extreme detached dust layers near martian volcanoes evidence for dust transport by mesoscale circulations forced by high topography modeling suggests that thermal circulations over mars s highest volcanoes transport water vapor and dust from the surface into the middle atmosphere forming detached layers in these constituents intense vertical mixing also takes place in regional and global dust storms which can generate detached layers that are extreme in both altitude and magnitude here we employ observations by the mars climate sounder mcs on board mars reconnaissance orbiter taking advantage of improved vertical coverage in mcs s aerosol retrievals to discover a new class of extreme detached dust layers eddls observed during minimal dust storm activity and furthermore distinguished by their potentially large and measurable horizontal extent 1000 km these eddls cluster near olympus mons and the tharsis montes from which they likely originate the existence of these eddls suggests that vertical mixing by topographic circulations can be much stronger than previously modeled and more frequent than previously observed

the oceanographic and biological tidal cycle succession in shallow sea fronts in the north sea and the english channel abstract two major discrepancies between existing theories concerning the positioning and development of shallow shelf sea fronts and available data are that these models 1 do not adequately describe the neap spring adjustment of frontal position and 2 these theories fail to explain the relatively small changes in the location of the front despite the rapid seasonal decay of the stabilizing surface heat flux in the model presented here these weaknesses in earlier attempts to describe frontal position are overcome this is achieved partly by incorporating wind stirring in the heat and mixing balance for the frontal area and partly by allowing for a cross frontal circulation this new model defines two types of shallow sea fronts tidal surface and tidal bottom fronts surface fronts are dominated by tidal and bottom fronts by wind stirring the front type determines the circulation pattern on the stratified side of the front the circulation pattern in turn governs the biological activity the model describes the persistent but temporally varying redistribution of water from the bottom and surface layers to the thermocline layer which supplies nutrients from the deep light limited to the shallow regions where phytoplankton activity is stimulated this new interpretation of the spring neap adjustment also implies that during the heating season an increasing part of the potential energy to be overcome during a tidal cycle stems from the mixing of nutrient rich dense bottom water and light surface water imported from offshore the existence of this mechanism is supported by field observations

on the true antiquity of eoarchean chemofossils assessing the claim for earth s oldest biogenic graphite in the saglek block of labrador abstract a recent claim to have found traces of earth s earliest life 3 95 ga utilising isotopically light carbon in graphite bearing metapelites from the saglek block of northern labrador canada is re evaluated applying rigorous geological and geochronological criteria the establishment of these criteria in previous evaluations of early life claims from southern west greenland and northern canada is reviewed in order to provide a backdrop to discussion of the saglek claim in particular we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims which are considerably easier to demonstrate from large relatively less tectonised supracrustal remnants like the isua greenstone belt than they are from smaller isolated enclaves of the kind found on akilia or the highly tectonised and imbricated unit that is found in the saglek block unambiguous field relationships between ca 3 9 ga tonalitic gneiss and the graphite bearing metasediments have not been demonstrated in the literature that the saglek claim relies upon and earlier u pb hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a mesoarchean to neoarchean time of deposition we conclude that irrespective of the validity of the carbon isotopic evidence field relationships and geochronological evidence fail to demonstrate an age of 3 95 ga for the potential traces of life

characterization of vertical mixing at a tidal front on georges bank studies of mixing were done at the northern flank of georges bank in the summer and autumn of 1988 two time series of the evolution and intensity of microstructure were examined over a tidal period in the context of tidal forcing and the evolution of the density and velocity field at the site from the ctd adcp and microstructure observations epsonde on georges bank several interesting features of the mixing processes were found high dissipation and diffusivity regions appear near the bottom of the bank turbulence near the bottom is highest in intensity and reaches farthest from the bottom at peak tidal flow and diminishes in intensity and vertical extent as the flow decreases the thickness of the bottom turbulent layer has its maximum value when the flow is strongest and the stratification is weakest characterization of the dissipation rate and turbulent diffusivities in respect to buoyancy frequency n current shear s richardson number ri and n2 was done dissipation and t showed little dependence on shear or n2 but decreased at larger ri t was found to be higher in regions of higher n2 and increased as n2 increased kt k and k were all highest near the bottom in excess of 10 2m2s 1 and decreased towards the surface there was little suggestion of a dependence of mixing efficiency on s2 ri or n2 but some indication that decreases with decreasing n2

zooplankton distributions and transport across the northeastern tidal front of georges bank tidal fronts are important sites of contact and mixing between zooplankton populations on and off georges bank in the northwest atlantic this study a component of the us globec program examined crossfrontal distributions of zooplankton especially the copepod calanus finmarchicus across the northeastern tidal front of georges bank relative to the tidal cycle frontal structure flow field and day night cycle a unique feature was a precisely executed sampling strategy designed to isolate spatial tidal and day night effects vertically stratified zooplankton sampling was done along tidally coordinated crossfrontal transects and during time series studies at specific locations zooplankton taxa differed in their spatial and vertical distributions relative to the front and the hydrographic structure of the water column there were significant associations between zooplankton assemblages and spatial location depth and water mass zooplankton distributions generally tracked water masses but usually showed crossbank abundance gradients consistent differences between day and night distributions were not apparent for most taxa meroplankton larvae of benthic animals were derived primarily from onbank populations the zooplankton community was advected with the tidal flow crossfrontal distributions characteristic of onbank and offbank flow regimes clearly differentiated tidal phases and interacted with the flow field and topography salinity a proxy for water mass and depth interacted in controlling vertical distributions as tidal forcing sloshed water masses up and down the bank slope some taxa followed isohalines through depth changes while other taxa maintained themselves within a relatively limited depth or salinity regime surface convergence and downwelling tidal upwelling of bottom water and mid depth tidal pumping influenced crossfrontal exchange

