evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

a model for changes in channel form at a river confluence a model for changes in river channel geometry at a confluence is deduced by substituting the hydraulic geometry relationships into the continuity equation of flow changes in the hydraulic geometry variable g can be described by g x 0 g x 1 g x 2 where the subscripts 0 1 and 2 denote the receiving stream major and minor tributaries the exponent may be a system wide parameter bar x or may be computed for individual junctions x in the first case bar x is expected to be equal to the reciprocal of the exponent of the hydraulic geometry relationship between g and water discharge in the latter case x may be found by solving the above equation iteratively or from two nomographs presented in this paper using data from the literature richards 1980 and our own measurements of widths and slopes in a miniature drainage network we show that although average morphometric changes at a junction may be adequately described by bar x the behavior of individual junctions

junction angles in drainage networks the systematics of tributary junction angles are an important link between the topology of a drainage network and its topography because the angle of junction depends on the relative slopes of the intersecting tributaries i present here a model for stream junction angle as a function of position link magnitude within the network which quantitatively predicts the observed down network increase of junction angle for tributaries joining progressively larger recipient links good agreement between model and data exists for mature drainage networks mapped at varying scales additionally the dependency of junction angle on relative tributary slopes suggests that as individual links within the network modify their slopes to evolve toward graded longitudinal profiles there exists a central tendency within the network modulated by structural and lithologie controls for evolution to an equilibrium or graded planimetric pattern extraction of process related as opposed to purely topological information from network planimetric pattern is important in terrestrial and planetary remote sensing applications

reconstruction of primary productivity from the barium contents in surface sediments of the south atlantic ocean abstract the aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity for this purpose 190 surface sediment samples from the south atlantic ocean were analysed for their barium and aluminium concentrations biogenic barium is estimated by subtracting the calculated terrigenous barium obtained from the terrigenous ba al ratio and the amount of al in the sample from the total ba content in the sample based on the accumulation rates of biogenic barium export production is estimated using three different algorithms proposed by paleoceanography 7 1992 163 global biogeochem cycles 9 1995 289 geomar report 38 1995 105 primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters only the primary productions calculated on the basis of the algorithm of paleoceanography 7 1992 163 yield productivity values comparable to those existing in ocean surface waters this study further reveals that it is not sufficient to use a constant generally applicable organic carbon biogenic barium ratio as is postulated by global biogeochem cycles 9 1995 289 this ratio has to be assessed regionally for the sediments of the cape basin in the eastern south atlantic ocean a new algorithm is developed which gives plausible primary productivities for the overlying surface waters

structural elements of the southern tethyan himalaya crust from wide angle seismic data a deep seismic common midpoint cmp profile shot across the southern margin of the tethyan himalaya as the first stage of project international deep profiling of tibet and the himalaya indepth imaged the top main himalayan thrust or mht and bottom moho of the indian continental crust underthrusting the himalaya we used portable seismographs to record the cmp profile shots at a wide range of offsets up to 155 km our short offset data corroborate the cmp profile data while our large offset data are dominated by a band of reflectivity we interpret the bright onset of this reflective band as the basal detachment of the south tibetan detachment system and a phase at the end of the reflective band as the mht we used the cmp profile first break data to model the uppermost 2 km in detail the depth of young extensional basins of the yadong rift system along the cmp profile was constrained to a maximum of 2 km yielding a throw of 4 6 km across the eastern flank of the rift and a rough estimate of approximately 1 5 east west extension by normal faulting in the tethyan himalaya the wide angle data were used to construct a crustal seismic velocity model down to the mht the south tibetan detachment system stds basal detachment reflector is observed dipping 12 5 nne from a depth of about 6 km beneath the surface under the south end of the cmp profile to a depth of 22 km then flattening to a dip of only 2 5 nne thus our observations suggest that the stds basal detachment is a deep rooted basement fault for the mht we observe a dip of 7 5 nne from 20 km depth below sea level at the crest of the high himalaya crystalline sheet to 36 km below sea level 40 km beneath the surface at a distance of about 70 km south of the indus yarlung suture iys from geometrical arguments we suggest that indian crust underthrusts the surface expression of the iys within the crust but does not form the lower crust of central and northern tibet

ferrimagnetic iron sulfide formation and methane venting across the paleocene eocene thermal maximum in shallow marine sediments ancient west siberian sea

tributary confluences and discontinuities in channel form and sediment texture rio chama nm numerous morphological changes can occur where two channels of distinct sediment and flow regimes meet including abrupt shifts in channel slope cross sectional area planform style and bed sediment size along the receiving channel along the rio chama between el vado and abiquiu dams northern new mexico arroyo tributaries intermittently deliver sediment from erodible sandstone and shale canyon walls to the mainstem channel much of the tributary activity occurs in flash floods and debris flows during summer thunderstorms which often load the channel with sand and deposit coarser material at the mainstem confluence in contrast mainstem channel flow is dominated by snowmelt runoff to examine tributary controls we systematically collected cross section elevation and bed sediment data upstream and downstream of 26 tributary confluences along a 17 km reach data from 203 cross sections were used to build a one dimensional hydraulic model for comparing estimated channel parameters at bankfull and low flow conditions at these sites as compared to intermediate reaches confluences primarily impact gradient and bed sediment size reducing both parameters upstream of confluences and increasing them downstream cross section area is also slightly elevated above tributary confluences and reduced below major shifts in slope and bed sediment size at confluences appear to drive variations in sediment entrainment and transport capacity and the relative storage of sand along the channel bed the data were analyzed and compared to models of channel organization based on lateral inputs such as the network variance model and the sediment link concept at a larger scale hillslope channel coupling increases in the downstream third of the study reach where the canyon narrows resulting in steeper slopes and more continuous coarse bed material along the mainstem and thus limiting the contrast with tributary confluences however channel form and sediment characteristics are highly variable along the study reach reflecting variations in the size and volume of sediment inputs related to the surface geology in tributary watersheds morphology of the rio chama at the junction i e bends confinement and the relative magnitude and location of past depositional events copyright 2014 john wiley sons ltd

seismic imaging of the waltham canyon fault california comparison of ray theoretical and fresnel volume prestack depth migration abstract near vertical faults can be imaged using reflected refractions identified in controlled source seismic data often theses phases are observed on a few neighboring shot or receiver gathers resulting in a low fold data set imaging can be carried out with kirchhoff prestack depth migration in which migration noise is suppressed by constructive stacking of large amounts of multifold data fresnel volume migration can be used for low fold data without severe migration noise as the smearing along isochrones is limited to the first fresnel zone around the reflection point we developed a modified fresnel volume migration technique to enhance imaging of steep faults and to suppress noise and undesired coherent phases the modifications include target oriented filters to separate reflected refractions from steep dipping faults and reflections with hyperbolic moveout undesired phases like multiple reflections mode conversions direct p and s waves and surface waves are suppressed by these filters as an alternative approach we developed a new prestack line drawing migration method which can be considered as a proxy to an infinite frequency approximation of the fresnel volume migration the line drawing migration is not considering waveform information but requires significantly shorter computational time target oriented filters were extended by dip filters in the line drawing migration method the migration methods were tested with synthetic data and applied to real data from the waltham canyon fault california the two techniques are applied best in combination to design filters and to generate complementary images of steep faults

a 2 d ray based depth migration method for deep seismic reflections summary although accurate depth migration is essential for the correct geometrical interpretation of deep reflection data migration by the standard numerical methods has generally not been satisfactory the alternative method presented in this paper provides an efficient migration for deep reflection profiles in the form of a digitized line drawing assuming a general 2 d velocity structure migration of a digitized line drawing reduces the effects of low signal noise ratios the calculation is rapid allowing migrations to be calculated for a large number of alternative velocity structures and iterative improvement of the velocity structure the migration has been applied to 3 d structures observed on intersecting profiles collected by birps british institutes reflection profiling syndicate

diagenetic barium cycling in black sea sediments a case study for anoxic marine environments high resolution sedimentary records of major and minor elements al ba ca sr ti total organic carbon toc and profiles of pore water constituents view the mathml sourceso42 ch4 ca2 ba2 mg2 alkalinity were obtained for two gravity cores core 755 501 m water depth and core 214 1686 m water depth from the northwestern black sea the records were examined in order to gain insight into the cycling of ba in anoxic marine sediments characterized by a shallow sulfate methane transition smt as well as the applicability of barite as a primary productivity proxy in such a setting the ba records are strongly overprinted by diagenetic barite baso4 precipitation and remobilization authigenic ba enrichments were found at both sites at and slightly above the current smt transport reaction modeling was applied to simulate the migration of the smt during the changing geochemical conditions after the holocene seawater intrusion into the black sea based on this sediment intervals affected by diagenetic ba redistribution were identified results reveal that the intense overprint of ba and baxs ba excess above detrital average strongly limits its correlation to primary productivity these findings have implications for other modern and ancient anoxic basins such as sections covering the oceanic anoxic events which ba is frequently used as a primary productivity indicator our study also demonstrates the limitations concerning the use of baxs as a tracer for downward migrations of the smt due to high sedimentation rates at the investigated sites diagenetic barite fronts are buried below the smt within a relatively short period thus relict barite fronts would only be preserved for a few thousands of years if at all

unlocking the barite paleoproductivity proxy a new high throughput method for quantifying barite in marine sediments abstract the accumulation rate of barite baso4 in marine sediments is a powerful tracer of paleo export productivity due to the refractory nature of barite in sediments and its ability to record ecosystem wide productivity it has been used to reconstruct past export productivity and to infer the effects of ocean circulation and nutrient supply on ocean ecosystems however the difficulty in generating large datasets of reliable barite accumulation rates has limited the use of this proxy here we present a new method for quantifying barite in marine sediments that promises a dramatic increase in sample throughput while also improving recovery of barite in a wide variety of marine sediments this method relies on selective dissolution of barite using a chelating ligand that avoids liberating ba from silicate phases and it can therefore work reliably on small 250 mg sediment aliquots rigorous evaluation of the method reveals that it quantitatively extracts barite and is largely unaffected by other ba sources due to the relatively simple analytical requirements of this barite extraction method and the small sample amount needed it can also be used to rapidly generate paleoproductivity records both during and after coring expeditions altogether this new technique should expand the study of the marine biologic system in the past by dramatically increasing analytical efficiency and widening the range of questions that are accessible using barite accumulation rate as a paleoproductivity proxy

morphometric response of late aptian planktonic foraminiferal communities to environmental changes a case study of paraticinella rohri at poggio le guaine central italy abstract the aptian albian boundary interval represents a pivotal time of pronounced climate ocean changes associated with oceanic anoxic event 1b oae 1b that seriously influenced marine pelagic ecosystems the sedimentary successions record the first and most significant species turnover in the evolutionary history of planktonic foraminifera consisting of the selective extinction disappearance of large aptian species and resulting in inter specific size reduction the post extinction dwarfing as in the lilliput effect in the earliest albian assemblages in this study we investigate how the planktonic foraminifera respond to crucial environmental stress and if any intra specific decrease in test size predate the extinction pre extinction dwarfing paraticinella rohri was chosen as the most representative of the aptian planktonic foraminiferal taxa and its test size evolution was compared with the high resolution record of redox and productivity geochemical proxies through the uppermost aptian lowermost albian pelagic succession of poggio le guaine central italy our geochemical data document long term fluctuations characteristic of a system with transitions among different states oligotrophic mesotrophic eutrophic and oxic dysoxic anoxic the strong correlation of the large test size changes of pa rohri during its total range with higher productivity increased baexcess concentrations suggests that the trophic regime was the key factor controlling robust abundance and size variability in this taxon the progressive increase in surface water fertility during the late aptian offered optimum ecological conditions for planktonic foraminifera that increased in abundance size and specific diversity after the transient anoxic 113 jacob sub event smaller to dwarfed specimens of pa rohri and depauperate planktonic foraminifera assemblages faced high selective pressures due to more oligotrophic conditions transition to a more eutrophic ocean in the latest aptian irreversibly eroded their resilience and aptian dwarfed k strategist taxa became extinct the planktonic foraminifera turnover has been therefore the result of an irreversible and drastic regime shift toward a more eutrophic ecosystem that promoted the thriving of small sized opportunistic disaster planktonic foraminiferal species and the siliceous radiolaria

millennial scale variability in the productivity signal from the alboran sea record western mediterranean sea primary productivity of the alboran sea western mediterranean is reconstructed for the time interval 48 000 to 28 000 years bp using the following geochemical proxies calcium carbonate barium excess baexcess total organic carbon toc and alkenone concentration the records show a consistent pattern that indicates enhanced productivity during dansgaard oeschger d o warm periods at the same time deep water conditions were favorable for organic matter preservation in the sediments as indicated by the variation in redox sensitive elements such as mn cr or v and by the 13c record of benthic foraminifers the alboran sea productivity variation during d o cycles is explained by the succession of two climatic scenarios that led to latitudinal shifts of the westerly wind system this mechanism would also account for the variability in the deep water ventilation of the western mediterranean sea

the occurrence of obtuse junction angles and changes in channel width below tributaries along the mekong river south east asia classic descriptions of drainage patterns suggest that confluence angle is determined by the shape of the drainage basin unless constraining factors such as the geological structure affect stream flow downstream changes in channel width below tributary junctions have long been associated with tributary inputs of flow and sediment analysis of tributary junction geometry and channel width changes in large rivers and over large reaches is sparse the lower mekong basin exhibits a generally dendritic drainage network despite flowing through a diverse array of geological settings publicly available spot 5 imagery from google earth was used to identify and catalogue junction geometries and downstream changes in channel width below tributary junctions along a 2200 km reach of the mekong river of the 284 junctions identified the majority 66 2 were acute however 12 4 8 were found to be normal 90 and 75 30 were found to be obtuse this latter number is in contrast to previous studies over similar spatial scales which found little evidence of obtuse junctions meander extension of the incoming tributary and deflection of the tributary across bedrock shoulders were found to be the dominant geomorphological causes of obtuse tributary junctions the relationship between the width of the tributary channels and the width of the mainstem upstream and downstream of the confluences was analysed it was observed that over the whole reach a slight narrowing occurred immediately below tributary junctions although the changes themselves were small the slight net narrowing is shown to be statistically significant the observed relationship is shown to vary considerably with geology the geological control suggests that complex factors play important roles in determining changes to channel width across large systems and that simple cause effect relationships do not hold in such complicated geological settings

10be and ba concentrations in west african sediments trace productivity in the past particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles in order to determine the influence of these processes on the depositional fluxes of10be and barium we have performed high resolution measurements on sediment core geob 1008 3 from the congo fan because the core is dominated by terrigenous matter supplied by the congo river it has a high average mass accumulation rate of 6 5 cm kyr biogenic10be and ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10be and ba which are assumed to be proportional to the flux of al2o3 the meanba al weight ratio of the terrigenous component was determined to be 0 0045 the unusualy high terrigenous10be concentrations of 9 1 109 atoms g al2o3 are either due to input of particles with high10be content by the congo river or due to scavenging of oceanic10be by riverine particles the maxima of biogenic10be and ba concentrations coincide with maxima of the paleoproductivity rates time series analysis of the10be and of ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr which also controls the rates of paleoproductivity in this core during the maxima of productivity the flux of biogenic ba is enhanced to a larger extent than that of biogenic10be applying a model for coastal scavenging we ascribe the observed higher sensitivity of ba to biogenic particle fluxes to the fact that the ocean residence time of ba is approximately 10 times longer than that of10be

