time scales of sedimentary transfer and weathering processes from u series nuclides clues from the himalayan rivers abstract in order to define time scales of erosion and sedimentary transfer in the himalaya 238 u 234 u 230 th disequilibria have been analysed in river bank sediments and in bedloads collected along the kali gandaki river one of the main nepalese rivers and in the ghaghara and gandak rivers two major plain tributaries of the ganges the th activity ratios and u th ratios in river sediments of the two ganges tributaries constantly decrease from upstream to downstream this is related to the maturation of sediments by weathering during their transfer to the plain the u series data allow to calculate a transfer time for the sediments in the alluvial gangetic plain from the chain front to the confluence with the ganges of about 100 kyr for both rivers the kali gandaki river sediment data highlight a decrease of both the th isotopic and u th ratios which is explained by a mixing between two sources with similar u th ratios but having suffered a different u th fractionation history interpretation of the u series data in the frame of this scenario gives long time scales of weathering of several 100 s kyr for the himalayan terranes the results imply that himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers in addition occurrence of similar u sr signatures in dissolved i e

chemistry of uranium thorium and radium isotopes in the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal abstract the most comprehensive data set on uranium thorium and radium isotopes in the ganga brahmaputra one of the major river systems of the world is reported here the dissolved 238u concentration in these river waters ranges between 0 44 and 8 32 1 and it exhibits a positive correlation with major cations na k mg ca the 238 u cations ratio in waters is very similar to that measured in the suspended sediments indicating congruent weathering of uranium and major cations the regional variations observed in the 234 u 238 u activity ratio are consistent with the lithology of the drainage basins the lowland tributaries chambal betwa ken and son draining through the igneous and metamorphic rocks of the deccan traps and the vindhyan bundelkhand plateau have 234 u 238 u ratio in the range 1 16 to 1 84 this range is significantly higher than the near equilibrium ratio 1 05 observed in the highland rivers which drain through sedimentary terrains the dissolved 226ra concentration ranges between 0 03 and 0 22 dpm 1 the striking feature of the radium isotopes data is the distinct difference in the 228ra and 226ra abundances between the highland and lowland rivers the lowland waters are enriched in 228ra while the highland waters contain more 226ra this difference mainly results from the differences in their weathering regimes the discharge weighted mean concentration of dissolved 238u in the ganga at patna and in the brahmaputra at goalpara are 1 81 and 0 63 1 respectively the ganga brahmaputra river system constitutes the major source of dissolved uranium to the bay of bengal these rivers transport annually about 1000 tons of uranium to their estuaries about 10 of the estimated global supply of dissolved uranium to the oceans via rivers the transport of uranium by these rivers far exceeds that of the amazon although their water discharge is only about 20 of that of the amazon the high intensity of weathering of uranium in the ganga brahmapura river system can also be deduced from the 232 th 238 u and 230 th 238 u activity ratios measured in the suspended sediments 230th is enriched by about 19 in the suspended sediments relative to its parent 238u the flux of excess 230th supplied to the bay of bengal via these river sediments is 980 1012 dpm a about six times more than its in situ production from seawater in the entire bay of bengal

uranium series isotopes in river materials insights into the timescales of erosion and sediment transport abstract the uranium series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed these signatures are a function of both time and weathering induced fractionation between the different nuclides provided appropriate models can be developed they can be used to constrain the residence time of river sediment this chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile storage in temporary sediment deposits e g floodplain and transport in the river this approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years iceland to several hundreds of thousands of years lowlands of the amazon various factors control how long sediment resides in the watershed the longest residence times are observed on stable cratons unaffected by glacial cycles or more generally climate variability and human disturbance shorter residence times are observed in active orogens andes or fast eroding recently glaciated catchments iceland in several cases the residence time of suspended sediments also corresponds to the time since the last major climate change the u series isotope composition of rivers can also be used to predict the river sediment yield assuming steady state erosion is reached by comparing this estimate with the modern sediment yield obtained by multi year sediment gauging it is clear that steady state is seldom reached this can be explained by climate variability and or human disturbance steady state is reached in those catchments where sediment transport is rapid iceland or where the region has been unaffected by climate change and or human disturbance u series are thus becoming an important tool to study the dynamics of erosion

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

martin brasier s contribution to the palaeobiology of the ediacaran cambrian transition martin brasier s work spanned almost the entire geological column but the origin of animals and the nature of the cambrian explosion were areas of particular interest to him martin adopted a holistic approach to the study of these topics that considered the interplay between multiple geological and biological phenomena and he sought to interpret the fossil record within the broad context of geological biogeochemical and ecological changes in the earth system here we summarize martin s main contributions to this area of research and assess the impact of his findings on the development of this field karl popper would have said that palaeontology is not real science because you can t go out and sample it i think absolutely the opposite i think this is actually where science is it s trying to guess what lies over the hill and map terra incognita when people come in and colonize that s just technology martin brasier 2013 excerpt from a phone interview in moor 2016

a record of late quaternary continental weathering in the sediment of the caspian sea evidence from u th sr isotopes trace element and palynological data abstract this study presents combined mineralogical chemical isotopic 87sr 86sr and u th disequilibria series and palynological analyses on bulk sediments and on distinct mineral phases carbonates and clays from a 10 m long core drilled in the southern caspian sea and containing a late pleistocene and early holocene record the data allowed identifying 1 the main variations in sedimentation 2 the processes causing these variations 3 the modification of erosion vs weathering and 4 the influence of climatic and or caspian sea level changes in the region since the late pleistocene the chemical and mineralogical results allowed the division of the sedimentary sequence into three main units and a transition zone the lower unit unit u1 primarily consists of silicate and carbonate rich detritus sedimentation characteristics including observation of detritus in secular equilibrium are relatively constant within this unit and reflect mechanical erosion in a cold climate unit u1 probably corresponds to a glacial period when the vegetation cover was sparse and wind and river transport of pollen were strong subsequently global increase in temperatures has greatly modified the sedimentation in the caspian sea south basin biogenic sedimentation is higher in units u2 and u3 and detrital inputs varied from unit u1 variations in detrital input are likely to be caused by decreasing aeolian contribution and by relative changes in river volumes and origins the study of the bulk sediments clays and carbonates reflects an increase in chemical weathering since about 10 14c ka bp ago base of unit u2 in line with an increase in the vegetation cover our results suggest an evolution of continental weathering conditions in the catchment area of the caspian sea from dominantly mechanical physical erosion during the cold period to a continuous increase in weathering since the lateglacial period as climate improved which illustrates the strong relation between climate vegetation cover and weathering processes this paper also highlights how the u th disequilibrium series proxy combined with other geochemical and palynological studies could be a useful tool to record and constrain the modification of weathering regime in response to climatic and environmental changes at regional scale

an integrated stratinomic model for the genesis and concentration of small shelly fossil style phosphatic microsteinkerns in not so exceptional conditions abstract carbonate fluorapatite filled millimeter scale microsteinkerns are a key element of the small shelly fossil ssf fauna generally associated with the late proterozoic cambrian the purported disappearance of these fossils and this style of phosphatic preservation during the cambrian stage 2 botomian extinction has been linked to changes in ocean chemistry and or changes to the physical environment due to the evolution of metazoans as well to the changing role of metazoans in the marine environment through a qualitative meta analysis of existing literature we document a broader temporal distribution for ssf style preservation than previously recognized we examine these occurrences for common patterns of depositional conditions and we offer a conceptual model for both the origin of the phosphatic microsteinkerns and their eventual concentration in certain sediments based on known biogeochemical diagenetic processes of authigenic calcium fluorapatite precipitation phosphogenesis and subsequent concentration of phosphatic sedimentary particles this polycyclic phosphogenic condensation ppc model makes predictions as to how and where this form of preservation might occur in cambrian and non cambrian marine strata alike

seismic structure and segmentation of the axial valley of the mid cayman spreading center we report the results of a two dimensional tomographic inversion of marine seismic refraction data from an array of ocean bottom seismographs obss which produced an image of the crustal structure along the axial valley of the ultraslow spreading mid cayman spreading center mcsc the seismic velocity model shows variations in the thickness and properties of the young oceanic crust that are consistent with the existence of two magmatic tectonic segments along the 110 km long spreading center seismic wave speeds are consistent with exhumed mantle at the boundary between these two segments but changes in the vertical gradient of seismic velocity suggest that volcanic crust occupies most of the axial valley seafloor along the seismic transect the two spreading segments both have a low velocity zone lvz several kilometers beneath the seafloor which may indicate the presence of shallow melt however the northern segment also has low seismic velocities 3 km s in a thick upper crustal layer 1 5 2 0 km which we interpret as an extrusive volcanic section with high porosity and permeability this segment hosts the beebe vent field the deepest known high temperature black smoker hydrothermal vent system in contrast the southern spreading segment has seismic velocities as high as 4 0 km s near the seafloor we suggest that the porosity and permeability of the volcanic crust in the southern segment are much lower thus limiting deep seawater penetration and hydrothermal recharge this may explain why no hydrothermal vent system has been found in the southern half of the mcsc

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

trace fossils and biofabrics at the precambrian cambrian boundary interval in western mongolia a small suite of trace fossils from the zavkhan basin govi altay includes many of the ichnotaxa typical of the nemakit daldynian tommotian and younger cambrian stages and other indeterminate forms the traces are almost entirely from the sandstone intervals of the large scale alternations of carbonate and siliciclastic sediments thus emphasizing the facies and taphonomic controls on trace fossil distribution and hence the inherent difficulties in using trace fossils in detailed global correlation especially in using first appearances the assemblage of traces and biofabrics is seen as resulting from the partly non uniformitarian non actualistic sedimentary environments that pertained over the boundary interval rather than as evolutionary failures and subsequently vacated environments

the erosion response to quaternary climate change quantified using uranium isotopes and in situ produced cosmogenic nuclides abstract studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates but also predict the consequences of future climate change on soil resource availability recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion this work reviews the principles of the application of in situ produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10 kyr scale at the million year scale it is more difficult to untangle the role of climate and tectonics a strong coupling exists at the 10 kyr to 100 kyr scales between climatic cycles and the transfer time of regolith from source to sink this coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale or by the storage of sediments on continental shelves at a larger scale although further analytical developments are required for these tools to reach their full potential existing works suggest that in the near future they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations climatic tectonic and or anthropic

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

effects of bioturbation on carbon and sulfur cycling across the ediacaran cambrian transition at the gssp in newfoundland canada the initiation of widespread penetrative bioturbation in the earliest phanerozoic is regarded as such a significant geobiological event that the boundary between ediacaran and cambrian strata is de

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

a complex trace fossil from the spitskop member terminal ediacaran lower cambrian of southern namibia streptichnus narbonnei igen et isp nov a new trace fossil from the upper part of the spitskop member of the urusis formation southern namibia consists of clusters of unidirectionally curved radial elements in which individual elements typically are composed of imbricated sickle shaped segments somewhat comparable to those of treptichnus pedum such complex trace fossils generally are found only in cambrian or younger strata this opens to question the position of the ediacaran cambrian boundary in the nama group suggesting that it may locally be within the uppermost part of the urusis formation rather than at the base of the nomtsas formation

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

uranium colloidal transport and origin of the 234u 238u fractionation in surface waters new insights from mount cameroon in order to evaluate the spatial variability of the 234u 238u fractionation and its origin in surface waters 234u 238u activity ratios a r and trace element concentrations were measured in 34 streams and springs draining mount cameroon volcano in addition tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes u concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element even in diluted and organic poor waters such influence which may be related to the chemical and physical nature of colloids does induce fractionations between u and sr along with filter size 234u 238u a r variations among the different ultrafiltered fractions of a sample remain small or even negligible only two samples exhibit a significant isotopic difference between colloid associated and solute u colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters at the scale of the volcano the 234u 238u a r in surface waters is large ranging from 1 038 to 1 397 small 234u 238u fractionations between 1 038 and about 1 10 are attributed to surface and meteoric weathering of the bedrock by contrast the highest 234u 238u fractionations up to 1 397 are associated with groundwater contributions thus variations in the 234u 238u a r could correspond to the water source and not to the surface weathering processes consequently 234u 238u disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters

priapulid worms pioneer horizontal burrowers at the precambrian cambrian boundary the major evolutionary events that characterize the precambrian cambrian transition are accompanied by profound ecological changes in the composition of benthic communities the nature of the substrate and the occupation of marine ecospace the increased animal activity on and within the substrate is attested to by numerous trace fossils such as the cosmopolitan treptichnus pedum whose fi rst appearance is used as the global stratotype section and point gssp to mark the base of the cambrian in spite of its major importance in biostratigraphy the maker of treptichnus trace fossils and more generally of treptichnids has long remained an enigma treptichnids were subhorizontal burrow systems produced in the subsurface and had a worldwide distribution throughout the cambrian here we show by using experimental ichnology that the treptichnid burrow systems were most probably produced by priapulid worms or by worms that used the same locomotory mechanisms as the recent priapulids e g priapulus their typical three dimensional morphology with repeated arcuate probing branches suggests that their function was related to the feeding strategy of the worm such as predation or scavenging upon small epibenthic or endobenthic invertebrates this interpretation is strongly supported by the preserved gut contents of cambrian priapulids from the burgess shale lagerstatte that contain effectively a variety of small epibenthic prey the antiquity of treptichnids would designate priapulids as one of the earliest infaunal colonizers of the substrate that possibly interacted with epibenthic communities thus playing a leading role 1 in the construction of the early marine food chain and 2 as important subhorizontal bioturbators in the early stages of the cambrian substrate revolution

the evolution of weathering profiles through time new insights from uranium series isotopes the sustainability of soil is a major issue for society in principle the evolution of soil resources can be constrained by comparing the rates of soil erosion and production cosmogenic isotopes provide one measurement of soil erosion rates they can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production this implies that the evolution of soil resources thinning thickening or constant since soil thickness is assumed to be constant with time here we utilise an independent method to estimate soil production rates using uranium series u series isotopes the study of a site in temperate australia undisturbed by human activity shows that soil production rates inferred from u series isotopes are similar to erosion rates derived from beryllium 10 10be measurements implying that at this site there is no net accumulation or loss of soil saprolite production rates the migration rate of the weathering front into the bedrock are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady state this study demonstrates that the combination of u series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles preliminary observations suggest that the rate of bedrock weathering i e saprolite production in temperate australia is of the same order of magnitude as that inferred for laterites in tropical climates this may suggest that for thick weathering profiles although the extent of weathering strongly differs between temperate and tropical climates the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

major ion chemistry of the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal the ganga brahmaputra one of the world s largest river systems is first in terms of sediment transport and fourth in terms of water discharge a detailed and systematic study of the major ion chemistry of these rivers and their tributaries as well as the clay mineral composition of the bed sediments has been conducted the chemistry of the highland rivers upper reaches of the ganga the yamuna the brahmaputra the gandak and the ghaghra are all dominated by carbonate weathering ca mg and hco3 account for about 80 of the cations and anions in the lowland rivers the chambal the betwa and the ken hco3 excess over ca mg and a relatively high contribution of na k to the total cations indicate that silicate weathering and or contributions from alkaline saline soils and groundwaters could be important sources of major ions to these waters the chemistry of the ganga and the yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions illite is the dominant clay mineral about 80 in the bedload sediments of the highland rivers kaolinite and chlorite together constitute the remaining 20 of the clays in the chambal betwa and ken smectite accounts for about 80 of the clays this difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins the highland rivers weather acidic rocks whereas the others flow initially through basic effusives the ganga brahmaputra river system transports about 130 million tons of dissolved salts to the bay of bengal which is nearly 3 of the global river flux to the oceans the chemical denudation rates for the ganga and the brahmaputra basins are about 72 and 105 tons km yr 1 respectively which are factors of 2 to 3 higher than the global average the high denudation rate particularly in the brahmaputra is attributable to high relief and heavy rainfall

tracing the distribution of erosion in the brahmaputra watershed from isotopic compositions of stream sediments bank sediments and suspended loads of the brahmaputra river and its important tributaries were collected from the himalayan front to bangladesh along with most of the important tributaries chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin overall isotopic compositions range from 0 7053 to 0 8250 for sr and nd from 20 5 to 6 9 this large range derives from the variable proportions of sediments from himalayan formations with high sr isotopic ratios and low nd and transhimalayan plutonic belt with lower sr isotopic ratios and higher nd the latter are exposed to erosion in the tsangpo and in the eastern tributary drainages overall erosion of the himalayan rocks is dominant representing ca 70 of the detrital influx compositions of the brahmaputra main channel are rather stable between 0 7177 and 0 7284 for sr and between 14 4 and 12 5 for nd throughout its course in the plain from the siang tsangpo at the foot of the himalayan range down to the delta this stability despite the input of large himalayan rivers suggests that the siang tsangpo river represents the major source of sediment to the whole brahmaputra geochemical budget implies that erosion of the namche barwa zone represents about 45 of the total flux at its outflow before confluence with the ganga from only 20 of the mountain area higher erosion rates in the eastern syntaxis compared to the other himalayan ranges is related to the rapid exhumation rates of this region possibly triggered by higher precipitation over the far eastern himalaya and the high incision potential of the tsangpo river due to its very high water discharge

