stages in the life of a magnetic grain sediment source discrimination particle size effects and spatial variability in the south african karoo mineral magnetic properties are commonly used to trace the sources of river and lake sediments with the aim of identifying areas of a catchment experiencing the high rates of soil loss in some areas however there is poor understanding of how magnetic properties are able to discriminate between potential sediment sources the basis for mineral magnetic source discrimination between topsoils and subsurface material over different rock parent materials was investigated in the south african karoo the effects of particle size soil development alteration and diagenesis processes on the source discrimination were investigated good discrimination up to 100 correct discrimination was found between the sedimentary sources soils and subsurface and dolerite soils due to much higher concentrations of magnetic minerals in the dolerite there was little evidence of alterations to magnetic properties in subsurface sediments resulting in only a small ability of properties to differentiate between surface and subsurface sources it was found that during the development of topsoils there was a loss of high coercivity weakly magnetic grains and an enrichment in small superparamagnetic and stable single domain grains particle size was strongly related to magnetic properties with almost all of the secondary superparamagnetic and stable single domain grains created during soil formation being concentrated in the 125 m fractions the small ability of sediment to discriminate between surface and subsurface sources was based upon the dissolution of primarily fd and arm carrying grains in the 32 m fractions and to a lesser extent the 63 32 m fraction as a result it was recommended that source tracing with the 32 m fraction of soils and sediment should be treated separately from the 63 32 m and larger particle size fractions the relationships between particle size and magnetic properties were fairly consistent in all soil samples in each source group raising the possibility that reliable correction factors could be developed for each source group

the estuarine geochemical reactivity of zn isotopes and its relevance for the biomonitoring of anthropogenic zn and cd contaminations from metallurgical activities example of the gironde fluvial estuarine system france abstract zinc stable isotopes measurements by mc icp ms validated by laboratory intercalibrations were performed on wild oysters suspended particles and filtered river estuarine water samples to provide new constraints for the use of zn isotopes as environmental tracers the samples selected were representative of the long range 400 km transport of metal zn cd etc contamination from former zn refining activities at decazeville i e 66 zn 1 and its phasing out recorded during 30 years in wild oysters from the gironde estuary mouth rno rocch sample bank the study also addresses additional anthropogenic sources urban and viticulture and focuses on geochemical reactivity of zn in the turbidity gradient and the maximum turbidity zone mtz of the fluvial gironde estuary in this area dissolved zn showed a strong removal onto suspended particulate matter spm and progressive enrichment in heavy isotopes with increasing spm concentrations varying from 66 zn 0 02 at 2 mg l to 0 90 at 1310 mg l these signatures were attributed to kinetically driven adsorption due to strongly increasing sorption sites in the turbidity gradient and mtz of the estuary oysters from the estuary mouth contaminated sediments from the lot river and spm entering the estuary showed parallel historical evolutions 1979 2010 for zn cd ratios but not for 66 zn values oysters had signatures varying from 66 zn 1 43 in 1983 to 1 18 in 2010 and were offset by 66 zn 0 6 0 7 compared to past 1988 and present spm from the salinity gradient isotopic signatures in river borne particles entering the gironde estuary under contrasting freshwater discharge regimes during 2003 2011 showed similar values 66 zn 0 35 0 03 1sd n 15 i e they were neither related to former metal refining activities at least for the past decade nor clearly affected by other anthropogenic sources therefore the zn isotopic signatures in gironde oysters reflect the geochemical reactivity of zn in the estuary rather than signatures of past metallurgical contaminations in the watershed as recorded in contaminated river sediments the study also shows that the isotopic composition of zn is strongly fractionated by its geochemical reactivity in the gironde estuary representative of meso macrotidal estuarine systems

links between bulk sediment particle size and magnetic grain size general observations and implications for chinese loess studies using a combination of particle size analysis magnetic measurements scanning electron microscopy and transmission electron microscopy imaging this study shows that in a wide range of depositional environments there is a strong link between particle size classes and magnetic response especially below the upper limit of stable single domain magnetic behaviour ferrimagnetic grain assemblages dominated by stable single domain magnetosomes regularly have peak susceptibility and remanence values in coarser grades than do those containing finer grained viscous and superparamagnetic secondary magnetic minerals formed during pedogenesis this effect is despite the fact that there is a one to two orders of magnitude size difference between the particle size boundaries at 1 or 2 m and key domain state transitions mostly below 0 05 m the implications of these results are explored using samples spanning 22 myr of loess accumulation on the chinese loess plateau the results from the loess sections complemented by data from low temperature magnetic experiments show that there are subtle distinctions in mean ferrimagnetic grain size between the pleistocene and miocene parts of the record thus allowing more refined rock magnetic interpretations of the fine grained ferrimagnetic mineral assemblages arising from the effects of weathering pedogenesis and possibly diagenesis in the sections studied

geomorphological palaeoenvironments of the sneeuberg range great karoo south africa sedimentary sequences within headwater valleys on the landward side of the great escarpment of south africa are elucidated and their significance as indicators of environmental change is assessed this study focuses on the sneeuberg range the most prominent mountain range in the semi arid central great karoo valley fills of a hitherto unrecognised complexity and of a greater age than any previously recorded in the central great karoo are reported three phases of deposition spanning the late pleistocene up to the present are documented from sites where gully erosion has incised the valley fills the earliest depositional phase is represented by deeply weathered calcretised gravel deposits which probably were emplaced by debris flow and fluvial processes in the form of a fan these deposits subsequently were buried by finer grained largely unconsolidated sediment with much of this emplacement occurring during the holocene there is evidence for phases of landscape stability and instability within this facies finally sheetwash has removed fine grained sediments from valley flanks and has deposited it either on valley bottoms or in presently active gullies this process appears to be ongoing and is the subject of current investigation the sedimentary deposits are interpreted as representing a wide range of palaeoenvironmental conditions that have prevailed within the central great karoo since the penultimate glaciation copyright 2003 john wiley sons ltd

variability in the mineral magnetic properties of soils and sediments within a single field in the cape fold mountains south africa implications for sediment source tracing abstract when tracing the sources of sediment reference to potential source groups is a necessity however the range of uncertainty present in source fingerprinting outputs has been shown to be increased by a high within source group variability the environmental factors controlling the magnetic properties of samples within a single field were examined on a particle size specific basis to continue building on the recent studies of the spatial variability associated with different tracer types the study area was in the eastern cape of south africa over a quartzite geology and was covered by rough grassland topsoils and gully walls were intensively sampled within the study field further samples were collected from weathering bedrock exposures burnt soil and wetland soil in order to explore potential controlling factors on source magnetism each sample was fractionated to 125 63 m 63 32 m 32 25 m 25 10 m and topographic position was a major controlling factor on soil magnetism with saturated lowland gully walls and wetland topsoils being less magnetic than dry topsoils especially within the little difference was found between the low frequency magnetic susceptibility of quartzite topsoils and the shale and sandstone topsoils found elsewhere in the eastern cape however igneous sources such as dolerite are far more magnetic than the quartzite topsoils discrimination between sediment sources can be highly variable within the

a role for scavenging in the marine biogeochemical cycling of zinc and zinc isotopes zinc zn and cadmium cd are important biologically active trace metals in the ocean to date the marine distributions of these elements have been understood primarily in terms of biological assimilation by growing phytoplankton and regeneration of sinking biological material initial studies of zn and cd concentrations and stable isotope ratios 66zn and 114cd have therefore focused on their use as simple tracers of assimilation and regeneration in the oceans however these two processes are insufficient to explain new data on the marine distribution of zn and 66zn here using the first high resolution paired marine depth profiles of zn cd 66zn and 114cd we suggest that scavenging of zn onto organic matter plays a major yet largely unconsidered role in the marine cycling of zn this hypothesis is supported by culture experiments which show that zn released from degrading phytoplankton is rapidly scavenged back onto organic matter and that adsorbed zn is isotopically heavier than the dissolved phase by 0 58 in contrast very little cd or phosphate was scavenged and cd isotopes were not significantly fractionated during degradation our hypothesis is further supported by one dimensional modeling which reproduces observed marine 66zn profiles with 1 of zn adsorbed to particles understanding how zn cycling in the oceans is a balance between assimilation scavenging and regeneration is necessary in order to investigate 66zn as a tracer of marine productivity we anticipate that paired analyses of 66zn and 114cd will prove to be valuable new tools in constraining patterns of global primary productivity providing key information for the marine carbon cycle during periods of past and present global climate change

magnetic imprints of pedogenesis in planosols and stagnic alisol from bulgaria iron in its diverse forms in soils is an important indicator of a number of past and or ongoing environmental processes iron oxide mineralogy pertinent to a particular soil type could be obtained with high resolution using magnetic methods planosols are a particular soil type characterized by surface water logging leading to development of great textural differentiation with depth this study presents magnetic investigation on three planosols and a stagnic alisol from bulgaria combined with conventional chemical extraction methods oxalate citrate dithionite and soil property characterization ph grain size fractions and organic carbon content extensive magnetic analyses on high resolution sampled depth profiles have been done for determination of iron oxyhydr oxide minerals their stability and variations of effective magnetic grain size with depth zones of intensive iron oxides dissolution the topmost soil horizons are magnetically depleted in the content of strongly magnetic minerals but the paramagnetic content detected through the ratio 77 293 is enhanced accordingly bleached eluvial horizons however contain stable crystalline magnetite maghemite fraction albeit in a trace amounts we ascribe this to the processes of lessivage and translocation of magnetite maghemite from the topmost humus horizon the main results on iron oxide mineralogy suggest that thin humus horizons are enriched with pedogenic maghemite thermal behavior of magnetic susceptibility suggests that ferrihydrite is present in humus and eluvial horizons of all profiles in illuvial horizons of planosols the common mineral is lepidocrocite while in alisol al substituted hematite planosols and stagnic alisol contain both poorly crystalline and well crystalline iron oxides increasing precipitation and weathering leads to higher proportion of well crystalline pedogenic phases in the alisol profile

long term studies of land degradation in the sneeuberg uplands eastern karoo south africa a synthesis for the past 15 yr the sneeuberg uplands in the eastern karoo south africa have been a focus for research on land degradation by the above authors and other colleagues earlier work in the karoo emphasised vegetation change whereas we concentrate on physical changes to the landscape at the small catchment scale e g bare degraded areas badlands and gully donga systems analysis of sedimentation in farm dams allows for reconstruction of environmental histories using 210pb 137cs geochemical and mineral magnetic properties of the sediments erosion rates on badlands are monitored using arrays of erosion pins sediment source tracing within small catchments points to the importance of hillslope sources and the relative erosional inactivity of gully systems sediment supply from hillslope and colluvial sources is maintained by high rates of weathering on mudstones and sandstones current degradation should be viewed in the context of a c 200 yr history of overgrazing by european style stock farming and limited areas of former cultivation in the valleys grazing pressures are now much reduced but the loss of soils and vegetation suggests that landscape recovery will require several decades additional drivers of past degradation are likely to have been periods of drought and fire natural and managed and a gradual increase in both rainfall intensity and the frequency of extreme rainfall events the future of the degraded sneeuberg landscape will depend on future farming practices desirable options include more sustainable livestock practices adoption of wildlife farming and other more benign regimes involving mixes of agriculture tourism and wildlife protection together with landscape rehabilitation measures

