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

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

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

heavy metal migration at a landfill site sarnia ontario canada 2 metal partitioning and geotechnical implications abstract heavy metal profiles below a 15 year old sanitary landfill overlying a 30 m thick natural clay deposit are presented results indicate that unlike soluble species such as cl and na which have migrated distances up to 130 cm cu zn and pb have migrated only up to 10 cm the extent of fe migration is estimated to be 20 cm highly reducing conditions at the interface e h 130mv coupled with the alkaline nature of the clay pore waters have resulted in the precipitation of migrated heavy metals as carbonates at the clay waste interface 88 84 and 80 of the excess fe zn and pb respectively are present as secondary carbonates this is confirmed by selective chemical dissolution analyses which also show that fe zn pb and to a greater extent cu are present in solid organic forms at the interface batch equilibrium studies clearly show that cu and pb removal from leachate is significantly increased by the presence of carbonates in the soil for example 75 more pb is removed by the carbonate rich bulk soil than the carbonate free soil the batch studies also show that when the ph 5 2 removal of metal increases significantly due to precipitation as carbonates from the results it is concluded that the presence of metal sludges in landfills lined naturally or artifically by a carbonate rich clayey barrier reduces the rate of migration of numerous toxic transition metals and may also decrease the barrier porosity by precipitation the decreases in porosity will be beneficial to the performance of the barrier due to reductions in both advection and diffusion

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

stratification of the f 2 layer observed in southeast asia this paper describes the latitudinal variation in f2 stratification balan and bailey 1995 as observed by a number of oblique and vertical ionosondes operating in southeast asia during 1997 stratification of the f2 layer was seen at dip latitudes from 4 s to 18 s on the southern side of the magnetic equator but did not occur at the closest reflection point to the magnetic equator dip latitude 2 3 n the observed transient cusp vertical ionosonde or additional nose oblique ionosonde was defined as an f3 layer or an f1 5 layer depending on whether it occurred above or below the layer which maintained continuity with the normal f2 layer peak within the zone of occurrence the transient layer was commonly seen as an f3 layer at reflection points closest to the magnetic equator but invariably as an f1 5 layer at reflection points farther from the magnetic equator these observations suggest that the distortion in the equatorial electron density profile associated with the phenomenon moved toward the base of the f2 layer as magnetic field lines descended with increasing latitude stratification of the f2 layer commenced at the same local time e g 0845 lt in november 1997 throughout the longitudinal range of coverage and was associated with a rapid rise in f2 layer height following sunrise the stratification ended at times varying from 1300 lt to sunset and was associated with a fall in the height of f2 peak electron density the region of maximum f2 layer stratification lay between the magnetic equator and the peak of the southern equatorial anomaly

an additional layer in the low latitude ionosphere in indian longitudes total electron content observations and modeling 1 the paper presents the observations and modeling of an additional layer in the low latitude ionosphere in indian longitudes the signatures of the additional layer are observed as ledges or humps between the equatorial ionization anomaly trough and crest eia in the latitudinal profiles of total electron content tec measured using a single ground based beacon receiver located at trivandrum 8 5 n 77 e dip 0 5 n in india the ground based ionograms also show the presence of the so called f 3 layer for a short duration corresponding to these signatures and the layer is found to drift upward to the topside ionosphere the study provides first observational evidence that the so called humps in the latitudinal variation of tec are nothing but the upward propagating f 3 layer this conclusion is supported by theoretical modeling using the sheffield university plasmasphere ionosphere model it is shown that upward exb drift and strong equatorward neutral wind perturbed by atmospheric waves can produce the humps in the latitudinal variation of tec through the reduction in the downward diffusion of ionization along geomagnetic field lines the model results also show that the f 3 layer drifts to the topside and forms topside ledges

additional stratifications in the equatorial f region at dawn and dusk during geomagnetic storms role of electrodynamics 1 the role of electrodynamics in producing additional stratifications in the equatorial f region f 3 layer at dawn and dusk during geomagnetic storms is discussed two cases of f 3 layer at dawn 0600 0730 lt on 5 october 2000 and 8 december 2000 and one case of f 3 layer at dusk 1600 1730 lt on 5 october 2000 are observed for the first time by the digital ionosonde at the equatorial station trivandrum 8 5 n 77 e dip 0 5 n in india the unusual f 3 layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field b z suggesting that eastward prompt penetration electric field could be the main cause of the f 3 layers the dawn f 3 layer on 5 october is modeled using the sheffield university plasmasphere ionosphere model by using the e x b drift estimated from the real height variation of the ionospheric peak during the morning period the model qualitatively reproduces the dawn f 3 layer while the existing f 2 layer rapidly drifts upward and forms the f 3 layer and topside ledge a new layer forming at lower heights develops into the normal f 2 layer

distribution characteristics of topside ionospheric density irregularities equatorial versus midlatitude regions 1 the global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between 35 geographic latitudes has been studied with the rocsat data during moderate to high solar activity years of 1999 to 2004 the result indicates that the global occurrence distribution of the intermediate scale 0 1 to 50 km density irregularities can be grouped into two different populations one in the equatorial region and the other in the middle to subauroral latitude region the global seasonal longitudinal s l distribution of equatorial irregularities in the current report reproduces the result of mcclure et al 1998 obtained with the ae e observations of the mesoscale 50 to 1000 km plasma bubble structures during high solar activity years of 1978 to 1980 two solar cycles ago this implies that the density irregularities of different scales from multistage cascading process of the large scale 1000 km gravitational rayleigh taylor instability have manifested in same global s l distribution pattern furthermore global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years in addition the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about 30 a different kind of midlatitude irregularity distribution starts to fill in from this dip latitude in other words the equatorial density irregularity inside a depleted flux tube can only rise on statistical average to an apex height of 2000 km different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes the occurrence frequency of equatorial density irregularities increases with solar flux intensity whereas the midlatitude density irregularity is more likely to occur during low solar activity period the equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times on the other hand the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity the local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate scale density irregularities

features of the f3 layer in the low latitude ionosphere at sunset the f 3 layer is a common feature within 10 degrees of the magnetic equatorial ionosphere in the daytime according to balan et al 1998 the f 3 layer occurs mainly during the morning noon period due to the combined effect of the upward e x b drift and the neutral wind that provides upward plasma drifts at and above the f 2 layer the f 3 layer occurrence rate is higher in summer and decreases with increasing solar activity in this study the characteristic of the sunset f 3 layer is first investigated using a solar cycle of ionosonde data 1995 2010 from the magnetic equatorial station at jicamarca and compared with the features derived from the four subtropical stations at sao luis fortaleza kwajalein and vanimo evidence shows that the local time distribution of the occurrence of the f 3 layer can extend to the postsunset time 1800 2100 local time the sunset f 3 layer has a strong seasonal dependence occurring mainly during the summertime unlike the daytime f 3 layer the occurrence of the sunset f 3 layer clearly increases and the virtual height of the bottom side of the f 3 layer statistically increases from 620 to 1000 km with increasing solar activity in addition the occurrence of the sunset f 3 layer at the other stations is much less than that at jicamarca these features of the dependence on the season solar activity and latitude are clearly related to the geomagnetic control of the evening prereversal enhancement of the equatorial zonal electric field and geomagnetic configuration

evidence of silicic acid leakage to the tropical atlantic via antarctic intermediate water during marine isotope stage 4 antarctic intermediate water aaiw and subantarctic mode water samw are the main conduits for the supply of dissolved silicon silicic acid from the deep southern ocean to the low latitude surface ocean and therefore have an important control on low latitude diatom productivity enhanced supply of silicic acid by aaiw and samw during glacial periods may have enabled tropical diatoms to outcompete carbonate producing phytoplankton decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pco2 this mechanism is known as the silicic acid leakage hypothesis salh here we present records of neodymium and silicon isotopes from the western tropical atlantic that provide the first direct evidence of increased silicic acid leakage from the southern ocean to the tropical atlantic within aaiw during glacial marine isotope stage mis 4 60 70 ka this leakage was approximately coeval with enhanced diatom export in the nw atlantic and across the eastern equatorial atlantic and provides support for the salh as a contributor to co2 drawdown during full glacial development

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

constraining oceanic dust deposition using surface ocean dissolved al we use measurements of ocean surface dissolved al a global biogeochemical elemental cycling bec ocean model and the global dust entrainment and deposition dead model to constrain dust deposition to the oceans our al database contains all available measurements with best coverage in the atlantic vertical profiles and seasonal data exist in limited regions observations show that surface dissolved al is distributed similarly to the dust deposition predicted by dead and other models there is an equatorial atlantic al maximum that decreases toward higher latitudes there are high al concentrations in the mediterranean sea and the arabian sea and low concentrations in the pacific and the southern ocean the ocean basins maintain more distinct al profiles than fe profiles in the upper ocean consistent with a weaker biological influence on al than fe the bec predicted surface dissolved al compares relatively well with observations the al distribution reflects the combined effects of al input from dust and al removal by particle scavenging and biological uptake by diatoms model observed biases suggest a southward shift of maximum dust deposition compared to current dust model predictions dead appears to overestimate deposition north of 30 n in the pacific and to underestimate deposition south of 30 n observed al concentrations and the ocean model predicted surface al lifetime provide a semi independent method to estimate oceanic dust deposition this technique indicates that dead may overestimate dust deposition to the north equatorial atlantic but underestimate in other atlantic regions the southern ocean and the arabian sea however spatial variations in aerosol al solubility may also contribute to the model observation mismatch our results have implications for all dust borne ocean nutrients including fe and demonstrate the potential of marine geochemical data to constrain atmospheric aerosol deposition fields

dayside ionospheric response to the intense interplanetary shocks solar wind discontinuities observations from the digisonde global ionospheric radio observatory 1 the digisonde global ionospheric radio observatory giro has recorded strong ionospheric responses to two powerful interplanetary shock passages on 7 november 7 2004 and 21 january 2005 both events provide excellent opportunities to study geospace response processes to strong interplanetary forward shocks in great detail the angle between the normal of the 7 november 2004 shock front and the sun earth line was 3 0 degrees indicating that the shock hit the equatorial magnetosphere at 1200 lt local noon the subsequent dayside shock induced ionospheric phenomena were found to have marked longitudinal and latitudinal distributions comparative studies of the intense interplanetary shocks including the sheath associated with northward and southward interplanetary magnetic fields imf revealed their different geospace effects on the dayside ionospheric region the equatorial ionosphere responds to the interplanetary shock rather quickly and if the shock is associated with southward imf the plasma electrons in the equatorial ionosphere are rapidly uplifted during the 7 november 2004 event the averaged uplift velocity was close to 67 m s and the ionospheric total electron content tec increased from the original 16 tec units tecu to 38 tecu 1 tecu 1016 m 2 which may be due to the shock effect when the interplanetary shock including the magnetosheath is associated with northward imf the plasma in the equatorial ionosphere moves downward causing a sudden drop in the total electron content during the 21 january 2005 event the averaged downward velocity was 120 m s and the tec dropped from 75 tecu to 22 tecu at the time of maximum solar wind dynamic pressure and then recovered to 68 tecu in about 2 h after the shock passage the middle and high latitude ionospheric tec enhancements may be due to particle ion and electron precipitation ionization losses caused by the strong impacts of the interplanetary shocks

theoretical study of new plasma structures in the low latitude ionosphere during a major magnetic storm 1 theoretical model simulations for an intense magnetic storm show the creation of a low latitude electron density arch aligned along the geomagnetic field created by strong uplift of the f2 layer that is driven by the penetration electric field when the arch forms during the day a new f2 layer is created at the original altitude by photoionization and a density hole can be created between this new f2 layer and the arch when the arch forms during the night the f2 layer is not recreated and no hole forms in a vertical profile of electron density the daytime elevated ionospheric layer can appear distinctly from the recreated f2 layer in which case the elevated layer is called the f3 layer a latitude cut through the night side arch shows the characteristics of an equatorial electron density trough bounded to the north and south by enhanced densities associated with plasma that has diffused down along the geomagnetic field from the elevated layer it is pointed out that the signature of the night side arch has been seen in low earth orbit spacecraft data but has not been previously associated with an arch structure

an empirical model of the earth s horizontal wind fields hwm07 1 the new horizontal wind model hwm07 provides a statistical representation of the horizontal wind fields of the earth s atmosphere from the ground to the exosphere 0 500 km it represents over 50 years of satellite rocket and ground based wind measurements via a compact fortran 90 subroutine the computer model is a function of geographic location altitude day of the year solar local time and geomagnetic activity it includes representations of the zonal mean circulation stationary planetary waves migrating tides and the seasonal modulation thereof hwm07 is composed of two components a quiet time component for the background state described in this paper and a geomagnetic storm time component dwm07 described in a companion paper

first simultaneous observations of f3 layer and e b drift in indian sector and modeling 1 although the basic mechanism of an additional stratification of the daytime equatorial f region called the f3 layer has been explained two observational aspects that remain not so clear are its summer winter differences and simultaneous occurrences on either side of the geomagnetic equator in this paper we address these two aspects using simultaneous observations of the f3 layer and vertical e b drift velocity made for the first time in an indian station gadanki 6 5 n mag lat during the low solar activity period 2008 2009 and making model computations using the sheffield university plasmasphere ionosphere model the observations confirm the frequent occurrence of the f3 layer in summer months compared to winter months the observations also reveal the occasional occurrence of clear and distinct f3 layer in winter although such a layer is frequent in summer in addition a threshold vertical plasma velocity involving the e b drift and neutral wind and its time integrated value are found to be important for the formation of the f3 layer the model results qualitatively reproduce the observations and show that irrespective of season the formation of the f3 layer is centered on that side of the equator where the equatorward neutral wind reduces the downward field aligned flow of plasma and the latitude band of the layer can extend to the opposite hemisphere especially when the vertical plasma velocity is large

neutral wind effect in producing a storm time ionospheric additional layer in the equatorial ionization anomaly region 1 a new type of the additional layer is predicted to occur in the low latitude f region ionosphere during evening hours of a major magnetic storm by using the coupled ncar thermosphere ionosphere electrodynamic general circulation model tiegcm and sheffield university plasmasphere ionosphere model supim runs during the major magnetic storm of 29 30 october 2003 we present a new type of additional layer which occurs at the equatorial ionization anomaly eia by a new physical mechanism this mechanism requires that storm time meridional neutral wind surges travel from high to low latitudes and cross into the opposite hemisphere the wind surges modify the field aligned plasma velocities in the eia regions significantly after uplift of the ionospheric layer by a penetration electric field and interact with the downward field aligned plasma velocities of the enhanced equatorial fountain the combined storm effects of the enhanced plasma fountain and the neutral wind surges result in plasma convergence in altitude and form the additional layers underneath the eia crests

magnetic conjugate observation of the f3 layer using the sealion ionosonde network 1 results from the meridional ionosonde network located in southeast asia sealion demonstrate the interesting nature of the f3 layer showing its generation mechanism ionograms obtained on 16 november 2004 and 31 march 2005 at chiang mai cmu geographic latitude 18 8 n geographic longitude 98 9 e and magnetic latitude 13 2 n chumphon cpn 10 7 n 99 4 e and 3 2 n and kototabang ktb 0 2 s 100 3 e and 10 1 s showed significant differences between cpn near the magnetic equator and cmu and ktb in the magnetic low latitude region the simultaneous magnetic conjugate observations of the f3 layer achieved using the sealion ionosonde network data showed clear dependences of the f3 layer on the magnetic latitude it is suggested that these magnetic latitude dependences of the f3 layer can be explained by considering the plasma diffusion effects along the magnetic field lines in the magnetic low latitude region

f3 layer during penetration electric field 1 the occurrence of an additional layer called f3 layer in the equatorial ionosphere at american indian and australian longitudes during the super double geomagnetic storm of 7 11 november 2004 is presented using observations and modeling the observations show the occurrence reoccurrence and quick ascent to the topside ionosphere of unusually strong f3 layer in australian longitude during the first super storm 8 november and in indian longitude during the second super storm 10 november all with large reductions in peak electron density nmax and total electron content gps tec the unusual f3 layers can arise mainly from unusually strong fluctuations in the daytime vertical e b drift as indicated by the observations and modeling in american longitude the strongest upward e b drift or eastward prompt penetration electric field ppef ever recorded at jicamarca produces unusually strong f3 layer in the afternoon hours 1400 1600 lt of ppef with large reductions in nmax and tec the layer also reappears in the following evening 1700 1800 lt owing to an unusually large downward drift at night when the drift is unusually upward and strong the f region splits into two layers

silicon isotope variations in central siberian rivers during basalt weathering in permafrost dominated larch forests abstract this work is devoted to the characterization of natural mechanisms of silicon isotope fractionation within siberian watersheds and predicting the climate warming effect on si fluxes from the land to the arctic ocean to unravel the different sources of silica generated by basalt weathering in central siberia under permafrost and larch deciduous forest conditions we measured the si isotopic composition of large and small rivers surface flow interstitial soil solutions plant litter and soils the average annual discharge weighted 30 si values of the second largest tributary of the yenissei river nyzhnaya tunguska and its main northern tributary kochechum are equal to 1 08 0 10 and 1 67 0 15 respectively while their average annual si concentrations are very similar 3 46 and 3 50 mg l respectively during summer baseflow the dissolved si isotope composition of both large rivers and a small stream ranges between 1 5 and 2 5 this is much heavier compared to the source basaltic rocks but similar to the fresh litter of larix gmelinii the dominating tree species in this region it could be consistent with litter degradation in the uppermost soil horizons being the dominant source of solutes annually exported by central siberian rivers during spring flood accounting for 60 80 of annual si flux the 30 si of the large rivers dissolved load decreases by 1 1 5 thus approaching the value of the bedrock and the silicate suspended matter of the rivers rsm this may reflect the dissolution of the silicate suspended load at high water mineral ratio the winter 30 si values of the large river dissolved load range between 1 0 and 2 5 during this period contributing to 10 of the annual si chemical flux the interaction between bedrock porous tuffs and deep ground waters occurs at a very high solid solution ratio leading to the precipitation of isotopically light secondary minerals and enrichment of 30 si in the fluids that feed the river through the unfrozen flowpaths results of this study imply that more than a half of the silica transported by siberian rivers may transit through the biogenic pool and that like in other stable basaltic regions bedrock water interactions account for a lesser fraction of the silica flux as a result of projected future climate warming and weathering increases in boreal regions the 30 si isotopic composition of large siberian rivers is likely to shift towards less positive values

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

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

equatorial f region vertical plasma drifts seasonal and longitudinal asymmetries in the american sector longitudinal and seasonal asymmetries in the evening ionospheric f region plasma vertical drift v sub z enhancements between two longitudinally separated stations situated along the magnetic equator in the american sector are investigated under solar maximum conditions based on results obtained from the analysis of ionosonde data for these stations the two stations are huancayo peru and fortaleza brazil which have markedly different magnetic declination angles the observed asymmetries are interpreted using a detailed numerical simulation of the e and f region electrodynamic coupling process that takes into account also its asymmetry about the magnetic equator arising from the finite magnetic declination angle the results of the simulation show in agreement with observations that the occurrence time of the evening f region vertical drift prereversal peak and its seasonal variation at a station are controlled by the magnetic declination angle at that station which determines the seasonal variation of the sunset times and hence the integrated pedersen conductivity longitudinal gradient at its magnetic conjugate e layers the amplitude of the prereversal peak on the other hand undergoes the influence of the magnetic declination angle as well as of the thermospheric zonal wind component

coupled al si geochemistry in an ocean general circulation model a tool for the validation of oceanic dust deposition fields 1 we introduced the marine cycle of al into a geochemical ocean general circulation model in order to assess the potential of modeled surface ocean dissolved al as a tracer for dust input to the world ocean the geochemical cycles of si and al have been coupled through the dynamics of diatoms aluminum is added to the ocean by deposition of continental aerosols followed by partial dissolution of the aluminosilicate fraction dissolved al interacts with suspended siliceous particles according to a reversible first order scavenging reaction dust fields are provided by atmospheric general circulation models a database of marine dissolved al measurements was collated the ocean model results for two different dust deposition fields andersen et al 1998 mahowald et al 1999 are compared with these data the model confirms a solubility of the aluminosilicate fraction between 1 5 and 3 0 modeled levels of dissolved al in surface waters mirror the delivery of dust to the surface ocean comparing dissolved al distributions resulting from the two different dust fields yields significant differences in ocean dissolved al distributions which reflect a differential contribution of individual source regions model results suggest a significant overestimation of the north african arabian desert source region for the dust field after mahowald et al 1999 dust input to the central pacific ocean appears to be of the correct order of magnitude this approach allows for a direct comparison of dust fields and proposes an independent check of the underlying parameterization of uplift transport and deposition of dust

silica cycling over geologic time the earth s long term silica cycle is intimately linked to weathering rates and biogenic uptake changes in weathering rates and the retention of silica on land have altered silica availability in the oceans for hundreds of millions of years

importance of flood plain sedimentation for river sediment budgets and terrigenous input to the oceans insights from the brahmaputra jamuna river recent authors have suggested that a significant proportion of the worldwide terrigenous sediment budget is trapped landward of the river ocean boundary in high load tectonically active basins to test this idea modern flood plain sediment accumulation rates were determined along a 110 km reach of the brahmaputra locally jamuna river using 137cs geochronology of sediment cores and geographic information system gis extrapolation to adjacent areas 137cs accumulation rates decrease exponentially away from the channel from 4 cm yr on the natural levees to 1 cm yr within a few tens of kilometers basinward important controls on sedimentation in this area include proximity to distributary channels local topography and interannual variability of the flood pulse model results indicate that an average of 23 m t yr of sediment are sequestered in this section of the flood plain extrapolated basinwide as much as 39 71 of the river sediment budget may be trapped landward of the ganges brahmaputra mouth

effects of disturbed electric fields in the low latitude and equatorial ionosphere during the 2015 st patrick s day storm the 2015 st patrick s day geomagnetic storm with sym h value of 233 nt is an extreme space weather event in the current 24th solar cycle in this work we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the asian australian sector and the american sector during this super storm event the results reveal that the disturbed electric fields which comprise penetration electric fields pefs and disturbance dynamo electric fields ddefs play a decisive role in the ionospheric storm effects in low latitude and equatorial regions pefs occur on march 17 in both the american sector and the asian australian sector the effects of ddefs are also remarkable in the two longitudinal sectors both the ddefs and pefs show the notable local time dependence which causes the sector differences in the characteristics of the disturbed electric fields this differences would further lead to the sector differences in the low latitude ionospheric response during this storm the negative storm effects caused by the long duration ddefs are intense over the asian australian sector while the repeated elevations of hmf2 and the eia intensifications caused by the multiple strong pefs are more distinctive over the american sector especially the storm time f3 layer features are caught on march 17 in the american equatorial region proving the effects of the multiple strong eastward pefs

silicate weathering in the ganges alluvial plain the ganges is one of the world s largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny weathering and global climate change three regions can be recognised in the ganges basin with the himalayan orogeny to the north and the plateaus of peninsular india to the south together delimiting the ganges alluvial plain despite constituting approximately 80 of the basin weathering processes in the peninsula and alluvial plain have received little attention here we present an analysis of 51 water samples along a transect of the alluvial plain including all major tributaries we focus on the geochemistry of silicon and its isotopes area normalised dissolved si yields are approximately twice as high in rivers of himalaya origin than the plain and peninsular tributaries 82 51 and 32 kmol sio2 km 2 yr 1 respectively such dissolved si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the dsi mobilised during the initial weathering process is retained in secondary clay minerals however the silicon isotopic composition of dissolved si expressed as delta si 30 varies from 0 8 parts per thousand in the ganges mainstem at the himalaya front to 3 0 parts per thousand in alluvial plain streams and appears to be controlled by weathering congruency i e by the degree of incorporation of si into secondary phases the higher delta si 30 values therefore reflect decreasing weathering congruency in the lowland river catchments this is exploited to quantify the degree of removal using a rayleigh isotope mass balance model and consequently derive initial silica mobilisation rates of 200 150 and 107 kmol sio2 km 2 yr 1 for the himalaya peninsular india and the alluvial plain respectively because the non himalayan regions dominate the catchment area the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment 41 and 34 respectively c 2015 the authors published by elsevier b v less

equatorial plasma fountain and its effects over three locations evidence for an additional layer the f 3 layer the equatorial plasma fountain and equatorial anomaly in the ionospheres over jicamarca 77 w trivandrum 77 e and fortaleza 38 w are presented using the sheffield university plasmasphere ionosphere model under magnetically quiet equinoctial conditions at high solar activity the daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer the f3 layer at latitudes within about plus or minus 10 of the magnetic equator in each ionosphere the maximum plasma concentration of the f3 layer which occurs at about 550 km altitude becomes greater than that of the f2 layer for a short period of time before noon when the vertical e b drift is large within the f3 layer the plasma temperature decreases by as much as 100 k the ionograms recorded at fortaleza on january 15 1995 provide observational evidence for the development and decay of an f3 layer before noon the neutral wind which causes large north south asymmetries in the plasma fountain in each ionosphere during both daytime and nighttime becomes least effective during the prereversal strengthening of the upward drift during this time the plasma fountain is symmetrical with respect to the magnetic equator and rises to over 1200 km altitude at the equator with accompanying plasma density depletions in the bottomside of the underlying f region the north south asymmetries of the equatorial plasma fountain and equatorial anomaly are more strongly dependent upon the displacement of the geomagnetic and geographic equators jicamarca and trivandrum than on the magnetic declination angle fortaleza

features of additional stratification in ionospheric f2 layer observed for half a solar cycle over indian low latitudes 1 the features of the additional stratification in the ionospheric f2 layer often referred to as the f3 layer observed over an indian low latitude station waltair 17 7 n 83 3 e magnetic latitude mag lat 8 2 n during the period 1997 2003 are presented along with the data from two other indian stations the observations grossly confirm those reported earlier in the occurrence and seasonal variability of the f3 layer from an analysis of half a solar cycle ionosonde data 1997 2003 it is observed that the layer appeared more frequently during the summer solstice months of low solar activity period and persisted for longer durations during this season compared with equinox and winter solstice the best stratification is seen between 10 12 hours ist the occurrence of the f3 layer does not seem to depend on magnetic activity but the percentage of occurrence decreased with increasing solar activity the solar activity dependence over waltair confirms the model predictions of balan et al 1998 that the layer becomes less distinct and less frequent as solar activity increases the ionosonde data for the same period 1997 2003 from an equatorial station trivandrum 8 4 n 76 9 e mag lat 0 47 n and another low latitude station shar 14 n 80 e mag lat 6 8 n are also analyzed with a view to examine the effect of the equatorial plasma dynamics on the occurrence of such events

role of river suspended material in the global carbon cycle the reaction of ca derived from silicate weathering with co2 in the world s oceans to form carbonate minerals is a critical step in long term climate moderation ca is delivered to the oceans primarily via rivers where it is transported either as dissolved species or within suspended material the relative importance for climate moderation of riverine dissolved ca vs suspended ca transport stems from the total ca flux and its climate dependence data in the literature suggest that within uncertainty global riverine dissolved ca flux is equal to suspended material ca flux to determine how these fluxes depend on temperature and rainfall a 40 yr field study was performed on 4 catchments

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

peculiar features of ionospheric f 3 layer during prolonged solar minimum 2007 2009 we present the seasonal and local time occurrence of ionospheric f 3 layer over tirunelveli geographic longitude 77 8 e geographic latitude 8 7 n dip 0 7 during extremely low and prolonged solar activity period 2007 2009 canadian advanced digital ionosonde observations from this station are used in the present study we find that the occurrence of f 3 layer is nearly 3 times higher during 2009 48 as compared to that during 2007 16 the increase of this order just within the low solar activity period is unusual in earlier studies similar increase in f3 occurrence has been reported when solar activity changes from high f10 7 182 t o low f10 7 72 the other important feature is the presence of postnoon f3 layers which are observed dominantly during summer solstice of 2009 such occurrence of postnoon f3 layers was nearly absent during summer solstice of the previous solar minimum 1996 over nearby dip equatorial station trivandrum we take equatorial electrojet eej as a proxy for eastward electric field it is noticed that the eej strength and the maximum rate of change of eej are higher for f3 days as compared to those on non f3 days we find that the peak occurrence of prenoon f3 layer closely coincides with the time of maximum rate of change of eej it is in general accordance with the theory proposed by balan et al 1998 that suggests the formation of f3 through vertically upward e b drift in presence of equatorward neutral wind the present study reveals that the rate of change of eastward electric field de dt as well plays an important role in the formation of f3 layer

silicon isotopes in meteorites and planetary core formation the silicon si isotope compositions of 42 meteorite and terrestrial samples have been determined using mc icpms with the aim of resolving the current debate over their compositions and the implications for core formation no systematic 30si differences are resolved between chondrites 30si 0 49 0 15 2 sd and achondrites 30si 0 47 0 11 2 sd although enstatite chondrites are consistently lighter 30si 0 63 0 07 2 sd in comparison to other meteorite groups the data reported here for meteorites and terrestrial samples display an average difference 30sibse meteorite 0 15 0 10 which is consistent within uncertainty with previous studies no effect from sample heterogeneity preparation chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets the heavier composition of the bulk silicate earth is consistent with previous conclusions that si partitioned into the metal phase during metal silicate equilibration at the time of core formation fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor e 0 89 the 30sibse meteorite value from this study indicates that the earth core contains at least 2 5 and possibly up to 16 8 wt si

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

observations of daytime f2 layer stratification under the southern crest of the equatorial ionization anomaly region 1 ionospheric vertical sounding observations are being carried out at sao jose dos campos 23 2 s 45 9 w dip latitude 17 6 s brazil under the southern crest of the equatorial ionization anomaly eia since august 2000 in this paper we present and discuss the observations of daytime f2 layer stratification near the crest of eia for the first time under magnetically quiet high solar activity conditions three examples and a year of statistics are presented the f2 layer stratification and f3 layer were observed between 10 40 and 11 45 ut on 31 december 2000 between 13 30 and 14 30 ut on 1 january 2001 and between 13 15 and 15 15 ut on 11 february 2001 the statistics during september 2000 to august 2001 shows that the f3 layer occurs only for 66 days 18 occurrence and it occurs only during september february spring summer with maximum occurrence in september october and longest duration in february the f2 layer stratification seems to be associated with gravity waves gws which have periods of about 30 60 min downward phase velocities of about 60 140 m s and vertical wavelengths of about 200 500 km the presence of powerful gravity waves in a vertically extended f layer seems to stratify the f2 layer and produce the f3 layer because the stratifications are observed during geomagnetically quiet periods the source of the gravity waves are most likely to be associated with local tropospheric disturbances and not with high latitude disturbances

physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere a physical mechanism and the location and latitudinal extent of an additional layer called the f3 layer that exists in the equatorial ionosphere are presented a statistical analysis of the occurrence of the layer recorded at the equatorial station fortaleza 4 s 38 w dip 9 s in brazil is also presented the f3 layer forms during the morning noon period in that equatorial region where the combined effect of the upward e b drift and neutral wind provides a vertically upward plasma drift velocity at altitudes near and above the f2 peak this velocity causes the f2 peak to drift upward and form the f3 layer while the normal f2 layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region the peak electron density of the f3 layer can exceed that of the f2 layer the f3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar activity and to become less distinct as the solar activity increases ionograms recorded at fortaleza in 1995 show the existence of an f3 layer on 49 of the days with the occurrence being most frequent 75 and distinct in summer as expected during summer the layer occurs earlier and lasts longer compared to the other seasons on the average the layer occurs at around 0930 lt and lasts for about 3 hours the altitude of the layer is also high in summer with the mean peak virtual height being about 570 km however the critical frequency of the layer f0f3 exceeds that of the f2 layer f0f2 by the largest amounts in winter and equinox f0f3 exceeds f0f2 by a yearly average of about 1 3 mhz

stable runoff and weathering fluxes into the oceans over quaternary climate cycles the effect of glacial interglacial cycles on surface weathering rates has been unclear a beryllium based proxy for weathering shows minimal variations in the input of silicate weathering products to the oceans for the past two million years

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

trace metal speciation in landfill leachate commentaire des articles de yanful e k nesbitt h w et quigley r m 1988a heavy metal migration at a landfill site sarnia ontario canada i thermodynamic assessment and chemical interpretations appl geochem 3 523 533 et de yanful e k quigley r m et nesbitt h w 1988b ii metal partitioning and geotechnical implications appl geochem 3 623 629

comparison between nighttime ionosonde incoherent scatter radar ae e satellite and hf doppler observations of f region vertical electrodynamic plasma drifts in the vicinity of the magnetic equator 1 nighttime f region vertical drifts were made using an ionosonde for the equatorial station ibadan 7 4 n 3 9 e 6 s dip from 1 year of data during 1957 1958 international geophysical year igy that corresponds to a period of solar maximum for undisturbed condition we compare the seasonal vertical drifts with measurements made by incoherent scatter radar ae e satellite and hf doppler for equatorial f region vertical drifts we find a comparable variability pattern during periods of high f layer heights during equinox and the december solstice and the opposite behavior occurs during june solstice the drifts are predominantly downward between 2000 and 0500 lt intervals ionosonde drifts are smaller in values by either a factor of two or three than other methods except for consistent june solstice ionosonde and satellite magnitudes the equinoctial average prereversal enhancements measured by the four techniques are roughly comparable about 36 m s and occur at the same local time 1900 lt for all the seasons the evening reversal times are similar apart from june solstice that exhibits large variations the morning reversal times are also in accord except for the equinoctial jicamarca drift our observations indicate that ionosonde drifts measurements are in better agreement with vertical drifts results at other equatorial stations

changes in silicate utilisation and upwelling intensity off peru since the last glacial maximum insights from silicon and neodymium isotopes we combine the stable silicon isotope composition 30si of diatoms and the radiogenic neodymium isotope compositions end of past seawater extracted from the authigenic fraction of the sediments mn fe coatings of particles and benthic foraminifers as well as the radiogenic isotope compositions nd sr of the detrital material itself to reconstruct silicic acid utilisation water mass mixing and upwelling intensity from the same marine sediments in the peruvian upwelling region during the past 20 000 years additionally the sedimentary signals were compared to the water column isotope compositions along the peruvian shelf the dissolved end in the water column ranges from 5 7 to 0 6 the corresponding end signatures of the coatings and the benthic foraminifers of the surface sediments range from 4 5 to 1 8 and from 2 5 to 2 2 respectively the detrital end 87sr 86sr signatures range from 6 3 to 0 0 70508 0 71049 all phases show a trend from more radiogenic end values in the north towards less radiogenic values in the south broadly reflecting local weathering inputs and hinterland geology the end signatures of the coatings extracted from sediment core so147 106kl located in the present day centre of coastal upwelling near 12 s have been essentially constant 1 5 throughout the past 20 000 years while the detrital end 87sr 86sr varied between values of 0 7 0 70620 during the last glacial and 4 5 0 70849 during the late holocene reflecting changes in the origin of the sediment and current transport from a more northerly towards a more southerly source and from overall stronger to weaker upwelling the 30si signature of both total biogenic opal 30siopal and of hand picked diatoms 30sidiatom ranged from 0 3 last glacial to 1 4 late holocene confirming large variations in upwelling intensity driving silicic acid utilisation by diatoms our combined bsio2 mar 30siopal and detrital radiogenic isotope results demonstrate that the strongest el nino southern oscillation conditions off peru of the past 20 ka have prevailed during the past 5 ka

a simulation study on the impact of altitudinal dependent vertical plasma drift on the equatorial ionosphere in the evening we carry out a simulation study on the impact of altitudinal dependent plasma drift on the equatorial ionosphere in the evening under geomagnetically quiet conditions our study used the vertical plasma drift velocity data measured by an incoherent scatter radar at jicamarca 11 95 s 76 87 w the data covered the local sunset period on 15 and 16 november 2004 the plasma drift had significant altitudinal variations in the vertical component which is perpendicular to the magnetic field we employed sami2 sami2 is another model of the ionosphere to evaluate the effect of the altitude dependent ion drift on the equatorial ionosphere three types of plasma drift velocity inputs were used in our simulations the first input is calculated from an empirical model the second is a height averaged drift obtained from the observed drift velocity and the third one corresponds to the observed altitudinal dependent drift data a strong equatorial ionization anomaly occurred in the results of all numerical experiments additional layers f3 layers in electron densities over the equatorial f region and arch latitudinal structures extending to lower middle latitudes were seen in the simulations driven by the observed altitudinal dependent drift we further show that neutral winds do not have a significant effect on the simulated f3 layers the results of our numerical experiments suggest that the simulated additional ionospheric layers and arch structures are associated with the altitudinal gradients in the vertical plasma drift velocity

an empirical model of the occurrence of an additional layer in the ionosphere from the occultation technique preliminary results about 8 year electron density profile edp data from the cosmic formosat 3 satellites radio occultation technique were used to investigate the additional stratification of the f2 the so called f3 layer layer over the equatorial and low latitude ionosphere on a global scale for both the bottomside and topside ionosphere the f3 layer was recognized through the altitude differential profile featured by two maxima existing from the selected edp profile there were 37 000 bottomside and 25 000 topside cases of f3 layer selected out of 1 27 million occultation events at equatorial and low latitude areas during the period of april 2006 to august 2014 the statistical results for the bottomside ionosphere resemble that reported in zhao et al 2011a while in the topside the highest occurrence of f3 layer shows a 3 4 h delay depending on the altitude range of the stratification the magnetic latitude distribution shows different dependence with a tendency to form a single crest toward high altitude also the seasonal variation is weaker in the topside ionosphere compared to the bottomside one especially in the high altitude then we build up an empirical model of the f3 layer occurrence using the bottomside statistics based on empirical orthogonal function eof decomposition as it gets the inherent characters inside the data set and converges quickly the model well grasps the main features of the f3 occurrence e g the f3 occurrence s sensitivity on the magnetic latitude further in order to accommodate the ground observation a corrected factor was introduced as f3 layer is an important phenomenon in the low latitude ionosphere we have made an attempt to describe its feature with a consecutive function although future work needs to be done for an overall expression of this structure

global characteristics of occurrence of an additional layer in the ionosphere observed by cosmic formosat 3 global observations of electron density profile edp from the cosmic formosat 3 satellites were used to investigate for the first time the additional stratification of the f2 layer over the equatorial ionosphere on a global scale which is called f3 layer the f3 layer in edp was recognized through the altitude differential profile featured by two maxima existing from 220 km to the peak height of the electron density there were similar to 9 400 cases of f3 layer selected out of similar to 448 000 occultation events at low and equatorial areas during the period of april 2006 september 2010 statistical results show that the highest occurrence of f3 layer appears at dip latitude 7 similar to 8 degrees 7 similar to 8 degrees for northern southern hemisphere and is more pronounced during summer months at 10 00 14 00 lt the occurrence also has a clear longitude dependence during boreal summer with relatively higher occurrence at 80 similar to 100 degrees 20 similar to 20 degrees 80 similar to 120 degrees and 160 similar to 170 degrees longitudes that is possibly associated with the wavenumber 3 diurnal tide de3 the results support the principle of the f3 layer proposed by balan et al 1998 which in turn validate the accuracy of the retrieval of the cosmic edp data citation zhao b w wan x yue l liu z ren m he and j liu 2011 global characteristics of occurrence of an additional layer in the ionosphere observed by cosmic formosat 3 geophys res lett 38 l02101 doi 10 1029 2010gl045744

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

terrestrial ecosystems and the global biogeochemical silica cycle 1 most research on the global si cycle has focused nearly exclusively on weathering or the oceanic si cycle and has not explored the complexity of the terrestrial biogeochemical cycle the global biogeochemical si cycle is of great interest because of its impact on global co2 concentrations through the combined processes of weathering of silicate minerals and transfer of co2 from the atmosphere to the lithosphere a sizable pool of si is contained as accumulations of amorphous silica or biogenic silica bsi in living tissues of growing plants known as phytoliths and after decomposition of organic material as remains in the soil the annual fixation of phytolith silica ranges from 60 200 tmol yr 1 and rivals that fixed in the oceanic biogeochemical cycle 240 tmol yr 1 internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical si cycle challenging the ability of weathering models to predict rates of weathering and consequently changes in global climate consideration must be given to the influence of the terrestrial bsi pool on variations in the global biogeochemical si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles

day to day variability of the equatorial ionization anomaly and scintillations at dusk observed by guvi and modeling by sami3 1 the day to day variability in ionospheric irregularity generation giving rise to equatorial scintillation has remained an unresolved issue over many decades we take a fresh look at the problem utilizing the global imagery provided by the global ultraviolet imager guvi instrument on nasa s thermosphere ionosphere mesosphere energetics and dynamics satellite guvi has been acquiring images of 135 6 nm emission in the earth s ionosphere thermosphere system since 2001 these guvi disk images at dusk have been used to identify cases where the equatorial ionization anomaly eia crests lie near the magnetic equator over a relatively narrow longitude range so that the anomaly looks collapsed a 16 month period of guvi data collected during evening at solar maximum is used to study the morphology of these so called collapses since the eia collapse is shown to be linked to the suppression of equatorial plasma bubbles and scintillations in particular we look at the june solstice during which the atlantic and pacific show very different climatology and eia collapses are most frequent in the guvi data on the other hand eia collapses are a relatively rare occurrence during the equinox period when scintillations are most prevalent globally we obtained a few dramatic examples of day to day variability in eia behavior and scintillations over india the sami3 is also a model of the ionosphere sami3 model was used to investigate the conditions during the evening collapse of the anomaly in the indian longitude sector where measurements of total electron content tec and scintillations and estimates of the daytime vertical drifts and those at dusk were available results from sami3 show that the observed collapse of the anomaly at dusk can be simulated by a reversal of the upward vertical drift in midafternoon in agreement with the drift estimates from magnetometer observations such reversed vertical drifts at this time of the day are generally seen during counterelectrojet events introduction of neutral winds into sami3 better approximates the dusk behavior of tec at low latitude stations in india this study reveals that the longitudinally confined eia collapse may explain some of the differences in day to day variability of scintillations at different locations around the globe