what does ophiomorpha irregulaire really look like ophiomorpha irregulaire is a surprisingly poorly characterised ichnotaxon given its common occurrence in hydrocarbon reservoir facies current debate surrounds the ichnotaxobases suitable for ichnospecific level identification and also the palaeogeographic distribution of the taxon o irregulaire is conventionally characterised in core by the presence of spikey fine grained wall lining pellets since the horizontal meander maze that is also part of the ichnospecific diagnosis cannot normally be demonstrated to resolve ichnotaxonomic issues concerning the validity of pellet morphology as a primary ichnotaxobase material from the type locality is re described cretaceous book cliffs utah with an emphasis on burrow wall morphology comparative neoichnological studies using the callianassid crustacean neotrypaea californiensis were additionally conducted to understand the behaviour of modern taxa that produce burrows closely resembling o irregulaire high resolution three dimensional morphological models were created from specimens of ophiomorpha from the type locality of o irregulaire in utah usa and from eocene deep marine turbidites of the juncal formation california comparison of the morphological features from these two localities as well as specimens observed in core from offshore newfoundland demonstrate conclusively that o irregulaire is not restricted to the cretaceous western interior seaway o irregulaire may have a stratigraphic range from jurassic to recent and occur in palaeoenvironmental settings ranging from shallow marine to continental slope settings the flame like pellet morphology is considered characteristic of the type material and is a valid criterion for identifying o irregulaire in core

u series disequilibria in minerals from gandak river sediments himalaya abstract the impact of mineralogical sorting on u th ra variations observed for the ganga sediments in the himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the u series isotopic system in himalayan minerals for this purpose u th and ra concentrations along with 238u 234u 230th 226ra radioactive disequilibria were measured in several pure mineral fractions i e monazite zircon titanite ilmenite rutile garnet magnetite quartz biotite and muscovite separated from one bank sediment of the gandak river ganga basin the data obtained through a double digestion protocol in high pressure and high temperature vessels confirm that u and th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in u and th namely monazite titanite and epidote as well as major minerals with lower u and th concentrations such as quartz more importantly the data clearly indicate that similarly to whole rock samples the u th ra systematics of individual minerals have likely recorded complex u th ra fractionations in particular the 226ra enrichment observed in the biotite and clay fractions likely results from the affinity of ra for phyllosilicate minerals while the 230th and to a lesser extent the 234u enrichment observed for the other minerals might result from a nuclide adsorption process on the fe oxide coatings present on the grains or directly on the mineral surface the linear trend outlined by the minerals phases in the 226ra 230th vs 230th 234u diagram suggests that 230th is probably mobilized from zircons the only fraction presenting 230th 234u

the importance of planolites in the cambrian substrate revolution abstract early cambrian subtidal shelf substrates were characterized by low water content and steep chemical gradients conditions likely facilitated by the presence of microbial mats as reflected by an abundance of microbially mediated sedimentary structures in lower cambrian strata such substrate conditions would have been unfavourable for burrowing by benthic metazoans a combination of environmental restrictions and a lack of adaptations to vertical burrowing likely prevented most benthic metazoans from burrowing infaunally in early cambrian subtidal shelf substrates the eventual acquisition of burrowing adaptations by benthic metazoans later in the cambrian promoted an increase in the depth and intensity of bioturbation and initiated a transition toward well hydrated substrates in which extensive infaunal activity was possible siliciclastic units of the lower cambrian succession in the white inyo mountains eastern california contain abundant horizontal bioturbation on bedding planes as documented by bedding plane bioturbation indices but little vertical bioturbation as shown by ichnofabric indices and x radiography planolites a simple horizontal trace fossil represents the dominant type of bioturbation in these units planolites is found in a range of diameters indicating that more than one species of tracemaker likely produced this type of trace although these planolites do not have a vertical component their abundance on bedding planes indicates that the activities of planolites tracemakers had a significant impact on subtidal shelf substrates represented by lower cambrian units in the white inyo mountains early in the cambrian substrate revolution

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

application of acoustic full waveform inversion to retrieve high resolution temperature and salinity profiles from synthetic seismic data 1 recent works show that multichannel seismic mcs systems are able to provide detailed information on the oceans fine structure the aim of this paper is to analyze whether 1 d full waveform inversion algorithms are suitable to recover the extremely weak acoustic impedance contrasts associated to the oceans fine structure as well as their potential to image meso scale objects such as meddies we limited our analysis to synthetic noise free data in order to identify some methodological issues related to this approach under idealistic conditions e g 1 d wave propagation noise free data known source wavelet we first discuss the influence of the starting model in the context of the multi scale strategy that we have implemented then we show that it is possible to retrieve not only sound speed but also salinity and temperature contrasts within reasonable bounds from the seismic data using neural network relationships trained with regional oceanographic data sets potentially the vertical resolution of the obtained models which depends on the maximum frequency inverted is of the order of 5 10 m whereas the root mean square error of the inverted properties is shown to be 0 5 m s for sound speed 0 1 c for temperature and 0 06 for salinity to conclude this study we have inverted synthetic data simulated along an oceanographic transect acquired during the eu funded geophysical oceanography go project the results demonstrate the applicability of the method for synthetic data as well as its potential to define oceanographic features along 2 d transects at full ocean depth with excellent lateral resolution

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

increasing chemical weathering in the himalayan system since the last glacial maximum continental chemical weathering is central in earth s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans however the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained in this work a composite sediment record from the bay of bengal is used to document the evolution of chemical weathering in the himalayan system himalayan range and indo gangetic floodplain the world largest sediment conveyor to the oceans since the last glacial maximum lgm the degree of weathering of the sediments is documented using mobile to immobile ratios such as k si and h2o si as well as detrital calcite abundance robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition the bay of bengal record is also further compared to the chemical composition of modern river sediments from the ganga and brahmaputra basin weathering proxies all indicate that the sediments exported by the ganga and brahmaputra rivers became increasingly weathered over the past 21 kyr whereas sr nd and major elements suggest a constant sediment provenance in the system over the last 21 kyr these changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the indian ocean during the lgm was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the ganga and brahmaputra responds to late quaternary climate changes

clay minerals and sr nd isotopic composition of the bay of bengal sediments implications for sediment provenance and climate control since 40 ka abstract grain size clay mineralogy 87sr 86sr end and ams14c analyses of deep sea sediments cored in the central bay of bengal are used to reconstruct the evolution of provenances and climate control since the last glacial period clay minerals 87sr 86sr ratios and end values indicate a mixture of sediment from the himalaya source indian peninsula and indo burman ranges irrawaddy river these analyses show that the himalaya mountains and indo burman ranges irrawaddy river are the main suppliers before the holocene period while the inputs of erosion materials from the indo gangetic floodplain and the indian peninsula increase during the holocene period accompanied by reduced inputs of the indo burman ranges and irrawaddy sediments the regional source sink process is significantly controlled by the climate warm and wet climate conditions and enhanced indian monsoon intensity during the holocene period are responsible for the increase of sediment input from the indo gangetic plain and the indian peninsula by improving erosion in the source area input from the floodplain to the river channel and the intensity of the southwest monsoon current in contrast due to the decrease of the northeast monsoon current during the holocene period sediment input from the irrawaddy and indo burman ranges are constrained deposition center transition between the submarine fan and shelf during the last glacial period and the holocene period occurs in response to the sea level change which controls the sedimentary mode and thus significantly influences the transportation and deposition processes in the bay of bengal bob

c4 plants decline in the himalayan basin since the last glacial maximum abstract the bengal fan turbiditic system supplied by the ganga brahmaputra river system provides an integrated record of himalayan system erosion thanks to minor inputs of marine organic matter and almost complete preservation of riverine organic carbon organic matter buried in bengal fan sediments is a proxy of himalayan basin paleo vegetation the active channel levee system of the middle fan documents the last 19 ka and allows the reconstruction of vegetation change in the himalayan basin since the last glacial maximum lgm we measured 13 c of organic carbon c org in order to track changes in the proportions of c3 and c4 plants in the himalayan basin from lgm to mid holocene 13 c of bulk c org shifts of 3 4 towards more negative values relative abundance of individual n alkanes reveals that terrestrial higher plant inputs represent a dominant fraction of c org buried in bengal fan sediments 13 c of higher plants biomarkers mimic that of bulk c org showing that the later can be used as continental paleo vegetation proxy 13 c negative shift from lgm to mid holocene mostly indicates the transition from a dominant input of c4 plants to a dominant input of c3 plants and therefore reveals that c4 plants were more abundant in the basin under glacial conditions vegetation repartition in the basin simulated using the caraib dynamic vegetation model is consistent with c org data the model indicates a dominance of c4 plants in the gangetic plain during the lgm while eastern part of the basin remains dominated by c3 plants the comparison between our data and proxies of regional paleo climate suggests that the large decline of c4 plants after the lgm was due to combined increase of atmospheric co 2 and humidity levels integrated record of himalayan basin paleo vegetation suggests more arid conditions during the lgm than during the mid holocene and agrees with reconstructions of the monsoon indicating stronger sw monsoon during interglacial and stronger ne monsoon during glacial periods

mobility of u series nuclides during basalt weathering an example from the deccan traps india abstract u series disequilibria have been measured by tims and mc icp ms in dissolved phases and suspended sediments of the main basaltic rivers from the deccan trap region india for dissolved phases 234u 238u 230th 238u and 226ra 238u range between 1 11 and 1 28 between 0 03 10 2 and 1 35 10 2 and between 0 02 and 0 16 respectively for suspended particles 234u 238u 230th 238u and 226ra 238u range from 0 95 to 1 1 from 1 23 to 1 59 and from 0 1 to 0 47 respectively 230th 238u and 234u 238u systems show direct links with indices of weathering the data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady state because present day physical erosion rates are higher than expected 100 400 mm ka versus 40 mm ka for a steady state process for the narmada and the tapti basins models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40 90 ka in contrast with basins located in northern latitudes erosion in the deccan basins does not appear to have been significantly disturbed during the last glaciation 234u 238u disequilibria in the dissolved phase of large rivers world wide are also explored and two key factors are highlighted in this study the dissolution of carbonates releasing dissolved u at secular equilibrium and the silicate weathering rate estimated at the scale of the watershed in particular a positive correlation is found between dissolved 234u 238u and basaltic weathering rates for the deccan that could reflect a key role of physical erosion since for silicate monolithological basins chemical erosion rates are primarily controlled by physical erosion rates for basins with mixed lithology dissolved organic matter also plays a key role in the preferential release of 234u from silicate minerals 234u 238u ratios in rivers therefore reflect a mixture between u released by carbonate dissolution and 234u preferentially released from silicate minerals located in high physical erosion and peat areas

first evidence for locomotion in the ediacara biota from the 565 ma mistaken point formation newfoundland evidence for locomotion in the precambrian fossil record is scant reliable ediacaran trace fossils are all younger than 560 ma and consist of relatively simple horizontal burrows and trails from shallow water deposits here we describe an assemblage of macroscopic locomotory traces from deep water environments at mistaken point southeastern newfoundland canada dated to ca 565 ma these trails extend the record of complex trace fossils back into the earliest avalonian biota our new evidence for large motile organisms on the seafloor at this time suggests that at least some of these early ediacaran organisms whose biological affinities are widely debated could have been muscular and of metazoan grade

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

crustal structure of the southwest indian ridge at 66 e seismic constraints the southwest indian ridge represents a slow spreading end member of the global mid ocean ridge system and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges we present results from a wide angle seismic experiment conducted at the ridge axis at 66 e in a region that has comprehensive swath bathymetric gravity and magnetic data coverage and where the full spreading rate is 12 mm yr 1 based on these data the experiment traversed four spreading segments crustal thickness and velocity structure were determined along three intersecting profiles each 100 km long using shots from a 10 gun 71 l tuned airgun array towed at 15 m depth and fired at 40 s intervals recorded on three ocean bottom hydrophones on each profile obh data show high amplitude arrivals from oceanic layer 2 lower amplitude arrivals from layer 3 and wide angle reflections from the moho forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1 5 2 5 km thick layer 2 with a high velocity gradient and a 0 5 3 0 km thick layer 3 with a low velocity gradient and a crustal thickness of 2 2 5 4 km additional constraints on the models come from 2 d modelling of gravity data along the profiles corrected for 3 d effects of off line bathymetry along axis the thickness of layer 2 varies little but layer 3 is thick at segment centres and very thin at segment boundaries along a flowline profile crustal thickness varies by up to 75 per cent from its minimum value in 3 myr the reduced crustal thickness is consistent with observations from very slow spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle the large along axis variations in layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation flowline variations in crustal thickness may result from episodicity of melt supply on timescales of 3 myr and by tectonic extension during amagmatic periods velocities at the top of layer 2 are poorly correlated with crustal age based on magnetic anomalies suggesting also that episodicity is decoupled between adjacent segments

eolian contribution to soils on mount cameroon isotopic and trace element records abstract we determined sr nd and pb isotopic compositions and major and trace element compositions of soil samples recovered from three soil profiles developed on relatively young pyroclastic deposits less than 10 000 years old of the mount cameroon volcano the time elapsed since then is short compared with the half lives of rb sm u and th suggesting that radioactive decay can be neglected we therefore assumed that i these polyphase soils developed on isotopically homogenous substrates and ii any isotopic variation within such profiles cannot be due to selective dissolution of primary minerals with isotopic compositions different from that of bulk bedrock any sr nd or pb isotopic variation would therefore record an allochtonous input occurring either in a solid or dissolved state sr and nd isotope compositions change systematically with depth in the soil profile in particular 87 sr 86 sr ratios are far greater in the humic horizons than in the underlying horizons except in the ca9h soil profile although smaller than for sr isotope compositions 143 nd 144 nd ratios displayed an inverse relationship with a decrease towards the uppermost organic rich horizons these isotope shifts correlate with changes in sr and nd contents suggesting an allochtonous input characterized by significantly different isotope compositions and slightly lower sr contents but higher nd contents pb isotopic compositions and pb concentrations in the uppermost horizons were distinct from those of underlying horizons in the soil sequences since an anthropogenic contamination such as from leaded petrol cannot explain the pb shifts a natural source has to be invoked for pb as well as for sr and nd considering both the wind paths in west africa and the isotopic shifts registered in the uppermost horizons saharan dust appears to be the best candidate to explain such variations these inputs probably happened in winter when dry warm harmattan wind blows from the northeast a maximum of 8 of saharan dust accretion was calculated for the mount cameroon soils this corresponds to average dust deposition rates between 1 3 and 0 8 g cm 2 ka 1 values that are much higher than those found in hawaiian soils by kurtz et al kurtz a c derry l a chadwick o a 2001 accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances geochim cosmochim acta 65 1971 1983 and are probably related to shorter distance between dust source and deposit any elemental mass balance calculated in the upper organic rich horizons should be then corrected from allochtonous wind borne saharan dust before evaluating weathering linked chemical mobilities

triple oxygen isotope signatures in evaporated water bodies from the sistan oasis iran natural samples from water bodies in the arid and semiarid environment of the sistan oasis iran demonstrate a systematic evolution of 17o excess and 18o as a result of nonequilibrium fractionation during extreme evaporation residual water samples exhibit a significant and systematic decrease of 17o excess with progressive evaporation loss reaching values of 160 per meg over a 35 range of 18o waters from heavily evaporated volume limited natural bodies with limited or no recharge fall on the theoretically predicted isotopic evolution curve in agreement with ambient relative humidity of 30 to 35 recharged water bodies appear to follow a different trend these new results demonstrate the potential of 17o excess for the estimation of evaporation loss and ambient conditions in an arid environment

aggregate scale spatial heterogeneity in reductive transformation of ferrihydrite resulting from coupled biogeochemical and physical processes abstract iron hydr oxides are ubiquitous in soils and sediments and play a dominant role in the geochemistry of surface and subsurface environments their fate depends on local environmental conditions which in structured soils may vary significantly over short distances due to mass transfer limitations on solute delivery and metabolite removal in the present study artificial soil aggregates were used to investigate the coupling of physical and biogeochemical processes affecting the spatial distribution of iron fe phases resulting from reductive transformation of ferrihydrite spherical aggregates made of ferrihydrite coated sand were inoculated with the dissimilatory fe reducing bacterium shewanella putrefaciens strain cn 32 and placed into a flow reactor the reaction cell simulates a diffusion dominated soil aggregate surrounded by an advective flow domain the spatial and temporal evolution of secondary mineralization products resulting from dissimilatory fe reduction of ferrihydrite were followed within the aggregates in response to a range of flow rates and lactate concentrations strong radial variations in the distribution of secondary phases were observed owing to diffusively controlled delivery of lactate and efflux of fe ii and bicarbonate in the aggregate cortex only limited formation of secondary fe phases were observed over 30 d of reaction despite high rates of ferrihydrite reduction under all flow conditions tested ferrihydrite transformation was limited in the cortex 70 85 mol fe remained as ferrihydrite because metabolites such as fe ii and bicarbonate were efficiently removed in outflow solutes in contrast within the inner fractions of the aggregate limited mass transfer results in metabolite fe ii and bicarbonate build up and the consummate transformation of ferrihydrite only 15 40 mol fe remained as ferrihydrite after 30 d of reaction goethite lepidocrocite and minor amounts of magnetite formed in the aggregate mid section and interior at low lactate concentration 0 3 mm after 30 d of reaction under high lactate 3 mm concentration magnetite was observed only as a transitory phase and rather goethite lepidocrocite and siderite were the dominant secondary mineralization products our results illustrate the importance of slow diffusive transport of both electron donor and metabolites concentrations and concomitant biogeochemical reactions within soils and sediments giving rise to heterogeneous products over small spatial m scale

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

the early paleozoic development of bioturbation evolutionary and geobiological consequences abstract bioturbation the physical and chemical mixing of sediments by burrowing animals is a critical engineering process in modern seafloor environments and exerts an important control on not only benthic ecology and sediment properties but also ocean wide biogeochemical cycling well mixed sediments have long been assumed to appear at the precambrian cambrian boundary with the first occurrence of the index fossil and three dimensional burrow treptichnus pedum however field based analyses synthesizing ichnological stratigraphic sedimentological and taphonomic data collected from lower paleozoic successions worldwide indicate that sediment mixing in marine shelf environments remained limited until at least the late silurian 120 million years after the precambrian cambrian transition in spite of early advances in complex modes of infaunalization mixed layer development had a much more gradual trajectory and did not occur in step with the first appearance of three dimensional burrows the delayed appearance of modern style intensities of sediment reworking postdating both the cambrian explosion and the great ordovician biodiversification event may be due to the late stage radiation of mobile infaunal deposit feeders biological bulldozers the organisms which are in modern marine settings the most efficient bioturbators the protracted development of the sediment mixed layer holds important implications for contemporaneous biogeochemical cycling and oxygenation of the ocean atmosphere system the delayed development of intensive sediment mixing may have also mediated exceptional preservation of both fossilized soft tissues and shallow tier trace fossils and may therefore have contributed to the anomalous preponderance of konservat lagerstatten and exceptionally preserved shallow tier trace fossils and bioglyphs characteristic of the lower paleozoic stratigraphic record

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

broadband spectra of seismic survey air gun emissions with reference to dolphin auditory thresholds acoustic emissions from a 2120 cubic in air gun array were recorded through a towed hydrophone assembly during an oil industry 2 d seismic survey off the west wales coast of the british isles recorded seismic pulses were sampled calibrated and analyzed post survey to investigate power levels of the pulses in the band 200 hz 22 khz at 750 m 1 km 2 2 km and 8 km range from source at 750 m range from source seismic pulse power at the 200 hz end of the spectrum was 140 db re 1 pa2 hz and at the 20 khz end of the spectrum seismic pulse power was 90 db re 1 pa2 hz although the background noise levels of the seismic recordings were far in excess of ambient due to the proximity of engine propeller and flow sources of the ship towing the hydrophone seismic power dominated the entire recorded bandwidth of 200 hz 22 khz at ranges of up to 2 km from the air gun source even at 8 km range seismic power was still clearly in excess of the high background noise levels up to 8 khz acoustic observations of common dolphins during preceding seismic surveys suggest that these animals avoided the immediate vicinity of the air gun array while firing was in progress i e localized disturbance occurred during seismic surveying although a general pattern of localized disturbance is suggested one specific observation revealed that common dolphins were able to tolerate the seismic pulses at 1 km range from the air gun array given the high broadband seismic pulse power levels across the entire recorded bandwidth and known auditory thresholds for several dolphin species we consider such seismic emissions to be clearly audible to dolphins across a bandwidth of tens on kilohertz and at least out to 8 km range

microbial mats implicated in the generation of intrastratal shrinkage synaeresis cracks intrastratal shrinkage often termed synaeresis cracks are commonly employed as diagnostic environmental indicators for ancient salinity stressed transitional fluvial marine or marginal marine depositional environments despite their abundance and use in facies interpretations the mechanism of synaeresis crack formation remains controversial and widely accepted explanations for their formation have hitherto been lacking sedimentological ichnological petrographic and geochemical study of shallow marine mudstone beds from the ordovician beach formation of bell island newfoundland has revealed that crack development cf synaeresis cracks on the upper surface of mudstone beds is correlated with specific organic geochemical and sedimentological parameters contorted sinuous sand filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds cracks are absent in highly bioturbated mudstone and are considered to pre date firmground assemblages of trace fossils that include planolites and trichophycus the tops of cracked mudstone beds contain up to 2 1 wt total organic carbon relative to underlying mudstone beds that contain around 0 5 wt total organic carbon high resolution carbon isotope analyses reveal low 13corg values 27 6 on bed tops compared with sandy intervals lacking cracks 24 4 to 24 9 cracked mudstone facies show evidence for microbial matgrounds including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section non cracked mudstone lacks evidence for development of microbial mats microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the precambrian and early phanerozoic in later phanerozoic strata matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface grazing macrofauna unless evidence of synaeresis i e contraction of clay mineral lattices in response to salinity change can be independently demonstrated the general term intrastratal shrinkage crack is proposed to describe sinuous and tapering cracks in mudstone beds

wide area imaging from obs multiples the subseafloor structure offshore western canada was imaged using first order water layer multiples from oceanbottom seismometer obs data and the results were compared to conventional imaging using primary reflections this multiple migration mirror imaging method uses the downgoingpressurewavefieldjustabovetheseafloor which is devoid of any primary reflections but consists of receiverside ghosts of these primary reflections the mirror imaging method employs a primaries only kirchhoff prestack depth migration algorithm to image the receiver ghosts the additional travel path of the multiples through the water layer is accountedforbyasimplemanipulationofthevelocitymodel and processing datum the receivers lie not on the seabed but on a sea surface twice as high as the true water column migration results show that the multiple migrated image provides a much broader illumination of the subsurface than is possible for conventional imaging using the primaries especially for the very shallow reflections and sparse obs spacing the resulting image from mirror imaging has illumination comparable to the vertical incidence surface streamer single channelreflectiondata

accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs including sea salt aerosols and asian dust we use soil mineralogy and radiogenic isotope geochemistry sr and nd to evaluate the impact of asian dust on a chronosequence of hawaiian soils dust becomes an important constituent of soils 20 ky and older near surface 50 cm depth horizons contain as much as 30 quartz a mineral absent from local parent material basaltic sr and nd isotope signatures in these horizons have been completely overprinted by asian dust signatures with 87sr 86sr ratios as high as 0 723 and end values as low as 7 ree patterns in these soils are indistinguishable from that of average upper continental crust quartz abundance and nd isotopes provide two independent tracers of long term dust additions to the chronosequence soils the two tracers indicate a minimum long term average dust accretion rate of 125 mg cm 2 ky 1 at the 150 ka chronosequence site roughly a factor of three higher than holocene dust accumulation rates estimated from north pacific sediment cores we find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older requiring a loss mechanism for accreted dust on the basis of the geomorphic stability of these sites observed preferential loss of dust derived mica relative to quartz and estimates of soil si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils dust has a profound effect on the budgets of elements that are susceptible to leaching losses dust becomes the dominant source of soil nutrients si and p in the oldest most intensely weathered soils we calculate a dust derived p input flux of 0 8 mg cm 2 ky 1 and a dust derived si input flux of 35 mg cm 2 ky 1 si leaching fluxes are high 1400 mg cm 2 ky 1 in the youngest 2 ka soils and drop systematically with soil age to a value that closely balances the dust derived si input flux by 4100 ka extremely refractory elements such as nb which are concentrated in soils by residual enrichment processes are much less readily impacted by dust addition although dust has had a pronounced impact on sr and nd isotopic budgets and on soil mineral composition at the 150 ka site dust cannot have contributed 4 of the total nb contained in this soil profile

bioinfiltration irrigation driven transport of clay particles through bioturbated sediments abstract the transport of clay size particles into the pore networks of siliciclastic sediments by the bioirrigating activities of burrowing organisms is demonstrated here for the first time in a laboratory microcosm evidence for the advection of kaolinite particles into pore spaces was observed at the interface between two buried sand layers the clay transport was caused by the burrow irrigating behavior of the mud shrimp upogebia pugettensis the term bioinfiltration is introduced herein to describe the process of particulate transport into pore water networks given the abundance of bioirrigating organisms in shallow marine environments bioinfiltration may be a widespread phenomenon affecting the properties of large sediment volumes the introduction of clay mineral particles into otherwise clay free sediments through bioirrigation would affect the sediment characteristics significantly in particular bioinfiltrated clays would occlude pore space could act as precursors to the production of authigenic cements and inhibit the growth of quartz cements all of which would reduce the permeability and porosity of hydrocarbon reservoirs and groundwater aquifers bioinfiltration therefore needs to be considered when assessing the origins and diagenetic effects of clay particles in bioturbated sediments and is likely also to be of significance to sediment biogeochemistry and infaunal ecology this is because of the potential influence of sedimentary pore occlusion on pore water exchange and through the introduction of particulate organic nutrients into the sediment as flocs or colloids such changes are likely to affect microbial productivity which is thought to be the food source of many burrowing micro and macro organisms

protracted development of bioturbation through the early palaeozoic era mobile organisms first appeared in the fossil record prior to the precambrian cambrian transition sediment textures indicate that the degree of sediment mixing by animal activity remained low for 120 million years following the transition

the neoproterozoic lower cambrian in northern govi altay western mongolia regional setting lithostratigraphy and biostratigraphy this paper provides a framework for the stratigraphy and correlation of key sections through the neoproterozoic lower cambrian of the govi altay area western mongolia studied by members of igcp project 303 dzabkhan formation volcanic rocks at the base of the succession are suggested to have formed during rifting of the baydrik tarbagatay microcontinent in the baykalian interval c 850 650 ma pre vendian accretion transformed this rift and adjacent microcontinental fragments into a marginal marine back arc basin in which accumulated relatively uninterrupted neoproterozoic and earliest palaeozoic sediments lithostratigraphy for the latter is defined with detailed maps at five sections bayan gol northern and southern blocks tsagaan gol salaany gol and tayshir the tsagaan oloom formation lower member contains two diamictites of possible sturtian age middle and upper carbonates are compared with the yudomian vendian of siberia the former ending with a unit of boxonia grumulosa stromatolites in the upper member phosphorite shale is followed mainly by limestones with the earliest skeletal fauna the bayan gol formation comprises siliciclastic limestone intercalations with increasingly diverse invertebrate biota correlation of lithological marker beds across the study area produces a composite stratigraphic section to which the earliest skeletal fossil occurrences are tied the anabarites trisulcatus zone of the nemakit daldynian stage begins near the base of the tsagaan oloom formation upper member unit 11 the purella antiqua zone begins near the base of the bayan gol formation unit 18 assemblages typical of the nochoroicyathus sunnaginicus zone base of the tommotian stage range through units 19 to 23 elements typical of the dokidocyathus regularis zone middle tommotian appear in unit 24 near the top of the bayan gol formation

u series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains the case of the himalayan rivers 238u 234u 230th radioactive disequilibria were analyzed in suspended sediments collected at different depths from the ganges river and one of its main tributaries the narayani gandak river results associated with bedload sediment data suggest that uranium series u series disequilibria in river sediments of the ganges basin vary with grain size and sampling location the range of observed u series disequilibria is explained by a mixing model between a coarse grained sediment end member represented by bedload and bank sediments and a fine grained end member that both originate from himalaya but undergo different transfer histories within the plain the coarse grained sediment end member transits slowly i e several 100 s ky in the plain whereas the fine grained sediment end member is transferred much faster 20 25 ky as indicated by the absence of significant variations in th isotope composition of the fine grained sediment end members these results show that u series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference or more between the transfer time of suspended and bedload sediments this underlines that a good knowledge of the proportion of suspended vs bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors

uranium series isotopes in colloids and suspended sediments timescale for sediment production and transport in the murray darling river system abstract we have measured 238u 234u 230th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the murray darling river basin se australia continuous compositional trends of composition are observed for 234u 238u and 230th 238u activity ratios over the range of size fractions studied they are explained by the variable contribution of two components detrital alumino silicates and organic matter using relationships between activity ratios and the loss on ignition it is possible to estimate the u series isotopic composition of both detrital and organic end members which provide the best estimate for the composition of the true detrital phase i e controlled only by the removal of elements through weathering and the true dissolved phase i e colloid free respectively when performing a single filtration at 0 2 m it appears that size fractions and

petrographic evidence for ediacaran microbial mat targeted behaviors from the great basin united states abstract the lower member of the wood canyon formation contains a variety of simple late precambrian early cambrian dwelling and grazing traces typical of early bioturbators the traces dominated by planolites and palaeophycus are heavily abundant throughout sections comprised of higher energy siliciclastics two of which chicago pass and boundary canyon california usa also contain evidence for targeted microbial mat penetration by undermat mining organisms below and into the pc c boundary the co occurence of penetrative burrows within microbial microlaminae surficial trails and dwelling structures is characteristic of the complex ediacaran trace fossil lamonte trevallis as described by meyer et al 2014 from the dengying formation of south china the specimens from the wood canyon formation present similar structures resulting from altering behaviors of epibenthic locomotion undermat mining and dwelling meyer et al 2014 principally grain size analyses confirmed the presence of microbial mat targeted mining seemingly resistant to overprinting by co occurring horizontal traces therefore we have classified the traces present in the california sections as lamonte trevallis broadening the geographic extent of this particular mining behavior in the precambrian and the instances of complex bioturbating behaviors preceding the cambrian substrate revolution

engineering the cambrian explosion the earliest bioturbators as ecosystem engineers by applying modern biological criteria to trace fossil types and assessing burrow morphology complexity depth potential burrow function and the likelihood of bioirrigation we assign ecosystem engineering impact eei values to the key ichnotaxa in the lowermost cambrian fortunian surface traces such as monomorphichnus have minimal impact on sediment properties and have very low eei values quasi infaunal traces of organisms that were surficial modifiers or biodiffusors such as planolites have moderate eei values and deeper infaunal gallery biodiffusive or upward conveying downward conveying traces such as teichichnus and gyrolithes have the highest eei values the key cambrian ichnotaxon treptichnus pedum has a moderate to high eei value depending on its functional interpretation most of the major functional groups of modern bioturbators are found to have evolved during the earliest cambrian including burrow types that are highly likely to have been bioirrigated in fine grained or microbially bound sedimentary environments trace makers of bioirrigated burrows would have had a particularly significant impact generating advective fluid flow within the sediment for the first time in marked contrast with the otherwise diffusive porewater systems of the proterozoic this innovation is likely to have created significant ecospace and engineered fundamentally new infaunal environments for macrobiotic and microbiotic organisms alike

baseline determination of the atmospheric pb sr and nd isotopic compositions in the rhine valley vosges mountains france and the central swiss alps abstract the pb sr and nd isotopic compositions of biomonitors lichen moss bark and soil litter from different regions in the rhine valley as well as of

increasing the number and signal to noise ratio of obs traces with supervirtual refraction interferometry and free surface multiples summary the theory of supervirtual interferometry is modified so that free surface related multiple refractions can be used to enhance the signal to noise ratio snr of primary refraction events by a factor proportional to ns where ns is the number of post critical sources for a specified refraction multiple we also show that refraction multiples can be transformed into primary refraction events recorded at virtual hydrophones located between the actual hydrophones thus data recorded by a coarse sampling of ocean bottom seismic obs stations can be transformed in principle into a virtual survey with p times more obs stations where p is the order of the visible free surface related multiple refractions the key assumption is that the refraction arrivals are those of head waves not pure diving waves the effectiveness of this method is validated with both synthetic obs data and an obs data set recorded offshore from taiwan results show the successful reconstruction of far offset traces out to a source receiver offset of 120km the primary supervirtual traces increase the number of pickable first arrivals from approximately 1600 to more than 3100 for a subset of the obs data set where the source is only on one side of the recording stations in addition the head waves associated with the first order free surface refraction multiples allow for the creation of six new common receiver gathers recorded at virtual obs station located about half way between the actual obs stations this doubles the number of obs stations compared to the original survey and increases the total number of pickable traces from approximately 1600 to more than 6200 in summary our results with the obs data demonstrate that refraction interferometry can sometimes more than quadruple the number of usable traces increase the source receiver offsets fill in the receiver line with a denser distribution of obs stations and provide more reliable picking of first arrivals apotentialliabilityofthismethodisthatlong offsetrefractionarrivalsextractedbyinterferometry might not necessarily be head waves from deeper refraction interfaces the extracted arrivals might be from a shallower interface and so only supply redundant information about that portion of the subsurface nevertheless our tomography example shows the value of these arrivals in reducing artefacts and increasing resolution in the tomogram

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

burrowing below the basal cambrian gssp fortune head newfoundland the range of treptichnus pedum the index trace fossil for the treptichnus pedum zone extends some 4 m below the global standard stratotype section and point for the base of the cambrian period at fortune head on the burin peninsula in southeastern newfoundland the identification of zigzag traces of treptichnus isp even further below the gssp than t pedum in the fortune head section and in other terminal proterozoic successions around the globe supports the concept of a gradational onset of three dimensional burrowing across the proterozoic cambrian boundary although t pedum remains a reasonable indicator for the base of the cambrian period greater precision in the stratotype section can be achieved by a detailed re evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the t pedum zone

crustal structure of the propagating tammar ridge segment on the mid atlantic ridge 21 5 n active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time the mechanisms controlling ridge propagation however are poorly understood at the slow spreading mid atlantic ridge at 21 5 n a seismic refraction and wide angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply thus crust is thickest 8 km at the domed segment center and decreases in thickness toward both segment ends however thicker crust is formed in the direction of ridge propagation suggesting that melt is preferentially transferred toward the propagating ridge tip further while seismic layer 2 remains constant along axis seismic layer 3 shows profound changes in thickness governing variations in total crustal thickness this feature supports mantle upwelling at the segment center thus fluid basaltic melt is redistributed easily laterally while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply the onset of propagation seems to have coincided with the formation of thicker crust suggesting that propagation initiation might be due to changes in the melt supply after a rapid initiation a continuous process of propagation was established the propagation rate seems to be controlled by the amount of magma that reaches the segment ends the strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

importance of atmospheric inputs and fe oxides in controlling soil uranium budgets and behavior along a hawaiian chronosequence abstract a long term budget of uranium calculated for a chronosequence of hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway a series of sequential extractions on the same soils shows a strong association between fe oxides and uranium especially the amorphous and poorly crystalline fe oxides present in intermediate age soils 20 and 150 ka despite intense leaching conditions soil u contents were found to increase between 20 fold and 60 fold with age along the chronosequence from 9 to 17 g cm 2 at 0 3 ka sites to 344 543 g cm 2 at the 4100 ka site an open system transport function comparing u to an index least mobile element nb was used to calculate u gains or losses from the soils relative to the input from parent material weathering results show small losses or small gains 25 of u occurred in soils 150 ka and younger while gains of up to 531 u occurred in soils older than 150 ka highlighting the importance of mineral aerosol dust deposition at the older sites in the 4100 ka soil the ratio of dust derived u inputs to parent material derived u inputs is approximately 6 sequential extractions show that between 0 1 and 1 2 of the total u in the soils was extracted with ammonium acetate indicating that very little u exists as exchangeable cations a second extraction employing citrate dithionate and hydroxylamine hydrochloride was performed to extract u associated with fe oxides average percent occurrences for u in the fe oxide extraction for each profile are 7 at 0 3 ka 53 at 20 ka 40 at 150 ka and 17 at 4100 ka illustrating an increase with age to 20 ka followed by a subsequent decline this parallels the trend observed in amorphous fe oxide content suggesting that adsorption on amorphous fe oxides is the dominant control on u in the intermediate age soils

isotopic tracing of the dissolved u fluxes of himalayan rivers implications for present and past u budgets of the ganges brahmaputra system u activity ratios have been measured in the dissolved loads of selected rivers from the himalayan range in central nepal and from the bangladesh as well as in some rain waters a few european and asian rivers have also been analyzed for their u activity ratios the data confirm the negligible effect of rainwater on the budget of dissolved u in river waters the results also indicate that rivers on each himalayan structural unit have homogeneous and specific u isotope compositions i 234u 238u activity ratios slightly lower than unity in the dissolved load of the streams draining the tethyan sedimentary series tss ii values slightly higher than unity for waters from the high himalaya crystalline hhc and the lesser himalaya lh iii systematically higher 234u 238u activity ratios for waters from the siwaliks thus u activity ratios in association with sr isotopic ratios can be used to trace the sources of dissolved fluxes carried by these rivers coupling of u with sr isotope data shows 1 that the u carried by the dissolved load of the himalayan rivers mainly originates from u rich lithologies of the tss in the northern formations of the tibetan plateau and 2 that the elemental u and sr fluxes carried by the himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the himalayan chain rivers flowing on the indian plain define a different trend from that of the himalayan rivers in the u sr isotopic diagram indicating the contribution of a specific floodplain component to the u and sr budgets of the ganges and the brahmaputra the influence of this component remains limited to 10 to 15 percent for the u flux but can contribute 35 to 55 of the sr flux the variations of the sr and u fluxes of the ganges brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high altitude terrains during glacial episodes this scenario would significantly decrease the dissolved u flux of the ganges brahmaputra river system and increase its u activity ratio such a climatic dependence of the himalayan u flux could induce a periodic variation of the mean u activity ratio of the world rivers on glacial interglacial timescales

seismic structure of the upper oceanic crust revealed by in situ q logs in situ seismic attenuation is computed through 2 1 km of the upper oceanic crust in the vicinity of odp hole 504b the results strongly tie crustal properties to seismic measurables and observed geological structures we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity the in situ attenuation q 1 consists of both intrinsic and scattering contributions but is dominated by the scattering attenuation unless porosities are near zero when it approaches typical estimates from seismic refraction studies the attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic vp log and is found to decrease step wise in relatively homogeneous layers from q 25 to q 300 between the top of seismic layer 2a and a sharp discontinuity at 1 3 km depth these changes correspond with heterogeneities at 1 0 1 3 m and at 5 6 10 0 m wavelengths that we interpret to be associated with fracturing and structure of pillow basalts and lava flows in seismic layers 2a and 2b although seismic velocity studies suggest that the layer 2 3 boundary also occurs at about 1 3 km the large variation in q 140 to 460 below this depth indicates that a seismically homogeneous and uniform layer 3 has not been reached in hole 504b we derive an empirical relationship between attenuation and porosity q 1 q0 1 e where q0 1 0 004 and 25 that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models

segmentation and melt supply at the southwest indian ridge the segmentation of mid ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust we present seismic refraction results from three segments of the very slow spreading southwest indian ridge from the absence of oceanic layer 3 beneath segment boundaries and from significant variations in maximum crustal thickness between segments we infer that melt generation and or delivery is focused at segment midpoints and varies between segments anomalously thin crust 2 0 2 5 km beneath the nontransform segment boundaries and 3 5 6 0 km beneath the segment mid points is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center

gyrolithes from the ediacaran cambrian boundary section in fortune head newfoundland canada exploring the onset of complex burrowing abstract the beginning of the cambrian explosion is characterized by the onset of infaunalization and the appearance of systematic patterns of burrowing the trace fossil gyrolithes is common in the ediacaran cambrian global stratotype section and point where it shows a higher diversity and burrow depth than previously reported from any cambrian spiral vertical burrows two ichnospecies are present g scintillus isp nov exhibits a small burrow radius to whorl radius ratio whereas g gyratus hofmann exhibits an exceptionally large ratio that formerly led to its identification as an aberrant ichnospecies of skolithos the helical morphology of these cambrian gyrolithes is interpreted as having served two purposes as an anchor in a relatively high energy environment and as an optimal shape for maximizing surface area for bacterial gardening these shallow marine gardening burrows share a similar feeding strategy as graphoglyptids suggesting that shallow early cambrian sediments may have been relatively poor in organics and thus further supporting the model for onshore offshore migration of gardening burrows

ichnological analysis the common ground between ichnofacies workers and ichnofabric analysts abstract this work compares the approaches taken by the two schools of applied ichnology ichnofacies and ichnofabric analysts and proposes an integrated approach for future improvement there are many similarities between the two approaches which mainly differ in the resolution at which ichnological analysis is undertaken ichnofabric work is by necessity performed on a bed by bed basis and interpreted at a scale consistent with that of the sedimentologist s facies ichnofacies studies in contrast seldom report bed by bed changes in ichnological content instead highlighting bedset to facies association scale changes in ichnology through comparison with summary models of ichnological trends this work highlights the fact that workers from both schools utilize the same tools with slightly different emphases to reach their ichnological conclusions contention between the two schools has in recent years centred upon which is the better method for studying trace fossils an artificial construct that detracts from the fact that both are extremely useful to sedimentological and palaeoenvironmental studies ichnofabric analysis is in addition a highly effective means of documenting bioturbated sedimentary fabrics as the basis for petrophysical studies ichnofacies and ichnofabric analysts use the same toolbox of ichnological methods to determine ancient palaeoenvironments the scale of observation is generally different but the results closely comparable the recognition of key stratal surfaces on ichnological grounds is also possible with both methodologies ichnofacies analysts look to recognise glossifungites ichnofacies firmgrounds in offshore mudstones ichnofabric studies would look to recognise overprinting of fabrics created by groups of organisms from different palaeoenvironments but associated with the same key surface it is proposed that the terms ichnofacies method and ichnofabric analysis are abandoned in favour of a neutral ichnological analysis method encompassing the approaches and terminology of both to produce a unified scientifically rigorous user friendly methodology

sediment transit time and floodplain storage dynamics in alluvial rivers revealed by meteoric 10be quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes biogeochemical cycling and improving watershed management but measuring sediment transit time is challenging here we provide the first systematic test of measuring cosmogenic meteoric beryllium 10 10bem in the sediment load of a large alluvial river to quantify sediment transit times we take advantage of a natural experiment in the rio bermejo a lowland alluvial river traversing the east andean foreland basin in northern argentina this river has no tributaries along its trunk channel for nearly 1 300 km downstream from the mountain front we sampled suspended sediment depth profiles along the channel and measured the concentrations of 10bem in the chemically extracted grain coatings we calculated depth integrated 10bem concentrations using sediment flux data and found that 10bem concentrations increase 230 from upstream to downstream indicating a mean total sediment transit time of 8 4 2 2 kyr bulk sediment budget based estimates of channel belt and fan storage times suggest that the 10bem tracer records mixing of old and young sediment reservoirs on a reach scale 10bem transit times are shorter where the channel is braided and superelevated above the floodplain and longer where the channel is incised and meandering suggesting that transit time is controlled by channel morphodynamics this is the first systematic application of 10bem as a sediment transit time tracer and highlights the method s potential for inferring sediment routing and storage dynamics in large river systems

silicified horodyskia and palaeopascichnus from upper ediacaran cherts in south china tentative phylogenetic interpretation and implications for evolutionary stasis horodyskia is one of the earliest known macroscopic life forms with a fossil record dating from c 1 4 ga palaeopascichnus represents a key ediacaran element with world wide distribution however their body constructions and affinities are poorly understood partly because previously described species are mostly preserved as casts and moulds in siliciclastic rocks silicified specimens from the upper ediacaran liuchapo formation in eastern guizhou south china are described as horodyskia minor sp nov and palaeopascichnus jiumenensis sp nov their taxonomic assignments are based on their uniserial arrangement of spheroidal or discoidal units which are connected by a filament and surrounded by a quartz halo they are unlikely to be brown algae animal traces faecal pellets colonial metazoans or giant sulphide oxidizing bacteria instead we propose that horodyskia and palaeopascichnus may be phylogenetically related and their collective morphologies allow tentative comparison with agglutinated foraminifers the segments can be compared with cytoplasm filled chambers connecting filament with small passage between chambers and quartz haloes with agglutinated tests however their ontogeny appears to be distinct from that of modern foraminifers the occurrences of horodyskia fossils in mesoproterozoic and ediacaran rocks indicate an extremely long range c 900 ma and echoes the proposition of extended evolutionary stasis in the proterozoic

long term storage and age biased export of fluvial organic carbon field evidence from west iceland terrestrial organic carbon oc plays an important role in the carbon cycle but questions remain regarding the controls and timescale s over which atmospheric co remains sequestered as particulate oc poc motivated by observations that terrestrial poc is physically stored within soils and other shallow sedimentary deposits we examined the role that sediment storage plays in the terrestrial oc cycle specifically we tested the hypothesis that sediment storage impacts the age of terrestrial poc we focused on the efri haukadalsa river catchment in iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon based determinations of poc storage duration by supplying pre aged petrogenic poc our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial scale storage times comparison between the sample types suspended and deposits suggested an age offset between transported suspended sediments and stored deposits poc at the time of sampling which is predicted by theory for the sediment age distribution in floodplains we also observed that poc in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data which suggested an additional role for oc cycling consistent with this we observed interparticle heterogeneity in the composition of poc by imaging our samples at the microscale using x ray absorption spectroscopy specifically we found that particles within individual samples differed in their sulfur oxidation state which is indicative of multiple origins and or diagenetic histories altogether our results support recent coupled sediment storage and oc cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling

weathering of the ganga alluvial plain northern india implications from fluvial geochemistry of the gomati river abstract in the northern part of the indian sub continent the ganga alluvial plain gap feeds its weathering products to the ganga brahmaputra river system one of the world s largest fluvial systems the authors present a geochemical study of the gap weathering products transported by the gomati river the ganga river tributary to understand weathering processes of an alluvial plain in a humid sub tropical climate a total of 28 sediment samples were collected during the monsoon season and were analysed by x ray fluorescence spectrometry for 25 major and trace elements bulk chemistry of the channel flood and suspended sediments mostly consists 90 80 and 75 respectively of three elements al si and fe major element concentrations normalised with respect to upper continental crust ucc show strong depletion of highly mobile elements na ca and enrichment of immobile elements ti si silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples trace element ba cr cu nb ni pb v and zn concentrations were strongly correlated with major element si al fe mn and k concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments the gap weathering products were geochemically distinguished as arkose to litharenite in rock classification chemical mobility normalised with respect to tio 2 in ucc indicates that si na zr ba and sr mainly derived from feldspar muscovite and biotite are lost during weathering iron and zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine grained sediment fractions the chemical index of alteration indicates that the gap has experienced chemical weathering of incipient to moderate intensity the gap weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made up of the himalayan derived sediments a model has been proposed to better understand weathering processes and products of the gap in temporary storage of 50 ka in a humid sub tropical climate

chemical and physical weathering in south patagonian rivers a combined sr u be isotope approach abstract we investigated physical and chemical weathering in south patagonia encompassing both the tectonically active andes with alpine glaciers and the quiescent seaboard plain with arid climate chemical denudation rates determined from riverine dissolved major elements were 0 07 5 10 5 tons year 1 and the long term rates of co 2 consumption by alkaline earth silicates were 0 03 0 5 10 5 mol km 2 year 1 commensurate with the average global co 2 consumption rate 0 25 10 5 mol km 2 year 1 unradiogenic strontium isotope ratios indicated that the source of silicate weathering was volcanic sedimentary cover basin average total denudation rates based on 10 be measured in active streambed sediments ranged from 0 009 to 0 6 mm year 1 uranium series disequilibria suggested that there is significant redistribution of nuclides between the dissolved and suspended material when applying the simultaneous gain and loss model to the u series data of the suspended load sediment residence times of 10 150 ky were obtained comparison of the dissolved load based chemical denudation rate and 10 be based total denudation rate revealed that some basins are dominated by chemical and some by physical denudation

uranium thorium chronometry of weathering rinds rock alteration rate and paleo isotopic record of weathering fluids abstract the potential of 238u 234u 230th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old costa rican alluvial terrace eighteen subsamples were collected from one section of the clast by drilling cores 4 mm in diameter and 5 mm depth along two transects straddling the core rind boundary variations of major and trace element concentrations along the two transects point out a intense loss of alkaline and alkaline earth elements b conservative behaviour of elements such as zr hf and th and c external input of u into the rind without any evidence of u loss during basalt weathering in addition variations in u concentrations along the transects show that the main u th fractionation process associated with the weathering of the basaltic clast is an external input of u without addition nor loss of th in the basalt rind transition zone and that once deposited u is immobile in the weathering rind in the frame of this interpretation scheme a weathering rate of 0 5 0 2 mm kyr can be calculated for the studied clast which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces in addition the variations in the 234u 238u ratio along the analysed transects as well as the increase in sr isotopic ratios within the weathering rind are best explained by temporal variation of the u activity ratios and sr isotope ratios of the soil solutions brought into the rind this work highlights how well detailed u th chronological studies of weathering rinds can 1 constrain the formation rates of weathering systems and 2 record the time variation of isotopic composition of weathering fluids

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to