diatom inferred salinity in palaeolakes an indirect tracer of climate change lakes in arid and semi arid regions respond to climatic change through shifts in lake water volume and ionic concentration because diatom distribution is highly correlated with lake hydrochemistry diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates here we critically examine the use of diatom inferred salinity as a climate proxy with examples taken from both modern waterbodies and sedimentary profiles sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages furthermore threshold effects salinity regulation via groundwater seepage antecedent conditions and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing these complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages as well as evaluation of potential non linearities in the salinity climate relationship through comparison of diatom inferred salinity with other proxy records of hydrochemical change historical documents where they exist and with other regional lake systems

biological remains geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north sahara abstract a detailed reconstruction of the holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the great western erg algeria it is based on a multidisciplinary approach using numerous techniques the chronological framework is established on the basis of 23 14 c dates on inorganic calcite and mollusc shells ranging from ca 9300 to 3000 yr b p and on an isolated 14 c age of ca 10 900 yr b p the significance of various environmental markers is discussed biological remains are abundant and diversified about 150 taxa including diatoms chrysophytes charophytes ostracods molluscs and foraminifera they provide information on the water chemistry total salinity ionic ratio ph and partial pressure of carbon dioxide and on the habitats water depth development of aquatic plants ephemerality etc emphasis is given to the total salinity individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine like and even hypersaline waters attention is drawn to the possible colonization of marine organisms in continental basins without any connection with the sea stable isotope analyses have been carried out on both inorganic and biogenic mollusc and ostracod shells charophyte gyronites carbonates the 18 o contents demonstrate wide changes in the hydrological balance fluctuations in 13 c contents inform on water mixing residence time and on the importance of biological effects on total dissolved inorganic carbon geochemical analyses were performed on inorganic calcite and on ostracod shells high contents in fe and mn indicate reducing environments during stages of high salinity sr ca and mg ca are used as salinity indicators zn mg ratios suggest that the major water supply was the aquifer of the great western erg today down to about 50 m rather than surface waters from the atlas mountains salinities resulted from the balance between inputs from the water table evaporation rate leaching of pre existing salt crusts and seepage through the lake bottom a reconstruction of the evolution of the palaeosystems through time and space has been attempted with special attention to the instability of the water and ionic balances for different time scales short term fluctuations occurring during a single hydrological cycle annual or seasonal are deduced from the apparent discrepancies between the various indicators found in a given sample long term 10 2 10 3 years changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 and may be from ca 10 900 yr b p to 3000 yr b p for the first time the salinity deduced from diatoms and the 18 o content of the water deduced from that of the carbonate at 25 c are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the holocene major environmental trends recorded from ca 9300 to 3000 yr b p do not show a clear synchronims between the different profiles studied therefore they cannot be interpreted in terms of palaeoclimate but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater a common situation in arid and semi arid zones

hydrological response of an east saharan palaeolake nw sudan to early holocene climate we present a palaeolimnological record in western nubia nw sudan eastern sahara isolated from large scale artesian systems and thus reflecting hydrological changes resulting from local rainfall past changes in lake water chemistry and water depth are interpreted by combining results from biological indicators diatoms ostracodes and charophytes with geochemical and stable oxygen and carbon isotope data palaeolake gureinat existed from 10 9 cal ka bp until at least 7 1 cal ka bp and is devided into five lake phases for the first two millennia the lake was shallow and fluctuating salinity conditions prevailed 500 to 3000 s cm after 8 7 cal ka bp the lake level rose waters became fresh to oligosaline and the lake had a minimum depth of at least 8 m both transgressive and stable lake phases were punctuated by at least three drops in lake level with complete desiccation at around 10 9 and 8 9 cal ka bp the third of these regressions may be coeval with the 8 2 ka climatic event sediments

stable isotope composition of middle miocene carbonates of the frontal cordillera and sierras pampeanas did the paranaense seaway flood western and central argentina abstract the geographic extent interconnectedness and chronology of the miocene paranaense epeiric seaway have been the subject of considerable debate understanding the timing and location of this marine incursion is significant for documenting sea level changes paleoclimatic changes and surface uplift or subsidence stable isotope analyses of carbonate strata in the flat slab segment of the sierras pampeanas and high andean cordillera previously purported to be of marine origin provide evidence that the parana seaway did not inundate this portion of west and central argentina no unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units the saguion formation ese of salinas grande in cordoba province anta formation in the quebrada de la yesera and lerma valley of salta province del buey and del abra formations within the famatina ranges la rioja province and chinches formation in the manantiales foreland basin in the frontal cordillera san juan province paleontologic and lithologic features in support of a marine origin are reconsidered herein instead a lacustrine origin is inferred for these formations their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting until more substantive evidence of marine deposition is found we reject correlation of these units with the parana seaway preserved in the chaco pampean plain subsurface and discourage mapping of the seaway as extending into the central sierras pampeanas or andean foreland our findings suggest the miocene andean foreland was elevated above sea level although it remained as a largely flat lying area west of the pampas and chaco plains which were inundated by the parana seaway

paleohydrology of the upper miocene bicorb lake eastern spain as inferred from stable isotopic data from inorganic carbonates abstract the hydrological evolution of the upper miocene bicorb lake eastern spain has been established based on sedimentology carbonate mineralogy and stable isotopic data from inorganic carbonates from the upper lacustrine unit three main subunits have been recognized in subunit a the sedimentological features suggest a hydrologically closed lake although the isotopic data are not conclusive the isotopic compositions of primary carbonates from subunit b display a distribution typical for a hydrologically closed lake and allow the differentiation of four developmental stages the first stage was defined by the deposition of gypsum lmc dolomite and aragonite indicating large variations in the water mg ca ratio during the second and third stages only lmc was deposited suggesting only minor mg ca fluctuations the 13c and 18o pattern of the second stage shows a well defined covariance r 0 8 while in the third stage the isotopic data display a negative covariant relationship r 0 8 changes in dissolved inorganic carbon composition are considered the main factor to explain the variations from positive to negative covariance in the fourth stage the lake waters underwent large mg ca fluctuations as indicated by the deposition of lmc hmc and aragonite the isotopic trend is also covariant finally subunit c is characterized by lmc and minor amounts of hmc suggesting minor changes in mg ca ratio the isotopic features of this subunit are more congruent with a hydrologically open lake the lowest 13c and 18o values are very similar in the three subunits reflecting the most dilute conditions of the lake water and a uniform isotopic composition of the local meteoric water during these periods the changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates were controlled by the diapiric evolution and changes in the e p balance

oxygen and carbon isotope trends and sedimentological evolution of a meromictic and saline lacustrine system the holocene medicine lake basin north american great plains usa abstract the succession of sedimentary subfacies in the medicine lake basin south dakota usa documents numerous changes in total dissolved solids tds lake level and the nature and stability of meromixis during the holocene the lake formed about 10 6 ka b p when coarse clastics and massive fine grained muds were deposited in a well mixed freshwater system subsequently the lake became saline and meromictic depositing a finely laminated unit composed of variegated and gray muds organic facies gypsum and aragonite laminae phototrophic bacterial plate communities bpc at the chemocline were significant producers of pelagic organic matter relatively high lake levels at least 10 m higher than today but saline and meromictic conditions predominated through the early to mid holocene at the end of this phase lake level dropped and stable meromixis was disrupted sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities bmc spread over the lake replacing bpcs as the predominant microbial producers when lake level rose again massive and laminated units were deposited under less saline conditions another pronounced episode of low lake level and high salinity followed the most recent sediments are sapropels massive or banded muds and littoral sands the 18o record from authigenic aragonite shows more positive values during the arid mid holocene interval with an abrupt increase at the end of this period followed by generally more negative values during the cooler and wetter late holocene the small range of 18o values 3 suggests that the lake was well buffered against high frequency climatic cycles probably as a result of groundwater modulation we postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system either bpcs or bmcs exerted major controls on the carbon cycle and consequently on the development of isotopic trends linking 13c and 18o values more negative 13c values during the early and mid holocene are interpreted as a consequence of the predominance of 12c enriched microbial carbon from bpcs examples of both covariant and inverse covariant isotope trends between 13c and 18o values occur in the medicine lake record inverse covariant trends characterize long periods of stable meromixis when bpcs predominate over bmcs we hypothesize that well developed water stratification caused by greater influx of relatively 18o depleted freshwater induces higher productivity at the chemocline and consequently more positive 13c values in the precipitated carbonates unstable phases of meromixis or holomictic conditions are characterized by a predominance of bmcs or other biogenic producers over bpcs and a number of other factors combine to promote covariant trends the medicine lake record is consistent with the paleoclimatic interpretation for the northern great plains of a more arid early and middle holocene followed by an abrupt transition to a relatively cooler and wetter late holocene the sedimentary succesion in medicine lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late holocene climate punctuated by arid episodes

weathering of hornblende plagioclase and chlorite in meta dolerite australia abstract weathering of meta dolerite by alteration of hornblende plagioclase feldspars and chlorite occurs in a west australian saprolite chlorite alters to randomly interstratified chlorite vermiculite and goethite lath shaped goethite crystals are arranged in a hexagonal pattern on chlorite flakes with the b axis of chlorite parallel to the c axis of the goethite feldspars and hornblende dissolve congruently producing etchpits within which halloysite crystallizes from solution ilmenite alters to microcrystalline anatase via pseudorutile the contents of fe mg ca na si k mn ba sr cu ni zn and co decrease whereas those of al ti cr v and ga remain approximately constant during weathering these chemical changes are explained by reference to the properties of the primary minerals and their various alteration products

feedbacks between climate and surface water in northern africa during the middle holocene observations indicate that the area of lakes and wetlands in northern africa was considerably greater during the middle holocene than at present simulations are designed to examine whether expanded surface waters may have had a significant impact on the strength of the summer monsoon of northern africa three experiments with the national center for atmospheric research community climate model ncar ccm3 are analyzed a modern and two middle holocene 6000 years before present simulations one with and one without prescribed expanded surface water there is a significant increase in the strength of the summer monsoon in the middle holocene simulation due to the enhanced seasonal insolation cycle the addition of surface waters result in a june july and august mean increase in the net surface radiation 5 an increase in the latent heat flux 30 a decrease in the sensible heat flux 10 and an increase in the near surface specific humidity 5 compared to the middle holocene simulation without surface water changes the changes in these simulated climate variables are comparable in scale to changes due to orbital forcing alone the expanded surface waters result in a cooling of the atmosphere and anticyclonic flow over the large water bodies in summer relative to the simulation without surface water changes the combination of increased atmospheric moisture and altered circulation results in significant changes to the precipitation distribution in northern africa including a small increase in the zonal mean july precipitation to the north of the lakes and a decrease to the south the geographic distribution of the precipitation with surface water changes is qualitatively in better agreement with observations than the distribution with orbital forcing alone but still does not fully match the expansion implied by observations nor the expansion required to produce the simulated middle holocene surface waters used in this study the results of this study suggest that surface waters were an important factor in the climate of northern africa during the middle and early holocene and that they must be included for accurate simulation of this climate

late quaternary floods and droughts in the nile valley sudan new evidence from optically stimulated luminescence and ams radiocarbon dating our results show that the late pleistocene nile in northern sudan was shifting position and actively aggrading at 145 20 kyr 83 24 kyr 32 8 kyr and 20 7 0 2 kyr and indicate for the first time a phase of high energy flow in the white nile at 27 8 3 2 kyr with still high but somewhat reduced flow in that river at 13 3 kyr 10 kyr and 4 8e4 0 kyr beach ridges associated with a 386 m strandline of the white nile have osl ages of 27 5 2 7 kyr and 14 5 1 6 kyr the holocene terraces and former channels of the main nile have ages of 11 kyr 6 5e5 0 kyr and 4 8e4 0 kyr after which there was a general decline in flood discharge the now arid main nile valley in northern sudan was significantly wetter during the early to middle holocene with a lake up to 450 km 2 in area fed by an overflow channel from the early holocene nile between 9 5 kyr and 7 5 kyr previously stable late pleistocene dunes were reactivated at intervals during the holocene with five samples from the white nile valley indicating brief phases of holocene dune activity at 9 9 2 0 kyr 9 0 2 8 kyr 6 6 0 9 kyr 4 8 0 9 kyr and 2 9 0 5 kyr the earliest of which occurred within periods of generally wetter climate and higher nile flow the youngest freshwater shells on the khor abu habl alluvial fan west of the white nile correspond to a time of regionally wetter climate between 1 7 and 1 0 kyr our results suggest that millennial scale climatic instability may have been characteristic of holocene climates in this region 2010 elsevier ltd all rights reserved

subsurface dissolution of heavy minerals frio formation sandstones of the ancestral rio grande province south texas abstract oligocene sandstones frio formation of south texas deposited by the ancestral rio grande contain a heavy mineral assemblage that is markedly altered even in the shallowest sample examined approximately 1 km depth the weight fraction of detrital heavy minerals in the modern rio grande is approximately 1 whereas subsurface frio sandstones contain as little as 0 1 or less heavy minerals modern rio grande sand contains a diverse and unstable heavy mineral assemblage dominated by pyroxenes amphiboles and an assortment of iron titanium oxides minor components include apatite garnet sphene epidote and zircon heavy minerals in subsurface oligocene sandstones include abundant apatite zircon and titanium oxide among detrital components minor iron rich garnet also survives in the subsurface to depths of about 3 km other very stable but also very minor heavy minerals include tourmaline chromite spinel and monazite locally the heavy mineral fraction in the frio is dominated by authigenic components such as siderite pyrite or barite authigenic sphalerite is also observed among the heavy minerals feldspars and rock fragments suggest that sediment deposited by the ancestral rio grande was more strongly dominated by volcanic detritus than is the modern river it is reasonable to infer that the initial heavy mineral assemblage of the frio formation was likewise more unstable than that which appears today in the rio grande thus heavy mineral loss determined by comparing subsurface frio sandstones with the sand in the modern rio grande is conservative comparison of sr and nd concentrations in modern rio grande sands and subsurface frio sandstones suggests that heavy mineral destruction in the subsurface does not cause major perturbations of whole rock isotopic systems involving these elements

transformation of a subtropical river into a hyperhaline estuary the casamance river senegal paleogeographical implications abstract located in the south of senegal in the transitional guineo congolian soudanian zone the casamance system is composed of a small river supplied by a narrow drainage basin and followed by a 250 km long estuary the continuous rainfall decrease experienced by the region since the beginning of the century drastically affected the casamance river this climatic change suddenly accelerated during 1968 1970 which led to a four stage succession before 1968 1970 stage 1 rainfall 1300 mm the casamance estuary showed the customary negative upstream salinity gradient between 1968 1970 and 1978 stage 2 rainfall 1000 mm under decreasing rainfall the casamance estuary shifted to a hyperline inverse estuary it showed well defined physical chemical and biological zonations fresh groundwater resources supplying the upper reaches of the river during the dry season were progressively depleted between 1978 and 1986 1987 stage 3 the salinity peak of the end of the dry season progressively rose and shifted upward the ecological zonation showed the increasing marine influence until 1986 between 1986 1987 and 1990 stage 4 the dry season fresh water input had weakened with the depletion of groundwater resources a slight increase in rainfall was insufficient to repulse the hyperhaline waters and the restricted area extended upstream and downstream the physical chemical and biological zonations strongly widened and flattened in the future the return to stage 1 conditions by an increase in average rainfall would take decades inevitably it will be much longer than the transformation into a hyperhaline estuary thus a slight decrease of the average rainfall in the region provoked a drastic change of the hydrological and biological patterns of the ria the result seems to be a major crisis in the region and might be recorded in the fossil records we discuss these observations in terms of possible misreadings of paleoenvironments and in relation with long term climate trends

rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations 1 the late quaternary a working hypothesis is proposed to account for the present accumulation of brines in isolated pockets of the ocean floor and for the formation of the underlying organic and metal rich sediments these are the tyro and bannock basins in the east mediterranean the red sea deeps and the orca basin in the northern gulf of mexico initiation of brine derived deposition in the red sea deeps and orca basin occurred between 12 000 and 8000 years b p this time bracket also encompasses the formation of the latest east mediterranean sapropel and the wettest global climate since the last glacial maximum this wet period first appeared in the tropics around 12 000 years b p then in the subtropical and middle latitudes during the same period the 23 000 year precession cycle brought the summer insolation of the northern hemisphere to its peak at 11 000 years b p with retreating northern hemisphere ice sheets the red sea deeps and the orca basin became anoxic during this humid period and metal rich sapropel deposition then began in contrast the tyro and bannock basins began accumulating a brine long before and persisted beyond this climatic stage the hypothesis involves two propositions 1 as in the eastern mediterranean sea marine anoxia was mainly the consequence of the large influx of continental runoff and local precipitation longer residence time of bottom waters so called stagnation in silled rimmed basins would have resulted from lower salinity at the sea surface in areas of deep water formation in the eastern mediterranean the red sea and the gulf of mexico and 2 miocene or older evaporites underlie these basins or outcrop on their flanks leaching from these evaporites was an ongoing process before the quasi stagnation phase but the initial leachate much less saline than the present brines was continuously flushed by bottom circulation the climate induced quiescence of bottom waters in these basins enabled the leachate to accumulate the feedback of stagnation by increased density progressively raised the salinity of entrapped bottom waters to the present brine concentration the high density has resisted brine removal by bottom circulation until present time long after cessation of the initiating wet period the brines therefore are stagnant fossil waters

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

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

weathering profiles and clay mineralogical developments bornholm denmark abstract saprock saprolite associations were studied by field and laboratory methods optical microscopy x ray powder diffraction scanning electron microscopy electron microprobe in order to describe regolith development in the crystalline rocks of the nygard kaolin pit bornholm denmark the clay sequences and stages of porosity development are similar to those observed for reservoir rocks from the utsira high riber et al 2016 the weathering of the parent granite began before the end of the mesozoic two stages of syn pre burial alteration followed by diagenesis during burial and then post uplift weathering have been recognized in stage plagioclase and some biotite biotite vermiculite kaolinite reacted to form elongate booklets of highly ordered kaolinite or smaller blocky pseudohexagonal crystals stage ii represented more extreme weathering developed along local fracture systems the higher potential for fluid flow in the fractures caused highly ordered kaolinite to alter to halloysitic poorly ordered kaolinite plagioclase biotite and k feldspar continued to interact with formation water and formed additional quantities of secondary clay minerals as a consequence of inter intra and trans granular cracks the original rock fabric deteriorated and porosity increased during later burial diagenetic siderite crystals filled voids and chlorite formed at the expense of kaolinite after re exposure calcite precipitated in localized zones smectite formed on a broader scale wherever high ionic strength groundwaters were present a mineralogical key to predicting where high porosity and permeability zones occur is the recognition of halloysitic poorly ordered kaolinite highly ordered kaolinite is abundant but it is ubiquitous siderite chlorite and smectite adversely affect permeability riber l dypvik h sorlie r and ferrell r 2016 clay minerals in deeply buried paleoregolith profiles norwegian north sea clays clay miner doi 10 1346 ccmn 2016 064036

holocene climatic evolution at 22 23 n from two palaeolakes in the taoudenni area northern mali abstract in the endorheic hydrologically isolated taoudenni depression nowadays receiving 5 mm mean annual rainfall continuous sections in fossiliferous holocene deposits from two palaeolakes with different geological evolutions have been compared the following climatic sequence is determined 9000 8300 yr b p the beginning of a humid phase 8300 6700 yr b p climatic optimum from 6700 yr b p onwards a change in regime 4500 4000 yr b p end of humid phase aridification begins the data imply a 600 km northern shift of the monsoonal belt which confirms models described in the literature

processes affecting groundwater quality in arid zones the case of the bou areg coastal aquifer north morocco abstract the coastal aquifer of bou areg morocco has been studied to identify the main processes causing groundwater salinization using a multi tracer general chemistry and isotopes 2h 18o 13c 15nno3 18ono3 geochemical approach groundwater is characterized by the widespread occurrence of brackish waters tds no 3 reaching a maximum of 208 mg l 1 and cl lagoon samples represent a mixture of fresh water and sea water showing a na cl ratio in agreement with that of sea water and an excess of ca the high ca values represent the main peculiarity of the groundwater lagoon water system two types of groundwater could be identified i freshwater separated from the whole system and located at the limit of the irrigated area characterized by low tds depleted isotopic composition and relatively high quality and ii water mainly recharged by mountain runoff interacting with local recharge and acquiring salinity from different sources thus creating a complex system of dilute waters hydrochemical results confirm that the high salinity of the aquifer is caused by the coexistence of dissolution of evaporate rocks and carbonates from miocene strata water rock interaction and human impacts due to agricultural return flows the latter represents the main contribution to groundwater salinization especially in the central part of the aquifer as well as one of the main causes of the general increase in no 3 concentrations only locally in the southern part of the aquifer close to the city of kariat arkmane the high salinization observed may be attributed to the presence of lagoon water intrusion the isotopic composition of dissolved no 3 indicates manure and septic effluents especially in urban areas and in the central part of the plain and synthetic fertilizers in the agricultural zone as the main drivers for human induced pollution the study shows that agricultural return flow has significantly modified the chemistry of the system and it is a prime example of the human induced changes over coastal environments saline water intrusion from the lagoon in the shallow aquifer is negligible while discharge of polluted groundwater into the lagoon has been found to partially alter its quality

palaeoenvironmental reconstruction of a pleistocene lacustrine sequence from faunal assemblages and ostracode shell geochemistry baza basin se spain abstract we have reconstructed the early pleistocene palaeoenvironmental record for a shallow lacustrine sequence from the baza basin southern spain using faunal assemblages and the shell chemistry of two ostracode species candona sp and cyprideis torosa in the ne basin sector orce area a lower pleistocene lacustrine sequence up to 10 m thick contains alternating phases of two fossil assemblages which differ in their salinity requirements the faunal assemblages record phases of 1 slightly saline ca bicarbonate rich water when freshwater organisms predominated and 2 saline nacl dominant water in which marine like organisms lived the ostracode shells from intervals with a saline fauna have higher 18o values than those from intervals with a freshwater fauna this feature corresponds to a parallel alternation of positive and negative hydrologic balances 18o and 13c values from ostracode calcite display a covariant trend which indicates that the ostracodes lived in a closed lacustrine system the isotopic data show a better agreement with the inferences from faunal assemblages than with trace element trends the trace element data although locally fitting well with the isotope and faunal interpretations suggest overall that trace element contents in ostracodes are not consistent indicators of salinity and or temperature variations but depend on a set of complex factors which may not be easily deciphered the alternation of concentration dilution phases recorded in the studied section can be correlated with climatic cycles described from synchronous ocean basin records from the late matuyama chron

use of charophyte carbonates as proxy indicators of subtle hydrological and chemical changes in marl lakes example from the miocene bicorb basin eastern spain abstract the lower alluvial unit in the miocene bicorb basin contains several metric scale limestone intervals which record episodic shallow lacustrine environments in an alluvial setting developed during the early stage of the basin s evolution five main carbonate facies have been differentiated in the lacustrine limestones although calcite charophyte incrustations predominate and constitute the most striking features of these deposits the thinnest limestone intervals correspond to deposits from charophyte meadows in ponded shallow depressions in floodplains the thickest limestone intervals are mainly formed by banded limestones and usually correspond to diverse types of regressive sequences that have been interpreted as resulting from the infill of shallow lakes the sedimentological features and sequences show noticeable differences in the gradient of the littoral zones and the amount of palustrine deposits with models proposed for marl lakes charophytic carbonates from the best preserved facies show similar microtextures to those from recent charophyte incrustations the variations in stable isotopes 13 c 18 o for these primary carbonates occur in parallel with luminescence variations and correspond to hydrological changes and variations in solute composition and eh ph status in the lake waters the carbonates that display moderate to strong diagenetic modifications show a diverse degree of compaction aggrading neomorphism strong cementation and nodulization the isotopic values for these are arranged in diverse clusters there is a correlation between the degree of luminescence and the 13 c this suggests that hydrological and hydrochemical variations both in the lacustrine and diagenetic environments are being recorded in parallel we emphasize the need for further comparative studies between recent and ancient charophytic carbonates as these carbonates have been used in palaeoenvironmental reconstructions special attention must be paid to the diagenetic changes in ancient charophytic marls

ages of quaternary pluvial episodes determined by uranium series and radiocarbon dating of lacustrine deposits of eastern sahara abstract as documented by radiocarbon dating and geoarchaeological investigations the now hyperarid northwestern sudan and southwestern egypt experienced a period of greater effective moisture during early and middle holocene time about 10 5 ka we have used the uranium series technique to date lacustrine carbonates from bir tarfawi bir sahara east wadi hussein oyo depression and the great selima sand sheet localities results indicate five paleolakeforming episodes occurred at about 320 250 240 190 155 120 90 65 and 10 5 ka four of these five pluvial episodes may be correlated with major interglacial stages 9 7 5e and 1 the 90 65 ka episode may be correlated with substage 5c or 5a our results support the contention that past pluvial episodes in north africa corresponded to the interglacial periods farther north ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka age results and field relationships suggest that the oldest lake and ground water deposited carbonates were much more extensive than those of the younger period and that carbonate of the latest wet periods were geographically localized within depressions and buried channels

desert wetlands in the geologic record abstract desert wetlands support flora and fauna in a variety of hydrologic settings including seeps springs marshes wet meadows ponds and spring pools over time eolian alluvial and fluvial sediments become trapped in these settings by a combination of wet ground conditions and dense plant cover the result is a unique combination of clastic sediments chemical precipitates and organic matter that is preserved in the geologic record as ground water discharge gwd deposits gwd deposits contain information on the timing and magnitude of past changes in water table levels and therefore are a potential source of paleohydrologic and paleoclimatic information in addition they can be important archeological and paleontological archives because desert wetlands provide reliable sources of fresh water and thus act as focal points for human and faunal activities in some of the world s harshest and driest lands here we review some of the physical sedimentological and geochemical characteristics common to gwd deposits and provide a contextual framework that researchers can use to identify and interpret geologic deposits associated with desert wetlands we discuss several lines of evidence used to differentiate gwd deposits from lake deposits they are commonly confused and examine how various types of microbiota and depositional facies aid in reconstructing past environmental and hydrologic conditions we also review how late quaternary gwd deposits are dated as well as methods used to investigate desert wetlands deeper in geologic time we end by evaluating the strengths and limitations of hydrologic and climatic records derived from gwd deposits and suggest several avenues of potential future research to further develop and utilize these unique and complex systems

seasonal variation in equatorial mesospheric temperatures observed by sme abstract observations made by the solar mesosphere explorer sme satellite from 1982 through 1986 are used to examine the seasonal variation of temperature in the equatorial mesosphere between 58 5 and 90 km near the equator seasonal variability is dominated by a strong semiannual oscillation sao whose amplitude increases from about 3 k in the lower mesosphere to 7 3 k near 80 km above 80 km the amplitude of the oscillation decreases to a minimum at 83 km but increases again sharply above that level reaching 16 6 k at 90 km the highest level observed the structure of the temperature sao is consistent with previous observations of the saos in temperature and zonal wind although the very large amplitude at 90 km may be due in part to contamination by the diurnal tide just below 80 km temperatures are warm cold near the solstices equinoxes implying westerly easterly accelerations above the behavior at 58 5 km lap that at 80 km by about 2 months there is evidence in the data for a seaso

pleistocene paleoenvironmental evolution at continental middle latitude inferred from carbon and oxygen stable isotope analysis of ostracodes from the guadix baza basin granada se spain abstract a representative paleoenvironmental reconstruction of continental middle latitude from ca 2 my to the upper part of middle pleistocene 279 77 ky was obtained from the carbon and oxygen stable isotopes analyzed in ostracode shells cyprideis torosa recovered in the guadix baza basin se spain an intramontaneous closed depression filled by alluvial and lacustrine sediments this study was performed along a 356 m thick composite section dated previously by paleomagnetism and the amino acid racemization method 13c and 18o profiles reflected changes in temperature the evaporation infill ratio in the water bodies and the amount of rain 13c is also affected by changes in plant biomass periods with high 13c and 18o values are associated with warm and dry regimes and with less vegetation which in some cases coincide with the development of displacive gypsum crystals whereas low 13c and 18o values correlate with cold and humid episodes which cause more vegetation biomass and therefore increasing the input of isotopically light carbon intermediate 18o values are linked to temperate dry or humid episodes when they coincide with high or low 13c values respectively 86 paleoclimatic events were distinguished in the pleistocene record from the 13c and 18o profiles from both the statistical analysis of the geochemical data and the geological observations four cold and humid long periods low 18o and four warm and dry long periods high 18o were defined this differs with respect to the paleoclimatological behavior established for the northern hemisphere where during cold periods glacial no water was available while permafrost conditions persisted whereas in warm episodes interglacial higher precipitation rates occurred good correspondences between the guadix baza basin paleoclimatic record and a marine oxygen isotope sequence two continental cores and other long mediterranean paleoenvironmental records pollen sequences from israel were found which suggested that climate changes in the guadix baza basin were in tune with global climatic changes

estimation of long term diffuse groundwater discharge in the northern sahara using stable isotope profiles in soil water stable isotope profiles in soil water in the sahara have been interpreted in terms of the hydrologic processes taking place at one site where the water table is at about 10 m depth the isotope data are interpreted using the model of barnes and allison to show that the water loss by evaporation from the water table is about 2mm a 1 at another site nearby there is a net evaporative loss from the water table but the evaporation is interrupted by occasional infiltration events it is suggested that accumulation of chloride in soil water can give information about the onset of aridity in the sahara

dendroecological evidence of lake level changes during the last three centuries in subarctic quebec abstract dendroecological analysis of black spruces picea mariana mill bsp near the shore of clearwater lake in the eastern canadian subarctic yielded information on water level over the past three centuries tree positions growth froms layering tree ring patterns and ice scars provide direct evidence of a major rise in lake level from the 17th century to the present trees on the lake shore were progressively submerged by the increasing water level which reached a maximum in this century higher levels began around the mid 18th century when shoreline spruce trees started to lean because extensive wave erosion caused landward shore displacement high mortality of leaning spruce and abundant ice scar formation occurred during this century the rise in water level in inferred to have been caused by snowier winters occurring parallel to climatic warming

experimental study of dissolution rates of actinolite in cacl2 hcl h2o up to 400 c abstract actinolite dissolution rates in aqueous hcl and aqueous cacl2 hcl have been measured at temperatures ranging from 25 to 400 c at 23 mpa under conditions far from equilibrium using a flow through reactor the relative release rates for the various elements in actinolite due to breaking of metal oxygen bonds are all different all of the release rates vary with temperature and ph the release rate of si increases as temperature rises from 20 to 300 c but then decreases at higher temperatures other metallic ions such as ca mg fe and al are all released more rapidly than si at temperatures between 25 and 300 c but more slowly at temperatures 300 c the ratios of mca msi and mmg msi in the effluent at 200 c are close to the stoichiometric proportions of these elements in actinolite but differ from stoichiometric values at temperatures both above and below 200 c actinolite dissolution in aqueous hcl at 100 c yields element ratios close to stoichiometric values in the mineral but again the ratios vary at higher and lower temperatures our experiments demonstrate that under conditions far from equilibrium dissolution rates depend on both ph and the activity of aqueous mi zi where zi is the valence of the metal mi in the mineral and mi zi refers to ca2 mg2 or al3 the ion exchange model suggests that metal hydrogen ion exchange reaction on the surface is a key step for mineral dissolution and that a si rich mi deficient precursor complex is formed on the mineral surface at temperatures r a exp e a rt a h zi a mi zi where is the order with respect to the ratio of activity of hydrogen ions and dissolved metal ions mi the mg h exchange coefficient is 0 55 at 100 c the apparent activation energy ea 14 4 kj mol and a 0 23 10 3 m cm 2 s 1 our results show that water properties vary strongly within the critical region which affects the dissolution mechanism thus the release rates for various elements at temperatures above 300 c are different from those at temperatures below 300 c high resolution transmission electron microscopy hrtem and scanning electron microscopy sem studies indicate that a fe al rich si deficient layer is formed on the surface at temperatures above 300 c x ray photoelectron spectroscopy xps analyses show that the surface layer that formed by reaction with aqueous hcl at 400 c is a hydrated silicate composed of si oh and mi oh as well as si o and mi o bonds

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

humidity changes in southern tunisia during the late pleistocene inferred from u th dating of mollusc shells abstract calcareous deposits mainly consisting of mollusc shell accumulations which have been dated by the u th disequilibrium method mark the shorelines of paleolake highstands in the great chotts area of southern tunisia the 5 sites studied consist of discontinuous accumulations of fossils of marine like organisms e g cerastoderma glaucum melania tuberculata melanopsis praemorsa cerithium rupestre u th isochron plots and age frequency histograms for 39 shell samples are reported here limited variations for u content and 234u 238u activity ratios ar of shells support the hypothesis of closure of the geochemical system with respect to this element it is remarkable that 234u 238u ar of shells collected in chott fejej or chott jerid are clustered around different values reflecting probably different groundwater recharge from the continental intercalaire ci or complexe terminal ct aquifers furthermore waters collected near wadi el akarit show 234u 238u ar values comparable to those observed for shells 14c determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka bp the u th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30 95 100 130 150 and 180 200 ka for the episode centred around 30 ka the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event moreover the comparison of 13c and 18o with those of older humid pleistocene phases when very large palaeolakes have been recorded suggests that these young carbonate shells are not related to a true highstand lake it is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place it should be noted that this period was less arid than the present

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

diagenetic alteration of detrital biotite in proterozoic sedimentary rocks from sweden abstract the alteration of detrital biotites has been studied in two clastic proterozoic sedimentary sequences from southern and central sweden namely the visingso group and the dala sandstone respectively the results showed that the biotites are altered through replacement by clay minerals illite chlorite and less commonly kaolinite biotite may either be replaced pseudomorphically one biotite crystal by one clay mineral crystal or by fine crystalline clay minerals which merge into the interstitial clay matrix in reddish rocks biotite may be associated with pigmentary hematite and in greenish rocks with pyrite the variation in the two textural habits of biotite alteration i e pseudomorphic or fine crystalline is related to physico chemical properties of pore solution and nature of biotite particles

freshwater to marine like environments from holocene lakes in northern sahara continuous swamp and lacustrine sequences from the northern edge of the great western erg are dated at 9 300 3 000 yr bp diatoms ostracods molluscs and foraminifera show great changes in salinity ranging from freshwater to marine like environments the stable isotope composition of carbonates indicates that the evaporation of groundwater supplies is responsible for these fluctuations in salt contents until now evidence for the occurrence of a wet climatic phase of early and middle holocene age in the north north west sahara was based on a restricted number of 14c dates from scattered lacustrine deposits and paleosols1 8 the present study complements this knowledge and highlights the importance of local hydrological factors in interpreting palaeolimnological changes

natural weathering mechanisms of almandine garnet the weathering of almandine garnet in the oxidized vadose zone of saprolite near otto north carolina begins at grain boundaries and along fractures traversing the garnet grains radially oriented fibrous intergrowths of gibbsite and goethite form layers of uniform thickness which grow by centripetal replacement as weathering proceeds the contact between the garnet surface and the layer of weathering products is sharp and smooth and garnet corners are rounded large well defined etch pits are absent on the underlying garnet surface these observations suggest that diffusion transport of reactants and or products through the gibbsite goethite layer is the rate limiting step in the weathering of almandine garnet in the oxidizing environment of the saprolite in soils overlying the saprolite garnet surfaces are directly exposed to weathering solutions without the intervening surface layer such unprotected grains in soils and stream sediments exhibit numerous large well developed etch pits surface reaction control rather than transport control prevails during weathering of almandine garnet in the absence of protective surface layers of weathering products biochemical or biological processes in the soil apparently prevent the gibbsite goethite layer from forming or persisting soil solutions can therefore react directly with the garnet surface rather than requiring reactants or products to diffuse through the gibbsite goethite coating as in the saprolite the mechanism and rate limiting step of almandine garnet weathering is apparently strongly dependent on the chemical environment in which weathering occurs

constraining the timing of alluvial fan response to late quaternary climatic changes southern tunisia the evolution of the oued es seffaia alluvial fan during the last 50 000 years is analyzed in the light of chronometric data derived from ams radiocarbon and optical dating techniques these ages have enabled the temporal constraint of the distalward progression of the intersection point the upper limit of the locus of deposition which has resulted in the telescopic segmentation of the fan comparison of these data with the late quaternary climatic history of the maghreb highlights several problems of interpretation due both to our limited knowledge of regional palaeoclimates and of the response of dryland fluvial systems to climate changes however our data provide some evidence to suggest that fan incision and formation of telescopic segments occurs in response to changes from arid to less arid conditions supporting the conclusions of some of the work from other areas

direct observation of reactant product interfaces formed in natural weathering of exsolved defective amphibole to smectite evidence for episodic isovolumetric reactions involving structural inheritance abstract high resolution transmission electron microscopy hrtem and energy dispersive x ray microanalysis eds were employed to characterize grain boundary structures in naturally weathered amphibole our observations provide insights into the submicroscopic mineralogic controls on transport of solutions to and from reaction sites finely exsolved amphibole anthophyllite gedrite in outcropping slightly weathered gedrite gneiss transforms isovolumetrically to smectite stacking faults and exsolution lamellar boundaries focus weathering reactions whereas chain width defects decrease the susceptibility of the surrounding amphibole to alteration relatively al poor anthophyllite lamellae alter slightly more readily than those of al rich gedrite large quantites of mg fe si and al are removed from reaction sites however smectite compositions directly reflect

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

scanning electron microscope study of zircons and rutiles from a podzol chronosequence at cooloola queensland australia summary a chronosequence of four soils derived from aeolian deposited sand at cooloola along the eastern coast of southern queensland has been studied to document predepositional and pedogenic changes in zircon and rutile grains in the fine sand fraction 53 125 m with increasing soil age the soils range in age from holocene typic quartzipsamment presently drifting sand to mid pleistocene spodic quartzipsamment 90 ka bp parent materials are moderately stratified sands with irregular depositional layers in places having high concentration of heavy mineral grains scanning electron microscopy was used to characterize grain surface microtexture by a semi quantitative method based on frequency of occurrence of ten classes of surface microtextures pedogenic changes were quantified and differences between horizons in the same profile between different profiles within the same horizon and between minerals were tested for significance using the 2 distribution zircon grains are less chemically weathered during pedogenesis than rutile possibly due to the higher iron content of the latter however zircons tend to show more features which can be associated with physical alteration rutile grains in the illuviated b2hs horizons of the troporthod and spodic quartzipsamments are more thickly coated with poorly crystalline iron and alumino silicate compounds compared with zircon grains and the presence of grain coatings does not seem to have prevented the surfaces of grains from developing solution pits with increasing age there is a preponderance of weathering features on grains of both minerals the presence of various shapes and surface microtextures on grains from the different soils suggests a multiple grain source for zircon and rutile and a complex predepositional history for these minerals nevertheless microtextural features of zircons and rutiles are valuable for the interpretation of the extent and nature of post depositional weathering

supergene alteration of sulphides vii distribution of elements during the gossan forming process abstract the results of many electrochemical weathering experiments involving sulphide ore assemblages are brought together and discussed in terms of the mechanism of reaction of sulphide minerals through to oxide gossan minerals the observed reactions of various sulphide minerals are compared and the behaviour of the resultant cations and anions in solution including their hydrolysis and adsorption properties as they effect the types of gossan materials that are formed is given the interaction between the ph of the reactions and the amount of fe in the sulphides is shown to be the dominant feature which controls the mobility of many elements the experiments demonstrate how a low ph develops near to the surface of oxidising sulphides giving a scale at this point that is depleted in most base metals relative to gossan materials that precipitate from solution gangue minerals are shown to be leached and the elements leached play a role in the formation of gossan materials there is a complex interaction between the fe ii fe iii couple and the cu i cu ii couple that should favour the formation of hematite native cu at low ph and malachite hematite goethite at ph above 6 5

weathering reactions and hyporheic exchange controls on stream water chemistry in a glacial meltwater stream in the mcmurdo dry valleys 1 in the mcmurdo dry valleys antarctica dilute glacial meltwater flows down wellestablished streambeds to closed basin lakes during the austral summer during the 6 12 week flow season a hyporheic zone develops in the saturated sediment adjacent to the streams longer dry valley streams have higher concentrations of major ions than shorter streams the longitudinal increases in si and k suggest that primary weathering contributes to the downstream solute increase the hypothesis that weathering reactions in the hyporheiczonecontrolstreamchemistrywastestedbymodelingthedownstreamincreasein solute concentration in von guerard stream in taylor valley the average rates of solute supplied from these sources over the 5 2 km length of the stream were 6 1 10 9 mol si l 1 m 1 and 3 7 10 9 mol k l 1 m 1 yielding annual dissolved si loads of 0 02 1 30 mol si m 2 of watershed land surface silicate minerals in streambed sediment were analyzed to determine the representative surface area of minerals in the hyporheic zone subject to primary weathering two strategies were evaluated to compute sediment surface area normalized weathering rates the first applies a best linear fit to synoptic data in order to calculate a constant downstream solute concentration gradient dc dx constant weathering rate contribution crc method the second uses a transient storage model to simulate dc dx representing both hyporheic exchange and chemical weathering hydrologic exchange he method geometric surface area normalized dissolution rates of the silicate minerals in the stream ranged from 0 6 10 12 mol si m 2 s 1 to 4 5 10 12 mol si m 2 s 1 and 0 4 10 12 mol k m 2 s 1 to 1 9 10 12 mol k m 2 s 1 these values are an order of magnitude lower than geometric surface area normalized weathering rates determined in laboratory studies and are an order of magnitude greater than geometric surface area normalized weathering rates determined in a warmer wetter setting in temperate basins despite the cold temperatures lack of precipitation and lack of organic material these results suggest that the continuous saturation and rapid flushing of the sediment due to hyporheic exchange facilitates weathering in dry valley streams index terms 1045 geochemistry low temperature geochemistry 1806 hydrology chemistry of fresh water 1625 global change geomorphology and weathering 1824 1886 1890 hydrology wetlands keywords hyporheic zone chemical weathering flow path antarctica stream

palaeogeographic palaeoclimatic palaeohydrological and chemical biochemical controls on accumulation of late eocene coastal lacustrine palustrine limestones southern england abstract the shallow lacustrine to palustrine hatherwood limestone member of the late eocene headon hill formation southern england is in a succession of fluvio estuarine and laguno lacustrine siliciclastic sediments it is up to 8 0 m thick and is subdivided by metric scale palaeotopographic erosion surfaces one of which underlies lenticular humic siliciclastic mudstones the limestone lithologies are 1 biocalcilutites biocalcisiltites 2 biocalcarenites 3 calcirudites and 4 laterally discontinuous laminar and fenestral crusts lithology 1 accumulated in a low energy shallow lacustrine depositional environment indicated by the predominance of pulmonate pond snails in the mollusc assemblage in contrast lithologies 2 and 3 are the coarse grained fills of shallow palaeotopographic lows and also form basal lags and palaeokarst fills lithology 3 comprises remnant autochthonous lags that resulted from selective erosion during lacustrine transgressions brecciation and proto intraclast to nodular differentiation in lithology 2 suggests derivation of the intraclasts from subaerially and pedogenically modified shallow lacustrine muds laminar crusts 4 0 5 6 0 cm in thickness display irregularly undulose and anastamosing laminations 150 400 m thick that display differential staining by iron oxide the abundance of rootlet traces and calcitised parenchymatic root tissues in the fenestral crusts suggests a palustrine depositional environment for these lithologies the siliciclastic mudstone lithologies are 1 black humic muds and 2 blue green silty muds and marls the high c n molar ratio of the black muds suggests derivation of the humic content mainly from vascular plants in local wetlands the mollusc assemblage of the blue green siliciclastic muds resembles floodplain lake assemblages from the regional palaeogene succession mean stable isotopic values for limestone samples of 4 75 1 53 for 13 c and 2 77 1 47 for 18 o indicate a relatively heavy tendency within published spectra for tertiary lacustrine carbonates the 18 o isotope results agree with published subtropical surface water palaeotemperatures for this succession the total characteristics of the member therefore suggest a shallow lacustrine to palustrine depositional environment pedogenic overprinting and mechanical reworking displayed by the limestones are attributable to seasonal fluctuations of water level during lake highstands but in this marine influenced setting major long term lake lowstands resulting in deep erosion and palaeokarsting of the limestones may be attributable to falls in relative sea level the biocalcilutites and humic muds accumulated during the ensuing lake transgressions

amalgamated accumulations resulting from climatic and eustatic changes akchar erg mauritania the akchar erg of the sahara of western mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late pleistocene and holocene deposits that reflect eustatic and climatic events mapping trenching and dating by 14c methods and artefacts show that the prominent complex linear dunes draas of the akchar erg are actually composite features showing at least three constructional and two destructional phases the constructional phases are represented by three convex up layers i a modern veneer moulded into superimposed crescentic dunes which partially mantle the larger linear bedforms ii a middle partly root turbated sand deposited sometime during the last 4000 years and iii a core of linear dune sand formed during the last glacial period 13 000 20 000 yr bp which today shows relict relief intense root turbation and pedogenesis these constructional phases are separated by super bounding surfaces that coincide with erg destructional phases surface 2 bounds the middle aeolian sand and is marked by a lag surface of small granules surface 1 is a very prominent surface with an abundance of neolithic artefacts and represents stabilization of the linear dunes during the humid interglacial period 4000 11 000 yr bp interdraa deposits originated during the interglacial period and consist of continental lacustrine limestones and sandstones humic sands deposited in marshes and sabkhas on the coast the sabkhas originated during interglacial highstand of sea level when interdraa areas were marine embayments and subsequently dried during regression the draa and interdraa sequences therefore in spite of being adjacent facies actually represent different events and were not formed simultaneously the upwind sand sheet margin of the akchar erg shows exposures of the middle and core aeolian sands which were previously protected from deflation by vegetation being progressively cannibilized in the current phase of erg construction and revealing a crystalline basement rock in this proximal area conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record

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

organic matter preservation on continental slopes importance of mineralogy and surface area abstract theoretical considerations calculations and data reported by keil et al 1994a were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix linked organic carbon content of continental margin sediments occurs as a monolayer or monolayer equivalent coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains mayer 1994 results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments even in the sand and silt fractions but rather the presence of nonspherical high surface area to volume particles primarily clays but also oxy hydroxides and ultra structured nonorganic bioclasts such as diatom frustules analysis of the mineralogy organic carbon content surface area and in situ microfabric of continental margin sediments off california in the mendocino and san luis obispo areas and off the state of washington indicate a strong correlation between the amount of matrix linked organic matter present and the suite of clay minerals data indicate that organic carbon appears to be preferentially sequestered in smectite rich sediments compared to those whose clay fractions are dominated by chlorite we suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior our data also indicate that organic carbon preservation on the three california transects all of which have a suboxic oxygen minimum zone that impinges on the sea floor is not significantly influenced by differences in bottom water oxygen concentration and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin

micropore surface area variation with grain size in unweathered alkali feldspars implications for surface roughness and dissolution studies abstract the bet and micropore surface areas of different size fractions of four different unweathered alkali feldspars were calculated using gas adsorption data bet surface area decreased with increasing grain size micropore surface area accounted for 12 84 of bet surface area for two of the feldspars eifel sanidine and kb14 micropore surface area increased with decreasing grain size with regard to the other two perth perthite and keystone microcline surface area varied randomly with grain size micropores have diameters 2 nm which is on the same scale as alkali feldspar unit cell dimensions as there are no channels in the alkali feldspar crystal structure it was concluded that the measured micropore surface area was due to cracks generated during the grinding involved in sample preparation values for the surface roughness of the grains were calculated assuming that micropore surface area equaled internal surface area values varied between 1 and 7 for the different grain size fractions of perth perthite keystone microcline and eifel sanidine for kb14 surface roughness varied between 7 and 24 it is possible that the dissolution of feldspars proceeds at different rates down cracks and at the outer surface of grains if this is the case then the present data suggest that care must be taken when comparing surface area normalised dissolution rates obtained from chemically similar samples which have been ground for different periods of time with different degrees of force

diatom inferred salinity and carbonate oxygen isotopes in holocene waterbodies of the western sahara and sahel africa abstract thirteen holocene palaeolakes in the western sahara and sahel have provided diatom records with carbonate oxygen isotope profiles available from eight of them most of these palaeolakes were groundwater fed lake water chemistry is reconstructed using diatom transfer functions lake water salinity and 18 o records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15 000 yr data are in general agreement with climate simulations using coupled atmosphere ocean vegetation models which show a mid holocene wetting over the whole of northwest africa and a rapid drying by 6 4 ka the lake record also shows that at many sites the major lake infilling lags the end of the younger dryas by 1 2 ka regional differences also appear in the timing of the lake hydrological optimum ca 10 5 8 5 and 7 5 4 5 ka in the northern sahara 10 8 5 ka in the air tenere 10 5 7 or 4 5 in the sahel and 7 5 ka in lake chad the whole of the holocene is punctuated by short term drying events changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity during the wet holocene period however the very low values in the southern sahara also imply changes in the moisture transport pattern or rainfall mechanisms data suggest an apparent decrease in 18 o content of precipitation along the monsoon flow in contrast with modern patterns changes in water availability and quality have driven population migrations in and out of the sahara sahel but relationships between climate and cultures are complex short term dry events might have driven inventive adaptations in the sahara drying at 5 4 5 ka coincides with both the collapse of the classical neolithic civilization and the settlement of new cultures

ostracodological studies in archaeological settings a review during recent years geo archaeology has been revitalised and much effort has been made to combine geo archaeological sciences like geomorphology sedimentology micropalaeontology and geochemistry with the cultural archaeological approach the first definition of geo archaeology dates back to the late seventies and since then has evolved to include more and more multidisciplinary studies in this perspective ostracods play a crucial role since they are valuable palaeoenvironmental indicators in all aquatic habitats from freshwater to marine despite their potential the integration of ostracods in archaeological studies has not been immediate only in the nineties did ostracod geo archaeology seem to finally take its place including a systematic approach to ostracod studies in archaeological excavations since then many geo archaeological investigations have included ostracod analyses exploiting their potential as multitask tools the research papers composing this special issue demonstrate that ostracods can be used in the most diverse settings as tracers of archaeological material provenance proxies for palaeoenvironmental and palaeoclimate reconstructions as well as clues to human impact

surface characterization of acid leached olivines by x ray photoelectron spectroscopy abstract x ray photoelectron spectroscopy was used to study the surface alteration of olivines fayalite and forsterite during acid dissolution 0 05 mol 1 1 h 2 so 4 at room temperature for one day the abundances of fe and mg relative to si in the near surface of olivines decreased after acid dissolution the divalent cations in the fayalite were removed more readily than those in the forsterite after acid dissolution the si 2s and o 1s spectra of fayalite were deconvoluted to contributions from unleached and leached phases the si 2s and o 1s binding energies were higher for the leached phase relative to the unleached phase and comparable to those of silicon dioxide quartz and silica gel indicating that sio 2 n h 2 o formed on the mineral surface scanning electron micrographs of the leached olivine grains showed a harshly etched structure for fayalite and a locally etched structure for forsterite indicating that the dissolution proceeded heterogeneously

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

composition of inland waters with marine like fauna and inferences for a miocene lake in spain abstract knowledge of the ionic composition of athalassic non marine waters where marine like organisms currently live allows to evaluate their ionic and total dissolved solid tds tolerance ranges together with other ecological data these chemical parameters can be used to interpret hydrological and hydrochemical conditions in ancient lake basins where analogous faunas have been recorded an example of such a utilization of the biological record is presented for miocene clastic dominated lacustrine deposits of the tertiary ebro basin of ne spain which contain marine like fossils comparison of the deduced chemical parameters for the miocene lake waters with the chemical composition of recent groundwaters and saline springs in the same area allows us to infer a similar mode of solute acquisition this implies that there were important cl so 4 saline inflows to the miocene lakes

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

paleoclimatic reconstruction in northern oman based on carbonates from hyperalkaline groundwaters abstract a paleoclimatic reconstruction for the past 35 000 years for northern oman is based on an unusual approach using travertines and fracture calcites associated with hyperalkaline springs high ph groundwaters ph up to 11 9 discharge from the mantle sequence of the oman ophiolite as the product of modern low temperature serpentinization under arid climatic conditions hyperalkaline discharge occurs at the surface uptake of atmospheric co 2 precipitates characteristic laminated travertines accompanied by strong kinetic depletion of 13 c and 18 o pluvial climates supporting a shallow bicarbonate groundwater flow system and vegetation are recorded by fracture calcites with equilibrium stable isotope contents and calcite replaced roots and stems all such carbonates have modern initial 14 c contents allowing radiocarbon dating and paleoclimatic reconstruction for the late pleistocene and holocene our reconstruction shows a dominantly wet late pleistocene up to 19 000 yr b p when a phase of climatic deterioration began leading to a period of hyperaridity which dominated from ca 16 300 to 13 000 yr b p the early holocene pluvial occurred from 12 500 to ca 6500 yr b p and was followed by renewed climatic deterioration and the current phase of hyperaridity comparison of this paleoclimatic reconstruction with that for lacustrine deposits from the a rub al khali of central saudi arabia and the summer insolation driven monsoon record of east africa and the arabian sea is remarkably good

environmental controls on perennial and ephemeral carbonate lakes the central palaeo andean basin of bolivia during late cretaceous to early tertiary times abstract in the central palaeo andean basin potosi basin of bolivia the up to 450 m thick el molino formation campanian maastrichtian to early tertiary includes two main carbonate lacustrine episodes each of different extent and duration these episodes are separated by an extensive development of alluvial facies floodplain deposits and limestones deposited in playa lakes or isolated ponds the el molino formation has been investigated in six different areas of the potosi basin and affords the opportunity to document climatic and tectonic controls on the development of perennial and ephemeral carbonate lacustrine systems the first lacustrine episode originated predominantly in perennial lakes with wave dominated low gradient ramp type margins outer marginal lacustrine facies include winnowed oolitic grainstones ostracod packstones and microbialite bioherms more agitated areas were characterized by nearshore oolitic bars with associated thrombolite mounds the fossil content and the lack of significant precipitation of evaporites indicate that the lake waters were of low salinity probably oligohaline during most of this period after the first lacustrine episode the widespread development of terrigenous alluvial facies floodplain deposits with interspersed playa lakes or isolated ponds in depressions on the floodplain may indicate a change in the net water budget the second lacustrine episode is dominated by dark micritic limestones and to a lesser extent microbialite bioherms which developed under semiarid climatic conditions this episode includes the deposits of ephemeral saline locally alkaline shallow lakes which were characterized by low energy low gradient ramp type margins the stable isotopic analysis of 163 carbonate samples covering the different facies and depositional settings displays a wide range of values 14 2 18 o and 12 9 13 c that is typical of nonmarine environments the distribution of the values indicates that during the deposition of the el molino formation the basin was hydrologically closed and experienced no strong hydrogeographical changes except for variations in the palaeolake level related to fluctuations in the regional p e ratio these are recorded by two main isotopic trends related to changes between perennial and ephemeral lacustrine conditions lacustrine sedimentation was controlled predominantly by climatically driven hydrological changes with repeated oscillations of the water level expansion and contraction and subsequent fluctuations in the width of lacustrine facies belts

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

the age and origin of groundwater in the great western erg sub basin of the north western sahara aquifer system insights from krechba central algeria abstract the north western sahara aquifer system nwsas forms an important transboundary groundwater resource whose properties remain to be fully understood across its whole extent for example groundwater flow in the cretaceous continental intercalaire ci unit of the nwsas is well characterised in the northern part of its range around the algeria tunisia libya borders and in the great eastern erg sub basin immediately to the south to the southwest however the ci of the great western erg sub basin has been much less studied the present paper reports hydrogeochemical data from a wellfield in central algeria which will contribute to a better understanding of this sector of the nwsas in terms of the age and origin of groundwater within it groundwater pumped from five deep boreholes in the ci aquifer overlying the krechba gas field has been studied using a variety of environmental tracers including hydrochemistry environmental isotopes and reactive and noble gases the latter being reported for the first time for this sub basin all the waters were dilute sec 460 600 s cm contained detectable o2 6 3 7 5 mg l showed evidence of evaporation relative enrichment in 18o gave late pleistocene 14c model ages 13 5 19 3 ka and yielded lower than present day noble gas recharge temperatures 14 3 17 6 c various lines of evidence suggest that these waters are the product of mixing between water recharged direct to the ci and leakage from the neogene quaternary erg aquifer the results support the long held concept of regional flow from a palaeo recharge area to the northwest finally while the krechba gas field carboniferous has been since 2004 the site of a pilot carbon capture and storage ccs project the data revealed no evidence for leakage of fluids gas or brines into the overlying ci aquifer at the time of sampling october 2014

stable carbon and oxygen isotopic composition of early holocene gastropods from wadi mansurab north central sudan the 8500 to 7000 year old shell bearing sediments in shallow claypans near wadi mansurab in north central sudan appear to reflect a widespread and much wetter period in northeast africa stable carbon and oxygen isotope analysis of the wadi mansurab gastropods reveals that the shell carbonate is highly depleted in 13c and 18o the highly depleted oxygen isotopic composition is indicative of rainfall derived from a distant oceanic source the highly negative nature of the oxygen isotopes indicates that the region had significantly less evaporation than today while the extreme variability in isotopic composition up to 6 7 pdb is consist ent with a seasonal rainfall regime characterized by a high degree of inter annual variability the dominance of semi aquatic and swamp dwelling gastropods over truly aquatic species implies that the region was probably a seasonally flooded grassplain similar to the toich lands of south central sudan today our data support the inference that towards 8500 7000 bp there

predicting radioactive accessory mineral dissolution during chemical weathering the radiation dose at the solubility threshold for epidote group detrital grains from the yangtze river delta china abstract the influence of heavy ion irradiation on the solubility of natural epidote group minerals has been investigated the epidote group are calc silicate minerals that if soluble during terrestrial chemical weathering are capable of consuming atmospheric co 2 over geologic time the experimental design was to calculate the particle dose for actinide rich epidote group grains from the deltas of rivers draining large watersheds large watersheds should yield a suite of epidote group grains spanning the range of compositions and radiation damage commonly observed in nature these grains would be resistant to dissolution during chemical weathering and survive into fluvial sediments therefore the highest particle dose calculated for the suite of grains represents a minimum solubility threshold for the epidote group minerals during chemical weathering the particle dose is calculated from the 232 th 238 u and 235 u content of a grain and the grain s date actinide rich epidote group grains were isolated from yangtze river and river nile delta sediments although only the grains from the yangtze delta had 232 th 208 pb ratios sufficiently high to yield meaningful dates a total of 28 th rich epidote group grains were isolated from the yangtze delta sediment with 24 being classified as allanite and four being classified as ree rich epidote isotopic ratio data were measured by laser ablation microprobe inductively coupled plasma mass spectrometry dates and ree patterns revealed at least 17 different sources of grains with tho 2 contents ranging from 0 057 to 4 37 wt and dates ranging from 32 2 to 3788 263 ma the calculated maximum particle dose of the suite of yangtze delta epidote group mineral grains and hence the minimum solubility threshold is 7 1 10 15 decay mg 1 therefore to predict the likelihood of an epidote group mineral dissolving during chemical weathering at a specific study site the particle dose may be calculated from the date of the rock even if estimated and the radioactinide concentrations of the mineral a calculated particle dose below 7 1 10 15 decay mg 1 likely reflects epidote group mineral stability during chemical weathering calculation of the particle dose may be superior to microscopic identification of accessory the particle dose at the solubility threshold of 7 1 10 15 decay mg 1 value is approximately double that reported for zircon therefore relative to zircon epidote group minerals can withstand more radiation prior to becoming soluble this finding is surprising considering the chemical durability of zircon this substantial variance in solubility threshold radiation doses between the two minerals cannot be explained by the bond strengths of non tetrahedral cations and structural oxygen which are lower in allanite than zircon

paleohydrologie isotopique de l oued el akarit sud tunisie au pleistocene superieur et a l holocene previous data from gobert and howe coque and page on the upper quaternary deposits from wadi el akarit are expanded through radiocarbon and stable isotope measurements 18c and 13c from carbonates 18o and 34s from sulphates these new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late pleistocene vegetal dams linked to spring activity caused water stagnation fine grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels in the upper part of the typical cross section located on the left bank about 100 m upstream of the railway bridge pleistocene deposits show a palustrine carbonate crust this formation is attributed to the drainage induced by the major marine regression at glacial maximum during the holocene stagnation periods still occurred interrupted by flood episodes marked by bottom transport and coarse deposits in the typical cross section deposits are well drained and oxidized however downstream a stagnation episode takes place with a rich malacofauna dominated by cerastoderma cardium the uneven distribution of rainfall is documented by the alternation of fine and coarse deposits and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr bp later exceptional floods removed sediments from the wadi channel cutting the upper quaternary deposits for about 13 m along the central course of the wadi radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon atmospheric co2 organic activity in the basins and aquifer supply a simple exchange mixing model between aquifer bicarbonate and atmospheric co2 is used for calculations of initial carbonate activities chemical isotopic and hydraulic steady states are assumed in this treatment present day 14c and 13c contents of aquifer bicarbonate are thus adopted for past values the model leads to acceptable corrected ages for pleistocene carbonate samples corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels however holocene samples appear overcorrected their crystallization appears to be controlled by modern organogenic co2 holocene 14c ages are probably correct 18o contents of carbonate indicate a continental origin for environmental water nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers they show 18o contents compatible with present day stable isotope compositions of artesian springs this supports the concept of a good time constancy in stable isotope contents shell carbonate values indicate a variable evaporation rate from the same initial waters 18o and 34s contents of gypsums are also compatible with aqueous sulphates from springs the data set falls within the range of permo triassic sulphate and suggests deep hydraulic connections within aquifer systems the entire formation appears to be of continental origin the occurence of cardium rich basins can only be attributed to the initial salinity of springs waters 7 9 g 1 1 sightly increased by evaporation this signifies that one can not deduce any tectonic uplift from their location with respect to sea level

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

the calcareous tufa in the tadrart acacus mt sw fezzan libya an early holocene palaeoclimate archive in the central sahara article i nfo the palaeoclimatic signi cance of calcareous tufa deposits found in the presently hyperarid tadrart acacus massif central sahara sw libya is discussed on the basis of u th datings stable isotope analyses and the regional hydrogeological context these deposits formed mostly between c 9600 and 8100 yr bp the isotopic signature 18 o and 13 c of this tufa indicates that the water forming these carbonates originated as isotopically depleted rainwater as a result of the northward expansion of the sw african monsoon system during the early holocene carbonate sedimentation in the tadrart acacus decreased shortly before c 8000 yr bp attributed to a drop in precipitation in the course of the well known 8 2 kyr event a correlation between tufa development and climate change in n and e africa and in the mediterranean basin during the early holocene is discussed the effect of early holocene climate change on both the landscape and the dwellers living in the tadrart acacus region is also evaluated

on the ages of humid holocene and late pleistocene phases in north africa remarks on late quaternary climatic reconstruction for the maghreb north africa by p rognon discussion sur l article de rognon p paru en 1987 dans palaeogeogr palaeoclimatol palaeoecol vol 58 p 11 34

contrasted effect of aluminum on the serpentinization rate of olivine and orthopyroxene under hydrothermal conditions abstract olivine and pyroxene are the major minerals of ultramafic rocks the hydrothermal alteration of these rocks leads to the serpentinization reaction that mainly forms serpentine and variable amounts of talc brucite and magnetite as well as hydrogen the serpentinization kinetics of pyroxene under hydrothermal conditions has been very little studied in comparison with olivine and both have been evaluated experimentally only in simple aqueous fluids here we evaluate the effect of aluminum on the serpentinization rate of olivine and orthopyroxene at 200 c 340 c and 200 mpa to simulate natural hydrothermal conditions we used low pressure diamond anvil cells lp dac and time resolved x ray diffraction to monitor in situ the progress of the serpentinization reaction in four experiments we also performed two long lasting additional experiments with orthopyroxene for six days at 340 c and 200 mpa for which in situ monitoring was not possible at 340 c in presence of al olivine conversion into lizardite is extremely fast half time reaction t1 2 7 h while orthopyroxene did not react much even after 6 days 11 in contrast to olivine orthopyroxene conversion to serpentine was faster without al 48 in 6 days magnetite was also observed in the run with olivine only at 340 c in experiments run with orthopyroxene only we observed the exclusive formation of proto serpentine instead of lizardite we propose that the contrasted effect of al on the serpentinization rate of olivine and orthopyroxene results from the complexation of al in the solution that reacts differently with the different mineral surfaces during their dissolution the positively charged olivine surface allows the adsorption of the dominant negatively charged al oh 4 complex while the neutral surface of orthopyroxene does not this adsorption process could facilitate both the dissolution of olivine and the nucleation growth of an al enriched lizardite

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

micromorphology and chemical weathering of a k rich trachyandesite and an associated sedimentary cover parkes se australia abstract the micromorphology composition and sequential formation of weathering products of a k rich trachyandesite and an associated sedimentary cover at parkes se australia are described this has been achieved using a combination of mineralogical optical x ray microdiffraction and sem edxa microanalysis and chemical x ray fluorescence techniques the weathering profiles have developed in a low altitude relatively flat landscape setting the following processes are dominant a weathering of primary minerals notably feldspars and micas to transient poorly crystalline oxyhydroxide patches smectite and ultimately kaolinite b weathering of sediments in a zone of intense solution activity and redox reactions resulting in development of mottles and ferruginous nodules and c greater thickness up to 350 cm and compositional differences between the soil over weathered sediments and in situ weathered bedrock the dominant profile microsystems evolved under a past humid climate later modified by the prevailing semi arid to arid conditions

holocene palaeoecology of the eastern sahara selima oasis buried lake sediments and tufa strandlines of holocene age provide evidence of a former lake in the selima oasis depression northwest sudan near the hyperarid core of the eastern sahara desert a 1 6 m section through laminated lake muds and precipitates has been analysed in detail and 30 radiocarbon dates geochemical sedimentological diatom and pollen analyses indicate that the unit dated from approximately 8 4 ka bp to 6 ka bp and was deposited in an environment that passed through one arid to humid to arid cycle the arid episodes from prior to 8 4 ka bp and subsequent to 7 ka bp are characterized by high caco3 deposition abundant diatom indicators of relatively high salinity and well developed periphytic communities and pollen spectra dominated by desert grassland indicators a humid period centered on 8 ka bp is indicated by low caco3 deposition high organic rich mud and al and si oxide deposition diatoms indicating a fresh deep 20 m lake and a pollen assemblage with maximum values for sahelo sudanian tree taxa suggesting a thorn tree and shrub savanna vegetational mosaic this reconstruction concordant with higher groundwater table and precipitation between 9 3 and 7 ka bp agrees with palaeoecological inferences from other sites in north africa

european lakes as palaeohydrological and palaeoclimatic indicators abstract lakes in temperate regions have fluctuated in size on annual to millenial time scales changes in water level or depth can be reconstructed by applying geomorphic sedimentological and biostratigraphic methods to transects of sediment cores this systematic field based approach has been used at a number of sites in europe it is illustrated by reconstructions of water level changes in lakes from southern sweden these lakes have shown broadly synchronous changes during the holocene such synchronous changes in water levels at several sites in a region reflect climatic changes inferences about the mechanism of the climatic change depend on identifying both synchronous changes within a region and differences in the direction magnitude and timing of lake level changes between regions continental compilations of lake level data are therefore necessary detailed studies are not available from enough sites but the interpretation of already existing stratigraphic and biostratigraphic data in terms of lake level changes has greatly increased the number of sites available for a continental scale synthesis in europe this eclectic literature based approach is illustrated with examples from southern europe lakes in the mediterranean region were high around the glacial maximum and again somtime during the first half of the holocene in the western mediterranean the peak of late glacial aridity occurred at 15 000 16 000 bp and the lakes were already high again by 12 000 bp in the east maximum late glacial aridity occurred between 13 000 and 11 000 bp and there was a gradual return to wetter climates during the early holocene such that 9000 bp was more arid than 6000 bp most lakes in the mediterranean region were high at 6000 bp the transition to arid conditions after 5000 bp occurred abruptly in the west and more gradually in the east the lakes of southern sweden indicate drier climates in the early holocene ca 9000 bp more effective moisture between 8000 and 6000 bp increased aridity between 5500 and 3500 bp and increased effective moisture again during the last 3000 years the history of lake level changes in southern sweden and the mediterranean region can be partially explained in terms of changes in the direct effects of insolation and in the strength and position of the westerlies and of the subtropical high pressure cell during the late quaternary

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

etch pit formation on iron silicate surfaces during siderophore promoted dissolution abstract understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria promoted dissolution on mineral surfaces although siderophores are known to promote mineral dissolution their effects on mineral surfaces are not well known we have utilized atomic force microscopy afm x ray photoelectron spectroscopy xps and mirau vertical scanning interferometry vsi to investigate surfaces after incubation with the siderophore desferrioxamine b mesylate dfam and under colonies of bacteria iron silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore producing bacteria bacillus sp for 46 days with parallel abiotic experiments conducted with and without 240 m dfam with and without 0 01 g l 1 of microbially produced extracellular polysaccharides eps alginate or xanthan gum some glass planchets were protected by dialysis tubing from direct contact with the eps weekly sampling and analysis of all filtered sample solutions showed negligible fe and al release in the control experiments and significant release of fe and al in the presence of dfam with negligible changes in ph concentration of fe in the filtered solutions after incubation with bacteria was below detection consistent with uptake of fe by cells release of fe al and si in control xanthan only and alginate only experiments was negligible release of these elements was enhanced in all experiments containing dfam and greatest in alginate dfam experiments afm and vsi analyses reveal widespread small etch pits and greater root mean squared roughness on siderophore exposed surfaces and fewer localized larger etch pits on bacteria exposed surfaces this is the first documented case of etch pit development during siderophore promoted dissolution roughness was not affected by the growth medium alginate or xanthan gum alone the roughness trends among samples correlate with trends in fe depletion documented by xps enhanced dissolution and roughness cannot be attributed to direct contact with eps because no significant chemical or physical differences were observed between surfaces directly exposed to eps and those protected by dialysis tubing acetate released from the eps may have enhanced the siderophore promoted dissolution siderophores produced by bacillus sp may be responsible for some of the biopits the difference in size and distribution of the biopits may be related to colonization