late pleistocene and holocene depositional systems and the palaeogeography of the dogger bank north sea 3d seismic data from the dogger bank north sea allow the mapping of late pleistocene and holocene depositional systems in unprecedented detail the data demonstrate that glacial processes resulted in the development of incised tunnel valley systems during the weichselian and that these were subsequently modified by fluvial processes in a pro glacial setting subsequently the dogger bank formed an emergent plain during the holocene with a complex meandering river system associated tributary or distributary channels and lakes dominating the region prior to the sea level rising sufficiently to submerge the dogger bank around 7500 yr ago the meandering river system was replaced by a dendritic channel network of potential fluvial estuarine or intertidal origin as the holocene depositional features bear no systematic relationship to the bathymetry this study demonstrates that previously published bathymetry based models for the holocene palaeogeographic development of the north sea require modification

residual mean solutions for the antarctic circumpolar current and its associated overturning circulation abstract residual mean theory is applied to the streamwise averaged antarctic circumpolar current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated residual meridional overturning circulation

impact of the late glacial eruption of the laacher see volcano central rhineland germany abstract within a period of a few weeks toward the end of the allerod interstadial the major plinian eruption of the laacher see volcano produced some 20 km3 of eruptiva covering and preserving the late glacial landscape in the german central rhineland over an area of more than 1000 km2 correlation of terrestrial archives with the greenland ice core records and improved calibration of the radiocarbon timescale permit a precise accurate age determination of the laacher see event some 200 yr before the onset of the younger dryas cold episode carbonized trees and botanical macrofossils preserved by laacher see tephra permit detailed regional paleoenvironmental reconstruction and show that open woodland were typical for the cool and humid hemiboreal climatic conditions during the late allerod this woodland provided the habitat for a large variety of animal species documented at both paleontological and final paleolithic archeological sites preserved below laacher see deposits of special interest are numerous animal tracks intercalated in middle laacher see deposits at the south of the neuwied basin this knowledge may help to evaluate possible supraregional impacts of this volcanic event on northern hemispheric environment and climate during the late allerod

drifter observations of submesoscale flow kinematics in the coastal ocean fronts and eddies identified with aerial guidance are seeded with drifters to quantify submesoscale flow kinematics the lagrangian observations show mean divergence and vorticity values that can exceed five times the coriolis frequency values are the largest observed in the field to date and represent an extreme departure from geostrophic dynamics the study also quantifies errors and biases associated with lagrangian observations of the underlying velocity strain tensor the greatest error comes from under sampling even with a large number of drifters a significant bias comes from inhomogeneous sampling of convergent regions that accumulate drifters within a few hours of deployment the study demonstrates a lagrangian sampling paradigm for targeted submesoscale structures over a broad range of scales and presents flow kinematic values associated with vertical velocities o 10 m h 1 that can have profound implications on ocean biogeochemistry

improved age modelling and high precision age estimates of late quaternary tephras for accurate palaeoclimate reconstruction abstract the role of tephrochronology as a dating and stratigraphic tool in precise palaeoclimate and environmental reconstruction has expanded significantly in recent years the power of tephrochronology rests on the fact that a tephra layer can stratigraphically link records at the resolution of as little as a few years and that the most precise age for a particular tephra can be imported into any site where it is found in order to maximise the potential of tephras for this purpose it is necessary to have the most precise and robustly tested age estimate possible available for key tephras given the varying number and quality of dates associated with different tephras it is important to be able to build age models to test competing tephra dates recent advances in bayesian age modelling of dates in sequence have radically extended our ability to build such stratigraphic age models as an example of the potential here we use bayesian methods now widely applied to examine the dating of some key late quaternary tephras from italy these are the agnano monte spina tephra amst the neapolitan yellow tuff nyt and the agnano pomici principali app and all of them have multiple estimates of their true age further we use the bayesian approaches to generate a revised mixed radiocarbon varve chronology for the important lateglacial section of the lago grande monticchio record as a further illustration of what can be achieved by a bayesian approach with all three tephras we were able to produce viable model ages for the tephra validate the proposed 40 ar 39 ar age ranges for these tephras and provide relatively high precision age models the results of the bayesian integration of dating and stratigraphic information suggest that the current best 95 confidence calendar age estimates for the amst are 4690 4300 cal bp the nyt 14320 13900 cal bp and the app 12380 12140 cal bp

direct observations of microscale turbulence and thermohaline structure in the kuroshio front 1 direct observations of microstructure near the kuroshio front were conducted in august 2008 and october 2009 these show negative potential vorticity pv in the mixed layer south of the front where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind scaling these elevated dissipation rates scale better with an empirical scaling which considers local wind and ekman buoyancy flux driven by downfront wind near zero pv in the thermocline under the kuroshio mainstream is observed at 200 300 m depth with dissipation exceeding open ocean thermocline values by factors of 10 100 overall the large turbulent dissipation rates measured in the kuroshio can be categorized into two groups one characterized by low richardson number along the kuroshio front thermocline and the other characterized by high stratification away from the kuroshio mainstream the former is attributed to mixing by unbalanced frontal ageostrophic flows and the latter is attributed to internal wave breaking on average both groups appear in regions of large horizontal density gradients observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues extending upward from 500 to 100 m depth in a narrow 20 km zone suggesting down and upwelling driven by geostrophic straining which is confirmed by quasigeostrophic omega equation solutions this implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the kuroshio front likely contribute to npiw formation

sea ice melt onset associated with lead opening during the spring summer transition near the north pole in the central arctic ocean autonomous observations of the ocean mixed layer and ice documented the transition from cold spring to early summer in 2011 ice motion measurements using gps drifters captured three events of lead opening and ice ridge formation in may and june satellite sea ice concentration observations suggest that locally observed lead openings were part of a larger scale pattern we clarify how these ice deformation events are linked with the onset of basal sea ice melt which preceded surface melt by 20 days observed basal melt and ocean warming are consistent with the available input of solar radiation into leads once the advent of mild atmospheric conditions prevents lead refreezing we use a one dimensional numerical simulation incorporating a local turbulence closure scheme to investigate the mechanisms controlling basal melt and upper ocean warming according to the simulation a combination of rapid ice motion and increased solar energy input at leads promotes basal ice melt through enhanced mixing in the upper mixed layer while slow ice motion during a large lead opening in mid june produced a thin low density surface layer this enhanced stratification near the surface facilitates storage of solar radiation within the thin layer instead of exchange with deeper layers leading to further basal ice melt preceding the upper surface melt

between warm and cold impact of the younger dryas on human behavior in central europe abstract following a thorough review of high resolution environmental archives this paper aims at discriminating the factors determining the heterogeneous repercussion of the lateglacial younger dryas in central europe when examining the archaeological implications of human adaptation to the subsequent changes in the natural environment two divergent biotic regions are of special interest the north european plain and adjacent areas and the alpine foothills and surrounding mountain ranges in these regions two different archaeological technocomplexes traditions are found the tanged point complex and the curve backed point groups considering the distribution of the archaeological sites witnessing changes in the material culture and subsistence pattern the intensity of the environmental changes caused by the younger dryas is a decisive element settlement discontinuity during the younger dryas is questioned moreover the potential existence of established social networks between the two regions expressed by comparable developments such as microlithization is considered finally the authors assess whether the younger dryas acted as an accelerator or a brake in the process of regional diversification prior to the early mesolithic

frontogenesis subduction and cross front exchange at upper ocean fronts the subduction of water at upper ocean fronts driven by internal instabilities is investigated a simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation a nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front frontogenesis generates vertical velocities of 0 30 m d and ageostrophic cross front flows of 3 to 5 cm s the subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front the subducted parcels quickly develop a circulation through balanced adjustment anticyclonic for deep surface layers and cyclonic for shallow surface layers the characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices the horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer the total permanent subduction rate is estimated to be 20 m yr for frontal subduction driven by frontogenesis although local and temporary subduction can be much larger the subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices thus forming a baroclinic dipole that propagates at approximately 3 5 cm s and is capable of transporting the anomalous water properties far from the region of formation similarities with observations in the north atlantic subtropical convergence zone and near the gulf stream are discussed

intensification of ocean fronts by down front winds abstract many ocean fronts experience strong local atmospheric forcing by down front winds that is winds blowing in the direction of the frontal jet an analytic theory and nonhydrostatic numerical simulations are used to demonstrate the mechanism by which down front winds lead to frontogenesis when a wind blows down a front cross front advection of density by ekman flow results in a destabilizing wind driven buoyancy flux wdbf equal to the product of the ekman transport with the surface lateral buoyancy gradient destabilization of the water column results in convection that is localized to the front and that has a buoyancy flux that is scaled by the wdbf mixing of buoyancy by convection and ekman pumping suction resulting from the cross front contrast in vertical vorticity of the frontal jet drive frontogenetic ageostrophic secondary circulations ascs for mixed layers with negative potential vorticity the most frontogenetic ascs select a preferred cross front width and do not translate with

a mid european decadal isotope climate record from 15 500 to 5000 years b p oxygen isotope ratios of precipitation delta18op inferred from deep lake ostracods from the ammersee southern germany provide a climate record with decadal resolution the record in detail shows many of the rapid climate shifts seen in central greenland ice cores between 15 000 and 5000 years before the present b p negative excursions in the estimated delta18op from both of these records likely reflect short weakenings of the thermohaline circulation caused by episodic discharges of continental freshwater into the north atlantic deviating millennial scale trends however indicate that climate gradients between europe and greenland changed systematically reflecting a gradual rearrangement of north atlantic circulation during deglaciation

friction frontogenesis and the stratification of the surface mixed layer the generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air sea fluxes and stresses in this paper it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum a classic example is the strong restratification resulting from frontogenesis in regions of confluent flow frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving ekman flows that differentially advect density ekman flow induced during spindown always tends to restratify the fluid while wind driven ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front i e directed against or with the geostrophic shear respectively scalings are constructed for the relative importance of friction versus frontogenesis in the restratification of the mixed layer and are tested using numerical experiments of mixed layer fronts forced by both winds and a strain field the scalings suggest and the numerical experiments confirm that for wind stress magnitudes mixed layer depths and cross front density gradients typical of the ocean wind induced friction often dominates frontogenesis in the modification of the stratification of the upper ocean the experiments reveal that wind induced destratification is weaker in magnitude than restratification because the stratification generated by up front winds confines the turbulent stress to a depth shallower than the ekman layer which enhances the frictional force ekman flow and differential advection of density frictional destratification is further reduced over restratification because the stress associated with the geostrophic shear at the surface tends to compensate a down front wind stress

bio and chronostratigraphy of the lateglacial in the eifel region germany abstract in recent years enormous success has been achieved in varve counting of the eifel maar lakes but a detailed correlation with the biostratigraphy has been missing in this paper we present new palynological results of the lateglacial sequences from holzmaar lake and meerfelder maar lake based on annually laminated sediments in particular the meerfelder maar has great potential because in contrast to the holzmaar the sequence between the ulmener maar tephra 11 000 varve years bp and the laacher see tephra 12 880 varve years bp including the younger dryas is undisturbed and complete therefore we currently use the meerfelder maar chronology brauer et al 1999b as an independent varve calendar for the biostratigraphy of the lateglacial the palynological signals of both maar lakes are in good agreement and can easily be correlated with one another and with type sections type regions in northwestern germany and jutland the sequences of the eifel maar lakes have the quality of hypostratotypes with regional biozones based on an absolute time scale

evolution and environmental impacts of the eruption of laacher see volcano germany 12 900 a bp abstract laacher see volcano lsv 40 km south of bonn explosively erupted ca 6 3 km 3 of chemically strongly zoned phonolite magma probably during spring 12 900 years ago the resulting eruption column having reached at least 20 km in height the bulk of the laacher see tephra lst was deposited east of the volcano within the neuwied basin lst ashes form the most important stratigraphic marker in lateglacial deposits over much of central europe a minimum of 2 mt total s was calculated to have been released during the eruption by comparing pre and posteruptive volatile concentrations in glass inclusions and pumice matrix glasses because a separate s bearing vapor phase was probably present in the magma prior to eruption as indicated by the high melt h 2 o contents close to saturation level the actual amount of s released during the eruption could have significantly exceeded 2 mt the sulfuric acid aerosol layer resulting from the massive stratospheric s input probably resided in the stratosphere for years and most likely had a significant impact on climate and thus the environment increased precipitation in central europe and or impairment of the vegetation cover for several years is suggested by several proxies especially increased sediment supply into lakes proximal to the eruptive center major environmental impacts include an initial blast that felled trees up to 4 km away from vent fast deposition of huge tephra volumes led to the complete disruption of rhine river within the lower neuwied basin and the damming up of a lake ca 140 km 2 in areal extent sudden collapse of the temporary tephra dam caused a catastrophic flood wave downstream whose deposits are recognized as far north as bonn reworking of unconsolidated tephra deposits over several 100 km 2 was widespread and extensive generating abundant lahars and flood plain deposits a braided river system established in neuwied basin probably persisted up to several years recent reports for a second eruption of lsv are not confirmed

eddy transport and mixing in a wind and buoyancy driven jet on the sphere abstract the baroclinically unstable wind and buoyancy driven flow in a zonally reentrant pie shaped sector on a sphere is numerically modeled and then analyzed using the transformed eulerian mean tem formalism mean fields are obtained by zonal and time averaging performed at fixed height the very large latitudinal extent of the basin 50 7 s latitude to the equator allows the latitude variation of the coriolis parameter to strongly influence the flow persistent zonal jets are observed in the statistically steady state reynolds stress terms play an important role in redistributing zonal angular momentum convergence of the lateral momentum flux gives rise to a strong eastward jet with an adjacent westward jet equatorward and weaker multiple jets poleward an equally prominent feature of the flow is a strong and persistent eddy that has the structure of a kelvin cat s eye and generally occupies the zonal width of the basin at latitudes 15 10 s a strongly mixed surface diabatic zone overlies the

ocean fronts trigger high latitude phytoplankton blooms 1 density fronts are ubiquitous features of the upper ocean here numerical simulations show that restratification at fronts inhibits vertical mixing triggering phytoplankton blooms in low light conditions the stability of the water column at fronts is set by a competition between frontal instabilities which restratify the upper ocean and turbulent mixing which acts to destroy this stratification recent studies have found that frontal instabilities can restratify the upper ocean even in the presence of strong surface cooling and destabilizing winds during winter at high latitudes primary production by phytoplankton is generally limited by low ambient light levels and deep turbulent mixing when the turbulent mixing inhibited by frontal restratification becomes smaller than a critical turbulence threshold a phytoplankton bloom can develop the finding that fronts can trigger phytoplankton blooms by reducing mixing provides an explanation for satellite observations of high chlorophyll concentrations at high latitude fronts

paleo environment and radiocarbon calibration as derived from lateglacial early holocene tree ring chronologies abstract we present an overview of the extended hohenheim oak chronology hoc and the dendrochronologically dated preboreal pine tree ring chronology ppc both provide an absolute annual time frame of the holocene extending into the younger dryas yd back to 11 919 bp two floating pine and larch chronologies are 14 c dated covering large parts of the lateglacial dendro ecological parameters such as ring width and stable isotope variation are used to infer past environmental conditions 14 c analyses on decadal sections provide a high precision high resolution data set for calibration of the radiocarbon time scale based on a marked change in ring width and growth pattern the yd termination is clearly identified in the german pine chronology its absolute age of 11 570 bp appears synchronous within the errors of the respective chronologies to related signals in the greenland ice cores grip gisp2 and in lacustrine varve sequences the 14 c age of the laacher see tephra lst is determined from a series of decadal tree ring samples to 11 063 12 14 c bp the calibrated range is 13 010 13 200 cal bp the climatic impact of the lst is reflected in the growth pattern of our tree ring chronologies

accumulation and subduction of buoyant material at submesoscale fronts abstractthe influence of submesoscale currents on the distribution and subduction of passive buoyant tracers in the mixed layer is examined using large eddy simulations submesoscale eddies are generated through an ageostrophic baroclinic instability associated with a background horizontal buoyancy gradient the simulations also include various levels of surface cooling which provides an additional source of three dimensional turbulence submesoscales compete against turbulent convection and restratify the mixed layer while generating strong turbulence along a submesoscale front buoyant tracers accumulate at the surface along the submesoscale front where they are subducted down into the water column the presence of submesoscales strongly modifies the vertical tracer flux even in the presence of strong convective forcing the correlation between high tracer concentration and strong downwelling enhances the vertical diffusivity for buoyant tracers

when mixed layers are not mixed storm driven mixing and bio optical vertical gradients in mixed layers of the southern ocean author s carranza mm gille st franks pjs johnson ks pinkel r girton jb abstract 2018 american geophysical union all rights reserved mixed layers are defined to have homogeneous density temperature and salinity however bio optical profiles may not always be fully homogenized within the mixed layer the relative timescales of mixing and biological processes determine whether bio optical gradients can form within a uniform density mixed layer vertical profiles of bio optical measurements from biogeochemical argo floats and elephant seal tags in the southern ocean are used to assess biological structure in the upper ocean within the hydrographically defined mixed layer the profiles show significant vertical variance in chlorophyll a chl a fluorescence and particle optical backscatter biological structure is assessed by fitting chl a fluorescence and particle backscatter profiles to functional forms i e gaussian sigmoid exponential and their combinations in the southern ocean which characteristically has deep mixed layers only 40 of nighttime bio optical profiles were characterized by a sigmoid indicating a well mixed surface layer of the remaining 60 that showed structure 40 had a deep fluorescence maximum below 20 m depth that correlated with particle backscatter furthermore a significant fraction of these deep fluorescence maxima were found within the mixed layer 20 80 depending on mixed layer depth definition and season results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification we hypothesize that periods of quiescence between synoptic storms which we estimate to be 3 5 days depending on season allow bio optical gradients to develop within mixed layers that remain homogeneous in density

frontal circulation and submesoscale variability during the formation of a southern ocean mesoscale eddy observations made in the scotia sea during the may 2015 surface mixed layer evolution at submesoscales smiles research cruise captured submesoscale o 1 10 km variability along the periphery of a mesoscale o 10 100 km meander precisely as it separated from the antarctic circumpolar current acc and formed a cyclonic eddy 120 km in diameter the meander developed in the scotia sea an eddy rich region east of the drake passage where the subantarctic and polar fronts converge and modifications of subantarctic mode water samw occur in situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy a mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series under certain simplifying assumptions appropriate for the current dataset upwelling downwelling rates of o 100 m day 1 are found in the northern southern eddy sector favorable conditions for submesoscale instabilities are found in the mixed layer particularly at the beginning of the survey in the vicinity of density fronts shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front evolution in temperature salinity t s space indicates modification of water mass properties in the upper 200 m over 2 days modifications along 27 27 2 kg m 3 have climate related implications for mode and intermediate water transformation in the scotia sea on finer spatiotemporal scales than observed previously

environmental and climatic changes in the jura mountains eastern france during the lateglacial holocene transition a multi proxy record from lake lautrey this paper presents a multi proxy reconstruction of the climatic and environmental changes during the last glacial interglacial transition as recorded by a sediment sequence from lake lautrey jura eastern france this reconstruction is based on analysis of pollen chironomid organic matter oxygen isotope mineralogical magnetic susceptibility and inferred lake level data at a high temporal resolution the chronology is derived using ams radiocarbon dates the position of the laacher see tephra lst and of correlation between the lautrey and grip oxygen isotope records this data set reveals a detailed sequence of environmental changes in the jura mountains from greenland stadial 2a to the early holocene biotic and abiotic indicators allow the recognition of major abrupt changes associated with the gs 2a gi 1e gi 1a gs 1 and gs 1 preboreal transitions and other minor fluctuations related to the cold events gi 1d gi 1b and the preboreal oscillation pbo they also suggest additional cooling spells at ca 14 550 and 14 350 cal yr bp intra bolling cold periods at ca 13 500 and ca 12 700 cal yr bp just before the gs 1 onset and at ca 11 350 cal yr bp just before the pbo as well as an intriguing brief warming episode within gs 1 at ca 12 080 cal yr bp summer temperature increased by ca 5 c at the start of gi 1e and by 1 5 3 c at the holocene onset while it decreased by ca 3 4 c at the beginning of gs 1 major changes in local hydrology and in seasonality appear to be also associated with the gs 2a gi 1e gi 1a gs 1 and gs 1 preboreal transitions pollen and abiotic indicators suggest a greater sensitivity of the vegetation cover to climatic oscillations during the first part of the lateglacial interstadial than during the second part after ca 13 700 cal yr bp when a closed forest had been restored in this area by contrast the restoration of forest cover took less than 300 yr after the end of gs 1 at the beginning of gi 1e and gs 1 no lag occurs within the sampling resolution of 20 50 yr in the responses of aquatic chironomids and terrestrial pollen ecosystems while at the onset of the holocene the response of the vegetation appears slightly delayed in comparison with that of the chironomid community finally the recognition of two successive tephra layers which were deposited just before the lst at ca 12 950 cal yr bp and which originated from le puy de la nugere massif central france provides an additional tephrochronological tool for correlation between lateglacial european sequences

environmental response to climatic and volcanic events in central europe during the weichselian lateglacial this paper reviews major results of the recent german research priority program changes of the geo biosphere during the last 15 000 years a contribution to pages focussing on changes of the geo biosphere during the weichselian lateglacial different continental archives such as annually laminated lacustrine sediments floodplain sediments and speleothems were used to reconstruct environmental response to climatic changes and the laacher see eruption event at ca 12 900 cal bp special emphasis is paid to establish a reliable time control using varve counting high precision radiocarbon dating of tree ring series and ams radiocarbon dating of terrestrial plant macrofossils recovered from lacustrine sediments to correlate and synchronize large scale environmental changes and events in central europe

abrupt transitions in submesoscale structure in southern drake passage glider observations and model results abstractenhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior these dynamics are of particular importance in the southern ocean where the ventilation of many density classes occurs here we present results from an observational field program in southern drake passage a region preconditioned for submesoscale instability due to its strong mesoscale eddy field persistent fronts strong down front winds and weak vertical stratification two gliders sampled from december 2014 through march 2015 upstream and downstream of shackleton fracture zone sfz the acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities the regions flanking the sfz displayed remarkably different characteristics despite similar surface forcing mixed layer depths were nearly twice as deep and horizontal buoyancy

late glacial climatic oscillations as recorded in swiss lake sediments regional pollen assemblage zones for the late glacial period of the swiss plateau are introduced and defined they include four major zones artemisia juniperus hippophae betula pinus paz with several subzones pollen and oxygen isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000 9500 yr bp the first oscillation termed the aegelsee fluctuation occurs shortly before 12 000 yr bp and varve counts suggest its duration was ca 100 yr it is characterised by a short decrease in the oxygen isotopes as well as a short increase in nap associated with a depression in birch pollen values the second oscillation which occurs in the 18o record shortly before the deposition of the laacher see tephra ca 11 000 yr bp is termed the gerzensee fluctuation it occurs during a pine dominated phase and its vegetational effects cannot be determined palynologically the most prominent regressive phase is the younger dryas biozone ca 10 700 10 000 yr bp characterised by an increase in heliophilous nap and low 18o values the younger dryas biozone can often be subdivided palynologically into two parts a first part rich in grasses and juniper and a second part with higher filipendula and birch values during the preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios comparison of the swiss oxygen isotope profiles with the greenland dye 3 record suggests that not only the three major shifts in the 18o curves but also the minor ones are closely comparable suggesting some common climatic control

enhancement in vertical fluxes at a front by mesoscale submesoscale coupling oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean submesoscale o 1 rossby number dynamics are particularly relevant for inducing the vertical and lateral flux of buoyancy and tracers in the mixed layer but how these couple with the stratified pycnocline is less clear observations show surface fronts often persist beneath the mixed layer here we use idealized three dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities we contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper for the deeper fronts we examine the effect of density stratification on the vertical coupling our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts the vertical enhancement of tracer fluxes through the mesoscale submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean a further implication for wind forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior the discrepancies are most severe within the mixed layer suggesting a potential role for ekman layer dynamics absent in existing submesoscale parameterizations

recharge mode and mineralization of groundwater in a semi arid region sidi bouzid plain central tunisia groundwater is the most important source of water supply in sidi bouzid plain located in central tunisia proper understanding of the geochemical evolution of groundwater is important for sustainable development of water resources in this region a hydrogeochemical survey was conducted on the mio plio quaternary aquifer system using stable isotopes radiocarbon tritium and major elements in order to evaluate the groundwater chemistry patterns and the main mineralization processes occurring in this system the chemical data indicate that dissolution of evaporate minerals and evaporation are the main processes controlling groundwater mineralization the isotopic data show that groundwater in the study area is a mixture of recent shallow waters located upstream and along wadi al fakka bed and paleowaters located towards plain limits and discharge areas low 3h and 14c contents are observed in major part of the plain indicating that recharge of the aquifer occurs mainly through direct infiltration at wadi al fakka while there is no evidence of significant recharge in major part of the plain and mountains piedmonts

using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in an arid region a case study in the upper middle reaches of the shule river basin northwestern china understanding sources of recharge and mechanisms for hydrogeochemical evolution of groundwater in the shule river basin an arid inland river basin in northwestern china is essential for successful water resource management sources of water and associated groundwater recharge processes were investigated using hydrogeological hydrogeochemical and isotopic methods the study area was divided into three parts changma the shule valley and the beishan area the hydrogeochemical and isotopic analysis results show that groundwater in changma mainly originated from precipitation in the qilian mountains lateral flow from changma is the primary recharge mechanism while direct recharge by infiltration of precipitation can be neglected in the shule valley minerals within the aquifer material e g halite calcite dolomite and gypsum dissolve into water that recharges the groundwater system therefore strong linear relationships were found between na and cl and between ca2 mg2 and so42 hco3 with stoichiometry ratios of approximately 1 1 in both cases groundwater quality in the region is generally good with low salinity and hardness local precipitation is the only source of recharge for groundwater in the beishan area low groundwater velocity and dry climatic conditions suggest that dissolution of minerals from the aquifer material controls hydrogeochemical evolution water rarely travels from the beishan area to the shule valley therefore almost all the groundwater in the shule valley originates from changma global climate change and loss of glaciers in the qilian mountains may lead to a reduction of recharge therefore the groundwater resources of the shule valley must be managed to sustain water supply

parameterization of mixed layer eddies part i theory and diagnosis ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean in this paper a parameterization for the restratification driven by finite amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal the streamfunction is proportional to the product of the horizontal density gradient the mixed layer depth squared and the inertial period hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence in this paper the parameterization is theoretically motivated confirmed to perform well for a wide range of mixed layer depths rotation rates and vertical and horizontal stratifications it is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification two companion papers discuss the numerical implementation and the climate impacts of this parameterization

destruction of potential vorticity by winds the destruction of potential vorticity pv at ocean fronts by wind stress driven frictional forces is examined using pv flux formalism and numerical simulations when a front is forced by downfront winds that is winds blowing in the direction of the frontal jet a nonadvective frictional pv flux that is upward at the sea surface is induced the flux extracts pv out of the ocean leading to the formation of a boundary layer thicker than the ekman layer with nearly zero pv and nonzero stratification the pv reduction is not only active in the ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low pv from the ekman layer with high pv from the pycnocline extraction of pv from the pycnocline by the secondary circulations results in an upward advective pv flux at the base of the boundary layer that scales with the surface nonadvective frictional pv flux and that leads to the deepening of the layer at fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux f b the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for pv

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

varve chronology and palynology of the lateglacial in northwest germany from lacustrine sediments of hamelsee in lower saxony abstract the hamelsee is a dolina from a collapse over permian salt that occurred at the end of the pleniglacial it is more than 21 m deep and contains 18 m of lacustrine sediments the lateglacial sediments in the central part of the lake consist of 2 7 m from 15 to 17 7 m of sapropelic and silty muds which are mostly laminated and partly bioturbated minor slumps occur mainly during colder phases the subdivision of the lateglacial is based on pollen zones multidisciplinary studies of the microfacies from thin sections geochemistry pollen analysis and rotifer analysis allow environmental and palaeoclimatological interpretations to be made the biozones of the lateglacial and their boundaries are marked by rapid climatic changes though covering comparatively short periods of time they are not monotonous but characterized by minor internal climatic fluctuations the sediments of the meiendorf and oldest dryas are not laminated the allerod sediment is seasonally varved without interruption and encompasses 625 yr the eruption of the laacher see volcano 12 900 cal bp had an impact on the climate during the following 10 yr the transition from the allerod to the younger dryas took some 50 yr and it occurred 200 yr after the eruption of the laacher see volcano the abrupt change at the end of the lateglacial 11 560 cal bp occurred within less than 10 yr however many proxies reacted with a time lapse of 15 to 30 yr

geostrophic adjustment and restratification of a mixed layer with horizontal gradients above a stratified layer abstract the restratification of a mixed layer with horizontal density gradients above a stratified layer is considered solutions are obtained on the assumption that the width across this front is much larger than the local radius of deformation bh f based on the buoyancy change across the front b mean mixed layer depth rdh and the coriolis parameter f where b is defined as rd g 0 0 but the fractional change in the mixed layer depth is not required to be small for an initially quiescent mixed layer created by homogenizing a fluid of constant stratification to a depth that varies horizontally the isopycnals in the mixed layer tilt about their intersections with the top surface in the adjusted state and the base of the mixed layer flattens slightly in the frontal region other cases considered include mixed layer fronts with initial momentum out of geostrophic balance created by vertical mixing of a layer with horizontal gradients previously in thermal wind balance for a wide front the

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

mesoscale to submesoscale transition in the california current system part ii frontal processes this is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized equilibrium subtropical eastern boundary upwelling current system similar to the california current the emergent upper ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives using a combination of composite averaging and separation of distinctive subregions of the flow the initiating dynamical process for the transition is near surface frontogenesis the frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics i e uniform interior potential vorticity q and diagnostic force balance in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field however there are significant behavioral differences compared to the surface dynamics model wind stress acts on fronts through nonlinear ekman transport and creation and destruction of potential vorticity the strain induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events submesoscale vortices and excitation of even smaller scale flows intermittent submesoscale breakdown of geostrophic and gradient wind force balance occurs during the intense frontogenesis and frontal instability events

submesoscale impacts on horizontal and vertical transport in the gulf of mexico 1 impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions 1 and 5 km the focus is the northwestern gulf of mexico where the loop current eddies are surrounded by smaller vortices by increasing the horizontal resolution the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities inside the coherent eddies and at their peripheries increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer but are found at all depths horizontal velocities on the contrary are similar lagrangian isobaric tracers are deployed close to the surface and at 100 m and three dimensional neutrally buoyant particles are released close to the surface at the base of the mixed layer and at 100 m the modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution close to the ocean surface however convergence zones generated by submesoscale ageostrophic motions and resolved at 1 km resolution influence the details of the tracer distributions vertical dispersion increases by several folds for increasing resolution at all depths explored with the largest differences found close to the surface therefore submesoscales processes play a fundamental role in driving vertical transport in eddy dominated flows both within and below the mixed layer for times comparable to the eulerian time scale

inner shelf intrusions of offshore water in an upwelling system affect coastal connectivity 1 poleward warm water flows around point conception are an important transport mechanism linking biogeographic provinces along central california these flows are initiated by relaxation of upwelling winds the intensity and duration of upwelling is an important driver of how far north warm water penetrates against the prevailing surface currents in this paper we present evidence of offshore surface water intrusions to the inner shelf 130 km north of point conception near cambria california satellite observations show that the intrusions originate as eddies generated offshore along the upwelling front these eddies may form as submesoscale instabilities or by interaction of upwelling centers with offshore waters the intrusions move southern waters to the central california inner shelf further northward than is typical for a coastal relaxation plume and therefore inner shelf connectivity and ecology may be governed over timescales and distances longer than those set by the intermittent relaxation of upwelling winds

the eco geomorphological roles of rocky deep crevices for water supply on arid zone mountain slopes case study mehriz yazd iran rocky slope landscapes are widely distributed on arid land mountains where there are many restoration and forestation project sites this study has surveyed the role of deep crevices and stone layers for supplying water to establish trees and shrubs on arid lands rocky slopes the research was concentrated on the layers of limestone granite and andesite in central iran thirty transects each 25 m long were randomly laid out throughout the studied lithologic units correlation of vegetation cover percentage and abundance of trees were studied along with crack dimensions including width depth and length categorized into three classes large medium and low sizes along those transects the amount of ec ph organic materials caso4 caco3 soil texture n p k and soil moisture content in the cracks were also measured the results showed significant differences p 0 05 among crack properties vegetation cover and life forms in the study units also the vegetation cover and life forms have positive significant relationships with increase of crevice width as well as length and depth of each lithologic unit the findings demonstrated that the inherent potential of limestone rocky slopes with deep layers is higher than other rocky units for restoration and afforestation in this arid region

mixed layer instabilities and restratification abstract the restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied the lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical these instabilities which are referred to as mixed layer instabilities mlis differ from instabilities in the ocean interior because of the weak surface stratification spatial scales are o 1 10 km and growth time scales are on the order of a day linear stability analysis and fully nonlinear simulations are used to study mlis and their impact on mixed layer restratification the main result is that mlis are a leading order process in the ml heat budget acting to constantly restratify the surface ocean climate and regional ocean models do not resolve the scales associated with mlis and are likely to underestimate the rate of ml restratification and consequently suffer from a bias in sea surface temperatures and ml depths in a

groundwater recharge in semi arid carbonate aquifers under intensive use the estepa range aquifers seville southern spain quantifying groundwater recharge in carbonate aquifers located in semi arid regions and subjected to intensive groundwater use is no easy task one reason is that there are very few available methods suitable for application under such climatic conditions and moreover some of the methods that might be applied were originally designed with reference to non carbonate aquifers in addition it is necessary to take into account the fact that in any given aquifer groundwater recharge is modified by the groundwater exploitation here we focus on four methods selected to assess their suitability for estimating groundwater recharge in carbonate aquifers affected by intensive exploitation the methods were applied to the estepa range aquifers of seville southern spain which are subjected to different degrees of exploitation two conventional methods were used chloride mass balance and daily soil water balance these results were compared with the results obtained by means of two non conventional methods designed for application to the carbonate aquifers of southern spain the aplis and eras methods the results of the different methods are analogous comparable to those obtained in nearby non exploited carbonate aquifers confirming their suitability for use with carbonate aquifers in either natural or exploited regimes in a semi arid climate

a global perspective on langmuir turbulence in the ocean surface boundary layer the turbulent mixing in thin ocean surface boundary layers osbl which occupy the upper 100 m or so of the ocean control the exchange of heat and trace gases between the atmosphere and ocean here we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the osbl in global climate models which then leads to biases in sea surface temperature one reason we argue is that current parameterizations are missing key surface wave processes that force langmuir turbulence that deepens the osbl more rapidly than steady wind forcing scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing and against buoyancy forcing a global perspective on the occurrence of wave forced turbulence is developed using re analysis data to compute these parameters globally the diagnostic study developed here suggests that turbulent energy available for mixing the osbl is under estimated without forcing by surface waves wave forcing and hence langmuir turbulence could be important over wide areas of the ocean and in all seasons in the southern ocean we conclude that surface wave forced langmuir turbulence is an important process in the osbl that requires parameterization citation belcher s e et al 2012 a global perspective on langmuir turbulence in the ocean surface boundary layer geophys res lett 39 l18605 doi 10 1029 2012gl052932

sub mesoscale lateral density structure in the oceanic surface mixed layer 1 we investigate the horizontal scales of density variability in the surface mixed layer sml in the north pacific subtropical front stf during a period of highly variable atmospheric forcing wavelet analysis shows that horizontal density variability is not restricted to scales l 10 km as previously suggested but extends to l 2 km the limiting l varies strongly with location and corresponds to a local internal rossby radius that accounts for transient stratification above the seasonal thermocline density compensation in the sml achieved when temperature and salinity effects cancel in their effect on density occurs at 30 n at the climatological front associated with the northern boundary of the stf where large thermohaline gradients are observed at 28 n however temperature gradients within the sml are not compensated by salinity and horizontal density gradients result in 2 l 10 km our observations suggest dynamic processes restratify the sml at scales rarely resolved by numerical models of the sml

volcanic ash layers from the last glacial termination in the ngrip ice core the tephrochronological record of the 1400 1640 m depth 10 000 16 000 calendar ice core years before present of the ngrip ice core has been established by particle screening of selected samples ash was identified in 20 samples correlation with ice marine and terrestrial records from volcanic source regions in the northern hemisphere positively identifies the saksunarvatn ash and the vedde ash ash zone 1 major element chemistry of the remaining identified ash layers mainly points towards an icelandic origin this tephrochronological record provides new important marker horizons for correlating the timing of the climatic changes associated with the last glacial termination within the north atlantic region as well as outlining more details concerning the frequency and composition of volcanic eruptions occurring at this deglaciation copyright 2005 john wiley sons ltd

rapid changes in mixed layer stratification driven by submesoscale instabilities and winds 1 submesoscale eddies generated by baroclinic instability of upper ocean fronts lead to rapid restratification of the mixed layer on a time scale of days this restratification can be opposed by a down front wind stress acting in the direction of the geostrophic velocity that drives a surface ekman flow from the dense side to the light side of the front to arrest the slumping of isopycnals a scaling diagnostic is suggested to determine whether the effect of eddies or wind dominates under different conditions using a numerical model we investigate the juxtaposition of submesoscale eddies and down front winds acting on the mixed layer by estimating the eddy induced overturning stream function in the mixed layer we separate the along and cross isopycnal fluxes of buoyancy associated with submesoscale mixed layer eddies and demonstrate the need for parameterization of the advective along isopycnal flux though the cross front transport of buoyancy induced by the down front component of the wind opposes restratification by mixed layer eddies it becomes diminished as the eddies and growth of the frontal instability disrupt alignment between the wind and frontal axis

environmental impact of the laacher see eruption at a large distance from the volcano integrated palaeoecological studies from vorpommern ne germany abstract in order to investigate the environmental impacts of the laacher see eruption 12 900 calendar years b p and the deposition of the laacher see tephra at a large distance from the volcano fine resolution micromorphological diatomological and palynological investigations were carried out on closely spaced areas in the reinberg basin vorpommern ne germany the lower part of the lst originates from atmospheric input whereas the upper part was washed in from the surrounding basin slopes diatom populations expanded as a consequence of the input of silica while simultaneously dissolution of dead diatoms decreased the tephra provided a favourable habitat for many epilithic and epipelic diatom taxa the diatoms show a slight acidification and an increase in specific conductivity of the water whereas no eutrophication is indicated these effects disappeared after lake sediments covered the tephra the pollen diagrams from the reinberg basin as well as many other pollen diagrams from ne germany indicate that the vegetation reacted to rising temperatures after the cooler gerzensee fluctuation to a short lived water level rise connected with higher precipitation after the volcanic eruption and to minor openings in the forest vegetation due to increased fire probably connected with severe thunderstorms

restratification of the upper ocean after the passage of a tropical cyclone a numerical study abstractthe role of baroclinic instability in the restratification of the upper ocean after the passage of a tropical cyclone tc is determined by means of numerical simulations using a regional ocean model the regional ocean modeling system roms a high resolution three dimensional simulation that includes the process of baroclinic instability and is initialized with moderate amplitude eddy structures reproduces the satellite observed decay rate of the tc induced sea surface temperature sst anomaly and is also in qualitative agreement with published observations after the passage of hurricane fabian in 2003 that showed decaying cold and warm anomalies located in the climatological mixed layer cml and upper thermocline respectively the model ocean is restratified after approximately one month with a net heat gain in the water column due to anomalous air sea heat fluxes the model shows however that vertical heat fluxes associated with baroclinic instability dominate over air sea heat fluxes i

generation of subsurface anticyclones at arctic surface fronts due to a surface stress isolated anticyclones are frequently observed below the mixed layer in the arctic ocean some of these subsurface anticyclones are thought to originate at surface fronts however previous idealize

mesoscale to submesoscale transition in the california current system part i flow structure eddy flux and observational tests in computational simulations of an idealized subtropical eastern boundary upwelling current system similar to the california current a submesoscale transition occurs in the eddy variability as the horizontal grid scale is reduced to o 1 km this first paper in a series of three describes the transition in terms of the emergent flow structure and the associated time averaged eddy fluxes in addition to the mesoscale eddies that arise from a primary instability of the alongshore wind driven currents significant energy is transferred into submesoscale fronts and vortices in the upper ocean the submesoscale arises through surface frontogenesis growing off upwelled cold filaments that are pulled offshore and strained in between the mesoscale eddy centers in turn some submesoscale fronts become unstable and develop submesoscale meanders and fragment into roll up vortices associated with this phenomenon are a large vertical vorticity and rossby number a large vertical velocity relatively flat horizontal spectra contrary to the prevailing view of mesoscale dynamics a large vertical buoyancy flux acting to restratify the upper ocean a submesoscale energy conversion from potential to kinetic a significant spatial and temporal intermittency in the upper ocean and material exchanges between the surface boundary layer and pycnocline comparison with available observations indicates that submesoscale fronts and instabilities occur widely in the upper ocean with characteristics similar to the simulations

hydrochemical and isotopic study of groundwater in wadi el hechim garaa hamra basin central tunisia hydrochemical and isotopic study of miocene and mio plio quaternary m p q aquifers in wadi el hechim garaa hamra basin central tunisia was undertaken in order to investigate recharge mode and processes leading to mineralization of groundwater as well as interaction between both systems the results revealed striking differences between the two aquifer systems while the miocene aquifer contains recently recharged waters with generally low mineralization around 0 5 g l 1 stemming mainly from dissolution of carbonate minerals the m p q aquifer reveals tds values reaching 3 g l 1 controlled mainly by dissolution of evaporitic minerals isotopic data indicate that the miocene aquifer contains water recharged in past several decades bomb tritium and bomb radiocarbon detected the m p q system appears to be much slower with time scales of groundwater flow possibly reaching some thousands of years sharp discontinuity of hydrochemical and isotope characteristic of groundwater observed across the major tectonic fault separating the miocene and m p q aquifers supports the idea of very limited if any hydraulic interconnection between both studied systems this in turn calls for revision of existing conceptual models of groundwater flow in the region postulating significant groundwater fluxes crossing the fault in the direction of m p q aquifer and adjacent aquifers in the wadi al fakka plain

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

late and post glacial evolution of lake steisslingen i abstract piston cores from lake steisslingen a small hard water lake situated 20 km west of lake constance and mainly supplied by submerged springs reveal a record of the environmental history going back to 15 600 cal yr bp from the bolling to ca 9800 cal yr bp the sediments are partly seasonally laminated from then on up to the middle ages the lamination is mostly undisturbed providing a high resolution time scale by varve counting a detailed pollen profile and 13 ams 14c analyses all investigated sediment sections show evidence of exceptionally high sulphate concentrations in the lake water which lead to the fixation of all reactive iron as pyrite the high sulphur content can only be attributed to s rich water supplied by the submerged springs thus indicating the miocene obere meeres molasse as the dominant aquifer since soil erosion is strongly reduced due to increased forest density in the preboreal boreal the submerged springs are the main nutrient source for the lake mixing or stable stratification of the waterbody mostly depends on nutrient availability and bioproductivity in the epilimnion the controlling external factors are wind and changes in water level and or temperature at the end of the younger dryas and during the preboreal low water levels and precipitation rates are assumed the water column mixing was regular bioproductivity high and high degradation activity of organic matter led to a redissolution of calcite the hydrological separation of a northern shallow lake seeried probably fell into the period of this low lake level around 10 450 10 300 cal yr bp the lake level rose but never again reached the former level between 10 000 and 7200 cal yr bp non glacial varves are even found in shallower parts of the lake indicating a stable anoxic monimolimnion with a rising chemocline bioproductivity was low wind protection provided by the now dense forestation and favourable climatic conditions e g high summer temperatures and low winter temperatures seem to be the responsible factors fluctuating human impact is identified from about 7400 cal yr bp until permanent settlement by the alemanni about 1500 cal yr bp strong human influence is observed from 2000 cal yr bp onwards when agricultural activity and clearing of the shoreline vegetation by man caused high soil erosion rates and thus eutrophication at about 800 cal yr bp the recent hydrological conditions were established by implementing an artificial drain thus lowering the lake level

the early lateglacial re colonization of britain new radiocarbon evidence from gough s cave southwest england abstract gough s cave is still britain s most significant later upper palaeolithic site new ultrafiltered radiocarbon determinations on bone change our understanding of its occupation by demonstrating that this lasted for only a very short span of time at the beginning of the lateglacial interstadial greenland interstadial 1 gi 1 bolling and allerod the application of bayesian modelling to the radiocarbon dates from this and other sites from the period in southwest england suggests that re colonization after the last glacial maximum took place only after 14 700 cal bp and is therefore more recent than that of the paris basin and the belgian ardennes on their own the radiocarbon determinations cannot tell us whether re colonization was synchronous with just prior to or after lateglacial warming isotopic studies of humanly modified mammalian tooth enamel may be one way forward

a nonhydrostatic mesoscale ocean model part i well posedness and scaling abstract the incompressibility and hydrostatic approximations that are traditionally used in large scale oceanography to make the hydrodynamic equations more amenable to numerical integration result in the primitive equations these are ill posed in domains with open boundaries and hence not well suited to mesoscale or regional modeling instead of using the hydrostatic approximation the authors permit a greater deviation from hydrostatic balance than what exists in the ocean to obtain a system of equations that is well posed with the specification of pointwise boundary conditions at open or solid boundaries these equations formulated with a free surface boundary model the mesoscale flow field accurately in all three dimensions even the vertical it is essential to retain the vertical component of the coriolis acceleration in the model since it is nonhydrostatic

parameterization of mixed layer eddies part ii prognosis and impact abstract the authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts the parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient mixed layer depth and inertial period the parameterization has remarkable skill for an extremely wide range of mixed layer depths rotation rates and vertical and horizontal stratifications in this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations the parameterization proves accurate in predicting changes to the buoyancy the climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations the mixed layer depth is estimated from climatology and the buoyancy gradients are from satellite altimetry the vertical fluxes are comparable to monthly mean air sea fluxes in large areas of

determination of the origins and recharge rates of the sfax aquifer system southeastern tunisia using isotope tracers the origin of groundwater in the sfax aquifer system was studied using environmental isotopic tracers 18o 2h and 3h in total 164 water samples were analyzed for stable isotopes 73 collected from the sfax shallow aquifer 63 from the sfax middle aquifer and 28 from the sfax deep aquifer recent recharge of the groundwater in the sfax aquifer was identified using tritium concentrations in 82 groundwater samples from different depths the isotopic ratios of the shallow aquifer range from 5 55 to 1 59 for 18o and from 38 38 to 14 19 for 2h and the isotopic ratios in the middle aquifer range from 6 86 to 2 97 for 18o and from 44 18 to 22 38 for 2h the deep aquifer exhibited markedly lower isotopic values ranging from 6 70 to 5 70 for 18o and from 42 40 to 38 89 for 2h the mixing proportions inferred from stable isotopic mass balance calculations suggest that the deep aquifer contributes significantly to the middle aquifer through geologic structures and may reach 100 in the menzel chaker region the isotopic mass balance model also indicates that the middle groundwater aquifer may contribute up to 100 of the shallow plio quaternary aquifer particularly in the western and northeastern parts of the study area between bir ali ben khalifa and djebeniana the tritium data support the existence of recent recharge the tritium and stable isotope data clearly indicate the presence of mixing processes especially in the northwestern and coastal portions of the study area a conceptual model is established explaining the pressure differences that generate vertical leakage which is a reasonable mechanism for flow between the aquifers

global estimates of lateral springtime restratification abstractsubmesoscale frontal dynamics are thought to be of leading order importance for stratifying the upper ocean by slumping horizontal density gradients to produce vertical stratification presented here is an investigation of submesoscale instabilities in the mixed layer mixed layer eddies mles as a potential mechanism of frontal slumping that stratifies the upper ocean during the transition from winter to spring when wintertime forcings weaken but prior to the onset of net solar warming observations from the global argo float program are compared to predictions from a one dimensional mixed layer model to assess where in the world s oceans lateral processes influence mixed layer evolution the model underestimates spring stratification for 75 25 of the world s oceans relationships between vertical and horizontal temperature and salinity gradients are used to suggest that in 30 20 of the oceans this excess stratification can be attributed to the slumping of horizontal density fronts fin

mesoscale eddy buoyancy flux and eddy induced circulation in eastern boundary currents abstracta dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the eastern boundary currents off california and peru chile based on regional equilibrium simulations the eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean as such they serve to balance mean offshore air sea heating and coastal upwelling in the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy induced velocity advecting mean buoyancy and tracers furthermore with a suitable gauge choice the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient consistent with the advective parameterization scheme of gent and mcwilliams the associated diffusivity is surface intensified matching the vertical stratification profile the fluxes span the across shore band of high eddy energy but their alongshore structure is unresolved because of sampling limitations in the sur

oceanic restratification forced by surface frontogenesis potential vorticity pv conservation implies a strong constraint on the time evolution of the mean density at a given depth the authors show that on an f plane and in the absence of sources and sinks of pv it only depends on two terms namely the time evolution of the product between density anomaly and relative vorticity and the vertical pv flux this primitive equation result which applies at any depth suggests that the ageostrophic dynamics induced by baroclinic eddies strongly affect the mean oceanic stratification profile this result is illustrated for two simple initial value simulations of a baroclinic balanced jet for initial situations propitious to surface frontogenesis the simulations show a restratification over the whole water column characterized by the amplification in time of the brunt vaisala frequency in the upper oceanic layers in the absence of surface frontogenesis such as when the jet is initialized at the middepth of the water column the restratification is much weaker and slower because both simulations have similar kinetic energy and growth rate of baroclinic instability the results clearly reveal that the restratification is driven by surface frontogenesis in the first case and by vertical pv flux in the interior in the second case the authors also point out that the dynamics of the interior pv is tightly related to the surface dynamics because of total mass conservation

submesoscale fronts in the antarctic marginal ice zone and their response to wind forcing submesoscale flows in the ocean are energetic motions o 1 10 km that influence stratification and the distributions of properties such as heat and carbon they are believed to play an important role in sea ice impacted oceans by modulating air sea ice fluxes and sea ice extent the intensity of these flows and their response to wind forcing are unobserved in the sea ice regions of the southern ocean we present the first submesoscale resolving observations in the antarctic marginal ice zone miz collected by surface and underwater autonomous vehicles for 3 months in austral summer we observe salinity dominated lateral density fronts occurring at sub kilometer scales surface winds are shown to modify the magnitude of the mixed layer density fronts revealing strongly coupled atmosphere ocean processes we posture that these wind front interactions occur as a continuous interplay between front slumping and vertical mixing which leads to the dispersion of submesoscale fronts such processes are expected to be ubiquitous in the southern ocean miz

the surface branch of the zonally averaged mass transport circulation in the troposphere the near surface branch of the overturning mass transport circulation in the troposphere containing the equatorward flow is examined in isentropic and geometric coordinates a discussion of the zonal momentum balance within isentropic layers shows that the equatorward flow at a given latitude is confined to isentropic layers that typically intersect the surface at that latitude as a consequence of mass transport within the surface mixed layer much of the equatorward flow occurs in layers with potential temperatures below the mean surface potential temperature in the conventional transformed eulerian mean formulation for geometric coordinates the surface branch of the overturning circulation is represented in an unrealistic manner streamlines of the residual circulation do not close above the surface a modified residual circulation is introduced that is free from this defect and has the additional advantage that its computation unlike that of the conventional residual circulation does not require division by the static stability which may approach zero in the planetary boundary layer it is then argued that cold air advection by the residual circulation is responsible for the formation of surface inversions at all latitudes in idealized gcms with weak thermal damping also included is a discussion of how a general circulation theory for the troposphere must be built upon a theory for the near surface meridional mass fluxes

reconstruction of postglacial to early holocene vegetation history in terrestrial central europe via cuticular lipid biomarkers and pollen records from lake sediments high resolution environmental reconstruction for central european terrestrial settings is commonly based on sediment records that provide the pollen spectra used for reconstruction of vegetation development alternatively complementary algal insect or zooplankton assemblages and or carbonate 18 o isotope profiles are investigated in this study vegetation development and inferred climate evolution were documented on a decadal to centennial scale by analysis of land plant cuticular lipids accumulated in lake sediments wax lipids derived from land plant leaves and blades are source diagnostic and highly resistant to degradation their aerodynamic properties favor short distance eolian transport but they are less affected by possible long distance transportation that complicates pollen analysis the late glacial to holocene sedimentary record of lake steisslingen southwestern germany was investigated by palynological and geochemical means tundra type pioneer vegetation consisting predominantly of grasses and herbs is dominated by nc31 alkanes whereas pioneer birch forest yields preferentially nc27 alkanes establishment of a pine assemblage is recognizable by elevated nc29 and nc31 leaf wax alkanes and the diversification of deciduous forests in the holocene is marked by nc29 dominance lipid geochemistry and palynology resolve decadal excursions such as the mid younger dryas event centennial climate trends with pronounced and rapid changes at the oldest dryas bolling allerod younger dryas and younger dryas preboreal transitions are consistently reflected by molecular and palynological proxy data the demonstrated suitability of an independent molecular proxy in paleoenvironmental studies provides new perspectives in high resolution climate reconstruction

a nonhydrostatic mesoscale ocean model part ii numerical implementation abstract the nonhydrostatic model with a free surface is numerically implemented in boundary fitted curvilinear coordinates to model the mesoscale circulation in an ocean basin with natural topography a semi implicit numerical scheme is used and the directional inhomogeneity in the elliptic equation for pressure is exploited to speed up the computation of its solution while using the multigrid method the model is used to simulate the circulation in the gulf of mexico we observe the formation of the loop current and several eddies the flow is very strongly controlled by the topography and our numerical experiments reveal that in the bottom layers the flow along topographic contours is in the opposite direction of the anticyclonic circulation in the top layers

topographic influence on submesoscale dynamics in the southern ocean topography influences the circulation in the southern ocean generating stationary meanders in the lee of topographic features triggering hot spots of mesoscale eddy kinetic energy and modifying jets and fronts however the relationship between topography and submesoscale flows with length scales of order 1 10 km has not yet been explored the first submesoscale resolving 1 80 resolution ocean model with realistic topography of the indian sector of the southern ocean is used to investigate this interaction the results show spatial inhomogeneity in submesoscale activity that is correlated with topography topographic influence is primarily indirect topography controls mesoscale flows which in turn generate submesoscale activity mesoscale eddy kinetic energy and strain rate can be used to first order to infer submesoscale vertical velocity implying a possible route to parameterise submesoscale activity in coarser resolution models

downfront winds over buoyant coastal plumes abstractdownfront or downwelling favorable winds are commonly found over buoyant coastal plumes it is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport spatial extent and stability of the plumes in the present study the interaction of the ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets the approach makes use of an idealized numerical model and scaling theory it is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near zero average potential vorticity q for weak to moderate winds the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratificati

submesoscale instabilities in mesoscale eddies submesoscale processes have been extensively studied in observations and simulations of fronts recent idealized simulations show that submesoscale instabilities also occur in baroclinic mesoscale cyclones and anticyclones the instabilities in the anticyclone grow faster and at coarser grid resolution than in the cyclone the instabilities lead to larger restratification in the anticyclone than in the cyclone the instabilities also lead to changes in the mean azimuthal jet around the anticyclone from 2 km resolution but a similar effect only occurs in the cyclone at 0 25 km resolution a numerical passive tracer experiment shows that submesoscale instabilities lead to deeper subduction in the interior of anticyclonic than cyclonic eddies because of outcropping isopycnals extending deeper into the thermocline in anticyclones an energetic analysis suggests that both vertical shear production and vertical buoyancy fluxes are important in anticyclones but primarily vertical buoyancy fluxes occur in cyclones at these resolutions the energy sources and sinks vary azimuthally around the eddies caused by the asymmetric effects of the ekman buoyancy flux glider transects of a mesoscale anticyclone in the tasman sea show that water with low stratification and high oxygen concentrations is found in an anticyclone in a manner that may be consistent with the model predictions for submesoscale subduction in mesoscale eddies

parameterization of mixed layer and deep ocean mesoscales including nonlinearity abstractin 2011 chelton et al carried out a comprehensive census of mesoscales using altimetry data and reached the following conclusions essentially all of the observed mesoscale features are nonlinear and mesoscales do not move with the mean velocity but with their own drift velocity which is the most germane of all the nonlinear metrics accounting for these results in a mesoscale parameterization presents conceptual and practical challenges since linear analysis is no longer usable and one needs a model of nonlinearity a mesoscale parameterization is presented that has the following features 1 it is based on the solutions of the nonlinear mesoscale dynamical equations 2 it describes arbitrary tracers 3 it includes adiabatic a and diabatic d regimes 4 the eddy induced velocity is the sum of a gent and mcwilliams gm term plus a new term representing the difference between drift and mean velocities 5 the new term lowers the transfer of mean potential energy to mesoscales 6 th

influence of late glacial climate changes on sediment transport in the river werra thuringia germany abstract several sedimentary records of floodplain sediments in the werra valley and subcatchments have been analysed in order to investigate the influence of climate change on the sediment transport system in the sedimentary records of the subcatchments a marked change from minerogenic to organic sediments is visible about 14 6 ka b p according to palynological analyses this change occurs in a period with a remarkable rise in arboreal pollen percentages in addition a reverse change from organic to clastic sediments takes place shortly after 12 9 ka b p coincident with a decrease in arboreal pollen percentages during this period assuming that changes in vegetation cover are mainly caused by climate the variations in the sedimentary records are also attributed to these climatic oscillations in contrast to the subcatchments comparable changes in the sedimentary record are not detectable in the floodplain of the werra river this points to the conclusion that the influence of late glacial climate fluctuations on the sedimentary regime of a river with a relatively large and a more heterogeneous catchment seems less pronounced