paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

modeling of water rock interaction in the mackenzie basin competition between sulfuric and carbonic acids the chemical weathering of continental surfaces depends on many factors including lithology climate tectonics erosion soil or vegetation and also on the availability of acidity in the weathering zone in addition to atmospheric soil co2 sulfide oxidation may represent a significant source of protons for weathering reactions as shown previously for the rivers draining the mackenzie basin canada here a numerical model describing continental weathering reactions based on laboratory kinetic laws is coupled to a dynamic global vegetation model the b witch model in order to estimate the impact of sulfide oxidation on chemical weathering and associated atmospheric co2 consumption fluxes throughout the mackenzie river basin the model is first calibrated with available field data for the various sub basin of the mackenzie watershed and then a simulation is performed removing the contribution of sulfuric acid to the weathering system the main outcomes of the present study are that 1 the dissolved silica export is controlled by secondary minerals reactivity in soils and the calcium and magnesium fluxes are driven by the dissolution of calcite and dolomite 2 the atmospheric co2 consumption by weathering processes in the mackenzie river is limited accounting for only 24 of the total bicarbonate flux to the ocean confirming a previous study and suggesting that weathering processes in the mackenzie watershed acts as a source of carbon to the atmosphere at the geological timescale and 3 the presence of sulfide minerals within a river basin decreases the apparent net consumption of atmospheric co2 through chemical weathering we show that the actual production of sulfuric acid in the weathering zone of the mackenzie basin reduces the net atmospheric co2 consumption by about two compared to what the atmospheric co2 consumption should be in the absence of sulfide minerals sulfuric acid driven chemical weathering has thus to be considered at both short and long timescales when looking at the link between chemical weathering and the global carbon cycle

paleomagnetic evidence for tectonic rotations in the variscan mountain belt a critical review of devonian and carboniferous paleomagnetic results from the hercynian basement of western and central europe reveals large rotations their geographic distribution suggests a tectonic origin for large scale curved structures such as the ibero armorican arc or the variscan v together with a coherence of the hercynian domain with time geodynamic models which can have produced the observed rotation pattern are discussed it appears that lateral compression due to the accommodation of continental collision as in the india asia convergence is a plausible mechanism although variscan and himalayan mountain belts have different characteristics

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

modelling weathering processes at the catchment scale the witch numerical model a numerical model of chemical weathering in soil horizons and underlying bedrock witch has been coupled to a numerical model of water and carbon cycles in forest ecosystems aspects to simulate the concentration of major species within the soil horizons and the stream of the strengbach granitic watershed located in the vosges mountains france for the first time simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis the model is able to reproduce the concentrations of most major species within the soil horizons as well as catching the first order seasonal fluctuations of aqueous calcium magnesium and silica concentrations however the witch model underestimates concentrations of mg2 and silica at the spring of the catchment stream and significantly underestimates ca2 concentration the deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock however the resulting increased ca2 release yields important smectite precipitation in the deepest model layer in contact with the bedrock and subsequent removal of large amount of silica and magnesium from solution in contrast the model accurately accounts for the concentrations of major species ca mg and silica measured in the catchment stream when precipitation of clay minerals is not allowed the model underestimation of mg2 and h4sio4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming in agreement with observations on other watersheds draining granitic rocks this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the strengbach watershed

tracing silicon cycling in the okavango delta a sub tropical flood pulse wetland using silicon isotopes chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction potentially confounding estimates of silicate weathering rates silicon isotopes d30si may be a useful tool to investigate these processes here we present 82 d30si measurements from surface waters pore waters biogenic silica bsi clays sand and vegetation from the okavango delta botswana a freshwater sub tropical flood pulse wetland hydrologically the okavango is dominated by evapotranspiration water losses to the atmosphere it receives an annual pulse of water that inundates seasonal floodplains while river baseflow is sufficient to maintain a permanent floodplain d30si in dissolved silica dsi in surface waters along a 300 km transect at near peak flood show a limited range 0 36 1 19 imply ing the delta is well buffered by a balance of processes adding and removing dsi from the surface water a key control on dsi concentrations is the uptake production of bsi and recycling of si by aquatic vegetation although the net isotopic effect is necessarily small since all bsi re dissolves on short timescales in the sediments bsi d30si n 30 ranges from less

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

sensitivity of carbonate weathering to soil co2 production by biological activity along a temperate climate transect abstract we investigated the controls on carbonate weathering in a well drained pure carbonate area subject to strong environmental gradients the jura mountains western europe the water chemistry of sampled springs and resurgences is dominated by ca 2 87 to 96 eq of the cationic charge and hco 3 90 to 97 eq of the anionic charge reflecting the overwhelming imprint of calcium carbonate dissolution by atmospheric biogenic co 2 ca 2 concentration which directly gives access to the amount of calcium carbonate dissolved per unit of water runoff shows a gradual two fold decrease from 3000 to 1400 mol l along the elevation gradient from 300 to 1200 m after discussing the possible influence of each environmental parameter on the observed water chemistry gradient a decreasing soil pco 2 the main source of acidity with increasing altitude appears as the most likely explanation as no spatial and temporal record of soil pco 2 are available for the jura mountains we performed soil pco 2 modeling using the ecological and hydrological aspects model that allows reconstructing carbon and water exchange fluxes between the vegetation soil and atmosphere modeling results suggest that soil pco 2 decreases with altitude in response to both the change in vegetation species from deciduous dominated forest in the lowlands to evergreen dominated forest above 800 m responsible for 65 of the variation and the change in climate and soil properties responsible for 35 of the variation carbonate weathering would thus be strongly sensitive to the type of vegetation which drives both temporal and spatial variations of soil carbon and water budgets based on field observations we show that carbonate weathering rates are 20 30 higher under deciduous vegetation cover than under conifers at a given water runoff value in agreement with modeling results chemical denudation rates of carbonate in the jura mountains vary from 152 to 375 t km 2 yr corresponding to 60 150 mm ka of carbonate being removed carbonate weathering within the 10 000 km 2 of the study area accounts for an atmospheric co 2 consumption of 0 3 tgc yr showing that carbonate rocks have an enhanced capacity of atmospheric co 2 neutralization at least transiently this study demonstrates that carbonate weathering is sensitive to the ecosystem dynamics a conclusion that might be much more general and suggests that carbonate weathering and associated co 2 consumption fluxes quickly react to any global change or land use modification

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

paleomagnetic results from volcanic rocks of the shelve inlier wales evidence for a wide late ordovician iapetus ocean in britain abstract in order to shed light on the early paleozoic paleogeography of iapetus bordering continents samples for paleomagnetic study were collected from middle ordovician llanvirn volcanic rocks of the shelve inlier welsh borderland the folding within the inlier is constrained to late ordovician ashgill time thus offering a fold test with tight age brackets after stepwise thermal demagnetization a pre folding characteristic magnetization component was isolated from 95 specimens the mean of 11 site mean directions for the characteristic magnetization gives d i 116 4 67 9 k 89 4 95 4 9 and the paleomagnetic pole lies at 26 6 n 36 1 e p 6 9 m 8 2 the fold test is positive at the 99 confidence level indicating that the magnetization was acquired during the middle or late ordovician we infer from geological considerations that this magnetization is not a primary thermoremanence but was acquired during late ordovician time as a result of a thermochemical disturbance of that age our results indicate that southern britain was near 51 south latitude during the later ordovician since middle to late ordovician paleolatitudes for gondwana and laurentia are well determined our result can be used to test various scenarios for the closure of iapetus we conclude that 1 the iapetus ocean in britain still had a latitudinal width of some 30 at this time thus rendering a postulated ordovician closure unlikely 2 southern britain avalon and hercynian europe all occupied high latitudes during the later ordovician consistent with a paleoposition near the northern margin of gondwana and 3 the acadian orogeny in britain and north america could have been the result of the collision of gondwana and laurentia during early devonian time

development and origin of the microgranular structure in latosols of the brazilian central plateau significance of texture mineralogy and biological activity brazilian latosols are characterized by low activity clay little horizon differentiation a weak macrostructure and a strong microgranular structure the development of the latter that was reported as being possibly related to the fine material mineralogy location in the landscape parental material and biological activity is still under discussion the aim of this study is to discuss the origin of the microgranular structure of latosols located in the brazilian central plateau ten latosols l developed in different parent materials were selected along an approximately 350 km long regional toposequence across the south american surface sas l1 to l4 and velhas surface vs l5 to l10 the structure of the latosols was studied in the field and samples of the diagnostic bw horizons were collected for laboratory analyses basic soil characterization was performed on the 2 mm material according to the brazilian standard procedures the sio2 al2o3 fe2o3 and tio2 content was determined after dissolution with 1 1 h2so4 the microstructure was studied in optical microscopy and electron scanning electron microscopy in thin sections the results showed a varying compound strong microgranular structure and weak to moderate medium sub angular blocky structure independently of the landscape position as well as the parental material except for the latosol developed on a quartzite the latosols were classified as gibbsitic sexquioxic kaolinitic non sesquioxid or kaolinitic sesquioxid latosols but there was no relationship between their composition and the development of the microgranular structure in the bw horizon the analyses showed indeed several types of microgranular structure with different proportions but without any relationship with their texture and mineralogy actually the types of microgranular structure appeared to be mainly related to soil bioturbation by termites and eventually secondarily by ants thus the weak macrostructure and strong microgranular structure of the latosols studied the little vertical differentiation of the horizons and the lack of a clear relationship between their landscape position and parent material characteristics would result from long term biotic action with high probability

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

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

geologic constraints on the glacial amplification of phanerozoic climate sensitivity the long term carbon cycle depends on many feedbacks silicate weathering consumes atmospheric co2 but is also enhanced by the increased temperatures brought about by this important greenhouse gas the long term sensitivity t2x of climate to co2 doubling modulates the strength of this negative feedback we update the model experiment of royer and others 2007 by estimating an empirical probability density function pdf of t2x for the phanerozoic by using an improved geocarbsulf carbon cycle model to predict a larger recalibrated set of proxy co2 measurements from the present day to 420 ma the new geocarbsulf parameterizes the rapid weathering of volcanic rocks relative to plutonic rocks updates to the carbon cycle model and the proxy co2 data set induce opposing model responses as a result our experiment maintains an agreement with t2x estimates based on numerical climate models and late cenozoic paleoclimate for a climate sensitivity t2x that is uniform throughout the phanerozoic the most probable value is 3 to 4 c geocarbsulf fits the proxy co2 data equally well and with far more parameter choices if t2x is amplified by at least a factor of two during the glacial intervals of the paleozoic 260 340 ma and cenozoic 0 40 ma relative to non glacial intervals of earth history for glacial amplification of two the empirical pdfs for glacial climate sensitivity predict t2x g 2 0 c with 99 percent probability t2x g 3 4 c with 95 percent probability and t2x g 4 4 c with 90 percent probability the most probable values are t2x g 6 to 8 c this result supports the notion that the response of earth9s present day surface temperature will be amplified by the millennial and longer term waxing and waning of ice sheets

palaeomagnetism and age of mid palaeozoic ring complexes in niger west africa and tectonic implications summary detailed af and thermal demagnetization has been performed on 185 oriented samples from 36 sites in a series of basic to acid subvolcanic ring complexes in central and southern niger published rb sr whole rock and mineral isochrons from acid members of the suite range north to south from 480 ma to 320 ma two distinct vectors have been isolated by linear regression of demagnetization data 1 a due north and negative inclination rarely reversed component is present in basic rocks from 12 sites in four distinct northern complexes published rb sr ages date from 480 ma to 420 ma this component is carried by single domain magnetite occasionally also by titanhaematite and is considered to be primary acquired during the terminal stages of this igneous episode 40ar 39ar dating of biotite from three samples used in the palaeomagnetic study yield 435 myr ages 2 a due north and positive inclination rarely reversed direction is present as a second component in some of the basic suite sites and is the predominant component in most sites in syenitic to granodioritic rocks primary fe ti oxides are rare in these acid igneous rocks and the feldspars within them are usually pervasively altered this component carried primarily by haematite is believed to be either a modern day vrm or a secondary tcrm acquired during hydro thermal activity associated with widespread quaternary volcanism in the southern air the primary component gives a mean south pole at 43 4s 08 6e n 12 k 50 95 6 2 located southwest of cape town this position requires a rapid southward excursion of the gondwana a p w path in early silurian time but it is generally consistent with most of the other silurian devonian palaeomagnetic data from gondwana these however are from the lachlan fold belt in southeast australia whose relation to the australian craton in early palaeozoic time is not clear integrating all stratigraphic palaeoclimatic tectonic and other palaeomagnetic data for early and middle palaeozoic time suggests that the proto atlantic closed during the silurian period if a late devonian early carboniferous gondwana pole in central africa is valid the ocean may have opened again before the final closure and reassembly of gondwana in the late carboniferous

palaeomagnetism and 40ar 39ar dating of the tregor dolerites armorican massif france abstract the tregor doleritic dike swarm intrudes the cadomian north tregor batholith armorican massif france alternating field a f and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin section examinations three magnetization components were recognized in addition to a low temperature component with a viscous origin the most frequently observed component named b d 214 i 10 was isolated at 12 sites independent upon their magnetic mineralogy and appears to be a remagnetization negative baked contact test a carboniferous age of acquisition is inferred from comparison of the b component palaeopole 29 s 318 e with an armorican apwp at some sites after demagnetization of the b component higher temperature and coercivity components were isolated their directions fall into two groups named a and c positive baked contact tests were obtained for both the c component d 174 i 50 whose magnetic carrier is magnetite yields a palaeopole 11 s 2 e which is similar to some ordovician palaeopoles from the british isles south of the lapetus suture or eocambrian palaeopoles from the tregor area the a component d 191 i 33 whose magnetic carrier is magnetite partly transformed to leucoxene yields a palaeopole 58 s 336 e and a palaeolatitude 17 9 n not previously observed for the armorican massif in palaeozoic time comparison between this palaeolatitude and those for northern spain from ordovician to carboniferous time suggests a late silurian remanence age this interpretation supports the model of apparent polar wander for gondwana between ordovician and devonian time proposed by van der voo and bachtadse and briden 40 ar 39 ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the a and c components

modelling the primary control of paleogeography on cretaceous climate the low thermal gradients and clement winters characterizing climates of the cretaceous period reveal that the climate system has modes of behaviour quite different from the present recent proxy data analyses suggest that some aspects of climate change within the cretaceous appear to be decoupled from co2 evolution at the geological time scale here we investigate the impact of paleogeography on the global climate with the climate model foam using a early mid and late cretaceous continental configuration we find that changes in geography from the early to mid to late cretaceous cause a large decrease of the seasonal cycle first order identified processes are the decreased continentality of the mid to high latitudes from the mid cretaceous and the increase of the latent heat transport into the winter hemisphere which induce a wetter and a cloudier atmosphere capable of diminishing the winter cooling of the continents owing to the modifications of the seasonal cycle in response to the tectonic forcing the equator to pole thermal gradient is reduced from the early to mid to late cretaceous we nevertheless still do not succeed in simulating warm enough polar temperatures and a definitive theory still waits for an integrated approach explicitly accounting for each factor influencing the thermal gradient ocean dynamics stratospheric clouds and vegetation our study also suggests a mechanism that can weaken the correlation between co2 and climate changes during the cretaceous as evolving geography from the early to late cretaceous through the response of the water cycle and the changes in the thermal gradient results in a 3 8 c global warming at a constant atmospheric co2 level this demonstrates that the paleogeography may affect the relation between pco2 and the climate history and consequently has to be accounted for when linking the atmospheric co2 evolution and the climate record at geological time scales

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

paleohydrologic response to continental warming during the paleocene eocene thermal maximum bighorn basin wyoming abstract geologically rapid global warming occurred during the paleocene eocene thermal maximum petm 56 ma several studies have argued that important changes occurred in the hydrological cycle during the petm but results have been inconsistent ranging from global increases in humidity to drier conditions changes in paleosols during the petm in the southeastern bighorn basin document major drying during the body of the event paleosol changes also suggest transitional episodes of climate change that both preceded and followed the petm qualitative semi quantitative and fully quantitative analyses of a 70 m thick interval of paleosols provide a high resolution record of changes in soil moisture and precipitation those changes are compared to changes in temperature determined from 18o values of tooth enamel from the mammal coryphodon a distinct shift to drier soils occurred just prior to the petm a conclusion that is consistent with previous observations that warming began before the onset of the negative carbon isotope excursion associated with the petm paleosols show a progressive drying trend into the lower part of the petm and become even drier in the upper part of the body of the petm purple red paleosols that appear during the recovery phase of the petm indicate wetter soils although they are better drained than paleosols below the onset the purple red paleosols continue for 15 m above the recovery and indicate that wetter soil conditions persisted after the recovery it is not clear whether changes in the paleosols that preceded and followed the petm reflect global forcing factors like orbital cycles or release of carbon that lacks an isotopic label however such mechanisms would provide a unifying explanation for shifts seen in continental and marine environments

modelling of atmospheric co2 consumption by chemical weathering of rocks application to the garonne congo and amazon basins modelling of atmospheric co consumption by chemical weathering of rocks application to the garonne congo and amazon basins

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

topographic controls on the depth distribution of soil co2 in a small temperate watershed accurate measurements of soil co2 concentrations pco2 are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon c cycle while there have been many studies of soil pco2 most sample or model only one or at most a few landscape positions and therefore do not account for complex topography here we test the hypothesis that soil pco2 distribution can predictably vary with topographic position we measured soil pco2 at the susquehanna shale hills critical zone observatory sshczo pennsylvania where controls on soil pco2 e g depth texture porosity and moisture vary from ridge tops down to the valley floor between planar slopes and slopes with convergent flow i e swales and between north and south facing aspects we quantified pco2 generally at 0 1 0 2 m depth intervals down to bedrock from 2008 to 2010 and in 2013 of the variables tested topographic position along catenas was the best predictor of soil pco2 because it controls soil depth texture porosity and moisture which govern soil co2 diffusive fluxes the highest pco2 values were observed in the valley floor and swales where soils are deep 0 7 m and wet resulting in low co2 diffusion through soil profiles in contrast the ridge top and planar slope soils have lower pco2 because they are shallower 0 6 m and drier resulting in high co2 diffusion through soil profiles aspect was a minor predictor of soil pco2 the north i e south facing swale generally had lower soil moisture content and pco2 than its south i e north facing counterpart seasonally we observed that while the timing of peak soil pco2 was similar across the watershed the amplitude of the pco2 peak was higher in the deep soils due to more variable moisture content the high pco2 observed in the deeper wetter topographic positions could lower soil porewater ph by up to 1 ph unit compared to porewaters equilibrated with atmospheric co2 alone co2 is generally the dominant acid driving weathering in soils based on our observations models of chemical weathering and co2 dynamics would be improved by including landscape controls on soil pco2

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

superimposed recent permo carboniferous and ordovician palaeomagnetic remanence in the builth volcanic series wales the builth volcanic series of llanvirnian age in llanelwedd quarries mid wales carries three components of natural remanent magnetisation component p regarded as primary is a thermochemical remanence directed at d 181 7 i 54 5 95 4 4 relative to bedding component s is a secondary component with in situ d 178 7 i 6 7 95 5 4 and is believed to be a low temperature chemical remanence crm of permo carboniferous age component r is directed close to the present geomagnetic field and is believed to be a recent viscous remanence vrm the results are of interest for three reasons first they are an unusually good example of multi component nrm analysis the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges second they provide evidence of a permo carboniferous event possibly a mild thermal or hydrothermal pulse promoting crm acquisition some 40 km north of the hercynian orogenic front third they illustrate very clearly the importance of detailed demagnetisation this work revises the pole position for these rocks by 10 and removes an obstacle to the palaeomagnetic recognition of the 1000 km wide iapetus ocean cutting britain in ordovician time

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

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

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

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

paleomagnetism of the late ordovician thouars massif vendee province france a paleomagnetic study of the late ordovician thouars massif reveals very steep components of magnetization directed upward and downward and with thermally discrete blocking temperatures of 550 to 580 c this characteristic magnetization yields a mean direction with declination inclination 340 83 with k 27 and 95 7 based on 15 sites the south paleopole is located at 34 n 5 e a partial magnetic overprint of hercynian age is observed in sites close to the south armorican shear zone the 15 sites are spread over some 20 by 30 km and the internal consistency of the site mean directions indicates that differential tilting of parts of the massif has not occurred since it appears to be unlikely that the entire massif was tilted we infer that the characteristic direction is primary and that the paleopole adequately represents the local magnetic field in the late ordovician the paleopole for the thouars massif supports previously obtained results from other ordovician rocks in hercynian europe but it is the first result to pass modern reliability criteria the high ordovician paleolatitudes of the armorican massif other areas in hercynian europe and north western africa indicate that gondwana and armorica were adjacent until late ordovician time low paleolatitudes for late devonian time suggest that armorica underwent rapid northward motion at a rate of about 10 cm yr during the silurian and earliest devonian armorica is thought to have collided with north america and the baltic shield russian platform before or during the early devonian forming the old red continent the collisions produced the caledonian and acadian orogenic belts

impact of atmospheric co2 levels on continental silicate weathering 1 anthropogenic sources are widely accepted as the dominant cause for the increase in atmospheric co2 concentrations since the beginning of the industrial revolution here we use the b witch model to quantify the impact of increased co2 concentrations on co2 consumption by weathering of continental surfaces b witch couples a dynamic biogeochemistry model lpj and a process based numerical model of continental weathering witch it allows simultaneous calculations of the different components of continental weathering fluxes terrestrial vegetation dynamics and carbon and water fluxes the co2 consumption rates are estimated at four different atmospheric co2 concentrations from 280 up to 1120 ppmv for 22 sites characterized by silicate lithologies basalt granite or sandstones the sensitivity to atmospheric co2 variations is explored while temperature and rainfall are held constant first we show that under 355 ppmv of atmospheric co2 b witch is able to reproduce the global pattern of weathering rates as a function of annual runoff mean annual temperature or latitude for silicate lithologies when atmospheric co2 increases evapotranspiration generally decreases due to progressive stomatal closure and the soil co2 pressure increases due to enhanced biospheric productivity as a result vertical drainage and soil acidity increase promoting co2 consumption by mineral weathering we calculate an increase of about 3 of the co2 consumption through silicate weathering mol ha 1 yr 1 for 100 ppmv rise in co2 importantly the sensitivity of the weathering system to the co2 rise is not uniform and heavily depends on the climatic lithologic pedologic and biospheric settings

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana