submarine groundwater discharge sgd into the yellow sea revealed by 228ra and 226ra isotopes implications for global silicate fluxes abstract we estimated the magnitude of submarine groundwater discharge sgd into the yellow sea which is one of the largest continental shelves in the world using 226 ra and 228 ra isotopes on the basis of 228 ra and 226 ra mass balance models we estimated the advective flux of 226 ra through sgd to be 270 10 12 dpm yr 1 in the yellow sea using this 226 ra flux and the measured 226 ra activity in coastal groundwater the submarine discharge of groundwater mostly brackish groundwater was calculated to be at least 40 of the river water input 2 3 10 11 m 3 yr 1 then on the basis of the sgd and the concentration of si in coastal groundwater we estimated the flux of si through sgd to be 20 100 of that associated with river discharge 23 10 9 mol yr 1 this large sgd is likely to be due to the high tidal range up to 10 m and the wide distribution of sandy sediments in the yellow sea favorable for the recirculation of seawater through bottom sediments and rocks this result from such a large area implies that the si flux through sgd may be significant on a global scale

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

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

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

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

emplacement of clay into sand by infiltration abstract laboratory experiments and examination of core samples from holocene sand bodies demonstrate that vadose infiltration of muddy water through sand is an effective mechanism by which clay can be emplaced into sand infiltrated clays form coatings on and bridges between sand sized framework grains clay particles are mostly oriented parallel to grain surfaces in coatings and perpendicular to grain surfaces in bridges the amount of clay emplaced as coatings and bridges is determined by complexly interacting variables that include sand grain size clay particle size clay particle shape concentration of suspended sediment and clay composition of these relatively coarse sand sizes and high concentrations of suspended sediment appear to promote the emplacement of the most infiltrated clay in holocene sands the formation of grain coatings and bridges is facies dependent infiltrated clays are most effectively emplaced in environments characterized by high suspended sediment concentrations fluctuating water levels and minimal sediment reworking e g point bar and delta plain in contrast virtually no infiltrated clay is emplaced in environments characterized by low suspended sediment concentration and continuous reworking e g beach and tidal delta

silicon isotope variations accompanying basalt weathering in iceland abstract this study presents the si isotope compositions of the dissolved phase of twenty rivers from across iceland the high resolution nu plasma 1700 mc icp ms was used to provide interference free 30 si 28 si ratios with an average limiting precision of 0 04 30 si the si isotope composition ranges from 0 08 to 1 46 30 si averaging 0 63 0 38 1 sd this is only slightly lighter than the recently reported mean of 0 84 0 19 for high precision measurements of swiss rivers but is significantly lighter than compositions reported for some other rivers the si isotope composition varies slightly with the amount of glacial cover which provides evidence that the continental si input to the oceans may be climatically controlled and thus likely to vary over glacial interglacial cycles after correction for the dissolution of trace amounts of calcite in one region a correlation is found between 30 si and ca si this is consistent with the measured basaltic composition and indicates isotopic fractionation during si removal from the dissolved phase by calculating the fraction of si that is removed from the dissolved phase into secondary phases it is possible to use the riverine si flux si denudation rate to calculate the initial dissolution rate of si within these catchments from this study we estimate that the total amount of si being dissolved by primary weathering of iceland is 3 million tons per yr however only 50 of this si is released into the rivers and contributes to denudation using the si isotope composition and the si fraction removed it is possible to model the si isotope fractionation as either a rayleigh type fractionation or a steady state system both yield similar fractionation factors of about 1 5 30 si implying only one underlying fractionation process that is controlled by external parameters such as runoff furthermore rivers that follow a steady state fractionation model are associated with higher si dissolution rates and lower 30 si values indicating an inverse relationship between weathering rates and dissolved riverine si isotope composition

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

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

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

δ30si systematics in a granitic saprolite puerto rico granite weathering and clay mineral formation impart distinct and interpretable stable si isotope 30 si signatures to their solid and aqueous products within a saprolite clay minerals have 30 si values 2 0 more negative than their parent mineral and the 30 si signature of the bulk solid is determined by the ratio of primary to secondary minerals mineral specific weathering reactions predominate at different depths driving changes in differing 30 si pore water values at the bedrock saprolite interface dissolution of plagioclase and hornblende creates 30 si pore water signatures more positive than granite by up to 1 2 these reactions are the main contributor of si to stream water and determine its 30 si value throughout the saprolite biotite weathering releases si to pore waters but kaolinite overgrowth formation modulates its contribution to pore water si the influence of biotite on 30 si pore water is greatest near the bedrock where biotite derived si mixes with bulk pore water prior to kaolinite formation higher in the saprolite biotite grains have become more isolated by kaolinite overgrowth which consumes biotite derived si that would otherwise influence 30 si pore water because of this isolation which shifts the dominant source of pore water si from biotite to quartz 30 si pore water values are more negative than granite by up to 1 3 near the top of the saprolite

silicon isotope study on rice plants from the zhejiang province china silicon isotope compositions of 16 rice plants from the zhejiang province were studied and potential applications of silicon isotope study to agriculture and biogeochemistry were discussed it is found that the sio2 contents in ignited rice plant samples increased generally from roots through sl stem and leaves to husks large and systematic silicon isotope fractionation was observed among different organs of individual rice plants their 30si values show a generally increasing trend from roots and sl through husks to grains which can be explained by kinetic isotope fractionation in a rayleigh process a 30si value of 6 1 was obtained from two samples of rice grains which is much greater than the highest value 3 4 observed in terrestrial samples before it is inferred that monosilicic acid is the major silicon bearing component taken up by rice roots passive uptake of silicon is important for rice and evapotranspiration may be the major mechanism for the transportation and precipitation of silicon in rice plants it is suggested that silicon isotope study has a potential application to agriculture and biogeochemistry

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

pliocene 3 2 2 4 ma ostracode faunal cycles and deep ocean circulation north atlantic ocean ostracode assemblages from deep sea drilling project sites 607 western mid atlantic ridge and 610 southeast rockall plateau show rapid systematic shifts during late pliocene glacial interglacial cycles that reflect deep sea environmental change progressive decreases in north atlantic deep water taxa and increases in southern ocean taxa occur from 3 4 to 2 4 ma and high amplitude faunal cycles begin near 2 8 ma four ostracode assemblages each with a characteristic phase relative to 41 k y obliquity glacial interglacial 18 o cycles characterize the benthic faunal record at site 607 cross spectral analysis shows that the site 607 glacial assemblage has a 41 k y periodicity significant at the 95 level other assemblages show a less significant but still obvious concentration of variance at 41 k y faunal patterns suggest climatically controlled reorganization of deep sea benthic communities during glacial interglacial cycles due to oscillating deep sea environments

late quaternary paleoceanography of the eurasian basin arctic ocean we reconstructed late quaternary deep 3000 4100 m and intermediate depth 1000 2500 m paleoceanographic history of the eurasian basin arctic ocean from ostracode assemblages in cores from the lomonosov ridge gakkel ridge yermak plateau morris jesup rise and amundsen and makarov basins obtained during the 1991 polarstern cruise modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant relatively species rich benthic ostracode assemblages in part reflecting the influence of high organic productivity and inflowing atlantic water in contrast deep arctic eurasian basin assemblages have low abundance and low diversity and are dominated by krithe and cytheropteron reflecting faunal exchange with the greenland sea via the fram strait major faunal changes occurred in the arctic during the last glacial interglacial transition and the holocene low abundance low diversity assemblages from the lomonosov and gakkel ridges in the eurasian basin from the last glacial period have modern analogs in cold low salinity low nutrient greenland sea deep water glacial assemblages from the deep nansen and amundsen basins have modern analogs in the deep canada basin during termination 1 at intermediate depths diversity and abundance increased coincident with increased biogenic sediment reflecting increased organic productivity reduced sea ice and enhanced inflowing north atlantic water during deglaciation deep nansen basin assemblages were similar to those living today in the deep greenland sea perhaps reflecting deepwater exchange via the fram strait in the central arctic early holocene faunas indicate weaker north atlantic water inflow at middepths immediately following termination 1 about 8500 7000 year b p followed by a period of strong canada basin water overflow across the lomonosov ridge into the morris jesup rise area and central arctic ocean modern perennial sea ice cover evolved over the last 4000 5000 years late quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold deepwater of greenland sea origin and warmer and middepth atlantic water to the eurasian basin through the fram strait and export of arctic ocean deepwater

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

silicon in the earth s core the earth and moon have iron isotopic abundances that are slightly heavy compared with mars the asteroid vesta and primitive meteorites the reason for this has been unclear one suggestion is that there was a large scale isotopic equilibration during the giant impact the collision between the early earth and a mars sized body that created the moon but this has seemed unlikely given that most other elements do not show the effect an analysis of 44 meteorite and terrestrial samples now shows that the silicon isotopic compositions of basaltic rocks from the earth and moon are also distinctly heavy the finding is consistent with the isotopic equilibration model and it suggests that silicon was already partitioned into the earth s core as a light element before the moon formed the silicon isotopic compositions of basaltic rocks from the earth and moon are distinctly heavy the similar isotopic composition of the bulk silicate earth and the moon are consistent with large scale isotopic equilibration during the giant impact indicating that si was already a light element in the earth s core before the moon formed small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation or from evaporative losses during accretion as the planets were built up basalts from the earth and the moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from mars vesta and primitive undifferentiated meteorites1 2 3 4 chondrites explanations for these differences have included evaporation during the giant impact that created the moon when a mars sized body collided with the young earth however lithium5 and magnesium6 lighter elements with comparable volatility7 8 9 reveal no such differences rendering evaporation unlikely as an explanation here we show that the silicon isotopic compositions of basaltic rocks from the earth and the moon are also distinctly heavy a likely cause is that silicon is one of the light elements in the earth s core we show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate the similar isotopic composition of the bulk silicate earth and the moon is consistent with the recent proposal11 that there was large scale isotopic equilibration during the giant impact we conclude that si was already incorporated as a light element in the earth s core before the moon formed

naturally weathered feldspar surfaces in the navajo sandstone aquifer black mesa arizona electron microscopic characterization abstract naturally weathered feldspar surfaces in the jurassic navajo sandstone at black mesa arizona was characterized with high resolution transmission and analytical electron microscope hrtem aem and field emission gun scanning electron microscope feg sem here we report the first hrtem observation of a 10 nm thick amorphous layer on naturally weathered k feldspar in currently slightly alkaline groundwater the amorphous layer is probably deficient in k and enriched in si in addition to the amorphous layer the feldspar surfaces are also partially coated with tightly adhered kaolin platelets outside of the kaolin coatings feldspar grains are covered with a continuous 3 5 m thick layer of authigenic smectite which also coats quartz and other sediment grains authigenic k feldspar overgrowth and etch pits were also found on feldspar grains these characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems and may partially contribute to the commonly observed apparent laboratory field dissolution rate discrepancy at black mesa feldspars in the navajo sandstone are dissolving at 10 5 times slower than laboratory rate at comparable temperature and ph under far from equilibrium condition the tightly adhered kaolin platelets reduce the feldspar reactive surface area and the authigenic k feldspar overgrowth reduces the feldspar reactivity however the continuous smectite coating layer does not appear to constitute a diffusion barrier the exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer leached layer versus re precipitated silica which is uncertain at present however the nanometer thin layer can be detected only with hrtem and thus our study raises the possibility of its wide occurrence in geological systems rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface

cenozoic deep sea benthic foraminifers tracers for changes in oceanic productivity from late middle eocene through earliest oligocene high latitude regions cooled and by the end of the period continental ice sheets existed in antarctica diversity of planktonic microorganisms declined and modern groups of terrestrial vertebrates originated coeval faunal changes in deep sea benthic foraminifers have been related to cooling of deep waters and increased oxygenation cooling however occurred globally whereas species richness declined at high latitudes and not in the tropics the late eocene and younger lower diversity high latitude faunas typically contain common epistominella exigua and alabaminella weddellensis opportunistic phytodetritus exploiting species that indicate a seasonally fluctuating input of organic matter to the sea floor we speculate that the species richness gradient and increase in abundance of phytodetritus exploiting species resulted largely from the onset of a more unpredictable and seasonally fluctuating food supply especially at high latitudes

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

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

analog modeling of divergent and convergent transfer zones in listric normal fault systems transfer zones are common features in passive margins such as the gulf of mexico where deformation between adjacent listric normal faults is accommodated by the formation of complex secondary fault systems two common types of transfer zones are 1 convergent with the main faults dipping toward each other and 2 divergent with the main faults dipping away from each other analog clay models are developed for both convergent and divergent transfer zones with the faults approaching each other offset by 90 and overlapping each other during extension the structures initiate as symmetric grabens and later transform into asymmetric half grabens the main fault and associated synthetic faults form a narrow zone of deformation whereas the antithetic faults develop into a broader zone constituting evenly spaced discrete fault segments orientations lengths densities and sizes of connected fault clusters vary with the type of transfer zone structural position relative to the fault offsets and total extension the experiments show that the antithetic faults in convergent transfer zones and synthetic faults in divergent transfer zones tend to be more consistent in orientation and connect easily synthetic faults in convergent transfer zones and antithetic faults in divergent transfer zones change orientation along strike toward the transfer zone fault connectivity in approaching transfer zones is generally higher than in normal offset or overlapping configurations the analog models provide important insights on the geometry of fault patterns and possible fluid migration pathways within transfer zones

an oblique slant on deep sea biodiversity organisms at the bottom of the deep seas might seem to be immune from large scale shifts in climate not so according to a study which reveals links between benthic biodiversity glaciation and the earth s obliquity

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

submarine groundwater discharge revealed by 228 ra distribution in the upper atlantic ocean submarine groundwater discharge estimated from a 228ra inventory across the upper atlantic ocean provides a flux of 2 4 1013 m3 yr 1 equivalent to 80 160 of the influx from rivers into the atlantic ocean