circulation about the mediterranean tongue an analysis of an eof based model ocean abstract absolute circulation about the mediterranean salt tongue is examined using a combined box spiral inverse model applied to a model ocean constructed from a limited number of degrees of freedom the region of analysis is inside a square box 12 on each side centered about 30 n 25 w the resulting flow field is consistent with other recent analyses of the region but demonstrates that no mixing is required to obtain a consistent picture the circulation consists of an anticyclonic flow order 1 cm s 1 above the thermocline and a cyclonic flow order 1 mm s 1 near the core of the mediterranean water these findings imply that an efficient description of the hydrographic structure of the ocean retains essential features of the ocean from which circulation can be determined

terminal pleistocene and early holocene archaeology and stratigraphy of the southern nejd oman abstract archaeological geoarchaeological and geochronological research at the terminal pleistocene early holocene sites of al hatab ghazal and khamseen rockshelters help to elucidate our understanding of the late palaeolithic occupation of the south arabian highlands the stone tools found at these sites are attributed to the nejd leptolithic tradition an autochthonous lithic culture found across dhofar oman the artifact bearing deposits excavated within these rockshelters have been chronologically constrained by optically stimulated luminescence dating while sedimentological analyses help to establish site formation processes together these methods provide a chronological anchor for some nejd leptolithic archaeological findspots across dhofar the archaeological evidence from surface and stratified sites indicates a technological continuum across the shifting climatic regimes of the terminal pleistocene early holocene this technological continuity points to possible population persistence within one of the posited south arabian refugia

palaeoenvironmental signals in ancient urban setting the heavy rainfall record in sumhuram a pre islamic archaeological site of dhofar s oman a case study of an archaeological section in the pre islamic town of sumhuram third century bc fifth century ad dhofar s oman is presented sedimentological and palynological data have been integrated with the aim of finding signals of natural processes within an urban context two main types of deposits alternate in the section anthropogenic ad and waterlain wd the presence of laminated sediments within the wd deposits suggesting short term street flooding from heavy rainfall encouraged the development of a targeted palynological sampling strategy the results of the pollen analysis in these laminated sediments e g high pollen concentrations particularly of wet environment plants clumped pollen grains long distance pollen grains and overall good state of preservation suggest a stronger monsoonal influence than the present and high precipitation in s oman between 204 bc and ad 130

south asian monsoon climate change and radiocarbon in the arabian sea during early and middle holocene the 1 4 c ages of planktonic foraminifers globigerinoides sacculifer bracketing the younger dryas in a 1 8 o record of globigerinoides ruber from a laminated sediment core on the pakistani continental margin suggest thatsurface reservoir ages in the arabian sea were in excess of 1000 years during the deglaciation a least squares error fit of a detailed 1 4 c chronology to the atmospheric tree ring record gave variable early holocene reservoir ages between 780 and 1120 years well above the prebomb value of 640 years mid holocene reservoir ages are less well constrained but were probably closer to the prebomb value the method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans which in the core presented here were taken from each section s range of 1 4 c ages a significant 220 year quasi oscillation was present in the 1 8 o record during the early holocene but not thereafter this frequency and amplitude pattern resembles an early holocene 207 to 227 year oscillation previously observed in the atmospheric 1 4 c record which is generally interpreted as reflecting solar irradiance variability an early holocene climate event at 8150 8400 calendar years b p observed elsewhere within the asian east african monsoon system was again found in our record suggesting a reduction in precipitation over nw india and pakistan

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

integral measurements of mass transport and heat content in the strait of gibraltar from acoustic transmissions three view the mathml sourceacoustic transceivers were deployed on either side of the eastern entrance of the strait of gibraltar during april may 1996 to determine the feasibility of using acoustic methods to make routine rapidly repeated horizontally integrated measurements of flow and temperature in straits reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths transmissions directly across the strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path the geometry was selected to provide ray paths that only sample the lower layer mediterranean water so that the feasibility of monitoring the mediterranean outflow using the various methods could be evaluated the acoustic scintillation method did not yield useful current estimates but the experimental parameters were not optimized for this approach since the low frequency variability in log amplitude was found to be highly correlated at receivers view the mathml source apart it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results the horizontal deflection method gave encouraging results at the time of neap tides but less so during spring tides for this approach both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal wave induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured further work is needed to determine if a smaller horizontal spacing and higher signal to noise ratios would yield better results reciprocal travel time measurements diagonally across the strait performed the best of the three methods giving absolute flow estimates consistent with those derived from current meter data the fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0 017 i e 98 of the a priori tidal transport variance was resolved the spatial scales of the sub tidal flow are thought to be significantly shorter than those of the tidal flow however which means that a more elaborate monitoring network is required to achieve the same performance for sub tidal variability finally sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

a new tracer advection scheme for bryan and cox type ocean general circulation models in regions with large tracer gradients and or velocities the advection scheme that is used in many ocean models leads to significant under and overshoot of the tracer values in the case of the u k fine resolution antarctic model fram this lead to unphysical negative surface temperatures in some regions and overheating in others in this paper a new advection scheme is proposed and tested in the ocean model context using a limited area model centered on a region where problems occurred in fram

speleothem evidence from oman for continental pluvial events during interglacial periods growth periods and stable isotope analyses of speleothems from hoti cave in northern oman provide a record of continental pluvial periods extending back over the past four of earth s glacial interglacial cycles rapid speleothem growth occurred during the early to middle holocene 6 10 5 ka b p 78 82 ka b p 120 135 ka b p 180 200 ka b p and 300 325 ka b p the speleothem calcite deposited during each of these episodes is highly depleted in 18 o compared to modern speleothems the d 18 o values for calcite deposited within pluvial periods generally fall in the range of 24 to 28 relative to the vienna peedee belemnite standard whereas modern speleothems range from 21 to 23 the growth and isotopic records indicate that during peak interglacial periods the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record the association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions and not changes in solar radiation are the primary control on continental wetness on glacial interglacial time scales

holocene lakes from ramlat as sab atayn yemen illustrate the impact of monsoon activity in southern arabia abstract paleoecology and paleohydrology of the ramlat as sab atayn southern arabia are reconstructed from a comparative study including sedimentology mineralogy stable isotope ratios of carbonates and palynology of lacustrine sediments recovered from the al hawa depression the section dates from 8700 to 7200 yr b p and records an early phase of flooding followed by distinct lacustrine development from 7800 to 7200 yr b p coeval with maximum activity of the indian monsoon comparison of the pollen record with modern pollen deposition suggests that regional vegetation was then already of desert type and was related to strong seasonal trade winds

late holocene lake levels of the dead sea this work presents a high resolution lake level record of the late holocene dead sea a hypersaline terminal lake whose drainage basin encompasses both mediterranean and hyperarid climatic zones the lake level curve reflects the regional hydrologic variations in the drainage basin which in turn represent the levant paleoclimates the curve is based on 46 radiocarbon ages of organic remains from well exposed sedimentary sequences along the dead sea shores these sequences record fluvial and lacustrine depositional environments the paleolakeshores are marked by shore ridges coarse sand units and aragonite crusts in the modern dead sea such features indicate the exact elevation of the shore the late holocene dead sea level fluctuated within the range of 390 to 415 m below sea level mbsl for most of the time the lake was below the topographic sill 402 mbsl separating the northern and southern basins of the dead sea and was confined to the deep northern basin nevertheless short term rises in the late holocene dead sea level caused the flooding of the shallow and flat southern basin highstands occurred in the second and first centuries b c and the fourth century a d during the roman and early byzan tine periods respectively in the eleventh and twelfth centuries a d during the crusader period and at the end of the nineteenth century a d the rises mark a significant change in the annual rainfall in the region which likely exceeded the instrumentally measured modern average the curve also indicates drastic drops that exposed the sedimentary sequences to erosion the oldest and probably deepest drop in the lake level culminated during the fifteenth and fourteenth centuries b c after a retreat from a higher lake stand the longest lowstand occurred after the byzantine period and continued at least until the ninth century a d this arid period coincided with the invasion of moslem arab tribes into the area during the seventh century a d the dramatic fall of the dead sea level during the twentieth century is primarily artificial and has been caused by the diversion of runoff water for the drainage basin but the magnitude is not considered exceptional for the late holocene although the past drops in the lake never exceeded the modern artificial drop rates they do represent extreme arid conditions that occurred frequently over the past several thousand years

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

numerical modelling of tidally generated internal wave radiation from the andaman sea into the bay of bengal abstract semi diurnal internal waves generated by tides in a high resolution numerical model that includes the andaman sea archipelago are found to propagate into the central bay of bengal and reach the coasts of india and sri lanka the waves are also present in subsurface temperature records from rama moorings and their propagation speed across the bay of bengal agrees well with satellite remote sensing from modis imagery in spite of the hydrostatic nature of the model the internal waves are simulated by a fully coupled ocean atmosphere prediction system exchanging surface fluxes between the air and sea at high frequency and at high resolution for the ocean a hydrostatic model including diurnal and semi diurnal tides provides a 2 km resolution representation of the entire bay of bengal in the ocean model simulations the semi diurnal internal waves interact with the mesoscale circulation and surface waves and modify the flow and the stratification by comparing coupled ocean wave model runs with tides and without tides but forced by identical surface fluxes from the atmosphere it is demonstrated that the inclusion of diurnal and semi diurnal tides act to cool the core of the thermocline while increasing the temperature above and below it along the pathway of the internal waves a result that likely is due to vertical mixing by the waves

a multi proxy analysis of the holocene humid phase from the united arab emirates and its implications for southeast arabia s neolithic populations an early to mid holocene humid phase has been identified in various arabian geo archives although significant regional heterogeneity has been reported in the onset duration and stability of this period a multi proxy lake and dune record from wahalah in the united arab emirates uae documents significant variations in hydrology biological productivity and landscape stability during the first half of the holocene these data reveal that post last glacial maximum dune emplacement continued into the earliest part of the holocene with the onset of permanent lacustrine sedimentation at the site commencing 8 5 ka cal bp a long term shift towards more arid conditions is inferred between 7 8 and 5 9 ka cal bp with intermittent flooding of the basin and distinct phases of instability throughout the catchment area this transition is linked to the southwards migration of the intertropical convergence zone itcz and associated weakening of monsoon rains a peak in landscape instability is recorded between 5 9 and 5 3 ka cal bp and is marked by a pronounced increase in regional dune emplacement these variations are considered alongside the record of human settlement raising important questions about the interactions between population demographics climate and environment in southeast arabia during the neolithic

reactivation of the pleistocene trans arabian wadi ad dawasir fluvial system saudi arabia during the holocene humid phase the wadi ad dawasir fluvial system in central saudi arabia is investigated using remote sensing and sedimentology in combination with bio proxy analyses molluscs and ostracods age control is provided by radiocarbon as well as luminescence dating using both quartz and feldspar grains it is shown that the fluvial system was active from the asir mountains across the partially sand covered interior of the arabian peninsula to the arabian gulf during the holocene humid period sedimentology and faunal analysis reveal the presence of perennial streams and a permanent freshwater lake in the distal reach of the dawasir system that are synchronous with fluvial accumulation in the headwaters of its major tributary wadi tathlith the increased runoff during the holocene led to a re activation of streams that largely followed pre existing late pleistocene courses and eroded into older sediments the absence of holocene lakes in most of the rub al khali implies that trans arabian rivers were mainly fed by precipitation in the asir mountains monsoonal rainfall was apparently stronger there as well as in the northern south eastern and southern part of the arabian peninsula southern yemen and oman but it apparently did not directly affect the interior during the holocene the palaeoenvironmental reconstruction shows a narrow trans arabian green freshwater corridor as the result of phases of sustained flow lasting up to several centuries the permanent availability of water and subsistence for wildlife provided a favourable environment for human occupation as documented by neolithic stone tools that are found all along wadi ad dawasir

paleoclimatic investigations using isotopic signatures of the late pleistocene holocene groundwater of the stratified aquifers in kuwait abstract the study attempts to determine paleo recharge sources for two major aquifer units of kuwait namely kuwait group aquifer kga and dammam formation dfm were collected and analyzed for major ions and isotopes the obtained results of 14c ages indicated that the groundwater samples ranged from 31 9 ky to 3 9 ky in dfm and 23 3 ky to 0 8 ky in kga the variations in 18o signatures with respect to age reflected that there were two major sources of rainwater such as enriched southern indian ocean monsoon and the depleted northwesterly mediterranean vapor source the samples found to show variations during the three marine isotopic stages mis 1 3 heinrich events bolling allerod and dryas time periods in both aquifers a shift in the intertropical convergence zone itcz was witnessed in the groundwater of both aquifers from 7 8 ky to 4 9 ky the variation in the holocene climate is mainly due to the influence of tropospheric easterly jet governed by the swing in itcz as a result of holocene volcanism the mean 18o values of dfm samples 3 81 reflect depletion than kga samples 2 31 and the d excess values were 0 95 and 2 05 respectively thus it could be inferred that the dfm was predominantly recharged during late pleistocene period and it was in a relatively cooler climate than recharge environment of kga the study infers that kga was recharged between 26on and 26 8on latitudes and that of the dfm was noted to be recharged south west of kuwait in two different patches between 25 8on and 26 2on and 26 7on 27 4on latitudes during late pleistocene the kga was concluded to be recharged between 25 5on and 27 5on latitudes south of kuwait during holocene

global ocean circulation during 1992 1997 estimated from ocean observations and a general circulation model 1 a three dimensional oceanic state is estimated for the period 1992 1997 as it results from combining large scale ocean data sets with a general circulation model at the cost of increased computational load the estimation assimilation method is chosen specifically so that the resulting state estimate is consistent with the model equations having no artificial sources or sinks to bring the model into close agreement with observations its initial temperature and salinity conditions are permitted to change as are the time dependent surface fluxes of momentum heat and freshwater resulting changes of these control vectors are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses the assimilation procedure is able to correct for many of the traditional shortcomings of the flow field by changing the surface boundary conditions changes in the resulting flow field are predominantly on the gyre scale and affect many features that are often poorly simulated in traditional numerical simulations such as the strengths of the gulf stream and its extension the azores current and the anticyclonic circulation associated with the labrador sea tests of the results and their consistency with prior error assumptions show that the constrained model has moved considerably closer to the observations imposed as constraints but has also moved closer to independent data from the world ocean circulation experiment not used in the assimilation procedure in some regions where the comparisons remain indeterminate not enough ocean observations are available and it is difficult to ascribe the residuals to either the model or the observations although problems remain a useful first solution to the global time dependent ocean state estimation problem has been found the estimates will continue to improve through the evolution of numerical models computer power increases more data and more efficient estimation methods

holocene soils and sediments around ma rib oasis yemen further sabaean treasures the ancient cultures of southern arabia are increasingly recognised as playing as major a role in the heritage of mankind as the early cultures of egypt mesopotamia or the indus valley the beginning of the widely known sabaean culture dates back to the end of the second millennium bc whereas undoubtedly its wealth came mainly from the trade along the incense road the backbone of its economy was irrigated agriculture since agriculture is based on soil and water resources and hence land availability the buried soils and sediments of the area surrounding the ma rib oasis have been investigated both as an archive of holocene soil development in pre sabaean times and as natural treasures as for example ores or alabaster are defined the natural buried holocene soils around ma rib are rich in phosphate organic material and volcanic ashes in a few places they demonstrate cultivation before the great dam of ma rib was built in the first millennium bc most important are those soils that formed d

epipalaeolithic occupation and palaeoenvironments of the southern nefud desert saudi arabia during the terminal pleistocene and early holocene the transition from the terminal pleistocene to the early holocene is poorly represented in the geological and archaeological records of northern arabia and the climatic conditions that prevailed in the region during that period are unclear here we present a new record from the site of al rabyah in the jubbah basin southern nefud desert saudi arabia where a sequence of fossiliferous lacustrine and palustrine deposits containing an archaeological assemblage is preserved sedimentological and palaeoenvironmental investigations both at al rabyah and elsewhere in the jubbah area indicate phases of humid conditions during which shallow lakes developed in the basin separated by drier periods at al rabyah the end of a terminal pleistocene phase of lake expansion has been dated to 12 2 ka using optically stimulated luminescence osl with a mid holocene humid phase dated to after 6 6 ka palaeoecological reconstructions based primarily on non marine molluscs and ostracods from the younger lacustrine deposits indicate a relatively shallow body of freshwater surrounded by moist well vegetated environments a lithic assemblage characterized by bladelets and geometric microliths was excavated from sediments attributed to a drier climatic phase dated to 10 1 ka the lithic artefact types exhibit similarities to epipalaeolithic industries of the levant and their occurrence well beyond the core region of such assemblages and at a significantly later date has important implications for understanding interactions between levantine and arabian populations during the terminal pleistocene early holocene we suggest that the presence of foraging populations in the southern nefud during periods of drier climate is due to the prolonged presence of a freshwater oasis in the jubbah basin during the terminal pleistocene early holocene which enabled them to subsist in the region when neighbouring areas of northern arabia and the levant were increasingly hostile

reconstructed mediterranean salt lens trajectories the existence of energetic anticyclonic mid depth vortices of mediterranean water meddies questions the validity of a conventional advective diffusive balance in the eastern atlantic subtropical gyre a mesoscale experiment in the azores madeira region reveals a link of these meddies to large scale subsurface meanders for the first time it is shown that meddies may have strong surface vorticity indicative of a generation process involving the azores current a deep reaching near surface jet

a model analysis of the behavior of the mediterranean water in the north atlantic abstract the behavior of the mediterranean water in the north atlantic ocean sector of a global ocean general circulation model is explored starting from its entry point at the strait of gibraltar the analysis focuses primarily on one experiment in which explicit watermass exchange between the mediterranean sea and the atlantic at the strait of gibraltar is permitted the model produces an exchange rate of approximately 1 sv sv 106 m3 s 1 this is comparable to estimates derived from field measurements the density of the mediterranean outflow however is lower than observed mainly because of its high temperature more than 2 c higher than in reality the lower density of the outflow and the model s inadequate representation of the entrainment mixing in the outflow region cause the mediterranean water to settle in a depth range 800 1000 m in the north atlantic about 200 m shallower than observed here an interesting current system forms in response to the intrusion of the mediterranean water i

thermohaline structure of an eddy resolving north atlantic model the influence of boundary conditions abstract a t s volumetric census with a resolution of 0 2 c and 0 1 psu for years 20 25 of the world ocean circulation experiment community modeling effort eddy resolving simulation of the equatorial and north atlantic ocean reveals how the thermohaline character of the model has changed from the initial conditions which were taken from the levitus climatology any changes in the thermohaline structure other than stirring mixing or geostrophic adjustment of smoothed climatology must be due to the boundary conditions which are imposed at the surface and at four sponge layers northern boundary southern boundary labrador sea and mediterranean sea where water temperature and salinity are nudged toward climatological conditions several unrealistic thermohaline features appear in the solution which can be traced to these surface and lateral sponge boundary conditions 1 water masses from the arctic ocean are overrepresented in the model the volume transport across the northern sponge is twice

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

early to middle pleistocene homo dispersals from africa to eurasia geological climatic and environmental constraints abstract although the pleistocene human dispersal is one of the most intriguing issues of paleoanthropology large sectors along possible migratory pathways are ill defined or poorly documented this paper examines the geological geomorphological climatic and environmental conditions that could have determined hominin transit from their motherland in the east africa rift valley toward the circummediterranean regions or arabian peninsula for their choice between these two alternatives the danakil depression afar rift immediately north of the ethiopian rift valley was a strategic area they could reach the mediterranean sea through a northern route along the nile river valley the trans sahara megalake or riverine belts or else navigate an eastern route into the arabian peninsula after crossing the bab el mandeb straits it is assumed all of these pathways were possible during convenient geological climatic and or sea level conditions hominins had three additional choices once they arrived at the mediterranean sea shores with eurasia as their final destination the levant corridor was the easiest because walkable and almost always under favorable conditions two other possibilities were the sicily channel and the strait of gibraltar these sea arms are located in two extremely active settings of the mediterranean sea and were at their embryonic stage during the early pleistocene because large scale deformations of the sea floor were associated to prolonged sea level lowstands these two sea arms could have been passable similar considerations apply to the bab el mandeb straits region for the trans mediterranean and the trans red sea routes of dispersal the environmental conditions and the submarine and terrestrial topographical features that could have made sea transit possible even without land bridges are underscored to track the dispersal relations between northern africa and eurasia three schematic maps are presented to show the distribution of the archeological and paleoanthropological sites currently regarded as reliably dated they document three groups of dates between 2 0 and 1 5 ma 1 4 and 0 7 ma and 0 6 and 0 1 ma their distribution is matched with coeval paleoclimatic proxies such as marine oxygen isotope stages aeolian dust in the atlantic ocean and sapropels levels and dust fluxes in the eastern mediterranean dispersal was temporally arranged into cycles of four major exodus waves 2 0 1 6 ma 1 4 1 2 ma 1 0 0 8 ma and 0 6 0 1 ma controlled by climatic and environmental changes

the paradox of tropical karst morphology in the coral reefs of the arid middle east despite differences in reef growth between the arabian gulf and the red sea a common distinctive pattern of polygonal sills surrounding ponded depressions consistently occurs in shallow water viewed from a satellite these seafloors are reticulated and maze like despite little current rainfall this patterning is best explained by karst dissolution of limestone during periods of lower sea level this is a paradox since such fine scale karstification is confined to areas with considerably more precipitation than currently observed in arabia we resolve this apparent contradiction by developing a pleistocene holocene chronology of sea level and climate for the red sea and arabian gulf and through the use of pattern analysis and computer simulation reveal the mechanism of formation for these structures we demonstrate that this patterning can be taken as a quaternary signature of paleohumidity in the now hyperarid red sea and arabian gulf

unravelling the influence of water depth and wave energy on the facies diversity of shelf carbonates carbonate sequence stratigraphy is founded on the principle that changes in relative sea level are recorded in the rock record by the accumulation of sediment with relative water depth dependent attributes while at the scale of a shelf to basin transect facies clearly arrange by water depth the relation blurs for depths 40 m the most vigorous zone of carbonate production the reason for this change in behaviour is two fold firstly in shallow water the intrinsic processes of storm and wave reworking influence the seabed through submarine erosion and sediment redistribution secondly facies diversity tends to be greater in shallow water than deep water because of a greater diversity in grain producers remote sensing imagery field observations and hydrodynamic models for two reef rimmed shore attached carbonate platforms in the red sea show neither water depth nor energy regime to be reliable indicators of facies type when considered in isolation considered together however the predictive power of the two variables rises significantly the results demonstrate it to be an oversimplification to assume a direct link between palaeo water depth and depositional diversity of subtidal lithofacies while highlighting the importance of hydrodynamics in directing the accumulation of carbonate sediments in the shallow photic zone while the size distributions of facies extents in the two focus areas follow power laws no direct relation between the lateral continuity of the facies belts and water depth or wave height is reported the work is relevant for the interpretation of metre scale subtidal carbonate cycles throughout the geological record by demonstrating how caution must be applied when inferring palaeowater depths from depositional facies

holocene climate and cultural evolution in late prehistoric early historic west asia abstract the precipitation climatology and the underlying climate mechanisms of the eastern mediterranean west asia and the indian subcontinent are reviewed with emphasis on upper and middle tropospheric flow in the subtropics and its steering of precipitation holocene climate change of the region is summarized from proxy records the indian monsoon weakened during the holocene over its northernmost region the ganges and indus catchments and the western arabian sea southern regions the indian peninsula do not show a reduction but an increase of summer monsoon rain across the holocene the long term trend towards drier conditions in the eastern mediterranean can be linked to a regionally complex monsoon evolution abrupt climate change events such as the widespread droughts around 8200 5200 and 4200 cal yr bp are suggested to be the result of altered subtropical upper level flow over the eastern mediterranean and asia the abrupt climate change events of the holocene radically altered precipitation fundamental for cereal agriculture across the expanse of late prehistoric early historic cultures known from the archaeological record in these regions social adaptations to reduced agro production in both dry farming and irrigation agriculture regions are visible in the archaeological record during each abrupt climate change event in west asia chronological refinement in both the paleoclimate and archaeological records and transfer functions for both precipitation and agro production are needed to understand precisely the evident causal linkages

geochemical evidence for a subducted infant arc in franciscan high grade metamorphic tectonic blocks some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites as validated by the geochemical data presented from high grade metamorphic tectonic blocks from the franciscan complex in central california combining major elements compatible and incompatible trace element concentrations and ratios and sr nd pb isotopic data we conclude that the protoliths of these high grade franciscan blocks were arc lavas with no continental crust derived components although the protoliths underwent high grade metamorphism at the inception of subduction the tectonic affi nity of the protoliths can be distinguished by our geochemical study our geochemical data are signifi cant in that 1 the protoliths of these high grade blocks have a different mode of origin than the protoliths of the lower grade franciscan basalts that were incorporated later in the subduction complex as the latter generally are considered to be oceanic island basalts oib and derived from mid oceanicridge basalt morb and 2 the protoliths of the high grade blocks were formed over a pre franciscan subduction zone based on the geochemical evidence for an arc origin and the available geochronological data we suggest that protoliths of the highgrade tectonic blocks of the franciscan complex and the coast range ophiolite formed in the same infant arc setting initiation of franciscan subduction in this arc crust led to the formation of the high grade blocks beneath the young coast range ophiolite continued subduction resulted in subduction of older oceanic crust of morb and oib origin some of which was scraped off to form lower grade metabasites within the franciscan complex

atmospheric summer circulation and coastal upwelling in the arabian sea during the holocene and the last glaciation accumulation rates of biogenic and lithogenic components were studied in 39 turbidite free well dated sediment cores from the northern indian ocean to define the proportions of fluvial and eolian input and to reconstruct quaternary patterns of coastal upwelling the majority of dust deposited in the western arabian sea during the holocene about 100 106 t yr 1 is advected from arabia by northwesterly winds which overlie the low level southwest monsoon the glacial increase in dust flux to 160 106 t yr 1 culminated in the northern arabian sea most probably due to i entrainment of dust rich in chlorite dolomite and lithogenic carbonate in the then dry persian gulf and ii a southward shift of the mean position of the southwest monsoon during glacial summer this shift is recorded in reduced accumulation rates of biogenic opal and increased rates of marine carbonate off somalia and oman both the terrigenous and biogenic sediment records show that the northwesterly winds and the southwest monsoon persisted over the last 27 000 yr as well as the asian continental summer heat low however the glacial seasonal time span of the southwest monsoon season was much reduced most likely because of a delay in the seasonal onset of the southwest monsoon

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

a numerical model of the ventilated thermocline abstract a steady state numerical solution is found for an idealized rectangular ocean basin driven by wind and surface buoyancy flux a three dimensional primitive equation model is used in agreement with recent analytical modeling the thermocline in the numerical solution consists of three regions quite distinct in their ventilation characteristics forming the greater part of the subtropical thermocline is an unventilated pool zone located in the core of the subtropical gyre and a ventilated zone to the east the unventilated shadow zone lies farther east and toward the equator analysis of potential vorticity on constant density surfaces is used to study the structure of the thermocline a small but intense zone of convection located in the western boundary outflow caused by rapid heat loss to the atmosphere produces source water for the ventilated zone this water of extremely low potential vorticity mode water is distributed widely into the subtropical thermocline the pool forms equ

tracking three meddies with sofar floats abstract three meddies were tracked for up to two years in the canary basin using neutrally buoyant sofar floats these meddies have cores of warm salty mediterranean water and are approximately 100 km in diameter 800 m thick and are centered at a depth of 1100 m meddy 1 was tracked for two years 1984 86 with five floats as it drifted 1090 km southward with a mean velocity of 1 8 cm s 1 four shipboard surveys made during these two years revealed the nearly total decay of meddy 1 by gradual mixing processes meddy 2 drifted 530 km southwestward over 8 5 months with a mean velocity of 2 3 cm s 1 until it collided with hyeres seamount near 31 n 29 w the floats trapped in this meddy then stopped looping abruptly implying a major disruption of this meddy meddy 3 drifted 500 km southwestward for a year and a half with a mean translation velocity of 1 1 cm s 1 a comparison of the velocity of meddies to the velocity of nearby floats at 1100 m depth outside of the meddies shows clearly that all three m

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

holocene vegetation dynamics in the northeastern rub al khali desert arabian peninsula a phytolith pollen and carbon isotope study the holocene vegetation history of the arabian peninsula is poorly understood with few palaeobotanical studies to date at awafi ras al khaimah uae a 3 3 m lake sediment sequence records the vegetation development for the period 8500 cal yr bp to 3000 cal yr bp 13 c isotope pollen and phytolith analyses indicate that c3 pooid grassland with a strong woody element existed during the early holocene between 8500 and 6000 cal yr bp and became replaced by mixed c3 and c4 grasses with a strong c4 panicoid tall grass element between 5900 and 5400 cal yr bp an intense arid event occurred at 4100 cal yr bp when the lake desiccated and was infilled by aeolian sand from 4100 cal yr bp the vegetation was dominated by c4 chloridoid types and cyperaceae suggesting an incomplete vegetation cover and aeolian dune reactivation owing to increased regio nal aridity these data outline the ecosystem dynamics and carbon cycling in response to palaeomon soon and north westerly variability during the holocene copyright 2004 john wiley sons ltd

late holocene climates of the near east deduced from dead sea level variations and modern regional winter rainfall the dead sea is a terminal lake of one of the largest hydrological systems in the levant and may thus be viewed as a large rain gauge for the region variations of its level are indicative of the climate variations in the region here we present the decadal to centennialresolution holocene lake level curve of the dead sea then we determine the regional hydroclimatology that affected level variations to achieve this goal we compare modern natural lake level variations and instrumental rainfall records and quantify the hydrology relative to lake level rise fall or stability to quantify that relationship under natural conditions rainfall data pre dating the artificial dead sea level drop since the 1960s are used in this respect jerusalem station offers the longest uninterrupted pre 1960s rainfall record and jerusalem rains serve as an adequate proxy for the dead sea headwaters rainfall principal component analysis indicates that temporal variations of annual precipitation in all stations in israel north of the current 200 mm yr 1 average isohyet during 1940 1990 are largely synchronous and in phase 70 of the total variance explained by pc1 this station also represents well northern jordan and the area all the way to beirut lebanon especially during extreme drought and wet spells we a determine the modern and propose the past regional hydrology and eastern mediterranean em climatology that affected the severity and length of droughts wet spells associated with multiyear episodes of dead sea level falls rises and b determine that em cyclone tracks were different in average number and latitude in wet and dry years in jerusalem the mean composite sea level pressure and 500 mb height anomalies indicate that the potential causes for wet and dry episodes span the entire em and are rooted in the larger scale northern hemisphere atmospheric circulation we also identified remarkably close association within radiocarbon resolution between climatic changes in the levant reflected by level changes and culture shifts in this region

early holocene paleosols at the southwestern ramlat as sab atayn desert margin new climate proxies for southern arabia abstract the ramlat as sab atayn desert margin near ma rib yemen displays well preserved early holocene paleosols which are documented by ams 14 c and soil data mollusc shells in bwk and bwkb horizons are dated to the period between 12 5 and 9 5 ka cal bp the buried soils above ahb and abwkb horizons indicate a period of soil formation mainly between 8 8 and 8 3 ka cal bp between 8 3 and 6 7 ka cal bp more weakly developed soils occur and those only locally in contrast to the paleosols sandy cover sediments c horizons without considerable signs of pedogenesis appear due to increasing aridity wind activity and sheet wash after 8 3 ka cal bp these results indicate an arid climate and the formation of cover sediments at the desert margin of yemen in the rain shadow of the yemen highlands after the main period of soil formation which ended around 8 3 ka cal bp at the latest at 6 7 ka cal bp calculating from geochemical data a palaeoprecipitation of about 400 to 600 mm a 1 235 mm a 1 from 12 5 to 8 3 ka bp is proposed accordingly after 8 3 ka cal bp the summer position of the itcz and the associated monsoon rain belt carrying rain to northeastern yemen is assumed not to go beyond 15 n in its whole amplitude

low latitude holocene hydroclimate derived from lake sediment flux and geochemistry this study investigates hydrological responses to climatic shifts using sediment flux data derived from two dated palaeolake records in south east arabia flux values are generally low during the early holocene humid period ehhp 9 0 6 4k cal a bp although several short lived pulses of increased detrital input are recorded the most prominent of which is dated between 8 3 and 7 9k cal a bp the ehhp is separated from the mid holocene humid period mhhp 5 0 4 3k cal a bp by a phase of increased sediment flux and aridity which began between 6 4 and 5 9k cal a bp and peaked between 5 2 and 5 0k cal a bp the termination of the mhhp is marked by a phase of high detrital sediment flux between 4 3 and 3 9k cal a bp while long term shifts in climate are most likely linked to changes in the summer position of the intertropical convergence zone and associated indian and african monsoon systems it is noted that the abrupt short term phases of aridity observed in both records are coeval with intervals of rapid climate change globally which triggered non linear widespread landscape reconfigurations throughout south east arabia

the early holocene humid period in nw saudi arabia sediments microfossils and palaeo hydrological modelling abstract an early to mid holocene humid period has been inferred from a variety of geo bio archives of the sahara and the arabian peninsula including sabkhas palaeo lakes sand dunes wadis speleothems or marine sediments on the arabian peninsula most of these records are located in the southern and southeastern part studies from the northern part are rare this paper presents the first results from palaeo environmental and hydrological investigations on the sabkha basin of the tayma oasis northwestern saudi arabia sedimentary characteristics micro and macrofauna a digital elevation model based on dgps measurements and 14 c ams data indicate the presence of a perennial lake with a minimum depth of 13 m a stored water volume of 1 16 10 7 m 3 and a surface of 18 45 km 2 between 10 000 9000 cal bp foraminiferal test malformations and the shape of sieve pores on ostracod valves were used to detect trends in palaeo salinity and ecological stress conditions the reconstructed gradual contraction of the lake at least after 8500 cal bp reflects the long term aridisation trend on the arabian peninsula after the early holocene based on the hydrological water balance equation quantitative data on minimum palaeo rainfall during the early holocene humid period were calculated input parameters for the equation are the minimum lake level lake surface and lake volume during the peak of the early holocene humid period as well as palaeo evapotranspiration groundwater infiltration and surface runoff accordingly a perennial lake in the endorheic basin of the modern sabkha with a lake level at the same elevation as the uppermost shoreline deposit would have required a minimum annual precipitation of 150 25 mm this value amounts to c 300 of recent precipitation rates

early middle holocene environmental changes and pre neolithic human occupations as recorded in the cavities of jebel qara dhofar southern sultanate of oman abstract numerous palaeoenvironmental and archaeological studies from southern arabia yemen and oman have revealed strong relations between phases of human settlements and climate change linked to the indian monsoon system analyses on speleothems cave fills lacustrine deposits and palaeo mangroves have shown that during the early to mid holocene a humid optimum culminated around 9000 8000 cal bp new results on inland speleothems and cave sediments from the jebel qara southern oman are crucial in our depiction of early and mid holocene climatic evolution and cultural dynamics of the region these aspects are discussed here based on new archaeological surveys excavations geoarchaeological and micromorphological studies aiming to better understand connections with terminal pleistocene and early holocene autochthonous cultures of southern arabia our results suggest that the final pleistocene was marked by strong aridity which promoted a widespread thermoclastism within rock shelter and deposition of aeolian sand in contrast the transition towards the holocene is marked since c 12 000 cal bp by a progressive increasing in environmental humidity which permitted the formation of thick strata of peridesert loess after this phase the environmental humidity of the jebel increased and permitted the existence of a large community of land snails the latter were exploited by early holocene hunter gatherers who lived in the rock shelters between c 10 500 9500 cal bp and left consistent accumulations of land shells escargotieres the maximum of holocene humidity was reached between 9000 and 8000 cal bp regional aquifer were recharged and the deposition of calcareous tufa at the entrance of caves started lasting up to c 4500 cal bp c and o stable isotopes from calcareous tufa highlights in accordance with several regional records the progressive decline of the intensity of the indian ocean monsoon and the transition towards arid conditions in this phase the area was abandoned and archaeological communities possibly relocated along the coast of central and southern oman where they exploited the mangrove environment

effects of increased horizontal resolution in a simulation of the north atlantic ocean global mean and eddy fields from a four year experiment with a 1 6 1 5 horizontal resolution implementation of the cme north atlantic model are presented the time averaged wind driven and thermohaline circulation in the model is compared to the results of a 1 3 2 5 model run in very similar configuration in general the higher resolution results are found to confirm that the resolution of previous cme experiments is sufficient to describe many features of the large scale circulation and water mass distribution quite well while the increased resolution does not lead to large changes in the mean flow patterns the variability in the model is enhanced significantly on the other hand however not all aspects of the circulation have improved with resolution the azores current frontal zone with its variability in the eastern basin is still represented very poorly particular attention is also directed toward the unrealistic stationary anticyclones north of cape hatteras and in the gulf of mexico

the subpolar mode water of the north atlantic ocean abstract the warm waters of the subtropical and subpolar basins of the north atlantic have tight regional temperature salinity relationships and are conventionally called the regional central waters a volumetric census of the temperature salinity characteristics of the north atlantic by wright and worthington 1970 shows that waters characterized by certain segments of the t s relationships have large volumes compared to those of other segments volumetric mode waters such mode waters appear as layers with increased vertical separation between isopycnals pycnostads the present study reports on the existence of pycnostads in the central and eastern north atlantic these subpolar mode waters are formed by deep winter convection in the subpolar north atlantic and participate in the upper water circulation of the northern north atlantic the seasonal outcropping of the pycnostads occurs within and adjacent to the north atlantic current the irminger current the east and west greenland currents and

a ventilated middepth circulation model for the eastern north atlantic abstract a mechanism is proposed based on the assumption of ventilation to explain the middepth northward flow observed in the north atlantic the main feature of the solution is that the outcropping line of an intermediate layer at high latitudes is uniquely specified by the condition that the middepth waters of the subtropical gyre may be sucked up by the positive ekman pumping of the subpolar gyre this model is consistent with several characteristic features of the circulation in that region such as its vertical structure the greater northward extension of the warm waters in the eastern ocean and the shape of the large scale mediterranean water plume in a variation of the model the observed isopycnal slopes along the eastern coast are included in the analysis this implies the existence of eastern boundary layers

climate change at the 4 2 ka bp termination of the indus valley civilization and holocene south asian monsoon variability 1 planktonic oxygen isotope ratios off the indus delta reveal climate changes with a multi centennial pacing during the last 6 ka with the most prominent change recorded at 4 2 ka bp opposing isotopic trends across the northern arabian sea surface at that time indicate a reduction in indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south asia the 4 2 ka event is coherent with the termination of urban harappan civilization in the indus valley thus drought may have initiated southeastward habitat tracking within the harappan cultural domain the late holocene drought cycles following the 4 2 ka bp event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14c production rates this suggests that solar variability is one fundamental cause behind holocene rainfall changes over south asia

upwind weighted advection schemes for ocean tracer transport an evaluation in a passive tracer context centered in space centered in time integration has generally been used for the advection of scalars in ocean models an assessment is made of the implications of centered leapfrog integration in the context of two dimensional passive tracer advection within a stommel 1948 gyre nonphysical ripples in the tracer field grow to alarming levels in purely advective integrations diffusive parameterizations of eddy mixing moderate these ripples but it is found that laplacian diffusion greatly reduces the peak amplitude of the tracer field while biharmonic or weaker laplacian diffusion allows ripples of large amplitude several forward in time upwind weighted schemes are found to provide better solutions smolarkiewicz s 1984 multi dimensional positive definite advection and transport algorithm mpdata scheme is slightly superior for an integration at moderate resolution within which the western boundary current is poorly resolved in typical fashion third order upwind based schemes exhibit little sensitivity to the details of multidimensional treatment for this problem of passive tracer advection with results nearly as good as for mpdata

geothermal prospecting by geochemical methods in the quaternary volcanic province of dhamar central yemen abstract this paper deals with geothermal prospecting carried out in the quaternary volcanic field of dhamar which is located almost in the centre of the main oligo miocene basaltic trap plateau of yemen by applying geochemical and thermometric techniques in domestic wells producing water from the shallow unconfined aquifer in the area which is prevalently hosted inside the quaternary volcano clastic material a closed thermal anomaly associated with the quaternary volcanic activity was well delineated although the aquifer s has a ca na hco 3 composition that is typical of shallow groundwater there are several chemical anomalies in the hotter central area compared to typical aquifers i the p h is lower and consequently the calculated partial pressure of co 2 in solution is higher ii the electrical conductivity is higher iii the total salinity is higher and iv the fluoride ion concentration is higher such chemical anomalies in the hotter part of the aquifer do not seem to be generated by the rising and or mixing of deep hydrothermal components rising into the shallow aquifer but rather produced by enhanced water rock interaction processes resulting from the higher temperature of the aquifer and its greater acidity by applying some speculative calculations based on the likely temperature of rainfall in the area and the depth and temperature of individual wells the local thermal gradients in the area have been calculated the thermal gradient varies from less than the average earth gradient at the periphery of the delimitated thermal anomaly to more than 250 c km within an extensive area exceeding 200 km 2 where the gradient is greater than 100 120 c km

rivers of the hadramawt watershed yemen during the holocene clues of late functioning recent geoarchaeological investigations have been carried out in the thick silty alluvial formations of the northern hadramawt basin yemen field data was combined with a compilation of palaeoenvironmental records published from arabia and its margins research performed on palaeolakes and other geoarchives indicates that the wet early mid holocene has little in common with the continuously arid period after 5 2 ka bp the data sets allowed the authors to propose new perspectives for the holocene palaeohydrological evolution of southern arabia which was more variable and contrasting than previously thought furthermore these data provide the opportunity to go deeper into the holocene and to present a new precise palaeohydrological framework for the late holocene 7 0e2 0 ka bp river functioning fluvial aquifer dynamics and types of sedimentation are closely related to the rainfall regime length and intensity monsoonal or mediterranean and to the landscape conditions in the upper watersheds the development of fluvial palaeosols in the hadramawt valleys often associated with high levels of the aquifers is clearly in phase with humid periods until the beginning of the first millennium bc 5 9e5 3 4 8e4 5 and 4 0e2 7 ka river behaviour also impacted neolithic and early bronze age settlement systems in this lowland area as phases of floodplain stability favour site location the hyperarid 4 2 ka event is associated with an unprecedented fluvial detrital phase with high energy discharges in the entire hadramawt basin

impacts of the mesoscale ocean atmosphere coupling on the peru chile ocean dynamics the current induced wind stress modulation the ocean dynamical responses to the surface current wind stress interaction at mesoscale are investigated in the south east pacific using a high resolution regional ocean atmosphere coupled model two simulations are compared one includes the surface current in the wind stress computation while the other does not in the coastal region absolute wind velocities are different between the two simulations but the wind stress remains very similar as a consequence the mean regional oceanic circulation is almost unchanged on the contrary the mesoscale activity is strongly reduced when taking into account the effect of the surface current on the wind stress this is caused by a weakening of the eddy kinetic energy generation near the coast by the wind work and to intensified offshore eddy damping we show that above coherent eddies the current stress interaction generates eddy damping through ekman pumping and eddy kinetic energy dissipation through wind work this alters significantly the coherent eddy vertical structures weakening the temperature and vorticity anomalies and increasing strongly the vertical velocity anomalies associated to eddies

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

holocene paleoclimates of southern arabia from lacustrine deposits of the dhamar highlands yemen abstract this paper presents new evidence from the dhamar highlands yemen of paleohydrologic response to fluctuations in holocene climate stratigraphic geochemical and chronological analyses of highland peat and lacustrine deposits contribute to knowledge of the timing of early holocene moisture changes on the arabian peninsula providing a backdrop to understanding early cultural development in the arabian highlands the location of the dhamar highlands characterized by intermontane valleys surrounded by the highest mountains on the arabian peninsula and adjacent to the indian ocean is ideal for examining the influence of the indian ocean monsoon iom on the moisture history of this region fluctuations in the lacustrine and paleosol records of the dhamar highlands reflect both local changes in paleohydrology and regional influences on the holocene paleoclimatic conditions in southwest arabia in addition a peat deposit with a radiocarbon age of 10 253 10 560 cal yr bp documents some of the earliest holocene high moisture conditions on the arabian peninsula

mechanisms for spreading of mediterranean water in coarse resolution numerical models different processes have been proposed to explain the large scale spreading of mediterranean water mw in the north atlantic however no systematic study comparing the efficiency of different processes is yet available here the authors present a series of experiments in a unified framework that is designed to quantify the effects of several physical processes on the spreading of mw in an idealized model of the north atlantic the common technique of restoring temperature and salinity to an observed distribution near the mediterranean inflow fails to produce an adequate amount of mw because the eastern boundary region near the mw inflow is rather quiescent in models diapycnal processes like double diffusion and cabbeling turn out too inefficient to alone account for the large scale mw anomaly however with a preexisting anomaly double diffusion leads to a considerable northward and zonal redistribution of mw the density anomaly induced by cabbeling curtails the zonal spreading of mw while it increases the northward spreading with isopycnal mixing and the weak mean flow that prevails in the outflow region a spatial distribution of the mw anomaly is obtained that is inconsistent with observations unrealistically high diffusion coefficients would be necessary to reproduce the observed salt flux into the atlantic the most effective process in the experiments is the volume flux associated with the atlantic mediterranean exchange the current system that is established in response to the inflow of mw into the atlantic carries the anomaly almost 30 of longitude into the basin and along the eastern margin up to the northeastern corner of the domain and farther along the northern boundary

radiocarbon dating of minute gastropods and new constraints on the timing of late quaternary spring discharge deposits in southern arizona usa abstract gastropod shells are commonly preserved in quaternary sediments but are often avoided for radiocarbon dating because some taxa incorporate 14 c deficient carbon during shell formation recently brennan and quade 1997 quat res 47 329 336 found that some minute taxa vallonia pupilla and succineidae appear to yield reliable 14 c ages for late pleistocene samples a more rigorous evaluation of the 14 c inventory of minute gastropods is presented here which involved measuring the 14 c activity of specimens collected live in two geologic settings that maximize the potential for ingestion of old carbon 1 alluvium dominated by paleozoic carbonate rocks and 2 adjacent to extant springs with highly 14 c deficient water present at the surface we found that several minute taxa including vallonia incorporate significant and variable amounts of old carbon 2 to 30 during shell formation the 14 c activities of the land snails pupilla blandi and euconulus fulvus however are indistinguishable from the 14 c activity of live plants the 14 c activity of the semi aquatic gastropod catinella sp family succineidae deviates from modern values in the presence of 14 c deficient water by an amount equivalent to 10 of the local carbon reservoir effect these results imply that at least some minute gastropods can provide reliable 14 c ages even when 14 c deficient carbon is readily available to demonstrate an application of our findings we 14 c dated shells from p muscorum e fulvus and succinea sp family succineidae recovered from the coro marl a late pleistocene spring fed marsh deposit exposed at the murray springs paleoindian site in the san pedro valley of southern arizona usa radiocarbon ages obtained from the minute gastropods show that the unit was deposited between 25 000 and 13 000 14 c years ago the marl is situated 15 m above the modern water table at murray springs and is similarly positioned in discontinuous outcrops along a 150 km stretch of the san pedro valley thus the 14 c ages of minute gastropods presented here may be used to infer the timing of high water table levels throughout the valley

performance of a second order moments advection scheme in an ocean general circulation model 1 the reliability of ocean general circulation models ogcms strongly depends on the quality of their tracer advection schemes for the sake of simplicity and computing time tracer advection schemes most commonly used in large scale ogcms tend to be low order schemes which suffer from spurious numerical diffusion and dispersion that result in distorted solutions the application of high order schemes would reduce numerical errors but at a considerable cost in terms of computing time an alternative to the use of high order methods is the implementation of algorithms that take into account the sub grid distribution of tracers one such method is the second order moments som scheme of prather 1986 which is more accurate than a fourth order scheme but at the time consumption of a second order algorithm this article presents results from coarse resolution global ocean simulations with very low explicit diapycnal mixing in which active and passive tracers were advected with the som method we compare the performance of the method with that of more traditional schemes namely the fct flux corrected transport and quicker quadratic upstream interpolation for convective kinematics schemes in general the use of the som method significantly improves tracer distributions and transports compared to fct and quicker thus leading to a better representation of ocean currents notably boundary currents and frontal systems while model simulations employing the fct and quicker schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes the som simulations admit a circulation pattern closer to that known as the reconfigured conveyor belt toggweiler and samuels 1993 in which the bulk of the global ocean upwelling occurs in the southern ocean

on the influence of dsow in a numerical model of the north atlantic general circulation abstract two numerical experiments regarding the north atlantic circulation are compared the two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland sea one case climatological wind stress sst and surface salinity to force the model it reproduces the atlantic circulation found in similar studies a change in surface boundary values in the iceland sea in order to improve the denmark strait overflow water properties in the second case results in major changes in circulation and hydrography the meridional overturning more than doubles and horizontal gyres are intensified recirculation cells occur north and south of the gulf stream the gulf stream separates from the north american coast in a more realistic manner and its further course is in better agreement with observations than in the reference run the subsurface temperature distribution differs by several kelvin in the subpolar and

mechanisms forcing abrupt fluctuations of the indian ocean summer monsoon during the last deglaciation abstract a piston core from the somali upwelling area has been studied at high resolution for its dinoflagellate cyst content variations in cyst association are inferred to reflect changes in indian ocean summer monsoon intensity several abrupt fluctuations in monsoon intensity are detected for the interval between 20 and 10 ka bp comparison of these fluctuations with changes in contrast between 30 n and 30 s july insolation and the 18 o grip ice core values suggests that different mechanisms influenced monsoon intensity at different time intervals a general trend in monsoon intensity follows variations in insolation contrast lagging minimum contrast by 7 5 ka and maximum contrast by 4 ka this phase lag difference can be explained by assuming the existence of an ice snow cover over central asia tibet during glacial times between 18 7 and 12 5 ka bp fluctuations in sw monsoon intensity may be forced by variations in the thermohaline ocean circulation a rapid transition towards strong sw monsoons at 12 5 ka bp is possibly the result of variations in atmospheric circulation and melting of the snow ice fields in central asia tibet variations in glacial interglacial boundary conditions related to temperature change at northern latitudes are likely to have influenced sw monsoon intensity between 12 5 and 10 ka bp whereas after 10 ka bp variations in tropical land surface boundary conditions may be the dominant forcing factor 0 1997 elsevier science ltd all rights reserved