the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

a combined in situ oxygen silicon isotopic and fluid inclusion study of a chert sample from onverwacht group 3 35ga south africa new constraints on fluid circulation abstract cherts have been extensively used as a proxy of precambrian environmental conditions of the earth cherts from the onverwacht group 3 5 3 3 ga south africa are considered as well preserved and have been used as a paleo oceanic thermometer especially by using the oxygen isotope composition of the chert nevertheless there are several lines of evidence size and number of the quartz veins that fluids circulated through these cherts after their deposition these cherts are composed mainly of microquartz with large quartz veins 100 m and iron carbonates as ankerites a combined study of oxygen silicon and fluid inclusions study have been performed on one chert sample from the onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions in situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector cameca 1270 ion probe crpg cnrs nancy moreover fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation three types of primary fluid inclusions carbonic lc aqueous carbonic fluids lc w and aqueous fluid lw have been observed the temperatures of this fluid circulation were determined by microthermometry and using quartz carbonate pair thermometry at between 200 c and 300 c fluid pressures range from 100 mpa to 30 mpa which correspond to depths ranging from 4 to 1 km this evolution could be related to an exhumation associated with a dilution and cooling process this fluid circulation could be associated to the gold bearing fluid responsible of the sheba fairview ore system dated at 3074 6 ma the results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz the 18 o value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold bearing fluid the silicon isotope composition of the microquartz allows determination of the initial 30 si of the microquartz precursor this precursor has been precipitated from a hydrothermal fluid hence a combined 18 o 30 si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined

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

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

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

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

biogenic silica dissolution in sediments of the southern ocean i solubility abstract a stirred flow through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the indian sector of the southern ocean antares i cruise the results show that the apparent silica solubility in the cores may decrease increase or remain constant with depth the silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material by combining the solubility data with measured dissolved silica profiles it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility in sediments that experience a significant detrital input the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica the principal oceanographic control on pore water silica build up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels

conversion of diatoms to clays during early diagenesis in tropical continental shelf muds in coastal muds downdrift from the amazon river mouth marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial the dominant neoformed crystalline phases have a composition similar to k smectite yield electron diffraction patterns characteristic of clay minerals and exhibit a range of crystal sizes and morphologies including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica a poorly crystalline or microcrystalline k rich and fe rich aluminosilicate material also replaces the siliceous frustules the conversion process is not always complete leaving relics of the original frustule in the resulting authigenic aluminosilicate phases laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20 23 months the conversion of biogenic silica to authigenic clays occurs throughout the amazon deltaic deposits and presumably takes place in other comparable depositional settings biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation rich aluminosilicate phases and it may prove to be important for oceanic geochemical cycles as a sink for si k and other elements incorporated in the authigenic aluminosilicates rapid formation of authigenic k smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis

what controls silicon isotope fractionation during dissolution of diatom opal abstract the silicon isotope composition of opal frustules from photosynthesising diatoms is a promising tool for studying past changes in the marine silicon cycle and indirectly that of carbon dissolution of this opal may be accompanied by silicon isotope fractionation that could disturb the pristine silicon isotope composition of diatom opal acquired in the surface ocean it has previously been shown that dissolution of fresh and sediment trap diatom opal in seawater does fractionate silicon isotopes however as the mechanism of silicon isotope fractionation remained elusive it is uncertain whether opal dissolution in general is associated with silicon isotope fractionation considering that opal chemistry and surface properties are spatially and temporally i e opal of different age diverse in this study we dissolved sediment core diatom opal in 5 mm naoh and found that this process is not associated with significant silicon isotope fractionation since no variability of the isotope effect was observed over a wide range of dissolution rates we can rule out the suggestion that back reactions had a significant influence on the net isotope effect similarly we did not observe an impact of temperature specific surface area or degree of undersaturation on silicon isotope partitioning during dissolution such that these can most likely also be ruled out as controlling factors we discuss the potential impacts of the chemical composition of the dissolution medium and age of diatom opal on silicon isotope fractionation during dissolution it appears most likely that the controlling mechanism of silicon isotope fractionation during dissolution is related to the reactivity or potentially aluminium content of the opal such a dependency would imply that silicon isotope fractionation during dissolution of diatom opal is spatially and temporally variable however since the isotope effects during dissolution are small the silicon isotope composition of diatom opal appears to be robust against dissolution in the deep sea sedimentary environment

pure siliceous ooze a diagenetic environment for early chert formation the formation of marine opal ct nodules or layers as early diagenetic deposits has been documented only in antarctic deep sea sediments in contrast porcellanites and cherts in land sections and deep sea drilling project and ocean drilling program drill sites are usually found in sediment sections of miocene age and older during r v polarstem cruises ant ix 3 and 4 young porcellanites were recovered for the first time in contact with their host sediment in two cores from the atlantic sector of the southern ocean chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation in both cores the porcellanites are embedded in sediments rich in opal a with extremely low levels of detrital minerals an environment that seems conducive to a rapid transformation of biogenic silica into porcellanites

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

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

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

a young porcellanite occurrence from the southwest indian ridge a porcellanite layer probably younger than 0 6 0 4 ma of a nearly monomineralic composition of opal ct was sampled on the southwest indian ridge during polarstern cruise ant vi 3 the intense cementation of the rock together with recent findings by the ocean drilling program legs 113 and 120 and the occurrence of a unique older porcellanite from eltanin core 47 15 provides evidence of very early silica precipitation in pure diatom oozes of the southern ocean such porcellanites occur in shallowly buried young sediments and provide a contrast to the established concepts of porcellanite formation

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

a petrographic and isotopic criterion of the state of preservation of precambrian cherts based on the characterization of the quartz veins the coupled o and si isotope variations of precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on earth however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation even pristine samples show evidence of fluid circulation by the presence of various quartz veins here we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples these samples show different quartz habit and texture in their quartz veins which imply various fluid system conditions therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz these observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins we have demonstrated that in some conditions fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo environmental conditions the petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz

the framework of hydrocarbon generation and migration gulf of mexico continental slope abstract the occurrence of large volumes of crude oil and thermogenic gas in plio pleistocene reservoirs and holocene seeps of the gulf of mexico slope argues that the process of hydrocarbon generation and migration continues at the present time calculated thermal maturity models based on deep seismic stratigraphy indicate that cretaceous and possibly early tertiary sediments are presently within the thermal maturity range for generation and expulsion of crude oil the cretaceous and early tertiary sediments are suggested as possible source rocks because younger sediments are thermally immature for crude oil generation and older sediments are thermally overmature for crude oil preservation the calculated thermal maturity profile for the slope indicates that some generation of crude oil could occur as shallow as 6 km and that liquid hydrocarbons could be preserved as deep as 9 km thermogenic gas is stable at even greater depths migration with a strong vertical component must be invoked to explain thermogenic hydrocarbons in shallow reservoirs and seeps of the slope the great depth at which liquid hydrocarbons and dry gas can be preserved in reservoirs of the gulf of mexico salt basin indicates that huge volumes of prospective section remain essentially unexplored

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

aluminium and silicic acid in water and sediments of the enderby and crozet basins abstract the distributions of dissolved aluminium a1 and silicic acid h 4 sio 4 were investigated in the indian sector of the southern ocean along a hydrographic section from 52 s in the eastern enderby basin to 39 s in the western crozet basin going northward there is a clear increase of a1 concentrations at all depths starting with a nearly featureless profile a1 generally below 1 nm at the southernmost station to profiles with maxima of 4 09 nm at 500 m and 3 57 nm in bottom waters at the northern stations the general trend is interrupted by intrusions of antarctic intermediat water with low a1 coinciding with oxygen maxima 260 m in the surface layer h 4 sio 4 ranged from 1 5 to m and increased continuously with depth to 158 7 m in the southernmost bottom waterand to 141 m in the bottom water of the crozet basin there is a linear relation between h 4 sio 4 and salinity except inthe eastern enderby basin where the bottom water is enriched in h 4 sio 4 at the eastern flank of the crozet plateau a western boundary current was observed carrying deep water withabout 2 3 nm al and 155 156 m h 4 sio 4 this indicates a net input of h 4 sio 4 and no net input of al during water transport from the western weddell basin to the crozet kerguelen passage at the southernmost station all al concentrations ranging from 0 84 to 1 2 nm with minima of 0 3 0 5 nm between 500 and 1000 m are below the values reported for the western weddell sea where the water originates also the al values in the salinity maximum of the circumpolar deep water are lower than expected from the intrusion of saline al rich north atlantic deep water both observations suggest that scavenging of a1 possibly by diatoms occurs in parts of the southern ocean in the crozet basin deep waters h 4 sio 4 behaves conservatively while increased concentrations of dissolved a1 in bottom water are due to input from the sediments in the eastern endergy basin excess h 4 sio 4 in bottom waters indicates input from the sediment without such an input of a1 this is explained by the higher concentrations of h 4 sio 4 and lower dissolved a1 in pore water of the enderby basin compared to the crozet basin the a1 si ratios of biogenic silica in the sediment were 0 0027 in the enderby basin and 0 011 in the crozet basin both higher than in diatoms from surface waters it is suggested that the biogenic silica rich surface sediment in the enderby basin acts as a filter for a1 diffusiong upward from pore water whereas the low a1 si ratioss in biogenic silica support a marked h 4 sio 4 flux into the bottom waters in the crozet basin sediments with less biogenic silica and more a1 releasing terrigenous material some dissolved a1 can escape into bottom water but the higher al si ratios in biogenic silica suppress release of h 4 sio 4

sequences of silica phase transitions effects of na mg k al and fe ions abstract hydrothermal experiments on silica phase transitions in artificial solutions tested the effects of dissolved aluminum and iron on the evolution of solution composition and the mineral compositions of solid phases dissolved aluminum and iron come from detrital aluminosilicate phases including their coatings aluminosilicate phases retard formation of opal ct a metastable silica phase but they may promote transformation of opal ct to quartz the stable phase the four starting solutions all had alkalinity greater than seawater either 90 or 8 mm the four solutions were dominated by different cations one solution contained magnesium ion one magnesium and sodium ions one potassium ion and one sodium ion compositions of the solutions changed within 4 weeks quartz formed under a higher initial alkalinity of 90 mm with or without magnesium minor phases including opal ct clay minerals and melanophlogite were also observed in some experiments opal ct formed in the presence of magnesium and an initial alkalinity above 8 mm dissolved aluminum or iron reduced concentrations of dissolved silica causing precipitation of an initially more ordered opal ct in the presence of magnesium and quartz in its absence these effects influence the rates and sequences of silica diagenesis

seismicity of the panama block i magnitudes and spatial distribution of epicentres abstract an earthquake catalogue for panama has been compiled to cover the period of measurements from 1904 to 83 vergara munoz 1987 most magnitudes are determined from records at uppsala since as early as 1904 the catalogue is complete for magnitudes above 6 0 for at least 60 yrs but magnitudes of about 5 2 are completely reported only during the most recent 20 yrs the earthquake population with homogenized magnitudes was used for noncumulative frequency magnitude analysis the results showed that a stable b value can be determined when the population size reaches 300 events in a natural sequence for magnitudes larger than 4 7 reported in the catalogue the spatial distribution of epicentres leads to a delineation of an aseismic block surrounded by three very active shallow sources the panama coiba fracture zone at the boundary between the nazca and cocos plates the panama south america suture zone between the panama block and the south american plate and the panama deformed belt which defines the boundary of the caribbean plate with the panama block

influence of rock composition on kinetics of silica phase changes in the monterey formation santa barbara area california the diatomaceous deposits of the monterey formation along the santa barbara coast were subjected to temperatures and burial depths that increased westward after deposition as a result the silica phase in originally similar laterally equivalent strata generally changed westward from biogenous opal a to diagenetic opal ct to diagenetic quartz in individual beds at the same stratigraphic level however silica phases changed at various temperatures producing a transformation or kinetic sequence related to variations in the ratio of silica to detrital minerals compared to silica in rocks with minor detrital minerals in rocks with progressively more abundant detrital minerals 1 opal ct formed progressively later 2 the initial d spacing of opal ct was progressively smaller and 3 quartz formed progressively earlier comparison of associated calcareous dolomitic and carbonate free rocks with equal silica detrital ratios shows that carbonates did not significantly affect rates of silica diagenesis in the vast majority of rocks in carbonate bearing rocks with silica detrital ratios 8 however some 2 to 10 quartz formed early in diagenesis probably prior to opal ct formation in addition atypical quartz cherts formed locally in carbonate bearing strata by pore filling and replacement early in diagenesis possibly before widespread opal ct formation

low temperature opal ct precipitation in antarctic deep sea sediments evidence from oxygen isotopes porcellanite has been found in antarctic deep sea sediments of shallow burial depth at four different sites in host sediments younger than 4 ma oxygen isotope analysis shows that the opal ct samples are extremely rich in18o 18o 41 2to44 7 rel smow according to the quartz water fractionation the calculated isotopic opal ct formation temperature is in the range of 0 to 4 c this agrees well with sediment temperature measurements the low opal ct precipitation temperature contrasts with current ideas about later diagenetic formation of opal ct at higher temperatures of 18 to 56 c

experimental constraints on kinetic and equilibrium silicon isotope fractionation during the formation of non biogenic chert deposits silicon isotopic compositions 30si of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments but interpretations are hampered by the lack of experimentally validated fractionation factors here we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch reactors at low temperature 10 35 c and near neutral ph 7 5 8 5 as analogue for non biogenic chert formation instantaneous fractionation factors derived from 30si values of the total dissolved sitd silica and mass balance computations with inst 30sippt 1000 30sitd 1000 decrease with progressive precipitation and reduced reaction rates this suggests that silica deposition in the batch reactors is kinetically dominated at the start of the experiments but approaches a metastable equilibrium after ca 400hours modelled kinetic fractionation factors range from 0 9965 at 10 c to 0 9976 at 20 c and 0 9993 at 35 c and ph8 5 whereas equilibrium isotope effects are smaller and range from 0 9995 at 10 c to 1 000 at 20 c and 1 0005 at 35 c our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated implying that the kinetic conditions of non biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks with particular relevance for those preserved in the archean chert record

reflector pc a prominent feature in the maud rise sediment sequence eastern weddell sea occurrence regional distribution and implications to silica diagenesis condensed pelagic deposits characterize the uppermost sediment sequence of the maud rise in the eastern weddell sea the comparison of subbottom profiling data with the sediments drilled at two odp sites shows that lithologic unit ib deposited during late miocene to late pliocene times is generally characterized by acoustic transparency a prominent reflector with strongly increased amplitude called reflector pc occurs within this mainly homogeneous diatom ooze unit and is correlated with the porcellanite horizon found at odp site 689 as evidenced by sediment findings reflector pc marks a distinct stratigraphic layer of early pliocene age the development of the reflector pc and its amplitude of reflection in the echosounding records is most likely caused by the silica cementation of porcellanite within this layer the analyses of the sediment echosounder profiles from five r v polarstern cruises make it possible to map the regional distribution of reflector pc in large areas of the central maud rise a patchy but also continuous distribution of reflector pc was found the reflector does not occur in sediments below 3000 meters of water on average reflector pc is detected around 11 and 12 meters below seafloor but depth occurrence is highly variable within the range of 8 m to more than 18 m the apparent amplitude of reflector pc is higher in those areas where the diatom ooze unit ib reaches greater thicknesses which may be caused by local enhanced tectonic subsidence in general a good correlation of reflector pc amplitude and thickness of lithologic unit ib is observed this may imply that the diatom ooze unit ib serves as a silica source since the extent of silica cementation of the porcellanite layer is controlled by the amount of dissolved silica supply other possible interpretations for silica sources include silica rich interstitial water ascending from deeper sediment levels using tectonic structures for migration

secular distribution of biogenic silica through the phanerozoic comparison of silica replaced fossils and bedded cherts at the series level abstract the modern marine silica cycle is dominated by silica secreting phytoplankton principally diatoms but this cycle has evolved considerably during the phanerozoic we analyzed the temporal distribution of silica replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies trends in silica replacement of fossils match faunal radiations during the ordovician and siluro devonian with peaks in silica replacement in the late ordovician and middle devonian corresponding to peaks in the abundance and diversity of siliceous sponges sharp drops in the abundance of silica replaced fossils and or bedded cherts coincide with four of the five major mass extinctions no discernible decrease marks the extinction at the end of the triassic we expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling orogenic episodes which may trigger continental glaciati

silicon isotopes in 3 8 ga west greenland rocks as clues to the eoarchaean supracrustal si cycle we report mc icp ms si isotopic measurements 29si recalculated as 30si on micro subsamples at 500 m scale from several major rock types of the 3 8 ga old isua greenstone belt igb southern west greenland and surrounding eoarchaean terrains with a large overall range of variations 2 80 30si 0 68 they demonstrate strong involvement of surface fluids enriched with dissolved si the resistance of si isotopes to metamorphic resettings and metasomatic overprints is also established metabasaltic pillows and metasediments display similar 30si enriched signatures suggesting that emergent surfaces of the eoarchaean protocrust were composed of slightly weathered hydrothermally altered mafic ultramafic bodies isua magnetite quartz banded iron formation bif is strongly depleted in 30si relative to all coeval rocks this depletion supports rayleigh controlled precipitation from seafloor vented hydrothermal fluids in contrast banded quartz pyroxene rocks from akilia island some 150 km southwest of the igb which some authors have identified as bif related yield quartz with si isotopic composition 30si 0 36 similar to metamorphic derived quartz 30si 0 50 this supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers