solute transport modelling in karst conduits with slow zones during different hydrologic conditions summary eight tracer tests conducted under different flow conditions in two well developed karst connections in the santa eufemia erenozar karst unit basque country northern spain are considered for transport characterization lithium tracer was injected as an impulse release for obtaining well defined tracer breakthrough curves tbcs in order to simulate curve evolution and obtain transport characteristic parameters two approaches have been applied a standard analytical solution and a new numerical one that accounts for transient storage processes the numerical model simulates the effect of localized variations in conduit geometry that give way to dead zones or slow zones in the transport process this model fits the experimental curves well and explains their tails mainly by diversion processes from the main flow line towards stagnant zones an increase in dispersion coefficient values is observed in both conduits as flow rates and average flow velocities increase stagnation and tracer transient retention processes which may be quantified by means of the numerical solution are more important as circulating flow rates decrease and travel times increase the diversion processes in the two connections follow a similar evolution with significant changes in the magnitude of the retentions registered depending on the hydrodynamic state of the traced system consequently there is a clear hydrodynamic controlled tailing effect

major pathways of atlantic water in the northern north atlantic and nordic seas toward arctic 1 the major pathways of near surface atlantic water in the northern north atlantic and nordic seas are identified as current speeds above 30 cm s using 1014 lagrangian drifters combined with previously published hydrography the inflow over the scotland greenland ridge and establishment of the two branch norwegian atlantic current nwac are described in light of the circulation in the northern north atlantic the western branch of the nwac appears as a jet in the polar front topographically guided from the iceland faroe front through the nordic seas toward fram strait the eastern branch starts as a shelf edge current above the irish scottish continental shelf and after passing through the faroe shetland channel it continues northward along the norwegian shelf edge toward the arctic with a branch bifurcating into the barents sea the nwac appears to maintain its two branch structure throughout the nordic seas with the atlantic water confined to a 200 600 km wide wedge

a reduced order successive linear estimator for geostatistical inversion and its application in hydraulic tomography national natural science foundation of china 51779179 51609173 51479144 51522904 crdf daa2 15 61224 1 tianjin normal university from the thousand talents plan of tianjin city special fund for public industry research from ministry of land and resources of china 201511047

a pseudo genetic stochastic model to generate karstic networks summary in this paper we present a methodology for the stochastic simulation of 3d karstic conduits accounting for conceptual knowledge about the speleogenesis processes and accounting for a wide variety of field measurements the methodology consists of four main steps first a 3d geological model of the region is built the second step consists in the stochastic modeling of the internal heterogeneity of the karst formations e g initial fracturation bedding planes inception horizons etc then a study of the regional hydrology hydrogeology is conducted to identify the potential inlets and outlets of the system the base levels and the possibility of having different phases of karstification the last step consists in generating the conduits in an iterative manner using a fast marching algorithm in most of these steps a probabilistic model can be used to represent the degree of knowledge available and the remaining uncertainty depending on the data at hand the conduits are assumed to follow minimum effort paths in a heterogeneous medium from sinkholes or dolines toward springs the search of the shortest path is performed using a fast marching algorithm this process can be iterative allowing to account for the presence of already simulated conduits and to produce a hierarchical network the final result is a stochastic ensemble of 3d karst reservoir models that are all constrained by the regional geology the local heterogeneities and the regional flow conditions these networks can then be used to simulate flow and transport several levels of uncertainty can be considered large scale geological structures local heterogeneity position of possible inlets and outlets phases of karstification compared to other techniques this method is fast to account for the main factors controlling the 3d geometry of the network and to allow conditioning from available field observations

optimization of the hydrodynamic characteristics of a karst conduit with cfpv2 coupled to ostrich the final publication is available at elsevier via https dx doi org 10 1016 j jhydrol 2018 10 050 2018 this manuscript version is made available under the cc by nc nd 4 0 license https creativecommons org licenses by nc nd 4 0

response of norwegian sea temperature to solar forcing since 1000 a d abstract we report on a 1000 year long oxygen isotope record in sediments of the eastern norwegian sea which we argue represents the temperature and transport of warm atlantic waters entering the nordic sea basin via the north atlantic drift and the large scale meridional overturning circulation the single sample resolution of the record is 2 5 10 years and age control is provided by pb 210 and cs 137 dating identification of historic tephra and a c 14 wiggle match dating method in which the surface reservoir 14c age in the past is constrained rather than assumed thereby eliminating a large source of chronological uncertainty the oxygen isotope results indicate decade to century scale temperature variations of 1 2 degrees c in the shallow similar to 50 m deep subsurface which we find to be strongly correlated with various proxies of past solar activity the correlations are synchronous to within the timescale uncertainties of the ocean and solar proxy records which vary among the records and in time with a range of about 5 30 years the observed ocean temperature response is larger than expected based on simple thermodynamic considerations indicating that there is dynamical response of the high latitude ocean to the sun correlations of our results with a gridded temperature reconstruction for europe are greater in central europe than in coastal regions suggesting that the temperature and transport of warm atlantic waters entering the nordic basin and the pattern of temperature variability over europe are both the proximate responses to a change in the atmospheric circulation consistent with a forced shift in the primary modes of high latitude atmospheric variability less

surface changes in the eastern labrador sea around the onset of the little ice age despite the relative climate stability of the present interglacial it has been punctuated by several centennial time scale climatic oscillations the latest of which are often colloquially referred to as the medieval climatic anomaly mca and the little ice age lia the most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and or volcanic activity and amplified by ocean atmosphere sea ice feedbacks as such changes in the strength of the atlantic meridional overturning circulation amoc are widely believed to have been involved in the amplification of such climatic oscillations the labrador sea is a key area of deep water formation the waters produced here contribute approximately one third of the volume transport of the deep limb of the amoc and drive changes in the north atlantic surface hydrography and subpolar gyre circulation in this study we present multi proxy reconstructions from a high resolution marine sediment core located south of greenland that suggest an increase in the influence of polar waters reaching the labrador sea close to mca lia transition changes in freshwater forcing may have reduced the formation of labrador sea water and contributed towards the onset of the lia cooling

transient hydraulic tomography in a fractured dolostone laboratory rock block experiments 1 the accurate characterization of fractured geologic medium imaging of fracture patterns and their connectivity have been a challenge for decades recently hydraulic tomography has been proposed as a new method for imaging the hydraulic conductivity k and specific storage ss distributions of fractured geologic media while encouraging results have been obtained in the field the method has not been rigorously assessed in a controlled laboratory setting in this study we assess the performance of transient hydraulic tomography tht in a fractured dolomitic rock block the block is characterized through flow through tests and multiple pumping tests the pumping test data were then analyzed with the tht code of zhu and yeh 2005 to image the fracture patterns and their connectivity through the delineation of k and ss distributions or tomograms results show that the tht analysis of pumping tests yields high k and low ss zones that capture the fracture pattern and their connectivity quite well and those patterns become more vivid as additional pumping test data are added to the inverse model the performance of the estimated k and ss tomograms are then assessed by 1 comparing the tomograms obtained from synthetic to real data 2 comparing the tomograms from two different pumping configurations 3 comparing the estimated geometric mean of the hydraulic conductivity kg from the k tomogram to the effective hydraulic conductivity keff estimated from the flow through tests and 4 predicting five independent pumping tests not used in the construction of the k and ss tomograms the performance assessment of the k and ss tomograms reveals that tht is able to image high k and low ss zones that correspond to fracture locations in the fractured rock block and that the tomograms can be used to predict drawdowns from pumping tests not used in the construction of the tomograms with reasonable fidelity

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

episodic reductions in bottom water currents since the last ice age although north atlantic deep water formation was greatly reduced during the last glacial maximum bottom water currents were as vigorous as at present however they were weakened during periods of north atlantic surface freshening a strong correlation can be seen between bottom water current strength and greenland air temperature records thus confirming a close connection between ocean circulation and abrupt climate change past changes in the freshwater balance of the surface north atlantic ocean are thought to have influenced the rate of deep water formation and consequently climate1 2 although water mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3 there has been little dynamic evidence to support this linkage here we present a 25 000 year record of variations in sediment grain size from south of iceland which indicates vigorous bottom water currents during both the last glacial maximum and the holocene period together with reconstructions of north atlantic water mass distribution vigorous bottom currents suggest a shorter residence time of northern source waters during the last glacial maximum relative to the holocene period the most significant reductions in flow strength occur during periods that have been associated with freshening of the surface north atlantic the short term deglacial oscillations in bottom current strength are closely coupled to changes in greenland air temperature with a minimum during the younger dryas cold reversal and a maximum at the time of rapid warming at the onset of the holocene our results support a strong connection between ocean circulation and rapid climate change

turbulent and laminar flow in karst conduits under unsteady flow conditions interpretation of pumping tests by discrete conduit continuum modeling due to the duality in terms of 1 the groundwater flow field and 2 the discharge conditions flow patterns of karst aquifer systems are complex estimated aquifer parameters may differ by several orders of magnitude from local borehole to regional catchment scale because of the large contrast in hydraulic parameters between matrix and conduit their heterogeneity and anisotropy one approach to deal with the scale effect problem in the estimation of hydraulic parameters of karst aquifers is the application of large scale experiments such as long term high abstraction conduit pumping tests stimulating measurable groundwater drawdown in both the karst conduit system as well as the fractured matrix the numerical discrete conduit continuum modeling approach modflow 2005 conduit flow process mode 1 cfpm1 is employed to simulate laminar and nonlaminar conduit flow induced by large scale experiments in combination with darcian matrix flow effects of large scale experiments were simulated for idealized settings subsequently diagnostic plots and analyses of different fluxes are applied to interpret differences in the simulated conduit drawdown and general flow patterns the main focus is set on the question to which extent different conduit flow regimes will affect the drawdown in conduit and matrix depending on the hydraulic properties of the conduit system i e conduit diameter and relative roughness in this context cfpm1 is applied to investigate the importance of considering turbulent conditions for the simulation of karst conduit flow this work quantifies the relative error that results from assuming laminar conduit flow for the interpretation of a synthetic large scale pumping test in karst

what does hydraulic tomography tell us about fractured geological media a field study and synthetic experiments abstract between 2005 and 2010 japan atomic energy agency conducted four long term independent pumping tests in a fractured granite formation at the mizunami underground research laboratory miu site in mizunami city central japan during these tests drawdowns were monitored at different depths along several deep boreholes these tests become one of the few if not the only hydraulic tomographic survey conducted in the world over thousands of meters in a fractured geologic medium with several fault zones we analyzed the drawdown time data set associated with each pumping test independently and then the data sets from all pumping tests jointly to derive the spatial distributions of hydraulic conductivity k and specific storage s s of the medium these estimated distributions revealed some large scale high k and low k zones while the low k zones corroborated well with known low permeable layers and fault based on geological investigations there were no clear geological features that can be related to the large scale high k zones in order to understand and substantiate these high and low k zones we simulated a hydraulic tomographic survey in a synthetic fractured aquifer which bears similar geologic features i e formations fractures and faults at the miu site with exception that the hydraulic properties fracture and fault distributions were known exactly results of the simulation show that not only are the identified high k zones related to fracture networks connected with pumping and observation locations of each pumping test but also their values reflect the degree of connectivity of the network afterward we investigated the extent of the improvement of characterization of the fault and fractures through the use of deploying dense monitoring intervals and late time flux measurements

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

morphometric analysis of three dimensional networks of karst conduits the main idiosyncrasy of a typical karst system is the presence of a three dimensional network of conduits behaving as drains in the system and being responsible of both the quick response of karst springs to rainfall events and the complex distribution of solutes in the system a morphometric analysis of the three dimensional geometry of conduits provides quantitative measures that can be used in a range of applications these morphometric parameters can be used as descriptors of the underground geomorphology they provide information on speleogenesis processes they can be correlated with karst denudation ratios they can be used to control the simulation of realistic stochastic karst networks of conduits and they can be correlated with hydrogeologic behaviour of the karst system the main purpose of this paper is to define describe and illustrate a range of morphometric indexes and morphometric functions that can be calculated nowadays because the availability of three dimensional topographies provided by speleological work and the availability of the computational and graphical power provided by modern computers some of the morphometric parameters describe the existence of preferential directions of karstification others describe the kartification along the vertical and the possible presence of inception horizons other indexes describe the shape complexity of the karstic network whilst other indexes describe spatial variability of the conduit geometry and other parameters give account of the connectivity of the three dimensional network the morphometric analysis is illustrated with a three dimensional karstic network in southern france

north atlantic climate and deep ocean flow speed changes during the last 230 years variations in the near bottom flow speed of iceland scotland overflow water isow are documented in a 230 year long deep sea sediment record from the eastern flank of reykjanes ridge in the subpolar north atlantic at sub decadal time scales for recent decades the isow palaeocurrent reconstructions show similarities with observational hydrographic data furthermore recent isow flow changes fall mostly within the range of its variability of the past 230 years the record also reveals a hitherto unrecognized coupling of deep flow speeds in the subpolar north atlantic with the north atlantic oscillation nao index with more less vigorous isow flow during negative positive phases of the nao our results suggest that the changes in isow vigor are largely controlled by the transport and characteristics of labrador sea water rather than variations in the overflow itself with implications for the meridional overturning of the atlantic ocean and climate

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

evidence for a widespread climatic anomaly at around 9 2 ka before present 1 the 8 2 ka event was triggered by a meltwater pulse mwp into the north atlantic and resultant reduction of the thermohaline circulation thc this event was preceded by a series of at least 14 mwps their impact on early holocene climate has remained almost unknown a set of high quality paleoclimate records from across the northern hemisphere shows evidence for a widespread and significant climatic anomaly at 9 2 ka b p this event has climatic anomaly patterns very similar to the 8 2 ka b p event cooling occurred at high latitudes and midlatitudes and drying took place in the northern tropics and is concurrent with an mwp of considerable volume 8100 km3 as the 9 2 ka mwp occurs at a time of enhanced baseline freshwater flow into the north atlantic this mwp may have been despite its relatively small volume sufficient to weaken thc and to induce the observed climate anomaly pattern

freshwater outburst from lake superior as a trigger for the cold event 9300 years ago paleoclimate proxy records reveal a pervasive cooling event with a northern hemispheric extent approximately 9300 years ago coeval changes in the oceanic circulation of the north atlantic imply freshwater forcing however the source magnitude and routing of meltwater have remained unknown located in central north america lake superior is a key site for regulating the outflow of glacial meltwater to the oceans here we show evidence for an approximately 45 meter rapid lake level fall in this basin centered on 9300 calibrated years before the present due to the failure of a glacial drift dam on the southeast corner of the lake we ascribe the widespread climate anomaly approximately 9300 years ago to this freshwater outburst delivered to the north atlantic ocean through the lake huron north bay ottawa river st lawrence river valleys

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

integrated high resolution geophysical investigations as potential tools for water resource investigations in karst terrain karstic aquifers can be particularly vulnerable to both pollution from surface activities and large scale dewatering from mineral winning operations this is because of the enhanced vertical and lateral flow paths resulting from the dissolution of carbonate species by rainfall often this process results in the development of voids that can range in size from several centimetres to several tens of metres to date groundwater vulnerability maps for england and wales including karst areas have been produced using a methodology that does not consider the presence of karst features the uncertainties that are presented by the potential for pollution by the presence of water carrying conduits in karst areas where there are proposed or existing limestone quarries require new techniques for detecting and delineating underground cave systems in order for any mapping technique to provide an acceptable assessment of vulnerability the location and spatial distribution of high permeability flow paths need to be established of the available geophysics techniques that may allow for the identification of such features microgravity and resistivity imaging are likely to be the most successful microgravity surveying has the potential to identify the presence and location of such voids and with the integration of electrical tomographic work can provide targets for the location of monitoring boreholes whilst these techniques are intensive and may not be cost effective on a regional scale they do have the potential to provide high resolution data over smaller areas which would be invaluable to any site or area specific assessment of vulnerability

lumped and distributed approaches to model natural recharge in semiarid karst aquifers summary recharge is a key component of the hydrological balance in groundwater systems particularly in arid and semiarid settings recharge often takes place during isolated rainfall episodes as a result pumping might cause groundwater levels to fall continuously for long periods of time even if yearly abstractions remain below the average replenishment rate recharge is naturally difficult to quantify as it depends on a complex variety of factors indirect techniques based on mathematical models have long since been advocated as valuable means to estimate recharge this paper presents a methodology to estimate groundwater recharge in quick response semiarid karst aquifers lumped and distributed models are used to evaluate the fraction of rainfall that ultimately results in aquifer recharge as well as the correlation between the magnitude of rainfall events and infiltration rates modelling results are then compared with direct observations of the recharge processes and discussed to evaluate the implications of time scales this study is demonstrated through a case specific application to the ventos aquifer an intensively exploited carbonate system located in one of the driest areas of peninsular spain overall both approaches perform similarly although the lumped model exhibits a better agreement with field records results reflect the nonlinear nature of the rainfall recharge ratio the fraction of rainfall that ultimately recharges the aquifer seems to increase exponentially with the magnitude of the event

holocene thinning of the greenland ice sheet the nature of the response of the greenland ice sheet gis to climate change is a fundamental question in climate science but the details remain unclear during the holocene climate optimum 6 000 to 9 000 years ago for example many locations in the northern hemisphere showed unusual warmth yet changes in greenland appeared inconsistent vinther et al now standardize the previously disparate seeming ice core records of climate history on the gis during the holocene and find consistent evidence for a stronger climate optimum than was previously recognized together with elevation reduction and marginal ice sheet thinning the response of the greenland ice sheet gis to changes in climate remains uncertain during the holocene climatic optimum an unusually warm period from about 9 000 to 6 000 years ago changes in greenland appear inconsistent with the rest of the northern hemisphere here the greenland holocene temperature history and the evolution of gis surface elevation are extracted from four gis locations the results may resolve this disparity on entering an era of global warming the stability of the greenland ice sheet gis is an important concern1 especially in the light of new evidence of rapidly changing flow and melt conditions at the gis margins2 studying the response of the gis to past climatic change may help to advance our understanding of gis dynamics the previous interpretation of evidence from stable isotopes 18o in water from gis ice cores was that holocene climate variability on the gis differed spatially3 and that a consistent holocene climate optimum the unusually warm period from about 9 000 to 6 000 years ago found in many northern latitude palaeoclimate records4 did not exist here we extract both the greenland holocene temperature history and the evolution of gis surface elevation at four gis locations we achieve this by comparing 18o from gis ice cores3 5 with 18o from ice cores from small marginal icecaps contrary to the earlier interpretation of 18o evidence from ice cores3 6 our new temperature history reveals a pronounced holocene climatic optimum in greenland coinciding with maximum thinning near the gis margins our 18o based results are corroborated by the air content of ice cores a proxy for surface elevation7 state of the art ice sheet models are generally found to be underestimating the extent and changes in gis elevation and area our findings may help to improve the ability of models to reproduce the gis response to holocene climate

near surface circulation in the northern north atlantic as inferred from lagrangian drifters variability from the mesoscale to interannual 1 the near surface circulation of the nordic seas is basically cyclonic and consists of jets and recirculation cells which are tightly linked to the bottom topography variable forcing by the large scale rotation of the wind leads to a modulation in the strength of the gyres and their interconnecting jets this is seen in drifter and altimeter data currents are stronger during winter and during phases of high north atlantic oscillation index the exchanges between the north atlantic and the nordic seas do not seem to be directly affected by this variable forcing the narrow boundary currents and the intergyre jets are subject to instability causing mesoscale current fluctuations which contribute to the stirring and mixing of polar and atlantic water masses

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

a dye tracing test as an aid to studying karst development at an artesian limestone sub aquifer zagros zone iran a dye tracing test is employed to study the karst development and flow regime at an artesian limestone sub aquifer the khersan3 dam site zagros zone iran tracer breakthrough curves showed an early dominant peak followed by a pronounced tailing effect the peak concentration was a response to induced pressure during dye injection the results suggest that the dye was pushed into the small pores and fissures around the injection point during dye injection hence the dye moved out as a result of matrix and fissure diffusive processes and created a long pronounced tailing the maximum flow velocity in the upper artesian sub aquifer ranged from 0 97 to 2 9 m h however the mean tracer velocity ranged from 0 19 to 0 51 m h based on the mean residence tracer time consequently the flow regime in the artesian sub aquifer was determined to be mainly diffusive the results reveal 1 a low hydraulic gradient from upstream of the dam axis to downstream consequently there is no considerable flow 2 poor karst development and diffuse flow at the tracing test area 3 a discharge zone at a location downstream of the dam axis which is the main terminal of general flow direction at the dam site

a geostatistical inverse method for variably saturated flow in the vadose zone a geostatistical inverse technique utilizing both primary and secondary information is developed to estimate conditional means of unsaturated hydraulic conductivity parameters saturated hydraulic conductivity and pore size distribution parameters in the vadose zone measurements of saturated hydraulic conductivity and pore size distribution parameters are considered as the primary information while measurements of steady state flow processes soil water pressure head and degree of saturation are regarded as the secondary information this inverse approach relies on the classical linear predictor cokriging theory and takes the advantage of the spatial cross correlation between the soil water pressure head and each of the following degree of saturation saturated hydraulic conductivity and a pore size distribution parameter using an approximate perturbation solution for steady variably saturated flow under general boundary conditions the cross covariances between the primary and secondary information are derived the approximate solution is formulated on the basis of a first order taylor series expansion of a discretized finite element equation the sensitivity matrix in the solution is evaluated by an adjoint state sensitivity approach for flow in heterogeneous media under variably saturated conditions through several numerical examples the inverse model demonstrates its ability to improve the estimates of the spatial distribution of saturated hydraulic conductivity and pore size distribution parameters using the secondary information

the impact of glacier ice contact and subglacial hydrochemistry on evolution of maze caves a modelling approach summary labyrinth and maze cave networks are a conspicuous feature in formerly glaciated stripe karst in scandinavia often found in topographically impossible situations their genesis is attributed to glacial ice contact conditions this is further supported by observing that individual networks may either be influent effluent or through flow depending on the attitude of the host rock and former glacier directions the ice contact hypothesis is tested by using a finite difference fracture network model where chemical and hydrological conditions can be varied subglacial chemistry alone low partial pressure of co 2 low temperature is not sufficient to favour mazes over linear caves however when coupled with high input saturation ratio high and varied hydraulic gradients and glacial hydrology the model produced cave patterns comparable in scale and complexity to our field examples

measurement of the sortable silt current speed proxy using the sedigraph 5100 and coulter multisizer iie precision and accuracy the weight percentage and mean size of the 10 63 m terrigenous silt fraction termed sortable silt of fine grained marine sediments have recently been argued to be proxies for near bottom palaeocurrent intensity this paper details the accuracy and precision of the sedigraph 5100 and the coulter multisizer iie in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep ocean sediments three sortable silt standards and a fine silt clay diluent were generated from natural deep sea sediment of the iceland basin these were used to produce silty clays with standard additions of sortable silt ranging from 1 to 20 of the total by weight accuracy was measured relative to the size of the pure spike measured by each instrument at sortable silt abundance 5 the sedigraph and the coulter multisizer estimate both weight percentage and mean grain size precisely and accurately while below 5 instrumental noise degrades their performance on the basis of the sedigraph results a series of percentage errors is defined which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near bottom current reconstruction it is suggested that ideally palaeocurrent reconstructions should be based on sedigraph determinations of the 0 63 m grain size distribution yielding both mean and abundance or on coulter multisizer determinations of the sortable silt grain size mean the coulter multisizer may prove especially useful in regions of very depleted 5 sortable silt component

hydrography and sedimentation under the deep western boundary current on bjorn and gardar drifts iceland basin abstract the interaction of northern component waters overflowing the iceland scotland ridge with sediments further south in the iceland basin has been responsible for highly variable sedimentation patterns since at least the late miocene 5 million years ago and has formed both bjorn and gardar drifts presently this process is maintained by a flux of approximately 3 sv measured along the south icelandic slope of iceland scotland overflow water isow with a mean regional flow direction to the sw which affects sediment transport on bjorn drift below 1350 m depth and over most of the gardar drift coriolis deflection of isow flow causes it to move as a deep western boundary current dwbc in the iceland basin which redistributes sediments under the influence of seabed topography here hydrographic and sedimentological data are used to illustrate the effects and extent of the vigorous dwbc activity the study presents a baseline against which inferred changes in the past can be assessed north of 58 n bottom nepheloid layers with heavy suspended sediment loads are often encountered particularly under the denser and faster flowing core of isow on the eastern slope of gardar drift in this area the presence of migrating regular sedimentary waves is seen on 3 5 khz profiles this is consistent with both the near bottom flow velocity probably in excess of 10 cm s 1 estimated by the geostrophic method and the presence of well sorted foraminiferal sand on the seabed south of 58 n dilute bottom nepheloid layers associated with more tranquil conditions near the seabed inferred from photographic and sedimentological evidence show that currents on gardar drift are weak although most of the south iceland basin is mantled by both regular and irregular non migratory mudwaves the absence of migrating structures with the exception of current intensification associated with discrete basement highs indicates that more sluggish currents characterise this region throughout the region highly variable mean holocene sediment accumulation rates are inferred 2 40 cm ka 1 as a result of the dwbc interacting with the topography

a decadal scale holocene sea surface temperature record from the subpolar north atlantic constructed using diatoms and statistics and its relation to other climate parameters 1 a sediment core from reykjanes ridge has been studied at 10 to 50 year time resolution to document variability of holocene surface water conditions in the western north atlantic and to evaluate effects of holocene ice rafting episodes diatom assemblages are converted to quantitative sea surface temperatures sst using three different transfer functions spectral and scale space methods are also applied on the records to explore variability at different timescales diatom assemblage and sst records clearly show that decaying remnants of the laurentide ice sheet strongly influenced early holocene climate in the western north atlantic this overrode the predominance of milankovitch forcing which played a key role in the development of holocene climate in the eastern north atlantic and nordic seas superimposed on general holocene climate change is high frequency sst variability on the order of 1 3 c the record also documents climatic oscillations with 600 to 1000 1500 and 2500 year periodicities with a time dependent dominance of different periodicities through the holocene a clear change in variability occurred about 5 ka bp the sst record also provides evidence for holocene cooling events hce that in some cases correlate to documented southward intrusions of ice into the north atlantic

measured volume heat and salt fluxes from the atlantic to the arctic mediterranean 1 the flow of warm and saline atlantic water towards the arctic crosses the greenland scotland ridge in three current branches since the mid 1990 s extensive monitoring with quasi permanent moorings and regular ctd cruises has been in operation on three sections crossing the branches averaged over the years 1999 to 2001 values of volume heat relative to 0 c and salt flux due to the total atlantic inflow across the greenland scotland ridge into the nordic seas are estimated as 8 5 sv 1 sv 106 m3 s 1 313 1012 w and 303 106 kg s 1 in this period the average temperature and salinity of the atlantic inflow were 8 5 c and 35 25 respectively within the observational uncertainty we do not find any significant seasonal variation of the volume flux but a negative correlation between the inflow flux through the faroe shetland channel and through the other two gaps was indicated

characterization of aquifer heterogeneity using transient hydraulic tomography 1 hydraulic tomography is a cost effective technique for characterizing the heterogeneity of hydraulic parameters in the subsurface during hydraulic tomography surveys a large number of hydraulic heads i e aquifer responses are collected from a series of pumping or injection tests in an aquifer these responses are then used to interpret the spatial distribution of hydraulic parameters of the aquifer using inverse modeling in this study we developed an efficient sequential successive linear estimator ssle for interpreting data from transient hydraulic tomography to estimate three dimensional hydraulic conductivity and specific storage fields of aquifers we first explored this estimator for transient hydraulic tomography in a hypothetical one dimensional aquifer results show that during a pumping test transient heads are highly correlated with specific storage at early time but with hydraulic conductivity at late time therefore reliable estimates of both hydraulic conductivity and specific storage must exploit the head data at both early and late times our study also shows that the transient heads are highly correlated over time implying only infrequent head measurements are needed during the estimation applying this sampling strategy to a well posed problem we show that our ssle can produce accurate estimates of both hydraulic conductivity and specific storage fields the benefit of hydraulic tomography for ill posed problems is then demonstrated finally to affirm the robustness of our ssle approach we apply the ssle approach to a hypothetical three dimensional heterogeneous aquifer

two millennia of north atlantic seasonality and implications for norse colonies 18o values of mollusks recovered from near shore marine cores in northwest iceland quantify significant variation in seasonal temperature over the period from 360 b c to a d 1660 twenty six aragonitic bivalve specimens were selected to represent intervals of climatic interest by using core sedimentological characteristics carbonate powder was sequentially micromilled from shell surfaces concordant with growth banding and analyzed for stable oxygen 18o and carbon 13c isotope values because 18o values record subseasonal temperature variation over the lifetime of the bivalves these data provide the first 2 000 year secular record of north atlantic seasonality from ca 360 cal yr b c to cal yr a d 1660 notable cold periods 360 b c to 240 b c a d 410 and a d 1380 to 1420 and warm periods 230 b c to a d 140 and a d 640 to 760 are resolved in terms of contrast between summer and winter temperatures and seasonal temperature variability literature from the viking age ca 790 to 1070 during the establishment of norse colonies and later in iceland and greenland permits comparisons between the 18o temperature record and historical records thereby demonstrating the impact of seasonal climatic extremes on the establishment development and in some cases collapse of societies in the north atlantic

rapid holocene deglaciation of the labrador sector of the laurentide ice sheet abstract retreat of the laurentide ice sheet lis following the last glacial maximum 21 000 yr bp affected regional to global climate and accounted for the largest proportion of sea level rise although the late pleistocene lis retreat chronology is relatively well constrained its holocene chronology remains poorly dated limiting our understanding of its role in holocene climate change and sea level rise here new 10be cosmogenic exposure ages on glacially deposited boulders are used to date the final disappearance of the labrador sector of the lis ls lis these data suggest that following the deglaciation of the southeastern hudson bay coastline at 8 0 0 2 cal ka bp the southwestern margin of the ls lis rapidly retreated 600 km in 140 yr and most likely in 600 yr at a rate of 900 m yr 1 with final deglaciation by 6 8 0 2 10be ka the disappearance of the ls lis 6 8 10be ka and attendant reduction in freshwater runoff may have induced the formation of labrador deep seawater while the loss

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

gulf stream density structure and transport during the past millennium the gulf stream transports approximately 31 sv 1 sv 10 6 m 3 s 1 of water and 1 3 10 15 w of heat into the north atlantic ocean the possibility of abrupt changes in gulf stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries given the limited length of the instrumental record our knowledge of gulf stream behaviour on long timescales must rely heavily on information from geologic archives here we use foraminifera from a suite of high resolution sediment cores in the florida straits to show that the cross current density gradient and vertical current shear of the gulf stream were systematically lower during the little ice age ad 1200 to 1850 we also estimate that little ice age volume transport was ten per cent weaker than today s the timing of reduced flow is consistent with temperature minima in several palaeoclimate records implying that diminished oceanic heat transport may have contributed to little ice age cooling in the north atlantic the interval of low flow also coincides with anomalously high gulf stream surface salinity suggesting a tight linkage between the atlantic ocean circulation and hydrologic cycle during the past millennium

simulating long term nitrate n contamination processes in the woodville karst plain using cfpv2 with umt3d summary a research version of cfp conduit flow process code cfpv2 is applied with umt3d to simulate long term 1966 2018 nitrate n contamination transport processes in the woodville karst plain wkp northern florida where karst conduit networks are well developed groundwater flow in the wkp limestone porous matrix is simulated using darcy s law and non laminar flow within conduits is described by darcy weisbach equation nitrate n conduit transport and advective exchanges of groundwater and nitrate n between conduits and limestone matrix are calculated by cfpv2 and umt3d instead of modflow and mt3dms since reynolds numbers for flows in conduits are over the criteria of laminar flow the developed numerical model is calibrated by field observations and then applied to simulate nitrate n transport in the wkp the numerical simulations verify the theories that two sprayfields near the city of tallahassee and septic tanks in the rural area are major nitrate n point sources within the wkp high nitrate n concentrations occur near lost creek sink and conduits of wakulla spring and spring creek springs where aquifer discharge groundwater conduit networks control nitrate n transport and regional contaminant distributions in the wkp as nitrate n is transported through conduits rapidly and spread over large areas

reconstruction of the late holocene changes in the sub arctic front position at the reykjanes ridge north atlantic changes in the dynamics of the north atlantic subpolar gyre are involved in the modulation of the northward salinity and heat transport in the northern north atlantic via the north atlantic current nac variations in the strength of the east greenland current egc can influence the gyre dynamics by impacting deep convection in the labrador sea oxygen isotope data of three planktonic foraminiferal species surface water neogloboquadrina pachyderma dextral coiling and globigerina bulloides thermocline recorder globorotalia inflata from a site located close to the present sub arctic front at the reykjanes ridge suggest significant strengthening or shifting of the sub arctic front throughout the late holocene the oxygen isotope based inferences are supported by mg ca derived temperature reconstructions from neogloboquadrina pachyderma dextral coiling alkenone derived sea surface temperature and other paleoclimatic proxy data the late holocene strengthening shift of the sub arctic front appears caused

holocene climate variability reflected in diatom derived sea surface temperature records from the subpolar north atlantic a detailed investigation of two deep sea sediment cores from the gardar drift in the subpolar north atlantic cores md99 2251 and md99 2252 has been undertaken to determine the extent of holocene

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

cenozoic sediment accumulation on drifts in the northern north atlantic the northern north atlantic sediment drifts have a much greater areal extent than has previously been indicated the northern north atlantic extends from the east greenland to european continental shelves and from the charlie gibbs fracture zone to the greenland scotland ridge within this region are seven major sediment drifts containing some of the best clues for pre quaternary bottom water circulation in the north atlantic feni drift is the oldest in the region originating near the eocene oligocene boundary it was followed by accumulation of bjorn gardar hatton and snorri drifts from the late early to middle miocene eirik drift may have started to accumulate in the late miocene and gloria drift in the early pliocene through analysis of the cenozoic sediment mass age distribution and the change in area of seafloor for the study area through time it can be concluded that there is a deficit of oligocene to miocene sediment in the region this could be explained by sediment cycling the erosion of older sediment to produce young sediment the mass of pliocene to quaternary sediment is much larger than that of the older sediment and may have been derived primarily through the erosion of eocene and oligocene sediment by deepwater currents the sediment drift portion of the total regional sediment mass increased significantly three times during the cenozoic each of these drift growth phases lasted between 3 and 4 m y the first two growth phases from the late eocene to early oligocene and from the early to middle miocene were accompanied by northward shifts of depocenters to the newly forming sediment drifts the third and largest growth phase occurred between 7 and 3 ma and may have been preceded by the initiation of accumulation on eirik drift 8 7 ma this most recent growth phase was accompanied by high apparent accumulation rates over southern gardar drift as it expanded to the east erosion rates over reykjanes ridge may have been higher during the mid pliocene than in the quaternary there are two main sites where dense water could have formed and flowed south through the rockall trough to start the accumulation of feni drift if the atmosphere were cool enough dense water could have formed in the norwegian sea or on the faeroe shelf dense water may also have formed on the rockall plateau due to salinity increase through evaporation in an arid climate as an alternative to the hypothesis of arctic ocean water overflowing the iceland faeroe ridge in the early middle miocene it is suggested that dense water formed along shallow segments of the iceland faeroe ridge and then flowed into the south iceland basin to begin accumulation of bjorn gardar and snorri drifts the initiation of sediment drift formation in the south iceland basin may have been caused by a decreased rate of deep water production combined with a higher rate of detrital input due to a middle miocene uplift event along the iceland faeroe ridge a second uplift event along the greenland scotland ridge and a decreased production of northern component water in the early pliocene may be responsible for increased sediment accumulation in bjorn gardar and eirik drifts

development of a discrete continuum vdfst cfp numerical model for simulating seawater intrusion to a coastal karst aquifer with a conduit system a hybrid discrete continuum numerical model variable density flow and solute transport conduit flow process vdfst cfp is developed to simulate seawater intrusion to a coastal karst aquifer with a conduit network the darcy weisbach equation is applied to simulate the nonlaminar groundwater flow in the conduit system that is conceptualized as pipes while the darcy equation is used for laminar groundwater flow in the continuum porous medium density dependent groundwater flow with appropriate additional density terms in the conduit is analytically derived the flow and transport equations are coupled and numerically solved by the finite difference method with an implicit iteration procedure two synthetic benchmarks are developed to compare the vdfst cfp model results with other numerical models such as the variable density seawat constant density continuum modflow mt3dms and constant density discrete continuum cfpv2 umt3d models the vdfst cfp model compares reasonably well with the other model results in both conduit and porous medium domains and well describes water and salt exchange between the two systems under turbulent flow conditions within the conduit the darcy weisbach equation calculates the flow rate more accurately without overestimation by the darcy equation sensitivity analysis indicates that conduit diameter friction factor matrix hydraulic conductivity and effective medium porosity are important parameters in the vdfst cfp model the pros and cons of the vdfst cfp model are discussed including the model assumptions and simplifications limitations of the discrete continuum modeling method and the convergence criteria in general the newly developed vdfst cfp model provides a new numerical modeling method for simulating seawater intrusion in a coastal karst aquifer with conduits

rapid early holocene deglaciation of the laurentide ice sheet the demise of the laurentide ice sheet during the early holocene epoch is the most recent and best constrained disappearance of a large ice sheet in the northern hemisphere and thus allows an assessment of rates of ice sheet decay as well as attendant contributions to sea level rise here we use terrestrial and marine records of the deglaciation to identify two periods of rapid melting during the final demise of the laurentide ice sheet when melting ice contributed about 1 3 and 0 7cm of sea level rise per year respectively our simulations with a fully coupled ocean atmosphere model suggest that increased ablation due to enhanced early holocene boreal summer insolation was the predominant cause of laurentide ice sheet retreat although the surface radiative forcing in boreal summer during the early holocene is twice as large as the greenhouse gas forcing expected by the year 2100 the associated increase in summer surface air temperatures is very similar we conclude that our geologic evidence for a rapid retreat of the laurentide ice sheet may therefore describe a prehistoric precedent for mass balance changes of the greenland ice sheet over the coming century

detection of karst conduit patterns via hydraulic tomography a synthetic inverse modeling study abstract karst aquifers are geologic media that consist of enlarged solution conduits the geometry of such conduits is very difficult to delineate and its hydraulic parameters such as hydraulic conductivity k and specific storage ss are difficult to estimate hydraulic tomography ht shown to successfully map the hydraulic heterogeneity of porous and fractured rock aquifers may also be effective in karst aquifers in this study we utilize two synthetic karst aquifers with the branchwork and network conduit patterns bcp and ncp to explore the ability of ht to map conduit patterns and their hydraulic parameters prior to the ht analysis forward models are built to study groundwater flow through synthetic karst aquifers to generate data and to gain insights into flow patterns forward modeling results reveal that 1 flow rates at the outlet of the bcp model is higher than the ncp model even when the same water injection rates are applied at the wells 2 the location of the injection well has a significant impact on the flow rate at the outlet and 3 flow lines mainly follow the conduit pattern these results suggest that the accurate mapping of karst networks is critical next the ht analysis of these data is conducted using a set of six models bcp and ncp to map the heterogeneity of different conduit systems under steady state and transient flow conditions inverse modeling results reveal that the estimated k maps become more reliable as the number of injection tests and the corresponding monitoring data are increased in the ht analysis moreover the results of the bcp model is closer to the original conduit pattern in comparison to the ncp model a comparative study of three inverse models with different number of observation wells and assuming the availability of geological data has shown that a more reliable pattern of the conduits can be mapped by adding geological information even when a small number of observation wells are available overall our results confirm the feasibility of ht in detecting major conduits as well as being able to differentiate between the bcp and ncp network types

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event abstract during the last deglaciation of north america huge proglacial lakes formed along the southern margin of the laurentide ice sheet the largest of these was glacial lake agassiz which formed about 11 7 14 c kyr and drained into hudson bay about 7 7 14 c kyr 8 45 cal kyr overflow from these lakes was variably directed to the mississippi st lawrence and mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events when the ice dam across hudson bay finally was breached a massive flood drained lake agassiz which was routed through hudson strait to the labrador sea in terms of stored water volume the largest reservoir was associated with the kinojevis level of lake agassiz for this maximum filling the impounded water volume available to produce floods is estimated as 40 000 151 000 km 3 depending on the location of the ice margin and route used the timing of this rapid release of stored freshwater just precedes the early holocene cooling event at 8 2 cal kyr bp we use the spring hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are 5 sv and 0 5 yr multiple fillings and floods are possible as are single floods having a complex multipulse structure modelling results suggest that the outburst flood from lake agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established connecting lake agassiz to the tyrrell sea

an algorithm for 3d simulation of branchwork karst networks using horton parameters and application to a synthetic case this paper presents a method to stochastically simulate 3d karstic networks and more specifically branchwork pattern cave systems considering that they can be compared with 3d fluvial networks the topological classification of strahler and the corresponding ratios of horton are used to define three morphometric parameters these parameters are integrated in an algorithm that computes branches hierarchically to obtain a final network organized around the main observed inlet and outlet with a branching complexity controlled by the user each branch corresponds to a low cost path between two points calculated with the a graph search algorithm speleogenetic information on inception horizons palaeo water tables and fractures is accounted for by adapted definitions of the searching functions of a the method is demonstrated on a 3d synthetic case with discrete fractures networks inception horizons and a palaeo water table the simulated karsts have a realistic geometry and are geologically consistent simulation

dynamics of north atlantic deep water masses during the holocene high resolution flow speed reconstructions of two core sites located on gardar drift in the northeast atlantic basin and orphan knoll in the northwest atlantic basin reveal a long term decrease in flow speed of northeast atlantic deep water neadw after 6 500 years benthic foraminiferal oxygen isotopes of sites currently bathed in neadw show a 0 2 depletion after 6 500 years shortly after the start of the development of a carbon isotope gradient between neadw and norwegian sea deep water we consider these changes in near bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the holocene deep ocean circulation and attribute the changes to a weakening of neadw flow during the mid to late holocene that allowed the shoaling of lower deep water and deeper eastward advection of labrador sea water into the northeast atlantic basin

on the importance of geological data for three dimensional steady state hydraulic tomography analysis at a highly heterogeneous aquifer aquitard system abstract hydraulic tomography ht has been shown to map subsurface heterogeneity accurately through the joint interpretation of multiple pumping tests previous research has shown that smooth hydraulic conductivity k estimates are obtained beyond where pumping observation data are available using the geostatistical inversion approach when the inversion begins with a homogeneous k and when data densities are not high however geological data are typically available through outcrops and borehole logs to provide geological variability therefore we investigate the usefulness of geological data for ht analysis at a highly heterogeneous field site by 1 comparing calibrated geological models of two different resolutions to two homogeneous and four highly parameterized geostatistical inverse models in terms of both model calibration and validation performances as well as correspondence of estimated k values with permeameter estimated k profiles along boreholes and 2 using geological models as prior information for the geostatistical inversion approach results reveal that the simultaneous calibration of geological models to seven pumping test data yields k values that correctly reflect the general patterns of vertical distributions of permeameter estimated k we also find that the geostatistical inversion approach using a geological model as prior information performs better for both model calibration and validation than using a homogenous k as a prior and more importantly improves the correspondence of k estimates to permeameter test results along wells as well as in preserving geological features where drawdown measurements are lacking overall our results suggest the joint use of both geological and pumping test data for ht analysis when accurate geological data are available

age of the fjord stade moraines in the disko bugt region western greenland and the 9 3 and 8 2 ka cooling events abstract retreat of the western greenland ice sheet during the early holocene was interrupted by deposition of the fjord stade moraine system the fjord stade moraine system spans several hundred kilometers of western greenland s ice free fringe and represents an important period in the western greenland ice sheet s deglaciation history but the origin and timing of moraine deposition remain uncertain here we combine new and previously published 10 be and 14 c ages from disko bugt western greenland to constrain the timing of fjord stade moraine deposition at two locations 60 km apart at jakobshavn isfjord the northern of two study sites we show that jakobshavn isbrae advanced to deposit moraines ca 9 2 and 8 2 8 0 ka in southeastern disko bugt the ice sheet deposited moraines ca 9 4 9 0 and 8 5 8 1 ka our ice margin chronology indicates that the greenland ice sheet in two distant regions responded in unison to early holocene abrupt cooling 9 3 and 8 2 ka as recorded in central greenland ice cores although the timing of fjord stade moraine deposition was synchronous in jakobshavn isfjord and southeastern disko bugt within uncertainties we suggest that jakobshavn isbrae advanced while the southeastern disko bugt ice margin experienced stillstands during the 9 3 and 8 2 ka events based on regional geomorphology and the distribution of 10 be ages at each location the contrasting style of ice margin response was likely regulated by site specific ice flow characteristics jakobshavn isbrae s high ice flux results in an amplified ice margin response to a climate perturbation both warming and cooling whereas the comparatively low flux sector of the ice sheet in southeastern disko bugt experiences a more subdued response to climate perturbations our chronology indicates that the western greenland ice sheet advanced and retreated in concert with early holocene temperature variations and the 9 3 and 8 2 ka events although brief were of sufficient duration to elicit a significant response of the western greenland ice sheet

centennial to millennial scale holocene climate deep water linkage in the north atlantic the stability of the earth s climate is strongly linked to the steadiness of the atmospheric and oceanic transport of heat from low to high latitudes high resolution holocene sedimentary records from the subpolar north atlantic demonstrate a persistent link between centennial to millennial scale fluctuations in the flow speed of iceland scotland overflow water inferred from granulometric proxy data and both the surface north atlantic current component of the meridional overturning circulation moc and published fennoscandian atmospheric temperature variability however these data also highlight the difficulties associated with the interpretation of geochemical and sedimentological deep water circulation proxies as they do not provide a wholly consistent picture the similarity between surface and deep ocean 13c records suggests a preformed component in the deep ocean 13c signal on the other hand the sedimentological results display a strong linkage to changing conditions in the overlying surface ocean waters suggesting this proxy better reflects the activity of the moc at least during the holocene of this region the most widespread oceanic perturbation coincides with an interval of possibly global climatic deterioration around 2700 years ago these results firmly establish deep water production in the nordic seas as a key component in the assortment of variables affecting climate during the holocene

structure and function of a conduit aquifer the hydrological function of a karst aquifer is strongly controlled by its physical structure observations of physical hydrology and water chemistry along with artificial tracing allow aquifer behaviour to be identified so that hypothetical aquifer structures can be envisaged such structures can be evaluated in terms of the geology and geomorphology of the area in question and explained in terms of the geomorphological evolution of the aquifer a study of the castleguard meadows karst aquifer rocky mountains alberta is used to demonstrate this four karst springs one perennial the remainder intermittent in one group are considered to be the outlets to a number of conduits the oldest and largest of these is possibly choked by sediment and acts as a reservoir rather than a conduit while others convey overflow floods from the upper saskatchewan glacier the aquifer is of great antiquity but has been disrupted by glaciation the aquifer model developed is a simple hypothesis but provides a valuable

speleogenesis geometry and topology of caves a quantitative study of 3d karst conduits karst systems are hierarchically spatially organized three dimensional 3d networks of conduits behaving as drains for groundwater flow recently geostatistical approaches proposed to generate karst networks from data and parameters stemming from analogous observed karst features other studies have qualitatively highlighted relationships between speleogenetic processes and cave patterns however few studies have been performed to quantitatively define these relationships this paper reports a quantitative study of cave geometries and topologies that takes the underlying speleogenetic processes into account in order to study the spatial organization of caves a 3d numerical database was built from 26 caves corresponding to 621 km of cumulative cave passages representative of the variety of karst network patterns the database includes 3d speleological surveys for which the speleogenetic context is known allowing the polygenic karst networks to be divided into 48 monogenic cave samples and classified into four cave patterns vadose branchwork vb water table cave wtc looping cave lc and angular maze am eight morphometric cave descriptors were calculated four geometrical parameters width height ratio tortuosity curvature and vertical index and four topological ones degree of node connectivity and graph indices and ramification index respectively the results were validated by statistical analyses kruskal wallis test and pca the vb patterns are clearly distinct from am ones and from a third group including wtc and lc a quantitative database of cave morphology characteristics is provided depending on their speleogenetic processes these characteristics can be used to constrain and or validate 3d geostatistical simulations this study shows how important it is to relate the geometry and connectivity of cave networks to recharge and flow processes conversely the approach developed here provides proxies to estimate the evolution of the vadose zone to epiphreatic and phreatic zones in limestones from the quantitative analysis of existing cave patterns

on the use of multiple point statistics to improve groundwater flow modeling in karst aquifers a case study from the hydrogeological experimental site of poitiers france abstract limestone aquifers often exhibit complex groundwater flow behaviors resulting from depositional heterogeneities and post lithification fracturing and karstification in this study multiple point statistics mps was applied to reproduce karst features and to improve groundwater flow modeling for this purpose mps realizations were used in a numerical flow model to simulate the responses to pumping test experiments observed at the hydrogeological experimental site of poitiers france the main flow behaviors evident in the field data were simulated particularly i the early time inflection of the drawdown signal at certain observation wells and ii the convex behavior of the drawdown curves at intermediate times in addition it was shown that the spatial structure of the karst features at various scales is critical with regard to the propagation of the depletion wave induced by pumping indeed i the spatial shape of the cone of depression is significantly affected by the karst proportion in the vicinity of the pumping well and ii early time inflection of the drawdown signal occurs only at observation wells crossing locally well developed karst features

three dimensional imaging of aquifer and aquitard heterogeneity via transient hydraulic tomography at a highly heterogeneous field site abstract previous studies have shown that geostatistics based transient hydraulic tomography tht is robust for subsurface heterogeneity characterization through the joint inverse modeling of multiple pumping tests however the hydraulic conductivity k and specific storage ss estimates can be smooth or even erroneous for areas where pumping observation densities are low this renders the imaging of interlayer and intralayer heterogeneity of highly contrasting materials including their unit boundaries difficult in this study we further test the performance of tht by utilizing existing and newly collected pumping test data of longer durations that showed drawdown responses in both aquifer and aquitard units at a field site underlain by a highly heterogeneous glaciofluvial deposit the robust performance of the tht is highlighted through the comparison of different degrees of model parameterization including 1 the effective parameter approach 2 the geological zonation approach relying on borehole logs and 3 the geostatistical inversion approach considering different prior information with without geological data results reveal that the simultaneous analysis of eight pumping tests with the geostatistical inverse model yields the best results in terms of model calibration and validation we also find that the joint interpretation of long term drawdown data from aquifer and aquitard units is necessary in mapping their full heterogeneous patterns including intralayer variabilities moreover as geological data are included as prior information in the geostatistics based tht analysis the estimated k values increasingly reflect the vertical distribution patterns of permeameter estimated k in both aquifer and aquitard units finally the comparison of various tht approaches reveals that differences in the estimated k and ss tomograms result in significantly different transient drawdown predictions at observation ports

identifying flow networks in a karstified aquifer by application of the cellular automata based deterministic inversion method lez aquifer france the distributed modeling of flow paths within karstic and fractured fields remains a complex task because of the high dependence of the hydraulic responses to the relative locations between observational boreholes and interconnected fractures and karstic conduits that control the main flow of the hydrosystem the inverse problem in a distributed model is one alternative approach to interpret the hydraulic test data by mapping the karstic networks and fractured areas in this work we developed a bayesian inversion approach the cellular automata based deterministic inversion cadi algorithm to infer the spatial distribution of hydraulic properties in a structurally constrained model this method distributes hydraulic properties along linear structures i e flow conduits and iteratively modifies the structural geometry of this conduit network to progressively match the observed hydraulic data to the modeled ones as a result this method produces a conductivity model that is composed of a discrete conduit network embedded in the background matrix capable of producing the same flow behavior as the investigated hydrologic system the method is applied to invert a set of multiborehole hydraulic tests collected from a hydraulic tomography experiment conducted at the terrieu field site in the lez aquifer southern france the emergent model shows a high consistency to field observation of hydraulic connections between boreholes furthermore it provides a geologically realistic pattern of flow conduits this method is therefore of considerable value toward an enhanced distributed modeling of the fractured and karstified aquifers

interpretation of pumping tests in a mixed flow karst system a long duration pumping test performed in the conduit of a mixed flow karst system mfks is analyzed and interpreted it constitutes a unique experiment of catchment wide response of a karst system with drawdowns measured both in the pumped conduit and in the matrix a modeling approach is proposed for this interpretation the developed double continuum model consists of two reservoirs karst conduits and the surrounding carbonate rocks between which flow exchange is modeled using the superposition principle and the hypothesis of darcian flow in the matrix considered as an equivalent porous media the karst conduits are assumed to have an infinite hydraulic conductivity model calibration results in a very good match relative root mean square rrms 2 3 with drawdown measured at the pumping well karst conduit it shows that the matrix hydrodynamic parameters hydraulic conductivity and storativity have a greater influence on the drawdown than the storage capacity of the conduit the accuracy of the model relies mostly on a very good knowledge of both pumping rate and natural discharge at the spring with and without pumping this type of approach represents an advance in double continuum modeling of karst systems it also provides a methodology for the management of water resources from karst aquifers

faroe bank channel overflow 1995 2005 abstract the faroe bank channel fbc is the deepest passage across the greenland scotland ridge and there is continuous overflow of dense water through the channel which provides a substantial contribution to the global thermohaline circulation since november 1995 acoustic doppler current profilers adcps have been moored at the sill of the channel continuously except for short annual servicing periods in addition to this ctd sections have regularly been occupied across the channel and instruments recording temperature and salinity have been moored for short periods these observations demonstrate that the channel due to its small width can be monitored fairly well with only one adcp mooring and they allow the generation of a time series of overflow volume flux for the 1995 2005 period the kinematic overflow flux defined solely from the velocity field was found to have an average value of 2 1 0 2 sv with a seasonal and with inter annual variations of approximately 10 amplitude but with no discernible trend for the whole period the average flux of water with potential density exceeding 27 8 kg m 3 was found to be 1 9 0 3 sv with average properties 0 25 c s 34 93 28 01 kg m 3 for this period no evidence was found for a weakening overflow flux but the salinity of the fbc overflow especially the warmer parts has increased substantially during the period which implies an increased average density on the order of 0 01 kg m 3 previous observations of a helical cross channel circulation are confirmed and may explain the thin pycnocline layer but shear instability induces intensive mixing that puts a lower limit on the layer thickness critical control at one point of the sill cross section rather than friction seems to govern the overflow dynamics and simple layered models can explain much of the observed features as well as the seasonal variation but potential vorticity as defined for a single overflow layer is not conserved a previously suggested 1950 2000 weakening of the fbc overflow which was partly based on a subset of the data presented here is not supported by the full dataset

three dimensional interference test interpretation in a fractured aquifer using the pilot point inverse method a geostatistical conditional simulation routine that employs both parametric and nonparametric geostatistics is coupled to the pilot point inverse method to simulate the spatial distribution of conductivities in a dolomitic aquifer which is fractured in some areas and unfractured in others an inversion is then conducted to obtain the conductivities within the fractured and nonfractured parts of the aquifer to match interference data from a series of three dimensional pumping tests for an ensemble of 100 conditional simulations the calibrated and predicted drawdowns match the observed drawdowns well a comparison to the transmissivity interpreted from a single well pumping test compared very well to the geometric mean transmissivity calculated from the calibrated realizations in this study the results from this study indicate that conditioning the conductivity fields to the geologic facies data in this case fractured unfractured categorical data as well as to ample transient hydraulic data can lead to robust groundwater flow models which can adequately predict the response to other hydraulic interference tests

on the importance of geological data for hydraulic tomography analysis laboratory sandbox study abstract this paper investigates the importance of geological data in hydraulic tomography ht through sandbox experiments in particular four groundwater models with homogeneous geological units constructed with borehole data of varying accuracy are jointly calibrated with multiple pumping test data of two different pumping and observation densities the results are compared to those from a geostatistical inverse model model calibration and validation performances are quantitatively assessed using drawdown scatterplots we find that accurate and inaccurate geological models can be well calibrated despite the estimated k values for the poor geological models being quite different from the actual values model validation results reveal that inaccurate geological models yield poor drawdown predictions but using more calibration data improves its predictive capability moreover model comparisons among a highly parameterized geostatistical and layer based geological models show that 1 as the number of pumping tests and monitoring locations are reduced the performance gap between the approaches decreases and 2 a simplified geological model with a fewer number of layers is more reliable than the one based on the wrong description of stratigraphy finally using a geological model as prior information in geostatistical inverse models results in the preservation of geological features especially in areas where drawdown data are not available overall our sandbox results emphasize the importance of incorporating geological data in ht surveys when data from pumping tests is sparse these findings have important implications for field applications of ht where well distances are large

simulation of the development of karst aquifers using a coupled continuum pipe flow model 1 this paper is intended to provide insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems the dissolution kinetics and the associated temporal decrease in flow resistance karst water hydraulics is strongly governed by the interaction between a highly conductive low storage conduit network and a low conductive high storage rock matrix under variable boundary conditions only if this coupling of flow mechanisms is considered can an appropriate representation of other relevant processes be achieved e g carbonate dissolution transport of dissolved solids and limited groundwater recharge here a parameter study performed with the numerical model carbonate aquifer void evolution cave is presented which allows the simulation of the genesis of karst aquifers during geologic time periods cave integrates several important features relevant for different scenarios of karst evolution 1 the complex hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix 2 laminar as well as turbulent flow conditions 3 time dependent and nonuniform recharge to both flow systems 4 the widening of the conduits accounting for appropriate physicochemical relationships governing calcite dissolution kinetics this is achieved by predefining an initial network of karst conduits protoconduits which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions the increase in conduit transmissivity is associated with an increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time the importance of various parameters controlling karst genesis is demonstrated in a parameter study covering the recharge distribution the upgradient boundary conditions for the conduit system and the hydraulic coupling between the conduit network and the rock matrix in particular it is shown that conduit diameters increase in downgradient or upgradient direction depending on the spatial distribution local versus uniform of the recharge component which directly enters the conduit system

holocene multidecadal to millennial scale variations in iceland scotland overflow and their relationship to climate the nordic seas overflows are an important part of the atlantic thermohaline circulation while there is growing evidence that the overflow of dense water changed on orbital time scales during the holocene less is known about the variability on shorter time scales beyond the instrumental record here we reconstruct the relative changes in flow strength of iceland scotland overflow water isow the eastern branch of the overflows on multidecadal millennial time scales the reconstruction is based on mean sortable silt ss from a sediment core on the gardar drift 60 19 n 23 58 w 2081 m our ss record reveals that the main variance in isow vigor occurred on millennial time scales 1 2 kyr with particularly prominent fluctuations after 8 kyr superimposed on the millennial variability there were multidecadal centennial flow speed fluctuations during the early holocene 10 9 kyr and one prominent minimum at 0 9 kyr we find a broad agreement between reconstructed isow and regional north atlantic climate where a strong weak isow is generally associated with warm cold climate we further identify the possible contribution of anomalous heat and freshwater forcing respectively related to reconstructed overflow variability we infer that ocean poleward heat transport can explain the relationship between regional climate and isow during the middle to late holocene whereas freshwater input provides a possible explanation for the reduced overflow during early holocene 8 10 kyr

capturing aquifer heterogeneity comparison of approaches through controlled sandbox experiments 1 groundwater modeling has become a vital component to water supply and contaminant transport investigations these models require representative hydraulic conductivity k and specific storage ss estimates or a set of estimates representing subsurface heterogeneity currently there are a number of approaches for characterizing and modeling k and ss heterogeneity in varying degrees of detail but there is a lack of consensus for an approach that results in the most robust groundwater models with the best predictive ability the main goal of this study is to compare different heterogeneity modeling approaches e g effective parameters geostatistics geological models and hydraulic tomography when input into a forward groundwater model and used to predict 16 independent cross hole pumping tests we first characterize a sandbox aquifer through single and cross hole pumping tests and then use these data to construct forward groundwater models of various complexities both homogeneous and heterogeneous distributions two effective parameter models are constructed 1 by taking the geometric mean of single hole test k and ss estimates and 2 calibrating effective k and ss estimates by simultaneously matching the response at all ports during a cross hole test heterogeneous models consist of spatially variable k and ss fields obtained via 1 kriging single hole data 2 calibrating a geological model and 3 conducting transient hydraulic tomography zhu and yeh 2005 the performance of these parameter fields are then tested through the simulation of 16 independent cross hole pumping tests our results convincingly show that transient hydraulic tomography produces the smallest discrepancy between observed and simulated drawdowns

comparative study of transient hydraulic tomography with varying parameterizations and zonations laboratory sandbox investigation the final publication is available at elsevier via http dx doi org 10 1016 j jhydrol 2017 09 045 2017 this manuscript version is made available under the cc by nc nd 4 0 license http creativecommons org licenses by nc nd 4 0

characterization and development of subsurface fractures observed in the marcellus formation appalachian plateau north central pennsylvania we report on subsurface deformation features measured from recently acquired core and image logs from the marcellus formation in north central pennsylvania supplemented with data collected from valley and ridge outcrop the subsurface data are from an area that bridges a gap in existing published data between outcrop in the valley and ridge province of pennsylvania and the relatively undeformed outcrop exposed on the appalachian plateau of new york we find one distinct set of vertical veins that strike orthogonal to appalachian plateau fold axes and an associated set of low angle reverse faults and stylolites that strike parallel to fold axes as the trend of the fold axes changes around the pennsylvania salient the trend of associated mesostructures changes to maintain kinematic compatibility with the shortening direction change along the salient these structures are interpreted to support a single although possibly protracted strain event during alleghanian deformation as opposed to multiple events previously interpreted to occur in different regions and stratigraphic levels within the fold belt the veins are associated with clusters of bedding plane slip surfaces which are found at distinct mechanical stratigraphic positions where the competence contrast between shale and stiff limestone units is greatest we interpret this association to indicate that bed parallel shear within detachment zones provides the sufficient driving force required to nucleate vertical veins and the decoupling of beds accommodates extension orthogonal to the shortening direction although these veins are oriented orthogonal to the present day maximum principal horizontal stress they remain propped open by crystalline cements homogenization temperatures of fluid inclusions trapped in the veins combined with one dimensional burial and thermal history models suggest that the pervasive vein set formed during the late pennsylvanian permian during the alleghanian orogeny

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

links between early holocene ice sheet decay sea level rise and abrupt climate change the beginning of the holocene interglacial was marked by ice sheet melting and sea level rise a review of sea level and climate records identifies two sea level jumps associated with the final drainage of glacial lake agassiz and links them to an abrupt cooling event

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

the arctic atlantic thermohaline circulation abstractthe atlantic ocean s thermohaline circulation is an important modulator of global climate its northern branch extends through the nordic seas to the cold arctic a region that appears to be particularly influenced by climate change a thermohaline circulation is fundamentally concerned with two degrees of freedom this is in particular the case for the inflow of warm and saline atlantic water through the nordic seas toward the arctic that is balanced by two branches of outflow the authors present an analytical model rooted in observations that constrains the strength and structure of this arctic atlantic thermohaline circulation it is found maybe surprisingly that the strength of atlantic inflow is relatively insensitive to anomalous freshwater input it mainly reflects changes in northern heat loss freshwater anomalies are predominantly balanced by the inflow s partition into estuarine and overturning circulation with southward polar outflow in the surface and dense overflow at depth res

variations in the strength of the north atlantic bottom water during holocene abstract we report here on a multi proxy study of the changes in the dynamics and the properties of bottom water mass in the subpolar north altantic during holocene magnetic properties coupled with sortable silt and benthic carbon isotopes are investigated for holocene marine sedimentary sequences located in the charlie gibbs fracture zone 53 n and in central 57 n and southern gardar drift 59 n all the cores are located at water depths bathed by the iceland scotland overflow water isow mixed at the southernmost locality with southern sourced water masses the long term variations in measured proxies are fitted with similar polynomial curves an early holocene event characterized by a shutdown shoaling of the bottom circulation at the deepest sites is most likely related to the main deglacial freshwater inputs it is followed by a progressive strengthening deepening of the overflow water which culminates around 6 kyr in coincidence with the holocene thermal maximum after 6 kyr corresponding to a drastic hydrological reorganization in the north atlantic a general decline in the bottom flow strength is observed until about 2 kyr b p when it reached its present day state after detrending several short periods of reduced bottom flow strength and sedimentary transport from the northern detrital sources are observed with a periodicity of around 600 yr with no clear relationship at this time scale between surface and deep ocean