microbially induced cementation to control sand response to undrained shear current methods to improve the engineering properties of sands are diverse with respect to methodology treatment uniformity cost environmental impact site accessibility requirements etc all of these methods have benefits and drawbacks and there continues to be a need to explore new possibilities of soil improvement particularly as suitable land for development becomes more scarce this paper presents the results of a study in which natural microbial biological processes were used to engineer a cemented soil matrix within initially loose collapsible sand microbially induced calcite precipitation micp was achieved using the microorganism bacillus pasteurii an aerobic bacterium pervasive in natural soil deposits the microbes were introduced to the sand specimens in a liquid growth medium amended with urea and a dissolved calcium source subsequent cementation treatments were passed through the specimen to increase the cementation level of the sand particle matrix the results of both micp and

silicic acid dynamics in the glacial sub antarctic implications for the silicic acid leakage hypothesis 1 the silicic acid leakage hypothesis salh purports that changes in silicon and nitrogen depletion ratios in the glacial antarctic created a large pool of unused silicic acid that was transported to lower latitudes in subantarctic mode water samw where it enhanced diatom productivity lowering atmospheric pco2 however increased opal accumulation beneath the sub antarctic during glacial times implies significant consumption of silicic acid in subantarctic surface waters that may have significantly diminished or eliminated si leakage to test how nutrient dynamics in the sub antarctic affected the si oh 4 content of samw during the last glacial period we produced 30si opal records for cores from the subantarctic and subtropical zones of the indian ocean spanning the last 50 000 years comparison with diatom bound 15n records shows that subantarctic surface waters were enriched in si relative to n during the last glaciation consistent with the salh the record from the subtropics does not show this enrichment in si during the last glacial period suggesting that subantarctic surface waters were mainly incorporated into samw rather than being transported across the subtropical front isotope mass balance calculations were used to test for si leakage from the sub antarctic the results show that silicic acid concentration in samw would more than double during the last glaciation if upwelling and northward ekman drift in the antarctic were similar to the present day circulation calculations that assume increased stratification in the glacial antarctic eliminate si leakage but they do not produce the known increase in opal burial in the glacial sub antarctic reconciling the isotope data and opal burial records with a highly stratified antarctic requires the addition of a large local source of silicic acid in the glacial sub antarctic that is inconsistent with present day circulation and nutrient distributions including such a source in our calculations results in significant opal burial in the sub antarctic but it does not enhance si leakage over that occurring in the holocene resolving past changes in the circulation of the southern ocean is clearly vital to future tests of the salh

the δ15n of nitrate in the southern ocean consumption of nitrate in surface waters we report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern indian and pacific sectors of the southern ocean focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton the 15n of nitrate increases by 1 2 from deep water into the antarctic summertime surface layer due to kinetic isotopic fractionation during nitrate uptake estimation of the nitrate uptake isotope effect from antarctic depth profiles yields values in the range of 5 6 in east indian sector and 4 5 in the east pacific sector surface transect data from the pacific sector also yield values of 4 5 the major uncertainty in the profile based estimation of the isotope effect involves the 15n of nitrate from the temperature minimum layer below the summertime antarctic surface layer which deviates significantly from the predictions of simple models of isotope fractionation for the subantarctic surface it is possible to distinguish between nitrate supplied laterally from the surface antarctic and nitrate supplied vertically from the subantarctic thermocline because of the distinctive relationships between the 15n and concentration of nitrate in these two potential sources our subantarctic samples collected during the summer and fall indicate that nitrate is supplied to the subantarctic surface largely by northward transport of antarctic surface water isotopic data from the pacific sector of the subantarctic suggest an isotope effect of 4 5 indistinguishable from the antarctic estimates in this sector

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

pore scale network modeling of microbially induced calcium carbonate precipitation insight into scale dependence of biogeochemical reaction rates the engineering of microbially induced calcium carbonate precipitation micp has attracted much attention in a number of applications such as sealing of co2 leakage pathways soil stabilization and subsurface remediation of radionuclides and toxic metals the goal of this work is to gain insight into pore scale processes of micp and scale dependence of biogeochemical reaction rates this will help us develop efficient field scale micp models in this work we have developed a comprehensive pore network model for micp with geochemical speciation calculated by the open source phreeqc module a numerical pseudo 3d micromodel as the computational domain was generated by a novel pore network generation method we modeled a three stage process in the engineering of micp including the growth of biofilm the injection of calcium rich medium and the precipitation of calcium carbonate a number of test cases were conducted to illustrate how calcite precipitation was influenced by different operating conditions in addition we studied the possibility of reducing the computational effort by simplifying geochemical calculations finally the effect of mass transfer limitation of possible carbonate ions in a pore element on calcite precipitation was explored this article is protected by copyright all rights reserved

relevance of silicon isotopes to si nutrient utilization and si source assessment in antarctic waters 1 we analyzed 29si of dissolved silicate for eight water column profiles across the southern ocean south of australia in spring 2001 from the seasonal ice zone siz north to the subantarctic zone saz including the first isotopic compositions measured for si depleted seawaters all profiles display mixed layer enrichments in heavy si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms as silicate levels decrease from the siz northward across the polar front zone pfz to the saz surface and mesopelagic 29si signatures generally become progressively heavier but the most si depleted saz waters do not exhibit 29si values heavier than in the pfz this intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters and by applying a steady state open system model we estimate a fractionation factor 29 between diatoms and seawater of 0 45 0 17 independently of zones and phytoplankton community though encouraging these results are related to latitudinal changes in mesopelagic 29si values complexity in surface silicate 29si correlations and differences from previous studies which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations

strength stiffness and microstructure characteristics of biocemented calcareous sand calcareous sands are known as problematic soils in nature and challenge geotechnical engineers in many practical projects microbially induced calcite precipitation micp is an innovative soil imp

silicon isotope fractionation during nutrient utilization in the north pacific the distribution of silicon in the north pacific is controlled by the utilization of silicic acid by diatoms a process that fractionates silicon si isotopic compositions silicon isotope variations are presented for six water column profiles from the surface mixed layer down to the deep waters of the north pacific ocean although the observed si isotopic variations display an apparently simple inverse relationship with dissolved nutrient concentrations in fact they reflect mixing of surface waters undergoing active si isotope fractionation and deep waters with more uniform concentrations and isotope compositions samples from the surface of the subtropical gyre have the lowest dissolved si concentrations and heaviest si isotope compositions of marine waters measured thus far fractionation in the surface waters follows a typical rayleigh type distillation curve for a closed surface water reservoir resulting from stratification of the surface layer in the subarctic region in contrast an open system prevails within the subtropical gyre where there is significant recycling of silicic acid in the upper water column and lateral transport of silicon within surface currents for deep waters the si isotope composition distinguishes between the northern north pacific deep water npdw and the southerly derived bottom water the relatively light si isotope compositions measured from waters within the subarctic gyre provides evidence for isolation of the nutrient pool in the north pacific

factors controlling the silicon isotope distribution in waters and surface sediments of the peruvian coastal upwelling we present the first systematic study of the silicon isotope composition in the water column 30sisi oh 4 and in diatoms 30sidiatom from the underlying surface sediments in a coastal upwelling region the surface waters upwelling on the shelf off peru are mainly fed by southward flowing subsurface waters along the coast which show a mean 30sisi oh 4 of 1 5 the concentration of dissolved silicic acid si oh 4 increases towards the south in these waters and with increasing water depth suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica bsio2 in the water column and in the surface sediments surface waters in the realm of the most intense upwelling between 5 s and 15 s have only marginally elevated 30sisi oh 4 values 30sisi oh 4 1 7 with respect to the source si isotope composition whereas further north and south where upwelling is less pronounced surface waters are more strongly fractionated 30sisi oh 4 up to 2 8 due to the stronger utilisation of the smaller amounts of available si oh 4 the degree of si oh 4 utilisation in the surface waters along the shelf estimated from the si oh 4 concentration data ranges from 51 to 93 the 30sidiatom values of hand picked diatoms in the underlying surface sediments vary from 0 6 to 2 0 which is within the range of the expected fractionation between surface waters and diatoms the fractionation signal in the surface waters produced during formation of the diatoms is reflected by the 30sidiatom values in the underlying sediments with the lowest 30sidiatom values in the main upwelling region the silicon isotope compositions of bsio2 30sibsio2 from the same surface sediment samples are generally much lower than the 30sidiatom signatures indicating a significant contamination of the bsio2 with biogenic siliceous material other than diatoms such as sponge spicules this shift towards lighter 30sibsio2 values by up to 1 3 compared to 30sidiatom signatures for the same surface sediment samples potentially biases the interpretation of 30si paleorecords from sediments with low bsio2 concentrations and thus the reconstruction of past si oh 4 utilisation in surface waters

simulating the modern δ30si distribution in the oceans and in marine sediments the 30si of biogenic silica inline image in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past the application of this proxy however requires an understanding of the modern 30si distributions and their controlling mechanisms here we present results from a modern climate simulation with a coupled ocean sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation in agreement with previous studies biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated 30si a new finding is a distinct seasonal cycle of 30si in the surface ocean which is inversely related to that of silicic acid concentration and mixed layer depth we also provide the first simulation results of sedimentary 30si which reveal that 1 the inline image distribution in the surface sediment reflects the exported inline image signal from the euphotic zone and 2 the dissolution of biogenic silica in the sediment acts as a source of relatively light 30si into the bottom waters of the polar oceans while it is a source of heavier 30si to the subtropical south atlantic and south pacific

tracing silicon cycling in the okavango delta a sub tropical flood pulse wetland using silicon isotopes chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction potentially confounding estimates of silicate weathering rates silicon isotopes d30si may be a useful tool to investigate these processes here we present 82 d30si measurements from surface waters pore waters biogenic silica bsi clays sand and vegetation from the okavango delta botswana a freshwater sub tropical flood pulse wetland hydrologically the okavango is dominated by evapotranspiration water losses to the atmosphere it receives an annual pulse of water that inundates seasonal floodplains while river baseflow is sufficient to maintain a permanent floodplain d30si in dissolved silica dsi in surface waters along a 300 km transect at near peak flood show a limited range 0 36 1 19 imply ing the delta is well buffered by a balance of processes adding and removing dsi from the surface water a key control on dsi concentrations is the uptake production of bsi and recycling of si by aquatic vegetation although the net isotopic effect is necessarily small since all bsi re dissolves on short timescales in the sediments bsi d30si n 30 ranges from less

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

cementation of porous materials using calcite a technique of cementing porous materials artificially using calcite is presented the technique depends on flushing a mixture of chemical solutions through a porous medium leading to precipitation of calcite due to reaction of the solution ingredients the experimental procedures adopted to achieve the cementation are described the response of different soils to the cementation process is analysed the bond mechanism created by the calcite is illustrated using optical and electron micrographs the paper discusses the factors that appear to determine the response of geomaterials to calcite treatment it has been found that the strength of a calcite treated material increases with a intrinsic strength of the individual grains b density c decreasing particle size of the host grains d pre coating of grains with calcite and e roundness and non angularity of soil grains the cementation technique described in this paper has proved successful in reproducing the mechanical behaviour of natural

cementation of sand soil by microbially induced calcite precipitation at various degrees of saturation a newly emerging microbiological soil stabilization method known as microbially induced calcite precipitation micp has been tested for geotechnical engineering applications micp is a promising technique that utilizes the metabolic pathways of bacteria to form calcite precipitation throughout the soil matrix leading to an increase in soil strength and stiffness this paper investigates the geotechnical properties of sand bio cemented under different degrees of saturation a series of laboratory experiments was conducted including sieve analysis permeability unconfined compressive strength consolidated undrained triaxial and durability tests the results indicate that higher soil strength can be obtained at similar caco3 content when the treatment is performed under a low degree of saturation the experimental results are further explained with a mathematical model which shows that the crystallization efficiency i e actual volume of crystals forming at the contact point where they contribute the most to strength can be calculated from the degree of saturation and grain size fine sand samples exhibited higher cohesion but lower friction angle than coarse sand samples with similar caco3 content the results also confirm the potential of micp as a viable alternative technique for soil improvement in many geotechnical engineering applications including liquefiable sand deposits slope stabilization and subgrade reinforcement the freeze thaw and acid rain resistance of micp treated sand has also been tested

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

diatom silicon isotopes as a proxy for silicic acid utilisation a southern ocean core top calibration despite a growing body of work that uses diatom 30si to reconstruct past changes in silicic acid utilisation few studies have focused on calibrating core top data with modern oceanographic conditions in this study a microfiltration technique is used to divide southern ocean core top silica into narrow size ranges separating components such as radiolaria sponge spicules and clay minerals from diatoms silicon isotope analysis of these components demonstrates that inclusion of small amounts of non diatom material can significantly offset the measured from the true diatom 30si once the correct size fraction is selected generally 2 20 m diatom 30si shows a strong negative correlation with surface water silicic acid concentration r2 0 92 highly supportive of the qualitative use of diatom 30si as a proxy for silicic acid utilisation the core top diatom 30si matches well with mixed layer filtered diatom 30si from published in situ studies suggesting little to no effect of either dissolution on export through the water column or early diagenesis on diatom 30si in sediments from the southern ocean however the core top diatom 30si shows a poor fit to simple rayleigh or steady state models of the southern ocean when a single source term is used the data can instead be described by these models only when variations in the initial conditions of upwelled silicic acid concentration and 30si are taken into account a caveat which may introduce some error into quantitative reconstructions of past silicic acid utilisation from diatom 30si

the applicability of microbially induced calcite precipitation micp for internal erosion control in gravel sand mixtures seepage induced internal erosion in earth filled embankment dams has been attracting the attention of civil engineering researchers and practitioners for decades microbially induced carbonate prec

silicic acid leakage from the southern ocean a possible explanation for glacial atmospheric pco2 1 using a simple box model we investigate the effects of a reduced si n uptake ratio by antarctic phytoplankton on the marine silica cycle and atmospheric pco2 recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added hutchins and bruland 1998 takeda 1998 franck et al 2000 the southern ocean may have supported diatoms with reduced si n uptake ratios compared to today during the dustier glacial times petit et al 1999 a similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as phaeocystis our model shows that reduced si n export ratios in the southern ocean create excess silicic acid which may then be leaked out to lower latitudes any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes caco3 production and therefore diminishes the carbonate pump in our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric co2 from the interglacial 277 to 230 242 ppm depending on where the excess silicic acid is consumed by comparison the atmospheric pco2 sensitivity of general circulation models to carbonate pump forcing is 3 5 fold greater which combined with carbonate compensation can account for peak glacial atmospheric pco2 we discuss the importance of the initial rain ratio of caco3 to organic carbon on atmospheric pco2 and relevant sedimentary records that support and constrain this silicic acid leakage scenario

impact of iron on silicon utilization by diatoms in the southern ocean a case study of si n cycle decoupling in a naturally iron enriched area abstract biogenic silica stocks and fluxes were investigated in austral summer over the naturally iron fertilized kerguelen plateau and in nearby high nutrient low chlorophyll hnlc off plateau surface waters the kerguelen plateau hosted a large diatom bloom with high levels of biogenic silica bsi but relatively low silicic acid si oh 4 uptake rates 1100 600 mmol m 2 and 8 4 mmol m 2 d 1 respectively diatoms of the naturally iron enriched area presented high affinities for silicic acid allowing them in combination with a beneficial nutrient vertical supply to grow in low silicic acid waters

using silicon isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate abstract the growth of siliceous phytoplankton mainly diatoms in the southern ocean influences the preformed nutrient inventory in the ocean on a global scale silicic acid use by diatoms and deep circulation combine to trap dissolved si in the southern ocean resulting in high levels of silica production and expansive diatom oozes in southern ocean sediments the analysis of the silicon isotope composition of biogenic silica or opal and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the southern ocean in nutrient supply and carbon drawdown both in the modern and in the past silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column together these and other proxies reveal variations in the northward flow of southern ocean intermediate and mode waters and how changes in southern ocean productivity altered their preformed nutrient content we present a new hypothesis the silicic acid ventilation hypothesis savh to explain the geographical variation of opal based proxy records in particular the contrasting patterns of opal burial change found in the low and high latitudes by understanding the silicon isotope systematics of opal and silicic acid in the modern we will be able to use opal based proxies to reconstruct past changes in the southern ocean and so investigate its role in global carbon cycling and climate

vertical budgets for organic carbon and biogenic silica in the pacific sector of the southern ocean 1996 1998 abstract during the 1996 1998 antarctic environment and southern ocean process study aesops a component of us jgofs we obtained seasonal or longer term data sets on the rates of production vertical transport remineralization and burial of particulate organic carbon poc and biogenic silica bsio2 in the southern ocean at 170 w between 55 s and 68 s the aesops data records enable us to construct vertical c and si budgets for the water column and upper sediments with all estimates derived from direct measurement of the relevant fluxes we constructed annual c and si budgets for each of four ecologically distinct zonal bands within the system for both poc and bsio2 the greatest annual delivery to the sea floor 200 and 1400 mmol m 2 yr 1 respectively and burial 6 and 160 mmol m 2 yr 1 respectively were observed in the southern antarctic circumpolar current acc between 61 5 s and 65 5 s that pattern is consistent with our observation that a diatom bloom propagated southward through the southern acc during the spring and summer of 1997 1998 following the receding ice edge and that this bloom was the main source of both poc and bsio2 in the system on an annual basis in the other zones the annual fluxes of poc and bsio2 to the sea floor ranged from 19 to 67 of those in the zone traversed by the summer diatom bloom the higher benthic fluxes of both poc and bsio2 in the southern acc imply that blooms similar to the one we observed in 1997 1998 occur commonly in the southern acc and that their high productivity signature is transmitted to the sea floor the data show preferential preservation of bsio2 over poc throughout the water column and upper seabed in the four zonal bands we consider bsio2 and poc are produced in mole ratios of 0 1 0 4 exported from the upper 100 m in ratios of 0 2 0 6 arrive at 1000 m in ratios of 1 5 4 5 reach the sea floor in ratios of 2 2 7 6 and are buried in ratios of 11 6 28 despite the preferential preservation of bsio2 accumulation of opal rich sediments beneath the acc does not result from unusually efficient preservation of siliceous material the estimated bsio2 preservation efficiency burial production ranges from 1 2 to 5 5 indistinguishable from the global average of 3 instead opal rich sediment accumulation in this region reflects very high annual rates of bsio2 production in surface waters along with very low accumulation rates of other sedimentary components e g caco3 and detrital material the observed high ratios of bsio2 production to poc production in surface waters are consistent with the known tendency for the si c ratio of diatoms to increase when fe is low if greater fe availability during the last glacial maximum permitted diatoms in the southern ocean to grow with lower more normal si c ratios export of diatom produced poc could have occurred at 2 3 times its present rate a corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65 s under present conditions thus even large increases in poc production and export during glacial periods would not be reflected in the opal accumulation record

silicon isotope and silicic acid uptake in surface waters of marguerite bay west antarctic peninsula the silicon isotope composition 30si of dissolved silicon dsi and biogenic silica bsi provides information about the silicon cycle and its role in oceanic carbon uptake in the modern ocean and in the past however there are still questions outstanding regarding the impact of processes such as oceanic mixing export and dissolution on the isotopic signature of seawater and the impacts on sedimentary bsi this study reports the 30si of dsi from surface waters at the rothera time series rats site ryder bay in a coastal region of the west antarctic peninsula wap the samples were collected at the end of austral spring through the end of austral summer beginning of autumn over two field seasons 2004 5 and 2005 6 broadly for both field seasons dsi diminished and 30si of dsi increased through the summer but this was accomplished during only a few short periods of net nutrient drawdown during these periods the 30si of dsi was negatively correlated with dsi concentrations the si isotope fractionation factor determined for the net nutrient drawdown periods uptake was in the range of 2 26 to 1 80 when calculated using an open system model and 1 93 to 1 33 when using a closed system model these estimates of are somewhat higher than previous studies that relied on snapshots in time rather than following changes in 30si and dsi over time which therefore were more likely to include the effects of mixing of dissolved silicon up into the mixed layer results highlight also that even at the same station and within a single growing season the apparent fractionation factor may exhibit significant temporal variability because of changes in the extent of biological removal of dsi nutrient source siliceous species and mixing events paleoceanographic studies using silicon isotopes need careful consideration in the light of our new results

the transport of the weddell gyre across the prime meridian abstract current meter observations from instruments moored from 1996 to 2001 are used to determine the structure and the volume transport of the weddell gyre on the greenwich meridian the record length mean volume transport is 45 sv to the east within the northern and 56 sv to the west within the southern limb of the weddell gyre thus 11 sv must leave the gyre circulation west of the greenwich meridian and enter it east of it the deficit occurs with 8 sv mainly in the circumpolar deep water cdw warm deep water wdw layer surface waters and weddell sea deep water wsdw contribute 2 and 3 sv respectively in contrast to this 3 sv more of weddell sea bottom water wsbw flow in the northern limb to the east than in the southern limb to the west which suggests a wsbw formation rate of about 3 sv in the western weddell sea in the southern boundary of the antarctic circumpolar current acc a volume transport of 35 16 sv was found using geostrophic calculations adjusted with two measurements of the absolute velocity the large heat content of the northern limb points to an extensive heat exchange between the weddell sea and the world ocean west of the prime meridian hence the heat transport as well as the volume transport suggest that the water masses of the weddell gyre are not exclusively replaced in its eastern part but that there is a significant exchange of water masses to the west of the prime meridian

deconvolving the controls on the deep ocean s silicon stable isotope distribution abstract we trace the marine biogeochemical silicon si cycle using the stable isotope composition of si dissolved in seawater expressed as 30 si open ocean 30 si observations indicate a surprisingly strong influence of the physical circulation on the large scale marine si distribution here we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the 30 si distribution in the deep oceanic interior by parsing dissolved si into its preformed and regenerated components we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth we show that the systematic meridional 30 si gradient observed in the deep atlantic ocean is primarily determined by the preformed component of si whose distribution in the interior is controlled solely by the circulation we also demonstrate that the 30 si value of the regenerated component of si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large i e primarily in the southern ocean s opal belt the global importance of this regionally dynamic si cycling helps explain the observed strong physical control on the oceanic 30 si distribution since most of the regenerated si present within the deep atlantic and indo pacific oceans is in fact transported into these basins by deep waters flowing northward from the southern ocean our results thus provide a mechanistic explanation for the observed 30 si distribution that emphasizes the dominant importance of the southern ocean in the marine si cycle

fractionation of silicon isotopes during biogenic silica dissolution abstract silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization primarily by diatoms in surface waters of the ocean interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean as driven by biological fractionation during the production of biogenic silica bsio2 or opal by diatoms however other factors which may influence the silicon isotope composition of diatom opal particularly post formational aging and maturation processes remain largely uninvestigated here we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bsio2 suspended in seawater under closed conditions this fractionation acts counter to that occurring during bsio2 production and at about half its absolute magnitude with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor edsi bsi of 0 55 corresponding to a fractionation factor 30 28 of 0 99945 the enrichment factor did not vary with source material indicating the lack of a significant species effect or with temperature from 3 to 20 c thus the dissolution of bsio2 produces dissolved silicon with a 30si value that is 0 55 more negative than its parent bsio2 an effect that must be accounted for when interpreting oceanic 30si distributions the 30si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed implying a measurable 0 1 change in the relative 30si of opal samples whenever the difference in preservation efficiency between them is 20 this effect could account for 10 30 of the difference in diatom 30si values observed between glacial and interglacial conditions it is unlikely however that the inferred maximum possible change in b30sio2 of 0 55 would be manifested in situ as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

silicic acid leakage from the southern ocean opposing effects of nutrient uptake and oceanic circulation 1 studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of southern ocean intermediate waters comparison of 30si records from the southern ocean to records of biogenic opal accumulation from low latitude upwelling systems indicates a strong negative correlation between the silicic acid content of southern ocean surface waters and opal deposition at low latitudes during the last 80 kyr bp this implies that si leakage was somehow rendered ineffective when the silicic acid content of southern ocean waters was high comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods this indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of antarctic surface waters in determining the magnitude of silicic acid leakage from the southern ocean

iron and silicic acid concentrations regulate si uptake north and south of the polar frontal zone in the pacific sector of the southern ocean abstract we investigated the relative roles of fe and silicic acid availabilities in regulating si uptake rates across the polar frontal zone in the pacific sector of the southern ocean 59 68 s 170 w during the us jgofs antarctic environment southern ocean process study aesops meridional gradients in silicic acid concentration si oh 4 of about 0 25 0 56 m km 1 were observed in this area during austral spring and summer 1997 1998 with si oh 4 ranging from 32 si oh 4 and 15 no 3 uptake rates biogenic silica concentrations and si oh 4 no3 uptake ratios fe addition had little or no effect on si uptake rates in enrichments conducted in the low si waters north of the si gradient however fe addition increased si uptake rates 3 5 times over controls in enrichments conducted in the high si waters south of the gradient in both spring and summer fe addition decreased si oh 4 no3 uptake ratios by 2 5 times largely due to stimulation of no3 uptake rates zn addition had no effect on si oh 4 and no3 uptake rates short term 24 h si additions had varying effects on si uptake rates depending on season and location in spring additions of 40 m si to water from bottle enrichments conducted north of the si gradient in situ si oh 4 15 m did not increase si uptake rates initially but did increase uptake rates after 8 days in the summer enrichment north of the si gradient in situ si oh 4 5 m 50 m si additions doubled in situ si uptake rates in the initial water collected for the enrichment and increased si uptake rates as much as 16 fold during the experiment south of the si gradient where in situ si oh 4 was 40 m in both spring and summer si addition had no effect on in situ si uptake rates in the initial enrichment water nor on any si uptake rates measured during the experiment our results indicate that both fe and si availabilities regulate si uptake rates and silica production in the southern ocean along 170 w fe limitation appears to restrict si uptake rates south of the si gradient and plays a role in preventing si depletion south of the acc where ambient si oh 4 never fell below 40 m during 1997 1998 our experiments in the seasonal ice zone at 62 suggest that si uptake in this area switched from being fe limited in the spring when in situ si oh 4 was 40 m to si limited in the summer when si oh 4 was

laurentide ice streaming on the canadian shield a conflict with the soft bedded ice stream paradigm elucidating the controls on the location and vigor of ice streams is crucial to understanding the processes that lead to fast disintegration of ice flows and ice sheets in the former north american laurentide ice sheet ice stream occurrence appears to have been governed by topographic troughs or areas of soft sediment geology this paper reports robust evidence of a major paleo ice stream over the northwestern canadian shield an area previously assumed to be incompatible with fast ice flow because of the low relief and relatively hard bedrock a coherent pattern of subglacial bedforms drumlins and megascale glacial lineations demarcates the ice stream flow set which exhibits a convergent onset zone a narrow main trunk with abrupt lateral margins and a lobate terminus variations in bedform elongation ratio within the flow set match theoretical expectations of ice velocity in the center of the ice stream extremely parallel megascale glacial lineations tens of kilometers long with elongation ratios in excess of 40 1 attest to a single episode of rapid ice flow we conclude that while bed properties are likely to be influential in determining the occurrence and vigor of ice streams contrary to established views widespread soft bed geology is not an essential requirement for those ice streams without topographic control we speculate that the ice stream acted as a release valve on ice sheet mass balance and was initiated by the presence of a proglacial lake that destabilized the ice sheet margin and propagated fast ice flow through a series of thermomechanical feedbacks involving ice flow and temperature

silicon isotope constraints on sources and utilization of silicic acid in the northern south china sea the stable silicon isotopic composition 30si of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of si in the major ocean basins here we present the first si isotope data set from the northern south china sea nscs a large marginal sea system in the western north pacific to examine sources and utilization of silicic acid si oh 4 during two cruises in july august 2009 summer and january 2010 winter samples for isotope measurements of dissolved si oh 4 30sisi oh 4 and of biogenic silica 30sibsi in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep water slope as well as at the south east asian time series study seats station located in the nscs basin surface 30sisi oh 4 generally increased from values 2 3 on the inner shelf to 2 8 above the deep basin suggesting an increasing utilization of dissolved si oh 4 reflecting the transition from eutrophic to oligotrophic conditions the 30sibsi values were systematically lower than the corresponding 30sisi oh 4 in the euphotic zone above 100 m on the shelf and slope in contrast at station seats in the nscs basin 30sibsi signatures in both seasons were within error equal to 30sisi oh 4 in the surface mixed layer above 50 m and 30sibsi in waters below were significantly higher than the corresponding 30sisi oh 4 by comparing the field data with the si isotope fractionation revealed by the rayleigh or the steady state models we demonstrate the existence of variable si oh 4 origins in different areas of the nscs surface waters on the inner shelf were largely fed by nutrients from the pearl river input while the primary source of si oh 4 for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters the si oh 4 in the surface mixed layer of the nscs basin might have originated from horizontal mixing with other highly fractionated surface waters as a consequence the si isotope dynamics in the nscs are largely controlled by variable biological fractionation of si in waters from different sources with different initial si isotopic compositions rather than any single source water

extensive phytoplankton blooms in the atlantic sector of the glacial southern ocean 1 the sources and sinks of atmospheric carbon dioxide over glacial interglacial cycles are under debate variation in productivity of the antarctic circumpolar current acc could potentially play a significant role but current interpretations of sedimentary geochemical proxies suggest that glacial productivity was not higher than today we present areal and down core distribution patterns of previously overlooked diatom resting spores that indicate the occurrence of extensive phytoplankton blooms across the entire atlantic sector of the acc particularly in the seasonal ice zone siz linked to higher iron input during the last glacial sea ice acts as an effective transporter of iron and enhances its bioavailability the dominance of the deep living radiolarian cyladophora davisiana in glacial siz sediments indicates that organic carbon export to mesopelagic depths was at least tenfold higher than today

the silicon isotopic composition of the ganges and its tributaries abstract the silicon isotopic composition si 30 of the headwaters of the ganges river in the himalaya ranged from 0 49 0 01 to 2 17 0 04 at dissolved silicon dsi concentrations of 38 to 239 m both the concentration and isotopic composition of dsi in the tributaries increased between the highest elevations to where the ganges leaves the himalayas at rishikesh the tributaries exhibit a linear correlation between si 30 and dsi that may represent mixing between a low dsi low si 30 e g 40 m 0 5 component potentially reflecting fractionation during adsorption of a small fraction of silicon onto iron oxides and a high dsi high si 30 component e g 240 m 1 7 produced during higher intensity weathering with a greater proportional sequestration of weathered silicon into secondary minerals or biogenic silica on the ganges alluvial plain in the ganges and the yamuna gomati and their tributaries dsi ranged from 122 to 218 m while si 30 ranged from 1 03 0 03 to 2 46 0 06 highest values of si 30 occurred in the gomati and its tributaries in general the lower dsi and higher si 30 of dsi in these rivers suggests control of both by removal of dsi by secondary mineral formation and or biogenic silica production a simple 1 dimensional model with flow through a porous medium is introduced and provides a useful framework for understanding these results

biological fractionation of silicon isotopes in southern ocean surface waters 1 the fractionation of silicon isotopes by diatoms during silicification i e opaline cell wall formation provides a new tool for paleoceanographic studies of the silicon cycle here we examine the natural variations of the 30si 28si ratio in silicic acid si oh 4 and biogenic silica bsio2 in surface waters of the antarctic circumpolar current acc along 170 w the results provide direct evidence of biologically mediated fractionation of silicon isotopes with an enrichment factor of between 1 1 and 1 9 depending on the model employed comparison of the mass flux of bsio2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles we calculated si oh 4 no3 utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4 1 in this region this work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative si oh 4 utilization in surface waters

the silicon isotopic composition of surface waters in the atlantic and indian sectors of the southern ocean abstract we report here the silicon isotopic composition 30si of dissolved silicon dsi from 42 surface water samples from the drake passage the weddell gyre other areas south of the southern boundary of the antarctic circumpolar current acc and the acc near the kerguelen plateau taken between the beginning of february and the end of march 2007 from the beginning to end of the cruise antxxiii 9 dsi diminished in the antarctic by 50 mol l 1 while concentrations of nitrate nitrite and phosphate showed no net decline indicating that the high seasonal si n removal ratios well known for the southern ocean may be more related to the strength of the silicate pump in the southern ocean than to the instantaneous si n uptake ratio of diatoms the 30si of dsi in samples containing more than 20 m dsi were strongly negatively correlated to dsi concentrations supporting the use of 30si as a proxy for dsi removal the open system fractionation observed e 1 2 0 11 agrees well with results from previous work in other areas and the estimate of the initial 30si of dsi of 1 4 is not far off observations of the 30si of dsi in winter water ww in this area results were used to model dsi draw down in the past from the 30si of sediment cores although isotopic fractionation during silica dissolution appeared to influence the 30si of some surface water samples inviting further study of this phenomenon

opal burial in the equatorial atlantic ocean over the last 30 ka implications for glacial interglacial changes in the ocean silicon cycle 1 the silicic acid leakage hypothesis salh suggests that during glacial periods excess silicic acid was transported from the southern ocean to lower latitudes which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric co2 downcore records of 230th normalized opal fluxes and 231pa 230th ratios from seven equatorial atlantic cores were used to reconstruct diatom productivity over the past 30 ka where a is years and to test the salh downcore records of 231pa 230th ratios and opal fluxes are highly correlated suggesting that they constitute a production based record of opal flux opal flux records support the salh in that glacial opal burial exceeded holocene burial by 1 8 gt opal ka in the area 0 40 w and 5 n 5 s earlier results from the eastern equatorial pacific ocean showed the opposite trend with greater holocene than glacial opal burial but approximately the same magnitude of difference between glacial and interglacial opal burial we suggest four nonexclusive scenarios to explain the data from both basins 1 increased upwelling in the equatorial atlantic and el nino like conditions suppressing upwelling in the eastern equatorial pacific altered the overall nutrient supply to both basins 2 si leaked from the southern ocean because of fe fertilization but was prevented from upwelling in the equatorial pacific because of el nino like conditions during the lgm 3 si leaked from the southern ocean because diatom productivity was limited by increased sea ice extent and was again prevented from upwelling in the equatorial pacific or 4 changes in ocean circulation related to the decreased input of agulhas water to the glacial south atlantic provided excess dissolved si to the equatorial atlantic ocean a deglacial opal pulse is seen in both the atlantic and pacific oceans all four scenarios and the presence of the common deglacial opal pulse suggest a common driver in the southern ocean

biogrouting coarse materials using soil lift treatment strategy this paper investigates the feasibility of using a soil lift biogrouting treatment strategy to improve the mechanical properties of coarse sand with the view of applying it to stone columns sand piles and rammed earth columns type of applications a two phase percolation approach was adopted in this study that included percolating a bacterial suspension sporosarcina pasteurii in the first phase and a cementation solution in the second phase this process was repeated every two treatments the study reveals that an increase in the number of soil lifts negatively influenced the mechanical properties of the biocemented coarse sand however the minimum strength and stiffness achieved 2 8 mpa in this study was sufficient to mitigate slumping of a soil column that may occur during installation or excessive radial expansion furthermore it is shown that a single lift treatment can lead to a very high increase in strength and stiffness up to 8 9 mpa and 2 3 gpa respectively however calcite distribution

heavy silicon isotopic composition of silicic acid and biogenic silica in arctic waters over the beaufort shelf and the canada basin the silicon isotopic composition of silicic acid 30si oh 4 and biogenic silica 30si bsio2 were measured for the first time in marine arctic waters from the mackenzie river delta to the deep canada basin in the late summer of 2009 in the upper 100 m of the water column 30si oh 4 signals 1 82 to 3 08 were negatively correlated with the relative contribution of mackenzie river water the biogenic si isotope fractionation factor estimated using an open system model 30e 0 97 0 17 agrees well with laboratory and global ocean estimates nevertheless the 30si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30e between 0 33 and 0 41 depending on how the contribution of sea ice diatoms is incorporated in the upper 400 m 30si bsio2 values were among the heaviest ever measured in marine suspended bsio2 2 03 to 3 51 a positive correlation between 30si bsio2 and sea ice cover implies that heavy signals can result from isotopically heavy sea ice diatoms introduced to pelagic assemblages below the surface bsio2 production zone the 30si oh 4 distribution followed that of major water masses vertical 30si oh 4 profiles showed a minimum average of 1 84 0 10 in the upper halocline 125 200 m composed of modified pacific water and heavier average values 2 04 0 11 in atlantic water 300 500 m deep in the canada basin deep water below 2000 m 30si oh 4 averaged 1 88 0 12 which represents the most positive value ever measured anywhere in the deep ocean since most si oh 4 enters the arctic from shallow depths in the atlantic ocean heavy deep arctic 30si oh 4 signals likely reflect the influx of relatively heavy intermediate atlantic waters a box model simulation of the global marine 30si oh 4 distribution successfully reproduced the observed patterns with the 30si oh 4 of the simulated deep arctic ocean being the heaviest of all deep ocean basins

dissolved silicon and its isotopes in the water column of the bay of bengal internal cycling versus lateral transport abstract the concentration of dissolved si and its isotope composition are measured in the bay of bengal bob region of the northern indian ocean the isotope data are the first data set from the northern indian ocean the measurements are made in eight depth profiles closely along the 87 e transect gio1 section of the international geotraces program and in a few samples from the northern shelf of the bay dissolved si in the water column varies from 0 6 to 152 5 mol kg whereas the 30si data cover a range 1 2 to 3 6 the depth profiles of dissolved si show generally lower values in the surface increasing with depth whereas the pattern reverses in the case of 30si these vertical distribution patterns of si and 30si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing si biogeochemistry in the upper layers top 1500 m in contrast dissolved si in near surface waters of the northern shelf and the southernmost station is exceptionally high these results indicate a continental supply of dissolved si from the ganga brahmaputra river system g b and submarine groundwater discharge sgd to the shelf region and an intrusion of high salinity waters from the arabian sea in the southern bay the 30si values of 1 34 0 10 for deep bottom waters of the bob depth 1500 m are similar to those reported for the deep southern ocean and indicate the dominant control of water mass mixing the dissolved si concentrations in the bottom waters of the bob are generally higher than those of the water mass endmembers which suggest the need for an additional source of si in situ particle dissolution and or benthic release in the central bay seem to be the potential candidate the annual si budget in the top 100 m of the bob seems to suggest that meso scale eddies frequently occurring during non monsoon periods can supply at the most 2 6 g si m2 year which is about 33 of the si requirement to support new production in the bay the supply of dissolved si 1 3 0 5 1011 mol year from the g b river system and sgd has been calculated based on the distributions of dissolved si concentration and 30si in the northern shelf waters a comparison of this supply with the reported si flux upstream of the estuarine zone indicates about 40 removal of dissolved si in the g b estuary the mass balance of si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess si in the bay

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

mechanical behavior of sands treated by microbially induced carbonate precipitation abstractthe mechanical behavior of sands treated using microbially induced carbonate precipitation micp has been investigated at the macroscale and the microscale triaxial and confined compression tests with embedded shear and compression wave s wave and p wave sensors were conducted on two micp treated sands ottawa 50 70 and 20 30 silica sands triaxial compression tests were conducted at three different confining pressures 25 50 and 100 kpa tests were also performed at calcium chloride cacl2 concentrations of 0 1 and 0 3 m resulting in specimens with average calcium carbonate caco3 content ranging from 1 5 to 2 5 for the 50 70 sand and from 1 to 1 6 for the 20 30 sand in contrast to previous research the results of triaxial tests presented in this paper show an increase of the soil strength even at 1 calcium carbonate content after the tests samples taken from the specimens were utilized to measure the caco3 content and to perform analysis using scanning electron microscopy sem

bio cementation of sandy soil using microbially induced carbonate precipitation for marine environments this study proposes and describes a novel approach for cementing sandy soils in marine environments by modifying the promising technique of microbially induced carbonate precipitation micp in contrast to the usual micp technique described in the literature the method proposed herein relies on the calcium ions dissolved in seawater as the sole source of calcium for calcite formation this proposed method involves flushing high salinity tolerant urease active bacteria followed by a mixture of urea and seawater through a porous sandy soil leading to bacterial carbonate release from the urease reaction and precipitation of insoluble and semi soluble carbonate salts including calcium carbonate and magnesium carbonate trihydrate this precipitation method resulted in a physical stabilisation of sand that reached an unconfined compressive strength of up to 300 kpa which is about two fold higher with same amount of crystals produced than that of the micp treatment in which highly concentrated calcium and urea solutions are used permeability was retained at about 30 for all micp treated samples suggesting good drainage ability this new exploration of micp technology provides a high potential for using bio cementation in marine environments for applications such as mitigation of submarine sediment liquefaction and prevention of beach sand erosion and cliffs scouring