burial and degradation of organic carbon in louisiana shelf slope sediments abstract the primary focus of this paper is to better understand carbon burial on the louisiana continental margin using spatial scales that covered various shelf depositional areas far field and near field sediment and organic carbon inputs relative to river mouth proximity and covering a variety of sedimentation rates box cores samples were collected in july 2003 cores were collected along two depositional transects extending westward and southward from the southwest pass sw pass a key difference between the two transects sampled in this study was the greater occurrence of mobile muds derived from spillover from shallower regions along the westward 50 m isobaths the dominant mechanism for mixing in the surface active zone saz on the inner louisiana shelf was due to physical not biological forces burial efficiencies for organic carbon 57 2 91 5 and total nitrogen 44 2 86 9 ranged widely across all shelf stations lower burial efficiencies for bulk organic carbon total nitrogen and pigment biomarkers were associated with mobile muds west of southwest pass chlorophyll a concentrations were significantly higher than pheopigments at depth at the mississippi river and southwest pass stations making up 40 4 and 77 4 of total pigment concentrations in the saz and 46 2 and 63 2 in the accumulation zone az respectively these results are in agreement with earlier plant pigment studies which showed that a large fraction of the phytodetritus delivered to the inner shelf was derived from coastal and river diatoms the amount of lignin preserved with depth decreased with increasing residence time in the saz and diagenetic zone dz along the canyon transect but not along the western transect trends for lignin concentration followed previously identified surface sediment trends indicating overall lower burial of refractory terrestrial material at depth with greater distance offshore

a model for microbial phosphorus cycling in bioturbated marine sediments significance for phosphorus burial in the early paleozoic abstract a diagenetic model is used to simulate the diagenesis and burial of particulate organic carbon c org and phosphorus p in marine sediments underlying anoxic versus oxic bottom waters the latter are physically mixed by animals moving through the surface sediment bioturbation and ventilated by burrowing tube dwelling organisms bioirrigation the model is constrained using an empirical database including burial ratios of c org with respect to organic p c org p org and total reactive p c org p reac burial efficiencies of c org and p org and inorganic carbon to phosphorus regeneration ratios if p org is preferentially mineralized relative to c org during aerobic respiration as many previous studies suggest then the simulated p org pool is found to be completely depleted a modified model that incorporates the redox dependent microbial synthesis of polyphosphates and p org termed the microbial p pump allows preferential mineralization of the bulk p org pool relative to c org during both aerobic and anaerobic respiration and is consistent with the database results with this model show that p burial is strongly enhanced in sediments hosting fauna animals mix highly labile p org away from the aerobic sediment layers where mineralization rates are highest thereby mitigating diffusive po 4 3 fluxes to the bottom water they also expand the redox niche where microbial p uptake occurs the model was applied to a hypothetical shelf setting in the early paleozoic a time of the first radiation of benthic fauna results show that even shallow bioturbation at that time may have had a significant impact on p burial our model provides support for a recent study that proposed that faunal radiation in ocean sediments led to enhanced p burial and possibly a stabilization of atmospheric o 2 levels the results also help to explain c org p org ratios in the geological record and the persistence of p org in ancient marine sediments

carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration lake superior to understand carbon and oxygen dynamics in sediments with deep oxygen penetration we investigated eight locations 160 318 m depth throughout lake superior despite the 2 4 weight percent organic carbon content oxygen penetrated into the sediment by 3 5 to 12 cm at all locations such deep penetration is explained by low sedimentation rates 0 01 0 04 cm yr 21 high solubility of oxygen in freshwater and a shallow 2 cm bioturbation zone in response mainly to oxygen variations in the bottom waters the sediment oxygen penetration varied seasonally by as much as several centimeters suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged the oxygen uptake rates 4 4 7 7 mmol m 22 d 21 average 6 1 mmol m 22 d 21 and carbon mineralization efficiency 90 of deposited carbon were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates the reactivity of organic carbon was found to decrease with age similarly to the power law documented in marine environments the burial flux of carbon into the deep sediment 0 7 mmol m 22 d 21 was 2 5 of the previously estimated primary production maximum volume specific carbon degradation rates were 0 3 1 5mmol cm 23 d 21 bioturbation coefficient near the sediment surface was 3 8 cm 2 yr 21 these results indicate that carbon cycling in large freshwater systems conforms

for the deep biosphere the present is not always the key to the past what we can learn from the geological record microbial life below the earth s surface the deep biosphere has probably varied significantly since the archaean reconstructing changes in deep biosphere activity over geological timescales is necessary to understand its role in biogeochemical cycling even for the last few million years such changes are often not captured by studying the distribution of present activity however several studies using samples from scientific drilling have revealed mineralogical geochemical isotopic and fossil organic molecule imprints in the sedimentary record that document rather different past deep biosphere conditions changing deep biosphere conditions can also be simulated using geochemical models while some processes occurring in the past can be understood by comparing them with the present deep biosphere others lack any modern analogue they are defined as non actualistic a non actualistic consideration of the deep biosphere is therefore essential for a better understanding of how earth and life co evolved through time

the seafloor as the ultimate sediment trap using sediment properties to constrain benthic pelagic exchange processes at the goban spur the benthic diagenetic model omexdia has been used to reproduce observed benthic pore water and solid phase profiles obtained during the omex study in the goban spur area n e atlantic and to dynamically model benthic profiles at site omex iii 3660 m depth with the sediment trap organic flux as external forcing the results of the dynamic modelling show that the organic flux as determined from the lowermost sediment trap 400 metres above the bottom at omex iii is insufficient to explain the organic carbon and pore water profiles the best fitting was obtained by maintaining the seasonal pattern as observed in the traps while multiplying the absolute values of the flux by a factor of 1 85 the inverse modelling of diagenetic processes resulted in estimates of total mineralisation rate and of degradability of the organic matter at the different stations these diagenetic model based estimates are used to constrain the patterns of lateral and vertical transports of organic matter using the observed degradability as a function of depth we show that the observed organic matter fluxes at the different depths are consistent with a model where at all stations along the gradient the same vertical export flux occurs at 200 m and where organic matter sinks with a constant sinking rate of around 130 m d 1 if sinking rates were higher in the order of 200 m d 1 the observations could be consistent with an off slope gradient in export production of approximately a factor of 1 5 between the shallowest and deepest sites the derived high degradability of the arriving organic matter and the consistency of the mass fluxes at the different stations exclude the possibility of a massive deposition on the margin of organic matter produced on the shelf or shelf break however other hypotheses to explain the patterns found in the sediment trap data of both omex and other continental margin study sites also suffer from different inconsistencies further close examination of the flow patterns at the margin will be needed to examine the question

storm induced circulation in lunenburg bay of nova scotia observations and numerical simulations abstract an extreme weather event hurricane juan made landfall in nova scotia canada in september 2003 the storm produced an 70 cm storm surge and 40 cm s 1 coastal currents in lunenburg bay registered by a coastal observing system a fine resolution 60 m coastal circulation model is used to examine the response of lunenburg bay to hurricane juan the model is forced by local wind stress at the sea surface and tides and remotely generated waves specified at model open boundaries the model performance is assessed in terms of 2 the variance of the model errors normalized by the observed variance the model reproduces very well the observed surface elevations with 2 values of less than 0 05 and reasonably well the observed currents with 2 values between 0 2 and 1 1 in the bay during hurricane juan the model data comparisons demonstrate that the coastal circulation in the bay is significantly affected by local wind associated with the storm the model results are also used to demonstrate the

cyclic 100 ka glacial interglacial migration of subseafloor redox zonation on the peruvian shelf the coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood we examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial interglacial cycles in a sedimentary section drilled on the peruvian shelf ocean drilling program leg 201 site 1229 multiple and distinct layers of diagenetic barite and dolomite i e minerals that typically form at the sulfate methane transition smt occur at much shallower burial depth than the present smt around 30 meters below seafloor these shallow layers co occur with peaks of 13c depleted archaeol a molecular fossil of anaerobic methane oxidizing archaea present day non steady state distributions of dissolved sulfate also suggest that the smt is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the smt reaction transport modeling substantiates our hypothesis that shallow smts occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales e g glacial interglacial periods however our study demonstrates how dynamically sediment biogeochemistry of the peru margin has responded to glacial interglacial change and how these changes are now preserved in the geological record such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling

numerical study of circulation dispersion and hydrodynamic connectivity of surface waters on the belize shelf a nested grid ocean circulation modeling system is used to examine the circulation dispersion and hydrodynamic connectivity of surface waters on the belizean shelf the nested grid system consists of a coarse resolution 19 km outer model of the western caribbean sea an intermediate resolution 6 km middle model of the southern meso american barrier reef system mbrs and a fine resolution 2 km inner model of the belizean shelf the nested system is forced by climatological monthly mean surface forcing and integrated over 5 years the near surface circulation on the belize shelf produced by the inner model is characterized by a strong and persistent northwestward flow as a direct influence of the caribbean current on the northwestern shelf and a weak and spatially variable flow on the inner and southern shelf the monthly mean model currents are used to calculate retention and dispersion of conservative near surface particles carried by the ocean currents the near surface dispersion is relatively higher in areas seaward east of lighthouse and glovers reef atolls and lower on the inner shelf particularly within the inner channel and in the vicinity of south water cay to examine hydrodynamic connectivity of reefs in the surface waters of the belize shelf we calculate upstream and downstream retention areas for coral reefs at turneffe islands and glovers reef atolls the potential sources of passive near surface particle supply reaching these two reef atolls within 30 days include both the shallow waters surrounding the two sites the deep waters between them and the coastal waters of the bay islands honduras the 30 day downstream retention areas of the turneffe and glovers reef atolls cover the central and southern belize shelf respectively

impact of suboxia on sinking particulate organic carbon enhanced carbon flux and preferential degradation of amino acids via denitrification fluxes of particulate organic carbon poc through the oxygen deficient waters in the eastern tropical north pacific were found to be relatively less attenuated with depth than elsewhere in the eastern north pacific the attenuation coefficient b for the flux was found to be 0 40 versus the composite value of 0 86 determined by martin et al 1987 to examine this further sinking poc was collected using sediment traps and allowed to degrade in oxic and suboxic experiments using a kinetic model it was found that degradation proceeded at similar rates roughly 0 8 day 1 under oxic and suboxic conditions but a greater fraction of bulk poc was resistant to degradation in the suboxic experiments 61 vs 23 amino acids accounted for 37 of poc collected at 75m but following degradation the value dropped to 17 and 16 in the oxic and suboxic experiments respectively poc collected from 500m was 10 amino acids the non aa component of poc collected at 75m was not degraded under suboxic conditions while under oxic conditions it was these results suggest that microbes degrading oc under suboxic conditions via denitrification preferentially utilize nitrogen rich amino acids this preferential degradation of amino acids suggests that 9 more nitrogen may be lost via water column denitrification than is accounted for when a more redfieldian stoichiometry for poc is assumed

pore water geochemistry in shallow sediments from the northeastern continental slope of the south china sea abstract the northeastern continental slope of the south china sea is one of the promising areas for gas hydrates which is characterized by its variable topography and complex tectonics in this paper two gravity piston cores were sampled from the shallow sediments in the vicinity of the region where various morphologies of gas hydrate was recovered during the second gas hydrate expedition of guangzhou marine geological survey conventionally the identification of gas hydrate was primarily dependent on the bottom simulating reflector bsr apart from bsr the anomalies in pore water geochemistry which are highly related to various microbially mediated processes and features of fluid from deep hydrocarbon reservoir are also helpful for the gas hydrate exploration in this study upward methane flux may be responsible for distinct characteristics of pore waters observed at the two sample sites a reactive transport model was accordingly established to compare contrastive fluid features and to estimate the upward methane flux since fluid migration is typically associated with complex tectonic settings of the study area it is proposed that heterogeneous distribution of gas hydrates along the continental margin should be well considered

a revised global estimate of dissolved iron fluxes from marine sediments literature data on benthic dissolved iron dfe fluxes mol m 2 d 1 bottom water oxygen concentrations o2bw m and sedimentary carbon oxidation rates cox mmol m 2 d 1 from water depths ranging from 80 to 3700 m were assembled the data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic dfe fluxes inline image where 170 mol m 2 d 1 is the maximum flux for sediments at steady state located away from river mouths this simple function unifies previous observations that cox and o2bw are important controls on dfe fluxes upscaling predicts a global dfe flux from continental margin sediments of 109 55 gmol yr 1 of which 72 gmol yr 1 is contributed by the shelf 2000 m of 41 21 gmol yr 1 is unsupported by empirical data previous estimates of benthic dfe fluxes derived using global iron models are far lower approximately 10 30 gmol yr 1 this can be attributed to i inadequate treatment of the role of oxygen on benthic dfe fluxes and ii improper consideration of continental shelf processes due to coarse spatial resolution globally averaged dfe concentrations in surface waters simulated with the intermediate complexity university of victoria earth system climate model were a factor of 2 higher with the new function we conclude that i the dfe flux from marginal sediments has been underestimated in the marine iron cycle and ii iron scavenging in the water column is more intense than currently presumed

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

sedimentary organic matter distributions burrowing activity and biogeochemical cycling natural patterns and experimental artifacts abstract the coupling between biogenic reworking activity and reactive organic matter patterns within deposits is poorly understood and often ignored in this study we examined how common experimental treatments of sediment affect the burrowing behavior of the polychaete nephtys incisa and how these effects may interact with reactive organic matter distributions to alter diagenetic transport reaction balances sediment and animals were recovered from a subtidal site in central long island sound usa the upper 15 cm of the sediment was sectioned into sub intervals and each interval separately sieved and homogenized three initial distributions of sediment and organic substrate reactivity were setup in a series of microcosms 1 a reconstituted natural pattern with surface derived sediment overlying sediment obtained from progressively deeper material to a depth of 15 cm natural 2 a 15 cm thick sediment layer composed only of surface derived sediment rich and 3 a 15 cm thick layer composed of uniformally mixed sediment from the original 15 cm sediment profile averaged the two last treatments are comparable to that used in microcosms in many previous studies of bioturbation and interspecific functional interaction experiments sediment grain size distributions were 97 5 silt clay and showed no depth dependent patterns sediment porosity gradients were slightly altered by the treatments nepthys were reintroduced and aquariums were x rayed regularly over 5 months to visualize and quantify spatial and temporal dynamics of burrows the burrowing behaviour of adult populations having similar total biovolume biomass abundance and individual sizes differed substantially as a function of treatment burrows in sediment with natural property gradients were much shallower and less dense than those in microcosms with altered gradients the burrow volume biovolume ratio was also lower in the substrate with natural organic reactivity gradients variation in food resources or in sediment mechanical properties associated with treatments the latter in part coupled to remineralization processes such as exopolymer production may explain the burrowing responses in addition to demonstrating how species may respond to physical sedimentation events substrate homogenization and patterns of reactive organic matter redistribution these experiments suggest that infaunal species interactions in microcosms including the absolute and relative fluxes of remineralized solutes may be subject to artifacts depending on exactly how sediments are introduced experimentally nonlocal transport and cylinder microenvironment transport reaction models readily demonstrate how the multiple interactions between burrowing patterns and remineralization rate distributions can alter relative flux balances decomposition pathways and time to steady state

global scale quantification of mineralization pathways in marine sediments a reaction transport modeling approach 1 the global scale quantification of organic carbon corg degradation pathways in marine sediments is difficult to achieve experimentally due to the limited availability of field data in the present study a numerical modeling approach is used as an alternative to quantify the major metabolic pathways of corg oxidation cox and associated fluxes of redox sensitive species fluxes along a global ocean hypsometry using the seafloor depth sfd as the master variable the sfd dependency of the model parameters and forcing functions is extracted from existing empirical relationships or from the noaa world ocean atlas results are in general agreement with estimates from the literature showing that the relative contribution of aerobic respiration to cox increases from 80 in deep sea sediments sulfate reduction essentially follows an inversed sfd dependency the other metabolic pathways denitrification mn and fe reduction only adding minor contributions to the global scale mineralization of corg the hypsometric analysis allows the establishment of relationships between the individual terminal electron acceptor tea fluxes across the sediment water interface and their respective contributions to the corg decomposition process on a global average simulation results indicate that sulfate reduction is the dominant metabolic pathway and accounts for approximately 76 of the total cox which is higher than reported so far by other authors the results also demonstrate the importance of bioirrigation for the assessment of global species fluxes especially at shallow sfd most of the teas enter the sediments via bioirrigation which complicates the use of concentration profiles for the determination of total tea fluxes by molecular diffusion furthermore bioirrigation accounts for major losses of reduced species from the sediment to the water column prohibiting their reoxidation inside the sediment as a result the total carbon mineralization rate exceeds the total flux of oxygen into the sediment by a factor of 2 globally

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

numerical study of tidal circulation and nonlinear dynamics in lunenburg bay nova scotia 1 a three dimensional ocean circulation model is used to study the barotropic tidal circulation and nonlinear tidal dynamics in lunenburg bay nova scotia the model performance is first assessed using the observed bottom pressures and currents made in septembers of 1991 and 2003 the model results reproduce reasonably well the observed tidal circulation in the study region the simulated tidal circulation is dominated by the semidiurnal m2 tide and an intense jet like flow through a narrow channel connecting lunenburg bay and two shallow coves to demonstrate the importance of nonlinear tidal dynamics in the region the first two harmonics of m2 i e m4 and m6 are computed from the model results forced by the m2 tide at the model open boundaries both m4 and m6 are relatively large in western lunenburg bay and vicinity indicating the importance of the nonlinear tidal dynamics over these areas to quantify the role of various nonlinear terms in generating the intense jet and associated residual flows in the region of the jet both transient and tidally averaged time mean momentum equations are examined the transient and time mean momentum balances demonstrate that the nonlinear interaction of the tidal currents with local bathymetry is balanced primarily by the horizontal pressure gradients momentum advection and dissipation the coriolis effect plays a minor role in the study region

organic matter oxidation in deep sea sediments distribution in the sediment column and implications for calcite dissolution abstract this study uses pore water profiles of no3 and o2 from 34 deep sea study sites from five oceanic regions encompassing broad ranges of sediment composition and organic matter oxidation rate to analyze the depth distribution of organic matter oxidation within the sediment column while the results are consistent with those of hammond et al 1996 and hales 2003 the broader range of environmental variables considered in this study allows us to show that the depth of oxidation varies systematically with the integrated sedimentary organic matter oxidation rate the scale length for oxidation varies from large values of about 4 cm when the organic matter oxidation rate is 5 mol cm2 y to about 0 35 cm at oxidation rates above 20 mol cm2 y we have included this variability of organic matter oxidation depth in models of calcite dissolution our results show that variations in oxidation depth at a constant degree of supersaturation may result in changes in metabolic dissolution efficiency that are similar in magnitude to those resulting from basin wide variations in the degree of supersaturation with respect to calcite the model shows that when the ratio of sedimentary organic matter oxidation rate to calcite rain rate is held fixed at 0 8 mol mol metabolic dissolution is likely to be significant at all bottom water saturation levels and may result in dissolution of up to 50 of the calcite rain at the calcite saturation horizon

caco3 dissolution in california continental margin sediments the influence of organic matter remineralization in situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of caco3 on the continental rise adjacent to central california usa the porewater distributions and benthic fluxes of o2 no3 ta ca2 13c and tic are interpreted using a numerical simulation of organic matter remineralization and caco3 dissolution in marine sediments the processes considered in the simulation include organic matter oxidation by o2 no3 and so42 caco3 dissolution and precipitation hs and nh4 oxidation and sediment mixing and sediment accumulation calculated benthic fluxes of o2 no3 ta tic 13c and ca2 porewater concentrations of o2 no3 and nh4 and sediment distributions of organic carbon caco3 excess 210pb and 14c agree well with the measurements benthic fluxes of alkalinity and inferred caco3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation if the influence of metabolically produced co2 is included benthic fluxes are fully reconciled however this is in agreement with benthic chamber ca2 and 13c results that independently imply substantial caco3 dissolution in these sediments the above observations are in contrast to those reported by jahnke et al 1994 for the west african continental rise and the western equatorial pacific where 1 g diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers we conclude that the extent of metabolic caco3 dissolution may vary regionally numerous factors such as the depth of metabolic co2 production and caco3 dissolution kinetics are known or predicted to influence metabolic dissolution among the factors that should be considered in reconciling these observations are l the extent to which sulfate reduction and reoxidation reactions may influence acid base properties in surface sediments and 2 the total sedimentary caco3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface

sedimentary organic matter preservation a test for selective degradation under oxic conditions the authors report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water sediment interface oxygen penetration depths organic element compositions and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the washington continental shelf slope and adjacent cascadia basin individual amino acid sugar and pollen distributions were analyzed for a 11 to 12 cm horizon from each core and sup 14 c based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites sediment accumulation rates decreased and dissolved o sub 2 penetration depths increased offshore along the sampling transect as a result oxygen exposure times oet increased seaward from decades mid shelf and upper slope to more than a thousand years outer cascadia basin organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location in particular organic carbon surface area ratios decreased progressively offshore and with increasing oet three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded both concentrationmore and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible oxic effect oet helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere less

biogeochemical impact of submarine ground water discharge on coastal surface sands of the southern baltic sea this study evaluates the effect of submarine ground water discharge sgd on biogeochemical processes of sandy sediments of hel bight poland in the shallow southern baltic sea using stirred benthic chambers combined to seepage meters deep pore water profiles and a reactive transport model the main impacts of fresh anoxic groundwater seepage are due to 1 the efflux of methane 2 the efflux of phosphate and silicate 3 the efflux of dissolved organic carbon doc of aquifer origin methane from sgd is assumed to be only slightly oxidized within the sediments and potentially reach the atmosphere at a maximum rate of 30 mmol ch4 m 2 d 1 silicate and phosphate supplied by sgd promote a seep site net community production rate that is more than twice as compared to adjacent non seeping sites 70 and 30 mmol c m 2 d 1 respectively however oxygen uptake rates at the seep site during the night 30 mmol o2 m 2 d 1 are lower than those observed at the reference sites 50 mmol o2 m 2 d 1 we hypothesize that autogenic relatively labile doc is available at the reference site leading to higher oxygen uptake rates as compared to the seep sites where it is being replaced by less reactive doc originating from the ground water

coupled organic and inorganic carbon cycling in the deep subseafloor sediment of the northeastern bering sea slope iodp exp 323 abstract we studied microbially mediated diagenetic processes driven by carbon mineralization in subseafloor sediment of the northeastern bering sea slope to a depth of 745 meters below seafloor mbsf sites u1343 u1344 and u1345 were drilled during integrated ocean drilling program iodp expedition 323 at water depths of 1008 to 3172 m they are situated in the high productivity green belt region with organic carbon burial rates typical of the high productivity upwelling domains on western continental margins the three sites show strong geochemical similarities the downward sequence of microbially mediated processes in the sediment encompasses 1 organoclastic sulfate reduction 2 anaerobic oxidation of methane aom coupled to sulfate reduction and 3 methanogenesis the sediment contains two distinct zones of diagenetic carbonate formation located at the sulfate methane transition zone smtz and between 300 and 400 mbsf the smtz at the three sites is located between 6 and 9 mbsf the upward methane fluxes into the smtz are similar to fluxes in smtzs underlying high productivity surface waters off chile and namibia our bering sea results show that intense organic carbon mineralization drives high ammonium and dissolved inorganic carbon dic production rates 4 2 mmol m 3 y 1 in the uppermost 10 mbsf and strongly imprints on the stable carbon isotope composition of dic driving it to a minimum value of 27 vpdb at the smtz pore water calcium and magnesium profiles demonstrate formation of diagenetic mg rich calcite in the smtz below the smtz methanogenesis results in 13c enrichment of pore water dic with a maximum value of 11 9 the imprint of methanogenesis on the dic carbon isotope composition is evident down to a depth of 150 mbsf below this depth slow or absent microbially mediated carbon mineralization leaves dic isotope composition unaffected ongoing carbonate formation between 300 and 400 mbsf strongly influences pore water dic and magnesium concentration profiles the linked succession of organic carbon mineralization and carbonate dissolution and precipitation patterns that we observe in the bering sea slope sediment may be representative of passive continental margin settings in high productivity areas of the world s ocean

elemental and major biochemical changes across an oxidation front in a relict turbidite an oxygen effect abstract the elemental and major biochemical compositions of the relict f turbidite sampled in two cores from the madeira abyssal plain were determined this fine grained distal sequence occurs at ca 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change oxygen diffused downward through sediments above this interface and in ca 10 kyr destroyed 80 of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca 140 kyr these deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates in both cores a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers the unoxidized sequences of both turbidities contained 0 93 1 02 wt organic carbon oc and 0 10 0 11 wt total nitrogen tn approximately 20 of the initial oc and 40 of the initial tn remained in the oxidized horizons with a consequent decrease in atomic c n ratio from ca 11 to ca 5 all samples gave very low yields of lignin phenols and comparable oc normalized yields of total aldoses and amino acids and indicated predominantly marine organic matter om and nonselective oxic degradation of these biochemical classes compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments except that the oxidized horizons yielded markedly elevated 3 5x percentages of nonprotein amino acids this study clearly demonstrates that prolonged exposure to oz can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone in comparison to early diagenesis however alteration of the measured biochemicals was largely nonselective such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep ocean environments

a coupled ice ocean modeling study of the northwest atlantic ocean 1 a coupled ice ocean modeling system is developed for the northwest atlantic ocean based on the second version of the los alamos sea ice model and a regional ocean circulation model the coupled ice ocean system differs from other coupled systems for the same region mainly in two ways first the semi prognostic method suggested by sheng et al 2001 is used in the ocean component this method adjusts the momentum equation of the ocean component to reduce drift of the modeled ocean state allowing us to carry out a multiyear simulation second the sea ice component uses the elastic viscous plastic ice rheology developed by hunke and dukowicz 1997 and winton s 2000 three layer thermodynamics the coupled system is forced by climatological monthly mean atmospheric forcing at the atmosphere ocean atmosphere ice interface and oceanic forcing at the model open boundaries the system is integrated for 3 years model results from the third year compare favorably with the observations in the region the coupled system reproduces reasonably well the phase and magnitude of the annual cycle of sea ice we demonstrate the effect of the ice heat capacity previously unaccounted for in earlier model results of this region in delaying the springtime sea ice melt on the labrador and newfoundland shelves

the influence of organic matter diagenesis on caco3 dissolution at the deep sea floor abstract in situ benthic flux chamber measurements were performed at seafloor locations in the atlantic and pacific oceans where the bottomwaters were supersaturated approximately saturated and undersaturated with respect to calcite there was no evidence of significant dissolution at or above the saturation horizon below the saturation horizon however significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution where observed the rate of dissolution is consistent with a rate expression of reaction order 4 5 and a rate constant in the range of 0 05 to 0 5 per day a numerical simulation was constructed to represent organic matter and caco3 diagenesis at the seafloor at the supersaturated and near saturation sites the model over predicts the influence of metabolically produced co2 on the extent of caco3 dissolution it is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments the primary factors to be considered include 1 the oxidation state of the carbon being oxidized 2 the vertical distribution of oxidation rates including the role of doc in sediment remineralization 3 the influence of nondiffusive transport of near surface porewaters and 4 the influence of episodic depositional events on organic and inorganic carbon cycling

the global ocean flux of particulate organic carbon areal distribution and magnitude the magnitude and distribution of the particulate organic carbon poc rain rate to the seafloor in the atlantic pacific and indian ocean basins between 61 n and 61 s has been estimated from benthic oxygen flux estimates for water depths 1000 m only the calculation utilizes the extensive data sets of sedimentary organic carbon caco3 and accumulation rate to extrapolate between individual benthic flux measurement sites using an empirically derived correlation between the seafloor oxygen flux and these parameters the poc flux through the 1000 m depth horizon was then estimated from published correlations between sediment trap determined fluxes and water depth total oxygen utilization in the deep ocean is estimated to be 1 2 1014 mol o2 yr 1 a value that agrees well with previous estimates which were based on surface water primary productivity sediment trap and depth relationships and with deep water respiration rates estimated from apparant oxygen utilization aou 14c relationships on the basis of the derived global ocean flux distribution it is concluded that 1 dissolved organic carbon doc inputs are not required to account for estimated deep water respiration rates 2 the majority of the poc input to the deep ocean occurs within 30 of the equator 3 the proportion of primary production that reaches the deep sea does not vary greatly with latitude 4 gyre and continental margin regions contribute roughly equally to the deep poc flux with a relatively minor contribution from the equatorial divergence region 5 of the estimated 7 2 1013 mol c yr 1 of poc that sinks below the 1000 m depth horizon 45 3 3 1013 mol c yr 1 reaches the seafloor where it is oxidized 6 when normalized to basin area average deep flux rates in the atlantic and pacific are similar while highest rates are observed in the indian ocean and 7 the results can be fully reconciled only if the benthic flux of doc is significantly less than the benthic o2 flux

instability of a buoyancy driven coastal jet the gaspe current and its st lawrence precursor the discharge of the st lawrence river drives a buoyant jet which extends along the south shore of the lower st lawrence estuary and combining with the northwest gulf of st lawrence cyclonic gyre becomes the gaspe current in both the estuarine jet and the gaspe current strong lateral and vertical shears are present satellite thermal imagery has shown that the gaspe current is unstable its instabilities are manifested as prominent wave disturbances and eddy features we use satellite thermal images from 3 different years to show that unstable disturbances are also characteristic of the estuarine jet we update previous work to show that gaspe current is both baroclinically and barotropically unstable our model studies also show that this conclusion is valid for the estuarine jet as well finally we discuss a sequence of satellite images showing the development of an unstable disturbance apparently triggered by an eastward wind pulse

quantification of fluvial wood using uavs and structure from motion abstract fluvial wood is recognized as one of the key elements of riverine environments its importance role and potential related risks are widely considered and analysed still to characterize and measure wood elements in the field a large investment in terms of time and cost is normally required these activities are often carried out in complex and potentially dangerous environments in this context the use of unmanned aerial vehicles uavs provides a powerful alternative technology to traditional approaches in this work we propose a novel approach combining uav surveys with structure from motion sfm photogrammetry to quantify fluvial wood the proposed methodology was tested in the blanco river southern chile which was severely affected by the chaiten volcanic eruption in 2008 that delivered large quantities of wood to the channel an area of about 1 2 ha was surveyed using uav and field measurements a comparative analysis between the traditional field based and uav surveys was carried out to quantify the accuracy of the latter and to better understand the strengths and potential limitations of the proposed methodology results showed that differences between the two methodologies were about 13 for both single large wood elements and wood jam volumes moreover a sensitivity analysis applied to 12 different scenarios allowed us to define the measurement precision achieved using the orthomosaics and dems our results showed that the proposed approach is effective in terms of field and laboratory activities saving around half the time this study demonstrates that the combination of uav and sfm can yield accurate results covering large areas in less time and providing a powerful tool for performing repeated surveys and thus acquiring relevant information for wood budgets and wood dynamics in rivers

cross front mixing and frontal upwelling in a controlled quasi permanent density front in the gulf of st lawrence abstract ctd data obtained from three transects across a controlled quasi permanent density front in the gulf of st lawrence were analyzed for the purpose of investigating cross front mixing mechanisms for frontal convergence secondary circulation induced by the front and relationship between surface mixed layer properties and frontal structure water mass analysis indicates that mixing takes place mainly in the ambient water from the lower boundary of the frontal layer down to 100 m on the side of heavier water there is a region of low surface temperature the water masses have a distribution suggestive of upwelling in the low surface temperature region the thickness of the surface mixed layer varies across he front outside the frontal zone there is a well developed mixed layer of a thickness of about 25 m it disappears completely in the low surface temperature zone and is re established in the frontal layer with a reduced thickness horizontal intrusions below the frontal layer and interleavin

diffusive transfer of oxygen from seamount basaltic crust into overlying sediments an example from the clarion clipperton fracture zone highlights geochemical evidence confirms exchange between sediments and the basaltic crust fluids o2 exchange between the sediment and the basaltic crust produces a fully oxic sediment column transport reaction modeling was performed to quantify the exchange of o2 and nutrients abstract the clarion clipperton fracture zone ccfz in the pacific ocean is characterized by organic carbon starved sediments and meter scale oxygen penetration into the sediment furthermore numerous seamounts occur throughout its deep sea plain which may serve as conduits for low temperature hydrothermal seawater circulation through the oceanic crust recent studies in deep sea environments of the pacific and atlantic oceans have suggested and presented evidence of dissolved constituent exchange between the seawater flowing in the basaltic crust and the pore water of the overlying sediments through high resolution pore water oxygen and nutrient measurements we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of teddy bare a small seamount in the ccfz at three sites located 1000 700 and 400 m away from the foot of the seamount we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m toward the basaltic basement nitrate view the mathml sourceno3 concentrations mirror the oxygen concentration profiles as they increase with sediment depth but decrease towards the basement these profiles suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments and a downward diffusion of view the mathml sourceno32 from sediment pore water into the basaltic crust at one site we determined that the 87sr 86sr ratios of the bottom water and of the deep sediment near the basaltic crust are similar further supporting diffusive exchange between basaltic crust fluids and sediment pore water transport reaction modeling performed at two of the study sites revealed that 1 the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and 2 the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders of magnitude lower rates than between the upper sediment and the overlying bottom water these results further show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount

organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source we compared the burial efficiency of organic carbon buried oc deposited oc in a diverse set of 27 different sediments from 11 lakes focusing on the potential effects of organic matter source oxygen exposure and protective sorption of oc onto mineral surfaces average oc burial efficiency was high mean 48 and it was particularly high in sediments receiving high input of allochthonous organic matter mean 67 further oc burial efficiency was strongly negatively related to the oxygen exposure time again particularly so in sediments receiving high allochthonous loads on the other hand oc burial efficiency was not related to the mineral surface area which was used as a proxy of the sorption capacity of the mineral phase for oc the high oc burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes as well as to a strong dependence of oc burial efficiency on oxygen exposure time this study demonstrates that the carbon sink in lake sediments alters the oc export from the continents to the sea and that the fate of oc in lake sediments burial vs mineralization to carbon dioxide and methane is highly sensitive to environmental conditions

settling dissolution and burial of biogenic silica in the sediments off somalia northwestern indian ocean abstract particle fluxes of biogenic silica through the water column silica burial fluxes into the sediments and the flux of dissolved silica across the sediment water interface estimated from pore water profiles are used to assess the behaviour of biogenic silica at two stations 80 and 270 km offshore along a transect off the somali coast in the northwestern indian ocean particulate biogenic silica fluxes varied from 0 3 mmol m 2 day 1 in the non upwelling season to 6 mmol m 2 day 1 during upwelling on the somali slope fluxes were lower in the somali basin from 0 2 to 2 3 mmol m 2 day 1 evaluation of the dissolution curves derived by wet chemical leaching in sediment trap and sediment samples shows that the km values the apparent reactivity rates in alkaline medium are higher for the shallow sediment traps than for deep trap and boxcore sediments modelling of pore water profiles shows that in the sediment most dissolution occurs in the top halfcentimetre and pore water effluxes are in close agreement with those from in situ benthic incubations our results show that less than 10 of the biogenic silica arriving on the somali margin is buried in the sediments giving a burial efficiency lower than the approximately 20 reported from the open arabian sea

the distribution of large woody debris accumulations and pools in relation to woodland stream management in a small low gradient stream this paper focuses upon the natural dynamics of large woody debris lwd the impact of management on lwd dynamics and the impact of lwd removal and channelization on the distribution and size of pools in a british second to third order headwater catchment the study stream is rather different from those subject to lwd accumulations which have been studied in north america the most important contrast is that it is surrounded by predominantly deciduous rather than coniferous woodland in terms of its width 1 8 4 5 m and gradient 0 013 m m 1 it falls within the lower range of channels studied in north america nevertheless there are similarities in lwd dam and pool spacing with some north american studies the information on lwd dynamics during a period without management and on recovery of lwd dams after clearance covers a 16 year period 1982 1997 the paper illustrates that seven to eight years after clearance the total number of lwd dams has recovered but the most hydraulically active dam type has not recovered to pre clearance levels an analysis of geomorphological maps of the channel surveyed in 1982 and 1996 97 shows an overall decrease in the number and size of pools along the section that was cleared of lwd dams the magnitude of the decrease and the associated adjustments in pools through changes in their size and location differ according to location with respect to a section of the study stream which was channelized in c 1966 and which has subsequently incised its bed copyright 1998 john wiley sons ltd

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

river forced estuarine plumes abstract the development maintenance and dissipation of river forced estuarine plumes with and without seaward sloping bottom are studied by use of a three dimensional primitive equation model inside the estuary discussion is focused on how the coriolis force induces lateral asymmetries in the circulation four physical processes dictate the transient as well as the quasi steady circulation in moderately stratified estuaries 1 upper layer convergence during the transient spinup phase 2 upward entrainment leading to the two layer steady circulation pattern 3 upward stretching of the lower layer vortex during the spindown period and 4 in the presence of a seaward bottom slope the left bounded tendency of the landward undercurrent for all phases of development deviations from the laterally averaged circulation pattern caused by these processes are discussed over the shelf various types of plumes are defined according to the visual appearance of the surface salinity field the four types of p

transport and deposition of large woody debris in streams a flume experiment abstract large woody debris lwd is an integral component of forested streams of the pacific northwest and elsewhere yet little is known about how far wood is transported and where it is deposited in streams in this paper we report the results of flume experiments that examine interactions among hydraulics channel geometry transport distance and deposition of floating wood these experiments were carried out in a 1 22 m wide 9 14 m long gravel bed flume using wooden dowels of various sizes as surrogate logs channel planforms were either self formed or created by hand and ranged from meanders to alternate bars floating pieces tended to orient with long axes parallel to flow in the center of the channel pieces were deposited where channel depth was less than buoyant depth typically at the head of mid channel bars in shallow zones where flow expanded and on the outside of bends we hypothesize that the distance logs travel may be a function of the channel s debris roughness a dimensionless index incorporating ratios of piece length and diameter to channel width depth and sinuosity travel distance decreased as the ratio of piece length to both channel width and radius of curvature increased but the relative importance of these variables changed with channel planform large pieces can move further than our debris roughness models predict if greater than 50 of the active channel area is deeper than the buoyant depth of the piece or if momentum is high enough to carry pieces across shallows our debris roughness model allows first order prediction of the amount of wood transport under various channel geometries

early diagenetic remineralization of sedimentary organic c in the gulf of papua deltaic complex papua new guinea net loss of terrestrial c and diagenetic fractionation of c isotopes oceania supplies 40 of the global riverine flux of organic carbon approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems the gulf of papua on the south coast of the large island of new guinea is one such deltaic clinoform complex it receives 4 mt yr 1 particulate terrestrial organic carbon with initial particle corg loading 0 7 mg m 2 corg loading is reduced to 0 3 mg m 2 in the topset upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus and high net sediment accumulation rates of 1 4 cm yr 1 carbon isotopic analyses 13c 14c of co2 and corg demonstrate rapid 100 yr remineralization of both terrestrial 13c 28 6 and marine corg 13c 20 5 ranging in average age from modern bomb 14c 60 to 1000 yr 14c 140 efficient and rapid remineralization in the topset upper foreset zone is promoted by frequent physical reworking bioturbation exposure and reoxidation of deposits the seafloor in these regions particularly 20 m apparently functions as a periodically mixed suboxic batch reactor dominated by microbial biomass although terrestrial sources can be the primary metabolic substrates at inshore sites relatively young marine corg often preferentially dominates pore water co2 relative to bulk corg in the upper foreset thus a small quantity of young rapidly recycled marine organic material is often superimposed on a generally older less reactive terrestrial background whereas the pore water co2 reflects both rapidly cycled marine and terrestrial sources terrestrial material dominates the slower overall net loss of corg from particles in the topset upper foreset zone i e recycled marine corg leaves little residue preferential utilization of corg subpools and diagenetic fractionation of c isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics early diagenetic fractionation of c isotopes relative to the bulk sedimentary corg composition can produce changes in 14c activity independent of radioactive decay in the gulf of papua topset upper foreset 14c of pore water co2 averaged 300 greater than corg sediment between 1 3 m depth in deposits diagenetic fractionation and decomposition aging of sedimentary corg compromises simple application of 14c for determination of sediment accumulation rates in diagenetically reactive deposits

fluid inclusion and stable isotopic constraints on fluid sources and evolution of the luojiahe cu deposit in the southern margin of the north china craton abstract the luojiahe cu deposit in the zhongtiaoshan region is located in the southern margin of the north china craton the orebodies are hosted in the mafic volcanic sedimentary sequences of the metamorphosed greenschist facies neoarchean songjiashan group the luojiahe cu mineralization can be divided into the primary volcanogenic massive sulfide vms mineralization stage stage i banded or stockwork ores and the subsequent metamorphic remobilization stage stage ii coarse vein ores three types of quartz selected for fluid inclusion fi studies were collected from the stage i banded q1 and stockwork q2 ores and stage ii coarse vein q3 ores four types of fis were identified 1 liquid rich fis l type 2 pure vapor and vapor rich fis v type 3 daughter mineral bearing fis s type and 4 ch4 h2o fis c type systematical microthermometric and h o isotopic studies show that the stage i ore forming fluids consist predominantly of high salinity evolved seawater 125 220 c 23 9 27 9 wt nacl equiv and some magmatic hydrothermal fluids 249 339 c 34 5 42 2 wt nacl equiv the two fluid end members are represented by the l type fis in q1 and the s and v type fis in q2 the temperature and salinity variation trends of the l type fis in q1 indicate a mixing process between the hot evolved seawater and cold seawater at stage i furthermore the v and s type fi coexistence in q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at stage i in contrast the stage ii ore forming fluids consist of ch4 rich metamorphic fluids 192 350 c 10 6 43 2 wt nacl equiv carbon isotopic analysis of the stage ii calcite 4 58 to 10 83 and graphite 32 01 to 39 16 in the ore hosting chlorite schist indicates that the metamorphic ore forming fluids had exchanged carbon isotope with graphite the generation of ch4 may have resulted from the interaction between h2o released by metamorphic devolatilization and graphite the continuous consumption of h2o in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the ch4 nacl h2o system in addition the vms metallogenic environment is generally favorable for microbial communities it is considered that the graphite at luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems as also supported by carbon isotopic data we propose that at stage i the main mineralization may have been resulted from 1 fluid mixing of hot evolved seawater and cold seawater in the near surface environment and 2 fluid unmixing caused by the percolation of magmatic fluids into syn volcanic faults forming the stockwork ores at stage ii the interaction between h2o and graphite may have resulted in the reduction of ore forming fluids and cu precipitation and fluid unmixing in the ch4 nacl h2o system may have further promoted the cu mineralization

patterns and processes of wood debris accumulation in the queets river basin washington field surveys in the 724 km 2 queets river basin on the west slope of the olympic mountains in nw washington reveal basin wide patterns of distinctive wood debris wd accumulations that arise from different mechanisms of wd recruitment hydraulic geometry and physical characteristics of wd individual pieces of wd in an accumulation or jam can be separated into key racked and loose members ten types of wd accumulations are identified based on the mode of recruitment and the orientation of key racked and loose debris relative to the channel axis although some types of wd accumulation have few geomorphic effects others form stable in stream structures that influence alluvial morphology at both subreach and reach length scales ranging from less than 1 to greater than 10 channel widths in the queets river stable accumulations of wd directly influence channel anabranching planform geometry flood plain topography and establishment of long term riparian refugia for old growth forest development the classification of wood debris accumulations in the queets river basin is based on physical observations that offer a template potentially applicable to other forested mountain regions d 2002 elsevier science b v all rights reserved

a high toc shale in a low productivity world the late mesoproterozoic arctic bay formation nunavut abstract the latest mesoproterozoic arctic bay formation borden basin nunavut canada is up to 1130 m thick and contains a significant proportion of unusually organic rich black shale up to 12 3 wt total organic carbon insofar as increased biological productivity is related to organic matter burial this organic rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the proterozoic to better understand the conditions leading to development of this organic rich unit we explore the redox geochemistry of the arctic bay formation using a multi proxy approach nitrogen isotopes iron speciation total organic carbon total sulphur and trace metal abundances redox proxy data support a stratified water column with oxic surface waters underlain by intermittently euxinic waters which are in turn underlain by persistently ferruginous deeper waters the highly alkaline restricted marine basin in which the arctic bay formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals resulting in an excess of sulphur that resulted in sulphurisation of organic matter estimates for organic matter burial rates during deposition of the arctic bay formation suggest that they were perhaps 5 6 times mid proterozoic average values although there are permissible scenarios in which it was extremely productive underscoring that such organic rich sedimentary rocks could be produced in a low productivity world

numerical investigation of baroclinic instability in the gaspe current using a frontal geostrophic model the gaspe current situated in the western gulf of st lawrence is an intense buoyancy driven jet which is fed by freshwater runoff from the st lawrence river this coastal current is known to exhibit frequent episodes of instability culminating in backward breaking waves as well as filaments and eddies we investigate the observed mesoscale variability in a series of numerical simulations employing a two layer frontal geostrophic model that focuses on baroclinic dynamics unlike previous studies we allow for large amplitude variations of the interface depth with the possibility of outcroppings as well as two way coupling of the dynamic pressures in each layer it is found that the growth rates and length scales associated with the early evolution of disturbances compare favorably with those observed moreover the mesoscale features resulting from highly nonlinear interactions closely resemble those seen in satellite data

temperature dependence of organic matter remineralization in deeply buried marine sediments abstract assuming that the temperature dependence of sediment organic matter remineralization can be described by the arrhenius equation organic matter that is highly refractory at seafloor temperatures 2 3 c should become more reactive at sediment depths of several hundred meters due to burial and heating by the natural geothermal gradient results obtained using a coupled non linear reactive transport model support this suggestion for deeply buried marine sediments i e those found 100s of meters below the seafloor model results predict the occurrence of a deep zone of methanogenesis that is separated by a relatively thick region in which methane diffuses upwards to be oxidized by downward diffusing sulfate this depth zonation of biogeochemical processes in deeply buried sediments is in sharp contrast to that observed in nearshore marine sediments where sulfate reduction and methanogenesis generally occur in much closer vertical proximity model results presented here also provide a simple mechanistic explanation for the occurrence of linear pore water sulfate profiles that are common in many continental margin sediments linear sulfate gradients are often taken as an indirect indicator of gas hydrate occurrence although results presented here suggest that this does not necessarily always have to be the case application of the model to gas hydrate containing sediments on the blake ridge yields results that are in good agreement with sediment and pore water data and microbial rate studies in these sediments model results also suggest that in situ methanogenesis in and or near the gas hydrate stability zone may play an important role in supplying the methane found in blake ridge gas hydrates

evolution of organic matter degradation in cretaceous black shales inferred from authigenic barite a reaction transport model abstract a reaction transport model was used to infer the long term evolution of anaerobic organic matter degradation in cretaceous black shales from the distribution of authigenic barite in sediments drilled at demerara rise odp leg 207 site 1258 in these sediments sulfate reduction and methanogenesis are the major pathways of organic matter decomposition and the depth distribution of authigenic barite serves as an indicator for the temporal evolution of the sulfate methane transition zone smtz the strength of the biogenic methane flux and ultimately the organic matter reactivity in the black shales over geological timescales organic matter degradation is described according to the reactive continuum model approach and parameters values are determined by inverse modeling based on present day porewater and authigenic barite profiles fully transient simulations were performed over a period of 100 myrs and indicate that important features of the biogeochemical dynamics are associated to changes in the boundary forcing hiatuses in sediment accumulation rate result in quasi steady state conditions and lead to distinct accumulations of authigenic barites in the smtz the inversely determined parameters reveal that the reactivity of the organic matter was already low apparent first order rate constant k 10 4 year 1 10 6 year 1 at the time of its deposition in the cretaceous the geochemical characteristics of sediments drilled at demerara rise as well as the presence of specific biomarkers suggest that this low reactivity is most likely due to the euxinic palaeo conditions which favored the sulfurization of the organic matter simulation results predict average initial organic carbon contents between 8 1 and 9 5 wt implying a high preservation efficiency of the organic matter between 79 and 89 calculated mass accumulation rates between 0 43 and 0 5 g c m 2 year 1 compare well with estimations for the western basin of the cretaceous southern north atlantic simulation results thus indicate that the enhanced preservation of organic matter under euxinic conditions may have been the main cause for the formation of organic rich cretaceous black shales at demerara rise

the floodplain large wood cycle hypothesis a mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the north pacific coastal ecoregion abstract a floodplain large wood cycle is hypothesized as a mechanism for generating landforms and influencing river dynamics in ways that structure and maintain riparian and aquatic ecosystems of forested alluvial river valleys of the pacific coastal temperate rainforest of north america in the cycle pieces of wood large enough to resist fluvial transport and remain in river channels initiate and stabilize wood jams which in turn create alluvial patches and protect them from erosion these stable patches provide sites for trees to mature over hundreds of years in river valleys where the average cycle of floodplain turnover is much briefer thus providing a future source of large wood and reinforcing the cycle different tree species can function in the floodplain large wood cycle in different ecological regions in different river valleys within regions and within individual river valleys in which forest composition changes through time the cycle promotes a physically complex biodiverse and self reinforcing state conversely loss of large trees from the system drives landforms and ecosystems toward an alternate stable state of diminished biogeomorphic complexity reestablishing large trees is thus necessary to restore such rivers although interactions and mechanisms may differ between biomes and in larger or smaller rivers available evidence suggests that large riparian trees may have similarly fundamental roles in the physical and biotic structuring of river valleys elsewhere in the temperate zone

significant role of organic sulfur in supporting sedimentary sulfate reduction in low sulfate environments abstract dissimilatory sulfate reduction dsr is a major carbon mineralization pathway in aquatic sediments soils and groundwater which regulates the production of hydrogen sulfide and the mobilization rates of biologically important elements such as phosphorus and mercury it has been widely assumed that water column sulfate is the main sulfur source to fuel this reaction in sediments while this assumption may be justified in high sulfate environments such as modern seawater we argue that in low sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate using a reaction transport model we investigate the production of sulfate from sulfur containing organic matter for a range of environments the results show that in low sulfate environments 50 of sulfate reduction in well oxygenated systems porewater sulfate profiles often exhibit sub interface peaks so that sulfate fluxes are directed out of the sediment our measurements in lake superior the world s largest lake corroborate this conclusion offshore sediments act as sources rather than sinks of sulfate for the water column and sediment dsr is supported entirely by the in sediment production of sulfate sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter rate co regulation by sulfate availability becomes appreciable below 500 m level the results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low sulfate environments including several of the world s largest freshwater bodies deep subsurface and possibly the sulfate poor oceans of the early earth

assessment of ba ca in arctica islandica shells as a proxy for phytoplankton dynamics in the northwestern atlantic ocean abstract despite its major role as a global climate regulator little is known about the recent evolution of the north atlantic ocean especially prior to the fifties principally because of the lack of long term instrumental data moreover the north atlantic ocean is undergoing rapid changes at the physical scale leading to modifications at the biological scale increasing interest to monitor the environment the phytoplankton is the base of the ocean life and its perturbation can lead to further changes in the food chain recording its dynamics implies to observe both its spatial and temporal variations this study brings forward the use of the ba ca ratio from shells of the long living bivalve arctica islandica collected in the northwestern atlantic ocean near saint pierre and miquelon spm as a recorder of the past and present phytoplankton dynamics a high inter and intra reproducibility and synchrony between ba cashell profiles were demonstrated allowing the reconstruction of a 124 years annually resolved ba cashell master chronology from 1893 to 2016 this master chronology was positively correlated to the surface chlorophyll a measured by satellite sensor at regional around spm and global scales northwestern atlantic

an intermediate nepheloid layer associated with high microbial metabolic rates and denitrification in the northwest indian ocean extensive optical physical chemical and biochemical measurements made simultaneously in the northwest indian ocean reveal the occurrence of an intermediate nepheloid layer inl invariably associated with the secondary nitrite maximum maxima in particulate protein and the activity of the respiratory electron transport system ets are also found within the inl since the inl persists long distances from the continental margin with an offshore intensification it may not be related to the transport of material resuspended along the continental margin an apparent correlation of the inl with the previously reported subsurface maximum in bacterial abundance suggests that a local increase in the abundance of bacteria may be responsible for the increased turbidity positive correlations of the beam attenuation anomaly with nitrite and nitrate deficit suggest that most of these bacteria may be denitrifiers the organic carbon demand within the denitrifying layer computed from the observed ets activity appears to be severalfold higher than the sinking carbon fluxes to the denitrifying layer requiring additional modes of supply of the biodegradable organic matter it is proposed that a bacterial maximum could be maintained with efficient utilization of the dissolved organic matter within the denitrifying waters

diagenesis of organic matter in a 400 m organic rich sediment core from offshore namibia using solid state 13c nmr and ftir abstract although rates and mechanisms of early diagenesis have been well studied the effects of microbial metabolism on the molecular composition of the sedimentary organic matter som over long periods of time need more investigation in this study we characterize the early diagenesis of marine som from organic rich sediments of the ocean drilling program site 1082 located off namibia in the vicinity of the benguela coastal upwelling system we used both fourier transform infrared spectroscopy ftir and nuclear magnetic resonance spectroscopy 13 c nmr to assess the quantitative partitioning of the organic carbon into major compound classes aliphatic aromatic ester carboxylic amide and carbons from carbohydrates then we calculate the som composition in the main biomolecules proteins carbohydrates lipids and lignin on the basis of previous 13 c nmr based estimates of the molecular composition of the organic mixtures results show that the som is still labile at 7 m below the seafloor mbsf and composed of about 25 proteins and 15 carbohydrates with increasing depth the protein content exponentially decreases to 13 at 367 mbsf whereas the carbohydrate content decreases linearly to 11 the lignin and lipid content consistently represent around 10 and 40 of the som respectively and show an increase with depth due mostly to selective enrichment as the more labile components are lost by degradation thus these components of the som are considered refractory at the depth scale considered the calculated remineralization rates are extremely slow ranging from 5 6 mol c m 3 ky 1 at the top of the core to 0 2 mol c m 3 ky 1 according to the organic carbon flux to the seafloor knowing the labile carbon losses we propose a method to calculate the initial toc before the diagenesis took place

oxidative sulfide dissolution on the early earth recent evidence suggests that the biological production of oxygen in earth s surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more however the potential effects of early oxygen production on surface ocean chemistry have remained little explored and questions persist regarding the locus of oxidation of crustal material i e subaerial and or submarine settings here we revisit the notion of spatially restricted oxygen oases in the archean surface ocean by employing a simple steady state box model of the surface ocean in a coastal upwelling system using pyrite as an example we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere we find that it is possible to establish strong air sea gas exchange disequilibrium with respect to o 2 however in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering transport in the subaerial realm despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric po 2 relative to those attainable in productive regions of the surface ocean however broad scale subaerial weathering of sulfides would likely have required either local o 2 production at the site of weathering or transient increase in atmospheric po 2 above the most plausible levels characteristic of a pervasively reducing atmosphere

the evolution of early diagenetic signals in bering sea subseafloor sediments in response to varying organic carbon deposition over the last 4 3 ma abstract transient pore water and solid phase signatures in deep subseafloor marine sediments resulting from changes in both the amount and the quality of the organic matter input are common but often difficult to interpret we combined high resolution pore water and solid phase data from integrated ocean drilling program iodp expedition 323 site u1341 bowers ridge bering sea with inverse reaction transport modeling to examine the evolution and potential preservation of diagenetic signals in these deep subseafloor sediments we explore how these signals reflect major changes in the deposition and reactivity of organic matter to the seafloor at bowers ridge results of the inverse model approach reveal that 2 51 2 58 ma ago a high deposition flux of extremely labile organic matter probably linked to increased surface water primary productivity affected this site associated elevated organoclastic sulfate reduction rates facilitated low sulfate concentrations the onset of methanogenesis and consequently sulfate reduction coupled to the anaerobic oxidation of methane aom sulfate depletion caused the dissolution of biogenic barite reflected by a sedimentary interval with low ba al ratios two sulfate methane transition zones smtzs evolved where high rates of aom controlled sulfate consumption which was sustained by the influx of sulfate from seawater above and a deep source below the positions of both smtzs shifted non synchronously over the subsequent 130 000 yrs until methanogenesis and aom declined the present day sulfate concentration and sulfur isotope profiles still reflect the impact of the reactive organic matter pulse they also record a period of very low reactive organic matter deposition during the middle to late pleistocene probably linked to very low primary productivity resulting in little microbial carbon turnover in the sediment our study shows that combining biogeochemical signatures recorded in the solid phase of deep subseafloor sediments with the analysis of transient pore water signals by inverse reaction transport modeling yields new insights into past deep biosphere processes and the paleoproductivity of marine basins

an autumn instability event in the gasp aacute current abstract the gaspe current is a buoyant jet driven primarily by the freshwater discharge from the st lawrence estuary previously many incidents of unstable wave development in the3 gaspe current have been reported under summer high runoff conditions here we present satellite infrared images and discuss the occurrence and evolution of an autumn episode of current instability in the fall runoff is lower this event is notable in that the pattern observed differs considerably from that of summer events apparently due to the offshore shift that of the core of the jet in early autumn a stability analysis suggests that the gaspe current instabilities may grow slightly faster in autumn conditions the possible role of wind forcing in triggering wave growth is discussed

wood storage within the active zone of a large european gravel bed river wood storage within the active zone of the dynamic gravel bed fiume tagliamento italy was investigated at eight sites along the river s main stem the quantity nature and mode of wood storage revealed a number of trends related to active zone morphology cover type and distance from the river s source relatively small quantities of wood were stored on open gravel surfaces estimates ranged from 1 to 21 t ha 1 intermediate quantities were associated with established islands 24 186 t ha 1 and large quantities were associated with pioneer islands 293 1664 t ha 1 thus variations in the geomorphological style of the river which are associated with changes in these three cover types are reflected in variations in the amount of wood that is stored in different reaches in addition although wood was found in many locations within the active zone it was preferentially stored in three specific locations i bar crests the main open gravel location for wood accumulations and pioneer islands ii the margins and iii surfaces of established islands the proportion of the stored wood that was living sprouting increased downstream and was higher on the open gravel than in association with established islands there was a downstream gradient in the dominant type of wood accumulation individual logs predominated at the most upstream site thereafter on the open gravel whole shrubs and trees dominated the more confined sites in the headwaters and middle reaches whereas jams were the most frequent form of accumulation in the downstream reaches jams were the most frequent type of accumulation associated with established islands throughout the river in contrast to small streams where debris dams constitute the major type of wood accumulation complex patterns and trends of wood storage were revealed along the tagliamento although further studies are needed it is clear that erosion of woody vegetation its subsequent transport and deposition play a major role in structuring the geomorphological and ecological character of this relatively natural large european river system insight into the mechanisms underlying the observed spatial patterns will contribute to a better understanding of the dynamic processes involved and is essential for more effective management of river ecosystems

logjam controls on channel floodplain interactions in wooded catchments and their role in the formation of multi channel patterns the role of wood in the formation and maintenance of complex dynamic floodplain surfaces is important and to date has received relatively limited attention compared to in channel habitat processes this paper explores the role of logjams as important agents of channel floodplain interaction we draw on a specific case study as well as examples from the literature to show that although the processes of interaction differ the resulting dynamic floodplain patchwork is a common feature of rivers with logjams in addition we contend that the presence of logjams is an important factor in the evolution and maintenance of multiple channel patterns in both montane and lowland river environments these observations have important implications for the definition of reference targets for river restoration the specific results of this research show 1 the presence of a range of types of multiple channel network dissecting the floodplains of low order channels that are strongly associated with the presence of logjams 2 the relatively rapid formation of floodplain channels following logjam formation 3 the dynamic nature of logjams within headwater channels on both seasonal and annual timescales that lead to a highly dynamic habitat mosaic on the floodplain surface 4 an increased frequency of overbank flooding and high rates of floodplain sediment accumulation upstream of logjams and along floodplain channel networks that create the complex topography observed in the case study forested floodplains

the intrusion of a density current along the coast of a rotating fluid when light rotating fluid spreads over heavier fluid in the vicinity of a vertical wall coast a boundary jet of width forms the leading edge or nose of which propagates with speed along the coast a certain fraction 8 of the boundary transport is not carried by the nose but is deflected backwards detrained and left behind the propagating nose theoretical and experimental results for and are given for a quasi equilibrium constant regime over longer time intervals the laboratory observations suggest that the nose slows down and stagnates whereupon the trailing flow separates from the coast and an intermittent boundary current forms these processes may be relevant to the mixing of oceanic coastal currents and the maintenance of the mean current

using ammonium pore water profiles to assess stoichiometry of deep remineralization processes in methanogenic continental margin sediments 1 in many continental margin sediments a deep reaction zone exists which is separated from remineralization processes near the sediment surface here methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate however the methane sources that drive this anaerobic oxidation of methane aom in the sulfate methane transition zone smt may vary among sites in particular these sources can be thought of as either i internal sources from in situ methanogenesis regardless of where it occurs in the sediment column that are ultimately coupled to organic matter deposition and burial or ii external sources such as hydrocarbon reservoirs derived from ancient source rocks or deeply buried gas hydrates both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface using a modeling approach we examine the relationship between different methane sources and pore water sulfate methane dissolved inorganic carbon dic and ammonium profiles we show that pore water ammonium profiles through the smt represent an independent tracer of remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with aom and carbonate precipitation i e dic methane and sulfate pore water dic profiles also show an inflection point in the smt based on the type of deep methane source and the presence absence of accompanying upward dic fluxes with these results we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments

recent deep sourced methane mud discharge at the most active mud volcano in the western mediterranean abstract active mud volcanism in the west alboran basin wab is closely associated with tectonically mobilized overpressurized shales and shale diapirism this appears to control mud expulsion at carmen mud volcano a cone shaped structure 65 m high and 1 km in basal diameter the presence of gas rich mud breccia living chemosynthetic fauna the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of carmen mv for the lowermost interval the depletion of major elements i e ca2 and mg2 and the enrichment of trace species i e li and b in the pore water all indicate a deep fluid source the 18opw 5 7 vsmow and dpw 10 vsmow of pore water in the lowermost interval correspond with smectite dehydration as the main pore water freshening mechanism water formation temperatures calculated with empirical geo thermometers k na k mg 18opw dpw and dissolved b reveal that fluids were generated at temperatures of 140 20 c taking a regional geothermal gradient for the wab of 25 27 c km this points to a fluid source from 5 1 km sediment depth this is not only consistent with the depth of overpressurized shales and megabreccia of lower to middle miocene age but it also fits nicely with the upper middle miocene seawater value for the porewater 87sr 86sr derived from dissolving carbonates the stable carbon and hydrogen isotopic composition of methane 13cmethane 59 4 vpdb and dmethane 184 vsmow for the deepest samples of summit core gp05pc is consistent with the mentioned deep origin mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing leading to rapid instantaneous replacement of pore fluid by bottom water in the upper sediments the absence of oxidized sediment draping the seawater like pore water composition in the uppermost part of the mud breccia interval and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion the distinctively kink shaped pore water cl profile in core gp05pc has been used in a numerical transport reaction model to derive the timing for this event this eruptive event appears to have taken place very recently namely 12 5 yrs prior to the 2012 coring thus in the year 2000 ce

the role of large wood in retaining fine sediment organic matter and plant propagules in a small single thread forest river abstract this paper investigates associations among large wood accumulations retained sediment and organic matter and the establishment of a viable propagule bank within a forested reach of a lowland river the highland water uk a wood survey within the 2 km study reach illustrates that the quantity of wood retained within the channel is typical of relatively unmanaged river channels bordered by deciduous woodland and that the wood accumulations jams that are present are well developed typically spanning the river channel and comprised of wood that is well decayed sediment samples were obtained in a stratified random design focusing on nine subreaches within which samples were aggregated from five different types of sampling location two of these locations were wood associated within and on bank faces immediately adjacent to wood jams and the other three locations represented the broader river environment gravel bars bank faces floodplain the samples were analysed to establish their calibre organic and viable plant propagule content the gravel bar sampling locations retained significantly coarser sediment containing a lower proportion of organic matter and viable propagules than the other four sampling locations the two wood related sampling locations retained sediment of intermediate calibre between the gravel bar and the bank floodplain samples but they retained significantly more organic matter and viable propagules than were found in the other three sampling locations in particular the jam bank samples areas of sediment accumulation against bank faces adjacent to wood jams contained the highest number of propagules and the largest number of propagule species these results suggest that retention of propagules organic matter and relatively fine sediment in and around wood jams has the potential to support vegetation regeneration further sediment retention and as a consequence landform development within woodland streams although this process is arrested by grazing at the study site these results also suggest that self restoration using wood is a potentially cost effective and far reaching river restoration strategy but that its full effects develop gradually and require the establishment of a functioning wood budget coupled with grazing levels that are in balance with vegetation growth

large woody debris physical process and riparian forest development in montane river networks of the pacific northwest abstract we present a conceptual biogeomorphic model of riparian forest development in montane river networks the role of physical process in driving the structure composition and spatial distribution of riparian forests is examined we classify the drainage network into disturbance process based segments including 1 debris flow and avalanche channels 2 fluvial and debris flow channels and 3 fluvial channels riparian forests are shown to be significant in the development of channel morphology through the stabilization of active floodplains and as sources of large woody debris olwd lwd is operationally defined as wood 0 1 m diameter and 1 m length lwd plays a key role in the development of montane riparian forests lwd deposited in the active channel and floodplain provides sites for vegetation colonization forest island growth and coalescence and forest floodplain development riparian forest patterns parallel the distribution of hillslope and fluvial processes through the network riparian forest structure composition and spatial distribution through the network are driven by the major disturbance processes including 1 avalanches 2 debris flows and 3 flooding riparian forest patterns also reflect the action of lwd in the organization and development of forested floodplains in gravel bedded montane river networks the focus of our examples are montane river networks of the pacific northwest usa

influence of logjam formed hard points on the formation of valley bottom landforms in an old growth forest valley queets river washington usa abstract field surveys and radiocarbon dating of buried logjams in the floodplain of an old growth forest river demonstrate the formation of erosion resistant hard points on the floodplain of the queets river washington these hard points provide refugia for development of old growth forest patches in frequently disturbed riparian environments dominated by immature forest our surveys show that local bed aggradation associated with logjams not only influences channel patterns and profiles but leads to development of a patchwork of elevated landforms that can coalesce to form portions of the valley bottom with substantial i e 1 to 4 m relief above the bankfull elevation in addition logjam formed hard points promote channel avulsion anastomosing morphology and growth of mature patches of floodplain forest that in turn provide large logs needed to form more logjam formed hard points hence our findings substantiate the potential for a feedback mechanism through which hard points sustain complex channel morphology and a patchwork floodplain composed of variable elevation surfaces conversely such a feedback further implies that major changes in riparian forest characteristics associated with land use can lead to dramatic simplification in channel and floodplain morphology

mechanism for burgess shale type preservation exceptionally preserved fossil biotas of the burgess shale and a handful of other similar cambrian deposits provide rare but critical insights into the early diversification of animals the extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft bodied fossil assemblages has remained enigmatic since walcott s initial discovery in 1909 here we demonstrate the mechanism of burgess shale type preservation using sedimentologic and geochemical data from the chengjiang burgess shale and five other principal burgess shale type deposits sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation low sulfate concentrations in the global ocean and low oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability subsequently rapid entombment of fossils in fine grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments a permeability barrier provided by bed capping cements that were emplaced at the seafloor is a feature that is shared among burgess shale type deposits and resulted from the unusually high alkalinity of cambrian oceans thus burgess shale type preservation of soft bodied fossil assemblages worldwide was promoted by unique aspects of early paleozoic seawater chemistry that strongly impacted sediment diagenesis providing a fundamentally unique record of the immediate aftermath of the cambrian explosion

mechanistic controls on diverse fates of terrestrial organic components in the east china sea terrestrial carbon transferred from the land to sea is a critical component of the global carbon cycle a range of geochemical proxies has been developed to fingerprint the fate of terrestrial organic matter tom in marine sediments however discrepancies among different proxies limit our ability to quantify and interpret the terrestrial signals in marine sediments with consequences for the investigation of both the modern carbon cycle and past environmental change to mechanistically understand these discrepancies we examined the distributions of a range of terrestrial proxies and their aquatic counterparts i e marine proxies in the yangtze river east china sea yr ecs shelf system where tom experiences extensive modification during transport and burial tom proxies in the yr ecs system collectively fit a power law model but with distinct attenuation rates the a values for individual molecular proxy groups among a range of tom proxies the modeled a values decrease in the order soil marker bhps triterpenols lignin hmw n alkanols branched gdgts hmw n alkanes for biomarkers and rsoil bit tomiso for proxies tracing tom rapid loss of tom components through dissociation in the narrow estuary followed by oxidation over the wide open shelf are best described by power curves inherent chemical reactivity i e the number of functional groups responses to hydraulic sorting and in situ production regulate the individual attenuation rates of them chemical reactivity plays the most important role on proxy behavior supported by a strong correlation between a values and standard molal gibbs energies both physical protection and chemical reactivity fundamentally control the overall behavior of tom components with the relative importance being setting dependant the former is relatively important in the estuary whereas the later is the primary control over the open shelf moreover regional variation of different marine counterparts is also significant over the river ecs shelf system seemingly regulated by regional nutrient distributions therefore for tom estimates using molecular ratio approaches the specific behavior of individual terrestrial components and marine counterparts and the physical biological and chemical characteristics of depositional settings all need to be considered

organic carbon burial efficiencies in sediments the power law of mineralization revisited the reactivity of marine organic carbon toward oxidation has been described as a simple power law function that decreases with carbon age and is applicable from fresh phytoplankton to nearly kerogen we demonstrate that a correction to this law that accounts for a slowing in carbon mineralization under anoxic conditions makes the law consistent with observed carbon burial efficiencies which are known to be higher in anoxic environments our analyses of data from freshwater lakes reveal that the reactivity of autochthonous organic carbon follows similar trends in freshwater and marine environments the similarity suggests that the power law may be applicable to environments with diverse water chemistries such as the anoxic or low sulfate oceans of the geologic past using this power law we calculate the theoretical burial efficiencies for organic carbon under a range of conditions including those in anoxic ancient oceans the calculations suggest that in abyssal sediments the buried fraction of deposited organic carbon may have been 6 40 times higher in precambrian anoxic oceans than today whereas for coastal areas the difference was much smaller oxygenation of the deep ocean then would have caused a large scale decrease in organic carbon burial with a corresponding negative feedback on rates of atmospheric oxygen accumulation

understanding the causes and consequences of past marine carbon cycling variability through models abstract on geological time scales the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments exerts a dominant control on atmospheric co 2 and hence variability in climate this is a highly complex system involving a myriad of inter connected biological chemical and physical processes for this reason alone linking observations often highly abstracted in the form of proxies to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions is challenging the past few decades have seen a progressive improvement in theoretical and process based understanding of the various components that make up the marine carbon cycle and hand in hand with this the development of numerical model representations of the complete system models have also been designed and or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long term geologic evolutionary trends in global climate and possible feedbacks between them however we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle and indeed whether in the geological context we really know what these dynamics are here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics to this end we first provide an overview of commonly used paleoclimate models and some of their associated paleo applications drawing from a wide range of global carbon cycle box models and earth system models of intermediate complexity emics secondly we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models namely biological productivity at the ocean surface remineralisation dissolution of particulate carbon within the water column and the benthic pelagic coupling at the seafloor we generate and employ illustrative examples using the model genie to show how different parameterisations of water column and sediment processes can lead to significantly different model projections our compilation reveals the prevalence of static parameterisations of marine carbon cycling among existing paleoclimate models which are commonly empirically derived from present day observations although such approaches tend to represent carbon transfer in the modern ocean well they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events particularly those characterised by environmental boundary conditions that differ fundamentally from today finally we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed

toward a parameterization of global scale organic carbon mineralization kinetics in surface marine sediments an empirical function is derived for predicting the rate depth profile of particulate organic carbon poc degradation in surface marine sediments including the bioturbated layer the rate takes the form of a power law analogous to the middelburg function the functional parameters were optimized by simulating measured benthic o2 and no3 fluxes at 185 stations worldwide using a diagenetic model the novelty of this work rests with the finding that the vertically resolved poc degradation rate in the bioturbated zone can be determined using a simple function where the poc rain rate is the governing variable although imperfect the model is able to fit 71 of paired o2 and no3 fluxes to within 50 of measured values it further provides realistic geochemical concentration depth profiles no3 penetration depths and apparent first order poc mineralization rate constants the model performs less well on the continental shelf due to the high heterogeneity there when applied to globally resolved maps of rain rate the model predicts a global denitrification rate of 182 88 tg yr 1 of n and a poc burial rate of 107 52 tg yr 1 of c with a mean carbon burial efficiency of 6 1 these results are in very good agreement with published values our proposed function is conceptually simple requires less parameterization than multi g type models and is suitable for non steady state applications it provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in earth system models

effect of sea level on organic carbon preservation in the okinawa trough over the last 91 kyr abstract the burial of marine organic carbon primarily synthesized by marine autotrophic organisms into sediment removes co2 from the ocean atmosphere system however linkages between export productivity and organic carbon burial and the controlling factors remained ambiguous especially for marginal seas in this study we measured redox sensitive trace metal concentrations and planktic foraminifer fauna assemblages for a sediment core collected from the middle okinawa trough spanning the past 91 kyr through compilation with other published data we found that the export productivity was decoupled from organic carbon burial variability of manganese and total sulfur contents suggested that both bottom water and surface sediment had experienced significant redox change in the last glacial cycle by using the ratio of marine organic carbon to reactive phosphorus i e ocmarine preact we reconstructed organic carbon burial efficiency which showed relatively higher values during the last glacial and low values during the middle late holocene after synthesizing our new and reported data we conclude that the middle okinawa trough bottom water redox state was controlled by deep water ventilation induced by sea level change and related kuroshio current intrusion instead of by the oxygen consumption owing to local export productivity

quantifying manganese and nitrogen cycle coupling in manganese rich organic carbon starved marine sediments examples from the clarion clipperton fracture zone extensive deep sea sedimentary areas are characterized by low organic carbon contents and thus harbor suboxic sedimentary environments where secondary autotrophic redox cycling becomes important for microbial metabolic processes simulation results for three stations in the eastern equatorial pacific with low organic carbon content 20 m concentrations

on the oxidation and burial of organic carbon in sediments of the iberian margin and nazare canyon ne atlantic abstract as a contribution to the ec omex ii program sediment carbon and nitrogen budgets are presented for the iberian margin northeastern atlantic the budgets for degradable organic carbon and associated nitrogen were calculated from sediment and pore water properties using a steady state version of a numerical coupled diagenetic model omexdia data were collected throughout the major upwelling period along five transects four of which were located on the open margin and one positioned in a major submarine canyon the nazare canyon a comparison of in situ oxygen profiles measured with monocathodic microelectrodes and with clark type microelectrodes showed that monocathodic electrodes overestimate the oxygen concentration gradient near the sediment water interface this artifact probably results from the loss in sensitivity of the monocathodic microelectrode during profiling shipboard time course measurements with clark type electrodes demonstrated transient conditions upon sediment retrieval on deck and indicated enhanced rates of oxygen consumption in the surface sediment presumably as a result of lysis or exudation of oxidisable substrates by infauna as a result oxygen fluxes calculated from shipboard oxygen profiles overestimated in situ fluxes by up to a factor of 5 for water depths 1000 m the sediments from the canyon and from a depositional area on the shelf were enriched in organic carbon 3 4 5 wt relative to the open margin stations 0 5 2 wt and showed c n ratios exceeding redfield stoichiometry for marine organic matter indicating there was deposition of organic carbon of terrestrial origin in these areas the oxidation of organic carbon on the open margin declined from 11 gcm 2 y 1 on the shelf to 2 gcm 2 y 1 at 5000 m water depth and was dominated by aerobic oxidation the reactivity of the degradable organic carbon at the time of deposition was 1 on the shelf and declined to 1 offshore the burial of refractory organic carbon at the stations along the open margin transects also declined with increasing water depth from 5 gcm 2 y 1 on the shelf to 2 y 1 at 2000 m depth whereas the burial of particulate inorganic carbon declined from 20 gcm 2 y 1 to 2 y 1 a comparison of the estimated total organic carbon deposition and predicted delivery for the shelf suggest that 58 to 165 gcm 2 y 1 is oxidized in the water column laterally advected or focused into one of the canyons anaerobic oxidation denitrification and therefore total oxidation of organic carbon was enhanced within the canyon relative to the open margin total organic carbon oxidation decreased with water depth from 22 gcm 2 y 1 at the head of the canyon to 3 gcm 2 y 1 over its fan the reactivity of the organic carbon deposited in the canyon was lower than those of the shelf stations suggesting that the canyon is being enriched in older laterally advected organic matter the burial of refractory organic carbon in sediments from the nazare canyon was considerably higher than in the sediments from the open margin it also decreased with depth from 20 gcm 2 y 1 at 343 m to 2 5 gcm 2 y 1 at 4298 m water depth the burial of particulate inorganic carbon was slightly lower than that of refractory organic carbon the burial of refractory organic carbon and the deposition of degradable organic carbon were both positively correlated with the sedimentation rates for the iberian margin and indicated burial efficiencies were 0 6 to 48 a single trend for burial efficiency versus sedimentation rate for both the canyon and the open margin indicates that the sedimentation rate was the master variable for the geographical distribution of organic carbon oxidation and carbon preservation on the nw iberian margin

modeling studies of dissolved organic matter cycling in santa barbara basin ca usa sediments abstract here we describe new reaction transport models for the cycling of dissolved organic matter dom both dissolved organic carbon doc and dissolved organic nitrogen don in anoxic marine sediments and apply these models to data from santa barbara basin sediment cores maximum depth of 4 6 m model results show that most organic carbon and nitrogen flow in the sediments occurs through reactive dom intermediates that turn over rapidly to produce inorganic remineralization end products refractory dom is also produced and the vast majority of this refractory dom is not remineralized and either escapes as a benthic flux across the sediment water interface or is buried except near the sediment surface refractory dom represents 95 of the total pore water dom pore water dom appears to be consistently depleted in nitrogen as compared to its source organic matter which may be the result of differential production of carbon versus nitrogen containing refractory dom during remineralization refractory doc docr in santa barbara basin sediment pore waters is largely produced from degradation of sediment particulate organic carbon poc in addition there is an upward basal flux of docr that is strongly depleted in 14c 810 the 14c value of docr varies according to its source ranging from 60 a component of surface sediment poc enriched with radiocarbon from nuclear weapons testing in the 1960 s to 810 the basal doc flux each contributes to the docr benthic flux which has a weighted average 14c value of 40 the model determined docr benthic flux is roughly half of the total doc benthic flux consistent with observations in the literature that sediments are a source of both labile and refractory doc to bottom waters these results support previous arguments that sediment benthic fluxes represent an important source of refractory doc to the oceans the benthic flux of refractory doc from these sediments may also contribute pre aged doc to the water column if the different sub components of the anoxic pore water docr pool with differing radiocarbon ages have differing reactivities in the oxic marine water column

baroclinic eddy generation at a sharp corner in a rotating system laboratory experiments were conducted to investigate the generation of anticyclonic gyres by separation of a surface current from a coast in a rotating two layer system the experiments were motivated by the hypothesis that the flow of coastal currents around capes can generate oceanic eddies as well as by the observation of gyres at the mouths of various straits in the experiments the gyre is formed when the current which flows with the coast to its right if one is oriented in the downstream direction encounters a sharp convex corner the current overshoots the corner loops to the right and reattaches to the coast downstream of the corner between the current loop and the coast is an anticyclone whose width grows with time if a countercurrent flows under the surface current a similar separation in the lower layer results in the generation of a cyclone as well under some circumstances the cyclone and anticyclone advect each other away from the coast as a heton previous studies on related systems found that the corner must be sufficiently sharp for a gyre to form i show that for a very sharp corner the angle made by the corner must be above a critical value of between 40 and 45 for a gyre to form this is in contrast to nonrotating flows of comparable rayleigh number which will separate from a sharp corner at virtually any angle for angles below the critical value the current profile downstream of the corner changes as a function of corner angle indicating that it is the stagnation of the flow nearest the wall which causes the anticyclone to form this stagnation is reminiscent of the two dimensional nonrotating picture of viscous boundary layer dynamics forcing separation of a boundary current however the gyre grows more slowly when the lower layer is much thicker than the upper layer indicating that baroclinic processes are at least quantitatively important in the generation of the gyre by varying the initial condition of the current it is shown that the gyre formation is not a product of the interaction of the nose of the current with the corner in conclusion the experiments indicate that the basic mechanism of gyre formation may be viscous boundary effects as in nonrotating systems but that rotation tends to inhibit eddy generation while baroclinic effects tend to enhance it

the impact of alkenone degradation on uk 37 paleothermometry a model derived assessment the u37k proxy for past sea surface temperature sst is based on the unsaturation ratio of c37 alkenones it is considered a diagenetically robust proxy but biases have been invoked because the index can be altered by preferential degradation of the c37 3 alkenone resulting in higher reconstructed sst however alkenone degradation rate constants are poorly constrained making it difficult to evaluate the plausibility of such a bias therefore we quantitatively assessed the effect of 1 different alkenone degradation rate constants 2 differential degradation factors between di and tri unsaturated c37 alkenones 3 and initial u37k values on the u37k paleothermometer for two depositional environments shelf and upper slope by means of a reaction transport model rtm rtm results reveal that preferential degradation of c37 3 can potentially alter the original signal of the u37k paleothermometer but sst biases sst are largely within u37k calibration error sst 1 5 c are associated with marked downcore decreases in alkenone concentration consequently we caution against the interpretation of u37k indices when extensive degradation results in very low alkenone concentrations 5 ng g 1

dynamics of a buoyancy driven coastal jet the gaspe current a primitive equation ocean model is applied to the process study of the gaspecurrent and cyclonic circulation over the northwestern gulf of st lawrence nwg the model is driven by river discharge and barotropic boundary flows two types of model domains are used an idealized basin with a flat bottom and piecewise coastline and a realistic basin with model resolved nwg bathymetry the model domains are initially filled with horizontally uniform but vertically stratified waters the river discharge is expressed in terms of lower salinity and a weak barotropic inflow in the upper waters at the estuary head the early developments of the estuarine plume and coastal current system driven by the river discharge are qualitatively similar in both basins after a short period adjustment a buoyant plume is developed near the estuary mouth with a surface intensified coastal current advecting the estuarine water seaward in the direction of kelvin wave propagation the coastal current initially follows the coastline closely but later becomes unstable with backward breaking waves developed along the outer edge of the current the kinetic energy analysis reveals that the plume current system is baroclinically unstable with the transient motions resulting primarily from the mean available potential energy with the river discharge at the head as the only driving force the offshore front of the estuarine plume expands continuously seaward leading to a large scale anticyclonic circulation over the nwg the addition of a barotropic westward jet along the quebec shore however is able to restrain the seaward expansion of the offshore front of the plume and therefore form a large scale cyclonic motion over this region

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms

geochemical and microbiological zonation of the middendorf aquifer south carolina we consider the nature and origin of the chemical and microbiological zonation of the cretaceous middendorf aquifer in the coastal plain of south carolina in light of the chemical evolution of groundwater along a flow path there some types of microbial activity most notably aerobic respiration and iron reduction lead to a net generation of acid as co2 but reaction of groundwater with minerals in the aquifer consumes the acid driving up ph and controlling the evolution of the groundwater s major ion chemistry the thermodynamic energy available to the various functional groups of microbes except the autotrophic hydrogen oxidizing methanogens is everywhere along the flow path sufficient to drive microbial growth the concentrations of acetate formate and lactate do not vary systematically from zone to zone along the flow path as might be expected if each zone were dominated by a single functional group of microbes the overall rate of microbial respiration in the aquifer appears to be limited by the rate of the initial fermentation of organic matter we found no compelling evidence indicating that one functional group of microbes excludes others from any of the aquifer s zones sulfate reduction for example may be non existent in the high iron zone associated with iron reduction or may account for up to 90 of the respiration occurring there zonation of the groundwater into high iron and low iron facies may result from a minor change along the aquifer in the balance between the activities of iron and sulfate reducers

nearshore euxinia in the photic zone of an ancient sea abstract evidence for photic zone euxinia pze in ancient oceans is widespread and commonly associated with global climate change and biotic events however evidence of pze in marginal settings is commonly extrapolated to infer basin scale characteristics to explore the relationship between euxinia in marginal settings and the wider basin in one palaeozoic context we have conducted a detailed spatial and temporal examination the palaeowater column redox state from the polish sector of the southern permian basin spb during deposition of the zechstein lopingian second carbonate main dolomite cycle sediments from toe of slope slope and hypersaline lagoonal settings of the northeastern margin of the spb contain abundant chlorobactane isorenieratane and their likely degradation products c 15 to c 31 2 3 6 aryl isoprenoids these indicate that part of the photic zone was euxinic during significant intervals of the main dolomite deposition further evidence for strongly anoxic conditions includes the occurrence of c 28 bisnorhopane high concentrations of pentakishomohopanes i e high homohopane indices and the occurrence of framboidal and native sulphur crystals in slope facies however a lack of these signatures in basinal settings indicates that strongly reducing conditions were restricted to the slope and toe of slope locations even in these settings however highly variable homohopane indices and concentrations of isorenieratene and chlorobactene derivatives as well as the presence of benthic fauna and bioturbation indicate that anoxia fluctuated throughout deposition of the main dolomite likely on a variety of timescales sterane distributions are also variable suggesting a dynamic ecosystem likely exhibiting a strong response to environmental forcing overall it appears that high but episodic primary bioproductivity of organic matter was concentrated on the northeastern slope and restricted lagoons leading to the formation of source rocks for petroleum however the temporal and geographical restriction of anoxia appears to have prevented the accumulation of large or more widespread quantities of organic matter and in fact toc contents exhibit poor correlation with ecological and anoxia indicators this model indicates that strong evidence for pze in shelf and slope facies need not be associated with widespread basin scale anoxia with implications for organic matter burial carbon cycling and biotic crises

modeling detailed sedimentary 210pb and fallout 239 240pu profiles to allow episodic events an application in chesapeake bay dependable sediment chronologies are essential to the interpretation of the sedimentary record of past environmental change in the present article we use high resolution chemical and radiochemical data as the basis for a numerical simulation of sediment accumulation bioturbation and episodic deposition or erosion in a dynamic estuary we simulate episodic events by employing a time dependent sedimentation rate that we solve by finding a set of model parameters that describes depth profiles of both excess 210pb and fallout 239 240pu we apply the model to depth distributions of these tracer nuclides in cores from upper mid and lower bay sites in chesapeake bay at the upper bay site combining chemical and radiochemical data permits us to recognize and to quantify the sediment deposition due to tropical storm agnes 1972 at the lower bay site we demonstrate nonsteady sedimentation and propose plausible scenarios to account for it given adequate data our model can provide information that is not available from steady state models