illite neoformation in plagioclase during weathering evidence from semi arid northeast brazil illite minerals in soils and weathered rocks are usually interpreted as secondary clays derived from the transformation of primary rock forming minerals with similar properties e g muscovite or as direct products of hydrothermal alteration after feldspar i e sericite with the help of detailed investigations involving x ray diffraction scanning electron microscopy and x ray element mapping this study testifies to the neoformation of illite inside plagioclase feldspar crystals after their partial dissolution in the weathering zone of luvisols in semi arid northeast brazil our results thus reveal that meteoric weathering can produce illite not only from mica i e by a transformation process but also within non alkali feldspar by a neoformation process at least under the semi arid conditions that have long prevailed in the study area given that illite in weathering mantles has recurrently been attributed often by default to hydrothermal processes results shown here provide new focus to an alternative pathway of illite formation primarily controlled by climate

coastal land loss in texas an overview abstract nearly 1 500 acres of prime coastal real estate and productive wetlands are destroyed each year in texas along the gulf shoreline near bay margins and within alluvial valleys primarily as a result of erosion and submergence wetland losses constitute about 75 of the total land losses historical analyses of maps and aerial photographs indicate that land losses are accelerating and that human activities are either directly or indirectly responsible for the increased losses natural decreases in sediment supply have been exacerbated by 1 river basin projects that reduce the volume of sediment transported to the coast and 2 coastal structures and navigation projects that prevent redistribution of littoral sediments along the coast erosion caused by high wave and current energy and an inadequate supply of sediment is responsible for higher local rates of land loss than submergence erosion losses are also more perceptible especially after major storms when the greatest losses occur boat traffic causes substantial erosion along some navigation channels the principal components of submergence are subsidence and the eustatic rise in sea level submergence converts uplands to wetlands and wetlands to open water these surficial changes occur mostly on the coastal plain but are also observed on barrier islands on bayhead deltas and within entrenched valleys recent accelerated submergence is induced by extraction of shallow ground water or production of hydrocarbons at moderate depths faults activated by the withdrawal of these fluids concentrate the subsidence near fault planes coastal land losses directly caused by dredging marinas subdivisions canals rig locations are less than those caused by erosion and submergence but they constitute a growing percentage of total land losses in texas

the localized role of base level lowering on channel adjustment of tributary streams in the trinity river basin downstream of livingston dam texas usa the purpose of this study is to describe and explain river channel cross sectional change in tributary streams within the 60 km scour zone of the trinity river texas downstream of livingston dam this study investigates changes within the tributary systems to base level changes triggered by incision associated with scour and channel migration within the dammed trunk stream the dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment geomorphological effects of the lower trinity river tributaries were investigated through three different types of data resurveys of bridge cross sections planform change as measured from aerial photographs and field mapping of changes in channel geometry using fallen trees undercut banks and other geomorphic indicators of change while field evidence of channel change was found at sites well upstream of tributary confluences with the trinity these were not consistent with responses to lower base level downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches this suggests distance decay and lag times in transmission of responses upstream and or masking of dam induced effects by other processes and controls the most dynamic locations of geomorphic change are near the confluences of the tributaries with the trinity river where the response to dam induced incision is strongly influenced by local trunk stream morphology indicating a high degree of historical and geographical contingency

dam to delta sediment inputs and storage in the lower trinity river texas livingston dam on the trinity river in se texas usa disrupts the transport of sediment to the lower trinity river and the trinity bay galveston bay estuary however a sediment budget of the lower basin shows that the effects of this disruption are undetectable in the lower river sediment trapped in lake livingston is partly offset by channel erosion downstream of the dam and by inputs from the lower basin most importantly however the lower coastal plain reaches of the trinity are characterized by extensive alluvial storage and are a bottleneck that buffers the bay from effects of upstream changes in sediment flux storage is so extensive that the upper trinity basin and the lowermost river reaches were essentially decoupled in the sense that very little upper basin sediment reached the lower river long before the dam was constructed whereas sediment storage in lake livingston is extensive alluvial storage on the trinity flood plain is even more extensive dam related sediment starvation effects are noted for about 52 km downstream and the sediment budget suggests that a majority of the sediment in this reach is likely derived from channel scour and bank erosion the capacious alluvial storage in the lower trinity not only limits flux to the bay but the large amount of remobilizable alluvium also allows the system to adjust to localized sediment shortages as illustrated in the dam to romayor reach internal adjustments within the lower trinity river valley thus buffer the bay from changes in sediment supply upstream

review of recent developments in mineral magnetism of the chinese loess mineral magnetism of the chinese loess has been investigated for more than 20 years although there is a consensus that the neoformation of fine grained maghemite particles in the superparamagnetic sp and single domain sd grain size regions accounts for the magnetic enhancement in the chinese paleosols quantitative retrieval of paleoclimatic signals in terms of rock magnetic proxies is still a subject of debate the ambiguities arise from the inherent complexities of magnetic proxies as well as the multiple factors that control the pedogenic processes therefore a better description of the magnetic assemblage including its mineralogy grain size distribution and stoichiometry of two distinct origins pedogenic and eolian can help us better understand mechanisms behind variations in magnetic proxies at different timescales in order to link them to the paleoclimatic processes this review focuses on recent developments in loess magnetism and carefully evaluates merits and limitations of rock magnetic proxies furthermore several currently unsolved problems are addressed

paleomagnetic directions and 40ar 39ar ages from the tatara san pedro volcanic complex chilean andes lava record of a matuyama brunhes precursor the same volcanic sequence overlain by flows with normal polarity the 40 ar 39 ar incremental heating experiments on lavas within the two sections provide nine independent age determinations and yield a weighted mean of 791 7 3 0 ka 2s for the paleomagnetic transition the sections are linked by geological mapping the precise radioisotopic dating and geochemical correlations alternating field and thermal demagnetization studies rock magnetic analyses and petrographic observations indicate that the magnetization is primary and carried by titanomagnetite the polarity change is characterized by a jump from reverse poles to a quasi stationary cluster of virtual geomagnetic poles over australia followed by a jump to normal polarity latitudes magnetization of these lavas is thus consistent with either a brief period when the field was dominated by a subequatorial dipole or a more complex nondipolar field that may reflect the influence of a long lived regional lower mantle control over a weakened dynamo the quebrada turbia lavas are circa 16 kyr older than those dated by exactly the same methods which record a later more complex portion of the reversal at haleakala volcano maui moreover the 792 ka radioisotopic age of these chilean lavas is older than most astronomical estimates for the matuyama brunhes reversal suggesting that this section may in fact record a precursor to the actual field reversal that is expressed by low paleointensities in more than a dozen well studied marine sediment cores index terms 1535 geomagnetism and paleomagnetism reversals process timescale magnetostratigraphy 1030 geochemistry geochemical cycles 0330 9360 information related to geographic region south america 9604 information related to geologic time cenozoic keywords paleomagnetism reversal

using a structural approach to identify relationships between soil and erosion in a semi humid forested area south india biogeochemical and hydrological cycles are currently studied on a small experimental forested watershed 4 5 km 2 in the semi humid south india this paper presents one of the first data referring to the distribution and dynamics of a widespread red soil ferralsols and chromic luvisols and black soil vertisols and vertic intergrades cover and its possible relationship with the recent development of the erosion process the soil map was established from the observation of isolated soil profiles and toposequences and surveys of soil electromagnetic conductivity em31 geonics ltd lithology and vegetation the distribution of the different parts of the soil cover in relation to each other was used to establish the dynamics and chronological order of formation results indicate that both topography and lithology gneiss and amphibolite have influenced the distribution of the soils at the downslope the following parts of the soil covers were distinguished i red soil system ii black soil system iii bleached horizon at the top of the black soil and iv bleached sandy saprolite at the base of the black soil the red soil is currently transforming into black soil and the transformation front is moving upslope in the bottom part of the slope the chronology appears to be the following black soil bleached horizon at the top of the black soil streambed bleached horizon below the black soil it appears that the development of the drainage network is a recent process which was guided by the presence of thin black soil with a vertic horizon less than 2 in deep three distinctive types of erosional landforms have been identified 1 rotational slips type 1 2 a seepage erosion type 2 at the top of the black soil profile 3 a combination of earthflow and sliding in the non cohesive saprolite of the gneiss occurs at midslope type 3 types 1 and 2 erosion are mainly occurring downslope and are always located at the intersection between the streambed and the red soil black soil contact neutron probe monitoring along an area vulnerable to erosion types 1 and 2 indicates that rotational slips are caused by a temporary watertable at the base of the black soil and within the sandy bleached saprolite which behaves as a plane of weakness the watertable is induced by the ephemeral watercourse erosion type 2 is caused by seepage of a perched watertable which occurs after swelling and closing of the cracks of the vertic clay horizon and within a light textured and bleached horizon at the top of black soil type 3 erosion is not related to the red soil black soil system but is caused by the seasonal seepage of saturated throughflow in the sandy saprolite of the gneiss occurring at midslope c 2006 elsevier b v all rights reserved

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

the characterization of steady state in earth surface systems findings from the gradient modelling of an indian climosequence abstract gradient modelling of a steep ecocline in south india has highlighted a suite of landscape units which appear to be entirely ordained by a simple water balance index this univariate climofunction is reflected in vegetation landform weathering and soil patterns although the vegetation successions are the most spectacular consistent pedogeomorphological trends were identified at several embedded scales of reality at the microscopic scale the existence of illite rich saprolite situated between the kaolinite dominated and smectite dominated core areas of the climosequence so far a missing mineralogical link only predicted by geochemical theory was effectively verified by field evidence at the megascopic scale the variation in topographic texture from polyconvex to polyconcave land systems via a convexo concave transition zone also suggests a strong relationship between hydroclimatic budgets and landforms such an exceptional environmental cabinet is well suited to reexamining other benchmark climosequences of the tropics and to readdressing the debated theories of climatic geomorphology by a discussion on the applicability of climax and steady state to tropical landforms

present past and potential denudation rates is there a link tentative evidence from fission track data river sediment loads and terrain analysis in the south indian shield abstract ahnert ahnert f 1970 functional relationships between denudation relief and uplift in large mid latitude drainage basins am j sci 268 243 263 proposed that a denudation rate d could be predicted by the relation d 0 1535 h where h is local relief a substitute for mean slope the present study seeks to explore the value of this simple equation as a normative geomorphological tool in two successive and complementary stages its principles are applied to the western dharwar craton of south india first the robustness of local relief as a reliable substitute for mean slope is examined following the demonstration that the equation can successfully be used in the region of interest pending correction factors for rock resistance a potential denudation map of the western dharwar craton is established the pattern of potential denudation values is compared with known measurements of both long term 10 2 ma denudation rates inferred from apatite fission track data and modern 10 5 ma denudation rates derived from available sediment load measurements in relevant river basins unexpectedly close similarities between long term short term and potential denudation rates prompt a discussion on the merits of potential denudation as a possible standard in the appraisal of measured rates and as a tool for establishing permissible denudation rates when assessing the impact of man induced erosion a landscape specific basal metabolic rate of erosion is defined as a new concept the methodological pitfalls in extrapolating denudation rates from the measurement of sediment loads in rivers are reviewed and suggestions for tighter investigation are made

water balance modelling in a tropical watershed under deciduous forest mule hole india regolith matric storage buffers the groundwater recharge process accurate estimations of water balance are needed in semi arid and sub humid tropical regions where water resources are scarce compared to water demand evapotranspiration plays a major role in this context and the difficulty to quantify it precisely leads to major uncertainties in the groundwater recharge assessment especially in forested catchments in this paper we propose to assess the importance of deep unsaturated regolith and water uptake by deep tree roots on the groundwater recharge process by using a lumped conceptual model comfort the model is calibrated using a 5 year hydrological monitoring of an experimental watershed under dry deciduous forest in south india mule hole watershed the model was able to simulate the stream discharge as well as the contrasted behaviour of groundwater table along the hillslope water balance simulated for a 32 year climatic time series displayed a large year to year variability with alternance of dry and wet phases with a time period of approximately 14 years on an average input by the rainfall was 1090 mm year 1 and the evapotranspiration was about 900 mm year 1 out of which 100 mm year 1 was uptake from the deep saprolite horizons the stream flow was 100 mm year 1 while the groundwater underflow was 80 mm year 1 the simulation results suggest that i deciduous trees can uptake a significant amount of water from the deep regolith ii this uptake combined with the spatial variability of regolith depth can account for the variable lag time between drainage events and groundwater rise observed for the different piezometers and iii water table response to recharge is buffered due to the long vertical travel time through the deep vadose zone which constitutes a major water reservoir this study stresses the importance of long term observations for the understanding of hydrological processes in tropical forested ecosystems c 2009 elsevier b v all rights reserved

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

late quaternary alluvial history of the northwestern deccan upland region continental and oceanic sediments from tropical africa australia and south america have recorded analogous changes in climate and stream behaviour during the late quaternary1 4 recently data from north central india have provided evidence for late quaternary changes in river regime in response to global changes in climate in the intertropical zone4 5 here we report evidence for late quaternary river metamorphosis6 from the northwestern deccan upland region of india radiocarbon dates combined with stratigraphic data have allowed the identification of major episodes of erosion and deposition7 8 supporting evidence from the arabian sea2 3 demonstrates that the successive phases of aggradation and incision are linked to late quaternary climatic changes7 8

soils and climatic geomorphology on the karnataka plateau peninsular india abstract as a result of extensive field reconnaissance in southern india a geographical traverse across the planation surface of the karnataka uplands which lie in the rainshadow of the western ghats was selected to assess the consequences of a steep rainfall gradient on soil and landform patterns the study of the traverse described in detail reveals a characteristic progression of soil type and wash divide form from the high rainfall to the semi arid end of the continuum and invites a critical reappraisal of classical notions in climatic geomorphology and the soil sciences the assumption that alternations of wet and dry climates are necessary to achieve planation is given special consideration and the often held belief that kaolinite minerals are diagnostic of humid palaeoclimates when found in a semi arid climate is challenged the findings for the indian traverse are set against the better known data for west africa where the rainfall range between the gulf of guinea and the sahel is comparable with that observed in the south western deccan differences outweighing similarities attention is drawn to the possibility that tectonic regime which also controls the degree of landscape stripping is a major key to understanding the climatic geography of soils the tread of planation surfaces and the shape of hillslopes from one region of the tropics to another

geomagnetic dipole moment and 10be production rate intercalibration from authigenic 10be 9be for the last 1 3 ma 1 a synthesis of authigenic 10be 9be ratio records constructed from sedimentary sequences deposited over the last 300 ka in the north east atlantic and over the 500 1300 ka bp interval in the west equatorial pacific ocean is compared with proxies of the geomagnetic moment variations over the same time intervals these proxies consist in 1 relative paleointensity records obtained from the same sediments 2 a seafloor magnetization record derived from magnetic anomalies and 3 an absolute virtual dipole moment vdm record constructed from an updated database of absolute paleointensities the authigenic 10be 9be ratio records document the occurrence of the paleomagnetic events excursions and reversals reported for these time intervals by paleomagnetic studies reconstructions of 10be 9be derived magnetic moment records based on theoretical and empirical relationships between 10be 9be ratio and absolute vdm allow establishing 10be derived vdm records that exhibit the same overall structures as the geomagnetic proxy records

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

mineral magnetic properties of loess paleosol couplets of the central loess plateau of china over the last 1 2 myr 1 we have conducted a multiparameter investigation of 15 loess paleosol couplets s0 l1 to s14 l15 from the jiaodao section in the central loess plateau of china using environmental magnetic approaches coupled with soil science techniques the magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis high temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the lishi formation to the holocene black loam possibly indicating a decrease in weathering intensity this may reflect a long term increase in aridity and or a general long term cooling trend of the interior of the asian continent from 1 2 ma to the present several samples display wasp waisted hysteresis loops these are most pronounced in moderately enhanced paleosols less pronounced in the practically unaltered loess and subdued in the well developed paleosols but wasp waistedness reappears in the most developed paleosols this wasp waistedness sequence suggests that the composition concentration and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess paleosol sequence but each factor has a different effect at different stages of pedogenesis

regolith mass balance inferred from combined mineralogical geochemical and geophysical studies mule hole gneissic watershed south india the aim of this study is to propose a method to assess the long term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics geochemistry and mineralogy we initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the karnataka plateau peninsular india in the transition sub humid zone of this climatic gradient we have studied the pristine forested small watershed of mule hole 4 3 km2 mainly developed on gneissic substratum mineralogical geochemical and geophysical investigations were carried out i in characteristic red soil profiles and ii in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots in addition 12 electrical resistivity tomography profiles ert with an investigation depth of 30 m were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles the location of the ert profiles is based on a previous electromagnetic survey with an investigation depth of about 6 m the soil cover thickness was inferred from the electromagnetic survey combined with a geological pedological survey taking into account the parent rock heterogeneity the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices loss on ignition weathering index of parker chemical index of alteration comparing these indices with electrical resistivity logs it has been found that a value of 400 ohm m delineates clearly the parent rocks and the weathered materials then the 12 inverted ert profiles were constrained with this value after verifying the uncertainty due to the inversion procedure synthetic models based on the field data were used for this purpose the estimated average regolith thickness at the watershed scale is 17 2 m including 15 2 m of saprolite and 2 m of soil cover finally using these estimations of the thicknesses the long term mass balance is calculated for the average gneiss derived saprolite and red soil in the saprolite the open system mass transport function indicates that all the major elements except ca are depleted the chlorite and biotite crystals the chief sources for mg 95 fe 84 mn 86 and k 57 biotite only are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18 the ca accumulation can be attributed to the precipitation of caco3 from the percolating solution due to the current and or the paleoclimatic conditions overall the most important losses occur for si mg and na with 286 106 mol ha 62 of the total mass loss 67 106 mol ha 15 of the total mass loss and 39 106 mol ha 9 of the total mass loss respectively al fe and k account for 7 4 and 3 of the total mass loss respectively in the red soil profiles the open system mass transport functions point out that all major elements except mn are depleted most of the oligoclase crystals have broken down with a loss of 90 the most important losses occur for si na and mg with 55 106 mol ha 47 of the total mass loss 22 106 mol ha 19 of the total mass loss and 16 106 mol ha 14 of the total mass loss respectively ca al k and fe account for 8 6 4 and 2 of the total mass loss respectively overall these findings confirm the immaturity of the saprolite at the watershed scale the soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric co2

an integrated study of the grain size dependent magnetic mineralogy of the chinese loess paleosol and its environmental significance 1 to investigate the grain size dependent properties of the magnetic minerals in chinese loess paleosol samples touxiangdao xining qinghai province china magnetic extracts were divided into two size fractions by gravitational settling on the basis of hysteresis measurements thermal demagnetization of low temperature saturation isothermal remanent magnetization and nonmagnetic studies sem and xrd we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples the results show that the oxidation degree nonstoichiometry of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered in ascending order of the oxidation degree of our samples we find 1 loess coarse particles lc are multidomain md magnetite with slight oxidation 2 paleosol coarse pc particles are also md magnetite but with a higher oxidation degree compared to lc 3 loess fine lf particles are pseudo single domain psd magnetite with a high oxidation degree and 4 paleosol fine pf particles are psd maghemite single domain sd and superparamagnetic sp maghemite mainly stay in the residues further thermomagnetic analysis of pf psd maghemite revealed that this natural maghemite has a curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50 after a 700 c heating cooling cycle these new results identify the sources of multicomponent nrm in chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components

pedogenic carbonates on precambrian silicate rocks in south india origin and paleoclimatic significance abstract calcrete in alfisols on precambrian silicate rocks is studied here in an attempt to reconstruct paleorainfall trends in semiarid south india facies of observed carbonate accumulation are pseudomycelium calcrete nodules nodular horizons hardpan and laminar horizons based on geochemical and sr isotope tracing we relate the presence of calcrete to local geologic conditions within the precambrian basement notably discrete ca rich metamorphic outcrops and rule out eolian input micromorphologic analysis reveals a multi stage origin of the calcretes involving soil colluvial geochemical and biologic processes mostly in vadose environments preliminary u th ages of 190 200 ka obtained on calcrete nodules suggest a major phase of ca release rock weathering and precipitation during marine oxygen isotope stage 7 the calcrete thus indicates fairly stable semiarid conditions at the 200 ky time scale despite fluctuations around an annual rainfall average close to current values 700 mm the stability of the western ghats barrier as a shield to the southwest monsoon explains the long term average stability of the pedoclimatic environment in the rainshadow zone the radiometric ages suggest that weathering may occur massively at discrete intervals of geologic history during spikes of humidity in the paleoclimatic record the rest of the time is spent recycling and redistributing the products of weathering incrementally at landscape scale within seasonal cycles of a relatively stable semi arid climate dating the calcrete therefore does not equate with dating the soil but instead the residence times of nodules in a dynamic soil system in which different constituents have different turnover rates

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

a precursor to the matuyama brunhes reversal in chinese loess and its palaeomagnetic and stratigraphic significance summary we report high resolution palaeomagnetic results across the lower part of s8 in the luochuan area northwest of china using parallel subsets samples a palaeomagnetic anomaly with low palaeointensity and disordered magnetic direction was identified 21 ka prior to the matuyama brunhes mb polarity reversal and is consistent with the mb precursor observed from marine sediments and lava flows this is the first convincing report of the mb precursor in terrestrial material chinese loess which attests the global feature of the mb precursor constrained by the stratigraphic position of both the mb polarity reversal and the mb precursor the loess unit l8 and palaeosol unit s8 of the chinese loess sequence are undoubtedly tied to the marine isotope stages 18 and 19 respectively this correlation is critical for an accurate understanding of the correspondences of palaeomagnetic records between the chinese loess and marine sediments

a comparison of factors controlling sedimentation rates and wetland loss in fluvial deltaic systems texas gulf coast abstract submergence of coastal marshes in areas where rates of relative sea level rise exceed rates of marsh sedimentation or vertical accretion is a global problem that requires detailed examination of the principal processes that establish maintain and degrade these biologically productive environments using a simple 210 pb dating model we measured sedimentation rates in cores from the trinity lavaca navidad and nueces bayhead fluvial deltaic systems in texas where more than 2000 ha of wetlands have been lost since the 1950s long term average rates of fluvial deltaic aggradation decrease southwestward from 0 514 0 008 cm year 1 in the trinity 0 328 0 022 cm year 1 in the lavaca navidad to 0 262 0 034 cm year 1 in the nueces the relative magnitudes of sedimentation and wetland loss correlate with several parameters that define the differing fluvial deltaic settings including size of coastal drainage basin average annual rainfall suspended sediment load thickness of holocene mud in the valley fill and rates of relative sea level rise there is some evidence that upstream reservoirs have reduced wetland sedimentation rates which are now about one half the local rates of relative sea level rise the extant conditions indicate that fluvial deltaic marshes in these valleys will continue to be lost as a result of submergence and erosion

relative geomagnetic paleointensity of the brunhes chron and the matuyama brunhes precursor as recorded in sediment core from wilkes land basin antarctica abstract paleomagnetic and rock magnetic investigation was performed on the 35 m long md03 2595 cado coring adelie diatom oozes piston core recovered on the continental rise of the wilkes land basin east antarctica analysis of the characteristic remanent magnetization chrm inclination record indicates a normal magnetic polarity for the uppermost 34 m of the sequence and a distinctive abrupt polarity change at the bottom of the core this polarity change which spans a 27 cm thick stratigraphic interval represents a detailed record of the matuyama brunhes m b transition and it is preceded by a sharp oscillation in paleomagnetic directions that may correlate to the m b precursor event paleomagnetic measurements enable reconstruction of geomagnetic relative paleointensity rpi variations and a high resolution age model was established by correlating the cado rpi curve to the available global reference rpi stack indicating that the studied sequence reaches back to ca 800 ka with an average sedimentation rate of 4 4 cm ka orbital periodicities 100 ka and 41 ka were found in the chrm inclination record and a significant coherence of chrm inclination and rpi record around 100 ka suggests that long term geomagnetic secular variation in inclination is controlled by changes in the relative strength of the geocentric axial dipole and persistent non dipole components moreover even if the relatively homogeneous rock magnetic parameters and lithofacies throughout the recovered sequence indicates a substantial stability of the east antarctic ice sheet during the middle and late pleistocene influence of the 100 ka and 41 ka orbital periodicities has been detected in some rock magnetic parameters indicating subtle variations in the concentration and grain size of the magnetic minerals linked to orbital forcing of the global climate

modelling the chemical weathering fluxes at the watershed scale in the tropics mule hole south india relative contribution of the smectite kaolinite assemblage versus primary minerals we investigate the chemical weathering processes and fluxes in a small experimental watershed sew through a modelling approach the study site is the mule hole sew developed on a gneissic basement located in the climatic gradient of the western ghats south india the model couples a lumped hydrological model simulating the water budget at the watershed scale to the witch model estimating the dissolution precipitation rates of minerals using laboratory kinetic laws forcing functions and parameters of the simulation are defined by the field data the coupled model is calibrated with stream and groundwater compositions through the testing of a large range of smectite solubility and abundance in the soil horizons we found that despite the low abundance of smectite in the dominant soil type of the watershed 4 vol their net dissolution provides 75 of the export of dissolved silica while primary silicate mineral dissolution releases only 15 of this flux overall smectites modelled as montmorillonites are not stable under the present day climatic conditions furthermore the dissolution of trace carbonates in the saprolitic horizon provides 50 of the calcium export at the watershed scale modelling results show the contrasted behavior of the two main soil types of the watershed red soils 88 of the surface are provider of calcium while black soils smectite rich and characterized by a lower drainage consumes calcium through overall carbonate precipitation our model results stress the key role played by minor accessory minerals and by the thermodynamic properties of smectite minerals and by the drainage of the weathering profiles on the weathering budget of a tropical watershed c 2010 elsevier b v all rights reserved

spatial variations of soil weathering processes in a tropical mountain environment the baturite massif and its piedmont ceara ne brazil abstract strong spatial variations in soil weathering processes were identified in the uplands and piedmont zones of the baturite massif ceara ne brazil a medium elevation tropical mountain characterized by a steep ecocline between humid climate of the summit zone and semi arid conditions of the erosional piedmont with the combined help of field surveys and laboratory analyses involving micromorphological observations as well as geochemical and mineralogical investigations this study demonstrates a close spatial correlation between soil weathering processes and present day water balance wbi variations i in the humid massif wbi 500 mm y 1 monosiallitization is exclusive in soil and upper saprolite horizons and leads to the neoformation of 1 1 clay minerals well crystallized kaolinite with minor amounts of gibbsite ii in the subhumid peripheral area of the massif 50 1 monosiallitization coexists with bisiallitization in the soil profiles producing a mixture of 1 1 kaolinite and 2 1 illite clay minerals along with lower weathering intensity iii in the semiarid piedmont 0 1 bisiallitization becomes the dominant weathering pathway conducive to grus formation and fersiallitic pedogenesis with the most vulnerable primary minerals weathered into high charge 2 1 clay minerals smectite and illite given that soils and weathering mantles are integrators of average bioclimatic conditions that prevail at timescales of 1 to 10 myr the distribution of soil weathering signatures in the study area is probably reflective of inevitable long term quaternary to neogene climatic fluctuations around median values close to the present ones paleoclimatic legacies inherited from the paleogene lateritization event are rare because of the denudation pulse occurring in the late tertiary and favoring soil stripping to the paleoweathering front on the erosional piedmont

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

spatial heterogeneity of land cover response to climatic change in the nilgiri highlands southern india since the last glacial maximum fourteen hillslope soil profiles were sampled under natural vegetation i e grassland or forest and plantations in the nilgiri highlands southern india delta 13c ratios were measured at different depths and 14c ages determined for six profiles in these highland soils where the turnover rate of organic matter is extremely low the 13c ratios of entire soil profiles have recorded signatures of past land cover by correlating the data with results previously obtained from peat bogs and with knowledge concerning the history of human settlement we distinguish three contrasting trajectories of palaeoenvironmental history and landscape change since the last glacial maximum in the central nilgiris between 18 and 10 ka bp forest expansion occurred due to the conjunction of a wetter climate the maximum of southwest monsoon related humidity occurring at ca 11 ka bp and higher temperatures since 10 ka bp the reversal towards grassland vegetation is attributed to drier conditions in the western nilgiris where strong southwest monsoon winds permanently restrict forest patches to sheltered valley sites steady but limited expansion of forest from 18 ka bp to the present is recorded and attributed to rising temperatures the southern and eastern nilgiris where the northeast monsoon contributes 20 of the annual rainfall are the less sensitive to fluctuations in the southwest monsoon in these areas rapid and extensive expansion of forest occurred mainly as a consequence of higher temperatures from 18 ka bp to the present massive deforestation by badaga cultivators and europeans planters followed after the 16th century ad as a result and in contrast with the western nilgiris where the land cover mosaic has remained remarkably stable in the last 18 ka bp the current landscape differs sharply from the land cover pattern detected by the soil record

late archean manganese mineralization and younger supergene manganese ores in the anmod bisgod region western dharwar craton southern india geological characterization palaeoenvironmental history and geomorphological setting abstract the field setting mineralogical geochemical genetic and depositional features of late archean metasedimentary manganese as well as cenozoic supergene manganese and ferromanganese ores along the 75 km long linear tract of the anmod bisgod region in the western dharwar craton southern india are described the 2700 ma metasedimentary manganiferous formation is underlain by stromatolitic limestone dolomite and quartzite arenite and also occurs within the former the older quartzite arenite which contains redox sensitive detrital pyrite and uraninite as well as thucolite indicates sedimentation under anoxic to oxic transitional palaeoenvironmental condition whereas the younger stromatolitic limestone dolomite indicates development of oxygen in the depositional environment the manganese ore types have been classified into three categories one of which is a mn rich duricrust once capping the late archean manganiferous formation it formed during a cenozoic lateritic weathering event and was eroded in response to crustal uplift perhaps uniquely clastic deposits of the eroded mn duricrust occur in mineable quantities in the form of fluvial palaeochannel fills these cryptomelane rich supergene ores are also suitable for radiometric dating knowing their age would help to constrain the timing of mn duricrust formation and indirectly the regional weathering event responsible for the extensive laterite occurrences of southern india

characterization of groundwater chemistry under the influence of lithologic and anthropogenic factors along a climatic gradient in upper cauvery basin south india hydrogeological and climatic effect on chemical behavior of groundwater along a climatic gradient is studied along a river basin semi arid 500 800 mm of mean annual rainfall sub humid 800 1 200 mm year and humid 1 200 1 500 mm year are the climatic zones chosen along the granito gneissic plains of kabini basin in south india for the present analysis data on groundwater chemistry is initially checked for its quality using nicb ratio 40 of pollution and eliminated for the characterization piper s diagram with mean concentrations indicates the evolution of canahco3 semi arid from cahco3 humid zone along the climatic gradient carbonates dominate other anions and strong acids exceeded weak acids in the region mule hole sew an experimental watershed in sub humid zone is characterized initially using hydrogeochemistry and is observed to be a replica of entire sub humid zone with 25 wells extension of the studies for the entire basin 120 wells showed a chemical gradient along the climatic gradient with sub humid zone bridging semi arid and humid zones ca na molar ratio varies by more than 100 times from semi arid to humid zones semi arid zone is more silicaceous than sub humid while humid zone is more carbonaceous ca cl 14 along the climatic gradient groundwater is undersaturated humid saturated sub humid and slightly supersaturated semi arid with calcite and dolomite concentration depth profiles are in support of the geological stratification i e 18 m of saprolite and 25 m of fracture rock with parent gneiss beneath all the wells are classified into four groups based on groundwater fluctuations and further into deep and shallow based on the depth to groundwater higher the fluctuations larger is its impact on groundwater chemistry actual seasonal patterns are identified using recharge discharge concept based on rainfall intensity instead of traditional monsoon non monsoon concept non pumped wells have low na cl and ca cl ratios in recharge period than in discharge period dilution few other wells which are subjected to pumping still exhibit dilution chemistry though water level fluctuations are high due to annual recharge other wells which do not receive sufficient rainfall and are constantly pumped showed high concentrations in recharge period rather than in discharge period anti dilution in summary recharge discharge concept demarcates the pumped wells from natural deep wells thus characterizing the basin

ferrolysis induced soil transformation by natural drainage in vertisols of sub humid south india in sub humid south india recent studies have shown that black soil areas vertisols and vertic intergrades located on flat valley bottoms have been rejuvenated through the incision of streambeds inducing changes in the pedoclimate and soil transformation joint pedological geochemical and geophysical investigations were performed in order to better understand the ongoing processes and their contribution to the chemistry of local rivers the seasonal rainfall causes cycles of oxidation and reduction in a perched watertable at the base of the black soil while the reduced solutions are exported through a loamy sand network this framework favours a ferrolysis process which causes low base saturation and protonation of clay leading to the weathering of 2 1 then 1 1 clay minerals maximum weathering conditions occur at the very end of the wet season just before disappearance of the perched watertable therefore the by products of soil transformation are partially drained off and calcareous nodules then further downslope amorphous silica precipitate upon soil dehydration the ferrolysed area is fringing the drainage system indicating that its development has been induced by the streambed incision the distribution of c 14 ages of caco3 nodules suggests that the ferrolysis process started during the late holocene only about 2 kyr b p at the studied site and about 5 kyr b p at the watershed outlet the results of this study are applied to an assessment of the physical erosion rate 4 8x10 3 m kyr since the recent reactivation of the erosion process c 2010 elsevier b v all rights reserved

how does chinese loess become magnetized despite nearly three decades of paleomagnetic research on the extensive chinese loess deposits a convincing explanation has yet to be developed for how chinese loess becomes magnetized to address this problem we conducted re deposition experiments using weakly consolidated holocene chinese loess that was disaggregated in the laboratory and compare our results with published paleomagnetic data we simulated a depositional remanent magnetization drm associated with dry deposition of eolian sediments and a post depositional remanent magnetization pdrm in which the sediment was water saturated after deposition the simulated drm faithfully records the declination of the applied field but with systematic inclination flattening addition of minor water slightly improves recording of the applied field but inclination flattening persists reliable recording of the applied field occurs for pdrm simulation in water saturated sediment our synthesis of paleomagnetic data from chinese loess indicates that time averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole gad field but in many cases inclinations are shallower than for a gad field we conclude that the chinese loess is magnetized by a combination of drm and pdrm mechanisms with water content providing the dominant control on which mechanism aligns the detrital mineral fraction where pedogenesis causes neoformation of magnetic minerals an additional chemical remanent magnetization crm will occur the magnetization of chinese loess therefore appears to be controlled by a complex time varying combination of drm pdrm and crm mechanisms

determination of magnetic carriers of the characteristic remanent magnetization of chinese loess by low temperature demagnetization abstract the chinese loess paleosol sequences can provide excellent paleomagnetic records for detailed studies on both magnetostratigraphy and secular variations of the earth s magnetic field however the nature of the loess paleosol characteristic remanent magnetization chrm isolated by thermal demagnetization from the natural remanent magnetization still remains in debate in this study we directly measure the thermal alternating field and low temperature demagnetization cooling warming cycle of remanence in zero field between 300 and 50 k spectra of chrm of the loess samples at touxiangdao xining pr china the results show that coarse grained pseudo single domain psd multi domain md magnetite and psd maghemite particles are the main magnetic carriers of the loess and paleosol chrms respectively by comparing the chrm characteristics across a loess paleosol transition zone we have found that during pedogenesis the thermally separable chrm original detrital remanent magnetization carried by the psd md magnetite in loess is gradually overprinted by the chemical remanent magnetization carried by psd maghemite even moderate pedogenesis e g for the pedogenically altered loess can strongly affect the loess chrm thus more attention has to be focused on the paleosol chrm separated solely by thermal demagnetization especially when constructing a continuous paleomagnetic record covering both loess and paleosol units

inverse models to analyze the spatiotemporal variations of chemical weathering fluxes in a granito gneissic watershed mule hole south india water rock reactions are driven by the influx of water which are out of equilibrium with the mineral assemblage in the rock here a mass balance approach is adopted to quantify these reactions based on field experiments carried out in a granito gneissic small experimental watershed sew mule hole sew similar to 4 5 km 2 quartz oligoclase sericite epidote and chlorite are identified as the basic primary minerals while kaolinite goethite and smectite are identified as the secondary minerals observed groundwater chemistry is used to determine the weathering rates in terms of mass transfer coefficients mtcs of both primary and secondary minerals weathering rates for primary and secondary minerals are quantified in two steps in the first step top red soil is analyzed considering precipitation chemistry as initial phase and water chemistry of seepage flow as final phase in the second step minerals present in the saprolite layer are analyzed considering groundwater chemistry as the output phase weathering rates thus obtained are converted into weathering fluxes q weathering using the recharge quantity spatial variability in the mineralogy observed among the thirteen wells of mule hole sew is observed to be reflected in the mtc results and thus in the weathering fluxes weathering rates of the minerals in this silicate system varied from few 10 mu mol l in case of biotite to 1000 s of micromoles per liter calcite similarly fluxes of biotite are observed to be least 7 5 mol ha yr while those of calcite are highest 1265 791 mol ha yr further the fluxes determined annually for all the minerals are observed to be within the bandwidth of the standard deviation of these fluxes variations in these annual fluxes are indicating the variations in the precipitation hence the standard deviation indicated the temporal variations in the fluxes which might be due to the variations in the annual rainfall thus the methodology adopted defines an inverse way of determining weathering fluxes which mainly contribute to the groundwater concentration c 2011 elsevier b v all rights reserved