chromite in the platreef bushveld complex south africa occurrence and evolution of its chemical composition the regional distribution and chemical composition of massive and disseminated chromitites through a platreef sequence and along a strike distance of over 20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the bushveld complex the chromitite layers and seams of the platreef form two main chromite bearing zones the upper chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the platreef top contact and the lower chromitite that is composed of multiple seams in feldspathic harzburgite pyroxenite and norite close to the bottom contact of the platreef with footwall electron micro probe analyses reveal that the chemical composition of chromite depends on the host rock type norite and pyroxenite host chromite with the highest cr2o3 content while harzburgite hosted chromites are cr and mg poor the wide range in chromite compositions is explained by the influence of late magmatic processes including post cumulus growth and re equilibration interaction with fluid and sulphide saturated magmatic liquid and contact metamorphism each of these processes is characterised by its own distinct geochemical signature but generally they lead to a decrease in mg and al and an increase in both di and tri valent fe in the chromite the occurrence of chromitite locally on the different distance from the contact between the upper platreef sills and the overlying main zone magma suggests erosion of the upper platreef by the main zone as it was emplaced the localisation of chromitites supports an independent development of the northern limb during the critical zone emplacement although the chemical composition of chromite and co existing silicates from ultramafic rocks suggest a critical zone affinity with the eastern and western limbs of the bushveld complex

the effect of a storm on the 1992 summer sea ice cover of the beaufort chukchi and east siberian seas for the summer beaufort chukchi and east siberian seas a variety of active and passive microwave satellite data are used to determine the response of the ice edge and interior to a storm specifically the study concentrates on a low pressure system that passed over the region between august 14 and 20 1992 with peak geostrophic winds of about 18 m s 1 through the use of the ers 1 imaging radar and the special sensor microwave imager the ice response is examined at two locations at the ice edge for a nearly stationary location in the chukchi sea and in the ice interior along three swaths in the beaufort chukchi and east siberian seas the ice edge observations show that the storm fractures the large floes into small floes some of which are advected into the adjacent warm water the ice interior observations show that the storm caused an increase in the open water amount and a shift in the floe size distribution toward smaller floes in the ice interior application of the cumulative number distribution n d where n is the number of floes per unit area that are no smaller than some floe diameter d shows that for d 1 km n behaves like d where lies in the range 1 8 2 9 our analysis also shows that this slope is not affected by the storm and is slightly larger near the ice edge

circulation and thermohaline variability of the hanna shoal region on the northeastern chukchi sea shelf we analyzed velocity and hydrographic data from 23 moorings in the northeast chukchi sea from 2011 to 2014 in most years the eastern side of hanna shoal was strongly stratified year round while weakly stratified regions prevailed on the shelf south and west of the shoal stratification differences cause differential vertical mixing rates which in conjunction with advection of different bottom water properties resulted in seasonally varying along isobath density gradients in agreement with numerical models we find that bottom waters flow anticyclonically around the shoal whereas most of the shelf responded barotropically to wind forcing there was a strong baroclinic component to the flow field northeast of hanna shoal resulting in no net vertically integrated transport on average in contrast there is a net eastward transport from west of the shoal which implies convergence north of the shoal convergence and along isobath density gradients may foster cross shelf exchange north of hanna shoal modal analyses indicate that the shelf south of the shoal and barrow canyon responded coherently to local and remote winds whereas the wind current response around hanna shoal was less coherent barotropic topographic waves of 3 day period were generated episodically northeast of the shoal and propagate clockwise around hanna shoal but are blocked from entering barrow canyon and are possibly scattered by the horizontally sheared flow and converging isobaths on the western side of the shoal analysis of water properties on the western side of hanna shoal suggests that these include contributions from the western and southern portions of the chukchi sea

beaufort chukchi ice margin data from seasat ice motion the motion of sea ice in the summer and fall marginal zone of the beaufort and chukchi seas is observed by tracking ice features in sequential seasat synthetic aperture radar images four examples of ice motion fields are shown and discussed with translation observation density finer than one observation per 5 km for the most part in this region the components of proportional relative motion are shown to commonly have values in the range 0 1 to 0 25 day throughout the marginal zone which itself has a scale of the order of 100 km

the chromites of the bushveld complex in the nietverdiend area a geologic map and stratigraphic column of the nietverdiend basin are presented based on recent mapping and two boreholes drilled in the area the floor of this part of the bushveld complex is undulous with two approximately north south trending structures composed of metasedimentary rocks flanking the lower and critical zones the floor is confined to a north south aligned kidney shaped basin the marginal zone is composed of gabbroic and noritic rocks showing erratic variability in mafic mineral composition and reversed zoning in the plagioclases the lower and critical zones consist of bronzitites alternating with olivine and or chromite bearing rock types throughout these two zones the compositions of the silicates remain approximately constant the chemistry of the lg 1 to lg 4 chromitite layers was investigated in detail and the nature of the cation substitutions assessed there is an increase in cr 2 o 3 abundance from layer lg 1 to lg 4 with al 2 o 3 and or fe 2 o 3 being substituted in the spinel structure principal component analysis is used to group and differentiate among the four chromitite layers a dissimilarity between the chemistry of the chromitite layers from the southern and northern sectors of the basin is illustrated extensive assimilation of floor rocks by the bushveld magma especially during the earlier part of the intrusion is postulated

early summer water mass transformation in the eastern chukchi sea abstract pacific water entering the chukchi sea through bering strait can be modified by physical processes such as surface heating and cooling and internal mixing here we document the existence of a previously unobserved water mass in the eastern chukchi sea during the early summer using hydrographic and velocity data from the june july 2010 icescape survey the water mass in question is as salty as pacific winter water pww but is much warmer with temperatures typical of summer waters we label the water mass dense chukchi summer water dcsw which is dense enough to ventilate the upper halocline of the western arctic ocean normally ventilated by the much colder pww satellite sea surface temperature data long term mooring data from bering strait and the beaufort shelfbreak and atmospheric reanalysis fields are used to characterize the spatial and temporal variability of the dcsw and explore potential forcing scenarios that can lead to its formation in 2010 the dcsw did not enter the chukchi sea through bering strait hence it had to be formed locally on the shelf two distinct forcing mechanisms are considered 1 mixing of very warm alaskan coastal water with the cold pww when the two water masses come in contact in the energetic alaskan coastal current and 2 warming of pww when it experiences solar heating within the coastal polynya in the northeastern chukchi sea during the spring melt season finally we explore the conditions under which the springtime polynya has formed over the past three decades as it relates to the formation of dcsw

topographic preconditioning of open ocean deep convection abstract evidence for oceanic convection over maud rise in the weddell sea suggests that bottom topography may select the location and scale of deep convecting oceanic chimneys forced by seasonal large scale atmospheric cooling in this paper the role of bottom topography in open mean deep convection is studied using an idealized three dimensional primitive equation model a barotropic mean flow impinges on a gaussian shaped seamount in a stratified domain generating a taylor cap a region of topographically trapped fluid uniform surface cooling is applied throughout the domain when the taylor cap is tall enough to interact with the surface mixed layer the local isolation from the advection of heat by the mean flow forms a conduit into the deep water convection within this region is significantly enhanced relative to ambient levels away from the seamount and to similar numerical simulations performed without bottom topography given uniform background stratification domed isopycnals are not importa

the cenomanian turonian boundary event oae2 and palaeoenvironmental change in epicontinental seas new insights from the dinocyst and geochemical records abstract organic walled dinoflagellate cyst dinocyst and geochemical records across the cenomanian turonian boundary ctb are presented for the nw european reference section at eastbourne england dinocyst and nannofossil fertility indexes indicate that an upwelling driven productivity pulse accompanied a eustatic sea level fall that preceded by at least 40 kyr the global rise in 13 c values that marks the onset of oceanic anoxic event 2 oae2 and the deposition of black shales in the deep ocean a marine productivity collapse in the latest cenomanian is evidenced by the falling absolute and relative abundance of peridinioid dinocysts believed to be the product of heterotrophic dinoflagellates this biotic change accompanied transgression and sharply rising sea surface temperatures following an atlantic wide episode of short lived cooling geochemical tracers provide evidence of caribbean colombian plateau volcanism the increase in water depth caused by the latest cenomanian transgression resulted in less eutrophic waters in epicontinental seas where ctb biotic turnover was driven largely by water mass changes rather than oxygen depletion the species richness absolute abundance ratio of dinocysts is proposed as a water mass stability proxy

circulation and water mass transformation in a model of the chukchi sea 1 the circulation and water mass transformation in a regional ocean ice model of the chukchi sea are discussed the model has horizontal resolution of o 4 km is forced by fluxes derived from daily ncep reanalysis fields and has seasonally varying transport temperature and salinity imposed at bering strait many of the observed characteristics of the mean circulation and seasonal cycle in the chukchi sea are reproduced the discussion focuses on the branching of the inflow transport into pathways following herald canyon central channel and the alaskan coast the pattern of ice melt and the water mass transformation and formation of winter water and hypersaline water the ice melt pattern and timing is strongly influenced by advection through bering strait high frequency forcing results in a larger region of ice melt particularly over the shoals and in the northern chukchi sea compared to monthly mean forcing in the model the seasonal cycle of salinity in the southern and central chukchi sea is dominated by advection through bering strait while local atmospheric forcing and brine rejection are more important north of herald and hanna shoals and in barrow canyon however since the residence time in the chukchi sea is generally less than 1 year interannual variability in the bering strait salinity will be reflected in the salinity across the chukchi sea and at barrow canyon

lateral mixing across ice meltwater fronts of the chukchi sea shelf summer and fall hydrographic sections in the northeastern chukchi sea frequently capture 5 20 m thick intrapycnocline lenses or horizontal plumes of warm moderately salty summer bering sea water flowing northward from bering strait these features occur within the shallow 20 m depth pycnocline separating cold dilute surface meltwater from near freezing salty winter formed waters beneath the pycnocline an idealized numerical model suggests that the features arise from eddies and meanders generated by instability of the surface front separating meltwater from bering sea water warm bering sea water is transported across the front and into the pycnocline by the cross frontal velocities associated with the instabilities the accompanying lateral eddy heat fluxes may be important both in summer for promoting ice melt and in fall by delaying the onset of ice formation over portions of this shelf lateral heat flux magnitudes depend upon the stratification of the bering sea water

ocean circulation and fronts as related to ice melt back in the chukchi sea the work reported herein was supported in part by the arctic submarine laboratory naval ocean systems center san diego california under project order nos 000007 and 00164

circulation on the north central chukchi sea shelf abstract mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central chukchi shelf previous studies indicated that pacific waters crossed the chukchi shelf through herald valley in the west and barrow canyon in the east we find a third branch through the central channel onto the outer shelf the central channel transport varies seasonally in phase with bering strait transport and is 0 2 sv on average although some of this might include water entrained from the outflow through herald valley a portion of the central channel outflow moves eastward and converges with the alaskan coastal current at the head of barrow canyon the remainder appears to continue northeastward over the central outer shelf toward the shelfbreak joined by outflow from herald valley the mean flow opposes the prevailing winds and is primarily forced by the sea level slope between the pacific and arctic oceans current variations are mainly wind forced but baroclinic forcing associated with upstream dense water formation in coastal polynyas might occasionally be important winter water mass modification depends crucially on the fall and winter winds which control seasonal ice development an extensive fall ice cover delays cooling limits new ice formation and results in little salinization in such years bering shelf waters cross the chukchi shelf with little modification in contrast extensive open water in fall leads to early and rapid cooling and if accompanied by vigorous ice production within coastal polynyas results in the production of high salinity 33 shelf waters such interannual variability likely affects slope processes and the transport of pacific waters into the arctic ocean interior

the summer hydrographic structure of the hanna shoal region on the northeastern chukchi sea shelf 2011 2013 abstract we used shipboard and towed ctd current meter and satellite tracked drifter data to examine the hydrographic structure in the northeastern chukchi sea in august september of 2011 2012 and 2013 in all years the densest winter water was around and east of hanna shoal in 2012 and 2013 a 15 m deep layer of cold dilute meltwater overlaid the dense water north of the shelf region between 71 2 and 71 5 n a front extends from the southwest side of hanna shoal toward the head of barrow canyon separated meltwaters from warmer saltier bering sea summer waters to the south stratification was stronger and the surface density variances in the meso and sub mesoscale range were higher north of the front than to the south no meltwater or surface fronts were present in 2011 due to a very early ice retreat differences in summer ice cover may be due to differences in the amount of grounded ice atop hanna shoal associated with the previous winter s regional ice drift along the north side of hanna shoal the model predicted clockwise barotropic flow carrying waters from the western side of the shoal appears to converge with a counterclockwise baroclinic flow on the northeast side the baroclinic tendency is confined to the upper 30 m and can include waters transported from the shelfbreak the inferred zonal convergence implies that north of the shoal a near surface waters are a mixture of waters from the western and eastern chukchi sea and b the cross isobath pressure gradient collapses thereby facilitating leakage of upper layer waters northward across the shelf

halo of ice deformation observed over the maud rise seamount 1 a distinctive halo of sea ice deformation was observed above the maud rise seamount in the eastern weddell sea in the winter of 2005 the deformation halo is coincident with a halo of low mean ice concentration that is often observed in the region monthly mean ice vorticity estimates for the months july through november reveal the deformation zone most clearly in an arc about 100 km northwest of the seamount where there is a strong gradient in the bathymetry at depths of 3000 5000 m the deformation was computed from satellite based ice motion vectors derived from envisat synthetic aperture radar backscatter images the deformation halo is evidence of a taylor cap circulation over the seamount which has been described and analyzed with modeling studies and concurrent oceanographic observations obtained during an extensive field campaign

circulation of winter water on the chukchi shelf in early summer abstract using a variety of data sources we investigate the properties and pathways of pacific origin winter water as it spreads across the eastern chukchi shelf in early summer the focus is on the time period june july 2011 during which an extensive shipboard hydrographic velocity survey was undertaken as part of the impacts of climate on ecosystem and chemistry of the arctic pacific environment icescape program a revised circulation scheme is constructed revealing that the transport pathways on the chukchi shelf are more complex than previously thought notably the well known branch progressing northward from the central channel bifurcates as it reaches hanna shoal flowing around both sides of the shoal and dividing into smaller filaments that continue towards barrow canyon mass is conserved in the circulation scheme with approximately 1 sverdrup flowing poleward across the chukchi shelf within these pathways then exiting barrow canyon the salinity of the winter water varied on the shelf in 2011 with saltier water found in the upstream portion of what is defined as the central pathway using sea ice concentration data and atmospheric reanalysis fields we argue that salinization of the winter water in the central pathway occurred via brine rejection as the parcels progressed north and passed through the cape lisburne polynya this demonstrates that winter water pervading the interior shelf can be transformed by convective overturning north of bering strait presumably stirring up nutrients from the sediments and thereby influencing primary productivity in the region of hanna shoal

contrasting platinum group element concentration patterns in cumulates of the bushveld complex chromite rich lithologies in both the lower and the critical zones of the bushveld complex in the potgietersrus area display flat chondrite normalized platinum group element pge concentration patterns whereas those of associated sulphide bearing but chromite poor rocks are considerably steeper the low pt pd os ir ru ratio in the chromite bearing rocks is maintained irrespective of the amount of sulphide or chromite in the rock this feature suggests that the partitioning of the individual pge into pge bearing phases during conditions in the magma under which crystallization of chromite in excess of the normal cotectic amounts was favoured differed from conditions under which an immiscible sulphide liquid separated from the same magma in the absence of enhanced chromite crystallization these changes in the partitioning coefficients of the individual pge are considered to reflect changes in the solubility of these elements in response to variations in the intensive parameters in the magma necessary to bring about the enhanced crystallization of chromite

seafloor control on sea ice abstract the seafloor has a profound role in arctic sea ice formation and seasonal evolution ocean bathymetry controls the distribution and mixing of warm and cold waters which may originate from different sources thereby dictating the pattern of sea ice on the ocean surface sea ice dynamics forced by surface winds are also guided by seafloor features in preferential directions here satellite mapping of sea ice together with buoy measurements are used to reveal the bathymetric control on sea ice growth and dynamics bathymetric effects on sea ice formation are clearly observed in the conformity between sea ice patterns and bathymetric characteristics in the peripheral seas beyond local features bathymetric control appears over extensive regions of the sea ice cover across the arctic ocean the large scale conformity between bathymetry and patterns of different synoptic sea ice classes including seasonal and perennial sea ice is identified an implication of the bathymetric influence is that the maximum extent of the total sea ice cover is relatively stable as observed by scatterometer data in the decade of the 2000s while the minimum ice extent has decreased drastically because of the geologic control the sea ice cover can expand only as far as it reaches the seashore the continental shelf break or other pronounced bathymetric features in the peripheral seas since the seafloor does not change significantly for decades or centuries sea ice patterns can be recurrent around certain bathymetric features which once identified may help improve short term forecast seasonal outlook and decadal prediction of the sea ice cover moreover the seafloor can indirectly influence the cloud cover by its control on sea ice distribution which differentially modulates the latent heat flux through ice covered and open water areas

loitering of the retreating sea ice edge in the arctic seas each year the arctic sea ice edge retreats from its winter maximum extent through the seasonal ice zone siz to its summer minimum extent on some days this retreat happens at a rapid pace while on other days parts of the pan arctic ice edge hardly move for periods of days up to 1 5 weeks we term this stationary behavior ice edge loitering and identify areas that are more prone to loitering than others generally about 20 25 of the siz area experiences loitering most often only one time at any one location during the retreat season but sometimes two or more times the main mechanism controlling loitering is an interaction between surface winds and warm sea surface temperatures in areas from which the ice has already retreated when retreat happens early enough to allow atmospheric warming of this open water winds that force ice floes into this water cause melting thus while individual ice floes are moving the ice edge as a whole appears to loiter the time scale of loitering is then naturally tied to the synoptic time scale of wind forcing perhaps surprisingly the area of loitering in the arctic seas has not changed over the past 25 years even as the siz area has grown this is because rapid ice retreat happens most commonly late in the summer when atmospheric warming of open water is weak we speculate that loitering may have profound effects on both physical and biological conditions at the ice edge during the retreat season

a comparison between late summer 2012 and 2013 water masses macronutrients and phytoplankton standing crops in the northern bering and chukchi seas abstract survey data from the northern bering and chukchi sea continental shelves in august september 2012 and 2013 reveal inter annual differences in the spatial structure of water masses along with statistically significant differences in thermohaline properties chemical properties and phytoplankton communities we provide a set of water mass definitions applicable to the northern bering and chukchi continental shelves and we find that the near bottom bering chukchi summer water bcsw was more saline in 2012 and alaskan coastal water acw was warmer in 2013 both of these water masses carried higher nutrient concentrations in 2012 supporting a larger chlorophyll a biomass that was comprised primarily of small

physical and morphological properties of sea ice in the chukchi and beaufort seas during the 2010 and 2011 nasa icescape missions abstract physical and morphological properties of sea ice in the chukchi and western beaufort seas were measured during the 2010 and 2011 june july icescape impacts of climate on the eco systems and chemistry of the arctic pacific environment missions aboard the uscgc healy observations of ice conditions including ice thicknesses types and concentrations of primary secondary and tertiary categories were reported at 2 h intervals while the ship was in transit using the antarctic sea ice processes and climate aspect protocol on ice surveys of ice thickness melt pond depth and ice properties including profiles of internal temperature salinity and isotopic composition were conducted at 21 total ice stations 12 in 2010 and 9 in 2011 comparison to historical ship based observations confirms a multi decadal transition from a multiyear to thinner first year dominated seasonal sea ice pack in the region with much earlier ice retreat than in past decades the ice encountered was predominantly 98 first year with un deformed ice thicknesses ranging from 0 73 to 1 2 m pond coverage was extensive averaging 29 in 2010 and 19 in 2011 resulting in considerable light absorption in the ice and transmission of light to the ocean enhanced melting near the ice edge is consistent with transport of pacific origin heat and or ice albedo feedbacks sediment entrainment was visible in 7 5 and 10 9 of the ice in 2010 and 2011 respectively overall the results indicate a regime shift in the characteristics of the ice cover has occurred in the region over recent decades with substantive implications for thermodynamic forcing light availability in the upper ocean and biological and biogeochemical processes in the ice and water column beneath the results presented have applications for the interpretation of optical and biological measurements in the chukchi sea and serve as record of ice conditions for assessing long term change

assessing the role of oceanic heat fluxes on ice ablation of the central chukchi sea shelf abstract we investigate the role of oceanic heat flux convergence in the summertime retreat of sea ice in the presence of a mean background flow prior studies indicated that eddies generated along the marginal ice zone front carry substantial quantities of heat laterally beneath the ice but the direct importance of these fluxes to the summertime retreat of sea ice was not well established we use the regional ocean model system roms with an idealized configuration of the chukchi sea shelf and without wind forcing to 1 determine the contributions to ice melt from the oceanic heat flux compared to that from the net atmospheric heat flux through the ice surface 2 evaluate the role of mesoscale fluctuations versus the mean background flow in providing this sub surface heat flux and 3 evaluate the role of the underlying bathymetry in modifying the subsurface heat flux to the ice analyses show that the three main water masses melt water bering sea water and winter water establish frontal systems the shelf water front and the ice edge melt water front that control baroclinic processes which in turn regulate the quantity of heat fluxed laterally and vertically to the ablating ice edge we identify and characterize distinct oceanic zones associated with these fronts the shelf water transition zone and the melt water transition zone that are delineated by flow dynamics and hydrographic structure the integrated heat flux along the modeled ice edge is significant lateral eddy transport of heat is as large as 2 tw of which 50 is fluxed upwards into the near surface melt water we introduce a parameterization for the vertical heat transport to the ice through the near surface meltwater lens that is a function of the along front current velocity and the cross front temperature gradient such a parameterization could improve coarse scale ocean and climate models that lack the resolution necessary for reproducing such small scale processes the results of this study may be informative to investigations of ice edge biological processes

the effect of possible taylor columns on the summer ice retreat in the chukchi sea the chukchi sea topography consists of a broad flat 50 m deep plain with two prominent shoals herald and hanna which have horizontal dimensions of about 100 km and rise to depths of about 30 m herald shoal in particular is a prominent isolated seamount which because of the warm water flux in summer through bering strait has a warm 0 1 m s 1 flow incident upon it examination of active and passive microwave imagery of the region for 1992 1994 shows that as the general ice cover recedes the ice remains preferentially over herald shoal for 3 4 weeks after the surrounding ice has melted a scale analysis suggests that the cause of the ice persistence is the formation of a taylor column over the shoal which traps cold water and ice above it a similar trapping of ice occurs over hanna shoal and on its eastern slope but the response is complicated by a very different topography and the northward flow down barrow canyon

summertime circulation in the eastern chukchi sea abstract the transport of pacific origin water across the eastern chukchi sea during summer is studied using shipboard hydrographic and acoustic doppler current profiler data from multiple surveys the study identifies two major transport pathways in this region the well known alaskan coastal current acc flows poleward along the alaska coast while a more recently discovered and slower current flows northward through the central channel between herald and hanna shoals the two currents separate past point hope and appear to merge again in barrow canyon via previously unresolved pathways flowing west to east across the chukchi sea the collective flow that transports pacific water from bering strait to barrow canyon is termed the bering to barrow current system bbcs the coastal branch of the bbcs which encompasses the acc is a weakly baroclinic flow with a maximum speed in excess of 80 cm s inside barrow canyon the central channel branch of the bbcs is considerably weaker with a maximum flow speed under 20 cm s a portion of the pacific water that flows towards herald canyon to the west may also contribute to the bbcs the amount but the amount is within the margin of error of our data the difference in the advective time scales of the various branches allows water masses of different seasonal characteristics to be transported simultaneously by the bbcs summer water masses occupy most of the southern and central chukchi sea while winter water masses occupy hanna shoal and shelfbreak regions in the northern chukchi sea during summer the total bbcs volume transport in summer is measured at four different locations in the eastern chukchi sea and found to be generally consistent the overall mean transport is 0 86 sv compared to the summer bering strait transport of 1 08 sv this study suggests that the bbcs is the main transport pathway delivering pacific water masses into the western arctic basin

transit time of river water in the bering and chukchi seas estimated from δ 18 o and radium isotopes abstract seawater samples for the measurements of 226ra 228ra and stable oxygen isotope 18o were collected from the bering and chukchi seas in the summer of 2014 the fractions of meteoric water fmw and sea ice melted water fsim were estimated based on the mass balance of salinity and 18o with a three end member mixing model our results showed that the average fmw increased northward from the bering basin to the canada basin while the fsim distributed homogeneously the lowest fmw and 228ra 226ra a r values were found in the upper bering basin with little terrestrial input the highest fmw but low 228ra 226ra a r appeared in the northern chukchi sea and the canada basin ascribing to the current driven accumulation of freshwater and its long residence time more abundant sea ice melted water was found on the pack ice edge indicating the trap of earlier melted waters by the ice pack based upon the linear relationships between 228ra 226ra a r and fmw in the bering shelf and the chukchi shelf the transit time for the pacific inflow was constrained the transit time of river water from the bering shelf to the chukchi shelf was estimated as 0 2 4 4 years with an average of 1 6 1 5 years while that from the chukchi shelf to the canada basin was 10 2 13 2 years with an average of 11 8 1 1 years the spatial variation of the transit time was mainly affected by the current intensity our study highlights the importance of in depth evaluation for the subarctic arctic exchange