seasonal cycles of particle and solute transport processes in nearshore sediments 222rn226ra and 234th238u disequilibrium at a site in buzzards bay ma radiochemical measurements leading to the estimation of particle mixing rates and irrigation rates in the sediments of a site in buzzards bay massachusetts have been made over two nearly full seasonal cycles both particle mixing and sediment irrigation show significant seasonal variability at this site the variability is consistent with biologically driven transport mechanisms 234th238u disequilibrium measurements were used to estimate particle mixing rates based on three cores taken from december through march and four taken from june through october a cold season average mixing rate of 5 6 cm2 yr and a warm season average rate of 17 cm2 yr were found excess 234th inventories in the upper 5 cm of the sediments also varied seasonally with a cold season average value of 1 8 dpm cm2 significantly less than both the warm season value of 3 4 dpm cm2 and the predicted average value for buzzards bay 3 dpm cm2 222rn226ra disequilibrium in sediment pore waters was used to estimate sediment irrigation based on a total of seven cores three seasonal groupings of results were made 1 december through march when 222rn deficits could be explained by vertical molecular diffusion alone 2 early summer june when irrigation was important to at least 20 cm the maximum depth of the sampled region and 3 early fall when irrigation was important to about 10 cm a nonlocal exchange vertical molecular diffusion model emersonet al 1984 was used to obtain quantitative estimates of irrigation rates when the exchange parameter was allowed to vary exponentially with depth below the sediment water interface model rn deficit profiles fit measured profiles quite well

roles of sorption and tube dwelling benthos in the cycling of phosphorus in bering sea sediments adsorption of dissolved phosphate onto iron hydroxides has been shown to be one of the primary regulators of phosphorus cycling in sediments bioturbation and bioirrigation by benthic infauna modify this cycling by accelerating the transport of dissolved and particulate phosphorus and by changing rates of reactions that occur in the sediment such as the adsorption of phosphate by amorphous iron hydroxides hydrographic processes vary regionally in the bering sea and nutrient exchange between the sediments of the broad shallow shelf and overlying water may influence water column productivity these characteristics make the bering sea a good study site for examining the processes that influence sedimentary cycling of phosphorus to examine these processes we collected samples in four domains southern middle shelf southern outer shelf southern off shelf consisting of the continental slope and bering sea basin and northern bering shelf based on hydrographic regime at each station we directly measured phosphate flux and sediment oxygen consumption using whole core incubations we also measured infaunal burrow abundances amorphous iron hydroxide concen trations and phosphate sorption we found that three out of the four domains had a high affinity for trapping phosphate in the sediment as indicated by their adsorption coefficients 6 59 81 81 however the measured phosphate fluxes could not be explained by the adsorption capacity of the sediment alone the results indicated that on the middle shelf the phosphate flux positively co varied with infaunal burrow abundances the high number of organisms in this domain 10 32 burrows per 50 cm

the phosphorus cycle in coastal marine sediments the concentration of dissolved phosphate in sediment box cores from the laurentian trough in the gulf of st lawrence increased sharply across the sediment water interface from 2 0 mol po4 1 in the bottom water to 6 3 mot po4 1 in the top cm remained almost constant at this value down to 5 15 cm depth and then increased rapidly with further depth in the region of constant concentration phosphate is buffered by sorption equilibria with the sediment the production rate of phosphate the sorption capacity of the sediment and the thickness of the diffusive boundary layer at the sediment water interface appear to control the shape of the pore water profile even though the buffering places an upper limit on the concentration gradient across the sediment water interface and hence on the flux the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment a model is proposed to relate sediment chemistry to phosphorus fluxes

estimation of authigenic alteration of biogenic and reactive silica in pearl river estuarine sediments using wet chemical digestion methods the extent of authigenic alteration of biogenic and reactive silica in pearl river estuarine sediments has been estimated using wet chemical digestion methods results show relatively constant distributions of biogenic and reactive si horizontally and vertically based on three core measurements the biogenic and total reactive si average 77 91 and 264 77 mol si g 1 respectively their extents of authigenic alteration are correspondingly estimated as 55 6 and 70 6 the average biogenic si accumulation rate is calculated as 1 91 109 mol si year 1 which translates into storage of 7 15 of the annual riverine dissolved silica input by contrast the total reactive si accumulation rate is as high as 6 49 109 mol si year 1 improving annual riverine silicic acid storage to 24 19 detailed investigation is required for a good understanding of early diagenetic process of biogenic and reactive silica in this subtropical area

stratigraphic controls on fluid and solute fluxes across the sediment water interface of an estuary shallow stratigraphic features such as infilled paleovalleys modify fresh groundwater discharge to coastal waters and fluxes of saltwater and nutrients across the sediment water interface we quantify the spatial distribution of shallow surface water groundwater exchange and nitrogen fluxes near a paleovalley in indian river bay delaware using a hand resistivity probe conventional seepage meters and pore water samples in the interfluve region outside the paleovalley most nitrate rich fresh groundwater discharges rapidly near the coast with little mixing of saline pore water and nitrogen transport is largely conservative in the peat filled paleovalley fresh groundwater discharge is negligible and saltwater exchange is deep 1 m long pore water residence times and abundant sulfate and organic matter promote sulfate reduction and ammonium production in shallow sediment reducing iron rich fresh groundwater beneath paleovalley peat discharges diffusely around paleovalley margins offshore in this zone of diffuse fresh groundwater discharge saltwater exchange and dispersion are enhanced ammonium is produced in shallow sediments and fluxes of ammonium to surface water are large by modifying patterns of groundwater discharge and the nature of saltwater exchange in shallow sediments paleovalleys and other stratigraphic features influence the geochemistry of discharging groundwater redox reactions near the sediment water interface affect rates and patterns of geochemical fluxes to coastal surface waters for example at this site more than 99 of the groundwater borne nitrate flux to the delaware inland bays occurs within the interfluve portion of the coastline and more than 50 of the ammonium flux occurs at the paleovalley margin

fresh and saline groundwater discharge to the ocean a regional perspective 1 studies of groundwater flow in coastal aquifers often focus strongly on freshwater and investigate flow in a narrow 5 km zone surrounding the coastline this work was designed to place coastal flow in a regional context and to compare fresh and saline submarine groundwater discharge sgd for regional flow systems numerical flow and transport models were developed to estimate sgd associated with topography driven flow seawater recirculation and geothermal convection in a passive margin setting simulations were based on two cross sections of north carolina and sensitivity studies were used to explore the impact of varying hydraulic and transport parameters results suggest that saline flow associated with seawater recirculation and geothermal convection should be considered in studies of sgd studies limited to shallow topography driven flow may be justified in using small study areas but flow systems contributing to sgd may span at least 20 km surrounding the coast and continental slope

influence of water column anoxia on the elemental fractionation of carbon and phosphorus during sediment diagenesis abstract organic carbon oxidation and benthic total phosphorus flux measurements from sites covering a range of bottom water oxygen concentrations have been compiled the data show that p regeneration relative to carbon oxidation is much more extensive in sediments overlain by oxygen depleted waters these results are consistent with studies showing that low oxygen bottom water concentrations correlate with reduced total phosphorus burial efficiencies and enhanced organic carbon burial efficiencies based on these observations a positive feedback is proposed between water column anoxia enhanced benthic phosphorus regeneration and marine productivity this feedback may help explain the widespread accumulation of organic rich marine sediments from anoxic waters observed in the geologic record

intercomparison of submarine groundwater discharge estimates from a sandy unconfined aquifer summary recent studies suggest that chemical loading from submarine groundwater discharge sgd may rival other major sources such as rivers in many coastal areas sgd can occur as terrestrially derived typically fresh groundwater discharge or by seawater circulation through aquifer sediments because both terrestrial and recirculated seawater sgd can be significant sources of chemical loading to coastal waters appropriate methods are needed to properly assess and quantify the rates and distribution of these different inputs although many techniques exist to quantify sgd each method samples different components of sgd e g fresh versus saline discharge over different temporal and spatial scales the result is that confusion exists about how to use the different methods in this study we investigate the applicability of a variety of techniques for estimating sgd from an unconfined sandy coastal aquifer physical methods are first used to understand the spatial variability of discharge this information is then used with physical and geochemical data to estimate sgd investigative methods include direct measurement via seepage meters hydrogeologic estimation using darcy s law and tracer based estimates using radon and radium isotopes at our field site on cape cod massachusetts nearshore landward topography appears to exert a significant control on the spatial variability of fresh groundwater discharge presumably through its effects on evapotranspiration and hence recharge rates the darcy estimate of fresh sgd 4 0 m 3 m 1 d 1 is similar to a previous estimate made using a seepage meter transect that spans the seepage face radon serves as a valuable estimator of total sgd fresh plus saline 5 6 m 3 m 1 d 1 the difference between the total and fresh sgd estimates is in good agreement with the radium based sgd estimate 0 56 m 3 m 1 d 1 which primarily measures saline circulation through coastal sediments thus we conclude that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total sgd at our field site

discussion of results presented by hhndorf and dill some aspects of lead lead model ages of ore mineralizations from the western edge of the bohemian massif f r germany mineral deposita 21 329 336 1986

sedimentary iron phosphorus cycling under contrasting redox conditions in a eutrophic estuary abstract phosphorus p is often a limiting nutrient within freshwater and estuarine systems thus excess inputs of p from anthropogenic activities dominantly agriculture can induce eutrophication in receiving water bodies the sequestration of p within estuarine sediments is controlled by sorption and precipitation processes which are impacted by local redox conditions and burial environment despite the global spread of oxygen depletion in coastal marine systems p burial under reducing conditions remains poorly understood we investigated p cycling in relation to iron fe redox chemistry in sediments from the eutrophic peel harvey estuary in western australia using a combination of porewater analysis sequential chemical p extractions synchrotron based micro scale x ray fluorescence mapping and fe k edge x ray absorption spectroscopy and po43 sorption experiments the sediments represented redox regimes varying from strongly reducing organic rich sediments with little or no reactive fe iii oxyhydr oxides to oxygenated sediments that were enriched in reactive fe iii phases organic p and fe associated p were the main p burial phases and the latter was quantitatively important even in sediments with an overall strongly reducing character we attribute this to adsorption of p onto micro scale fe iii oxyhydroxide enrichments and or fe bearing clay minerals the organic rich sediments showed a strong decline in p contents with depth p was released from organic matter and fe phases but apatite precipitation was apparently inhibited in these sediments despite greater and stronger po43 sorption capacity the oxic sediments contained relatively little p due to a lack of the primary p source in marine sediments organic matter our results provide detailed insight into p burial in dynamic estuarine sediments and show that micro scale spectroscopic analyses greatly advance our understanding of p sequestration processes

lead isotope and pb pb model age determinations of ores from central europe and their metallogenetic interpretation the paper presents lead isotope data from 211 samples from phanerozoic lead bearing ore occurrences in central europe particularly from the southern part of the former german democratic republic the data are interpreted in terms of amov s dynamic model of continuous lead isotope evolution the relationships between thoro genic and urano genic model ages and the source of lead in different regional units are discussed we observed differences in lead isotope evolution in the hercynian internides and externides within the moldanubian and saxothuringian zones we distinguish five main lead bearing ore associations 1 cambrian stratiform base metal hermsdorf waldsassen 206pb 204pb 17 50 17 70 2 devonian vein type sb bearing metamorphogene neumuhle hartmannsdorf 17 80 18 00 3 upper carboniferous permian polymetallic including tin vein type kutna hora freiberg altenberg 18 00 18 20 4 triassic jurassic pb ba vein type st ibro halsbrucke 18 20 18 60 5 cenozoic polymetallic vein type riftogene roztoky banska stiavnica 18 80 19 10 pb isotope characteristics from ores of the montagne noire and the brioude massiac district correspond to this subdivision ore associations from the rheno hercynian zone display higher 207pb 204pb ratios which can be explained by more evolved and less metamorphosed source rocks mineralizations of the eastern harz strasberg neudorf belong to the permian association those from the western harz clausthal bad grund to the triassic jurassic because of pb isotope agreement the stratabound rammelsberg and the vein bound ramsbeck i mineralization are presumed to be isogenetic pb isotope identity of distinct mineralizations in the basement zone halsbrucke bad grund and in the triassic sediments gorny slask mechernich bleiglanzbanke suggests a strong genetic coherence pb isotope conformity between the upper carboniferous permian triassic ore associations and hercynian postkinematic granitoids and lamprophyric rocks also favours a close relationship pb isotope and other data indicate crustal sources as the age of the ore associations decreases crustal influences generally increase apart from the roztoky mineralization

sea bottom anoxia in the archipelago sea northern baltic sea implications for phosphorus remineralization at the sediment surface abstract the effects of the hydrophysical environment on oxygen conditions and p remineralization at sediment surface were studied in the archipelago sea northern baltic sea at 94 sites the water column was profiled for conductivity temperature and oxygen and the topmost sediment was collected for p and c fractions and oxy hydroxide fe analysis the near bottom water and sediment parameters as well as visual observations on the vertical colour distribution in the sediment cores were used to statistically classify the sediment surfaces into oxic anoxic and suboxic fluctuating oxygen conditions bottoms the anoxic bottoms occurred in the basins with water depths less than 47 m and the suboxic bottoms were most common in the basins with depths between 20 60 m while the oxic bottoms dominated in the depths over 60 m it was concluded that the preferential development of oxygen deficiency in the shallow basins results from the combined effects of complex topography and seasonal temperature stratification on both vertical and lateral bottom water exchange in the anoxic bottoms c p ratios suggest enhanced release of p compared to the oxic and suboxic bottoms in the suboxic bottoms high inorganic p concentrations and the strong coupling between p and fe suggest active transformation of organic p to the inorganic p phases probably due to the fluctuating oxygen conditions differences in the spatial distribution of the p c and fe were insignificant except for detrital p which decreased seawards

phosphorus speciation and availability in intertidal sediments of the yangtze estuary china abstract in order to better understand p cycling and bioavailability in the intertidal system of the yangtze estuary both surface 0 5 cm and core 30 cm long sediments were collected and sequentially extracted to analyze the solid phase reservoirs of sedimentary p loosely sorbed p fe bound p authigenic p detrital p and organic p the total sedimentary p in surface and core sediments ranged from 14 58 36 81 mol g 1 and 17 11 24 55 mol g 1 respectively and was dominated by inorganic p the average percentage of each fraction of p in surface sediments followed the sequence detrital p 54 9 fe bound p 23 7 organic p 14 3 authigenic p 6 3 loosely sorbed p 0 8 whereas in core sediments it followed the sequence detrital p 61 7 fe bound p 17 0 authigenic p 13 1 organic p 7 5 loosely sorbed p 0 7 post depositional reorganization of p was observed in both surface and core sediments converting organic p and fe bound p to authigenic p the accumulation rates and burial efficiencies of the total p in the intertidal area ranged from 118 70 904 98 mol cm 2 a 1 and 80 29 88 11 respectively high burial efficiency of the total p is likely related to the high percentage of detrital p and the high sediment accumulation rate in addition the bioavailable p represented a significant proportion of the sedimentary p pool which on average accounted for 37 4 and 25 1 of the total p in surface and core sediments respectively this result indicates that the tidal sediment is a potential internal source of p for this p limiting estuarine ecosystem

a multi level pore water sampler for permeable sediments abstract the construction and operation of a multi level piezometer multisampler designed to collect pore water from permeable sediments up to 230 cm below the sediment water interface is described multisamplers are constructed from 1 inch schedule 80 pvc pipe one quarter inch flexible pvc tubing leads from eight ports at variable depths to a 1 inch tee fitting at the top of the pvc pipe multisamplers are driven into the sediment using standard fence post drivers water is pumped from the pvc tubing with a peristaltic pump field tests in banana river lagoon florida demonstrate the utility of multisamplers these tests include collection of multiple samples from the permeable sediments and reveal mixing between shallow pore water and overlying lagoon water

recoil emanation theory applied to radon release from mineral grains abstract in this paper the recoil emanation theory is presented as a major mechanism for rn release from solid materials the basic theory is developed for rn emanation from geometrical shapes spheres and cylinders having variable thickness of precursor ra containing material subsequently a sphere pore model of emanation from mineral grains like those in rocks soils etc is developed in this model a surface of the grain is constructed as a superposition of in and out directed spherical caps and cylindrical pores the model incorporates processes like rn embedding pore overlap water effect and emanation from the edges using such a geometrical model it is shown how the rn emanating power measurements can give a clue to the ra distribution and surface properties of the samples in particular it is shown that for grains reasonably larger than the recoil range and for surface ra distribution the emanating power is independent of the grain size but the total ra concentration varies with the reciprocal of the grain size the opposite is true for ra distributed uniformly throughout the volume of the grain it is shown how the rn emanating power is related to the surface porosity of the grain also implications are made with respect to u transport in nature and with respect to low emanating power from lunar soil finally a failure of the emanation theory is discussed to reproduce emanation curves for the samples having high specific surface areas and ra distributed throughout the volume

estimating the groundwater contribution into florida bay via natural tracers 222rn and ch4 groundwater may represent a significant pathway for nutrients and other dissolved solutes into florida bay especially near the keys where wastewater disposal practices add large amounts of nitrogen and phosphorus to the subsurface each year previously we suggested that high water column inventories of the tracers 222rn and ch4 may be indicative of groundwater discharge in this study we employed mass balance calculations to determine that the total benthic fluxes required to maintain the measured water column tracer inventories were significantly larger than diffusive fluxes and varied between 4 2 5 6 dpm m 2 min 1 and 5 8 15 4 nmoles m 2 min 1 for 222rn and ch4 respectively independent estimates of the diffusive flux and porewater activities concentrations allowed us to calculate an advective groundwater velocity assuming that the difference between the total benthic flux given above and the diffusive flux is driven by seepage driven porewater advection these calculated velocities ranged from 0 2 to 4 3 cm d 1 for all sites tracers and sampling periods with a best estimate of approximately 1 7 cm d 1 these estimates of groundwater velocities compare very well with previous measurements of groundwater flux 1 3 cm d 1 at the same sites via seepage meters

estimating groundwater discharge into the northeastern gulf of mexico using radon 222 abstract submarine groundwater discharge sgd may provide important chemical constituents to the ocean but the dispersed nature of this process makes locating and quantifying its input extremely difficult since groundwater contains 3 4 orders of magnitude greater radon than seawater 222rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated we report development of a sgd tracing tool based on radon inventories in a coastal area of the northeastern gulf of mexico we evaluated factors that influence the concentration of radon in the water column i e production decay horizontal transport and loss across the pycnocline using a linked benthic exchange horizontal transport model total 222rn benthic fluxes 2420 dpm m 2 day 1 measured with in situ chambers are of the magnitude required to support measured sub pycnocline 222rn inventories while estimates of molecular diffusion show that this input is relatively small 230 dpm m 2 day 1 using this model approach together with measurements of the radon inventory we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 sec 1 into the 620 km2 study area this discharge equivalent to an upward advective velocity of approximately 2 10 cm day 1 dispersed over this entire study area is equivalent to approximately 20 first magnitude springs

iron sulfur phosphorus cycling in the sediments of a shallow coastal bay implications for sediment nutrient release and benthic macroalgal blooms we conducted a study to determine the seasonal relationship between iron sulfur and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay from april 1999 to september 2000 sediment cores were collected from rehoboth bay delaware analyses of the sediments in the upper 4 cm revealed that redox conditions controlled fe s p concentrations in the sediments pore waters and overlying water monthly sampling showed a marked decrease in the reactive solid phase p pool ascorbate leachable fraction asc p and sharp increases in soluble p measured as po in pore waters and overlying waters as the conditions became more reducing through32 4 out the summer months these changes were paralleled by decreases in the amorphous fe iii ascorbate leachable fraction asc fe and total fe iii oxyhydroxide pools dithionite extracted fraction fe iii oxide and increases in solid fes fes2 the release of soluble p from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid fes fes2 production which indicates that fe iii oxides act as a barrier to diffusive p flux during these anoxic conditions the regenerative p appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer by the late fall and into early spring sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline fe iii oxides and decreases in fes fes2 concentrations during this period increasing asc fe concentrations correlated with increases in asc p concentrations and decreases in pore water po the seasonal correlations 32 4 between fe s p indicate that fe redox chemistry controls sediment p flux to the overlying water column

the role of adsorption in sediment water exchange of phosphate in north sea continental margin sediments the effect of adsorption on the sediment water exchange of po was investj gated in sediments from four different types of sedimentary environments in the southern and eastern north sea in august 1991 and february 1992 nonlinear adsorption isotherms for oxidized sediment from eight stations indicate that north sea sediments differ widely in their capacity to adsorb po a good correlation between the value of the adsorption coefficient and nh oxalate extractable fe was observed a combination of the adsorption data with porewater po profiles solid phase results and measured and calculated rates of sediment water exchange of po for 15 stations in both august and february indicates that adsorption plays an important role in controlling sediment water exchange of po during at least a part of the year in three of the four north sea environments at most stations po adsorption constrains the flux of po to the overlying water at one station in the depositional environment of the skagcrrak however desorption is responsible for the maintenance of a flux of po to the overlying water a one dimensional reactiondiffusion model describing porewater po and solid phase p profiles was de deloped and applied to results for two stations the model results show that both enhanced retention and enhanced release of po can be adequately described when simultaneous equilibrium and first order kinetic reversible adsorptive reactions are assumed

the diagenesis of phosphorus in a nearshore marine sediment abstract chemical analyses have been made for total inorganic phosphorus total organic phosphorus and organic carbon in plankton and in anoxic sediments from long island sound u s a when combined with laboratory experiments the measurements of other studies on the same sediments and mathematical modelling the results indicate that 1 1 a large proportion of dissolved phosphate in the upper 10 cm of sediment zone of bioturbation is provided by the release of adsorbed phosphate during the reduction of ferric oxyhydroxides 2 2 below the zone of bioturbation phosphate is liberated to solution solely via organic matter decomposition and the release is stoichiometrically coupled to bacterial sulfate reduction and ammonia formation 3 3 a steady state diagenetic model can be used to predict the profile of organic phosphorus from that of dissolved phosphate at depths below the zone of bioturbation 4 4 the flux of dissolved phosphate out of the sediment must arise largely from phosphate liberation occurring very close 5 5 there is a large preferential loss of phosphorus relative to carbon from organic matter at the time of or prior to burial whereas after burial phosphorus is not lost preferentially at this location

evaluating the source and seasonality of submarine groundwater discharge using a radon 222 pore water transport model article i nfo pore water radon 222 rn distributions from indian river lagoon florida are characterized by three zones a lower zone where pore water 222 rn and sediment bound radium 226 ra are in equilibrium and concentration gradients are vertical a middle zone where 222 rn is in excess of sediment bound 226 ra and concentration gradients are concave downward and an upper zone where 222 rn concentration gradients are

evolution of uranogenic and thorogenic lead 1 a dynamic model of continuous isotopic evolution a dynamic model of the continuous isotopic evolution of uranogenic and thorogenic lead based on the minimum possible number of assumptions is proposed here the model is based on the simple assumption that the rate of change of the ratios 238 u 204 pb and w 232 th 204 pb present day values increased initially and after attaining a maximum started decreasing because of surface cooling of the earth the time t m when the and w rates of change attained their maximums is a model parameter depending on the age of formation of the different layers of the crust or in mixing processes on the participation of materials from the older or younger crust and mantle mathematically this is suitably described by a two parameter function which is the product of a linear and an exponential function the deduced equations of the model are calibrated according to the isotopic data on ore lead from the isotopic ratios of a lead sample it is possible to determine the two model ages uranogenic and thorogenic and the model parameter t m or another parameter t related to t m the values of the model parameters t m or t indicate the time of the enrichment ancient or younger in u pb which is an extremely important index for identifying the pb source type the use of these model parameters provide the possibility of a better theoretical description of the pb isotopic characteristics in the different types of sources

phosphorus speciation in the sediment and mass balance for the central region of the great barrier reef continental shelf australia abstract solid phase p speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the great barrier reef gbr lagoon this region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore seawards of which biogenic carbonate sediment predominates phosphorus speciation results show large variations along this transect organic p and authigenic apatite p are the major chemical forms of phosphorus in the central gbr continental shelf sediments post depositional reorganization of p was also observed converting organic p and iron bound p fe p to authigenic apatite p phosphorus burial rate was estimated from measurements of total p concentration and excess 210pb sediment mass accumulation rates burial efficiency varies significantly over the shelf inshore areas showed significant p remobilization from sediments to the water column up to 50 the mid and the outer shelf showed little evidence for remobilization except for coral reef platform sediments with more of the sediment p being in the less reactive authigenic apatite phases an appreciable fraction of this non labile authigenic apatite phase was identified as fish bone p sources and sinks over the central part of the gbr shelf were quantified using a mass balance approach this showed that coral sea shelf edge upwelling events are essential to satisfy the large p nutrient demand of the whole gbr lagoon p inputs due to upwelling events were greater than those contributed by local rivers over an average year

modelling the effect of salinity on radium desorption from sediments abstract the desorption of the four naturally occurring radium isotopes 223 ra 224 ra 226 ra and 228 ra from estuarine sediments is investigated these isotopes are created within sediments by the radioactive decay of insoluble thorium parents due to competition from other ions for the occupation of adsorption sites on the sediment grains radium desorption is a function of salinity a model is developed to describe radium desorption as it is affected by salinity grain size and sediment concentration it is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters one of these parameters is the concentration of radium available for exchange on the sediment the other depends on major ion and radium exchange coefficients desorption experiments performed on sediments from the bed of the bega river australia are used to validate the model and to evaluate the parameters needed for model application

using ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the bega river estuary has provided information on the types and rates of transport processes operating in the lagoon sediments the measurement techniques included ra mass budgets of the lagoon ra fluxes into benthic chambers and modelling of the pore water and solid phase ra profiles in a sediment core the sediment profile of 210pb and the solid phase and pore water profiles of the longer lived ra isotopes 228ra half life 5 7 years and 226ra half life 1600 years indicate bioturbation to a depth of 10 cm a diffusion bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic flux of ra the flux of the shortest lived isotope 224ra half life 3 7 days is not significantly enhanced by bioturbation and its flux is consistent with diffusion controlled release however bioturbation enhances the 228ra flux by a factor of more than two over the flux due to molecular diffusion alone modelled pore water profiles and flux calculations are consistent with a bioturbation time scale between 0 5 and 2 years the measured benthic flux of 226ra is much greater than can be accounted for by the modelled profile and may be due to slow 226ra desorption from the sediment variable sediment accumulation rates or groundwater flow based on 226ra pore water and flux measurements at the time of this study groundwater flow has an upper limit of 0 3 cm d 1

isotopic evolution of the mantle the role of magma mixing trace element and isotopic data for magmas from the two major mantle reservoirs appear to be inconsistent the incompatible elements and sr and nd isotopes show that abyssal tholeiities morb are form a reservoir that has current and time integrated depletions of the elements that are fractionated into a melt morb however have206pb 204pb and207pb 204pb ratios suggesting long term enrichment in u pb i e future single stage ages alkali basalts and tholeiities from continents and oceanic islands are derived from lile and u pb enriched reservoirs sr and nd isotopic ratios however appear to indicate that some of these basalts are derived from unfractionated reservoirs and others from reservoirs with time integrated depletions the apparent stages of the mantle reservoirs are much younger than the ages of continental shields these inconsistencies can be reconciled by assuming that oceanic and continental basalts are mixtures of magmas from depleted and enriched reservoirs morb are slightly contaminated depleted magmas while ocean island and continental basalts are mixtures of morb or a depleted picritic parent magma and an enriched end member having trace element patterns similar to high k magmas such as kimberlites or nephelinites the mixing relations are such that mixtures can be enriched in u pb rb sr nd sm 206pb 204pb relative to primitive mantle yet appear to have time integrated depletions in143nd 144nd and87sr 86sr a small amount of contamination by material from an enriched reservoir can explain the pb results for morb depleted basalts are more sensitive to pb than to rb or nd contamination the238u 204pb of uncontaminated morb may be about 7 compared to 7 9 for the primary growth curve and 10 for the enriched reservoirs similarly continental and ocean island basalts may represent mixtures of enriched and depleted magmas if so the ages of mantle reservoirs have been underestimated we propose that both of the major mantle magmas source are in the upper mantle and that both are global in extent the shallower one 200 km depth is inhomogenous having been enriched metasomatised at various time by fluids from the depleting layer it may also be the sink of subducted sediments and hydrothermally altered oceanic crust the deeper source 220 670 km depth is more homogenous more garnet rich and provides depleted magmas which however become contaminated as they rise through the shallow encriched layer partial melting and diapiric ascent originated in the thermal boundary layer between the two chemically distinct layers magma mixing prior to eruption is an inevitable consequence of a chemically layered mantle with depleted magmas rising through an enriched uppermost mantle this allows quantification of the heretofore nebolous concept of mantle metasomatism

simultaneous measurement of o2 mn fe i and s ii in marine pore waters with a solid state voltammetric microelectrode a solid state au hg voltammetric microelectrode was used to measure simultaneously and with millimeter spatial resolution the vertical distributions of o 2 mn 2 fe 2 hs and i in the pore water of sediments from the canadian continental shelf and slope the electrode was used shipboard to analyze undisturbed sediment cores and in a sediment mesocosm to determine the three dimensional distribution of redox species in the sediment surrounding an actively irrigated worm burrow in cores from emerald basin and cabot strait o 2 disappeared and mn 2 fe 2 and hs appeared in the vertical sequence predicted on the basis of thermodynamics in a core from the scotian slope o 2 disappeared at 14 mm depth but mn 2 was not detected over the 50 mm depth examined with the probe in all three cores i was detected in the pore water below the oxygen penetration depth in none of the cores did the distributions of o 2 and mn 2 overlap the thickness of the layer within which neither o 2 nor mn 24 could be detected ranged from 8 to 40 mm we suggest that nitrate not oxygen is used to oxidize mn 2 the upward directed mn 2 gradients of 3 5 and 10 mol m 4 in the emerald basin and cabot strait sediments respectively drive manganese fluxes that are morc than one order of magnitude lower than the o 2 fluxes which are estimated at 4 5 mmol m 2 d 1 the three dimensional distributions of solutes in the pore water in the sediment surrounding an irrigated worm burrow demonstrate that both the o 2 penetration depth and the depth where mn 2 is first detected are deeper in the vicinity of the worm burrow than away from it this is consistent with the notion that irrigation brings oxygenated water into the worm tube allowing oxygen to diffuse across the burrow wall into the sediment and react with mn 2 and other reduced porewater constituents three dimensional heterogeneity in porewater composition cannot be detected by conventional one dimensional coring and slicing techniques because these latter techniques average the porewater composition in a large volume of sediment they may indicate that the distributions of individual species overlap when in fact they do not

trace element cycling in a subterranean estuary part 2 geochemistry of the pore water abstract submarine groundwater discharge sgd is an important source of dissolved elements to the ocean yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers the net flux of elements from sgd to the coastal ocean is dependent on biogeochemical reactions in the groundwater seawater mixing zone recently termed the subterranean estuary this paper is the second in a two part series on the biogeochemistry of the waquoit bay coastal aquifer subterranean estuary the first paper addressed the biogeochemistry of fe mn p ba u and th from the perspective of the sediment composition of cores charette et al charette m a sholkovitz e r hansell c m 2005 trace element cycling in a subterranean estuary part 1 geochemistry of the permeable sediments geochim cosmochim acta 69 2095 2109 this paper uses pore water data from the subterranean estuary along with bay surface water data to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of fe mn u ba and sr in coastal aquifers nine high resolution pore water groundwater profiles were collected from the head of the bay during july 2002 there were non conservative additions of both ba and sr in the salinity transition zone of the subterranean estuary however the extent of sr release was significantly less than that of its alkaline earth neighbor ba pore water ba concentrations approached 3000 nm compared with 25 50 nm in the surface waters of the bay the pore water sr salinity distribution suggests a 26 elevation in the amount of sr added to the subterranean estuary the release of dissolved ba to the mixing zone of surface estuaries is frequently attributed to an ion exchange process whereby seawater cations react with ba from river suspended clay mineral particles at low to intermediate salinity results presented here suggest that reductive dissolution of mn oxides in conjunction with changes in salinity may also be an important process in maintaining high concentrations of ba in the pore water of subterranean estuaries in contrast pore water u was significantly depleted in the subterranean estuary a result of sgd driven circulation of seawater through reducing permeable sediments this finding is supported by surface water concentrations of u in the bay which were significantly depleted in u compared with adjacent coastal waters using a global estimate of sgd we calculate u removal in subterranean estuaries at 20 106 mol u y 1 which is the same order of magnitude as the other major u sinks for the ocean our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by sgd associated processes

pore water geochemistry of eastern mediterranean mud volcanoes implications for fluid transport and fluid origin abstract the pore water chemistry of mud volcanoes from the olimpi mud volcano field and the anaximander mountains in the eastern mediterranean sea have been studied for three major purposes 1 modes and velocities of fluid transport were derived to assess the role of upward advection and bioirrigation for benthic fluxes 2 differences in the fluid chemistry at sites of milano mud volcano olimpi area were compiled in a map to illustrate the spatial heterogeneity reflecting differences in fluid origin and transport in discrete conduits in near proximity 3 formation water temperatures of seeping fluids were calculated from theoretical geothermometers to predict the depth of fluid origin and geochemical reactions in the deeper subsurface no indications for downward advection as required for convection cells have been found instead measured pore water profiles have been simulated successfully by accounting for upward advection and bioirrigation advective flow velocities are found to be generally moderate 3 50 cm y 1 compared to other cold seep areas depth integrated rates of bioirrigation are 1 2 orders of magnitude higher than advective flow velocities documenting the importance of bioirrigation for flux considerations in surface sediments calculated formation water temperatures from the anaximander mountains are in the range of 80 to 145 c suggesting a fluid origin from a depth zone associated with the seismic decollement it is proposed that at that depth clay mineral dehydration leads to the formation and advection of fluids reduced in salinity relative to sea water this explains the ubiquitous pore water freshening observed in surface sediments of the anaximander mountain area multiple fluid sources and formation water temperatures of 55 to 80 c were derived for expelled fluids of the olimpi area

iron limitation in the western interior seaway during the late cretaceous oae 3 and its role in phosphorus recycling and enhanced organic matter preservation the sedimentary record of the coniacian santonian oceanic anoxic event 3 oae 3 in the north american western interior seaway is characterized by a prolonged period of enhanced organic carbon oc burial this study investigates the role of fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the coniacian santonian aged niobrara formation in the usgs 1 portland core iron speciation results indicate the development of a reactive fe limitation coeval with reduced bioturbation and increased organic matter preservation suggesting that decreased sulfide buffering by reactive fe may have promoted enhanced organic matter preservation at the onset of oae 3 an fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling additionally our results demonstrate inconsistencies between fe based and trace metal redox reconstructions iron indices from the portland core indicate a single stepwise change whereas the trace metal redox proxies indicate fluctuating redox conditions during and after oae 3 using fe speciation to reconstruct past redox conditions may be complicated by a number of factors including fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited fe supply and high levels of export production

cable bacteria generate a firewall against euxinia in seasonally hypoxic basins seasonal oxygen depletion hypoxia in coastal bottom waters can lead to the release and persistence of free sulfide euxinia which is highly detrimental to marine life although coastal hypoxia is relatively common reports of euxinia are less frequent which suggests that certain environmental controls can delay the onset of euxinia however these controls and their prevalence are poorly understood here we present field observations from a seasonally hypoxic marine basin grevelingen the netherlands which suggest that the activity of cable bacteria a recently discovered group of sulfur oxidizing microorganisms inducing long distance electron transport can delay the onset of euxinia in coastal waters our results reveal a remarkable seasonal succession of sulfur cycling pathways which was observed over multiple years cable bacteria dominate the sediment geochemistry in winter whereas after the summer hypoxia beggiatoaceae mats colonize the sediment the specific electrogenic metabolism of cable bacteria generates a large buffer of sedimentary iron oxides before the onset of summer hypoxia which captures free sulfide in the surface sediment thus likely preventing the development of bottom water euxinia as cable bacteria are present in many seasonally hypoxic systems this euxinia preventing firewall mechanism could be widely active and may explain why euxinia is relatively infrequently observed in the coastal ocean

evolution of uranogenic and thorogenic lead 2 some differences in the variations of the206pb 204pb and208pb 204pb ratios abstract on the basis of a dynamic model of a continuous pb isotope evolution the variations in the isotopic ratios of uranogenic and thorogenic pb in a number of young ore deposits ocean sediments and volcanic rocks from mid ocean ridges oceanic islands island arcs and continents are evaluated the deviations in the th u ratio from its model values are calculated a model parameter t which indicates a more ancient enrichment in u pb crustal type pb sources or younger enrichment in u pb mantle type pb sources is also calculated the relative change in the th u ratio is close to 1 for mantle type pb sources lower t values and decreases for crustal type pb sources higher t values an interpretation of these changes could be connected with the differences in the u and th geochemical behaviours in metamorphic processes under the conditions of a deeper mantle or crustal source owing to the differences in their valence states in some cases they have similar u 4 and th 4 migration abilities in others u 6 and th 4 considerably different

benthic iron and phosphorus fluxes across the peruvian oxygen minimum zone benthic fluxes of dissolved ferrous iron fe2 and phosphate tpo4 were quantified by in situ benthic chamber incubations and pore water profiles along a depth transect 11 s 80 1000 m across the peruvian oxygen minimum zone omz bottom water o2 levels were 2 mol l 1 down to 500 m water depth and increased to 40 mol l 1 at 1000 m fe2 fluxes were highest on the shallow shelf maximum 316 mmol m 2 yr 1 moderate 15 4 mmol m 2 yr 1 between 250 m and 600 m and negligible at deeper stations in the persistent omz core continuous reduction of fe oxyhydroxides results in depletion of sedimentary fe al ratios tpo4 fluxes were high maximum 292 mmol m 2 yr 1 throughout the shelf and the omz core in association with high organic carbon degradation rates ratios between organic carbon degradation and tpo4 flux indicate excess release of p over c when compared to redfield stoichiometry most likely this is caused by preferential p release from organic matter dissolution of fish debris and or p release from microbial mat communities while fe oxyhydroxides can only be inferred as a major p source on the shallow shelf the benthic fluxes presented here are among the highest reported from similar oxygen depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean the shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of fe2 and tpo4

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

isotopic composition of lead in proterozoic anoxic metasedimentary and volcanogenic rocks from the bohemian massif czech republic with metallogenetic implications abstract the isotopic composition of lead in normal and metal rich black shales and associated volcanogenic rocks has been studied in the barrandian upper proterozoic of the bohemian massif which is a part of the central european variscan belt these rocks played an important role in the metallogeny of the bohemian massif the late proterozoic age of metamorphism is evidenced by the slope of the secondary isochron of the uranogenic lead corresponding to the age of 635 45 ma and by the model age of 540 590 ma of thorogenic lead in the samples with low th u ratios in most of the samples the leached pb is less radiogenic than the residual pb no correlation was established between the presence of more radiogenic samples and the distance towards variscan granitoids the samples are mostly associated with the upper proterozoic volcano sedimentary sequences far from variscan intrusions and only some occur in the exocontacts of variscan plutons this suggest that most of the uranium was introduced into rocks during cadomian metamorphism rather than variscan tectonomagmatic phases however a possible contribution of radiogenic lead in connection with the latter events as well as some heterogeneity in u pb and th pb ratios from the time between sedimentation and metamorphism cannot be excluded the comparatively low 208 pb 204 pb ratios indicate that thorium was not mobilized by the metamorphic fluids and testifies that th is less mobile under metamorphic conditions the 207 pb 204 pb values 15 57 15 63 of less radiogenic lead in the rocks are close to those of galenas from different variscan polymetallic mineralizations in the bohemian massif which suggests a common source from a well mixed reservoir as observed in ore leads in mature island arcs or at active boundaries between continental and oceanic plates

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr

in situ evaluation of nearshore marine and fresh pore water transport into flamengo bay brazil abstract transport between pore waters and overlying surface waters of flamengo bay near ubatuba brazil was quantified using natural and artificial geochemical tracers 222rn cl and sf6 collected from multi level piezometers installed along a transect perpendicular to the shore eight sampling ports positioned along the length of the piezometers allowed sampling of pore waters at discrete depth intervals from 10 to 230 cmbsf centimeters below seafloor small volume samples were collected from the piezometers using a peristaltic pump to obtain pore water depth profiles pore water 222rn is deficient in shallow sediments allowing application of a diffusion corrected 222rn exchange rate this model estimates the magnitude of pore water exchange rates to be about 130 419 cm day an sf6 saturated fluorescein dye tracer was gently pumped into deep pore waters and exchange rates estimated from this method range from 29 to 185 cm day while absolute rates are higher using 222rn than sf6 rates are of similar magnitudes and the trends with distance from shore are the same flow is greatest 6 m from shore and decreases by more than 50 further offshore a cl mass balance indicates the greatest fraction of fresh sgd occurs along an apparent preferential flow path in sediments within 5 7 m of the shoreline 87 recirculating bay waters through sediments dominate pore water advection at 10 m offshore where only 4 of the flow can be attributed to a freshwater source both fresh and marine sources combine to make up submarine groundwater discharge to coastal water bodies the magnitude of fresh aquifer discharge is often a spatially variable and minor component of the total discharge

speciation bioavailability and preservation of phosphorus in surface sediments of the changjiang estuary and adjacent east china sea inner shelf abstract the speciation potential bioavailability and preservation of phosphorus p in surface sediments of the changjiang yangtze river estuary and adjacent east china sea ecs inner shelf were investigated through the analyses of p fractions and sediment bulk properties a sequential extraction method sedex was used to separate and quantify the following six sedimentary p reservoirs exchangeable p ex p authigenic p au p detrital p de p organic p or p refractory p re p and fe bound p fe p total p tp in surface sediments ranged from 15 0 to 21 4 mol g 1 and was highest near the changjiang river mouth the average contribution of each form of p to tp was 55 6 de p 17 8 re p 16 1 or p 5 5 au p 2 5 ex p and 2 5 fe p respectively de p showed relatively higher concentrations in the river mouth and the ecs shelf region off the changjiang estuary high concentrations of or p were found mainly in mud areas showing a similar distribution pattern with silt sediment surface area ssa and total organic carbon toc re p was mainly distributed near the estuarine area and the zhe min coast bioavailable p bap accounted for 9 5 32 0 of tp with a mean of 21 2 and showed a similar distribution pattern to that of or p de p ssa and toc ssa loadings both decreased with increasing of ssa while or p ssa loadings varied little with ssa indicating that or p may have reached an adsorption desorption equilibrium on mineral surfaces toc to total organic p top sum of re p and or p ratios less than the redfield ratio 84 in average may have indicated efficient remineralization of organic matter in mobile muds of the changjiang estuary and adjacent ecs inner shelf furthermore the relatively high toc or p ratios 72 422 with a mean of 188 likely suggest a higher degree of preferential regeneration of labile or p over toc in sediments

the transport of u and th series nuclides in sandy confined aquifers a comprehensive evaluation of the transport of u th ra and rn nuclides of the 238 u and 232 th decay series in an unconfined sandy aquifer long island ny was conducted groundwater data are compared with results of a theoretical transport model of weathering of aquifer grains and interaction with surface coatings to establish relationships between the concentrations of the radionuclide activities in the water and flow line distance the data provide estimates for geochemical parameters including weathering rates and chemical reactivities in both the vadose zone and the aquifer a theoretical treatment of the transport is presented that considers the reaction between the water and a reactive surface layer it is found that a model with chemical exchange between the surface layer and the water for all species is not valid and that the effects of saturation and irreversible precipitation of th is required the water table shows a relatively wide range in u activities the only element in the u th series for which vadose zone input is significant in the aquifer high weathering of u and recoil inputs of 234 u to the water occur in the upper 3 m of the vadose zone while lower weathering and removal of u from the water occur below the deeper aquifer has variable 238 u activities that can be accounted for by input from the vadose zone and is not a result of non conservative behavior the isotopic composition of u is shown to be directly related to the recoil rate relative to the weathering rate the wide range of 238 u in the aquifer waters is a reflection of diverse vadose zone inputs showing that dispersive mixing is not a dominant effect the higher values of 234 u in the aquifer reflect the recoil weathering input ratios from within the aquifer where the weathering rate is lower than the vadose zone both high u activities and high 234 u values cannot be obtained in the vadose zone or within reasonable flow distances in the aquifer radium isotopes are found to be in exchange equilibrium with the surface layer 224 ra 228 ra and 226 ra have comparable activities throughout the aquifer in the vadose zone the dominant input of ra to groundwater is weathering and recoil as found elsewhere the 222 rn in the water is a large fraction 5 of the rn produced in the aquifer rock this cannot be due to ra precipitation onto surface coatings in the aquifer as supported by present weathering with th in exchange equilibrium with the surface layer it is found that th is saturated in the waters under oxidizing conditions so that the weathering input is irreversibly precipitating onto surfaces however it is shown that under somewhat reducing conditions th activities are much higher and the th u ratio in the solution is approximately that of the rock we propose that under oxidizing conditions the source of rn is a surface coating enriched in 232 th and 230 th this th was precipitated in an earlier phase during rapid dissolution of readily weathered phases that contain 10 of the u th inventory of the rock with the associated u carried away in solution therefore the previously precipitated 230 th and 232 th produce daughter nuclides in the surface coating which are the dominant contributors of ra and rn to the ground water in particular rn is provided by very efficient losses by diffusion or recoil from the surface coating this then does not require recent large recoil losses from the parent rock or the presence of nanopores in the rock the first data of both long lived 232 th and short lived 234 th and 228 th in ground water is reported the th isotope activities indicate that desorption kinetics are slow and provide the first estimate based on field data of the th desorption rate from an aquifer surface the mean residence time of th in the surface coating is 3000 y while in the water it is 1 h ra is in partition equilibrium with the aquifer surface layer however the strong fixation of th on surface coatings is very susceptible to changes in oxidation state as is shown by a comparison of two adjacent aquifers this makes it difficult to define with certainty the retentive characteristics in natural systems in general it is shown that the distributions of naturally occurring nuclides can be used to calculate values for transport parameters that are applicable to the transport of anthropogenic nuclides