chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

the oxygen isotopic compositions of silica phytoliths and plant water in grasses implications for the study of paleoclimate abstract information about climatic conditions during plant growth is preserved by the oxygen isotope composition of biogenic silica phytoliths deposited in grasses the oxygen isotope composition of phytolith silica is dependent on soil water 18 o values relative humidity and evapotranspiration and temperature during plant growth phytolith and plant water 18 o values for c3 a breviligulata and c4 c longifolia grasses from natural and greenhouse sites in southwestern ontario were used to compare the isotopic fractionation between biogenic silica and water in various parts of these living plants for non or weakly transpiring tissues rhizomes stems sheaths in both grass species the 18 o silica plant water remained constant at 34 and the 18 o and d values of plant water collected from pre dawn and mid day samplings showed little variation these plant waters were only slightly enriched in 18 o and d relative to water provided to the grasses isotopic temperatures calculated from the silica and plant water isotopic data matched measured growing temperatures for the region by comparison the upper leaf water was extremely enriched in oxygen 18 and deuterium at maximum rates of transpiration relative to water from non transpiring tissues as were the calculated steady state values for leaf water 18 o and d silica produced in the transpiring tissues leaf inflorescence has higher 18 o values than silica from non transpiring tissues but the enrichment is modest compared to upper leaf water under mid day conditions leaf phytoliths have formed from plant water typical of average conditions in the lower leaf where the extreme 18 o enrichment is not encountered c longifolia was also collected from alberta and nebraska where growing conditions are different from southwestern ontario phytoliths at all three sites have a similar pattern of 18 o values within the plants but the isotopic separation between leaf and stem silica increases from 4 to 8 as average relative humidity decreases the difference between actual growing temperature and that calculated using measured 18 o values for stem silica and local meteoric water became progressively larger as relative humidity decreased likely because of evaporative 18 o enrichment of soil water such effects are most pronounced in arid environments and pertinent in grasslands where much of the active rooting zone can be situated at the shallower depths most affected by the 18 o enrichment of soil water

evolution of seawater 87sr86sr over the last 400 ka the absence of glacial interglacial cycles abstract fluctuations in the 87 sr 86 sr ratio of seawater accompanying glacial interglacial climate changes have been reported in recent studies but remain highly controversial to investigate these potential fluctuations we present very high precision 13 ppm 87 sr 86 sr measurements on planktonic foraminifera from indian ocean and pacific cores 87 sr 86 sr ratios from three different foraminiferal species are indistinguishable from one another in both core top and 50 ka samples demonstrating that changes due to diagenesis or contamination do not influence the measurements average 87 sr 86 sr ratios for pacific ocean and indian ocean samples are also indistinguishable at three intervals core top 50 ka and 300 370 ka demonstrating that the oceans have remained well mixed with respect to sr also 87 sr 86 sr ratios are not affected by changes in the precleaning of samples measurements from pacific core v28 238 that used in the study of dia et al 1 do not reproduce the cycles seen in the previous study and are statistically well explained by a linear increase in the seawater 87 sr 86 sr ratio it seems likely that an analytical artefact caused the cycles in the previous study measurements from two key sections of core from odp site 758 that used in the study of clemens et al 2 do not reproduce the largest shifts seen in the previous study despite the considerably better precision reported here the apparent cyclicity in the clemens et al 2 data is also suggested to be due to an unknown analytical artefact the 48 measurements presented here are a statistically good fit to a straight line defined by 87sr ppm 0 0613 age ka at the 13 ppm level of precision no evidence for a glacial interglacial variation in the seawater 87 sr 86 sr ratio is seen glacial interglacial variation may still exist but this data constraints its maximum amplitude to 6 9 ppm which would correspond to changes in the riverine sr flux of 30

absolute accumulation and isotope fractionation of si and fe during dolomite weathering and terra rossa formation abstract the behavior of silicon si and iron fe and their isotope fractionation during carbonate weathering and terra rossa formation are unclear here we focus on a 4 05 m thick terra rossa profile developed on dolomite in the yunnan guizhou plateau china we measured the mineral and geochemical composition magnetic susceptibility and si and fe isotopic composition of the terra rossa ferromanganese concretions fmcs and the dolomite and its insoluble residues the results indicate that there is absolute accumulation of si and fe during dolomite weathering and terra rossa formation due to the extraneous input of dissolved si and fe this means that the insoluble residues of dolomite are not the only source for terra rossa formation in addition we suggest that the high frequency dependent magnetic susceptibility fd and rubification of the terra rossa are the result of the oxidation of extraneous dissolved fe and the neoformation of finely dispersed fe oxides overall the terra rossa is characterized by the enrichment of light si and fe isotopes compared with insoluble dolomite residues and bulk dolomite the extraneous input of dissolved fe is characterized by a light isotopic signature whereas the si isotopic signature of the extraneous source is unclear due to modification by late desilicification si isotope fractionation in the terra rossa is mainly controlled by chemical weathering and associated mobilization and redistribution of si the 30 si values of terra rossa decrease with increasing weathering intensity while the re precipitation of mobilized si leads to the significant enrichment of isotopically heavy si in the basal terra rossa the 30 si values of terra rossa are not significantly affected by the adsorption of the fe and al oxyhydr oxides by contrast the major process that fractionates fe isotopes in the dolomite terra rossa weathering system is unclear in the terra rossa profile the 56 fe values of the fmcs are significantly lower than those of the surrounding terra rossa matrix these low 56 fe signatures of the fmcs reflect the preferential translocation of the light fe isotope while the fmcs may be able to record the low 56 fe characteristics of pore water

reappraising the geomorphology ecology link this thematic virtual special issue highlights a personal selection of 18 recent 2007 2009 contributions to earth surface processes and landforms these papers provide a flavour of recent research that is concerned with furthering our understanding of the many ways in which the biosphere interacts with the physical and chemical processes of sediment transfer transformation much of this research has focused on understanding the mechanics by which the biota can modulate sediment transport and the strength of earth surface materials often with the aim of applying that knowledge to enhance bioremediation methods of erosion control this work continues to be fundamentally important in enhancing our understanding of earth surface processes but often treats the biosphere and physical world as uncoupled entities this selection therefore also provides samples of work that point to an ongoing but significant disciplinary reappraisal in which it is the interactions between ecological and geomorphological realms that are of primary interest

airborne dust and its significance to soils abstract movement of mineral dust by the atmosphere has been underway for ages but efforts to assess its magnitude and significance have come largely in the last century attempts to identify and understand effects of the dust on soils are even more recent mostly in the last 30 years airborne movements have several forms dust is transported by large storms that can be spectacular dust is also moved by smaller pulses and in a rather continuous flux most movement is within continents or from continents to nearby oceans but fine dust fine silt and clay is carried from one continent to another prime sources of the dust are the deserts and semi arid regions of the world for example large quantities of dust are carried out of the sahara every year quantities of dust carried by individual pulses from the small ones to the massive storms extend from a few thousand to 150 million metric tons during the course of a year the total quantities can be large one example is an estimate that on the average 770 million metric tons originating in the western united states are carried 2300 km each year that amount would add roughly 3000 kg ha 1 yr 1 to soils between the rocky mountains and the mississippi river small pulses and the near continuous flux add between 100 and 200 kg ha 1 yr 1 to soils of the eastern united states on the average comparable quantities are being added to soils in other parts of the world as well airborne accessions of dust have a variety of effects in soil development and on soils after they have been formed dust dominates processes of genesis in a few soils affects horizon differentiation appreciably in many more is a primary source of constituents essential to some horizons and layers in a number of soils provides the bulk of the material for silty surface horizons in many soils and can contribute supplies of nutrient elements for plants examples of all such effects are covered in this review first among these effects to be recognized even though not understood were the silty surface layers that failed to match underlying horizons later recognized was airborne dust as a source of calcium carbonate for calcic horizons that the dust could add significant quantities of nutrient elements as for example south of the sahara came into the picture even later the beneficial effects of airborne dust to many soils raises an interesting question is the dust more or less useful in its new location than it was in its source area an adequate answer to that question would provide an improved basis for assessing the effects of wind erosion

complex kame belt morphology stratigraphy and architecture the development of glacier karst at the margins of melting ice sheets produces complex glaciofluvial sediment landform assemblages that provide information on ice sheet downwasting processes we present the first combined geomorphological sedimentological and geophysical investigation of the brampton kame belt an important glaciofluvial depositional zone at the centre of the last british irish ice sheet ground penetrating radar gpr data allow the broad scale internal architecture of ridges eskers and flat topped hills ice walled lake plains to be determined at four sites in combination with sediment exposures these provide information on lateral and vertical variations in accretion styles depositional boundaries and grain size changes building on existing work on the subject we propose a refined model for the formation of ice walled lake plains resulting from the evolution and collapse of major drainage axes into lakes as stable glacier karst develops during deglaciation the internal structure of esker ridges demonstrates variations in sedimentation that can be linked to differences in ridge morphologies across the kame belt this includes low energy flow conditions and multiple accretion phases identified within large s n oriented esker ridges and fluctuating water pressures hyperconcentrated flows and significant deformation within a fragmented sw ne oriented esker ridge in combination with updated geomorphological mapping this work allows us to identify two main styles of drainage within the kame belt 1 major drainage axes aligned broadly s n that extend through the entire kame belt and collapsed into a chain of icewalled lakes and 2 a series of smaller fragmented sw ne aligned esker ridges that represent ice marginal drainage as the ice sheet receded south eastwards up the vale of eden our study demonstrates the importance of integrated geomorphological sedimentological and geophysical investigations in order to understand complex and polyphase glaciofluvial sediment landform assemblages

diagenetic fractionation of ge and si in reducing sediments the missing ge sink and a possible mechanism to cause glacial interglacial variations in oceanic ge si the average ge si ratio in the ocean is determined by the budgets for each of these elements previous budget formulations have assumed that the only important sink for both elements is burial as opal based on studies of the si cycle and the close oceanic coupling observed between inorganic ge and si distributions however these budgets implied two paradoxes 1 hydrothermal flow through ocean ridges is smaller than predicted by other tracers and 2 the lower ge si ratio of opal deposited during glacial times compared to that deposited during interglacial times required enhanced weathering during cooler drier climates both paradoxes could be resolved if a significant sedimentary sink for ge other than opal burial could be identified and the objective of this study was to search for one two pore water profiles collected in equatorial pacific sediments show that ge and si behave similarly in the upper 10 cm of sediment indicating no evidence for a significant non opal sink for ge in oxic sediments by contrast profiles in several cores from the california margin demonstrate that in reducing sediments ge diagenesis is poorly coupled to si diagenesis significant ge removal is evident both downcore and sometimes in the near surface benthic flux chamber measurements at three continental slope stations all with an oxic layer less than 1 cm thick and large iron gradients in near surface pore waters showed that 55 9 of the ge released by opal dissolution is sequestered however at two locations with anoxic sediments but little pore water fe 2 in the upper 2 cm flux measurements indicated little fractionation from the oceanic ratio during diagenesis implicating the importance of iron for fractionating ge from si during diagenesis if the ge sequestration observed in the iron rich ca margin sediments is typical of all slope sediments using a depth range of 200 1000 m then the ge sink is sufficient to bring the hydrothermal budget based on ge into concurrence with that based on other tracers the temporal variation in oceanic ge si could be explained if ge and si inputs remain constant and the effective diagenetic fractionation of ge increases by a factor of 2 3 during glacial times increased fractionation would require that glacial periods are characterized by increased opal dissolution in iron rich reducing sediments this could be caused by 1 thinning of the oxygenated sediment layer in response to decreased bottom water oxygen concentrations or increased rain of organics to the sea floor 2 increased rain of iron rich detrital sediments in areas receiving high opal rain 3 increased rain of opal to sediments in margin areas if the oceanic ge si ratio reflects increased rain of diatom opal or organic carbon in margin areas during glacial periods it may indicate an increase in the efficiency of the biological pump for co2 during glacial times

the occurrence and significance of biogenic opal in the regolith abstract biogenic opal produced by vascular plants diatoms and siliceous sponges have been found in soils and terrestrial sediments of all continents except antarctica since the middle of the 19th century the opal particles range in size from fine silt to fine sand almost all soils contain detectable opal up to levels of 2 3 and a significant number contain values in excess of 5 even higher values have been found from soils and sediments of all continents in a wide range of soil types the most important factor is poor soil drainage and seasonal to permanent water logging this encourages the proliferation of silica producing organisms such conditions have been found in the soils and aquatic sediments of the monsoonal tropics tropical rain forests temperate forests tropical savanna tropical islands semi arid grasslands and savanna and temperate woodland and grassland the presence of a volcanic substrate also appears favourable in some cases but is not necessary biogenic opal preferentially collects in the a horizon of soils and to a lesser extent in the b horizon this preferential distribution facilitates identification of palaeosols in stacked soil sequences biogenic opal is also a component of windblown dust even in arid environments biogenic opal is significant to regolith processes in a number of ways firstly as in the case in marine environments it is likely to be important in silica cycling and storage because of its greater lability compared to quartz secondly dissolution and reprecipitation of opal a as opal ct or micro quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete thirdly the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution finally some forms of biogenic silica in particular sponge spicules can present a health hazard their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust

global chemical erosion during the last glacial maximum and the present sensitivity to changes in lithology and hydrology geographically based calculations for 18 000 years ago 18 ka and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial interglacial timescales runoff fields were derived from general circulation model predictions of precipitation minus evaporation then global lithologic maps were prepared so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates assuming that significant chemical erosion does not occur beneath ice sheets we find that chemical erosion in ice free areas at 18 ka was only slightly greater than today this result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff the small 20 increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates which weather faster than average data on glacial interglacial movements of the calcite compensation depth ccd are inconclusive but seem to indicate little change whereas even a 20 increase in the riverine bicarbonate flux should cause an observable deepening if the ccd did not indeed change then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means such as increased carbonate productivity about half of the observed decrease in atmospheric pco2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice free areas consideration of the maximum possible effect of meltwater at ice margins leads to an 80 increase in the global chemical erosion rate at 18 ka such an increase would lead to a larger and faster drop in atmospheric pco2 but also to an excessive deepening of the global ccd this scenario seems unrealistic our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of quaternary marine sr isotopic ratios therefore if these records are indeed globally representative and entirely due to increased silicate chemical erosion as opposed to changes in source areas or hydrothermal inputs then other factors such as a shift in the global weathering regime which we do not explicitly consider here may be involved sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non spatially resolved calculation

opaline and al si phytoliths from a tropical mire system of west malaysia abundance habit elemental composition preservation and significance abundant and unique opaline and al si phytoliths and opaline bioliths have been discovered in holocene peat deposits of tropical tasek bera in peninsular malaysia these are secreted mainly by plants higher plants and algae and incorporated in the sediment cycle during the rapid biomass turnover many wetland plants have entire skeletons secreted the study shows that differing plant parts of same species as well as same parts of differing plant species incorporate different amounts of elements whilst deciduous trees of the swamp forest have phytoliths enriched in al most phytoliths of palms sedges and grasses of the littoral swamp are composed of opaline silica incorporation of al and many other elements into relatively stable phytoliths is the result of high evapotranspiration rate and ample plant available elements and can serve to detoxify the soil solution because of the abundance of such opaline and al si structures spongillid and algal remains in acid peat accumulating environments tropical peatlands represent repositories of si and al which together with variable amounts of other elements could provide the prime material for silicate neoformation during later diagenetic processes neoformation of minerals from bioliths would explain the scarcity of biogenic remains in carboniferous and tertiary coal deposits furthermore incorporation of major elements in phytoliths may limit the rate of leaching of these elements in an environment where biomass turnover is rapid thus reducing the loss of nutrients and other plant essential elements most nutrients of tropical peats are recycled by plants within the top 150 cm and an upward migration of plant essential elements such as mg ca or p but also of si and al occurs during peat accumulation such elemental cycling strongly influences the geochemical composition of the peats during mire evolution utilizing al for normalization of major and minor elements of tropical peats for paleoclimatic and paleodepositional analyses may thus result in incorrect interpretations

stable isotope ratios of carbon in phytoliths as a quantitative method of monitoring vegetation and climate change the 13c 12c ratios of occluded carbon within opal phytoliths from the northern great plains show potential as a basis for paleoclimatic reconstruction a significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths the 13c 12c ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of c3 and c4 plants variations in the 13c values of phytoliths with soil depth are caused by a variety of processes burial of soil surface by dust bioturbation and possible illuviation by percolating water also contributions of phytoliths by dust and roots have unknown isotopic effects the 13c values of phytoliths from soils increase with decreasing 14c age suggesting that the proportion of c4 plants in this region has increased during the holocene phytoliths of apparent mid holocene age suggest exclusive dominance by c4 plants which agrees with paleoclimatic interpretations of an arid middle holocene climate

effect of the advent and diversification of vascular land plants on mineral weathering through geologic time the origin of vascular land plants in the silurian and their subsequent diversification have had a major effect on mineral weathering through geologic tune the presence of vascular plants reduces the stability of soil minerals through a net export of ions from soil waters and through the release of complexing organic acids by root mycorrhizae additional factors that dictate the nature of plant induced mineral weathering are 1 the differences in nutrient dynamics between evergreen and deciduous species 2 the role of specific nutrient sinks biomass storage and secondary soil mineralization and outputs runoff etc in plant ecosystems and 3 the effect of long and short term ecosystem disturbances first order increases in overall mineral weathering probably took place in the middle paleozoic and early tertiary following the initial colonization and diversification of land plants and the radiation of deciduous angiospenns second order fluctuations would typify time intervals where paleoecosystem disturbances were maximized such as periods of climatic instability

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

87sr 86sr ca sr and ge si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment central pennsylvania usa plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems hydrologic and other losses are offset by inputs from atmospheric deposition and weathering this multi tracer study investigates these effects using 87sr 86sr ca sr and ge si ratios from solid soil profiles and bedrock biological leaves and sap waters and water pore ground stream waters samples to study ca sr ge and si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central pennsylvania leaves and sap waters were found to have similar ge si ratios 1 mol mol which is consistent with biological fractionation occurring at the root soil water interface ge si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of si over ge ca sr ratios in oak leaves 1360 40 mol mol were significantly higher than those in maple leaves 650 20 mol mol ca sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential sr adsorption or sr uptake during transpiration 87sr 86sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of sr and ca although there are site to site differences additionally 87sr 86sr ratios in plant tissues were most similar to shallow pore waters 0 50 cm suggesting that mineral nutrients are obtained by trees from the near sub surface 87sr 86sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing sr derived from shale and atmospheric deposition valley bottom soil reservoirs and stream and groundwater samples include sr and ca derived from dissolution of two isotopically distinct generations of carbonates a preliminary estimate of the sr and ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca 300 m myr 1 a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and u series isotopes the data for ca sr si and ge in soil soil solutions and stream waters reflect the interaction of slower weathering processes with rapid biologically driven cycling between soils and biomass

biogenic silica deposition in lake malawi east africa over the past 150 000years abstract we report a 150 000 year record of the abundance of biogenic silica bsi in drill cores from the northern and central basins of lake malawi the periods of highest diatom burial at both sites occurred around 65 69 and 51 60 thousand years ago ka after the termination of prolonged intense drought in the region these peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin the droughts that preceded these bsi depositional episodes occurred at precessional frequency corresponding to times of minimum austral spring or summer insolation in the region prior to 60 ka subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity however biogenic silica records do not show significant sensitivity to precessional forcing the bsi burial flux in the north basin displays strong millennial scale variability since 50 ka with peak values occurring during cold times in the northern hemisphere as reflected in the greenland ice core records we observe no relationship between bsi and deposition of volcanic tephras in this system the central basin core also displays millennial scale variability in bsi abundance during the past 50 ky of comparable magnitude to that in the north basin but without systematic correlation to the north basin bsi or greenland ice core records in general we conclude that bsi profiles from the north and probably south basins are more readily interpreted in terms of regional climate dynamics than are bsi profiles from more central locations in long narrow tropical rift lakes diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low latitude lacustrine systems

32si in precipitation evaluation of temporal and spatial variation and as dating tool for glacial ice abstract the use of32si half life about 140 yr as a dating tool requires knowledge of its temporal and spatial distribution previously reported and new data are similar to the patterns of other cosmogenic radionuclides latitude and seasonal variation anti correlation to solar activity contribution from nuclear weapons testing highest natural concentrations in antarctica compiled and estimated32si si ratios show that rain snow and ice samples of low si concentration could be measured by accelerator mass spectrometry a significant single event32si deposition concentration higher by a factor of 4 was detected counting in fresh snow on the jungfraujoch switzerland due to scavenging from the atmosphere by an abnormally high atmospheric dust content

a test of ge si opal as a paleorecorder of ge si seawater late pleistocene variations of germanium to silicon ratios in marine diatom shells from sediment cores ge si opal are coherent with the global isotope record of glacial to interglacial climate change these variations are thought to reflect changes in ge si seawater driven by climate modulated alterations in oceanic ge si sources and sinks however an important criterion for interpreting ge si opal as a monitor of whole ocean ge si seawater is that the opal burial ratio be insensitive both to local diatom production and surface ocean silica concentrations so called biological fractionation effects and to differential dissolution artifacts so called diagenesis offsets here we test these assumptions by comparing model ocean sediment ge si opal distributions with data from holocene and glacial sediments across the high latitude indian antarctic ocean siliceous ooze belt in contrast to the model the data show no gradients in either holocene or glacial ge si opal values across productivity zones displaying dramatic changes in biosiliceous production opal burial and dissolution this evidence supports the contention that fractionation effects are small and that observed down core variations in ge si opal faithfully record secular changes in ge si seawater

climatically linked carbon isotope variation during the past 430 000 years in southern ocean sediments we use the isotopic composition of carbon from organic matter enclosed within diatom frustules as a proxy for paleoproductivity and paleo dissolved carbon dioxide concentrations in ocean surface waters a southern ocean record from south of the antarctic polar front and spanning 430 000 years of carbon isotopic variation in diatomaceous organic matter is presented for the first time the most refractory diatomaceous organic matter fraction was extracted and analyzed to avoid problems associated with diagenesis the results clearly indicate cyclic changes in organic carbon isotopic ratios with 13c depleted values associated with all of the last five glacial periods reflecting changes in surface water properties and primary productivity changes in dissolved carbon dioxide concentrations are the most probable cause of these cycles but the possible effect of seawater ph changes cannot be excluded

a combined oxygen and silicon diatom isotope record of late quaternary change in lake el gygytgyn north east siberia determining the response of sites within the arctic circle to long term climatic change remains an essential pre requisite for assessing the susceptibility of these regions to future global warming and arctic amplification to date existing records from north east russia have demonstrated significant spatial variability across the region during the late quaternary here we present diatom 18o and 30si data from lake el gygytgyn russia and suggest environmental changes that would have impacted across west beringia from the last glacial maximum to the modern day in combination with other records the results raise the potential for climatic teleconnections to exist between the region and sites in the north atlantic the presence of a series of 2 3 decreases in 18odiatom during both the last glacial and the holocene indicates the sensitivity of the region to perturbations in the global climate system evidence of an unusually long holocene thermal maximum from 11 4 7 6 ka bp is followed by a cooling trend through the remainder of the holocene in response to changes in solar insolation this is culminated over the last 900 years by a significant decrease in 18odiatom of 2 3 which may be related to a strengthening and easterly shift of the aleutian low in addition to possible changes in precipitation seasonality

holocene climatic change in swedish lapland inferred from an oxygen isotope record of lacustrine biogenic silica holocene climatic variability was studied in a 9500 year lake sediment sequence from the abisko region in swedish lapland using the oxygen isotope ratio in diatom biogenic silica d 18 osi oxygen and hydrogen isotope ratios of waters from the abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation the hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity inflow and evaporation show that the downcore diatom d18osi record is primarily controlled by changes in the summer isotopic composition of the lake water the overall 3 5 depletion in d 18 osi since the early holocene is interpreted as an increase in the influence of the arctic polar continental air mass that carries depleted precipitation we estimate that this change is associated with a 2 5 4 c cooling that has occurred since the early holocene in general the diatom d 18 osi record resembles the average annual air temperature reconstructed for the greenland ice core gisp2 especially during the past 4000 years with a pronounced cooling starting at 2000 years bp

weathering and the global carbon cycle geomorphological perspectives abstract the links between the weathering of silicate minerals the global carbon cycle and climate change are complex and involve a whole range of often inter related tectonic climatic geomorphological and biological factors silicate weathering rates are known to have varied through geological time and increasingly sophisticated models now link such variation to the long term carbon cycle and climate change in order to improve the modelling further one key research need is for more and better datasets on weathering and the factors that control it as a contribution to this we review the nature of the weathering carbon cycle system the sources of evidence used to understand it and the challenges that remain in quantifying the feedbacks between weathering and the global carbon cycle we propose that increased understanding of geomorphological diversity of landscapes and processes and especially the dynamic relationships between weathering and erosion will aid clearer portrayal of the links between weathering and the long term carbon cycle

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

plant induced weathering of a basaltic rock experimental evidence abstract the active role of higher plants in the weathering of silicate minerals and rocks is still a question for debate the present work aimed at providing experimental evidence of the important role of a range of crop plants in such processes in order to quantitatively assess the possible effect of these diverse plant species on the weathering of a basaltic rock two laboratory experiments were carried out at room temperature these compared the amounts of elements released from basalt when leached with a dilute salt solution in the presence or absence of crop plants grown for up to 36 days for si ca mg and na plants resulted in an increase in the release rate by a factor ranging from 1 to 5 in most cases ca and na seemed to be preferentially released relative to other elements suggesting that plagioclase dissolved faster than the other constituents of the studied basalt negligible amounts of fe were released in the absence of plants as a consequence of the neutral ph and atmospheric po2 that were maintained in the leaching solution however the amounts of fe released from basalt in the presence of plants were up to 100 to 500 fold larger than in the absence of plants for banana and maize the kinetics of dissolution of basalt in the absence of plants showed a constantly decreasing release rate over the whole duration of the experiment 36 days no steady state value was reached both in the absence and presence of banana plants however in the latter case the rates remained at a high initial level over a longer period of time up to 15 days before starting to decrease for fe the maximum rate of release was reached beyond 4 days and this rate remained high up to 22 days of growth of banana the possible mechanisms responsible for this enhanced release of elements from basalt in the presence of plants are discussed although these mechanisms need to be elucidated the present results clearly show that higher plants can considerably affect the kinetics of dissolution of basalt rock therefore they need to be taken into account when assessing the biogeochemical cycles of elements that are major nutrients for plants such as ca mg and k but also micronutrients such as fe and nonessential elements such as si and na

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

oxygen isotopic composition of opaline phytoliths potential for terrestrial climatic reconstruction opaline mineralized bodies are produced by many terrestrial plants and accumulate in certain soils and archaeological sites analyses of the oxygen isotopic compositions of these so called phytoliths from stems and leaves of wheat plants grown in a greenhouse showed a linear relationship with stem and leaf water isotopic compositions and hence indirectly rain water isotopic composition analyses of wheat plants grown in fields showed that stem phytoliths isotopic composition directly reflects the seasonal air temperature change whereas leaf phytoliths isotopic composition reflects both temperature and relative humidity temperature and the oxygen isotopic composition of stem phytoliths were related by an equation similar to that proposed for marine opal oxygen isotopic compositions of fossil phytoliths and in particular those from stems could be valuable for reconstructing past terrestrial climate change

late quaternary changes in carbon cycling on mt kenya east africa an overview of the δ13c record in lacustrine organic matter abstract the 13 c signature of sedimentary organic matter acts as a tracer for past changes in the terrestrial and aquatic carbon cycles compound specific 13 c analysis enables these two signals to be disentangled we compare the molecular isotopic records from a transect of five lakes between 1820 and 4595 m a s l on mt kenya east africa spanning the last 100 000 years with multiproxy palaeoenvironmental data in order to determine the mechanisms underlying late quaternary variations in carbon cycling at different altitudes the amplitude of down core variations in the 13 c values of total organic carbon varied from 4 8 to 17 4 the 13 c signals in compounds of terrestrial aquatic macrophyte and algal origin were tightly coupled reflecting strong atmospheric control during the early glacial and the last glacial maximum the effects of sub ambient co 2 and enhanced moisture stress reinforced each other resulting in carbon starvation and very high 13 c values in contrast smaller millennial scale shifts in 13 c during the late glacial and holocene were driven primarily by fluctuations in monsoon precipitation however the impact of late quaternary atmospheric changes were complex involving variations in biomass burning vegetation cover soil erosion lake levels and water chemistry in addition to direct physiological responses by higher plants and algae

geochemistry of large river suspended sediments silicate weathering or recycling tracer this study focuses on the major and trace element composition of suspended sediments transported by the world s largest rivers its main purpose is to answer the following question is the degree of weathering of modern river borne particles consistent with the estimated river dissolved loads derived from silicate weathering in agreement with the well known mobility of elements during weathering of continental rocks we confirm that river sediments are systematically depleted in na k ba with respect to the upper continental crust for each of these mobile elements a systematics of weathering indexes of river borne solids is attempted a global consistency is found between all these indexes important variations in weathering intensities exist a clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands however no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones an inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments finally chemical and physical weathering are compared through the use of a simple steady state model we show that the weathering intensities of large river suspended sediments can only be reconciled with the silicate derived dissolved load of rivers by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles a simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins finally even if orogenic zones produce weakly weathered sediments we emphasize the fact that silicate chemical weathering rates and hence co2 consumption rates by silicate weathering are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher hence the parameters that control chemical weathering rates would be those that control physical denudation rates

direct effect of ice sheets on terrestrial bicarbonate sulphate and base cation fluxes during the last glacial cycle minimal impact on atmospheric co2 concentrations chemical erosion in glacial environments is normally a consequence of chemical weathering reactions dominated by sulphide oxidation linked to carbonate dissolution and the carbonation of carbonates and silicates solute fluxes from small valley glaciers are usually a linear function of discharge representative glacial solute concentrations can be derived from the linear association of solute flux with discharge these representative glacial concentrations of the major ions are f25 of those in global river water a 3 d thermomechanically coupled model of the growth and decay of the northern hemisphere ice sheets was used to simulate glacial runoff at 100 year time steps during the last glacial cycle 130 ka to the present the glacially derived fluxes of major cations anions and si over the glaciation were estimated from the product of the glacial runoff and the representative glacial concentration a second estimate was obtained from the product of the glacial runoff and a realistic upper limit for glacial solute concentrations derived from theoretical considerations the fluxes over the last glacial cycle are usually less than a few percent of current riverine solute fluxes to the oceans the glacial fluxes were used to provide input to an oceanic carbon cycling model that also calculates changes in atmospheric co2 the potential change in atmospheric co2 concentrations over the last glacial cycle that arise from perturbations in glacial solute fluxes are insignificant being 1 ppm d 2002 elsevier science b v all rights reserved

pedogenesis significantly decreases the stability of water dispersible soil colloids in a humid tropical region abstract the stability of soil colloids influences soil physicochemical properties soil development and transfer of nutrients and contaminants to surface and ground waters a better understanding of soil colloids stability dynamics during soil evolution is important for the evaluation of soil s capacity to retain nutrients and or accommodate toxic contaminants this study was aimed to determine changes in the stability of water dispersible soil colloids that accompany mineral transformation and surface charge evolution during pedogenesis using a well characterized chronosequence derived from basalt in the humid tropical region of hainan island south china the results demonstrated that the ph dependent colloid stability decreased significantly with tropical soil development which we attribute to the substantial changes in clay mineral compositions and colloid surface charge properties clay minerals in the studied chronosequence were characterized by an increase of kaolinite gibbsite and fe oxides and a decrease of quartz and halloysite towards more advanced stages of weathering which resulted in the decline of permanent negative charges in the older soils the point of zero charge phpzc increased while ph decreased across the tropical soil chronosequence being in good agreement with the observed lower colloid stability in aged soils dominated by kaolinitic minerals our study of colloid stability at long term pedogenic time scale suggests young tropical soils

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

solute flux and mineral mass balance approaches to the quantification of plant effects on silicate weathering the weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric co 2 over geologic time and vascular plants may have played an important role in this process we have examined the role of plants in weathering in western iceland by two methods a solute flux approach and a mineral mass balance approach using the chemistry of weathering products and of waters draining adjacent areas of basaltic rocks that are either barren with a partial cover of mosses and lichens or populated by trees the study area was chosen to maximize vegetational differences and to minimize differences in microclimate slope and lithology while avoiding hydrothermal waters and anthropogenic acid rain solute flux results including data on cation uptake by growing trees indicate that the rate of weathering release of ca and mg to streams is about 4 times higher in vegetated areas than in bare areas mineral mass balance results show that trees increase plagioclase weathering by a factor of 2 and pyroxene weathering by a factor of 10 possibly as a result of preferential weathering of pyroxene by the vegetation biomass normalized weathering fluxes from the birch trees are greater than those from the conifers suggesting that angiosperms enhance weathering to a greater degree than gymnosperms results indicate a perpetual short term sink for atmospheric c as tree induced leakage of hco super 3 from the forest ecosystem to rivers and streams in addition to the finite sink of c in forest biomass the findings also suggest a major role for vascular plants as they invaded upland portions of the continents between 410 and 360 my ago in accelerating weathering and thereby lowering atmospheric co 2

early diagenesis of germanium in sediments of the antarctic south atlantic in search of the missing ge sink pore water and solid phase geochemistry profiles were obtained from several cores between 41 degree s and 53 degree s in the atlantic sector of the southern ocean pore water nitrate manganese and iron profiles delineate standard redox zones in these sediments and help characterize those with classic vs burn down behaviors pore water si and ge profiles demonstrate that ge released during opal dissolution is removed pervasively throughout the uppermost interval of silicate release and also downwards into the suboxic zone by as yet unidentified precipitation mechanisms these results indicate that early diagenesis of ge is uncoupled from that of opal solid phase extractions fe mn u mo ge cu ni co v and cd in a few cores suggest that anthigenic ge removal in the suboxic zone is not associated with peaks in authigenic mn cycling mno sub 2 and related metals but rather with processes deeper in the sediments perhaps fe or u diagenesis below the interval of ge removal pre water ge increases linearly with depth by over two orders of magnitude indicating a deep below recovery source of large magnitude the fraction of opal derived ge precipitated authigenically in these sediments ranges from approximately 1 to 96 and correlates strongly with the detrital fraction asmore well as the detrital to opal ratio both of which generally decrease from north to south the ge sink observed in these sediments would need to be globally representative to account for the entire missing ge sink in today s oceanic ge balance which seems unlikely benthic fluxes of ge and si estimated from these pore water profiles and from measurements in three benthic flux chamber experiments at high carbon rain continental margin sites demonstrate that the ge si rate released from the seafloor in locations with high benthic silicate and carbon fluxes is congruent with holocene opal dissolution ge si approximately 0 7 x 10 sup minus 6 in contrast ge si flux ratios in areas with low silicate fluxes are ge depleted ge si 0 5 x 10 sup minus 6 the authors speculate that glacial interglacial changes in oceanic ge si as recorded in diatoms may be due in part to variations in this authigenic ge sink perhaps related to shifts in siliceous productivity from open ocean antarctic siliceous oozes during interglacials to areas of higher detrital input sub antarctic continental margins and possibly also to generally more reducing conditions in sediments during glacials less

late quaternary changes in ecosystems and carbon cycling on mt kenya east africa a landscape ecological perspective based on multi proxy lake sediment influxes abstract the degree to which different lakes within a landscape respond coherently in unison to external drivers such as climate change and soil development is uncertain presentation of multi proxy geochemical and palaeoecological data from individual lakes in the form of fluxes minimizes distortions resulting from variable sedimentation rates and changes in sediment composition we use the accumulation rates of magnetic minerals total organic c and n terrestrial and aquatic biomarkers graminoid epidermis pollen green algae diatoms and diatom c in four small lakes situated between 2350 and 4595 m a s l on the ne flank of mt kenya east africa to reconstruct changes in c cycling over the last 38 ka the results conflict with earlier models of landscape and lake development showing 1 that glacial interglacial changes in vegetation cannot be interpreted as simple altitudinal shifts in the modern vegetation belts and 2 that limnological changes were not coherent rapid variations in climate water level erosion and nutrient input overwhelmed long term successional trends in lake sedimentation and c accumulation since the last glacial maximum the results also reveal previously unrecognized features of the palaeoenvironmental record such as the rapid degradation of organic matter in diatoms and the occurrence of a productive fire prone montane grassland during the highly seasonal monsoonal climate of the lateglacial and early holocene

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

terrestrial paleorecords of ge si cycling derived from lake diatoms abstract modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium silicon ge si ratios compared to regions with less extensive chemical weathering thought to be the result of ge fractionation in 2 1 clays temporal variations in ge si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering however this relationship has not been adequately ground truthed by a terrestrial based paleoclimate study utilizing ge si ratios in lake diatoms to this end a sediment core was extracted from dry lake el 2763 m in the headwaters of the santa ana river of southern california in july 1996 the dry lake drainage basin is comprised of biotite muscovite gneiss and granite with sparse pine forests and relatively high relief the core had a basal ams 14c age of 8 350 60 ybp we successfully separated diatom samples large enough for cleaning dissolution and chemical analysis without contamination by detrital materials confirmed by trace metal analyses two dissolved opal samples were analyzed for ge and si concentrations along with modern water samples collected from waters within the drainage basin diatoms obtained from 8000 year old sediments near the bottom of the core had an opal ge si of 0 79 10 6 mol mol a composite sample of diatoms deposited within the past 100 years yielded a significantly lower ge si of 0 34 10 6 analysis of ge si was also performed from a variety of other materials in the dry lake basin including stream and lake water unweathered bedrock soils and mineral separates together with sedimentologic records from the lake sediments and ge si recorded in other basin materials it appears that the high ge si values recorded in the 8000 year opal sample were the result of preferential early weathering of high ge si biotite and muscovite minerals from the slopes thus the detailed examination of ge si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive ge si paleorecords in some settings

discovery of active hydrothermal venting in lake taupo new zealand the horomatangi geothermal system of lake taupo new zealand is a sub lacustrine equivalent of subaerial geothermal activity nearby in the taupo volcanic zone tvz the setting of this system is rare within the tvz as it is directly associated with an individual volcanic feature that of the 1 8 ka taupo eruption vent two distinct hydrothermal vent areas named te hoata and te pupu have been discovered during dives with the submersible jago venting of gases was seen at both sites and hot water up to 45degreesc discharges at the te pupu site dilute water samples have concentrations of so4 cl na and sio2 above ambient lake water values gas samples have compositions similar to other tvz geothermal systems gas geothermometers indicate the existence of a high temperature hydrothermal environment beneath the lake with reservoir temperatures in excess of 300degreesc chimney structures were found at the te pupu site they are up to 30 cm tall and mineralized by an epithermal suite of elements including s hg as sb and tl the walls of the chimneys are largely composed of diatoms and strands of silicified filamentous bacteria embedded in an amorphous silica groundmass bacterial mats are commonly associated with the gas vents and also occur at two hot springs close to the vents commonly perched on top of dead chimneys and or exposed outcrops are dense assemblages of what are probably a new species of sponge of the genus heterorotula the sponges host a notably diversified associated invertebrate fauna and represent a previously unseen biomass on the lake floor the sponges appear to have bored into the mineralized chimneys c 2002 elsevier science b v all rights reserved

enhanced chemical weathering of rocks during the last glacial maximum a sink for atmospheric co2 abstract it has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric co2 on the continents may have at least partly contributed to the low co2 concentrations during the last glacial maximum lgm variations in continental erosion could thus be one of the driving forces for the glacial interglacial climate cycles during quaternary times to test such an hypothesis a global carbon erosion model has been applied to a lgm scenario in order to determine the amount of co2 consumed by chemical rock weathering during that time in this model both the part of atmospheric co2 coming from silicate weathering and the part coming from carbonate weathering are distinguished the climatic conditions during lgm were reconstructed on the basis of the output files from a computer simulation with a general circulation model only the predicted changes in precipitation and temperature have been used whereas the changes in continental runoff were determined with an empirical method it is found that during the lgm the overall atmospheric co2 consumption may have been greater than today by about 20 mainly because of greater carbonate outcrop area related to the lower sea level on the shelves this does not however affect the atmospheric co2 consumption by silicate weathering which alone has the potential to alter atmospheric co2 on the long term silicate weathering and the concomitant atmospheric co2 consumption decreased together with a global decrease of continental runoff compared to present day both by about 10 nevertheless some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known the values we present refer to the ice free continental area only but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget although glacial runoff was considerably increased during lgm weathering under the ice sheets seems to be of minor importance

weathering plants and the long term carbon cycle the chemical weathering of ca mg silicate rocks is the principal process whereby co2 is removed from the atmosphere on long or multimillion year time scales at present vascular land plants exert an important influence on weathering by recycling water and by bringing about the secretion of acids and the buildup of high levels of co2 in soils before the spread of vascular plants on the continents during the mid paleozoic chemical weathering must have been achieved by higher levels of atmospheric co2 and or by the intercession of primitive land biota such as lichens and cyanobacteria from examination of several pre vascular weathering scenarios it is concluded that in order to prohibit unreasonably high levels of atmospheric co2 weathering during the precambrian and early paleozoic must have taken place under essentially closed system abiotic conditions linear feedback or via strong regulation by primitive biota preliminary examination of modern weathering by lichens over tens to thousands of years militates against the latter possibility however much more work needs to be done on the quantitative effect of both primitive biota and vascular plants on the rate of weathering and their response to changes in atmospheric co2 before anything more definitive can be said

the effect of organic matter on chemical weathering study of a small tropical watershed nsimi zoetele site cameroon abstract the effect of organic matter during soil water interaction is still a debated issue on the controls of chemical weathering in a tropical environment in order to study this effect in detail we focused on the weathering processes occurring in a small tropical watershed nsimi zoetele south cameroon this site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry the lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones unsaturated slope soils on the hills and or elevated areas and water saturated soils in the swamp zone which represent 20 of the basin surface the study present chemical analysis performed on water samples collected monthly from different localities between 1994 1997 and on soil samples taken during a well drilling in december 1997 the results suggest the existence of chemical and spatial heterogeneities of waters in the basin colored waters flooding the swamp zone have much higher concentrations of both organic matter i e doc and inorganic ions e g ca mg al fe th zr than those from springs and groundwater from the hills nevertheless these organic rich waters present cation concentrations na ca mg k which are among the lowest compared to that of most world rivers the main minerals in the soils are secondary kaolinite iron oxi hydroxides quartz and accessory minerals e g zircon rutile we mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through sem observations the textural characterization of weathered minerals such as kaolinite zircon rutile and the secondary recrystallization of kaolinite microcrystals within the soil profile water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin thus in order to explain the increase of element concentration in the organic rich waters we suggest that organic acids enhance dissolution of minerals such as kaolinite goethite and zircon and also favors the transport of insoluble elements such as al fe ti zr and ree by chemical complexation sio2 aq concentrations in these waters are above saturation with respect to quartz dissolution of phytholithes amorphous silica may be responsible for this relatively high sio2 aq concentration al mg ratios obtained for the soil and the mengong river waters show that a significant amount of al does not leave the system due to kaolinite recrystallisation in the swamp zone soils geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone quantitatively we propose that the increasing amount in total dissolved solid tds due to organic matter and vegetation effect is about 35 in summary this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements thus the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils

the messinian diatomite deposition in the mediterranean region and its relationships to the global silica cycle abstract diatomites constitute a widely represented lithology in the messinian sections of the circum mediterranean neogene marginal basins although traditionally interpreted as genuine evidence of the gradually restricted conditions that characterized the mediterranean just before the messinian salinity crisis their coeval occurrence with a global intensification of the opaline production in the world oceans late miocene early pliocene biogenic bloom suggests that an integrative analysis of the origins of these sediments is necessary a comprehensive analysis of the geological and paleontological records suggests that the synergistic intervention of abiotic tectonic and climate reconfigurations and biotic expansion of grass dominated opal rich biomes controlling factors may have promoted a remarkable enhancement of silica flux from continents to oceans which in turn can explain the opaline burst that occurred during the late miocene at both the global and mediterranean scale the finely laminated pattern and the rich fossil content of diatomaceous deposits that are usually considered to be byproducts of anoxic conditions are briefly discussed some studies seem to indicate that instead of anoxia the aggregation and sedimentation of diatom tests may play a critical role in these processes the lower messinian diatomites of the mediterranean region are generally interbedded with organic rich sediments sapropels clearly attesting prolonged precessionally controlled periods of basin stratification and bottom water anoxia or hypoxia a causal relationship between sapropel and diatomite deposition in the mediterranean is proposed considering the possible interplay between stratification adapted diatoms and anaerobic bacteria and their respective role in influencing the marine silica cycle

silicate rock weathering and atmospheric soil co2 uptake in the amazon basin estimated from river water geochemistry seasonal and spatial variations abstract using the data of the camrex project 1982 1984 on the water geochemistry of the amazon river and its main tributaries it was possible to assess the silicate rock weathering processes and the associated consumption of atmospheric soil co2 taking into account seasonal and spatial variations this study confirms the important role of the andes in the fluvial transport of dissolved and particulate material by the amazon and it shows for the first time that the silicate weathering rate and atmospheric soil co2 consumption are higher in the andes than in the rest of the amazon basin the seasonal variations exhibit the significant role of runoff as a major factor controlling silicate weathering processes and show that the chemical erosion rates vary greatly from low discharge to high discharge the average weathering rate estimated for the whole amazon basin 15 m my is comparable to other estimations made for other tropical equatorial environments a comparison between physical and chemical weathering rates of silicate rocks for the amazon basin and for each tributary basin show that in the andes and in the amazon trough the soil thicknesses are decreasing whereas in the shield the soil profiles are deepening

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

eolian evidence for spatial variability of late quaternary climates in tropical africa abstract study of the eolian fraction of late quaternary sediments from the tropical atlantic reveals that two modes of long term climate variability have existed in tropical africa during the last 150 000 yr tropical northwest africa i e the southwestern sahara and sahel was driest during glaciations and stades but wetter than at present during interglaciations and interstades this may be a response to ice sheets at higher latitudes via equatorward displacement of the westerlies and the subtropical high in contrast central equatorial africa southeast of the sahara was most arid during interstades and times of ice growth and most humid during deglaciation wet periods in this area correspond to insolation maxima in northern hemisphere summer a 23 000 yr precessional rhythm is suggested supporting a direct link between african monsoon intensity and orbitally modulated insolation the late holocene is the only time observed when both areas are arid during an interglacial episode this may reflect in part anthropogenic disturbance of late holocene climates

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

the role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams abstract by integrating the results of recent studies on the concentration and flux of si in european forest soils podzols and acid brown soils with published evidence on the amounts of si cycled through forest vegetation we conclude that phytoliths must be the principal immediate source and sink of silica in soil solution although mineral weathering is the ultimate source during the growing season forest vegetation takes up most of the si made available by weathering of soil minerals and by phytolith dissolution and converts it into phytoliths phytoliths will be the principal source of si leached from the soil during winter rains and spring snowmelt this work accounts for the plateau concentrations of si observed in laboratory equilibrations of bs horizon soils for the maximum si concentration reported for b horizons in the field for a deep sink for si in bs horizons and for the fact that the concentrations of si in soil drainage water and in streams all fall in or below the range expected for phytolith solubility concentrations of si in surface horizons can exceed these levels during droughts the common assumption that the concentration of si in soil solution is a result of a dynamic balance between si released by mineral weathering and that lost by leaching is untenable for bs horizons

a unified model for predicting earthquake induced sliding displacements of rigid and flexible slopes abstract permanent sliding displacement represents a common damage parameter for evaluating the seismic stability of slopes recently developed empirical models for the sliding displacement of shallow rigid sliding masses have demonstrated that including multiple ground motion parameters in the predictive model e g peak ground acceleration and peak ground velocity improves the displacement prediction and reduces it uncertainty a unified framework is developed that extends these empirical displacement models for application to flexible sliding masses where the dynamic response of the sliding mass is important this framework includes predicting the seismic loading for the sliding mass in terms of the maximum seismic coefficient kmax and the maximum velocity of the seismic coefficient time history k velmax the predictive models are a function of the peak ground acceleration pga peak ground velocity pgv the natural period of the sliding mass ts and the mean period of the earthquake motion tm the empirical predictive models for sliding displacement utilize kmax and k velmax in lieu pga and pgv and include a term related to the natural period of the sliding mass this unified framework provides a consistent approach for predicting the sliding displacement of rigid ts 0 and flexible ts 0 slopes

the distribution and relationship between palynomorphs in aerosols and deep sea sediments off the coast of northwest africa abstract palynomorphs in eolian dust and bottom sediments from the northeastern atlantic off the coast of northwest africa were studied primary examination was for terrestrial plant detritus pollen spores opal phytoliths and freshwater diatoms but observations about marine entities were also made three distinct geographic palynofloras were detected a northern palynoflora consisting of pollen derived primarily from the mediterranean basin a central or saharan palynoflora characterized by pollen derived from desert plants and a southern tropical equatorial palynoflora composed of pollen derived from tropical plants but including up to 50 allochthonous pollen from the drier interior of west africa palynomorph distributions are related closely to both source vegetation and to atmospheric and oceanic transport mechanisms and in many cases distribution patterns in bottom sediments and aerosols are complementary the quantity of pollen and spores per gram of bottom sediment ranges from greater than 2000 off the saharan coast in mauritania to less than 50 in deep ocean basins pollen and spores in mediterranean aerosols may exceed 40 per cubic meter of air during the summer and range between 4 to 6 for tropical aerosols during the winter tropical aerosols and bottom sediments contained the greatest abundance of fungal spores the distribution of opal phytoliths and freshwater diatoms in both aerosols and bottom sediments indicates that dust storms are a major transporting agent for these entities from the interior of west africa to the gulf of guinea these storms are also an important agent for the transport of pollen to the tropical atmosphere distances of transport may exceed 5000 km for mediterranean pollen and even greater distances for palynomorphs in eolian dust from interior west africa the abundance of dinoflagellates and microforaminifera in bottom sediments is directly related to the area of upwelling off the west african coast

direct effects of co2 and temperature on silicate weathering possible implications for climate control a critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity temperature and atmospheric co2 on silicate weathering here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic rich solutions is not directly affected by soil co2 but is very sensitive to temperature apparently co2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids the weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change when used as input in steady state carbon cycle and climate models

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

prolific organic sio2 precipitation in a solute deficient river rio negro brazil silicon extracted and precipitated by siliceous algae indicate that the dissolved silicon levels of the rio negro brazil are in part controlled by biological activity diatoms are the most prolific and adhesive eucaryotic microorganisms found in the study area wood leaves and rocks serve as both solid substrates for the adhesive microbes and as nutrient sources scanning electron micrographs of the wood samples revealed a siliceous gel precipitated on both the outer surface and within the submerged wood a biomineralization process was occurring leading to the silicification of the wood sample on a regional scale this process may have important implications for the freshwater silica cycle

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

a review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction diatom silica frustules are a form of biogenic opal and contain oxygen isotopes that are being increasingly used in palaeoclimate studies frustules are comprised of an inner tetrahedrally bonded silica skeleton si o si with an outer hydrous layer the hydrous layer is freely exchangeable and must be removed prior to oxygen isotope measurement using oxidising reagents and or high temperatures analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since extraction techniques will liberate oxygen from all the components in the sediment there is a generally acceptable protocol involving chemistry sieving and settling techniques and more recently laminar flow separation recent studies of lacustrine diatoms have shown that even a small proportion of contaminant can have a significant influence on the oxygen isotope value all lake sediments require their own specific procedure and every sample must be scrutinised by microscopy to check for the level of contamination prior to analysis where sediment cannot be purified sufficiently a semi quantitative assessment of the diatom content can facilitate mass balance techniques when robust preparation methods are used diatom silica offers an important palaeoclimate proxy providing an additional complementary rather than an alternative host of oxygen isotopes in carbonates in lacustrine isotope studies it is often assumed but rarely proven that diatom silica form in isotope equilibrium and hence empirically derived palaeotemperature equations usually made in vitro can be used most studies lack the detailed investigation of contemporaneous materials from natural waters of the region under study to test these equations further investigations need to be conducted in order to understand the systematic relationship between temperature water isotope composition and diatom isotope composition to enable quantitative interpretation of the sediment record experiments to test for vital effects and inter specific differences have so far revealed little variation it is useful to know when and where the diatom silica is formed in the modern environment so details of diatom ecology are important in the interpretation of isotope data here we show that valuable palaeoclimate data can be gained from the oxygen isotope composition of diatom silica 18odiatom especially since diatoms are abundant in many lakes sensitive to climate variation where other hosts e g carbonates are absent to date the most successful studies have been conducted in areas where the 18odiatom registers changes in the oxygen isotope composition of the lake water rather than temperature which is then related to other aspects of climate in these studies the range of values obtained is often greater than the error introduced by contaminating materials

the inorganic chemistry of peat from the maunachira channel swamp system okavango delta botswana abstract the okavango delta is a large 18000 km2 low gradient 1 3600 alluvial fan situated in the semi arid kalahari basin of northern botswana seasonal floodwaters from tropical angola disperse on the fan creating both perennial 6000 km2 and seasonal 7000 to 12000 km2 swamps ninety five percent of this water is lost annually by evapotranspiration organic rich sediment peat is a major sediment of the perennial swamps peat formation commences during senescence of the plants when certain nutrients are recycled while others are lost by rainwater leaching further changes in chemistry occur during subaqueous decay of the plants which involve both gains and losses of constituents decaying plants trap detrital mineral matter which becomes an integral part of the peat the main sources and forms of inorganic matter in the peat are allochthonous kaolinite 40 and quartz 20 and both allochthonous and autochthonous phytolithic silica 30 several inorganic components fe k p na ca and mg which make up the remaining 10 are associated with the organic fraction ion exchange plays only a minor part in their uptake and it seems that these metals are taken up during bacterial activity in the peat the weight proportion of inorganic matter ash decreases downstream mainly due to a decrease in allochthonous mineral matter volume percentage also decreases but is low throughout generally less than five percent this study has revealed that the low quantity allochthonous mineral matter is the main reason for the long term survival of this ecosystem uptake of soluble ions by the peat is important in off setting evaporative concentration of metals

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

late quaternary fires and grassland palaeoecology of mount kenya east africa evidence from charred grass cuticles in lake sediments abstract multi proxy evidence from sacred lake on mount kenya including pollen and compound specific carbon isotope data indicates grassland expansion at the last glacial maximum lgm circa 18 000 14c years bp but lacks the taxonomic resolution with which to derive ecological inferences about past grasslands analysis of grass cuticles preserved in east african lake sediments is presented here as a palaeoecological tool for addressing this problem sub fossil grass cuticles retain micro morphological features that often permit identification to subfamily supertribe tribe and in some instances genus or species level a method for isolating and identifying this novel resource is presented along with an analysis of late quaternary to recent age sediments from sacred lake mount kenya data derived from 43 sediment samples and over 5000 graminoid cuticles support isotopic evidence for the presence of grasslands at sacred lake during the lgm size class analysis of charred graminoid fragments indicates that relatively local low intensity fires occurred throughout the sequence current autoecological understanding of the taxa identified indicates that the grassland present at the lgm possessed specific adaptations to burning we conclude that fire was an important landscape feature on mount kenya reinforcing the impact of past variations in climate and atmospheric co2

biogenic silica storage in soils the role of plants in soil development has become of crucial interest in quantifying global weathering processes on the volcanic reunion island indian ocean a 15 cm thick phytolith rich horizon biogenic opal a developed at the expense of trachytic ashes between 3820 85 and 335 90 yr b p then 97 138 t km 2 yr of sio 2 were biogeochemically recycled from the weathering of parent rocks through a past forest of bamboos nastus borbonicus this rate is in the same range as the present chemical weathering rates of silica in the area considered to be among the highest in the world these results demonstrate that the storage of biogenic silica in soils may be significant and may retard the output of silica to rivers and ocean

the half life of 32si a new estimate based on varved lake sediments abstract cosmic ray produced 32 si has found several applications as a tracer in aqueous geochemical processes these applications require precise knowledge of the half life of 32 si the reported values show a wide range that varies from 101 18 to 330 40 years thus leaving the question of the half life of 32 si still wide open in this study we have determined the half life of 32 si to be 178 10 years by measuring the decrease of its activity with depth in an accurately dated varved sediment core from the kassjon lake north sweden this result reinforces the use of 32 si as a geochronometer to study earth surface processes that have occurred during the last millennium

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

terrestrial sedimentation and the carbon cycle coupling weathering and erosion to carbon burial this paper examines the linkages between the carbon cycle and sedimentary processes on land available data suggest that sedimentation on land can bury vast quantities of organic carbon roughly 1015 g c yr 1 to evaluate the relative roles of various classes of processes in the burial of carbon on land terrestrial sedimentation was modeled as a series of 864 scenarios each scenario represents a unique choice of intensities for seven classes of processes and two different global wetland distributions comparison was made with presumed preagricultural conditions the classes of processes were divided into two major component parts clastic sedimentation of soil derived carbon and organic sedimentation of autochthonous carbon for clastic sedimentation masses of sediment were considered for burial as reservoir sediment lake sediment and combined colluvium alluvium and aeolian deposits when the ensemble of models is examined the human induced burial of 0 6 1 5 1015 g yr 1 of carbon on land is entirely plausible this sink reaches its maximum strength between 30 and 50 n paddy lands stand out as a type of land use that warrants future study but the many faces of rice agriculture limit generalization in an extreme scenario paddy lands alone could be made to bury about 1 1015 g c yr 1 arguing that terrestrial sedimentation processes could be much of the sink for the so called missing carbon is reasonable such a hypothesis however requires major redesign of how the carbon cycle is modeled unlike ecosystem processes that are amenable to satellite monitoring and parallel modeling many aspects of terrestrial sedimentation are hidden from space

towards an understanding of late quaternary variations in the continental biogeochemical cycle of silicon multi isotope and sediment flux data for lake rutundu mt kenya east africa since 38 ka bp silicon is an essential nutrient for marine diatoms which dominate the export of organic carbon to the deep ocean despite the dominance of the oceanic si budget by fluvial inputs and the role of the land biosphere in controlling si losses from rocks and soils to rivers few studies have considered how continental biogeochemical si fluxes varied on an orbital timescale we reconstruct changes in si cycling by the catchment lake ecosystem of lake rutundu mt kenya 3078 m a s l over the last ca 38 ka using a novel combination of lake sediment fluxes and stable isotope 13c 15n 18o 30si data under glacial conditions 38 3 14 3 ka bp high diatom productivity was maintained by substantial losses of dissolved sio2 and soil nutrients from a sparse leaky terrestrial ecosystem during the following period of enhanced monsoon rainfall and seasonality 14 3 9 5 ka bp rapid si cycling by fire prone mesic grassland was associated with substantial aeolian transport of opal phytoliths by smoke plumes but greatly reduced nutrient losses in runoff invasion of tall subalpine shrubs after 9 5 ka bp further enhanced landscape stability leading to very low sediment fluxes of both phytoliths and diatoms this case study offers new insights into processes that may have operated at biome to continental scales during the late quaternary natural environment research council nerc copyright 2008 reproduced with the permission of nerc published by john wiley sons ltd

decadal record of climate variability spanning the past 700 yr in the southern tropics of east africa biogenic silica profiles in varved sediments from northern lake malawi nyasa east africa span the past 700 yr and reflect past primary productivity in the overlying waters on a centennial scale this has been influenced by lake level and a consequent shift in the location of high diatom productivity within the lake basin primary production was higher during the little ice age an arid period from about a d 1570 to 1850 when lake level was about 120 m lower than during the previous three centuries or the past 150 yr

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

natural controls of fluvial denudation rates in major world drainage basins we present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5 105 km2 in area these estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible chemical denudation rates have been calculated by deducting the nondenudational component of solute load mechanical denudation rates range from 1 mm kyr 1 for the st lawrence and dnepr basins to 670 mm kyr 1 for the brahmaputra basin chemical denudation rates vary from 1 mm kyr 1 kolyma niger nile and rio grande basins to 27 mm kyr 1 chiang jiang basin the kolyma basin has the lowest 4 mm kyr 1 and the brahmaputra basin the highest overall rate of denudation 688 mm kyr 1 relationships between denudation rates and a range of morphometric hydrologic and climatic variables are investigated through correlation and regression analysis morphometric variables such as mean local relief are accurately calculated for large basins for the first time by using the national geophysical data center 10 minute topographic database variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates with more than 60 of the variance in total denudation being accounted for by basin relief ratio and runoff basin area runoff variability and mean temperature however are only weakly associated with rates of denudation although direct comparisons cannot be made it appears that rates of basin denudation derived from present day mass flux estimates are not overall significantly different from estimates of long term rates based on sediment volume and thermochronologic data it therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes

river fluxes of dissolved silica to the ocean were higher during glacials ge si in diatoms rivers and oceans two centric marine diatom species thalassiosira oceanica and thalassiosira antarctica were grown in batch cultures to determine the incorporation of germanium ge and silicon si into siliceous shells opal the results were modeled as ge si isotope fractionation during exponential growth diatoms take up and incorporate ge si from solution without major discrimination against ge during stationary phase growth near silica limitation the antarctic species t antarctica discriminates slightly against ge but integrated ge si opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio these results confirm experiments using radioactive 68ge that showed absence of fractionation during diatom silica uptake azam and volcani 1981 in contrast to two box ocean models that invoke 50 ge discrimination by diatoms to explain the observed excess surface ocean germanium concentration murnane and stallard 1988 froelich et al 1989 and late pleistocene ocean sediment ge si opal records mortlock et al 1991 runs of a 10 box ocean ge and si model pandora with 50 discrimination reproduce the excess surface ocean ge but introduces curvature into the deep ocean ge versus si relationship that is not observed in the oceans thus 50 fractionation is not supported by either cultures or models if diatoms do not fractionate ge si then late pleistocene ge si opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization mortlock et al 1991 but rather by whole ocean changes in ge si seawater the marine ge si opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of murnane and stallard 1990 glacial periods are characterized by lower continental weathering intensity lower ge si riv and two fold higher dissolved silica river fluxes marine ge si opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering

glacial interglacial variability of atmospheric co2 due to changing continental silicate rock weathering a model study an 11 box model of the oceanic carbon cycle including sedimentary processes is used to explore the role chemical weathering of continental silicate rocks might play in driving atmospheric co2 levels on glacial interglacial timescales histories for the consumption of co2 by silicate rock weathering processes are derived from the marine ge si record taking the major uncertainties in the knowledge of the ge and si cycles into account several histories for the evolution of the riverine dissolved silica fluxes are calculated from this record the investigation of the systematics between riverine dissolved silica and bicarbonate fluxes under different weathering regimes leads us to the tentative conclusion that although there is no correlation between dissolved silica and total bicarbonate concentrations in the major rivers there may exist a negative correlation between weathering intensity and the ratio of dissolved silica to bicarbonate derived from silicate weathering alone with this correlation as a working hypothesis it is possible to interpret the dissolved silica fluxes in terms of equivalent co2 consumption rates the calculated histories indicate that glacial rates of co2 consumption by chemical silicate rock weathering could have been twice and possibly up to 3 5 times as high as they are today when used to force the carbon cycle model they are responsible for glacial interglacial pco2 variations in the atmosphere of typically 50 60 ppm and up to 95 110 ppm these variations are superimposed to a basic oscillation of 60 ppm generated by the model mainly in response to coral reef buildup and erosion processes the total pco2 signal has an amplitude of about 80 90 ppm and up to 125 135 ppm although these large amplitudes indicate that silicate weathering processes should be taken into account when studying glacial interglacial changes of co2 in the atmosphere it also raises new problems such as too high co2 levels during the period from 110 70 kyr b p requiring further study

promotion of chemical weathering by higher plants field observations on hawaiian basalts abstract an electron microprobe and petrographic microscope were used to study the biota rock interface beneath higher plant communities and lichens as well as unvegetated surfaces on hawaiian basalt flows ranging from a few years to several thousand years in age and currently receiving moderate to extremely high annual rainfall we find dramatic order of magnitude minimum acceleration of weathering rates beneath vascular plants on the youngest flows relative to adjacent non or primitively vegetated controls notably rocks colonized by the common fruticose lichen stereocaulon uulcani bulk dissolution of plagioclase accounts for most of the weathering observed beneath higher plants on these young basalts analysis of older flows under mature forests reveals that chemical weathering beneath vascular plant communities continues unabated for thousands of years after initial colonization moreover such weathering is not limited to exterior basalt surfaces but is commonly found adjacent to joints and vesicles within the rock where it is often associated with fine sub 2 mm diameter plant roots plagioclase dissolution was also observed on lichen encrusted and unvegetated flows a few hundred to thousands of years in age but it was typically much less advanced than beneath higher plants and rarely extended more than several millimeters from exterior rock surfaces chemical denudation rates on older flows with higher plants are at minimum ten times greater than those with only lichens or microbiota in the environments studied if s uulcani and associated microflora resemble the terrestrial precursors to vascular plant communities then the results of studies such as this have implications for the history of atmospheric co 2 and the earth s climate

diagenetic transformations and silcrete calcrete intergrade duricrust formation in palaeo estuary sediments the boteti palaeo estuary in northern botswana is located where the endoreic boteti river an overflow from the regional okavango river system enters the makgadikgadi pans the present work considers diagenetic silica and calcium carbonate dominated transformations the aims are to help identify precursor conditions for the origin of microcrystalline silcrete calcrete intergrade deposits while developing insight into pene contemporaneous silica and calcite matrix formation general precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan the pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater freshwater inflow into a saline pan drives the geochemistry of the system from freshwater to saline from neutral to high ph the geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan the source of most silica and localized caco3 is derived from the dissolution and precipitation of micro fossils while more general caco3 enrichment stems from saline pan based carbonate fractionation diagenetic change leads to colloidal then more consolidated bsio2 cao aggregate formation amorphous silica followed by transformations into opaline silica over time irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments inferring a source for local silica mobilization in intergrade deposits the distribution of calcareous micro fossils may have a similar converse effect diagenetic evidence from an intergrade deposit with a low sio2 cao ratio suggests that transformation occurred more into the pan while an intergrade deposit with a high sio2 cao ratio more likely formed closer to a land margin and was frequently inundated by freshwater pene contemporaneous silcrete calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in ph the continuing consolidation of bsio2 cao aggregates may be facilitated by the presence of increasing amounts of calcite it appears that caco3 may act as a catalyst leading to pene contemporaneous bsio2 cao aggregate formation however the processes involved require further work copyright 2013 john wiley sons ltd

present weathering rates in a humid tropical watershed nsimi south cameroon the study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks common for the temperate zone has not yet been widely applied to the tropics especially humid areas this paper presents an updated database for a six year period for the small experimental watershed of the mengong brook in the humid tropics nsimi south cameroon this watershed is developed on precambrian granitoids north congo shield and consists of two convexo concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter mineralogical and geochemical investigations were carried out in the protolith the saprolite the hillside lateritic soils and the swamp hydromorphic soils biomass chemical analyses were done for the representative species of the swamp vegetation the groundwater was analysed from the parent rock saprolite weathering front to the upper fringe in the hillside and swamp system the chemistry of the wet atmospheric and throughfall deposits and the mengong waters was monitored in the nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter these organic colloids contribute significantly to the mobilization and transfer of fe al zr ti and th in the uppermost first meter of the swamp regolith when the organic colloid content is low i e in the hillside groundwater th and zr concentrations are extremely low 3 pmol l icp ms detection limits strongly insoluble secondary thorianite tho 2 and primary zircon zrsio 4 crystals control their mobilization respectively this finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed biogenic recycling significantly influences the low cl input fluxes sodium is a good tracer of chemical weathering in the watershed the sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate even though low 2 8 mm kyr the chemical weathering rate predominates over the mechanical weathering rate 1 9 mm kyr compared to the rio icacos watershed the most studied tropical site the chemical weathering fluxes of silica and sodium in the mengong are 16 and 40 times lower respectively this is not only related to the protective role of the regolith thick in both cases but also to differences in the hydrological functioning this is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level

basalt weathering in central siberia under permafrost conditions chemical weathering of basalts in the putorana plateau central siberia has been studied by combining chemical and mineralogical analysis of solids rocks soils river sediments and suspended matter and fluid solution chemistry altogether 70 large and small rivers 30 soil pore waters and groundwaters and over 30 solids were sampled during july to august 2001 analysis of multiannual data on discharge and chemical composition of several rivers of the region available from the russian hydrological survey allowed rigorous estimation of mean annual major element concentrations and dissolved and suspended fluxes associated with basalt weathering for the rivers tembenchi and taimura that drain monolithologic basic volcanic rocks the mean multiannual flux of total dissolved cations tds c ca mg na k corrected for atmospheric input is 5 7 0 5 t km 2 yr for the largest river nizhniya tunguska draining essentially basic rocks the tds c is 6 1 1 5 t km 2 yr the overall co2 consumption flux associated with basalt weathering in the studied region 700 000 km 2 achieves 0 08 10 12 mol yr which represents only 2 6 of the total co2 consumption associated with basalt weathering at the earth s surface the fluxes of suspended matter were estimated as 3 1 0 5 9 0 0 8 and 6 5 2 0 t km 2 yr for rivers taimura eratchimo and nizhniya tunguska respectively based on chemical analyses of river solutes and suspended matter the relative dissolved versus particulate annual transport of major components is cinorg corg na k ca mg si fe mn ti al which reflects the usual order of element mobility during weathering according to chemical and mineralogical soil and sediment analyses alteration of basalt consists of 1 replacement of the original basaltic glass by si al fe rich amorphous material 2 mechanical desegregation and grinding of parent rocks leading to accumulation of primary hydrothermal trioctahedral smectite and 3 transformation of these trioctahedral oxy smectites and mixed layer chlorite smectite into secondary dioctahedral smectite accompanied by removal of ca mg and fe and enrichment in al no vertical chemical differentiation of fluid and solid phases within the soil profile was identified all sampled soil pore waters and groundwaters were found to be close to equilibrium with respect to chalcedony gibbsite halloysite and allophanes but strongly supersaturated with respect to goethite nontronite and montmorillonite over the annual cycle the contribution of atmospheric precipitation permafrost melting underground reservoirs litter degradation and rock and soil mineral weathering for the overall tds c transport in the largest river of the region nizhniya tunguska is 9 3 3 10 5 10 5 5 25 20 and 45 30 respectively in the summertime direct contribution of rocks and soil mineral weathering via solid fluid interaction does not exceed 20 the main unknown factors of element mobilization from basalt to the river is litter degradation in the upper soil horizon and parameters of element turnover in the vegetation copyright 2005 elsevier ltd

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

estimated storage of amorphous silica in soils of the circum arctic tundra region we investigated the vertical distribution storage landscape partitioning and spatial variability of soil amorphous silica asi at four different sites underlain by continuous permafrost and rep

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

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years