perturbation solution for dam break floods a solution for the complete and instantaneous collapse of a dam in a prismatic infinitely long sloping channel is found by using the method of matched asymptotic expansions the outer solution is a kinematic wave approximation and an inner solution that includes a depth gradient term in the momentum equation is calculated for the region near the moving kinematic shock front the reservoir is modeled with a point mass source and the solution is believed to become asymptotically valid after the shock front has advanced about four reservoir lengths downstream a comparison with experiment shows good qualitative agreement but more comparisons with experiment should be made in order to assess the qualitative accuracy of the solution

the dynamical control of subduction parameters on surface topography the long wavelength surface deflection of earth s outermost rocky shell is mainly controlled by large scale dynamic processes like isostasy or mantle flow the largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates having a mantle convection model with a free surface that is able to reproduce both realistic single sided subduction and long wavelength surface topography self consistently we are now able to better investigate this interaction we separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters results are diagnosed by splitting the topographic signal into isostatic and residual components and by considering various physical aspects like viscous dissipation during plate bending performing several systematic suites of experiments we are then able to quantify the topographic impact of the buoyancy rheology and geometry of the subduction zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws we identify slab dip and slightly less importantly slab buoyancy as the major agents controlling surface topography at subduction zones on earth only the island arc high and the back arc depression extent are mainly controlled by plate strength overall his modeling study sets the basis to better constrain deep seated mantle structures and their physical properties via the observed surface topography on present day earth and back through time

surface water temperature salinity and density changes in the northeast atlantic during the last 45 000 years heinrich events deep water formation and climatic rebounds we developed a new method to calculate sea surface salinities sss and densities ssd from planktonic foraminiferal 18o and sea surface temperatures sst as determined from planktonic foraminiferal species abundances sst sss and ssd records were calculated for the last 45 000 years for biogeochemical oceanic flux study bofs cores 5k and 8k recovered from the northeast atlantic the strongest feature is the dramatic drop in all three parameters during the heinrich ice rafting events we modelled the possibility of deepwater formation in die northeast atlantic from the ssd records by assuming that the surface waters at our sites cooled as they flowed further north comparison with modelled north atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45 000 and 30 000 14c years b p interrupted by iceberg meltwater input of heinrich event 3 and 4 at 27 000 and 38 000 14c years b p and during the holocene no amount of cooling in the northeast atlantic between 30 000 and 13 000 years could cause deep water to form because of the low salinities resulting from the high meltwater inputs from icebergs our records indicate that after each heinrich event there were periods of climatic rebound with milder conditions persisting for up to 2000 years as indicated by the presence of warmer and more saline water masses after these warm periods conditions returned to average glacial levels these short term cold and warm episodes in the northeast atlantic are superimposed on the general trend towards colder conditions of the last glacial maximum lgm heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the fennoscandian ice sheet we propose that meltwater input of heinrich event 1 significantly reduced north atlantic deep water formation reducing the heat exchange between the low and high latitudes thus delaying deglaciation by about 1500 radiocarbon years 2000 calendar years

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

late quaternary paleoproductivity and deep water circulation in the eastern south atlantic ocean evidence from benthic foraminifera abstract two late quaternary sediment cores from the northern cape basin in the eastern south atlantic ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition the locations of the cores were selected such that both of them presently are bathed by north atlantic deep water nadw and past changes in deep water circulation should be recorded simultaneously at both locations however the areas are different in terms of primary production one core was recovered from the nutrient depleted walvis ridge area whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high recent data served as the basis for the interpretation of the late quaternary faunal fluctuations and the paleoceanographic reconstruction during the last 450 000 years nadw flux into the eastern south atlantic ocean has been restricted to interglacial periods with the strongest dominance of a nadw driven deep water circulation during interglacial stages 1 9 and 11 at the continental margin high productivity faunas and very low epibenthic 13c values indicate enhanced fluxes of organic matter during glacial periods this can be attributed to a glacial increase and lateral extension of coastal upwelling the long term glacial interglacial paleoproductivity cycles are superimposed by high frequency variations with a period of about 23 000 yr enhanced productivity in surface waters above the walvis ridge far from the coast is indicated during glacial stages 8 10 and 12 during these periods cold nutrient rich filaments from the mixing area were probably driven as far as to the southeastern flank of the walvis ridge

millennial scale glacial variability versus holocene stability changes in planktic and benthic foraminifera faunas and ocean circulation in the north atlantic during the last 60 000 years two piston cores ds97 2p from the reykjanes ridge in the central north atlantic ocean 1685 m water depth and enam33 from southwest of the faeroe islands in the ne atlantic 1217 m water depth have been investigated for their planktic and benthic foraminiferal content ds97 2p is situated near the subarctic front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the holocene the productivity shows a clear decrease from an early holocene maximum to a late holocene minimum coeval changes in the benthic faunas indicate that the food supply changed from large irregular pulses during the early holocene to a more sustained flux during the late holocene presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer another feature of the late holocene is an increasing instability of the north atlantic thermohaline circulation regime nevertheless the changes in faunal composition and productivity during the holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial the glacial shifts were on a millennial time scale and correlate with the interstadial stadial phases of the dansgaard oeschger cycles in the greenland ice cores the faunas of the warm interstadial phases resembled the holocene faunas and both surface and bottom productivity was high the faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the nordic seas and generation of north atlantic deep water the planktic faunas during the cold stadials and heinrich events were completely dominated by the polar species neogloboquadrina pachyderma s and surface conditions were cold and the productivity low the benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content the water column was probably stratified with low saline cold surface water overlying poorly aerated intermediate water masses

biogenic carbonate and ice rafted debris heinrich layer accumulation in deep sea sediments from a northeast atlantic piston core abstract biogenic carbonate mass accumulation rates show distinct quaternary glacial interglacial changes in a mid latitude northeast atlantic deep sea core during the last 208 kyr these were generally higher during interglacials averaging 2 2 g cm 2 kyr than during glacial periods with a maximum average of 1 2 g cm 2 kyr intervals containing high ice rafted debris have very low biogenic carbonate accumulation rates paleo primary productivity estimates based on biogenic carbonate accumulation rates are generally higher during interglacials averaging 80 gc m 2 yr than during glacials which average 49 gc m 2 yr maximum estimate short intervals with possibly reduced primary productivity correspond to heinrich layers which are the result of pulses of massive iceberg production and rapid melting in the north atlantic we identified thirteen heinrich layers in isotopic stages 7 to 2 they have calendar ages of about 201 189 182 183 164 167 146 149 142 128 131 64 67 52 54 40 43 29 31 21 and 15 kyr these heinrich layers contain large amounts of ice rafted debris including detrital carbonate low amounts of coccoliths and planktic foraminifera mainly neogloboquadrina pachyderma sinistral the upper five heinrich layers are almost barren of coccoliths and the fourth layer is also free of foraminifera the virtual absence of coccoliths is interpreted to reflect reduced coccolithophorid productivity in the photic zone due to low salinity of surface waters and to blockage of light by ice and suspended materials shed off by melting icebergs considering the limited use of other proxies such as organic carbon diatom and benthic foraminifera due to their very low content abundance in the sediments biogenic carbonate mass accumulation rates offer a workable paleo primary productivity proxy in open ocean sediments of the carbonate dominated northeast atlantic

evaluation of the titan2d two phase flow model using an actual event case study of the 2005 vazcun valley lahar abstract titan2d is a depth averaged thin layer computational fluid dynamics cfd code suitable for simulating a variety of geophysical mass flows titan2d output data include flow thickness and flow momentum at each time step for all cells traversed by the flow during the simulation from this information the flow limit run out path flow velocity deposit thickness and travel time can be calculated results can be visualized in the open source grass gis software or with the built in titan2d viewer a new two phase titan2d version allows simulation of flows containing various mixtures of water and solids the purpose of this study is to compare simulations by the two phase flow version of titan2d with an actual event the chosen natural flow is a small ash rich lahar volume 50 000 m3 70 000 m3 that occurred on 12 february 2005 in the vazcun valley located on the north east flank of volcan tungurahua ecuador lahars and pyroclastic flows along this valley could potentially threaten the 20 000 inhabitants living in and near the city of banos a variety of data sources exist for this lahar including post event meter scale topography and photographic video seismic and acoustic flow monitoring afm records from during the event these data permit detailed comparisons between the dynamics of the actual lahar and those of the titan2d simulated flow in particular detailed comparisons are made between run up heights flow velocity inundation area and flow thickness simulations utilize a variety of data derived from field observations such as lahar volume solid to pore fluid ratio and pre event topography titan2d is important in modeling lahars because it allows assessment of the impact of the flows on buildings and infrastructure lifelines located near drainages that descend from volcanoes

