biogenic grooving on glass shards the occurrence of grooves on the surface of volcanic glass shards in marine and nonmarine environments suggests the existence of microorganisms that can dissolve silica substrates the etchings occur within a 30 cm thick marine turbidite bed in the miocene obispo tuff at shell beach central california and also in a tuff layer deep creek tuff in the miocene john day formation of eastern oregon the etchings occur in three different patterns with two subtypes depending on the formation in which they occur type a is an anastomosing pattern of semicircular grooves covering the entire grain subtypes of a are distinguished by the presence obispo tuff type a 1 or lack john day type a 2 of annulations type b has some sinuous branching grooves covering only part of the grain subtypes are the same as those of type a type c occurring only on obispo tuff grains is a series of straight nonbranching grooves on one side of platy shards the grooves range in size from 4 20 m in the obispo formation to 15 18 m in the john day formation the grooves are strikingly similar to grooves occurring on carbonate grains

mineralogical changes during intense chemical weathering of sedimentary rocks in bangladesh abstract bangladesh is situated in a subtropical to tropical climatic zone a recently weathered crust has developed on sedimentary bedrock sandstone siltstone shale and claystones of tertiary quaternary age weathered samples were collected from 16 sections totaling 68 samples and were analyzed mineralogically the main primary minerals identified in the weathered crust of sedimentary rocks are quartz plagioclase k feldspar biotite muscovite sparse carbonate and epidote the secondary minerals are kaolinite illite chlorite gibbsite and goethite weathering initiated along the grain boundaries and cleavage planes of the minerals forming small cloudy materials which were very difficult to identify in the advanced stage of weathering these cloudy materials have turned into secondary minerals in region 1 high rain fall 7100 mm yr and monsoonic climate resulted in a kaolinite gibbsite goethite suite through the weathering of feldspars and biotite the occurrence of gibbsite in the relatively elevated lands of sylhet and fe kaolinite throughout the study areas is indicative of a humid tropical climate during formation of the weathered crust

can we use pyroxene weathering textures to interpret aqueous alteration conditions yes and no pyroxene minerals are a significant component of shergottite nakhlite chassignite snc meteorites e g velbel 2012 and detected across large areas of mars surface e g mustard et al 2005 these minerals are associated with chloride sulfate and perchlorate salts that may represent briny waters present in mars history previous textural analyses by velbel and losiak 2010 comparing pyroxenes and amphiboles from various natural weathering environments showed no correlation between apparent apical angles describing the morphology of denticular weathering textures and mineralogy or aqueous alteration history in relatively dilute solutions however high salinity brines preferentially dissolve surface species potentially leading to different textures dependent on the brine chemistry in this study we performed controlled pyroxene dissolution experiments in the laboratory on a well characterized diopside to determine if aqueous alteration in different high salinity brines representative of potential weathering fluids on mars produce unique textural signatures following two months of dissolution in batch reactors we observed denticles on etch pit margins and pyroxene chip boundaries in all of the solutions investigated ultrapure water 18 m cm 1 a h 2 o 1 low salinity solutions containing 0 35 m nacl a h 2 o 0 99 0 35 m na 2 so 4 a h 2 o 0 98 and 2 m naclo 4 a h 2 o 0 9 and near saturated brines containing 1 7 m na 2 so 4 a h 2 o 0 95 3 m nacl a h 2 o 0 75 and 4 5 m cacl 2 a h 2 o 0 35 no systematic change in denticle length or apical angle was observed between any of the solutions investigated even when altered in brines with significantly different salinity activity of water and anion composition based on these and previous results from natural systems apical angle measurements are not a useful proxy for determining the extent or nature of aqueous alteration however since denticles form relatively slowly during weathering at circum neutral ph denticle length may be a useful proxy for chemical weathering duration all of the experimental solutions produced median denticle lengths 1 m likely due to the brief weathering experiments however perchlorate brines produced a significantly wider range of denticle lengths than those observed in all the other experimental solutions tested since perchlorate is likely a common constituent in martian soils glotch et al 2016 denticle length measurements should be used cautiously as proxies for extent of aqueous alteration on mars particularly in samples that also contain perchlorate

scale issues in weathering studies a review of the major scale issues in weathering studies reveals concerns over the fundamental spatio temporal distributions of weathering phenomena as well as issues of upscaling microscope based observations and linking different scales of observations in explanations of landform development various strategies are proposed which can be used to tackle these issues many rooted in non linear dynamical systems ideas as an initial step spatio temporal scale distributions are estimated here for weathering processes landforms and controls based on a range of empirical data two case studies of phytokarst in grand cayman and blistering and scaling of building stones are presented to illustrate the types of data that might be used to establish more convincing scale linkages in weathering investigations

use of scanning electron microscopy to investigate records of soil weathering preserved in lake sediment reconstruction of soil weathering is one of the more difficult problems for palaeoecologists interested in landscape scale environmental changes here we use scanning electron microscopy and energy dispersive x ray analysis sem eds to study the pitting of feldspars in soils and lake sediments at two sites in the great lakes region usa a semi quantitative feldspar weathering index fwi allows description of the weathering status for large numbers of individual grains and reveals differences among soil horizons and strata of lake sediment as well as between the two sites feldspars in upper soil horizons generally have greater fwi than those in lower horizons but analyses also suggest considerable weathering in the soil c horizon at the site with more rapid water infiltration at one site the fwi of upper soil horizons and surface lake sediment are similar implying that lake sediments have the potential to record changes in soil over time however differences between upper soil horizons and surface sediment at the other site highlight the need to examine more sites and more profiles within sites to better relate catchment and sediment characteristics we interpret increases in the fwi over time within lake sediments as evidence of progressive weathering of soils and conclude that the sites had different histories of soil development however the complexities of sediment deposition in lakes and the paucity of information relating fwi to soil characteristics prevent straightforward reconstruction of soil development variability within each record appears tied to initial grain chemistry depositions environment changes in lake level and progressive degradation of grains as more information relating micromorphology to soil characteristics becomes available this technique should become a more useful complement to chemical studies of lake sediment

chemical weathering in two adirondack watersheds past and present day rates rates of chemical weathering in two forested adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input output budgets based on precipitation and surface water chemistry long term rates of weathering have averaged about 500 to 600 eq ha yr for both watersheds since the glaciers retreated from the region about 14 000 yr ago present day denudation rates average 1679 eq ha yr in the panther lake watershed and only 198 eq ha yr in the woods lake watershed mineral weathering reactions in both watersheds involve primarily the dissolution of plagioclase potassium feldspar and hornblende mass balance calculations however indicate that hornblende weathers in disproportionately large amounts in the panther lake watershed relative to its abundance in soils and till cation exchange in the glacial till mantling the panther lake watershed may also play an important role in controlling surface water chemistry in the basin in the panther lake watershed the current rate of weathering which is about a factor of 3 greater than the long term average may reflect a recent adjustment for higher hydrogen ion fluxes brought about by acid deposition the capacity of the thick till deposits in the panther lake watershed to neutralize acidity is ultimately reflected by the circum neutrality of panther lake in the woods lake watershed the soils and thin till deposits cannot effectively neutralize current acid loadings and therefore woods lake is acidic with a ph below 5

denticles on chain silicate grain surfaces and their utility as indicators of weathering conditions on earth and mars abstract denticulated margins also known in older literature as sawtooth cockscomb or hacksaw terminations are a common feature of pyroxenes and amphiboles visible in optical and electron microscopes denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits the characteristic aqueous dissolution form of chain silicate minerals denticles occur widely in terrestrial near surface materials that have experienced low temperature aqueous alteration including chemically weathered regoliths soils in a variety of climatic and geomorphic settings sediments and sedimentary rocks ranges of dissolution forms and dimensions commonly tens of microns in length are identical on both pyroxenes and amphiboles in these materials denticles on terrestrial chain silicates are formed by low temperature aqueous alteration denticles on a soil mineral grain of unknown composition at the phoenix mars lander site and on terrestrial pyroxenes naturally weathered in terrestrial low temperature aqueous environments are broadly similar in size shape and orientation relationships between denticles however quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample such variations are too large to allow simple identification of minerals of unknown composition from morphology alone nevertheless the overall morphological similarities are consistent with a low temperature aqueous corrosion origin of denticles at the phoenix mars lander site

characterization of the weathering status of feldspar minerals in sandy soils of minnesota using sem and edx scanning electron microscopy was used to assess the weathering status of plagioclase and alkali feldspar grains isolated from sandy outwash soils located under three vegetation communities mixed red and white pine maple basswood and white cedar feldspars were selected for this study because they were the most common weatherable minerals in these sandy soils a morphological weathering index based on the percent of the surface of the feldspar grains covered with etch pits was used to evaluate weathering intensity results revealed that significant pedogenic weathering has occurred in the surface and subsurface horizons of these soils compared to the parent material plagioclases were found to be much more weathered than alkali feldspars within the plagioclase solid solution series the weathering index increased linearly with increasing ca content as determined by energy dispersive x ray analysis comparisons among the three sites showed that weathering environments under the pine and maple basswood communities were relatively similar being only slightly more intense under pine the most intense weathering occurred under the white cedar community particularly in the e horizon linear regression analyses showed that the weathering index for alkali feldspars was significantly related to soil ph r2 0 30 p 0 001 in the case of plagioclases multiple linear regression analyses showed that the weathering index was significantly dependent on both ca ca na molar ratio and ph r2 0 83 p 0 001 with 64 weight effect due to ca ca na ratio and 19 due to ph

microdenticles on naturally weathered hornblende abstract microdenticles with lengths in the micron submicron range rather than tens of microns are developed on the lateral surfaces of larger classic denticles on naturally weathered hornblende from weathered amphibolite of the carroll knob complex in western north carolina microdenticles share the shape and orientation of the larger more typical denticles producing arrays of microdenticles that give the larger host denticle the appearance of a surface covered with imbricate pointed or rounded scales the arrays of imbricate microdenticles are formed by low temperature aqueous alteration during weathering of the carroll knob complex hornblende they are later stage corrosion forms on already corroded surfaces of hornblende that show larger scale evidence of typical weathering in the carroll knob occurrence hornblende microdenticles are associated with dilute weathering solutions suggesting possible control by extreme undersaturation of solutions with respect to hornblende

kinetics of weathering and diagenesis weathering and diagenesis are discussed from a kinetic standpoint such a discussion depends upon an intimate interplay between field measurements laboratory experiments and mathematical theory because of the accessibility of soils and sediments models based on laboratory measurements and theoretical calculations can be used along with field data to calculate rate parameters which are otherwise difficult to measure in the laboratory also theoretical models can be tested in the field ways in which chemical kinetics can be applied to geological problems in the surficial environment are discussed and the author recomends extensive future study of such problems jmt

dating post metamorphic cooling of the eastern granulites in the mozambique belt of northern tanzania using the garnet sm nd method abstract granulites from the usambara wami river and uluguru areas in the northern part of the mozambique belt in tanzania yield sm nd garnet whole rock ages of between 580 and 634 ma with a mean of 609 11 ma 2 this mean age is only slightly younger than the previously published peak metamorphic age of 641 2 ma suggesting that contrary to some earlier arguments garnet sm nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes using published peak metamorphic temperatures of 810 c and cooling rates of 1 4 c ma the mean age translates into garnet closure temperatures of 690 to 780 c the similarity in garnet ages over widely separated areas coupled with the previously established similarity in equilibrium pt conditions indicate that isolated complexes that form the eastern granulites in the tanzanian sector of the mozambique belt share the same thermal history and were formed under the same geodynamic setting a few published garnet ages of between 525 and 545 ma indicate a younger less pervasive event of granulite facies metamorphism in the belt the bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of gondwana took place in two stages the 610 ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of india madagascar parts of eastern antarctica the kalahari craton the congo craton and the arabian nubian shield forming the imslek ans collage on the other hand the 525 545 ma ages may mark cooling from a thermal event associated with the collision of australo antarctica with the imslek ans collage

microtextural controls of weathering of perthitic alkali feldspars the relationship between the microtexture and dissolution behaviour of fresh hf acid etched and naturally weathered alkali feldspar phenocrysts from the lower devonian shap granite has been investigated by sem and tem a novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface further electron microscope work suggests that shap phenocrysts are representative of the alkali feldspar in the protolith of many soils fresh and unweathered shap feldspars have a complex microtexture comprising areas of pristine cryptoperthite and lamellar microperthite cross cut by volumes of microporous altered feldspar or patch perthites cryptoperthites are made up of 75 nm wide albite films the platelets are coherent but albite films have numerous edge dislocations along their interface with orthoclase 001 and 010 cleavage surfaces intersect 2 3 edge dislocations m2 in three dimensions these edge dislocations form an orthogonal net in the murchison plane of easy fracture close to 601 patch perthites are irregular semicoherent to incoherent intergrowths of albite and irregular microcline subgrains with 0 65 0 70 sub m to m sized pores m2 microporous patch perthites form by dissolution reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime dissolution of shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture especially by dislocations and exsolution lamellae the core and strain energy associated with dislocation outcrops on 001 and 010 cleavage surfaces promotes rapid dissolution at those sites and formation of nn sized etch pits after 30 s of laboratory etching with hf acid vapour with progressive hf etching crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite naturally weathered feldspars were collected from the glacial erratic boulders fine gravels surrounding exposed granite surfaces and from peat soil overlying the granite during natural weathering etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate 15 m below the cleavage surface forming an interconnecting ladder like grid of submicrometer wide channels in the murchison plane coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly but faster than orthoclase this is probably because the albite lamellae have significant elastic coherency strain but this is much less per unit volume of albite than the core and strain energy associated with edge dislocations patch perthites etch in hf vapour and weather rapidly in nature to produce a honeycomb like texture of interconnecting nanometer to micrometer sized pits which nucleate at preexisting micropores or incoherent subgrain boundaries the size and density of etch pits on microperthite surfaces which is determined by rates of growth and coalescence may be a useful progress variable for natural and experimental dissolution all alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale these microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains the controls on absolute dissolution rates and why they commonly change over time the nonlinear variation of dissolution rate with grain size the ratio of alkali ions released into solution and disparities between laboratory dissolution rates and those observed in the field

development of a feldspar weathering index and its application to a buried soil chronosequence in southeastern england abstract an index of feldspar weathering ifw has been developed based upon the assignment of individual grains to weathering classes defined in terms of progressive changes in surface morphology identifiable under a polarising microscope the index was used to test the hypothesis that profiles of a buried paleosol developed on the higher terraces of the river thames in southeastern england are more weathered than those formed on lower and younger terrace surfaces ifwr ratio between the ifw of soil and parent material and ifwd difference between the ifw of soil and parent material values support a trend of increased weathering with assumed terrace age although the pattern is less clear and conclusive towards the base of the terrace sequence anomalies in the dataset can be attributed to a range of factors including the possible variable composition and structure of the feldspars the nature of the parent material samples and probable fluctuations in soil processes and environments over time notwithstanding these problems some of which are specific to the study reported here the technique and general approach clearly have more widespread potential for the semi quantitative comparison of the relative weathering statuses of soils

anatomy and origin of a cretaceous phosphorite greensand giant egypt late cretaceous epicontinental phosphorites porcelanites cherts dark coloured shales glauconitic sandstones and bioclastic and fine grained carbonate rocks in egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics two main depositional realms are interpreted and correlated through sequence stratigraphic analysis 1 a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon rich shales biosiliceous sediments and thick phosphorites and 2 a relatively high energy depositional regime accompanying sea level fall during which deltas advanced glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis thereby promoting solution collapse phenomena in associated cherts lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments in eastern egypt the phosphorites form winnowed lag layers some of which may have been redeposited down slope in structural lows in the west these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning however direct desorption of phosphorus to pore waters from detrital iron oxyhydroxide phases also may have been important in the cretaceous setting the iron reduced in this process being available for incorporation in glauconites the main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters this study suggests a model for the common coexistence of glauconites and phosphorites in the geological record although upwelling is often advocated as the origin of nearly all giant phosphorite deposits we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and or reduction in bottom waters or pore waters

sem analysis of weathered grains pretreatment effects fresh microcline albite and almandine along with soil grains were treated with various traditional pretreatments prior to observation with scanning electron microscopy sem the grains were observed with sem and given ratings for each of several surface properties to determine which pretreatments produced clean surfaces on soil grains without laboratory induced damage on the fresh mineral grains gentle overnight shaking in 2 sodium bicarbonate at ph 9 5 produced the most effective cleaning of soil grains with the least amount of induced damage to fresh mineral samples this pretreatment was equivalent to that given the control samples shaken overnight in distilled water for observations of etch pits on fresh mineral samples within a 25 equivalence or negligible difference interval ultrasonification hydrogen peroxide and boiling hydrochloric acid caused the most damage to mineral samples mainly in the form of 0 5 m etch pits boiling hydrochloric acid boiling nitric acid and stannous chloride resulted in increased coated surfaces on soil grains

macrobenthic community structure in a high productivity region upper campanian mishash formation israel abstract a paleoecological study of molluscan fossils from the upper campanian mishash formation of southern israel was undertaken to provide new insights into the inference of bottom oxygen levels food availability and bottom water energy on high productivity seafloors quantitative data for molluscan analysis were collected from each of the main lithologies that comprises the high productivity facies mosaic of the mishash sea i e organic rich carbonates thin porcelanite and chert beds and loosely to densely packed shell beds diagenesis has obliterated faunal remains in carbonate layers pollution studies in modern environments have demonstrated the close connection between stressful conditions such as organic load toxicity and water and sediment aeration and benthic community structure as expressed through species richness evenness taxonomic composition body sizes and trace fossil assemblages these connections and behavior patterns were investigated in the varied benthic fossil assemblages of the phosphate member of the mishash formation bivalves and gastropods dominated the macrofauna in all rock types but in varying abundance and taxonomic compositions changing from diverse assemblages 15 22 species per sample to less rich ones 5 7 species and even barren intervals this range of abundance neatly describes the lateral mosaic and heterogeneous nature of the upwelling environment of the upper campanian and is comparable to the mosaic of sub facies that characterize modern upwelling facies belts today e g in the benguela upwelling system the variations shown for the macrobenthic assemblages within the mishash formation high productivity regime are associated with proximity to the high productivity center or to periods of intensified upwelling the inferred variable oxygen levels and organic load changes point up how dynamic these regions really are this is expressed in community structure and composition which vary between two extremes the low oxygen biofacies 1 is characterized by a smaller range of shell sizes infaunal deposit feeders and low species richness while the high oxygen biofacies 3 is characterized by greater ranges in shell size infaunal but also epifaunal suspension feeders and higher species richness the chert and porcelanite beds exhibit a variety of sub environments and seem to represent an intermediate facies that bridges the two extremes of biofacies 1 and 3 the above ecological criteria may provide evidence of the original carbonate environment for the thin chert and porcelanite layers the chert or porcelanite samples that contain a moderately even and rich assemblage of species intermediate to the values of the two end member facies are taken to represent a shallow water carbonate marine environment marginal to the upwelling center whereas layers of chert and porcelanite containing very low species richness and evenness are taken to characterize primary depositional environments proximal to the paleo upwelling cells

timing and origin for sand dunes in the green river lowland of illinois upper mississippi river valley usa abstract the recent increase in dune studies in north america has been heavily focused in the great plains while less attention has historically been given to the dune fields east of the mississippi river here we report ages and suggest a potential sediment source for sand dunes in the green river lowland illinois which may provide a better understanding of the dynamic interactions between eolian glacial lacustrine and fluvial processes that shaped the landscapes of the upper midwest seven coherent optically stimulated luminescence ages osl or optical ages obtained from four sites suggest that major dune construction in the green river lowland occurred within a narrow time window around 17 500 ago this implies either an enhanced aridity or an episodic increase of sediment supply at 17 500 years ago or combination of the both contrary to previous assertions that dune sand was sourced from the deflation of the underlying outwash sand deposited when the lake michigan lobe retreated from the area we propose that green river lowland dunes sand originated from the green bay lobe through the rock river specifically sediment supply increased in the rock river valley during drainage of glacial lake scuppernong which formed between 18 000 and 17 000 years ago when the green bay lobe retreated from its terminal moraine the lake drained catastrophically through the rock river valley providing glacial sediment and water to erode the preexisting sandy sediments throughout the remainder of the late pleistocene the laurentide ice sheet drained into larger more northerly glacial lakes that in turn drained through other river valleys therefore the dunes in the green river lowland formed only during the catastrophic drainage of glacial lake scuppernong but were stabilized through the remainder of the pleistocene this scenario explains the abrupt dune construction around 17 500 years ago and explains the lack of later dune activity up to the pleistocene holocene transition osl and radiocarbon ages also indicate that dunes were reactivated during the early middle and late holocene some eolian activation occurred within well defined dry intervals in the upper midwest suggesting that increased aridity may have been the primary driver in mobilizing sand however many ages do not correspond to drier periods in contrast to the relative coherency of the pleistocene osl ages from multiple study sites the holocene osl ages do not overlap from one site to another suggesting that increased aridity alone cannot explain the multiple phases of dune reactivation in the holocene therefore we conclude that the combined effect of localized disturbances and greater aridity acted in concert to increase eolian sand activity in the holocene the multiple periods of eolian activity during the holocene suggest a high potential for future sand activation in the region and these results are informative for environmental prediction and potential future mitigation

evolution of tethyan phosphogenesis along the northern edges of the arabian african shield during the cretaceous eocene as deduced from temporal variations of ca and nd isotopes and rates of p accumulation abstract the evolution of tethyan phosphogenesis during the cretaceous eocene is examined to try to explain fluctuations of phosphogenesis through time and whether or not they reflect long term changes in ocean circulation or in continental weathering twenty seven time stratigraphic phosphate levels in various tethyan sites covering a time span of about 90 myr from the hauterivian to the eocene were analyzed for 44 ca 42 ca and 143 nd 144 nd in their carbonate fluorapatite cfa fraction p and ca accumulation rates and bulk sedimentation rates were quantified throughout the cretaceous eocene negev sequence to examine how changes in 44 ca 42 ca and 143 nd 144 nd are reflected in the intensity of phosphogenesis a clear cut change occurs in e nd t and 44 ca and in the rates of p and ca accumulation and bulk sedimentation through the time analyzed e nd t is much lower in the hauterivian lower cenomanian 12 8 to 10 9 than in the upper cenomanian eocene 7 8 to 5 9 much lower 44 ca values occur in the hauterivian turonian 0 22 to 0 02 than in the coniacian eocene 0 23 to 0 40 p accumulation rates in the negev steeply increase from 2 k yr 1 in the albian coniacian to 1500 mol cm 2 k yr 1 in the campanian whereas a strong decrease is concomitantly recorded in the rates of ca accumulation and bulk sedimentation in addition distinct e nd t values are shown by the phosphorites of the negev 6 7 to 6 4 and egypt 9 1 to 7 6 during the campanian and by those of the negev 7 8 to 6 3 and north africa 10 1 to 8 9 during the maastrichtian eocene the culmination of p accumulation rates in the negev during the campanian occurring with a high in e nd t and 44 ca and a low in sedimentation rates indicates that paleoceanographic and paleogeographical factors mostly governed phosphorite accumulation in this area the abrupt e nd t rise after the cenomanian is attributed to increased incursion of pacific radiogenic water masses into the tethys driven by the late cretaceous global sea level rise the connection between north and south atlantic the global post santonian cooling and the progressive widening of the caribbean threshold all acting in combination to significantly intensify the tethyan circumglobal current tcc it also reflects a weakening of the continental nd signal due to a reduction of exposed landmasses caused by increased flooding of continental shelves high 44 ca values at those times also point to a decrease in weathering ca 2 fluxes and expansion of carbonate sedimentation in shelves both enriching seawater with isotopically heavy ca 2 deep ocean circulation intensified by the post santonian cooling of high latitudes increased p inventory in the tethys basin whereas the strengthened tcc and the folded shelf likely resulted in coastal and topographically induced upwelling supplying p rich intermediate waters to southeastern tethys shelves only in the paleocene eocene following major changes in global circulation produced by narrowing of tethys and widening of the atlantic did phosphogenesis shifts its locus of high intensity to the western atlantic tethys and west african atlantic coasts this change in paleocirculation is expressed by distinctly differing e nd t in the middle east and the north and west african phosphorites suggesting different oceanic p sources and current systems for these two major groups of phosphorites our nd isotope results further suggest a weaker tcc during the mid cretaceous becoming more intense in late cretaceous times they also point to the north pacific ocean as major source of deep water formation for the intermediate deep waters in the tethys basin during the late cretaceous

development of etch pit size distributions on dissolving minerals we have applied the theory of crystal size distributions to the evolution of etch pit size distributions psd s on dissolving minerals population balance equations are derived based on nucleation growth and annihilation of etch pits psd plots of log n number of pits per size per area vs w etch pit width are used to compare etch pit populations we have used psd analysis to study the development of etch pits on calcite dissolved in a rotating disk experiment far from equilibrium 25 c initial ph 8 6 0 7 m kcl a maximum is present on the psd plot in the early stages of dissolution and appears to result from nucleation of pits at all available sites on the initial cleaved surface over a finite time interval as pits grow larger become flat bottomed and annihilate the peak moves to larger sizes and decreases in magnitude to become insignificant after 17 5 hr after 0 5 hr the psd plots become linear at small pit widths marking a population balance as new pits nucleate within flat pits grow larger become flat bottomed and finally annihilate this population of short lived small 100 m etch pits possibly results from nucleation at point defect or impurity clusters a linear psd for long lived larger pits appears at 17 5 hr possibly reflecting etch pits at dislocations the overall psd plot at 17 5 hr is kinked due to these two populations calcite dissolution rates calculated from the coefficients of the psd are within a factor of two of the dissolution rate measured from solution chemistry such estimates will be more accurate if the duration of dissolution does not greatly exceed the mean lifetime of etch pits psd plots for hornblende and microcline grains from a loess soil that has weathered for 12 500 yr show similarities to the psd s from our calcite experiments the hornblende psd shows a peak and the microcline psd is linear or possibly kinked bulk dissolution rates for the grains calculated from the psd s are slower than rates measured for these minerals in the laboratory dissolution rates estimated from the psd coefficients for the hornblende and microcline soil grains are 3 4 and 1 2 orders of magnitude slower respectively than the corresponding laboratory rates

heavy mineral weathering under acidic soil conditions abstract the weathering of heavy minerals in acid soil profiles ph 4 5 has been studied in terms of their relative mineral proportions and by using the surface etching features of apatite and hornblende grains an increasing stability order of apatite titanite hornblende garnet epidote zircon is found in the 45 63 m coarse silt fraction of soils developed on tills in southwestern sweden in glaciofluvial deposits sorting processes during deposition have largely determined the variations in heavy mineral content of soils and only the dissolution of apatite is significant hornblende etching is more extensive in soils with low hornblende content indicating that the release of cations relative to the amount of hornblende is greater in such soils

etch pit size and shape distribution on orthoclase and pyriboles in a loess catena abstract etch pits on 50 100 m orthoclase and pyribole grains from the e b and c horizons of a catena or drainage sequence of well drained somewhat poorly drained and poorly drained loessal soils were analyzed using sem and digital image analysis the objective was to determine the influence of soil drainage conditions and the associated geochemical environments on dissolution of primary silicates as determined from the distribution of etch pits on the grain surfaces etch pit size frequency distributions indicated that the poorly drained soil contained the smallest and most numerous etch pits for both minerals this indicated low growth rates for etch pits in the poorly drained soil analysis of etch pit shapes indicated that etch pits on pyriboles were more elongated and irregular in the poorly drained soil possibly resulting from a greater growth rate whereas etch pits on orthoclase grains were more elongated and irregular in the well drained soil the larger more elongated and irregularly shaped etch pits on orthoclase from the well drained soil indicative of a greater pit growth rate is consistent with a flushing model of mineral weathering in soils the etch pit size distribution on pyriboles is consistent with the flushing concept but the shape data indicates that other mechanisms of weathering may be involved

hornblende etching and quartz feldspar ratios as weathering and soil development indicators in some michigan soils weathering can be used as a highly effective relative age indicator one such application involves etching of hornblende grains in soils etching increases with time duration and decreases with depth in soils and surficial sediments other variables related to intensity of weathering and soil formation are generally held as constant as possible so as to only minimally influence the time etching relationship our study focuses on one of the variables usually held constant climate by examining hornblende etching and quartz feldspar ratios in soils of similar age but varying degrees of development due to climatic factors we examined the assumption that the degree of etching varies as a function of soil development even in soils of similar age the spodosols we studied form a climate mediated development sequence on a 13 000 yr old outwash plain in michigan their pedogenic development was compared to weathering related data from the same soils in general soils data paralleled weathering data hornblende etching was most pronounced in the a and e horizons and decreased rapidly with depth quartz feldspar ratios showed similar but more variable trends in the two most weakly developed soils the q f ratio was nearly constant with depth implying that this ratio may not be as effective a measure as are etching data for minimally weathered soils our data indicate that hornblende etching should not be used as a stand alone relative age indicator especially in young soils and in contexts where the degree of pedogenic variability on the geomorphic surface is large d 2004 university of washington all rights reserved