fingerprinting upland sediment sources particle size specific magnetic linkages between soils lake sediments and suspended sediments accelerated erosion of fine grained sediment is an environmental problem of international dimensions erosion control strategies and targeting of mitigation measures require robust and quantitative identification of sediment sources here we use magnetic fingerprinting to characterize soils and examine their affinity with and contribution to suspended sediments transported within two subcatchments feeding bassenthwaite lake northwest england a high resolution soil magnetic susceptibility survey was made using a field susceptometer zh instruments sm400 probe combining the spatial and vertical down profile soil magnetic data a subset of soil profiles was selected for detailed laboratory based magnetic remanence analyses the magnetic properties of the catchment soils are highly particle size dependent magnetic analyses were performed on the 31 63 m fraction for particle size specific comparison both with the suspended sediments and lake sediments fuzzy cluster analysis groups the soil magnetic data into six clusters apparently reflecting variations in parent material and horizon type with three magnetically hard soils as unclassified outliers examination of the cluster affinity of the soils suspended sediments and lake sediments indicates that topsoils of the upper newlands valley and subsoils around keskadale beck are a major source of the newlands beck suspended load and the recent post nineteenth century sediments in the deep lake basin older lake sediments show strong affinity with a small number of the derwent suspended sediments and one of the glenderamackin soils a large number of derwent suspended sediments show no affinity with any of the soils or lake sediments instead forming a coherent discrete and statistically unclassified group possibly resulting from mixing between the magnetically hard subsoils of the medium to high altitude glenderamackin and troutbeck areas and softer lower altitude glenderamackin soils the lack of any affinity of these suspended sediments with the lake sediments may indicate deposition along the derwent flood plain and or in the shallow delta of lake bassenthwaite particle size specific magnetic fingerprinting is thus shown to be both highly discriminatory and quantitatively robust even within the homogeneous geological units of this catchment area such a methodological approach has important implications for small large scale catchment management where sources of sediment arising from areas with uniform geology have been difficult to determine using other approaches such as geochemical or radionuclide analyses

magnetic mineralogy of soils across the russian steppe climatic dependence of pedogenic magnetite formation formation of ferrimagnets in well drained buffered unpolluted soils appears to be related to climate and especially rainfall if robust this magnetism rainfall couple can be used to estimate past rainfall from buried soils particularly the multiple soils of the quaternary loess soil sequences of central asia however dispute exists regarding the role of climate vs dust flux for the magnetic properties of modern loessic soils here we examine the mineralogical basis of the magnetism rainfall link for a climate transect across the loess mantled russian steppe where critically dust accumulation is minimal at the present day magnetic and independent mineralogical analyses identify in situ formation of ferrimagnets in these grassland soils increased ferrimagnetic concentrations are associated with higher annual rainfall xrd and electron microscopy show the soil formed ferrimagnets are ultrafine grained 50 nm and pure ferrimagnetic contributions to ma ssbauer spectra range from 17 in the parent loess to 42 for a subsoil sample from the highest rainfall area total iron content varies little but the systematic magnetic increases are accompanied by decreased fe2 content reflecting increased silicate weathering for this region parent materials are loessial deposits topography is rolling to flat and duration of soil formation effectively constant the variations in soil magnetic properties thus predominantly reflect climate and its co variant organic activity statistical analysis identifies strongest relationships between rainfall and magnetic susceptibility and anhysteretic remanence this magnetic response correlates with that of the modern soils across the chinese loess plateau such correlation suggests that the rainfall component of the climate system not dust flux is a key influence on soil magnetic properties in both these regions

floodplains of large rivers weathering reactors or simple silos abstract large river sediments are mostly derived from tectonically active mountain belts but then undergo a series of sedimentation temporary storage and reworking episodes on their journey to the ocean the long transfer time of these sediments through active floodplains might result in a significant chemical maturation via weathering reactions which is of critical importance for biogeochemical cycles at the earth surface this study reports the chemical composition of river sediments from different locations throughout the courses of the main tributaries of the amazon basin sampling along river depth profiles yields access to the whole grain size and chemical composition range of river sediment here weathering intensities i e losses of na k mg and ca associated with chemical weathering in floodplains are 1 examined as a function of grain size and 2 integrated over the whole grain size range for three selected long river reaches flowing through the foreland and the lowland of the amazon basin the upper maranon the beni and the lower madeira rivers a relatively small na loss through plagioclase dissolution is observed in the madeira reach while an important ca loss due to carbonate dissolution occurs in the two foreland reaches maranon and beni no significant loss of k and mg is observed in any of the reaches showing the low alterability of primary k and mg bearing minerals and suggesting retention of k and mg in the particulate phase by secondary minerals the combination of these findings with previously reported data on the downstream change of dissolved na k mg and ca fluxes suggests that chemical weathering of stable alluvial deposits could also significantly contribute to theweathering flux generated in foreland and lowland areas the comparison between the three amazon reaches and the gangetic plain tends to show that the features observed in the amazon are valid on a global scale finally we show that although resulting in a relatively small change in the chemical composition of the river sediment silicate weathering in the lower madeira floodplain can lead to a co2 drawdown equivalent to ca 10 of the total co2 consumption flux of the whole madeira basin

chemical weathering and atmospheric soil co2 uptake in the andean and foreland amazon basins abstract this study is a geochemical investigation of the andean and foreland basins of the amazon river at high spatial and time resolution carried out within the framework of the hybam research program hydro geodynamics of the amazon basin monthly sampling was carried out at 27 gauging stations located in the upper tributaries of the amazon basin from north to south the napo maranon ucayali madre de dios beni and mamore rivers the aim of this paper is to estimate the present day chemical weathering rate cwr as well as the flux of co 2 consumption from total and silicate weathering in the andes and foreland amazon basins and to discuss their distribution as a function of geomorphic and structural parameters based on the forward method the napo and other ecuadorian basins present high silicate weathering rates in comparison with the other basins we confirm that the maranon and ucayali rivers control the amazon hydrochemistry due to the presence of salt rocks and carbonates in these basins the madre de dios beni and mamore basins do not contribute much to the amazon dissolved load this north to south cwr gradient can be explained by the combination of decreasing weatherable lithology surface and decreasing runoff rates from the north to the south the foreland part of the basins or mountain lowland transition accounts for nearly the same proportion of the amazon silicate chemical weathering and carbonate chemical weathering fluxes as the andean part this result demonstrates the importance of the sediment accumulation areas in the amazon basin weathering budget and can be explained by the occurrence of a higher temperature the deposition of fresh sediments from andean erosion and a higher sediment residence time than in the upper part of the basin with a total co 2 consumption rate of 744 10 3 moles km year 1 and a silicate co 2 consumption rate of 300 10 3 moles km year 1 the upper amazon river andes foreland part is the most intense part of the amazon basin in terms of atmospheric co 2 consumption by weathering processes it is an important co 2 sink by weathering processes but accounts for only somewhat more than half of the co 2 consumption by silicate weathering of the amazon basin this result points out the importance of the lowland part of the basin in the inorganic c silicate budget the upper amazon accounts for 2 4 of the world s silicate co 2 consumption which is the same proportion as for the southern and southern east himalaya and tibetan plateau

distinguishing sediments from the yangtze and yellow rivers china a mineral magnetic approach the yellow river and the yangtze river contribute 10 of the global fluvial sediment flux to the oceans proper characterization of the sediments of the two rivers is critical to the knowledge of their fate in the marginal seas of the west pacific ocean magnetic measurements have been made on bulk sediments as well as on separated particle size fractions of representative samples from the yellow and yangtze river estuaries with the purpose of providing an efficient means of differentiating both groups of river sediments from each other we found that on average the yangtze river estuary sediments have relatively higher ferrimagnetic mineral contents and ferrimagnetic to antiferromagentic ratios compared with the yellow river estuary sediments a diagram of saturated isothermal remanent magnetization sirm versus the demagnetization parameter s 100 can clearly distinguish sediments from the two rivers detailed magnetic measurements carried out on particle size fractions provide additional confirmation

magnetic enhancement in wildfire affected soil and its potential for sediment source ascription intense rainfall following wildfire can cause substantial soil and sediment redistribution with concern for the increasing magnitude and frequency of wildfire events research needs to focus on hydrogeomorphological impacts of fire particularly downstream fluxes of sediment and nutrients here we investigate variation in magnetic enhancement of soil by fire in burnt eucalypt forest slopes to explore its potential as a post fire sediment tracer low frequency magnetic susceptibility values lf of 10 m material sourced from burnt slopes c 8 0 10 4 10 6 m3 kg 1 are an order of magnitude greater than those of 10 m material derived from long unburnt areas 0 8 10 6 m3 kg 1 susceptibility of anhysteretic remanent magnetization arm and saturation isothermal remanent magnetization sirm values are similarly enhanced signatures are strongly influenced by soil and sediment particle size and storage of previously burnt material in footslope areas whilst observations indicate that signatures based on magnetic enhancement show promise for post fire sediment tracing problems arise with the lack of dimensionality in such data magnetic grain size indicators fd arm sirm and fd arm offer further discrimination of source material but cannot be included in numerical unmixing models owing to non linear additivity this leads to complications in quantitatively ascribing downstream sediment to source areas of contrasting burn severity since sources represent numerical multiples of each other indicating the need to involve additional indicators such as geochemical evidence to allow a more robust discrimination copyright 2005 john wiley sons ltd

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

rock magnetic signatures in diagenetically altered sediments from the niger deep sea fan 1 diagenesis has extensively affected the magnetic mineral inventory of organic rich late quaternary sediments in the niger deep sea fan changes in concentration grain size and coercivity document modifications of the primary magnetic mineral assemblages at two horizons the first front the modern iron redox boundary is characterized by a drastic decline in magnetic mineral content coarsening of the grain size spectrum and reduction in coercivity beneath a second front the transition from the suboxic to the sulfidic anoxic domain a further but less pronounced decrease in concentration and bulk grain size occurs finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment low and high temperature experiments were performed on bulk sediments and on extracts which have also been examined by x ray diffraction thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components their broad range of titanium contents reflects the volcanogenic traits of the niger river drainage areas diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition low titanium compounds are less resistant to diagenetic dissolution intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments at the fe redox boundary precipitation of authigenic possibly bacterial magnetite is documented the presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain grain size specific data argue for a gradual development of a superparamagnetic and single domain iron sulfide phase in this milieu most likely greigite

a rouse based method to integrate the chemical composition of river sediments application to the ganga basin 1 the ganga river is one of the main conveyors of sediments produced by himalayan erosion determining the flux of elements transported through the system is essential to understand the dynamics of the basin this is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions using acoustic doppler current profiler adcp acquisitions with sediment depth profile sampling of the ganga in bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments the model shows that ca 390 30 mt of sediments are transported on average each year through the ganga at haring bridge bangladesh modeled average sediment grain size parameters d 50 and d 84 are 27 4 and 123 9 mm respectively grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation the integrated sediment flux is characterized by low al si and fe si ratios that are close to those inferred for the himalayan crust this implies that only limited sequestration occurs in the gangetic floodplain the stored sediment flux is estimated to c a 10 of the initial himalayan sediment flux by geochemical mass balance the associated globally averaged sedimentation rates in the floodplain are found to be ca 0 08 mm yr and yield average himalayan erosion rate of ca 0 9 mm yr this study stresses the need to carefully address the average composition of river sediments before solving large scale geochemical budgets citation lupker m c france lanord j lave j bouchez v galy f metivier j gaillardet b lartiges and j l mugnier 2011 a rouse based method to integrate the chemical composition of river sediments application to the ganga basin

using magnetic susceptibility to facilitate more rapid reproducible and precise delineation of hydric soils in the midwestern usa abstract standard field indicators currently used for hydric soil delineations usda nrcs 1998 field indicators of hydric soils in the united states version 4 0 in g w hurt et al ed united states department of agriculture nrcs fort worth tx are useful but in some cases they can be subjective difficult to recognize or time consuming to assess magnetic susceptibility ms measurements acquired rapidly in the field with a portable meter have great potential to help soil scientists delineate and map areas of hydric soils more precisely and objectively at five sites in illinois from 5 to 15 ha in area with contrasting soil types and glacial histories the ms values of surface soils were measured along transects and afterwards mapped and contoured the ms values were found to be consistently higher in well drained soils and lower in hydric soils reflecting anaerobic deterioration of both detrital magnetite and soil formed ferrimagnetics at each site volumetric ms values were statistically compared to field indicators to determine a critical ms value for hydric soil delineation such critical values range between 22 10 5 and 33 10 5 si in silty loessal or alluvial soils in illinois but are as high as 61 10 5 si at a site with fine sandy soil a higher magnetite content and slower dissolution rate in sandy soils may explain the difference among sites with silty parent material the lowest critical value 22 10 5 si occurs in soil with low ph 4 5 5 5 since acidic conditions are less favorable to ferrimagnetic mineral neoformation and enhance magnetite dissolution because of their sensitivity to parent material properties and soil ph critical ms values must be determined on a site specific basis the ms of studied soil samples 0 5 cm depth is mainly controlled by neoformed ultrafine ferrimagnetics and detrital magnetite concentrations with a minor contribution from anthropogenic fly ash neoformed ferrimagnetics are present in all samples but based on high fd 5 to 10 are most prevalent in high ph mollisols of northeastern illinois scanning electron microscope images display significantly more detrital magnetite alteration in hydric soils substantiating that reductive dissolution of magnetite aided by microorganisms is a primary cause for lower ms fly ash comprises 8 50 of the 5 m strongly magnetic particles and typically accounts for 5 15 of the total ms signal the proportion of fly ash in 5 m strongly magnetic fractions is greater in hydric soils because of lower natural magnetite contents possibly combined with historical topsoil accumulation in lower landscapes magnetic fly ash particles are also more altered in low ms soils implying that significant magnetite dissolution can occur in less than 150 years

determining the sources of suspended sediment in a forested catchment in southeastern australia 1 knowledge is scarce regarding the relative contributions from harvested areas and unsealed roads to sediments in forested catchments we investigate the source composition of suspended sediment using sediment tracers and an improved multivariate mixing model sediment samples were collected during six rainfall events geochemical and radiometric tracer properties were corrected for particle size and organic content as well as conservativeness during erosion and sediment delivery the mixing model incorporates variability of the tracer properties using a monte carlo simulation technique mean sediment contribution from the undisturbed forest was 50 70 harvested areas gravel surfaced roads and ungravelled roads contributed 5 15 6 14 and 12 25 respectively the unsealed roads contribute 20 to 60 times more sediment than the undisturbed forest and about 10 times more sediment than the harvested areas on a per unit area basis harvested areas contribute 1 to 5 times greater sediment than the undisturbed forest these results support other studies that identify unsealed roads as important sediment sources in forested catchments

isotopic composition of zinc copper and iron in lunar samples abstract we determined by icp ms the concentrations and isotopic ratios of fe cu and zn in the ti rich lunar basalt 74275 in the lunar orange glass 74220 and in up to 10 lunar soils namely 14163 15231 64501 66041 68841 69941 70011 72501 75081 and 76501 two analyses of zinc in lunar basalt 74275 give 66 zn 0 17 and 0 75 values within the range of those measured in terrestrial basalts copper in lunar basalt 74275 has 65 cu 1 4 which is isotopically heavier than values observed in terrestrial basalts in the orange glass we measured 56 fe 0 24 65 cu 0 42 and 66 zn 3 6 these values of are more negative than those obtained for 74275 and for typical lunar basalts but for cu comparable to those observed in terrestrial sulfides and meteorites in lunar soils we found 0 11 56 fe 0 51 2 6 65 cu 4 5 and 2 2 66 zn 6 4 insofar as we can generalize from a small sample set s fe cu zn and cd show similar trends in isotopic fractionation on the moon lunar basalts have nearly terrestrial isotopic ratios relative to the lunar basalt 74275 the pyroclastic glass 74220 is enriched in the lighter isotopes of fe cu and zn and the soils are enriched in the heavier isotopes of fe cu and zn the patterns in the basalts are likely inherited from the source material the light isotope enrichments seen in the orange glass originated during lava fountaining or less probably during partial condensation of vapor and the heavy isotope enrichments in the lunar soils were likely created by a combination of processes that included micrometeorite vaporization and sputtering in the orange glass the light isotope enrichments relative to lunar basalts of zn are larger than those of cu if these enrichments reflect accurately the isotopic composition of the gas they suggest that cu is more volatile than zn in the liquid from which the gas derived a simple model built on the known flux of micrometeorites to the lunar surface and a published estimate that micrometeorites generate 10 times their own mass of vapor predicts heavy isotope enrichments comparable to those observed in soils but only if the regolith gardening rate is set at about one twentieth of the generally accepted value of 1 cm my this discrepancy may reflect the difference in the time constants for micrometeorite milling and decimeter scale gardening or the importance of sputtering

the detection of bacterial magnetite in recent sediments of lake chiemsee southern germany the sediments of lake chiemsee located in the alpine foreland in southern germany host a variety of magnetotactic bacteria mtb which contain intracellular crystals of magnetite arranged in linear chains to detect bacterial magnetite in the carbonate dominated surface sediments and further quantify its contribution to the magnetic signal of the sediments we conducted detailed rock magnetic measurements as well as complimentary non magnetic analyses electron microscopy powder x ray diffraction and sediment pore water analysis our results demonstrate that biogenic single domain magnetite characterized by bullet and truncated hexagonal prismatic shapes is the dominant ferrimagnetic component in the topmost few centimetres of the sediment the changes of magnetic properties with depth are due to the occurrence of live mtb and the downward increasing dissolution of biogenic magnetite moreover the ratios of remanence loss on warming through the verwey transition after field cooling and zero field cooling of saturation isothermal remanence dfc dzfc were determined as 1 47 and 1 25 for freeze dried and air dried sediment samples containing mtb respectively these low ratios suggest that the bacterial magnetite chains were disrupted to a large extent and or that the bacterial magnetosomes might have undergone partial

hydrogen peroxide treatment effects on the particle size distribution of alluvial and marsh sediments pretreatment of sediment with hydrogen peroxide to remove organic constituents and aid deflocculation is a common component of particle size analyses of terrestrial and marine sediments this study

establishing fire incidence in temperate soils using magnetic measurements abstract natural unburnt topsoil samples from the cotswolds region of england and samples from archaeological contexts in the uk and hungary some of which are known to have been affected by burning have been used to identify a distinctive magnetic signature linked to the effects of fire on magnetic properties a combination of bulk low field lf and frequency dependent fd magnetic susceptibility and normalized anhysteretic remanent magnetization arm measurements have been used on both unburnt and experimentally burnt samples from the cotswolds and on the archaeological samples complementary determinations were made for both sets of samples of the maximum potential susceptibility max and hence percentage fractional conversion conv which provides independent evidence of burning the results show that the ferrimagnetic mineral assemblages produced as a result of fire have a significantly finer grain size than those arising from weathering and soil formation alone burning thus gives rise to a distinctive envelope of values on a bilogarithmic plot of arm fd versus arm lf such a diagram provides a convenient basis for identifying magnetic signatures in soils archaeological materials and palaeosols that have arisen mainly through burning

fluvial connectivity and climate a comparison of channel pattern and process in two climatically contrasting fluvial sedimentary systems in south africa abstract the aim of this research was to investigate the dynamics of valley formation sediment delivery and channel pattern in two climatically contrasting fluvial sedimentary systems in south africa each system comprised a network of headwater valley fills and floodplains underlain by sedimentary karoo supergroup rocks that are intersected by resistant dolerite dykes and sills the seekoei river floodplain and gordonville valley fill site in the great karoo however experience less than half the annual precipitation of the nsonge river floodplain and hlatikhulu valley fill in the kwazulu natal drakensberg foothills furthermore rainfall is more variable in the karoo despite climatic differences headwater valley fills were geomorphically similar in contrast floodplains in the two regions were vastly different even when the same downstream control a resistant dolerite intrusion crossing the drainage line was considered upstream of a dolerite dyke the nsonge river is highly sinuous and located in a wide floodplain that has been carved by lateral planation of the underlying bedrock in comparison the seekoei river located upstream of a dolerite sill is discontinuous and characterized by floodouts and avulsing distributaries that undergo periods of bedrock incision followed by infilling it is likely that this disparity is caused by the inability of infrequent unsustained flows to develop meanders and thus adjust the channel planform to changes in discharge sediment load and valley slope flow variability thus exercises a strong control on channel pattern and causes floodouts in headwater settings and the semi arid karoo floodplain as a result sediment transport in the seekoei river is likely to be episodic and net retention of sediment in the semi arid floodplain is greater than in the sub humid nsonge river floodplain where sediment depth is limited

how old are the gullies dongas of the sneeuberg uplands eastern karoo south africa abstract the sneeuberg uplands in the eastern karoo are wetter than lowland sites with 400 mm of rainfall for over 150 years until recently they have been overgrazed largely by sheep the landscape consists of badlands less degraded areas and networks of gullies occupying the valley floors and hillside depressions these gullies have been relatively stable since at least 1945 the age of the gullies is controversial but various authors have speculated that they result from european incursion from the late 18th century onwards reconsideration of the relationship between vlei wetland deposits in the valley bottoms and gullies suggest that the gullies must post date late holocene development of vlei conditions incision by gullies would have resulted in a drying out of wetlands which occurred later and allowed cultivation of many valley bottoms for wheat and fodder crops gullies may have developed from deliberate attempts to drain wetlands for agriculture they are also likely to have formed from wagon tracks and a changed hydrology resulting from overgrazing and vegetation degradation on valley bottoms and hillslopes in other parts of south africa it has been suggested that gullies result from climatic impacts including instability in the little ice age this study should be seen in the context of an on going global debate about the role of climate and human mismanagement of the natural landscape

mineral magnetic characterization of sediment sources from an analysis of lake and floodplain sediments in the catchments of the old mill reservoir and slapton ley south devon uk research on suspended sediment transport in the catchments of the old mill reservoir and slapton lower ley south devon has attempted to discriminate changing catchment sources on the basis of downcore variations in the mineral magnetic properties of lake reservoir and floodplain sediments here we examine these downcore variations and also explore the variability in catchment sources and the influence of topographic controls on mineral magnetic signatures of topsoils and subsoils particle size controls on the mineral magnetic signatures are explored by an analysis of a fractionated sediment sample whilst the possible impact of diagenesis is assessed by an examination of the mn profiles in the lake and reservoir sediments from this analysis it is evident that the mineral magnetic signatures of well sorted floodplain deposits are more likely to reflect the particle size composition of the transported material by contrast the mineral magnetic record in the sediment of slapton ley appears to be most strongly influenced by dissolution of magnetic minerals the sediment of the old mill reservoir provides the only suitable record for the application of a simple mixing model which is developed in order to quantify changes in the relative contribution of topsoil and subsoil through time the research has important implications for attempting to reconstruct sediment sources in highly eutrophic lakes and emphasizes the uncertainty in the application of simple mixing models 1998 john wiley sons ltd

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

sediment fingerprinting in agricultural catchments a critical re examination of source discrimination and data corrections fine sediment source fingerprinting techniques have been widely applied in agricultural river catchments successful source discrimination in agricultural environments depends on the key assumption that land use source signatures imprinted on catchment soils are decipherable from those due to other landscape factors affecting soil and sediment properties in this study we re examine this critical assumption by investigating i the physical and chemical basis for source discrimination and ii potential factors that may confound source un mixing in agricultural catchments including particle size and organic matter effects on tracer properties the study is situated in the river tamar a predominantly agricultural catchment 920 km2 in south west england that has also been affected by mining source discrimination focused on pasture and cultivated land uses and channel banks monthly time integrated suspended sediment samples were collected across seven catchments for a 12 month period physical and chemical properties measured in source soils and sediment included fallout radionuclides 137cs excess 210pb major and minor element geochemical constituents total organic carbon and particle size source discrimination was entirely dependent on differences in tracer property concentrations between surface and sub surface soils this is based on fallout radionuclide concentrations that are surface elevated while many geochemical properties are surface depleted due to weathering and pedogenetic effects although surface soil contamination can reverse this trend however source discrimination in the study catchments was limited by i rotation of cultivated and pasture fields resulting in reduced differences between these two sources and ii the cultivated source signature resembling a mix of the pasture and channel bank sources for many tracer properties furthermore a combination of metal pollution from abandoned historic mines and organic enrichment of sediment from upland areas of peaty soils resulted in the non conservative behaviour of some tracer properties in several catchments differences in the particle size and organic carbon content of source soils could explain much of the variation in these properties in downstream sediment rather than selective transport effects inconsistent relationships between particle size organic carbon and tracer property concentrations further undermined the basis for the use of widely applied corrections to tracer datasets sensitivity analysis showed that correcting source tracer data for differences in organic matter can produce large changes to source contribution estimates that cannot be justified and such corrections should not be used confounding factors related to poor source discrimination and non conservative behaviour are highly likely to affect sediment fingerprinting studies in many agricultural catchments as a result estimates of source contributions in many fingerprinting studies may contain significant unquantified errors

diagenesis of magnetic minerals in the recent sediments of a eutrophic lake magnetic mineral assemblages in lake sediments are usulally interpreted as reflecting changing catchment erosion magnetic and chemical assemblages in a small eutrophic lake lough augher northern ireland suggest diagenetic dissolution of fine grained magnetite however probably as a result of changing redox conditions associated with eutrophication water column results show internal cycling of mn and fe from the sediments into the armxic hypolimnion calculations suggest that late summer hypolimnetic fe concentrations are controlled by formation of aulhigenic vivianite sedimentary pore water concentrations of total soluble fe and p suggest a similar process in the sediments the loss of mn from the lake by diffusion from the sediment and washout at overturn is indicated by mn budget calculations and by it marked decrease in its sedimentary concentration reflecting onset of eutrophication as indicated by diatom biostratigraph y the changes in the sediment mn profile are coincident with and in some cases identical to changes in several rnagnctic parameters the changes in magnetic stratigraphy suggest a coarsening of the magnetic grain size and reduction in the total concentration agreement with chemical profiles indicative of changing rcdox suggest that this coarsening results from dissolution of fine grained magnetite catchment stability together with other stratigraphic profiles of eutrophication and changing redox supports the conclusion that the magnetic profiles are reflecting in situ diagenetic changes rather than altered catchment processes with the data available it is not possible to identify the actual clissolution process involved or to determine whether it is chemically or bacterially controlled

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

evidence for the dissolution of magnetite in recent scottish peats abstract magnetic hydrological and geochemical data for 11 scottish peat cores confirm previous reports that anthropogenic pollutants constitute the principal source of ferrimagnets to such environments but indicate that postdepositional fe3o4 persistence may be more limited than formerly envisaged in all cores the lower stratigraphic limit of magnetic enrichment coincides with the water table below which negative mv values and enhanced ratios of fe mn and cu zn depict an abrupt transition to a strongly anoxic regime dissolution of fe3o4 in the reducing inundated zone is also signified by the more rapid down core loss of magnetic properties in cores from topographically depressed sphagnum lawns than in those from more elevated calluna hummocks as a consequence the peak and cumulative sirm values for cores from the former settings appear disproportionately low and the magnetostratigraphic records from adjacent but topographically variant sites are frequently asynchronous in contrast to most marine and lacustrine sediments dissolution of fe3o4 in peat induces a refinement of magnetic mineral assemblages highlighted by declining sirm arm ratios with depth and a hardening of remanence near the base of the magnetically enhanced strata sem micrographs of magnetic extracts from one scottish core indicate that this refinement results from preferential dissolution along crystallographic weaknesses within multidomain phases ultimately causing their disintegration and the adoption of single domain type magnetic behavior rates of dissolution are considerably greater than those reported for marine sediments probably due to the unusually high dissolved sulfide concentrations in mire waters

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

development of badlands and gullies in the sneeuberg great karoo south africa abstract the study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the sneeuberg uplands of the great karoo this is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation species diversity has declined and ground cover has been reduced leading to a positive feedback loop which exacerbates the degradation many foot slopes developed in shales clays and colluvium have extensive incipient badland development with closely spaced gullying up to 1 5 m deep in valley bottom and valley side depression locations gullies up to 8 m deep have developed usually cut to bedrock through valley fills of mainly holocene colluvium both badlands and gullies appear to have developed since european settlement and to be part of the same hydrological system with extensive areas of bare ground badlands feeding water to incising gullies experiments using simulated rainfall throw some light on current processes badland areas are active under high frequency low magnitude rainfall events major gullies are likely to be the result of occasional high magnitude events but these have not been observed overgrazing in the past is the most likely cause of the degradation

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

volcanology and petrology of mathews tuya northern british columbia canada glaciovolcanic constraints on interpretations of the 0 730 ma cordilleran paleoclimate petrological volcanological and geochronological data collected at mathews tuya together provide constraints on paleoclimate conditions during formation of the edifice the basaltic tuya was produced via pleistocene glaciovolcanism in northern british columbia canada and is located within the tuya volcanic field 59 195 n 130 434 w which is part of the northern cordilleran volcanic province ncvp the edifice comprises a variety of lithofacies including columnar jointed lava pillow lava massive dikes and volcaniclastic rocks collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by pleistocene ice as is typical for tuyas the volcaniclastic facies record multiple fragmentation processes including explosive quench and mechanical fragmentation all samples from mathews tuya are olivine plagioclase porphyritic alkali olivine basalts they are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the tuya volcanic field e g ash mountain south tuya tuya butte as well as the dominant ncvp rock type crystallization scenarios calculated with melts account for variations between whole rock and glass compositions via low pressure fractionation the presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0 6 gpa the eruption of mathews tuya occurred between 0 718 0 054 ma and 0 742 0 081 ma based on 40ar 39ar geochronology weighted mean age of 0 730 ma the age determinations provide the first firm documentation for large 700 m thick pre fraser wisconsin glaciers in north central british columbia 0 730 ma and correlate in age with glaciovolcanic deposits in russia e g komatsu et al geomorph 88 352 366 2007 and with marine isotopic evidence for large global ice volumes 0 730 ma

insights into iron sources and pathways in the amazon river provided by isotopic and spectroscopic studies abstract the present study investigated the weathering and transport mechanisms of fe in the amazon river a particular emphasis was placed on fe partitioning speciation and isotopic fractionation in the contrasting waters of the solimoes and negro rivers and their mixing zone at the beginning of the amazon river samples collected in the end member rivers and thirteen sites distributed throughout the mixing zone were processed through frontal vacuum filtration and tangential flow ultrafiltration to separate the different suspended solid fractions i e particulate p 0 45 m and p 0 22 m colloidal 0 22 m c 5 kda and truly dissolved elements td 57 fe values relative to irmm 14 while the particulate fractions yielded light isotopic compositions of 0 344 for p 0 22 m and 0 104 for p 0 45 m fractions the mineral particulate rich waters of the solimoes river had dissolved and colloidal fractions with light isotopic composition 0 532 and 0 176 respectively whereas the particulate fractions yielded 57 fe values close to those of the continental crust i e 0 029 for p 0 22 m and 0 028 for p 0 45 m ten kilometers downstream from the negro and solimoes junction the concentrations of colloidal and dissolved fe species deviate markedly from conservative mixing a maximum fe loss of 43 g l i e 50 of the dissolved and colloidal fe is observed 110 km downstream from the rivers junction the contrasting negro and solimoes rivers isotopic compositions along the pore sized water fractions is attributable to the biogeochemical processes involving different types of upland soils and parental materials for instance the isotopic composition of colloidal and dissolved fe from the negro river are consistent with fe oxidation and complexation mechanisms at the interface between waterlogged podzols and river networks as supported by strong organo fe complexes signals observed by epr conversely the particulate and colloidal fractions from the solimoes river have 57 fe consistent with strong mechanical erosion in the andean cordillera and upland soils as evidenced by high concentrations of fe 3 oxides sensu lato measured by epr the massive dissolved and colloidal fe removal is associated with the evolution of the physical and chemical composition of the waters i e ionic strength during mixing which influences organo fe 3 and fe 3 oxyhydroxides stability several models are discussed to explain fe non conservative behavior including dissociation of organo fe complexes and the subsequent formation of solid fe 3 oxyhydroxides and semiquinone free radicals as evidenced by epr spectra demonstrating that organo fe signals decrease as fe 3 oxyhydroxides and free radicals signals increase as in estuarine regions the mechanisms involving fe transfer and loss in the mixing zone has a negligible effect on the bulk water fe isotopic composition this result suggests that a tropical basin similar to the amazon river basin delivers to the ocean waters with an fe isotopic composition similar to that of the earth s continental crust

fate of particulate copper and zinc isotopes at the solimoes negro river confluence amazon basin brazil abstract the behaviour and fate of copper cu and zinc zn at river confluences is poorly understood although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean to identify and quantify the effect of such processes in river mixing zones particulate cu and zn isotope signatures 65cuspm and 66znspm were measured along cross sections and vertical profiles at the largest river confluence in the world the encontro das aguas mixing zone of the amazon river where the organic rich negro river meets the sediment rich solimoes river the negro river suspended sediments with highly variable cu and zn concentrations as well as 65cuspm and 66znspm are mostly influenced by organic matter and by the lateritic cover of the watershed the solimoes river suspended sediments more homogeneous in cu and zn concentration and isotope composition across the river section reflect the signature of weathered silicate rich sediments derived from the andes the solimoes river supplies the majority of the suspended cu and zn to the amazon river and despite important flux losses across the confluence 35 for cu and 27 for zn 65cuspm and 66znspm show a conservative behaviour during the mixing in the dissolved load cu concentrations and 65cudiss mostly supplied by the solimoes river behave conservatively whereas zn derived mainly from the negro river suffers an important loss 58 that can be attributed to zn adsorption onto the suspended sediments from the solimoes river this transfer does not induce a significant 66znspm shift in the amazon river suspended sediments therefore cu and zn isotope ratios in the suspended sediments behave conservatively through this confluence which mixes two very chemically contrasted rivers our findings thus suggest that the riverine isotopic information on the sources of particulate cu and zn is preserved during tributary mixing