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

modeling of calcite dissolution by oxic respiration in supralysoclinal deep sea sediments abstract pore water profiles of co2 ph ca2 and o2 in situ concentrations were measured at two stations on the upper continental slope off gabon the present study evaluates these measurements concerning their implications for the calcium carbonate system in deep sea sediments the model cotrem which was used to simulate the dynamics of this complex geochemical system revealed a strong dependence of calcite dissolution on oxic respiration at both sites all simulated calcite dissolution kinetics reached a dynamic equilibrium with almost equal calcite dissolution rates but different sub saturation states and ph values the latter are mainly dependent on boundary conditions and kinetic rate law parameters boundary conditions are of immense importance they define which ph deviations between measured data and the simulated equilibration instantaneous kinetics have still to be fitted by kinetic restrictions per rate law for the equilibration these ph deviations set up the range of possible values for rate constants in a given rate law to yield a well simulated ph a failure in the implementation of these boundary conditions may lead to non linearly flawed rate constants which fit only one usually the maximal ph deviation well the whole depth distribution of ph deviations has to be fitted very well by a kinetic rate law only this procedure secures that the used boundary conditions and rate laws constants are acceptable nevertheless several kinetic rate laws may be used for good ph fits featuring clear differences in parameter values for rate constants and reaction orders the connection between these rate laws with different parameters is examined and the dependence of the rate constant on the given form of the rate law is demonstrated this study achieves rate constants of 0 038 and 18 d for a dissolution rate law of 4 5th reaction order and sub saturation dependence from the term 1 these results are well within the range of rate constants obtained during former field studies for the same form of the dissolution rate law

evidence in support of first order dissolution kinetics of calcite in seawater abstract re examination of an experimental determination of the dissolution rate of reagent grade calcite in seawater shows that the original conclusion of rate law with 4 5 order dependence on undersaturation was very sensitive to uncertainties in the saturation state of the seawater with respect to calcite c in particular use of an erroneously high value of the calcite stoichiometric solubility product generated correspondingly low values of the saturation state extrapolation of the experimental measurements to a rate of zero dissolution indicates that the calcite solubility was about 20 lower than that used in the original study similar to more recent estimates if the lower solubity is used for recalulation of the experimental saturation states the dissolution rate rd c day 1 is adequately described by the rate experssion rd c 38 1 omega c 1 in situ measurements of ph in the pore waters of calcite rich sea floor sediments are more consistent with first order kinetics than with 4 5 order kinetics interpretation of pore water ph data using the 4 5 order rate expression requires dissolution rate constants that are different by at least two orders of magnitude and stoichiometric calcite solubility products that are different by several percent between two otherwise similar sites application of the first order dissolution reduces the variability in the rate constant to less than one order of magnitude and all in situ observations are consistent with a single estimate of calcite solubility first order kinetics do not reduce the discrepancy between the laboratory determined rate constants and those based on pore water measurements dissolution rate constants constrained by the in situ ph measurements are at least two orders of magnitude less than the laboratory results

caco3 dissolution in sediments of the ceara rise western equatorial atlantic we have used porewater sampling by in situ techniques including whole core squeezing as well as by shipboard sectioning and whole core squeezing to estimate the rates of sedimentary organic matter oxidation and caco3 dissolution at seven sites on the ceara rise in the western equatorial atlantic ocean porewater no3 profiles at all sites show a pattern indicative of active organic matter oxidation in the upper 15 20 cm of the sediments and in a buried organic rich layer the organic c oxidation rate generally decreases with increasing water depth from a value of 22 mol cm2 y at the shallowest site 3279 m to 14 mol cm2 y at the deepest site 4675 m over this depth range the bottomwaters vary from moderately supersaturated with respect to calcite to strongly undersaturated high resolution alkalinity profiles measured in porewaters collected by in situ whole core squeezing yield estimated ca2 fluxes of 11 mol cm2 y at a site located at the depth of the calcite saturation horizon and 7 6 zmol cm2 y at a moderately undersaturated site ca2 fluxes calculated from profiles in porewaters collected by relatively coarse resolution in situ sampling methods clearly indicate that there is caco3 dissolution above the calcite saturation horizon the dissolution of aragonite may contribute to the dissolution flux at the shallowest site these ca2 fluxes as well as fluxes estimated from a model of sedimentary organic matter oxidation and calcite dissolution indicate that 36 66 of the caco3 rain to the seafloor dissolves at sites at and above the calcite saturation horizon while 52 75 of the rain dissolves at sites below this depth when these results are incorporated into the oceanic caco3 budget of milliman 1993 they indicate that 35 of caco3 production is preserved in the deep sea they suggest a caco3 accumulation rate that is 27 lower than that estimated by milliman 1993 our corg oxidation caco3 dissolution model indicates that a large fraction of the caco3 dissolution that is occurring on the ceara rise is attributable to the neutralization of metabolic acids produced during organic matter oxidation the efficiency with which organic matter oxidation dissolves caco3 that is the ratio caco3 dissolution attributable to organic matter oxidation organic matter oxidation rate generally increases as degree of undersaturation of bottomwaters increases however there are deviations from the general trend that can be attributed to site to site variations in the kinetics of organic matter oxidation and calcite dissolution this result indicates that the dissolution of caco3 as a result of organic matter oxidation in the deep sea may mask the effects of variations in surface water caco3 productivity and bottomwater chemistry on the accumulation rate of caco3 in deep sea sediments

some effects of bioturbation by earthworms oligochaeta on archaeological sites abstract an experiment was devised to assess the extent to which small mammal bones may be dispersed in soil profiles by earthworm activity the results indicate that certain species of earthworms notably lumbricus terrestris displace small mammal bones considerable distances both vertically and horizontally these bones may also be broken and abraded and become mixed with bones from other sources other small objects are also likely to be moved in this way so the stratigraphic integrity of biological and cultural remains retrieved from soils where earthworms have been active should be cautiously appraised some features associated with earthworm activity are also described

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

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

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

computer simulation of deep sulfate reduction in sediments of the amazon fan pore water concentration profiles of sediments at a site on the amazon fan were investigated and simulated with the numerical model cotrem column transport and reaction model to reveal the biogeochemical processes involved the pore water profiles for gravity core geob 4417 7 showed a distinct sulfate methane transition zone in which deep sulfate reduction occurs only a small sulfide peak could be observed at the reaction zone due to high amounts of iron minerals the produced sulfide is instantaneously precipitated in form of iron sulfides we present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate furthermore sulfide iron ph pe calcium and total inorganic carbon tic were included in the simulation the program calculated mineral equilibria to mackinawite iron sulfides more stable than mackinawite iron hydroxides and calcite via saturation indices si by a module incorporating the program phreeqc parkhurst 1995 the measured sulfide and iron profiles are obtained in the simulation output by using a constant si 0 for mackinawite and calcite while a depth dependent si distribution is applied for the phreeqc phases pyrite and fe oh 3 a representing a composition and the kinetics of different iron sulfides and iron hydroxides these si distributions control the results of sulfide and iron pore water profiles especially conserving the sulfide profile at the reaction zone during the simulation the results suggest that phases of iron hydroxides are dissolved mackinawite is precipitated within and other iron sulfides are precipitated below the reaction zone the chemical reactivity of iron hydroxides corresponds to the rate of sulfide production the system h2o co2 caco3 is generally successfully maintained during the simulation deviations to the measured ph profile suggest that further processes are active which are not included in the simulation yet

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

an integrated study of dark earth from the alluvial valley of the senne river brussels belgium abstract the present article discusses the integration of urban geoarchaeological and archaeobotanical data of a series of dark earth deposits situated in the alluvial valley of the senne river in brussels belgium due to their homogeneous character their interpretation is a huge challenge for archaeologists through a case by case approach a detailed picture of the sequence of different activities and semi natural events leading to the build up of dark earth at each individual site has been obtained among the activities agricultural practices and waste management which are rarely archaeologically recorded in urban contexts have been identified despite being situated in a valley context none of the sites presented an excellent preservation of the organic plant remains however through the integration of geoarchaeological and botanical data the taphonomical history of the botanical remains could better be understood allowing us to document the evolution of the environment surrounding the sites additionally it has been demonstrated that the botanical study of dark earth units can also provide valuable information on vegetal consumption in particular fruits on a broader scale this integrated geoarchaeologial and archaeobotanical study sheds some light on the process of urbanisation of the senne alluvial valley between the 11th 12th and the 16th century ad

early diagenetic processes fluxes and reaction rates in sediments of the south atlantic abstract porewaters recovered from sediment cores gravity corers box corers and multicorers from various subrogions of the south atlantic amazon river mouth equatorial upwelling congo river mouth benguela coastal upwelling area and angola basin were investigated geochemically objectives included determination of eh ph oxygen nitrate sulfate alkalinity phosphate ammonium fluoride sulfide cal mg sr fe mn and si in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes porewater profiles from the eastern upwelling areas of the south atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation whereas organic matter degradation by sulfate reduction is restricted to the near surface sediments further a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m far below the zone of bioturbation or bioirrigation because other sedimentological reasons seem to fail as explanations an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed porewater profiles from the sediments of the amazon fan area are mainly influenced by reactions with fe iii phases the remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments in contrast to the upwelling sediments the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5 3 m probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals a comparison of organic matter degradation rates from the different areas of the south atlantic show the expected relationship to primary productivity oxygen is the dominating oxidant whereas organic matter degradation by nitrate mn iv and probably also fe iii reduction is several orders of magnitude lower sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic rich sediments but may also include methane oxidation reoxidation of ammonium and mn2 by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone but may contribute to the nitrate reduction in suboxic layers

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

calcite dissolution in sediments of the ontong java plateau in situ measurements of pore water o2 and ph we present in situ electrode measurements of sediment resistivity pore water oxygen and pore water ph from three stations between 2300 and 3000 m depth on the ontong java plateau in the western equatorial pacific one of these stations is also the site of a concurrent benthic chamber incubation experiment jahnke et al 1994 the pore water oxygen data and a steady state diffusion and reaction model constrain the depth dependent rate of oxic respiration in the sediments and imply a diffusive flux of oxygen to the sediments of 10 21 mol cm 2 yr 1 given these respiration rates the pore water ph data cannot be explained without calcite dissolution driven by metabolically produced co2 the dissolution necessary to explain the observations quantified by a statistical approach is 3 5 6 mol cm 2 yr 1 which corresponds to at least 20 40 of the calcite rain to these sediments over 65 of the total dissolution is driven by metabolic co2 oxygen fluxes and net calcite dissolution constrained by the electrode data are compatible with the benthic chamber measurements of jahnke et al 1994 the dissolution flux while a significant part of the early diagenesis of calcite in these sediments is less than would be predicted by earlier models of dissolution and jahnke et al 1994 probably could not distinguish it from zero with the benthic chamber technique the dissolution rates found in this study are lower than previous estimates because the respiration reaction is concentrated near the sediment water interface and the calcite dissolution rate constants are very small the statistical evaluation of the pore water ph data and model constrain the calcite dissolution rate constant to 0 005 0 16 d 1 following the general trend of lower values determined by in situ techniques rather than by laboratory methods

artifact preservation and post depositional site formation processes in an urban setting a geoarchaeological study of a 19th century neighborhood in detroit michigan usa a geoarchaeological study was carried out to assess levels of artifact deterioration occurring in a historic period urban soil during the 20th century the study site is a former house lot in a park created in 1919 by demolition of a residential community in detroit michigan usa the results show that despite nearly a century of burial in an urban soil impacted heavily by pollution and other anthropogenic activity many 19th century artifacts are remarkably well preserved the observed weathering stability sequence of glass bone mortar plaster paint is consistent with decreasing solubility product values of the corresponding principal mineral constituent glass apatite portlandite gypsum cerrusite even severely weathered 19th century nails and mortar can often be distinguished using optical petrographic and sem edax methods the excellent state of artifact preservation is attributed to a calcareous soil microenvironment and artificial compaction which limited the weathering effects of water and oxygen artifact preservation was further enhanced by burial beneath a thick biomantle created by the casting activity of an invasive species of earthworm however lumbricus terrestris may now pose the greatest threat to artifact preservation because casting and burrowing activities are decreasing bulk density and promoting the diffusion of air and water into the soil early excavation is recommended to recover artifacts in soils impacted by the combined effects of urban pollution and earthworm burrowing anthropogenic microparticles smaller than those normally classified as microartifacts were found to be useful indicators of human occupation

deposition and taphonomy of earthworm granules in relation to their interpretative potential in quaternary stratigraphy calcium carbonate granules up to 2 5 mm in size are commonly found in quaternary soils and sediments but have only rarely been used for any form of interpretation growing interest in recent years has focused on the concentration patterns in stratigraphy containing buried land surfaces and the possibility of dating the granules making sense of either of these approaches requires a basic understanding of granule types together with their modes of accumulation and destruction in stratigraphy details of the formation morphology deposition and post depositional changes are discussed along with the necessary ecological and pedological information on earthworm behaviour and effects then summarised into a framework for interpretations copyright 2006 john wiley sons ltd

a new boundary layer mixing scheme part i scheme description and single column model tests a new boundary layer turbulent mixing scheme has been developed for use in the ukmo weather forecasting and climate prediction models this includes a representation of nonlocal mixing driven by both surface fluxes and cloud top processes in unstable layers either coupled to or decoupled from the surface and an explicit entrainment parameterization the scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers details of the scheme and examples of its performance in single column model tests are presented

a horizon dynamics in a historical hedged landscape abstract in hedged landscapes the hedge structure induces a modification of soil organization at the landscape scale and another modification of soil geometry and properties at the slope scale in these environments the effects of landscape structures on soil pedogenesis and properties are identified yet the time scale on which the target processes are occurring remains unknown the aim of this paper is to constrain the space and time changes in hedge structure within a historical hedged landscape in order to better quantify the dynamics of a horizons geometry to achieve this goal we implemented various relative methods soil geometry analysis historical document analysis and absolute methods 14 c 137 cs results highlight the complexity of soil spatial organization within the landscape a horizon geometry is clearly influenced by landscape structures whereas deeper horizons appear not to be influenced by these structures the explanatory factors of this current organization are not only the present topography and hedgerow network pattern but also the position of former hedgerows organo mineral horizon changes clearly post date the establishment of the hedgerow network landscape which has controlled topsoil horizon geometry since 1833 the decrease in hedgerow network density has changed the spatial distribution of soil redistributions through diffusive transport including tillage translocation and water erosion processes locally it has increased soil erosion processes with respect to the increase in uphill drainage surface areas without changing general soil organization at the landscape scale acquired over the longer term

seamless stratocumulus simulation across the turbulent gray zone abstracta pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested the method blends a conventional boundary layer parameterization suitable for large grid lengths with a subgrid turbulence scheme suitable for large eddy simulation the key parameter for blending the schemes is the ratio of grid length to boundary layer depth the new parameterization is combined with a scale aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations the improved microphysical representation removes the correlation between precipitation rate and model grid length while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced

modeling the calcite lysocline a numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea model results are compared with lysocline data from 13 regions in the atlantic pacific and indian oceans the model lysocline shape is sensitive to the calcite dissolution rate constant the calcite organic carbon and refractory material rain rates and the rates of oxic versus anoxic organic carbon degradation in the sediment the model is able to reproduce the observed lysocline within the constraints of the sediment trap and calcite accumulation data using a calcite dissolution rate constant of 30 100 d 1 a molar ratio of organic carbon to calcite rain rates of 0 5 1 0 and an initial caco3 fraction of 90 excluding organic carbon this rate constant is consistent with microelectrode results presented by archer et al 1989 the model predicts that 30 50 of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration consistent with previous modeling studies the range in the best fit value of the organic inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material rather than from scatter in the data lysocline data from the western equatorial atlantic are anomalous to the rest of the data this anomaly may be explained by high rates of refractory material sedimentation from the amazon river

reconstructing taphonomic histories using histological analysis abstract recent years have seen rapid advances in the understanding of diagenetic changes to bone tissues and how these influence the chemistry microstructure and histological appearance of ancient bone it is now possible to recognise many characteristic features of diagenetically modified bone and this has led to the potential use of these parameters in estimating the potential survival of biogenic signals such as dna lipids proteins and stable isotopes these characteristic features also hold the potential for preserving a record of different post mortem environments in individual bones or assemblages of bones from the same site in sites where the burial conditions have changed over archaeological or geological timescales histological analyses can shed light on these different burial environments and permit the reconstruction of taphonomic histories of some bones examination of polished sections of bone using bse sem has been used to identify characteristic features attributed to aerobic soil bacteria cyanobacteria and sulphate reducing bacteria the approach shows promise for providing supplementary evidence when phasing complex sites such as graveyards which developed over several hundred years

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

quantification of diffusive benthic fluxes of nitrate phosphate and silicate in the southern atlantic ocean pore water data from surface sediments and bottom water data at 180 locations were used to create distribution maps of diffusive benthic release rates of nitrate phosphate and silicate in the southern atlantic ocean for water depths 1000 m geostatistical methods of variogram analysis and kriging were used for data interpolation since benthic fluxes are controlled by organic matter degradation and dissolution processes and therefore by the supply of organic matter to the sediment surface the distribution pattern of all investigated constituents should reflect a general coupling to surface water productivity this seems to be true for the most part of the oligotrophic central south atlantic and the upwelling area off southwest africa striking regional differences in this regard however exist for the eastern equatorial region where unexpectedly low fluxes were calculated and the argentine basin where benthic fluxes are very high the argentine basin obviously receives a higher input of degradable dissolvable material whereas in the eastern equatorial region the low benthic fluxes are possibly due to intense nutrient recycling within the water column estimated fluxes vary considerably over the entire region no3 10 180 mmol m 2 yr 1 po4 2 11 mmol m 2 yr 1 and si 0 0 65 mol m 2 yr 1 areas of highest release rates are the continental margins off southwest africa and off argentina the investigated area comprises a total of approximately 32 106 km2 between 8 n and 40 50 s our estimations reveal benthic release rates of 1 55 l012 mol no3 yr 1 3 5 l010 mol po4 yr 1 and 2 1 1012 mol si yr 1 open ocean areas with middle to low fluxes contribute about 70 80 of the total estimated fluxes the release of phosphate is probably underestimated because of core recovery artifacts and is therefore based on the assumption that fluxes are not lower than 0 25 mmol m 2 yr 1 at any location of the studied area the investigated area represents about one tenth of the world deep ocean without the arctic ocean assuming the south atlantic to be representative we calculate annual global release rates of 15 l012 mol no3 32 l010 mol po4 and 20 1012 mol si if our estimates are correct about 4 6 of silicate 3 4 of nitrate and 1 of phosphate annually produced in the surface waters are remineralized in surface sediments from water depths 1000 m

diagenetic alteration of magnetic signals by anaerobic oxidation of methane related to a change in sedimentation rate geochemical and rock magnetic investigations of sediments from three sites on the continental margin off argentina and uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane aom the western argentine basin represents a suitable sedimentary environment to study nonsteady state processes because it is characterized by highly dynamic depositional conditions mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals as a typical feature of these deposits distinct minima in magnetic susceptibility are observed pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate methane transition smt where hs is generated by the process of aom the released hs reacts with the abundant iron oxyhydr oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate sr which occurred during the pleistocene holocene transition we assume that the strong decrease in mean sr encountered during this glacial interglacial transition induced a fixation of the smt at a specific depth the stagnation has obviously enhanced diagenetic dissolution of iron oxyhydr oxides within a distinct sediment interval this assumption was further substantiated by numerical modeling in which the mean sr was decreased from 100 cm kyr 1 during glacial times to 5 cm kyr 1 in the holocene and the methane flux from below was fixed to a constant value to obtain the observed geochemical and magnetic patterns the smt must remain at a fixed position for 9000 yrs this calculated value closely correlates to the timing of the pleistocene holocene transition the results of the model show additionally that a constant high mean sr would cause a concave up profile of pore water sulfate under steady state conditions copyright 2005 elsevier ltd

the numerical representation of entrainment in parameterizations of boundary layer turbulent mixing abstract problems have been identified with parameterizations of convective boundary layers in particular with their numerical treatment of the capping inversion it is shown that the turbulence scheme can combine with the numerical representation of subsidence to produce unphysical entrainment as was also identified by lenderink and holtslag a correction is proposed for the lock et al boundary layer parameterization in which a discontinuous inversion is diagnosed from the mean thermodynamic profiles every time step this then allows a consistent treatment of subgrid scale processes in the region of the inversion in particular the parameterized turbulent entrainment flux can be adjusted to allow for the spurious entrainment arising from the conflicting representations of turbulent mixing and vertical advection it also allows a direct coupling between the turbulent and radiative fluxes to ensure that cloud top radiative cooling is correctly distributed between the inversion grid level and the mixed

a moist pbl parameterization for large scale models and its application to subtropical cloud topped marine boundary layers abstract a new general purpose boundary layer parameterization that permits realistic treatment of stratocumulus capped boundary layers scbls with coarse vertical resolution is described it combines a 1 5 order turbulent closure model with an entrainment closure at the boundary layer top three different implementations of the entrainment closure in which the boundary layer height is respectively prognosed reconstructed from thermodynamic values at the grid points or restricted to lie on a flux level of the host model grid are tested in a single column modeling framework at both fine and coarse vertical resolution the first two approaches permit a stratocumulus top and base to lie between grid levels and evolve continuously with time but are more complicated to implement in a three dimensional model the model performs very well in cases of dry convection whatever the inversion implementation and the vertical resolution with 15 mb or better vertical resolution all approaches properly simulate

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

micromorphological and chemical investigation of late viking age grave fills at hofstaðir iceland grave fills from seven human burials from a late viking age early medieval cemetery at hofsta ir m vatnssveit iceland were examined by soil micromorphology and organic chemical analysis detailed analysis of the weathering of the mineral constituents of the grave fills demonstrates a relationship between the extent of weathering of volcanic silicates within the fills and the presence of buried human remains gas chromatography mass spectrometry gc ms of extracts from the fills and controls of two graves revealed organic signatures dominated by plant derived organic matter with no evidence of degradation products of the body tissues transformation of n alkanes into n alkan 2 ones provides evidence for microbial activity within the fills gc ms analysis of the organic extract from under one of the skulls and pyrolysis gas chromatography of wood fragments found in that grave provide compelling evidence for the former presence of a conifer wood coffin the use of this non native wood in the burial provides evidence for either the import of foreign lumber or the utilisation of driftwood most likely originating from russia siberia

systematic cultivation of the swifterbant wetlands the netherlands evidence from neolithic tillage marks c 4300 4000 cal bc the middle neolithic 4300 4000 cal yr bc archaeological sites in the swifterbant area were typically regarded to represent a transitional phase between a hunter gatherer and agricultural subsistence the discovery of a tilled layer on one of the sites s4 site in 2007 during a renewed excavation campaign 2004 2007 was made possible by a close cooperation between archaeologists diatom specialists and micromorphologists intensive sampling and micromorphological investigation revealed that the swifterbant cultural layers typically consists of micro laminated deposits of phytoliths and charred plant remains with waste this led to the reinterpretation of the sites as middens rather than settlements at least five levels could be identified that had been tilled with some kind of hand tool these levels were separated by natural clay deposits and midden layers these results and reinterpretation of observations from two other sites in the swifterbant area s2 and s3 indicate that tillage and crop production formed a regular part of the subsistence of the inhabitants during the middle neolithic rather than a transitional phase the sites investigated probably should be regarded as traces of a fully agricultural society

observed lagrangian transition of stratocumulus into cumulus during astex mean state and turbulence structure aircraft measurements made during the first lagrangian of the atlantic stratocumulus transition experiment astex between 12 and 14 june 1992 are presented during this lagrangian experiment an air mass was followed that was advected southward by the mean wind five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below from the horizontal aircraft legs the boundary layer mean structure microphysics turbulence structure and entrainment were analyzed the vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds during the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli the total water flux the virtual potential temperature flux and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights to analyze the cloud top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top these jumps were compared with cloud top entrainment instability criteria discussed in the literature it is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus cumulus transition

same anthropogenic activity different taphonomic processes a comparison of deposits from los husos i ii upper ebro basin spain abstract sediments from archaeological deposits can be affected both by human activity and environmental agents after deposition as a result physical and chemical alterations can be recognized in the stratigraphical record this work focuses on characterizing and comparing the post depositional alterations that have affected prehistoric stall deposits from two neighbouring rockshelters located in the upper ebro basin los husos ii ca 4450 3790 cal bc and los husos i ca 2210 1620 cal bc micromorphological and fluorescence microscopy analyses applied on ash dung plant remains and geogenic parent material allow disentangling the origin and formation processes of physical mechanical and physicochemical taphonomic features from both sites identification of truncation interfacies compaction bioturbation and mechanical effects by water among physical alterations together with evidence of weathering redistribution and neoformation of phosphorous calcium iron and manganese rich features makes it possible to assess the dynamics of the stabling sediments after their accumulation

the effect of horizontal resolution on indian monsoon depressions in the met office nwp model monsoon depressions are synoptic scale features that are responsible for a significant fraction of the rain over northern india during the summer monsoon season and as such it is important to quantify their structure and behaviour in numerical weather prediction models it is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term as well as better representation of tropical cyclones here we explore the sensitivity of depressions to changes in resolution using the met office unified model seven nwp case studies of depressions from 2013 15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km and compared with data for the same events from trmm and era interim reanalysis we found that at the low resolution end of the spectrum increases in resolution led to improvements in the composite structure but with diminishing returns the model also persistently overestimated the depression intensity in particular the wind speed and the warm core aloft with the source appearing to originate in the mid troposphere the sensitivity of the diurnal cycle to resolution was also explored the stratiform component was found to be very well represented by the model whereas the convective component was described quite poorly improvement in most components of structure with increasing model resolution were marginal beyond n320 63 km and n512 39 km for dynamic and thermodynamic fields respectively

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

evaluation of the caco3 dissolution proxies in sediment cores from above the lysocline calcium carbonate analyses were carried out from two size fractions 63 m from the ocean drilling program sites 752 and 715 collected from above the lysocline depth in the indian ocean to evaluate the calcium carbonate size index as paleocarbonate ion proxy in these cores size index values varied from 69 to 81 at site 752 and showed a similar pattern with that of total carbonate content at this site but high values of size index and the location of this site far from any landmass suggest that only changes in productivity of overlying waters are responsible for the observed variations low values of size index at site 715 suggest supralysoclinal dissolution the inverse relation between size index variations and the total caco3 content shows that the total carbonate variations are controlled by the finer fraction composed of juvenile foraminifera foraminifera fragments and coccolithophores this further confirms the observed variations of caco3 content of the 63 m fraction which follows the trend of total caco3 content this study demonstrates that apart from factors such as breakup of shells after burial due to organic matter respiration and dilution due to terrigenous material changes in surface water productivity and coccolithophores might be a constraint to use size index as a paleocarbonate ion proxy for sediments from above the lysocline

a late glacial colluvial sequence at watcombe bottom ventnor isle of wight england in a dry valley near ventnor isle of wight thick subaerial slope deposits of devensian late glacial age overlie the chalk the deposits are crudely stratified chalk muds and rubble produced by frost shattering and moved downslope by the release of water from melting snow fields and frozen ground a laterally extensive humic horizon of a rendzina soil occurs within the sequence clearly reflecting a period of relative slope stability in places this horizon divides and its upper surface appears to have been disturbed possibly by cryoturbation micromorphological and other analyses confirm the pedogenic origin of this humic horizon but also demonstrate the occurrence of pedological features both above and below it the humic horizon therefore is not a buried soil in the strictest sense but is part of a vertical sequence representing a single complex soil with transported accretionary and welded components molluscan analyses reveal that the sequence can be divided into four local mollusc zones showing a progressive increase in faunal diversity throughout the profile this succession is broadly similar to other late glacial sequences described from south east england minute fragments of charcoal from the lower part of the humic horizon have yielded an ams date of 11690 120 vr bp demonstrating formation during the allerod phase of the late glacial interstadial this humic horizon is correlated provisionally with the pitstone soil even though existing dates from its type site in buckinghamshire are somewhat younger

benthic fluxes and pore water studies from sediments of the central equatorial north pacific nutrient diagenesis benthic exchange rates of radon 222 oxygen nitrate ammonia and silica were determined using an in situ benthic flux chamber and by modeling pore water profiles at three sites in the central equatorial north pacific a comparison of these results reveals several artifacts of pore water collection and processing whole core squeezer wcs silica profiles are influenced by adsorption during squeezing and yield calculated fluxes that are too large pore water ammonia profiles show near surface maxima that appear to be an artifact of core recovery near surface nitrate measurements may also be suspect due to oxidation of the ammonia released causing anomalously large nitrate gradients that yield overestimates of benthic exchange rates fluxes of radon oxygen and nitrate calculated from wcs profiles agree with chamber fluxes to better than 40 at all sites fluxes of silica and nitrate calculated from pore water data collected at coarser scales 1 cm intervals agree within 50 with chamber measurements previous flux estimates from pore water and solid phase models established at two of these sites using data collected 6 years prior to this work differ from these chamber measurements in some cases by up to a factor of 5 due to modeling uncertainties and temporal variabilities in the delivery of organic matter to a site the benthic oxygen consumption rates measured at these sites are similar they average 0 36 0 03 mmol m 2 day 1 and are consistent with a trend of oxygen uptake vs water depth previously established by others on a transect through the oligotrophic north pacific gyre

earthworm activity and archaeological stratigraphy a review of products and processes the activity of earthworms has long been understood to have significant effects on archaeological stratigraphy but many of the details have not been fully grasped by stratigraphers or by the specialists who advise them although worms are widely known to cause the burial of finds due to surface casting some of their other activities are less commonly recorded amongst these their active burial of stones and seeds their building of cairns and their production of calcium carbonate granules have mostly not been studied in any detail and have been little exploited from an interpretative point of view this paper reviews the existing knowledge as well as presenting some recent research carried out into various aspects of worm actions and their relationship to taphonomy

controls on modern carbonate preservation in the southern florida straits abstract the water masses in the florida straits and bahamas region are important sources for the northern atlantic surface ocean circulation in this study we analyse carbonate preservation in surface sediments located above the chemical lysocline in the florida straits and bahamas region and discuss possible reasons for supralysoclinal dissolution calcite dissolution proxies such as the variation of the foraminiferal assemblage fragmentation index benthic foraminifera index and resistance index displayed a good preservation in both areas the pteropod species limacina inflata showed very good preservation in sediments of inter platform channels from the great bahama bank providence channel exuma sound above the aragonite lysocline supralysoclinal aragonite dissolution however was observed at two water depth levels 800 1000 m and below 1500 m in the florida straits our observations suggest that the supralysoclinal dissolution in the florida straits is due to the degradation of organic material the presence of antarctic intermediate water aaiw may be a contributing factor for the significant aragonite dissolution in 800 1000 m the comparison of modern preservation patterns of the surface sediments with hydrographical measurements shows that the l inflata dissolution index ldx might be an adequate proxy to reconstruct paleo water mass conditions in an area which is highly saturated with respect to calcium carbonate

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

factors influencing cloud area at the capping inversion for shallow cumulus clouds large eddy simulations of marine cumulus clouds are performed aspects of the simulations are varied in order to identify those parameters that play a role in determining when the cumulus clouds spread out under the capping inversion into stratocumulus these include the strength of the surface fluxes which also affects the strength of the cumulus mass transport the strength of the capping inversion the moisture content of the free atmosphere the strength of cloud top radiative cooling and whether there is significant precipitation or sedimentation of cloud drops tests are also performed to evaluate sensitivity to the model resolution as in previous studies having sufficient resolution at the capping inversion is found to be important for robust simulation of any stratiform cloud that forms there in addition the cloud cover is found to be sensitive to many of the physical processes investigated in the end though a clear and robust dependence emerges on the cloud top entrainment instability parameter for large a typical small shallow cumulus cloud cover is seen for below the threshold for buoyancy reversal the cumulus spreads into a solid sheet of stratocumulus beneath the inversion consistent with previous studies there is no abrupt transition and the cloud cover decreases smoothly as increases the conclusion is reached that varying other parameters in the simulations affects the cloud cover through their impact on the size of the jumps across the inversion and thence the implications for turbulent mixing parametrizations in general circulation models are discussed crown copyright 2009 reproduced with the permission of hmso published by john wiley sons ltd

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

past changes in biologically mediated dissolution of calcite above the chemical lysocline recorded in indian ocean sediments abstract in a deep sea sediment core recovered from a site lying well above the local lysocline several organic geochemical proxies and two different calcite dissolution indicators are compared in order to evaluate the relationship between calcite dissolution and paleoproductivity over the past three glacial interglacial cycles the degree of foraminiferal break up and the caco3 particle size distribution both point to significant periods of dissolution every 22 kyr during glacial stages and substages these dissolution events are concomitant with periods of enhanced primary productivity as indicated by the abundance of several biomarkers alkenones cholesterol brassicasterol keto ol used here to indicate changes in paleoproductivity dissolution fluctuations are highly coherent and in phase with the estimated paleoproductivity variations providing strong evidence that the observed dissolution is due to organic matter remineralization within the sediments rather than to changes in co32 concentration in the overlying water column