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

barium peaks at glacial terminations in sediments of the equatorial atlantic ocean relicts of deglacial productivity pulses in sediment cores recovered from oligotrophic settings of the western and eastern north equatorial atlantic ocean ceara rise and sierra leone rise distinct peaks of excess barium were detected at glacial interglacial transitions these ba maxima are unrelated to any other potential productivity proxy e g organic carbon carbonate or opal moreover they coincide with minima in organic carbon contents despite the lack of correlation between excess barium and organic carbon we ascribe these ba spikes to pulses of productivity that occurred during glacial interglacial transitions the discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward progressing oxidation fronts during deglacial nonsteady state depositional conditions the oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced nadw production during interglacial periods the fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short time productivity events this assumption is supported by the depth arrangement of the solid phase peaks of ba fe and mn along the 6 5 oxygen isotope stage boundary in sediments of the ceara rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern mediterranean sediments it remains unclear whether productivity was proportional to the magnitude of the baexcess maxima possibly higher dissolved ba concentrations in intermediate and deep waters during glacial interglacial transitions caused the precipitation of barite to increase out of proportion of the postulated deglacial productivity pulses

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

in search of the late miocene early pliocene biogenic bloom in the atlantic ocean ocean drilling program sites 982 925 and 1088 1 we reconstruct paleoproductivity at three sites in the atlantic ocean ocean drilling program sites 982 925 and 1088 to investigate the presence and extent of the late miocene to early pliocene biogenic bloom from 9 to 3 ma our approach involves construction of multiple records including benthic foraminiferal and caco3 accumulation rates uvigerina counts dissolution proxies and geochemical tracers for biogenic and detrital fluxes this time interval also contains the so called late miocene carbon isotope shift a well known decrease in benthic foraminiferal 13c values we find that the timing of paleoproductivity maxima differs among the three sites at site 982 north atlantic benthic foraminifera and caco3 accumulation were both at a maximum at 5 ma with smaller peaks at 6 ma the paleoproductivity maximum was centered earlier 6 6 6 0 ma in the tropical atlantic site 925 in the south atlantic site 1088 paleoproductivity increased even earlier between 8 2 ma and 6 2 ma and remained relatively high until 5 4 ma we note that there is some overlap between the interval of maximum productivity between sites 925 and 1088 as well as the minor productivity increase at site 982 we conclude that the paleoproductivity results support hypotheses aiming to place the biogenic bloom into a global context of enhanced productivity in addition we find that at all three sites the 13c shift is accompanied by carbonate dissolution this observation is consistent with published studies that have sought a relationship between the late miocene carbon isotope shift and carbonate preservation

seawater mediated interactions between diatomaceous silica and terrigenous sediments results from long term incubation experiments reactors containing frustules of the cultured diatom thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the mississippi river delta or the congo river fan the diatom frustules were separated from the terrigenous sediments by a dialysis membrane thereby only allowing the exchange of dissolved species one series of incubations was carried out in the laboratory at room temperature 21 c and for a period of 10 months another series of reactors was deployed along a mooring in the mozambique channel at three water depths 500 1250 and 2000 m for a period of 22 months chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater mg and k and the sediments al fe mn p and ca to the frustules in the presence of the terrigenous sediments the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments in addition electron microscopy revealed the formation of new mineral precipitates these included amorphous deposits on the frustules containing si fe al mg k and p as well as euhedral clay crystallites differences were observed between the incubations performed in the laboratory and those deployed at sea likely as a result of differences in redox conditions temperature and reaction time overall the interactions between biogenic silica seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms these interactions take place on relatively short time scales months to years and affect not only the marine cycle of silicon but also those of other major and minor elements such as al fe mn k and mg

geophysical images and a crustal model of intrusive structures beneath the messum ring complex namibia the early cretaceous messum complex in namibia is an excellent example of the type of composite mafic and alkaline igneous ring complexes associated with continental rifts worldwide this study integrates reflection seismic gravity and aeromagnetic data from a profile across messum and reveals a heavily intruded root zone extending vertically to moho depths 40 km the crustal section to either side of the messum complex is characterized by a seismically transparent upper crust to 20 km depth and a laminar reflective lower crust below the crust below the messum complex is seismically transparent at all depths but it shows a combination of high seismic reflection strength and weak coherency relative to the unintruded crust and p velocities about 0 2 0 3 km s higher at all depths the seismic attributes are best explained by a network of mafic intrusions and this is supported by steep sided positive magnetic and gravity anomalies centered on the complex modelling indicates that about 30 gabbroic material in the crustal column can explain the velocity and potential field data an increase in reflective strength below 20 km suggests the degree of intrusion is greater in the lower crust but the difference is not resolved in the velocity or gravity models the new geometric and compositional constraints on the crust below the messum complex contradict the hypothesis that messum was the sole eruptive center for an estimated 8600 km3 of crustally derived rhyodacitic magmas that now form part of the regional parana etendeka volcanic sequences to our knowledge this is the first study to provide a seismic image of the whole crust below a typical igneous ring complex and our conclusion that the cross sectional area of the complex is the same in the lower as in the upper crust could not have been predicted from surface studies the data resolution is insufficient to define intrusion geometry in detail but the cylindrical 3 d shape throughout the crust and the lack of major breaks in the vertical distribution of intrusions indicate depth independent controls on the nature of the intruded zone the suggestion is that the magma transport and emplacement took place in a brittle fashion at all levels of the crust in response to conditions of high fluid magma pressure and a high magma flux

recent deep water sedimentation trace metal and radioisotope geochemistry across the southern ocean and northern weddell sea antarctica abstract a wide variety of deep water depositional environments were examined across 12 sampling stations in the southern ocean and northern weddell sea antarctica in order to provide a depositional context for the benthic biodiversity studies of andeep antarctic benthic deep sea biodiversity colonisation history and recent community patterns here we present a diverse dataset comprising sub bottom profiles seabed photographs sediment profile images and box and multicore samples in order to investigate the recent depositional environments across the region the sediments were investigated using texture structure trace metal mn fe u mo ba and pb stable 206pb 207pb and radionuclide excess 210pb isotopes the sediments of the agulhas basin are interpreted as siliceous pelagites the product of enhanced surface productivity the eastern weddell continental rise is characterised by diverse depositional environments with hemipelagic turbiditic and contouritic sedimentation in contrast the northern weddell abyssal plain is dominated by hemipelagites with fine grained turbidites a single sampling station from the central powell basin contained hemipelagites and a fine grained turbidite a core from the central bransfield strait recovered hemipelagites with a silt turbidite the seabed photographs display a seabed dominated by biogenic mud balls formed by cirratulid polychaetes trace metal pb isotope and excess 210pb data all reveal the central bransfield strait to be a region of enhanced productivity with a mass accumulation rate of 0 073 g cm2 two stations in the bellingshausen sea recovered hemipelagites and turbidites mn fe and mo data suggest a degradation of organic matter in the sediments leading to redox cycling

ferromanganese nodules from manop sites h s and r control of mineralogical and chemical composition by multiple accretionary processes abstract the chemical composition of ferromanganese nodules from the three nodule bearing manop sites in the pacific can be accounted for in a qualitative way by variable contributions of distinct accretionary processes these accretionary modes are 1 1 hydrogenous i e direct precipitation or accumulation of colloidal metal oxides in seawater 2 2 oxic diagenesis which refers to a variety of ferromanganese accretion processes occurring in oxic sediments and 3 3 suboxic diagenesis which results from reduction of mn 4 by oxidation of organic matter in the sediments geochemical evidence suggests processes 1 and 2 occur at all three manop nodule bearing sites and process 3 occurs only at the hemipelagic site h which underlies the relatively productive waters of the eastern tropical pacific a normative model quantitatively accounts for the variability observed in nearly all elements zn and na however are not well explained by the three end member model and we suggest that an additional accretionary process results in greater variability in the abundances of these elements variable contributions from the three accretionary processes result in distinct top bottom compositional differences at the three sites nodule tops from h are enriched in ni cu and zn instead of the more typical enrichments of these elements in nodule bottoms in addition elemental correlations typical of most pelagic nodules are reversed at site h the three accretionary processes result in distinct mineralogies hydrogenous precipitation produces mno2 oxic diagenesis however produces cu ni rich todorokite and suboxic diagenesis results in an unstable todorokite which transforms to a 7 a phase birnessite upon dehydration the presence of cu and ni as charge balancing cations influence the stability of the todorokite structure in the bottoms of h nodules which accrete dominantly by suboxic diagenesis na and possibly mn 2 provide much of the charge balance for the todorokite structure limited growth rate data for h nodules suggest suboxic accretion is the fastest of the three processes with rates at least 200 mm 106 yr oxic accretion is probably 10 times slower and hydrogenous 100 times slower since these rates predict more suboxic component in bulk nodules than is calculated by the normative analysis we propose that suboxic accretion is a non steady state process variations in surface water productivity cause pulses of particulate flux to the sea floor which result in transient mn reduction in the surface sediments and reprecipitation on nodule surfaces

crustal deformation of the lhasa terrane tibet plateau from project indepth deep seismic reflection profiles international deep profiling of tibet and the himalaya indepth deep reflection data in the yangbajain damxung graben of southern tibet yield evidence of magmatism and deformation beneath the southern lhasa terrane and yarlung zangbo suture shallow reflections and low velocity first arrivals indicate a thin quaternary graben fill of generally less than a few hundred meters thickness underlying stratified reflections extending to a maximum of about 11 5 km depth likely originate from deformed paleozoic cenozoic supracrustal strata of the lhasa terrane a prominent undulatory band of reflections within the crystalline basement extends beneath the length of the yangbajain damxung graben depth ranges from 12 km to 18 km this horizon has been interpreted to mark the top of a midcrustal partial melt zone underlying southern tibet the undulatory character of this horizon additionally suggests that it may be tectonically deformed a somewhat deeper subhorizontal wide angle reflection extends southward beneath the outcrop of the yarlung zangbo suture indicating that the suture is cut off or superposed by a younger structure at depth three gently north dipping reflections at 19 km 24 km and 27 km depth beneath the gangdese batholith are suggestive of a midcrustal duplex and may mark the northward downdip extension of the late oligocene early miocene gangdese thrust system a prominent 40 north dipping reflection imaged in the deep crust beneath the gangdese batholith between 40 and 60 km depth might mark the downdip expression of the yarlung zangbo suture or alternatively a younger reverse fault in the lower crust or both viewed in aggregate the reflection data are suggestive of moderate postcollisional shortening of the upper crust of the lhasa terrane accompanied by melting of the middle crust although the data are permissive of wholesale underthrusting or fluid injection of indian continental crust beneath the lhasa terrane they show no direct evidence for this having occurred

scavenged excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial pacific ocean we present results from chemical analyses of al and ti in surface sediment sampled along two cross equator latitudinal transects at 135w and 140w in the central equatorial pacific ocean although traditionally both al and ti are considered to reside exclusively within terrigenous phases in marine sediment these sediments present extremely high al ti ratios that are several times that of average shale and other potential crustal sources a sharp maximum in al ti is observed slightly south of the equator where sedimentary bulk accumulation rate bar is also highest reflecting elevated productivity in the overlying water caused by surface water divergence bulk al ti decreases sharply away 2 latitude to near crustal values at 4 north and south the latitudinal profiles of al ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of al and ti these results indicate the presence of a significant scavenged component of al sourced directly from seawater during particle settling the data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135w transect calculations of alexcess indicate that the highest al ti ratios correspond to 50 of the total al being unsupported by the small amount of terrigenous phases present these results are consistent with previous and ongoing studies of biogenic sediment suspended particulate matter and sediment trap material quantitative use of al as an index of terrigenous material may therefore lead to an overestimation by a factor of two of the true terrigenous load in marine sediment sedimentary rock and settling particles because bulk al ti appears to respond to sedimentary bar which in biogenic regimes is linked to surface water productivity downcore records of al ti in biogenic sediment may track productivity changes through time such al ti proxy records may be applicable in sediment of all ages unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr

active post depositional oxidation of the most recent sapropel s1 in sediments of the eastern mediterranean sea abstract over a wide area of the eastern mediterranean basin two mn rich layers have been observed above the most recent sapropel s1 one immediately above the sapropel top and one a few centimetres closer to the sediment surface different mechanisms have been proposed to explain the occurrence of these two mn peaks either both peaks have a diagenetic origin in which case the upper mn peak is actively forming or the lower peak is actively forming and the upper peak has a different formation mechanism high resolution porewater including a gel sampler used for the first time in marine sediments and solid phase data are now used to demonstrate that the oxidation front is located at the level of the lower mn peak which therefore is presently being formed a barium organic carbon relationship is used to calculate the initial organic carbon profile of the s1 sapropel the palaeoproductivity profiles generated by this method demonstrate that the original sapropel unit was bounded by the upper mn peak this implies that the interval between the two mn peaks where a low organic carbon content is now observed was originally part of the sapropel the initially deposited organic carbon has been oxidised by a progressive downwards moving oxidation front the penetration depth of this oxidation front i e the distance between the two mn peaks is mainly determined by the organic carbon content the sediment accumulation rate and the bioturbation depth the upper mn peak appears to have formed as a result of either mn precipitation upon oxygenation of previously anoxic eastern mediterranean deep water or preservation of a surficial mn peak at the end of the high productivity episode in either case the upper mn peak marks the end of sapropel formation as indicated by the ba profiles this means that formation of the s1 sapropel ceased more recently than is indicated by radiocarbon dating of the visible top of s1

short time scale variability in ventilation and export productivity during the formation of mediterranean sapropel s1 1 high resolution laser ablation inductively coupled plasma mass spectrometry la icp ms scanning of laminated sediments from the urania basin is used to investigate short time scale variability in export productivity and redox conditions during the formation of eastern mediterranean sapropel s1 sedimentary enrichments of molybdenum mo vanadium v and uranium u reflect deep water redox conditions most likely those near to the seawater brine interface while enrichment of ba is related to biogenic barite and hence to export productivity the enrichments of all four elements show strong variability on multidecadal to multicentennial time scales throughout s1 a partial decoupling of export productivity from redox conditions at the height of sapropel formation suggests that hydrographic changes i e a variable ventilation rate of the eastern mediterranean played an important role in determining deep water redox conditions a pronounced switch is observed in the enrichments of redox sensitive trace metals from dominantly 300 600 year variability during early s1 to dominantly 100 300 year variability during late s1 indicating a change in the mean frequency of variability in the ventilation rate the presence of a similar shift in the frequency of tropical and extratropical climate records at this time suggests that ventilation of the eastern mediterranean was coupled to global climate variability

inorganic geochemical indicators of glacial interglacial changes in productivity and anoxia on the california continental margin abstract evidence from sediments in cores collected from within the present oxygen minimum zone omz 600 1200 m on the central and northern california margins record several episodes during the last interstadial ois 3 ca 60 24 ka of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions e g mo ni zn and cu the presence of abundant well preserved organic matter as well as lack of bioturbation and the presence of elevated concentrations of mo and other trace elements all support the theory that the omz in the northeastern pacific ocean was more intense possibly anoxic at several times during the late pleistocene sediments of all ages in cores from the southern california margin contain elevated concentrations of mo suggesting that this area has always had higher rates of sulfate reduction than either the central or northern california areas most of the ba in sediments in all cores collected on the upper continental slope 200 2700 m off california and southern oregon is derived from detrital clastic material and this source did not change much in time however the amount of biogenic ba did vary with time and these variations closely follow the temporal variations in organic c corg mass accumulation rate using ba and corg mass accumulation rates as proxy variables for productivity all cores show that organic productivity under the california current upwelling system was highest during ois 3 and the holocene and lowest during the last glacial interval lgi ca 24 10 ka all paleoproductivity proxy variables indicate that the southern california area has always experienced higher productivity than other areas under the california current at least over the last 50 ky

fault systems of the 1971 san fernando and 1994 northridge earthquakes southern california relocated aftershocks and seismic images from larse ii we have constructed a composite image of the fault systems of the m 6 7 san fernando 1971 and northridge 1994 california earthquakes using industry reflection and oil test well data in the upper few kilometers of the crust relocated aftershocks in the seismogenic crust and larse ii los angeles region seismic experiment phase ii reflection data in the middle and lower crust in this image the san fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of 25 from near the surface at the northridge hills fault in the northern san fernando valley to the san andreas fault in the middle to lower crust it follows a prominent aseismic reflective zone below and northward of the main shock hypocenter interpreted upward splays off this decollement include the mission hills and san gabriel faults and the two main rupture planes of the san fernando earthquake which appear to divide the hanging wall into shingle or wedge like blocks in contrast the fault system for the northridge earthquake appears simple at least east of the larse ii transect consisting of a fault that extends 20 km southward at a dip of 33 from 7 km depth beneath the santa susana mountains where it abuts the interpreted san fernando decollement to 20 km depth beneath the santa monica mountains it follows a weak aseismic reflective zone below and southward of the main shock hypocenter the middle crustal reflective zone along the interpreted san fernando decollement appears similar to a reflective zone imaged beneath the san gabriel mountains along the larse i transect to the east in that it appears to connect major reverse or thrust faults in the los angeles region to the san andreas fault however it differs in having a moderate versus a gentle dip and in containing no mid crustal bright reflections

surface sediment distribution along the chilean continental slope related to upwelling and productivity abstract the coastal upwelling system of the peru chile current belongs to the most productive regions in the world oceans in spite of this remarkable fact only very little is known about the sediment distribution in its southern part off the coast of chile to increase the knowledge about this region a multiparameter study of the surface sediment distribution along the chilean continental slope between 27 and 42 s has been carried out detailed analyses of sedimentary data toc and carbonate contents 13corg 15n of the species composition of planktic foraminifera and of the stable isotope composition of the two planktic foraminifera species neogloboquadrina pachyderma dex and globigerina bulloides reveal a close relation to environmental conditions in the region the pattern of a southward increase of surface water productivity known from satellite data is closely reflected in the organic carbon and 15n data information about the water mass distribution especially about varying influences of upwelling waters and warmer subtropical surface waters can be derived from the species composition of the planktic foraminifera while the stable isotope data provide additional clues about water mass characteristics high productivity along the chilean margin north of 40 s is sustained by coastal upwelling while further to the south advection of nutrients with the antarctic circumpolar current and a significant riverine input fuel the highest productivity found in the study area this data set will provide the base for future paleoclimatic and paleoceanographic reconstructions of the southeast pacific region

geochemistry and isotopic study of southern bay of bengal sediments implications for provenance and paleoenvironment during the middle miocene abstract the bay of bengal bob the largest bay in the world is known to receive large amounts of fresh water and suspended particulate matter from different sources the sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies the miocene epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity redox conditions and provenance of sediments however limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the miocene here we present detailed geochemical and radiogenic sr nd isotopic results from predominantly middle miocene sediments of ocean drilling program odp site 758a located in the southern bob lat 5 23 n long 90 21 e water depth 2925 m to understand paleoredox conditions paleoproductivity and provenance the results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region the distribution of redox sensitive element proxies u v cr at interval 17 8 ma points to an oxic depositional environment the major trace and rare earth elements distribution supports prevalence of the indian summer monsoon ism at 12 10 ma productivity proxies babio porg show a decreasing trend during colder events east antarctic ice sheet expansion eais and advent of late miocene cooling lmc events the trace and rare earth element concentrations are generally low except at intervals where glass shards were found the 87sr 86sr and end 0 values indicate that the sediments belong to the high himalayan sedimentary series hhss throughout the middle miocene however prior to initiation of the ism these sediments underwent meteoric alteration in the lowlands before deposition in the bob our records suggest that sediments at odp site 758a may have been introduced into the bob through the irrawaddy yarlung tsangpo drainage system during the entire period of this study the irrawaddy yarlung tsangpo system drained the indus tsangpo suture zone southern tibet the eastern himalayas and the arakan coast before debouching in the bob

confluence effects in rivers interactions of basin scale network geometry and disturbance regimes 1 we reviewed 14 studies documenting the effects of tributaries on river morphology at 167 confluences along 730 km of river spanning seven orders of magnitude in drainage area in western united states and canada in both humid and semiarid environments the probability of observing significant confluence related changes in channel and valley morphology due to tributary influxes of sediment e g changes in gradient particle size and terraces etc increased with the size of the tributary relative to the main stem effects of confluences on river morphology are conditioned by basin shape and channel network patterns and they include the nonlinear separation of geomorphically significant confluences in river networks other modifying factors include local network geometry and drainage density confluence related landforms i e fans bars terraces etc are predicted to be dominated by older features in headwaters and younger features downstream a pattern driven by the frequency and magnitude of floods and punctuated sediment supply that scale with watershed size index terms 1824 hydrology geomorphology 1625 1815 hydrology erosion and sedimentation 1821 hydrology floods 1848 hydrology networks keywords confluences fluvial geomorphology river networks

a data driven model of the global calcite lysocline gridded maps of sediment calcium carbonate calcite concentration and overlying water saturation state archer 1996 are combined with maps of benthic oxygen fluxes and sediment accumulation rates from jahnke 1996 and cweink 1986 to drive a diagenetic model of calcium carbonate preservation in deep sea sediments the only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor so i use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor the predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data the model is sensitive to the ratio of organic carbon to calcite rain rates the rain ratio and within the constraint of pelagic sediment trap rain ratio data is unable to reproduce the calcite field without respiratory dissolution the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon however relative to variability in sediment trap data the required model rain ratio is insensitive to the extent of anoxic respiration the stoichiometric ratio of o c during respiration the bioturbation rate the dissolution rate constant the effect of borate chemistry and small offsets in the saturation state for caco3 the model is sensitive to the accumulation rate of non caco3 material and predicts an atlantic pacific difference in non caco3 rain rate which is consistent with observations the model predicts that the dissolution flux of caco3 from sediments is 25 40 1012 mol yr 1 roughly half of the total deep sea caco3 dissolution rate estimated from the water column alkalinity distribution broecker and peng 1987 archer and maier reimer 1994 wollast 1994 the model also predicts that only 20 30 of the flux of caco3 to the seafloor globally escapes dissolution

migration why doesn t it work for deep continental data summary conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free surprisingly deep data often subjectively appear to be best migrated at velocities which are up to 50 less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities the explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce during the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a smiley section which appears overmigrated since the lateral extent of individual smiles increases with increasing migration velocity increasing two way time and increasing seismic wavelength the effect is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data

inorganic geochemistry of the type caradoc series sandbian to middle katian upper ordovician onny valley shropshire uk the geochemistry and petrology of the type section of the caradoc series in the onny valley indicate that it was deposited on a marginal basin continental shelf similar to the western side of the present sea of japan the lower beds form a transgressive regressive sequence in which the rocks become less mature upwards all the coarser sediments above the basal quartzites and conglomerates are greywackes in which the apparent muddy and ferrous matrix is due to the breakdown of unstable minerals and particles higher values of na 2 o and na k ratios are found in the coarser shallow water sandstones of the horderley sandstone formation and decrease markedly in the succeeding beds accompanied by an increase in k 2 o higher values of carbonate corrected and hence related other major and minor elements like sio 2 cao p 2 o 5 mno and most trace elements correlate with the transgressive systems and maximum flooding surfaces of the three sequences recognized where they are related to condensation at those horizons chemical indices of alteration cia suggest that the horderley sandstone formation underwent greater predepositional physical weathering than lower and higher beds which is compatible with the petrography and were deposited during a cool phase within overall warm sandbian katian times trace element ratios suggest an oxic to suboxic depositional environment

water depth and diagenetic constraints on the use of barium as a palaeoproductivity indicator sediments underneath the high productivity areas of the coastal upwclling and equatorial divergence zones in the pacific and indian oceans are enriched in barium and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator analyses of sediments from the peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean but also show that this pattern cannot be extended to shallow water deposits sediments from the peru shelf lack any barium enrichment whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m this depth effect is further illustrated at site 682 located in the rapidly subsiding lima basin the quaternary sequence deposited at water depths 3000 m is enriched in organic carbon opal and biogenic barium the miocene sediments although deposited under highly productive waters associated with a coastal upwclling centre do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels a pattern that is consistent with deposition in shallow basins in addition to the effect of water depth on the barium distribution overprinting of primary signals can be observed in sediments from upwclling areas undergoing strong anoxic diagenesis microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the peru margin dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts the barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator however barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undcrsatur ates the interstitial waters with respect to barite

diverse patterns of ocean export productivity change across the cretaceous paleogene boundary new insights from biogenic barium 1 one of the best studied aspects of the k pg mass extinction is the decline and subsequent recovery of open ocean export productivity e g the flux of organic matter from the surface to deep ocean some export proxies including surface to deep water 13c gradients and carbonate sedimentation rates indicate a global decline in export productivity triggered by the extinction in contrast benthic foraminiferal and other geochemical productivity proxies suggest spatially and temporally heterogeneous k pg boundary effects here we address these conflicting export productivity patterns using new and compiled measurements of biogenic barium unlike a previous synthesis we find that the boundary effect on export productivity and the timing of recovery varied considerably between different oceanic sites the northeast and southwest atlantic southern ocean and indian ocean records saw export production plummet and remain depressed for 350 thousand to 2 million years biogenic barium and other proxies in the central pacific and some upwelling or neritic atlantic sites indicate the opposite with proxies recording either no change or increased export production in the early paleocene our results suggest that widespread declines in surface to deep ocean 13c do not record a global decrease in export productivity rather independent proxies including barium and other geochemical proxies and benthic community structure indicate that some regions were characterized by maintained or rapidly recovered organic flux from the surface ocean to the deep seafloor while other regions had profound reductions in export productivity that persisted long into the paleocene

the impact of sediment provenance on barium based productivity estimates abstract biogenic barium in marine sediments has been suggested to be a reliable proxy of export productivity from the surface ocean and algorithms have been developed to link these properties however problems arise when the proposed algorithms are applied to predominantly terrigenous sediments a major source of error is incorrect estimates of the terrigenous ba al ratio in normative calculations of the amount of biogenic barium in the sediment compared to an often used global average ba al ratio much better results can be obtained by estimating the terrigenous ba al ratio from exponential regression of the ba al ratios of surface sediments obtained from continental slope transects this method has been applied to surface sediments from the chilean continental slope the calculated regional terrigenous ba al ratios could be verified with purely terrigenous samples from chilean rivers the resulting accumulation rates of biogenic barium on the chilean continental slope reliably reproduce the regional pattern of primary productivity in the southern peru chile current indicating the potential of biogenic barium as a useful paleo productivity proxy

deep water sedimentary environments of the northwestern weddell sea and south sandwich islands antarctica abstract two transects have been sampled using short cores multi and box seabed photography video sequences and sediment profile images across the northwestern weddell sea and south sandwich forearc antarctica a total of 12 core stations were examined for sediment structure texture and composition to determine their depositional history four of the core stations from the weddell continental slope abyssal plain and south sandwich forearc and trench were further examined for trace metal analysis ba u mn and pb for sediment source and palaeoproductivity and 210pb for age profiles and depth of mixing by bioturbation the 3055 m station at the base of the weddell continental slope recovered laminated muddy silty contourites with a surface layer of hemipelagites with abundant ice rafted debris possibly originating from the 1995 larsen a and 2002 larsen b ice shelf disintegration this is interpreted as a moderate energy depositional environment influenced by weddell sea bottom water along slope current reworking sediment sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along slope transport reflected in the mn li and ba li ratios the 4760 m station on the weddell abyssal plain recovered hemipelagites interpreted as a low energy depositional environment with sediment supplied both from direct ice rafting and pelagic settling during productive seasonally ice free periods the 210pb profiles indicate a sedimentation rate of 0 024 cm year 1 with the 206 207pb isotope ratios suggesting a principle sediment source from ice rafting the 2292 and 6326 m stations from the slope and floor of the south sandwich forearc recovered pelagic and turbiditic sediments this is interpreted as a high energy down slope depositional environment with high surface biogenic productivity and sediment supply leading to localised down slope mass movements on the slope and floor high levels of mn li and elevated u li ratios demonstrate a strong concentration of mn in organic poor sediment the mn redoxcline is present at 10 cm depth the high ba li 500 ppm in the forearc floor core reflects some limited enhanced productivity source sediment for the forearc may in part be derived from the northwest weddell abyssal plain based on the 206 207pb isotope ratios reflecting transport by weddell sea bottom water

continuous estimation of 3 d reflector orientations along 2 d deep seismic reflection profiles abstract interpretation of 2 d deep seismic reflection data can be limited by the recording of out of plane reflections that cannot be readily distinguished from those originating beneath a seismic line here i present a method analogous to semblance velocity analysis that utilizes varying source receiver azimuths to derive continuous estimates of 3 d reflector orientations along onshore 2 d reflection profiles for each zero offset time within a common depth point supergather the semblance is calculated along 3 d travel time curves and the dip and strike of the most coherent reflection is determined relative errors in these angles are derived from the range of travel time curves that have semblance values greater than a specified fraction for example 90 of the maximum the method is illustrated using a section from line 10ga yu1 from the youanmi terrane of the yilgarn craton in australia in which the original field data have been replaced with synthetic in line and cross line reflections reflector orientations are generally well recovered where the range of available source receiver azimuths is greater than 20o but the method fails at lower ranges where the seismic line is almost linear and this behavior is also observed in analysis of the field data nevertheless when using a realistic 1 d velocity function the orientation of upper crustal shear zones can be readily determined and on unmigrated sections subhorizontal sills can be distinguished on the basis of their geometry from the mid crustal reflectivity in future surveys reflector orientations can be determined along the near linear sections of a seismic profile by deploying additional receivers perhaps as cross line recording spreads to supplement the limited range of azimuths available from the in line acquisition the method can in principle be extended to marine reflection surveys and more complex sub surface velocity models

crustal structure of the eastern dabie shan interpreted from deep reflection and shallow tomographic data abstract a 20 km long seismic line characterises the crustal reflection pattern of the easternmost dabie shan the archetypal ultrahigh pressure uhp orogen of eastern china the weak to non reflective upper crust 5 s two way travel time twt 15 km depth is interpreted to comprise uhp rocks thrust over lithologically similar but non uhp crust the tectonic contact although not imaged as a distinct reflector is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust thus the seismic pattern of the upper crust implies that mafic oceanic crust does not constitute a significant proportion the middle to lower crust 5 10 s twt 15 33 km depth probably represents cratonal yangtze basement unaffected by the uhp metamorphism the prominent lowermost reflectors 10 12 s twt 33 40 km depth are interpreted to trace the moho excluding the presence of a crustal root inherited from the uhp orogeny a tomographic p wave velocity model for the uppermost crust

biogenic barium and the detrital ba al ratio a comparison of their direct and indirect determination biogenic barium concentrations obtained from direct determination by a three step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion a comparison of the biogenic barium from the direct sequential extraction and the indirect normative calculation clearly shows that the detrital ba al ratio is the critical factor in the normative approach and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium overall the crustal average ba al ratio of 0 005 0 01 is much higher than our directly determined global average of 0 0037 this would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the ba flux obtained from the normatively calculated ba record using our ba al det ratio of 0 0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the atlantic pacific and indian ocean our directly determined regional ba al det ratios deviate from those calculated or assumed from hinterland ratios and our global average ba al det is lower than the one commonly reported therefore in sediments with a significant terrigenous fraction the sequential extraction technique is always required

advective excess ba transport as shown from sediment and trap geochemical signatures abstract we report the results of a geochemical study of sediment and trap material major and trace elements zr ba rare earth elements and th were analyzed on bulk sedimentary material collected along the ne atlantic margin our aim is to test the widespread use of ba barite as a proxy for paleoproductivity in a continental margin area this environment is of great interest because atmospheric oceanic exchanges are important in sediments the geochemical signatures remain close to an upper crust reference with flat shale normalized rare earth elements patterns and constant elementary ratios the calculated biogenic fraction of ba or excess ba 20 45 remains lower than the excess ba record in trap material 80 99 the evolution of the geochemical signature along the margin reflects variable dilution of a detrital post archean australian shale like component by a biogenic carbonaceous seawater derived component the trap material displays a wide range of variation in its trace element content e g ba 150 3000 ppm zr 2 100 ppm except for the abyssal site which is characterized by constant signature in the two other sites all of the trace element contents increase with water depth and present pronounced seasonal changes at each sampled water depth the amount of excess ba also increases in the deepest traps and its evolution throughout the year mimics the change of the other analyzed trace elements in contrast its relationships with particulate organic carbon are not obvious in term of fluxes two periods of enhanced excess ba fluxes are observed 1 excess ba flux increases with the detrital like elements like th especially during winter and 2 excess ba flux is enhanced without any change for the other trace elements during spring to explain the first case a supply through lateral advection is proposed such transient input of significant excess ba flux will have a great impact on the yearly averaged estimation of the export production indeed only the second case reflects a bloom in the biological productivity of the water column and must be taken into account in a mean calculation of the export production finally a normalization of the excess ba by detrital like element like th will help to discriminate between a real increase of the excess ba due to local productivity change high excess ba and high excess ba th ratio 10 000 and any input due to advection process high excess ba but low excess ba th ratio

paleoproductivity and paleoredox conditions during late pleistocene accumulation of laminated diatom mats in the tropical west pacific paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the parece vela basin of the eastern philippine sea the core consists of three units from youngest to oldest 1 laminated diatom mats ldm formed by ethmodiscus rex during the last glacial maximum similar to 18 28 kyr b p 2 diatomaceous clay dc and 3 pelagic clay pc elevated levels of export productivity during ldm deposition are indicated by high values for excess ba opal content and toc ti ratios estimated rates of organic carbon degradation ca 98 opal mass accumulation average 1322 g m 2 yr 1 and corrected organic carbon flux average 248 g m 2 yr 1 are comparable to high productivity regions of the modem ocean the ldm is also characterized by moderate enrichment of redox sensitive elements such as u mo cd and zn highly s 34 depleted pyrite sulfur isotopic compositions indicating bacterial sulfate reduction in a sulfate unlimited system and c s fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive fe availability these features suggest mainly suboxic conditions in bottom waters but development of sulfidic anoxic conditions at or close to the sediment water interface association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation the dc and pc accumulated under oxic to suboxic conditions our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought and thus the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously c 2012 elsevier b v all rights reserved

bioevents and redox conditions around the cenomanian turonian anoxic event in central mexico abstract the xilitla section of central mexico western margin of the proto north atlantic is characterized by pelagic sediments enriched in marine organic matter using biostratigraphic and radiometric data it was dated at the latest cenomanian earliest turonian transition we identified an interval coeval with the faunal turnover associated with the oceanic anoxic event 2 oae 2 recording the heterohelix shift and the filament event for the first time in mexico an integral analysis of sedimentary facies pyrite and geochemical proxies reveals vertically variable redox conditions with prevailing anoxic to dysoxic bottom waters along with phosphorous and manganese depletion the highest content of total organic carbon and of certain redox sensitive trace elements rstes is found during part of the anoxic event confirming more uniform and constant oxygen depleted conditions this interval is also characterized by a significant enrichment in biogenic barium and elevated toc ntot ratios suggesting a link between productivity and anoxia sulfur isotope fractionation has a maximum value within the anoxic event favored by the increase in the flux of organic matter and intensified through sulfur recycling highly bioturbated beds representing short lived episodes of oxic conditions are intermittent within the oae 2 and become more frequent in the early turonian this study proposes a model similar to that of modern upwelling regions high marine productivity controlled organic matter burial and oxygenation at the seafloor varying between anoxic laminated facies with small pyrite framboids and dysoxic conditions bedding parallel bioturbated facies with inoceramid bivalves and large pyrite framboids interrupted by short term well oxygenated episodes thoroughly bioturbated facies with common benthic foraminifera general low oxygen conditions led to the formation of glauconite and pyrite bacterially mediated the enrichment of redox sensitive trace elements in sediments cd zn v and cr scavenged by organic matter and ni mo pb co and re by pyrite and resulted in mn and p depletion

changes in channel size at river confluences with coarse bed material a field survey of thirty stream junctions from a small watershed together with data collected by miller 1958 allowed us to investigate morphometric adjustments occurring at confluences the model proposed by roy and woldenberg 1986 was slightly modified and used as a tool for morphometric analysis two parameters are necessary in order to evaluate the rate of change in channel size at a confluence the area ratio channel capacity above the confluence channel capacity below the confluence and the discharge ratio discharge of the minor tributary discharge of the major tributary our data show that total channel capacity tends to decline below most confluences a reduction in cross sectional area implies an increase in average flow velocity this interpretation is consistent with lyell s observations and with results from recent flume experiments best and reid 1984

barium accumulation in the arabian sea controls on barite preservation in marine sediments in this study a new sequential extraction technique is used to investigate the particulate barium ba phases in sediments from the arabian sea to determine the processes controlling ba accumulation in marine sediments the total solid phase ba concentration in arabian sea surface sediments increases with water depth from 200 ppm at 500 meters below sea surface mbss to 1000 ppm at 3000 mbss the sedimentary ba composition consists of three major fractions barite ba incorporated in aluminosilicates and ba associated with mn fe oxides accumulation of barite which is the most important ba fraction in sediments located below 2000 mbss increases gradually with water depth the ba al ratio of the terrigenous fraction varies significantly across the arabian basin as the result of differences in grain size and provenance of the terrigenous sediment ba associated with mn oxides is a relatively minor fraction compared with bulk ba concentrations and it only accumulates in well oxygenated sediments below the present day oxygen minimum zone the water depth dependent accumulation of barite in the arabian sea is not related to the continuous formation of barite in settling organic particles or ba scavenging by mn oxyhydroxides but is primarily controlled by differences in ba preservation upon deposition a good correlation between the barite saturation index and the barite accumulation rate in the upper 2000 m of the water column may indicate that the degree of barite saturation of the bottom water is the main environmental factor regulating the burial efficiency of barite organic matter degradation bioturbation diagenetic mn cycling and the crystallinity of the accumulating barite may play additional roles copyright 2001 elsevier science ltd

hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material a survey of flows was conducted at a river confluence with coarse bed material bridges were installed on both tributaries at the confluence and farther downstream on the receiving stream at these stations flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises at bankfull discharge average velocity is 1 6 times higher at the confluence than on either tributary flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence the development of a zone of high shear stress is also associated with the cell of high flow velocity flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs the acceleration is not related to the development of flow separation zones as observed by best and reid 1984 for wide junction angles nor is it explained by the reduction of the friction exerted by the banks acceleration is associated with the plan geometry of the confluence with the lateral slopes which permit water to converge and with a reduction in grain roughness at the confluence owing to the curvature of the tributary and to the acute angle of entry relative power losses through the confluence decrease with increasing stages

palaeoproductivity changes and upwelling variability in the galicia mud patch during the last 5000 years geochemical and microfloral evidence theholocenepalaeoclimatichistoryofthegaliciancontinentalshelfhasbeeninvestigatedthrough theanalysesofdiatomremains othersiliceouscompounds biogenicsilica bsi andmetalscontentinamul tiproxyapproachtoagravitycorerecoveredfromthegaliciamudpatch nwiberianpeninsula coveringthe last 5000 years downcore changes in diatom assemblages composition and abundance reflect changes in diatomproductionrelatedtolong short termvariationsinclimate regionaloceanography upwellingstrength and river influx off the coast of nw spain and portugal palaeoclimatic variability was related to the relative strengths and position of the azores high and iceland low pressure cells metals and microflora fluctuations are interpreted as changes in the riverine influence and upwelling intensity paced by oceanographic atmos pheric and climatic changes lack of diatoms between 4700 3300 and 1800 1200 cal yr bp could be linked to early diagenetic processes taking place in the sediment after burial biogenic barium baexcess metals and diatomassemblages showageneralincreaseofmarineproductivityforthelast1200cal yrbp between800 and500cal yrbphighproductionofthemicrofloraistriggeredbyinfluxofriver derivednutrientsundercon ditionsofswwindsandstormsresultingfromanaonegative likephase thebiosiliceousandgeochemical signatures of sediments from the last 500 cal yr bp indicate conditions of enhanced upwelling and increased

indepth international deep profiling of tibet and the himalaya multichannel seismic reflection data description and availability project indepth international deep profiling of tibet and the himalaya has collected over 300 km of multichannel deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the tibetan plateau the reflection profiles lie within the yadong gulu rift and were acquired in the summers of 1992 and 1994 data processing utilized typical industry tools and a new method was used to migrate the data both unmigrated and migrated sections are presented here in large format to facilitate further interpretations

barium accumulation in the atlantic sector of the southern ocean results from 190 000 year records extensive investigations of sedimentary barium were performed in the southern south atlantic in order to assess the reliability of the barium signal in antarctic sediments as a proxy for paleoproductivity maximum accumulation rates of excess barium were calculated for the antarctic zone south of the polar front where silica accumulates at high rates the correspondence between barium and opal supports the applicability of barium as a proxy for productivity within the antarctic zone north of today s average sea ice maximum interglacial vertical rain rates of excess barium are high with a maximum occurring during the last deglaciation and early holocene and during oxygen isotope chronozone 5 5 during these periods the maximum silica accumulation was supposedly located south of the polar front glacial paleoproductivity instead was low within the antarctic zone north of the polar front significantly higher barium accumulation occurs during glacial times the vertical rain rates however are as high as in the glacial antarctic zone therefore there was no evidence for an increased productivity in the glacial southern ocean

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

terrigenous sediment supply along the chilean continental margin modern regional patterns of texture and composition the regional patterns of texture and composition of modern continental slope and pelagic sediments off chile between 25 s and 43 s reflect the latitudinal segmentation of geological morphological and climatic features of the continental hinterland grain size characteristics are controlled by the grain size of source rocks the weathering regime and mode of sediment input eolian off northern chile vs fluvial further south bulk mineral assemblages reveal a low grade of maturity regional variations are governed by the source rock composition of the different geological terranes and the relative source rock contribution of the coastal range and andes as controlled by the continental hydrology the relative abundance of clay minerals is also predominantly influenced by the source rock composition and partly by continental smectite neoformation latitudinal variations of illite crystallinities along the chilean continental slope and west of the peru chile trench clearly reflect modifications of the weathering regime which correspond to the strong climatic zonation of chile

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence

barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization results for the north pacific abstract this study focuses on the fate of exported organic carbon in the twilight zone at two contrasting environments in the north pacific the oligotrophic aloha site 22 45 n 158 w hawaii studied during june july 2004 and the mesotrophic subarctic pacific k2 site 47 n 161 w studied during july august 2005 earlier work has shown that non lithogenic excess particulate ba baxs in the mesopelagic water column is a potential proxy of organic carbon remineralization in general baxs contents were significantly larger at k2 than at aloha at aloha the baxs profiles from repeated sampling five casts showed remarkable consistency over a period of three weeks suggesting that the system was close to being at steady state in contrast more variability was observed at k2 six casts sampled reflecting the more dynamic physical and biological conditions prevailing in this environment while for both sites baxs concentrations increased with depth at k2 a clear maximum was present between the base of the mixed layer at around 50 and 500 m reflecting production and release of baxs larger mesopelagic baxs contents and larger bacterial production in the twilight zone at the k2 site indicate that more material was exported from the upper mixed layer for bacterial degradation deeper compared to the aloha site furthermore application of a published transfer function dehairs f shopova d ober s veth c goeyens l 1997 particulate barium stocks and oxygen consumption in the southern ocean mesopelagic water column during spring and early summer relationship with export production deep sea research ii 44 1 2 497 516 relating oxygen consumption to the observed baxs data indicated that the latter were in good agreement with bacterial respiration calculated from bacterial production these results corroborate earlier findings highlighting the potential of baxs as a proxy for organic carbon remineralization the range of poc remineralization rates calculated from twilight zone excess particulate ba contents also compared well with the depth dependent poc flux decrease as recorded by neutrally buoyant sediment traps except in one case out of four this discrepancy could indicate that differences in sinking velocities cause an uncoupling of the processes occurring in the fine suspended particle pool from those affecting the larger particle pool which sustains the vertical flux thus rendering comparison between both approaches risky

kinetics of selenite interactions with boom clay adsorption reduction interplay the speciation of selenium se in clay rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration removal of selenite from formation water can be caused by reduction and adsorption reduction could potentially be inhibited or delayed by adsorption here the interplay of adsorption and reduction of selenite was investigated in batch experiments with boom clay and its separated size fractions in all experiments dissolved se concentrations seaq showed a fast initial decrease that was followed by a slower decline until removal was almost complete x ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of seaq followed by slower selenite reduction eventually almost all solid bound seiv became reduced to se0 in all experiments the progress of seaq removal and seiv reduction to se0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite this implies that the reduction of selenite to se0 is not significantly hindered or delayed by selenite adsorption on clay minerals pyrite is probably the most relevant reductant for selenite in boom clay although reduction by feii structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks

a novel selective extraction of barite and its application to eastern mediterranean sediments abstract a sequential extraction method basex was developed to determine the different ba fractions in sediments calibration of basex with pure barite and a home made mixed mineral standard showed that barite is separated from all other major ba containing mineral phases such as clay minerals and fe and mn oxides this method permits the direct determination of sedimentary barite content and of the ba al ratio of clay and is therefore a useful tool in palaeoclimatic studies application of basex to eastern mediterranean sediments including sapropel s1 revealed that the enrichment in ba concentrations observed in this sapropel consists predominantly of barite the relationship of barite ba with organic carbon in sapropel s1 sediments appears to be independent of water depth of organic matter fluxes and of the sedimentation rate for cores ranging from 2150 to 3300 m water depth apparently organic carbon remineralisation has been virtually absent in the water column deeper than 2150 m this might have been caused by suboxic to anoxic conditions present in the deep eastern mediterranean basin during the formation of sapropel s1 because of the constant relationship between barite and organic carbon in sapropelic sediments we suggest that the barite content can be used to quantify the initial organic carbon content of the sediment in the oxidised sapropel interval

a simple method for migrating narrow aperture noisy seismic reflection data and application to project indepth international deep profiling of tibet and the himalaya deep seismic profiles migration of deep seismic data is often hindered by a narrow recording aperture line length by record length and a low signal to noise ratio the severity of typical migration artifacts e g lateral smearing of discontinuous reflections into synforms smiles increases with travel time such that interpreters of deep seismic data have often substituted migrated line drawings for the actual sections as part of project indepth international deep profiling of tibet and the himalaya a new migration method was developed to address both the noise and migration issues the method works in the time space domain and uses the simple constant velocity straight ray path to perform the migration first only amplitudes within a given range are retained for migration thus avoiding high amplitude noise bursts and low amplitude background noise then the local dip of a reflection is found by automatically fitting a straight line to the highest amplitudes within a small window several time samples by several traces and calculating the dip of the line using a constant velocity finally using this dip the method migrates a selected amplitude value the dips lateral positions and depths of the migrated events compare very well with output from more conventional algorithms e g fk stolt finite difference etc the advantages of the new method include fewer artifacts fast computer run times low memory use and the ability to migrate long profiles and travel times e g 50 s the output of the method is a grid of migrated amplitudes not wavelets or dip values which is particularly effective for making small figures such as those needed for publication the principal disadvantage is the use of a constant migration velocity