silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

the relationship between silicon isotope fractionation in sponges and silicic acid concentration modern and core top studies of biogenic opal recent work has shown the silicon isotope composition denoted by 30si of deep sea sponges reflects the concentration of ambient silicic acid si oh 4 in seawater however existing calibrations are based predominantly on living sponges collected from the southern ocean these data cannot however be used to determine whether other parameters that correlate with silicic acid in the southern ocean such as temperature and salinity influence 30si of sponges furthermore the published data do not demonstrate whether disaggregated core top sedimentary spicules preserve the primary 30si signal recorded in living sponges here we address both of these issues we refine and widen the existing calibration by including a global distribution of modern sponges in addition we provide the first systematic calibration from spicules picked from core top sediments that covers sites from different ocean basins the relationship between si oh 4 and 30si in sponge spicules is the same in different ocean basins between specimens that grew in different temperature and salinity conditions our core top data agree well with the modern sponge calibration indicating there are no significant post depositional effects or early diagenetic overprints these two new datasets support the assertion that sponge 30si can be used as a proxy for silicic acid concentrations in the past

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

silicon isotopes in granulite xenoliths insights into isotopic fractionation during igneous processes and the composition of the deep continental crust the silicon si cycle is of great current interest but the isotopic composition of the continental crust has not been determined magmatic differentiation generates liquids with heavier si and the lower crust thought to be dominated by cumulates and restites is predicted to have a light isotopic composition this is borne out by the composition of many types of granite which appear to have relative light si for their silica content here we report the si isotopic compositions of two granulite facies xenolith suites from the chudleigh and mcbride volcanic provinces australia providing new constraints on deep crustal processes and the average composition of the deep continental crust the xenoliths display a range of isotopic compositions 30si 0 43 to 0 15 comparable to that measured previously for igneous rocks the isotopic compositions of the mcbride xenoliths reflect assimilation and fractional crystallisation afc and or partial melting processes silicon and o isotopes are correlated in the mcbride suite and can be explained by afc of various evolved parent melts in contrast the chudleigh xenoliths have si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates using the xenolith data and a number of weighting methods the si isotope compositions of the lower and middle crust are calculated to be 30si 0 29 0 04 95 s e and 0 23 0 04 95 s e respectively these values are almost identical to the composition of the bulk silicate earth implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir

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

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

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

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

temporal isotopic variations of dissolved silicon in a pristine boreal river abstract it has previously been concluded that the stable si isotopes are fractionated during geochemical and biogeochemical elemental transfers such as weathering and precipitation of clays and biogenic si which has opened up the possibility of using si as a tracer in natural terrestrial ecosystems furthermore quantification of the biogenic impact on the biogeochemical si cycle has attracted significant scientific interest since biological control has been suggested previous observations of seasonal variations in the dissolved si isotopic pattern further imply that high frequency riverine sampling during main hydrological events might provide important information about natural processes governing the fluvial biogeochemical si cycle therefore temporal variations in the isotopic composition of riverine dissolved si were investigated for the kalix river northern sweden the largest pristine river system in europe based on high frequency sampling during a period of 25 weeks from early april to early october 2006 temporal variations spanning 0 4 for 29si and 0 8 for 30si of dissolved si in the kalix river were observed during the period suggesting that the riverine dissolved si input to the oceans cannot be considered to have a constant si isotopic composition on a short time scale the results implicate biogeochemical si cycling via formation and dissolution of biogenic silica as processes significantly affecting the dissolved si transport in boreal systems during april to early october the si budget in the river system appeared to be controlled by relative si enrichment during high discharge events and relative si depletions in the subarctic mountainous and lake dominated areas the si enrichments and depletions were accompanied by decreasing and increasing riverine 29si and 30si respectively these isotope variations can be explained by release of plant derived silica depleted in heavier si isotopes during the spring snowmelt further increased volumetric contribution from the headwater and losses of dissolved si due to biogenic silica formation by diatoms in the subarctic lakes at a later period are expected to be responsible for the preferential losses of lighter isotopes as further verified by land cover analysis

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

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

germanium silicon fractionation in a tropical granitic weathering environment abstract germanium silicon ge si ratios were determined on quartz diorite bedrock saprolite soil primary and secondary minerals phytolith soil and saprolite pore waters and spring water and stream waters in an effort to understand ge si fractionation during weathering of quartz diorite in the rio icacos watershed puerto rico the ge si ratio of the bedrock is 2 mol mol with individual primary mineral phases ranging between 0 5 and 7 mol mol the ratios in the bulk saprolite are higher 3 mol mol than values measured in the bedrock the major saprolite secondary mineral kaolinite has ge si ratios ranging between 4 8 and 6 1 mol mol the high ge si ratios in the saprolite are consistent with preferential incorporation of ge during the precipitation of kaolinite bulk shallow soils have lower ratios 1 1 1 6 mol mol primarily due to the residual accumulation of ge poor quartz ge si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths spring water and baseflow stream waters have the lowest ge si ratios 0 27 0 47 mol mol reflecting deep initial weathering reactions resulting in the precipitation of ge enriched kaolinite at the saprolite bedrock interface mass balance calculations on saprolite require significant loss of si and al even within 1 m above the saprolite bedrock interface higher pore water ge si ratios 1 2 mol mol are consistent with partial dissolution of this ge enriched kaolinite pore water ge si ratios increase up through the saprolite and into the overlying soil but never reach the high values predicted by mass balance perhaps reflecting the influence of phytolith recycling in the shallow soil

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

relative sea level change in western istria croatia during the last millennium relative sea level change occurred along the western coast of the istrian peninsula in croatia sediments from two cores located in the bay of santa marina and soline bay have been analyzed for microfaunal and sedimentological investigations chronostratigraphical evidence was obtained from 14c dating of lagoonal shells gastropods from the sedimentological and paleo environmental analysis 4300 years of continuous deposition in a shallow marine environment was determined at around 550 cal bp marine sedimentation changed to continental consequently the initial sedimentation rate of 0 7 mm y changed to 2 36 mm y after 550 cal bp those upper deposits of continental origin are today partly submerged the data were further compared with a geophysical model which allows the prediction of the local sea level curve as the results from the cores are largely below the curve tectonic subsidence of the area is confirmed radiocarbon analysis allowed the age of the shells to be related to the age of the submerged tidal notch that occurs almost continuously some hundreds of meters south eastwards from the gulf of trieste to the dalmatian coast

