retardation of arsenic transport through a pleistocene aquifer holocene aquifers are the source of much arsenic poisoning in south and southeast asia whereas pleistocene aquifers are mostly safe here the delayed arsenic contamination of a pleistocene aquifer is described and modelled millions of people across southeast asia are exposed to arsenic contaminated drinking water drawn from holocene aquifers layers of sand deposited less than 5 000 years ago by contrast pleistocene aquifers deposited about 12 000 years ago have lower levels of contamination and are increasingly being exploited as safe sources of drinking water this study reports the gradual penetration of arsenic into a low arsenic pleistocene aquifer south of hanoi vietnam changes in groundwater flow and the redox state of the aquifer sands induced by pumping are introducing contamination from the high arsenic holocene aquifer contamination so far is limited owing to the absorption of arsenic onto aquifer sands which delays arsenic movement by decades groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast asia exposes an estimated population of over a hundred million people to toxic levels of arsenic1 holocene aquifers are the source of widespread arsenic poisoning across the region2 3 in contrast pleistocene sands deposited in this region more than 12 000 years ago mostly do not host groundwater with high levels of arsenic pleistocene aquifers are increasingly used as a safe source of drinking water4 and it is therefore important to understand under what conditions low levels of arsenic can be maintained here we reconstruct the initial phase of contamination of a pleistocene aquifer near hanoi vietnam we demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a holocene aquifer into a previously uncontaminated pleistocene aquifer we also find that arsenic adsorbs onto the aquifer sands and that there is a 16 20 fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period our findings suggest that arsenic contamination of pleistocene aquifers in south and southeast asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport

representational bias in phytoliths from modern soils of central north america implications for paleovegetation reconstructions abstract understanding localized patterns and community compositions of vegetation in an environment is critical to the reconstruction of climatic and ecological conditions across all spatiotemporal scales one of the most accurate and useful ways to characterize vegetation and therefore to describe the climatic and ecological conditions of a location is through the plant fossil record phytoliths plant silica microfossils are often preserved in the absence of other paleobotanical data and are becoming more widely used for deep time vegetation reconstructions significant work has been done to standardize the analytical methodology of phytolith extraction statistical analysis and interpretation but more detailed investigations are needed to understand how well a given soil assemblage represents the actual aboveground plant biomass for a given ecosystem we present results from paired soil phytolith assemblages and local vegetation assemblages across the central united states including temperate forest grassland and rangeland scrubland ecosystems phytolith assemblages obtained via extractions from soil a horizons were compared to percent cover of species and plant biomass estimates obtained via in situ field observations and aerial estimates of tree cover to analyze differences in the relative abundance of forest woody vegetation vs grasses soil phytolith assemblages from all sites average a 29 bias toward the grass morphotypes as compared to actual aboveground biomass observations and comparisons to percent cover yielded broadly comparable bias figures percent bias estimates do not show significant correlations to most environmental factors temperature precipitation local elevation however an extremely strong correlation p

the origin and spread of grass dominated ecosystems in the late tertiary of north america preliminary results concerning the evolution of hypsodonty abstract sediment samples were collected in northwestern nebraska from the following lithologic units 1 the upper oligocene monroe creek formation the lower miocene harrison formation and upper harrison beds of the arikaree group and 2 the lower miocene runningwater formation and the dawes clay member box butte formation of the ogallala group the samples were processed using heavy liquid extraction and analyzed for siliceous plant microfossils phytoliths all samples yielded well preserved and diverse assemblages dominated by grass phytoliths but included phytoliths produced by woody and herbaceous dicotyledons palms and sedges the abundance of woody dicotyledons and palms decreased up section the grass assemblages consisted mainly of festucoid c3 phytolith morphotypes an index assessing the amount of tree cover d p the ratio of dicotyledon phytoliths to grass phytoliths was employed to interpret the data according to this analysis there was open c3 dominated grassland in the study area between 25 and 17 ma at least 7 ma before supposed adaptations to grasslands in ungulates hypsodonty originate

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

the spread of grass dominated habitats in turkey and surrounding areas during the cenozoic phytolith evidence abstract the arrival of hipparionine horses in the eastern mediterranean region around 11 ma was traditionally thought to mark the simultaneous westward expansion of savanna vegetation across eurasia however recent paleoecological reconstructions based on tooth wear carbon isotopes and functional morphology indicate that grasses played a minor role in late miocene ecosystems of the eastern mediterranean which were more likely dry woodlands or forests the scarcity of grass macrofossils and pollen in miocene floras of europe and asia minor has been used to support this interpretation based on the combined evidence it has therefore been suggested that late miocene ungulate faunal change in the eastern mediterranean signals increased aridity and landscape openness but not necessarily the development of grass dominated habitats to shed new light on the miocene evolution of eastern mediterranean ecosystems we used phytolith assemblages preserved in direct association with faunas as a proxy for paleovegetation structure grassland vs forest we extracted phytoliths and other biogenic silica from sediment samples from well known early to late miocene 20 7 ma faunal localities in greece turkey and iran in addition a middle eocene sample from turkey yielded phytoliths and served as a baseline comparison for vegetation inference phytolith analysis showed that the middle eocene assemblage consists of abundant grass phytoliths grass silica short cells interpreted as deriving from bambusoid grasses as well as diverse forest indicator phytoliths from dicotyledonous angiosperms and palms pointing to the presence of a woodland or forest with abundant bamboos in contrast the miocene assemblages are dominated by diverse silica short cells typical of pooid open habitat grasses forest indicator phytoliths are also present but are rare in the late miocene 9 7 ma assemblages our analysis of the miocene grass community composition is consistent with evidence from stable carbon isotopes from paleosols and ungulate tooth enamel showing that c4 grasses were rare in the mediterranean throughout the miocene these data indicate that relatively open habitats had become common in turkey and surrounding areas by at least the early miocene 20 ma 7 million years before hipparionine horses reached europe and arid conditions ensued as judged by faunal data

chemistry of modern soils as a guide to interpreting precambrian paleosols precambrian paleosols are potentially one of our best sources of information about the composition of the atmosphere early in earth s history to be used this way however an alteration profile must be shown to be an in situ product of vadose processes data from modern soils show that the ratios of less mobile elements such as zr ti and al can be used as a preliminary screen to test for the in situ nature of a profile if the ti zr ratio departs from that of the host rock by more than 40 or the ti al ratio departs by more than 50 for moderate degrees of weathering then a significant allochthonous contribution to the profile is indicated this technique correctly separates in place from allochthonous horizons when applied to independent data from two modern profiles a review of published precambrian profiles identifies two often cited examples denison and pronto as suspect based on these criteria in the interpretation of paleoatmospheres it is also necessary to show that fe is controlled not b

miocene tectonics and climate forcing of biodiversity western united states ungulate and carnivore diversity patterns since 30 ma in different regions of the western united states suggest abrupt increased diversification at 17 17 5 ma followed by decreases ca 11 ma and stasis thereafter although global climate change presumably affects evolution we hypothesize that widespread extensional tectonism in the western u s also helped drive diversity increases ca 17 5 ma through a topographically induced increase in floral and habitat diversity the decreases in diversities ca 11 ma as well as the rapid increase in c4 ecosystems rice worldwide at 7 8 ma may have responded to climate teleconnections and increased seasonality linked to global cooling and growth of orogenic plateaus particularly the tibetan plateau between 13 and 8 ma thus biodiversity complexly responds both to climate and to tectonics

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

arsenic in groundwater of the red river floodplain vietnam controlling geochemical processes and reactive transport modeling abstract the mobilization of arsenic as to the groundwater was studied in a shallow holocene aquifer on the red river flood plain near hanoi vietnam the groundwater chemistry was investigated in a transect of 100 piezometers results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of fe oxides and methanogenesis enhanced pco2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone the concentration of as increases over depth to a concentration of up to 550 g l most as is present as as iii but some as v is always found arsenic correlates well with nh4 relating its release to organic matter decomposition and the source of as appears to be the fe oxides being reduced part of the produced fe ii is apparently reprecipitated as siderite containing less as results from sediment extraction indicate most as to be related to the fe oxide fractions the measured amount of sorbed as is low in agreement speciation calculations for a fe oxide surface suggest as iii to constitute only 3 of the surface sites while the remainder is occupied by carbonate and silica species the evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component a redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter apart from the upper meter a constant degradation rate of 0 15 c mmol l yr could explain the redox zonation throughout the aquifer modeling also indicates that the fe oxide being reduced is of a stable type like goethite or hematite arsenic is contained in the fe oxides and is first released during their dissolution our model further suggests that part of the released as is adsorbed on the surface of the remaining fe oxides and in this way may be retarded

high resolution isotopic record of c4 photosynthesis in a miocene grassland abstract the origin and spread of grasslands is one of the key evolutionary events in the cenozoic which is characterized by long term cooling and drying climatic conditions one way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols paleosols that preserve organic matter in the form of carbonaceous root traces provide direct evidence of the proportion of c3 to c4 biomass that grew in the soil in contrast with pedogenic carbonate 13c values which may also reflect complicating factors including mixing from atmospheric 13c in low productivity ecosystems a high temporal and spatial resolution reconstruction of past ecosystems was derived from thirty five paleosols in a thirty four meter section of the sixmile creek formation at timber hills montana usa that was deposited during the miocene 10 2 8 9 ma ago phytoliths were extracted from paleosol samples to compare vegetation assemblages to inferences based on isotopic compositions with both proxies giving similar results isotopic results from organic matter indicate both a small component of c4 photosynthetic plants locally prior to their regional expansion to dominance in the late miocene through the early pleistocene and large variation in the abundance of c4 plants 0 25 in this ecosystem both laterally and on a 100 kyr timescale in contrast pedogenic carbonate 13c values from this site indicate a high proportion of c4 photosynthesis that is at odds with both phytolith and 13corg results suggesting that the carbonate values are biased by diagenesis or diffusion of atmospheric co2 and that a similar issue may impact previous paleovegetation reconstructions based on pedogenic carbonates quantitative reconstructions of mean annual temperature and mean annual precipitation indicate little local variability through time and that the fluctuations in c4 proportion were not climatically driven instead the variable proportion of c4 photosynthesis is best explained by ecosystem scale variables such as succession fluvial avulsion and fire regime

processes governing arsenic retardation on pleistocene sediments adsorption experiments and model based analysis in many countries of south south east asia reliance on pleistocene aquifers for the supply of low arsenic groundwater has created the risk of inducing migration of high arsenic groundwater from adjacent holocene aquifers adsorption of arsenic onto mineral surfaces of pleistocene sediments is an effective attenuation mechanism however little is known about the sorption under anoxic conditions in particular the behavior of arsenite we report the results of anoxic batch experiments investigating arsenite 1 25 mol l adsorption onto pleistocene sediments under a range of field relevant conditions the sorption of arsenite was non linear and decreased with increasing phosphate concentrations 3 60 mol l while ph range 6 8 had no effect on total arsenic sorption to simulate the sorption experiments we developed surface complexation models of varying complexity the simulated concentrations of arsenite arsenate and phosphate were in good agreement for the isotherm and phosphate experiments while secondary geochemical processes affected the ph experiments for the latter the model based analysis suggests that the formation of solution complexes between organic buffers and mn ii ions promoted the oxidation of arsenite involving naturally occurring mn oxides upscaling the batch experiment model to a reactive transport model for pleistocene aquifers demonstrates strong arsenic retardation and could have useful implications in the management of arsenic free pleistocene aquifers

surface complexation modeling of groundwater arsenic mobility results of a forced gradient experiment in a red river flood plain aquifer vietnam abstract three surface complexation models scms developed for respectively ferrihydrite goethite and sorption data for a pleistocene oxidized aquifer sediment from bangladesh were used to explore the effect of multicomponent adsorption processes on as mobility in a reduced holocene floodplain aquifer along the red river vietnam the scms for ferrihydrite and goethite yielded very different results the ferrihydrite scm favors as iii over as v and has carbonate and silica species as the main competitors for surface sites in contrast the goethite scm has a greater affinity for as v over as iii while po43 and fe ii form the predominant surface species the scm for pleistocene aquifer sediment resembles most the goethite scm but shows more si sorption compiled as iii adsorption data for holocene sediment was also well described by the scm determined for pleistocene aquifer sediment suggesting a comparable as iii affinity of holocene and pleistocene aquifer sediments a forced gradient field experiment was conducted in a bank aquifer adjacent to a tributary channel to the red river and the passage in the aquifer of mixed groundwater containing up to 74 channel water was observed the concentrations of as

evidence of early c 4 grasses habitat heterogeneity and faunal response during the miocene climatic optimum in the mojave region abstract major changes to landscapes climate and mammalian faunas occurred at the regional scale in western north america during the miocene climatic optimum mco between 17 and 14 ma but few studies have looked at how the mco affected basin scale environments paleoenvironmental reconstructions coupled with mammalian fossil assemblages from the crowder and cajon valley formations in the mojave desert california contribute insights into local scale 10s of kilometers responses to warming during the hemingfordian and barstovian north american land mammal ages by integrating lithological descriptions phytolith analyses carbon isotope composition of preserved soil organic matter 13csom and elemental geochemistry of paleosols we provide reconstructions of depositional environments vegetation and precipitation through the mco phytolith and 13csom evidence suggest that paleoenvironments were predominately c3 grasslands with the earliest potential presence of c4 grasses in the region found within the crowder formation at 17 ma based on elemental geochemistry of paleosols mean annual precipitation estimates are 807 182 mm yr 1 at 17 ma in the crowder formation and 740 800 182 mm yr 1 from 16 to 15 ma in the cajon valley formation our multiproxy approach indicates that the crowder and cajon valley basins were stable large scale braided stream and floodplain systems with intermittent paleosol development and heterogeneity in vegetation and moisture conditions across spatial scales overall we find a signal of local paleoenvironmental stability during the mco however a significant drying trend in the crowder formation does correspond with faunal turnover and reduction in the number of taxa present high species diversity within faunal assemblages especially among small mammals and fewer shared species between the crowder and cajon valley assemblages than expected based on bootstrap analysis indicate that peak mammal diversity was accommodated at both local and regional scales during sustained tectonic activity and the mco in western north america

rayleigh distillation and the depth profile of 13c 12c ratios of soil organic carbon from soils of disparate texture in iron range national park far north queensland australia a depth and particle size specific analysis of soil organic carbon soc and its isotopic composition was undertaken to investigate the effects of soil texture or particle size on the depth profile of stable carbon isotopic composition of soc 13csoc in two tropical soils depth specific samples from two soil profiles of markedly different texture coarse grained and fine grained were separated into particle size classes and analyzed for the mass mass concentration of soc c and 13csoc within 1 m of the soil surface 13csoc in the coarse textured soil increases by 1 3 to 1 6 while 13csoc from the fine textured soil increase by as much as 3 8 to 5 5 this increasing depth trend in the coarse textured soil is approximately linear with respect to normalized c while the increase in the fine textured soil follows a logarithmic function with respect to normalized c a model of rayleigh distillation describing isotope fractionation during decomposition of soil organic matter som accounts for the depth profile of 13csoc in the fine textured soil but does not account for the depth profile observed in the coarse textured soil despite their similar climate vegetation and topographic position these results suggest that kinetic fractionation during humification of som leads to preferential accumulation of 13c in association with fine mineral particles or aggregates of fine mineral particles in fine textured soils in contrast the coarse textured soil shows very little applicability of the rayleigh distillation model rather the depth profile of 13csoc in the coarse textured soil can be accounted for by mixing of soil carbon with different isotopic ratios

geochemistry and mineralogy of shallow alluvial aquifers in daudkandi upazila in the meghna flood plain bangladesh the shallow alluvial aquifers of the delta plains and flood plains of bangladesh comprises about 70 of total land area are mostly affected by elevated concentrations of arsenic as in groundwater exposing a population of more than 35 million to as toxicity geochemical studies of shallow alluvial aquifer in the meghna flood plain show that the uppermost yellowish grey sediment is low in as 1 03 mg kg compared to the lower dark grey to black sediment 5 24 mg kg rich in mica and organic matter sequential extraction data show that solid phase as bound to poorly crystalline and amorphous metal fe mn al oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level 3 05 mg kg in the mica rich layers amount of as bound to sulphides and organic matter also peaks in the dark grey to black sediment vertical distributions of major elements determined by x ray fluorescence xrf show that iron fe2o3 aluminum al2o3 and manganese mno follow the general trend of distribution of as in the sediments concentrations of as mn fe hco3 so42 and no3 in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry mineralogical analysis by x ray diffraction xrd and scanning electron microscopy sem fitted with energy dispersive x ray spectrometer eds revealed dominance of crystalline iron oxides and hydroxides like magnetite hematite and goethite in the oxidised yellowish grey sediment amorphous fe oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of fe iii oxyhydroxides which in turn act as sink for as presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer

evolution of grasses and grassland ecosystems the evolution and subsequent ecological expansion of grasses poaceae since the late cretaceous have resulted in the establishment of one of earth s dominant biomes the temperate and tropical grasslands at the expense of forests in the past decades several new approaches have been applied to the fossil record of grasses to elucidate the patterns and processes of this ecosystem transformation the data indicate that the development of grassland ecosystems on most continents was a multistage process involving the paleogene appearance of c3 and c4 open habitat grasses the mid late cenozoic spread of c3 grass dominated habitats and finally the late neogene expansion of c4 grasses at tropical subtropical latitudes the evolution of herbivores adapted to grasslands did not necessarily coincide with the spread of open habitat grasses in addition the timing of these evolutionary and ecological events varied between regions consequently region by region investigations using both direct plant fossil

tertiary history of c4 biomass in the great plains usa we use the carbon isotope composition of paleosols to reconstruct the history of c4 biomass on the great plains from ca 23 to 1 ma the proportion of c4 biomass was uniform and moderate 12 34 throughout the miocene increased between 6 4 and 4 0 ma and reached modern levels by 2 5 ma ecological changes in great plains ungulates preceded the increase in c4 biomass the contrasts in the paleosol and ungulate records may indicate initial development of c3 grasslands after the middle miocene or a greater role for ecological interactions within communities in structuring ungulate faunas contrasts in paleosol records from different continents point to regional rather than global controls on the evolution of c4 grasslands

carbon isotope ratio of cenozoic co2 a comparative evaluation of available geochemical proxies 1 the carbon isotope ratio 13c of plant material is commonly used to reconstruct the relative distribution of c3 and c4 plants in ancient ecosystems however such estimates depend on the 13c of atmospheric co2 13cco2 at the time which likely varied throughout earth history for this study we use benthic and planktonic 13c and 18o records to reconstruct a long term record of cenozoic 13cco2 confidence intervals for 13cco2 values are assigned after careful consideration of equilibrium and non equilibrium isotope effects and processes as well as resolution of the data we find that benthic foraminifera better constrain 13cco2 compared to planktonic foraminiferal records which are influenced by photosymbiotes depth of production seasonal variability and preservation furthermore sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera 13c and 18o values indicate that these factors act to offset one another our reconstruction suggests that cenozoic 13cco2 averaged 6 1 0 6 1 while only 11 2 million of the last 65 5 million years correspond to the pre industrial value of 6 5 with 90 confidence here 13cco2 also displays significant variations throughout the record at times departing from the pre industrial value by more than 2 thus the observed variability in 13cco2 should be considered in isotopic reconstructions of ancient terrestrial plant ecosystems especially during the late and middle miocene times of presumed c4 grassland expansion

multi proxy evidence for tectonic control on the expansion of c4 grasses in northwest argentina the timing of the rise of c4 grasses and the climatic conditions driving their expansion are widely debated with recent studies suggesting that the expansion was more complex than previously thought proxy reconstructions from northwestern argentina record multiple increases and decreases in c4 abundance throughout the late miocene to early pliocene as well as variations in the timing of the spread of c4 grasses into the region here we present a multi proxy paleovegetation reconstruction of the proportions of c3 and c4 vegetation using organic carbon and phytoliths preserved in paleosols at entre rios in the santa maria basin of catamarca argentina between 6 2 and 3 3 ma both corg13 and phytolith assemblages indicate a c3 dominated ecosystem with no evidence for substantial amounts of c4 vegetation from the late miocene through mid pliocene despite their rise to dominance elsewhere during this time these data suggest that by 6 2 ma the santa maria basin floor was at an elevation high enough to create locally cool conditions that allowed c3 vegetation to outcompete c4 grasses phytolith assemblages show that later uplift of the sierra calchaquies and sierra aconquija ranges through the pliocene increased aridity and drove the expansion of c3 grasses at the expense of palms and other forest indicators this multi proxy paleovegetation reconstruction demonstrates that tectonics played an important role in controlling the late miocene expansion c4 grasses by suppressing their spread into northwest argentina

isotopic reconstruction of past continental environments abstract vertebrate fossils and continental sediments provide a rich record of variations in the isotopic composition of surface environments to interpret these records a greater understanding of isotopic sources as well as fractionations associated with animal physiology soil geochemistry and diagenesis has been essential tooth enamel and fish otoliths yield subannual records of surface environments whereas soil minerals may integrate signals over many thousands of years carbon isotope variations in fossil vertebrates and soils record changes in the structure of vegetation and the isotope composition and concentration of atmospheric co2 oxygen isotope variations may be indirectly related to climate through reconstruction of the oxygen isotope composition of meteoric water or directly related to temperature through application of oxygen isotope paleothermometry to soil minerals or otoliths in africa nitrogen isotope variations show promise as a proxy for rainfall abundance though the gen

geochemical study of arsenic release mechanisms in the bengal basin groundwater 1 to investigate arsenic mobility in the bengal basin groundwater we sampled water wells and sediments throughout the region there are strong correlations among high levels of dissolved arsenic and iron ammonia and methane especially in samples from a single site laxmipur no linkage is seen between as and agricultural tracers such as phosphate the association of as and fe occurs because arsenic strongly adsorbs onto feooh particles in river water they flocculate with other fine grained particles at the freshwater saltwater transition zone subsequent bacterially mediated reduction of feooh in the clay releases the adsorbed arsenic weathering of as bearing mica plays a significant role in the as budget the correlated presence of as ch4 and nh4 in water supply wells is the result of diffusion out of organic rich clay into the more permeable zones arsenic is mainly released from recent sediments at 50 m depth deposited in the gbr floodplain as sea level rose throughout the holocene

regional scale variability in the spread of grasslands in the late miocene abstract a new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle late miocene 11 2 9 5 ma of southwestern montana both sources of data indicate an open habitat c3 grassland ecosystem with a small c4 component

vertebrate preservation in fluvial channels abstract two taphonomic modes for attritional vertebrate assemblages in channels are proposed based on the sedimentary context of the vertebrate remains and taphonomic features of the bones themselves the channel lag mode includes bones that are buried with coarse lithologies near the bases of active channels the channel fill mode occurs in fine grained to mixed fills of abandoned channels the extreme for a channel lag assemblage would be a cluster of allochthonous abraded unidentifiable fragments and the extreme for a channel fill assemblage would be a cluster of autochthonous unbraded complete skeletons between these extremes there is a broad spectrum of possible taphonomic histories for bones in channels but distinct channel lag vs channel fill modes can be recognized in fluvial deposits in different tectonic and climatic settings throughout the phanerozoic physical and biological processes that affect the different modes produce different samples of vertebrate paleocommunities with bones in the channel lag mode representing transported remains from a variety of habitats whereas channel fill assemblages are more autochthonous and habitat specific channel facies channel pattern and alluvial architecture are used to develop hypotheses concerning how the taphonomic modes relate to different scales of fluvial processes fluvial systems with numerous abandoned channels provide more sites for preservation of relatively complete fossil vertebrates in channel fills while systems that continually rework sediments by lateral migration preserve more vertebrate remains as channel lags large scale physical controls on channel pattern and fluvial architecture probably have had significant effects on the quality and quantity of the vertebrate record throughout the history of land vertebrates taphonomic modes provide a basis for comparing faunas with similar preservational histories throughout the geologic record and they can help to minimize biases in important paleobiological parameters such as diversity estimates and the timing of appearance and extinction events

cenozoic paleoclimate on land in north america abstract paleotemperature and paleoprecipitation over the past 40 m yr can be inferred from the degree of chemical weathering and depth of carbonate nodules in paleosols of oregon montana and nebraska paleosol records show that late eocene 35 ma middle miocene 16 ma late miocene 7 ma and early pliocene 4 ma warm climatic episodes were also times of a wet climate in oregon montana and nebraska oregon and nebraska were humid during warm wet times but montana was no wetter than subhumid within the rain shadow of intermontane basins global warm wet paleoclimatic spikes steepened rather than flattened geographic gradients of rocky mountain rain shadows long lived mountain barriers created dusty dry basins with sedimentation rates high enough to preserve milankovitch scale 100 41 kyr global paleoclimatic variation in some sequences of paleosols greenhouse warm wet climates indicated by paleosols were also peaks of diversity for north american plants and animals and coincided with advance

mobility of arsenic in the sub surface environment an integrated hydrogeochemical study and sorption model of the sandy aquifer materials groundwater and aquifer materials have been characterized geochemically at a field site located in the chakdaha municipality of west bengal india sorption experiments were also carried out on a sandy aquifer material to understand the mobility of arsenic as in the sub surface environments the result shows that the areas associated with high groundwater as mean 1 8 m is typically associated with low eh mean 129 mv and high fe mean 0 11 mm where fe2 fe oh 3 couple is controlling groundwater redox potential analysis of the aquifer material total concentrations showed the dominance of as range 8 9 22 mg kg 1 fe range 3 0 9 7 as fe2o3 and mn range 0 05 0 18 as mno in the silt clay rich sediments whereas fine medium sand rich sediment contains considerably lower amount of as 8 1 mg kg 1 fe range 1 6 3 9 as fe2o3 and mn range 0 02 0 08 as mno the acid extractable as do not correlate with ascorbate extractable fe oxyhydroxide however fe oxyhydroxide is generally high in the sediments from low groundwater as areas chemical speciation computations indicated fe ii ca ii mg ii and mn ii to be at equilibrium with respect to calcite dolomite and rhodochrosite or slightly over saturated with respect to siderite these carbonate minerals may therefore participate to the as immobilization the measured total organic carbon 1 and groundwater temperature 26 32 c coupled with sorption studies strongly favors microbially mediated fe iii oxyhydroxide reduction as the dominant mechanism for the release of as in the groundwater oscillations of as mn and fe concentrations with depth reflected pco2 oscillations consecutive to microbial respiration intensity

using phytolith assemblages to reconstruct the origin and spread of grass dominated habitats in the great plains of north america during the late eocene to early miocene abstract this paper outlines a general approach for analyzing siliceous microfossils from plants phytoliths in geologic sediments and uses this method to reconstruct the evolution of grasslands in the central great plains during the late tertiary phytolith assemblage analysis is widely employed for reconstructing vegetation types in pleistocene holocene paleoecology and archaeology but no standardized methodology is currently available published phytolith studies are often system specific and tailored after specific modern analogues as a result they vary in extraction technique classification scheme and method of inference for ancient phytolith assemblages whose modern analogues are not well understood a more general analytic approach is necessary this analytic approach includes studying all size fractions 2 250 mm using a classification scheme based on the available literature and a comprehensive modern phytolith reference collection and comparing relative frequencies through time of all diagnostic phytoliths to interpret changes in habitat structure this general method was applied to late eocene to early miocene sediment samples from northwestern nebraska to deduce vegetation history and the results were compared to those obtained using a more specific technique stromberg paleogeogr paleoclimatol paleoecol 177 2002 59 in both studies phytoliths were extracted by means of heavy liquid separation from the following lithostratigraphic units 1 the late eocene chadron formation and the early oligocene brule formation white river group 2 the late oligocene early miocene monroe creek formation the early miocene harrison formation and anderson ranch formation arikaree group and 3 the early miocene runningwater formation and box butte formation ogallala group the two techniques general vs specific produced results that differ markedly data from the general approach indicate the presence of relatively closed habitats in late eocene to early oligocene typified by the presence of phytoliths from bambusoid grasses woody and herbaceous dicotyledons and palms the introduction and spread of modern open habitat grasses such as pooids arundinoids and panicoids occurred between the late oligocene and early miocene this resulted in a savanna or woodland type habitat by the early miocene in contrast analysis using the specific method suggested the presence of open habitats in northwestern nebraska from the late eocene onward the difference in interpretation based on the two approaches is principally due to inclusion of phytoliths 50 mm and the use of an expanded reference collection in the current study general analytic approach these results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record

environmental change in the great plains an isotopic record from fossil horses abstract carbon and oxygen isotope ratios of fossil horse tooth enamel from nebraska and texas show evidence for late neogene environmental changes in the great plains the earliest unambiguous c4 dietary signal among texas equids coincides with the age of the classic late hemphillian age coffee ranch fauna which we suggest is 6 6 ma based on volcanic ash correlations c4 vegetation was present in the diets of a small fraction of late hemphillian equids in nebraska and was thereafter ordinary in the diets of both nebraska and texas equids there is no unequivocal evidence for abundant c4 vegetation in the diets of pre late hemphillian equids and we suggest that the ensuing dietary change reflects c4 biomass expansion in the latest miocene carbon isotope ratios of post hemphillian horses in nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co occur with horse remains indicating that the two proxies carbon isotopes and presence absence of tort

new constraints on using paleosols to reconstruct atmospheric pco2 ecosystem and climatic changes due to anthro po genic emissions of co 2 are of increasing importance to society one way to predict these changes in the future is to study warming events in the geologic past the main control of earth s temperature over geologic time is the concentration of atmospheric co 2 however the precise relationship between co 2 and temperature is not fully understood therefore it is essential to be able to quantify atmospheric co 2 concentrations in the geologic past especially at times of rapid climate change analogous to current atmospheric changes one widely applied proxy relates the carbon isotopic composition of pedogenic carbonates to atmospheric pco 2 but one of the key variables soil respired co 2 s z is not well constrained this study presents a new proxy where soil respired co 2 is related to mean annual precipitation map by the following equation s z 5 67 map 269 9 r 2 0 59 se 681 ppm this proxy validated for modern atmospheric pco 2 levels constrains the primary source of uncertainty in the soil carbonate paleobarometer we apply this proxy to make atmospheric pco 2 reconstructions for examples from the cenozoic mesozoic and paleozoic and calculate co 2 estimates that are in better agreement with estimates from other independent proxies and model results than previous pedogenic carbonate reconstructions the implications of the uncertainties attributed to pedogenic carbonate paleobarometry as well as guidelines for using this method are discussed by making individual measurements rather than assumptions for each isotopic value and combining those analyses with an s z estimate for each paleosol considered using our new proxy described herein we can obtain increased precision for atmospheric pco 2 reconstructions

anion exclusion during transport through the unsaturated zone a 20 year chronological record of the flow of water and anions along 27 m in the unsaturated zone of a phreatic aquifer was reconstructed water was traced according to its tritium content using the difference between the environmental tritium content of rain and irrigation water anions were traced using data about their sequential input to the overlying cultivated field and their concentration in the profile evidences of anion exlcusion were found along a 10 m thick clay loam layer the vertical velocities of water molecules and anions so2 4 and cl were calculated to be 0 7 0 05 and 1 35 0 05m yr respectively the average thickness of the water films and the equivalent distance of exclusion at a 15 gravimetric water content were 25 and 12a respectively these field data fit and support the theoretical relationship between the water film thickness and the relative exclusion concentration proposed by bresler 1973

examining the spatial consistency of palaeosol proxies implications for palaeoclimatic and palaeoenvironmental reconstructions in terrestrial sedimentary basins terrestrial sediments are a significant archive of past conditions and the critical zone in particular is important for understanding past and ongoing interactions between the atmosphere biosphere and lithosphere understanding the details of a palaeolandscape or palaeocatena record can greatly improve palaeoenvironmental descriptions and palaeoclimate reconstructions during crucial time periods in the past this article presents multi proxy data including palaeosol descriptions major element geochemistry magnetic composition and stable isotope geochemistry from an early eocene palaeosol transect which show distinct trends in palaeosol features and geochemistry across the landscape particularly in terms of the differences between a horizon and b horizon samples a horizon geochemical and physical features are highly variable and reflect other not strictly pedogenic controls whereas b horizon composition is extremely consistent given that most palaeoclimatic or palaeoenvironmental proxies based on palaeosols use b horizon features these results highlight the robustness of those proxies at the same time the difference between a horizons and b horizons and the variability within a horizons of the same palaeosol in different geomorphic settings underscores the importance of palaeosol description detailed sampling and a multi proxy approach to produce both precise and accurate reconstructions of past environments and climates using terrestrial sedimentary records

groundwater arsenic concentrations in vietnam controlled by sediment age arsenic contamination of groundwater threatens the health of millions of people in southeast asia measurements in an arsenic contaminated aquifer in vietnam point to sediment age as a key determinant of groundwater arsenic concentrations

pedogenic carbonate stable isotope record of environmental change during the neogene in the southern great plains southwest kansas usa carbon isotopes and the evolution of c4 dominated grasslands fossiliferous strata in the meade basin southwest kansas preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from the clarendonian north american land mammal age nalma 12 0 9 0 ma early late miocene to the early irvingtonian nalma ca 2 5 ca 1 0 ma early pleistocene faunas from these sections document the evolution of the small mammal community of the modern grassland ecosystem of the region and the stable isotope composition of paleosol carbonates provides a means by which the environmental context of the evolution of the modern ecosystem may be documented we used the stable carbon isotope composition 13 c relative to vienna peedee belemnite vpdb of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of c 4 grass abundance in the meade basin paleosol carbonate 13 c values reflect the proportion of c 3 trees shrubs cool climate grasses and c 4 warm climate grasses plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats paleosol carbonate 13 c values record a three phase increase in the abundance of c 4 biomass during the neogene in the meade basin late miocene sections have mean 13 c values of 7 6 0 90 clarendonian and 6 5 0 31 hemphillian nalma 9 0 4 9 ma consistent with 17 and 26 c 4 biomass respectively miocene 13 c values from meade are statistically identical to published 13 c values for miocene paleosol carbonates elsewhere in the southern great plains supporting the widespread presence of 20 c 4 biomass on average in the region throughout the miocene the abundance of c 4 biomass increased between the end of the hemphillian section and the beginning of the early blancan nalma 5 0 3 0 ma early and middle blancan 3 0 2 5 ma carbonates have statistically identical 13 c values 4 9 0 90 and 5 0 1 10 respectively suggesting a stable ecosystem during the early pliocene although high 13 c variability in densely sampled intervals suggests a high degree of landscape scale variation in c 4 abundance the final phase geochronologically controlled by two well characterized ashes huckleberry ridge 2 10 ma cerro toledo b 1 47 1 23 ma and magnetostratigraphy is a trend to higher 13 c values from the late blancan to early irvingtonian ca 2 5 ca 1 0 ma from 4 at the base of the section to 1 at the top corresponding to an increase from almost 50 to 65 c 4 biomass the abundance of c 4 biomass first reaches modern levels for the region 78 10 9 around the level of the cerro toledo b ash indicating that a modern like grassland ecosystem first appeared in the region ca 1 3 ma although 13 c values do not remain consistently high through the rest of the section

arsenic migration to deep groundwater in bangladesh influenced by adsorption and water demand elevated concentrations of arsenic are common in shallow groundwater in many parts of south and southeast asia field experiments and model simulations suggest that adsorption of arsenic to sediments could help to limit contamination of deep groundwater

terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early eocene climatic optimum major changes in climate and ecology occurred during the early eocene climatic optimum sometime between 52 and 50 ma recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms and that traditional causal mechanisms may not adequately explain such differences we applied high resolution paleopedology geochemical analysis and phytolith biostratigraphy techniques to paleosol suites within the well described wind river formation of western wyoming usa this multiproxy record indicates a short 1 m y peak period of carbon isotopic enrichment up to 2 higher and elevated pco2 high temperatures up to 8 c higher increased precipitation up to 500 mm yr 1 higher and shifts in fl oral composition up to 10 terrestrial climatic and ecological changes of this kind during the early eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pco2 but a transient peak interval suggests that the cause of high atmospheric pco2 during the early eocene was likely not increased volcanism or decreased silicate weathering which operate on longer timescales instead terrestrial records from across western north america agree that early eocene climatic optimum changes may have been caused by other sources such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins the climatic and environmental changes exhibited by this and other north american terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events

new weathering index improves paleorainfall estimates from vertisols vertisols are clayey shrink swell soils that are widely recognized in the rock record thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations we assessed the weathering performance of the four major base forming oxides cao mgo na 2 o k 2 o along a modern vertisol climosequence spanning a strong precipitation gradient and discovered that the concentration of bulk soil cao and mgo yields the strongest correlation to mean annual precipitation map based on this fi nding we introduce the calmag weathering index defi ned as al 2 o 3 al 2 o 3 cao mgo 100 which improves rainfall estimates for vertisols relative to the wellestablished cia k chemical index of alteration minus potassium weathering index rather than documenting the hydrolysis of weatherable minerals common in many other soil orders in vertisols calmag principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate detrital clay and exchangeable ca 2 and mg 2 application to two mesozoic paleosols reveals that in drier climates cia k yields higher map estimates than calmag but that the reverse is true in wetter climates this work improves paleorainfall estimates from vertisols and suggests that a family of weathering indexes is needed for different paleosol types

sedimentogenesis and hydrobiogeochemistry of high arsenic late pleistocene holocene aquifer systems geogenic enrichment of arsenic as in groundwater has been a topic of worldwide concern over the past several decades due to the severe health threat to an estimated over one hundred million people despite the genetic diversity and the spatial variability of as concentrations the most common geogenic as enriched groundwater occurs in flat low lying river floodplains and fluvial lacustrine plains within river systems draining the himalaya tectonic movement sedimentological processes and palaeoclimatic optima after the last glacial maximum have created favorable conditions for the formation of high as aquifer systems mainly within the late pleistocene holocene deposits the co deposition of as bearing fe oxides and reactive organic matter within sediments of deltas floodplains or fluvial lacustrine plains forms the premise for as release under reducing geochemical environments however depending on the reactivity of organic matter and as bearing fe oxides as well as the availability of aqueous sulfate geochemical mechanisms controlling as enrichment in the groundwater differ between the coastal floodplains of south and southeast asia and the fluvial lacustrine plains of north and central china the indigenous geomicrobes thriving to adapt to specific aquifer environments may mediate fe s as biogeochemical cycling responsible for as speciation transformation dissolution desorption and hence mobilization hydrologic processes regulated by the local and regional groundwater flow regimes control as transport as well as its spatial distribution special attention must be paid to anthropogenic perturbation from intensive groundwater pumping and land use changes that affect the aquifer hydrological conditions and the biogeochemical processes thus altering groundwater as levels to decipher the patterns of as variation within the aquifers an integrated approach characterizing the major sedimentological hydrobiogeochemical and anthropogenic processes influencing as release and transport needs to be applied to ensure sustainable water supply for domestic and agricultural needs

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

arsenic in groundwater testing pollution mechanisms for sedimentary aquifers in bangladesh in the deltaic plain of the ganges meghna brahmaputra rivers arsenic concentrations in groundwater commonly exceed regulatory limits 50 m gl 21 because feooh is microbially reduced and releases its sorbed load of arsenic to groundwater neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution the most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat concentrations of ammonium up to 23 mg l 21 come from microbial fermentation of buried peat and organic waste in latrines concentrations of phosphorus of up to 5 mg l 21 come from the release of sorbed phosphorus when feooh is reductively dissolved and from degradation of peat and organic waste from latrines calcium and barium in groundwater come from dissolution of detrital and possibly pedogenic carbonate while magnesium is supplied by both carbonate dissolution and weathering of mica the 87 sr 86 sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall 0 711 6 0 001 and detrital carbonate 0 735

adsorption and desorption of arsenic to aquifer sediment on the red river floodplain at nam du vietnam abstract the adsorption of arsenic onto aquifer sediment from the red river floodplain vietnam was determined in a series of batch experiments due to water supply pumping river water infiltrates into the aquifer at the field site and has leached the uppermost aquifer sediments the leached sediments remain anoxic but contain little reactive arsenic and iron and are used in our experiments the adsorption and desorption experiments were carried out by addition or removal of arsenic from the aqueous phase in sediment suspensions under strictly anoxic conditions also the effects of hco3 fe ii po4 and si on arsenic adsorption were explored the results show much stronger adsorption of as v as compared to as iii full reversibility for as iii adsorption and less so for as v the presence or absence of hco3 did not influence arsenic adsorption fe ii enhanced as v sorption but did not influence the adsorption of as iii in any way during simultaneous adsorption of as iii and fe ii as iii was found to be fully desorbable while fe ii was completely irreversibly adsorbed and clearly the two sorption processes are uncoupled phosphate was the only solute that significantly could displace as iii from the sediment surface compiling literature data on arsenic adsorption to aquifer sediment in vietnam and bangladesh revealed as iii isotherms to be almost identical regardless of the nature of the sediment or the site of sampling in contrast there was a large variation in as v adsorption isotherms between studies a tentative conclusion is that as iii and as v are not adsorbing onto the same sediment surface sites the adsorption behavior of arsenic onto aquifer sediments and synthetic fe oxides is compared particularly the much stronger adsorption of as v than of as iii onto red river as well as on most bangladesh aquifer sediments indicates that the perception that arsenic phosphate and other species compete for the same surface sites of iron oxides in sediments with properties similar to those of for example a synthetic goethite probably is not correct a simple two component langmuir adsorption model was constructed to quantitatively describe the reactive transport of as iii and po4 in the aquifer

mobility of arsenic in a bangladesh aquifer inferences from geochemical profiles leaching data and mineralogical characterization aquifer geochemistry was characterized at a field site in the munshiganj district of bangladesh where the groundwater is severely contaminated by as vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer depth 150 m below a thick confining clay 119 to 150 m a sandy upper aquifer 3 5 to 119 m above this confining layer and a surficial clay layer 3 5 m in the deep aquifer and near the top of the upper aquifer aqueous as levels are low 10 g l but aqueous as approaches a maximum of 640 g l at a depth of 30 to 40 m and falls to 58 g l near the base 107 m of the upper aquifer in contrast solid phase as concentrations are uniformly low rarely exceeding 2 g g in the two sandy aquifers and never exceeding 10 g g in the clay layers solid phase as is also similarly distributed among a variety of reservoirs in the deep and upper aquifer including adsorbed as as coprecipitated in solids leachable by mild acids and reductants and as incorporated in silicates and other more recalcitrant phases one notable difference among depths is that sorbed as loads considered with respect to solid phase fe extractable with 1 n hcl 0 2 m oxalic acid and a 0 5 m ti iii citrate edta solution appear to be at capacity at depths where aqueous as is highest this suggests that sorption limitations may in part explain the aqueous as depth profile at this site competition for sorption sites by silicate phosphate and carbonate oxyanions appear to sustain elevated aqueous as levels in the upper aquifer furthermore geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of fe iii oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved as levels in the upper aquifer microprobe data indicate substantial spatial comapping between as and fe in both the upper and deep aquifer sediments and microscopic observations reveal ubiquitous fe coatings on most solid phases including quartz feldspars and aluminosilicates extraction results and xrd analysis of density magnetic separates suggest that these coatings may comprise predominantly fe ii and mixed valence fe solids although the presence of fe iii oxyhydroxides can not be ruled out these data suggest as release may continue to be linked to dissolution processes targeting fe or fe rich phases in these aquifers copyright 2004 elsevier ltd

mobilization of arsenic and iron from red river floodplain sediments vietnam abstract sediments from the red river and from an adjacent floodplain aquifer were investigated with respect to the speciation of fe and as in the solid phase to trace the diagenetic changes in the river sediment upon burial into young aquifers and the related mechanisms of arsenic release to the groundwater goethite with subordinate amounts of hematite were using mossbauer spectroscopy identified as the iron oxide minerals present in both types of sediment the release kinetics of fe as mn and po 4 from the sediment were investigated in leaching experiments with hcl and 10 mm ascorbic acid both at ph 3 from the river sediments most of the fe and as was mobilized by reductive dissolution with ascorbic acid while hcl released very little fe and as this suggests as to be associated with an fe oxide phase for oxidized aquifer sediment most fe was mobilized by ascorbic acid but here not much as was released however the reduced aquifer sediments contained a large pool of fe ii and as that is readily leached by hcl probably derived from an unidentified authigenic fe ii containing mineral which incorporates as as well extraction with ascorbic acid indicates that the river sediments contain both as v and as iii while the reduced aquifer sediment almost exclusively releases as iii the difference in the amount of fe ii leached from river and oxidized aquifer sediments by ascorbic acid and hcl was attributed to reductive dissolution of fe iii the reactivity of this pool of fe iii was quantified by a rate law and compared to that of synthetic iron oxides in the river mud fe iii had a reactivity close to that of ferrihydrite while the river sand and oxidized aquifer sediment exhibited a reactivity ranging from lepidocrocite or poorly crystalline goethite to hematite mineralogy by itself appears to be a poor predictor of the iron oxide reactivity in natural samples using the reactivity of synthetic fe oxides as a reference sediments were incubated both unamended and with acetate added and monitored for up to 2 months the river mud showed the fastest release of both fe and as while the effect of acetate addition was minor this suggests that the presence of reactive organic carbon is not rate limiting in the case of the river and aquifer sediments the release of fe and as was always stimulated by acetate addition and here reactive organic carbon was clearly the rate limiting factor the reduced aquifer sediment apparently can sustain slower but prolonged microbially driven release of as the highly reactive pools of fe iii and as in the river mud could be due to reoxidation of as and fe contained in the reducing groundwater from the floodplain aquifers that are discharging into the river deposition of the suspended mud on the floodplain during high river stages is proposed to be a major flux of as onto the floodplain and into the underlying aquifers

stable isotope records of hydrologic change and paleotemperature from smectite in cenozoic western north america abstract the oxygen and hydrogen isotopic composition of soil water 18ow and dw hereafter reflect the history of water through processes such as source evaporation precipitation and vapor recycling temperature humidity evaporation and post condensation processes can affect 18ow and dw as such isotope proxy records are often limited in their ability to constrain paleoclimate paleoecology or paleoelevation without independently corroborating data smectite preserves both the hydrogen and oxygen isotope signature of parent water and therefore provides critical insight into meteoric water line relationships and paleotemperature here we use in situ pedogenic smectite 18o and d records to characterize the evolution of the hydrologic cycle in cenozoic western north america we incorporate 192 samples 119 of which are previously unpublished from 11 cenozoic basins representing a range of environments in the basin and range rocky mountains and great plains our results indicate that the processes controlling smectite isotopic compositions vary both regionally and temporally in some localities such as oligocene to pleistocene western nebraska change in temperature is the primary control on smectite isotopic composition in other basins such as in miocene trapper creek id isotope values lie along the meteoric water line suggesting change in meteoric water composition is responsible for the variation in most basins especially those in the neogene basin and range smectite line slope suggests either evaporation of previously meteoric water or a combination of change in paleotemperature and meteoric water composition smectite geothermometry suggests mineral formation temperatures of 30 40 c in the middle miocene in the rocky mountains great plains and basin and range and a decrease of 10 15 c since the middle miocene climatic optimum consistent with clumped isotope and paleofloral temperature estimates

using fossil leaves as paleoprecipitation indicators an eocene example estimates of past precipitation are of broad interest for many areas of inquiry including reconstructions of past environments and topography climate modeling and ocean circulation studies the shapes and sizes of living leaves are highly sensitive to moisture conditions and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators most quantitative estimates of paleoprecipitation have been based on a multivariate data set of morphological leaf characters measured from samples of living vegetation tied to climate stations however when tested on extant forests this method has consistently overestimated precipitation we present a simpler approach that uses only the mean leaf area of a vegetation sample as a predictor variable but incorporates a broad range of annual precipitation and geographic coverage into the predictor set the significant relationship that results in addition to having value for paleoclimatic reconstruction refines understanding of the long observed positive relationship between leaf area and precipitation seven precipitation estimates for the eocene of the western united states are revised as lower than previously published but remain far wetter than the same areas today abundant moisture may have been an important factor in maintaining warm frost free conditions in the eocene because of the major role of water vapor in retaining and transporting atmospheric heat

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central new mexico usa in order to better calibrate pedogenic carbonate as a proxy for past environments we compared the stable isotopic composition of soil co 2 soil water and pedogenic carbonate in young soils from central new mexico usa seasonal changes in the 13 c value of soil co 2 the 18 o value of soil water and the soil temperature were monitored to establish the timing of isotopic equilibrium with the carbonate calcite solubility was calculated from measured temperatures and co 2 concentrations in the soil this approach allowed us to determine the conditions associated with pedogenic carbonate formation carbon isotope equilibrium oxygen isotope equilibrium and minimum calcite solubility all occurred simultaneously during warm dry conditions in may 2008 when soil co 2 concentrations were low it is therefore concluded that pedogenic carbonate forms during warm dry periods and does not record mean growing season conditions as typically assumed the seasonal bias in pedogenic carbonate formation may explain the occurrence of pedogenic carbonate in monsoon climates and its absence in regions where annual precipitation is more uniformly distributed the implications of the seasonal bias for stable isotope based paleoenvironmental reconstructions are that paleoelevations may have been previously over or underestimated paleoatmospheric co 2 concentrations likely have been signifi cantly overestimated and pedogenic carbonate provides a c 4 biased record of paleovegetation especially in dry soils