coarsening upward sequences which had 0 02 to 0 07 m of mud at the base and in some cases the deposits coarsened upwards to coarse sand on the top lee dunes composed of fine and very fine sand were deposited in the wake zone downstream from the trees the characteristic morphology of the dunes can be used to estimate some flood variables such as suspended sediment particle size minimum depth and critical shear velocity information about depositional processes during extreme floods is rare and therefore the results from this study aid in interpreting the record of terrace stratigraphy along other rivers

geochemical and isotopic characterization of the bodele depression dust source and implications for transatlantic dust transport to the amazon basin the bodele depression chad in the central sahara sahel region of northern africa is the most important source of mineral dust to the atmosphere globally the bodele depression is purportedly the largest source of saharan dust reaching the amazon basin by transatlantic transport here we have undertaken a comprehensive study of surface sediments from the bodele depression and dust deposits chad niger in order to characterize geochemically and isotopically sr nd and pb isotopes this dust source and evaluate its importance in present and past african dust records we similarly analyzed sedimentary deposits from the amazonian lowlands in order to assess postulated accumulation of african mineral dust in the amazon basin as well as its possible impact in fertilizing the amazon rainforest our results identify distinct sources of different ages and provenance in the bodele depression versus the amazon basin effectively ruling out an origin for the amazonian deposits such as the belterra clay layer by long term deposition of bodele depression material similarly no evidence for contributions from other potential source areas is provided by existing isotope data sr nd on saharan dusts instead the composition of these amazonian deposits is entirely consistent with derivation from in situ weathering and erosion of the precambrian amazonian craton with little if any andean contribution in the amazon basin the mass accumulation rate of eolian dust is only around one third of the vertical erosion rate in shield areas suggesting that saharan dust is consumed by tropical weathering contributing nutrients and stimulating plant growth but never accumulates as such in the amazon basin the chemical and isotope compositions found in the bodele depression are varied at the local scale and have contrasting signatures in the silica rich dry lake bed sediments and in the calcium rich mixed diatomites and surrounding sand material this unexpected finding implies that the bodele depression material is not pre mixed at the source to provide a homogeneous source of dust rather different isotope signatures can be emitted depending on subtle vagaries of dust producing events our characterization of the bodele depression components indicate that the bodele calcium rich component identified here is most likely released via eolian processes of sand grain saltation and abrasion and may be significant in the overall global budget of dusts carried out by the harmattan low level jet during the winter

cycling of lithogenic marine particles in the us geotraces north atlantic transect in this paper we present describe and model the first size fractionated 0 8 51 m 51 m water column particulate trace metal results from the us geotraces north atlantic zonal transect in situ pumping survey with a focus on the lithogenic tracer elements al fe and ti this examination of basin wide full depth distributions of particulate elements elucidates many inputs and processes some for bulk lithogenic material others element specific which are presented via concentration distributions elemental ratios size fractionation dynamics and steady state inventories key lithogenic inputs from african dust north american boundary interactions the mediterranean outflow hydrothermal systems and benthic nepheloid layers are described using the refractory lithogenic tracer ti we develop a 1 d model for lithogenic particle distributions and test the sensitivities of size fractionated open ocean particulate ti profiles to biotically driven aggregation disaggregation rates vertical sinking speeds and dust input rates we discuss applications of this lithogenic model to particle cycling in general and to poc cycling specifically

geochemistry of the xining xifeng and jixian sections loess plateau of china eolian dust provenance and paleosol evolution during the last 140 ka abstract loess and paleosol samples from three distant sections xining xifeng jixian of the chinese loess plateau were analyzed for chemical and nd sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka these three sections represent three different climatic conditions from arid in the west xining to more humid in the east jixian across the loess plateau isotopic analyses show that all the three sections display a restricted range of 143 nd 144 nd ratios or e nd 10 0 5 indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust the loess and paleosol samples have typical upper crustal 87 sr 86 sr ratios 0 714 0 718 and their variation can partly be attributed to different carbonate silicate ratios in the samples ree data from the three sections are indistinguishable from each other they show typical upper crust patterns characterized by la yb n 10 and eu eu 0 65 the high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes various chemical indicators have been used to monitor the pedogenetic intensity in these sections although results for the jixian section agree reasonably well with the known climatic conditions on the loess plateau the other two sections yield results not in agreement with some well accepted proxies like magnetic susceptibility or grain size distribution some chemical proxy indicators established in our previous work on the luochuan section such as the systematic u depletion and negative ce anomalies in ree patterns in soils are not clearly recorded in the present case the discrepancy between different climatic proxies could be related to the effective soil water budget temperature as well as the rate of development of each proxy