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

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

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

micrometer silicon isotope diagnostics of soils by uv femtosecond laser ablation this study presents the first si isotope data of the principle si pools in soils determined by a uv femtosecond laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer mc icp ms this method reveals accurate and precise si isotope data on bulk materials and at high spatial resolution on the mineral scale the following si pools have been investigated a the si source to soils on all major silicate minerals on thin sections from bedrock fragments in the soil profiles b bulk soils particle size 2 mm after fusion to glass beads with an iridium strip heater or pressed into powder pellets c separated clay fractions as pressed powder pellets and e separated phytoliths as pressed powder pellets multiple analyses of three rock standards bhvo 2 agv 1 and rgm 1 as fused glass beads and as pressed powder pellets reveal 30si values within the expected range of igneous rocks the mpi ding reference glass kl2 g exhibits the same si isotope composition after remelting by an iridium strip heater showing that this technique does not alter the isotope composition of the glass we used this approach to investigated two immature cambisols developed on sandstone and paragneiss in the black forest germany respectively bulk soils show a largely uniform si isotope signature for different horizons and locations which is close to those of primary quartz and feldspar with 30si values around 0 4 soil clay formation is associated with limited si mobility which preserves initial si isotope signatures of parental minerals an exception is the organic horizon of the paragneiss catchment where intense weathering leads to a high mobility of si and significant negative isotope signatures as low as to 1 00 in bulk soils biogenic opal in the form of phytoliths exhibits negative si isotope signatures of about 0 4 these results demonstrate that uv femtosecond laser ablation mc icp ms provides a tool to characterize the si isotope signature of the principle si pools left behind after weathering and si transport have altered soils these results can now serve as a fingerprint of the residual solids that can be used to explain the isotope composition of dissolved si in soil solutions and river water which is mostly enriched in the heavy isotopes

effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of geysir iceland detectible 30si variations in present day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents interpretations of 30si signatures remain problematic in view of potential post depositional changes of which magnitudes and underlying mechanisms are largely unknown a critical issue in the interpretation of isotope data from cherts concerns the extent to which early diagenetic processes modify original 30si signatures here we report 30si variations in opal a opal a ct and opal ct from fossil sinter deposits in an active discharge apron in the geysir geothermal field iceland opal a samples show an average 30si of 0 7 0 2 while opal ct samples are isotopically lighter with an average 30si of 2 0 0 4 implying a sizable shift of approximately 1 3 between the different phases this shift can be explained by repetitive dissolution re precipitation processes diffusive transport or temperature differences during phase transitions on average the fossil opal a tends to be less negative in 30si than amorphous silica that recently precipitated from the hydrothermal water the difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface or to a post depositional release of surface 28si at the onset of diagenetic formation our results corroborate the perception that original silicon isotope signatures of silica acquired during chemical precipitation from a saturated solution may not be preserved in the geological record and that post depositional changes must be taken into account when interpreting data from ancient chert deposits

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

primary silica precipitate at the precambrian cambrian boundary in the south oman salt basin sultanate of oman abstract an organic rich laminated chert silicilyte consisting of up to 90 microcrystalline quartz that formed at the precambrian cambrian boundary acts as a light oil reservoir in the subsurface of the south oman salt basin sultanate of oman fully encased in salt domes it was first discovered during the 1990 s hydrocarbon exploration activities of petroleum development oman because of the economic significance and the unconventional reservoir characteristics there is great interest in understanding the origin of the laminated chert its source of silica and mode of precipitation in an anoxic sulphur rich stagnant and highly saline basin the homogeneous distribution and high values of stable si isotope composition avg 30 si 0 83 0 28 coupled with a low molar ge si ratio 6 of the microcrystalline matrix quartz clearly reveal dissolved silica in the seawater as the si source whereas hydrothermal or biogenic e g sponge derived silica can be excluded silica precipitation from seawater was likely the result of a dramatic increase in salinity in response to salt dissolution atop and adjacent to the edges of transtensional depressions on the deep basin floor thus markedly reducing the solubility of amorphous silica in these brine filled seawater depressions this saturation triggered the formation of silica gel which accumulated at the basin floor forming a soft silica rich layer on bacterial mats giving rise to a laminated sediment the mean number of laminae is ca 32 per year suggesting that layering is non annual and controlled by processes such as fluctuations in nutrient supply lunar driven re mixing or diagenetic segregation the transformation of the silica gel to microcrystalline quartz occurred below 45 c indicating a less than 4 5 18 o composition of the pore water during microcrystalline quartz formation

the effect of permafrost vegetation and lithology on mg and si isotope composition of the yenisey river and its tributaries at the end of the spring flood abstract this work focuses on the behavior of the stable mg and si isotope compositions of the largest arctic river the yenisey river and 28 of its major and minor tributaries during the spring flood period samples were collected along a 1500 km latitudinal profile covering a wide range of permafrost lithology and vegetation despite significant contrasts in the main physico geographical climate and lithological parameters of the watersheds the isotope composition of both dissolved mg and si was found to be only weakly influenced by the degree of the permafrost coverage type of vegetation forest vs tundra and lithology granites basalts carbonates or terrigenous rocks this observation is generally consistent with the lack of chemical uptake of mg and si by soil mineral formation and vegetation during the early spring the radiogenic sr isotope composition of the yenisey and its tributaries varied within a narrow range 0 708 87 sr 86 sr 0 711 reflecting the dominance of phanerozoic rock weathering and or atmospheric deposition on these compositions the mg and si isotopic compositions of riverine samples reflect two main processes with distinct isotopic signatures first isotopically heavier mg 26 mg 1 0 0 2 and isotopically lighter si 30 si 1 0 0 25 are added to the waters by river suspended matter dissolution and leaching from vegetation biomass topsoil litter second isotopically lighter mg 26 mg 1 5 to 1 75 and isotopically heavier si 30 si 1 75 2 0 are delivered to the yenisey s tributaries from deep underground water feeding the rivers via taliks this lighter mg and heavier si isotopic composition is interpreted to originate from precambrian dolomite dissolution and aluminosilicate dissolution coupled with authigenic mineral precipitation respectively in deep underground water reservoirs taking account of the isotopic composition evolution over the course of the year established earlier on mono lithological watersheds of the yenisey basin the average annual isotopic signatures of the yenisey river arriving to the arctic ocean are estimated to be 26 mg 1 58 0 30 and 30 si 1 60 0 25 as the yenisey is the largest river feeding the arctic ocean and as it samples a large variety of environments and lithologies these values may be reasonable estimates for the average mg and si isotopic composition of the dissolved riverine flux to the arctic ocean

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

silica diatoms and a freshwater productivity maximum in atlantic coastal plain estuaries chesapeake bay abstract monthly data collected in 1981 from the james york and rappahannock rivers show that dissolved silica silicic acid exhibits two distinct types of longitudinal distributions in these estuaries during the year conservative behavior was observed during winter or when river discharge was high the second distribution occurs when uptake by planktonic diatoms reduces concentrations below the levels attributable to dilution by seawater in all three systems the non conservative behavior is characterized by rapid uptake of silicic acid in the tidal freshwater portion just upstream of the limit of salt intrusion and corresponds to a chlorophyll maximum shown to contain bloom densities of diatoms silicic acid is exhausted below detectable concentrations in the vicinity of the diatom maximum representing more than 98 removal of the riverine source non conservative distributions of total silica silicic acid biogenic silica in this same region indicate deposition of biogenic silica to the sediments at the same time the riverine source is depleted by the diatoms in the freshwater reaches silicic acid is resupplied to the water column in the oligohaline portion of the estuaries this resupply is shown to be significant nearly equal to the amount of silicic acid entering the estuary in river input suggesting complete mineralization of biogenic silica produced in the freshwater zone the spatial distributions described above were observed regularly in all three systems indicating that enhanced phytoplankton production and nutrient regeneration in the vicinity of the transition zone is a natural feature of these partially mixed estuarine rivers possible mechanisms contributing to the accumulation of phytoplankton biomass and the enhanced cycling of silicon are discussed

diatom induced silicon isotopic fractionation in antarctic sea ice 1 in this study we measure the silicon isotopic composition 29si relative to nbs28 quartz standard of dissolved silicon and biogenic silica collected by sequential melting from spring antarctic pack ice collected near 117 e 64 5 s this work aims to investigate the use of si isotopes to quantify the activity of sea ice diatoms in the different brine structures and the influence of sea ice diatoms on the spring ice edge blooms from three cores with contrasted physico chemical characteristics we report significant isotopic fractionations linked to diatom activity with distinct silicon biogeochemical dynamics between different brine structures the diatoms in snow ice and in brine pockets of frazil or congelation ice have the heavier silicon isotopic composition 0 53 to 0 86 indicating that they grow in a closed system and use a significant fraction of the small dissolved silicon pool available in the brine channels and skeletal layer diatoms display a relatively lower silicon isotopic composition 0 41 to 0 70 although it is still relatively positive compared to expected equilibrium fractionation this suggests that the diatoms have grown in a semi closed system where the dissolved silicon pool i e brine is partially replenished the silicon isotopic composition 0 63 of the sea ice diatoms is much heavier than the one of biogenic silica in the seasonal ice zone mixed layer 0 09 our results suggest that sea ice diatoms either contribute to an insignificant part of the whole diatom biomass in the upper water layer or that they are directly exported below the mixed layer

silicon fluxes and isotope composition of direct groundwater discharge into the bay of bengal and the effect on the global ocean silicon isotope budget abstract this study presents silicon isotope data for rivers and groundwater from the bengal basin variations of si isotope ratios were analyzed by means of high resolution mc icp ms using a nuplasma hr the rivers show typical positive 30 si values between 1 3 and 1 7 whereas the groundwater samples show decreasing 30 si values from 1 3 in shallow groundwater to 0 2 in the deeper groundwater beside a very distinctive isotope composition the concentration of dissolved si in these groundwater samples is 2 3 times higher than in river samples taken during dry season the resulting si flux by groundwater 9 3 10 10 mol yr 1 is on the order of the combined ganges brahmaputra si fluxes and equals 40 of the total river groundwater annual si flux into the bay of bengal given the significant large flux and distinctive isotope composition means that the overall isotopic input into the ocean is different from riverine values however a sound knowledge of all inputs into the ocean and of how these inputs might vary throughout time is mandatory in order to correctly interpret palaeo records of 30 si variations as recorded in sedimentary diatom opal from the last glacial maximum we extrapolate our results from the bengal basin onto a global scale and assess the ocean s sensitivity to changes in inputs as triggered by large scale events such as glaciation periods where the hydrological cycle might be out of steady state due to the build up of large continental ice shields in such a glaciation scenario riverine vs groundwater inputs can be shifted favoring isotopically lighter groundwater over heavier river inputs into the ocean the model proves impossible to change the biogenic output to a significant degree on time scales of a few thousand years by just changing the isotope inputs into the ocean

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

the riverine silicon isotope composition of the amazon basin we present here the first large scale study of riverine silicon isotope signatures in the amazon basin the amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle the 30si signature of the dissolved silicon dsi exported to the estuary weighted for dsi flux for the period considered is estimated at 0 92 a river cross section shows the homogeneity of the amazon river regarding dsi concentration and isotope ratio the biogenic silica bsi concentration measured in surface water from all rivers is generally small compared to the dsi reservoir but large variations exist between rivers very low isotope signatures were measured in the upper rio negro 30si 0 05 0 06 which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation the si isotope fractionation in the andean tributaries and the amazon main stem can be explained by clay formation and follow either a rayleigh or a batch equilibrium fractionation model our results also suggest that the formation of 2 1 clays induces a fractionation factor similar to that of kaolinite formation

chemical weathering processes in the great artesian basin evidence from lithium and silicon isotopes variations in lithium and silicon isotope ratios in groundwaters of the great artesian basin in australia and the causes of these variations have been explored the chemistries of li and si in groundwater are influenced by the dissolution of primary phases the formation of secondary minerals and the reaction of solid phases with dissolved constituents while isotopic variations are generated by uptake into clays which preferentially incorporate the light isotopes the lithium isotopic composition expressed as view the mathml source li7 of the groundwaters ranges from 9 to 16 16 and clearly reflects changes in aquifer conditions reaction transport modelling indicates that changes in li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals whereas view the mathml source li7 is affected by the extent of isotope fractionation during secondary mineral formation which is dependent on mineralogy the patterns of groundwater si concentrations and view the mathml source si30 values versus flow distance suggest that si is at steady state in the aquifer the view the mathml source si30 value of most of the groundwater samples is close to 1 1 which is significantly lower than the view the mathml source si30 value of the reservoir rocks 0 0 since precipitation of clays preferentially removes the light si isotopes from solution the most plausible explanation for these low groundwater view the mathml source si30 values is addition of si by dissolution of isotopically light secondary minerals these data together with model calculations show that li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer whereas si is not importantly the model suggests that even in large aquifers with long fluid residence times where steady state would be expected to be reached the concentrations and isotopic fractionation of trace elements are not controlled by li adsorption the model developed here provides a basis for using li isotopes measured in groundwaters and surface waters to constrain weathering processes

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field east pacific rise 18 s an inactive vent field comprised of dead chimneys was discovered on the ultrafast east pacific rise epr at 18 degrees s during the research campaign naudur with the r v le nadir in december 1993 one of these chimneys was sampled studied and found to be largely composed of silica mineralized bacterial like filaments the filaments are inferred to be the result of microbial activity leading to silica fe oxyhydroxide precipitation the chimney grew from the most external layer precipitated 226 4 yr b p towards the central chimney conduit hydrothermal activity ceased 154 13 yr b p and the chimney conduit was completely sealed mixing between an end member hydrothermal fluid and seawater explains the sr nd isotopic composition of the chimney seawater was the major source of sr to the chimney whereas the dominant nd source was the local mid ocean ridge basalt morb leached by the hydrothermal fluids the mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions the proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85 pb isotope data indicates that all of the pb in the chimney was derived from the underlying morb the precipitation temperatures of the chimney layers varied between 55 and 71 degrees c and were a function of the seawater end member hydrothermal fluid mixing ratio delta si 30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the si isotope composition of the chimney concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end member hydrothermal fluid the inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater hydrothermal fluid mixing ratio which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids the silica chimney is significantly enriched in u likely a result of bacterial concentration of u from the seawater dominated vent fluid the chimney is poor in rare earth elements ree it inherited its ree distribution patterns from the parent end member hydrothermal fluids the dilution of the hydrothermal fluid with over 85 seawater could not obliterate the particular ree features positive eu anomaly of the hydrothermal fluids

depositional environments and sequence stratigraphy of the late carboniferous early permian coal bearing successions shandong province china sequence development in an epicontinental basin abstract this study focuses on the late carboniferous early permian coal bearing successions in shandong province north china in order to understand the depositional processes and sequence stratigraphic framework in an epicontinental basin based on detailed analysis of eleven facies five facies assemblages fas were recognized in the studied succession fa1 3 are present mainly in the benxi and taiyuan formations and consist of mixed siliciclastic and carbonate lithofacies representing eluvial lagoon barrier lagoon and tidal flat environments fa4 occurs in the shanxi formation and consists mainly of interbedded medium to fine sandstone siltstone mudstone and coal lithofacies representing river dominated deltaic environments fa5 is characterized by interbeds of trough cross stratified coarse sandstone and silty mudstone mainly in the lower shihezi formation which was deposited in meandering river channel and floodplain three third order sequences were established based on the vertical arrangement of facies assemblages and identification of physical surfaces i e subaerial unconformity transgressive surface and regressive surface each sequence comprises a transgressive systems tract tst and a highstand systems tract hst tst of sequence 1 is composed of eluvial lagoonal deposits fa1 whereas hst formed in lagoon barrier and tidal flat settings fa2 and fa3 tst of sequence 2 formed in a barrier lagoon system fa2 whereas hst is characterized by repetitive accumulation of interbedded limestone sandstone mudstone and coal deposited under lagoonal and tidal flat settings fa2 and fa3 tst of sequence 3 comprises fa2 and hst mainly fa4 deposited in a river dominated shallow water delta system sequence 3 is overlain by a fluvial sequence fa5 the three third order sequences in the shandong region are generally correlated with those in the taebaeksan basin south korea the eastern part of the north china block the relative sea level curves established in the two regions show a generally similar long term rising trend

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

the silicon isotope composition of the upper continental crust the upper continental crust ucc is the major source of silicon si to the oceans and yet its isotopic composition is not well constrained in an effort to investigate the degree of heterogeneity and provide a robust estimate for the average si isotopic composition of the ucc a representative selection of well characterised continentally derived clastic sediments have been analysed using high precision mc icpms analyses of loess samples define a narrow range of si isotopic compositions 30si 0 28 to 0 15 this is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples the average loess 30si is 0 22 0 07 2 s d identical to average granite and felsic igneous compositions therefore minor chemical weathering does not resolvably affect bulk rock 30si and loess is a good proxy for the si isotopic composition of unweathered crystalline continental crust the si isotopic compositions of shales display much more variability 30si 0 82 to 0 00 shale si isotope compositions do not correlate well with canonical proxies for chemical weathering such as cia values but do correlate negatively with insoluble element concentrations and al si ratios this implies that more intensive or prolonged chemical weathering of a sedimentary source with attendant desilicification is required before resolvable negative si isotopic fractionation occurs shale 30si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine derived silica in such samples using the data gathered in this study combined with already published granite si isotope analyses a weighted average composition of 30si 0 25 0 16 2 s d for the ucc has been calculated

constraining the oceanic barium cycle with stable barium isotopes highlights we present a ba isotope data set of seawater river waters and biogenic particles ba isotope signatures of upper ocean waters are heavier than river and deep waters adsorption of lighter ba isotopes on biogenic particles induces the fractionation ba isotopes trace land sea interactions and ocean mixing processes decoupling of ba from major nutrients confirms ba to be a biointermediate element abstract the distribution of barium ba concentrations in seawater resembles that of nutrients and ba has been widely used as a proxy of paleoproductivity however the exact mechanisms controlling the nutrient like behavior and thus the fundamentals of ba chemistry in the ocean have not been fully resolved here we present a set of full water column dissolved ba dba isotope 137badba profiles from the south china sea and the east china sea that receives large freshwater inputs from the changjiang yangtze river we find pronounced and systematic horizontal and depth dependent 137badba gradients beyond the river influence characterized by generally light signatures 0 0 to 0 3 0 3 the 137badba values in the upper water column are significantly higher 0 9 0 9 than those in the deep waters 0 5 0 5 moreover 137badba signatures are essentially constant in the entire upper 100 m in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters combined with the decoupling of dba concentrations and 137badba from the concentrations of nitrate and phosphate this implies that the apparent nutrient like fractionation of ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization the subsurface 137badba distribution is dominated by water mass mixing the application of stable ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered clearly however ba isotopes show great potential as a new tracer for land sea interactions and ocean mixing processes

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

desilication in archean weathering processes traced by silicon isotopes and ge si ratios abstract the influence of continental weathering on oceanic seawater compositions in the precambrian is poorly understood here we combine si isotopes and ge si ratios of a mesoarchean paleosol 2 95 ga and overlying shales as proxies for weathering processes and si mass transfer at the early earth s surface results show that as with modern soils neoformation of secondary clay minerals in the paleosol was associated with the fractionation of si isotopes and ge si ratios in response to chemical weathering and soil desilication furthermore the loss of fe bearing minerals most likely fe bearing smectites produced additional controls on si and ge mobility through the coupled dissolution and re precipitation of clay minerals under reductive conditions opposite fractionation behaviors are observed neoformed secondary clay minerals acted as a 28 si and ge sink whereas the leaching of fe bearing minerals released 28 si and ge into soil solutions an attempt to estimate 30 si and ge si signatures of released dissolved elements into soil solution provided 30 si and ge si signatures of about 0 1 and 1 4 mol mol respectively furthermore shales deposited shortly after paleosol formation display 30 si and ge si compositions that may be explained as mixtures of the recognized paleosol components the shale record suggests that weathering induced desilication and leaching of fe bearing minerals to a lesser extent might have been widely effective during the mesoarchean for the transfer of si from the continents to the hydrosphere comparison of si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication relative to changes in hydrothermal and continent derived solute inputs to ocean and ocean cooling

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

silicon isotope composition of diatoms as a paleoenvironmental proxy in lake huguangyan south china abstract silicon is essential for the growth of diatoms which utilize dissolved silicic acid in lake water and form opaline silica sio 2 n h 2 o during the uptake of dissolved silicic acid there is a preferential incorporation of light silicon isotope 28 si into biogenic silica resulting in the enrichment of heavy silicon isotope 30 si in dissolved silicic acid the silicon isotope composition of diatom silica 30 si diatom may thus record changes in the percentage utilization of dissolved silicic acid by diatoms which can be then related to other aspects of climate environment with the aim of exploring the potential of 30 si diatom as an indicator of lacustrine environment here we made the first measurements of 30 si diatom in the sediment core from lake huguangyan a closed crater lake in china the result shows that 30 si diatom varies from 0 6 to 1 1 and displays broad similarities to variations in contents of biogenic silica and organic carbon throughout the sediment core 30 si diatom is a reliable paleotemperature proxy in lake huguangyan which is supported by good correlation between 30 si diatom and available temperature records heavier 30 si diatom indicates greater dissolved silicic acid utilization at higher temperature while lighter 30 si diatom reflects decreased utilization at lower temperature the most negative 30 si diatom values in the sediment core occur between ad 1580 and 1920 which suggests ad 1580 1920 was the coldest period in lake huguangyan over the past 2000 years thus providing evidence for the existence of the lia in tropical south china there are few means by which to reconstruct the history of temperature changes in tropical terrestrial region 30 si diatom in this study has proven to be a new promising paleotemperature proxy in lacustrine sediments and may play important role in reconstructing past temperature changes at low latitude in the future detailed investigations on the silicon isotopes of diatoms in more lakes would be desirable in further research

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract the recognized worldwide gap in bif deposition between 2 4 and 2 0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time however the recently dated 2 3 2 2 ga yuanjiacun banded iron formation bif in the north china craton ncc offers a possibility to redress these uncertainties shale normalized rare earth element yttrium ree y patterns of the bif and interlayered meta chert samples show features characteristic of other archean and paleoproterozoic bifs with hree enrichment relative to lree positive la and eu anomalies and superchondritic y ho ratios comparable to modern seawater very low al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd t 2 4 which obtained its signature through weathering of the nearby landmasses and associated with high si fluxes our findings suggest that the ree budget of the oceans prior to 2 3 ga was generally dominated by hydrothermal circulation of seawater through depleted mantle derived source rocks however where evolved local continental crust and or an enriched mantle source were present this positive mantle nd signal became discernible in the bifs by comparing the nd isotopic features of similar aged bifs and marine carbonates it is concluded that similar to modern oceans the early precambrian ocean was not well mixed with respect to its nd isotopic composition

the effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed tana river kenya abstract we present the first study of the si isotopic composition of dry season river waters from the tana river kenya data encompasses the entire river basin with samples collected from headwaters to the estuary thereby capturing a salinity gradient in the headwaters the isotopic signature is affected by climate as a result of its control on soil drainage and weathering the 30 si signatures in the basin range from 0 69 up to 2 23 signatures are clearly affected by dams an increase in 30 si ratios of 0 54 and a decrease in the dissolved si dsi concentration by 41 were observed downstream of the masinga dam the largest of a succession of 5 hydroelectric dams this reduction in si load is most likely due to increased diatom productivity as the corresponding change in 30 si signature is consistent with the known fractionation by these organisms the 30 si composition of waters entering the estuary is ca 2 and dsi concentrations are 349 m in the estuary the dsi concentrations decrease linearly following the salinity while the 30 si ratio remains stable indicating the absence of processes affecting the dsi pool for the studied range of salinity

evidence for a major change in silicon cycling in the subarctic north pacific at 2 73 ma the initiation of northern hemisphere glaciation in the subarctic north pacific at 2 73 ma was marked by an abrupt cessation of high opaline accumulation considered to result from an increased stratification of the water column that should have led to higher utilization of nutrients in the surface ocean we present a new stable si isotope based record of si utilization that is hard to reconcile with this model a drop in 30si 28si by 0 4 at 2 73 ma is coincident with an increase in bulk n isotope composition the contrasting utilization records cannot have been both caused by a hydrographic change alone excluding a change in the si n export ratio these results either imply a relative increase in silicic acid supplied to the surface waters or a change in its si isotope composition while it is impossible to distinguish between these two possibilities both imply a regional or global change in the si biogeochemical cycle potentially caused by an enhanced storage of si in the underlying deep waters of the pacific

coupling of the distribution of silicon isotopes to the meridional overturning circulation of the north atlantic ocean abstract the distribution of silicon isotopes within silicic acid 30 si oh 4 was examined along a section in the north atlantic from the cape verde islands off africa to cape cod massachusetts in north america surface water displayed elevated 30 si oh 4 associated with biological fractionation of si during silica production below 300 m variations in 30 si oh 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis confirming a tight relationship between 30 si oh 4 and the meridional overturning circulation in the atlantic a linear relationship between 30 si oh 4 and the inverse of silicic acid concentration supported control of si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the atlantic there was a suggestion of a weak local minimum in 30 si oh 4 in deep waters above the trans atlantic geotraverse hydrothermal zone on the mid atlantic ridge consistent with the light 30 si oh 4 of hydrothermal waters the lightest 30 si oh 4 values were observed in the deep western and deep eastern basins where antarctic bottom water aabw dominated the heaviest values in subsurface waters occurred in north atlantic deep water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the arctic ocean the concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end member water masses arise and how they influence si isotope distributions northern end member water masses are heavy due to the influence of the arctic ocean bottom topography prevents light deep waters from entering the arctic and the further removal of light isotopes through local biological productivity results in extremely heavy 30 si oh 4 within the arctic light aabw dominates the southern end member the southern ocean silicic acid trap distills heavier isotopes of si out of the southern ocean as preformed silicic acid within antarctic mode waters while retaining light isotopes that become incorporated into aabw the influence of the silicon isotope bipole is predicted to be strongest in the atlantic as the net flow of waters through the arctic ocean is from the pacific to the atlantic

the effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes lake myvatn iceland lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes but little is known on their behaviour in groundwaters and hydrothermal springs and how these sources might influence the weathering signal in surface waters this study presents lithium isotope compositions 7li for cold groundwaters 3 7 c and hydrothermal springs that were at geothermal temperatures 200 300 c but have cooled during transport 17 44 c both represent an important source of water and nutrients for lake myvatn iceland we also present a time series from the laxa river which is the single outflow from the lake the 7li values in the input springs to lake myvatn are highly variable 5 27 and correlate inversely with temperature and total dissolved solids these co variations imply that even in such waters the processes controlling 7li variations during weathering still operate that is the ratio of primary rock dissolution to secondary mineral formation where the latter preferentially incorporates 6li with a temperature dependent fractionation factor in high temperature geothermal waters 300 c secondary mineral formation is inhibited and has a low fractionation factor leading to little 7li fractionation even in waters that have cooled considerably over several months from their geothermal temperatures fractionation is still low and 7li values are similar to those reported from waters measured at 350 c in contrast cooler groundwaters promote relatively high proportions of clay formation which scavenge dissolved solids including 6li the time series on the laxa river the single outflow from lake myvatn shows little 7li variation with time over the 12 month sampling period 17 21 demonstrating that in contrast to tracers such as si isotopes li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake thus these results clearly illustrate that li isotopes are ideally suited to constrain silicate weathering processes because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant and because li isotopes are demonstrably unaffected by phytoplankton or plant growth

constraining modern day silicon cycling in lake baikal constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different time scales silicon isotope data of dissolved silica 30sidsi are presented here from lake baikal and its catchment in central siberia as well as being the world s oldest and voluminous lake lake baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the arctic ocean data from river waters accounting for 92 of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river 30sidsi composition the absence of a change in 30sidsi within the selenga delta through which 62 of riverine flow passes suggests a net balance between biogenic uptake and dissolution in this system a key feature of this study is the use of 30sidsi to examine seasonal and spatial variations in dsi utilization and export across the lake using an open system model against deepwater 30sidsi values from the lake we estimate that 20 24 of dsi entering lake baikal is exported into the sediment record while highlighting the impact that lakes may have upon the sequestration of continental dsi mixed layer 30sidsi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilization with lower rates in the north relative to south basin

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

enrichment of dissolved silica in the deep equatorial pacific during the eocene oligocene silicon isotope ratios expressed as 30si in marine microfossils can provide insights into silica cycling over geologic time here we used 30si of sponge spicules and radiolarian tests from the paleogene equatorial transect ocean drilling program leg 199 spanning the eocene and oligocene 50 23 ma to reconstruct dissolved silica dsi concentrations in deep waters and to examine upper ocean 30si the 30si values range from 3 16 to 0 18 and from 0 07 to 1 42 for the sponge and radiolarian records respectively both records show a transition toward lower 30si values around 37 ma the shift in radiolarian 30si is interpreted as a consequence of changes in the 30si of source dsi to the region the decrease in sponge 30si is interpreted as a transition from low dsi concentrations to higher dsi concentrations most likely related to the shift toward a solely southern ocean source of deep water in the pacific during the paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes sponge 30si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past

silicon isotope compositions of euhedral authigenic quartz crystals implications for abiotic fractionation at surface temperatures abstract silicon si isotopes have been demonstrated as proxies for the paleoenvironmental conditions of various silica deposits in an effort to investigate the petrogenesis and paleoenvironments of phanerozoic silica deposits a suite of euhedral megaquartz crystals in the cretaceous edwards formation from central texas was petrographically analyzed and their si isotopic compositions analyzed by bulk and in situ techniques petrographic analysis shows a close relationship between megaquartz and evaporite minerals the lithologic association of silica with evaporite bearing dolomitized carbonate strata suggests that the silicification probably developed in a back reef tidal flat environment in which quartz crystals formed after the primary calcite cementation and partial dissolution of evaporites silicon isotopic mapping across the megaquartz crystals show that 30si nbs28 values range from 2 90 to 2 94 these values span the majority of published silicon isotopic values observed in nature and indicate complex growth histories the negative 30si values are attributed to the dissolution of sponge spicules which likely act as the primary source of the authigenic megaquartz the observed range of 30si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re precipitated as megaquartz in a closed system during diagenesis this rayleigh type fractionation model also adds a new level of insight into the abiotic fractionation factor between dissolved and precipitated silica based on temperature estimates of 20 to 50 c for megaquartz precipitation the fractionation factor was determined to be between 1 8 and 2 1 the estimated average 30si value of early cretaceous seawater is 2 7 to 3 0 significantly higher than modern seawater

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

the dynamics of si cycling during weathering in two small catchments in the black forest germany traced by si isotopes abstract silicon stable isotopes have emerged as a powerful proxy to investigate weathering because si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition in this study we determined the si isotope compositions of the principle si pools within two small catchments located on sandstone and paragneiss respectively in the temperate black forest germany at both settings clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with 30si values of around 0 7 bulk soils rich in primary minerals are similar to bulk parental material with 30si values close to 0 4 topsoils are partly different because organic matter degradation has promoted intense weathering leading to 30si values as low as 1 0 water samples expose highly dynamic weathering processes in the soil zone 1 after spring snowmelt increased release of doc and high water fluxes trigger clay mineral dissolution which leads to 30si values down to 0 7 and 2 in course of the summer si uptake by the vegetation and secondary mineral formation drives dissolved si to typical positive 30si values up to 1 1 groundwater with 30si values of around 0 4 records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation an isotope mass balance approach reveals incongruent weathering conditions where denudation of si is largely driven by physical erosion erosion of phytoliths contributes 3 to 21 to the total si export flux which is in the same order as the dissolved si flux these results elucidate the si dynamics during weathering on catchments underlain of sedimentary origin prevailing on the earth surface and provide therefore valuable information to interpret the isotope signature of large river systems

texture specific si isotope variations in barberton greenstone belt cherts record low temperature fractionations in early archean seawater sedimentary cherts are unusually abundant in early archean pre 3 0 ga sequences suggesting a silica cycle that was profoundly different than the modern system previously applied for the purpose of paleothermometry si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration precipitation diagenesis and metamorphism early archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition cementation silicification and recrystallization to add a new layer of insight into the chemistry of early cherts we have used wavelength dispersive spectroscopy and then secondary ion mass spectrometry sims to produce elemental and si and o isotope ratio data from banded black and white cherts from the onverwacht group of the barberton greenstone belt south africa this geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations sims allows the comparison of si and o isotope ratios of distinct silica phases including black carbonaceous chert beds and bands many including well defined sedimentary grains white relatively pure chert bands including primary silica granules early cavity filling cements and later quartz filled veins including all chert types and textures analyzed the 30 si dataset spans a range from 4 78 to 3 74 with overall mean 0 20 median 0 51 and standard deviation 1 30 n 1087 most samples have broadly similar 30 si distributions but systematic texture specific 30 si differences are observed between white chert bands mean 0 60 n 750 which contain textures that represent primary and earliest diagenetic silica phases and later cavity filling cements mean 1 41 n 198 we observed variations at a 100 m scale indicating a lack of si isotope homogenization at this scale during diagenesis and metamorphism although fractionations during diagenetic phase transformations may have affected certain textures we interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated sims 18 o values fall in a range from 16 39 to 23 39 n 381 similar to previously published data from bulk gas source mass spectrometry of onverwacht cherts we observed only limited examples of texture related variation in 18 o and did not observe correlation of 18 o with 30 si trends this is consistent with hypotheses that si isotope ratios are more resistant to alteration under conditions of rock buffered diagenesis marin carbonne et al 2011 our results indicate that low temperature processes fractionated silicon isotopes in early archean marine basins a behavior that probably precludes the application of chert 30 si as a robust paleothermometer the values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 si rich than that expected for bulk silicate earth this is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 si enriched cherts and 30 si depleted iron formation lithologies precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 si depleted sink with the corollary of an important non carbonate alkalinity sink consuming cations released by silicate weathering

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

modeling novel stable isotope ratios in the weathering zone when rock is converted to weathering products the involved processes can be fingerprinted using the stable isotope ratios of metals for example li mg ca fe sr and metalloids b si here we construct a framework for interpreting these novel stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context the approach is applicable to any novel stable isotope system and is based on a simple steady state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays and uptake of nutrients by plants the model results show that the isotope composition of a given element in the weathering zone compartments depends on 1 the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material consisting of secondary precipitates and organic matter 2 the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants a relationship is established between isotope ratios isotope fractionation factors and indexes for chemical weathering such as chemical depletion fractions cdf and elemental mass transfer coefficients derived from simple elemental concentration measurements from this relationship isotope fractionation factors can be calibrated from chemical and isotope data measured on field material furthermore we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock water and sediment this calculation can be applied to samples from soils from rivers and from the sedimentary record and does not require knowing the isotope fractionation factors involved in the reactions finally we apply the model to the oceanic li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the cenozoic

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

sources and biological fractionation of silicon isotopes in the eastern equatorial pacific abstract silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4 n and 3 s latitude at 110 w longitude in the eastern equatorial pacific silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation the value of e estimated by applying a steady state isotope fractionation model to data from all stations between 4 n and 3 s was 0 77 0 12 std err when the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the equatorial undercurrent euc the resulting value of e was 1 08 0 27 std err similar to the value established in culture studies 1 1 when the non steady state rayleigh model was applied to the same restricted data set the resulting value of e was significantly more positive 0 61 0 16 std err to the extent that the equatorial system approximates a steady state these results support a value of 1 1 for the fractionation factor for isotopes of si in the sea without the assumption of steady state the value of e can only be constrained to be between 0 6 and 1 1 silicic acid in equatorial pacific deep water below 2000 m had a near constant 30 si of 1 32 0 05 that value is significantly more positive than obtained for north pacific deep water at similar depths at stations to the northwest of our study area 0 9 1 0 and it is slightly less positive than new measures of the 30 si of silicic acid from the silicic acid plume centered over the cascadia basin in the northeast pacific si oh 4 180 m 30 si 1 46 0 12 sd n 4 we show that the data from the equator and cascadia basin fit a general trend of increasing 30 si oh 4 with increasing silicic acid concentration in the deep sea but that the isotope values from the northeast pacific are anomalously light the observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution confirmation of such high variability in deep water 30 si oh 4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea

lithium magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in iceland this study presents lithium magnesium and silicon isotope ratios from pore waters and soils from a well characterised histic andosol in south west iceland the soil 7li composition ranges between values slightly lighter than basalt to those that are much heavier 1 1 to 26 8 and are possibly influenced by sea salt in contrast precipitation corrected dissolved pore water 7li values 1 8 10 0 appear to reflect preferential adsorption of 6li onto secondary minerals where allophane supersaturation results in high 7li values conversely low 7li together with high li are probably due to destabilisation of allophane at low ph and thus desorption of li when compared to icelandic river values it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters precipitation corrected pore water mg isotope ratios 26mg range between 0 46 and 0 12 and correlate with the amount of heavy mg adsorbed onto the soil exchange complex silicon isotopes in the soils are isotopically lighter 30si 0 91 to 0 53 than basalt 0 29 whereas pore waters are heavier 0 13 to 1 03 soil 30si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals dissolved si isotopes also respond to chemical weathering processes and show that isotopically heavy 30si corresponds to high cation fluxes and high secondary mineral formation however comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions this indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent in turn this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation and thus on the chemical weathering processes

isotopic composition of silicon in meteorites abstract samples of bulk meteorites show only mass dependent fractionation of silicon isotopes no isotopic anomalies were found the variation of the ratios 29 si 28 si and 30 si 28 si over the meteorite classes is small 1 per mass unit difference the average si isotopic composition for each class of meteorites is identical within analytical uncertainties this is quite unlike o whose anomalous isotopic abundances in bulk samples differentiate among the classes of meteorites the overlapping abundance ranges of si isotopes among many classes of meteorites suggest closed system behavior for this element prior to meteorite accretion and allow calculation of an average solar system si isotope composition

the influence of water mass mixing on the dissolved si isotope composition in the eastern equatorial pacific silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon si in this study the distribution of the si isotope composition of dissolved silicic acid 30si oh 4 was analyzed in the water column of the eastern equatorial pacific eep where one of the globally largest oxygen minimum zones omzs is located samples were collected at 7 stations along two meridional transects from the equator to 14 s at 85 50 w and 82 00 w off the ecuadorian and peruvian coast surface waters show a large range in isotope compositions 30si oh 4 2 2 to 4 4 with the highest values found at the southernmost station at 14 s this station also revealed the most depleted silicic acid concentrations 0 2 mol kg which is a function of the high degree of si utilization by diatoms and admixture with waters from highly productive areas samples within the upper water column and the omz at oxygen concentrations below 10 mol kg are characterized by a large range in 30si oh 4 which mainly reflects advection and mixing of different water masses even though the highly dynamic hydrographic system of the upwelling area off peru does not allow the identification of clear si isotope signals for distinct water masses therefore we cannot rule out that also dissolution processes have an influence on the 30si oh 4 signature in the subsurface water column deep water masses 2000 m in the study area show a mean 30si oh 4 of 1 2 0 2 which is in agreement with previous studies from the eastern and central pacific comparison of the new deep water data of this study and previously published data from the central pacific and southern ocean reveal substantially higher 30si oh 4 values than deep water signatures from the north pacific as there is no clear correlation between 30si oh 4 and silicic acid concentrations in the entire data set the distribution of 30si oh 4 signatures in deep waters of the pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct si isotope signatures including lower circumpolar deep water lcdw and north pacific deep water npdw

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

quantifying the impact of freshwater diatom productivity on silicon isotopes and silicon fluxes lake myvatn iceland diatom productivity in the oceans plays a crucial role in the carbon cycle but is strongly dependent upon the continental silicon supply however the relative influence of weathering and biological processes on continental si fluxes remains poorly constrained this study aims to quantify the impact of terrestrial diatom productivity on si fluxes to the ocean lake myvatn in north iceland is one of the most productive lakes in the northern hemisphere with nutrient rich waters almost uniquely sourced by groundwater the primary production is mainly controlled by diatom growth but also by cyanobacteria and the lake output is via a single river thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and si budget of lake waters silicon stable isotopes o 30si provide a tracer of this biocycling and have been measured in groundwater inputs to the lake and in time series monitoring of waters at the lake outlet the o 30si values at the outlet range from 0 70 0 08 to 1 42 0 06 which is significantly heavier than the groundwater input average cold and hot springs 0 50 0 17 2sd and consistent with the preferential uptake of light si isotopes by diatoms the o 30si value at the outlet increases by up to 0 9 in spring and autumn relative to the si isotope composition of the inflow these seasonal diatom blooms can be modeled by an open system of si uptake and affect si fluxes at the outlet of the lake by up to 79 or 53 integrated over the year in the summer a shift to lighter o 30si values is correlated with a higher ph which results in dissolution of diatoms releasing light si isotopes from mass balance this seasonal diatom dissolution affects si fluxes by up to 33 but is limited to 3 7 integrated over the year these results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived si they also demonstrate the ph dependency of diatom dissolution and or preservation which is likely to affect not only the continental si fluxes to the ocean but also the si recycling in the oceans themselves

lithium isotope behaviour during weathering in the ganges alluvial plain the ganges river system is responsible for the transportation of a large flux of dissolved materials derived from himalayan weathering to the oceans silicate weathering driven cooling resulting from uplift of the himalayas has been proposed to be a key player in cenozoic climate variation this study has analysed li isotope 7li ratios from over 50 ganges river waters and sediments in order to trace silicate weathering processes sediments have 7li of 0 identical to bulk continental crust however suspended sediment depth profiles do not display variations associated with grain size that have been observed in other large river systems dissolved 7li are low 11 in the ganges headwaters but reach a constant value of 21 1 6 within a relatively short distance downstream which is then maintained for almost 2000 km to the ganges mouth given that li isotopes are controlled by the ratio of primary mineral dissolution to secondary mineral formation this suggests that the ganges floodplain is at steady state in terms of these processes for most of its length low 7li in the mountainous regions suggest silicate weathering is therefore at its most congruent where uplift and fresh silicate exposure rates are high however there is no correlation between 7li and the silicate weathering rate in these rivers suggesting that li isotopes cannot be used as a weathering rate tracer although they do inform on weathering congruency and intensity the close to constant 7li values for the final 2000 km of ganges flow also suggest that once the size of the alluvial plain reached more than 500 km the flow distance after which riverine 7li stops varying the ganges exerted little influence on the changing cenozoic seawater 7li because riverine 7li attained a near steady state composition

silicon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water the silicon isotope composition d30si of biogenic opal provides a view of the silica cycle at times in the past reconstructions require the knowledge of silicon isotope fractionation during opal biomineralization the d30si of specimens of hexactinellid sponges and demosponges growing in the modern ocean ranged from 1 2 to 3 7 n 6 corresponding to the production of opal that has a d30si value 3 8 0 8 more negative than seawater silicic acid and a fractionation factor a of 0 9964 this is three times the fractionation observed during opal formation by marine diatoms and terrestrial plants and is the largest fractionation of silicon isotopes observed for any natural process on earth the d30si values of sponge spicules across the eocene oligocene boundary at ocean drilling program site 689 on maud rise range from 1 1 to 3 0 overlapping the range observed for sponges growing in modern seawater

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

modeling the modern marine δ30si distribution 1 the silicon isotopic composition of opal sediments can be used as a proxy for past si utilization and hence paleoproductivity for a fixed ocean circulation interpretation of this si utilization proxy is limited by our understanding of the present marine si isotope distribution observations show significant isotopic variations is silicic acid between deep water masses that are not captured by a previous global climate model gcm here simple box models of the global oceans for the si cycle are presented which can account for the observed 0 3 o difference between atlantic and pacific deep water 30 si values using typical opal export fluxes the 30 si gradient results from both a high 30 si of dissolved si overturning in the north atlantic and the dissolution of low 30 si of opal in the deep waters around the southern ocean that feed into the pacific

micro scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the 3 7 ga isua greenstone belt sw greenland pillow basalt and chert form integral lithologies comprising many archean greenstone belt packages to investigate details of these lithologies in the 3 7 ga isua greenstone belt sw greenland we measured silicon isotope compositions of quartz crystals by secondary ion mass spectrometry from a quartz cemented quartz amygdaloidal basaltic pillow breccia recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions the recrystallized chert chert clasts and quartz cement have overlapping 30si values while the 30si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age despite amphibolite facies metamorphism we suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting consistent with the pillow breccia origin on the basis of the present data we conclude that the geological context of each sample must be carefully evaluated when interpreting 30si values of quartz