redox dynamics in the chesapeake bay the effect on sediment water uranium exchange the effect of seasonal variations in productivity and redox dynamics on the sediment water exchange of uranium was investigated on a twelve cruise time series in the chesapeake bay the deep waters of the bay undergo seasonal anoxia in response to high primary productivity and water column stratification from late spring to early fall dissolved oxygen was used to monitor water column redox conditions and dissolved iron manganese and sulfide were measured to monitor sediment redox conditions dissolved 238u was measured in the water column and sediment porewaters to monitor water column sediment exchange uranium incorporation in bay sediments results from two distinct processes productivity dependent scavenging from the water column and redox dependent cycling of uranium between sediments and bottomwater uranium is removed from surface waters of the bay by scavenging with biodetritus during periods of high primary productivity bottomwater and sediment redox conditions determine whether this particle bound uranium is buried or released to overlying water particulate uranium is released to bottomwaters and porewaters during the degradation of biodetritus and oxidation of authigenic uranium low oxygen in bottomwaters in the summer results in minimal exchange of uranium between the sediments and bottomwater due to the stability of reduced u iv high bottomwater oxygen concentrations associated with bay turnover in the fall results in release of authigenic uranium by oxidation to the soluble vi form enrichment of uranium in fall bottomwater suggests that authigenic uranium is very labile when exposed to oxic environmental conditions this process is enhanced by physical mixing when anoxic sediments are resuspended into the oxic bottomwaters

anomalous accumulation of acid volatile sulphides avs in a coastal marine sediment saguenay fjord canada sulpha te reduction usually leads to the formation and accumulation of authigenic pyrite fes2 in organic rich coastal marine sediments iron monosulphides fesx are most often found as a minor constituent of the reduced sulphur pool in these sediments and are believed to be precursor phases which convert to pyrite during early diagenesis abnormally high ratios of acid volatile sulphides avs to pyrite however are found in the sediments of the saguenay fjord canada the avs s fes2 s ratio observed throughout the anoxic zone in three sediment cores collected along the main axis of the fjord generally exceeds 1 and reaches values as high as 7 at the head of the fjord to document and explain this anomaly solid phase components avs fes2 s reactive iron feno coro and total n as well as porewater constituents so4 fe and mn were analyzed degree of pyritization dop values were low 0 1 11 and generally decreased toward the mouth of the fjord in contrast degree of sulphidization dos esti mates were high and reached values of up to 43 at the head of the fjord these latter values indicate that pyrite formation is not limited by iron availability but a large excess of reactive iron may in fact inhibit the build up of reduced sulphur zh2s in the porewaters as a result of avs s precipitation in the absence of a strong eh2s gradient slow diffusion to the redox boundary and limited formation of soluble inter mediate reduced sulphur species may hinder the conversion of avs to pyrite

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

geochemistry of molybdenum in the chao phraya river estuary thailand role of suboxic diagenesis and porewater transport abstract the concentration of dissolved molybdenum mo was measured in the chao phraya river estuary thailand to examine the sources and geochemical behavior of mo water samples in the salinity range of 0 02 24 5 collected during periods of contrasting river discharge have mo concentrations of 3 2 117 nm in the estuary mo and salinity show strong linear correlations suggestive of conservative mixing however at salinity 5 mo concentrations are much higher than that can be achieved from mixing of river water and average seawater desorption from river borne and estuarine sedimentary particles contribute mo that is significantly less than the observed excess mo in the chao phraya estuary groundwater discharge is unlikely to be an important source of mo in the estuary reductive mobilization of mo in the sediments of the gulf of thailand its transport to the overlying water and finally to the estuary is likely to be the dominant mechanism that is responsible for the observed excess mo this inference is drawn from higher salinity and excess mo in deepwater of the gulf of thailand strong correlation between dissolved mo and 228 ra and the observation of suboxic diagenesis in the gulf sediments contribution of mo to the estuary via porewater transport is likely to be enhanced by higher water temperature tidal currents sediment re working and bottom water upwelling which are all documented in the northern gulf of thailand porewater flux of mo estimated using 228 ra flux is much higher than the average riverine mo flux temporal changes in redox condition and circulation pattern in the gulf seem to be responsible for the observed seasonal variation of mo abundances in the estuary

pangaean divergent margins historical perspective abstract pangaean divergent margins caused by the breakup by rifting of that supercontinent display three distinct evolutionary stages in their youth or rift stage divergent margins display a synsediment fault style that has considerable tectonic instability during the mature or drift stage seafloor spreading continental margins are relatively stable and are dominated by thermal subsidence these two stages rifting and drifting are separated in many areas by an unconformity coeval with the oldest associated oceanic crust the breakup unconformity in some areas the onset of seafloor spreading is marked by intense magmatic activity sediment facies the rifting stage range from continental clastics landward of the continental basement hinge and along the flanks of the basin to saline deposits and more open water carbonates in the axial zone of the rift lithologies emplaced during the rift or mature stage of margin development range from shallow clastics carbonates to deep water carbonate siliceous oozes turbidites and contourites some of which were deposited in an anoxic environment deposition then is controlled by changes in configuration of basins changes in sea level and plate migration through several climatic belts during the final or old age of margin development divergence gives way to convergence this stage which generally terminates with continental collision is characterized by tectonism magmatic activity and crustal shortening mesozoic divergent margins along the east west trending former tethys ocean have reached this stage in their evolution

remobilization of authigenic uranium in marine sediments by bioturbation uranium behaves as a nearly conservative element in oxygenated seawater but it is precipitated under chemically reducing conditions that occur in sediments underlying low oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon sites characterized by a range of bottom water oxygen bwo and organic carbon flux ocf were studied to better understand the conditions that determine formation and preservation of authigenic u in marine sediments our study areas are located in the mid latitudes of the northeast pacific and the northwest atlantic oceans and all sites receive moderate 0 5 g cm2 kyr to high 2 8 g cm2 kyr ocf to the sediments bwo concentrations vary substantially among the sites ranging from 3 to 270 m a mass balance approach was used to evaluate authigenic u remobilization at each site within each region studied the supply of particulate nonlithogenic u associated with sinking particles was evaluated by means of sediment traps the diffusive flux of u into sediments was calculated from pore water u concentration profiles these combined sources were compared with the burial rate of authigenic u to assess the efficiency of its preservation a large fraction one third to two thirds of the authigenic u precipitated in these sediments via diffusion supply is later regenerated even under very low bwo concentrations 15 m bioturbating organisms periodically mix authigenic u containing sediment upward toward the sediment water interface where more oxidizing conditions lead to the remobilization of authigenic u and its loss to bottom waters

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

uranium behavior during a tidal cycle in an estuarine system affected by acid mine drainage amd abstract the tinto river estuary is one of the most contaminated coastal systems in the world due to the high amounts of pollutants this acidic river transports the tidal influence and the fluvial discharges control the water mixing which include a salt induced process that is typical of all estuaries and an acid neutralization process ph induced mixing these processes affect the geochemical behavior of uranium which has a non conservative pattern despite the high concentrations of dissolved uranium in the tinto river 234u 238u shows that uranium stems mostly from seawater between 80 and 100 riverine uranium is adsorbed onto both fe and al precipitates when the acidic river water is mixed with seawater distribution coefficients kd show that dissolved and particulate uranium are controlled by adsorption desorption processes and the formation of carbonate complexes both depending on ph at low ph uranium tends to be dissolved and the adsorption by suspended particles is low as the ph increases the adsorption processes onto fe and al particles are more intense increasing particulate uranium which reaches a maximum at ph 5 5 where the uranium solubility minimum occurs at higher ph values dissolved uranium increases by carbonate complexation

using multiple environmental proxies and hydrodynamic modeling to investigate late holocene climate and coastal change within a large gulf of mexico estuarine system mobile bay alabama usa abstract a high degree of uncertainty exists for understanding and predicting coastal estuarine response to changing climate land use and sea level conditions leaving geologic records as a best proxy for constraining potential outcomes with the majority of the world s population focused in coastal regions understanding how local systems respond to global regional and even local pressures is key in developing mitigation adaptation and management plans the geomorphology of mobile bay in southeast alabama usa has evolved considerably e g bayhead delta back stepping over the late holocene in response to global and regional sea level and climate change smaller scale geomorphic changes e g spit and beach ridge development have also had a significant influence on the evolution of the estuary organic matter characteristics inorganic sediment geochemistry benthic microfossils and pollen in a 3500 cal yr bp sediment sequence recovered in a gravity core 20gc from bon secour bay a small sub bay in the southeast corner of mobile bay record time varying marine influence increases in marine influence during 3500 to 2300 cal yr bp and 1930 to 1160 cal yr bp are defined as zones with high density and pre dominantly calcareous foraminiferal species abundant sand 10 and more marine like geochemical signatures which contrast the low density and pre dominantly agglutinated foraminiferal and more terrestrially influenced estuarine muds observed in other intervals of the sedimentary record 2300 1930 and 1160 400 cal yr bp and the modern bay hydrodynamic models constrained by geomorphic boundary conditions for the time 3500 cal yr bp consistent with the most prominent marine influenced sediment provide insight to potential coastal configuration that might have permitted such marine water intrusion into the bay of several scenarios evaluated a breach in morgan peninsula produces tidal circulation within the basin supportive of persistent marine incursions in the bay between 3500 to 2300 cal yr bp the findings show that slight variations in coastal configuration can have broad scale effects on bays and estuaries with consequences that may relate to water quality vertebrate and invertebrate habitat and coastal vulnerability to episodic events like extra tropical storms

subsurface structure of a large basaltic maar volcano examined using geologically constrained potential field modelling lake purrumbete maar newer volcanics province southeastern australia abstract lake purrumbete maar lpm is situated in the late cenozoic intraplate basaltic newer volcanics province southeastern australia it is one of the largest maar volcanoes in the world with a near circular crater that is up to 2800 m in diameter containing a 45 m deep lake gellibrand marl accidental lithics which occurs to a maximum depth of 250 m below lpm represent the deepest excavated host rock unit present in the volcanic succession irregular clast shapes and peperitic textures observed in marl lithics suggest the host rock was poorly consolidated during the eruption high resolution lake and land based gravity and magnetic data were collected to conduct forward and inverse modelling of the subsurface architecture of the maar this is done to test the assumption based on lithics that the diatreme is limited to 250 m depth and identify the reasons behind lpm s large size the collection of gravity data presented a unique challenge due to the nature of measuring small changes in gravitational forces

the diagenetic geochemistry and contamination assessment of iron cadmium and lead in the sediments from the shuangtaizi estuary china sediment and pore water samples have been collected from the coastal tidal flat in the shuangtaizi estuary china in order to investigate the geochemical behavior of iron cadmium and lead during diagenesis and to assess the degree of contamination the calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of fe and pb in the study area whereas the distribution of cd has been closely influenced in this way the high percentage of exchangeable cd average of 56 34 suggests that cd represents a potential hazard to benthic organisms in the estuary the calculated diffusive fluxes of metals show that the most mobilized metal is fe 9 22 mg m 2 a 1 followed by cd 0 54 mg m 2 a 1 and pb 0 42 mg m 2 a 1 low fe2 contents in surface pore water alongside high chromium reducible sulfur contents and low acid volatile sulfur and elemental sulfur contents at 0 25 cm depth in sediments show that fe2 is formed by the reduction of fe oxides and is transformed first to a solid phase of iron monosulfides fes and eventually to pyrite fes2 the release of adsorbed pb due to reductive dissolution of fe mn oxides during early diagenesis could be a source of pb2 in pore water from the relatively low total organic carbon contents measured in sediments 0 46 1 28 with an average of 0 94 and the vertical variation of cd2 in pore water sulfide or fe mn oxides instead of organic matter are presumed to exert a significant influence on carrying or releasing cd by the sediments

early diagenesis of trace metals cd cu co ni u mo and v in the freshwater reaches of a macrotidal estuary vertical profiles from the water column including the maximum turbidity zone mtz to the consolidated sediment were sampled in september 2000 in the freshwater reaches of the gironde estuary during a complete neap tide spring tide cycle the vertical distributions of dissolved major redox parameters and metals mn fe cd cu v co ni mo and u were determined reactive particulate metal fractions were also determined from selective leaching the studied system is characterized by density layers functioning at different time scales consisting of two mobile layers i e the liquid lm and the soft mud sm overlying consolidated sediments cs this results in a three zone diagenetic regime where 1 o2 dynamics are fast enough to show depletion in the rapidly mixed lm sequence tidal time scale 2 denitrification occurs on the weekly time scale mixing sm sequence and 3 the mn fe and sulfate cycling occurs in the cs layer annual time scale the studied trace metals show differential behavior during early diagenesis 1 cd cu and v are released into pore water preferentially from organic matter in the sm 2 co ni and u are released in the cs from mn and fe oxides during reductive dissolution and 3 mo from both processes transient conditions i e oscillations of redox fronts and reoxidation processes due to the dynamics of the mobile layers strongly influence the trace metal distributions as inducing resolubilization cd cu and mo in the cs authigenic metal phases accumulate either by direct precipitation with sulfides cu cd or co precipitation with fe sulfides mo microbially mediated reduction of fe oxides is proposed to control u removal from pore water by reduction of u vi to u iv at depth however a significant fraction of the trace metals is trapped in the sediment in exchangeable forms and therefore is susceptible to be mobilized due to resuspension of estuarine sediment during strong river flood periods and or dredging activities

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

trace metal geochemistry of organic carbon rich watercourses draining the nw german coast abstract numerous small watercourses are draining the hinterland of the nw german coast the waters mainly originate from marsh and fen areas and have yellow to deep brownish color during their flow path the waters exhibit gradients in salinity 0 2 3 ph 6 2 8 8 particulate organic carbon 5 25 and iron oxides 7 12 which alter the concentrations of most dissolved and particulate trace metals for example dissolved fe is rapidly removed from solution at increasing salinities by flocculation whereas dissolved u is removed in the very low salinity zone by fe and organic rich colloids the waters at the flood gate of neuharlingersiel where a composite sample of the entire study area is collected before the freshwater is discharged into the marine dominated tidal flat area in front of the mainland dike have the following average trace metal concentrations dissolved fe 11 m mn 4 m mo 10 m u 1 8 m v 75 m and particulate fe 7 mg kg 1 mn 1200 mg kg 1 mo 2 mg kg 1 u 1 7 mg kg 1 v 140 mg kg 1 after passing the flood gate most mo and u salinity distributions fit well to the conservative mixing line that connects the seawater of the wadden sea to the low salinity river water entering the tidal flat area in contrast dissolved fe mn and v are removed from solution especially at intermediate salinities which may be due to mixing of freshwater with seawater depleted in these elements flocculation and or scavenging by particulate matter in general processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea compared to average river water the watercourses are enriched in dissolved and particulate fe mn mo u and v due to the fast removal of dissolved fe in the salinity gradient and the high concentrations of mo and u in seawater the watercourses only form a source for mn v terrestrial organic carbon and particulate fe for the wadden sea

reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments abstract sporadic direct observations over a 50 yr period inadequately characterize the history of seasonal hypoxia and anoxia in chesapeake bay a large estuary threatened by eutrophication here we undertake a reconstruction of 20th century oxygen depletion in this estuary using mo concentrations in 210pb dated sediments cu concentrations are used to control for anthropogenic influences cores from the central channel display mild mo enrichments above crustal backgrounds up to 5 g g and strong cu enrichments up to 35 g g temporally cu enrichment mostly anthropogenic began earlier and stabilized in the last two thirds of the 20th century in contrast mo enrichment has grown during the last two thirds of the century molybdenum enrichment is mostly hydrogenic except in a section of the channel that receives additional mo from erosion of early miocene shore deposits two geochemical mechanisms promote mo enrichment manganese refluxing concentrates dissolved moo42 at the sediment water interface and sulfide substitution into moo42 produces thiomolybdates which can be fixed by particles the mo enrichment mechanisms operate primarily during periods when bottom waters are anoxic and thiomolybdate formation can occur near the sediment water interface this implies a temporal coupling between water column anoxia and mo fixation even though fixation occurs only within sediments the mo enrichment profiles suggest that chesapeake bay has experienced growing o2 depletion since the first half of the 20th century but especially after 1960

estimation of azimuthal anisotropy from vsp data using multicomponent s wave velocity analysis observation of azimuthal shear wave anisotropy can be useful for characterization of fractures or stress fields shear wave anisotropy is often estimated by measuring splitting of individual shear wave events in vertical seismic profile vsp data however this method may become unreliable for zero offset marine vsp where the seismogram often contains no strong individual shear events such as direct downgoing shear wave but often contains many low amplitude ps mode converted waves we have developed a new approach for estimation of the fast and slow shear wave velocities and orientation of polarization planes based on the multicomponent linear traveltime moveout velocity analysis this technique is applicable to zerooffset vsp data and should take advantage of the presence of a large number of shear wave events with the same apparent velocity which for a horizontally layered medium should be close to the interval velocity the approach assumes that the vsp data are acquired in a vertical well drilled in an orthorhombic medium with a horizontal symmetry plane including horizontal transverse isotropy the main idea is to estimate the dominant apparent velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity the algorithm was tested on marine three component 3c vsp acquired in the north west shelf of australia and on land 3c vsp acquired with different sources in the same borehole located in otway basin victoria these tests show good agreement between anisotropy parameters magnitude and orientation derived from the vsp and cross dipole sonic log data