ca sr and 87sr 86sr ratios as tracers of ca and sr cycling in the rio icacos watershed luquillo mountains puerto rico abstract we investigated ca and sr cycling in a humid tropical forest by analyzing ca sr ratios and 87sr 86sr ratios in soil minerals soil exchangeable cations soil porewater and plant roots wood and leaves and calculating the relative contributions of sr from atmospheric inputs and weathering of local bedrock an unexpectedly large contribution of bedrock derived sr and presumably ca is cycled through the vegetation reflecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil this is surprising because over 99 of the ca and sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site and because there are large atmospheric inputs to the site of both sea salt and saharan dust substantial differences in ca and sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site measured ca sr ratios reflect fractionation between these elements during biogeochemical cycling fractionation was particularly evident between wood and foliar tissue but fractionation during soil exchange processes is also likely in comparing the ca sr ratios of plants exchangeable cations and bulk soils we found that foliar ca sr ratios were greater than exchangeable cation ca sr ratios which in turn were greater than soil ca sr ratios similar to patterns observed at other highly weathered tropical sites

stream terrace genesis implications for soil development genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting base level of erosion complex response threshold of critical power diachronous and synchronous response times and static and dynamic equilibrium climatic and tectonic stream terraces are major terraces below which flights of minor complex response degradation terraces can form these three types of terraces can be summarized by describing a downcutting aggradation renewed downcutting sequence for streams with gravell bedload by tectonically induced downcutting streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream terrace landform aggradation events record brief reversals of long term tectonically induced downcutting because they raise active channels they may be considered as major the result of climatic perturbations or minor the result of complex response model types of perturbations climatically controlled aggradation followed by degradation leaves an aggradation surface this type of fill terrace tread is the fundamental climatic stream terrace landform aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form first in alluvial fill and then in underlying bedrock along tectonically active streams these are complex response terraces because they result from interactions of dependent variables within a given fluvial system pauses in degradation to a new base level of erosion and or minor episodes of backfilling lead to formation of complex response fill cut and strath or of fill terraces fill cut terraces are formed in alluvium they are complex response terraces because they are higher than the base level of erosion good exposures and dating are needed to distinguish static equilibrium complex response minor strath terraces from dynamic equilibrium tectonic major straths strath terraces may be regarded as complex response terraces where degradation rates between times terrace tread formation exceed the long term uplift rate for the reach based on ages and positions of tectonic terraces late quaternary global climatic changes control aggradation events and even the times of cutting of major tectonic straths because the base level of erosion can not be attained during times of climatically driven aggradation degradation events most terrace soils form on treads of climatic and complex response terraces aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors climatic terraces of adjacent watersheds also should be roughly synchronous correlatable variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full glacial times and interglacial times soils on some complex response terraces may be diachronous within a given fluvial system and typically are diachronous between watersheds

sr nd elements and isotopes as tracers of dust input in a tropical soil chronosequence abstract although hainan island is located in one of the relative least dusty regions in china asian dust has been identified in hainan soils we used sr and nd isotopes of soils saprolite bedrocks and asian dust to not only identify long range transported dust but also quantify the dust contribution to hainan soils sr and nd isotopic compositions of the soils are clearly affected by dust deposition the soils in near surface horizons have 87 sr 86 sr values as high as 0 7193 that are more radiogenic than the basalts indicating involvement of asian dust whereas deep horizons show a dominantly basaltic signature 87 sr 86 sr 0 7038 the e nd 0 values for hainan soils range from 6 46 to 1 85 the higher value closely approximates the values measured for the hainan lava and the lower value indicates the fingerprint of the asian dust the 87 sr 86 sr and e nd 0 values for soils also show a progressive trend with time which implies that dust accumulates with soil age we calculated the dust accretion rates for sr based and nd based to quantify the dust contributions to soils the estimate of dust accreted shows an approximate increasing trend with the ages of soils for sr based and nd based methods the soils in the old sites are likely to have undergone long term dust input and accumulated high dust levels we also find excellent agreement in the amount of dust present and dust accretion rates based on the two tracers at the 300 ka site and the 1120 ka site respectively besides lower dust deposition in the pliocene epoch than the late quaternary over eastern eurasian the loss of mineralogical and geochemical tracers over million year time scales also may lead to underestimates of time averaged dust deposition rates in the old sites therefore the young sites 180 ka and 300 ka should provide more realistic estimates of average dust accretion rates the importance of dust in the tropical hainan soil chronosequence highlights the significance of dust accretion to soils globally

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

case study two dimensional model simulation of channel migration processes in west jordan river utah the paper presents the application of a two dimensional depth averaged numerical model to simulate the lateral migration processes of a meandering reach in the west jordan river in the state of utah a new bank erosion model was developed and then integrated with a depth averaged two dimensional hydrodynamic model the rate of bank erosion is determined by bed degradation lateral erosion and bank failure because bank material in the west jordan river is stratified with layers of cohesive and noncohesive materials a specific bank erosion model was developed to consider stratified layers in the bank surface this bank erosion model distinguishes itself from other models by relating bank erosion rate with not only flow but also sediment transport near the bank additionally bank height slope and thickness of two layers in the bank surface were considered when calculating the rate of bank erosion the developed model was then applied to simulate the processes of meandering migration in the study reach

