on the association between veining and index mineral distributions in barrow s metamorphic zones glen esk scotland the concept of index mineral based metamorphic zones was first introduced by george barrow in 1912 and the barrovian metamorphic zones continue to be used as a framework for describing regional met

10be and9be in south atlantic dsdp site 519 relation to geomagnetic reversals and to sediment composition a high resolution10be profile has been measured on dsdp site 519 central south atlantic in order to search for changes in the flux of10be into a pelagic sediment during geomagnetic reversals as compared with times of constant magnetic polarity the decay corrected10be concentrations range from 1 to 9 108 at g the average10be flux rate is 5 108 at cm 2 kyr 1 9be and caco3 were measured on the same samples the10be vs depth curve is overall similar to the9be vs depth curve while the caco3 vs depth curve anticorrelates the beryllium isotope curves closely implying that be is carried mainly by the non caco3 phases of the sediment the positive correlation between10be and9be along with the fact that the average 10be 9be ratio of 5 5 10 8 is within the range of 10be 9be ratios reported for deep ocean water leads us to conclude 1 that the beryllium isotopes come close to an equilibrium in the open ocean and 2 that the 10be 9be in deep sea sediments with a low detrital component monitors the 10be 9be of deep ocean water through time relative increases of the 10be 9be ratio are observed in the vicinity of the geomagnetic reversals but the relationship between reversals and higher 10be 9be remains doubtful other mechanisms which could alter the 10be 9be ratio are discussed input of10be by melting of glacial ice changing input of9be due to changes in continental erosion changing deep ocean circulation changing scavenging activity at ocean margins but as yet no conclusive explanation for the observed behavior of the beryllium isotopes can be given

relationships between co2 thermodynamic limits on silicate weathering and the strength of the silicate weathering feedback abstract recent studies have suggested that thermodynamic limitations on chemical weathering rates exert a first order control on riverine solute fluxes and by extension global chemical weathering rates as such these limitations may play a prominent role in the regulation of carbon dioxide levels p co 2 over geologic timescales by constraining the maximum global weathering flux in this study we develop a theoretical scaling relationship between equilibrium solute concentrations and p co 2 based on equilibrium constants and reaction stoichiometry relating primary mineral dissolution and secondary mineral precipitation we test this theoretical scaling relationship against reactive transport simulations of chemical weathering profiles under open and closed system conditions representing partially and fully water saturated regolith respectively under open system conditions equilibrium bicarbonate concentrations vary as a power law function of p co 2 y k x n where n is dependent on reaction stoichiometry and k is dependent on both reaction stoichiometry and the equilibrium constant under closed system conditions bicarbonate concentrations vary linearly with p co 2 at low values and approach open system scaling at high p co 2 to describe the potential role of thermodynamic limitations in the global silicate weathering feedback we develop a new mathematical framework to assess weathering feedback strength in terms of both 1 steady state atmospheric p co 2 concentrations and 2 susceptibility to secular changes in degassing rates and transient carbon cycle perturbations which we term 1st and 2nd order feedback strength respectively finally we discuss the implications of these results for the effects of vascular land plant evolution on feedback strength the potential role of vegetation in controlling modern solute fluxes and the application of these frameworks to a more complete functional description of the silicate weathering feedback most notably the dependence of equilibrium solute concentrations on p co 2 may represent a direct weathering feedback largely independent of climate and modulated by belowground organic carbon respiration

authigenic 10be 9be ratios and 10be fluxes 230thxs normalized in central baffin bay sediments during the last glacial cycle paleoenvironmental implications abstract authigenic 10 be 9 be ratios and 10 be fluxes reconstructed using the 230 th xs normalization proxies of the cosmogenic radionuclide 10 be production rate in the atmosphere have been measured in a sedimentary core from baffin bay north atlantic spanning the last 136 ka bp the normalization applied on the exchangeable authigenic 10 be concentrations using the authigenic 9 be isotope and 230 th xs methods yield equivalent results strongly correlated with sedimentological parameters grain size and mineralogy lower authigenic beryllium be concentrations and 10 be 9 be ratios are associated with coarse grained carbonate rich layers while higher authigenic be values are related to fine grained felspar rich sediments this variability is due to i sediment composition control over beryllium scavenging efficiency and ii glacial history that contributed to modify the 10 be concentration in baffin bay by input and boundary scavenging condition changes most paleo denudation rates inferred from the 10 be 9 be ratio vary weakly around 220 76 tons km 2 yr 1 0 09 0 03 mm yr 1 corresponding to relatively steady weathering fluxes over the last glacial cycle except for six brief intervals characterized by sharp increases of the denudation rate these intervals are related to ice surging episodes coeval with heinrich events and the last deglaciation period an average freshwater flux of 180 6 km 3 yr 1 0 006 sv consistent with recent models has been calculated in order to sustain glacially derived 10 be inputs into baffin bay it is concluded that in such environments the authigenic 10 be measured mainly depends on climatic effects related to the glacial dynamics which masks the 10 be production variation modulated by geomagnetic field changes altogether these results challenge the simple interpretation of 10 be concentration variation as a proxy of interglacial glacial interstadial stadial cycles in arctic and sub arctic regions they rather suggest the effect of higher frequency paleoclimatic changes and local glacial dynamics on 10 be signature

beryllium isotope signatures of ice shelves and sub ice shelf circulation abstract be isotopes are a useful tracer of sediment source and transport pathways but have not been widely tested in glacio marine environments we measured be isotopes in a range of depositional environments from open marine sub ice shelf and subglacial settings throughout prydz bay one of antarctica s largest ice drainage systems we find that strong sub ice shelf and bottom current circulations can advect 10be rich open marine sediments into an ice shelf cavity and 10be poor terrestrial sediments onto the continental shelf at the ice shelf outflow meaning that 10be concentrations reflect sub ice shelf circulation patterns rather than depositional environment however hcl extractable 10be 9be ratios can provide a more robust discrimination of sediment deposited in open marine and sub ice shelf settings thus be isotopes are a useful tracer of both environmental setting and sub ice shelf circulation strength in both modern and paleo ice sheet margins

reconstruction of global 10be production over the past 250ka from highly accumulating atlantic drift sediments in this study we present a reconstruction of the global 10be production rate over the past 250 000 years from three marine sediment cores located in high accumulation environments in the north northwest and south atlantic ocean odp sites 983 1063 and 1089 the 10be records are corrected for oceanic transport processes and principal component analysis pca is used to extract the common signal from the three records which we interpreted as variations of the global 10be production rate the reconstruction presented here may serve as i a record of past flux of galactic cosmic rays gcr ii a proxy for past geomagnetic dipole strength and iii as a global matching tool to synchronize marine archives with ice cores and terrestrial records

evolution of the global overturning circulation since the last glacial maximum based on marine authigenic neodymium isotopes abstract the global overturning circulation is linked to climate change on glacial interglacial and multi millennial timescales the understanding of past climate circulation links remains hindered by apparent conflicts among proxy measures of circulation here we reconstruct circulation changes since the last glacial maximum lgm based on a global synthesis of authigenic neodymium isotope records end we propose the bottom up framework of interpreting seawater and authigenic end considering not only conservative watermass mixing but also the preformed properties and the non conservative behavior of end both subject to sedimentary influences we extract the major spatial temporal modes of authigenic end using principal component analysis and make a first order circulation reconstruction with budget constrained box model simulations we show that during the lgm the source region of north atlantic overturning shifted southward which led to more radiogenic preformed end of glacial northern source water nsw considering this preformed effect we infer that glacial deep atlantic had a similar proportion of nsw as today although the overall strength of glacial circulation appears reduced from both north atlantic and southern ocean sources which increased the relative importance of non conservative behavior of end and may have facilitated accumulation of respired carbon in the deep ocean during the deglaciation we find that southern ocean overturning increased which offset suppressed north atlantic overturning and resulted in a net stronger global abyssal circulation faster global scale deglacial circulation reduced the relative importance of non conservative effects resulting in atlantic pacific convergence of abyssal end signatures variations of southern ocean overturning likely drove a significant fraction of deglacial changes in atmospheric co2 and oceanic heat budget

river fluxes to the sea from the oceanʼs 10be 9be ratio the ratio of the meteoric cosmogenic radionuclide 10be to the stable isotope 9be is proposed here to be a flux proxy of terrigenous input into the oceans the ocean s dissolved 10be 9be is set by 1 the flux of meteoric 10be produced in the atmosphere 2 the denudational flux of the rivers discharging into a given ocean basin 3 the fraction of 9be that is released from primary minerals during weathering meaning the 9be transported by rivers in either the dissolved form or adsorbed onto sedimentary particles and incorporated into secondary oxides and 4 the fraction of riverine 10be and 9be actually released into seawater using published 10be 9be data of rivers for which independent denudation rate estimates exist we first find that the global average fraction of 9be released during weathering into river waters and their particulate load is 20 and does not depend on denudation rate we then evaluate this quantitative proxy for terrigenous inputs by using published dissolved seawater be isotope data and a compilation of global river loads we find that the measured global average oceanic dissolved 10be 9be ratio of about 0 9 10 7 is satisfied by the mass balance if only about 6 of the dissolved and adsorbed riverine be is eventually released to the open ocean after escaping the coastal zone when we establish this mass balance for individual ocean basins good agreement results between 10be 9be ratios predicted from known river basin denudation rates and measured ocean 10be 9be ratios only in the south atlantic and the south pacific the 10be 9be ratio is dominated by advected be and in these basins the ratio is a proxy for ocean circulation as the seawater 10be 9be ratio is faithfully recorded in marine chemical precipitates the 10be 9be ratio extracted from authigenic sediments can now serve to estimate relative changes in terrigenous input into the oceans back through time on a global and on an ocean basin scale

the continental si cycle and its impact on the ocean si isotope budget abstract the silicon isotope composition of biogenic silica 30sibsi in the ocean is a function of the 30si of the available dissolved si dsi h2sio4 the degree of utilisation of the available dsi and for some organisms the concentration of dsi this makes 30sibsi in sediment archives a promising proxy for past dsi concentrations and utilisation at steady state mean 30sibsi must equal a weighted average of the inputs the majority of which are of continental origin variation in the functioning of the continental si cycle on timescales similar to the residence time of dsi in the ocean 10 ka may therefore contribute to downcore variability in 30sibsi on millennial or longer timescales the direction and magnitude of change in published 30sibsi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers they document glacial values that are typically 0 5 to 1 0 lower than interglacial values and together hint at coherent and predictable glacial interglacial variability in whole ocean 30si driven by a change in mean 30si of the inputs in this contribution we review the modern inputs of dsi to the ocean and the controls on their isotopic composition and assess the evidence for their variability on millennial plus timescales today 9 55 1012 mol yr 1 dsi enters the ocean of which roughly 64 and 25 are direct riverine inputs of dsi and dsi from dissolution of aeolian and riverborne sediment respectively the remainder derives from alteration or weathering of the ocean crust each input has a characteristic 30si with our current best estimate for a weighted mean being 0 74 although much work remains to be done to characterise the individual fluxes many aspects of the continental si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean 30si of dsi entering the ocean these changes relate to i a cooler drier glacial climate ii lowered sea level and the exposure of continental shelves iii the presence of large continental ice sheets and iv altered vegetation zonation using a simple box model with a monte carlo approach to parameterisation we find that a transition from a hypothesised glacial continental si cycle to the modern si cycle can drive an increase in whole ocean 30si of comparable rate and magnitude to that recorded in 30sibsi this implies that we may need to revisit our understanding of aspects of the si cycle in the glacial ocean although we focus on the transition from the last glacial our synthesis suggests that the continental si cycle should be seen as a potential contributory factor to any variability observed in ocean 30sibsi on millennial or longer timescales

impact of glacial activity on the weathering of hf isotopes observations from southwest greenland abstract data for the modern oceans and their authigenic precipitates suggest incongruent release of hafnium hf isotopes by chemical weathering of the continents the fact that weathering during recent glacial periods is associated with more congruent release of hf isotopes has led to the hypothesis that the incongruency may be controlled by retention of unradiogenic hf by zircons and that glacial grinding enhances release of hf from zircons here we study the relationship between glacial weathering processes and hf isotope compositions released to rivers fed by land terminating glaciers of the greenland ice sheet as well as neighbouring non glacial streams the weathered source rocks in the studied area mostly consist of gneisses but also include amphibolites of the same age 1 9 ga hafnium and neodymium isotope compositions in catchment sediments and in the riverine suspended load are consistent with a predominantly gneissic source containing variable trace amounts of zircon and different abundances of hornblende garnet and titanite glacially sourced rivers and non glacial streams fed by precipitation and lakes show very unradiogenic nd isotopic compositions in a narrow range nd 42 8 to 37 9 hafnium isotopes on the other hand are much more radiogenic and variable with hf between 18 3 and 0 9 in glacial rivers and even more radiogenic values of 15 8 to 46 3 in non glacial streams although relatively unradiogenic hf is released by glacial weathering glacial rivers actually fall close to the seawater array in hf nd isotope space and are not distinctly unradiogenic based on their abundance in rocks and sediments and their isotope compositions different minerals contribute to the radiogenic hf in solution with a decreasing relevance from garnet to titanite hornblende and apatite neodymium isotopes preclude a much stronger representation of titanite hornblende and apatite in solution such as might result from differences in dissolution rates than estimated from mineral abundance the strong contrast in hf isotope compositions between glacial rivers and non glacial streams results mostly from different contributions from garnet and zircon where zircon weathering is more efficient in the subglacial environment a key difference between glacial and non glacial waters is the water rock interaction time while glacial rivers receive continuous contributions from long residence time waters of distributed subglacial drainage systems non glacial streams are characterized by fast superficial drainage above the permafrost horizon therefore the increased congruency in hf isotope weathering in glacial systems could simply reflect the hydrological conditions at the base of the ice sheet and glaciers with zircon weathering contributions increasing with water rock interaction time

rare earth element and neodymium isotope tracing of sedimentary rock weathering abstract chemical weathering plays an important role in sequestering atmospheric co2 but its potential influence on global climate over geological timescales remains debated to some extent this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record two types of rocks having presumably different impact on the long term carbon cycle in this study we investigate the use of rare earth element ree and neodymium isotopes end in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents a new index called concavity index ci is defined for measuring the degree of mid ree enrichment in geological samples which enables the determination of the source of iron oxides in sediments such as seawater derived fe oxyhydroxide phases ancient marine fe oxides derived from the erosion of sedimentary rocks and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation using this index we demonstrate that the end difference between paired fe oxide and detrital fractions in river sediments defined here as end feox det directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering while rivers draining old cratons and volcanic provinces display near zero end feox det values indicative of dominant silicate weathering 0 5 1 1 n 30 multi lithological catchments hosting sedimentary formations yield systematically higher values 2 7 1 2 n 44 showing that sedimentary rock weathering can be traced by the occurrence of riverine fe oxides having more radiogenic nd isotope signatures compared to detrital fractions this assumption is reinforced by the evidence that calculated end feox det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins examining the influence of climate and tectonics on measured nd isotopic compositions we find that end feox det is strongly dependent on temperature in lowlands following an arrhenius like relationship that reflects enhanced alteration of silicate rocks and formation of secondary fe oxides in warmer climates in contrast in high elevation catchments end feox det defines striking correlation with maximum basin elevation which we also interpret as reflecting the intensification of silicate weathering and associated fe oxide formation as elevation decreases due to the combined effects of thicker soils and warmer temperature overall our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high elevation environments while silicate weathering dominates in floodplains this novel approach combining ree and nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record which could be used in future studies to reassess the causal relationships between mountain uplift erosion and climate throughout earth s history

geomagnetic moment instability between 0 6 and 1 3 ma from cosmonuclide evidence 1 the reliability of paleomagnetic records as proxies of the geomagnetic field intensity is still a matter of controversy since volcanic materials hardly provide continuous records and marine sediments are suspected to carry a remanence biased by post depositional realignments and or by overprints such long standing debate emphasizes the need for the development of methods independent from paleomagnetism to decipher geomagnetic intensity variations high resolution measurements of authigenic 10be 9be along with a detailed sedimentary record of directional and relative paleointensity variations evidence over the 0 6 1 3 ma time interval frequent and recurrent excursions or short events in the late matuyama and the early brunhes epochs among which two brunhes matuyama reversal precursors and an intra jaramillo excursion the results of this study confirm the idea of a highly unstable geomagnetic field as suggested by paleomagnetic evidences

timing and hydrological conditions of sapropel events in the eastern mediterranean as evident from speleothems soreq cave israel this paper explores the connection between the timing of sapropel events in the eastern mediterranean sea and the time of low 18o events in speleothems of the soreq cave israel a new well dated 18o and 13c profile of the soreq cave speleothems for the last 140 kyr reveals five prominent low 18o events dated at 124 to 119 kyr with the main peak at 122 kyr event v 108 to 100 kyr with peaks at 107 and 102 kyr event iv 85 to 79 kyr with peak at 80 kyr event iii 55 to 52 kyr with a peak at 54 kyr event ii and 8 5 to 7 kyr with peaks at 8 5 and 7 kyr event i these events are characteristic of period of enhanced rainfall in the eastern mediterranean area particularly so for events v and i where the marked decrease in 18o and the correspondingly marked increase in 13c indicate very wet conditions the chronology of the low 18o events v iv iii and i match the ages estimated for the formation of sapropels s5 s4 s3 and s1 which are also associated with high hydrological activity thus it is suggested that the low 18o events in the soreq cave speleothems constrain the maximum duration of the sapropel formation pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that sapropels s5 and s1 were deposited during very wet periods when there were major decrease in the sea surface salinity coupled with frost free winters and drought free summers sapropels s4 and s3 were formed during periods of increased precipitation but ones in which the general climate was mediterranean semi arid

glacial interglacial variation in catchment weathering and erosion paces the indian summer monsoon during the pleistocene abstract silicate weathering via co2 consumption is considered to regulate the long term climate change discriminating silicate versus carbonate weathering from sedimentary records is helpful to understand the coupling relationship between weathering and past climate however owing to the paucity of well dated and continuous records of weathering and erosion the relationship between silicate and carbonate weathering and their links with mountain uplift and glacial interglacial climate remain elusive geochemical analysis of a 666 m core from the heqing basin a paleolake situated at the southeastern margin of the tibetan plateau demonstrates silicate versus carbonate weathering and erosion changes under indian summer monsoon ism conditions spanning the entire pleistocene via respective tracing between rb sr and al na ratios the rb sr record is interpreted as reflecting a competition between clayey silicate origin rb versus carbonate origin sr fluxes whereas al na ratio is a balance between weathering and erosion of silicate minerals both pacing the glacial interglacial cycles of the ism during interglacials strong silicate weathering intensity reflected by low al na values is mainly controlled by warmer temperatures with the reduced variability during 1 82 0 92 ma recalling the pacing of the arabian sea surface temperature amplitudes within glacials the weak carbonate leaching and clay erosion within the older and younger intervals are interrupted by strong clay erosion within the middle interval in conclusion the heqing rb sr and al na records reveal the sensitivity of catchment weathering and erosion to ism changes over glacial interglacial timescales with warmer temperature enhancing silicate weathering and clay formation during interglacials and greater physical erosion during glacials

northern hemisphere climate control of the bengali rivers discharge during the past 4 ma abstract nd isotopes are useful tracers for paleoceanography due to the short nd residence time in seawater and the large differences between the isotopic signatures of various geological reservoirs therefore nd variations reflect the geological history of individual oceanic basins using a differential dissolution technique which extracts nd isotopes of seawater trapped in mno2 coatings and carbonates in marine sediment we measured almost two hundred samples from odp sites 758 and 757 in the northern bay of bengal covering the last 4 ma for the first time we have shown a covariation between nd and 18o over at least the last 800 ka we also show that from 4 ma to 2 6 ma nd is almost constant and starts to fluctuate at 2 6 ma when northern glaciations increased from 2 6 ma to 1 ma the fluctuation period is close to 40 ka while from 1 ma to present it is dominantly 100 ka we attribute these findings to mixing between himalayan river water that ultimately originates as indian summer monsoon rain and normal bay of bengal seawater previous studies on seawater using nd 18o analyzed on planktonic foraminifera and sedimentary data can be integrated into this model a simple quantitative binary mixing model suggests that the summer monsoon rain was more intense during interglacial than glacial periods during last glacial episode the monsoon trajectory was deviated to the east at a large scale the indian monsoon is fully controlled by the variations in northern hemisphere climate but with a complex response function to this forcing our study clearly establishes the large potential of nd isotope data to evaluate the hydrological river regime during the quaternary and its relationship with climate fluctuations particularly when the sediment archive is sampled close to sediment sources

margarite corundum phyllites from the appalachian orogen of south carolina mineralogy and metamorphic history phyllites from the charlotte belt central piedmont of south carolina contain porphyroblasts of black corundum in a matrix of margarite and minor muscovite the margarite corundum phyllites formed during ordovician amphibolite facies metamorphism of an aluminous protolith with a probable mineralogy of pyrophyllite diaspore and calcite through reactions such as 6dia prl 2cal 2mrg 2co 2 2h 2 o and 2dia crn h 2 o the protolith was probably a hydrothermally altered felsic tuff of late proterozoic to cambrian age based on a whole rock chemical analysis of margarite corundum phyllite the protolith is believed to have consisted of about 7 mol calcite 23 mol pyrophyllite and 70 mol diaspore a t x co 2 plot of pertinent reactions in the system cao al 2 o 3 sio 2 h 2 o co 2 shows that in the presence of an h 2 o rich fluid x co 2 0 08 margarite began forming by the reaction 6dia prl 2cal 2mrg 2co 2 2h 2 o at about 350 c at 5 kbar this reaction continued along a univariant path until pyrophyllite and calcite were consumed the remaining diaspore was converted to corundum by the reaction 2dia crn h 2 o at about 430 c for this h 2 o rich fluid at 5 kbar the upper temperature limit of this assemblage is constrained by the reaction mrg crn an h 2 o which should occur at about 575 c under this pressure the absence of plagioclase in these rocks suggests that the conditions of this reaction were never achieved fluids associated with subsequent alleghanian greenschist facies metamorphism reacted with co rundum to form fine grained margarite at corundum margins

retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism staurolite kyanite schists from the dalradian of scotland and ireland show two types of retrograde alteration in the irish examples staurolite and kyanite are replaced by muscovite and paragonite often with margarite and sometimes chloritoid plagioclase in these samples may be albitized in contrast one scottish sample shows replacement of both staurolite and plagioclase by muscovite retrogression resulted from infiltration of fluid but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration it is demonstrated by the use of activity diagrams showing the relationships between al silicate white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down temperature from a granitic or arkosic source in contrast if growth of retrograde albite is actually accompanied by na metasomatism then the infiltrating fluids are likely to have been moving up temperature through pelitic lithologies large fluid rock ratios of around 10 1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids

in phase anomalies in beryllium 10 production and palaeomagnetic field behaviour during the iceland basin geomagnetic excursion increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie points for correlation between records of past climate from marine and terrestrial archives we have investigated the relative timing of variations in 10be production rate and the corresponding palaeomagnetic signal during one of the largest pleistocene excursions the iceland basin ib event ca 190 kyr as recorded in two marine sediment cores odp sites 1063 and 983 with high sedimentation rates variations in 10be production rate during the excursion were estimated by use of 230thxs normalized 10be deposition rates and authigenic 10be 9be resulting 10be production rates are compared with high resolution records of geomagnetic field behaviour acquired from the same discrete samples we find no evidence for a significant lock in depth of the palaeomagnetic signal in these high sedimentation rate cores apparent lock in depths in other cores may sometimes be the result of lower sample resolution our results also indicate that the period of increased 10be production during the ib excursion lasted longer and most likely started earlier than the corresponding palaeomagnetic anomaly in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition the lack of evidence in this study for a significant palaeomagnetic lock in depth suggests that there is no systematic offset between the 10be signal and palaeomagnetic anomalies associated with excursions and reversals with significance for the global correlation of climate records from different archives

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

increasing chemical weathering in the himalayan system since the last glacial maximum continental chemical weathering is central in earth s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans however the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained in this work a composite sediment record from the bay of bengal is used to document the evolution of chemical weathering in the himalayan system himalayan range and indo gangetic floodplain the world largest sediment conveyor to the oceans since the last glacial maximum lgm the degree of weathering of the sediments is documented using mobile to immobile ratios such as k si and h2o si as well as detrital calcite abundance robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition the bay of bengal record is also further compared to the chemical composition of modern river sediments from the ganga and brahmaputra basin weathering proxies all indicate that the sediments exported by the ganga and brahmaputra rivers became increasingly weathered over the past 21 kyr whereas sr nd and major elements suggest a constant sediment provenance in the system over the last 21 kyr these changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the indian ocean during the lgm was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the ganga and brahmaputra responds to late quaternary climate changes

glacial interglacial variations in the neodymium isotope composition of seawater in the bay of bengal recorded by planktonic foraminifera this study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from odp site 758 in the southernmost reaches of the bay of bengal in the north east indian ocean cleaning experiments using oxidative reductive techniques suggest that diagenetic fe mn oxyhydroxide coatings can be effectively removed and that the measured nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated modern core top pulleniatina obliquiloculata and globorotalia menardii give end values of 10 12 0 16 and 10 28 0 16 respectively indistinguishable from recent direct measurements of surface seawater in this area a high resolution nd isotope record obtained from g menardii for the past 150 kyr shows systematic variations end 3 on glacial interglacial timescales the timing of those variations shows a remarkable correspondence with the global oxygen isotope record which suggests a process controlling the nd isotope composition that responds in phase with global climate cycles palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the indian subcontinent and a decrease in the flux of river water delivered to the bay of bengal thus changes in the riverine input of nd a change in either flux or composition most likely caused the isotope variations although changes in dust source or local ocean circulation may have also played a role these results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans and illustrate the potential of nd isotopes in foraminifera for high resolution palaeoceanographic reconstruction

close mass balance of long term carbon fluxes from ice core co 2 and ocean chemistry records on geological timescales carbon dioxide enters the atmosphere through volcanism and organic matter oxidation and is removed through mineral weathering and carbonate burial an analysis of ice core co2 records and marine carbonate chemistry indicates a tight coupling between these processes during the past 610 000 years which suggests that a weathering feedback driven by atmospheric co2 leads to a mass balance between co2 sources and sinks on long timescales feedbacks controlling long term fluxes in the carbon cycle and in particular atmospheric carbon dioxide are critical in stabilizing the earth s long term climate it has been hypothesized that atmospheric co2 concentrations over millions of years are controlled by a co2 driven weathering feedback that maintains a mass balance between the co2 input to the atmosphere from volcanism metamorphism and net organic matter oxidation and its removal by silicate rock weathering and subsequent carbonate mineral burial1 2 3 4 however this hypothesis is frequently challenged by alternative suggestions many involving continental uplift and either avoiding the need for a mass balance or invoking fortuitously balanced fluxes in the organic carbon cycle5 6 7 8 9 here we present observational evidence for a close mass balance of carbon cycle fluxes during the late pleistocene epoch using atmospheric co2 concentrations from ice cores10 11 12 we show that the mean long term trend of atmospheric co2 levels is no more than 22 p p m v over the past 610 000 years when these data are used in combination with indicators of ocean carbonate mineral saturation to force carbon cycle models the maximum imbalance between the supply and uptake of co2 is 1 2 during the late pleistocene this long term balance holds despite glacial interglacial variations on shorter timescales our results provide support for a weathering feedback driven by atmospheric co2 concentrations that maintains the observed fine mass balance

lithium isotopes in speleothems temperature controlled variation in silicate weathering during glacial cycles terrestrial chemical weathering of silicate minerals is a fundamental component of the global cycle of carbon and other elements past changes in temperature rainfall ice cover sea level and physical erosion are thought to affect weathering but the relative impact of these controls through time remains poorly constrained this problem could be addressed if the nature of past weathering could be constrained at individual sites in this study we investigate the use of speleothems as local recorders of the silicate weathering proxy li isotopes we analysed 7li and li in speleothems that formed during the past 200 ka in two well studied israeli caves soreq and tzavoa as well as in the overlying soils and rocks leaching and mass balance of these soils and rocks show that li is dominantly sourced from weathering of the overlying aeolian silicate soils speleothem 7li values are ubiquitously higher during glacials 23 than during interglacials 10 implying more congruent silicate weathering during interglacials where congruent means a high ratio of primary mineral dissolution to secondary mineral formation these records provide information on the processes controlling weathering in israel consideration of possible processes causing this change of weathering congruency indicates a primary role for temperature with higher temperatures causing more congruent weathering lower 7lispeleo the strong relationship observed between speleothem 7li and climate at these locations suggests that li isotopes may be a powerful tool with which to understand the local controls on weathering at other sites and could be used to assess the distribution of weathering changes accompanying climate change such as that of pleistocene glacial cycles

pluvial lakes in the great basin of the western united states a view from the outcrop abstract paleo lakes in the western united states provide geomorphic and hydrologic records of climate and drainage basin change at multiple time scales extending back to the miocene recent reviews and studies of paleo lake records have focused on interpretations of proxies in lake sediment cores from the northern and central parts of the great basin in this review emphasis is placed on equally important studies of lake history during the past 30 years that were derived from outcrop exposures and geomorphology in some cases combined with cores outcrop and core records have different strengths and weaknesses that must be recognized and exploited in the interpretation of paleohydrology and paleoclimate outcrops and landforms can yield direct evidence of lake level facies changes that record details of lake level fluctuations and geologic events such as catastrophic floods drainage basin changes and isostatic rebound cores can potentially yield continuous records when sampled in stable parts of lake basins and can provide proxies for changes in lake level water temperature and chemistry and ecological conditions in the surrounding landscape however proxies such as stable isotopes may be influenced by several competing factors the relative effects of which may be difficult to assess and interpretations may be confounded by geologic events within the drainage basin that were unrecorded or not recognized in a core the best evidence for documenting absolute lake level changes lies within the shore nearshore and deltaic sediments that were deposited across piedmonts and at the mouths of streams as lake level rose and fell we review the different shorezone environments and resulting deposits used in such reconstructions and discuss potential estimation errors lake level studies based on deposits and landforms have provided paleohydrologic records ranging from general changes during the past million years to centennial scale details of fluctuations during the late pleistocene and holocene outcrop studies have documented the integration histories of several important drainage basins including the humboldt amargosa owens and mojave river systems that have evolved since the miocene within the active tectonic setting of the great basin these histories have influenced lake levels in terminal basins many pre late pleistocene lakes in the western great basin were significantly larger and record wetter conditions than the youngest lakes outcrop based lake level data provide important checks on core based proxy interpretations we discuss four such comparisons in some cases such as for lakes owens and manix outcrop and core data synthesis yields stronger and more complete records in other cases such as for bonneville and lahontan conflicts point toward reconsideration of confounding factors in interpretation of core based proxies

soil liquefaction case histories in italy abstract in order to collect adequate data for the evaluation of the soil liquefaction hazard during seismic events in italy a preliminary investigation was carried out aimed at identifying the phenomena that can be attributed to possible soil liquefaction associated with historical earthquakes the more important works on historical seismicity were examined mainly seismic catalogues and specific studies of the main earthquakes the analysis of the data collected so far indicates that soil liquefaction is generally characterized by limited phenomena that occurred within the epicentral areas of events having intensity i ix mcs at the site

frequency dependent landscape response to climatic forcing 1 whereas the existence of pronounced orbitally controlled periodicities is a major feature of earth climate its impact on landscape dynamics remains poorly understood we use a landscape evolution model lem to systematically investigate the response of landscapes to a range of periodic oscillations in precipitation the resulting sediment flux evolution displays a pronounced sensitivity to the period of the input precipitation signal such that for a given erodibility a specific periodicity maximizes the amplitude of the response this optimal period of resonance scales as the inverse of the erodibility but is progressively filtered out of the response when the intensity of hillslope diffusion increases this frequency dependent landscape behavior displayed by our model provides a mechanistic perspective on molnar s 2004 proposition that ubiquitous changes in late cenozoic continental denudation could result directly from modifications in the spectral content of the climatic signal

denudation rates on cratonic landscapes comparison between suspended and dissolved fluxes and 10be analysis in the nyong and sanaga river basins south cameroon south cameroon is located in a tropical and tectonically quiescent region with landscapes characterized by thick highly weathered regolith indicative of the long term predominance of chemical weathering over erosion currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources we analysed two of the main south cameroon rivers the nyong river and sanaga river the sanaga catchment undergoes a contrasted tropical climate from sub humid mountainous and humid climate and is impacted by deforestation agriculture damming mining and urbanization especially in the mbam sub basin draining the highly populated volcanic highlands by contrast the nyong catchment only under humid tropical climate is preserved from anthropogenic disturbance with low population except in the region of yaounde mefou sub basin moreover the nyong basin is dam free and less impacted by agriculture and logging we explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment averaged erosion and denudation datasets the first one consists of an 11 year long gauging dataset while the second one comes from cosmogenic radionuclides crns here beryllium 10 be 10 from sand sampled in the river mainstreams timescale of tens to hundreds of thousands of years modern fluxes estimated from gauging data range from 5 to 100m ma 10 to 200t km 2 yr our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60 and 40 respectively of the total denudation beryllium 10 denudation rates and sediment fluxes range from 4 8 to 40 3m ma or 13 to 109t km 2 yr respectively after correction for quartz enrichment these fluxes are slightly less than the modern fluxes observed in cameroon and other stable tropical areas the highest be 10 derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact copyright c 2016 john wiley sons ltd

long term stability of global erosion rates and weathering during late cenozoic cooling reports that the geological record of erosion indicates a fourfold increase in global sedimentation rates during the past 5 million years merited a global explanation explanations offered include an increased rate of mountain formation and global cooling now jane willenbring and friedhelm von blanckenburg reanalyse original data sets and use ocean records of beryllium isotope ratios to obtain an indirect measure of past erosion rates and rock weathering they find no clear evidence either for an increase in global erosion or for a pulse of weathered material in the oceans during the past 12 million years they suggest that cenozoic global cooling must have some other cause and the mountain building that did occur had little effect on erosion or weathering flux globally accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates and by inference in erosion rates or weathering fluxes during the past 5 million years here proxy evidence is provided that indicates stable weathering fluxes in the late cenozoic era it is proposed that processes different from an increase in denudation caused cenozoic global cooling over geologic timescales co2 is emitted from the earth s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial this balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1 changes in this balance have been attributed to changes in topographic relief where varying rates of continental rock weathering and erosion1 2 are superimposed on fluctuations in organic carbon burial3 geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1 2 4 widespread observations of a recent 0 5 myr fourfold increase in global sedimentation rates require a global mechanism to explain them4 5 6 accelerated uplift and global cooling have been given as possible causes2 4 6 7 but because of the links between rates of erosion and the correlated rate of weathering8 9 an increase in the drawdown of co2 that is predicted to follow may be the cause of global climate change instead2 however globally rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4 10 moreover proxy records of past atmospheric co2 provide no evidence for this large reduction in recent co2 concentrations11 12 here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13 as evidence we recast the ocean dissolved 10be 9be isotope system as a weathering proxy spanning the past 12 myr ref 14 this proxy indicates stable weathering fluxes during the late cenozoic era the sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean we conclude that processes different from an increase in denudation caused cenozoic global cooling and that global cooling had no profound effect on spatially and temporally averaged weathering rates

enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands a sink for atmospheric co2 atmospheric co2 and global climate are closely coupled since 800 ka co2 concentrations have been up to 50 higher during interglacial compared to glacial periods because of its dependence on temperature humidity and erosion rates chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition cooler and drier conditions and lower non glacial erosion rates suppressed in situ chemical weathering rates during glacial periods however using systematic variations in major element geochemistry sr nd isotopes and clay mineral records from ocean drilling program sites 1143 and 1144 in the south china sea spanning the last 1 1 ma we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials we attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for 9 of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial interglacial variation of 80 ppmv as a result if similar magnitudes can be identified in other tropical shelf slope systems the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric co2 during cold stages of the quaternary this in turn would have caused an intensification of glacial cycles

synsedimentary deformation of pleistocene glaciolacustrine deposits in the albese con cassano area southern alps northern italy and possible implications for paleoseismicity abstract we describe and interpret the well exposed soft sediment deformations preserved in a mid pleistocene proglacial sequence outcropping at albese con cassano near como southern alps n italy the observed deformations affect six distinct stratigraphic intervals within the same depositional sequence the stratigraphy of this sequence the geomorphic setting and the careful geological mapping of the area clearly show that these deformations are not the result of glaciotectonics or cryoturbation but are due to six separate events of synsedimentary liquefaction in particular the 3 basal deformed units include sand and gravel volcanoes that formed at the bottom of a proglacial lake we examined several hypothesis of possible causative mechanism for the formation of these features taking into account all the information available on the local geology and sedimentary environment it is possible to assess that the liquefaction features at albese con cassano had a coseismic origin to our knowledge this would be the first report of similar evidence of paleoseismicity in the lombardia southern alps also there is no record of historical liquefaction triggered by seismic events in an area of ca 70 km of radius near albese con cassano including the town of como the large metropolitan area of milano and most of w lombardia that is one of the largest industrial districts in europe therefore the interpretation of the observed deformations can have critical impact for understanding the current seismicity level in the region and mitigating the associated risk in this paper we show that this interpretation is not a simple process liquefaction features must be treated with caution in order to derive suitable information for seismic hazard characterization even if their coseismic origin has been definitely proved it is necessary to understand the amount of seismic shaking for instance in terms of local macroseismic intensity that can have produced that specific feature at that specific site the location of the causative seismic event epicenter and focal depth and eventually its size in terms of epicentral intensity and or earthquake magnitude in the case of albese con cassano the implications in terms of seismic hazard of the observed possible earthquake paleoliquefactions can only be assessed through the definition of several scenarios taking into account a different thresholds of shaking for triggering the described synsedimentary structures and b different seismotectonic frameworks related to the mid pleistocene to holocene evolution of the s alps including seismicity induced by isostatic glacial rebound we conclude that it is worth checking these scenarios in the field with further paleoseismological research on other possible evidence of past earthquakes there are no unequivocal sedimentological indicators of paleoseismicity however when carefully interpreted within the proper local seismotectonic context and compared with other earthquake induced features soft sediment deformations can offer information of critical value for seismic hazard assessment

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

the chemical index of alteration cia as a proxy for climate change during glacial interglacial transitions in earth history abstract the glacial interglacial transitions of ice ages are associated with large rapid changes in climate which potentially can be reconstructed from stratigraphic profiles of the chemical index of alteration cia here we present a case study based on high resolution cia profiles of neoproterozoic glacial deposits from south china paleolatitude 30 n as a record of the climate transition at the termination of the sturtian glaciation 663 ma drillcore zk2115 gaodi deposit exhibits a progressive upsection increase in mean cia over a 50 m interval from 58 range 52 65 note all ranges are given as 16th 84th percentiles in the synglacial upper tiesi ao formation to 67 range 64 69 in the deglacial basal datangpo formation cap carbonates and to 68 range 66 70 in the postglacial datangpo formation black shales a coeval section from the lijiawan deposit also exhibits an increasing cia trend upsection within shallow water mn carbonate facies these cia patterns show no relationship to lithology and are interpreted to reflect climatic warming and intensified chemical weathering during the sturtian deglaciation similar large increases in postglacial cia values are seen in paleoproterozoic and late paleozoic successions and at least moderate increases in cia values are recorded during warm stages of the late quaternary e g the bolling allerod and holocene relative to the cold stages e g the last glacial maximum and younger dryas the rapidity of cia changes in quaternary systems 1 to 10 kyr suggests that weathering intensity may have changed at similarly rapid rates in more poorly dated deep time glacial successions with potential implications for snowball versus slushball models of cryogenian glaciations these examples show that cia can be a robust proxy for climate changes i e enhanced chemical weathering intensity during glacial interglacial transitions of both ancient and recent ice ages

a high resolution authigenic 10be 9be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the portuguese margin a high resolution study of authigenic be isotopes 10be and 9be combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the portuguese margin n e atlantic covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10be production rate variations a carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10be 9be ratio eight intervals of significant authigenic 10be 9be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions some being already admitted others being proposed as new geomagnetic features since contrarily to sedimentary magnetic remanence the authigenic 10be 9be records dipole moment variations without significant acquisition delay it provides better constraints on their timing comparison of 10be 9be and benthic 18o records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes with a quasi period of 100 ka

insights into shallow level processes of mountain building from the northern apennines italy recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify the early stages of mountain building similar on land evidence is comparatively sparse as a result of difficulties of exposure and of early features having been obscured here we describe aspects of three structures from the northern apennines of italy that corroborate these notions of complex shallow level orogenic processes the upper cretaceous castiglioncello melange formed during the subduction phase of apennine orogenesis the major cervarola thrust and the smaller intra formational balduini thrust both of tortonian age represent the ensuing collisional phase all three structures formed at burial depths less than 2 or 3 km and are syn diagenetic although differing in detail all three show features such as clay fabrics and mineralized veins that record complex histories of fluid flow overpressure and deformation the clays show varying types of scaliness some of which had to involve pore dilation and collapse operating in cycles the veins typically evolved from networks of bedding parallel and bedding perpendicular veinlets through dilation breccias and mosaic textures to shear and extensional fibre growth during hydrofracture such observations imply fluctuating physical conditions and dynamic tectonic processes operating on shallow level sediments in a way that is highly heterogeneous with depth areally and through time

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

late quaternary variability of mediterranean outflow water from radiogenic nd and pb isotopes mediterranean outflow water mow is characterised by higher temperatures and salinities than other ambient water masses mow spreads at water depths between 500 and 1500 m in the eastern north atlantic and has been a source of salinity for the atlantic meridional overturning circulation in the north atlantic we used high resolution nd and pb isotope records of past ambient seawater obtained from authigenic ferromanganese coatings of sediments in three gravity cores at 577 1745 and 1974 m water depth in the gulf of cadiz and along the portuguese margin complemented by a selection of surface sediments to reconstruct the extent and pathways of mowover the past 23 000 years the surface and downcore nd isotope data from all water depths exhibit only a very small variability close to the present day composition of mowbut do not reflect the present day nd isotopic stratification of the water column as determined from a nearby open ocean hydrographic station in contrast the pb isotope records show significant and systematic variations which provide evidence for a significantly different pattern of the mow pathways between 20 000 and 12 000 years ago compared with the subsequent period of time

the depositional flux of meteoric cosmogenic 10be from modeling and observation abstract meteoric cosmogenic 10be is a powerful tracer to quantify dates and rates of earth surface processes over timescales of 103 105 yrs a prerequisite for its applications is knowledge of the flux at which 10be produced in the atmosphere is delivered to the earth surface four entirely independent approaches are available to quantify this flux 1 general circulation models gcm combined with 10be production functions and aerosol dynamics 2 10be in precipitation collections 3 10be inventories in dated soil profiles 4 riverine 10be exported in solid and dissolved forms we compiled and reprocessed published globally distributed 10be flux data from each of these methods and compared them with each other after normalization to a common atmospheric production rate based on precipitation records we propose a simple framework to discriminate between two delivery effects on 10be fluxes in the additive effect water vapor and 10be are continuously accumulating during long distance transport leading to an increase in 10be flux with precipitation rate in the dilution effect the 10be flux is delivered from proximal vapor sources limited by the rate of 10be introduction from the stratosphere and independent of precipitation rate both effects are mostly present in combination and the relative weight of either effect depends on vapor condensation rate and on the ratio of vapor condensation area to precipitation area a comparison between precipitation derived fluxes and gcm derived fluxes shows that half of the precipitation estimates are 2 times greater than gcm derived fluxes by comparison soil and gcm derived fluxes agree within a factor of 2 for more than half 57 of the dataset and the remaining soil estimates 43 are much lower than gcm derived fluxes 71 of 10be flux estimates from riverine export using 10be meteoric 9be ratios also agree with gcm derived fluxes within a factor of 2 we explain the precipitation derived fluxes that commonly exceed all other estimates by short term stochasticity in precipitation events that might introduce a measurement time interval bias towards higher fluxes this bias is not present over longer term 103 105 yrs flux estimates like those from soil profiles soil derived fluxes might still present an underestimation when retention of 10be in soil is incomplete we recommend producing more 10be depositional flux data from soil inventories with full be retention as these generate in our view the most relevant estimates for applications of meteoric 10be on millennial scale earth surface processes

the geological significance of cosmogenic nuclides in large lowland river basins abstract measured from a bag of sand taken in large rivers the concentration of cosmogenic nuclides such as in situ produced 10be 26al and 14c can be used to constrain the mean sediment flux of the headwaters and assess the duration of sediment storage from source to sink we revisit these principles with examples from the amazon and ganga basins we identify two end member cases controlling the concentration of cosmogenic nuclides in lowland river sediment 1 if the time scale of floodplain sediment storage is short compared to the half life of the nuclide in situ cosmogenic nuclide concentrations are not significantly altered in lowland basins in this case the concentration of e g in situ produced 10be in the sediment taken in the lowland basin equals in most cases that exported from the sediment source area but the variability in nuclide concentrations between headwater streams is significantly averaged out thus to convert the measured river sediments in situ cosmogenic nuclide concentration into a catchment wide denudation rate production rates are scaled to include those of the sediment producing mountainous areas only rather than the entire catchment area nuclide production in the lowlands where no sediment is being produced is hence excluded this correction is termed floodplain correction 2 if sediment buried for periods of the order of the nuclides half life is episodically re entrained into the active river paired nuclides for example the ratio of 26al 10be through the differential decay of these isotopes constrain the the storage duration and re mixing of floodplain sediment as in situ cosmogenic nuclides measure denudation rates over longer time scales typically integrating between 103 and 105 years their combination with modern estimates of sediment fluxes from river load gauging offers a rich potential of deciphering the controls of earth surface fluxes across large basins from this combination we can assess how river systems react to external perturbations such as climate change or human interference and how such signals are transmitted through the alluvial reaches of lowland basins the most important results in both basins are that i lowland amazon and ganga nuclide concentrations indeed broadly reflect the spatial average of andean and himalayan denudation respectively ii the thus calculated sediment fluxes are within uncertainty of modern fluxes from sediment gauging indicating complete sediment delivery to the sea without net loss into the basin and iii only the amazon basin given its size contains a substantial fraction of sediment buried temporarily in the quaternary finally we provide a short introduction to the use of meteoric cosmogenic nuclide 10be and its ratio to stable 9be released by rock weathering in lowlands in the amazon basin we show that 10be concentrations in fine grained suspended loads depth integrated over the water column using al si ratios provide erosion rates whereas the 10be 9be ratio on be adsorbed to particles provides the denudation rate both are similar to denudation rates from in situ 10be we show that this system responds more sensitively to sediment storage than the in situ system and both accumulation and decay of meteoric concentrations may thus be used to determine sediment residence time

glacial weathering sulfide oxidation and global carbon cycle feedbacks connections between glaciation chemical weathering and the global carbon cycle could steer the evolution of global climate over geologic time but even the directionality of feedbacks in this system remain to be resolved here we assemble a compilation of hydrochemical data from glacierized catchments use this data to evaluate the dominant chemical reactions associated with glacial weathering and explore the implications for long term geochemical cycles weathering yields from catchments in our compilation are higher than the global average which results in part from higher runoff in glaciated catchments our analysis supports the theory that glacial weathering is characterized predominantly by weathering of trace sulfide and carbonate minerals to evaluate the effects of glacial weathering on atmospheric pco 2 we use a solute mixing model to predict the ratio of alkalinity to dissolved inorganic carbon dic generated by weathering reactions compared with nonglacial weathering glacial weathering is more likely to yield alkalinity dic ratios less than 1 suggesting that enhanced sulfide oxidation as a result of glaciation may act as a source of co 2 to the atmosphere back of the envelope calculations indicate that oxidative fluxes could change ocean atmosphere co 2 equilibrium by 25 ppm or more over 10 ky over longer timescales co 2 release could act as a negative feedback limiting progress of glaciation dependent on lithology and the concentration of atmospheric o 2 future work on glaciation weathering carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals

the laschamp geomagnetic dipole low expressed as a cosmogenic 10 be atmospheric overproduction at 41 ka authigenic 10be 9be ratiomeasurementswere performed at high resolution along a portuguese margin deep sea core 37 48 n 10 09w spanning the 20 50 ka time interval in order to reconstruct variations in atmospheric cosmogenic 10be production rates and derive the related geomagnetic dipole moment modulation a complementary approach consisting in 230thxs normalized 10be deposition rate determination on selected samples confirmed the reliability of the authigenic 10be 9be record this study constitutes the first successful comparison of the two widely used normalization techniques of 10be concentrations for both methods the presented results herein evidence a factor of 2 cosmogenic nuclide overproduction linked to a minimum dipole moment associated with the laschamp excursion the latter is stratigraphically constrained beneath heinrich event 4 its age is estimated at 41 ka on the basis of direct correlation between the series of rapid paleoclimatic events recorded in the portuguese margin sediments and in the greenland ice sheet and is confirmed by calibrated radiocarbon dating carried out on the same sediments the remarkable agreement between the authigenic 10be 9be and the greenland ice cores 10be deposition rate records attests to their global significance this new authigenic 10be 9be record has been combined with that previously obtained at the same site to produce a stacked record that is calibrated using absolute values of virtual dipole moment determined on lava flows this provides a reconstruction of dipole geomagnetic moment variations over the 20 50 ka interval independent frompaleomagnetically constrainedmethods that documents the laschamp dipole low but fails to express any dipole low related to t e mono lake excursion this high resolution record responds to the necessity of supplementing the knowledge of the atmospheric 14c variations in the 30 45 ka interval during which the 14c calibration curve suffers from a lack of accurate data and during which a discrepancy of about 5500 yr between the 14c and u th ages is due to the laschamp geomagnetic dipole low such new high resolution datasets from records obtained from different latitudes will be required to make significant advances in understanding the causes of atmospheric 14c variations

stable runoff and weathering fluxes into the oceans over quaternary climate cycles the effect of glacial interglacial cycles on surface weathering rates has been unclear a beryllium based proxy for weathering shows minimal variations in the input of silicate weathering products to the oceans for the past two million years

resolving the integral connection between pedogenesis and landscape evolution abstract soil is central in the terrestrial ecosystem linking and providing feedback responses to the other components i e water atmosphere and vegetation however the role of soil in landscape evolution is usually not well acknowledged in modeling landscape evolution soil is only treated as a residue of weathering that is transported and redistributed along the hillslope weathering is considered as a process that produces clays and generates unconsolidated materials available for erosion while pedology has been debating the form of qualitative factorial models for 75 years models for soil water heat solute gas and chemical reactions in a profile have matured as soils are distributed continuously in three dimensions across landscapes the profile models need to consider lateral fluxes this review outlines the role of soil in landscape modeling first we review the role of soil in the current landscape evolution models we then review data and models on soil weathering rates and transport processes we discuss soil profile models that simulate soil formation processes and combined soil landscape evolution models finally we discuss how the models can be tested and validated in the real world and suggest how both soil scientists and landscape modelers can work together to address the grand challenges in modeling earth surface processes

the competition between coastal trace metal fluxes and oceanic mixing from the 10be 9be ratio implications for sedimentary records at an ocean margin site 37 s offshore chile we use the meteoric cosmogenic 10be 9be ratio to trace changes in terrestrial particulate composition due to exchange with seawater we analyzed the marine authigenic phase in surface sediments along a coast perpendicular transect and compared to samples from their riverine source we find evidence for growth of authigenic rims through coprecipitation not via reversible adsorption that incorporate an open ocean 10be 9be signature from a deep water source only 30 km from the coast overprinting terrestrial 10be 9be signatures together with increasing 10be 9be ratios particulate bound fe concentrations increase which we attribute to release of fe rich pore waters during boundary exchange in the sediment the implications for the use of 10be 9be in sedimentary records for paleodenudation flux reconstructions are the following in coast proximal sites the authigenic record will likely preserve local riverine ratios unaffected by exchange with seawater whereas sites beneath well mixed seawater will preserve global flux signatures

critical zone structure controls concentration discharge relationships and solute generation in forested tropical montane watersheds concentration discharge c q relationships are poorly known for tropical watersheds even though the tropics contribute a disproportionate amount of solutes to the global ocean the luquillo mountains in puerto rico offer an ideal environment to examine c q relationships across a heterogeneous tropical landscape we use 10 30 years of weekly stream chemistry data across ten watersheds to examine c q relationships for weathering products sio2 aq ca2 mg2 na and biologically controlled solutes dissolved organic carbon doc dissolved organic nitrogen don nh4 no3 po43 k so42 we analyze c q relationships using power law equations and a solute production model and use principal component analysis to test hypotheses regarding how the structure of the critical zone controls solute generation volcaniclastic watersheds had higher concentrations of weathering solutes and smaller tributaries were approximately 3 fold more efficient at generating these solutes than larger rivers lithology and vegetation explained a significant amount of variation in the theoretical maximum concentrations of weathering solutes r2 0 43 0 48 and in the c q relationships of po43 r2 0 63 and sio2 aq r2 0 47 however the direction and magnitude of these relationships varied across watersheds various forms of n and p displayed variable c q relationships while doc was consistently enriched with increasing discharge results suggest that po43 may be a useful indicator of watershed function relationships between c q and landscape characteristics indicate the extent to which the structure and function of the critical zone controls watershed solute fluxes

a test of the cosmogenic 10be meteoric 9be proxy for simultaneously determining basin wide erosion rates denudation rates and the degree of weathering in the amazon basin we present an extensive investigation of a new erosion and weathering proxy derived from the 10be meteoric 9be stable ratio in the amazon river basin this new proxy combines a radioactive atmospheric flux tracer meteoric cosmogenic 10be with 9be a trace metal released by weathering results show that meteoric 10be concentrations 10be and 10be 9be ratios increase by 30 from the andes to the lowlands we can calculate floodplain transfer times of 2 30 kyr from this increase intriguingly however the riverine exported flux of meteoric 10be shows a deficit with respect to the atmospheric depositional 10be flux most likely the actual area from which the 10be flux is being delivered into the mainstream is smaller than the basin wide one despite this imbalance denudation rates calculated from 10be 9be ratios from bed load suspended sediment and water samples from amazon rivers agree within a factor of 2 with published in situ 10be denudation rates erosion rates calculated from meteoric 10be measured from depth integrated suspended sediment samples agree with denudation rates suggesting that grain size induced variations in 10be are minimized when using such sampling material instead of bed load in addition the agreement between erosion and denudation rates implies minor chemical weathering intensity in most amazon tributaries indeed the be specific weathering intensity calculated from mobilized 9be comprising reactive and dissolved fractions that are released during weathering is constant at approximately 40 of the total denudation from the andes across the lowlands to the amazon mouth therefore weathering in the amazon floodplain is not detected

surface ages and weathering rates from 10be meteoric and 10be 9be insights from differential mass balance and reactive transport modeling abstract to reconstruct the dynamic evolution of weathered material at the earth s surface requires techniques that record the fluxes of material into and out of the weathering zone cosmogenic isotopes formed in situ within the earth s surface or in the atmosphere meteoric are important chronometers for both time and fluxes meteoric 10 be has been used to evaluate surface ages soil residence times and rates of sediment transport however the meteoric 10 be dating tool relies on complete retention of 10 be on mineral surfaces within the weathered regolith an assumption that will not be valid where soil ph is low or mineral surface area available for sorption is lacking here we evaluate whether the stable isotope of 9 be that is released upon weathering reactive be can be used to correct for 10 be mobility this approach relies on the inventory of reactive 9 be in the weathering zone as a measure of the mass depletion whereas the 10 be inventory is a function of age to explore the spatial and temporal relationship between the be isotopes we use a multi component reactive transport model framework to evaluate a set of differential mass balance equations for the 10 be 9 be composition of the bulk solid and sorbed reservoirs on non eroding surfaces we first find that the 10 be 9 be reac is inversely proportional to the long term chemical weathering rate even in cases of incomplete be retention second we find that coupling 10 be and 9 be to provide a tool for surface age dating 10 be is complex because incomplete retention affects each isotope to a different extent beryllium 10 is supplied at the surface of soil where sorption onto secondary minerals is more extensive while 9 be is released from primary minerals at the weathering front where secondary minerals are less abundant if 9 be loss is less than about 15 we find 9 be inventories can be used to reliably correct 10 be inventories to within 10 the error associated with the correction also increases with increasing age and thus the correction is more favorable for younger surfaces over longer timescales the error on the age introduced by the loss correction using 9 be is magnified whereas the error associated with uncorrected ages decreases because the 10 be is more strongly retained as chemical weathering progresses we also find that the reactive transport model confirms a set of simple mass balance descriptions providing guidance for future data collection collectively the modeling results suggest that measurement of be isotopes in weathered regolith provides a unique approach to determine both surface ages and chemical weathering rates

glacial erosion dynamics in a small mountainous watershed southern french alps a source to sink approach in this study we used major element composition neodymium isotopes ratios endend and concentration of ree to track and quantify the sediment routing in the var sedimentary system from source southern french alps to sink ligurian sea over the last 50 ka our data reveal that changes in sediment sources over that period associated with concomitant changes in the hyperpycnal i e flood generated turbidity currents activity in the var submarine canyon were mainly driven by paleoenvironmental conditions in the upper basin and in particular by the presence of glaciers during the last glacial period based on this evidence we determined when and how glacier derived sediments were produced then excavated and transferred to the ocean allowing us to ultimately tune offshore sedimentary records to onshore denudation rates in contrast to large glaciated systems we found that sediment export from the var river to the mediterranean sea directly responded to climate induced perturbations within the basin finally we estimated that sediment fluxes in the var routing system were 2 5 times higher during the last glacial maximum than today thus confirming that glacier denudation rates exceed fluvial rates and that such a pattern also governs the interglacial glacial sediment flux cycle in other small mountainous basins

earth surface erosion and weathering from the 10be meteoric 9be ratio abstract the isotope ratio of the meteoric cosmogenic nuclide 10 be to the mineral derived stable isotope 9 be discloses both the earth surface denudation rate and its weathering intensity we develop a set of steady state mass balance equations that describes this system from a soil column over the hillslope scale to an entire river basin the prerequisites making this new approach possible are 1 the 9 be concentration of parent rock typically 2 5 0 5 ppm in granitic and clastic sedimentary lithologies is known 2 both be isotopes equilibrate between the fluids decomposing rock and reactive solids formed during weathering and 3 a critical spatial scale is exceeded at which the fluxes of both isotopes into and out of the weathering zone are at steady state over the time scale of weathering typically 10 kyr for these cases the isotope ratios can be determined in bulk sediment or soil on leachates from the reactive adsorbed and pedogenic mineral bound phase in sediment or soil and even on the dissolved phase in river water the 10 be 9 be ratio offers substantial advantages over the single isotope system of meteoric 10 be the latter system allows to directly determine erosion rates only in the case that 10 be is fully retentive in the weathering zone and that riverine sorting has not introduced grain size dependent 10 be concentration gradients in sediments we show the feasibility of the 10 be 9 be tracer approach at the river scale for sediment and water samples in the amazon basin where independent estimates of denudation rates from in situ produced 10 be exist we furthermore calculate meaningful denudation rates from a set of published 10 be 9 be ratios measured in the dissolved load of globally distributed rivers we conclude that this isotope ratio can be used to reconstruct global paleo denudation from sedimentary records

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to

an emp and tem aem study of margarite muscovite and paragonite in polymetamorphic metabauxites of naxos cyclades greece and the implications of fine scale mica interlayering and multiple mica generations

the relationship between 10be and geomagnetic field strength records in central north atlantic sediments during the last 80 ka abstract a profile of leached 10 be 9 be has been measured in an 80 ka sediment core from the northeast atlantic ocean this profile shows a significant inverse correlation with the palaeomagnetic intensity profile inferred from nrm arm measurements in the same core particularly between 40 and 50 ka where a rapid decrease in field intensity is accompanied by a rapid increase in 10 be 9 be the extent to which this increase in 10 be 9 be at 40 ka is associated with a 10 be peak seen in polar ice cores at about the same time remains speculative