the relationship between current and paleo groundwater base levels abstract base levels are basically divided into two categories oceanic and marine base levels and continental terminal endorheic ones the marine domain serves as the ultimate base level for both surface and groundwater drainage systems continental inland depressions very often serve as terminal endorheic base levels to both surface drainage and groundwater systems many of the current both marine and inland base levels served also in the past as paleo base levels fluctuations of base level elevations affect the adjoined groundwater system in several ways such as changes in groundwater levels location of divides depths of fresh saline water interfaces and the amount of flow to the base levels this paper provides examples from many different parts of the world that describe the relationship between current and paleo base levels the examples include marine base levels as well as inland endorheic ones both currently above and below sea level close to and distant from the sea

fresh pumice from the central indian basin a krakatau 1883 signature the occurrence of pumice in the central indian basin cib has been a controversy some are of the opinion that these pumices are erupted from a volcano along the ninetyeast ridge while others suggest the indonesian arc volcanism as their probable source recently it is also suggested that the pumice are the products of in situ silicic volcanism in the cib this study was initiated in order to resolve this controversy about the origin of the fresh pumice from the cib the fresh pumice pieces of different sizes and shapes were collected from the surficial sediments of the cib from an average depth of about 5000 m 12 s 76 e glass is the main phase comprising about 95 volume and resembles the matrix glass in krakatau 1883 dacitic pumice which is also very poor 5 in the modal mineral composition the mineral and glass analyses were carried out with the help of the electron microprobe the orthopyroxenes clinopyroxenes plagioclases and spinels are the mineral phases present in these pumices the pyroxenes have similar wo en and fs components to those previously reported from the 1883 krakatau eruption products geochemically these fresh pumices have a high sio2 content 71 and high total alkali content 8 similar to 1883 eruption products of krakatau thus based on the similarities in mineralogy and glass chemical composition it is inferred that these fresh pumices are the products of the fierce 1883 eruption of the krakatau volcano in the sunda strait of the indonesian arc

understanding and modelling extra tropical teleconnections with the indo pacific region during the northern winter teleconnections originated by anomalous heat sources over the tropical indo pacific oceans are investigated in this paper comparing observational results with numerical simulations run as part of the re forecast set for the latest ecmwf seasonal forecasting system system 4 we show that the traditional methodology of linearly relating circulation anomalies to sst anomalies while appropriate for signals originated in central and east pacific fails to adequately identify the response to anomalous heating over the west pacific and most of the indian ocean because of the relatively weak or even negative correlation between sst and rainfall anomalies in these regions instead if teleconnections are computed from covariances with rainfall anomalies a stronger consistency is found between observed and modelled patterns as well as between diagnostics derived from seasonal and intra seasonal time scales the main mode of inter annual variability in indo pacific rainfall associated with planetary scale teleconnections is a tri polar structure with two positively correlated centres in the western indian ocean and the central pacific and a third centre around the maritime continents which is anti correlated with the other two in the extratropical response positive rainfall anomalies over the western and central indian ocean wcio are connected with a negative height anomaly centred over alaska and a positive north atlantic oscillation nao signal in a way reminiscent of the cold ocean warm land pattern this teleconnection cannot be explained by the enso forced component of indian ocean rainfall and is in phase with signals associated with the madden julian oscillation results from the system 4 re forecasts show that the ecmwf coupled model reproduces the broad features of tropical and extratropical teleconnections with a good degree of fidelity however the model significantly over estimates the correlation between rainfall anomalies in the wcio with those over the western and central pacific the impact of this deficiency on the extratropical flow is to weaken the relationship between the nao and indian ocean rainfall on the seasonal scale and to affect the projection of the enso response on the nao finally we argue that reproducing the correct relationship between sst and rainfall anomalies in different part of the indo pacific basin is also crucial for the correct simulation of inter decadal variability particular care should be taken in interpreting results of agcm simulations with prescribed sst where the absence of feedbacks between convection and sst over the warm pool region affects the simulation of rainfall anomalies

contribution of synoptic transients to the potential predictability of pna circulation anomalies el nino versus la nina abstractthe potential predictability pp of seasonal mean 200 hpa geopotential height z200 anomalies in the pacific north american pna region is examined for el nino and la nina separately by using 50 ensemble members of twentieth century agcm simulations observed sea surface temperature sst is prescribed for the period 1870 2009 and 14 el nino and la nina years after 1900 are selected for the present study the domain averaged value of pp for z200 in the pna region as measured by the signal to noise ratio for el nino is about 60 larger than that of la nina such a large pp is mainly due to a larger signal and partly to less noise during el nino compared to that during la nina the transient eddy feedback to the pna circulation anomalies is stronger during el nino events about 50 than that during la nina and this difference in the transients contributes significantly to the different z200 signals in the pna region the noise variance of the transients during el nino is about 17 smaller t

petrologic evolution of krakatau indonesia implications for a future activity abstract krakatau volcano is located along a n35e volcanic lineament running through the sunda straits indonesia its last activity has been characterized by successive phases each beginning with the construction of a cone and ending with its destruction and the formation of a caldera the two last pre and post 1883 cycles are well known but the more ancient ones are not so clearly defined lavas of krakatau belong to an andesitic series in which fractional crystallization plays the most important role the petrologic evolution is characterized by a cyclicity in good agreement with the structural evolution the succession is regular basalts basic andesites acid andesites dacites a gap between acid and basic andesites occurs in each cycle the destructive stages correspond to the occurrence of dacitic terms the anak cycle was characterized from 1927 to 1979 by basalts and basic andesites the 1981 eruption involved a more differentiated magma close to dacitic detailed study of the petrologic evolution since 1883 emphasizes the predominant role of fractional crystallization this process occurred during a very short period between 1979 and 1981 separation of labradorite augite olivine and magnetite from parental basic andesite may generate the dacitic descendant in a shallow reservoir p h 2 o estimated about 0 5 kbar implications for a future activity are considered

multiple flow regimes in the northern hemisphere winter part i methodology and hemispheric regimes abstract recurrent and persistent flow patterns are identified by examining multivariate probability density functions pdfs in the phase space of large scale atmospheric motions this idea is pursued systematically here in the hope of clarifying the extent to which intraseasonal variability can be described and understood in terms of multiple flow regimes bivariate pdfs of the northern hemisphere nh wintertime anomaly heights at 700 mb are examined in the present paper using a 37 year dataset the two dimensional phase plane is defined by the two leading empirical orthogonal functions eofs of the anomaly fields pdfs on this plane exhibit synoptically intriguing and statistically significant inhomogeneities on the periphery of the distribution it is shown that these inhomogeneities are due to the existence of persistent and recurrent anomaly patterns well known as dominant teleconnection patterns that is the pacific north american pna pattern its reverse and zonal and blocked phases of the

a mechanistic model of the northern annular mode 1 the northern annular mode nam is the leading mode of variability in the northern hemisphere the nam consists of a meridional dipole of the zonal mean zonal wind that extends into the stratosphere during the winter season during the high index state of the nam high latitude westerly winds are stronger than normal leading to a stronger and colder polar vortex over the past few decades observations show a trend toward higher nam index values to investigate whether this trend is a result of natural variability or is related to greenhouse warming a mechanistic model is developed that provides a simple analogue of the nam the model represents the interaction between the zonally symmetric flow and planetary waves for two meridional modes of variation greenhouse warming is specified in the model by varying the radiative equilibrium meridional temperature gradient variations in the second meridional mode qualitatively resemble those of the observed nam model runs that correspond to larger greenhouse gas concentrations yield on average higher nam index values

decadal interactions between the western tropical pacific and the north atlantic oscillation the relationship between interdecadal variations of tropical sea surface temperature sst in the last 120 years and circulation anomalies related to the north atlantic oscillation nao is investigated in this study using an atmospheric general circulation model agcm we confirm observational evidence that variations in the sst gradient in the western tropical pacific are related to the nao anomalies on decadal timescale and may be contributing to the shift towards the positive nao phase observed in the late 20th century the role played by the indian ocean nao teleconnection advocated in recent studies focused on the last 50 years is also assessed in the context of the 120 year long record it is suggested that a positive feedback between the pacific sst and the hemispheric circulation pattern embedding the decadal nao signal may act to enhance the internal variability of the coupled ocean atmosphere system and justify the stronger teleconnection found in observational data than in sst forced agcm experiments

mechanisms of zonal index variability in an aquaplanet gcm abstract data from a 2025 day geophysical fluid dynamics laboratory aquaplanet gcm integration are used to examine the temporal evolution of the zonal index defined as the principal component of the first eof of the zonally and vertically integrated absolute angular momentum this eof represents meridional displacements of the subtropical jet positive negative values of the zonal index correspond to poleward equatorward displacements of the subtropical jet and are referred to as the high low zonal index composites of various quantities are used to examine the temporal evolution of both the zonal mean zonal winds and the eddy fields associated with either index the high index is initiated by an enhancement in the equatorward wave activity propagation which causes the subtropical jet to move in a poleward direction the low index is led by a weakening in the equatorward propagation which results in an equatorward displacement of the subtropical jet more importantly for the high index the corr

a zonal wavenumber 3 pattern of northern hemisphere wintertime planetary wave variability at high latitudes abstracta prominent pattern of variability of the northern hemisphere wintertime tropospheric planetary waves referred to here as the wave3 pattern is identified from the ncep ncar reanalysis it is worthy of attention because its structure is similar to the linear trend pattern as well as the leading pattern of multidecadal variability of the planetary waves during the past half century the wave3 pattern is defined as the second empirical orthogonal function eof of detrended december february mean 300 hpa meridional wind v300 and denotes a zonal shift of the ridges and troughs of the climatological flow although its interannual variance is roughly comparable to that of eof1 of v300 which represents the pacific north america pna pattern its multidecadal variance is nearly twice as large as that of the pna wave3 is not completely structurally or temporally distinct from the northern annular mode nam but for some attributes the linkage of the observed trend to wave3 is clearer than to nam th

application of avhrr to monitoring a climatically sensitive playa case study chott el djerid southern tunisia the importance of monitoring changes in the levels of lakes within endorheic basins using remotely sensed data as a means of assessing changes in regional aridity is noted large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patterns and which commonly do not have extensively managed catchments to explore the application of high temporal frequency monitoring of salt playas using remote sensing the chott el djerid a large salt playa situated in southern tunisia was targeted a short time series of 39 advanced very high resolution radiometer avhrr resolution 1 1km at nadir images of the chott el djerid spanning 36 months between 1987 and 1990 were compiled along with climate information from a weather station at tozeur using image histogram manipulation lake areas were extracted from the time series a good level of agreement was observed between recorded rainfall events and the presence of surface water on the playa and for a limited sample of large flood events it was found that there were significant relationships between rainfall evaporation and estimated lake areas r 2 98 5 p50 001 overall these data suggest that contemporary lake formation is largely controlled by temporal changes in effective precipitation within the basin in addition it was found that the coefficient of variation of the time series and a combination of temporal reflectance profiles extracted from it could be used to give a direct indication of which sedimentary surfaces on the playa are affected by large flood events and the extent to which these events may be preserved within the recent sedimentary record at these sites copyright 1999 john wiley sons ltd

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

a possible submarine volcano near the central part of ninety east ridge indian ocean abstract floating pumice masses have been observed several times near the central part of ninety east ridge indian ocean in 1879 and in 1883 the author is of the opinion that the source of these pumice masses cannot have been the volcano krakatau in the sunda strait as has been suggested earlier by other authors but that they originated in situ examples of floating pumice observed over submarine volcanoes lying at depths of 1500 m or even more are mentioned briefly by other authors suggesting that eruptions in deep water may indeed lead to its formation the suspected submarine volcano may coincide with an elongated but rather flat elevation a little north of the central part of ninety east ridge the center of this elevation is located approximately at 6 05 s 89 10 e a roughly nw se profile of this area shows the presence of this shield like feature very near the site where floating pumice was seen on three occasions the inferred submarine volcanic center is lying within the belt of heat flow anomaly called belt i a particular hot spot with an unusually high heat flow value 7 61 hfu is located southeast of the feature in question the heat flow values measured in belt i are well above the world average this belt may be the result of the presence of linearly arranged magma chambers at a relatively shallow depth probably within the oceanic crust this belt has a length of 1100 km the seismic activity of the area investigated is similarly remarkable numerous powerful shocks have taken place there belt i represents the axis of the seismic zone extending from points b to c fig 5 the seismic belt is regarded as a zone of weakness within which fractures can originate permitting rock melts to emerge from depth towards the surface

simulated impacts of the south atlantic ocean dipole on summer precipitation at the guinea coast an intermediate complexity atmospheric general circulation model has been used to investigate the influence of the south atlantic ocean sao dipole saod on summer precipitation over the guinea coast of west africa two ensemble integrations in which idealized but realistic saod type sea surface temperature sst anomaly is prescribed only in the sao and then globally are performed and inter compared consistently above below the average precipitation is simulated over the guinea coast during the positive negative phase of the saod comparison of the two set of experiments reveal that in its active years the saod is a dominant mechanism that shapes the spatial character of summer precipitation at the guinea coast the global sst variability merely slightly moderate its effects during the saod cool sst anomaly in the extra tropical sao off the brazil uruguay argentina coast gives rise to suppressed convection and mass divergence in turn the subsidence tends to amplify the sub tropical arm of anomalous hadley type circulation and consequently large scale convection and mass flux convergence in the equatorial atlantic ocean gulf of guinea region bordering on the coastal fringes of west africa precipitation is therefore increased at the guinea coast

sources of sulphur in gypsiferous sediments and crusts and pathways of gypsum redistribution in southern tunisia southern tunisia contains one of the most extensive gypsum accumulations in africa comprising triassic cretaceous eocene and mio pliocene marine evaporites spring deposits playa sediments aeolian sands and gypsum crusts sulphur isotope analysis delta 34 s of bedrock samples groundwater playa brines playa sediments and gypsiferous crusts provides insight into the sources of gypsum in the region and sheds light on the processes that lead to gypsum crust formation results suggest that recycling of marine gypsum is the most likely source of the sulphate in the groundwater playa sediments and crusts the low ps values found in eocene and mio pliocene samples suggest that this recycling has been going on for millions of years though bedrock appears to be the ultimate source of the gypsum in the crusts transport of this sulphate to playas concentration therein and subsequent dispersal across the landscape by aeolian processes provides the most likely pathway for surticial gypsum crust formation comparison of these results with those from australia chile and namibia suggests that although the source of the sulphur varies from region to region the processes of surficial crust formation appear to be similar copyright c 2004 john wiley sons ltd

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

wave maintained annular modes of climate variability abstract the leading modes of month to month variability in the northern and southern hemispheres are examined by comparing a 100 yr run of the geophysical fluid dynamics laboratory gcm with the ncep ncar reanalyses of observations the model simulation is a control experiment in which the ssts are fixed to the climatological annual cycle without any interannual variability the leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward this fluctuation is strongest during the winter months the structure and amplitude of the simulated modes are very similar to those derived from observations indicating that these modes arise from the internal dynamics of the atmosphere dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere while

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

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

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

planetary scale variability in the northern winter and the impact of land sea thermal contrast this paper investigates the impact of land sea thermal contrast on large scale modes of variability during the northern hemisphere winter with specific attention to those modes which explain a large proportion of the observed inter decadal changes in the late 20th century two possible mechanisms are considered where the land sea contrast plays either a passive or an active role namely the introduction of zonal asymmetries in an eddy driven annular mode and variations in the thermal balance of a planetary wave pattern with zonal wavenumber 2 in addition to diagnostics based on re analysis data two long perpetual winter simulations of an intermediate complexity agcm are performed imposing surface boundary conditions with either realistic or artificially reduced land sea thermal contrast on the basis of previous studies indices derived from surface fields are defined to represent the two mechanisms outlined above atmospheric patterns co varying with these indices are first computed from ncep ncar re analysis and compared with traditional definitions of dominant modes of variability such as eofs and the cold ocean warm land cowl pattern subsequently the indices are applied to fields from the simulations with realistic and reduced land sea contrast in order to highlight the impact of thermal asymmetries on the structure of annular and planetary wave modes our results show that the zonal thermal contrast on the western border of the north atlantic is responsible for the localization of a dipolar structure in sea level pressure and height fields which bears a strong resemblance to the north atlantic oscillation nao in this region the response to thermal land sea contrast is evident on both annular and planetary wave patterns with a nao like dipole being the dominant regional feature in the north pacific on the other hand diabatic forcing is balanced primarily by meridional temperature advection because here contrarily to the north west atlantic region the surface temperature gradient is stronger in the meridional than in the zonal direction therefore the modification of the annular mode by zonal temperature advection at the asia north pacific border produces a different pattern from that associated with the thermal balance of a cowl like planetary wave we conclude that thermally balanced wave mode is a dynamically appropriate description for the pattern previously identified as the second eof of low frequency 500 hpa height variability or empirically defined as the cold ocean warm land pattern since this mode accounts for a large proportion of the upper air inter decadal variations in the second half of the 20th century we suggest that such variations are dynamically consistent over the hemispheric domain therefore they should be understood in terms of planetary scale dynamics rather than by the casual superposition of regional effects

ash layers and pumice in the central indian basin relationship to the formation of manganese nodules abstract manganese nodules in areas of biosiliceous sedimentation in the central indian basin cib have three main types of nuclei 1 fragments of altered ash layer 2 felsic pumice and 3 indurated residual sediments red clay type morphological variations among nodules are due to the form of the nuclei the type of accretionary process diagenetic or hydrogenetic accretion and thickness of the crust piston cores in the same areas sampled up to four tephra layers interstratified in red clay all of which underlie condensed series of pliocene to recent siliceous oozes the in situ tephra layers and fragments of tephra in the core of the nodules are similar in composition they are completely altered to smectite and smectite zeolite tephra horizons are 10 35 mm thick and have sharp contacts with the red clay rare earth element ree patterns zr hf and nb content and ti al ratios strongly suggest that this altered volcaniclastic material originated as distal fallout from the indonesian volcanic arc iva and was deposited in late neogene time positive ce ce anomalies indicate that alteration of this tephra may have occurred at the sediment water interface prior to burial under hydrogenous conditions rhyolite pumicelapilli are water laid and are variably replaced by fe mn oxides inducing a higher metal content in the bulk nodules pumice constitutes one of the main types of nuclei of diagenetic nodules that are found in the deep sea troughs and plains where biosiliceous deposits are thick petrological characteristics of the pumice suggest that volcanic eruptions from toba in northern sumatra are the most likely source tectonic deformation of the indian plate may have affected the sedimentary sequence causing uplift and outcrop of ash layers on the topographic highs and slopes eroded fragments of these ash layers and waterlaid pumice fragments have contributed significantly to the large extent of mn nodules in the cib

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

low frequency variability of the indian monsoon enso relationship and the tropical atlantic the weakening of the 1980s and 1990s abstract the indian monsoon el nino southern oscillation enso relationship according to which a drier than normal monsoon season precedes peak el nino conditions weakened significantly during the last two decades of the twentieth century in this work an ensemble of integrations of an atmospheric general circulation model agcm coupled to an ocean model in the indian basin and forced with observed sea surface temperatures ssts elsewhere is used to investigate the causes of such a weakening the observed interdecadal variability of the enso monsoon relationship during the period 1950 99 is realistically simulated by the model and a dominant portion of the variability is associated with changes in the tropical atlantic ssts in boreal summer in correspondence to enso the tropical atlantic ssts display negative anomalies south of the equator in the last quarter of the twentieth century and weakly positive anomalies in the previous period those anomalies in turn produce heating anomalies which excit

groundwater evaporation from salt pans examples from the eastern arabian peninsula summary the major groundwater resources of the arabian peninsula are stored in the large sedimentary basins in its eastern part evaporation from continental salt pans playas is an important process in water resources assessments of its upper principal aquifers the upper mega aquifer system as it constitutes a significant sink however literature values on evaporation rates vary widely and usually report about coastal salt pans where seawater evaporation is assumed the present study applies different methods to provide a comprehensive picture of groundwater evaporation from salt pans of the upper mega aquifer system a remote sensing approach provided the spatial distribution and total salt pan area of about 36 500 km 2 hydrochemical and isotopic investigations revealed that from about 10 3600 km 2 1600 km 2 of the mapped salt pan area seawater evaporates to estimate the groundwater evaporation rate from continental salt pans a laboratory column experiment was set up implying a mean annual evaporation rate of about 42 mm 13 mm in situ analysis of water table fluctuations in the field suggested about 3 mm a 1 originate from recently infiltrated rainwater leading to an annual net groundwater evaporation of 39 mm 13 mm relating this number to the mapped salt pan area from which groundwater evaporates provides a total annual groundwater loss of 1 3 km 3 0 5 km 3 for the upper mega aquifer system

a pleistocene lacustrine episode in southeastern libya fossiliferous lacustrine deposits of middle pleistocene age have been found in central fezzan libya 1 on each side of the east west depressions of the wadi shati south of the precambrian qarqaf anticline1 8 they represent the shoreline of a former brackish water lake fig 1 which some 130 000 yr bp covered an area about 100km long and 10 20 km wide at present the area may be classified as hyperarid since 1978 a collaborative study of the fossil assemblages has been undertaken it included macro and micropalaeontological determination th u dating stable isotope and chemical analyses in parallel with sedimentological and archaeological studies of the sites here we consider the th u dating and the isotopic palaeohydrology of the lacustrine episode

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

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

magnetic lineations fracture zones and seamounts in the central indian basin abstract magnetic and bathymetric data collected in the central indian basin between 8 s and 16 s latitudes and 71 e and 82 e longitudes have been studied the inferred fracture zones at 73 e 76 30 e and 79 e demarcate the central indian basin into zones depicting right lateral offsets of the magnetic lineations a21 to a26 multibeam sonar surveys carried out over a part of the study area depict bathymetric evidence for the 79 e fracture zone several seamounts are mapped in this region based on echosounding and multibeam sonar data it is observed that these seamounts cluster along the fracture zones the tectonic implications of the seamount distribution in relation to the fracture zones are discussed