mega meander paleochannels of the southeastern atlantic coastal plain usa abstract paleodischarge estimates based on the slope area method and channel boundaries determined from stratigraphic cross sections indicate that large terminal pleistocene meandering paleochannels mega meanders in river valleys of the southeastern atlantic coastal plain of the united states represent bankfull flows that were at least double the magnitude of those on modern rivers correlation of radiocarbon and luminescence dated paleomeanders with previously reported pollen and eolian sedimentary records suggests that greater discharge was driven by seasonally wetter conditions resulting from dynamic changes to regional precipitation and runoff that occurred in association with global warming at the end of the pleistocene while reflecting larger channel forming flows the exceptionally large widths and radii of curvature of mega meanders were nonetheless maintained by a relatively modest discharge magnitude that was between two and four times larger than modern bankfull flow and within the size range of the present two to five year flood despite their large planform the paleochannels conveyed relatively modest bankfull discharges because a wide shallow shape limited their cross sectional area and hydraulic radius within the late quaternary evolution of fluvial systems in the region scrolled mega meanders constitute a transitional meandering planform that remained influenced by large volumes of sandy bedload following the sand bed braided channels of the late wisconsin interval circa 30 17 ka in addition to greater discharge the large planimetric dimensions of paleochannels reflect a lack of cohesive vertical accretion facies on paleomeander floodplains a sediment regime that transported large quantities of bedload sand and the influence of these factors on channel boundary composition bank stability and channel shape findings underscore the importance of reconstructing channel cross sectional dimensions and slope when estimating discharge for infilled paleomeanders this approach reduces uncertainties surrounding channel cross sectional area gradient and boundary composition inherent to studies lacking subsurface data that rely upon meander geometry to retrodict discharge

sources sinks and storage of river sediment in the atlantic drainage of the united states the history of sediment and its movement in the atlantic drainage demonstrate some of the difficulties of modeling sediment on a river basin scale soil erosion was accelerated by a factor of at least 10 when european settlers cleared forests and planted crops although increasing soil conservation practice and decreasing crop farming have since reduced the rates of erosion large quantities of eroded material are still stored on hillslopes and in stream valleys where they continue to augment the sediment loads of the rivers the sediment from this episode of erosion that is largely past can be expected to emerge from storage for many decades and perhaps even several centuries to come the reservoirs that have been built on many of the major rivers trap significant portions of the moving sediment which in some places may be remobilized by large floods essentially all the river sediment that reaches the atlantic coastal zone is trapped in estuaries and coastal marshlands probably less than 5 is deposi

the source of alluvium in large rivers of the lower coastal plain of north carolina summary despite high erosion rates in the north carolina piedmont and extensive sediment delivery to coastal plain reaches of piedmont draining streams sediment budget studies and estuarine sedimentology suggest that relatively little piedmont derived sediment reaches the lower coastal plain dominant sources of alluvium in the lower coastal plain reaches of piedmont draining rivers were inferred from alluvial floodplain soils piedmont derived alluvial soils are characterized by mixed mineralogy and presence of mica flakes alluvial soils derived from erosion of coastal plain uplands are characterized by siliceous mineralogy and an absence of mica flakes the coastal alluvial soils which occur in fluvial estuarine transition zones being drowned by rising sea level are organic or buried organic soils the distribution of these soils along the tar and neuse rivers in the lower coastal plain shows almost complete domination of fluvial estuarine transition zones by soils of the coastal group and by soils of the coastal plain and coastal groups for 40 km or more inland of the upper estuaries no soils of the piedmont group were found in fluvial estuarine transition zones or lowermost fluvial reaches this suggests that the piedmont is a relatively insignificant source of alluvium in the lower reaches of the rivers the absence of piedmont sediment in the lower coastal plain is attributed to extensive sediment storage upstream dilution by coastal plain sediments and coastal plain erosion rates which may be higher than was previously believed

submesoscale physical biogeochemical coupling across the ligurian current northwestern mediterranean using a bio optical glider a bio optical autonomous underwater gliding vehicle equipped with a conductivity temperature depth ctd sensor an oxygen sensor backscatter meters chromophoric dissolved organic matter cdom and chlorophyll fluorometers and upwelling and downwelling radiometers was developed to characterize the biogeochemical response to physical forcing at the submesoscale 1 10 km winter data obtained in the northwestern mediterranean basin in a 50 km transect that crossed the permanent along coast ligurian current show tight physical biogeochemical coupling across the ligurian current not observed using other sampling strategies at the scale of the transect the various biogeochemical parameters displayed independent behaviors while at the submesoscale there was a coherent covariance of these variables especially in the frontal zone near isopycnal tongues of elevated fluorescence and oxygen concentration were recorded down to a depth of 180 m and are the likely signature of a converging horizontal and downwelled water flow local anomalies in a horizontal section well below the mixed layer depth are likely representative of downwelled waters from the euphotic layer intrusions of elevated cdom concentrations together with signatures of smaller particles are the likely features of a local divergence and upwelled waters from subjacent aphotic layers these are particularly apparent in the form of local anomalies in a horizontal section within the mixed layer similar tongues were observed in data from subsequent glider deployments such biogeochemical signatures enable the identification of upward and downward physical motions not observed by other technologies reinforcing the need for coupled high resolution physical biogeochemical studies not only for investigating biogeochemical processes themselves but also for resolving physical processes at these scales

quantification of holocene asian monsoon rainfall from spatially separated cave records abstract a reconstruction of holocene rainfall is presented for southwest china an area prone to drought and flooding due to variability in the east asian monsoon the reconstruction is derived by comparing a new high resolution stalagmite 18 o record with an existing record from the same moisture transport pathway the new record is from heshang cave 30 27 n 110 25 e 294 m and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature heshang lies 600 km downwind from dongge cave which has a published high resolution 18 o record wang y j cheng h edwards r l he y q kong x g an z s wu j y kelly m j dykoski c a li x d 2005 the holocene asian monsoon links to solar changes and north atlantic climate science 308 854 857 by differencing co eval 18 o values for the two caves secondary controls on 18 o e g moisture source moisture transport non local rainfall temperature are circumvented and the resulting 18 o signal is controlled directly by the amount of rain falling between the two sites this is confirmed by comparison with rainfall data from the instrumental record which also allows a calibration of the 18 o proxy the calibrated 18 o record provides a quantitative history of rainfall in southwest china which demonstrates that rainfall was 8 higher than today during the holocene climatic optimum 6 ka but only 3 higher during the early holocene significant multi centennial variability also occurred with notable dry periods at 8 2 ka 4 8 4 1 ka 3 7 3 1 ka 1 4 1 0 ka and during the little ice age this holocene rainfall record provides a good target with which to test climate models the approach used here of combining stalagmite records from more than one location will also allow quantification of rainfall patterns for past times in other regions

sources and flow patterns of deep ocean waters as deduced from potential temperature salinity and initial phosphate concentration through the deconvolution of ocean chemical data it has been shown that the ratio of o2 utilization to phosphate production in the sea is 175 6 rather than 138 as proposed by redfield et al 1963 we find no evidence that this ratio changes significantly with location or depth in the sea we have used this new ratio to calculate the initial phosphate concentrations for the waters sampled during the geosecs and tto programs the important application of these new results is in constraining the origin and flow patterns of deep waters in the ocean we believe that a strong case can be made that antarctic salinity maximum water i e the common water of montgomery 1958 is produced by the mixing of waters entering the antarctic from mid depths in the indian pacific and atlantic with weddell sea bottom water southern component antarctic common water consists of about 45 weddell sea bottom water 30 intermediate waters from the pacific and indian oceans and 25 deep water originating from the northern atlantic

quantitative estimates of pedogenic ferromagnetic mineral formation in chinese loess and palaeoclimatic implications abstract the quasi continuously deposited loess sediments of the central chinese loess plateau constitute one of the most complete and long lasting records of quaternary palaeoclimate on land much of this record has been uncovered by measurement and interpretation of the magnetic properties of the loess the polarity of natural remanent magnetization has provided absolute age dates by correlation with the geomagnetic polarity time scale magnetic low field susceptibility has been used to reconstruct the cyclic changes of pleistocene palaeoclimates although the origin of the signal is still poorly understood from comparison with the 10 be concentration throughout the sequence we are able to determine the contributions to susceptibility from inherited and authigenic sources for the last 130 ky since the production of authigenic magnetite is directly related to regional precipitation annual palaeoprecipitation rates can be calculated palaeoprecipitation rates similar to present day rates are derived for oxygen isotope stage 5 including the last interglacial in contrast cold periods corresponding to oxygen isotope stages 2 and 4 experienced negligible precipitation the beginning and end of the time interval represented by oxygen isotope stage 3 however was characterized by short episodes when precipitation exceeded the holocene average

tracing the sources fate and recycling of fine sediments across a river delta interface abstract deltaic floodplains are thought to be long term depositional environments however there remains a limited understanding regarding timescales of depositional and erosional events sediment delivery pathways and sediment storage this study uses sediment concentration and sediment fingerprinting to examine the contribution of surface and subsurface sources to suspended sediment transiting the lower roanoke river north carolina united states the lower roanoke is disconnected from its high gradient uplands in the piedmont and appalachian mountains by a series of dams which effectively restricts suspended sediment delivery from the headwaters accordingly sediments from the lower roanoke river basin are the primary source of suspended sediment downstream of the dams the fingerprinting method utilized fallout radionuclide tracers 210pbxs and 137cs to examine the spatial variation of sediment source contributions to suspended sediment samples n 79 three end member sources were sampled 1 surface sources floodplains and topsoils n 60 2 subsurface sources channel bed and banks n 66 and 3 deltaic sources delta front and prodelta n 11 the results demonstrate that with decreasing river slope and increasing influence of estuarine driven flow dynamics the relative contribution of surface sediments to the suspended sediment load increases from 20 2 in the upper reach to 67 1 in the roanoke bayhead delta bhd at the river mouth the surface sediment contribution decreases and the delta front and prodelta sediments contribute 74 1 to the suspended load these results indicate that during the delta transgression erosion of the lower delta provides an additional source of sediment to the upper delta at the same time the lower deltaic plain considered a sediment sink and long term sediment storage site becomes erosional the lower river and distributary network of the delta plain which were thought to only disperse sediments in a seaward direction may also have an important landward directed sediment dispersal component that provides nourishment and fortification to the upper bhd at the cost of the eroding lower delta recognition of these contrasting sediment pathways in the roanoke river highlights that these complex bidirectional processes may exist in other eroding deltas understanding these bidirectional processes will be necessary for the ongoing management of deltaic environments under increasing anthropogenic stress such as land use change and accelerating sea level rise

hydrology geomorphology and vegetation of coastal plain rivers in the southeastern united states rivers of the coastal plain of the southeastern united states are characteristically low gradient meandering systems that develop broad floodplains subjected to frequent and prolonged flooding these floodplains support a relatively unique forested wetland bottomland hardwoods which have received considerable ecological study but distinctly less hydrogeomorphic study the hydroperiod or annual period of inundation largely controls the development of characteristic fluvial landforms sediment deposition and vegetation distribution patterns order of magnitude differences in wetted perimeter width depth suspended sediment load and hydraulic roughness may exist between dry in channel seasons and the hydyoperiod substantial sediment and adsorbed contaminants retention and storage through lateral and bertical accretion is common where not heavily impacted by flow regulation along these coastal pi n rivers the present chapter summarizes our current understanding of the hydrology fluvial geomorphology general and local sedimentation patterns and related plant ecological patterns of these coastal plain bottomlands

modeling floodplain inundation using an integrated gis with radar and optical remote sensing abstract synthetic aperture radar images from multitemporal l band jers 1 and c band ers 1 satellites a landsat thematic mapper tm time series and gis coverages were used in an integrative approach to model the potential of flood inundation within the lower roanoke river floodplain north carolina a digital elevation model dem with one meter vertical resolution was developed for the region from scan digitized mylar separates of contour lines on usgs 7 5 min quadrangles several models representing potential wetness and potential flood inundation were generated from the dems using both raster grid and vector network analyses the potential inundation surfaces were derived from regression models that related known flood elevations to river position and floodplain location the gis models were assessed by comparison to classifications of flood change detection achieved through the radar data statistical results indicate that the gis derived models successfully identified flooded areas as mapped by the radar change detections further statistical tests assessed the ability of individual radar and optical landsat tm images to discriminate flooding as predicted by the gis models both jers 1 and ers 1 images identified areas of inundation at different flood levels

geomorphic and vegetative characteristics along three northern virginia streams geometry sediment and woody vegetation data were collected from bottomland geomorphic surfaces at valley sections along three gaged perennial streams of northern virginia the basins of the streams differ widely in topography and physiography mean discharges vary from 0 196 to 323 m3 per sec prevalent surfaces identified were the depositional bar the active channel shelf the flood plain and terraces the stages corresponding to active channel shelf levels were equivalent to flow durations of 5 to 13 stages corresponding to flood plain levels were equivalent to discharges with 1 4 to 2 0 yr recurrence intervals the discharge data and statistical tests of geomorphicsurface sediment and vegetative data suggest that the various alluvial features are formed and maintained by hydraulically controlled sorting processes analysis of woody plant and geomorphic data shows that each surface supports characteristic species some of which are nearly unique to a surface tests of sediment type with species distribution showed lower correlation than that of geomorphic surface with species it is inferred that plant distributions largely are controlled by flow frequency and intensity and that plants may help to identify geomorphic levels and potential for flood damage

intensification of ocean fronts by down front winds abstract many ocean fronts experience strong local atmospheric forcing by down front winds that is winds blowing in the direction of the frontal jet an analytic theory and nonhydrostatic numerical simulations are used to demonstrate the mechanism by which down front winds lead to frontogenesis when a wind blows down a front cross front advection of density by ekman flow results in a destabilizing wind driven buoyancy flux wdbf equal to the product of the ekman transport with the surface lateral buoyancy gradient destabilization of the water column results in convection that is localized to the front and that has a buoyancy flux that is scaled by the wdbf mixing of buoyancy by convection and ekman pumping suction resulting from the cross front contrast in vertical vorticity of the frontal jet drive frontogenetic ageostrophic secondary circulations ascs for mixed layers with negative potential vorticity the most frontogenetic ascs select a preferred cross front width and do not translate with

orbital scale stratigraphy and high resolution analysis of biogenic components and deep water oxygenation conditions in the japan sea during the last 640 kyr abstract paleoceanographic conditions in the japan sea changed drastically during the late quaternary in association with orbital and suborbital cycles and glacio eustatic sea level changes however few studies have examined long term sediment records older than 160 ka because of the lack of long and continuous cores and the difficulty in constructing high resolution age models in this study we first constructed an orbital scale stratigraphy of the sediments covering the last 640 kyr by comparing the specmap 18o curve and the 18o curve of planktonic foraminifera 18opf in the japan sea together with the combination zones of dominant planktonic foraminifera species then we reconstructed temporal variations in biogenic silica biosil and biogenic carbonate biocarb contents calculated from high resolution major element analysis data determined by an xrf microscanner in order to evaluate the deep water oxygenation conditions we also defined excess s as the difference between the content of s in the sediments and that for normal marine sediments calculated from the organic carbon content corg estimated from lightness l by assuming a total sulfur content stotal corg ratio of 0 36 and examined its variability biosil shows orbital scale variation and is generally low except during interglacial maxima when global sea level was high enough to allow a strong influx of the tsushima warm current biocarb shows orbital and suborbital scale variations and tends to be higher in dark layers deposited during glacial stages whereas the relationship is reversed during interglacial stages the low biocarb in dark layers deposited during interglacial stages is probably due to dissolution of biogenic carbonate caused by the increase in surface productivity as a result of the strong influx of nutrient rich east china sea coastal water and consequent increase in surface productivity and decomposition rate of organic matter in the intermediate water excess s tends to be higher in thicker dark layers deposited during glacial maxima when the sea level dropped below 90 m compared to adjacent light layers suggesting development of euxinic deep water conditions during these periods the results demonstrated the presence of orbital and suborbital scale variations in biosil and biocarb and their association with dark and light layers in the japan sea that reflect variations in the nature and strength of inflow through the tsushima strait controlled by both climatic and sea level changes whereas higher excess s reflects the development of euxinic deep water condition during glacial maxima in association with glacio eustatic sea level drops below 90 m during the last 640 kyr

factors affecting the spatial pattern of the deep chlorophyll maximum in the region of the azores front abstract results are given of two surveys of the deep chlorophyll maximum dcm in the vicinity of an oceanic front south west of the azores the surveys made in late april and may 1981 used mainly a towed undulating fish seasoar fitted with ctd and in situ fluorometer in the latter survey observations of underwater irradiance nitrate concentration and primary production were also made the front was baroclinic and is considered to be an arm of the gulf stream return flow in the north east atlantic the current associated with the front made a large meander through the area and during the observation period shed an eddy which later drifted westwards the front separated two distinct water types which were termed east atlantic water eaw and west atlantic water waw the dcm in waw was on average 20 m deeper and between 50 and 60 of the magnitude of that in eaw however the 1 light level coincided with the depth of the dcm in both water types and this fact together with observations on the different nitrate concentrations in the two water types led to the theory that the differences in the dcm could be explained by assuming an overall lower level of primary productivity in waw although differences in vertical mixing may have been a contributory factor the measurements of primary production imply that the specific growth rate of phytoplankton in the dcm was less than 0 1 day 1 in the front itself there was some evidence for small scale 20 km eddy like features which produced patchiness of the same scale in the dcm there was also evidence for advective intrusions along isopycnals of eaw across the front into waw in two cases this process produced deep subsidiary maxima below the dcm with chlorphyll a concentrations of 0 1 mg m 3 as deep as 170 m in the late april survey two of the seasoar sections showed higher phytoplankton chlorophyll concentrations on the eaw side of the front but in general on both surveys there was little evidence for increased phytoplankton biomass associated with the front

methyl mercury distributions in relation to the presence of nano and picophytoplankton in an oceanic water column ligurian sea north western mediterranean recent findings on the distribution of methylated mercury mehgt in waters have highlighted the importance of organic carbon remineralization on the production of these compounds in the open ocean here we present the first time series 20 monthly samplings between july 2007 and may 2009 of high resolution vertical profiles 10 12 depths in a 2350 m water column of mehgt distributions in an open ocean environment the ligurian sea north western mediterranean sea concentrations varied within the sub picomolar range general mean 0 30 0 17 pmol l 1 n 214 with the lowest values at the surface increasing with depth up to the oxygen minimum zone and decreasing slowly at greater depth concentrations in the surface waters never exceeded 0 15 pmol l 1 while the highest concentrations up to 0 82 pmol l 1 were associated to the hypoxycline during the autumn bloom a detailed vertical mehgt profile reveals a double peak pattern coincidental with the two microbial layers described by tanaka and rassoulzadegan 2002 the so called microbial food web in the euphotic zone 100 m temporal variations in the mehgt abundance and distribution in the water column were linked to seasonality the highest mehgt concentrations were found in the oxygen minimum zone during the period of stratification and coincide with the greatest abundance of nano and picophytoplankton cyanobacteria nanoflagellates etc in the euphotic layer none of our deep mehgt measurements 100 m above the sea bottom revealed a significant sedimentary source of mehgt we explored the correlation between mehgt concentrations and the apparent oxygen utilization a proxy of organic matter remineralization over the study period results of this study strengthen the hypothesis that net mercury methylation in the open ocean occurs in the water column is linked to organic matter regeneration and is promoted by the presence of small sized nano and picophytoplankton that dominate under oligotrophic conditions 2010 elsevier ltd all rights reserved

variations of east asian summer monsoon since the last deglaciation based on mg ca and oxygen isotope of planktic foraminifera in the northern east china sea 1 variations in mg ca based sea surface temperature and oxygen isotope ratio 18o of the surface water in the northern east china sea ecs were reconstructed with high resolution during the last 18 kyr using planktic foraminifera millennial scale variations between warmer more saline surface water and cooler less saline surface water were recognized during the early deglacial period and the holocene suggesting changes in the mixing ratio between the kuroshio water and the changjiang diluted water stronger east asian summer monsoon easm precipitation events in south china are identified at 10 5 8 8 7 0 5 3 4 7 2 9 1 7 and 0 5 ka based on sea surface salinity sss records of the northern ecs weaker easm precipitation events are also detected at 9 3 8 3 7 3 6 0 3 3 2 3 0 7 and 0 4 ka during the holocene these events agree with the maxima in 18o records of stalagmites from various parts of the changjiang yangtze river drainage this agreement supports that our sss record properly captures the millennial scale dry less easm precipitation events over the drainage basin of the changjiang river during the holocene these dry events are also in good agreement with north atlantic ice rafted events suggesting a teleconnection between north atlantic climate and the easm during the holocene

evolution of atchafalaya lacustrine deltas south central louisiana abstract the entrenched pleistocene holocene mississippi alluvial valley was cut during the last low sea level stand and alluviation of the entrenched valley occurred during the holocene sea level rise regional gulf coast structures salt domes and faults have strongly influenced the morphology of the valley valley paleotopography has controlled depositional patterns and locations of major river courses as well as partially isolated large interdistributary basins within the alluvial deltaic plain interdistributary basin formation is further enhanced by distributary channel progradation switching and abandonment in conjunction with compactional subsidence of the alluvial plain surface geomorphology and sediment cores from the atchafalaya basin louisiana revealed that this basin has alternately accumulated fine grained organic rich sediments and terrigenous clastic dominated meander belt and lacustrine deposits basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary fill prograding lacustrine deltas of the atchafalaya river are the primary mechanism for filling the atchafalaya basin these upward coarsening deltas are relatively thin 3 m but are areally extensive tens to hundreds of kilometers and because they occur so frequently during the infilling process they account for a significant volume of the basin sequence the atchafalaya basin sequence documents lacustrine deltaic episodes dating from the early holocene maringouin and teche deltas to the present atchafalaya delta continued development of the atchafalaya river will rework portions of the backswamp and lacustrine deposits into meander belt deposits upon complete filling of the atchafalaya basin sediment will mostly bypass the basin and become deposited in the marine delta in atchafalaya bay

日本海堆積物と東アジア モンスーン変動 iodp日本海 東シナ海掘削に向けて

plio pleistocene climate evolution trends and transitions in glacial cycle dynamics abstract we describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5 3 myr in particular we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic 18 o when the exponential trend in 18 o variance is removed its spectral power exhibits strong proportional responses to amplitude modulations in orbital forcing over most of the plio pleistocene precession responses correlate with modulations in forcing for the last 5 myr but 41 kyr response is sensitive to obliquity modulation only before 1 4 myr where responses are sensitive to modulations in forcing we demonstrate that glacial cycles are orbitally forced rather than being self sustained or paced by orbital changes the shapes of glacial cycles have several nonlinear properties which may be indicative of glacial interglacial differences in climate sensitivity or response time the saw tooth asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation and the relative duration of interglacial stages decreases at 1 4 myr collectively trends in the shape of glacial cycles and the sensitivity of 18 o to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2 5 and 1 4 myr but show no significant change associated with the appearance of strong 100 kyr cycles during the mid pleistocene transition

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

biological and chemical signs of upward motions in permanent geostrophic fronts of the western mediterranean upward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the western mediterranean while physical evidence of such upward advections is seldom reported the goal of this study is to define biological and chemical criteria which can be used to localize such upward motions zones we use a one dimensional time dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass nutrients and dissolved oxygen simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth in conformance with the continuity equation boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer low upward advections 3 m d led to a shallowing and sharpening of the nitracline oxycline and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass by confining the phytoplankton nutrient system in the surface mixed layer higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer the nitracline and the oxycline being then at the top of the pycnocline data collected during the prolig 2 cruise may 1985 on the heavy side of the liguro provenqal front are interpreted as an illustration of these numerical results computed primary production rates are compared with measurements conducted in the almer a oran front during the almofront i cruise april 1991 in a similar situation in both fronts upward advections of 1 2 m d would be sufficient to account for the observed vertical distributions and the increased primary production ecological implications for the phytoplankton nutrient system are discussed particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts 1 introduction the liguro provengal front b thoux et prieur 1983 sournia et al 1990 and the almerfa oran front cheney and doblar 1982 tintor et al 1988 are permanent geostrophic structures in the western mediterranean both fronts are areas of high plankton biomass in contrast with two adjacent oligotrophic type systems lohrenz et al 1988 tintor et al 1988 sournia et al 1990 prieur et al 1993 claustre et al 1994b and show net change in both the specific composition of the autotrophic claustre et al 1994b and the heterotrophic boucher et al 1987 thibault et al 1994 communities several features indicate that the two fronts are sites of enhanced production in response to geostrophic frontal dynamics in addition to the main geostrophic flow a secondary circulation exists i e the ageostrophic flow which is characterized by enhanced vertical advections as shown for example by dewey et al 1991 in the california current it is thought that upward advections in the frontal zone upwell nutrients into the photic zone and enhance primary production lohrenz et al 1988 fideau et al 1994 the resulting high biomass is then downwelled by the conver paper number 98jc01537 0148 0227 98 98 jc 01537509 00 gent part of the ageostrophic circulation along the isopycnals claustre et al 1994a b fideau et al 1994 however physical evidence for upward advection in the two fronts is not well documented see however tintor et al 1991 and l iudez and tintor 1996 and poorly understood this paper defines criteria from the vertical distribution of parameters involved in primary production which can be used to localize with some confidence the upward motions in such frontal zones in a previous modeling study zakardjian and prieur 1994 two types of phytoplankton nutrient systems resulting from high and low turbulent regimes htr and ltr respectively were described with the ltr representing the ecological conditions in the waters adjacent to the liguro provengal and almerga oran fronts where the vertical structure is controlled by diffuse vertical fluxes we use a modified version of this previous one dimensional vertical model presented in section 2 to test the sensitivity of the ltr system when an upward advection term is added in section 3 we describe the steady state vertical distribution of phytoplankton biomass nutrients and dissolved oxygen obtained with upward advection values ranging from 0 to 5 m d effects of upward advection on the vertical distribution of primary production on phytoplankton growth and finally on the productivity indexes of the water column are detailed in section 4 we present multiparametric 27 849

riparian vegetation and fluvial geomorphic processes abstract riparian vegetation and fluvial geomorphic processes and landforms are intimately connected parts of the bottomland landscape relations among vegetation processes and landforms are described here for representative streams of four areas of the united states high gradient streams of the humid east coastal plain streams great plains streams and stream channels of the southwestern united states vegetation patterns suggest that species distributions in the humid east are largely controlled by frequency duration and intensity of floods along channelized streams vegetation distribution is largely controlled by variation in fluvial geomorphic processes cycles of degradation and aggradation in response to increases in channel gradient associated with channelization similarly riparian vegetation of great plains streams may be controlled by fluxes in sediment deposition and erosion along braided streams patterns of riparian vegetation in semi arid regions may be most closely related to patterns of water availability unlike most other streams in more humid environments channel equilibrium conditions control stability of the coincident fluvial landform and attendant vegetation pattern throughout the continent in most situations riparian vegetation patterns are indicative of specific landforms and thus of ambient hydrogeomorphic conditions

reconstruction of floodplain sedimentation rates a combination of methods to optimize estimates reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics and thereby provides a basis for efficient and sustainable floodplain management we compared the results of four independent reconstruction methods optically stimulated luminescence osl dating caesium 137 cs 137 dating heavy metal analysis and flood bed interpretation applied at three embanked floodplain sites along lower rhine river distributaries in the netherlands all methods indicate significant sedimentation rates on the floodplains varying between 2 7 mm a in the distal zones and 3 9 mm a in the proximal zones on a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm a were found except for some minor inconsistencies in cs 137 dating results all methods show decreasing sedimentation rates with increasing distance from the river channel intercomparison of the results of the different dating methods revealed the potential errors associated with each method particularly where disagreement among the results were found uncertainties may arise due to 1 grain size dependent downward migration of cs 137 2 smoothing of the vertical heavy metal and cs 137 profiles 3 delayed sediment associated input of cs 137 in addition to direct atmospheric fall out 4 overestimation of the burial age in os l dating due to incomplete resetting of the osl signal or 5 non linear relationships between sediment deposition and flood magnitude in the count from the top correlation between sediment lamination and past observed flood records still taking the uncertainties associated with each method into account the results are generally in good agreement using the results we indicate the optimal spatial range of application of each method depending on sediment texture and sedimentation rate the optimal spatial and temporal ranges differ for each method but show significant overlap a combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

the spatial variability of overbank sedimentation on river floodplains abstract river floodplains are important landforms and information on rates and patterns of overbank sediment deposition is essential for understanding their development such information is also needed for validating existing overbank floodplain flow and sediment transport and deposition models the use of bomb derived 137cs for documenting rates of overbank sediment deposition on river floodplains offers the potential for assembling data for a large number of points on a floodplain and therefore for investigating the spatial patterns involved this approach also overcomes some of the operational and sampling problems encountered in traditional monitoring techniques the study reported uses 137cs measurements on sediment cores to investigate the spatial variability of sediment deposition on selected floodplain reaches of five british lowland rivers attention is given both to the overall reach and to more detailed investigations of small areas the observed spatial patterns of sediment deposition are interpreted in terms of the major controls involved which include the floodplain microtopography and morphology the relationship between the grain size composition of deposited sediment and sedimentation rate is also considered

two dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large scale deformation 1 ageostrophic secondary circulation asc in frontal data sets have been previously diagnosed using only adiabatic versions of omega equations this study includes the effect of vertical mixing on such diagnostics modified quasi geostrophic and semigeostrophic balance equations are examined for the existing azores front data sets and idealized fronts in the presence of diabatic vertical mixing as well as a large scale deformation field the relevant dimensionless parameter representing the relative importance of large scale deformation and vertical mixing is the inverse of the ekman number based on the deformation rate h2 ao where is large scale deformation rate h is vertical scale of the front and ao is the maximum vertical eddy viscosity when this parameter is smaller than o 102 the influence of vertical mixing may be important other relevant parameters include burger number bu and prandtl number pr the vertical velocity in ascs caused by diabatic mixing in all diagnostics solutions except for large bu cases shows good agreement with the garrett loder scaling the structure of ascs is found to be sensitive to pr the diagnostics of ascs for the azores front data set indicates that the vertical mixing intensifies the asc in the upper ocean 100 m and should not be neglected when the large scale deformation rate is o 10 6 s 1 or smaller the magnitude of vertical velocity in ascs due to mixing contributions reaches o 10 m d 1 with downwelling on the dense side and upwelling on the less dense side of azores front

millennial scale climate change and oceanic processes in the late pliocene and early pleistocene we generated 200 500 year resolution records of oceanic processes in the north atlantic ocean drilling program site 983 60 24 n 23 38 w 1983 meters water depth for intervals in the latest pliocene 1 86 1 93 ma and the earliest pleistocene 1 75 1 83 ma in order to examine the linkages between millennial scale variations in the ocean and background glacial interglacial climate change within glacial intervals we find evidence for variations similar to those observed in the late pleistocene we find discrete ice rafted debris ird events that reoccur every 2 5 kyr these events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters the timing of ird events in the late pliocene and early pleistocene intervals is similar to that of dansgaard oeschger cycles but we find no ird events comparable in timing to late pleistocene heinrich events although interglacial intervals are much more stable we do find evidence for low amplitude variations in deep water properties that reoccur every 2 kyr within interglacial intervals the similarity between our late pliocene early pleistocene records and late pleistocene records implies that the mechanism driving millennial scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late pleistocene

development and application of a pollen based paleohydrologic reconstruction from the lower roanoke river basin north carolina usa we used pollen assemblages to reconstruct late holocene paleohydrologic patterns in floodplain deposits from the lower roanoke river basin north carolina southeastern usa using 120 surface samples from 38 transects we documented statistical relationships between pollen assemblages vegetation and landforms backswamp pollen assemblages long hydroperiods are dominated by nyssa tupelo and taxodium cypress and have high pollen concentrations sediments from elevated levees and seasonally flooded forests shorter hydroperiods are characterized by dominant pinus pine pollen variable abundance of hardwood taxa and low pollen concentrations we apply the calibration data set to interpret past vegetation and paleohydrology pollen from a radiocarbon dated sediment core collected in a tupelo cypress backswamp indicates centennial scale fluctuations in forest composition during the last 2400 years backswamp vegetation has occupied the site since land clearance began 300 years ago recent dam empl

spatial and temporal variability in sedimentation rates associated with cutoff channel infill deposits ain river france floodplain development is associated with lateral accretion along stable channel geometry along shifting rivers the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence which exhibit specific overflow patterns in this contribution the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river ain river france the sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137cs and unsupported 210pb sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain sedimentation rates range between 0 65 and 2 4 cm a1 over a period of 10 to 40 years the data provide additional information on the role of distance from the bank overbank flow frequency and channel geometry in controlling the sedimentation rate channel infills lower than adjacent floodplains exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain additionally channel degradation aggradation and bank erosion which reduce or increase the distance between the main channel and the cutoff channel aquatic zone affect local overbank flow magnitude and frequency and therefore sedimentation rates thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills last the dendrogeomorphic and 137cs approaches are cross validated for estimating the sedimentation rate within a channel infill

patterns of recent sediment accumulation in chesapeake bay virginia maryland u s a tributaries abstract sediment accumulation rates obtained from 39 cores in 10 tributaries of the western shore of chesapeake bay average 0 30 cm yr 1 since european settlement compared with pre settlement rates in three tributaries of 0 14 cm yr 1 drainage areas of six of the tributaries extend from the coastal plain into the piedmont and four are restricted to the coastal plain rates are highly variable but higher in the upper parts of the tributaries and lower at the mouths during the period of early agriculture when pre settlement rates are based on unpublished reports of 14c dates ranging from 670 to 5330 yr b p of peat layers from three western shore tributaries where sediment deposition is believed to have been continuous post settlement rates are average rates calculated between a maximum of five pollen horizons the pollen horizons represent historically documented changes in the regional vegetation and include initial land clearance 1634 1650 1720 and 1730 depending on the tributary shift to intensive agriculture 1780 and 1840 beginning of the chestnut blight 1910 demise of chestnut 1930 and the beginning of large scale urbanization 1960

sedimentation patterns across a coastal plain floodplain the importance of hydrogeomorphic influences and cross floodplain connectivity abstract the floodplains of large coastal plain rivers in the southeastern u s are important long term storage sites for alluvial sediment and nutrients yet considerable uncertainty surrounds sediment dynamics on many large river floodplains and in particular the local scale factors that affect the flux of sediment and nutrients from rivers onto their floodplains and their subsequent deposition this research quantifies short term rates of sediment deposition from 2012 to 2014 on floodplain sites at congaree national park using feldspar pads sediment deposition rates ranged from 0 1 to 15 6 cm median 1 46 cm and were closely associated with inundation frequency and geomorphic position cross floodplain distributary channels served as particularly important conduits for moving sediment onto the floodplain physical and chemical analyses of soil samples demonstrated that the most flood exposed sites had higher major nutrient and micronutrient levels especially of phosphorus and more diverse nutrient compositions this research advances current understandings of lateral floodplain connectivity by demonstrating the complex effects of regional hydrology and local floodplain environmental characteristics on the supply of sediment and nutrients

geomorphic adjustment to hydrologic modifications along a meandering river implications for surface flooding on a floodplain abstract responses of large regulated rivers to contemporary changes in base level are not well understood we used field measurements and historical analysis of air photos and topographic maps to identify geomorphic trends of the lower white river arkansas usa in the 70 years following base level lowering at its confluence with the mississippi river and concurrent with flood control by dams incision was identified below a knickpoint area upstream of st charles ar and increases over the lowermost 90 km of the study site to 2 m near the confluence with the mississippi river mean bankfull width increased by 30 m 21 from 1930 to 2010 bank widening appears to be the result of flow regulation above the incision knickpoint and concomitant with incision below the knickpoint hydraulic modeling indicated that geomorphic adjustments likely reduced flooding by 58 during frequent floods in the incised lowermost floodplain affected by backwater flooding from the mississippi river and by 22 above the knickpoint area dominance of backwater flooding in the incised reach indicates that incision is more important than flood control on the lower white river in altering flooding and also suggests that the mississippi river may be the dominant control in shaping the lower floodplain overall results highlight the complex geomorphic adjustment in large river floodplain systems in response to anthropogenic modifications and their implications including reduced river floodplain connectivity

sedimentation in the riparian zone of an incising river this paper focuses on one aspect of riparian zone change associated with channel incision it presents information on sedimentation within the riparian zone of the river garonne france derived from both historical and contemporary sources the riparian zone investigated is morphologically complex containing a remnant island and cut off channel information drawn from historical and contemporary sources permits the reconstruction of channel planform and cross profile change as well as an assessment of changes in riparian zone sedimentation rates and patterns very significant changes are identified over the last 50 years a change from a multiple thread to single thread channel planform high rates of riparian zone sedimentation of the order of 0 5 to 2 5 cm a 1 clear spatial patterns in over bank sedimentation reflecting topographic and vegetational variations and flood event magnitude and a reduction in morphological and vegetational diversity through the period which is expected to continue as the zone is transformed into an increasingly homogeneous sediment sink copyright 2001 john wiley sons ltd

fluvial processes and vegetation glimpses of the past the present and perhaps the future abstract most research before 1960 into interactions among fluvial processes resulting landforms and vegetation was descriptive since then however research has become more detailed and quantitative permitting numerical modeling and applications including agricultural erosion abatement and rehabilitation of altered bottomlands although progress was largely observational the empiricism increasingly yielded to objective recognition of how vegetation interacts with and influences geomorphic process a review of advances relating fluvial processes and vegetation during the last 50 years centers on hydrologic reconstructions from tree rings plant indicators of flow and flood frequency parameters hydrologic controls on plant species regulation of sediment movement by vegetation vegetative controls on mass movement and relations between plant cover and sediment movement extension of present studies of vegetation as a regulator of bottomland hydrologic and geomorphic processes may become markedly more sophisticated and widespread than at present research emphases that are likely to continue include vegetative considerations for erosion modeling response of riparian zone forests to disturbance such as dams and water diversion the effect of vegetation on channel and bottomland dynamics and rehabilitation of stream corridors research topics that presently are receiving attention are the effect of woody vegetation on the roughness of stream corridors and hence processes of flood conveyance and flood plain sedimentation the development of a theoretical basis for rehabilitation projects as opposed to fully empirical approaches the effect of invasive plant species on the dynamics of bottomland vegetation the quantification of below surface biomass and related soil stability factors for use in erosion prediction models and the effect of impoundments on downstream narrowing of channels and accompanying encroachment of vegetation bottomland vegetation partially controls and is controlled by fluvial geomorphic processes the purposes of this paper are to identify and review investigations that have related vegetation to bottomland features and processes to distinguish the present status of these investigations and to anticipate future research into how hydrologic and fluvial geomorphic processes of bottomlands interact with vegetation

temporal and spatial patterns of sedimentation within the batture lands of the middle mississippi river usa abstract sediment deposition and storage are important functions of batture lands the land between the channel s low water elevation and the flood mitigation levee however sedimentation processes within these areas are not fully understood in this paper we explore the spatiotemporal patterns rates and volume of sedimentation within the batture lands along the middle mississippi river mmr between the confluence of the missouri and ohio rivers using three approaches 1 comparison of historical to modern elevation data in order to estimate long term 100 yr sedimentation rates 2 estimation of medium to short term 70 attributed largely to sediment trapping by dams during the second half of the twentieth century the increase in mmr batture land sedimentation rates are attributed to at least two potential mechanisms 1 the above average frequency and duration of low magnitude floods 2 yr and 5 yr flood during the short term assessment periods which allowed for more suspended sediment to be deposited within the batture lands and 2 the construction of levees that substantially reduced the floodplain area 75 available for storage of overbank deposits increasing the vertical accumulation and consequently the detectability of a given volume of sediment the gcd estimated batture land sediment volumes were 9 0 of the suspended load at st louis this substantial storage of sediment 8 5 mt yr 1 along the mmr suggests batture lands are an important sink for suspended sediments

soil property trends and classification of alluvial floodplains south carolina coastal plain abstract alluvial soils derived from sediments of piedmont origin occur in the atlantic coastal plain of the southeastern u s in south carolina high order rivers receiving sediment are savannah sv congaree cn wateree wa santee st lynches ly and great pee dee pd this study investigated distribution of soil properties on natural levee valley flat and backswamp landforms within and among the six river floodplains and compared results to the taxonomic classifications used historically during soil survey production this study also investigated the contribution of sediment sources to soil characteristics data were compiled for 65 soil pedons sampled from 16 soil survey areas during a 41 year time period pedons were located in three major land resource areas mlras and the results should aid in development of both land resource area and ecological site concepts data analyses centered on clay and silt contents mineralogy base saturation bs and cation exchange activity cea classes evaluation of data from the six river floodplains revealed common properties among soils on fluvial landscapes across multiple mlras four soil systems each derived from a predominant rock type composed the piedmont drainage basin of each river the absolute area of each soil system differed between rivers but sediment supplied typically did not result in great soil texture variation per landform the land area proportion of the carolina slate belt soil system had a close relationship to silt contents of the alluvial soils on the valley flat landform of five floodplains particle size control section pscs median clay content ranged only between 302 g kg 1 and 405 g kg 1 among all valley flats median pscs silt content ranged between 361 g kg 1 and 511 g kg 1 slight positive downriver trends for clay were observed on sv wa and pd and for silt on sv and wa although local bs variation could be high between pedons slight positive downriver bs trends on valley flats were observed on sv wa and st negative bs trends were seen on ly and pd kaolinite was the dominant clay mineral in soils of all landforms on the six floodplains weatherable primary minerals in sand and silt fractions resulted in higher bs except for the ly soils where resistant minerals composed sand and silt fractions the semiactive cea class occurred predominantly on all three landforms taxonomic analysis supported soil assignment to the suborders aquepts or udepts on all floodplains great group level eutrudepts in south carolina are first reported here and they occur extensively enough on sv cn wa and pd to warrant recognition in soil map units because most soil properties showed little variability for soils occurring in floodplains for all six rivers botanical assessments will be needed to determine the magnitude of the role the properties have toward defining ecological sites ultimately the alluvial systems may be aggregated as a single land resource area within the coastal plain

correlation of the magnetic susceptibility stratigraphy of chinese loess and the marine oxygen isotope record chronological and palaeoclimatic implications abstract the thick deposits of wind blown silt of the loess plateau in central china constitute arguably the most important terrestrial record of the late pliocene and pleistocene climate magnetic susceptibility stratigraphy has proven extremely useful for quantifying the palaeoclimatic record of chinese loess with intervals of high magnetic susceptibility corresponding to warm humid interglacials and intervals of low susceptibility to cooler drier glacials several workers have noted that the magnetic susceptibility stratigraphy strongly resembles the marine oxygen isotope record for which a high resolution chronology is available from tuning to the frequencies of earth orbital parameters attempts have been made to correlate the two records statistically and we use this approach to generate a new magnetic susceptibility time series for the central chinese loess plateau our results suggest that the two records are sufficiently similar over the interval ca 0 1 5 my bp for this approach to yield a useful age model for loess accumulation spectral analysis of the resulting magnetic susceptibility time series clearly shows that the growth of 100 ky variance which occurs in the marine oxygen isotope record after about 1 my bp also occurs synchronously in the magnetic susceptibility record reinforcing recent suggestions of a strong linkage between the high latitude northern hemisphere and the asian climate we also show that a significant improvement in the strength of the loess magnetic susceptibility marine oxygen isotope correlation occurs after about 1 5 my bp which corresponds to a significant increase in the rate of sediment accumulation across the loess plateau

asynchronous holocene optimum of the east asian monsoon abstract the spatial and temporal distribution of summer monsoon precipitation or effective moisture during the holocene has been reconstructed on the basis of geological data including lake levels pollen profiles and loess paleosol records in addition the summer july precipitation increment effective precipitation and monsoon strength index have been obtained from numerical modeling experiments both geological data and numerical modeling indicate that the holocene optimum as defined by peak east asian summer monsoon precipitation was asynchronous in central and eastern china reaching a maximum at different times in different regions e g ca 10 000 8000 yr ago in northeastern china 10 000 7000 yr ago in north central and northern east central china ca 7000 5000 yr ago in the middle and lower reaches of the yangtze river and ca 3000 yr ago in southern china in southwestern china the maximum appeared ca 11 000 yr ago but probably was related to the maximum landward extension of the indian summer monsoon the regional shift in the maximum precipitation belt from northwest to southeast over the past 10 000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate the southeastward shift of the east asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the holocene and the east asian monsoon index declined while the early maximum in southwestern china matches the maximum of the indian monsoon index

sediment fabrics oxygenation history and circulation modes of japan sea during the late quaternary abstract the quaternary hemipelagic sediments of the japan sea are characterized by centimeter to decimeter scale alternations of heavily bioturbated organic poor light layers and finely laminated organic rich dark layers these dark and light layers which can be correlated throughout the deeper part of the basin are considered to reflect changes in bottom water oxygenation however depositional mechanisms responsible for individual dark layers and causes of oxygen depletion are not fully understood two possible settings for deposition of organic rich dark layers are recognized one is a stratified ocean like the black sea where limited vertical advection results in euxinic conditions in the deeper parts of the basin the other is an upwelling region like the arabian sea where the oxygen minimum zone is intensified by enhanced nutrient upwelling and surface productivity in this study fabrics of hemipelagic sediments deposited in the japan sea during the last 160 ka were studied in three cores collected from different latitudes and water depths fabrics were classified into 4 classes based on corresponding bottom water oxygenation level five dark layer types were defined based on the distribution patterns of sediment fabric classes within each dark layer comparison of dark layer types at shallower and deeper sites in a chronologic framework provided evidence for five different modes of deep water circulation these circulation modes which reflect relative intensities of upwelling and density stratification appear to be related to global sea level changes as well as climate impact on the nature of water influx through the tsushima strait

spreading of levantine intermediate waters by submesoscale coherent vortices in the northwestern mediterranean sea as observed with gliders since 2007 gliders have been regularly deployed in the northwestern mediterranean sea a crucial region regarding the thermohaline circulation of the mediterranean sea it revealed for the first time very warm 10 48c and saline 10 1 submesoscale anticyclones at intermediate depth characterized by a small radius 5 km high rossby 0 3 and burger 0 7 numbers they are likely order of 10 to be formed each year have a life time order a year and certainly contribute significantly to the spreading of the levantine intermediate waters liw toward the whole subbasin thus potentially impacting wintertime vertical mixing through hydrographical and dynamical preconditioning they could be mainly formed by the combined action of turbulent mixing and flow detachment of the northward flow of liw at the northwestern headland of sardinia upwelling conditions along the western coast of sardinia associated with a southward geostrophic flow within the upper layers seem to play a key role in their formation process

neogene sapropels in the mediterranean a review for 50 years the existence of sapropels organic carbon rich sediments deposited within plio pleistocene sediments of the mediterranean sea has been known initially research concentrated on material recovered in relatively short gravity piston cores taken from the eastern basins where sequences were found to be well developed preserved and had extensive spatial coverage in the main previous studies concentrated upon establishing a workable stratigraphy spatial correlation of individual layers and determining the probable depositional mechanisms however despite a plethora of research papers some issues still remain unresolved this is in part due to a lack of agreement between investigators sampling and analytical short comings restricted sample size and the fact that in many instances like was not being compared with like recently the limit of sapropels in the western basin has been further extended as a result the palaeoceanographic palaeoclimate models which had previously been developed for deposition of sapropels in the eastern basin have been modified most recently strong links have been established between astronomical cyclicity and sapropel formation this review paper provides a summary of sapropel research to date and ongoing sapropel research in the mediterranean some of which appears in this thematic issue of marine geology it is fitting that this thematic issue of marine geology be dedicated to the memory of colette vergnaud grazzini and rob kidd who in many ways helped to initiate the resurgence in sapropel studies in the 1970s in the mediterranean perhaps in 50 more years we will know all of the answers 1999 elsevier science b v all rights reserved