experimental and numerical study of granular flow and fence interaction dense snow avalanches are regarded as dry granular flows this paper presents experimental and numerical modelling of deposition processes occurring when a gravity driven granular flow meets a fence a specific experimental device was set up and a numerical model based on shallow water theory and including a deposition model was used both tools were used to quantify how the retained volume upstream of the fence is influenced by the channel inclination and the obstacle height we identified two regimes depending on the slope angle in the slope angle range where a steady flow is possible the retained volume has two contributions deposition along the channel due to the roughness of the bed and deposition due to the fence the retained volume results only from the fence effects for higher slopes the effects of slope on the retained volume also showed these two regimes for low slopes the retained volume decreases strongly with increasing slope for higher slopes the retained volume decreases weakly with increasing slope comparison between the experiments and computed data showed good agreement concerning the effect of fence height on the retained volume

return period calculation and passive structure design at the taconnaz avalanche path france this paper aims to show how recent knowledge developed in the field of avalanche research can be used for a real case study the taconnaz avalanche path where passive structures already existed but had to be improved first a morphological analysis of the site is done and historical data are analysed second each recorded event is back calculated using a numerical model of dense flow avalanches for each surveyed avalanche parameters at the entry of the runout zone upstream of the defence structures are defined third a statistical analysis of these parameters allows characterization of 100 year return period events fourth physical and numerical models of dense avalanches interacting with defence structures are combined in order to design the most effective passive structure able to contain the reference scenarios finally physical and numerical modelling of the interaction between the powder avalanche and the designed defence structure is performed to show that the proposed improvements do not increase the residual risk due to the powder part in areas downstream of the defence structures

newtonian fluid mechanics treatment of debris flows and avalanches calculations are carried out first for steady fully developed laminar flow on a slope application of this result to field data published in a previous paper by another author suggests that many debris flows are laminar then singular perturbation techniques are used to obtain first order solutions for unsteady non uniform flow from a point source on a steep slope solutions are obtained for both laminar and turbulent flows and it is found that the laminar flow model gives the best description of laboratory data obtained by a previous investigator a number of differences are observed in the behavior of the outer kinematic wave solutions for laminar and turbulent flow and it is suggested that it is probably not possible to describe all debris flows and avalanches with laminar flow models it is also suggested that more comparisons need to be made between theoretical results and field data measurements

composition and origin of volcanic ash zones in late quaternary sediments from the reykjanes ridge evidence for ash fallout and ice rafting abstract seven quaternary volcanic ash zones including the well known ash zones az i and az ii have been investigated in five well dated sediment cores from the reykjanes ridge between 59 and 60 n the zones were defined on their geochemical composition morphological variation of the particles and stratigraphic position the ash zones az i and az ii show a mixture of colorless bubble wall shards and brown blocky glasses whereas ash zones vz 1 to 5 are composed of dark to light brown volcanic glasses ranging in shape and texture from blocky nonvesicular shards to highly vesicular pumiceous types the dark brown glass shards of ash zones vz 1 to 5 represent subalkalic and low k subalkalic basaltic series geochemical correlation of the ash zones with possible source areas indicates that they all derived from iceland the colorless shards in az i and az ii are of rhyolitic composition the eastern volcanic zone on iceland is the most important source with abundant volcaniclastics being produced by subaerial subglacial eruptions and secondary erosion of volcanic rocks explosive volcanism on iceland is manifested by marine ash layers az i and az ii most of the volcaniclastic zones vz 1 to 5 coincide chronologically with peak abundances of coarse ice rafted material ird indicating that most of this volcanic glass was incorporated into the icelandic ice sheet and released to the ocean by iceberg calving and subsequent melting however two layers in vz 1 and vz 5 have no correlatives in the ird record suggesting that they were probably transported by local sediment gravity flows from this we must infer that most of the ice rafting episodes bearing volcanic particles as observed in sediments from reykjanes ridge monitor glacier fluctuations on iceland

thermohaline instability in the north atlantic during meltwater events stable isotope and ice rafted detritus records from core so75 26kl portuguese margin a benthic isotope record has been measured for core so75 26kl from the upper portuguese margin 1099 m water depth to monitor the response of thermohaline overturn in the north atlantic during heinrich events evaluating benthic 18o in ts diagrams in conjunction with equilibrium c fractionation implies that advection of mediterranean outflow water mow to the upper portuguese margin was significantly reduced during the last glacial 15 compared to 30 today the benthic isotope record along core so75 26kl therefore primarily monitors variability of glacial north atlantic conveyor circulation the 14c accelerator mass spectrometry ages of 13 54 07 and 20 46 12 ka for two ice rafted detritus ird layers in the upper core section and an interpolated age of 36 1 ka for a third ird layer deeper in the core are in the range of published 14c ages for heinrich events h1 h2 and h4 marked depletion of benthic 13c by 0 7 1 1 during the heinrich events suggests reduced thermohaline overturn in the north atlantic during these events close similarity between meltwater patterns inferred from planktonic 18o at site 609 and ventilation patterns inferred from benthic 13c in core so75 26kl implies coupling between thermohaline overturn and surface forcing as is also suggested by ocean circulation models benthic 13c starts to decrease 1 5 2 5 kyr before heinrich events hl and h4 fully increased values are reached 1 5 3 kyr after the events indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor s full strength well after the events benthic 13c changes in the course of the heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation a distinct feature of thermohaline instability in numerical models inferrred gradual spin up of thermohaline circulation after hl and h4 is in contrast to abrupt wanning in the north atlantic region that is indicated by sudden increases in greenland ice core 18o and in marine faunal records from the northern north atlantic from this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the greenland ice core record

trophic control of benthic foraminiferal abundance and microhabitat in the bathyal gulf of lions western mediterranean sea surface sediment was sampled at two bathyal sites in the southwestern gulf of lions in the western mediterranean sea in february and august 1997 to study the distribution and microhabitat of living rose bengal stained deep sea benthic foraminifera both standing stock and diversity of the faunas and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments in the axial channel of the lacaze duthiers canyon organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer the corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity and a well developed deep infauna in addition to freshly deposited phytodetritus more degraded organic matter seems to be an important food source in contrast at the open slope organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon the corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow infaunal species with much lower standing stocks and clear differences between february and august in august standing stocks are higher and the average living depths of most species shift towards the sediment surface these differences can be attributed to patchiness or represent a seasonal trophic signal

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

rapid changes in the mechanism of ocean convection during the last glacial period high amplitude rapid climate fluctuations are common features of glacial times the prominent changes in air temperature recorded in the greenland ice cores1 2 are coherent with shifts in the magnitude of the northward heat flux carried by the north atlantic surface ocean3 4 changes in the ocean s thermohaline circulation are a key component in many explanations of this climate flickering5 here we use stable isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate change events deep water was generated more or less continuously in the nordic seas during the latter part of the last glacial period 60 to 10 thousand years ago but by two different mechanisms the deep water formation occurred by convection in the open ocean during warmer periods interstadials but during colder phases stadials a freshening of the surface ocean reduced or stopped open ocean convection and deep water formation was instead driven by brine release during sea ice freezing these shifting magnitudes and modes nested within the overall continuity of deep water formation were probably important for the structuring and rapidity of the prevailing climate changes

recirculating components to the deep boundary current of the northern north atlantic abstract the meridional overturning system of the north atlantic is conventionally thought of as transporting warm water 4 c to high latitudes and covd water to low latitudes the northern cold water sources for this system are dense overflow from the nordic seas and less dense water from the labrador sea lsw the overflow components are carried westward by a deep northern boundary current dnbc that is shown here actually to begin southeast of the overflow regions the dnbc lies along the southern side of a system of islands and submarine ridges that divides the nordic seas from the subpolar basins the overflows act to increase the transport of the dnbc and this transport is further increased through the entrainment of warm waters once inside the labrador sea the dnbc is joined by lsw before continuing south to low latitudes as a cold deep western boundary current dwbc however published transport estimates are larger than can be explained by warm water entrainment alone thus indicating entrainment also of recirculating cold components the subject of this paper their sources are shown to be the lsw and cold abyssal waters originating from the southern hemisphere the lsw is entrained downward into the denser part of the dnbc and laterally into the upper part of the dwbc whereas the cold abyssal waters are supplied by an eastern intensified northward flow in the eastern atlantic that serves as the initial source of the dnbc and by a similar northward flow in the western atlantic the meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system the cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin but their influence in the northwestern atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the newfoundland and labrador basins this influence is detected by a deep silicate maximum extending poleward from the mid latitude western atlantic through the two gyres with eastern concentration and recirculating back diluted toward the south in the dwbc within the respective basins such a maximum also extends northward through the irminger basin into the dnbc but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin or if it comes from the mid latitude eastern basin by way of westward flow through the charlie gibbs fracture zone it is perhaps consequence of both estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north

field experiments and numerical modeling of mass entrainment in snow avalanches 1 snow entrainment alters the speed and hence the run out distance of avalanches yet little is known about this significant process we studied entrainment in snow avalanches using observations from 1 the swiss vallee de la sionne test site 2 the italian pizzac site 3 catastrophic avalanches that occurred during the winter 1998 1999 in switzerland and 4 a medium sized spontaneous avalanche that occurred in 2000 in davos switzerland we determined mass and energy balances for 18 avalanche events on average the mass increased by a factor of 4 the primary mode of entrainment appeared to be frontal ploughing although entrainment behind the avalanche front was also observed step entrainment where a snow cover layer fractures and is entirely consumed by the avalanche also occurred basal erosion was negligible mass availability and snow cover structure were the limiting factors governing entrainment other factors such as track topography and avalanche dimension played a secondary role using the experimental results we introduced an entrainment model into a saint venant type flow model where the internal shear deformation of the avalanche is governed by a bagnold law and the shear stress at the basal layer is treated as a voellmy fluid the model with entrainment not only improves the prediction of the velocities and flow heights in comparison to measurements but also reproduces the variations in run out distances which characterize avalanches with similar terminal velocities but different masses

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

scaling relationships for constant volume snow avalanches we present simple scaling relationships that allow us to predict the main dynamical characteristics height length and velocity of constant volume snow avalanches with minimal input data in particular we show that both avalanche height and length can be expressed as a function only of the avalanche volume and the distance travelled by the front independently of the specific rheological parameters of the snow these scaling relationships are derived from a large time asymptotic solution to the avalanche dynamic equations corresponding to a balance between gravity and voellmy friction the kinematic wave approximation numerical simulations of the complete hydraulic shallow flow equations confirm that this asymptotic solution and the predicted scaling relationships capture the main avalanche characteristics even for relatively small travel distances though derived under restrictive assumptions we argue that the scaling relationships described in this paper may constitute useful tools for avalanche engineering and design of protective structures

paleoceanographic reconstructions of surface ocean conditions in the greenland iceland and norwegian seas through the last 14 ka based on diatoms prior reconstructions of the last glacial maximum and the holocene did not include detailed paleoclimate data from the greenland iceland and norwegian gin seas which cover sensitive areas for oceanographic boundaries sea ice distribution and deep water formation in order to fill this gap we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface gin seas at 2000 year intervals since 13 400 bp based on the diatom record results show that a n s extending sea ice free corridor had opened along norway already at 13 400 bp indicating a northward flow of a branch of the north atlantic drift possibly caused by a change in the jet stream flow due to the decreased height of the north american ice sheet a major change of climatic conditions occurred over the gin seas as the insolation anomaly reached its maximum around 9000 bp when the sea ice cover and the oceanic fronts retreated to a northwesterly position along greenland and the ocean temperatures rose the first half of the holocene is recorded as the warmest period during the last 13 400 bp in the gin seas the duration of this climatic optimum decreased both toward the north and toward the margins of the area the second half of the holocene is characterized by a cooling trend in step with the decreasing insolation these results support the proposal of imbrie et al 1992 that the gin seas region acts as an initial responder to insolation changes