late quaternary geological history of the dead sea area israel abstract a 34 5 m borehole which was drilled near the dead sea coast altitude 394 m in the southern part of the fan delta of wadi zeelim reveals the geological history of that area from the latest pleistocene to present the depositional time frame is based on six 14c dates and two u th dates an erosional or nondepositional period is implied by the hiatus between 21 100 yr b p u th age depth 33 m and 11 315 yr b p 14c age depth 32 m a subsequent arid phase is recorded by a 6 5 m thick layer of halite based on 14c dates this phase relates to the abrupt younger dryas cold period reported in temperate to polar regions the fragility of the environment in this region is indicated by the fact that the region experienced such a severe short aridification phase less than 1000 yr evidence of which is found widely in the desert fringes of the middle east and north africa the aragonite found in most of the holocene section indicates that the well site was covered by the lake for most of the holocene exceptions are the intervals at 0 3 and 10 14 m depths which represent low stands of the lake

active circulation of saline ground waters in carbonate platforms evidence from the great bahama bank measurements of salinity temperature and ground water discharges within the great bahama bank provide evidence of active circulation of near normal ocean water beneath north andros island elevated salinity waters 38 42 derived by density refulx from the great bahama bank flow eastward beneath the island and mix with normal salinity cold ground waters 19 20 c from deeper than 250 m in the adjacent oceans east to west flow of water from the straits of florida may be driven by head differences generated by the florida current which impinges on the western margins of the platform alternatively density differences between the reflux and tongue of the ocean sea waters may cause more local circulation on the eastern flanks these flows have important diagenetic implications particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the bahamas

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

sedimentology and stable isotopes from a lacustrine to palustrine limestone deposited in an arid setting climatic and tectonic factors miocene pliocene opache formation atacama desert chile abstract field relations petrography gis and geochemistry of a late miocene pliocene limestone the opache formation calama basin atacama desert chile shows that depositional facies diagenetic histories and isotopes differ from eastern to western parts of the basin we recognize two facies lacustrine characterized by ostracod diatom mudstone and wackestone and palustrine characterized by both shallow water and pedogenic features gis analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west with the calama constriction restricting flow between east and west petrography and geochemical results confirm that the calama constriction influenced depositional parameters ponding groundwater to the east forming a shallow lake and creating palustrine conditions to the west our analyses indicate that lake waters were less enriched in 13c or 18o than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history derived from groundwater rich in dissolved minerals this seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the pacific ocean and was less evolved whereas eastern limestones precipitated from water which had become depleted in 18o during transport from the east today moisture from the pacific very rarely if ever reaches inland as far as calama indicating that during the late miocene mid pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal pacific current allowed moisture from the pacific to reach inland this changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the andes allowing it to reach farther westward into the atacama region during the late miocene pliocene together these moisture sources were sufficient to produce carbonate rich palustrine and lacustrine environments in the calama basin

thermal anomalies associated with forced and free ground water convection in the dead sea rift valley the dead sea rift valley is a left lateral transform along which several rhombshaped grabens were formed at the sea of galilee which is one of these rhomb shaped grabens ambiguous heat fluxes were measured 70 80 mw m 2 at the central part of the lake 36 mw m 2 at the lake s southern coast 10 km apart and most surprising about 135 mw m 2 at the southern golan heights 6 8 km east of the graben margin a detailed geologic cross section traversing the entire sedimentary basin was constructed the hydrodynamics in this cross section were analyzed quantitatively using a two dimensional finite element code that solves the coupled variable density ground water flow and conductive convective heat transfer equations on the basis of numerical simulations different mechanisms of basin scale groundwater convection are suggested for the two sides of the rift that could influence the transport of heat 1 forced convection gravitydriven flow of hot brines from deeper aquifers to the land surface at the western side and 2 large scale free convection buoyancy driven flow of deep ground water at the eastern side the different heat fluxes within the rift valley are attributed to the different lithologies and to the locations of specific conduits through which the hot ground waters ascend from deeper horizons these simulations also explain the different salinities of the hot springs on the two sides of the rift

spatial delineation of groundwater salinity using deep time domain electromagnetic geophysical measurements a feasibility study 1 the objective of this study was to delineate the spatial distribution of fresh brackish and saline groundwater bodies as well as brine beneath the judea desert israel we used the deep time domain electromagnetic tdem geophysical system cycle 5m which provides the maximum exploration capability to depths 1 5 2 km below land surface the study was conducted at 21 locations at the desert plateau where fresh groundwater flows through the upper aquifers from the replenishment area at the judea mountains toward the dead sea springs in deeper aquifers brackish and saline groundwater bodies exist which originated from lakes and lagoons that existed at the dead sea rift valley during the pleistocene and pliocene respectively it was found that within carbonate formations fresh groundwater c 103 ppm total dissolved solids tds is characterized by a resistivity of more than 15 m brackish groundwater 103 c 104 ppm tds is characterized by 5 15 m saline groundwater 104 c 105 ppm tds is characterized by 0 5 5 m and brine 105 c ppm tds is characterized by less than 0 5 m

paleohydrologic record from lake brine on the southern high plains texas the timing of changes in the stage and salinity of double lakes of lynn county texas was estimated using dissolved chloride profiles across an underlying shale layer lake conditions over the past 30 to 50 ka can be inferred from the chloride profiles by using the advective velocity of the pore water through the shale and an appropriate coefficient of molecular diffusion the profiles suggest that net evaporative conditions existed over the southern high plains for the past 50 ka a period of increasing salinity in the lake began at 20 ka and reached current levels at 5 ka in addition deflationary conditions were present for at least 4 ka and likely began or were accelerated during the most recent altithermal period at 5 ka this type of lake brine record may also exist in many other saline lake environments throughout the great plains of north america

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

saline versus freshwater contribution to the thermal waters of the northern jordan rift valley israel abstract temperature and salinity differences between thermal waters at equal depths on both sides of lake tiberias reveals that the hotter eastern system consists of heated fresh groundwater whereas the western system consists of heated saline groundwater this difference seems to effect exceptional fluorine enrichment in the fresh thermal carbonate aquifer through temperature controlled equilibrium solubility of fluorite and calcite mixing models based on oxygen isotopic composition and other chemical parameters suggest two distant recharge areas for the mezar hammat gader thermal system as well as a rift valley salinity source hydrogeological considerations and chemical constraints i e oxygen and strontium isotopic ratios suggest that heat in the western system is attained through circulation under a normal geothermal gradient while in the eastern system a higher than usual geothermal gradient controls the heat balance

internal structure of the dead sea leaky transform rift in relation to plate kinematics abstract the structures along the dead sea transform rift are related to the motions of the sinai and arabia plates which border it and to the irregularities of their boundaries the total slip was 105 km left lateral but the present structures were formed mainly during the last 40 km of slip which probably occurred in the plio pleistocene along the southern half of the transform the strike slip motion takes place on en echelon faults this produces rhomb shaped grabens or pull aparts which are sometimes composite and in which there is local crustal separation thus much of the transform is leaky these structures occur in a morpho tectonic rift valley delimited by normal faults which express a small component of transverse extension along a few segments the shape of the transform is such that lateral motion produces local transverse compression the geometric relations of the structures along the transform define an eulerian pole of relative plate motions at 32 8 n 22 6 e 0 5 the older motion was somewhat different and is described by a pole located about 5 west of the above then the component of transverse extension and crustal separation was much smaller than now while local transverse compression was more important the northern half of the dead sea transform has an irregular shape and the bordering plates did not remain rigid as lateral motion continued here transverse compression is often important

basin scale migration of continental rift brines paleohydrologic modeling of the dead sea basin it was suggested that brine of the dead sea rift has originated from a residual product of intensively evaporated seawater that invaded the rift precipitated halite and later interacted through dolomitization with the host rock during subsurface migration detection of this brine in many deep wells located at distances as far as 100 km away from the rift was attributed to long distance migration of the brine the physical feasibility of such migration which probably spanned the past 3 6 m y is quantitatively tested and verified in this study by using paleohydrologic modeling the structural formation of the rift is described by a chronological sequence of geologic cross sections serving as the basis for hydrodynamic calculations which assess the effects of the structure on fluid migration salinity redistribution and heat transport across the sedimentary basin results indicate that two basin scale ground water systems one atop the other but with opposite flow directions coexisted in the dead sea rift valley the first is a topography driven flow of meteoric water from the surrounding highlands toward the rift through relatively shallow aquifers 1 km the second is a density driven migration of the dead sea brine through deep aquifers 4 5 km in the opposite direction the configuration of these flow systems has changed during the structural evolution of the dead sea rift illustrating the interrelationships among basin formation paleohydrology and paleogeochemistry

freshwater limestone in an arid rift basin a goldilocks effect abstract the olduvai basin 3 s situated just west of the east african rift system in northern tanzania contains a two million year record of paleoclimate and paleoenvironmental change as well as a rich archive of vertebrate paleontology including hominins milankovitch precession cycles 20 kyr regulate the mean annual precipitation 250 700 mm yr and the 2500 mm yr evapotranspiration results in a negative hydrologic balance despite persistent aridity extensive deposits of freshwater limestone punctuate the stratigraphic record between 2 and 1 ma lake olduvai occupied the basin and its sediments are a proxy for climate driven cycles three limestones 1 84 1 80 and 1 36 ma in age which formed within lake margin floodplains adjacent to the lake were studied to determine their depositional environment using field relations sedimentary structures fossils petrography and stable isotope and major element geochemistry the three limestones are similar in that they contain peloidal micrite siliciclastic detrital grains and rhizoconcretions abundant faunal remains gastropods ostracods charaphytes and fish indicate that ponded water was relatively fresh and alkaline geochemical and stable isotope data indicate two types of groundwater sourced carbonate producing waters a deeper sourced fluid that was enriched in iron and manganese due to extended water rock interactions and a shallower groundwater that traveled through alluvial fan deposits regional faults tapped the deeper groundwater producing carbonate at spring sites while seeps associated with basinward changes in alluvial fan slopes drew on shallower groundwater sources isotope compositions indicate that fault related waters experienced some evaporation as water moved away from the spring sites while compositions of the seep related carbonate remained relatively constant pedogenic alteration and meteoric calcite cementation affected the carbonate when the spring and seep sites dried out secondary strontium rich dolomite precipitated within the limestones during burial under paleo lake olduvai sediments and fluids during periods of lake expansion integrating these data within the geological context of regional paleoclimatic and local environmental change indicates that the freshwater carbonates formed periodically when the conditions were just right i e a goldilocks effect carbonates near the basin center formed from groundwater flowing under a hydraulic head from faults or fractures during the falling limbs of milankovitch cycles when the lake was in recession carbonates near the basin margin formed from an increased rate of groundwater seepage that occurred only on the rising limbs of cycles in both contexts the continuous flow of groundwater with surface evaporation and co2 degassing optimizes the conditions for limestone formation in this arid environment these results help explain the formation of freshwater limestones in the rift basin link the carbonates to specific portions of milankovitch cycles and document that localized sources of potable water were present for use by hominins in olduvai gorge during an important period of human evolution

88 sr 86 sr fractionation and calcite accumulation rate in the sea of galilee abstract this study used the sea of galilee lake kinneret northern israel as a natural laboratory to investigate the fractionation of the stable sr isotope ratio 88 sr 86 sr during precipitation of inorganic primary calcite from the lake s water it was found that the absolute value of the 88 sr 86 sr fractionation factor 88 86 sr increases as a function of calcite accumulation rate 88 86 sr 0 05 to 0 042 log r mol m 2 d 1 where r is the accumulation rate furthermore the 87 sr 86 sr and 88 sr 86 sr ratios in the freshwater and brines that enter the lake were used to calculate the contributions of these sources to the lake sr budget the 87 sr 86 sr and 88 sr 86 sr ratios were measured in primary calcite aragonite shells of live melanopsis lake water and various water sources to the lake while the lake s 87 sr 86 sr ratios are determined by the mixture of freshwater of the jordan river and saline springs the 88 sr 86 sr ratios of the lake reflect a more complex mass balance that includes the effect of isotopic fractionation associated with the precipitation of primary calcite data analysis suggests that long term accumulation of inorganic calcite depleted in the heavy isotope 88 sr results in an increase of the 88 86 sr value of the lake water by 0 05 in contrast to the primary inorganic calcite biogenic aragonite of the melanopsis shells show a rather constant 88 sr 86 sr water caco 3 fractionation of 88 86 sr 0 21 similar 88 86 sr values were reported for the precipitation of coralline and inorganic aragonite from seawater and the precipitation of inorganic calcite from various continental waters the 88 86 sr value of inorganic calcite is modulated by the rate of carbonate precipitation as noted above and shown by precipitation experiments massive precipitation of primary calcite with a wide spread of accumulation rates occurs during the spring phytoplankton bloom in lake kinneret the bloom dictates the degree of calcite saturation in the lake and hence the stable sr isotopic composition of the precipitating calcite the correlation between 88 sr 86 sr and the calcite accumulation rate can be therefore used as a tool to reconstruct environmental and ecological variations in the historical lake by analyzing the 88 sr 86 sr ratio in the primary caco 3 recovered from the lake s sedimentary archives

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

hypersaline brine diagenesis and evolution in the dead sea lake lisan system israel abstract water soluble salts were extracted from 57 aragonite and detrital laminae sampled from the pleistocene lake lisan sediments the solutions and solids were analysed for ca mg na k sr cl so 4 and bicarbonate average na cl mg ca sr ca mg cl ca cl sr cl k cl k na and k mg ratios calculated for seven stratigraphie elevations in the lisan column indicate that the dead sea rift sediment brine system has remained practically closed for 70000 years the soluble salts reflect both the hydrologie evolution of the lake as well as diagenetic processes mg ca ratios systematically decrease in younger sediments and this decline is accompanied by a 1 5 4 fold sr enrichment in aragonite solubles relative to salts extracted from detrital laminae the major process responsible for these observations is dolomitization of sr rich aragonite by interstitial brine diagenesis was triggered by slow desiccation of the lake s sediments after the lisan waters started to recede due to a change in climate very sharp chemical gradients which developed in the sediments after deposition have been preserved because further diagenesis was quenched by the advancing dry out h 2 o loss by evaporation induced difrusional migration of na and cl from aragonite laminae into adjacent detrital layers this lowered the na cl ratio in the aragonite and raised it in the detritus relative to the initial dead sea na cl ratio of 0 26 the early lisan brine was similar in its chemical composition and total salinity to that of the present dead sea

free thermohaline convection in sediments surrounding a salt column complex groundwater convection patterns are possible near salt domes because groundwater is subject to both lateral heat and salinity gradients in order to assess the mechanisms responsible for driving convection near salt domes we use dimensional analysis and numerical simulations to investigate coupled heat and salt transport in homogeneous sediments surrounding a cylindrical salt column the dimensional analysis does not require the boussinesq assumption the coupled heat solute and groundwater transport equations are controlled by three dimensionless parameters the rayleigh number the lewis number and the buoyancy ratio the buoyancy ratio is the ratio of salinity to temperature effects on groundwater density and it directly affects the groundwater flow equation a finite difference numerical multigridding algorithm is used to iteratively solve the coupled transport equations the multigridding technique was about 3 times faster than a point wise successive overrelaxation solution boundary conditions for the numerical simulations were adjusted to represent different contrasts in the thermal gradient between the salt and the overlying sediments the contrast in thermal gradient is parameterized by the thermal conductivity ratio and is responsible for isotherms being elevated near the salt the analysis suggests that a wide range of convective flow patterns are possible with flow occurring either up or down along the salt flank the sense of convection is dependent mainly on the value of the buoyancy ratio and how sharply isotherms are pulled up near the salt these factors in turn depend on the regional salinity variation the time since diapirism and the thermal conductivity of water saturated sediments while this analysis provides useful insight into the mechanisms driving free convection near salt domes the assumptions about medium and fluid properties may limit the applicability of dimensional analysis in simulating flow in specific geologic settings

a numerical study on the influence of fractured regions on lake groundwater interaction the lake kinneret sea of galilee case increased lake salinity is a growing problem in arid and semi arid regions operational management which is based on a reliable hydrological understanding has the potential to reduce the lake salinity this is the case of the salinity in lake kinneret sea of galilee where saline water flows into the lake through on shore and off shore springs here we present a time dependent flow and transport numerical model that successfully reproduces the monitored groundwater level discharge and salinity of the lake springs the model utilizes a continuum approach and describes the flow through a confined saline carbonate aquifer which interacts with the discharge lake through fractures and faults in particular the model investigates the hydrology around two groups of springs fuliya and tabgha along the western shore of the lake based on seasonal characteristics of the springs and measured boundary conditions the two springs groups were defined as lake dominated springs lds fuliya and aquifer dominated springs ads tabgha the models of the two groups differ from each other in the distribution of fractured regions that link between the lake and the underlying aquifers the numerical solution shows how the fractured regions affect groundwater head spring discharge and spring salinity the different behavior of the lds and ads was reproduced with an excellent agreement between measured and calculated patterns differences in salinity within a group of springs are shown to be dependent on the depth of the fractured region it was revealed that in addition to the appropriate distribution of fractured regions some adjustment in the up stream boundary conditions is necessary to fully reproduce the system hydrology the model has improved our understanding and provides a prediction tool for future management strategies

the reconstruction of quaternary freshwater lakes from the chemical and isotopic composition of gastropod shells the dead sea rift israel abstract sr ca ba ca and 87sr 86sr ratios in fossil freshwater aragonitic gastropods melanopsis praemorsum and melanoides tuberculata have been used to reconstruct the water sources of quaternary lakes in the jordan river rift valley the reconstruction is based on the assumption that the same waurces which supply freshwater into the rift valley in the present i e carbonate water basaltic water and rift brines were active throughout the quaternary the distribution coefficients for sr and ba were obtained from live gastropods which were collected from modern water bodies in the region by using these distribution coefficients the composition of the ancient waters could be derived from the fossil shells chemistry altogether four ancient sites were sampled of which mishmar ha yarden and gesher benot ya akov formations are located in the upper jordan valley and erq el ahmar and ubeidiya formations are in its central part low salinity carbonate waters 300 mg l comprised the main h2o source into the hula basin in the upper jordan valley these solutions display a 87sr 86sr ratio of 0 7082 0 0002 which is significantly higher than the corresponding modern ratio 0 7070 suggesting that the ancient drainage system was initially fed by upper eocene carbonate aquifers and shifted later to a jurassic carbonate system the erq el ahmar lake in the central jordan valley was supplied by approximately equal proportions of carbonate and basaltic waters resulting in a water body of low salinity 300 mg l the water source of lake ubeidiya also in the central jordan valley was predominantly basaltic yet its chemistry bears a strong rift brine signature resulting in a salinity of about 1000 1300 mg l the values obtained are in good accord with the biological evaluation of the fossil biota

boron isotope geochemistry of thermal springs from the northern rift valley israel a boron isotope study was carried out on groundwaters from the northern rift valley israel the relatively high salinity 18 000 mgcl l 1 boron content 3 9 mg l 1 and 11 b value 44 and the low b cl ratios of the saline tiberias hot spring suggest dilution of deepseated trapped cacl2 brines with a 11b 44 which is consistent with previous interpretations based on na cl 0 58 and 18o 3 values the 11b values of the springs in hammat gader decrease gradually from 11b of 41 9 to 20 9 with increasing salinity 70 to 497 mgcl l 1 temperature 25 50 c and boron content 0 1 0 3 mg l 1 the 11b values suggest that boron is derived from leaching of calcium carbonate or desorption of clay minerals of a 11b 14 and mixing with meteoric water of a high 11b value the relatively low 11b values of two low salinity springs 1240 and 2700 mgcl l 1 along the sea of galilee 11b 31 6 and 31 8 and the lake itself 24 suggest that a significant fraction of boron in these waters is also derived from rock leaching the influence of boron that is leached from the rocks is limited and can be recognized mainly in boron depleted groundwater the variations of 11b na cl and 18o parameters in the hammat gader springs demonstrate the sensitivity of each element to different sources whereas oxygen isotope compositions reflect the meteoric source the major ions e g na cl and br cl ratios record the influence of the deep seated brines and boron isotopes in diluted groundwater are sensitive to water rock interaction

increased northeast pacific climatic variability during the warm middle holocene 1 we use a pair of decadally resolved planktonic foraminiferal 18o records from santa barbara basin sbb to show that the warmest interval of the current interglacial the middle holocene was characterized by increased decadal to centennial scale climatic variability modern relationships between the pacific decadal oscillation pdo the el nino southern oscillation enso and upper water conditions in sbb also suggest a preference of the northeast ne pacific climate during the warm mid holocene toward the warm phase of the pdo and more intense enso warm events el ninos in the context of future climate this study suggests increasing decadal to centennial scale climatic variability in a globally warmed world

evidence for local shifting of the main fault and changes in the structural setting kinarot basin dead sea transform two new high resolution seismic reflection profiles from the kinarot valley dead sea rift analyzed with older conventional oil exploration profiles throw light on the quaternary evolution of the african arabian transform type plate boundary in the area the present en echelon main faults of the dead sea transform are observed in the seismic data as wide zones of deformation rather than as distinct fault planes a similar zone of deformation is observed in the center of the kinarot basin and may be associated with an extinct trace of the main fault the zemah structure previously mapped and drilled is shown by the new data to be an inactive anticline compression within the basin is suggested as being the result of motion along the now inactive main fault observed in the center of the basin compression and shortening started some 2 m y ago and ended late in the quaternary as a result of changes in the geometry of the transform at present the kinarot basin is subsiding

role of salinity derived variable density flow in the displacement of brine from a shallow regionally extensive aquifer portions of many regional scale aquifers in midcontinent sedimentary basins exhibit large salinity gradients that significantly impact the velocity field and solute distribution through time a two dimensional numerical transport model was constructed to examine the role of salinity derived variable density flow on changes in the velocity field and solute distributions in a near surface regionally extensive aquifer as brine is displaced by infiltrating meteoric water the silurian devonian carbonate aquifer in the western portion of the appalachian basin was used as a framework to insure that realistic flow velocities and salinities were used in the assessment the variable density effects on brine displacement are observed by examining the differences in the velocity fields and solute distributions produced by uniform density and variable density simulations the effects include the change from an intraformational displacement pattern to a cross formational displacement pattern with the development of flow reversals and partitioning of regional flow cells into smaller flow cells variable density effects also are manifest in the solute distributions by slowing the displacement of brine and influencing the magnitude of the salinity gradient a sensitivity analysis used to examine the influence of flow and transport parameters on the transient development and migration of salinity gradients shows that increasing cross formational leakage into the regional aquifer causes flow velocities to decrease which magnifies the influence of the variable density behavior by slowing the displacement of brine the sensitivity analysis also shows that increasing the value of dispersivity causes an increase in the variable density effects however the effects of variable density flow are relatively insensitive to changes in values of horizontal and vertical anisotropy assigned to the aquifer or to the presence of an overlying transmissive layer

variable density flow in the midcontinent basins and arches region of the united states basins in the midcontinent basins and arches region of the united states are typical of many sedimentary basins containing a diverse assemblage of lithologies that have been deformed at various stages in their geologic history and now transmit fluids of varying density on the basis of the synthesis of geologic data from more than 500 wells specific gravity data from more than 1800 samples fluid pressure data from nearly 40 deep wells and hydraulic conductivity data from core analyses of more than 35 wells the midcontinent basins and arches region is conceptualized as a layered heterogeneous hydrologically mature region containing 11 hydrostratigraphic units that have undergone various periods of deformation and comprise a sequence of aquifers and confining layers with fluid density variations ranging from freshwater to brine an interpretative threedimensional steady state variable density finite difference flow model was constructed to organize the field data and to evaluate various controls on regional flow patterns simulation results indicate that flow directions in the shallow hydrostratigraphic units are controlled by local surface elevations the deeper flow systems however show the influence of regional structural features such as the cincinnati arch that control the locations of regional groundwater divides the amount of downward cross formational flow into the older hydrostratigraphic units is greatest along the area of the cincinnati and findlay arches where the mt simon sandstone is relatively shallow and fracturing and faulting are more common spatial variations in fluid density are a significant influence on vertical cross formational flow in regions where salt bearing units are present

radiocarbon dating of ohalo ii archaeological and methodological implications abstract charcoal samples from the early epipalaeolithic submerged fisher hunter gatherers site of ohalo ii have been dated by three laboratories the samples derive from huts hearths a grave and a stone installation twenty five date range between 17 500 21 050 bp and average c 19 400 bp the thickness of deposits the perishable building material of the huts the spatial organization of the camp and its rapid cover by sand and water suggest several occupational episodes lasting not more than tens of years together the discrepancy between the range of 14 c dates and the real length of occupation are due to the statistical limitations of the dating method

transient groundwater lake interactions in a continental rift sea of galilee israel the sea of galilee located in the northern part of the dead sea rift is currently an intermediate fresh water lake it is postulated that during a short highstand phase of former lake lisan in the late pleistocene saline water percolated into the subsurface since its recession from the kinarot basin and the instantaneous formation of the fresh water lake the sea of galilee the previously intruded brine has been flushed backward toward the lake numerical simulations solving the coupled equations of fluid flow and of solute and heat transport are applied to examine the feasibility of this hypothesis a sensitivity analysis shows that the major parameters controlling basin hydrodynamics are lake water salinity aquifer permeability and aquifer anisotropy results show that a highstand period of 3000 yr in lake lisan was sufficient for saline water to percolate deep into the subsurface because of different aquifer permeabilities on both sides of the rift brine percolated into aquifers on the western margin whereas percolation was negligible on the eastern side in the simulation after the occupation of the basin by the sea of galilee the invading saline water was leached backward by a topographydriven flow it is suggested that the percolating brine on the western side reacted with limestone at depth to form epigenetic dolomite at elevated temperatures therefore groundwater discharging along the western shores of the sea of galilee has a higher calcium to magnesium ratio than groundwater on the eastern side

strontium isotopic chemical and sedimentological evidence for the evolution of lake lisan and the dead sea precise strontium isotope ratios combined with chemical analyses and sedimentological information are used to monitor the water sources and the evolution of the dead sea and its late pleistocene precursor lake lisan 70 18 kyr b p the materials analyzed include bulk aragonite water leached soluble salts and residual aragonite and gypsum from the lisan formation in the perazim valley near the sw shore of the dead sea the residual aragonite and the associated soluble salts display systematic fluctuations in 17sr86sr ratios between 0 70803 and 0 70806 and from 0 70805 to 0 70807 respectively in individual soluble salt residual aragonite pairs the soluble salt displays a higher 87sr86sr ratio gypsum samples yield 17sr86sr ratios similar to the soluble salts from adjacent layers in the section this shows that in individual samples the source of sr in aragonite was distinct from that in soluble salts and the gypsum the sterility of the lisan sediments their strictly nonbioturbated fine lamination and their high content of chloride salts indicate that lake lisan was a saline or even hypersaline water body in the absence of alternative sources of hco3 and s042 the abundance of primary aragonite and gypsum in the lisan column reflects an import of very large volumes of freshwater into the otherwise saline lake resulting in a density stratification of this water body the history of the upper water layer and that of the lower brine is reflected in the chemical and strontium isotope composition of the aragonite and in that of the associated soluble salts and in the gypsum samples respectively whereas the bicarbonate and much of the ca2 required for aragonite crystallization were supplied by the freshwater the complementary ca2 and sr 2 were added by the lower brine the upper water layer of lake lisan acted as a so42 capacitor during the lake s rise periods it was removed therefrom as prominent gypsum beds upon climatic induced drier period mixing or even complete overturn of the lake the evolution of lake lisan took place between two distinct modes the first was characterized by an extensive supply of freshwater and resulted in a rise of the lake s level a density layered structure and precipitation of aragonite the second mode was marked by a diminishing freshwater input resulting in mixing or complete overturn of its water and precipitation of gypsum these two modes reflect the climatic evolution of the region in the late pleistocene which fluctuated between drier and wetter periods the transition to the holocene is accompanied by the dry up of lake lisan and its contraction to the present dead sea

lacustrine system evolution during early rifting el castellar formation galve sub basin central iberian chain abstract a detailed sedimentological study of the first synrift deposits recorded in the galve sub basin of ne spain during the early cretaceous aided in the reconstruction of climatic and tectonic influences the el castellar formation is composed of siliciclastics and carbonates unit 1 claystones gypsum and carbonates unit 2 and marls and limestones unit 3 unit 1 facies formed in alluvial and palustrine plains a local alluvial fan and low energy shallow lake subenvironments low energy lacustrine facies characterised unit 2 in contrast palustrine low and high energy lake facies were identified for unit 3 the spatial and temporal distribution of lithofacies representing the several environments were used to propose a general lacustrine system evolution from isolated ponds and marshes to low energy shallow lakes and finally to an extensive high energy lake lake evolution is related to changes in the subsidence pattern from local to basin scale subsidence which was ultimately related to the transition from the initial rift to rift climax stage shallowing upwards lacustrine successions have been interpreted as climatically forced and linked to sediment and water supply both tectonics and climate determined the change from an overfilled to a balanced fill lake basin through time

calibration of the 14c time scale to 40 ka by 234u 230th dating of lake lisan sediments last glacial dead sea a new comparison of 14c dates with 234u 230th ages is presented of aragonites from lake lisan the last glacial dead sea between 20 52 cal ka bp the lisan data are coincident with the coral based 14c calendar age calibration through the continuous portion of the curve to 23 5 cal ka bp and with the additional checkpoints at 30 and 40 cal ka bp the agreement with the corals provides evidence for the accuracy of the u th and 14c ages and indicates that lisan aragonites can potentially be used to generate a nearly continuous record of the atmospheric 14c variations through this crucial time interval the lisan data are compared with other records older than 25 cal ka bp from lake suigetsu japan north atlantic foraminifera south african cave deposits and tufa from spain over some age intervals the records show broad agreement over other intervals they diverge all agree that 14c ages were 2 5 0 5 ka younger than calendar ages between 20 32 cal ka bp for ages 32 cal ka bp the lake suigetsu data indicate small differences between 14c ages and calendar ages of less than 1 5 ka the other records broadly agree that 14c ages are too young by 3 1 ka from 32 39 cal ka bp at 40 cal ka bp the foraminifera show equal 14c and calendar ages while the corals lisan aragonites and the spanish tufa indicate a large age difference of 4 5 ka a recent paleomagnetic study of the lisan formation indicates that the high calendar 14c age difference at 40 cal ka bp may be associated with the laschamp paleomagnetic excursion

development of coarse grained facies in lacustrine rift basins examples from east africa detailed studies of high resolution and multifold seismic reflection data from the two largest east african rift lakes malawi and tanganyika reveal a complex suite of coarse grained depositional facies these facies occur within specific regions of the controlling half grabens that compose the rift lakes sand prone environments include subaqueous channels and small drowned fluvial complexes channel systems range from large erosional canyons to deep water turbidite channel levee systems lowstand and highstand deltas of axial and shoaling side rivers are volumetrically important coarse grained facies fan deltas develop along the base of major border faults during lake lowstands subaqueous talus fan deposits occur along the base of the border faults during lake highstands lowstand deltas are the best preserved progradational facies in these rift lakes in addition to simple tectonic control drastic tectonically or climatically induced lake level change significantly regulates the production of coarse grained lacustrine synrift deposits

chemical tracing of salinity sources in lake kinneret sea of galilee israel lake kinneret is a freshwater lake in northern israel that receives a major part of its salt input from unmonitored springs that discharge through the lake s bottom we attempt to characterize the nature of these springs by estimating their chemical composition while the springs around lake kinneret are subject to wide spatial and temporal variations in their ionic concentrations specific sodium na potassium magnesium mg strontium bromine and lithium to chlorine cl ion ratios are almost constant within individual springs and spring groups the radium cl ratio and the d 18 o cl relationship confirm the notion that the spring waters result from recent mixing between saline brines and freshwater available compositional data from past years along with new analyses of the lake and its known springs allow identification of the salinity source that causes the observed deficit in the lake s salt budget e g 91 93 chloride the relative contributions from these saline springs are different for different ions this contribution is highest for bromide 95 decreases to 84 for na and is less than 50 for mg two independent approaches have been used for balancing the salts in the lake and they are as follows 1 an annual mass balance between salt removal and supply of the different ions assuming a steady state lake and 2 simulation of the lake s evolution from 1964 the beginning of salt removal from the lake via the salinity diversion channel until the present both methods predict very similar ionic ratios for the yet unknown average saline spring s testifying to the reliability of both approaches the ionic ratios so obtained closely resemble fuliya 6 tabgha type waters excluding the tiberias and eastern shore springs as significant salt sources this inferred composition of the average unmonitored springs depends strongly on present day diversion of saline springs this diversion thus prevents their flow into the lake the different ionic ratios that identify the various spring groups reflect the respective compositions of the brine pockets that feed them our simulation also shows that the layered structure of lake kinneret enhanced the evolution rate of the lake after the implementation of the salt diversion program in 1964 lake kinneret lk lake tiberias or the sea of galilee is a lake of immense historical and religious significance it is also the only freshwater lake in israel and the main water reservoir for the national water carrier nwc the water supply system that transfers freshwater to central and southern israel about 30 of the country s water supply was pumped from lk in 1995 furthermore with the water shortage in the middle east becoming more and more acute lk is also a waterbody of great political significance the lake is monomictic it is stratified between mid march and december the mixing of a fully stratified to a fully mixed lake takes about 4 5 months though it is a flowthrough lake it is significantly saltier than the jordan river which is its major water source table 1 the composition

lake level variation in the lahontan basin for the past 50 000 years selected radiocarbon data on surficial materials from the lahontan basin nevada and california provide a chronology of lake level variation for the past 50 000 yr a moderate sized lake connected three western lahontan subbasins the smoke creek black rock desert subbasin the pyramid lake subbasin and the winnemucca dry lake subbasin from about 45 000 to 16 500 yr b p between 50 000 and 45 000 yr b p walker lake rose to its sill level in adrian valley and spilled to the carson desert subbasin by 20 000 yr b p lake level in the western lahontan subbasins had risen to about 1265 m above sea level where it remained for 3500 yr by 16 000 yr b p lake level in the western lahontan subbasins had fallen to 1240 m this recession appears synchronous with a desiccation of walker lake however whether the walker lake desiccation resulted from climate change or from diversion of the walker river is not known from about 15 000 to 13 500 yr b p lake level rapidly rose so that lake lahontan was a single body of water by 14 000 yr b p the lake appears to have reached a maximum highstand altitude of 1330 m by 13 500 yr b p a condition that persisted until about 12 500 yr b p at which time lake level fell 100 m no data exist that indicate the level of lakes in the various subbasins between 12 000 and 10 000 yr b p during the holocene the lahontan basin was the site of shallow lakes with many subbasins being the site of one or more periods of desiccation the shape of the lake level curve for the three western subbasins indicates that past changes in the hydrologic balance and hence climate of the lahontan basin were large in magnitude and took place in a rapid step like manner the rapid changes in lake level are hypothesized to have resulted from changes in the mean position of the jet stream as it was forced north or south by the changing size and shape of the continental ice sheet

reevaluation of the lake sediment chronology in the dead sea basin israel based on new 230th u dates the dead sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes lake samra and lake lisan the sedimentary history of these lakes was recon structed by means of 230th234u ages of 30 samples mostly of argonite laminae from 8 columnar sections up to 110 km apart the general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency agreement with stratigraphic order and concordance with 14c ages in the south only the part of the samra formation older than 170 000 yr is exposed while the aragonite detritus rhythmites found in the central and northern region are generally younger than 120 000 yr the lisan formation started accumulating about 63 000 yr b p with the clay and aragonite beds in the south central area reflecting a rise in water level to at least 280 m the upper part of the lisan formation the aragonite rich white cliff member started accumulating about 36 000 yr b p the lake probably reached its highest level sometime after this based on the ages of lisan sediments preserved in the southernmost reaches of the basin

paleohydrology of variable density ground water flow systems in mature sedimentary basins example of the palo duro basin texas usa abstract the paleohydrology of mature sedimentary basins undergoing uplift and tilting deposition and erosion is studied by numerically modeling the flow of variable density ground water in regional flow systems consisting of both shallow freshwater aquifers and deep saline aquifers complex ground water flow patterns can exist when buoyancy forces dominate the hydraulic head gradients arising from topographic relief the relative magnitude of the two driving forces hydraulic gradient and buoyancy changes during the hydrodynamic development of a sedimentary basin that is undergoing topographic changes the relationship between the two driving forces and their effect on flow and transport is studied through coupled flow and solute transport simulations using an idealized model scenario modeling shows that increases in recharge rates can rapidly change hydrodynamic conditions from buoyancy dominated flow governed by variable density to gravity driven flow governed by topography because the dissolved mass responds much slower to changes in hydrologic boundary condition than does fluid pressure transient conditions occur whereby the solute distribution may not correspond to the simulated ground water flow pattern hydrodynamic scenarios were examined in the palo duro basin texas where observed fluid densities range between 1000 and 1150 kg m 3 the palo duro basin was affected by cenozoic uplift deposition and erosion causing modifications in topography and major changes in regional hydrodynamics during the last 15 myear although fluid pressures have largely equilibrated with topographic changes in the recent geologic past the distribution of dissolved mass evolves over a much longer time and the observed solute distribution in the basin as suggested by the geochemistry of the deep basin brines may represent some past state of the hydrodynamic system paleohydrologic simulations describe buoyancy dominated flow phenomena in the recent geologic past when the topographic relief across the basin was much lower than it is today prior to maximum basin uplift simulated flow patterns in the deep aquifers are characterized by convection cells in the western part of the basin where fluid densities increase with depth convection cells show clockwise fluid motion restricting recharge of shallow ground water to the upper part of the deep aquifers in the eastern part of the basin where fluid densities decrease with depth convection cells are characterized by counterclockwise fluid motion and reflect thermohaline convection with enhanced recharge due to continued basin uplift and subsequent erosional events buoyancy dominated flow phenomena diminish and the gravity driven flow component arising from topographic relief becomes dominant this study demonstrates that although present ground water flow circulation in the basin is not affected by the observed fluid densities the flow pattern history during cenozoic basin development is much more complex than that which would be expected from a gravity driven flow conceptualization assuming uniform fluid densities more important the model scenarios describe the interrelationship between geologic processes regional hydrodynamics and transport phenomena and its importance in explaining hydrologic and geochemical phenomena in sedimentary basins

salinization and dilution history of ground water discharging into the sea of galilee the dead sea transform israel abstract the mechanism governing salinization of ground water discharging into the sea of galilee in israel has been the subject of debate for several decades because the lake provides 25 of the water consumed annually in israel correct identification of the salt sources is essential for the establishment of suitable water management strategies for the lake and the ground water in the surrounding aquifers existing salinization models were evaluated in light of available and newly acquired data including general chemistry and o h c and cl isotopes based on the chemical and isotopic observations the proposed salt source is an ancient intensively evaporated brine 21 to 33 fold seawater which percolated through the valley formations from a lake which had formed in the rift valley following seawater intrusion during the late miocene low na cl and high br cl values support the extensive evaporation whereas high ca cl and low mg cl values indicate the impact of dolomitization of the carbonate host rock on the residual solution based on radiocarbon and other isotope data the dilution of the original brine occurred in two stages the first took place 30 000 a ago by slightly evaporated fresh to brackish lake water to form the sea of galilee brine the second dilution phase is associated with the current hydrological regime as the sea of galilee brine migrates upward along the rift faults and mixes with the actively circulating fresh ground water to form the saline springs the spatially variable chemical and isotopic features of the saline springs suggest not only differential dilution by fresh meteoric water but also differential percolation timing of the original brine into the tectonically disconnected blocks registering different evaporation stages in the original brine consequently various operations to reduce the brine contribution to the lake may be differentially effective in the various areas

geophysical time domain electromagnetic model delineation of a shallow brine beneath a freshwater lake the sea of galilee israel the sea of galilee is a freshwater lake into which saline water emerges through onshore and offshore springs and through flux from the lake s sediments the novel surface marine modification of the time domain electromagnetic method was used to map the spatial distribution of brines in the sediments below the lake results indicate that electrical resistivities of 1 0 and 0 5 ohm m are detected at depths of 10 m below the lake bottom in most of the lake area which are equivalent to 11 000 and 22 000 mgcl l respectively relatively fresh groundwater was detected beneath most of the shoreline faulting controls the vertical interfaces between saline and fresh groundwaters it is hypothesized that salt transport is dominated by molecular diffusion in the central part of the lake and by advection from regional aquifers in the margins

transient salt transport modeling of shallow brine beneath a freshwater lake the sea of galilee israel during a lake highstand phase in the late pleistocene the former saline lake lisan covered the topographic depression of kinarot basin currently occupied by the freshwater lake sea of galilee it was hypothesized that during this period the dense saline waters of lake lisan percolated into the sediment the recession of the saline lake from the basin and the rapid formation of a freshwater lake triggered solute transport from the sediment into the lake a one dimensional numerical model of solute transport that considers sediment compaction was developed to simulate chloride transport from the sediment into the lake simulation results were compared with measured chloride concentration profiles in sediment cores on the basis of a sensitivity analysis results are in agreement with the hypothesis that lake lisan solutes are currently discharged into the sea of galilee the calculated upward water velocity in the sediment ranges between 9

sea surface elevation and bottom pressure anomalies due to thermohaline forcing part i isolated perturbations sea surface elevation and bottom pressure anomalies due to thermohaline forcing are examined through analytical and numerical models including boussinesq and non boussinesq models it is shown that boussinesq approximations can introduce noticeable errors depending on the spatial and temporal scales of the perturbations according to the theory of geostrophic adjustment when the initial perturbations have horizontal scales comparable to the barotropic radius of deformation the initial pressure perturbations will be basically retained through the adjustment on the other hand if the initial perturbations have horizontal scales much smaller than the barotropic radius of deformation the initial pressure perturbations will be largely lost precipitation has horizontal scales on the order of 10 100 km much smaller than the barotropic radius of deformation thus for timescales longer than days the contribution from individual precipitation events to the local free surface elevation and bottom pressure is small and is difficult to identify from satellite data on the other hand thermal forcing has horizontal scales comparable to the barotropic radius of deformation so its long term contribution to sea surface height anomaly is noticeable and is easily identified from satellite data because boussinesq models induce faulty sea surface height and bottom pressure signals the errors introduced by these models are noticeable for anomalies in large scale o 1000 km thermohaline forcing

controls of sedimentation in the malawi rift valley central africa abstract the malawi rift is the southermost classic rift valley trough in the cenozoic african rift valley system much of its floor is flooded by lake malawi which is approximately 570 km long 60 km across and up to 500 m deep this rift basin is a potentially useful analogue of many economically important ancient sedimentary troughs rift valley structures are extremely important in controlling both the distribution of sediments in the basin as a whole and also details of sedimentation on a local scale on the basin wide scale an important association between the type of rift boundary structure and amount of rift floor subsidence is recognised this association appears to apply to several section of the african rift system pre rift topography is major control on the position of major rivers and their deltas or turbidite fans along the basin margin the area of pre rift drainage network captured in the course of rift movements is a significant factor in the hydrologic budget of the basin climate is however the dominant control on the hydrologic budget and at present the basin is flooded to overflow level prevailing winds are important in beach ridge and back beach aeolian dune development climatic conditions produce thermal stratification of the lake with an oxic epilimnion down to 250 m depth overlying an anoxic hypolimnion the oxic bottom waters in the shallower parts of the lake appear to promote iron precipitation in the sediments