sea ice models for climate study retrospective and new directions scientific observations of sea ice began more than a century ago but detailed sea ice models appeared only in the latter half of the last century the high albedo of sea ice is critical for the earth s heat balance and ice motion across the ocean s surface transports fresh water and salt the basic components in a complete sea ice model must include vertical thermodynamics and horizontal dynamics including a constitutive relation for the ice advection and deformational processes this overview surveys topics in sea ice modeling from the global climate modeling perspective emphasizing work that significantly advanced the state of the art and highlighting promising new developments

the peraluminous aurumina granite suite in central brazil an example of mantle continental crust interaction in a paleoproterozoic cordilleran hinterland setting abstract peraluminous granites and tonalites granodiorites of the aurumina suite crop out in the external zone of the brasilia fold belt on the western margin of the sao francisco craton rocks of this suite intrude retrograde medium grade schists and paragneisses of the ticunzal formation and display a magmatic muscovite biotite garnet assemblage and discrete graphite lamellae the suite is divided into six members au1 through au6 which grade from synkinematic to post kinematic in character with ages spanning from 2 11 to 2 16 ga according to zircon u pb data compositional features of the aurumina suite and associated coeval metaluminous mafic to intermediate plutons suggest an origin by source hybridization processes characterized by reaction between metasedimentary rocks and basaltic melts that would have taken place at shallow depths pressure

the quality of sea ice velocity estimates several aspects of observed and estimated velocities for arctic sea ice are explored the long term mean ice motion field and associated variance are derived using 15 years over 85 000 observations of daily buoy motions the mean field is separated into the part due to long term currents and internal ice stress and the part due to mean winds the velocity variance is partitioned into linear and nonlinear parts the linear response has no trend over the 15 years examined for a 5 year subset of the buoy data an ice ocean model with constant air drag parameters is used to estimate daily ice velocities at the buoy locations and then the linear relationship between the modeled velocities and the geostrophic winds is examined compared with buoy velocities the modeled velocities have a larger variance and larger linear response to the winds in a parameter study the effects of seasonally varying air drag coefficients increased ice strength and different ocean currents are examined seasonally varying air drag with smaller average magnitude than the standard value significantly improves the match of total linear and nonlinear variances between observed and modeled velocities the model simulations with varying air drag also have a smaller root mean square daily velocity error comparison of daily large scale deformations shows that the day to day deformations are not modeled well but generally have means and standard deviations within 50 of the observed values no one version of the model is consistently better than the others at matching the means and standard deviations of the observed daily divergence vorticity and shear ice thickness is sensitive to the model parameters varied with 5 year average thicknesses ranging from 2 2 to 3 4 m for the various simulations

the younger dryas termination and north atlantic deep water formation insights from climate model simulations and greenland ice cores results from the gisp2 and grip ice cores show that the termination of the younger dryas yd climate event in greenland was a large and extremely fast climate change a reinitiation of north atlantic deep water formation following a shutdown and its associated winter release of heat to the atmosphere has been suggested as the most likely cause of this climate transition to test this idea two general circulation model experiments using genesis have been completed for yd time 12 000 calendar years ago one with low heat flux in the nordic seas 10 w m deep water shutdown and one with high nordic sea heat flux 300 w m active deep water formation comparison of greenland climate differences between these experiments with the ice core records shows that when deep water is turned on much of the yd termination warming is achieved the increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation which obscures the dominantly wintertime response to the specified forcing the winter storm track shift toward greenland contributes much of the climate change at the yd termination

the middle cambrian mount roosevelt formation new of northeastern british columbia evidence for rifting and development of the kechika graben system the eastern margin of the kechika graben in the vicinity of muncho and moose lakes northeastern british columbia contains middle cambrian rift related strata herein named the mount roosevelt formation the formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late early cambrian gataga group bonnia olenellus zone and is conformably overlain by a thick unnamed carbonate sequence of middle cambrian age plagiura poliella zone the mount roosevelt formation is subdivided into three members the lowermost member is characterized by ooid bearing siltstone and sandstone interbedded with dolostone limestone and hematitic conglomerate conformably above this the middle member is a thick sequence of polymict cobbly pebble conglomerate the upper member includes karstified dolostone calcareous cemented conglomerate and sandstone and limestone collectively the mount roosevelt formation reflects alluvial fan delta progradation into a transgressive mar

oxygen facies and secular controls on the appearance of cryogenian and ediacaran body and trace fossils in the mackenzie mountains of northwestern canada the causes behind the appearance of abundant macroscopic body and trace fossils at the end of the neoproterozoic era remain debated iron geochemical data from fossiliferous ediacaran successions in newfoundland suggested that the first appearances correlated with an oxygenation event a similar relationship was claimed to exist in the mackenzie mountains canada although later stratigraphic studies indicated that the sections analyzed for geochemistry were incorrectly correlated with those hosting the fossils to directly connect fossil occurrences with geochemistry in the mackenzie mountains we conducted a multiproxy iron carbon sulfur and trace element geochemical analysis of stratigraphic sections hosting both the cryogenian twitya discs at bluefish creek as well as ediacaran fossils and simple bilaterian traces at sekwi brook there is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows however some change in environment a potential partial oxygenation is correlated with increasing burrow width higher in the blueflower formation data from sekwi brook suggest that these organisms were periodically colonizing a predominantly anoxic and ferruginous basin this seemingly incongruent observation is accommodated through accounting for differing time scales between the characteristic response time of sedimentary redox proxies versus that for ecological change thus hypotheses directly connecting ocean oxygenation with the appearance of macrofossils need not apply to all areas of a heterogeneous ediacaran ocean and stably oxygenated conditions on geological time scales were not required for the appearance of these avalon assemblage ediacaran organisms at least in the mackenzie mountains the appropriate facies for fossil preservation appears to be the strongest control on the stratigraphic distribution of macrofossils

modeling global sea ice with a thickness and enthalpy distribution model in generalized curvilinear coordinates abstract a parallel ocean and ice model poim in generalized orthogonal curvilinear coordinates has been developed for global climate studies the poim couples the parallel ocean program pop with a 12 category thickness and enthalpy distribution ted sea ice model although the poim aims at modeling the global ocean and sea ice system the focus of this study is on the presentation implementation and evaluation of the ted sea ice model in a generalized coordinate system the ted sea ice model is a dynamic thermodynamic model that also explicitly simulates sea ice ridging using a viscous plastic rheology the ted model is formulated such that all the metric terms in generalized curvilinear coordinates are retained following the pop s structure for parallel computation the ted model is designed to be run on a variety of computer architectures parallel serial or vector when run on a computer cluster with 10 parallel processors the parallel performance of the poim is close to that of a correspon

palaeoenvironment of braided fluvial systems in different tectonic realms of the triassic sherwood sandstone group uk abstract fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional compressional and strike slip settings as well as on slowly subsiding passive basin margins a major rifting phase affected nw europe during the triassic and resulted in the generation of numerous sedimentary basins in the uk much of the fill of these basins is represented by fluvial and aeolian successions of the sherwood sandstone group additionally regions that experienced slow rates of mesozoic subsidence unrelated to triassic rifting also acted as sites of accumulation of the sherwood sandstone group one well exposed example being the eastern england shelf the fluvial depositional architecture of deposits of the sherwood sandstone group of the eastern england shelf a shelf edge basin is compared with similar fluvial deposits of the st bees sandstone formation eastern irish sea basin a half graben the two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors climate sediment source delivery style differences in preserved sedimentary style principally reflect their different tectonics settings analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel fill elements that are recorded predominantly by downstream accreting sandy barforms the different tectonic settings in which the two braided fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long term preservation potential on the eastern england shelf the vertical stacking of pebbly units and the general absence of fine grained units reflect a slow rate of sediment accommodation generation 18 19 4 m myr in this shelf edge basin successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved by contrast in the eastern irish sea basin of west cumbria the rate of sediment accommodation generation was substantially greater 119 m myr such that space was available to preserve complete fluvial cycles including silty drape units that cap the channelized deposits

an ice ocean coupled model an ice model an ocean model and a method of coupling the models are described the ice model is a synthesis with variations and extensions of previous modeling ideas ice thickness concentration velocity and internal energy are prognostic variables the ice thermodynamics are represented by temperatures at the snow surface ice surface the interior and the bottom surface melting and freezing rates are calculated at the ice atmosphere ice ocean and atmosphere ocean interfaces a prescribed portion of summer meltwater can be stored on the surface and refrozen in the fall the ocean model includes a second moment turbulence closure submodel and enables one to solve for oceanic heat flux the interfacial stress and subsurface properties in this paper the model is applied to one dimensional simulations but the equations are cited in a form for implementation by two and three dimensional models in a companion paper kantha and mellor this issue the model is used for two dimensional vertical plane simulations in the bering sea several one dimensional sensitivity studies are performed in the case where the ice model is decoupled from the ocean here the oceanic heat flux and sea surface temperature are prescribed constants the studies reveal the role and sensitivity of surface trapped meltwater ice concentration and ice divergence with the coupled ice ocean model the seasonally varying oceanic heat flux and mixed layer properties are determined by the model some comparisons with observations in the central arctic ocean are possible the role of the molecular sublayer immediately adjacent to the ice is examined frazi ice production is related to the large disparity in the molecular diffusivities for temperature and salinity the mixed layer model contains empirical constants which are known from turbulence data the molecular sublayer parameterization requires one empirical parameter b which is uncertain but from this study is assuredly greater than zero the value implicit in previous models the ice model requires the empirical parameters ci f and ci u to quantitatively account for freezing or melting processes in open leads their values are also uncertain but we present reasoning and sensitivity studies to suggest specific values finally an empirical parameter g is introduced it is the ratio of the value of the ice thickness used to represent average ice volume in the dynamic and thermodynamic equations to the value of the thickness needed in the heat conduction equation estimates of g are made from observed thickness distribution functions sensitivity studies show it to be an important parameter

sea ice circulation around the beaufort gyre the changing role of wind forcing and the sea ice state sea ice drift estimates from feature tracking of satellite passive microwave data are used to investigate seasonal trends and variability in the ice circulation around the beaufort gyre over the multi decadal period 1980 2013 our results suggest an amplified response of the beaufort gyre ice circulation to wind forcing especially during the late 2000s we find increasing anticyclonic ice drift across all seasons with the strongest trend in autumn associated with increased ice export out of the southern beaufort sea into the chukchi sea a flux gate analysis highlights consistency across a suite of drift products despite these seasonal anticyclonic ice drift trends a significant anticyclonic wind trend occurs in summer only driven in part by anomalously anticyclonic winds in 2007 across all seasons the ice drift curl is more anticyclonic than predicted from a linear relationship to the wind curl in the 2000s compared to the 1980s 1990s the strength of this anticyclonic ice drift curl amplification is strongest in autumn and appears to have increased since the 1980s up to 2010 in spring and summer the ice drift curl amplification occurs mainly between 2007 and 2010 these results suggest non linear ice interaction feedbacks e g a weaker more mobile sea ice pack enhanced atmospheric drag and or an increased role of the ocean the results also show a weakening of the anticyclonic wind and ice circulation since 2010 this article is protected by copyright all rights reserved

evolution of the early paleozoic cordilleran margin of laurentia tectonic and eustatic events interpreted from sequence stratigraphy and conodont community patterns several field seasons in the canadian cordillera have allowed the measurement description and sampling of over 20 000 m of lower paleozoic strata from 26 stratigraphic sections across four platform to basin transects with the recovery of over 100 000 conodonts from more than 1200 4 5 kg samples this work was part of the lithoprobe slave northern cordillera lithospheric evolution snorcle project but is also being extended through a pan lithoprobe project to understand the tectonic and eustatic response of much of the laurentian plate through the early paleozoic based on the abundant field data the complex stratigraphic framework is interpreted in terms of sequence stratigraphy and a derived relative sea level curve using detailed conodont taxonomic and biostratigraphic results cluster analysis of conodont distributional data identified an evolving series of conodont communities through space and time these communities were partitioned across the platform to basin gradient and provide an additional sensitive indicator of relative sea level change these two independent approaches generated comparable eustatic curves for this cordilleran laurentian margin during much of the early paleozoic and identified some global eustatic events noted by earlier workers this part of laurentia was not a simple passive margin during the early paleozoic but rather was affected by four main tectonic events complicated by six principal eustatic changes some success was achieved in filtering the global and regional tectonic eustatic effects and in proposing causes for some of the events

importance of convective mixing in seasonal ice margin simulations a two dimensional ice ocean model applicable to the greenland sea is constructed and used to examine the role of penetrative convection in the seasonal advance and retreat of the sea ice margin the standard version of the model is based on the full dynamic and thermodynamic equations of sea ice and on the primitive equations discretized on a vertical slab for the ocean it includes a special treatment of upper ocean vertical mixing based on a parameterization of entrainment at the bottom of the ocean mixed layer the simulation results with this standard version of the model show that when ice advection is included with ice moving outward over open water penetrative convection delays and greatly reduces the ice edge advance the resulting seasonal ice distribution yields better agreement with observation than a simulation that does not allow mixed layer entrainment when lateral heat transport is neglected in the ocean the ice margin again moves too a great extent outward this result demonstrates that in addition to vertical mixing a heat source must exist in the ocean mixed layer in order to resist and ultimately stop the advection of ice finally in the absence of ice motion penetrative convection has a less pronounced effect on the ice edge location

comparison of sea ice simulations with interactive and monthly averaged forcing models a dynamic thermodynamic sea ice model is developed and coupled with the 21 wave 9 level melbourne university general circulation model to simulate the seasonal cycle of the global sea ice distribution we have run the coupled system and obtained a creditable seasonal simulation of global sea ice when monthly averaged atmospheric data taken from the mean of the coupled run are used to force the sea ice model the seasonal cycle of sea ice extent to the outer ice edge is quite similar to that simulated in the interactive run however the actual sea ice covered area i e excluding leads differs considerably between the two simulations sea ice is more compact in the monthly averaged forced run than in the interactive run throughout the year in both hemispheres the sea ice thickness distribution also differs between the two runs in general the sea ice is more open and thicker in the seasonal ice zone of the two polar regions for the interactive coupled case than for the mean forcing we have also run the model forced with daily atmospheric data and the simulated sea ice distribution differs significantly from both the interactive model and the monthly averaged forcing results these differences highlight the dangers of undertaking studies with sea ice models forced with prescribed atmospheric conditions rather than using a fully interactive atmosphere sea ice system

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

evaluating new fault controlled hydrothermal dolomitization models insights from the cambrian dolomite western canadian sedimentary basin fault controlled hydrothermal dolomitisation in tectonically complex basins can occur at any depth and from different fluid compositions including deep seated crustal or basinal brines nevertheless many studies have failed to identify the actual source of these fluids resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitisation with development of new concepts in hydrothermal dolomitisation the study aims in particular to test the hypothesis that dolomitising fluids were sourced from either seawater ultramafic carbonation or a mixture between the two by utilising the cambrian mount whyte formation as an example here the large scale dolostone bodies are fabric destructive with a range of crystal fabrics including euhedral replacement rd1 and anhedral replacement rd2 since dolomite is cross cut by low amplitude stylolites dolomitisation is interpreted to have occurred shortly after deposition at a very shallow depth 1 km at this time there would have been sufficient porosity in the mudstones for extensive dolomitisation to occur and the necessary high heat flows and faulting associated with cambrian rifting to transfer hot brines into the near surface while the 18owater and 87sr 86sr ratios values of rd1 are comparable with cambrian seawater rd2 shows higher values in both parameters therefore although aspects of the fluid geochemistry are consistent with dolomitisation from seawater very high fluid temperature and salinity could be suggestive of mixing with another hydrothermal fluid the very hot temperature positive eu anomaly enriched metal concentrations and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks potentially as a result of interaction between the underlying proterozoic serpentinites and co2 rich fluids this study highlights that large scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses providing a potential explanation for the mass balance problem often associated with their genesis

integrated ediacaran chronostratigraphy wernecke mountains northwestern canada abstract terminal neoproterozoic ediacaran ca 600 543 ma strata of the upper part of the windermere supergroup are well exposed in the mackenzie mountains and the wernecke mountains of northwestern canada windermere strata in the mackenzie mountains contain an exceptional ediacaran biostratigraphic and isotopic c and sr record while the sequence stratigraphic record is subtle throughout this predominantly deep water succession coeval strata in the wernecke mountains can be correlated with the succession in the mackenzie mountains on a formational level in contrast to the deep water setting of the mackenzies succession the werneckes succession preserves a predominantly shallow water succession amenable to detailed sequence stratigraphy integrated lithostratigraphy chemostratigraphy biostratigraphy and sedimentology partitions the wernecke mountains succession into five depositional sequences and constrains correlation with strata in the mackenzie mountains the siliciclastic and carbonate sediments deposited along the margin of the proto pacific ocean record accumulation in continental slope neritic and terrestrial paleoenvironments distinctive temporal chemostratigraphic and biostratigraphic attributes allow these strata to be correlated with other ediacaran successions worldwide integrating sequence stratigraphy with these attributes is a robust yet under utilized correlation tool and is a step toward erecting a more detailed working neoproterozoic chronostratigraphy

paleotectonic and stratigraphic implications of the kingston peak noonday contact in the panamint range eastern california geologic mapping in parts of the southern panamint range eastern california has documented critical stratigraphic relationships at the upper proterozoic contact between the kingston peak formation and noonday dolomite these relations bear significantly upon the timing and style of development of the latest proterozoic and paleozoic continental margin of western north america they include 1 an unconformity below wildrose diamictite interpreted as glaciogenic the uppermost kingston peak unit 2 diamictite similar to wildrose diamictite interbedded within carbonate strata of the overlying basal noonday dolomite and 3 interfingering between rocks of the kingston peak and noonday formations the kingston peak noonday contact is thus complex and variable in character and does not represent a regional unconformity instead it marks a transition from rift to post rift sedimentation without any hiatus and shows there was no regional doming during rifting the stratigraphic relations also demonstrate

glacioeustasy meteoric diagenesis and the carbon cycle during the middle carboniferous middle carboniferous carbonates in the western u s have undergone pleistocene bahamas style meteoric diagenesis that may be associated with expanding late paleozoic ice sheets fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the middle carboniferous unconformity these sections contain top negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers desiccation cracks fabric destructive recrystallization or extensive root systems the timing of the diagenesis is synchronous with similarly scaled top negative carbon isotope excursions observed by others in england kazakhstan and china the mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle carboniferous platform carbonates is a significant component of the global carbon cycle we present a simple carbon box model to illustrate that the 13c of dissolved inorganic carbon in the ocean could be elevated by 1 4 as isotopically light carbon from the weathering of terrestrial organic matter reacts with exposed platforms before reaching the ocean and atmosphere these results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high 13c of the late paleozoic icehouse

effects of rheology and ice thickness distribution in a dynamic thermodynamic sea ice model realistic treatment of sea ice processes in general circulation models is needed to simulate properly global climate and climate change scenarios as new sea ice treatments become available it is necessary to evaluate them in terms of their accuracy and computational time here several dynamic ice models are compared using both a 2 category and 28 category ice thickness distribution simulations are conducted under normal wind forcing as well as under increased and decreased wind speeds it is found that the lack of a shear strength parameterization in the cavitating fluid rheology produces significantly different results in both ice thickness and ice velocity than those produced by an elliptical rheology furthermore use of a 28 category ice thickness distribution amplifies differences in the responses of the various models while the choice of dynamic model is governed by requirements of accuracy and implementation it appears that in terms of both parameterization of physical properties and computational time the elliptical rheology is well suited for inclusion in a gcm

a diagnostic ice ocean model abstract a coupled ice ocean model suitable for simulating ice ocean circulation over a seasonal cycle is developed by coupling the dynamic thermodynamic sea ice model of hibler with a multilevel baroclinic ocean model bryan this model is used to investigate the effect of ocean circulation on seasonal sea ice simulations by carrying out a simulation of the arctic greenland and norwegian seas the ocean model contains a linear term that damps the ocean s temperature and salinity towards climatology the damping term was chosen to have a three year relaxation time equivalent to the adjustment time of the pack ice no damping however was applied to the uppermost layer of the ocean model which is in direct contact with the moving pack ice this damping procedure allows seasonal and shorter time scale variability to be simulated in the ocean but does not allow the model to drift away from ocean climatology on longer time scales for the standard experiment an initial integration of five years was per

covariance of optimal parameters of an arctic sea ice ocean model abstractthe uniqueness of optimal parameter sets of an arctic sea ice simulation is investigated a set of parameter optimization experiments is performed using an automatic parameter optimization

a dynamic and thermodynamic sea ice model for subpolar regions a mesoscale dynamic thermodynamic sea ice model is presented the model differs from the large scale sea ice model hibler 1979 parkinson and washington 1979 in both the dynamic and thermodynamic parts in the dynamic calculation a constitutive relation based on ice floe collision is used for the determination of the internal ice force in the marginal ice zone miz while in the thermodynamic calculation a three level approach for the ice thickness distribution is adopted instead of the common two level approach hibler 1979 the simulation results from the mesoscale sea ice model show that the three level approach is more sensitive to geophysical forcing than the two level approach thus it is demonstrated that the mesoscale sea ice model has the potential of a more accurate description of the dynamics and the thermodynamics than the previous models in the simulation of sea ice processes in subpolar regions giving the exact initial conditions and the atmospheric and oceanic forcings this model can be used as a sea ice forecasting model in subpolar regions