magnetic mineral inventory of equatorial atlantic ocean marine sediments off senegal glacial and interglacial contrast summary in complex sedimentary environments with different sources of the magnetic particles it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources and diagenesis may alter the primary magnetic mineral assemblage rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages in this study of core retrieved sediments from the last 150 kyr off the coast of senegal nw africa the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant in glacial stages ferrimagnetic iron and iron titanium oxides magnetically dominate with few signs of a diagenetic overprint low and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low temperature ac susceptibility measurements and sem observations in contrast the interglacial samples appeared to have been severely affected by diagenesis with pyrite making up over 50 per cent of the iron bearing phases the verwey transition observed at 110 k confirms the presence of magnetite the glacial and interglacial contrast reveals the close link between northwest african climate and the magnetic mineralogy of the sediments

flood plain and channel dynamics of the quinault and queets rivers washington usa abstract comparison of historic channel migration rates modern planform conditions and overall sediment wood and flow conditions and interactions for the quinault river and queets river in the western olympic peninsula washington reveals decadal to century scale interactions between gravel bed channels and forested flood plains in temperate maritime environments the downstream alluvial portions of these two rivers can be divided into three reaches of different slope flow sediment and wood regimes i the upper quinault river is aggrading behind lake quinault a natural lake that traps most sediment and wood transported from the olympic mountain headwaters ii the lower quinault river downstream of lake quinault transports only sediment and wood derived from reworking of flood plain deposits and contributed from valley margins iii the queets river has unimpeded movement of sediment and water from the mountainous headwaters to the pacific ocean measurements of channel planform characteristics and historic migration rates and patterns show that these three reaches have correspondingly distinct channel and flood plain morphologies and dynamics the aggrading and sediment rich upper quinault river has the widest flood plain widest active channel greatest number of low flow channels and flanking gravel bars and an average channel migration rate of 12 7 3 3 m year between 1900 and 1994 the comparatively sediment poor lower quinault river has the narrowest flood plain narrowest active channel and lowest channel migration rate 4 0 1 2 m year and most flow is through a single channel with few adjacent gravel bars the queets river has attributes intermediate between the lower and upper quinault rivers including an average channel migration rate of 7 5 2 9 m year flood plain turnover rates are similar for all three reaches with channels eroding the flood plain at the rate of about 0 2 of the flood plain area per year and with corresponding flood plain half lives of 300 to 500 years observations from this study and previous studies on the queets river show that channel and flood plain dynamics and morphology are affected by interactions between flow sediment and standing and entrained wood some of which likely involve time frames similar to 200 500 year flood plain half lives on the upper quinault river and queets river log jams promote bar growth and consequent channel shifting short distance avulsions and meander cutoffs resulting in mobile and wide active channels on the lower quinault river large portions of the channel are stable and flow within vegetated flood plains however locally channel spanning log jams have caused channel avulsions within reaches that have been subsequently mobile for several decades in all three reaches log jams appear to be areas of conifer germination and growth that may later further influence channel and flood plain conditions on long time scales by forming flood plain areas resistant to channel migration and by providing key members of future log jams appreciation of these processes and dynamics and associated temporal and spatial scales is necessary to formulate effective long term approaches to managing fluvial ecosystems in forested environments

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

connection between south mediterranean climate and north african atmospheric circulation during the last 50 000 yr bp north atlantic cold events abstract high resolution clay mineralogical analyses were performed on sediment deposited during the last 50 000 yr in the alboran sea odp site 976 the clay mineral record is compared with pollen assemblages and with annual precipitation pann and mean temperatures of the coldest month mtco reconstructed with the modern analog technique mat enhanced contribution of palygorskite a typical wind blown clay mineral characterizes the north atlantic cold climatic events coeval development of the semi arid vegetation artemisia rich associated with a drastic fall of reconstructed precipitations and temperatures suggest cold and arid continental conditions in the west mediterranean area during north atlantic cold events the clay mineral association especially the palygorskite content and the illite to kaolinite ratio indicate western morocco as one of the major source of the clay size fraction during the north atlantic cold events the maximum abundance of artemisia associated with the presence of argania pollen both indicate morocco as the main origin for pollen during these cold periods the comparison of these pollen and clay mineral specific features allows us to pinpoint western morocco as the dominant source of wind blown particles during north atlantic cold events these specific mineralogical composition and palynological assemblages reveal enhanced aridity over north africa and intensification of winds favouring dust erosion and transport from north africa toward the alboran sea during the north atlantic cold events according to atmospheric models such a meridian transport 1 likely results from the development of strong and stable anticyclonic conditions over the tropical atlantic and north africa similar to today s summer meteorological configuration and 2 implies a northward position of the westerly winds during north atlantic cold events finally the synoptic situation over the west mediterranean during the north atlantic cold events is compared with the north atlantic oscillation nao suggesting that during the cold atlantic events weather regimes over europe and north africa may have been systematically shifted towards a positive nao situation

is there a close spatial relationship between faults and plutons abstract using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults our initial analyses of populations of faults and carboniferous plutons in the armorican massif france and faults and alleghanian plutons in the southern appalachians usa indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults maxima of integrated pluton areas occur at 1 4 appalachians and 1 2 armorican of the distance between the average fault spacing in these orogens although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts