controls on the geometry of transfer zones in the suez rift and northwest red sea implications for the structural geometry of rift systems two main types of transfer zones are mapped in the suez rift and the northwestern red sea these are transfer zones between indi vidual faults fault to fault transfer zones f f tz and transfer zones between half grabens with different fault domains half graben to half graben transfer zones h h tz relay ramps and linking transfer faults are two main recognized types of f f tz different varieties of these f f tz have the form of en echelon fault belts with single or mixed polarity tilted fault blocks at rift offsets and zigzag fault arrays at major fault or rift jumps relay ramps transfer throw between overlapping normal faults of the same polarity linking transfer faults join normal faults that have the same polarity with no overlapping segments leading to a characteristic zigzag pattern if the linked faults are listric local anticlines and synclines develop at the corners of the zigzag fault array three h h tz in the suez rift and northwest red sea show the effect of prerift structures both faults and major folds on the location orientation and style of deformation of the transfer zones of continental rifts the widest one of these three h h tz 40 60 km has a broad antiformal structure lying between the overlapping ends of the listric faults bounding the two linked half grabens and is deformed by dip slip normal faults of opposite polarity the other two h h tz reactivate prerift faults by strike slip faulting due to the angular relationship between these faults and the direction of extension in the rift

dry cave deposits and their palaeoenvironmental significance during the last 115 ka sodmein cave red sea mountains egypt the sodmein cliff foot cave is the present day remnant of an ancient cavity probably of karstic origin physical breakdown of the limestone bedrock rather than solution has governed its subsequent evolution long before 115 ka bp an estimated 8000 m 3 of debris came off the weathered roof and mixed with contemporaneous cliff rockfall over 4 m of sediments have since accumulated wet conditions outside the cave during isotopic stage 5e are documented by sedimentary properties of the j complex and by its detailed botanical and faunal content these wet conditions were of regional significance shortly after 115 ka bp further subsidence of the roof of the ancient cave led to the present day cave form the cave interior has remained dry up to the present but the deposits indicate an increase of animal passage and plant growth around 25 ka bp and during the holocene interglacial the latter period was rather arid in absolute terms receiving less precipitation under a less regular pluvial regime compared with the interglacial during isotopic stage 5e r 2002 elsevier science ltd all rights reserved

natural discharge a key to sustainable utilization of fossil groundwater summary rising demands for fresh water supplies in arid lands are leading to excessive exploitation and unsustainable mining of non renewable fossil groundwater using the nubian aquifer of saharan africa as a test site we demonstrate an integrated approach to identify areas of discharge that could have gone undetected and to model extraction that is sustained by natural discharge using isotopic and geochemical analyses along with field and remote sensing data we show that discharge of the nubian aquifer is occurring on a larger scale primarily through deep seated fault systems and that ascending groundwater discharges into relatively thick alluvial aquifers proximal to the fault complex that defines the river nile and the gulf of suez we develop a hydrologic model to assess the discharge and to constrain sustainable extraction in the asyuti area along the river nile and to demonstrate a replicable model for similar reservoirs a two dimensional groundwater flow model was constructed and calibrated against head data from 19 wells results point to a significant contribution to the asyuti groundwater system from rising nubian groundwater 3 19 10 7 m 3 yr 75 of incoming flow and a modest contribution 1 08 10 7 m 3 yr 25 of incoming flow from surface runoff approximately 2 5 10 7 m 3 yr of groundwater could be used in a sustainable manner in asyuti assuming geologic settings and discharge rates similar to those at asyuti we estimate that 70 10 7 m 3 yr of groundwater may be available for sustainable development in similar settings around the river nile and the gulf of suez

low oxygen and high inorganic carbon on the vancouver island shelf 1 recently independent concerns about declining oxygen and ph conditions in the coastal ocean have emerged in coastal upwelling regions hypoxia can be driven by onshore advection of oxygen depleted offshore waters as well as by local biological consumption triggered by high productivity as both mechanisms can also decrease ph and carbonate saturation states coupled studies of oxygen and carbon are imperative a quasi two dimensional model coupling carbon oxygen and nitrogen was developed for the summer wind driven upwelling region off southern vancouver island using the regional ocean modeling system the physical model is coupled to an ecosystem module that tracks 11 state variables and allows nonfixed c n ratios for detritus and dissolved organic matter given uncertainties in sediment parameterizations in biophysical models three sediment models are compared and discussed results demonstrate that sediment associated processes play a dominant role in consuming oxygen from and releasing inorganic carbon to the bottom waters over the shelf this study also examines the unique characteristics of the southern vancouver island shelf two key features distinguish this region from other shelves in the california current system and protect inner shelf waters from severe hypoxia and corrosive i e undersaturated in aragonite conditions first the near shore vancouver island coastal current provides a source of oxygen and nutrients and forms a barrier that prevents upwelled waters depleted in oxygen and rich in carbon from penetrating the inner shelf second the greater width of the shelf dilutes these upwelled offshore waters and reduces their penetration onto the shallower shelf region

potential and timescales for oxygen depletion in coastal upwelling systems a box model analysis a simple box model is used to examine oxygen depletion in an idealized ocean margin upwelling system near bottom oxygen depletion is controlled by a competition between flushing with oxygenated offshore source waters and respiration of particulate organic matter produced near the surface and retained near the bottom upwelling supplied nutrients are consumed in the surface box and some surface particles sink to the bottom where they respire consuming oxygen steady states characterize the potential for hypoxic near bottom oxygen depletion this potential is greatest for faster sinking rates and largely independent of production timescales except in that faster production allows faster sinking timescales for oxygen depletion depend on upwelling and productivity differently however as oxygen depletion can only be reached in meaningfully short times when productivity is rapid hypoxia thus requires fast production to capture upwelled nutrients and fast sinking to deliver the respiration potential to model bottom waters combining timescales allows generalizations about tendencies toward hypoxia if timescales of sinking are comparable to or smaller than the sum of those for respiration and flushing the steady state will generally be hypoxic and results indicate optimal timescales and conditions exist to generate hypoxia for example the timescale for approach to hypoxia lengthens with stronger upwelling since surface particle and nutrient are shunted off shelf in turn reducing subsurface respiration and oxygen depletion this suggests that if upwelling winds intensify with climate change the increased forcing could offer mitigation of coastal hypoxia even as the oxygen levels in upwelled source waters decline

numerical modelling of lithosphere asthenosphere interaction in a subduction zone we developed a new 2 d numerical approach to study solid fluid coupling applied to subduction zones the lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid the temperature effects are ignored and viscosity and density are constant in time the solid and the fluid problem are discretised by the finite elements method fem the same solid code used in hassani et al hassani r jongmans d chery j study of plate deformation and stress in subduction processes using two dimensional numerical models j geophys res 1997 102 17951 17965 has been used to compute the solution of the solid problem the stokes problem is solved by a direct solver with a stabilisation procedure we used a very simple staggered coupling method where the fluid domain is regularly re meshing we observed numerical instabilities when the time step is not sufficiently small especially when strong coupling between the solid and the fluid occurs we have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry the topography and the stress regime in the plates are affected by the viscous resistance of the mantle we observed that the asthenosphere viscosity is a fundamental parameter in the subduction process for subduction with an extensional regime in the upper plate we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity

flexural rigidity of fennoscandia inferred from the postglacial uplift the earth s response to glacial loading unloading offers exceptional promise for the study of the physical properties of the lithosphere in particular tilting of paleoshorelines is very sensitive to the lithosphere rigidity to determine the flexural rigidity the isostatic response to deglaciation in fennoscandia is modeled using an earth model with a layered mantle viscosity overlain by an elastic lithosphere the flexural rigidity and asthenosphere viscosity is allowed to vary to get a match between theoretical and observed present rate of uplift and tilting of paleoshorelines five different ice thickness models are used for a relatively thin ice 2500 m in central areas the resulting flexural rigidity is more or less uniform over fennoscandia with a value of 1023 n m elastic thickness te 20 km this is regarded as minimum for the flexural rigidity of central fennoscandia the pattern of the present rate of uplift and the tilts of the paleoshorelines of the area also sets an upper bound of the flexural rigidity 2 5 1025 n m te 110 km in more central areas of fennoscandia the flexural rigidity at the western coast of norway does not seem to exceed 1023 n m te 50 km the most likely glacier model gives a flexural rigidity of 1023 n m te 20 km at the norwegian coast increasing to above 1024 n m te 50 km in central parts of fennoscandia

an integrated approach for identifying aquifers in transcurrent fault systems the najd shear system of the arabian nubian shield groundwater potentials were assessed for major transcurrent fault systems in basement complex terrains using the najd transcurrent shear system of the arabian nubian shield as a test case spatial analysis within a geographical information system environment of relevant data sets field remote sensing geophysical and digital elevation revealed a spatial correlation between existing well locations and distribution of najd transcurrent shear system structural elements i e faults shear zones melange dykes within highly fractured domains suggesting a causal relationship specifically at 1 intersection of najd brittle shear zones and fault systems 2 intersections of two or more fault systems 3 intersections of najd related dykes with the main streams within catchment areas and 4 within tectonic melange for each of the former aquifer types aquifer specific field and satellite based characteristics were identified and used in conjunction with complementary data e g watershed morphology precipitation soil moisture to target aquifers of similar settings postulated well locations were then successfully tested and refined using very low frequency technologies results could potentially highlight the important role of major transcurrent fault systems in providing settings conducive for hosting and transmitting groundwater and could potentially serve as a replicable model for regional assessment of groundwater potentials in similar tectonic and hydrologic settings

space geodetic constraints on glacial isostatic adjustment in fennoscandia analysis of global positioning system gps data demonstrates that ongoing three dimensional crustal deformation in fennoscandia is dominated by glacial isostatic adjustment our comparison of these gps observations with numerical predictions yields an earth model that satis es independent geologic constraints and bounds both the average viscosity in the upper mantle 5 x 10 20 to 1 x 10 21 pascal seconds and the elastic thickness of the lithosphere 90 to 170 kilometers we combined gps derived radial motions with fennoscandian tide gauge records to estimate a regional sea surface rise of 2 1 0 3 mm year furthermore ongoing horizontal tectonic motions greater than 1 mm year are ruled out on the basis of the gps derived three dimensional crustal velocity field

inference of mantle viscosity from grace and relative sea level data summary gravity recovery and climate experiment grace satellite observations of secular changes in gravity near hudson bay and geological measurements of relative sea level rsl changes over the last 10 000 yr in the same region are used in a monte carlo inversion to infer mantle viscosity structure the grace secular change in gravity shows a significant positive anomaly over a broad region 3000 km near hudson bay with a maximum of 2 5 gal yr 1 slightly west of hudson bay the pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ice 5g deglaciation history strongly suggesting a postglacial rebound origin for the gravity change we find that the grace and rsl data are insensitive to mantle viscosity below 1800 km depth a conclusion similar to that from previous studies that used only rsl data for a mantle with homogeneous viscosity the grace and rsl data require a viscosity between 1 4 1021 and 2 3 1021 pa s an inversion for two mantle viscosity layers separated at a depth of 670 km shows an ensemble of viscosity structures compatible with the data while the lowest misfit occurs for upper and lower mantle viscosities of 5 3 1020 and 2 3 1021 pa s respectively a weaker upper mantle may be compensated by a stronger lower mantle such that there exist other models that also provide a reasonable fit to the data we find that the grace and rsl data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle

palaeohydrology and its impact on groundwater in arid environments gebel duwi and its vicinities eastern desert egypt abstract from ancient times access to water in arid and hyper arid regions has been crucial for life increased knowledge about groundwater conditions is therefore extremely important for the future development of desert areas gebel duwi a limestone ridge sited in the today hyper arid eastern desert of egypt was chosen for testing current hypotheses concerning the interdependence of palaeohydrologic features and geologic settings for groundwater investigations two sub basins are identified in affecting gebel duwi wadi nakheil and wadi sodmein both have extraordinary high stream order indicating the reality of a very old drainage system several additional palaeohydrologic features such as wadi terraces alluvial fans and playa deposits are also recorded in these sub basins these features as well as finds from an important archaeological site sodmein cave provide information about palaeoenvironmental changes and wetter climatic periods in the younger geologic past mainly in the late pleistocene and holocene which have affected the occurrence of present day groundwater this study combines new results of geological and geomorphological field studies examinations of digital elevation models as well as of rock and water analyses in an integrated approach resulting maps comprise landforms lithology elevation drainage lines slope structural lineaments and flow directions the hydrogeologic condition indicates suitable locations for groundwater recharge and accumulation the research presented is interdisciplinary in approach and is recommended for future studies with regard to groundwater exploration sustainable groundwater use and the protection of archaeological sites in arid lands

boundary element modeling of two plate interaction at subduction zones scaling laws and application to the aleutian subduction zone this work uses the boundary element method bem to explore the dynamics of subduction of a dense lithospheric plate subducting plate sp beneath an overriding plate op for simplicity the model is two dimensional the plates are purely viscous and the ambient fluid is infinitely deep the negative buoyancy of the slab is the only driving force of the system first we study the sp kinematics focusing on two characteristic instantaneous velocities the convergence speed vconv of the descending slab and the horizontal plate speed usp of the flat portion of the sp we find that vconv is entirely controlled by the slab s geometry by the width of the lubrication layer d2 separating the sp and the op and by the sp s flexural stiffness st turning to usp we find that this parameter depends not only on d2 and st but also on the lengths lsp and lop of the two plates the dependence of usp on lsp is exactly logarithmic both with and without an op next we explore the deformation of the op which occurs by a combination of extension compression and bending the op deformation is compression dominated close to the trench and bending dominated along the remaining portion of the op that undergoes significant deformation for a positively buoyant op backarc extension is also observed finally we estimate the subduction interface viscosity si of the central aleutian subduction zone running our bem model with the appropriate geometry according to lallemand et al 2015 https doi org 10 1029 2005gc000917 we find si 0 96 1 72 1020 pa s

one million year old groundwater in the sahara revealed by krypton 81 and chlorine 36 measurements of 81 kr kr in deep groundwater from the nubian aquifer egypt were performed by a new laser based atom counting method 81 kr ages range from 2 10 5 to 1 x 10 6 yr correlate with 36 cl cl ratios and are consistent with lateral flow of groundwater from a recharge area near the uweinat uplift in sw egypt low 2 h values of the 81 kr dated groundwater reveal a recurrent atlantic moisture source during pleistocene pluvial periods these results indicate that the 81 kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrolugic problems

dynamics of fossil fuel co2 neutralization by marine caco3 a detailed model of the ocean circulation and carbon cycle was coupled to a mechanistic model of caco3 diagenesis in deep sea sediments to simulate the millennium scale response of the oceans to future fossil fuel co2 emissions to the atmosphere and deep sea simulations of deep sea injection of co2 show that caco3 dissolution is sensitive to passage of high co2 waters through the atlantic ocean but caco3 dissolution has a negligible impact on atmospheric pco2 or the atmospheric stabilization co2 emission in the coming centuries the ultimate fate of the fossil fuel co2 will be to react with caco3 on the seafloor and on land an initial caco3 dissolution spike reverses the net sedimentation rate in the ocean until it is attenuated by an enhanced vertical gradient of alkalinity after about 1000 years the magnitude of the initial spike is sensitive to assumptions about the kinetics for caco3 dissolution but subsequent behavior appears to be less model dependent neutralization by seafloor caco3 occurs on a timescale of 5 6 kyr and is limited to at most 60 70 of the fossil fuel release even if the fossil fuel release is smaller than the seafloor erodible inventory of caco3 additional neutralization by terrestrial caco3 restores a balance between caco3 weathering and seafloor accumulation on a timescale of 8 5 kyr while the deficit of seafloor caco3 the lysocline is replenished with an e folding timescale of approximately 18 kyr the final equilibrium with caco3 leaves 7 8 of the fossil fuel co2 remaining in the atmosphere to be neutralized by the silicate rock cycle on a time frame of hundreds of thousands of years

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

a review of palaeoclimates and palaeoenvironments in the levant and eastern mediterranean from 25 000 to 5000 years bp setting the environmental background for the evolution of human civilisation abstract the southern levant has a long history of human habitation and it has been previously suggested that climatic changes during the late pleistocene holocene stimulated changes in human behaviour and society in order to evaluate such linkages it is necessary to have a detailed understanding of the climate record we have conducted an extensive and up to date review of terrestrial and marine climatic conditions in the levant and eastern mediterranean during the last 25 000 years we firstly present data from general circulation models gcms simulating the climate for the last glacial maximum lgm and evaluate the output of the model by reference to geological climate proxy data we consider the types of climate data available from different environments and proxies and then present the spatial climatic picture for key climatic events this exercise suggests that the major northern hemisphere climatic fluctuations of the last 25 000 years are recorded in the eastern mediterranean and levantine region however this review also highlights problems and inadequacies with the existing data

episodic growth of normal faults as recorded by syntectonic sediments july oil field suez rift egypt the july oil field is a major normal fault bounded structural block in the suez rift basin egypt it is adjacent to a major structural transfer zone which has controlled sediment influx to the rift basin center for the past 20 m y the lower to middle miocene upper rudeis formation part of the synrift stratigraphic sequence records deformation of the july structural block the formation contains abrupt lateral changes in thickness and facies which record earlier phases of fault movement and deformation in the july field area that do not conform to the present day structural configuration the upper rudeis formation was deposited as turbidites in a submarine fan system sourced from the western rift shoulder it was deposited over and around bathymetric highs created by coeval fault displacement in the july field area by studying thickness and facies patterns we have determined that the present day main bounding fault to the july block consisted of a series of unlinked fault segments which linked after upper rudeis deposition a subsidiary fault west of the block exerted the most control on thickness patterns not the present day main bounding fault thus commonly used models of deposition in active half grabens are difficult to apply at july field

viscous flow model of a subduction zone with a faulted lithosphere long and short wavelength topography gravity and geoid the essential features of observed topography free air gravity and the geoid over subduction zones have been reproduced with a dynamically self consistent viscous flow model of a subduction zone with a faulted lithosphere a fault between the overriding and subducting plates is crucial to understanding lithospheric deformation and gravity in a subduction zone the topography for models with a fault is characterized by a deep and narrow trench and a broad and shallow back arc depression quite different from viscous models without faults which only have a broad and large depression over subducting slabs for a model in which a slab is 600 km long and 80 myr old the resulting trench is approximately 3 km deep and 100 km wide and the back arc basin is about 1 5 km deep and 600 km wide the model is characterized by a long wavelength geoid high of about 30 m over the slab a local geoid low of 10 m over the trench and a 150 mgal gravity low over the trench the fact that trench depth and width are reproduced in models with a small resisting shear stress 50 bars suggests that convergent plate boundaries are probably weak

complexity of saharan paleoclimate reconstruction and implications for modern human migration abstract chronological and geochemical data from scarp and scarp foot depression sediments ssds have been used for deciphering saharan paleoclimates and patterns of migration of anatomically modern humans amh u th dating of thick accumulations of ssds indicates prevalent deposition during long lasting cool glacial periods marine isotope stage mis 6 8 10 and 12 and limited deposition during warm interglacial periods mis 5 7 and 13 in contrast quaternary sediments associated with short residence hydrologic systems in sinai yielded mis 2 osl ages of 27 7 to 10 2 ka the lack of ssds of mis 2 ages and the wide range of warm and cool stages in eastern sahara is attributed to 1 lengthy travel times of groundwater prior to discharge and deposition of ssds from deep groundwater originating from distant sources and 2 sampling and dating of ssds from depressions where natural discharge has been apparently continuous through wet and dry periods given that the previously dated depression ssds could have been deposited during dry periods and that the ages of ssds reflect the timing of groundwater discharge rather than the ages of the wet periods during which recharge occurred we suggest that earlier interpretations of saharan paleoclimate and amh migrations that heavily relied on dating of ssds should be revisited

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

lower miocene nukhul formation gebel el zeit egypt model for structural control on early synrift strata and reservoirs gulf of suez the aquitanian early burdigalian lower miocene nukhul formation at gebel el zeit egypt was deposited during early stages of gulf of suez rifting the unit dips 8 15 less than underlying prerift strata indicating that significant rotation and extension preceded subsidence of the gebel el zeit fault block the nukhul formation at gebel el zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit the formation varies considerably along strike because of syndepositional differential movement of small fault bounded blocks the lower clastic unit at south gebel el zeit contains poorly sorted conglomeratic marly sandstone that commonly displays grading and bouma sequences beds were deposited below storm base by sediment gravity flows thicker intervals are inferred to fill small structurally controlled submarine gullies that funneled sand and gravel southwestward to a half graben basin in contrast an inferred correlative thin basal conglomeratic unit in north gebel el zeit was deposited in a shallow marine setting the presence of basement clasts in nukhul strata indicates early syndepositional uplift due to structural tilting the upper carbonate unit consists of bioclast peloid and intraclast packstone wackestone and grainstone with minor floatstone rudstone and coral algal boundstone carbonate strata were deposited variously in deep marine low energy peritidal and subtidal and reefal environments deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition reefs and shallow marine bioclast shoals formed on higher submerged blocks nukhul strata show that synrift reservoir prediction in the gulf of suez the red sea and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis robert winn is a consulting geologist he received a ph d from the university of wisconsin madison in 1975 and joined marathon oil company as a research geologist in 1977 he left marathon in 1994 for the university of papua new guinea where he was senior lecturer and then associate professor he was head of the geology department from 1996 to 2000 his primary interests are in clastic sedimentology and sequence stratigraphy paul crevello is a consulting geologist and technical director of petrexasia which he formed in 1997 he received an m s degree from the university of miami 1978 and a ph d from colorado school of mines 1989 he was employed by marathon oil as a research geologist from 1978 to 1994 and by the university of brunei as senior lecturer from 1994 to 1997 his specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3 d reservoir models william bosworth is employed by marathon oil company he joined marathon in 1984 and has worked principally on international exploration and development projects from 1980 to 1984 he taught structural geology and tectonics at colgate university in hamilton new york his research interests are principally in extensional tectonics continental stress field evolution and the paleogeodynamics of the african plate

the role of a longshore pressure gradient in pacific northwest coastal dynamics abstract in this paper we demonstrate the importance of the seasonal barotropic longshore pressure gradient force to pacific northwest coastal dynamics values of the seasonal longshore pressure gradient corrected for gauge height relative to a level surface hickey and pola 1982 and for year to year variations enfield and alien 1980 were included in the two dimensional time dependent baroclinic finite difference model of hamilton and rattray 1978 as an external force observed wind stress stratification and bottom topography were included in the model and comparisons were made with current meter data in each of the three seasonal situations observed in the northwest pressure gradient force southward opposing local wind stress winter pressure gradient force northward opposing local wind stress summer and pressure gradient force and local wind stress both southward spring three important features of the seasonal circulation are shown to depend on the existence of the pressure gradient

the calcite lysocline as a constraint on glacial interglacial low latitude production changes we investigate the response of the calcite lysocline to changes in the export production of the low latitude surface ocean the combined equatorial tropical and subtropical regions we employ different caco3 throughput schemes in a time dependent ocean carbon cycle model to separate the caco3 production lysocline balance from the other components of the model response and to estimate the effect of dissolution driven by organic matter decomposition in surface sediments with this model we carry out three experiments based on previous hypotheses for the cause of glacial interglacial atmospheric co2 variations 1 a simple increase in low latitude production 2 a decrease in its caco3 organic carbon rain ratio and 3 an increase in the depth of organic carbon regeneration none of these changes when taken alone can lower atmospheric co2 to glacial levels without violating observations of the glacial lysocline depth if a low latitude production increase were accompanied by a decrease in the caco3 organic carbon rain ratio then these simultaneous changes could lower atmospheric co2 to observed glacial levels without causing a significant change in the ocean average depth of the lysocline however even with these simultaneous changes the required increase in production is 50 or more of the modern rate of production

carbon and oxygen cycles sensitivity to changes in environmental forcing in a coastal upwelling system 1 biogeochemical cycles in the coastal ocean are changing and will continue to change in response to a changing climate effects on the oxygen and carbon cycles are particularly important as either episodic or permanent shifts toward lower oxygen and or higher inorganic carbon conditions can impact coastal ecosystems negatively here we study the sensitivity of these cycles to changes that may occur in the coastal ocean focusing on a summer wind driven upwelling region off southern vancouver island shelf we use a quasi 2 d configuration of the regional ocean modeling system roms to perform six sensitivity experiments results indicate that carbon and oxygen cycles in this region may be significantly affected by an altered upwelling season a shallower offshore oxygen minimum zone and a carbon enriched environment combinations of these scenarios suggest a potentially increasing risk for the development of coastal hypoxia and corrosive conditions in the region

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

influence of lithosphere asthenosphere interaction on the overriding lithosphere in a subduction zone numerical modeling 1 several factors influence the topography of the overriding plate in a subduction zone mantle wedge flow caused by motion of the descending slab lithosphere elasticity and interaction between the solid lithosphere and the viscous asthenosphere this paper presents the results of new subduction modeling that incorporates both elastic elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere the effects of fluid solid coupling and interaction were considered using an arbitrary lagrangian eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate modeling results show that thicker lithosphere which is harder to deform contributes to the development of broadly shallower back arc basins as lithospheric strength decreases its ability to withstand outside force fluid load resulting from flow in the mantle wedge also declines leading to the development of deep and narrow basins large scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern which in turn increases the fluid load and thereby enhances topographic variation generally the effect of interaction is enhanced with lithospheric deformation the elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere similarly to previous studies back arc basins are deeper when mantle viscosity is higher for a 50 km thick elastic lithosphere the lithosphere asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 1019 pa s increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins

integration of geological investigations with multi gis data layers for water resources assessment in arid regions el ambagi basin eastern desert egypt water resources assessment in arid lands is of a particular interest not only in scientific terms but also under the concept of sustainable development the study area el ambagi basin represents a remote arid area with a scarcity of data the measured effective porosity and permeability revealed high potentialities of the nubian sandstone and oligocene sediments the hydrogeologic setting indicates favorable conditions for groundwater accumulation where there is an opportunity of recharge and thus the sustainability of the groundwater the research approach is based on geological investigations as well as several data layers in the gis environment including landforms lithology slope soil morphometric parameters structural lineaments and geophysical data several specific sites were suggested as targets for future groundwater explorations the studied basin contains four sub basins which have an intensive drainage network the morphometric parameters revealed that they have potential for surface runoff occurrence and also for groundwater recharge two of the investigated sub basins show high hazard degrees and should be protected to avoid flash floods the estimated runoff volume from a single rainfall event 27 mm can reach 3316 42 103 m3 hence management of rainwater harvesting and protection measures against flash floods are necessary a future water resources management strategy based on results of the multidisciplinary study is proposed

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

on the coupling of benthic and pelagic biogeochemical models abstract mutual interaction of water column and sediment processes is either neglected or only crudely approximated in many biogeochemical models we have reviewed the approaches to couple benthic and pelagic biogeochemical models it is concluded that they can be classified into a hierarchical set consisting of five levels differing in the amount of detail given to the sediment processes the most complex approach level 4 fully couples water column processes to a vertically resolved biogeochemical sediment model first simplification is achieved by using a vertically integrated dynamic sediment model level 3 next is a reflective type of boundary level 2 where particulate material arriving at the sediment surface is instantaneously transformed into dissolved components then comes a set of models in which either the bottom water concentration of dissolved substances or the sediment water exchange is imposed level 1 finally in some biogeochemical models the bottom is plainly ignored level 0 we have tested these various approaches in a coupled physical pelagic benthic biogeochemical model for oxygen nitrogen and carbon cycling in continental shelf areas we discuss the various model approaches with respect to their impact on the pelagic system and point out some of the inconsistencies hidden in certain formulations we conclude that lower boundary types in which sediment fluxes or concentrations are imposed level 1 are especially badly designed because they fail to assure conservation of mass finally we suggest as best choice a level 3 approach in which the evolution of sedimentary particulate matter is part of the solution and where the bottom fluxes of dissolved constituents are parameterised based on mass budget considerations these simplified formulations represent the best balance between computational demand and attained accuracy

timing and characteristics of late pleistocene and holocene wetter periods in the eastern desert and sinai of egypt based on 14c dating and stable isotope analysis of spring tufa deposits abstract there is very little dated evidence on wet periods in the eastern desert and sinai peninsula of egypt during the late pleistocene and holocene to obtain such information we have studied the petrography isotope geochemistry and ams radiocarbon ages of mostly relict tufas deposited by springs draining perched ground water bodies in metamorphic and volcanic rocks the tufas unconformably overly precambrian basic igneous rocks basalt diabase and gabbro as the ages of tufa carbonate are frequently older than the true ages of the deposits because of the incorporation of old 14c dead carbon we have dated both the carbonate matrix and insoluble organic material of the tufas these ages show that the tufas were largely formed during two broad time periods the most recent from 12 058 to 6678 cal yr bp african humid period and the other from 31 200 22 500 cal yr bp with preferential growth during the coldest times of this period namely during heinrich events 2 and 3 h2 and h3 and the last glacial maximum lgm the time span between 19 000 9000 cal yr bp including the yd and h1 appears to have been relatively more arid than the earlier lgm or h2 periods or the later holocene the late pleistocene tufas are depleted in 18o relative to the holocene tufas and were deposited at a lower temperature 14 0 20 8 c vs 18 4 23 4 c we believe that the holocene tufas in the sinai were formed by rainfall from the mediterranean and those in the southern part of the eastern desert by african monsoon rainfall derived from the red sea gulf of aden and indian ocean in contrast the moisture that fed the late pleistocene tufas which are depleted in 18o relative to holocene deposits and progressively depleted from north to south was probably brought by the westerlies from the atlantic mediterranean sea when the westerly circulation was pushed southwards during the coldest periods of the late pleistocene periods of tufa deposition correlate with major documented paleoclimatic events in north africa during the late pleistocene and holocene such as the nile floods high sea level and the formation of sapropels in the mediterranean

continuous gps measurements of postglacial adjustment in fennoscandia 1 geodetic results 1 data collected under the auspices of the bifrost gps project yield a geographically dense suite of estimates of present day three dimensional 3 d crustal deformation rates in fennoscandia johansson et al 2002 a preliminary forward analysis of these estimates milne et al 2001 has indicated that models of ongoing glacial isostatic adjustment gia in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3 d velocity field in this study we revisit our previous gia analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the bifrost rate estimates to establish insight into the physics of the gia response in the region we begin by decomposing a forward prediction into the three contributions associated with the ice ocean and rotational forcings from this analysis we demonstrate that recent advances in postglacial sea level theory in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin involve peak signals that are larger than the observational uncertainties in the bifrost network the forward analysis is completed by presenting predictions for a pair of fennoscandian ice histories and an extensive suite of viscoelastic earth models the former indicates that the bifrost data set provides a powerful discriminant of such histories the latter yields bounds on the assumed constant upper and lower mantle viscosity nu um nu lm specifically we derive a 95 confidence interval of 5 x 10 20 less than or equal to nu um less than or equal to 10 21 pa s and 5 x 10 21 less than or equal to nu lm less than or equal to 5 x 10 22 pa s with some preference for elastic lithospheric thickness in excess of 100 km the main goal of the bayesian inverse analysis is to estimate the radial resolving power of the bifrost gps data as a function of depth in the mantle assuming a reasonably accurate ice history we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle we conclude that the bifrost data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer however these data provide little constraint on viscosity in the bottom half of the mantle finally elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

glacial isostatic adjustment and the radial viscosity profile from inverse modeling 1 a formal inverse procedure is used to infer radial mantle viscosity profiles from several observations related to the glacial isostatic adjustment process the data sets consist of late pleistocene and holocene sea level data from scandinavia the barents sea central europe canada and the far field as well as observations of changes in the earth s rotation and gravitational field and present day uplift and gravity changes in scandinavia inferences of mantle viscosity are robust against assumptions such as the a priori viscosity model and model discretization however the quality of ice sheet reconstruction remains crucial for the inverse inference the importance to discuss regional mantle viscosity models in view of the lateral variability in mantle properties has been evident our inference suggests a two order of magnitude increase of mantle viscosity with depth and volume averaged upper and lower mantle viscosities around 7 1020 and 2 1022 pa s respectively mantle viscosity does not need to increase sharply across the 660 km seismic discontinuity the viscosity profiles suggested are also able to reconcile the large scale geoid anomaly related to mantle convection

multi emission luminescence dating of heated chert from the middle stone age sequence at sodmein cave red sea mountains egypt sodmein cave in egypt is one of the rare archaeological sites in north eastern africa conserving human occupation remains of a period most relevant for the out of africa ii hypothesis this underlines the need for establishing a chronological framework for the more than 4 m of stratified sediments ranging from the middle stone age msa to the neolithic the lowest layer j hosts huge fireplaces from which we report luminescence ages of heated chert fragments unearthed from different depths the multi emission dating approach using both the blue and red tl of each specimen as well as the osl emission of one sample allowed identifying the most reliable ages samples yield ages between 121 15 ka maximum age and 87 9 ka these data evidence human presence at the site during mis 5 for integrating sodmein cave into the actual discussion of the dispersal patterns of modern humans and to identify potential connections with other sites in the nile valley and in the middle east a sound and reliable chronology is indispensable

dynamic topography above retreating subduction zones dynamic topography provides a measure of stresses within the earth s interior dense slabs induce an upper mantle flow that deflects the surface of the earth downward above them by combining a simple theoretical stokeslet model of subduction gravity modeling and seismic tomography i show that a significant fraction as much as 2000 m of the topographic variations observed above the scotia mariana and hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction

glacial isostatic adjustment of scandinavia and northwestern europe and the radial viscosity structure of the earth s mantle summary during the last glacial maximum lgm large ice sheets covered scandinavia the barents sea and the northern british isles subsequent to the lgm the ice sheets disappeared and the solid earth readjusted towards a new isostatic equilibrium the glacial isostatic adjustment process is documented in numerous observations for example palaeoshorelines covering the last deglaciation phase and ongoing crustal deformations monitored by gps stations for example the bifrost project in this study we use palaeoshoreline data from scandinavia the barents sea and northwestern europe nw as well as radial crustal velocities from the bifrost campaign to infer the radial viscosity structure of the earth s mantle underneath scandinavia and nw europe a global inverse procedure based on the neighbourhood algorithm allows us to explore the hypothesis of a low viscosity zone in the upper mantle which has been proposed in the literature our results indicate a low viscosity zone underneath the barents sea with viscosities between 1019 and 1020 pa s in a depth interval of 160 200 km no such low viscosity zone is found underneath scandinavia and no clear indication for such a zone underneath nw europe the thickness of the rheological lithosphere increases from 60 to 70 km underneath nw europe and the barents sea towards values exceeding 120 km underneath scandinavia

combination of remote sensing gis and palaeohydrologic remarks for promoting the exploitation of water resources in the sahara cases from the red sea coast egypt the current work presents an integrated approach to maximize groundwater exploitation under sahara conditions the approach includes investigation of palaeohydrology remote sensing data field verification and geophysical datasets gravity aeromagnetic and geoloelectrical data results reveal the existence of palaeohydrologic remarks such as palaeodriange terraces tufas gorges and alluvial fans the hydrogeological investigation reflects that the quaternary groundwater existed under free water table conditions with water depths range between 5 06 and 50 m from the ground while the miocene aquifer has water depths vary between 22 m of shallow wells and 108 m of deep wells the study illustrates the presence of the tectonic fallen zones tfz where the precambrian rocks show throw down blocks associated with a set of faults and covered by thick sedimentary succession the tfz with their faults nw se and ne sw are attributed to the regional red sea rift where these faults can provide groundwater recharge opportunities the groundwater occurrences in the tfz indicate saturation thickness ranges between 75 and 175 m for miocene aquifer and between 50 and 180 m for nubian sandstone aquifer a total of 22 basins were analyzed where their morphometric parameters indicate the possibility of groundwater accumulations the obtained stable isotopes compositions of hydrogen d and oxygen 18o reveal that aquifers were recharged throughout the previous wetter climate with reasonable contributions during intervening arid periods due to the continued and urgent need for water in the sahara the obtained results can be utilized to develop applied recommendations for future groundwater exploration

improved interpretation of wireline pressure data modern wireline pressure data can have resolution and reproducibility sufficient to detect small fluid density changes and pressure barriers yet these features are commonly overlooked on conventional pressure depth plots the large pressure variation caused by weight of subsurface fluids hides these subtle features excess pressure is the pressure left after subtracting the weight of a fluid from the total pressure this concept is applied to wireline pressure data to remove effects of weight and emphasize subtle pressure differences caused by density variations and pressure barriers fluid density changes of 0 02 g cm3 or less can be resolved and within well pressure barriers in the order of 5 kpa 0 7 psi can be detected using good quality data effects of reservoir capillary displacement pressure can be detected by offset of the free water level from the petroleum water contact this effect can be used to estimate reservoir wettability subsurface fluid density measurements can also be used to evaluate oil or gas quality on a bed by bed scale in traps having variable oil or gas composition to detect compartmentalization by small petroleum density differences to verify quality of samples for pvt pressure volume temperature analysis and estimate salinity or temperature of unsampled water zones data quality limits barrier and fluid contact resolution thus quality control is essential pressure measurement errors on the 3 kpa 0 5 psi scale can be detected from behavior of the buildup pressure tests having the potential for small amounts of supercharge are identified from the overbalance and formation mobility examples illustrate identification of free water levels and fluid contacts fluid identification supercharge identification and water zone compartmentalization

mantle flow due to internal vertical forces abstract morgan s and blake and chwang s image system gives the flow due to a point force in a viscous fluid in the neighbourhood of a plane rigid wall i show that it can provide a good simple approximation for the flow in the earth s mantle and the forces on the surface plates in two cases of geophysical interest the uplift associated with cessation of subduction and the ridge push driving mechanism for plate tectonics for the latter case the method overcomes difficulties due to gibbs phenomenon in the alternative method of fourier analysis the method also predicts the distortion of a sinking slab detached from its oceanic plate if the viscosities in it and outside it are the same but viscosity contrasts are more likely to make the conclusions unreliable than in the other cases

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

holocene environment and subsistence patterns near the tree shelter red sea mountains egypt abstract the tree shelter site dates to the early to mid holocene 8000 to 4900 14c yr bp present conditions around the site are hyperarid but charcoal remains indicate less severe aridity at the time of its occupation the environment around the site then supported a rich wadi vegetation which allowed hunting during the epipaleolithic and herding during the neolithic occupation although more favorable than today the environmental conditions also displayed a desert character and seem to have limited the range of domestic herbivores introduced in the area

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

dynamical causes for incipient magma chambers above slabs using a regional upper mantle model with an unprecedented spatial resolution of 100 m we have investigated at multiple resolutions the character of incipient magma chambers forming under oceanic arcs the magma chambers are formed from wave like structures propagating upward along descending slabs and consist of compositionally buoyant hydrated partially molten subducted crustal and mantle material these wave structures are 300 500 c colder than the mantle wedge and may have an upward velocity of 1 m yr inverted temperature structures and transitory bimodal magmatism are plausible consequences of finger like penetration of relatively cold hydrated material of the incipient magma chambers into the hot mantle wedge apart from forming and periodic feeding of the magma chambers cold waves may also transport upward thousands of cubic kilometers of subducted material and may cause the rapid exhumation of ultrahigh pressure rocks along slabs

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

geochemical cycles in an ocean general circulation model preindustrial tracer distributions a state of the art report is given of the hamburg model of the oceanic carbon cycle the model advects geochemical tracers important to the carbon cycle by the currents of a general circulation model the geochemical cycling is driven by a michaelis menten type production kinetics the model is an extension of the bacastow and maier reimer 1990 model it is based on a more realistic current field and includes a mechanism of lysocline sediment interaction principal variables are co2 alkalinity phosphate oxygen and silicate the carbon variables are defined for 12c 13c and 14c separately in addition to these carbon isotopes 39a and 18o of dissolved oxygen are predicted the model predicts realistic global patterns of tracer distribution in the equatorial eastern pacific however the structures are exaggerated due to a strong upwelling which is a common feature of coarse resolution models of the general circulation of the ocean

quantifying biological carbon export for the northwest north atlantic continental shelves 1 it has been suggested that continental shelf systems contribute disproportionately to the oceanic uptake of atmospheric co2 but the magnitude of this flux and the relative contributions of different underlying mechanisms are poorly quantified a biological continental shelf pump mechanism has been implied however the magnitude of this export depends on advective transport of carbon rich water off the shelf a process that is difficult to observe directly here we use a physical biogeochemical model for the northeastern north american continental margin to estimate the uptake of atmospheric co2 the fraction of this uptake that results from biological processes and the transport of organic carbon off the shelf our results suggest that there is no systematic difference in the area normalized co2 uptake between the shelf regions and the adjacent deep ocean the advective transport of carbon rich water off the shelf is insufficient to drive a continental shelf pump in this region

multiscale dynamics of the tonga kermadec subduction zone our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle while geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle the influence of short wavelength variations in viscosity is still poorly understood we present 2 d and 3 d finite element models of mantle flow including strong lateral viscosity variations and local sources of buoyancy owing to both thermal and compositional effects we first use generic 2 d models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure in particular the slab and lower mantle viscosity and the presence of a low viscosity region in the mantle wedge above the slab we find that 1 the strain rate provides a strong constraint on the absolute viscosity of the slab 10 23 pa s 2 stress orientation within the slab is sensitive to the relative viscosity of the slab lower mantle and the wedge and 3 the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy in particular the state of stress in the overriding plate changes from compression to extension with the addition of a low viscosity wedge we then use observations of strain rate stress orientation dynamic topography and the geoid for the tonga kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3 d regional dynamic model together these observations are used to develop a self consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics over a broad range of length scales the presence of a low viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc and down dip compression in the shallow portion of the slab these results suggest that a low viscosity wedge plays an important role in controlling the presence of backarc spreading however for a model with a low viscosity and low density region that provides a good fit to the observed topography we find that a reduction of the slab density by a factor of 1 3 relative to the reference density model is required to match the observed geoid these results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long wavelength modelling of the geoid

red sea rifting controls on aquifer distribution constraints from geochemical geophysical and remote sensing data highly productive wells in the central eastern desert of egypt are tapping groundwater in subsided blocks of jurassic to cretaceous sandstone taref formation of the nubian sandstone group and oligocene to miocene sandstone nakheil formation now occurring beneath the red sea coastal plain and within the proximal basement complex aquifer development is related to red sea rifting 1 rifting was accommodated by vertical extensional displacement on preexisting nw se to n s trending faults forming a complex array of half grabens and asymmetric horsts and 2 subsided blocks escaped erosion accompanying the red sea related uplift subsided blocks were identifi ed and verifi ed using satellite data geologic maps and fi eld and geophysical investigations interpretations of very low frequency vlf measurements suggest the faults acted as conduits for ascending groundwater from the subsided aquifers stable isotopic compositions d 19 3 to 53 9 18 o 2 7 to 7 1 of groundwater samples from these aquifers are interpreted as mixtures of fossil up to 70 and modern up to 65 precipitation groundwater volumes in subsided blocks are large within the central eastern desert basement complex alone they are estimated at 3 10 9 m 3 and 10 10 9 m 3 for the

effective elastic thickness variations along the andean margin and their relationship to subduction geometry 1 we present a new map of the spatial variations in effective elastic thickness te along the andes estimated using bouguer coherence the te variations reflect interactions between subducting slab and preexisting terrane structure in the forearc conductive cooling of the continent by the subducting slab exerts primary control on rigidity resulting in te that is highest 40 km where the oceanic lithosphere is oldest and coldest 20 s in the central andes te is relatively low 20 km along the volcanic chain and the altiplano and puna plateaus we interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes a shallow and hot asthenosphere and a very weak lower crust throughout this region east of the plateaus high te delineates underthrusting of the brazilian shield finally north and south of the plateaus flat subduction areas are characterized by high te high shear wave velocity thick thermal lithosphere and low heat flow indicating that continental lithosphere there is thicker colder and stronger on the basis of these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere in particular we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge which control the suction moment responsible for the subduction angle at depths 70 100 km for example when oceanic lithosphere subducts beneath thin continental lithosphere the low viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles however when oceanic lithosphere subducts close or beneath thick and strong continental lithosphere the asthenospheric wedge narrows and corner flow drags high viscosity mantle from the base of the thick 150 km cold continent into the wedge suction forces increase with both narrowing of the wedge and its increasing viscosity we estimate the asthenospheric viscosity underneath thick cold continents to be 1020 pa s sufficient to induce flat subduction later after prolonged hydration and weakening of the continent s base asthenospheric flow into the wedge may resume allowing the slab to sink again into the mantle at normal angles

coefficients for sea surface wind stress heat flux and wind profiles as a function of wind speed and temperature surface layer coefficients for wind profiles wind stress and heat flux in typical open sea conditions are briefly reviewed businger dyer flux gradient relationships and a charnock wind stress formula fit the empirical data and are dimensionally consistent these have been solved by an iterative method and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data

thermoluminescence dating of a middle palaeolithic occupation at sodmein cave red sea mountains egypt abstract the palaeolithic sequence from sodmein cave in the egyptian eastern desert near quseir contains seven stratified archaeological levels from the middle and upper palaeolithic the lowest archaeological level middle palaeolithic level 5 consists of huge fireplaces associated with a lithic assemblage in which nubian levallois technology and bifacial tools are present from this level six fragments from two large burned flint blocks have been dated by thermoluminescence a mean age estimate of 118 8 ka for the six dates all of which fall between 109 and 127 ka puts the use of this hearth at the beginning of oxygen isotope stage 5 most likely during episodes e or d this age is close to that of other egyptian middle palaeolithic sites with very similar lithic assemblages

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

topography of the overriding plate during progressive subduction a dynamic model to explain forearc subsidence overriding plate topography provides constraints on subduction zone geodynamics we investigate its evolution using fully dynamic laboratory models of subduction with techniques of stereoscopic photogrammetry and particle image velocimetry model results show that the topography is characterized by an area of forearc dynamic subsidence with a magnitude scaling to 1 44 3 97 km in nature and a local topographic high between the forearc subsided region and the trench these topographic features rapidly develop during the slab free sinking phase and gradually decrease during the steady state slab rollback phase we propose that they result from the variation of the vertical component of the trench suction force along the subduction zone interface which gradually increases with depth and results from the gradual slab steepening during the initial transient slab sinking phase the downward mantle flow in the nose of the mantle wedge plays a minor role in driving forearc subsidence

numerical simulations of a buoyancy driven coastal countercurrent off vancouver island abstract a three dimensional prognostic numerical model has been developed to study the ocean circulation around vancouver island british columbia in a series of simulations the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of vancouver island the forcing is provided by the fraser river discharge into the strait of georgia an estuarine circulation establishes itself in juan de fuca strait from which a distinctive right bounded current is formed and advances along the coast sensitivity studies are conducted to determine the robustness of this current to initial conditions opposing wind enhanced vertical mixing and grid resolution finally various characteristics of the numerically modeled coastal flow are compared with observations

persistently declining oxygen levels in the interior waters of the eastern subarctic pacific abstract fifty years of measurements at ocean station papa osp 50 n 145 w show trends in the interior waters of the subarctic pacific that are both impacted by short term few years to bi decadal atmospheric or ocean circulation oscillations and by persistent climate trends between 1956 and 2006 waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen on density surfaces found in the depth range 100 400 m 26 3 27 0 the ocean is warming at 0 005 0 012 c y 1 whereas oxygen is declining at 0 39 0 70 mol kg 1 y 1 or at an integrated rate of 123 mmol m 2 y 1 decrease of 22 over 50 years during this time the hypoxic boundary defined as 60 mol o 2 kg 1 has shoaled from 400 to 300 m in the alaska gyre the 26 2 isopycnal occasionally ventilates whereas at osp 26 0 has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify to interpret the 50 year record at osp the isopycnal transport of oxygenated waters within the interior of the subarctic pacific is assessed by using a slightly modified no parameter broecker w 1974 no a conservative water mass tracer earth and planetary science letters 23 100 107 the highest nitrate oxygen signature in interior waters of the north pacific is found in the bering sea gyre western subarctic gyre and east kamchatka current region as a consequence of winter mixing to the 26 6 isopycnal by mixing with low no waters found in the subtropics and okhotsk sea this signature is diluted as waters flow eastward across the pacific evidence of low no waters flowing north from california is seen along the coasts of british columbia and se alaska oxygen in the subsurface waters of the alaskan gyre was supplied 60 by subarctic and 40 by subtropical waters during woce surveys whereas such estimates are shown to periodically vary by 20 at osp other features discernable in the osp data include periods of increased ventilation of deeper isopycnals on an 18 year cycle and strong short term few month variability caused by passing mesoscale eddies the potential impacts of declining oxygen on coastal ecosystems are discussed

modeling the generation of the juan de fuca eddy 1 numerical simulations with the regional ocean modeling system are used to study the generation of the cyclonic juan de fuca eddy located off the entrance of juan de fuca strait in summer an initial simulation forced with average summer upwelling favorable winds tides and buoyancy boundary conditions that maintain an estuarine flow in juan de fuca strait produces an eddy and currents that are in reasonable agreement with observations sensitivity studies are then carried out to explore the importance of these three forcing mechanisms the relative proximity of dense water in the bottom estuarine flow entering the strait is shown to lead to enhanced upwelling off cape flattery when either wind or tidal forcing is applied this upwelled water then mixes with the estuarine outflow and is advected offshore the tidal upwelling arises through three mechanisms m2 vertical excursions of nearly 20 m at 50 m depth west of the cape on the flood tide strong tidally rectified vertical velocities west of the cape and the spilling of denser bottom water over the western wall of juan de fuca canyon on the ebb tide the cyclonic eddy is a consequence of geostrophic adjustment to the doming isopycnals that arise from the upwelling these model simulations refute an earlier hypothesis that the eddy is generated when california undercurrent water is drawn up tully canyon and onto the vancouver island shelf suggesting instead that these canyon dynamics play only a secondary role in maintaining the eddy once it is formed

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins