a record of changing redox conditions in the northern peru basin during the late quaternary deduced from mn fe and growth rate variations in two diagenetic manganese nodules abstract two diagenetic manganese nodules from the peru basin were investigated by thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium the tims concentrations for nodule 62kd 63kg vary as follows 0 12 1 01 ppb 0 06 0 59 230th 0 51 1 98 ppm 0 43 1 40 232th 0 13 0 80 ppb 0 09 0 49 234u and 1 95 13 47 ppm 1 66 8 24 238u both nodules have average growth rates of 110 mm per million years however from the variations of excess 230th with depth we estimate partial accumulation rates which range from 50 to 400 mm per million years the 234u dating method cannot be applied due to remobilization of u from the sediment and subsequent incorporation into the nodules crystal lattice reflected by decay corrected 234u values far above the ocean water value sections of fast nodule growth are related to those layers having high mn fe ratios up to 200 and higher densities as a possible explanation we develop a scenario that describes similar glacial interglacial trends in both nodules as a record of regional changes of sediment and or deep water chemistry

226 ra excess ba growth rates and u th ra ba systematic of baltic mn fe crusts abstract we analyzed 238u 234u 232th 230th and 226ra by thermal ionization mass spectrometry tims and ba by inductively coupled plasma optical emission spectrometry icp oes on eight mn fe crusts from the mecklenburg bay sw baltic and on one from the bothnian bay n baltic to test the 226raex ba ratio as potential geochronometer 226raex ba ratios decrease as a function of depth within the concretions in all analyzed profiles calculated diffusion coefficients are relatively low 9 10 7 cm2 yr for ra and 5 10 7 cm2 yr for ba and suggest that diffusion is negligible for the ra and ba record in addition 226raex ba ages are consistent and independent from the growth rate and growth direction within a crust thus the decline in 226raex ba ratio is most likely due to radioactive decay of 226raex although the influence of varying oxic conditions has still to be evaluated 226raex ba growth rates range from 0 021 to 0 0017 mm yr and tend to be lower than those calculated and based on stratigraphic methods 1 to 0 013 mm yr 226raex ba ages of concretions from shallow water environment 20 m depth mecklenburg bay sw baltic cover a time interval from 990 140 yr to 4310 310 yr bp corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago one sample from greater depth 70 m bothnian bay n baltic showed a higher 226raex ba age of 6460 520 yr bp

geochemical characteristics of deep sea sediments from the arabian sea a high resolution study abstract five deep sea cores from the arabian sea covering a time interval of 170 ky were sampled at high resolution and analysed for major and trace elements the stratigraphy of the cores was obtained by comparing the ba al ratio with the specmap data this method is based on the close correlation between ba concentrations or ba al ratios and oxygen isotope ratios 18o of foraminifera in arabian sea sediments ba al ratios used as a productivity proxy are variable but high in all deep sea cores of the arabian sea indicating a basin wide influence of nutrient rich water masses compared to glacial intervals the warmer periods are characterised by higher element al ratios of proxies which are directly ba ca sr p i e hard parts of organisms or indirectly u 230thex related to biological productivity 230thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the indus fan the mn distribution in a core from the western arabian sea shows enrichments during interglacial periods and may indicate mn export owing to the presence of a stronger oxygen minimum zone several processes limit the applicability of proxies intercalated turbidites in the deep sea cores can be identified by means of si al ti al and zr al ratios versus depth due to the enrichment of quartz ti minerals and zircon in the basal layer these elements are also important for the reconstruction of the history and extent of the eolian dust input diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and normal pelagic sedimentation and may mask or destroy primary signals

magnetic basement in the bay of bengal through werner deconvolution abstract marine magnetic data from 16 well distributed profiles over bay of bengal were analysed to delineate the magnetic basement which in general matches with the sediment thickness pattern a series of high and low reliefs ride over a basement level of 11 12 km the high associated with 85 e ridge extends northward with an obliteration around 13 n parallel isolated depressions in this region and a nw se trending high between 11 and 15 n latitudes and 86 and 89 e longitudes are some of the features observed in the basement the basement obtained from available seismic data across 13 latitude shows a good agreement with that derived from the present analysis the deep basement in the northeast conforms to the high sediment thickness the undulations in the basement might be due to the stresses acting on the ocean floor at the time of break up of indian plate from gondwanaland

debris fan constrictions and flood hydraulics in river canyons some implications from two dimensional flow modelling rapids in river canyons are frequently found at sites where debris fans constrict flow along the channel whereas some fans may have persisted in the same location with unchanging geometry for centuries to millennia others have changed in response to flow conditions imposed by successive floods such a change in boundary conditions may alter local flow hydraulics this paper utilizes two dimensional flow modelling to compare flood hydraulics along two alternative versions of an idealized reach of a river canyon one with uniform width gradient and cross section and a second perturbed by a prominent debris fan along the valley wall the flow pattern along the reach with the fan is far more complex than the pattern along the uniform reach maximum velocity along the debris fan reach is up to 50 per cent higher than along the uniform reach maximum bed shear stress is up to three or four times higher and an area of supercritical flow is predicted extending from the nose of the fan into the zone of flow expansion immediately downstream comparison of model output along longitudinal profiles of the two reaches indicates that the backwater effect of the fan extends several valley widths upstream predicted flows based on the same stage are as much as 190 to 230 per cent greater along the uniform reach than along the debris fan reach reconstruction of palaeoflood discharge based on remnant flood marks in the vicinity of the fan would be sensitive to assumptions about boundary conditions that existed in the past this effect relaxes over a longitudinal distance of several hundred metres furthermore there are significant cross stream gradients that change slope and direction several times in the vicinity of the fan calling into question the utility of one dimensional step backwater hydraulic models for predicting high water marks in areas of complex valley morphology

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

238u234u230th disequilibrium in hydrogenous oceanic femn crusts palaeoceanographic record or diagenetic alteration abstract a detailed tims study of 234uexc 238u 230th 232th and th u ratios have been performed on the outermost margin of ten hydrogenous fe mn crusts from the equatorial pacific ocean and west central indian ocean th u concentration ratios generally decrease from the crust s surface down to 0 5 1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in fe mn crusts from the peru basin and the west central indian ocean fe mn crusts from the same geographical area define a single trend in plots of ln 234uexc 238u vs ln 230th 232th and th u ratios vs age of the analysed fractions results suggest that l hydrogenous fe mn crusts remain closed systems after formation and consequently 2 the discrepancy observed between the 230th and 234u chronometers in fe mn crusts and the variations of the th u ratios through the margin of femn crusts are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation but rather to temporal variations of both th u and initial thorium activity ratios recorded by the fe mn layers implications of these observations for determination of fe mn crust growth rates are discussed variations of both th u and initial th activity ratios in fe mn crusts might be related to changes in particle input to seawater and or changes in ocean circulation during the last 150 ka

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions abstract keller keller e a 1971 areal sorting of bed load material the hypothesis of velocity reversal geological society of america bulletin 82 753 756 hypothesized that at high flow near bed velocities in pools exceed velocities in riffles and create pool scour pools however typically have larger cross sectional areas of flow at bankfull discharge this condition raises an inconsistency with keller s velocity reversal hypothesis and the one dimensional continuity of mass equation to address this problem a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit slopes of pools according to the model a narrow zone of high velocity occurs in the center of pools creating scour along the downstream end of pools an uphill climb of particles up the pool exit slope promotes sediment deposition the model is tested with field and flume measurements of velocity water surface elevation and size of bed sediments in recirculating eddy influenced pools local reversals of the water surface gradient were measured in the field and a velocity reversal was created in the flume the reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow the distribution of bed sediments collected in the field also support the proposed model of pool maintenance

simulation of flow over pool riffle topography a consideration of the velocity reversal hypothesis computer flow simulations using the hec 2 step backwater routine are used to demonstrate the effect of systematically varying river channel width riffle spacing and channel roughness on the shear velocity section mean velocity and energy slope in fixed bed pool riffle sequences initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the river severn subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored the purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence reversal such that pools scour at high flow whilst deposition occurs on riffles it is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal for prescribed conditions the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool riffle topography

climatic influences on the growth rates of mn crusts during the late quaternary abstract the concentration of 230 th in mn crust va13 2 146 w 9 25 s 4830 m from the pacific ocean was measured with high depth resolution 0 02 mm to a depth of 1 4 mm corresponding to an age of 226 ka b p the mean growth rate estimated from these results is 6 2 0 4 mm ma however the exponential curve obtained shows two drastic events in the 230 th concentration record with durations of 41 4 ka and 37 6 ka which are interpreted as growth standstills the decay corrected 230 th concentration is inversely correlated with those of mn fe and co indicating that the 230 th flux into the crust has been constant throughout time and that the observed fluctuations in 230 th are due to dilution by the main components by applying a constant 230 th flux model high resolution growth rates were derived from every sample investigated a detailed comparison of the growth pattern with the specmap time table reveals that the va13 2 growth rates tend to be higher during interglacial than during glacial periods in particular the observed growth standstills are associated with glacial stages 6 and 8 further investigation of mn crust 72 dk 9 taken from shallower water 170 35 w 20 13 n 1550 m tend to confirm that there is a link between the growth rates of hydrogenetic crusts and the climate of the late quaternary

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

osmium isotope variations in the oceans recorded by fe mn crusts this study presents osmium os isotope data for recent growth surfaces of hydrogenetic ferromanganese fe mn crusts from the pacific atlantic and indian oceans in general these data indicate a relatively uniform os isotopic composition for modern seawater but suggest that north atlantic seawater is slightly more radiogenic than that of the pacific and indian oceans the systematic difference in the os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high re os ratio in the north atlantic ocean consistent with the distribution of nd and pb isotopes this spatial variation in the os isotope composition in seawater is consistent with a residence time for os of between 2 and 60 kyr indian ocean samples show no evidence of a local source of radiogenic os which suggests that the present day riverine input from the himalaya tibet region is not a major source for os recently formed fe mn crusts from the tag hydrothermal field in the north atlantic yield an os isotopic composition close to that of modern seawater which indicates that in this area the input of unradiogenic os from the hydrothermal alteration of oceanic crust is small however some samples from the deep pacific 4 km possess a remarkably unradiogenic os isotope composition 187 os 186 os ratios as low as 4 3 the compositional control of os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles rather than indicating the presence of an unradiogenic deep water mass moreover this unradiogenic signal appears to be temporary and local and has had little apparent effect on the overall evolution of seawater these results confirm that input of continental material through erosion is the dominant source of os in seawater but it is not clear whether global os variations are due to the input of mantle or meteoritic material or simply indicate that the continental source itself is not uniform 1999 elsevier science b v all rights reserved

the influence of the carbonate dissolution rate on the growth and composition of co rich ferromanganese crusts from central pacific seamount areas abstract the metal composition of oceanic ferromanganese deposits occurring in seamount regions line islands chain and mid pacific mountains varies with water depth and age the results of metal determinations of carbonate plankton samples suggest that carbonate dissolution in the water column might have an important influence on the accretion and composition of hydrogenetic precipitates two ferromanganese crust generations of different age have been observed the precipitation of the older crust took probably place during early oligocene the younger crust began to form during middle miocene between the two crust generations periods of carbonate sedimentation and of phosphorite deposition occur the hydrogenetic formation of the crusts is controlled by the metal supply from the water column according to the laws of colloidal surface chemistry dissolution experiments with carbonate plankton samples show that the main fe source for the hydrogenetic crust formation are colloidal fe hydroxide particles being released in the water column from the dissolution of carbonate plankton skeletons in the case of mn maximum dissolved mn occurs in the oxygen minimum zone as the result of in situ break down of organic matter and the in situ reduction of mn bearing solid phases closely beneath the oxygen minimum zone a fe supply mobilized within the oxygen minimum zone has also to be taken into account in the water column below the oxygen minimum zone a mixture of colloidal particles of mn fe oxyhydroxide and colloidal al fe silicate precipitate together on the surface of substratum rocks the mixing ratio of these colloidal phases controlling the metal composition of the ferromanganese precipitates is depth dependent and shows also temporal variations in general mn fe ratio ni and co contents decrease with depth down to the calcite compensation depth the most probable mechanism for the ultimate removal of co and ni from the water column might be a surface reaction mno2 is specifically able to absorb hydrous co2 and ni2 ions because of the surface enrichment of co and the strong electrical field of mn iv a subsequent oxidation of co2 to co3 takes place leading to higher enrichment of co in comparison to ni the most important factor governing the high co enrichment in the ferromanganese crusts is the growth rate the lower the growth rate the higher the co content maximum values of up to 2 co occurring in samples from water depths between 1500 and 1100 m 1 are related to lower carbonate dissolution rates and corresponding lower fe supply the metal supply from the water column is strongly related to distinct environmental factors such as bio productivity range of lysocline and calcite compensation depth rate of carbonate dissolution and activity of the antarctic bottom water thus our model shows that the growth periods and the metal composition of hydrogenetic seamount crusts are controlled by changes in the paleoceanography and reflect distinct environmental conditions

flow around a cylinder shallow water modeling with diffusion dispersion the equations for two dimensional 2d horizontal flow are obtained by vertical depth integration of the continuity and momentum equations diffusion dispersion terms appear containing turbulent and dispersion stresses turbulent stresses are expressed with the eddy viscosity concept dispersion stresses are evaluated using velocity distributions along and across the curved streamlines the differential equations are solved using the maccormack scheme the numerical simulation was done for three runs the expected alteration of the flow field around the cylinder is evident notably the wake behind the cylinder for comparison two similar runs were performed in a laboratory channel the velocity vectors upstream from the cylinder and along its sides are in reasonably good agreement however very close to the cylinder the simulation sometimes underpredicts the velocities downstream from the cylinder agreement is satisfactory both inside and outside the wake the flow depths at the centerline both upstream and downstream from the cylinder are also in good agreement however along the cylinder circumference the simulated flow depths are lower than the observed ones

unifying criterion for the velocity reversal hypothesis in gravel bed rivers it has been hypothesized that velocity reversals provide a mechanism for maintaining pool riffle morphology in gravel bed rivers an important habitat for salmonids which are at risk in many places worldwide and that are the focus of extensive environmental legislation in europe and north america however the occurrence of velocity reversals has been controversial for over 3 decades we present a simple one dimensional criterion that unifies and explains previous disparate findings regarding the occurrence of velocity reversals results show that reversal depends critically on the ratio of riffle to pool width residual pool depth difference between pool and riffle elevations and on the depth of flow over the riffle suggesting that land management activities which alter channel form or divert water from the channel can have negative impacts on the sustainability of pool riffle habitat in gravel bed rivers

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

u th chronology and paleoceanographic record in a fe mn crust from the ne atlantic over the last 700 ka hydrogenetic ferromanganese crusts fe mn crusts provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes in this respect the acquisition of an absolute and reliable chronology in fe mn crusts is a prerequisite here we combine four different and complementary chronometers 10 be 230 thex 230 thex 232 th 234 u 238 u in a fe mn crust dredged at 2000 m depth in the east atlantic to first establish a reliable chronology over the quaternary period then we use eds chemical analysis to look for correlation between major element chemistry and climate changes 230 thex 230 thex 232 th and be data give very consistent growth rates in particular the good match between 230 thex and 230 thex 232 th data indicates that at the location of crust 121dk 230 th and 232 th fluxes in the water column change simultaneously and suggests that the normalization of 230 thex to 232 th makes 230 thex 232 th a better chronometer our best fit model suggests that crust 121dk experi enced changes in growth rates at 122 and 312 ka and a growth with a constant 230 th initial flux this chronology returns an age of 680 ka for the uppermost 1 5 mm the 234 u 238 u depth profile however was clearly affected by diffusion of 234 u in the porous crust and can therefore not be used to derive a reliable chronology one part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity on the basis of the 230 thex 232 th chronology major element chemistry is shown to be linked to climate change mn fe variations compare well with those in a fe mn crust from the pacific showing systematic maxima during glacial stages 2 and 4 high mn fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods resulting from higher bioproductivity in addition we note that the surface 230 th 232 th activity ratio of crust 121dk is entirely consistent with advection of deep water from the western toward the eastern atlantic basin copyright 2005 elsevier ltd

processes controlling the heavy metal distribution in pacific ferromanganese nodules and crusts the ferromanganese precipitates existing in deep sea waters of the pacific consist of two types of deposits 1 nodules mainly are distributed in pelagic basins beneath the ccd calcite compensation depth where the rate of sedimentation is low 2 polymetallic encrustations are formed on exposed seamount rocks where currents prevent normal sediment accumulation nodules being formed in areas bordering the equatorial zone of high biological productivity grow by two different processes a early diagenetic growth by supply of metals and metal compounds from pore water and b hydrogenetic growth by supply of colloidal particles from near bottom seawater these processes lead to different kinds of oxide and different metal contents

flow reversal over a natural pool riffle sequence a computational study a computational study is presented on the hydraulics of a natural pool riffle sequence composed of mixed cobbles pebbles and sand in the river lune northern england a depth averaged two dimensional numerical model is employed calibrated with observed data at the field site from the computational outputs the occurrence of longitudinally double peak zones of bed shear stress and velocity is found in particular at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope in addition to a secondary peak at the pool head as discharge increases the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope showing progressive equalization to the surrounding hydraulic profiles concurrently the secondary peak due to channel constriction appears to stand at the pool head with its value increasing with discharge and approaching or exceeding the primary peak over the riffle the existence of flow reversal is demonstrated for this specific case which is attributable to channel constriction at the pool head a dynamic equilibrium model is presented to reconstruct the pool riffle morphology a series of numerical modelling exercises demonstrates that the pool riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments it is concluded that channel constriction can but may not necessarily lead to competence reversal depending on channel geometry flow discharge and sediment properties

correlation of co concentration and growth rate a method for age determination of ferromanganese crusts abstract based on the analysis and comparison of metal content and growth rate determination by the 10be method an empirical law has been developed to calculate the growth rate evaluating after 520 chemical analyses we have shown that the flux of co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated the amount of co supplied per unit of time and seabottom area is nearly constant in the oceanic water column the layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates the most important indicator revealing the growth rate is the co enrichment of each crust layer the higher the growth rate the lower the co concentration therefore it is possible to recalculate the age and growth rate of each ferromanganese crust layer by g mm ma 1 1 28 co 0 24 based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18 12 ma so called older crust two additional effects have to be taken into account a the intense phosphorite impregnation causing dilution and b the chemical dissolution of the ferromanganese crusts caused by the phosphate rich pore water both reduce the co concentration and the relating co contents co x and co x have thus to be compensated a co x co x m mn co x mn co b b co x co x 1 0 05 p 1 where co x double corrected co concentration of the layer x of the older crust generation co x corrected co concentration of the layer x of the older crust generation concerning the phosphate dilution co x m measured co concentration of the layer x of the older crust generation mn co x mn co ratio measured of the layer x of the older crust generation mn co b mn co ratio measured of the boundary layer between younger and older crust generation and p difference between the apatite fraction of the older crust of the layer x and the average value of the younger crust generation this simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts

beryllium 10 thorium 230 and protactinium 231 in galapagos microplate sediments implications of hydrothermal activity and paleoproductivity changes during the last 100 000 years biogenic particle fluxes from highly productive surface waters boundary scavenging and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the galapagos microplate eastern equatorial pacific in order to evaluate the importance of these three processes throughout the last 100 kyr concentrations of the radionuclides 10be 230th and 231pa and of mn and fe were measured at high resolution in sediment samples from two gravity cores klh 068 and klh 093 high biological productivity in the surface waters overlying the investigated area has led to 10be and 231pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr however during periods of high productivity at the up welling centers off peru and extension of the equatorial high productivity zone a relative loss of 10be and 231pa may have occurred in these sediment cores because of boundary scavenging the effects of hydrothermal activity were investigated by comparing the 230thex concentrations to the mn fe ratios and by comparing the fluxes of 230th and 10be which exceed production the results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core klh 093 during isotope stages 2 and 3 the hydrothermal supply of mn was deposited elsewhere probably because of changes in current regime or deep water oxygenation a strong increase of the mn fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230thex concentration is interpreted to be an effect of mn remobilization and reprecipitation in the sediment

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

engineered channel controls limiting spawning habitat rehabilitation success on regulated gravel bed rivers in efforts to rehabilitate regulated rivers for ecological benefits the flow regime has been one of the primary focal points of management strategies however channel engineering can impact channel geometry such that hydraulic and geomorphic responses to flow reregulation do not yield the sought for benefits to illustrate and assess the impacts of structural channel controls and flow reregulation on channel processes and fish habitat quality in multiple life stages a highly detailed digital elevation model was collected and analyzed for a river reach right below a dam using a suite of hydrologic hydraulic geomorphic and ecological methods results showed that despite flow reregulation to produce a scaled down natural hydrograph anthropogenic boundary controls have severely altered geomorphic processes associated with geomorphic self sustainability and instream habitat availability in the case study given the similarity of this stream to many others we concluded that the potential utility of natural flow regime reinstatement in regulated gravel bed rivers is conditional on concomitant channel rehabilitation

spatial variations in surface sediment structure in riffle pool sequences a preliminary test of the differential sediment entrainment hypothesis dseh riffle pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles this preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows however the differential sediment entrainment hypothesis dseh postulates that differential entrainment can instead result from spatial sedimentological contrasts here we use a novel suite of in situ grain scale field measurements from a riffle pool sequence to parameterise a physically based model of grain entrainment field measurements include pivoting angles lift forces and high resolution dems acquired using terrestrial laser scanning from which particle exposure protrusion and surface roughness were derived the entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles this is because pool grains have looser packing hence greater exposure and lower pivoting angles conversely riffle and pool exit grains have denser packing lower exposure and higher pivoting angles a cohesive matrix further stabilises pool exit grains the resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2d computational fluid dynamics cfd model of the reach the cfd model predicts that under bankfull conditions pools experience lower shear stresses than riffles and pool exits however the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles resulting in differential entrainment under bankfull flows significantly this differential entrainment does not require a reversal in flow velocities or shear stress suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle pool sequences this finding has implications for the prediction of sediment transport and the morphological evolution of gravel bed rivers

the negative gravity field over the 85 e ridge an isopach map made from seismic reflection and refraction data in the bay of bengal shows two prominent n s trending features in the basement topography one is the northernmost portion of the ninetyeast ridge which is totally buried by sediments north of 10on the other buried ridge trends roughly n s for 1400 km at 85oe to the latitude of sri lanka and then curves toward the west it has basement relief up to 6 km two free air gravity anomaly profiles across the region show a strong gravity low 60 mgal over the 85oe ridge while the ninetyeast ridge shows a gravity high to understand the negative free air gravity anomaly over the 85oe ridge we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load a plausible formation history for a buried ridge consists of at least two major episodes the first is the formation of the ridge on a lithosphere with a flexural rigidity of d i at some later time the ridge is buried by an influx of sediments the lithosphere is cooler and the flexural rigidity has increased to d 2 the character of the gravity field depends primarily upon the initial and final values of flexural rigidity these d1 and d2 values are varied to obtain good agreement between the model and observed gravity anomalies best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial an approximate relationship between flexural rigidity and crustal age shows that the 85oe ridge was formed on relatively young lithosphere 5 15 m y old and that it was buried when the lithosphere was 40 80 m y old

new age for ferromanganese crust 109d c and implications for isotopic records of lead neodymium hafnium and thallium in the pliocene indian ocean 1 this study presents a high resolution record of osmium and thallium isotopes in a ferro manganese fe mn crust from the indian ocean antipode 109d c these results when combined with additional new os isotope data from odp hole 756b in the southeast indian ocean define a new best estimate for the age at the base of this crust of 6 5 ma which is significantly different from a previous estimate of 15 ma based on co flux modeling the tl isotope record obtained for the indian ocean resembles that for the pacific ocean with a small but well defined increase occurring over the last 5 myr this contrasts with two records from the atlantic ocean which do not have resolvable variations ocean basin scale tl isotope variation may be inconsistent with the inferred modern marine residence time for tl of 20 kyr but could be explained by an increase in ocean crust production rates in the pacific and indian oceans since 10 ma the improved age model for 109d c reveals that the hf isotope composition of indian ocean bottom waters has remained homogenous over the last 6 myr thus this isotope system does not bear any evidence that the influence of north atlantic deep water in the formation of indian ocean bottom waters has changed during that time however because of the lack of knowledge about hf isotopes as a tracer of ocean circulation we cannot conclude that export of nadw decreased over the last 6 myr

238u234u230th chronometry of femn crusts growth processes and recovery of thorium isotopic ratios of seawater abstract comparison of 234 u excess 238 u and 230 th 232 th activity ratios in oceanic fe mn deposits provides a method for assessing the closed system behaviour of 238u 234u 230th as well as variations in the initial uranium and thorium isotopic ratios of the precipitated metal oxides this approach is illustrated using a fe mn crust from lotab seamount marshall islands west equatorial pacific here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust and from two depth profiles of the outermost rim of the same botryoid the decrease of 234 u excess 238 u and 230 th 232 th activity ratio with depth in the two profiles gives mean growth rates for the last 238u 212th 150 ka of 7 8 2 mm ma and 6 6 1 mm ma respectively all data points surface and core samples but one define a linear correlation in the ln 230 th 232 th ln 234 u excess 238 u diagram this correlation indicates that for all points the u th system remained closed after the fe mn layer precipitated and that the different samples possessed the same initial uranium and thorium isotope ratios furthermore these results show that the preserved surface of this fe mn crust may not be the present day growth surface and that the thorium and uranium isotopic ratios of seawater in west equatorial pacific have not changed during the past 150 ka the initial thorium activity ratio is estimated from the correlation obtained between ln 230 th 232 th ln 234 u excess 238 u

thin oceanic crust below bay of bengal inferred from 3 d gravity interpretation abstract the bay of bengal bob lithosphere evolved due to rifting between india and antarctica and subsequent seafloor spreading process since the early cretaceous is known for the presence of thick bengal fan sediments covering the entire bob with maximum sediment accumulation of 22 km in the offshore bangladesh three dimensional crustal structure of the bob is investigated by integrating the sediment isopach map seismic reflection refraction and wide angle reflection data with the 3 d gravity modelling large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function 3 d gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from 5 mgal over the ninetyeast ridge near the equator to as low as 210 mgal at the apex of bengal fan in the bangladesh shelf region the residual crustal bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to moho in the bob region varying from 11 km in the south to as much as 35 km in the bangladesh shelf with moho depressed locally below both 85 e and ninetyeast ridges the total crust map obtained after subtracting the water and sediment layers from the moho depth map shows an unusually thin oceanic crust of 2 4 km in the deep basinal areas of bob and 10 12 km of thick crust below 85 e and ninetyeast ridges due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85 e and ninetyeast ridges we infer its formation during plume spreading interactions in the eastern indian ocean further higher flexural rigidity of lithosphere and cold mantle in the bob region indicates that huge amount of bengal fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the amazon fan

thorium and uranium isotopes in a manganese nodule from the peru basin determined by alpha spectrometry and thermal ionization mass spectrometry tims are manganese supply and growth related to climate thorium and uranium isotopes were measured in a diagenetic manganese nodule from the peru basin applying alpha and thermal ionization mass spectrometry tims alpha counting of 62 samples was carried out with a depth resolution of 0 4 mm to gain a high resolution230thexcess profile in addition 17 samples were measured with tims to obtain precise isotope concentrations and isotope ratios we got values of 0 06 0 59 ppb 230th 0 43 1 40 ppm 232th 0 09 0 49 ppb 234u and 1 66 8 24 ppm 238u the uranium activity ratio in the uppermost samples 1 6 mm and in two further sections in the nodule at 12 5 1 0 mm and 27 3 33 5 mm comes close to the present ocean water value of 1 144 0 004 in two other sections of the nodule this ratio is significantly higher probably reflecting incorporation of diagenetic uranium the upper 25 mm section of the mn nodule shows a relatively smooth exponential decrease in the230thexcess concentration tims the slope of the best fit yields a growth rate of 110 mm ma up to 24 5 mm depth the section from 25 to 30 3 mm depth shows constant230thexcess concentrations probably due to growth rates even faster than those in the top section of the nodule from 33 to 50 mm depth the growth rate is approximately 60 mm ma two layers in the nodule with distinct laminations 11 15 and 28 33 mm depth probably formed during the transition from isotopic stage 8 to 7 and in stage 5e respectively the mn fe ratio shows higher values during interglacials 5 and 7 and lower ones during glacials 4 and 6 a comparison of our data with data from adjacent sediment cores suggests a a variable supply of hydrothermal mn to sediments and mn nodules of the peru basin or b suboxic conditions at the water sediment interface during periods with lower mn fe ratios

rhythmic growth of pacific ferromanganese nodules and their milankovitch climatic origin the internal microstructure of a ferromanganese nodule 2392 from 154 37 52 w 9 37 56 n at water depth 5194 m was examined in detail on polished sections and radiometrically dated 230thex 232th along a high resolution 0 1 mm depth profile 0 1 3 mm spanning approximately 271 ka the fabric shows typical stromatolithic structure and exhibits four orders of basic cyclic growth pattern namely laminae bands laminae zones laminae groups and laminae pairs having average thicknesses of 402 454 185 206 58 67 and 15 18 m respectively a profile from the depth of 200 2661 m was selected to obtain the geochemical series using line scanning electron microprobe analyses which provide a record of paleoceanographic oscillations during the growth period power spectral analysis of the geochemical series for al mn fe and fe mn from the depth of 200 1220 m where no obvious discontinuity could be observed display conspicuous cyclicities the cycles of laminae zones laminae groups and laminae pairs are reflected in the spectral patterns as well the significant spectral peaks are located at 186 108 66 m together with the cycle of laminae bands the ratios of these cyclicities are close to those of milankovitch orbital cycles through tuning to orbital cycles a net growth rate of 4 5 mm ma is derived for the profile which is in perfect agreement with the growth rate of 4 6 mm ma determined by 230thex 232th dating therefore the rhythmic growth of ferromanganese nodules appears definitely associated with milankovitch cycles and the growth cyclicities may offer a new tool for estimating growth rates of ferromanganese nodules and paleoenvironmental reconstruction at substage resolution when supported by radiometric dating

velocity reversal hypothesis a model approach hydraulic modelling of a riffle pool riffle sequence in a straight reach of dry creek near winters california indicates that while at low flow the mean section average velocity at riffles exceeds that of an adjacent pool during high flow the mean velocity of the pool exceeds that of adjacent riffles this supports the velocity reversal hypothesis which is based on limited field measurements of near bed flow velocity a velocity reversal does not occur at the same discharge in both riffle pool sequences modelled and a more detailed explanation is necessary to account for complex aspects of channel form and process in riffle pool sequences nevertheless the velocity reversal hypothesis helps to explain many of the observations and processes that operate in the riffle pool environment

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

60 myr records of major elements and pb nd isotopes from hydrogenous ferromanganese crusts reconstruction of seawater paleochemistry we compare the time series of major element geochemical and pb and nd isotopic composition obtained for seven hydrogenous ferromanganese crusts from the atlantic indian and pacific oceans which cover the last 60 myr average crust growth rates and age depth relationships were determined directly for the last about 10 myr using be 10 be 9 profiles in the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial be 10 be 9 ratios due to the lack of additional information co contents have also been used previously to estimate growth rates in co rich pacific and atlantic seamount crusts puteanus and halbach 1988 a comparison of be 10 be 9 and co based dating of three co rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated be 10 be 9 based growth rates back to 60 ma our data show that the flux of co into co poor crusts has been considerably lower the relationship between growth rate and co content for the co poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits manheim 1986 the results suggest that the co content provides detailed information on the growth history of ferromanganese crusts particularly prior to 10 12 ma where the be 10 based method is not applicable the distributions of pb and nd isotopes in the deep oceans over the last 60 myr are expected to be controlled by two main factors a variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and b variability of supply rates or provenance of detrital material delivered to the ocean linked to climate change glaciations or major tectonic uplift the major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust consequently any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution this is because local processes appear to have dominated over more basin wide paleoceanographic effects in this study co is the only element which shows a relationship to pb and nd isotopes in pacific crusts a possible link to changes of pacific deep water properties associated with an enhanced northward advection of antarctic bottom water from about 14 ma is consistent with the pb but not with the nd isotopic results the self consistent profiles of the pb and nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts including phosphatization events have been insignificant for particle reactive elements such as pb be and nd isotope time series of pb and nd show no systematic relationships with major element contents of the crusts which supports their use as tracers of paleo seawater isotopic composition

palaeoceanographic conditions during the formation of a ferromanganese crust from the afanasiy nikitin seamount north central indian ocean geochemical evidence abstract a ferromanganese crust dredged from the summit of the afanasiy nikitin seamount in the north central indian ocean ncio has recorded neogene oceanographic events the substrate of the crust is composed of fresh water phreatic calcite cement terebratulinae casts rounded and ferruginised basalt clasts and weathered coralline algal fragments suggesting subaerial exposure during the oligocene 30 ma global sealevel drop the mineralogy major trace and ree element geochemistry and co model age estimates suggest three distinct accretionary environments during the crust growth 1 a period of contemporary precipitation of fe mn oxide and carbonate fluorapatite cfa indicate an intensified intermediate water oxygen minimum zone omz in the late miocene 2 a pulse of very high cfa deposition and detrital input in addition to fe mn oxide accretion at the close of miocene reflect a more intense omz and the erosion of the himalayas and 3 more oxidizing conditions of the ambient seawater due to contraction of the late miocene omz facilitated the accretion of almost pure fe mn oxide since the pliocene these interpretations solely depend upon the chronology based on an empirical relationship and are subjected to the confirmation by radiometric dating as a consequence of significant deposition of cfa in addition to fe mn oxide during the intensified omz the normally coherent behaviour of trivalent rare earth elements 3 ree is not observed in the crust la yb and lu show a positive association with the cfa phase elements ca and p and pr nd and sm with the mn oxide phase elements mn co and ni this leads to an abnormal incoherent behaviour of the 3 ree in multimineral authigenic system

effects of three gorges dam operation on spatial distribution and evolution of channel thalweg in the yichang chenglingji reach of the middle yangtze river china abstract the evolution of the channel thalweg and pool riffle sequences is a critical feature of morphological adjustments in fluvial systems and might have important effects on aquatic habitat quality channel stability and navigation based on the 220 cross sectional profiles from 2002 to 2016 residual depths and spatial statistical analyses were used to quantitatively analyse the vertical variations and longitudinal distribution of the pool riffle sequences and channel thalweg in the yichang chenglingji reach ycr of the middle yangtze river which consists of the yizhi zhijiang shashi gongan and shishou jianli sub reaches the results showed that the dramatic reduction in the sediment loads following the construction of the three gorges dam tgd has caused significant adjustments in the size morphological diversity and spatial distribution of pool riffle structures in the ycr the average depth of the pools in the yizhi zhijiang reach and the shishou jianli reach overall exhibited a nonlinear growth trend with the average residual depth increasing by 1 4 m and 1 9 m respectively however the average residual depth in the shashi gongan reach showed a decreasing trend with a reduction of approximately 1 2 m the longitudinal lengths of the riffles in all three reaches contracted to different degrees and the specific percentage of longitudinal river length occupied by riffles total riffle length total sub reach length decreased by 8 9 6 and 5 9 in the three sub reaches from upstream to the downstream in addition the morphological diversity of the thalweg elevation in the shashi gongan reach and shishou jianli reach weakened after the construction of the tgd while the bed topography in the yizhi zhijiang reach became more complex and diverse moreover the pool riffle structures in the ycr were rearranged and distributed regularly following the initiation of the tgd operation although the variation characteristics of the average pool spacing in the three sub reaches were quite different quantitative relationship between the average residual depth and the previous four year hydrological condition was proposed for the three sub reaches based on the delayed response model drm and verified using the measurements from 2013 to 2016

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

growth response of a deep water ferromanganese crust to evolution of the neogene indian ocean abstract a deep water ferromanganese crust from a central indian ocean seamount dated previously by 10 be and 230 th excess was studied for compositional and textural variations that occurred throughout its growth history the 10 be 9 be dated interval upper 32 mm yields an uniform growth rate of 2 8 0 1 mm ma frank m o nions r k 1998 sources of pb for indian ocean ferromanganese crusts a record of himalayan erosion earth planet sci lett 158 pp 121 130 which gives an extrapolated age of 26 ma for the base of the crust at 72 mm and is comparable to the maximum age derived from the co model based growth rate estimates this study shows that fe mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the eocene 53 ma until the late oligocene 26 ma this paucity probably was the result of a nearly overlapping palaeo ccd and palaeo depth of crust formation increased early eocene productivity instability and reworking of the surface rocks on the flanks of the seamount and lack of oxic deep water in the nascent indian ocean crust accretion began older zone with the formation of isolated cusps of fe mn oxide during a time of high detritus influx probably due to the early miocene intense erosion associated with maximum exhumation of the himalayas op cit this cuspate textured zone extends from 72 mm to 42 mm representing the early miocene period intense polar cooling and increased mixing of deep and intermediate waters at the close of the oligocene might have led to the increased oxygenation of the bottom water in the basin a considerable expansion in the vertical distance between the seafloor depth and the ccd during the early miocene in addition to the influx of oxygenated bottom water likely initiated fe mn crust formation pillar structure characterises the younger zone which extends from 40 mm to the surface of the crust i e 15 ma to present this zone is characterised by 25 higher content of oxide bound elements than in the older zone possibly corresponding to further increased oxygenation of bottom waters increased stability of the seamount slope and gradually reduced input of continental detritus from the erosion of the himalayas middle miocene antarctic glaciation which peaked 12 13 ma ago increased the oxic bottom water influx to the basin resulting in accretion of the crust with low detritus therefore the younger crust started to accrete in response to a shift in bottom water circulation towards the contemporary pattern which produced a uniform growth rate and pillar structure up to the present

nd isotope signature of holocene baltic mn fe precipitates as monitor of climate change during the little ice age neodymium nd isotope profiles were analyzed on two baltic mn fe precipitates 99 2 and tl1 from shallow water 20 m of the mecklenburg bay the age range of these mn fe precipitates determined by 226raex ba dating reaches from recent growth back to 4300 and 1000 yr bp respectively over this time range the nd isotope composition varies from end 0 13 1 to 17 5 in the selected baltic precipitates indicating substantial changes in the nd isotope composition of the baltic sea the lowest end values were recorded during the time interval of the little ice age lia ad 1350 to 1850 these minimum values indicate either an increase of the input of less radiogenic nd from scandinavian archean proterozoic sources end about 22 to the baltic sea or a decrease of the input of more radiogenic nd from continental european sources end about 12 and or north sea water end about 10 variations of both erosive continental input and north sea inflow may indicate a direct response of the nd isotope signal in the baltic sea to climate changes during the lia and be related to cyclic shifts in the atmospheric circulation triggered by the north atlantic oscillation nao another aspect that possibly influenced the input of trace elements and nd isotopes into the baltic sea is the population development in the circum baltic area during the lia the lowest end values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland the reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the baltic sea by rivers which in turn may have resulted in a change of the nd isotope composition of baltic sea water

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

magnetic crust in the bay of bengal abstract four marine magnetic profiles across the bay of bengal are analyzed and the magnetic nature of the upper part of the crust is investigated by inversion in the frequency domain the magnetization distribution at the average depth of the basement below the sediments is computed and shows high magnetization with alternating polarity the spectral decay provides a maximum estimate of the depth to basement of 8 and 10 km along profiles approximately parallel to 10 and 14 n respectively this suggests a thick pile of sediments in this region with thickness increasing towards the north the inversion of the recorded magnetic field along the 10 n profile at an average depth of 8 km provides alternating magnetization of approximately 1 2 10 3 cgs units corresponding to basic rocks this indicates a magnetic basement in the bay of bengal with alternating polarities typical of oceanic crust greater magnetization 3 10 3 cgs units over the 85 east ridge and the pattern of computed magnetization distribution on either side of this ridge indicate that it might have been a spreading ridge at sometime in the geological past which subsequently aborted

the hf isotope composition of global seawater and the evolution of hf isotopes in the deep pacific ocean from fe mn crusts this paper presents a revised chemical purification method of hf for the measurement of hf isotope ratios of fe mn crusts on a new generation of double focusing multiple collector plasma source mass spectrometer mc icp ms by measuring surface scrapings of hydrogenetic fe mn crusts distributed over the three major ocean basins the present day hf isotope distribution of oceanic deep water is assessed in detail the results show an ehf provinciality correlated with those of other radiogenic isotope tracers such as nd and pb in agreement with previous studies this supports the use of hf isotopes as tracer of element source provenance and water masses fe mn crusts display the same present day hf nd isotope array as given before for mn nodules the smaller isotopic variability of hf compared with nd may either be caused by a more efficient mixing of hf than nd in the ocean due to a longer residence time or may be a consequence of a systematically more radiogenic hf than nd isotope signature delivered to the oceans by weathering of continental crust a hf isotope time series was measured on crust va13 2 to assess the hf isotope compositions of the central pacific deep water over the past 26 ma no consistency is observed between the hf and pb isotope time series in contrast hf and nd isotope time series display similar patterns which are however apparently offset by 2 ma prior to 14 ma differential diffusion of hf and nd does not explain this offset the smaller amplitude in the hf isotope variations compared with the nd isotopes rather argues for more efficient mixing of hf in the ocean we suggest that both isotope systems have responded in a similar way to the processes affecting the dissolved radiogenic isotope composition of pacific deep water during this time interval the parallel increase in ehf and end observed between 14 and 3 ma may probably be attributed to the increased inputs of hf and nd into the central pacific ocean derived from the weathering of the pacific islands arcs over the past 3 ma an increased aeolian continental input derived from asia most likely caused negative shifts in end and ehf recorded by va13 2 the fact that hf and nd isotope compositions plot along the present day array for fe mn crusts and mn nodules over the entire past 26 ma suggests that aeolian supply of hf to the pacific ocean has been a long term important feature

modelling three dimensional flow structures and patterns of boundary shear stress in a natural pool riffle sequence fluid sediment interactions control river channel forms and processes analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour in this paper the hydraulic processes active in a natural pool riffle sequence are simulated using a three dimensional computational fluid dynamics cfd model methods employed for the prescription of model boundary conditions are outlined model calculations are assessed using comparisons with field observations acquired over a range of flows simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near bankfull and an intermediate flow event results are used to assess existing theories that seek to explain the development and maintenance of pool riffle sequences simulated results suggest that near bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases model simulations indicate that secondary flow acts to route near bed flow over the downstream side of riffles and into the pool head away from the centre of pools implications for sediment transport and pool maintenance are discussed

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

cobalt in ferromanganese crusts as a monitor of hydrothermal discharge on the pacific sea floor ferromanganese oxide crusts which accumulate on unsedimented surfaces in the open ocean1 6 derive most of their metal content from dissolved and particulate matter in ambient bottom water7 8 in proportions modified by the variable scavenging efficiency of the oxide phase for susceptible ions9 they differ in this respect from abyssal nodules much of whose metals are remobilized from host sediments here we present maps of cobalt concentration and inferred accumulation rate of ferromanganese crusts from the pacific ocean we propose that depletion of cobalt in pacific crusts measures the location and intensity of submarine hydrothermal discharge use of the cobalt chronometer an algorithm inversely relating cobalt content and crust growth rate permits mapping of the accumulation rate of ferromanganese crusts with only indirect recourse to radioactivity based dating methods these maps show that crusts in hydrothermal areas grow from two to more than four orders of magnitude faster than in the central pacific ocean cobalt enriched crusts are found where water masses are most isolated from continental coastal and hydrothermal sources of metals now and in the past this relationship can resolve the problem of cobalt enrichment in crusts without recourse to hypotheses invoking special cobalt sources or enrichment mechanisms

explaining discrepant depth profiles of 234u 238u and 230thexc in mn crusts abstract manganese encrustations are an important archive for the reconstruction of deep ocean circulation in the past however because of discordant growth rates derived from the decrease of the activity ratios of 234 u 238 u and of 230 th exc with depth their dating via the measurement of depth profiles of 230 th and 231 pa has been recently put into question chabaux et al 1997 in this study we present high precision depth profiles of uranium and thorium isotopes tims of a hydrogenous mn crust from the south china sea indeed the depth profiles of 234 u 238 u and 230 th exc deliver very different growth rates of 8 80 1 20 and 2 64 0 12 mm ma respectively we solve this discrepancy with a simple model which assumes exchange of uranium adsorbed in the mn crust with uranium dissolved in the pore water the best agreement to the data is obtained applying an exchange coefficient of 5 x 10 6 a 1 application of this model to the data set of chabaux et al 1997 reproduces very well their profiles of the 234 u we conclude that 234 u 238 u dating of mn encrustations is not reliable because of open system conditions for uranium

surface roughness for shallow overland flow on crushed stone surfaces results of laboratory experiments carried out to determine the effective surface roughness for shallow overland flow as a function of the runoff rate roughness element height and underlying soil condition are presented this work was conducted to extend the understanding of the mechanics of shallow overland flows based on the relationships between the reynolds number froude number and surface resistance over a wide range of conditions results exhibit a strong dependence of the effective roughness on the ratio between the depth of flow and the height of the roughness element and a strong inverse relationship was found between manning s n and the darcy weisbach friction factor f and the froude number three distinct subcritical flow regimes were identified 1 submerged flow r 1 200 and 3 transitional partially submerged flow 300 r 1 200 analysis and synthesis of laboratory data allowed relationships between the reynolds and froude numbers and manning s n

magnetostratigraphy of hydrogenetic manganese crusts from northwestern pacific seamounts abstract fine scale dating is crucial to understanding the growth and environments of formation of marine manganese deposits the paleomagnetic method of dating of manganese nodules and crusts has been attempted but with no success so far we measured remanent magnetization rm on up to 75 mm thick hydrogenetic crusts at intervals of 2 5 mm after mineralogical and chemical examination careful separation and alternative field demagnetization stable sharp rm and well correlated polarity change patterns between four samples made it possible to identify the pliocene quaternary magnetic chrons in them these data show that the three crusts have grown continuously at rates of 14 17 mm m y since the early pliocene this is three or four times faster than those for central pacific seamount hydrogenetic crusts estimates of growth rate based on determination of co flux are within this range magnetostratigraphy therefore appears to be a powerful method for dating hydrogenetic manganese crusts when supported by conventional dating methods these results encourage finer scale investigations which promise more detailed paleoceanographic reconstruction

error propagation for velocity and shear stress prediction using 2d models for environmental management pasternack g b gilbert a t wheaton j m buckland e m 2006 error propagation for velocity and shear stress prediction using 2d models for environmental management journal of hydrology 328 1 2 227 241 sponsorship us fish and wildlife service contracting entity for calfed bay delta ecosystem restoration program cooperative agreement dcn 113322g003