optimization of a sea ice model using basinwide observations of arctic sea ice thickness extent and velocity a stand alone sea ice model is tuned and validated using satellite derived basinwide observations of sea ice thickness extent and velocity from the years 1993 to 2001 this is the first time that basin scale measurements of sea ice thickness have been used for this purpose the model is based on the cice sea ice model code developed at the los alamos national laboratory with some minor modifications and forcing consists of 40 yr ecmwf re analysis era 40 and polar exchange at the sea surface poles data three parameters are varied in the tuning process ca the air ice drag coefficient p the ice strength parameter and the broadband albedo of cold bare ice with the aim being to determine the subset of this three dimensional parameter space that gives the best simultaneous agreement with observations with this forcing set it is found that observations of sea ice extent and velocity alone are not sufficient to unambiguously tune the model and that sea ice thickness measurements are necessary to locate a unique subset of parameter space in which simultaneous agreement is achieved with all three observational datasets

synoptic and seasonal variations of the ice ocean circulation in the arctic a numerical study abstract the circulations of the arctic ice cover and ocean are investigated using a coupled ice ocean model the coupling is strong and two way for synoptic time scales but is limited on seasonal time scales the geostrophic ocean currents are not changed by the computed heat and salt fluxes the ice drift motion ekman transports and the wind driven part of the barotropic circulation are examined for the months of february and august 1986 representing different atmospheric forcing ice thickness and ice strength regimes initial examination of the results revealed no significant seasonal dependence of ice drift response on the synoptic time scale other than larger velocities with larger wind stresses daily maximum ice drift velocities range from 20 40 cm s in february and 15 30 cm s in august the corresponding mean monthly maximum drifts were 11 and 9 cm respectively the drag associated with the geostrophic currents plays a much bigger role in the summer because of the lighter atmospheric stresses the well known reversal of the normally clockwise beaufort gyre to a cyclonic system in august takes place in a few days and lasts well into september in february the beaufort gyre varies between a large clockwise system covering all the canadian basin to a small tight gyre centered over the southern beaufort sea without any hint of reversal or disappearance large areas of strong divergence were found in the ekman transport patterns as well as the ice divergence fields indicating areas where ice thinning openings and new ice formation might occur in august this occurred in the chukchi sea and in february just north of novaya zemlya

age and stratigraphic tectonic significance of proterozoic diabase sheets mackenzie mountains northwestern canada diabase dikes and sills were intruded during a late proterozoic phase of extensional tectonics in the northern canadian cordillera this event is strongly expressed in the depositional pattern of formations immediately below and above the base of the windermere supergroup rb sr isochron dates of 766 24 and 769 27 ma have been obtained for two of the these diabase sheets which intruded the tsezotene formation of the mackenzie mountains supergroup the emplacement of the diabase sills and dikes may be related to either of two volcanic events in the region one documented the other inferred in either case emplacement of the diabase at approximately 770 ma is close to the boundary between the mackenzie mountains supergroup and the overlying windermere supergroup precedes a regional glaciation shezal tillite and reflects a strong accentuation of the west facing cordilleran miogeocline

isotopic geochemical and temporal characterization of proterozoic basement rocks in the quitovac region northwestern sonora mexico implications for the reconstruction of the southwestern margin of laurentia a detailed geochemical characterization of 19 representative proterozoic basement rocks in the quitovac region in northwestern sonora mexico has identified two distinct paleoproterozoic basement blocks that coincide spatially with the previously proposed caborca and north america blocks new u pb zircon geochronology revises their age ranges the caborca 1 78 1 69 ga and north america 1 71 1 66 ga blocks at quitovac and precludes a simple age differentiation between them in addition grenvillian age granitoids ca 1 1 ga spatially associated with the caborca block have been identified at quitovac nd isotopes and major and trace element geochemistry support the distinction of these paleoproterozoic blocks granitoids of the north america block are characterized by depleted end values 3 4 3 9 and younger nd model ages 1800 1740 ma and have lower k 2 o y rb ba th ree and fe mg values than coeval rocks of the caborca block the caborca block granitoids are likewise characterized by slightly less depleted end 0 6 2 6 and older nd model ages 2070 1880 ma despite the subtle differences granitoids from both the caborca and north america blocks exhibit island arc like affinities we propose that the proterozoic basement rocks from the quitovac region are an extension of the proterozoic crustal provinces in the southwestern united states specifically rocks of the caborca block exhibit an affinity to rocks of either the yavapai province or the mojave yavapai transition zone whereas rocks of the north america block have signatures similar to those of the mazatzal province or possibly the yavapai province of arizona the new isotopic ages and geochemical data do not support the existence of the late jurassic mojave sonora megashear at quitovac as originally proposed however the quitovac region accounts only for a small fraction of the proterozoic basement in sonora so these findings do not eliminate the possibility of a megashear elsewhere in northern sonora our new data create the possibility of alternative hypotheses for the distribution of paleoproterozoic crustal provinces in southwestern north america that affect reconstructions of the original southwestern margin of laurentia and reduce uncertainties in the configuration timing and existence of the proterozoic supercontinent rodinia

laboratory simulation of exchange through fram strait laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy driven and wind driven rotating fluids in lock exchange experiments water with one density flows into a second basin after a sliding gate is removed water of a second density flows back into the first basin the size and location of the currents for various values of density difference rotation rate and assorted sidewall geometries was recorded volume flux of the fluid was also measured and compared with a theory for lock exchange flow of a rotating fluid in a separate group of experiments with a passive upper layer easterly winds like those in the arctic ocean drive the upper level water into the arctic ocean and therefore oppose the buoyant exchange westerly winds would drive the water out of the arctic ocean this indicates that the exchange between the arctic ocean and the greenland norwegian sea is likely to be driven by buoyancy rather than by driven by wind crude estimates of the volumetric and fresh water exchange rate from the lock exchange formulas are compared with observed ocean fluxes and approximate agreement is found

thin ice impacts on surface salt flux and ice strength inferences from advanced very high resolution radiometer temperatures and albedos derived from satellite imagery are combined with a thermodynamic ice model to estimate thin ice thickness distributions over the beaufort and the northern greenland seas the study shows that thin ice thinner than 1 m occupied over half the area in the seasonal ice zones in november and december of 1990 but dropped significantly in april 1991 the beaufort shelf showed the largest seasonal change the aggregate properties of surface salt flux and compressive ice strength both strongly dependent on the thin end of the ice thickness distribution were estimated with data from advanced very high resolution radiometer to reveal the spatial and temporal variations over a large scale the salt flux from growing thin ice was 1 2 orders of magnitude larger on the beaufort and greenland shelves than in the deep basins on the shelves flux from thin ice accounted for over 90 of the total surface salt budget these satellite derived estimates provided detailed spatial information on salt flux which can be of great use in studies of surface patterns of salinity forcing and shelf basin interaction also revealed by the satellite data was the wide range in values of compressive strength which affects how freely the ice cover can deform strengths were low in early winter and in seasonal ice zones and nearly doubled in spring both thin ice fraction and compressive ice strength estimated from satellite imagery were in good agreement with those simulated by a coupled ice ocean model monitoring the seasonal and interannual behavior of sea ice properties with noaa satellites is both an opportunity and a challenge the opportunity arises from twice daily overpasses which provide excellent coverage of the polar ocean the challenge lies with the surface being sampled sea ice appears radiometrically similar in the imagery of advanced very high resolution radiometer avhrr making it difficult to monitor for example the mass of the sea ice cover compromises in which useful ice properties match what satellites can distinguish must be made one useful property is the surface area and thickness of thin ice defined here as ice with thickness 1 m in winter thin ice has a warmer surface and appears darker than the surrounding thicker ice these thermal contrasts form the basis for the approach followed in this paper an example of a clear sky avhrr image over the southern canada basin shows the leads as linear dark red and orange features that are 10o 20oc warmer than the thick floes plate 1 since thin ice in leads responds rapidly to changes in wind and temperature and is generally covered with no or little snow the thickness of thin ice not just its presence can be inferred directly from the radiative surface temperature detected by the satellite fortuitously several aggregate properties are sensitive to thin ice and can be largely or completely determined by thin strength of the ice cover figure 1 shows an annual cycle of salt flux from both thin and thick ice while the thick ice continues melting negative salt flux in september thin ice grows vigorously as the air cools the growing thin ice dominates the total salt budget until january when thicker ice reaches the peak of its growth cycle in the annual mean brine rejection from thin ice is about 8 times larger than that from thicker ice profoundly affecting the density structure of underlying water thin ice also controls the compressive strength p and thus the mechanical behavior of sea ice high percentages of thin ice reduce the stresses transmitted among ice floes and make pressure ridges more likely to form the compressive ice strength can be related explicitly to the thin end of the ice thickness distribution e g rothrock 1975

the stratigraphic expression of a large negative carbon isotope excursion from the ediacaran johnnie formation death valley the rainstorm member of the ediacaran johnnie formation of the southern basin and range us records a large negative excursion in carbon isotope ratios of carbonate strata 13 c vpdb 6 the character of the excursion raises fundamental questions about whether this isotopic pattern is accurately capturing the time series behavior of marine dissolved inorganic carbon dic or is a product of diagenesis to explore this issue we examined the expression of this isotopic excursion within the johnnie oolite a 2 m thick marker bed which records the highest rate of change in 13 c sedimentologically the oolite unit is thought to be time transgressive if the isotopic excursion reflects time series behavior of the carbon cycle its expression in the oolite across the basin should systematically align the sections according to their slight diachroneity detailed carbon and oxygen isotopic stratigraphy of the oolite at seven different locations indicates the magnitude of the excursion at the base of the oolite is spatially variable such that after a palinspastic reconstruction the sections align along a systematic north to south gradient in the isotopic data the oolite preserved in the old dad mountains the most southerly section measured is an outlier to this trend and the most difficult to ordinate accurately in the reconstruction showing the largest isotopic range between the onset of carbonate deposition and top of the oolite several hypotheses are congruent with these data but the sum of observations is best explained by a scenario wherein the oolite is time transgressive and deposited in a north to south manner throughout the onset of the isotopic excursion if correct this implies that the stratigraphic 13 c pattern reflects time series behavior of marine dic

the shuram and subsequent ediacaran carbon isotope excursions from southwest laurentia and implications for environmental stability during the metazoan radiation current understanding of secular changes in the carbon isotopic composition of mid to late ediacaran carbonates suggests a relatively long steady recovery of the global ocean from the shuram negative excursion followed by a smaller negative excursion at the precambrian cambrian boundary new radiometric stratigraphic and carbon isotope data from thick exposures of the upper johnnie formation in the panamint range of eastern california combined with data from carbonate rich facies of the stirling quartzite in the funeral mountains confirm an ediacaran age for these strata and provide a more complete record of isotopic variations during this time interval than previously determined from sw laurentia and other key sections around the globe a siltstone in the lower part of the johnnie formation yielded a detrital zircon grain with an age of 640 33 0 09 ma lowering the maximum radiometric age constraint on the johnnie formation by 400 m y consistent with an ediacaran age based on chemo and biostratigraphic data in contrast to previous c isotope compilations from this region which were generally based on relatively thin portions of the cordilleran miogeocline near its depositional hinge the more basinward exposures exhibit a recovery from values near 12 to 0 within the upper part of the johnnie formation details in the shape of the chemostratigraphic profile through the upper johnnie formation closely match those in profiles through the wonoka formation in south australia which lies above the basal ediacaran global stratotype section and point and the shuram buah interval in oman confirming temporal correlation and suggesting genesis through changes in the isotopic composition of the global ocean the shuram excursion in sw laurentia is followed by at least three smaller ediacaran to earliest cambrian isotopic excursions recorded within from oldest to youngest the uppermost johnnie formation the middle stirling quartzite and the lower wood canyon formation these data indicate that the negative excursion associated with the base of the cambrian is not a unique post shuram event and that post shuram pre cambrian animal evolution occurred in an environment of repeated large magnitude fluctuations in the carbon isotopic composition of the global ocean

palaeoecology of the archaeocyathan reefs from the lower cambrian harkless formation southern nevada western united states and carbon isotopic evidence for their demise abstract archaeocyath sponges the first group of reef building organisms have a globally distributed albeit restricted stratigraphic occurrence archaeocyathan reefs are well known from the western united states particularly in units like the montenegro poleta and harkless formations of the white inyo ranges and esmeralda county nevada while it is recognized that these organisms disappear at some time above the last occurrence of their reefs in the harkless formation the precise timing of their disappearance is unknown here we show that the last archaeocyathan reefs in the upper harkless formation near gold point nevada are overlain by 20 60 m of interbedded siltstone and carbonate that preserve a negative carbon isotopic excursion consistent with the aece event archaeocyath extinction carbon excursion point counts of reef thin sections and analysis of acid insoluble residues of reef and reef adjacent samples illustrate that these last reefs also harbor previously undescribed diversity and abundance of animals living in and among the reefs including an array of small shelly fossil taxa we confirm previous work that the reefs of the early cambrian were ecologically diverse and functioned in many ways like reefs of the later paleozoic even just prior to their global demise in the late early cambrian and that the extinction of archaeocyaths is recorded in strata of the harkless formation

comparison of geochemical and distinctive mineralogical features associated with the kinzers and burgess shale formations and their associated units article i nfo article history accepted 12 august 2007 this paper compares two north american occurrences of fossils that exhibit burgess shale type preservation bst the middle cambrian burgess shale formation of southeastern british columbia and the lower to middle cambrian kinzers fm of southeastern pennsylvania redox sensitive trace elements mo ni co v sc indicate that palaeo oxygenation conditions varied with respect to faunal assemblage and location ogygopsis bearing fauna in both the burgess shale and kinzers formations are associated with a dysoxic metal signature burgess shale type preservation in the kinzers fm was deposited in a fluctuating oxycline whereas the bst fossil beds of fossil ridge and mt stephen burgess shale fm yield a geochemical signature that consistently indicates oxic conditions thus sustained low oxygen conditions do not correlate with higher quality of fossil preservation at these sites ogygopsis is associated with dysoxic conditions in both localities and may therefore be a faunal indicator of such environments in middle cambrian strata carbonate units associated with both the burgess shale and kinzers formations contain large generally concordant near surface cavities that contain herringbone calcite cements that show no significant enrichment in the potential calcite inhibitor ions such as reduced fe sr s pb and zn the abundance of micro inclusions of dolomite suggests that mg may have been the calcite inhibitor ion responsible for herringbone cement in these cambrian occurrences another distinctive mineralogical feature is the presence of doubly terminated euhedral zoned authigenic quartz as a conspicuous part of a carbonate host rock replacement assemblage the association of both herringbone calcite and zoned quartz crystals in units associated with the burgess shale and kinzers formations suggest a common feature in the depositional and diagenetic environment of these two regions in which burgess shale type fossil preservation occur syn depositional brine flow is the likely cause of these distinct mineralogical features brine seeps in the depositional environment may have provided nutrients for stable high abundance faunal communities in a slope setting thereby accounting for the abundance of fossil material furthermore high salinity brine in the pore waters of carcass bearing cambrian muds may have further limited shallow burrowing scavenging and decomposition rates accounting for the exceptional preservation in these two localities

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

precambrian cambrian transition problem in western north america part i tommotian fauna in the southwestern great basin and its implications for the base of the cambrian system an assemblage of shelly fossils has been discovered in the lower member of the deep spring formation in esmeralda county nevada preliminary taxonomic analysis suggests that the fossils consist of a variety of hyoliths coleolids and other elements the fossils lie about 1 000 m down section from the first trilobites and appear to be slightly younger than or possibly coeval with wyattia the fossils show affinities to tommotian taxa from siberia and other continents thus partial stratigraphic equivalents to the tommotian stage whose base marks the precambrian cambrian boundary can be defined for the southern great basin

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

haline circulation induced by formation and melting of sea ice a three dimensional coupled ice ocean model is developed and basic numerical experiments for examining effects of freshwater budget associated with formation and melting of sea ice on the thermohaline circulation are performed motion of sea ice affects the freshwater budget in two ways first it horizontally transports freshwater in the form of sea ice second it alters the conditions of sea ice which influence heat budget at the sea surface and thus the rate of formation and melting to separate such effects of ice motion two experiments are designed in one case ice motion is removed and in the other case it is included there appear significant differences in the thermohaline circulation between the two cases these differences are due to the difference in horizontal distribution of salinity which is maintained by the cycle of formation transport and melting of sea ice and the corresponding freshwater budget experiments for investigating dependence of the thermohaline circulation on the distribution of winds and consequent ice motion are also carried out it is demonstrated that wind induced ice motion has major influences on how saline water produced by sea ice formation at high latitudes extends

subsidence analysis of the cordilleran miogeocline implications for timing of late proterozoic rifting and amount of extension tectonic subsidence of the cordilleran miogeocline in the western united states during late proterozoic through early paleozoic time is best explained by rifting and subsequent thermal contraction of the lithosphere the tectonic component of subsidence can be calculated by removing the isostatic effects of sediment and water loading the resultant subsidence curves indicate that rifting began about 600 m y ago and that extension of the lithosphere during rifting was in excess of a factor of 1 3

thermal subsidence and eustasy in the lower palaeozoic miogeocline of western north america we have recently developed a procedure for calculating tectonic subsidence in fully lithified basin sequences and applied it to cambrian and ordovician strata of tbe cordilleran miogeocline exposed in the southern canadian rocky mountains1 the cumulative tectonic subsidence curves were found to be mainly thermal in form as predicted by the passive margin model for the miogeocline2 in this paper we extend the analysis to cambrian and ordovician strata in a much larger segment of the miogeocline extending from the yukon territory to central nevada the results indicate that thermal contraction was the dominant subsidence mechanism for more than 2 000 km along the eastern or inner part of the miogeocline in addition we find a slight but persistent deviation in the subsidence curves from a purely thermal form that is consistent with a global sea level change proposed by vail and others3 for cambrian to middle ordovician time

chemostratigraphy of predominantly siliciclastic neoproterozoic successions a case study of the pocatello formation and lower brigham group idaho usa abstract isotopic chemostratigraphy has proven successful in the correlation of carbonate rich neoproterozoic successions in successions dominated by siliciclastic rocks chemostratigraphy can be problematic but if thin carbonates punctuate siliciclastic strata useful isotopic data may be obtained the upper pocatello formation and lower brigham group of southeastern idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions the 5000 m thick succession consists predominantly of siliciclastic lithologies with only three intervals that contain thin intercalated carbonates its depositional age is only broadly constrained by existing biostratigraphic sequence stratigraphic and geochronometric data the lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the pocatello formation the delta 13c values of these carbonates are distintly negative 5 to 3 similar to carbonates that overlie neoproterozoic glaciogenic rocks worldwide stratigraphically higher carbonates record a major positive delta 13c excursion to values as high as 8 8 within the carbonate member of the caddy canyon quartzite the magnitude of this excusion is consistent with post sturtian secular variation recorded elsewhere in the north american cordillera australia svalbard brazil and nambia and exceeds the magnitude of any post varanger delta 13c excursion documented to date in most samples sr isotopic abundances have been altered by diagenesis and greenschist facies metamorphism but a least altered value of approximately 0 7076 supports a post sturtian and pre marinoan varanger age for upper pocatello and lower brigham rocks that lie above the pocatello diamictite thus even though available chemostratigraphic data are limited they corroborate correlations of pocatello formation diamictites and overlying units with sturtian glaciogenic rocks and immediately post sturtian successions in western north america and elsewhere

simultaneous parameter optimization of an arctic sea ice ocean model by a genetic algorithm abstractimprovement and optimization of numerical sea ice models are of great relevance for understanding the role of sea ice in the climate system they are also a prerequisite for meaningful pred

some results of the experimental study of seismic anisotropy of sedimentary rocks using different types of waves summary the presence of anisotropy has been established and its parameters have been estimated by the combined use of compressional and shear sh and sv waves anisotropy has been studied in argillaceous and carbonate rocks to a depth of 2 km by combined surface and borehole observations along longitudinal and non longitudinal vertical lines with linear shot lines anisotropy in carbonate rocks has been found to be a function of their heterogeneity ordered fracturing and the amount of clay the highest anisotropy has been found in argillaceous rocks irrespective of their depth and degree of heterogeneity

modeling the formation and fate of the near surface temperature maximum in the canadian basin of the arctic ocean 1 a numerical model is used to investigate the time and space extent of the near surface temperature maximum nstm of the canadian basin of the arctic ocean over the years 2000 2009 the nstm is formed from local summertime absorption of solar radiation which in some circumstances descends through the fall and early winter to form a warm subsurface layer just below the winter mixed layer we find that winter survival of this layer is confined largely to the beaufort gyre of the canadian basin where ekman convergence and downwelling push the summer warm layer down below the winter mixing depth in recent years summer stratification has increased downwelling has accelerated and the nstm has warmed as the sea ice cover in the beaufort gyre has thinned the result is a strengthening nstm which contained enough heat by the end of winter 2007 2008 to melt about 20 cm of sea ice northwest of alaska the model also simulates a second deeper temperature maximum layer that forms from advection of saltier summer pacific water however this layer is difficult to adequately resolve and maintain given the model s resolution

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

the force balance of sea ice in a numerical model of the arctic ocean the balance of forces in the sea ice model of hibler 1979 is examined the model predicts that internal stress gradients are an important force in much of the arctic ocean except in summer when they are significant only off the northern coasts of greenland and the canadian archipelago a partition of the internal stress gradient between the pressure gradient and the viscous terms reveals that both are significant although they operate on very different timescales the acceleration term is generally negligible while the sum of coriolis plus sea surface tilt is small thus the seasonal average force balance in fall winter and spring is mostly between three terms of roughly equal magnitudes air drag water drag and internal stress gradients this is also true for the monthly average force balance however we find that there is a transition around the weekly timescale and that on a daily basis the force balance at a particular location and time is often between only two terms either between air drag and water drag or between air drag and internal stress gradients the model is in agreement with the observations of thorndike and colony 1982 in that the correlation between geostrophic wind forcing and the model s ice velocity field is high this result is discussed in the context of the force balance we show that the presence of significant internal stress gradients does not preclude high wind ice correlation a breakdown of the internal stress gradient into component parts reveals that the shear viscous force is far from negligible which casts strong doubt on the theoretical validity of the cavitating fluid approximation in which this component is neglected finally the role of ice pressure is examined by varying the parameter p we find a strong sensitivity in terms of the force balance as well as ice thickness and velocity

a comparison of sea ice dynamics models at high resolution abstract an elastic viscous plastic evp model for sea ice dynamics has recently been proposed as a computationally efficient alternative to the viscous plastic vp model widely in use the evp model features a fully explicit discretization that improves the model s efficiency particularly on high resolution grids and adapts easily to parallel computation comparison of two high resolution arctic sea ice simulations identical except for the ice dynamics indicates that the evp model reproduces the vp model behavior on timescales relevant to climate studies the ice concentration and thickness distributions over a 1 yr integration period are remarkably similar in the two models although the evp model responds more rapidly and accurately to strong synoptic weather systems than does the vp model compared to drifting arctic buoys a close look at rates of strain shows that elastic waves in the evp model do not significantly alter the ice behavior in highly compact areas where the waves most benefit nu

geochronological constraints on neoproterozoic rifting and onset of the marinoan glaciation from the kingston peak formation in death valley california usa

evidence for late rifting of the cordilleran margin outlined by stratigraphic division of the lower cambrian gog group rocky mountain main ranges british columbia and alberta the lower cambrian mcnaughton formation of the gog group occupies a stratigraphic position transitional between the rift related rocks of the underlying upper proterozoic miette group and the overlying paleozoic passive margin succession a major regional unconformity overlain by a distinctive orthoquartzite marker has been traced within the mcnaughton formation this unconformity has been shown to truncate normal faults active during the deposition of the lower mcnaughton formation the lower mcnaughton formation consists of mature coarse grained fluvial sediments accumulated in hanging wall half grabens of active normal faults these faults represent the final stage of rifting on the continental margin the unconformity on the footwall blocks of these faults can be traced into the hanging wall and is overlain by the shoreface sediments of the transgressive upper mcnaughton formation formal subdivision of the mcnaughton formation into four lithostratigraphic members is proposed in order to describ

stratigraphic investigations of carbon isotope anomalies and neoproterozoic ice ages in death valley california an unusual richness of biogeochemical events is recorded in neoproterozoic cambrian strata of the death valley region california united states eight negative carbon isotope d13c excursions are found in carbonate units between 1 08 ga and the precambrian cambrian boundary four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial cap carbonates including one or more of the following laminite with rollup structures apparent tube rocks seafloor precipitates and sheet crack cements however only two of these units the sourdough limestone member of the kingston peak formation and the noonday dolomite rest directly upon glacial strata the basal beck spring dolomite and the rainstorm member of the johnnie formation each contain negative excursions and cap carbonate like lithofacies but do not rest on known glacial deposits if the negative d13c excursions are assumed to record depositional processes two equally interesting hypotheses are possible 1 the death valley succession records four glacial pulses in neoproterozoic time but glacial units are not preserved at two stratigraphic levels 2 alternatively other global oceanographic processes can cause negative excursions and cap carbonate like facies in addition to or independent of glaciation

thrombolite fabrics and origins influences of diverse microbial and metazoan processes on cambrian thrombolite variability in the great basin california and nevada thrombolites are a common component of carbonate buildups throughout the phanerozoic that are usually described as microbialites with an internally clotted texture a wide range of thrombolite textures have been observed and attributed to diverse processes leading to difficulty interpreting thrombolites as a group interpreting thrombolitic textures in terms of ancient microbial and metazoan ecosystems requires understanding diverse processes specifically those due to microbial growth and metazoan activity many of these processes are reflected in thrombolites in the cambrian carrara bonanza king highland peak and nopah formations great basin california usa they comprise eight thrombolite classes based on variable arrangements and combinations of depositional and diagenetic components four thrombolite classes hemispherical microdigitate bushy coalescent columnar and massive fenestrated contain distinct mesoscale microbial growth structures that can be distinguished from surrounding detrital sediments and diagenetic features by contrast mottled thrombolites have mesostructures that dominantly reflect post depositional processes including bioturbation mottled thrombolites are not bioturbated stromatolites but rather formed from disruption of an originally clotted growth structure three thrombolite classes arborescent digitate amoeboid and massive contain more cryptic textures all eight of the thrombolite classes in this study formed in similar cambrian depositional environments marine passive margin overall this suite of thrombolites demonstrates that thrombolites are diverse in both internal fabrics and origin and that clotted and patchy microbialite fabrics form from a range of processes the diversity of textures and their origins demonstrate that thrombolites should not be used to interpret a particular ecological evolutionary or environmental shift without first identifying the microbial growth structure and distinguishing it from other depositional post depositional and diagenetic components furthermore thrombolites by definition are fundamentally different from stromatolites and dendrolites in which the laminae and dendroids reflect a primary growth structure because clotted textures in thrombolites do not always reflect a primary microbial growth structure this article is protected by copyright all rights reserved

conodont biostratigraphy of the latest cambrian early ordovician upper mckay group southeastern british columbia the age of the upper mckay group based on conodont biostratigraphy is latest cambrian cordylodus proavus zone to late early ordovician middle floian oepikodus communis zone a collection of 12 940 conodont elements was recovered from 306 samples of upper mckay group strata exposed in the western main ranges of the southern canadian rocky mountains southeastern british columbia the conodont fauna is assigned to 53 species representing 30 genera twelve zones are recognized two of which are cosmopolitan cordylodus proavus zone and iapetognathus zone seven midcontinent realm zones in ascending order include polycostatus falsioneotensis rossodus tenuis rossodus manitouensis low diversity interval scolopodus subrex and acodus kechikaensis zones and tropodus sweeti subzone of the oepikodus communis zone three atlantic realm zones in ascending order include cordylodus angulatus acodus deltatus and paroistodus proteus zones the zonation for the upper mckay group establishes correlation w

