dissolution kinetics of calcium carbonate in equatorial pacific sediments benthic chambers were deployed in the equatorial eastern pacific ocean on a transect along the equator between 103 w and 140 w and on a transect across the equator at 140 w in order to establish the rate of calcium carbonate dissolution on the seafloor dissolution was determined from the rate of alkalinity increase within an incubation chamber measured over an 80 120 hour incubation period dissolution rates were lowest at eastern pacific sites 0 2 0 4 mmol caco3 m2 d and highest at the equatorial 140 w sites 0 5 0 7 mmol m2 d both oxygen consumption rates and the degree of bottom water saturation govern dissolution rates measured dissolution and oxygen consumption rates are used with a numerical model to constrain the value of the dissolution rate constant k formulated according to the equation developed by keir 1980 dissolution rate k 1 n the observed dissolution fluxes are predicted by the model when k 5 to 100 d and n 4 5 this range of k values has important implications regarding the type of carbonate dissolving and its location within the sediment column at low values of k organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution at higher values of k the degree of bottom water undersaturation becomes more important dissolution of carbonate within equatorial pacific sediments can be adequately described with k 20 10 d a rate constant much lower than some previously used values dissolution rates do not vary significantly over chamber boundary layer thicknesses between 200 and 800 m indicating that dissolution is not controlled by hydrodynamic conditions chambers acidified with hcl yield very large dissolution rates but for a given degree of acidification the dissolution rate was constant for sites ranging from water depths of 3300 4400 m this implies that there are not more and less easily dissolved forms of caco3 arriving on the seafloor between these depths a budget for alkalinity in the deep pacific predicted by the dissolution model and based on the assumption that all carbonate dissolution takes place in the sediments is within 85 of the input required by a published box model alkalinity budget based on the distribution of nutrients and the water mass transport rates

calcite dissolution in sediments of the ceara rise in situ measurements of porewater o2 ph and co2 aq in situ electrode measurements of porewater oxygen and ph from four sites on the ceara rise in the western tropical atlantic unequivocally demonstrate that calcite dissolution driven by metabolic co2 produced within the sediments is a significant part of the diagenesis of sedimentary calcite dissolving at least 20 of the calcite rain to the seafloor the first ever in situ absorbance based measurements of porewater co2 aq support the dissolution estimates based on models of the observed ph distributions and are incompatible with scenarios excluding metabolic co2 driven dissolution our dissolution estimates are consistent with those estimated by an independent study of porewater calcium and alkalinity profiles martin and sayles 1996 at this location the consistency of these disparate porewater measurements and our ability to interpret them with simple models of respiration and dissolution implies that no special mechanism is required to describe this process the calcite dissolution rate constant in the sediments of the deepest station in this study is significantly higher than determined by in situ studies at other locations values approach those determined by early laboratory measurements the dissolution rate constant here must be at least two orders of magnitude higher than for calcite in sediments on the ontong java plateau based on in situ electrode measurements there hales and emerson 1996 the calcite solubility consistent with these data is not compatible with that consistent with the ontong java data the reason for this difference is currently unresolved but may be due to the high order of the empirical dissolution rate law assumed in this study

the migif hafafit gneissic complex of the egyptian eastern desert fold interference patterns involving multiply deformed sheath folds abstract the wadi hafafit complex whc is an arcuate belt of orthogneisses migmatites and other high grade metamorphic rocks which marks the boundary between the central eastern and the south eastern deserts of egypt in the whc gneissic meta gabbro outlines macroscopic fold interference patterns characterized by elliptical to irregular culminations cored by gneissic meta tonalite to meta trondhjemite the five main culminations of the whc have previously been labeled a most northerly b c d and e most southerly a detailed structural investigation of b c d and e reveals that these structures are a result of the interference of four macroscopic fold phases the first three of which may represent a single deformation event the first folding involved sheath like fold nappes which were transported to the n or nw assisted by translation on gently dipping mylonite zones the regional gneissosity and mineral extension lineations formed during this folding event the fold nappes were deformed by mainly open upright small macroscopic and mesocopic folds with approximately ne trending hinges as a probable continuation of the latter folding the sheaths were buckled into large macroscopic folds and monoclines with the same ne trends the fourth macroscopic folding resulted from shortening along the ne sw direction producing mainly nw se trending upright gently plunging folds gravitative uplift is disputed as a component of the deformation history of the whc the peculiarities of the fold interference pattern result from the interesting behaviour of sheath folds during their refolding

micro scale biogeochemical heterogeneity in sediments a review of available technology and observed evidence the hypothesis that reducing conditions exist in localized zones of high organic matter termed microniches was first suggested over a century ago but only relatively recently have high resolution techniques been available to investigate them in any sediment containing benthic fauna bioturbation affects the distribution of a number of redox sensitive components direct faecal deposition and the death of fauna may be expected to cause particles of labile organic matter microniches to be distributed heterogeneously within the sediment this review discusses the significance and future direction of microniche studies by considering exclusively data obtained on a sub mm scale that provide significant evidence for the existence and properties of microniches microelectrodes and planar optodes have shown the significant effect of burrowing organisms on localized o2 distributions and revealed distinct depletions in o2 due to microniches localized increases in pco2 and decreases in ph measured by optodes were attributed to elevated activity at microniches diffusive gradients in thin films have shown isolated supersaturation of metals and sulphide providing evidence for possible simultaneous oxidation of organic matter by sulphate and iron oxides the stochastic nature of these data and the lack of information for the same precise location hinders interpretation in terms of sediment diagenesis if microniches are known to account for a significant proportion of organic matter degradation re examination of the current understanding of sedimentary diagenesis may be needed further investigation on the distribution and frequency of microniches is required including a wider range of analytes in order to estimate their cumulative effect on element diagenesis immobilisation remobilisation processes and ultimately pollutant fate

the composition and diagenesis of interstitial solutions ii fluxes and diagenesis at the water sediment interface in the high latitude north and south atlantic studies of the major element composition of in situ sampled pore waters are reported for the north atlantic and southern ocean between africa and antarctica the pattern of diagenetic modification of pore water composition is similar throughout the entire atlantic enrichment of na ca2 and alkalinity and depletion of mg2 and k are nearly universal only siliceous oozes consistently provide very limited evidence of cation diagenesis the changes observed and the calculated fluxes across the seawater sediment interface are much the same as those reported previously for other areas of the atlantic and caribbean fluxes of the major cations across the interface continue to be indicated as a major factor in the geochemical cycling of these elements particularly na mg2 and k diagenetic modelling indicates that aerobic oxidation of organic matter and consequent dissolution of caco3 is a dominant reaction throughout the north atlantic the data indicate that o2 oxidation to at least 30 cm is prevalent at nearly all stations dissolution of caco3 in response to the introduction of metabolic co2 can lead to significant post depositional modification of the sediments the modelling also indicates a 1 1 stoichiometric relationship between na release and mg2 uptake by sedimentary components although dissolved silica concentrations in biogenic siliceous sediments are among the highest yet reported 700 m calculations demonstrate that solubility control cannot be through equilibria with the mineral sepiolite further the influence of cation silicate surface phases generally upon solubility is contraindicated by si oh 4 mg2 h relationships evaluation of the influence of fluid advection on pore water profiles indicates that at the stations studied in the north atlantic it is small a more general consideration of the potential contribution of fluid advection to shaping interstitial water profiles demonstrates that advection can be a dominant factor in such circumstances serious misinterpretation of the nature of diagenetic reactions may result from a lack of knowledge and consideration of fluid advection

the influence of tortuosity on molecular diffusion in freshwater sediments of high porosity abstract in sediments diffusive transport of ions and molecules is basically influenced by two sediment characteristics tortuosity and porosity for the first time the formation factor f which combines the effect of tortuosity and porosity on diffusion was quantified in freshwater sediments at submillimeter resolution sediment cores were treated with kcl and f was determined using a resistivity sensor and k selective electrodes f was determined in sediments from different water depths of the eutrophic lake zug switzerland in sandy sediments from a shallow site 12 m depth f increased by approximately 50 within a few millimeters below the sediment surface in clayey and silty sediments from the oxic 160 m depth f increased by only 10 just below the sediment surface values of f were correlated with the porosity at each depth we found close correlations of f 1 02 1 81 for clay silt sediments and f 1 04 1 21 for sandy sediments the exponents are considerably smaller in lake zug than found for marine sediments thus diffusive transport seems to be less affected by tortuosity in this freshwater system

biogenic budgets of particle rain benthic remineralization and sediment accumulation in the equatorial pacific abstract budgets of organic c corg cac03 and opal have been constructed for the palisades ny pacific equatorial region at 140 w between 5 n and 5 s measurements of the rain and benthic remineralization rate of biogenic materials have been adjusted and normalized to account for sampling biases sea surface temperature serves as a master variable in normalizing sediment trap and benthic remineralization data to average conditions the rain and remineralization rates for corg are nearly equal 0 40 0 05 and 0 46 0 06 mmol m 2 d 1 respectively thus only a minor fraction of this constituent is buried rain and dissolution rates for biogenic opal are similarly balanced 0 3 0 06 and 0 36 0 01 mmol m 2 d 1 and consistent with the value for opal burial 0 0 0 004 the cac03 budget appears to have changed during the holocene the best estimates of modern cac03 dissolution 0 58 0 03 mmol m 2 d 1 and rain rate 0 61 0 06 are consistent with230th normalized carbonate accumulation rates for the late holocene 0 1 mmol m 2 d 1 however the balance between dissolution and rain is not consistent with early holocene carbonate accumulation 0 3 mmol m 2 d 1 and this imbalance suggests 1 a recent increase in the rate of cac03 dissolution on the sea floor or 2 a decrease in the rain rate of carbonate particles modeling230th profiles in sediments from this region define the last 3000 years as the duration of increased dissolution or decreased particle rain 231pa 230th ratios in sediments indicate that particle rain rates have remained constant or possibly increased slightly through the holocene two potential causes for increased dissolution were investigated a change in deep water carbonate saturation or a change in co g cac03 rain ratios a model describing carbonate dissolution as a function of the degree of undersaturation and the amount of organic carbon oxidation within sediments indicates that the recent increase in dissolution is more likely due to changes in bottom water chemical composition we propose that pacific ocean bottom water carbonate ion concentration has decreased by 10 15 m over the last 3000 years

calcite dissolution driven by benthic mineralization in the deep sea in situ measurements of ca 2 ph pco 2 and o 2 abstract in situ measured microprofiles of ca 2 pco 2 ph and o 2 were performed to quantify the caco 3 dissolution and organic matter mineralization in marine sediments in the eastern south atlantic a numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate from the oxygen microprofiles measured at four stations along a 1300 m isobath of the eastern african margin and one in front of the river niger at a water depth of 2200 m the diffusive oxygen uptake dou and oxygen penetration depth opd was calculated dou rates were in the range of 0 3 to 3 mmol m 2 d 1 and showed a decrease with increasing water depth corresponding to an increase in opd the calculated amount of degradated organic matter is in the range of 1 to 8 5 gc m 2 a 1 the metabolic co 2 released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite supersaturated water fluxes across the sediment water interface calculated from the in situ ca 2 microprofiles were 0 6 mmol m 2 d 1 for two stations at a water depth of 1300 m the ratio of calcite dissolution flux and organic c degradation is 0 53 and 0 97 respectively the microprofiles indicate that co 2 produced within the upper oxic sediment layer dissolves up to 85 of the calcite rain to the seafloor modeling our o 2 ph and ca 2 profiles from one station predicted a calcite dissolution rate constant for this calcite poor site of 1000 mol kgw 1 a 1 mol per kg water and year which equals 95 d 1 this rate constant is at the upper end of reported in situ values

organic carbon oxidation and benthic nitrogen and silica dynamics in san clemente basin a continental borderland site organic carbon oxidation rates in san clemente basin were determined by benthic chamber experiments using the bottom lander along with studies of pore water chemistry non steady state diagenetic models are developed for interpreting concentration time data from the benthic chamber experiments o2 no3 and so42 are all important oxidants for organic carbon at our study site regenerated fixed nitrogen was consumed by no3 reduction there is a flux of no3 into the sediments and the benthic flux of nh4 is undetectable the total rate at which fixed nitrogen is removed from the oceans at this site is about twice the flux of pon to the sea floor sio2 fluxes calculated from interfacial pore water gradients are in satisfactory agreement with those determined using the lander most silica dissolution must therefore occur within the sediments although interstitial profiles show that little dissolution occurs below 1 cm depth

the variability of benthic fluxes and sedimentary remineralization rates in response to seasonally variable organic carbon rain rates in the deep sea a modeling study

coupling of near bottom pelagic and benthic processes at abyssal depths in the eastern north pacific ocean a time series site was established in the eastern north pacific ocean to study the coupling between near bottom pelagic and benthic processes at abyssal depths in an area with well documented seasonality in surface water productivity particulate total mass ptm particulate organic c poc particulate total n ptn and particulate calcium carbonate p caco fluxes were measured from sediment trap collections at 600 and 50 m above bottom from february 1990 to october 199 1 606 d the sea floor at 4 100 m was concurrently monitored with a time lapse camera over a 386 d period from july 1990 to july 199 1 during this study in situ measurements of sediment community oxygen consumption scoc were made and surface sediment samples collected seasonally with a free vehicle grab respirometer ptm poc ptn and p cac0 fluxes exhibited a strong seasonal signal with episodic peaks from spring through fall and lows in winter scoc increased sharply between winter and summer coinciding with spring peaks in particulate fluxes detrital aggregates appeared on the sea floor in pulses over a 6 month period from july to december 1990 coinciding with periods when particulate fluxes were elevated but delayed 1 5 months after the initial peaks in particulate flux mobile epibenthic megafauna monitored with time lapse photography consisted primarily of holothuroids and echinoids and were twice as active when detrital aggregates were observed on the sea floor as during the rest of the year our results show a temporal relationship between the flux of particulate matter entering the benthic boundary layer the arrival and residence of detritus on the sea floor and the activity of the sediment community and mobile epibenthic megafauna