a transect across australia s southern margin in the otway basin region crustal architecture and the nature of rifting from wide angle seismic profiling abstract the otway basin in southeastern australia formed on a triangular shaped area of extended continental lithosphere during two extensional episodes in cretaceous to miocene times which ultimately led to the separation of australia and antarctica the velocity structure and crustal architecture of the otway continental margin has been interpreted from offshore onshore wide angle seismic profiling data along a transect extending from near the northern otway basin margin with palaeozoic outcrop to the deep ocean basin under the southern ocean along this transect the otway continental margin ocm transect the onshore half graben geometry of early cretaceous deposition gives way to a 5 km thick basin sequence p wave velocity 2 2 4 6 km s extending down the continental slope offshore to at least 60 km from the shoreline at 120 km from the nearest shore sonobuoy data indicate a 4 5 km sedimentary sequence overlying 7 km of crustal basement rocks above the moho at 15 km depth water depth 4220 m conspicuous strong moho reflections are evident under the continental slope at about 10 2 s twt basement is interpreted to be attenuated faulted palaeozoic rocks of the delamerian and lachlan orogens intruded with jurassic volcanics that thin from 16 km onshore to about 3 5 km at 120 km from the nearest shore these rocks comprise a 3 km section that has a velocity of 5 5 5 7 km s overlying deeper basement with a velocity of 6 15 6 35 km s over the same distance the moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore and then to about 12 km in the deep ocean at the limits of the profile water depth 5200 m the continent ocean boundary cob is interpreted to be at a prominent topographic inflection point at the bottom of the continental slope in 4800 m of water p wave velocities in the lower crust are 6 4 6 8 km s above a transition to the moho with an upper mantle velocity of 8 05 km s there is no evidence of massive high velocity 7 km s intrusives underplate material in the lower crust nor any syn rift or early post rift subaerial volcanics indicating that the otway continental margin can be considered a non volcanic margin similar in many respects to some parts of the atlantic ocean margins e g the nova scotia newfoundland margin off canada and the galicia bank off the iberian peninsula

facies associations detrital clay grain coats and mineralogical characterization of the gironde estuary tidal bars a modern analogue for deeply buried estuarine sandstone reservoirs abstract estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry in order to better predict the reservoir potential of these geobodies this study provides a modern day reservoir analogue describing tidal bars in the inner and outer gironde estuary from the microscopic to the macroscopic scale the originality of this work lies in the multi scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high energy environment this work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars x ray diffraction and electron microscopes reveal that these coats are mainly composed of di octahedral smectite illite chlorite and kaolinite associated with other components such as diatoms or pyrite the best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale an optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth these conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones

the geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters pore water profiles of uranium and thorium isotopes in the muddy sediments of buzzards bay ma permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements uranium shows a pronounced minimum pore water concentration 1 2 dpm kg near the sediment water interface 0 3 cm which coincides with the pore water fe maximum u concentrations increase with depth to a broad maximum which is greater than the overlying seawater value then decrease laboratory sediment tank experiments maintained without macrofauna also show the near interface minimum and increase and sediment incubation experiments show removal of u from the pore water to a constant value by about 40 days the 234u238u activity ratio is equal to the seawater value in all samples oxidation state separations of pore water u in hcl solution using anion exchange demonstrate that u is present in the pore water as u vi the coincidence of the pore water uranium and fe maximum is consistent with the hypothesis of reduction of u vi to u iv and removal from solution at about the same depth or pe as fe is reduced increases in pore water u with depth appear to be related to release of authigenic seawater u vi to solution possibly as alkalinity and ph increase and organic matter with which the u is associated is oxidized lower concentrations at depth in the sediment column may be linked to increases of effective u reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna in contrast pore water profiles of the long lived th isotopes 232th and 230th show low activities 02 dpm kg and relatively little change with depth although values are greater than those in the overlying water solid phase activities are similarly constant with depth and the distribution may be controlled by a sorption equilibrium between pore water and the solid phase the short lived isotope 234th shows greatest pore water activities in the upper 5 cm and is present at virtually zero activities elsewhere in the sediment column the upper few centimeters of the sediment generally contain excess 234th scavenged from the overlying water column and distributed by bioturbation as well as pore water fe and mn profiles which show maxima indicating reduction and dissolution of fe and mn oxyhydroxides the pore water 234th profile can be explained by release of 234th from the solid phase as femn oxide surface coatings are reduced and dissolved

cycling of trace metals mn fe mo u v cr in deep pore waters of intertidal flat sediments trace metals mn fe mo u cr v were studied in pore waters of an intertidal flat located in the german wadden sea the study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year the samplers were either located close to the tidal flat margin or in central parts of the tidal flat despite dynamic sedimentological and hydrological conditions the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat seasonal sampling revealed that v and cr vary concurrent with seasonal changes in dissolved organic carbon doc concentration this effect is most notable close to the tidal flat margin where sulphate doc and nutrients vary with season down to some metres depth seasonal variations of mn fe mo and u are by contrast limited to the upper decimetres of the sediment their seasonal patterns depend on organic matter supply redox stratification and particulate matter deposited on sediment surfaces pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments during low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments owing to the enrichment of specific elements like mn in pore water compared to sea water seeping pore waters may have an impact on the chemistry of the open water column mass balance calculations reveal that the impact of deep pore waters on the mn budget in the open water column is below 4 mn deep pore water discharge of the whole wadden sea is estimated to be about 9 of the total dissolved riverine mn input into the southern north sea

transition from continental to oceanic crust on the wilkes adelie margin of antarctica the wilkes adelie margin of east antarctica a passive margin rifted in the early cretaceous has an unusually reflective moho which can be traced seismically across the continent ocean transition velocity models and depth sections were constructed from a combined set of u s and french multichannel seismic reflection lines to investigate the transition from continental to oceanic crust these data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and in places by a shoaling of moho the moho reflection is continuous across the edge of oceanic crust and gradually deepens landward under the continental edge

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

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

oxic anoxic oscillations and organic carbon mineralization in an estuarine maximum turbidity zone the gironde france the study of vertical particle dynamics in the highly turbid gironde estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap spring time scales fluid mud with suspended particulate matter spm concentrations between 50 and 500 g liter 21 has been observed during neap tides vertical profiles of biogeochemical parameters have been measured in the fluid mud anoxic conditions have been detected when spm concentration exceeded 50 g liter 21 in the upstream and 140 g liter 21 in the downstream parts of the maximum turbidity zone mtz at the downstream part of the mtz anoxic fluid mud was partitioned into a denitrification layer spm 5 140 250 g liter 21 intensively reworked at the tidal time scale and by an mn iv reduction layer spm 250 g liter 21 preferentially reworked at the neap spring time scale due to the alternation of sedimentation and resuspension periods most of the sediment experienced oxic anoxic oscillations throughout the neap spring cycle fluid mud resuspension occurred without any observable incidence on the surface water oxygenation an increase in total alkalinity was found in the fluid mud due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory co2 generation this phenomenon significantly affected the inorganic carbon budget of the estuary increasing the hco input to the coastal ocean and reducing the co2 flux to the atmosphere an 2 3 accumulation of labile dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter pom decomposition in the gironde mtz a net loss of refractory land derived pom occurs this system acts as an efficient oxic suboxic fluidized bed reactor similar to mobile deltaic muds

mercury methylation in the sediments of a macrotidal estuary gironde estuary south west france abstract diagenetic behaviour of mercury species in surface sediments of the highly turbid gironde estuary was investigated along 6 vertical profiles including the water sediment interface swi in dredged and non dredged zones sediment and pore water were collected for analyses of major redox parameters e g dissolved fe mn and sulphate dissolved and particulate mono methyl mercury mehg and divalent mercury hg ii concentrations of the dissolved hg species were clearly up to 70 times higher in pore water than in the overlying water column probably due to the dissolution or degradation of major hg carrier phases such as fe oxyhydroxides and particulate organic matter the different profiles showed maximum mehg d concentrations up to 10 pmol l 1 and mehg d hg d ratios up to 40 at a few cm below the swi suggesting intense in situ methylation processes for this zone depth related methylation potential was investigated by spiking the cores with 199 hg ii and their subsequent incubation methylation of isotopically labelled 199 hg ii was measurable in all cores suggesting that methylation in the studied sediments was efficient in the uppermost 20 cm below the swi the comparison between sediments retrieved inside and outside the navigation channel showed higher methylation potential in dredged sediments and in intertidal sediment suggesting stimulation of the bacterial activity either by inputs e g fresh organic matter sulphate due to pore water renewal or by redox oscillation the estimated mehg d sediment to water flux was greater in non dredged sediments probably due to favourable anoxic conditions at the swi at the whole estuary scale the estimated direct sediment to water transfer via passive diffusion was 1 1 2 7 mol yr 1 210 55 g yr 1 for mehg d and 3 8 4 0 mol yr 1 750 790 g yr 1 for hg ii d which is roughly equivalent to 15 50 and 1 3 1 g yr 1 for mehg d and 0 2 mol yr 1 41 g yr 1 for hg ii d further work should aim at understanding the impact of suspending dredged anoxic sediments in the water column on the estuarine hg cycle

pelagic molybdenum concentration anomalies and the impact of sediment resuspension on the molybdenum budget in two tidal systems of the north sea the seasonal dynamics of molybdenum mo were studied in the water column of two tidal basins of the german wadden sea sylt romo and spiekeroog between 2007 and 2011 in contrast to its conservative behaviour in the open ocean both losses of more than 50 of the usual concentration level of mo in seawater and enrichments up to 20 were observed repeatedly in the water column of the study areas during early summer mo removal by adsorption on algae derived organic matter e g after phaeocystis blooms is postulated to be a possible mechanism mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches mo in the pore water first 98 95mo data of the study area disclose residual mo in the open water column being isotopically heavier than momo mean ocean molybdenum during a negative mo concentration anomaly whereas suspended particulate matter shows distinctly lighter values based on field observations a mo isotope enrichment factor of e 0 3 has been determined which was used to argue against sorption on metal oxide surfaces it is suggested here that isotope fractionation is caused by biological activity and association to organic matter pelagic mo concentration anomalies exceeding the theoretical salinity based concentration level on the other hand cannot be explained by replenishment via north sea waters alone and require a supply of excess mo laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound mo into the water column

near surface fault detection using high resolution shear wave reflection seismics at the co2crc otway project site australia high resolution near surface shear wave reflection seismic measurements were carried out in november 2013 at the co2crc otway project site victoria australia with the aim to determine and if so where deeper faults reach the near subsurface from a previous p wave 3 d reflection seismic data set that was concentrated on a reservoir at 2 km depth we can only interpret faults up to 400 m below sea level for the future monitoring in the overburden of the co2 reservoir it is important to know whether and how the faults continue in the subsurface we prove that two regional fault zones do in fact reach the surface instead of dying out at depth individual first break signatures in the shot gathers along the profiles support this interpretation however this finding does not imply perforce communication between the reservoir and the surface in the framework of co2 injection the shear wave seismic sections image with high resolution better than 3 m vertically and complementary to existing p wave volumes different tectonic structures similar structures also outcrop on the southern coast of the otway basin both the seismic and the outcrops evidence the complex youngest structural history of the area

deep seismic images and the tectonic framework of early rifting in the otway basin australian southern margin abstract the otway basin is one of a number of basins formed along australia s southern continental margin during the late jurassic early cretaceous rifting of australia from antarctica it lies on a transitional area of extended lithosphere between the margin to the west where continental separation took place close to the australian craton and the region to the east where separation failed to develop through bass strait but instead switched to the south of tasmania seven agso regional deep seismic profiles recorded during 1992 across onshore parts of the basin provide images of synrift basin bounding faults that dip predominantly towards the continent basin sequences in half graben rift segments at two way times greater than 4 s and mid crustal detachments with ramp and flat geometry at mid crustal levels when combined with industry seismic data trend variations on faults indicate that extensional strain direction varied along the rift system probably locally controlled by pre existing palaeozoic geology but consistent with general north south lithospheric extension basement highs between some of the rift segments are interpreted as accommodation or transfer zones multiple basin bounding faults in some places mirror features within basement and may indicate a component of local strike slip during early rifting the syn rift segments form part of the first stage early cretaceous failed rift system in the otway bass strait region crustal thickness is interpreted to be 31 km 10 3 10 5 s twt near the northern basin margin thinning to about 25 km 9 s twt 40 reduction in thickness of palaeozoic crust seaward of the tartwaup fault zone this fault zone and the timboon fault system farther east are interpreted to be headwall faults landward limit of a late cretaceous second stage rifting system along a lower plate margin which ultimately separated australia from antarctica

tungsten molybdenum fractionation in estuarine environments abstract dissolved tungsten w and molybdenum mo concentrations were measured in surface waters and sediment pore waters of terrebonne bay a shallow estuary in the mississippi river delta to investigate the biogeochemical processes that fractionate these group 6 elements relative to one another during transit from weathering to sedimentary environments although many of the chemical properties of w and mo are similar the two elements behave autonomously and the fractionation mechanisms are only partly understood in sulfidic pore waters dissolved mo is depleted relative to river water seawater mixtures whereas dissolved w is 10 fold enriched reductive dissolution of poorly crystalline phases like ferrihydrite which is a preferential host of w relative to mo in grain coatings on river borne particles can explain the dissolved w enrichment dissolved w becomes increasingly enriched as h 2 s aq rises above about 60 m due to transformation of wo 4 2 to thiotungstates as well as to additional reductive dissolution of phases that host w in contrast as rising sulfide transforms moo 4 2 to thiomolybdates in pore waters dissolved mo is suppressed probably owing to equilibration with an fe mo s phase this putative phase appears to control the aqueous ion product q fe 2 mos 4 2 0 6 h 2 s 0 0 4 h 0 8 at a value of 10 7 78 concentrations of dissolved w and mo in pore waters bear no relation to concentrations in surface waters of the same salinity in surface waters dissolved mo is nearly conserved in the estuarine mixing zone dissolved w appears also to be conserved except for several cases where w may have been enhanced by exchange with underlying w rich pore waters with increasing salinity the molar mo w ratio rises from about 10 to about 1000 in surface waters whereas it is mostly vi is more susceptible to reduction than w vi but the latter is more prone to adopting octahedral coordination we propose that the first difference facilitates mo but not w precipitation in sulfidic sediments whereas the second explains tungsten s proportionately greater sequestration on river borne particles and its subsequent release to sulfidic pore waters after the particles are deposited in the delta and become subject to reductive dissolution during diagenesis

active tectonics of the southeastern upper rhine graben freiburg area germany the upper rhine graben has two plio quaternary depocentres usually interpreted as resulting from tectonic reactivation the southern basin near freiburg im breisgau germany contains up to 250 m of sediments beneath the younger alluvial deposits related to the current drainage system a former river network deeply entrenched in the substratum reveals a very low regional base level of early pleistocene age the offset of channels at faults allows us to infer a pleistocene reactivation of the syn rift fault pattern and the estimation of slip rates maximum vertical movements along the faults have not exceeded 0 1 mm yr since the middle pleistocene current activity is concentrated along the westernmost faults morphologic markers indicate late pleistocene reactivation of the rhine river fault and geophysical prospecting suggests a near surface offset of young sedimentary deposits the size of the fault segments potentially reactivated suggests that earthquakes with magnitude larger than mw 6 3mw 6 3 could be expected in the area with a return interval of about 8000 years extrapolated to the duration of the plio pleistocene the strain rate estimates reveal that the tectonic forcing may account for only one third to one half of the whole thickness of the plio pleistocene sediments of the basin fill thus other processes must be invoked to understand the growth of the plio pleistocene basin especially the piracy of the rhine river to the north during the early pleistocene could explain these effects

impact of water particle interactions on molybdenum budget in humid tropical rivers and estuaries insights from nethravati gurupur and mandovi river systems abstract the study presents the seasonal and inter annual monitoring of molybdenum mo distribution and variability in humid tropical riverine and estuarine systems nethravati gurupur and mandovi estuaries of west coast of india the study investigates the geochemical behaviour of mo in the river and estuaries and their ultimate fluxes into the ocean the riverine flux of dissolved mo dmo to the nethravati gurupur and mandovi estuaries are 1800 mol yr 1 4 88 mol day 1 195 mol yr 1 0 53 mol day 1 and 10 5 103 mol yr 1 28 mol day 1 respectively and the riverine particulate mo pmo flux to nethravati estuary is 10 8 103 mol yr 1 the dmo in river 30 to 40 is scavenged onto particles under oxidized acidic river water conditions and subsequently released in the estuary impacting the solute budget of mo to the sea in the estuaries under low salinity conditions dmo is sequestered onto particles during pre monsoonal season the dmo sequestration in the estuary is estimated to be 2 mol day 1 in the nethravati estuary and 1 9 mol day 1 in the mandovi estuary during this season sequestration in the estuary is higher than the riverine supply indicating the sequestration of both marine and river borne dmo however the mechanisms involved in the removal process are different in these estuaries viz oxidative adsorption process in the nethravati gurupur estuary and microbial utilization in the mandovi estuary the lower salinity region during monsoon and post monsoon season shows slight excess of dmo river borne particulate mo could release up to 3 to 4 nmol l 1 by desorption under alkaline higher ionic strength conditions at higher salinity 20 psu in both the estuaries and in all the seasons dmo gain is systematic 1 to 37 nmol l 1 mo release from river borne particles could contribute only up to 3 to 4 nmol l 1 which is not sufficient to balance the observed mo excess on the other hand the reductive mo remobilization from bottom sediments mo 4 mg kg 1 during sediment diagenesis and subsequent tidal activity release up to 28 nmol l 1 of dmo to the estuarine water mo release to water column is supported by the gradual enrichment of dmo with depth in the estuary therefore diagenetic release of dmo forms the potential source of dmo excess in the estuary

two stage early cretaceous rifting of the otway basin margin of southeastern australia implications for rifting of the australian southern margin the otway basin of southeastern australia rifted from antarctica in two distinct stages in the early cretaceous the first stage began around 140 ma and ended between 110 and 120 ma and formed an onshore rift the rift faults typically detached within basement and were oriented between east west and east northeast on the crayfish platform these first stage rift structures are preserved the second stage of rifting at about 95 ma just prior to sea floor spreading had fault trends between east southeast and southeast it was less extensive in area and the zone of highest extension was localized near the junction with oceanic crust geohistory and tectonic analyses indicate that both stages of rifting affected the whole southern margin in the west however the first rift began at about 160 ma and first stage trends persisted during the second rift

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

the geometry of the active strike slip el tigre fault precordillera of san juan central western argentina integrating resistivity surveys with structural and geomorphological data the geometry and related geomorphological features of the right lateral strike slip el tigre fault one of the main morphostructural discontinuities in the central western precordillera of argentina were investigated achievements of this survey include recognition of structural and geometrical discontinuities along the fault trace identification and classification of landforms associated with local transpressional and transtensional sectors observation of significant changes in the fault strike and detection of right and left bends of different wavelength in the central segment of the el tigre fault 2d electrical resistivity tomography surveys were carried out across the fault zone the resistivity imaging permitted to infer the orientation of the main fault surface the presence of blind fault branches along the fault zone tectonic tilting of the quaternary sedimentary cover subsurface structure of pressure ridges and depth to the water table based on this information it is possible to characterize the el tigre fault also as an important hydro geological barrier our survey shows that the main fault surface changes along different segments from a high angle to a subvertical setting whilst the vertical slip component is either reverse or normal depending on the local transpressive or transtensive regime induced by major bends along the trace these local variations are expressed as sections of a few kilometres in length with relatively homogeneous behaviour and frequently separated by oblique or transversal structures

dissolved mo and u in rivers and estuaries of india implication to geochemistry of redox sensitive elements and their marine budgets abstract dissolved molybdenum mo and uranium u concentrations were measured in five indian estuaries the narmada tapi mandovi and the mahi fall into the arabian sea and the hooghly falling into the bay of bengal riverine mo and u vary significantly in the range of 1 to 90 and 0 02 to 19 nmol kg respectively the lowest mo and u were observed in the mandovi river consistent with lateritic lithology exposed in its drainage higher runoff and lower water rock interaction the sabarmati has the highest u 19 nmol kg and sources from the groundwater having very high u concentration the highest mo 90 nmol kg is observed in the mahi river and probably results from anthropogenic sources hooghly river seems to have high concentrations of mo 14 nmol kg and u 8 nmol kg and are possibly derived from weathering of black shales of the himalaya behaviour of mo and u in all the estuaries analysed in this study is highly variable both mo and u in the narmada estuary during pre monsoon and monsoon seasons and u in the tapi estuary behave conservatively mo in the tapi shows its addition in the mid salinity ranges 4 to 12 and is sourced from anthropogenic activity probably related to effluent from industries situated along the estuary in this salinity range both dissolved mo and u show their removal at lower salinity ranges in the hooghly and the mandovi estuaries these estuaries are net sink of mo in which significant amount of oceanic mo along with riverine mo is being lost about 1 6 10 6 and 2 10 5 mol of mo are being removed annually in the hooghly and the mandovi estuaries uranium removal is order of magnitude lower compared to mo in both the hooghly and the mandovi estuaries the loss of mo and u in the estuaries are associated with mangrove swamps present in the estuaries which seems to be an important sink of the oceanic mo and could represent its significant sink in oceanic budget of mo the gulf of cambay has high dissolved mo concentration compared to that of global average seawater value which is attributed to its supply from anthropogenic sources industrial effluent waste waters along with polluted rivers supply 5 10 6 mol of mo annually to the gulf of cambay which could be derived from petrochemicals and pharmaceutical industries situated along the gulf coast

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

comparative geochemistry of cadmium rhenium uranium and molybdenum in continental margin sediments the concentrations of authigenic phases of cd re u and mo increase with depth in four 45 cm long sediment box cores collected along the axis of the laurentian trough gulf of st lawrence average authigenic accumulation rates estimated from element inventories are similar to rates in other continental margin environments strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of cd and re this suggests that slow precipitation kinetics controls the accumulation of cd and re in these sediments the accumulation rate of authigenic u is more variable it may be tied to the kinetics of microbially mediated u reduction and be controlled by the availability of reactive organic matter authigenic mo is distinguished by a sharp subsurface concentration minimum above which mo cycles with manganese mo released to pore water upon reduction of mn oxides diffuses downward and enriches the subsurface sediment mo accumulates most rapidly in the sediment with the highest sulfide content slow conversion of molybdate to thiomolybdate may explain the much slower mo accumulation rate in the less sulfidic sediments a component of authigenic mo accumulates with pyrite in an approximately constant mo fe ratio the accumulation rate of pyrite and associated mo is insensitive to avs abundance pyrite formation may be limited by the reactivity of iron oxide minerals

algal blooms and marine snow mechanisms that enhance preservation of organic carbon in ancient fine grained sediments abstract combined petrographic and geochemical methods are used to investigate the microfabrics present in thin sections prepared from representative organic carbon rich mudstones collected from three successions the kimmeridge clay formation the jet rock member of the whitby mudstone formation and the pebble shale and hue shale this study was initiated to determine how organic carbon rich materials were being delivered to the sediment water interface and what happened to them after deposition prior to deep burial analyses of the fabrics present shows that they exhibit many common attributes in particular they are all 1 highly heterogeneous on the scale of a thin section 2 organized into thin beds the organomineralic aggregates present in these mudstones are interpreted to be ancient examples of marine snow this marine snow likely formed in the water column particularly during phytoplankton blooms and was then transported rapidly to the seafloor the existence of the thin beds with homogenized tops and an in situ infauna indicates that between blooms there was sufficient oxygen and time for a mixed layer to develop as a result of sediment colonization by diminutive organisms using either aerobic or dysaerobic metabolic pathways these textures suggest that the constituents of these mudstones were delivered neither as a continuous rain of sediment nor were the bottom waters persistently anoxic in addition the presence of thin lags and sharp based beds suggests that the seafloor was being episodically reworked during deposition these fabrics indicate that conditions in the water columns and at the seafloors while these rocks were being deposited were very dynamic and episodic fluxes of high concentrations of organic carbon to the seafloor during phytoplankton blooms likely enhanced preservation of organic carbon

effect of estuarine sediment resuspension on early diagenesis sulfide oxidation and dissolved molybdenum and uranium distribution in the gironde estuary france abstract vertical distributions of dissolved major redox parameters mo and u from the water to the sediment were sampled in the salinity gradient of the gironde estuary in the water sulfate and mo are conservatively transported throughout the salinity gradient in contrast u shows a significant removal rate of 85 mol m 2 a 1 related to reductive sequestration of u in the anoxic sediment early diagenesis is highly transient and characterized by a three zone regime related to three density layers functioning at different timescales excess of sulfate up to 1 mm is observed in the soft mud layer and is assigned to sulfide oxidation by both abiotic mn and fe oxide reduction and bacterial nitrate reduction during early diagenesis mo and u are released in the sediment porewater from mn and fe oxides in not dredged anoxic sediments authigenic precipitation of mo and u with iron sulfides is proposed to explain their removal from solution in contrast in dredged sediments authigenic precipitation of u is inhibited probably owing to the less effective reduction of soluble u vi to insoluble u iv due to oxidizing transient conditions mixing of anoxic sediment porewater with oxygenated water due to i cyclical resuspension of the soft mud layer and ii dredging operations in the navigation channel induces addition of dissolved sulfate produced from diagenetic sulfide oxidation mo and u to the water this addition is equivalent to 2 sulfate 5 5 mo and 0 5 u of the dissolved fluvial inputs into the estuary for the natural resuspension and 0 4 mo less than 0 2 sulfate u for dredge induced resuspension addition of dissolved mo and u produced by oxidation of sulfide phases in the dredged sediments redeposited in dynamic environments are equivalent to 12 mo and 0 8 u of their respective fluvial inputs additional mo and u inputs into the water originate from diffusive outflow from muddy sediments and are equivalent to 2 mo and 0 3 u of their respective fluvial inputs however we show that all these dissolved inputs are scavenged by solid phases

salt marshes an important coastal sink for dissolved uranium abstract the global budget for marine uranium demands another geochemical sink other than deep sea systems and the coastal environment may host some or all of this missing sink in a previous paper sarin and church 1994 we have shown that some large subtidal estuaries are seasonal summer sinks at low salinities in this paper we show that intertidal salt marshes are even stronger sinks at all salinities if for somewhat different reasons uranium was sampled in dissolved and particulate fractions over several tidal cycles and seasons for a lower delaware bay salt marsh canary creek lewes delaware usa and uniquely during summer months the dissolved uranium is nonconservative moreover because uranium extraction is greater on higher tides and occurs over the entire salinity gradient this processing appears associated with surface of vegetated high marsh we hypothesize that either 1 uranium scavenging occurs during the process of tidal mixing and attendant flocculation of humic acids and iron oxides favoring this process is the presence of sulfonate complexes in salt marsh humic substances and iron coprecipitation during its extensive redox cycling in the salt marsh or 2 uranium extraction occurs at the marsh surface during extensive flooding of the salt marsh surface sediments favoring this process is the increase in sulfuric acidity at the summer salt marsh surface that could destabilize the tetracarbonate species of u vi the latter option is favored by both field observations of maximum removal at the surface during the spring and summer tide conditions and selective extraction of sediment phases where uranium is found as adsorbed and complexed forms in the ascorbate citrate and humic acid fractions respectively mass balance calculations show that under steady state conditions nearly two thirds of the uranium extracted from tidal waters is retained in the sediments while one third is exported as u enriched particles during ebbing tides independent confirmation of this balance comes from the measured accumulation rate of uranium buried at depth this represents the net inventory buried below the geochemically reactive surface responsible for the initial extraction and redistribution of uranium onto sediment or tidally exported phases extrapolated globally uranium burial in salt marshes alone or total marine wetlands including mangroves could comprise at least 10 and perhaps as much as 50 the total marine sink for uranium or on an area specific basis up to 50 times their marine areal extent