the beaufort gyre intensification and stabilization a model observation synthesis a model observation synthesis is conducted to investigate changes in the upper ocean circulation and stratification in the canada basin cb of the arctic ocean results show that the beaufort gyre bg has been generally intensifying during 1992 2015 in conjunction with changes in sea ice and the upper ocean including increasing sea surface height ssh sea ice and ocean speed ekman transport convergence and downwelling and freshwater content decreasing ice thickness and upper ocean salinity shoaling summer halocline and mixed layer and deepening winter halocline and mixed layer increasing ekman transport convergence draws more water from surrounding areas into the cb thus lowering ssh in those areas and raising ssh in the cb the rate of change in the cb began to decrease in 2008 and the bg circulation appears to be stabilizing if not relaxing slightly this is reflected in the general plateauing of ssh the intensity of the sea ice and ocean circulation and various measures of the cb thermohaline stratification the bg intensification and subsequent stabilization appear to have been strongly controlled by atmospheric changes in the cb characterized by generally increasing anticyclonic wind circulation and sea level pressure slp before 2008 and falling wind strength and slp to below average levels in some years after 2008 changes in slp are highly correlated with changes in ocean surface stress curl and downwelling since 2008 the magnitude of the stress curl and downwelling in much of the cb has declined contributing to bg stabilization the general leveling off of sea ice thickness also contributes to the stabilization by limiting melt water input to the cb that increases freshwater content temperatures in the near surface temperature maximum layer trended upward slightly over 1992 2015 which is closely correlated with decreasing sea ice thickness upper ocean heat content increased over the study period mainly due to strong temperature increases in the summer pacific water layer

new isotopic data and a preliminary age for volcanics near the base of the windermere supergroup northeastern washington u s a new sm nd and rb sr data are presented from the volcanic member of the huckleberry formation a unit near the base of the upper proterozoic windermere supergroup in northeastern washington the isotopic data are compatible with a rift setting during deposition of windermere strata a best fit line through all the data yields an age of 795 ma with a large error of 115 ma 2 however a preliminary sm nd mineral whole rock age of 762 44 ma 2 obtained from a least altered basalt is within error of other age information from the canadian cordillera that broadly constrains the initiation of windermere deposition to 750 30 ma a synthesis of isotopic data of the results of tectonic subsidence analyses and of geologic relations within the windermere supergroup and the immediately overlying uppermost proterozoic lower cambrian strata leads to the conclusion that the windermere supergroup does not represent the rift deposits that led directly to the initiation of the early paleozoic miogeocline in

error analysis and assimilation of remotely sensed ice motion within an arctic sea ice model new sea ice motion fields available from remotely sensed data are potentially useful for assessing and improving models of the polar ice pack here we investigate the error characteristics of the observed ice motions relative to drifting buoys and a dynamic thermodynamic ice model a data assimilation approach is then used to assess the potential of the motion data for reducing model biases as well as the potential of the model to serve as an interpolation tool to generate improved ice motion data sets special sensor microwave imager ssm i derived and model simulated ice motions for the years 1988 through 1993 are compared with ice displacement observations from drifting buoys variability and biases are summarized for seasonal and regional means ssm i motions are assimilated into the model using an optimal interpolation method that accounts for the modeled and ssm i motion error variances and the number and distribution of the ssm i motions modeled and ssm i derived motions are found to have comparable mean errors with some notable regional and seasonal differences assimilation substantially reduces the error standard deviation and improves the correlation of the simulated motions relative to the buoy observations but some biases remain in the model framework used here assimilation of the ssm i data substantially alters average ice thickness in some regions of the arctic and affects ice mass outflow through the fram strait but has a small effect on mean ice concentration the assimilation yields an increase in the spatial and temporal variability in ice deformation the observations are particularly suited for improving the simulation of specific synoptic events where substantial differences can occur between simulated and observed ice transport

an early cambrian rift to post rift transition in the cordillera of western north america the upper proterozoic and lower palaeozoic wedge of miogeoclinal strata in the north american cordillera is widely regarded as evidence for a proto pacific passive margin1 3 the rifting history of this margin appears to have been protracted possibly spanning 200 myr in a tectonic setting that is not well understood3 quantitative subsidence analyses of lower palaeozoic strata between british columbia and utah however provide indirect evidence that the transition from rifting to post rift cooling occurred within a relatively short interval of time although probably not synchronously between 600 and 555 myr refs 4 6 this age is significantly younger than that implied in previous studies1 2 we describe here new field data which together with published geological data provide direct evidence of a latest proterozoic or early cambrian age for the rift to post rift transition the data support recent arguments for widespread initiation of passive margins around the edge of the north american craton close to the cambrian precambrian boundary7

lower paleozoic stratigraphic and biostratigraphic correlations in the canadian cordillera implications for the tectonic evolution of the laurentian margin the ancient laurentian margin rifted in the latest neoproterozoic to early cambrian but appears not to have developed as a simple passive margin through a long post rift drift phase stratigraphic and conodont biostratigraphic information from four platform to basin transects across the margin has advanced our knowledge of the early paleozoic evolution of the margin in northeastern british columbia two northern transects span the macdonald platform to kechika trough and ospika embayment and a third transect spans the parautochthonous cassiar terrane in the southern rocky mountains new conodont biostratigraphic data for the ordovician succession of the bow platform is correlated to coeval basinal facies of the white river trough in total from 26 stratigraphic sections over 25 km of strata were measured and 1200 conodont samples were collected that yielded over 100 000 conodont elements key zonal species were used for regional correlation of uppermost cambrian to middle devonian strata along the cordillera the biostratigraphy temporally constrains at least two periods of renewed extension along the margin in the latest cambrian and late early ordovician alkalic volcanics associated with abrupt facies changes across the ancient shelf break intervals of slope debris breccia deposits and distal turbidite flows suggest the margin was characterized by intervals of volcanism basin foundering and platform flooding siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the peace river arch prominent hiatuses punctuate the succession including unconformities of early late ordovician sub llandovery possibly early to middle silurian and early devonian ages

conodont biostratigraphy of the lower to middle devonian deserters formation new road river group northeastern british columbia a new lower to middle devonian basinal unit of the road river group herein formally named the deserters formation contains argillaceous crinoidal limestone and black shale deposited in a linear sub basin of the ospika embayment southern kechika trough the abrupt lateral facies changes in the region facies thickness and occurrence of volcanics indicate a period of extensional tectonism a total of 53 4 5 kg each samples from the deserters formation yielded 7766 conodont elements assigned to 14 genera representing 33 species the formation ranges from the lochkovian eleanorae zone or lower part of the delta zone of the cordilleran region to eifelian australis zone the temporal constraints established by conodont biostratigraphy allow correlation to the grizzly bear formation a regionally restricted lower to middle devonian unit in the selwyn basin to the north

a review of sea ice and ocean modeling relevant to the labrador and newfoundland shelves numerical and analytical models of sea ice and ocean circulation are reviewed from a viewpoint of their application to the labrador and newfoundland shelves seasonal variabilities of sea ice and ocean circulation have not been reported in numerical models the difficulty comes from the importance of both stratification and bottom topography sea ice should be included for seasonal cycle calculations individual mechanisms of variabilities with time scales ranging from a day to a month have been studied extensively using various models so that the coupling of these models will make it possible to test the predictability of sea ice and ocean circulation the data collected during limex 87 and 89 provide verification of the models