the northeast arran trough the corrie conundrum and the highland boundary fault in the firth of clyde sw scotland two strikingly different successions of lower carboniferous mainly tournaisian sedimentary rocks are closely juxtaposed on the ne coast of the island of arran sw scotland near the village of corrie a thin succession 17 m of tournaisian rocks is preserved whereas in the neighbouring fallen rocks laggan area correlative rocks are 300 m in thickness these contrasting successions are separated by the laggan fault which is a landward extension of the submarine brodick bay fault marking the sw boundary of the northeast arran trough the contrasting thickness and stratigraphy of the two sequences of sedimentary rocks result from juxtaposition of shoulder and trough deposits along the laggan brodick bay fault although originally a normal basin defining fault later sinistral movements caused significant displacement of the ne arran trough together with a segment of the highland boundary fault from their original positions the most northerly occurrence of the highland boundary fault on arran is thought to be the truncated northern end of the corloch fault to the sw the surface trace of the highland boundary fault is largely obscured by a palaeogene granite body but it is present on the west side of the island near dougrie the highland boundary fault appears to be displaced to the south in kilbrannan sound by a series of nw trending sinistral transcurrent faults thus the anomalous trend of the highland boundary fault and narrowing of the midland valley of scotland in the firth of clyde area may be explained by later fault movements and intrusion of the palaeogene north arran granite pluton

dissolution of calcite in deep sea sediments ph and o2 microelectrode results abstract we present microelectrode profiles of oxygen ph and porosity measured at in situ depth across the sediment water interface from stations above and below the calcite saturation horizon in the equatorial atlantic resistivity electrode data indicate that the diffusion coefficient for solutes is attenuated by nearly a factor of two within the top centimeter of the sediment lattice oxygen and porosity data are used to estimate that the organic carbon respiration rate of the sediment is 10 15 mol cm 2 yr 1 in these locations comparison of the ph data with numerical model results reveals that sedimentary calcite dissolves in response to respiration in the top few centimeters of sediment even above the saturation horizon we calculate that the effective in situ dissolution rate constant for calcite required to explain these data is the neighborhood of 10 100 d 1 although more data are needed to confirm this result it appears to be at least an order of magnitude less than the rate observed in laboratory stirred reactor experiments

modelling the distribution of stable carbon isotopes in porewaters of deep sea sediments abstract we present a numerical transport reaction model that describes the interaction between oxic organic matter decay and caco3 dissolution in deep sea sediments the model simulates the distribution and isotopic composition of carbonate species h2co 3 hco3 co32 in sediment porewaters forcing is provided by organic matter degradation by o2 whereas calcium carbonate dissolution is included as a first order kinetic reaction with respect to porewater undersaturation modelling of the isotopic data strongly constrains rate estimates of tco2 production by organic carbon mineralization and caco3 dissolution compared to previous models this approach is independent of assumptions on reaction stoichiometry the validity of the model is tested by simulating tco2 13c and ca2 profiles for two sites taken from sayles and curry 1988 located in the eastern equatorial pacific c 0 64 and western north atlantic c 1 30 the model successfully reproduces observed porewater distributions of tco2 13c and ca2 compared to estimates of tco2 produced by organic matter respiration derived by sayles and curry 1988 model integrated rates of tco2 production are larger by a factor of 1 6 at the eastern equatorial pacific station and a factor of 4 1 at the western north atlantic station the model allows for a quantification of the alkalinity flux induced by metabolic co2 released to porewaters during organic matter decay caco3 dissolution driven by metabolic decay gives rise to a substantial alkalinity flux at the western north atlantic station and contributes up to 53 to total alkalinity exchange at the station from the eastern equatorial pacific model derived calcite dissolution rate constants are within a factor of 4 at the two sites this low variability contrasts with previous studies which reported dissolution rate constants differing by several orders of magnitude

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

radiocarbon studies of organic compound classes in plankton and sediment of the northeastern pacific ocean radiocarbon 14c and stable carbon isotopes 13c were measured in total hydrolyzable amino acid thaa total carbohydrate tcho total lipid and acid insoluble organic fractions that had been separated from phytoplankton zooplankton sediment floc and sediment samples from an abyssal site in the northeastern pacific ocean thaa tcho and lipid fractions accounted for 91 99 of the total organic carbon toc in phytoplankton and zooplankton 57 of toc in sediment floc and 18 38 of toc in sediment based on concentration profiles in sediment first order degradation rate constants below the bioturbation zone were calculated using a multi g model considering both labile and refractory organic fractions the calculated rate constants were in the order thaa tcho toc tn lipid indicating the relative reactivities of these compound classes in the sediment during early diagensis bioturbation affected the distribution of these compound classes in the top few centimeters of the sediment the 14c values of all organic fractions decreased in the order plantkon in surface water to sediment floc to sediments at 4100 m depth distinct differences of 14c exist among thaa tcho and lipid fractions in sediment floc and sediments the lipid fraction exhibited lower 14c signatures than thaa and tcho fractions differential decomposition of organic matter and sorption and or biological incorporation of old doc into sediment appear to be the major processes that likely control the observed 14c signatures and abundances 13c values of the organic compound classes in sediment are similar to their values in plankton indicating that organic matter input to sediment in the northeastern pacific is mainly from marine sources also distinct 13c signatures were found in each of the four organic fractions

two dimensional ph distributions and dynamics in bioturbated marine sediments the seafloor is the site of intense biogeochemical and mineral dissolution precipitation reactions which generate strong gradients in ph near the sediment overlying water interface these gradients are usually measured in one dimension vertically with depth two dimensional ph distributions in marine sediments were examined at high resolution 65 65 m pixel and analytical precision over areas of 150 to 225 cm2 using a newly developed ph planar fluorosensor dramatic three dimensional gradients complex heterogeneity and dynamic changes of ph occur in the surficial zone of deposits inhabited by macrofauna ph can vary by 2 units horizontally as well as vertically over millimeter scales ph minima zones often form in association with redoxclines within a few millimeters of inner burrow walls and become more pronounced with time if burrows remain stable and irrigated for extended periods microenvironmental ph minima also form locally around decaying biomass and relict burrow tracks and dissipate with time 5 d h concentrations and fluxes in sandy mud show complex acid base reaction distributions with net h fluxes around burrows up to 12 nmol cm 2 d 1 and maximum net reaction rates varying between 90 consumption to 120 production m d 1 90 nmol cm 1 d 1 burrow length acid producing zones that surround irrigated burrows are largely balanced by acid titration zones along inner burrow walls and outer radial boundaries the geometry and scaling of ph microenvironments are functions of diagenetic reaction rates and three dimensional transport patterns determined by sediment properties such as diffusive tortuosity and by benthic community characteristics such as the abundance mobility and size of infauna previously undocumented biogeochemical phenomena such as low ph regions associated with in filled relict biogenic structures and burrowing tracks are readily demonstrated by two dimensional and time dependent images of ph and sedimentary structure

carbonate dissolution in the guts of benthic deposit feeders a numerical model abstract we present a numerical model to quantify calcite dissolution in the guts of deposit feeding invertebrates deposit feeder guts were modeled as constantly stirred reactors cstrs following terminology from digestion theory saturation state and dissolution of calcium carbonate were calculated from changes in total dissolved carbon dioxide and alkalinity resulting from sediment passage through the digestive tract while accounting for dissolution of calcite and respiration of organic carbon typical dissolution rates for a gut volume of 1 ml ranged between 0 5 4 mg calcite d 1 sensitivity analysis revealed gut ph sediment organic matter om content and om reactivity to be the critical parameters determining calcite dissolution rate carbonate dissolution rate was inversely related to gut ph however calcite dissolution was found to be possible even at alkaline gut ph due to respiration by intestinal microbes the kinetics of calcite dissolution had only marginal influence on daily calcite dissolution rates varying the calcite dissolution rate constant by six orders of magnitude affected calcite dissolution rates by less than a factor of 10 calcite dissolution rates were calculated for 4 different hydrographic regimes that differed in their content of sedimentary calcite and om and furthermore in their om reactivity highest dissolution rates were calculated for the shallow water setting where relatively high om content facilitated high microbial respiration rates depressing gut ph however dissolution rates for the deep sea setting were only slightly lower due to greatly elevated ingestion rates resulting from low om content as a consequence of much higher faunal abundances shallow water benthos is likely to contribute the vast majority of gut mediated carbonate dissolution nevertheless the fraction of sedimentary calcite that dissolves during one gut passage is probably too small to be observable by simple gravimetric analysis this may explain the notable scarcity of evidence for gut mediated carbonate dissolution in the literature to date assuming depth dependent calcite dissolution rates and deposit feeder abundances we estimate gut mediated carbonate dissolution to contribute approximately 5 of the annual global sedimentary carbonate dissolution rate which corresponds to an average calcite dissolution rate of approximately 0 5 mg m 2 d 1 for the entire ocean floor

pore waters of the central pacific ocean nutrient results interstitial waters from the manop siliceous and calcareous ooze sites in the central pacific ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler the pore waters were analyzed for nutrients carbonate system species mn 2 ca 2 total alkaline earth metals and dissolved oxygen siliceous ooze site only in calcareous sediments the co 2 alkalinity po 4 3 and ca 2 results obtained from shipboard techniques are significantly different than those recovered in situ the other species measured as well as all of the data from the siliceous ooze site reveal no significant dependence on the method used to sample the pore water the relative changes in o 2 no 3 mn 2 alkalinity and co 2 indicate that respiration using o 2 no 3 and mno 2 as electron acceptors is the dominant reaction affecting the pore water species we develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments this model includes the effects of pore water diffusion oxygen and nitrate respiration sediment accumulation sediment mixing and changes in sediment porosity we observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles the major factor controlling the pore water profiles is the flux of organic carbon across the sediment water interface we conclude that the large intra site variability observed in the interstitial water nitrate profiles is probably caused by small scale variations in the flux of organic material

in situ measurements of calcium carbonate dissolution rates in deep sea sediments benthic fluxes of alkalinity carbon dioxide and oxygen have been measured using an in situ incubation chamber at three sites manop sites c and s and pacflux site sc in the central equatorial north pacific at two carbonate rich sites c and sc a budget for oxygen alkalinity and tco2 fluxes indicate a net caco3 dissolution rate of approximately 0 4 mmol m 2 day 1 this rate is only 20 of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep sea sediments and derived from in situ flux measurements in sediments of the southern california borderland organic matter oxidation in the sediment column provides the acid for 60 100 of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water at a low carbonate site s no carbonate dissolution in the sediment or on the sea floor is apparent although sediment traps indicate a rain of caco3 through 3400 m the rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve while organic carbon is largely oxidized before it can be physically removed

pore water profiles and numerical modelling of biogeochemical processes in peru basin deep sea sediments abstract an extensive set of pore water profiles including o 2 no 3 no 2 nh 4 po 4 3 fe 2 mn 2 so 4 2 sio 4 4 ph and alkalinity as well as solid phase data on organic carbon content and porosity in 18 sediment cores obtained from five areas in the peru basin rv sonne 106 expedition in 1996 is presented the pore water profiles indicate that early diagenesis progresses through oxygen respiration denitrification and manganese iv reduction while the subsequent redox states iron iii reduction sulphate reduction and methanogenesis cannot be observed from the dissolved phase in these deep sea sediments a numerical diagenetic model was applied to the data in order to derive parameters such as the depositional flux of metabolizable organic matter or the bioturbation intensity that control the geochemical conditions in the deep sea floor and could not be obtained directly by measurements moreover the numerical simulations show that even those pore water no 3 profiles that decline non linearly to zero concentration are not compatible with a steady state assumption they can be explained only by transient downward progression of a nitrate front

determination of mixing parameters from tracer distributions in deep sea sediment cores abstract computer optimization and approximate analytical procedures are developed for the determination of the depth of bioturbation mixing d and the mixing coefficient d for instantaneous tracer inputs the methods have been applied to observations of microtektites volcanic ash and pumice in sixteen deep sea sediment cores the values for d are well determined generally with confidence intervals of a few centimeters or less and with an average value of d 11 cm for the sixteen cores the values for d are determined to first figure accuracy at best with values of around 10 cm 2 ka 1 or greater

particle mixing rates in deep sea sediments determined from excess 210pb and 32si profiles particle mixing rates have been determined for 5 south atlantic antarctic and 3 equatorial pacific deep sea cores using excess 210 pb and 32 si measurements radionuclide profiles from these siliceous calcareous and clay rich sediments have been evaluated using a steady state vertical advection diffusion model in antarctic siliceous sediments 210 pb mixing coefficients 0 04 0 16 cm 2 y are in reasonable agreement with the 32 si mixing coefficient 0 2 or 0 4 cm 2 y depending on 32 si half life in an equatorial pacific sediment core however the 210 pb mixing coefficient 0 22 cm 2 y is 3 7 times greater than the 32 si mixing coefficient 0 03 or 0 07 cm 2 y the difference in 210 pb and 32 si mixing rates in the pacific sediments results from 1 non steady state mixing and differences in characteristic time and depth scales of the two radionuclides 2 preferential mixing of fine grained clay particles containing most of the 210 pb activity relative to coarser particles large radiolaria containing the 32 si activity or 3 the supply of 222 rn from the bottom of manganese nodules which increases the measured excess 210 pb activity relative to 226 ra at depth and artificially increases the 210 pb mixing coefficient based on 32 si data and pore water silica profiles dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32 si profile in the mixed layer deep sea particle mixing rates reported in this study and the literature do not correlate with sediment type sediment accumulation rate or surface productivity based on differences in mixing rate among three antarctic cores collected within 50 km of each other local variability in the intensity of deep sea mixing appears to be as important as regional differences in sediment properties

microelectrode studies of organic carbon degradation and calcite dissolution at a california continental rise site fine scale porewater profiles of resistivity o2 ph and pco2 were measured in situ with microelectrodes at a site on the california continental rise they are reported here with more traditional measurements of shipboard porewater no3 alkalinity and tco2 and sediment organic c and caco3 profiles a numerical model encompassing diffusion advection and multiple reaction terms is applied to these data to characterize organic carbon degradation and caco3 dissolution processes near the sediment water interface a benthic o2 flux of 58 mol cm 2 yr 1 over the top 1 mm sediment is estimated based on one measured sub mm depth scale o2 profile the numerical model indicates that this high o2 consumption rate is related to a very labile fraction of organic c which has a degradation rate constant of approximately 4 5 10 8 s 1 1 4 yr 1 and which is not mixed downward by biological mixing seasonality in the benthic o2 consumption rate of this site may be attributed to variations in the input rate of this highly labile organic c degradation rate constants of the bulk organic c by oxygen reduction and by nitrate reduction are estimated to be about the same 1 10 9 s 1 the organic c rain rate responsible for the bulk organic c is estimated to be 53 mol cm 2 yr 1 this estimate exceeds the average rain rate determined by sediment trap collections at this site saturation state profiles calculated from the in situ ph and pco2 data and from modelling the in situ o2 and ph data both indicate that the porewater is undersaturated with respect to calcite to a depth of at least 8 cm the dissolution rate constant of calcite in this caco3 poor station is determined to be 5 10 day 1 assuming the reaction order is 4 5 which is close to the minimum estimates for environments richer in calcite a uniform calcite dissolution rate constant therefore may be adequate for representing carbonate dissolution in global scale models of ocean chemistry

measurements of diffusivity near the sediment water interface with a fine scale resistivity probe fine scale profiles of the sedimentary formation factor f were obtained from measurements of resistivity at millimetre intervals in marine sediments these profiles demonstrated that rapid decreases in diffusivity occurred in the upper few centimeters of both compact intertidal sands and fine silty muds in the compact sands a 70 decrease in the formation factor was observed within 2 cm of the interface close correlations were found between the formation factor and sediment porosity for both sediment types profiles of 1f predicted from these correlations and porosity profiles were very similar to those obtained from measurements of resistivity these correlations also suggested that 3 is a better approximation of the formation factor in fine muds than the archie relation 2 in both sediments the interfacial value of the diffusion coefficient was estimated to be between 74 and 87 of its value in free solution a value much higher than is normally assumed

numerical modeling of benthic processes in the deep arabian sea abstract aerobic and anaerobic degradation of particulate organic carbon poc and carbonate equilibria in deep sea surface sediments were studied at five stations located in the western wast northern nast eastern east central cast and southern sast arabian sea in situ oxygen fluxes porewater profiles of dissolved oxygen nitrate and mn ph profiles and solid phase profiles of particulate organic carbon mn and fe were measured at each station an early diagenesis model was applied to simulate the degradation and dissolution processes and to determine the benthic fluxes of poc oxygen nitrate phosphate co2 hco3 and co32 the benthic data sets were evaluated to constrain the poc input and the kinetics of organic matter degradation used in the model the modeling showed that the poc rain rate to the seafloor is high at the western and northern stations and decreases towards the southeast at stations located in the vicinity of continental margins wast nast east 5 7 of the primary production sinks to the deep sea floor this unusually high poc rain is either caused by dust particles that accelerate and amplify the particle export from the euphotic zone or by rapid lateral transport processes at the more remote stations cast sast that receive lower dust inputs the rain efficiency decreases to 1 4 in the model organic matter was separated into three fractions 3 g model that differ considerably in reactivity at stations wast nast east and cast the bulk of organic matter is composed of extremely labile organic matter with a first order degradation constant k of 15 30 yr 1 the moderately labile fraction with a kinetic constant of 0 2 0 6 yr 1 dominates the poc input at the oligotrophic station in the southern arabian sea sast the third fraction that has a very low reactivity k 2 5 10 4 yr 1 is only a minor component of the poc rain at all investigated stations more than 95 of the organic matter is consumed in aerobic degradation processes denitrification and metal oxide reduction only contribute 1 2 to the total poc degradation at the western station wast a non negligible portion 2 of organic matter is consumed via sulfate reduction the modeling demonstrates that carbonate dissolution is a major process in the deep arabian sea 52 83 of the carbonate rain to the seafloor is dissolved within the surface sediments in the western arabian sea the monsoon systems produce a strong seasonality in the primary production non steady state modeling indicates that the benthic oxygen nutrient and inorganic carbon fluxes closely follow the seasonal dynamics in primary and export production this very close benthic pelagic coupling is established by the extremely labile organic matter fraction that dominates the poc rain to the seafloor the metabolically released co2 induces a seasonal change in carbonate dissolution and carbonate alkalinity fluxes

spatial variability of surface sediment porewater ph and related water column characteristics in deep waters of the northern south china sea abstract this study analyzes the ph of surface sediment porewater i e 2 3 cm below the water sediment interface and concentrations of caco3 and organic carbon oc in 1192 sediment cores from the northern south china sea in water depths ranging from 137 to 3702 m this is the first study in the literature to analyze the large scale spatial variability of deep water surface sediment ph over a large ocean basin the data showed strong spatial variations in ph the lowest ph values

modeling of subsurface calcite dissolution including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off gabon abstract mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off gabon by using microsensors to measure o2 ph pco2 and ca2 in situ pore water concentration profiles of no3 nh4 fe2 and mn2 and so42 ex situ as well as sulfate reduction rates derived from incubation experiments the transport and reaction model cotrem was used to simulate the degradation of organic matter by o2 no3 fe oh 3 and so42 reoxidation reactions involving fe2 and mn2 and precipitation of fes model application revealed an overall rate of organic matter mineralization amounting to 50 mol c cm 2 yr 1 of which 77 were due to o2 17 to no3 and 3 to fe oh 3 and 3 to so42 the best fit for the ph profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0 01 and 0 5 d 1 and accounting for different calcite phases in the sediment a reaction order of 4 5 was assumed in the kinetic rate law a caco3 flux to the sediment was estimated to occur at a rate of 42 g m 2 yr 1 in the area of equatorial upwelling the model predicts a redissolution flux of calcite amounting to 36 g m 2 yr 1 thus indicating that 90 of the calcite flux to the sediment is redissolved

fine scale distributions of porosity and particulate excess210pb organic carbon and caco3 in surface sediments of the deep equatorial pacific abstract sediment samples were recovered from the central equatorial pacific ocean sectioned at 1 mm intervals and analyzed for porosity organic carbon excess210pb and caco3 steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface similarly particulate organic carbon contents are highest at the sediment surface decrease sharply in the upper 1 cm and are relatively constant between 1 and 5 cm caco3 values on the other hand are lowest at the sediment surface and increase to a constant value below 5 10 mm depth at the carbonate ooze sites excess210pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates however extremely high excess210pb activities 100 dpm g are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing this apparent contradiction along with the major features of the caco3 and particulate organic carbon profiles can be explained by a particle selective feeding mechanism in which organic carbon excess210pb enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms

simulation of early diagenetic processes in continental slope sediments off southwest africa the computer model cotam tested abstract pore water of marine sediments recovered from two stations of the continental slope off southwest africa were investigated we present computer simulations of in situ and laboratory concentration profiles of oxygen as well as laboratory concentration profiles of nitrate calcium ph and alkalinity the simulations were carried out with help of a numerical model cotam to describe the transport and the reaction of dissolved species in sediments cotam is based on an operator splitting approach comprising the independent calculation of transport and chemical reaction the consumption rates of oxygen and nitrate were determined by optimal fits to the measured pore water profiles of these species it could be shown that measured concentrations of nitrate in pore water correspond to a decomposition of organic matter with c n ratios between 3 and 3 7 however artificially increased subsurface nitrate concentrations due to core recovery cannot be excluded but our results show much greater deviations from expected concentrations assuming redfield stoichiometry than previously reported from comparative studies i e martin and sayles 1996 oxygen consumption in situ was shown to be distinctively lower than measured in multicorer cores after recovery simulations with varying denitrification rates indicate reduced diffusive nitrate release into the bottom water by up to 50 compared to shipboard results effects of nitrification and denitrification on pore water ph carbonate alkalinity and calcium concentrations were simulated by recalculating concentrations of these species with regard to calcite equilibrium for these calculations we used the standard software phreeqe as a subroutine of cotam calcium and carbonate alkalinity increase due to solid phase calcite dissolution ph was calculated to be an order of magnitude lower within the zone of oxygen depletion than indicated by shipboard results this difference is mainly related to non equilibrium conditions during ph measurements