40 r139 r and k ar age constraints on shear zone evolution southern taltson magmatic zone northeastern alberta1 new 40ar 39ar analyses of hornblende muscovite biotite and k feldspar constrain the timing of deformation and cooling of the southern taltson magmatic zone which underwent lower granulite to upper amphibolite grade deformation in part synchronous with voluminous 1 99 1 92 ga magmatism new data are combined with existing k ar dates into a regional cooling framework to provide thermotemporal constraints on the deformational history 40ar 39ar hornblende ages of ca 1900 ma are interpreted to record relatively rapid cooling following ductile thrusting on the andrew lake shear zone and younger anatectic magmatism these data with published k ar and u pb data support relatively rapid cooling of the taltson magmatic zone from monazite closure temperature of 725 c at ca 1930 ma to 525 c at ca 1900 ma cooling rate estimates are about 7 c ma which suggests moderate exhumation rates during the high grade part of the deformational history a muscovite 40 r 39ar plateau age of 1803 f 11 ma is consistent with the mean muscovite k ar age of 1792 ma indicating regional cooling through about 350 c at ca 1800 ma 40ar 39ar ages from magmatic biotite of 1856 and 1799 ma also suggest slow cooling during greenschist grade deformation which can be no older than ca 1860 ma a k feldspar 40ar 39ar age of 1681 ma provides a lower limit for the time of greenschist grade deformation cooling rate estimates during amphibolite to greenschist grade deformation are 1 75 2 25 cima

very recent and wide spread basaltic volcanism on mars new spacecraft data provide increasing evidence for a dynamic environment on presenta day mars exogenic processes such as impact cratering mass wasting processes and active dune migration have all been observed to modify the surface no traces of current endogenic activity have been found yet but some studies point to very localized volcanism in the last few millions of years however no systematic study of young volcanic surfaces had been performed so far we present absolute model age determinations of plains volcanism on mars as derived from impact crater sizefrequency distributions extended areas in tharsis the largest volcanoa tectonic region on mars have been resurfaced by lava flows in the last few tens of millions of years we also present results on the rheologic properties of these lava flows inferred from morphometric measurements yield strengths are in the range of 100a 300 pa and viscosities reach values of 10 2 to 10 3 pa s indicating basaltic compositions the results imply that mars retained until recently and probably still retains enough internal heat to produce widea spread plaina style volcanism producing lowa viscosity lava flows throughout large parts of tharsis

development of late paleozoic volcanic arcs in the canadian cordillera an example from the klinkit group northern british columbia and southern yukon the late paleozoic volcanic rocks of the northern canadian cordillera lying between ancestral north america to the east and the accreted terranes of the omineca belt to the west record early arc and rift magmatism along the paleo pacific margin of the north american craton the mississippian to permian volcano sedimentary klinkit group extends discontinuously over 250 km in northern british columbia and southern yukon the two stratotype areas are as follows 1 in the englishman range southern yukon the english creek limestone is conformably overlain by the volcano sedimentary mount mccleary formation lower clastic member alkali basalt member and volcaniclastic member and 2 in the stikine ranges northern british columbia the screw creek limestone is conformably overlain by the volcano sedimentary butsih formation volcaniclastic member and upper clastic member the calc alkali nature of the basaltic volcaniclastic members of the klinkit group indicates a volcanic arc setting la yb n 2 77

impact of atmospheric forcing on antarctic continental shelf water masses the antarctic continental shelf seas feature a bimodal distribution of water mass temperature with the amundsen and bellingshausen seas flooded by circumpolar deep water that is several degrees celsius warmer than the cold shelf waters prevalent in the weddell and ross seas this bimodal distribution could be caused by differences in atmospheric forcing ocean dynamics ocean and ice feedbacks or some combination of these factors in this study a highly simplified coupled sea ice mixed layer model is developed to investigate the physical processes controlling this situation under regional atmospheric forcings and parameter choices the 10 yr simulations demonstrate a complete destratification of the weddell sea water column in winter forming cold relatively saline shelf waters while the amundsen sea winter mixed layer remains shallower allowing a layer of deep warm water to persist applying the weddell atmospheric forcing to the amundsen sea model destratifies the water column after two years and applying the amundsen forcing to the weddell sea model results in a shallower steady state winter mixed layer that no longer destratifies the water column this suggests that the regional difference in atmospheric forcings alone is sufficient to account for the bimodal distribution in antarctic shelf sea temperatures the model prediction of mixed layer depth is most sensitive to the air temperature forcing but a switch in all forcings is required to prevent destratification of the weddell sea water column

ocean circulation its effects on seasonal sea ice simulations a diagnostic ice ocean model of the arctic greenland and norwegian seas is constructed and used to examine the role of ocean circulation in seasonal sea ice simulations the model includes lateral ice motion and three dimensional ocean circulation the ocean portion of the model is weakly forced by observed temperature and salinity data simulation results show that including modeled ocean circulation in seasonal sea ice simulations substantially improves the predicted ice drift and ice margin location simulations that do not include lateral ocean movment predict a much less realistic ice edge

studies of the arctic ice cover and upper ocean with a coupled ice ocean model an ice ocean model has been developed by coupling the hibler ice model to a three dimensional ocean model that consists of a turbulent mixed layer model and an inverse geostrophic model the coupled model has a horizontal grid spacing of 127 km and has 17 vertical levels extending to the ocean bottom the model is forced with 12 hourly general circulation model derived atmospheric fluxes for the year 1986 for which good quality ice edge analyses and buoy tracks were available for comparisons the results are presented for the fifth year of a repetitive simulation with the 1986 fluxes at which time the system has reached a statistical equilibrium the seasonal and geographical variations of the ice cover and the upper few hundred meters of the ocean have been examined including the heat and salt budgets the computed heat fluxes and mixed layer depths mlds fall in the observed seasonal ranges with winter heat fluxes ranging from 15 w m 2 in the central arctic to about 500 w m 2 in the barents sea area and summer fluxes from about 5 w m 2 under the ice to about 30 w m 2 in the various marginal ice zone miz edge areas the corresponding winter mlds are found to be about 25 75 m in the central arctic to a deep 800 m in the greenland sea typical summer mlds are between 5 and 30 m in all seasons the miz was found to be the center of flux activity for both heat and salt with the processes of advection diffusion atmospheric forcing and vertical oceanic fluxes having their largest values here and of comparable magnitudes values of the heat flux components in the miz exceed those found under the ice by an order of magnitude or more and those in the open water region by a factor of 2 for salt the situation is similar except in the summer june through september when a lot of salt flux activity takes place under the ice comparisons are made with naval polar oceanography center npoc analyses for ice concentration and ice edge and with observed arctic buoy tracks in the same 1986 time period the computed ice edge positions have comparable accuracy to previous three dimensional coupled ice ocean studies with too much ice growth during the winter in the barents sea and too little ice east of greenland the ice thickness distributions however are much better with a monotonic increase of the ice thickness from the siberian coast east toward the canadian archipelago with maximum winter values of about 5 6 m

development of a simple upper ocean and sea lce model abstract results are described for three simple numerical models of the upper ocean and sea ice with prescribed atmospheric forcing the ability of each model version to simulate the observed sea surface temperature sst is assessed as a basis of comparison for future coupled experiments with atmospheric gcm s lie upper ocean model versions raw from a slab of fixed thickness to a variable depth mixed layer above a variable exponential temperature gradient representing the seasonal thermocline sea ice thickness is determined thermodynamically by local melting or accretion and the effects of ice transport and leads are neglected each version is tested by integrating to equilibrium with horizontal advection neglected using monthly climatological atmospheric data for a selected north south section in the mid pacific ocean and the results for different model versions are compared with each other and with available observations it is found that the fixed slab version gives realistic sea ice thickness a

