temporal geomagnetic and related attributes of kimberlite magmatism at ekati northwest territories canada abstract this paper outlines the development of a multi disciplinary strategy to focus exploration for economic kimberlites on the ekati property high resolution aeromagnetic data provide an over arching spatial and magnetostratigraphic framework for exploration and kimberlite discovery at ekati and hence also for this investigation the temporal geomagnetic spatial and related attributes of kimberlites with variable diamond content have been constrained by judiciously augmenting the information gathered during routine exploration with detailed laboratory based or field based investigations the natural remanent magnetisation of 36 ekati kimberlites has been correlated with their age as determined by isotopic dating techniques and placed in the context of a well constrained geomagnetic polarity timescale kimberlite magmatism occurred over the period 75 to 45 ma in at least five temporally discrete intrusive episodes based on current evidence the older kimberlites 75 to 59 ma have low diamond contents and are distributed throughout the property younger kimberlites 56 to 45 ma have moderate to high diamond contents and occur in three distinct intrusive corridors with nne to ne orientations economic kimberlite pipes erupted at 55 4 0 4 ma along the a154 lynx intrusive corridor which is 7 km wide and oriented at 015 and at 53 2 0 3 ma along the panda intrusive corridor which is 1 km wide and oriented at 038 the intrusion ages straddle a paleopole reversal at chron c24n consistent with the observation that the older economic kimberlites present as aeromagnetic low anomalies while the younger economic pipes are characterised as aeromagnetic highs the aeromagnetic responses for these kimberlites are generally muted because they contain volcaniclastic rock types with low magnetic susceptibility kimberlites throughout the ekati property carry a primary natural magnetic remanence nrm vector in ti bearing groundmass magnetite and it dominates over vectors related to induced magnetisation magnetostratigraphic correlation of ekati kimberlites may therefore present a powerful adjunct to existing exploration techniques mainly because the diamond content of ekati kimberlites apparently is related more to the age of eruption than to any other parameter investigated in this work

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

thickness distribution of antarctic sea ice 1 ship based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around antarctica the data set comprises 23 373 observations collected over more than 2 decades of activity and has been compiled as part of the scientific committee on antarctic research scar antarctic sea ice processes and climate aspect program the results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness surface ridging snow cover and local variability for each region and season a simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography to provide a best estimate of the total ice mass including the ridged component the long term mean and standard deviation of total sea ice thickness including ridges is reported as 0 87 0 91 m which is 40 greater than the mean level ice thickness of 0 62 m analysis of the structure function along north south and east west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100 300 km which we use as a basis for presenting near continuous maps of sea ice and snow cover thickness plotted on a 2 5 5 0 grid

studies of the antarctic sea ice edge and ice extent from satellite and ship observations abstract passive microwave derived ice edge locations in the antarctic are assessed against in situ observations from ships between 1989 and 2000 during the growth season march october the ship data agrees with satellite data very well with r2 values of 0 99 and 0 97 for the bootstrap and team algorithms respectively during the melt season november february the agreement is not so good with the passive microwave ice edge typically 1 2 of latitude south of the observations this is due to the low concentration and saturated nature of the ice and the r2 values for this period are 0 92 and 0 80 for the bootstrap and team algorithms respectively sensitivity studies show that such an offset in the summer ice edge location can cause significant errors in trend studies of the extent of sea ice cover in the southern ocean the passive microwave ice concentration at the ice edge observed by ship varies greatly averaging 14 for the team algorithm and 19 for the bootstrap comparisons between passive microwave data and sar landsat and ols data during the ice growth season show that while small scale details in ice edge location are lost the passive microwave data generally provide good and consistent representation of the higher resolution imagery

two anisotropic layers in the slave craton abstract four years of recording global earthquakes using a broadband seismometer located at the ekati diamond mine revealed variations with earthquake azimuth in the arrival of sks phases these variations can be modeled assuming two distinct layers of anisotropy in the lithosphere the lower layer probably lies in the mantle and the anisotropy aligns with both north american plate motion and the strike of mantle structures identified by previous conductivity and geochemical analyses at n50 e the upper layer is hypothesized to result from regional structures in the uppermost mantle and the crust these trends are distinct from the mantle trends

a low velocity zone atop the transition zone in northwestern canada 1 seismic studies over the past decade have identified an s wave low velocity zone lvz above the transition zone at various locations around the globe this layer is hypothesized to be a lens of dense hydrous silicate melt ponding atop the 410 km discontinuity beneath the silicate melt density crossover predicted to exist within the upper mantle we have assembled a p and s receiver function data set to quantify the physical properties and geographical extent of the layer in northwestern canada geographic profiles formed from 1 d migration of receiver functions computed for the canadian northwest experiment canoe and portable observatories for lithospheric analysis and research investigating seismicity polaris slave arrays reveal an lvz beneath many stations at a nominal depth of 340 km to constrain layer thickness and poisson s ratio we performed a grid search over a suite of 1 d velocity profiles to model the relative delay times of direct conversions and reverberations from the top of the lvz and 410 km discontinuity as recorded at the yellowknife array in addition we performed linearized inversion of transmission coefficient amplitudes to estimate s velocity contrasts at the bounding interfaces the lvz is characterized by a thickness of 36 km with an s velocity contrast of 7 8 and poisson s ratio of 0 42 taken at face value the two latter results require an increase in p velocity into the lvz the poisson s ratio lies well above the iasp91 average of 0 29 0 3 for this depth range and favors the presence of high melt or fluid fractions

how well does wind speed predict air sea gas transfer in the sea ice zone a synthesis of radon deficit profiles in the upper water column of the arctic ocean we present 34 profiles of radon deficit from the ice ocean boundary layer of the beaufort sea including these 34 there are presently 58 published radon deficit estimates of k the air sea gas transfer velocity in the arctic ocean 52 of these estimates were derived from water covered by 10 sea ice or more the average value of k collected since 2011 is 4 0 1 2 m d 1 this exceeds the quadratic wind speed prediction of weighted kws 2 85 m d 1 with mean weighted wind speed of 6 4 m s 1 we show how ice cover changes the mixed layer radon budget and yields an effective gas transfer velocity we use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k when the ocean surface is ice covered whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations the same parameterizations could not reproduce k from the sea ice zone we conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice the magnitude of k through gaps in the ice may reach high values as ice cover increases possibly as a result of focused turbulence dissipation at openings in the free surface these 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover as dissolved tracer budgets they reflect air sea gas exchange with no impact from air ice gas exchange

seismic anisotropy of the slave craton nw canada from joint interpretation of sks and rayleigh waves summary teleseismic events recorded at a 25 element array in nw canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in sks and skks waves as well as directional dependence of rayleigh wave phase velocities typical delays between orthogonally polarized sks waves are 0 8 1 2 s and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle anisotropy of rayleigh waves is generally consistent with models of layered vs anisotropies that increase with depth from 1 per cent at the moho to 9 per cent at 200 km but vary between subarrays consistency between the sks and rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton the upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north south coinciding with regional scale fold axes mapped at the surface the fast polarization direction of the deeper layer aligns with current north america plate motion but its correlation with trends of coeval kimberlite eruptions within the lac de gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km

the importance of diurnal processes for the seasonal cycle of sea ice microwave brightness temperatures during early summer in the weddell sea antarctica over the perennial sea ice in the western and central weddell sea antarctica the onset of summer is accompanied by a significant decrease of sea ice brightness temperatures tb as observed by passive microwave radiometers such as the special sensor microwave imager ssm i the summer specific tb drop is the dominant feature in the seasonal cycle of tb data and represents a conspicuous difference to most arctic sea ice regions where the onset of summer is mostly marked by a rise in tb data from a 5 week drift station through the western weddell sea in the 2004 05 austral summer ice station polarstern ispol helped with identifying the characteristic processes for antarctic sea ice in situ glaciological and meteorological data in combination with ssm i swath satellite data indicate that the cycle of repeated diurnal thawing and refreezing of snow freeze thaw cycles is the dominant process in the summer season with the absence of complete snow wetting the resulting metamorphous snow with increased grain size as well as the formation of ice layers leads to decreasing emissivity enhanced volume scattering and increased backscatter this causes the summer tb drop

which forces drive north america understanding the mechanism of plate tectonics is one of the most important problems in the geosciences are the tectonic plates pulled and pushed from the side as the orowan elsasser model suggests or does mantle convection play an active role in driving the plates this question can be addressed by studying the deformation of deep continental roots the application to north america shown here indicates that the deeper mantle moves at a higher velocity than the north american plate and that the mantle plays an important role in driving the plates i suggest that this finding 1 provides a natural explanation for why the motion of north america slowed down dramatically during the past 100 m y and 2 predicts that north american motion will eventually come to a halt

new constraints on the upper mantle structure of the slave craton from rayleigh wave inversion 1 rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the slave craton in the northwestern canadian shield we invert for phase velocities at periods between 20 s 142 s with greatest sensitivity at depths of 28 200 km using crossing ray paths from events recorded by the polaris broadband seismic network and the yellowknife array phase velocities obtained for the slave province are comparable to those from other cratons at shorter periods but exceed the global average by 2 at periods above 60 s suggesting that the slave craton may be an end member in terms of its high degree of mantle depletion the one dimensional inversion of phase velocities yields high upper mantle s wave velocities of 4 7 0 2 km s that persist to 220 65 km depth and thus define the cratonic lithosphere azimuthal anisotropy is well resolved at all periods with a dominant fast direction of n59 e 20 suggesting that upper mantle anisotropy beneath the slave craton is influenced by both lithospheric fabric and sub lithospheric flow

teleseismic studies of the canadian landmass lithoprobe and its legacy national research council of canada nrc research press http www nrc cnrc gc ca notices e html copyright permission to reproduce materials on this web site were produced and or compiled by the national research council for the purpose of providing canadians with direct access to information about the programs and services offered by the government of canada the material on this site is covered by the provisions of the copyright act by canadian laws policies regulations and international agreements such provisions serve to identify the information source and in specific instances to prohibit reproduction of materials without written permission

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

kimberlite trends in nw canada kimberlite deposits attract much interest as the host rock for diamonds but their eruption mechanisms and deep root structure remain poorly perceived independent geochronological and seismic shear wave anisotropy studies in nw canada near the world9s newest diamond mines revealed an unexpected correlation in structural trends age of eruption grouped 36 kimberlites into linear clusters forward modelling of shear wave delays indicated two distinct layers of seismic anisotropy with a boundary about 120 km deep the similarity of inferred age and trend direction at surface and at depth indicates that these kimberlites erupted along lithospheric scale fractures oriented by continental stress fields related to north american plate motions

lithoprobe s impact on the canadian diamond exploration industrythis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent earth sciences sector contribution 20070261 lithoprobe contribution 1479 lithoprobe aided and will continue to guide diamond exploration in canada via a number of diverse ways and at scales from cratons to individual ore bodies specific transects outlined the domainal nature geometry and age relations of crust subjacent mantle and diamond bearing roots in unusually rich detail for cratonic settings such as those of the superior and slave cratons as well as pericratonic shield settings such as those of the sask craton and buffalo hills terrane the superior and slave examples of craton construction by multiple underthrust layers as inferred from structural geometries observed in lithoprobe seismic and magnetotelluric surveys remain the clearest resolved to date the first definition of the sask craton occurred 10 15 years ago and helped fuel the diamond industry s ongoing revision of clifford s rule for exploration so as to include targets in proterozoic shield settings main lithoprobe transect results and supporting geoscience studies have helped to demonstrate the

synchroneity of cratonic burial phases and gaps in the kimberlite record episodic magmatism or preservational bias abstract a variety of models are used to explain an apparent episodicity in kimberlite emplacement implicit in these models is the assumption that the preserved kimberlite record is largely complete however some cratons now mostly devoid of phanerozoic cover underwent substantial phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the slave craton superior craton and cratonic western australia a similar pattern exists in the kaapvaal craton although its magmatic deposition and erosion history differs in key ways from the other localities one explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts even if kimberlites occurred during cratonic burial phases the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced in this model kimberlite magmatism was more continuous than the preserved record suggests implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects either way the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns

crossing the divide representation of channels and processes in reduced complexity river models at reach and landscape scales abstract reduced complexity models have considerable potential as tools for elucidating river behaviour over periods of 10 0 10 4 years and consequently for addressing fundamental questions concerning the scale dependent nature of explanation in geomorphology this paper proposes a simple subdivision of reduced complexity models of river behaviour into two categories that mirror methodological developments in fluvial geomorphology over the past 50 years first high resolution cellular approaches that are implemented within a framework that resolves process form feedbacks at small time and space scales second models that incorporate section averaged representations of channel geometry and processes and that are typically underpinned by regime theory and equilibrium concepts examples of both model types are presented here in the form of a cellular representation of stream braiding and a combined lattice network model of alluvial fan evolution simulations conducted using these models demonstrate how small scale process form interactions determine the emergence of larger scale channel and fan morphology and in so doing regulate system response to external forcing in this sense both models demonstrate that internal feedbacks play a critical role in controlling river responses to environmental change over historic and holocene timescales however both classes of model are characterised by uncertainty in their parameterisation of geomorphic processes such that internal feedbacks and thresholds for channel response to external forcing may vary substantially between competing models methods of refining both approaches are considered and hybrid models based on lattice network structures and mechanistic representations of channel process form interactions are identified as a means of addressing the shortcomings of existing strategies

sea ice melting processes inferred from ice upper ocean relationships in the ross sea antarctica 1 sea ice melting processes are inferred from various summer sea ice and upper ocean data obtained in the ross sea in january 1999 using spatially 30 km averaged continuous data an ice concentration water temperature plot ct plot shows that the temperature at a depth of 7 m increases as ice concentration decreases in the ice interior region the ct plot is explained by a simple ice upper ocean coupled model in which sea ice melting is caused only by heat input through open water the bulk heat transfer coefficient between ice and ocean kb is estimated to be 1 2 10 4 m s 1 these findings are supported by the relationship between ice concentration and heat content in the mixed layer estimated from conductivity temperature depth ctd data salinity at a depth of 7 m salt deficit in the mixed layer associated with sea ice melt estimated from ctd data and their relationships with ice concentration also suggest that melting is mainly caused by atmospheric heat input through open water and that local balance of salinity nearly hold the time evolution of ice concentration calculated from the ice upper ocean coupled model with the estimated kb corresponds with that derived from the special sensor microwave imager ssm i confirming the model s applicability and the value of kb

regional classes of sea ice cover in the east antarctic pack observed from satellite and in situ data during a winter time period abstract ice concentration data alone are often of limited use in many process and modeling studies as different ice regimes of approximately 100 concentration can have significantly different heat flux albedo and other surface properties current ice concentration algorithms perform poorly in regions of predominantly thin or highly fragmented ice which constitute a significant proportion of the pack in east antarctica the impact of the sea ice cover on high latitude air sea interactions and marine ecology depends not only on ice extent and concentration but also on the ice type composition of the pack an unsupervised ice classification scheme using data from four channels of the ssm i is presented and tested as a means of gaining important additional complementary information on surface type class interpretation is by comparison with avhrr ers 1 sar and near coincident digital aerial photography and in situ data the classification does a reasonable job at consistently differentiating the large scale constituent regimes including the outer marginal ice zone the interior pack and a transition zone separating the two given the short period of data analyzed the cluster maps appear to be generally coherent and consistent through time as the pack changes in response to synoptic scale atmospheric forcing although the robustness of the technique needs further testing over longer time periods an observed crossover in the meridional brightness temperature profiles is a dominant and consistent feature which marks the transition from unconsolidated and wet ice in the marginal ice zone to more consolidated ice with a thicker and drier snow cover in the interior pack ambiguities occur at the boundaries of some of these regimes due to sensor resolution limitations and the mixing of different ice types and open water also some ice classes like brash ice cannot be distinguished from forming pancake ice furthermore although the signature of the inner pack is usually distinct from that of the outer pack we observed one extraordinary swell propagation event which led to ice fracturing and surface wetting and significantly altered the surface classification the results of this multiparameter study underline the importance of using multisensor systems synergistically to improve interpretation of passive microwave data and better characterize the complex antarctic pack

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

mapping the mantle lithosphere for diamond potential using teleseismic methods abstract recent developments in seismic magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres to map its structures and through geological interpretations to assess its potential as a diamond reservoir several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns for example beneath the central slave craton seismic discontinuities at depths of 38 110 140 and 190 km appear to bound two distinct anisotropic layers tomographic 3 d inversions of seismic wave travel times and 2 d inversions of surface or scattered waves use arrays of stations and provide lateral coverage in combination and by correlation with electrical conductivity and xenolith petrology studies these techniques provide maps of key physical properties within parts of the cratons known to host diamonds beneath the slave craton the discontinuity at 38 km is the base of the crust the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite to date only some of these techniques have been applied to the slave and kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not as yet generally agreed

antarctic sea ice extents and concentrations comparison of satellite and ship measurements from international polar year cruises antarctic sea ice processes and climate aspect ship based ice observations conducted during the sea ice mass balance in the antarctic simba and sea ice physics and ecosystem experiment sipex international polar year ipy cruises september october 2007 are used to validate remote sensing measurements of ice extent and concentration observations include varied sea ice types at and inside the ice edge of west 908 w and east 1208 e antarctica time series of advanced microwave scanning radiometer earth observing system amsr e ice extents and us national ice center nic ice edges were obtained for the 2007 08 periods bracketing the period these cruises were conducted a comparison between passive microwave satellite imagery and aspect observations of sea ice concentration during two cruises was also performed in 908w regions the concentrated pack ice indicated good correlation between ship observations and passive microwave estimates of the ice concentration r2 0 80 in the marginal zone of west antarctica and over nearly the entire sea ice zone of east antarctica correlation dropped to r2 0 60 these findings are consistent with other studies comparing passive microwave and ship observations and further verify that the east antarctic sea ice zone is more marginal in character there are significant ice edge differences between amsr e and nic between late november 2007 and early march 2008 such that the amsr e sea ice extent estimate is 1 2 106 km2 less than the nic estimate

kimberlite eruptions as triggers for early cenozoic hyperthermals 1 the early cenozoic experienced at least three short but major hyperthermals associated with disruptions of the global carbon cycle the largest among those the paleocene eocene thermal maximum was associated with a negative carbon isotopic excursion of 2 5 that appears to be best explained by the thermal dissociation of methane hydrates due to an initial period of warming the cause of the initial warming has been attributed to a massive injection of carbon co2 and or ch4 into the atmosphere however the source of the carbon is as yet unknown the emplacement of a large cluster of kimberlite pipes at 56 ma in the lac de gras region of northern canada may have provided the carbon that triggered early warming in the form of exsolved magmatic co2 our calculations indicate that the estimated 900 1100 pg of carbon required for the initial 3 c of ocean water warming associated with the paleocene eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster the coeval ages of two other kimberlite clusters in the lac de gras field and two other early cenozoic hyperthermals indicate that co2 degassing during kimberlite emplacement is a plausible source of the co2 responsible for these sudden global warming events

summer and early fall sea ice concentration in the ross sea comparison of in situ aspect observations and satellite passive microwave estimates sea ice conditions were observed using the aspect observation protocol on three cruises in the ross sea spanning the antarctic summer season apis december 1999 february 2000 anslope 1 march april 2003 anslope 2 february april 2004 an additional dataset was analyzed from helicopter video surveys taken during the apis cruise the helicopter video was analyzed using two techniques first as an aspect dataset where it was sampled visually for ice concentration floe sizes and ice type on a point basis at 11 km intervals second computerized image processing on a subset of nine helicopter flights to obtain ice concentration on a continuous basis 1 s intervals for the entire flight this continuous sampling was used to validate the point sampling methods to characterize the ice cover the aspect sampling on the helicopter video and the use of the aspect protocol on the ship surveys the estimates for average ice concentration agreed within 5 for the continuous digitized data and point sampling at 11 km intervals in this comparison the ship and video in situ datasets were then compared with ice concentrations from ssm i passive microwave satellite data derived using the bootstrap and nasa team algorithms less than 50 of the variance in summer ice concentration observed in situ was explainable by satellite microwave data the satellite data were also inconsistent in measurement both underestimating and overestimating the concentration for summer conditions but improved in the fall period when conditions were colder this improvement was in the explainable variance of 70 although in situ concentration was underestimated albeit consistently by the satellite imagery in fall

physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record we assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 1200 m and by a comparison with intraplate monogenetic basaltic volcanic fields eroded kimberlite fields are composed of pipes or diatremes and dikes and within any one kimberlite field regardless of erosion level kimberlite bodies vary in area at earth s surface over 2 3 orders of magnitude typically 60 70 of the bodies are

stacked uppermost mantle layers within the slave craton of nw canada as defined by anisotropic seismic discontinuities 1 a 20 station seismic array in nw canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the mohorovicic and lehman discontinuities following wavefield decomposition groups of seismograms were source normalized through simultaneous deconvolution to estimate the near receiver impulse response and thus detect major discontinuities beneath each seismic station stations within the lac de gras kimberlite field display an unusually strong negative impulse on the radial component within the nw quadrant and two moderate impulses on the transverse component forward modeling of these impulses suggests a mantle layer dipping at 22 to the southeast with a mildly anisotropic 2 upper discontinuity at 120 135 km depth and another mildly anisotropic 2 discontinuity at about 170 km depth superimposed on these layers is another stronger anisotropic 4 layer between 110 and 180 km depths that dips to the west stations outside of the lac de gras field but within the southeastern slave craton display more numerous but weaker impulses the most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north the prominent negative impulse observed on the radial component is interpreted to arise from structural preferred orientation in the form of a stockwork of wehrlite dykes beneath the lac de gras field interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000 2900 ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 ma the layer beneath the se slave craton has a similar but distinct tectonic history of nw verging underthrusting associated with the 2635 2615 ma defeat suite of plutonism taken together these interpretations indicate that the slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 ma each block is 90 120 km thick and all four blocks abut across a steep suture boundary beneath mackay lake in the central slave craton significant amounts of carbon and eclogites could have become incorporated into the central slave mantle within the diamond stability field during the proposed underthrusting of lithosphere to explain their relatively common occurrence in kimberlite eruptions within the slave craton