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

sub surface small scale eddy dynamics from multi sensor observations and modeling the study of mesoscale and submesoscale hereafter sub mesoscale hydrodynamic features is essential for understanding thermal and biogeochemical exchanges between coastal areas and the open ocean in this context a glider mission was conducted in august 2008 closely co located and almost simultaneously launched with a jason 2 altimetric pass to fully characterize the currents associated with regional sub mesoscale processes regularly observed to the north of mallorca mediterranean sea a synoptic view from satellite remote sensing fields before and during the glider mission provided a descriptive picture of the main surface dynamics at the balearic basin scale to quantify the absolute surface geostrophic currents the coastal altimetry derived current computation was improved and cross compared with its equivalent derived from glider measurements model simulations were then validated both qualitatively and statistically with the multi sensor observations the combined use of modeling and multi sensor observational data reveals the baroclinic structure of the balearic current and the northern current and a small scale anticyclonic eddy observed northeast of the mallorca coast current 15 cm s 180 m deep this mesoscale structure partially intercepted by the glider and along track altimetric measurements is marked by relatively strong salinity gradients and not as is more typical temperature gradients finally the use of the validated model simulation also shows that the geostrophic component of this small scale eddy is controlled by sub surface salinity gradients we hypothesize that this structure contains recently modified atlantic water arriving from the strait of ibiza due to a northerly wind which strengthens the northward geostrophic circulation

seasonality and interannual variability of the westerly jet in the tibetan plateau region abstract in this study 40 yr ecmwf re analysis era 40 data are used for the description of the seasonal cycle and the interannual variability of the westerly jet in the tibetan plateau region to complement results based on the analysis of monthly mean horizontal wind speeds an occurrence based jet climatology is constructed by identifying the locations of the jet axes at 6 hourly intervals throughout 1958 2001 thus a dataset describing the highly transient and localized features of jet variability is obtained during winter and summer the westerly jet is located respectively to the south and north of the tibetan plateau during the spring and autumn seasons there are jet transitions from south to north and vice versa the median dates for these transitions are 28 april and 12 october the spring transition is associated with large interannual variations while the fall transition occurs more reliably within a 3 week period the strength of the jet exhibits a peculiar seasonal cycle during northw

floodplain sedimentation in the upper mississippi valley natural versus human accelerated abstract understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems here 14c and 137cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper mississippi river and adjacent tributaries in southwestern wisconsin and northwestern illinois the most important environmental change that influenced fluvial activity in this region during last 10 000 years involved the conversion of a late holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with euro american settlement results presented herein for the upper mississippi valley umv show that the shift from pre agriculture natural land cover to landscape dominance by agricultural land use of the last 175 200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude accelerated overbank flooding led to increased bank heights on tributary streams and in turn contributed to more frequent deep flows of high energy these high energy flows subsequently promoted bank erosion and lateral channel migration and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain the new historical floodplain serves as a flume like channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods floodplains on lower tributaries however continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950 this lower valley anomaly is explained by minimal development of historical agriculture period meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient in general long term pre agriculture rates of vertical accretion between about 10 000 and 200 years ago averaged about 0 2 mm yr 1 in tributary watersheds smaller than about 700 km2 and about 0 9 mm yr 1 on the floodplain of the upper mississippi river where the contributing watershed area increases to about 170 000 km2 on the other hand rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr 1 with short episodes of even higher rates during times of particularly poor land conservation practices significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid 19th century the hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture high resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper mississippi river the relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper mississippi river should experience increased frequencies of extreme hydrologic events including large floods with anticipated continued global warming instrumental and stratigraphic records show that coincident with global warming a shift to more frequent large floods occurred since 1950 on the upper mississippi river and these floods generally contributed high magnitudes of floodplain sedimentation

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

ocean processes underlying surface clustering ageostrophic ocean processes such as frontogenesis submesoscale mixed layer instabilities shelf break fronts and topographic interactions on the continental shelf produce surface divergent flows that affect buoyant material over time this study examines the ocean processes leading to clustering i e the increase of material density over time on the ocean surface the time series of divergence along a material trajectory the lagrangian divergence ld is the flow property driving clustering to understand the impacts of various ocean processes on ld numerical ocean model simulations at different resolutions are analyzed although the relevant processes differ patterns in clustering evolution from the deep ocean and the continental shelf bear similarities smaller scale ocean features are associated with stronger surface divergence and the surface material clustering is initially dominated by these features over time the effect of these small scale features becomes bounded as material traverses small scale regions of both positive and negative divergence lower frequency flow phenomena however continue the clustering as a result clustering evolves from initial small scale to larger scale patterns

evidence for enhanced mediterranean thermohaline circulation during rapid climatic coolings molecular biomarkers c37 alkenones n nonacosane and n hexacosanol and toc are used together with benthic 18o and 13c data to document the hydrographic response of the western mediterranean sea to rapid climatic variability these proxies are recorded in core md 95 2043 alboran sea affording the study of the dansgaard oeschger d o and heinrich he variability during the last glacial period the results suggest that rapid changes in the western mediterranean thermohaline circulation occurred in parallel to sea surface temperature oscillations enhanced deep water ventilation occurred during cold intervals he and d o stadials probably driven by a strengthening of north westerly wind over the north western mediterranean sea in contrast decreased intensity of the thermohaline circulation is detected during warm intervals d o interstadials which led to low oxygenated deep water masses and better preservation of the organic matter in the sediment

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

high frequency climate linkages between the north atlantic and the mediterranean during marine oxygen isotope stage 100 mis100 high resolution records of mediterranean and north atlantic deep sea sediments indicate that rapid changes in hydrology and climate occurred during marine oxygen isotope stage 100 mis100 at 2 52 ma which exhibits characteristics similar to late pleistocene dansgaard oeschger bond cycles and heinrich events as in the late pleistocene north atlantic oceanographic and atmospheric changes were probably transmitted into the mediterranean by some combination of surface water inflow and atlantic mediterranean atmospheric pressure gradients our data suggest that the mechanism s responsible for sub milankovitch and millennial scale climate oscillations were the same over at least the past 2 6 ma notwithstanding the transition from the 41 kyr dominated glacial cycles of the late pliocene to the 100 kyr cycles of the late pleistocene

paleoceanographic evolution of the japan sea abstract multidisciplinary studies on nearly continuous hemipelagic sequences recovered from the deeper parts of the japan sea during odp leg 127 128 in 1989 yielded new information on the paleoceanographic conditions of the sea since its establishment as a deep marginal basin of significant size approximately at 20 ma by synthesizing these new information on surface and bottom water conditions ccd biogenic silica accumulation and paleogeog raphy it is possible to reconstruct changes in paleo ocean circulation within the sea it is also possible to specify the ultimate cause of such changes by comparing their timing and nature with those of global paleoceanographic as well as local tectonic events the result of our synthesis suggests that the japan sea has been responding to the high frequency changes of global surface conditions in very sensitive manner throughout its history however the mode of its response changed from time to time the changes in response mode have been strongly influenced by such factors as sill depth position of channels and the degree of closure of the sea which were controlled either by local tectonism or global eustatic sea level changes

orbital and millennial scale variations in asian dust transport path to the japan sea abstract orbital and millennial scale variations in the asian monsoon and its probable association with the dansgaard oeschger d o cycles have been demonstrated by previous studies however the origin and nature of such variations are poorly understood the japan sea is located down wind of the arid areas of the eurasian continent and thus receives significant amounts of aeolian dust consequently the hemipelagic sediments of the japan sea are expected to record a continuous aeolian dust accumulation which may provide information about the past variations in asian monsoon grain size flux and provenance of aeolian dust in the sediments were examined using two sediment cores obtained from the northern and southern parts of the japan sea in order to understand the spatial variability of asian monsoon since the silt fraction of the detrital materials in hemipelagic sediments of the japan sea is composed predominantly of aeolian dust the grain size flux and provenance of the silt fraction were examined the taklimakan desert loess plateau and siberia northeast china areas were identified as possible source areas of aeolian dust in the sediments of the japan sea based on the electron spin resonance esr signal intensity and crystallinity of quartz moreover the relative contribution of aeolian dust from each source area was estimated based on the esr signal intensity and crystallinity of quartz aeolian dust fluxes from the two source areas at the southern site were then estimated grain size and flux of eolian dust at the northern site and fluxes of aeolian dust from two source areas at the southern site show orbital scale variations in harmony with the insolation change at 30 n in june these results indicate orbital scale changes in the extent of the arid area and or frequency of dust storms in the taklimakan desert loess plateau and siberia northeast china areas and suggest that southward northward shifts of the westerly jet axis occurred together with either intensified weakened winter monsoon decreased increased winter precipitation in the siberia northeast china area or southeastward northwestward shifts of summer monsoon limit during periods of smaller larger insolation at 30 n in june in addition to these orbital scale variations millennial scale variations possibly associated with the d o cycles are observed in the grain size record of aeolian dust at the two sites based on analogy with orbital scale variations these millennial scale variations are considered to reflect millennial scale changes in the position of the westerly jet axis together with the changes either in winter monsoon intensity winter precipitation or position of summer monsoon limit our results suggest n s oscillations in the westerly jet and summer monsoon limit and changes in the intensity of the winter monsoon on orbital and millennial timescales

millennial scale oscillations of the westerly jet path during the last glacial period abstract it has been established that east asian summer monsoon intensity varies with dansgaard oeschger cycles suggesting a connection between the climates of east asia and the north atlantic on a millennial timescale however the dynamics of such a connection are still unsolved here we demonstrate that temporal changes in the provenance of aeolian dust in japan sea sediments which we interpret to reflect changes in the westerly jet path over east asia exhibit variations in harmony with dansgaard oeschger cycles the dominance of dust with a mongolian gobi provenance during stadials suggests a westerly jet axis located to the south of the himalaya tibetan plateau throughout most of the year whereas the co dominance of dust from both the taklimakan desert and the mongolian gobi during interstadials suggests that the westerly jet axis jumped to the north of the tibetan plateau at latest in summer as the shift of the westerly jet axis to the north of the tibetan plateau is closely related to the onset of the east asian summer monsoon changes of the westerly jet path apparently critically affect the teleconnection between the climates of asia and north atlantic on a millennial timescale

delivery of upper basin sediment to the lower neuse river north carolina u s a extensive storage of upper basin piedmont sediment and apparent low sediment supply to streams in lower basin coastal plain areas generates questions as to the source of alluvium in lower reaches of rivers of the u s atlantic drainage this was investigated on the neuse river north carolina using a mineralogical indicator of sediment source areas the utility of mica flakes for discriminating between piedmont and non piedmont sources of sediment in the lower coastal plain reaches of the neuse was established on the basis of an examination of the u s national soils database and of 26 soil surveys of the north carolina coastal plain from the neuse river estuary to 48 km upstream there are no mica flakes in floodplain soils or in river bank and channel shelf sediments mica flakes become more common upstream this suggests that a very small proportion of the sediment eroded in the piedmont portion of the watershed is delivered to the river mouth the small amounts which presumably do reach the lower coastal plain are so diluted by coastal plain derived alluvium that no piedmont origin can be discerned this demonstrates a dominantly coastal plain source and underscores the importance of storage and discontinuous transport in fluvial sediment systems more importantly results suggest that upper and lower basin sediment dynamics are not only non linearly related but may be virtually decoupled

late holocene climate and ecosystem history from chesapeake bay sediment cores usa palaeoclimate records from late holocene sediments in chesapeake bay the largest estuary in the usa provide evidence that both decadal to centennial climate variability and european colonization

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

large scale dynamics of the meiyu baiu rainband environmental forcing by the westerly jet abstract meiyu baiu is the major rainy season from central china to japan brought by a zonally elongated rainband from june to mid july large scale characteristics and environmental forcing of this important phenomenon are investigated based on a reanalysis dataset the meiyu baiu rainband is accompanied by a trough of sea level pressure horizontal shears and sharp moisture gradients near the surface a westerly jet tilted northward with height and large northeastward moisture transport from the south the analysis here reveals the westerly jet as an important culprit for meiyu baiu along the rainband mean ascending motion corresponds well with a band of warm horizontal temperature advection in the midtroposphere throughout summer this adiabatic induction of upward motion originates from the advection of warm air by the westerlies from the eastern flank of the tibetan plateau the ascending motion both induces convection and is enhanced by the resultant condensational heating the westerly jet anch

land ocean linkages over orbital and millennial timescales recorded in late quaternary sediments of the japan sea late quaternary sediments of the japan sea are characterized by centimeter to meter scale alternations of dark and light layers which are synchronous basinwide high resolution analyses of the sediments from ocean drilling program site 797 reveal that deposition of the meter scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the tsushima strait the centimeter to decimeter scale alternations reflect millennial scale variations which are possibly associated with dansgaard oeschger d o cycles with each dark layer appearing to correspond to an interstadial this variability is attributed to the development of a humid climate in central to eastern asia and the consequent increase in discharge from the huanghe and changjiang rivers during interstadials this caused expansion of the east china sea coastal water ecscw which penetrated more strongly into the japan sea the increased influence of the lower salinity nutrient enriched ecscw reduced deep water ventilation and enhanced the surface productivity leading to the development of anoxic bottom waters and deposition of the dark layers thus the centimeter to decimeter scale alternations of the dark and light layers record wet and dry cycles in central to eastern asia possibly associated with d o cycles

the tsushima warm current through tsushima straits estimated from ferryboat adcp data abstract current structures across the tsushima straits are studied using results from long term acoustic doppler current profiler adcp observations by a ferryboat between hakata and pusan conducted since february 1997 two maxima of the northeastward current are observed in the central parts of the eastern and western channels and the maximum velocity in the western channel is stronger than that of the eastern channel downstream of the tsushima islands a southwestward countercurrent is observed associated with a pair of cyclonic and anticyclonic eddies in the western channel the deep countercurrent is observed pronouncedly on the bottom slope of the korean side from summer to winter the volume transport of the tsushima warm current through the straits has strong seasonal variation with a minimum in january and two maxima from spring to autumn double peaks the spring peak of the volume transport through the eastern channel is more pronounced than the autumn peak and the autumn peak of the west

saharan dust transport and high latitude glacial climatic variability the alboran sea record millennial to submillennial marine oscillations that are linked with the north atlantic s heinrich events and dansgaard oeschger cycles have been reported recently from the alboran sea revealing a close ocean atmosphere coupling in the mediterranean region we present a high resolution record of lithogenic fraction variability along images core md 95 2043 from the alboran sea that we use to infer fluctuations of fluvial and eolian inputs to the core site during periods of rapid climate change between 28 000 and 48 000 cal yr b p comparison with geochemical and pollen records from the same core enables end member compositions to be determined and to document fluctuations of fluvial and eolian inputs on millennial and faster timescales our data document increases in northward saharan dust transports during periods of strengthened atmospheric circulation in high northern latitudes from this we derive two atmospheric scenarios which are linked with the intensity of meridional atmospheric pressure gradients in the north atlantic region

patterns of floodplain sediment deposition along the regulated lower roanoke river north carolina annual decadal centennial scales abstract the lower roanoke river on the coastal plain of north carolina is not embayed and maintains a floodplain that is among the largest on the mid atlantic coast this floodplain has been impacted by substantial aggradation in response to upstream colonial and post colonial agriculture between the mid eighteenth and mid nineteenth centuries additionally since the mid twentieth century stream flow has been regulated by a series of high dams we used artificial markers clay pads tree ring dendrogeomorphic techniques and pollen analyses to document sedimentation rates amounts over short intermediate and long term temporal scales respectively these analyses occurred along 58 transects at 378 stations throughout the lower river floodplain from near the fall line to the albemarle sound present sediment deposition rates ranged from 0 5 to 3 4 mm y and 0 3 to 5 9 mm y from clay pad and dendrogeomorphic analyses respectively deposition rates systematically increased from upstream high banks and floodplain to downstream low banks reaches except the lowest reaches conversely legacy sediment deposition a d 1725 to 1850 ranged from 5 to about 40 mm y downstream to upstream respectively and is apparently responsible for high banks upstream and large wide levees along some of the middle stream reaches dam operations have selectively reduced levee deposition while facilitating continued backswamp deposition a gis based model predicts 453 000 mg of sediment is trapped annually on the floodplain and that little watershed derived sediment reaches the albemarle sound nearly all sediment in transport and deposited is derived from the channel bed and banks legacy deposits sources and regulated discharges affect most aspects of present fluvial sedimentation dynamics the lower river reflects complex relaxation conditions following both major human alterations yet continues to provide the ecosystem service of sediment trapping