did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

chemical weathering river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on luzon island the philippines abstract we investigated rates of chemical weathering of volcanic and ophiolitic rocks on luzon island the philippines luzon has a tropical climate and is volcanically and tectonically very active all factors that should enhance chemical weathering seventy five rivers and streams 10 draining ophiolites 65 draining volcanic bedrock and two volcanic hot springs were sampled and analyzed for major elements alkalinity and 87sr 86sr cationic fluxes from the volcanic basins are dominated by ca2 and mg2 and dissolved silica concentrations are high 500 1900 m silica concentrations in streams draining ophiolites are lower 400 900 m and the cationic charge is mostly mg2 the areally weighted average co2 export flux from our study area is 3 89 0 21 106 mol km2 yr or 5 99 0 64 106 mol km2 yr from ophiolites and 3 58 0 23 106 mol km2 yr from volcanic areas uncertainty given as 1 standard error s e this is 6 10 times higher than the current best estimate of areally averaged global co2 export by basalt chemical weathering and 2 3 times higher than the current best estimate of co2 export by basalt chemical weathering in the tropics extrapolating our findings to all tropical arcs we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments which amount to 1 of the global exorheic drainage area chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long term carbon cycle

effects of temperature on silicate weathering solute fluxes and chemical weathering in a temperate rain forest watershed jamieson creek british columbia article i nfo article history accepted 7 september 2009 chemical weathering of silicate minerals has long been known as a sink for atmospheric co2 and feedbacks between weathering and climate are believed to affect global climate while warmer temperatures are believed to increase rates of weathering weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice in this study chemical weathering of silicate minerals is investigated in a small temperate watershed the jamieson creek watershed is covered by mature coniferous forest and receives high annual precipitation 4000 mm mostly in the form of rainfall and is underlain by quartz diorite bedrock and glacial till analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende however a watershed scale solute mass balance indicates high relative fluxes of k and ca indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients when compared to the rio icacos basin in puerto rico a pristine tropical watershed with similar annual precipitation and bedrock but with dissimilar regolith properties fluxes of weathering products in stream discharge from the warmer site are 1 8 to 16 2 fold higher respectively and regolith profile averaged plagioclase weathering rates are 3 8 to 9 0 fold higher this suggests that the arrhenius effect which predicts a 3 5 to 9 fold increase in the dissolution rate of plagioclase as temperature is increased from 3 4 to 22 c may explain the greater weathering fluxes and rates at the rio icacos site however more modest differences in k and ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at jamieson creek our findings suggest that under conditions of high rainfall and favorable topography weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area

phanerozoic atmospheric co2 change evaluating geochemical and paleobiological approaches abstract the theory and use of geochemical modeling of the long term carbon cycle and four paleo pco 2 proxies are reviewed and discussed in order to discern the best applications for each method geochemical models provide pco 2 predictions for the entire phanerozoic but most existing models present 5 10 m y means and so often do not resolve short term excursions error estimates based on sensitivity analyses range from 75 200 ppmv for the tertiary to as much as 3000 ppmv during the early paleozoic the 13 c of pedogenic carbonates provide the best proxy based pco 2 estimates for the pre tertiary with error estimates ranging from 500 1000 ppmv pre devonian estimates should be treated cautiously error estimates for tertiary reconstructions via this proxy are higher than other proxies and models 400 500 ppmv and should not be solely relied upon we also show the importance of measuring the 13 c of coexisting organic matter instead of inferring its value from marine carbonates the 13 c of the organic remains of phytoplankton from sediment cores provide high temporal resolution up to 10 3 10 4 year high precision 25 100 ppmv for the tertiary to 150 200 ppmv for the cretaceous pco 2 estimates that can be near continuous for most of the tertiary its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short term excursions this method however must correct for changes in growth rate and oxygen level at elevated pco 2 750 1250 ppmv this proxy loses its sensitivity and is not useful the stomatal density and stomatal index of land plants also provide high temporal resolution 2 year high precision 10 40 ppmv for the tertiary and possibly cretaceous pco 2 estimates and so also is ideal for discerning short term excursions unfortunately this proxy also loses sensitivity at some level of pco 2 above 350 ppmv which currently is largely undetermined our analysis of the recently developed 11 b technique shows that it currently is not yet well constrained most importantly it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time in addition it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur a fifth co 2 proxy based on the redox chemistry of marine cerium has several fundamental flaws and is not discussed in detail here

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

physical and chemical weathering rates and co2 consumption in a tropical lateritic environment the upper niger basin the chemical composition of niger river water measured bimonthly at bamako mali during the period 1990 1992 provides an estimate of present weathering rates in the upper niger basin the dominant weathering process is kaolinite formation monosiallitization however seasonal variations promote gibbsite formation in the rainy season september and smectite development in the dry season may the results show that lateritic profiles continue to develop even during very dry episodes the rate of profile development calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface is about 1 3 to 3 7 m myr a comparison between 43 river basins of the world shows that for similar runoff the co2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones

relationship between weathering mountain uplift and climate during the cenozoic as deduced from the global carbon strontium cycle model abstract a global carbon strontium coupled model over the cenozoic is presented the carbon cycle includes improvements on the uplift parameter in the himalayan and tibetan regions and the hydrothermal flux at back arc basins from the carbon cycle model of kashiwagi and shikazono kashiwagi h shikazono n 2003 climate change during the cenozoic inferred from global carbon cycle model including igneous and hydrothermal activities palaeogeography palaeoclimatology palaeoecology 199 167 185 the strontium cycle incorporates groundwater flux basalt weathering and himalayan weathering the model result indicates that the paleocene to the late eocene is characterized by decreasing strontium weathering and gradually increasing isotopic value 87sr 86sr which is a result of the decrease in radiogenic silicate weathering and less radiogenic carbonate weathering caused by decline of the atmospheric temperature since then the 87sr 86sr continuously increases towards the present the increase in the sr isotopic value from 40 ma to 35 ma is attributed to appearance of hydrothermal calcite in the himalayan region the increasing patterns of the 87sr 86sr from that time to the early miocene roughly correspond to the continental glaciations in high latitudes such as those at the eocene oligocene boundary in the middle miocene and at the oligocene miocene boundary radiogenic weathering accelerated by physical erosion due to glaciations and the subsequent deglaciations as suggested by previous studies might be related however there are time lags between the 87sr 86sr and 18o signals which might have resulted from long residence time of sr in the ocean reactivity of minerals under such low temperature and other climatic conditions is still unclear more supportive works to assure a favourable environment for sufficient weathering and to constrain the timings and amplitudes of the glacial events during these periods would be necessary the presented model shows the increase in the sr weathering flux since the late miocene which would have induced the elevation of seawater 87sr 86sr whereas the evolution of the glaciations partially might have contributed to that some disagreement between other studies and the model is confirmed the evaluation of the palaeo sealevel variation and carbonate compensation depth might produce the uncertainties in the estimate of weathering flux moreover the basaltic province through which several major rivers drain and discrimination of these rivers and the ganges brahmaputra indus might be important for the strontium cycle because these rivers are likely to be influenced by the radiogenic upstream area and non radiogenic downstream province whose 87sr 86sr would have changed considerably during the cenozoic despite these uncertainties it is clear that we can not attribute the increase in the seawater 87sr 86sr values during the cenozoic entirely to the influence of the himalayan uplift and the seawater 87sr 86sr variation does not serve as a proxy for silicate weathering rate and atmospheric co2 consumption rate of atmospheric co2

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

the influence of aspect on the biological weathering of granites observations from the kunlun mountains china abstract bedrock and boulder weathering in the higher elevation permafrost areas of the qinghai tibetan plateau have usually been ascribed to frost action for short periods during two summers an attempt was made to better elucidate weathering processes in this region a combination of visual observation coupled with rudimentary data regarding removable weathered material from the four cardinal aspects of granite boulders at elevations of 4600 5000 m in the kunlun mountains qinghai tibetan plateau showed that there was a distinct aspect influence south west north east on weathering it was also observed that there were aspect constrained variations in chasmoendolithic communities and for lichen distribution to further aid understanding temperature data for the four aspects were collected for several weeks during the summers of 2 years with data recording at 1 min intervals using ultraresponsive 0 05 s high precision 0 1 c ultrasmall 0 15 mm diameter thermocouples humidity data were also collected at the rock face based on these data an attempt is made to evaluate the nature of granite weathering in this region to better understand what the impact of the lichens endolithic communities was and how this varies use is made of xrd and sem analyses on samples from each aspect in addition iron rich patinas were found on a number of the boulders and these too were analyzed in an attempt to see their relationship if any to the biological action

shield effect on continental weathering implication for climatic evolution of the earth at the geological timescale abstract a model sensitivity study is performed in order to explore the impact of the development of thick weathering profiles in the equatorial area on the geological evolution of climate long term climatic evolution is driven by the balance existing between solid earth degassing and continental silicate weathering the co 2 consumption through continental silicate weathering is here calculated as a function of climate based on the correlation existing with mean annual continental runoff for large drainage basins these correlations appear to be dependent on the climatic regime under mid latitude temperate climate the silicate weathering is 5 times more sensitive to continental runoff than in the warm humid equatorial belt these correlations emphasize the role of the thick weathering profiles existing in equatorial area shielding the bedrock from further weathering we use this set of relationships between runoff and silicate weathering as a function of the geoclimatic zone to explore the impact of this shield effect on the mesozoic evolution of climate using the geoclim model compared to previous work we suggest that atmospheric co 2 was generally underestimated in the past evidencing a complex interplay between the paleogeographic setting and the shield effect particularly calculated atmospheric co 2 during the early middle jurassic rises up to 1600 ppmv while it was limited to 700 ppmv when shield effect was neglected corresponding to a global warming of more than 4 c this study emphasizes the need for the building up of numerical models describing the growth of the weathering profiles that can be coupled to vegetation and climate models to investigate biogeochemical cycle and climate evolution even at the geological timescale

a theoretical model coupling chemical weathering rates with denudation rates uplift of the himalayas has been proposed to have locally accelerated chemical weathering thus leading to enhanced co2 sequestration and global cooling this hypothesis assumes that rapid erosion exposes fresh highly reactive minerals at earth s surface empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results where the nature of the relationship is dependent on the weathering regime of the sampled landscapes we derive a quantitative model that defines this relationship across the range of weathering regimes from supply limited to kinetically limited conditions the model matches trends in field data collected by others and reconciles apparently conflicting results the model also demonstrates that as erosion rates increase potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith we conclude that the relationship between weathering and erosion is one of diminishing returns in which increases in erosion rate lead to progressively smaller increases in weathering rate indeed at the highest erosion rates weathering rates may decline the ability therefore of accelerated uplift and erosion to stimulate greater co2 sequestration may be significant in landscapes eroding at rates of 10 102 t km 2 yr 1 however where erosion rates are greater than 102 t km 2 yr 1 increases in denudation may not be matched by increases in chemical weathering finally our results suggest that watersheds with regolith thicknesses of 0 5 m will yield the greatest solute fluxes

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

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

biotic enhancement of weathering atmospheric oxygen and carbon dioxide in the neoproterozoic 1 it has been suggested that biological colonization of the land surface began in the neoproterozoic 1000 544 million years ago ma we hypothesize that this colonization involved selective weathering of p from rocks as well as an amplification of overall weathering rates we show that two recent models despite differences in the feedback mechanisms represented predict that an increase in the weathering flux of p to the ocean would have caused a rise in atmospheric o2 in the neoproterozoic this in turn may have provided a necessary condition for the evolution of animals with hard skeletons seen in the cambrian explosion increased weathering of silicate rocks would also have caused a decline in atmospheric co2 which could have been a causal factor in the neoproterozoic glaciations

enhanced carbonate and silicate weathering accelerates recovery from fossil fuel co2 perturbations 1 increasing atmospheric co 2 and surface temperatures should increase carbonate and silicate weathering rates directly via warming and indirectly via the co 2 fertilization effect enhancing plant productivity enhanced weathering should in turn increase alkalinity input to the ocean and accelerate long term co 2 uptake we added silicate and carbonate weathering and carbonate sediments to an existing global carbon cycle and surface temperature model and subjected it to a range of long term fossil fuel emissions scenarios spanning 1100 15 000 gtc in total emissions of 7350 gtc dissolve all carbonate sediments and enhanced carbonate and silicate weathering accelerate subsequent co 2 removal from the atmosphere by up to a factor of 4 for 1100 4000 gtc emissions enhanced weathering accelerates co 2 removal by a factor of 1 5 2 5 however it takes 1 myr for silicate weathering to stabilize atmospheric co 2 if land use tends to suppress vegetation and weathering rates on this timescale then co 2 will stabilize above preindustrial levels

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

rock weathering by the lichenlecidea auriculata in an arctic alpine environment scanning electron microscopy reveals that lecidea auriculata penetrates rock surfaces detaching incorporating and expelling flakes of rock measurements on an age sequence of moraine ridges illustrate changes in the extent and intensity of weathering over time a minimum rate of surface lowering 0 0012 mm a 1 is 25 to 50 times faster than weathering due to other processes in the same area the spatial pattern of weathering is influenced by the narrow ecological range of the species surface rocks on dry exposed sites are probably weathering much more rapidly than those in adjacent damp hollows and beneath snowpatches

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

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

two stepping into the icehouse east antarctic weathering during progressive ice sheet expansion at the eocene oligocene transition in conjunction with increasing benthic foraminiferal 18 o values at the eocene oligocene transition eot ca 34 ma coarse grained ice rafted debris ird 425 m appears abruptly alongside fossil fish teeth with continentally derived neodymium nd isotope ratios e nd in kerguelen plateau southern ocean sediments increased antarctic weathering flux as inferred from two steps to less radiogenic e nd values coincides with two steps in benthic foraminiferal 18 o values these results indicate that two distinct surges of weathering were generated by east antarctic ice growth during the eot weathering by ice sheets during a precursor glaciation at 33 9 ma did not produce significant ird accumulation during the first e nd shift glacial weathering was sustained during a terrace interval between the two steps probably by small high elevation ice sheets a large increase in weathering signals the rapid coalescence of small ice sheets into an ice sheet of continental proportions ca 33 7 ma rapid ice sheet expansion resulted in a suppression of weathering due to less exposed area and colder conditions parallel changes in antarctic weathering flux and deep sea carbonate accumulation suggest that ice sheet expansion during the eot had a direct impact on the global carbon cycle possible mechanisms include associated changes in silicate weathering on the east antarctic craton and enhanced fertilization of southern ocean waters both of which warrant further investigation

continental drift runoff and weathering feedbacks implications from climate model experiments changes in atmospheric carbon dioxide have been proposed as a major regulator of climate during the last 570 million years continental weathering and its variation over time are hypothesized to be important for controlling atmospheric carbon dioxide continental weathering is altered by changes in total runoff as well as changes in the size and elevation of the land masses to be weathered when paleogeographic information for the phanerozoic 570 m y ago to present is used in a global climate model the model exhibits substantial variations in precipitation evaporation and runoff even with dramatically different land ocean distributions an increase in global surface temperature leads to an increase in global precipitation but not always an increase in global runoff during the early phanerozoic 514 342 ma when the continents are relatively small and widely distributed runoff depends on continental evaporation and temperature as the continents migrate into subtropical latitudes global runoff decreases as global land temperatures and evaporation increase as the continents shift to higher latitudes global runoff increases as global land temperatures and evaporation decrease during the middle to late phanerozoic 306 ma present when large continental land masses predominate runoff depends on continental precipitation experiments with increased atmospheric co 2 for the middle ordovician 458 ma when the paleocontinent of gondwana was centered at subtropical latitudes and the early silurian 425 ma when gondwana had shifted to middle and high latitudes also point to a correlation between land mass location and runoff global runoff increases 15 with increased co 2 at 425 ma but remains unchanged for 458 ma even though global mean temperature and precipitation increase comparable amounts for the two time periods these results imply that weathering feedback between temperature and runoff may be dependent on land ocean configuration

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

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

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

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

early earth snowballs limited by weathering a series of extreme cooling episodes starting 750 million years ago could have repeatedly turned the planet into an ice covered snowball carbon cycle modelling suggests that the timing of the glaciations can be explained by chemical weathering rates

varve formation during the past three centuries in three large proglacial lakes in south central alaska the sediments stored in the large deep proglacial lakes of south central alaska are largely unstudied we analyzed sediments in 20 cores up to 160 cm long from eklutna kenai and skilak lakes using a combination of repeated lamination counting radionuclide dating event stratigraphy and tephrochronology we show that the characteristically rhythmic layers were deposited annually most of these glacial varves consist of one coarse grained base and a fine grained top but varves composed of multiple coarse grained turbidite pulses are common too they are likely related to successive episodes of high sediment discharge during flooding and they become more frequent in all three lakes along with increased sedimentation rates during the nineteenth century late phase of the little ice age these flood turbidites were generated by rain events and intense melting of snow and ice other mega turbidites are a result of earthquake triggered slope collapses e g a d 1964 some event layers are present in all three lakes in addition the annual time series of varve thickness normalized annual sedimentation rate are significantly correlated among the three lakes p 0 27 p 0 001 differences between the varve thickness records can be attributed partly to the dam construction at eklutna lake and outbursts from an ice dammed lake at skilak lake geomorphologic differences among the catchments result in further differences in sedimentation patterns in the three lakes

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

silicate weathering and co2 consumption deduced from the seven chinese rivers originating in the qinghai tibet plateau abstract we present river chemical data for the seven chinese rivers the jinsha jiang yalong jiang min jiang dadu he lancang jiang nu jiang and huang he originating in the qinghai tibet plateau our sampling locations are near sites where the seven rivers flow down the plateau water samples were collected in both high water periods summer and low water periods winter our study shows that most of the ca mg and hco 3 in the seven rivers are derived from the carbonate weathering and only small fractions 10 of the cations are derived from silicate weathering the chemical erosion rates of silicate and carbonate range from 1 1 mm ka 1 to 3 4 mm ka 1 and from 7 5 mm ka 1 to 29 9 mm ka 1 respectively the long term co 2 consumption by silicate weathering in the seven chinese river basins ranges from 0 7 10 5 mol km 2 a 1 to 3 7 10 5 mol km 2 a 1 based on our analysis of chemical data from the seven chinese rivers and the previously published data of the ganges brahmaputra and indus the main ten rivers originating in the himalaya and qinghai tibet plateau consume 328 10 9 mol a 1 of atmospheric co 2 this is only 3 8 of the co 2 consumption derived from global silicate weathering 8700 10 9 mol a 1 indicating that the chemical weathering of the himalaya and qinghai tibet plateau makes a very small contribution to the reduction of the global atmospheric co 2 concentration

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

mechanics of graben evolution in canyonlands national park utah results of numerical models and field observations of regularly spaced grabens in canyonlands national park utah demonstrate that salt flow beneath a brittle overburden accommodated recent and ongoing westward gravity spreading erosion of the colorado river canyon differentially loaded the underlying viscous salt in our models the overlying brittle strata flexed downward toward the canyon initiating faults near the surface that propagated downward toward the salt contact modeled grabens developed sequentially away from the canyon eastward as salt was expelled from beneath undeformed strata after their eastern boundary faults broke through horst blocks tilted in the opposite direction of initial flexure resulting in increased symmetry of older grabens closer to the canyon continued extension formed a reactive diapir beneath each graben field observations show that multiple faults bound grabens indicating reactive diapirs beneath them topographic profiles and surveyed points along a stratigraphic layer show that horst blocks subsided as salt migrated toward the river canyon and into the diapirs field data from less evolved horsts imply that individual horst blocks responded to differential loading by progressive flexure and tilt similar to the models horst block flexures also vary along strike and localized folds and faults formed where fault displacement changes abruptly

geochemical evidence supporting t c chamberlin s theory of glaciation in 1899 t c chamberlin proposed that the co sub 2 content of the atomsphere decreased during times of enhanced continental erosion ultimately resulting in glacial epochs he ascribed the increase in the rate of chemical weathering relative to the rate of supply of co sub 2 from earth s interior to increased orogenic activity and globally higher average elevations which promoted rapid chemical erosion of silicates the oceanic record of strontium isotopes preserved in marine sediment supports his suggestion that glacial climates during the phanerozoic are in part linked to increases in the rate of global chemical erosion relative to outgassing from earth s interior further the close correspondence of the major tectonic episodes of the late cenozoic to times of increased continental erosion and glaciation suggests that chamberlin s hypothesis of the cause of glacial periods should be revived

contribution of carbonate rock weathering to the atmospheric co2 sink to accurately predict future co2 levels in the atmosphere which is crucial in predicting global climate change the sources and sinks of the atmospheric co2 and their change over time must be determined in this paper some typical cases are examined using published and unpublished data firstly the sensitivity of carbonate rock weathering including the effects by both dissolution and reprecipitation of carbonate to the change of soil co2 and runoff will be discussed and then the net amount of co2 removed from the atmosphere in the carbonate rock areas of mainland china and the world will be determined by the hydrochem discharge and carbonate rock tablet methods to obtain an estimate of the contribution of carbonate rock weathering to the atmospheric co2 sink these contributions are about 0 018 billion metric tons of carbon a and 0 11 billion metric tons of carbon a for china and the world respectively further by the dbl diffusion boundary layer model calculation the potential co2 sink by carbonate rock dissolution is estimated to be 0 41 billion metric tons of carbon a for the world therefore the potential co2 source by carbonate reprecipitation is 0 3 billion metric tons of carbon a

evolution of the cenozoic carbon cycle the roles of tectonics and co2 fertilization cenozoic carbon fluxes associated with rock weathering sediment burial and volcanic degassing are calculated from the mass balance equations coupling marine isotopic records of carbon both organic and inorganic strontium and osmium the result is confirmed by the good match between modeled carbonate sedimentation rates and carbonate sedimentation rates previously integrated from ocean basins worldwide the coevolution between weathering and burial of carbonate suggests that marine carbonate accumulation was regulated mainly by the recycling of carbonate rocks which mediated the bicarbonate ion concentration of the oceans a reduction in co2 effusion to the atmosphere caused by reduced volcanic degassing from 52 to 15 ma and tectonically enhanced organic rock exhumation since 15 ma is also observed these changes in co2 effusion are balanced by concomitant changes in co2 sequestration by silicate weathering and organic carbon burial importantly we demonstrate a clear decoupling of modeled silicate weathering rates from global climate over the last 15 ma this observation is inconsistent with temperature mediated mineral dissolution acting as the key mechanism facilitating the co2 silicate weathering feedback process however we instead observe a clear coupling positive correlation between modeled silicate weathering organic carbon burial and atmospheric co2 concentration we suggest that co2 fertilization effects on terrestrial biomass productivity and plant weathering could have represented a major negative feedback process helping to balance atmospheric co2 at least during the cenozoic ice house periods

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

chemical weathering in glacial environments do glaciers enhance or inhibit chemical weathering rates relative to other environments the importance of glaciers in the global carbon cycle and climate change hinges on the answer we show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge silica denudation rates are distinctly lower in glacier covered catchments than in their nonglacial counterparts because sediment yields are high from glaciers this suggests that water flux rather than physical erosion exerts the primary control on chemical erosion by glaciers potassium and calcium concentrations are high relative to other cations in glacial water probably due to dissolution of soluble trace phases such as carbonates exposed by comminution and cation leaching from biotite preferential weathering of biotite may result in higher 87 sr 86 sr in glacial runoff than expected from whole rock compositions thus although glaciers do not influence total chemical denudation rates at a given runoff they may yield compositionally distinctive chemical fluxes to the oceans disruption of mineral lattices by grinding increases dissolution rates this and high surface area should make glacial sediments exceptionally weatherable weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles

direct evidence of the feedback between climate and weathering long term climate moderation is commonly attributed to chemical weathering the higher the temperature and precipitation the faster the weathering rate weathering releases divalent cations to the

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

temperature dependence of silicate weathering in nature how strong a negative feedback on long term accumulation of atmospheric co2 and global greenhouse warming estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent arrhenius activation energy of 18 4 kcal mol 77 0 kj mol much higher than most laboratory values this finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought this result provides a stronger negative feedback on long term greenhouse warming than has been assumed in most models of global carbon cycling the present estimate was determined from the ratio of feldspar weathering rates determined by geochemical mass balance in the southern blue ridge mountains of north carolina united states temperature a function of elevation is the only factor that differs between the two catchments parent rock type aspect hillslope hydrology and vegetation type and successional stage are the same in both

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