tectonic affinity of nisutlin and anvil assemblage strata from the teslin tectonic zone northern canadian cordillera constraints from neodymium isotope and geochemical evidence we present geochemical and isotopic data for nisutlin assemblage metasedimentary rocks and anvil assemblage greenstones from the teslin tectonic zone of the northern canadian cordillera this study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed for the nisutlin assemblage metasedimentary rocks the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments one source region was felsic upper crustal material with nd isotopic compositions compatible with ultimate derivation from the north american continent depleted mantle model age tdm 2 5 2 8 ga the second source region is deduced to be chemically primitive crust basaltic andesitic with a short crustal residence history 0 9 ga for the anvil assemblage greenstones immobile trace element abundances are dissimilar to within plate oceanic arc and normal mid ocean ridge basalts and similar to calc alkaline basalts in active continental margin settings we interpret the paleosetting for the nisutlin assemblage to be at the outermost margin of the ancestral north american continent in an area which received detrital input from the distal north american craton in the early paleozoic however this area also received detritus from a chemically and isotopically juvenile magmatic arc source a source type not known from paleozoic metasedimentary rocks from the miogeoclinal sequence on the basis of similar nd isotopic relationships recorded elsewhere in deformed paleozoic rocks of the orogen we infer that these geochemical signals reflect tectonic processes of regional extent the trace element geochemistry of the anvil assemblage greenstones does not support a correclation with known paleozoic greenstones of the slide mountain terrane which some tectonic models have advocated

deep electrical structure of northern alberta canada implications for diamond exploration geophysical studies of upper mantle structure can provide constraints on diamond formation teleseismic and magnetotelluric data can be used in diamond exploration by mapping the depth of the lithosphere asthenosphere boundary studies in the central slave craton and at fort a la corne have detected conductors in the lithospheric mantle close to or beneath diamondiferous kimberlites graphite can potentially explain the enhanced conductivity and may imply the presence of diamonds at greater depth petrologic arguments suggest that the shallow lithospheric mantle may be too oxidized to contain graphite other diamond bearing regions show no upper mantle conductor suggesting that the correlation with diamondiferous kimberlites is not universal the buffalo head hills in alberta host diamondiferous kimberlites in a proterozoic terrane and may have formed in a subduction zone setting long period magnetotelluric data were used to investigate the upper mantle resistivity structure of this region magnetotell

nd isotopic evidence for crustal recycling in the ca 2 0 ga subsurface of western canada sm nd isotopic data are presented for 23 drill core samples from five aeromagnetically and geochronologically u pb zircon distinct domains of the precambrian basement of northern alberta the domains in question are the taltson 1 96 1 94 ga buffalo head 2 32 1 99 ga chinchaga 2 19 2 09 ga ksituan 1 99 1 90 ga and nova 2 81 ga these domains are truncated to the north and south by the great slave lake shear zone and the snowbird tectonic zone respectively initial end values are 5 0 to 9 7 for the taltson 0 2 to 6 3 for the buffalo head 0 6 to 1 8 for the chinchaga 1 8 to 2 1 for the ksituan and 5 6 for the nova crustal residence model ages fall in the 2 5 2 8 ga range the nd isotopic signatures may be viewed in terms of mixing a minimum of 10 archean continental crust with a depleted mantle component speculations on the tectonic history of the basement domains in question involve the assembly of archean crustal nuclei to form the buffalo head chinchaga composite domain a

regional mapping of precambrian basement beneath phanerozoic cover in southeastern trans hudson orogen manitoba and saskatchewan the northern edge of phanerozoic platformal rocks of the western canada sedimentary basin overlies the flin flon belt trans hudson orogen in manitoba and saskatchewan a program of regional mapping of the phanerozoic covered basement has been undertaken involving the integration of high resolution aeromagnetic and gravity data with extensive drill core information several major domains are recognized in the buried basement each with a distinct lithotectonic character and potential field anomaly pattern three lithotectonic domains in the buried basement clearwater athapapuskow and amisk lake domains are characterized by northerly trending positive gravity and aeromagnetic anomalies and correlate with the 1 92 1 83 ga volcanic and plutonic rocks of the exposed flin flon belt amisk collage and snow lake assemblage an upper amphibolite grade orthogneiss complex namew gneiss complex containing calc alkaline intrusive rocks ranging in age from 1 88 to 1 83 ga and screens derived from the older v

a note on chimney formation in ice edge regions wind driven ice ocean interaction in the marginal ice zone has been shown to contribute to enhanced entrainment when the winds are favorable for upwelling at the ice edge if a strong surface heat loss is present formation of a deep convective region is possible resulting from an ever decreasing buoyancy and a rapid deepening of the mixed layer i e a chimney this chimney does not extend to the bottom but loss of buoyancy would indicate eventual overturning to the bottom in the studied two layer ocean model where initially a fresh and cold freezing mixed layer lies above a stagnant warm and saline lower layer a catalyst for the convection is the ice and especially the existence of the ice edge which has both a dynamic and thermodynamic interaction with the ocean in the coupled ice ocean model the entrainment between the mixed layer and the lower layer consists of both the wind mixing component involving the frictional velocity at the ocean surface a kraus turner parameterization and the convectively induced entrainment no dissipation of convective turbulence the latter is necessary in producing the chimney the results are an extension to the earlier model results hakkinen 1987 which did not include the convective component of entrainment

2 8 1 7 ga history of the jiao liao ji belt of the north china craton from the geochronology and geochemistry of mafic liaohe meta igneous rocks abstract the assembly and long term evolution of the eastern block of the north china craton are poorly constrained here we use bulk rock geochronological and geochemical data from mafic meta igneous rocks hornblendites amphibolites and a metagabbro of the liaohe group to reconstruct the neoarchean to paleoproterozoic history of the jiao liao ji belt located between the longgang and nangrim blocks that together form the eastern block of the north china craton the mafic ultramafic meta igneous rocks have intrusive or tectonic contacts with the liaoji granitic rocks 2 2 2 0 ga which form the basement of the jiao liao ji belt the major and trace element data indicate that the protoliths had calc alkaline composition and formed along an active continental margin subduction zone the mafic rocks form a whole rock 176lu 177hf isochron with an age of 2 25 0 31 ga overlapping with u pb zircon ages for mafic and granitic rocks from the jiao liao ji belt and consistent with being the emplacement age of the mafic protoliths along the active continental margin in contrast the whole rock 147sm 144nd isochron age of 2 83 0 18 ga is likely to reflect the average age of the lithospheric mantle source from which the mafic ultramafic protoliths were extracted together with geological evidence we propose that the southwestern portion of the longgang block was an active continental margin since at least the early paleoproteorozic literature age data from metamorphic zircons show that peak granulite metamorphism took place at 1 96 1 88 ga resulting from the collisional event that fused the longgang and nangrim blocks into the eastern block of the north china craton our bulk rock 207pb 206pb age of 1824 19 ma and our 87rb 86sr age of 1671 58 ma reflect retrograde cooling stages during the exhumation of the jiao liao ji belt after the orogenesis

a cretaceous back arc basin in the coast belt of the northern canadian cordillera evidence from geochemical and neodymium isotope characteristics of the kluane metamorphic assemblage southwest yukon the coast belt of the northern cordillera in canada is the locus of the boundary between accreted and ancient north american margin rocks the largest exposure of metasedimentary rocks in the coast belt is the kluane metamorphic assemblage kma a northwest striking belt 160 km long of graphitic mica quartz schist and gneiss with minor interfoliated olivine serpentinite the kma does not appear to correlate with other sedimentary or metamorphic rock assemblages in the canadian cordillera to determine its tectonic setting and protolith provenance we analyzed trace element rare earth elements and neodymium isotope compositions of the kma of the adjacent pericratonic aishihik metamorphic suite ams of the yukon tanana terrane and of adjacent slates of the dezadeash formation df filling a late jurassic early cretaceous flysch basin the end 0 values of analyzed kma samples range from 1 4 to 5 6 and depleted mantle model ages t dm range from 1 16 to 1 45 ga kma samples are intermediate between more evolved ams samples average end 0 25 t dm 2 6 ga and more juvenile df samples end 0 1 9 t dm 0 95 ga the intermediate characteristics of the kma samples cannot be linked to a known source region and are interpreted to reflect homogeneous mixing from predominantly juvenile and minor evolved sedimentary sources a compatible tectonic setting is a back arc basin within influence of a continental source eastward subduction of the kma beneath ancient north america collapsed the back arc basin by latest cretaceous time

a coupled one dimensional sea ice ocean model a prognostic one dimensional mixed layer pycnocline model describing the vertical structure of the upper ocean is coupled to a thermodynamic sea ice model the coupled prognostic model is compared with the more usual sea ice model overlying a fixed mixed layer with constant oceanic heat flux and is then applied to investigate the effect of temporal and regional variations of the entrained oceanic heat flux on the sea ice cover in both polar regions in the southern ocean a standard simulation is compared with two perturbation experiments which both describe the occurrence of a polynya in the arctic ocean the effect of possible soviet river diversions is investigated finally the response of the coupled model to paleoclimatic forcing and boundary conditions is presented

a new mechanism for decreasing north atlantic deep water production rates during the pleistocene geological data suggest that north atlantic deep water nadw production rates decreased during the pleistocene we apply a coupled ice mixed layer model for vertical overturn in the marginal ice zone hakkinen 1987a to propose an explanation for such a decrease the model predicts that prevailing wind direction is important in preconditioning of the water column along the marginal ice zone miz because it affects rates of upwelling and mixing of surface and subsurface waters processes that result in deep water formation at present prevailing wind directions during the winter in the greenland sea are northeasterly because ice has a larger drag coefficient than water ekman transport is greater under the ice than in open water this configuration causes divergence along the miz mixing of surface and subsurface waters and vertical overturn during the glacial maximum however the prevailing winds along the miz polar front were predominantly westerly modeling studies suggest that ice age changes in the prevailing winds would have significantly increased the depth of the mixed layer and decreased entrainment equal to mixing rates in agreement with geological data mixing changes due to westerly winds are greater than mixing changes due to a lower salinity layer under the present wind regime changes due to winds are so large that further changes in boundary conditions low salinity surface layer result in only minor additional reductions in vertical mixing still the maximum reduction occurs when the two processes are considered together a situation that may be applicable to the younger dryas event during the last deglaciation when a meltwater lid may have developed in the subpolar north atlantic our study suggests that wind induced changes in mixed layer depth along the miz represent a viable mechanism for decreasing past nadw production rates

nd isotopic evolution of the taltson magmatic zone northwest territories canada insights into early proterozoic accretion along the western margin of the churchill province the taltson magmatic zone is a north trending belt of early proterozoic granitoids exposed over an area exceeding 18 000 km 2 in the northwest territories of canada three petrologically and geochronologically discrete intrusive suites comprise the taltson zone these are the 1986 ma deskenatlata suite the 1955 to 1925 ma slave suite and the 1935 ma konth suite the deskenatlata suite is composed of biotite hornblende diorite to granite and displays a more mafic geochemical character relative to the other two intrusive suites initial epsilon nd values for the deskenatlata suite range from 2 7 to 3 6 the slave and konth suites are compositionally heterogeneous to homogeneous leucocratic granitoids that contain numerous xenoliths of dominantly pelitic high grade paragneiss and display a peraluminous granite mineralogy and geochemistry initial epsilon nd values for the konth suite cluster within 4 7 and 5 5 and are completely overlapped by the 4 1 to 6 0 range of the pelitic xe

the evolution of scaling laws in the sea ice floe size distribution the sub gridscale floe size and thickness distribution fstd is an emerging climate variable playing a leading order role in the coupling between sea ice the ocean and the atmosphere the fstd however is difficult to measure given the vast range of horizontal scales of individual floes leading to the common use of power law scaling to describe it the evolution of a coupled mixed layer fstd model of a typical marginal ice zone is explicitly simulated here to develop a deeper understanding of how processes active at the floe scale may or may not lead to scaling laws in the floe size distribution the time evolution of mean quantities obtained from the fstd sea ice concentration mean thickness volume is complex even in simple scenarios suggesting that these quantities which affect climate feedbacks should be carefully calculated in climate models the emergence of fstds with multiple separate power law regimes as seen in observations is found to be due to the combination of multiple scale selective processes limitations in assuming a power law fstd are carefully analyzed applying methods used in observations to fstd model output two important sources of error are identified that may lead to model biases one when observing an insufficiently small range of floe sizes and one from the fact that floe scale processes often do not produce power law behavior these two sources of error may easily lead to biases in mean quantities derived from the fstd of greater than 100 and therefore biases in modeled sea ice evolution

geochemical and nd pb o isotope systematics of granites from the taltson magmatic zone ne alberta implications for early proterozoic tectonics in western laurentia the early proterozoic taltson magmatic zone tmz comprises the southern part of the taltson thelon orogenic belt of northwestern canada the tmz is dominated by 1 96 1 99 ga i type and 1 93 1 95 ga s type suites of granitoids previous workers attributed the formation of the i type granitoids to subduction of oceanic crust beneath the churchill craton in an andean type setting and generation of the s type granitoids to subsequent collision between the churchill craton and the buffalo head terrane to test this hypothesis we carried out a geochemical and isotopic nd pb and o study of granitoids and associated metasedimentary rocks and basement gneisses from the southern part of the tmz both i and s type suites of granitoids are characterized by relatively high sio2 content with the great majority of samples containing more than 64 wt sio2 the two suites also have strongly crustal end t values of 3 8 to 9 8 and an average tdm of 2 8 ga the initial pb isotope composition of the granitoids as determined from leached feldspar mineral separates forms a linear array that is bracketed by the isotopic compositions of basement orthogneisses and metasediments similarly 18o values of the granitoids overlap those of the orthogneisses and metasediments these isotopic data are consistent with derivation of the granitoids from a mixture of metaigneous and metasedimentary source rocks similar to those presently exposed in the tmz importantly the data suggest that both i and s type granitoids of the southern tmz had an exclusively intra crustal origin unlike phanerozoic examples of subduction related magmatism where a significant mantle contribution is easily recognized therefore we argue that the existing andean type model for early tmz magmatism is not viable the tmz granitoids show geochemical and isotopic similarities to the cordilleran interior granitoids of western north america which formed in response to crustal thickening in the distant hinterland of a convergent plate margin we propose that these hinterland granitoids are a more appropriate analogue for tmz magmatism than are the granitoids of plate margins the tectonic implication of this proposal is that the taltson thelon orogenic belt does not mark the location of the plate boundary in western laurentia at 1 9 2 0 ga rather the tectonic setting of the tmz at that time may have been more akin to the present day intra continental mountain belts of central asia e g tian shan mountains where compressional tectonism and crustal thickening are occurring well inboard from a convergent plate margin

a coupled ice ocean model for the greenland iceland and norwegian seas abstract simulations from a coupled ice ocean model that highlight the importance of synoptic forcing on sea ice dynamics are described the ocean model is a non hydrostatic primitive equation model coupled to a dynamic thermodynamic sea ice model the ice modelling sensitivity study presented here is part of an ongoing research programme to define the role played by sea ice in the energy balance of the greenland sea the different categories of sea ice found in the subpolar regions are simulated through the use of equations for thin ice thick ice and the marginal ice zone a basin scale numerical model of the greenland iceland and norwegian seas has a horizontal resolution of 20 km and a vertical grid spacing of 50 m this resolution is adequate for resolving the mesoscale topographic structures known to control the circulation in this region the spin up reproduces the main features of the circulation including the cyclonic gyres in the norwegian and greenland basins and iceland plateau topographic steering of the flow is evident the baroclinic rossby radius of deformation is between 5 and 10 km so that the model is not eddy resolving the coupled ice ocean model was run for a period of two weeks the influence of horizontal resolution of the atmospheric model was tested by comparing simulations using six hourly wind fields from the ecmwf with those generated using six hourly fields from a hirlam with horizontal resolutions of 1 and 0 18 respectively the simulations show reasonable agreement with satellite ice compactness data and data of ice transports across sections at 79 n 75 n and denmark strait

a coupled dynamic thermodynamic model of an ice ocean system in the marginal ice zone wind driven ice ocean processes in the marginal ice zone are studied when thermodynamics is included into a coupled ice ocean model the ocean is considered to be a two layer ocean where a fresh and cold freezing mixed layer is lying above the stagnant warm and saline lower layer the essential part of the thermodynamics is the entrainment between the mixed layer and the lower layer which is parameterized using the frictional velocity at the ocean surface a kraus turner type parameterization it is shown that upwelling at the ice edge which is generated by the difference in the air ice and air water surface stresses will give rise to a strong entrainment by bringing the pycnocline nearer to the surface furthermore the entrainment is negligible outside the areas affected by the ice edge upwelling when cooling at the top is included the heat and salt exchange are further enhanced in the upwelling areas because the entrainment depends inversely on the density difference between the layers new ice formation occurs in the area which is not affected by ice edge upwelling that is no entrainment of the warm lower layer fluid so the mixed layer temperature stays at the freezing point the ice growth will give rise to a large salt flux to the upper layer but the subsequent salinity change is not as large as that due to the entrainment of the lower layer fluid the mixed layer in the areas affected by the ice edge upwelling is only slightly less saline than the lower layer it is suggested that these high salinity mixed layer regions with a scale of a few rossby radii of deformation 10 km will overturn due to cooling possibly as a plume and may this contribute to the formation of deep water

tectonic setting of the taltson magmatic zone at 1 92 0 ga a granitoid based perspective the paleoproterozoic taltson magmatic zone is one of the key tectonic features of western laurentia the existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin followed by direct collision between formerly separate crustal blocks we tested this model by comparing the large geochemical and isotopic database available for taltson magmatic zone granitoids with similar databases for phanerozoic granitoid suites from different tectonic environments the comparison reveals that the early granitoid suite of the taltson magmatic zone which had been ascribed to the subduction phase of orogenesis lacks the mantle signature apparent in granitoids of phanerozoic continental margin arc settings instead both early and late suites appear to have an intracrustal origin similar to mesozoic and cenozoic granitoids of the cordilleran interior of western north america which formed in the distant hinterland of a convergent plate margin in light of thes

the seasonal variation of the mixed layer and the pycnocline under polar sea ice a model of the seasonal variation of the mixed layer and the pycnocline under polar sea ice is compared with observations during the arctic ice dynamics joint experiment aidjex 1975 1976 the model is of the kraus turner type extended to include mixed layer and pycnocline the forcing mechanisms are the mechanical mixing due to ice keel stirring and the surface buoyancy flux due to melting and freezing of sea ice the model is designed to be used for climate studies in conjunction with atmospheric and oceanic general circulation models and with sea ice models

a two dimensional thermodynamic model for sea ice advance and retreat in the newfoundland marginal ice zone a time dependent two dimensional ice ocean coupled thermodynamic model is developed to study the ice edge advance and retreat development of the oceanic mixed layer and oceanic response to ice movement ice melt and heat balance in the newfoundland marginal ice zone the model domain is a vertical section of the ocean along the direction of the ice velocity initially the ocean is free of ice and has a deep mixed layer formed by winter surface cooling an ice sheet then moves into the domain from the upstream boundary at constant velocity it melts from the bottom and a shallow mixed layer beneath the ice is developed heat and buoyancy fluxes at the air sea and ice water interfaces and at the bottom of the mixed layer determine the melt rate and change of the mixed layer properties the ice edge reaches a maximum distance and starts to retreat when the ice is being melted by the warm water faster than it is being advected into the area the oceanic properties change with the ice movement during the advancing phase of the ice movement the heat loss to the ice bottom from ice melting exceeds the heat gain by surface heating and entrainment causing the mixed layer temperature to drop but since the buoyancy created by the melting is not sufficient to overcome the effect of wind mixing the mixed layer deepens rapidly during the retreating phase the mixed layer becomes shallower and warmer because of the increasing surface heating and buoyance production by ice melting the most important factors controlling the melt rate and the excursion distance are the air temperature and the ambient water temperature higher wind speeds increase the mixed layer depth but do not have a strong effect on the melt rate

comment on a coupled dynamic thermodynamic model of an ice ocean system in the marginal ice zone by sirpa hakkinen the recent article by hiikkinen 1987 creatively addresses a number of important dynamic and thermodynamic processes in the marginal ice zone miz we believe that her study will stimulate further research on the coupled ice ocean system for now we would like to offer the following discussion of the need to augment the entrainment equation to include surface buoyancy flux for general application to the miz

sequence stratigraphy provenance c and o isotopic composition and correlation of the late paleoproterozoic early mesoproterozoic upper hornby bay and lower dismal lakes groups nwt and nunavut abstract late paleoproterozoic basins of canada s north including the thelon athabasca and hornby bay basins can be broadly correlated based on similarities in age sequence stratigraphy paleocurrents and detrital zircon provenance but their detailed evolution is poorly understood and their age and lateral equivalencies are poorly constrained this study contributes to the understanding of sedimentary basins during a critical interval of time representing the latest stages of amalgamation of the supercontinent laurentia nuna through a detailed study of the hornby bay basin the hornby bay basin strata have been divided into four unconformity bounded subsequences a1 a2 a3 a4 that belong to the first of three unconformity bounded post hudsonian proterozoic stratigraphic packages in northwestern canada subsequences a2 1600 ma and a3 analysis of sm nd isotopes on mudrocks yielded isotopic compositions in the range 8 2 to 2 3 nd 1600 ma in subsequence a2 and 9 8 to 5 0 nd 1400 ma in subsequence a3 an nd versus time graph illustrates two distinct groupings that plot within known isotopic evolution fields of the taltson thelon tectonic zone and the great bear magmatic zone sediment provenance was strongly influenced by the paleoproterozoic forward orogeny but there are no significant changes in sedimentary provenance across the a2 a3 sequence boundary stable isotope analyses of c and o in the dolomitic east river formation of subsequence a2 reveal a relatively monotonous signal with an average of 0 5 13 c and 8 76 18 o which is consistent with global values of other late paleoproterozoic carbonate successions enriched mn values and depleted sr values however suggest that the east river carbonate rocks experienced some degree of meteoric alteration it is unknown how much this alteration affected stable isotope compositions based on similarities in sedimentary provenance age and sequence stratigraphy subsequence a2 of the hornby bay group is correlated with the wernecke supergroup yukon the latter is interpreted to represent the deep water distal equivalent of the upper hornby bay group in a westward deepening basin deposition of subsequence a2 is recorded across north america and is mirrored by deep water sedimentation along the eastern margin of the north australian craton the tectonic evolution of subsequence a2 in laurentia is therefore remarkably similar to that of the north australian craton up to 1600 ma the age of the forward orogeny and this has implications for interpretation of the latest stages of accretion of the supercontinent nuna

space time variation of mixed layer properties heat and salt fluxes and ice melt in the newfoundland marginal ice zone oceanographic data collected in the newfoundland marginal ice zone in march 1992 are analyzed to study the variation of mixed layer properties across the ice zone the mixed layer depth decreases from 80 m in the interior of the pack ice to 25 m at the eastern ice edge over a distance of 120 km in the open ocean without ice cover the depth is considerably greater 150 200 m the temperature under the pack ice is near freezing a sharp increase in both the temperature and salinity occurs across the ice edge the variation of the mixed layer properties is simulated by integrating a one dimensional bulk mixed layer model coupled to sea ice from november 1991 to march 1992 the initial ocean conditions are obtained from an objective analysis of archived data of november 1991 the forcings are 6 hourly meteorological data from the european centre for medium range weather forecasts ice concentration and thickness are treated as prescribed functions of time extracted from daily ice maps since ice in the area is not locally formed the model simulation is in good agreement with the observations from the model results a detailed analysis of the roles of wind stress ice melt surface cooling and shortwave radiation in the water mass transformation of the upper ocean is carried out there are large differences in the melt rate and water ice heat flux between the times when ice enters the shelf and after the water has been cooled by the ice to near freezing 15 20 cm d 1 and 500 700 wm 2 and 0 2 cm a 1 and 20 50 wm 2 respectively a large portion of the heat in the water column is expended for ice melting from january to march calculations that use surface data such as surface wind and sea surface temperature without considering ice melt can underestimate the net ocean to air heat flux especially at the eastern ice edge in march

modeling the thermodynamics of a sea ice thickness distribution 2 sea ice ocean interactions in this paper we examine the coupling of an ice thickness distribution model with an ocean mixed layer model the annual cycle of the modeled mixed layer temperature salinity and depth are compared to observations from the drifting ice stations of the arctic ice dynamics joint experiment aidjex the role of the ice thickness distribution in determining the ice ocean coupling is examined we find that the ice thickness distribution is important for the exchange of heat salt and fresh water between the ice and the ocean especially with regard to the effects of thin first year ice several different parameterizations of the ice ocean turbulent heat exchange are compared the results indicate that the storage of heat in the ocean mixed layer is an important physical process for determining the annual average ice thickness double diffusion and the stabilizing effects of meltwater also impact the exchange of heat between the ocean and ice cover questions remain as to the relative importance of these processes and their accurate parameterization in ice ocean coupled models