the relationship of sea surface temperature to thermocline depth at annual and interannual time scales in the tropical atlantic ocean expendable bathythermograph data derived from sequal focal and toga sponsored volunteer observing ship programs in the tropical atlantic ocean are used to study the relationship between the sea surface temperature sst and the underlying thermocline time series up to 8 years long allow investigation of both the annual and interannual variability between 2n and 2s along selected shipping lanes for the annual harmonic a warm sst is correlated with a shallow thermocline everywhere except in the eastern equatorial region near the intertropical convergence zone itcz the 180 phase shift in the annual harmonic of both the sst and the thermocline depth occurs at slightly different latitudes over most of the tropical atlantic the annual cycle of sst is linked to the phase of the net surface heat flux and only in the eastern equatorial and coastal upwelling regions does heat flux across the thermocline become a significant factor for the annual cycle of the sst canonical correlation analysis of the interannual variability reveal two distinct modes with structures similar to the annual cycle the first in which a warm sst is correlated with a shallow thermocline is a dipole with centers of maximum amplitude off the equator at approximately 8s at 12n and a node at the itcz the second mode for which a warm sst is correlated with a deep thermocline peaks at the equator the predominance of the first mode suggests that for most of the tropical atlantic ocean the dynamical state beneath the surface mixed layer has only a limited influence on sst fluctuations

mg ca ratios of single planktonic foraminifer shells and the potential to reconstruct the thermal seasonality of the water column mg ca ratios of surface and subsurface dwelling foraminifera provide valuable information about the past temperature of the water column planktonic foraminifera calcify over a period of weeks to months therefore the range of mg ca temperatures obtained from single specimens potentially records seasonal temperature changes we present solution derived mg ca ratios for single specimens of the planktonic foraminifera species globigerinoides ruber pink globigerinoides ruber white and globorotalia inflata from a sediment trap off northwest africa 20 45 6 n 18 41 9 w cleaning of single specimens was achieved using a flow through system in order to prevent sample loss mg ca ratios of surface dwelling g ruber pink show strong seasonality linked to sea surface temperature mg ca ratios of g ruber white do not show such seasonality subsurface dwelling g inflata flux is largest during the main upwelling season but mg ca ratios reflect annual temperatures at intermediate water depths the sediment trap time series suggests that changes in the range of mg ca ratios exhibited by single specimens of g ruber pink and g inflata from the sedimentary record should provide information on the past temperature range under which these species calcified statistical analysis suggests detectable changes in the mg ca range are 0 80 mmol mol g ruber pink and 0 34 mmol mol g inflata for g ruber pink such changes would indicate changes in the seasonal sea surface temperature range 4 c or a shift in the main calcification and reproductive period for g inflata such changes would indicate 1 7 c changes in the thermocline temperature or a change in the depth habitat

nutrient irrigation of the north atlantic abstract the north atlantic as all major oceans has a remarkable duality in primary production manifested by the existence of well defined high and low mean primary production regions the largest region is the north atlantic subtropical gyre nastg an anticyclone characterized by bowl shaped isopycnals and low production the nastg is surrounded at its margins by smaller cyclonic high production regions where these isopycnals approach the sea surface the most extensive cyclonic regions are those at the latitudinal extremes i e the subpolar and tropical oceans though smaller ones do occur at the zonal boundaries in this article we review historical data and present new analyses of climatological data and a selected number of hydrographic cruises in the western northwestern and eastern southeastern boundaries of the nastg with the objective of investigating the importance of upward epipycnal advection of nutrient rich subsurface layers irrigation in maintaining high primary production in the euphotic layers in the north atlantic subpolar gyre naspg irrigation implies intergyre exchange caused by the outcropping extension of the gulf stream gs following the formation of the deep winter mixed layer in the eastern boundary of the nastg irrigation is attained through a permanent upwelling cell which feeds the canary upwelling current cuc in the southeastern corner irrigation occurs in fall when the guinea dome gd is reinforced and in winter when the cuc reaches its southernmost extension other characteristics of the north south extension of the gs cuc are the seasonal nutrient replenishing of subsurface layers spring restratification of naspg and winter relaxation of the gd and the maintenance of high levels of diapycnal mixing during the last phase of nutrient transfer to the euphotic layers off the mid atlantic bight the gs transports a total of about 700 kmol s 1 of nitrate with almost 100 kmol s 1 carried in the surface

eddy activity and mixing in upwelling systems a comparative study of northwest africa and california regions a comparative modeling approach is used to study the mesoscale dynamics of coastal transition zones in the upwelling systems of western sahara and central california it is shown that large differences in eddy activity and mixing exist between the two systems since upwelling favorable winds off central california and western sahara have similar amplitudes we suggest that observed and modeled differences in eddy activity are due to differences in configuration parameters it is argued that two of these parameters stratification and topography eventually affect the energy available for the mesoscales through baroclinic instability the large difference in stratification appears to be linked to the salinity structure formed by the large scale circulation and is directly related to available potential energy the shape of the continental shelf affects the position structure and scale of the ekman divergence hence upwelling intensity and frontal formation the potential impact of mesoscale eddy mixing and its variability on upwelling ecosystems is discussed

morphological response of the lower yellow river to the operation of xiaolangdi dam china abstract the morphodynamics of the lower yellow river lyr has been impacted by intense dam induced changes over the past few decades here we present results on the morphological impacts produced on the lyr by the construction and operation of xiaolangdi dam whose construction was completed in 1999 we used multi temporal remotely sensed images and geomorphological parameters and developed a systematic and universal river morphology extraction framework to investigate changes in the planform of the lyr between 1987 and 2017 the morphological changes were assessed by identifying seven sub reaches r1 to r7 along 860 km of the lyr the multichannel upper reaches of the lyr exhibited more marked changes in anabranching and braid channel ratio sinuosity and lateral migration rate than the lower reaches which are characterized by a single channel pattern sandbars in the upper reaches r1 r3 underwent degradation after the beginning of operation of xiaolangdi dam while the reverse was seen in the lower reaches r6 r7 the area of overlap between the 1987 channel mask and the channel mask at different times throughout the study decreased gradually over the study period indicating intense channel migration the operation of xiaolangdi dam prevented the river from running dry but also from suffering floods while extending the duration of the low medium stage flows these actions account for the morphological changes in the lyr despite the fact that effective discharge did not change significantly after the xiaolangdi dam began operating it became compressed into a shorter range of annual discharge variability i e larger low level discharges and shorter rising smaller floods stages our integrated assessment shows a significant amount of water flow energy has been consumed by multiple planform adjustments in the upper reaches of lyr while local siltation is the main issue in the sinuous but laterally stable lower reaches

channel variations of the different channel pattern reaches in the lower yellow river from 1950 to 1999 abstract the yellow river is widely known as a hanging river due to its high sedimentation rate the current channel bed is about 2 74 m higher than the overbank plains on average in the lower reach understanding the variations in channel patterns and their influencing factors is crucial to regulate potential flooding risk in this critical region changes in discharge suspended sediment delivery rate median grain size sedimentation rate channel planform and cross section and channel width depth ratio during 1950 1999 from gauging stations in huayuankou gaocun sunkou and lijin that represent the braided meandering and straight channel sections respectively were compared during this period frequent internal channel variations dominated in the braided channel section the channel bed was continuously raised as a whole but experienced strong erosion in certain times mainly influenced by the operation and closure of the sanmenxia dam the straight channel section was relatively stable due to restricted lateral migration by the solid artificial levees in both banks instead channel aggradation was the dominant process and erosion only happened when channel avulsions occurred in the river mouth the meandering channel section between the upstream braided and downstream straight channel sections was formed as a result of stream self adjustment rapid lateral migrations were observed during this period but the extent of the migration was restricted by artificial levees sediment aggradation on the channel bed and floodplain was apparent since 1970 the channel shrank remarkably in all sections due to the increased use of water resources and the decreased precipitation in the yellow river basin recently migration of the channel is limited and the lower portion of the braided section seems to be a meandering channel

predicting the morphodynamic response of silt laden rivers to water and sediment release from reservoirs lower yellow river china the high sediment load of the yellow river results in rapid infilling of its reservoirs when sediment is not regularly flushed simultaneously the downstream reaches of the yellow river experience extremely high siltation rates which are reduced when sediment is retained in its reservoirs to minimize siltation in the reservoirs and the downstream river bed water and sediment are released from the reservoir in a controlled way through flushing experiments in this paper we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water despite this flushing of sediment the reservoir will eventually fill up and more sediment released again into the lower yellow river the change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower yellow river compared to the predam situation

calcareous dinoflagellate cyst distributions in surface sediments from upwelling areas off nw africa and their relationships with environmental parameters of the upper water column abstract only very few studies have focussed on the spatial distribution and ecology of calcareous dinoflagellate cysts dinocysts in upwelling areas here distributions of individual calcareous dinocyst species in surface sediment samples from the coastal upwelling zone off nw africa and their relationships with known environmental parameters of the sub surface waters have been analysed in order to enhance our knowledge on their modern distribution patterns and to determine the ecological significance and palaeoenvironmental reconstruction potential of each calcareous dinocyst species within this exemplary high productivity region in addition to calculating relative cyst abundances well constrained sedimentation rates have allowed the calculation of dinocyst accumulation rates cysts cm 2 ka 1 for most of the surface sample positions thus providing a much more accurate estimation of actual calcareous dinocyst production in the upper water column than could be warranted in similar studies so far distinct differences in the geographic distribution patterns of nine species were observed in general high accumulation rates of calcareous dinocysts were found around and south of 29 n and offshore the westernmost canary island la palma with slightly lower accumulation rates in the upwelling influenced areas off cape yubi and in the upwelling filament area off cape ghir multivariate ordination techniques were applied in order to compare the cyst accumulation rates of individual species with physical parameters temperature salinity density mixed layer depth and the trophic state nitrate phosphate and chlorophyll concentrations of the overlying sub surface waters all species were found to relate significantly to one or more of the environmental parameters partly confirming previous results on cyst ecology but also providing new information which will be useful for future palaeoenvironmental reconstructions within upwelling areas

planform channel dynamics along the ningxia inner mongolia reaches of the yellow river from 1958 to 2008 analysis using landsat images and topographic maps channel planform dynamics were analyzed for the ningxia inner mongolia reaches of the yellow river map data were analyzed by means of gis and used to determine the planform characteristics and changes in width sinuosity and shoreline migration rates between 1958 and 2008 the study was based on topographic maps of 1958 and 1967 which had been derived from aerial photos and a field survey and from satellite imagery of 1977 1990 2000 and 2008 the channel planform dynamics were determined by comparing sequential changes in the position of the shorelines in these years sinuosity adjustments were small during the study range 1 34 1 45 the initial sinuosity 1 45 in 1958 gradually decreased to 1 34 in 1990 and then increased to 1 40 in 2008 channel contraction had been the dominant planform process but periodic floods resulted in channel expansion 1958 1990 the river s channel area expanded substantially from 1958 to 1990 by 42 1 then sharply contracted from 1990 to 2000 by 45 8 with no subsequent changes the bank erosion and accretion rates were severe the mean erosion rates ranged from 30 7 to 68 3 m year on the left side and 27 1 to 58 3 m year on the right side the mean accretion rates ranged from 44 4 to 68 3 m year on the left side and 30 5 to 60 4 m year on the right side the mean channel midline shifted by 57 8 m toward the right from 1958 to 2008 the mean channel midline moved 4 8 m to right from 1958 to 1967 54 3 m to the left from 1967 to 1977 44 2 m to the right from 1977 to 1990 64 3 m to the right from 1990 to 2000 and 2 8 m to the right from 2000 to 2008

shift in the species composition of the diatom community in the eutrophic mauritanian coastal upwelling results from a multi year sediment trap experiment 2003 2010 abstract a multiannual continuous sediment trap experiment was conducted at the mooring site cbeu cape blanc eutrophic ca 20 n ca 18 w trap depth 1256 1296 m in the high productive mauritanian coastal upwelling here we present fluxes and the species specific composition of the diatom assemblage and fluxes of biogenic silica bsi opal and total organic carbon toc for the time interval june 2003 feb 2010 flux ranges of studied parameters are i total diatoms 1 2 108 4 7 104 valves m 2 d 1 average 5 9 106 valves 1 4 107 ii bsi 296 0 5 mg m 2 d 1 average 41 1 53 5 mg m 2 d 1 and iii toc 97 1 mg m 2 d 1 average 20 5 17 8 mg m 2 d 1 throughout the experiment the overall good match of total diatom bsi and toc fluxes is reasonably consistent and reflects well the temporal occurrence of the main mauritanian upwelling season spring and summer are the most favorable seasons for diatom production and sedimentation out of the recorded 14 diatom maxima of different magnitude six occurred in spring and four in summer the diverse diatom community at site cbeu is composed of four main assemblages benthic coastal upwelling coastal planktonic and open ocean diatoms reflecting different productivity conditions and water masses a striking feature of the temporal variability of the diatom populations is the persistent pattern of seasonal groups contribution benthic and coastal upwelling taxa dominated during the main upwelling season in spring while open ocean diatoms were more abundant in fall and winter when the upper water column becomes stratified upwelling relaxes and productivity decreases the relative abundance of benthic diatoms strongly increased after 2006 yet their spring summer contribution remained high until the end of the trap experiment the occurrence of large populations of benthic diatoms at the hemipelagic cbeu site is interpreted to indicate transport from shallow waters via nepheloid layers we argue that a significant amount of valves bsi and toc produced in waters overlying the banc d arguin and the mauritanian shelf is effectively transported to the cbeu trap in intermediate waters at the outer mauritanian slope the impact of the intermediate and bottom near nepheloid layers driven transport in the transfer of valves and bulk particulates and its potential contribution to the export of biogenic materials from the shelf and uppermost slope might play a significant role in hemipelagial fluxes off mauritania

contribution of oxygen minimum zone waters to the coastal upwelling off mauritania the coastal upwelling off mauritania and its connection with the oxygen minimum zone omz in the tropical atlantic is investigated in an eddy resolving general circulation model two main supply routes for the upwelling are identified first a southern eastward pathway crossing 23 degrees w between 3 degrees n and 10 degrees n related to the equatorial zonal current system supplies up to 50 of the water upwelled in winter and about 30 in summer second another eastward pathway crossing 23 degrees w further north between 28 degrees n and 38 degrees n supplies 35 of the upwelled water in spring compared to 25 during the rest of the year most of the water of the northern pathway is entrained into the mixed layer already before reaching the upwelling region only the southern pathway contributes not recently ventilated waters to the upwelling the connection with the omz is very weak only about 1 of the upwelling waters originate here on the other hand if water from the omz reaches the surface mixed layer within 6 years this mostly 71 happens in the upwelling region

islands as eddy transformation and generation hotspots cabo verde case study abstract deep ocean islands have long been associated with the generation of oceanic eddies in their wakes however their interaction with incoming eddies has seldom been considered this study focuses on the characterization of background and locally generated mesoscale eddies in the cabo verde archipelago between 2003 and 2014 special attention is given to the interaction of incoming eddies with the bathymetry of the islands along with their impacts on the local generation of eddies island induced wind shear effects are also considered in addition some examples of the biological response to background and locally generated eddies are discussed this is achieved by combining remote sensing satellite observations for wind sea surface height and chlorophyll a chla surface concentrations the results show that the interaction between incoming background eddies and the archipelago is a recurrent phenomenon which results in eddy deflection splitting merging intensification and termination sorted by highest to lowest number of occurrences local island induced disturbances are also significant mainly due to atmospheric effects such processes result in the generation of island induced eddies and in wind mediated eddy intensification and confinement more often observed in the leeward group nonetheless it is strongly suggested that many of the locally generated eddies are a direct product or by product of the interaction of background eddies with the islands with respect to the biological realm a locally generated cyclonic eddy is observed to originate a pronounced phytoplankton bloom in the vicinity of the tallest island nonetheless background eddies generated off the african coast are often associated with enhanced chla concentrations when they intersect the archipelago such observations challenge the idea that local biological productivity in deep oceanic islands is exclusively driven by island induced mechanisms

response of reach scale bankfull channel geometry to the altered flow and sediment regime in the lower yellow river abstract the channel geometry in an alluvial river usually varies in response to the altered flow and sediment regime from upstream damming bankfull channel dimensions have significantly adjusted along the lower yellow river lyr because of recent channel degradation caused by the operation of the xiaolangdi reservoir which has led to longitudinal variability in the cross sectional geometry therefore describing the bankfull channel geometry of the lyr is more representative using reach averaged variables in this study an integrated approach is proposed for calculating reach scale bankfull channel dimensions based on the measured section scale bankfull geometry in the lyr and the proposed approach integrates a geometric mean based on the log transformation with a weighted average based on the spacing between two consecutive sections the post flood reach scale bankfull channel dimensions in three different channel pattern reaches of the lyr were then calculated annually during the period from 1999 to 2012 using the proposed approach and the surveyed post flood profiles at 91 sedimentation sections calculated results indicate that the channel geometry changed in the components of bankfull width and depth in the braided reach with an increase of about 400 m in the reach scale bankfull width over the past 13 years and the channel evolution occurred mainly in the component of bankfull depth in the transitional and meandering reaches with the increased reach scale bankfull depths of 2 4 and 1 8 m respectively finally empirical relationships for the three reaches of the lyr were developed between the reach scale bankfull channel dimensions and the previous four year average discharge and incoming sediment coefficient during flood seasons with higher correlations being obtained when calibrated by the measurements from 1999 to 2010 furthermore the developed relations were also verified by the observed data in 2011 and 2012 with encouraging results

ecological evidence from live dead comparisons of benthic foraminiferal faunas off cape blanc northwest africa abstract a comparison of live and dead benthic foraminiferal faunas in a five station sample transect off cape blanc suggests that interspecific differences in living dead l d ratios are to a large extent determined by seasonal differences in reproduction it is proposed that many epifaunal and superficial infaunal species show a highly opportunistic behaviour with a majority of the foraminifera being produced in a relatively short period of the year the density of deeper infaunal taxa on the contrary would be much more stable over time sites with large contrasts in l d ratios between deep intermediate infaunal and epifaunal shallow infaunal taxa should be subject to important seasonal variations in the downward flux of organic matter this study suggests that important ecological information can be gained from live dead comparisons

paleoceanography of the mauritanian margin during the last two climatic cycles from planktonic foraminifera to african climate dynamics abstract the last 220 ka of the md03 2705 18 05 81 n 21 09 19 w sedimentary sequence retrieved off the cape verde islands was investigated using a multiproxy approach planktonic foraminifera assemblage analyses coupled with isotopic measurements 18 o from benthic planulina wuellerstorfi and planktonic globigerinoides ruber foraminifera monospecific samples were conducted along the topmost 11 m of the sequence high resolution x ray fluorescence measurements 0 5 cm resolution giving access to major element ratio have completed the geochemical analyses along the core seasonal and annual past sea surface temperatures sst were quantitatively reconstructed local sea surface salinity sss changes were then estimated by coupling sst with planktonic 18 o data our data provide a set of both oceanic and continental markers of environmental changes along the north western african margin the major changes detected in our record are discussed in the light of the regional paleoceanographic and paleoclimatic history of the last 220 ka coupled oceanographic and atmospheric processes portray the climatic evolution of the area and show strong links among the regional oceanography water mass advection the upwelling dynamics and the intertropical convergence zone itcz migration an increased upwelling activity or influence of upwelling filaments is noted at the end of the two last glacial periods probably in response to a more southerly position of the itcz higher sss are recorded over the area during arid intervals and were tentatively interpreted as signing a southward shift of the cape verde frontal zone a detailed coupling between dust advection and sss values over the site of study was noted during mis6 5

three dimensional circulation in the nw africa coastal transition zone abstract high resolution data collected southeast of the canary islands during late winter 2006 are analyzed to describe the hydrography and three dimensional circulation in the coastal transition zone off nw africa the data are optimally interpolated over a regular grid the geostrophic velocity field is calculated and the q vector formulation of the omega equation is used to compute the quasi geostrophic qg mesoscale vertical velocity the coastal transition zone is divided into upwelling frontal and offshore regions with distinct physical and dynamic characteristics the upwelling region is characterized by cold and weakly stratified waters flowing towards the equator with a poleward undercurrent of approximately 0 05 m s 1 over the continental slope the frontal region exhibits a southwestward baroclinic jet associated with cross shore raising isopycnals the jet transport is close to 1 sv with maximum velocities of 0 18 m s 1 at surface decreasing to 0 05 m s 1 at 300 db vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet the qg mesoscale vertical velocity field is patchy estimated to range from 18 to 12 m day 1 with the largest absolute values corresponding to an anticyclonic eddy located south of fuerteventura island these values are significantly larger than estimates for other vertical velocities diapycnal vertical velocities associated with mixing in the frontal region a few meters per day and wind induced vertical velocities non linear ekman pumping arising from the interaction between the wind stress and the background vorticity maximum values of a few meters per day linear ekman pumping due to the divergence of ekman transport a fraction of a meter per day or the coastal constraint in the upwelling region about 0 7 m day 1 however the patchiness in both the qg mesoscale vertical velocity and the non linear ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal ekman transport 0 08 sv integrated linear ekman pumping 0 05 sv or diapycnal transfer about 0 1 0 2 sv the pattern of the near surface fluorescence field is a good indicator of these different contributions with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures

modern heterozoan carbonates from a eutrophic tropical shelf mauritania abstract heterozoan or foramol production is typical in extratropical carbonate sedimentary systems however under mesotrophic to eutrophic conditions heterozoan carbonates also form in tropical settings but such heterozoan tropical sedimentary systems are poorly understood nevertheless distinction between tropical and extratropical heterozoan carbonates in ancient successions is crucial for accurate paleoenvironmental and paleoclimate reconstructions here surficial holocene and pleistocene sediments of the northern mauritanian shelf are studied as an example of a tropical eutrophic carbonate depositional system 11 g l 1 chl a chlorophyll a upwelling nutrient rich waters push onto the wide mauritanian shelf where they can warm up to in excess of 25 c this condition favors production of heterozoan carbonates dominated by bivalves and foraminifers even in this tropical setting in addition sediments are provided by eolian input from the desertic hinterland the resulting sediments are carbonate and mixed carbonate siliciclastic facies in which the carbonates are characterized by a mixture of tropical and cosmopolitan taxa benthic photosynthetic biota are absent while suspension feeding organisms are dominant this foramol grain association on a shelf scale is reminiscent of cool water carbonates therefore recognition of warm water heterozoan carbonates relies on key taxa related to tropical waters within the biota assemblages associated with a highly productive environment

the mean flow field of the tropical atlantic ocean the mean horizontal sow eld of the tropical atlantic ocean is described between 20in and 20is from observations and literature results for three layers of the upper ocean tropical surface water central water and antarctic intermediate water compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent the wind driven ekman layer in the upper tens of meters of the ocean masks at some places the sow structure of the tropical surface water layer as is the case for the angola gyre in the eastern tropical south atlantic although there are regions with a strong seasonal cycle of the tropical surface water circulation such as the north equatorial countercurrent large regions of the tropics do not show a signi cant seasonal cycle in the central water layer below the eastward north and south equatorial undercurrents appear imbedded in the westward sowing south equatorial current the antarcic intermediate water layer contains several zonal current bands south of 3in but only weak sow exists north of 3in the sparse available data suggest that the equatorial intermediate current as well as the southern and northern intermediate countercurrents extend zonally across the entire equatorial basin due to the convergence of northern and southern water masses the western tropical atlantic north of the equator is an important site for the mixture of water masses but more work is needed to better understand the role of the various zonal under and countercurrents in cross equatorial water mass transfer 1999 elsevier science ltd all rights reserved

changes in the ventilation of the oxygen minimum zone of the tropical north atlantic changes in the ventilation of the oxygen minimum zone omz of the tropical north atlantic are studied using oceanographic data from 18 research cruises carried out between 28 5 and 23 w during 1999 2008 as well as historical data referring to the period 1972 85 in the core of the omz at about 400 m depth a highly significant oxygen decrease of about 15 mol kg 1 is found between the two periods during the same time interval the salinity at the oxygen minimum increased by about 0 1 above the core of the omz within the central water layer oxygen decreased too but salinity changed only slightly or even decreased the scatter in the local oxygen salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and or zonal jets acting on the background gradients here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the omz contribute to the ventilation of the omz a conceptual model of the ventilation of the omz is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the omz according to the model a weakening of zonal jets which is in general agreement with observed hydrographic evidences is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the north atlantic omz

elevation adjustments of paired natural levees during flooding of the saskatchewan river natural levees control the exchange of water between an alluvial channel and its floodplain but little is known about the spatial distribution and evolution of levee heights the summer 2005 flood of the saskatchewan river cumberland marshes east central saskatchewan inundated large areas of floodplain for up to seven weeks forming prominent new deposits on natural levees along main stem channels measurements of flood deposit thickness and crest heights of 61 levee pairs show that the thickest deposits occur on the lower pre flood levee in 80 of the sites though no clear relationship exists between deposit thickness and magnitude of height difference only 16 of the pairs displayed thicker deposits on the higher levee half of which occurred at sites where relatively clear floodbasin waters re entered turbid channels during general flooding difference in crest elevation e between paired levees is approximately log normally distributed both before and after the flood though with different mean values supplemental observations from tank experiments indicate that during near bankfull flows temporally and spatially variable deposition and erosion occur on levees due to backwater effects associated with nearby channel bars and irregular rises of the channel bed forced by channel extension during floods preferential deposition in lows tends to even out crest heights copyright 2009 john wiley sons ltd

on the temporal memory of coastal upwelling off nw africa we use a combination of satellite in situ and numerical data to provide a comprehensive view of the seasonal coastal upwelling cycle off nw africa in terms of both wind forcing and sea surface temperature sst response wind forcing is expressed in terms of both instantaneous local and time integrated nonlocal indices and the ocean response is expressed as the sst difference between coastal and offshore waters the classical local index the cross shore ekman transport reproduces reasonably well the time latitude distribution of sst differences but with significant time lags at latitudes higher than cape blanc two nonlocal indices are examined one of them a cumulative index calculated as the backward averaged ekman transport that provides the highest correlation with sst differences reproduces well the timing of the sst differences at all latitudes except near cape blanc the corresponding time lags are close to zero south of cape blanc and range between 2 and 4 months at latitudes between cape blanc and the southern gulf of cadiz the results are interpreted based on calculations of spatial and temporal auto and cross correlations for wind forcing and sst differences at temporal scales of 2 3 weeks the alongshore advection of alongshore momentum compensates for interfacial friction allowing the upwelling jet and associated frontal system to remain active we conclude that the coastal jet plays a key role in maintaining the structure of coastal upwelling even at times of relaxed winds by introducing a seasonal memory to the system in accordance with the atmospheric forcing annual cycle

stable oxygen isotopes from the calcareous walled dinoflagellate thoracosphaera heimii as a proxy for changes in mixed layer temperatures off nw africa during the last 45 000yr abstract the present study is the first study on the stable oxygen isotope composition of the photosynthetic calcareous walled dinoflagellate species thoracosphaera heimii off nw africa during the last 45 000 yr t heimii based temperature estimates of sediment core geob 8507 3 were compared with those obtained from the stable oxygen isotopes of the planktic foraminifera globigerina bulloides and globigerinoides ruber pink and the mg ca ratio of g ruber pink we show that the isotopic composition of t heimii and the temperature estimates based on the equation for inorganically precipitated calcite provide comparable results to those obtained from g ruber pink isotopes and mg ca ratios with exception of the early holocene and the younger dryas the recently proposed palaeotemperature equation of zonneveld et al 2007 however provides unrealistic temperature reconstructions that are about 16 c lower than those based on planktic foraminifera thus this equation needs to be revised the difference between t heimii and g bulloides isotopic and temperature reconstructions can be ascribed to differences in the ecology of both species especially with regard to their depth habitat and or seasonal production in the research area all temperature proxies suggest comparable conditions during the last glacial and holocene small differences between the reconstructed temperature values of t heimii and the other proxies can be explained by differences in seasonal production of the individual species the relatively low temperatures recorded by t heimii at about 15 000 to 8 000 yr bp are interpreted to reflect an increase in duration and or intensity of the upwelling in the vicinity of the core site in comparison to the last glacial with an abrupt and strong decrease of upwelling intensity and or duration during the early holocene and the younger dryas

oxygen minimum zone in the north atlantic south and east of the cape verde islands the open ocean oxygen minimum zone omz south and east of the cape verde islands is studied from ctd hydrography adcp velocities argo float trajectories and historical data with a focus on the zonal supply and drainage paths the strongest oxygen minimum is located north of the north equatorial countercurrent necc at about 400 to 500 m depth just above the boundary between central water and antarctic intermediate water aaiw it is shown that the necc the north equatorial undercurrent at 4 to 6 n and a northern branch of the necc at 8 to 10 n are the sources for oxygen rich water supplied to the omz in summer and fall a weak eastward necc at 200 m depth also exists in winter and spring as derived from argo floats drifting at shallow levels historical oxygen data from 200 m depth confirm this seasonality showing high low oxygen content in summer and fall spring within the supply paths compared to the strong oxygen supply at 150 to 300 m depth the ventilation of the omz at 300 to 600 m depth is weaker westward drainage of oxygen poor water takes place north of the guinea dome i e north of 10 n most pronounced at 400 to 600 m depth in july 2006 the total eastward transport of both necc bands above 27 1 kg m 3 at 23 w was about 13 sv 1 sv 106 m3 s 1 about half of this water volume circulates within the guinea dome or recirculates westward north of the guinea dome

river sedimentation and channel adjustment of the lower yellow river as influenced by low discharges and seasonal channel dry ups fluvial sedimentation can be classified as one of the three types in terms of different morphological units i e vertical overbank deposition vertical deposition on the riverbed and lateral deposition on the riverbank with the example of the lower yellow river the three types of sedimentation have been studied in relation to channel adjustment since 1986 the lower yellow river has entered a new period that is characterized by small discharges with seasonal desiccations dry ups and strong sedimentation has occurred based on the measurement of 36 cross sections along the lower yellow river the rates of the three types of deposition have been calculated and are reported in this study multiple regression has been used to express the relationship between the three rates of deposition and the channel shape adjustment index influenced by the difference in channel sediment and by downstream variations in water discharge and sediment load the indices of channel deposition rate show a clear trend of downstream variation controlled by this the manner of channel adjustment also shows clear trends downstream under the hydrological regime dominated by small discharges the tendencies of adjustment of riverbed slope and bankfull slope are inverse i e the channel slope tends to decrease but the slope at bankfull stage to increase at present the former tendency is dominant thus the sediment carrying capacity of the lower yellow river tends to decrease which intensifies channel sedimentation

river mouth bar formation riverbed aggradation and channel migration in the modern huanghe yellow river delta china this paper addresses the recent 1970s 1990s processes of river mouth bar formation riverbed aggradation and distributary migration in the huanghe river mouth area in the light of station based monitoring field measurements and remote sensing interpretation the results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load landforrn development such as bar progradation occurred in two phases i e before and after 1989 which correspond to faster and lower bar growth rates respectively fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation during 1976 1996 about 2 8 of the total sediment loads were deposited in the river channel on the upper to middle delta therefore the river water level rose by a few meters from 1984 to 1996 the frequent distributary channel migration which switched the radial channel pattern into the se directed pattern in the mid 1980s was linked with mouth bar formation marine conditions also constrain seaward bar progradation furthermore the history of river mouth bar formation reflects human impacts such as dredging and dyking in order to stabilize the coastal area c 2005 elsevier b v all rights reserved

a study of the scour fill threshold based on lane s equilibrium relation the lower yellow river the scour fill threshold in river channels is an expression of scour fill equilibrium which may be understood as the balance between the river s sediment carrying capability and the sediment load imposed by the drainage basin in the present study the above capability load relation is quantified using lane s relation gamma qj q s d 50 wherein q is water discharge j is channel slope q s is sediment transport rate d 50 is median size of bed material and gamma is the specific weight of water on this basis we have established scour fill thresholds for the lower yellow river based on data from 141 flood events the results show that the q s d 50 gamma qj relation may well distinguish between the scour and fill flood events the following equation has been established sdrfe 0 00099 gamma qj 0 90 q s d 50 0 60 wherein sdrfe is channel sediment delivery ratio from the equation the scour fill threshold is established as q s d 50 0 00001 gamma qj 1 5 starting from this formula four possible options are proposed for sediment management of the lower yellow river c 2015 elsevier b v all rights reserved

the ichthyoplankton assemblage of the canaries african coastal transition zone a review abstract in this paper we review information on the ichthyoplankton assemblage of the canaries african coastal transition zone c actz this ctz shows the singularity that the canary archipelago interrupts the main flow of the canary current and trade winds introducing large mesoscale variability in the form of island warm wakes and cyclonic and anticyclonic eddies downstream of the islands besides upwelling filaments stretch towards the archipelago from the african coastal upwelling transporting phytoplankton zooplankton and fish larvae they also interact with eddies shed from the islands to exchange water properties and biogenic material all these mesoscale features influence the composition structure abundance and distribution of the larval fish community lfc of the region the canary current cc and eddies shed from the islands drag larvae of island neritic fish species into the oceanic region and contribute along warm wakes to the horizontal distribution of fish larvae upwelling and upwelling filaments transport larvae of african neritic species into the oceanic region these larvae dominate the lfc and account for the relatively high average larval fish abundance found in the c actz during the summer upwelling season filaments originated in the region of cape juby cape bojador are entrained around a quasi permanent cyclonic eddy trapped between gran canaria island and the african coast forming a system through which most of the african neritic larvae may return to the african shelf however some larvae reach the eastern islands of the canary archipelago and they may be spread all over the neritic region of the archipelago by eddies shed from the islands also in summer the distribution of the lfc of the c actz is vertically stratified and fish larvae seem to carry out little or not diel vertical migration overall this study highlights the strong relationship between mesoscale oceanographic processes and the lfc in the c actz

least squares multiple tracer analysis of water mass composition we present an inverse method that uses measurements of the concentration of several tracer variables to find the mixture of source water types that best describes in a weighted least squares sense the composition of the water sample the solution includes two physically realistic constraints first that all sources together must sum to 100 and second that no source is present in negative amounts these constraints improve the statistical stability of the solution the method is particularly valuable in regions involving strong mixing among a relatively large number of source water types in contrast to conventional ts analysis more water types can be considered the assumption of mixing along isopycnals is unnecessary and sensitivity to errors in individual tracer measurements is reduced by averaging the influence of a larger number of tracers we demonstrate the method using data from two oceanographic regions analysis of summer data from the continental margin off vancouver island british columbia canada confirms and quantifies the importance of deep upwelling of california undercurrent water onto the continental shelf it also identifies localized outer shelf filaments of low salinity water originating from the vancouver island coastal current finally we reanalyze the north african data of tomczak and confirm his interpretation of diapycnal mixing at the frontal boundary between north atlantic central water and south atlantic central water masses

influence of lateral particle advection and organic matter degradation on sediment accumulation and stable nitrogen isotope ratios along a productivity gradient in the canary islands region abstract we compare total and biogenic particle fluxes and stable nitrogen isotope ratios 15n at three mooring sites along a productivity gradient in the canary islands region with surface sediment accumulation rates and sedimentary 15n higher particle fluxes and sediment accumulation rates and lower 15n were observed in the upwelling influenced eastern boundary region ebc compared to the oligotrophic sites north of gran canaria european station for time series in the ocean canary islands estoc and north of la palma lp the impact of organic matter degradation and lateral particle advection on sediment accumulation was quantified with respect to the multi year flux record at the estoc remineralisation of organic matter in the water column and at the sediment surface resulted in an organic carbon preservation of about 0 8 and total nitrogen preservation of about 0 4 of the estimated export production higher total and carbonate fluxes and accumulation rates in the lower traps and surface sediment compared to the upper traps indicated that at least 50 of the particulate matter at the estoc was derived from allochthonous sources low 15n values in the lower traps of the estoc and lp point to a source region influenced by coastal upwelling we conclude from this study that the reconstruction of export production or nutrient regimes from sedimentary records in regions with strong productivity gradients might be biased due to the mixture of particles originating from autochthonous and allochthonous sources this could result in an imprint of high productivity signatures on sedimentation processes in oligotrophic regions

the upwelling area off northwest africa a description of phenomena related to coastal upwelling abstract this paper presents a brief review of the oceanography along the upwelling coast of northwest africa the description contains information on seasonal variations of large scale winds ocean circulation and some gross of features of the hydrography characteristic meso scale patterns of currents during the upwelling season between 16 n and 26 n air and water temperature differences during upwelling season diurnal and semi diurnal variations of temperature atmospheric pressure wind and current regional peculiarities canyons and the banc d arguin

sedimentation within a regulated river channel change induced by tributary sediment injection to a regulated gravel bed river is described for the river rheidol u k eighteen years after dam closure morphological and sedimentological data are presented for an aggrading reach a primary lobate bar appears to have initiated flow separation and the subsequent construction of channel side berms hydraulic sorting has operated to produce both longitudinal and lateral particle size reduction and selective particle size dispersal appears to be an important process influencing channel change the channel is adjusting to a lower width depth ratio form of smaller capacity and with a steeper slope

alterations of channel parameters in response to river regulation works since 1840 on the lower tisza river hungary abstract in the last few years an increase in the frequency and magnitude of floods was detected on the tisza river endangering large areas of hungary the causes of these record floods were complex including both natural and human induced factors this paper focuses on river management works and their effect on planimetric and cross sectional parameters with special attention to the flood conductivity changes to the river channel during 19th century river regulation works half of the total length of the tisza river was altered by cut offs while in the 20th century mostly revetments and groynes were constructed subsequently horizontal and vertical channel parameters have changed considerably due to semi natural bed processes in order to reveal changes hydrological map series 1842 1890 1929 1957 1976 and 1999 and cross sectional surveys from the same dates were analysed prior to the intensive human interventions before 1890s the river s course was highly sinuous with some very sharp bends due to cut offs both the length and sinuosity of the tisza river decreased by 35 while the lengths of straight sections and the river s slope doubled as a consequence the river incised by up to 3 8 m until the 1929 survey resulting better flood conductivity which improved flood safety in the 1920s river management favoured bank stabilisation in order to stop the lateral migration of the channel despite these measures meander development has continued however in a distorted manner this is reflected by the opposing processes of lengthening centre line on the one hand and gradually decreasing radius of curvature on the other these processes can be explained by the continuous development of natural point bars on the convex bank and the lack of lateral retreat on the concave stabilised bank the width of the river decreased by 17 45 while its mean and maximum depth increased by 5 48 the area of cross sections influenced by revetments decreased by 6 19 resulting in a 6 15 decline in flood conductivity the non stabilised sections were influenced by upstream revetments therefore their parameters show similar changes but with a smaller rate at present the flood conductivity of the channel is worse than it was in its natural state in all it was found that the ongoing process of cross sectional distortion is a significant factor in increasing flood stage and hazard and high floods can be expected more frequently in the future partly due to this factor

morphology of fluvial levee series along a river under human influence maros river hungary abstract the development and morphometry of fluvial levees reflect the connection between channel and overbank processes which can be altered by various human activities the aims of this study are to investigate the morphology and spatial characteristics of fluvial levees and evaluate the role of some local and catchment scale human activities on their medium term 150 years development this study applies lidar data along a 53 km long reach of the maros river in hungary six fluvial levee types are identified based on the beginning and end of their evolution these levee types were generated by local nineteenth century channel regulation works cutoffs and mid twentieth century channel narrowing which was caused by gravel mining and water impoundment in the upstream sections however other human activities also influenced the development of active fluvial levees because their horizontal evolution could have been limited by embanked flood protection levees or the widening of low lying floodplain benches that were generated by channel narrowing additionally revetment constructions influenced their vertical parameters as higher fluvial levees developed along the fixed banks generally the older active fluvial levees are wider while the younger active levees are narrower with steeper slopes but not always lower on the low lying floodplain levels benches the youngest fluvial levees evolved quite rapidly and consist of coarser material currently only 9 8 to 38 year return period floods could cover the fluvial levees contributing to their evolution this fact and the development of fluvial levee series with two three members reflect a gradual decoupling of the channel from the floodplain

main eddy vertical structures observed in the four major eastern boundary upwelling systems in the four major eastern boundary upwelling systems ebus mesoscale eddies are known to modulate the biological productivity and transport near coastal seawater properties toward the offshore ocean however little is known about their main characteristics and vertical structure this study combines 10 years of satellite altimetry data and argo float profiles of temperature and salinity and our main goals are i to describe the main surface characteristics of long lived eddies formed in each ebus and their evolution and ii to depict the main vertical structure of the eddy types that coexist in these regions a clustering analysis of the argo profiles surfacing within the long lived eddies of each ebus allows us to determine the proportion of surface and subsurface intensified eddies in each region and to describe their vertical structure in terms of temperature salinity and dynamic height anomalies in the peru chile upwelling system 55 of the sampled anticyclonic eddies aes have subsurface intensified maximum temperature and salinity anomalies below the seasonal pycnocline whereas 88 of the cyclonic eddies ces are surface intensified in the california upwelling system only 30 of the aes are subsurface intensified and all of the ces show maximum anomalies above the pycnocline in the canary upwelling system 40 of the aes and 60 of the ces are subsurface intensified with maximum anomalies extending down to 800 m depth finally the benguela upwelling system tends to generate 40 50 of weak surface intensified eddies and 50 60 of much stronger subsurface intensified eddies with a clear geographical distribution the mechanisms involved in the observed eddy vertical shapes are discussed

relation between the erosion and sedimentation zones in the yellow river china sedimentation in the lower reaches of the yellow river is a major problem requiring implementation of large scale control measures in the upper and middle drainage basin for maximum benefit major sediment generation areas must be delimited for this purpose the upper and middle drainage basin of the yellow river has been divided into four major sediment and water source areas a series of databases of runoff and sediment yields from these source areas and for the corresponding quantities of sedimentation in the lower yellow river channel have been established on this basis a set of multiple regression equations has been established that define the relationships between sedimentation in the lower yellow river and the yearly or event based runoff and sediment yields from the four source areas or subsystems based on the regression equations obtained the contribution of the four major source areas to the sedimentation in the lower yellow river channel can be estimated the results obtained indicate that given other factors for each ton of sediment reduced from the coarse sediment producing area csa the sediment deposited in the lower yellow river would be reduced by 0 455 ton for each ton of sediment reduced from the fine sediment producing area fsa the sediment deposited in the lower yellow river would be reduced by 0 154 ton only therefore if limited erosion control measures are applied to the coarse sediment producing area the benefits for sedimentation reduction in the lower yellow river will be much larger than if similar resources are applied to the fine sediment producing area c 2002 elsevier science b v all rights reserved

tidal influence on particulate organic carbon export fluxes around a tall seamount as tall seamounts may be stepping stones for dispersion and migration of deep open ocean fauna an improved understanding of the productivity at and food supply to such systems needs to be formed here the 234th 238u approach for tracing settling particulate matter was applied to senghor seamount a tall sub marine mountain near the tropical cape verde archipelago in order to elucidate the effects of topographically influenced physical flow regimes on the export flux of particulate organic carbon poc from the near surface topmost 100 m into deeper waters the comparison of a suitable reference site and the seamount sites revealed that poc export at the seamount sites was 2 4 times higher than at the reference site for three out of five seamount sites the calculated poc export fluxes are likely to be underestimates if this is taken into account it can be concluded that poc export fluxes increase while the passing waters are advected around and over the seamount with the highest export fluxes occurring on the downstream side of the seamount this supports the view that biogeochemical and biological effects of tall seamounts in surface ocean waters might be strongest at some downstream distance from rather than centred around the seamount summit based on measured vessel mounted adcp and modelled regional flow field aviso internal tides at senghor mitgcm flow dynamics it is proposed that tidally generated internal waves result in a screen of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased poc export above the seamount 1 sudden increased upward transport of nutrients into the euphotic zone driving brief pulses of primary production of new particulate matter followed by the particles export into deeper waters and 2 pulses of increased shear driven aggregation of smaller slower settling into larger faster settling particles this study shows that under certain conditions there can be an effect of a tall seamount on aspects of surface ocean biogeochemistry with tidal dynamics playing a prominent role it is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic pelagic communities at and near tall seamounts

classification of geomorphological effects downstream of dams abstract the effects of dams on downstream geomorphology are reviewed and a typology is devised consisting of nine cases the classification can be seen as a further development of lane s balance between water discharge sediment load grain size and river slope depending on changes in released water flow and changes in released sediment load relative to the transport capacity of the flow it is possible to estimate resulting cross sectional geomorphology the longitudinal extent of changes and their variability with time and the tributary response to altered mainstream cross section changes are also discussed

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

distribution of major elements in atlantic surface sediments 36 n 49 s imprint of terrigenous input and continental weathering 1 numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations the choice and interpretation of climate proxies however differ from site to site here we map the concentrations of major elements ca fe al si ti k in atlantic surface sediments 36 n 49 s to assess the factors influencing the geochemistry of atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes high concentrations of terrigenous elements and low ca concentrations along the african and south american margins reflect the dominance of terrigenous input in these regions single element concentrations and elemental ratios including ca e g fe ca are too sensitive to dilution effects enhanced biological productivity carbonate dissolution to allow reliable reconstructions of terrestrial climate other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas the atlantic distribution of ti al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material the spatial distributions of al si and fe k reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas high biogenic opal input however influences the al si ratio fe k is sensitive to the input of mafic material and the topography of andean river drainage basins both ratios are suitable to reconstruct african and south american climatic zones characterized by different intensities of chemical weathering in well understood environmental settings

the canary basin contribution to the seasonal cycle of the atlantic meridional overturning circulation at 26 n this study examines the seasonal cycle of the atlantic meridional overturning circulation amoc and its eastern boundary contributions the cycle has a magnitude of 6 sv as measured by the rapid mocha wbts project array at 26 n which is driven largely by the eastern boundary the eastern boundary variations are explored in the context of the regional circulation around the canary islands there is a 3 month lag between maximum wind forcing and the largest eastern boundary transports which is explained in terms of a model for rossby wave generated at the eastern boundary two dynamic processes take place through the lanzarote passage lp in fall the recirculation of the canary current and the northward flow of the intermediate poleward undercurrent in contrast during the remaining seasons the transport through the lp is southward due to the canary upwelling current these processes are linked to the seasonal cycle of the amoc

the gephyrocapsa sea surface palaeothermometer put to the test comparison with alkenone and foraminifera proxies off nw africa abstract recently a new palaeotemperature proxy was developed based on the morphology of the coccolithophore genus gephyrocapsa here we apply and validate this new methodology by comparing gephyrocapsa derived palaeotemperature estimates with temperature estimates obtained from alkenone ratios mg ca ratios in foraminiferal calcite and planktic foraminifera assemblages we investigated a downcore record off cape blanc geob 1048 20 55 n 19 43 w spanning the last 28 000 years in the vicinity of bofs core 31k 19 00 n 20 10 w for which a detailed multiproxy palaeotemperature record has already been established our data show an especially good correspondence between the gephyrocapsa and alkenone derived palaeotemperatures and thus validate the great potential of the new gephyrocapsa proxy to reconstruct sea surface water temperatures ssts both indicate a glacial drop in sst of 3 c compared to holocene values similar to the mg ca palaeotemperature record although the latter is consistently 2 5 c cooler largest discrepancies both in terms of downcore amplitude changes and same age sample estimates are found with available planktic foraminifera transfer function data this can be due to fundamental differences in the season of production ecologic preferences and habitats and or preservation on the other hand it may be a result of differences inherent to the transfer functions used and their original calibration i e regional vs global approach or different temperature ranges covered in addition to the downcore record off cape blanc sediment samples corresponding to the last glacial maximum lgm were selected from a dispersed set of eighteen well dated gravity cores in order to derive palaeo sst maps off northwest africa during this time slice by applying a regional gephyrocapsa transfer function for the first time we show that mean ssts in the canary islands region during the lgm were 4 6 c colder than today we found lowest glacial temperatures 14 15 5 c offshore morocco and south of the volcanic canary islands likely due to intensified upwelling related to stronger trade winds consistent with estimates from climap and other palaeotemperature reconstructions for the same region in contrast for offshore cape blanc today s centre of perennial upwelling our temperature estimates for the lgm are significantly warmer t today lgm 2 5 4 c than proposed by climap t today lgm 6 10 c however our results consistently support temperature reconstructions based on alkenones that also indicate rather small temperature changes t today lgm 2 4 c in this area we show that gephyrocapsa morphotypes may also provide useful qualitative information about relative changes in primary productivity although no linear relationship is apparent in summary our palaeotemperature and palaeoproductivity data support the observation that enhanced glacial productivity appears geographically limited to the northernmost latitudes of the nw african upwelling system whilst upwelling intensity off cape blanc was reduced despite overall enhanced trade wind intensity

hydrographic conditions associated with the relaxation of an upwelling event off the galician coast nw spain during the galicia xi cruise froin may 10 to may 14 1991 the relaxation of a previous strong tipwelling event took place along the galician coast caused by a shift in the position of the azores high the coldest tenperatures and highest nitrate levels were forrod near the coast corresponding to the advection of subsurface waters particularly of eastern north atlantic water enaw on the basis of thermohaline properties enaw of polar enawp and tropical enawt origin are distinguished the highest salinity values found correspond to a saline wedge of enaw t located off the west coast of galicia the relaxation of tipwelling produced a shoreward migration of surface water from oceanic stations which crone into contact with upwelled eastern north atlantic water ti mn the coastal stations to tbrm a convergence front 28 lan off the coast meanwhile the saline wedge displaced towards both the north and the shore which is thvored by the compression of the tipwelling system to a narrow coastal strip outwardly limited by the convergence zone

northward penetration of antarctic intermediate water off northwest africa abstract in this article historical and climatological datasets are used to investigate the seasonal northward propagation of antarctic intermediate waters aaiw along the eastern margin of the north atlantic subtropical gyre a cluster analysis for data north of 26 n shows the presence of a substantial number of hydrographic stations with aaiw characteristics that stretch northeast along the african slope this water mass extends north during fall as shown both through the comparison of actual and climatological data and by applying a mixing analysis to normal to shore seasonal sections at both 28 5 and 32 n the mixing analysis is further used with several fall cruises between 32 and 36 n and shows that at these latitudes the core of aaiw propagates along the 27 5 isoneutral with contributions that reach as much as 50 at 32 5 n an idealized sverdrup type model is used in combination with climatological hydrographic and wind data to examine what forces this eastern boundary propagation it is fo

holocene millennial scale erosion and deposition processes in the middle reaches of inland drainage basins arid china in inland drainage basins many studies have been conducted regarding the geomorphologic evolution processes in the upper and lower reaches few of them have been done on the middle reaches which are the pivotal areas connecting the erosion effects of the upper reaches and the deposition processes in terminal areas in this study we presented a millennial scale study from middle reaches of the shiyang river and the shule river drainage basins during the holocene several sedimentary sections are selected to calculate millennial scale erosion and deposition rates results show that the middle reaches of the shiyang river and the shule river are characterized by deposition processes in the early and middle holocene but erosion processes prevailed during the late holocene at the same time the sedimentary facies grain size geochemical proxies and pollen proxies of the sediments from middle reaches of the two drainages were used to complement the erosion and deposition rates which are good for understanding the environment background during the geomorphologic evolution processes besides the main factor affecting the basin wide geomorphologic evolution is millennial scale climate change and the responses of the middle reaches to climate change are different from those of the upper and lower reaches

response of bankfull discharge to discharge and sediment load in the lower yellow river this paper presents a study on the response of bankfull discharge to incoming discharge and sediment load based on long term hydrological and bankfull discharge data measured on the lower reaches of the yellow river the analysis indicated that the bankfull discharge was a function of the preceding 5 to 6 years flow regimes revealing that bankfull channel dimensions were a result of the accumulative effect of several consecutive years flow discharge and sediment load conditions in other words the mean relaxation time was about 5 to 6 years for channel adjustment theoretical methods for the prediction of bankfull discharge were developed in which the response time or the relaxation time was fully considered testing of the methods using data observed at five hydrologic stations in the lower reaches of the yellow river from 1960 to 2003 showed that the proposed methods can predict the variation of bankfull discharge in response to changes in the incoming discharge and sediment load the proposed methods hold promise for predicting the magnitude and trend of channel response to other rivers undergoing aggradation or degradation from changes in hydrologic or sediment regime

productivity response in the north canary basin to climate changes during the last 250 000 yr a multi proxy approach we present results from the investigation of the primary productivity record over the last 250 kyr in the north canary basin 30 n off northwest africa two distinct productive systems interfere in this area the oligotrophic open ocean and the upwelling filament off cape ghir that occasionally carries offshore cool nutrient rich waters the following geochemical and micropaleontological paleoproductivity proxies have been used in our study calcium carbonate barium excess baexcess total organic carbon toc and diatoms time series analysis of these proxies indicates that paleoproductivity in the north canary basin underwent important changes following precession and eccentricity cycles while the precessional signal appears to be mainly related to trade wind strength superimposed peaks in baexcess toc and diatom records point to large productivity events at terminations i ii and iii lowering of the north atlantic sea surface temperatures by melt water discharges which in turn strengthened the azores high pressure center and increased trade wind velocities is postulated as the mechanism to explain the enhancement of the coastal upwelling and associated filaments at terminations additionally the canary current may play a role in transmitting cold melt waters and nutrients from higher latitudes to the north canary basin

the role of crustal contamination in the formation of ni cu sulfide deposits in eastern tianshan xinjiang northwest china evidence from trace element geochemistry re os sr nd zircon hf o and sulfur isotopes tulaergen xiangshan huangshan tianyu and baishiquan magmatic ni cu sulfide deposits which are all located in the eastern part of the central asian orogenic belt and associated with small mafic ultramafic intrusions with high grade cu and ni orebodies have similar ages of formation their magma sources are morb like and oib like depleted mantle modeling calculations of sr nd and zircon hf isotopic data show that about 10 15 of oceanic slab components are added into the source of baishiquan and tianyu intrusions but only less than 5 are in the three other intrusions the higher degree of subduction metasomatism in the baishiquan and tianyu magma sources is likely the result of the paleozoic multiple stage subduction moreover the relative enrichment of lile depletion of hfse and lower ce pb ratios indicate that these intrusions experienced crustal contaminations the effects of crustal contamination are also identified in the magmatic sulfide deposits by the higher zircon d 18 o higher sulfide cos and d 34 s values observed we propose a two stage crustal contamination model of the tulaergen xiangshan tianyu and baishiquan intrusions however the huangshan intrusion which has the highest degree of crustal contamination likely experienced only single stage upper crustal contamination 14 2 the addition of crustal s appears to be directly related to triggering s saturation in these deposits according to a simple s isotopic modeling calculation 11 2 9 1 2 and 2 1 of the crustal sulfur addition into the parental magma would be required to form the tianyu baishiquan xiangshan and tulaergen deposits respectively

temporal variability of mass transport in the canary current abstract the variability of the canary current is investigated using bimonthly expandable bathythermograph sections from gran canaria island to the african coast between november 1996 and september 1998 the geostrophic transport of the easternmost branch of the canary current is estimated by integrating the thermal wind equation using the layer of neutral density 27 3 roughly 600 m depth as the layer of no motion the yearly average geostrophic transport of this branch of the canary current is 1 8 1 4 10 9 kg s 1 southward approximately half of the transport flows through the channel between the islands of gran canaria and fuerteventura and the other half through the channel between fuerteventura and the african coast the total southward geostrophic transport shows significant seasonal variability ranging from 1 2 0 3 10 9 kg s 1 in may to 2 6 0 1 10 9 kg s 1 in january although november is the only month with considerable differences in geostrophic transport between 1996 and 1997 there is seasonal northward transport in both channels during may in the gran canaria fuerteventura channel and during november in the fuerteventura african coast channel this seasonal pattern is probably linked to autumn weakening of upwelling in the canary islands area and the offshore diversion of this flow at cape ghir

glacial interglacial variability of particle accumulation in the canary basin a time slice approach abstract particle supply and sediment accumulation in the canary basin are strongly modified by seasonal coastal upwelling and dust input from the sahara along an e w depth profile and fertility gradient north of the canary islands we demonstrate an increase in total mass accumulation rates from a low of 1 2 g cm 2 ka 1 in the outer oceanic domain to 6 12 g cm 2 ka 1 closer to the moroccan shelf in holocene sediments during the last glacial maximum lgm oxygen isotopic event 2 2 a steeper gradient was found with two to four times higher mass accumulation rates at the near shore sites total mass accumulation rates during the last interglacial climax lic isotopic event 5 5 and end of the penultimate glaciation epg isotopic event 6 2 resemble holocene values with lowered near shore maxima of 3 and 4 g cm 2 ka 1 respectively analyses of separate sediment components show that total organic carbon total carbonate lithogenic and diatom accumulation rates were generally enhanced during glacial times especially during the lgm in contrast glacial accumulation rates of biogenic carbonate particles coccoliths and planktic foraminifera shells remained constant or were lowered with respect to interglacial time periods at the site closest to the moroccan shelf despite a doubling of bulk carbonate accumulation this could indicate lowered supply from the productive euphotic zone or a latitudinal offshore shift of the main sedimentary depocentre however enhanced carbonate dissolution could by itself account for this decrease in biogenic carbonate accumulation increased glacial input of detrital carbonate derived from the exposed moroccan shelf appears to be responsible for the overall enhanced carbonate accumulation at the same sites east of lanzarote

multidecadal variability and late medieval cooling of near coastal sea surface temperatures in the eastern tropical north atlantic 1 multidecadal variations in atlantic sea surface temperatures sst influence the climate of the northern hemisphere however prior to the instrumental time period information on multidecadal climate variability becomes limited and there is a particular scarcity of sufficiently resolved sst reconstructions here we present an eastern tropical north atlantic reconstruction of ssts based on foraminiferal mg ca ratios that resolves multidecadal variability over the past 1700 years spectral power in the multidecadal band 50 to 70 years period is significant over several time intervals suggesting that the atlantic multidecadal oscillation amo has been influencing local sst since our data exhibit high scatter the absence of multidecadal variability in the remaining record does not exclude the possibility that sst variations on this time scale might have been present without being detected in our data cooling by 0 5 c takes place between about ad 1250 and ad 1500 while this corresponds to the inception of the little ice age lia the end of the lia is not reflected in our record and sst remains relatively low this transition to cooler ssts parallels the previously reconstructed shift in the north atlantic oscillation toward a low pre 20th century mean state and possibly reflects common solar forcing

the giant mauritanian cold water coral mound province oxygen control on coral mound formation abstract the largest coherent cold water coral cwc mound province in the atlantic ocean exists along the mauritanian margin where up to 100 m high mounds extend over a distance of 400 km arranged in two slope parallel chains in 400 550 m water depth additionally cwcs are present in the numerous submarine canyons with isolated coral mounds being developed on some canyon flanks seventy seven uranium series coral ages were assessed to elucidate the timing of cwc colonisation and coral mound development along the mauritanian margin for the last 120 000 years our results show that cwcs were present on the mounds during the last interglacial though in low numbers corresponding to coral mound aggradation rates of 16 cm kyr 1 most prolific periods for cwc growth are identified for the last glacial and deglaciation resulting in enhanced mound aggradation 1000 cm kyr 1 before mound formation stagnated along the entire margin with the onset of the holocene until today the mauritanian mounds are in a dormant state with only scarce cwc growth in the canyons live cwcs are abundant since the late holocene at least thus the canyons may serve as a refuge to cwcs potentially enabling the observed modest re colonisation pulse on the mounds along the open slope the timing and rate of the pre holocene coral mound aggradation and the cessation of mound formation varied between the individual mounds which was likely the consequence of vertical lateral changes in water mass structure that placed the mounds near or out of oxygen depleted waters respectively

effects of watershed management practices on the relationships among rainfall runoff and sediment delivery in the hilly gully region of the loess plateau in china abstract the relationships among rainfall runoff and sediment delivery are significant in predicting soil erosion and in evaluating the benefits of watershed management practices wmp on the hilly gully regions of the loess plateau hydrologic data 1987 to 2010 were analyzed for variations in precipitation runoff and sediment delivery at annual and event scale these data were obtained from the qiaozi east watershed qe and the qiaozi west watershed qw with and without wmp respectively results indicated that the runoff coefficients of the watersheds decreased significantly although the runoff coefficient of qe was less than half of that of qw sediment delivery decreased more in qe than in qw mainly because of the increase in vegetation cover in both watersheds in qe the relationship between runoff and sediment delivery did not change significantly from 1987 to 2006 although the variation was significant from 2007 to 2010 in qw the relationship between runoff and sediment delivery was not notably altered from 1987 to 2010 this finding suggested that vegetation practices and engineering measures on the hillslope did not strongly affect the relationship between runoff and sediment delivery in the watersheds but the construction of check dams reduced gully erosion and the suspended sediment concentration hence control practices for gully erosion significantly influenced the relationship between runoff and sediment delivery this study suggests that a combination of hillslope and gully erosion control practices effectively reduces erosion and sediment delivery in the hilly gully regions of the loess plateau