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

martian north polar cap summer water cycle abstract a key outstanding question in martian science is are the polar caps gaining or losing mass and what are the implications for past current and future climate to address this question we use observations from the compact reconnaissance imaging spectrometer for mars crism of the north polar cap during late summer for multiple martian years to monitor the summertime water cycle in order to place quantitative limits on the amount of water ice deposited and sublimed in late summer we establish here for the first time the summer cycle of water ice absorption band signatures on the north polar cap we show that in a key region in the interior of the north polar cap the absorption band depths grow until l s 120 when they begin to shrink until they are obscured at the end of summer by the north polar hood this behavior is transferable over the entire north polar cap where in late summer regions flip from being net sublimating into net condensation mode this transition or mode flip happens earlier for regions closer to the pole and later for regions close to the periphery of the cap the observations and calculations presented herein estimate that on average a water ice layer 70 microns thick is deposited during the l s 135 164 period this is far larger than the results of deposition on the south pole during summer where an average layer 0 6 6 microns deep has been estimated by brown et al 2014 earth planet sci lett 406 102 109

residual circulations due to bottom roughness variability under tidal flows abstract tidal flows over irregular bathymetry are known to produce residual circulation flows due to nonlinear interaction with gradients of depth using the depth averaged vorticity equations the generation of residual vorticity and residual flows due to variation of the frictional coefficient are examined the authors find that the contribution due to bottom roughness variations can be as large as that arising from gradients of depth and velocity specific cases are considered on the northern california shelf georges bank and the u s south atlantic bight the generation of residual vorticity is a strong function of the length scales at which roughness or depth vary length scales of bottom roughness variation are commonly within the range of greatest effect e g sand patchiness cobbly outcrops etc the site specific cases show that the bottom roughness variability can generate as much residual circulation as that expected from depth variability the implication for numerical modeling studies

the continental and oceanic crust and lithosphere in the archaean isostatic thermal and tectonic models simple isostatic models of the development of selected archaean basins and greenstone belts can be used to constrain models of the archaean lithosphere geophysical geochemical and thermal data imply that in southern africa the late archaean crust was probably comparable in thickness to the modern continental crust and the whole lithosphere was perhaps over 80 100 km thick two archaean belts studied may have developed by stretching of the order of 1 3 1 6 1 isostatic and thermal data and petrological assumptions allow the construction of simple models of the archaean oceanic lithosphere oceans were probably deep mid ocean ridges were probably subaqueous and fed by komatiitic liquids calc alkaline volcanism in the archaean may have been produced as a consequence of the subduction of hydrated komatiitic oceanic crust into a hot low viscosity asthensophere

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

incompatible element enrichment in archean basalts a consequence of contamination by older sialic crust rather than mantle heterogeneity high abundances of incompatible elements in mgo rich archean basalts from western australia which were thought to have been derived from an enriched mantle source are probably the result of contamination of primitive magmas by small amounts of older sialic crust because these lavas represent some of the more extreme cases of enrichment in incompatible elements observed in archean basalts it may be possible to explain much of the trace element variation previously ascribed to a heterogenous archean mantle by variable crustal contamination of mantle derived magmas recent calculations of thermal erosion rates predict that high temperature high mgo magmas abundant in the archean are most susceptible to contamination therefore a cautious reappraisal of isotopic and trace element data from archean greenstone belts is required before the real extent of archean mantle heterogeneity can be established

mars aerosol studies with the mgs tes emission phase function observations optical depths particle sizes and ice cloud types versus latitude and solar longitude 1 emission phase function epf observations taken in 1999 2001 by mars global surveyor thermal emission spectrometer mgs tes support the broadest study of martian aerosol properties to date tes solar band and infrared ir spectral epf sequences are analyzed to obtain first time seasonal latitudinal distributions of visible optical depths particle sizes and single scattering phase functions this combined angular and wavelength coverage enables identification of two distinct ice cloud types over 45 s 45 n type 1 ice clouds exhibit small particle sizes reff 1 2 m and a distinctive backscattering increase they are most prevalent in the southern hemisphere during aphelion but also appear more widely distributed in season and latitude as topographic and high altitude 20 km ice hazes type 2 ice clouds exhibit larger particle sizes reff 3 4 m a distinct side scattering minimum at 90 100 phase angles characteristic of a change in particle shape relative to the type 1 and appear most prominently in the northern subtropical aphelion cloud belt the majority of retrieved dust visible to ir optical depth ratios are indicative of reff 1 5 0 1 m consistent with pathfinder and viking mariner 9 reanalyses however increased ratios 2 7 versus 1 7 appear frequently in the northern hemisphere over ls 50 200 indicating substantially smaller dust particles sizes reff 1 0 0 2 m at this time in addition larger reff 1 8 2 5 m dust particles were observed locally in the southern hemisphere during the peak of the 2001 global dust storm detailed spectral modeling of the tes visible band pass indicates agreement of epf derived dust single scattering albedos 0 92 0 94 with the spectrally resolved results from pathfinder observations

the distribution composition and particle properties of mars mesospheric aerosols an analysis of crism visible near ir limb spectra with context from near coincident mcs and marci observations the compact reconnaissance imaging spectral mapper crism onboard the mars reconnaissance orbiter mro obtains pole to pole observations i e full mro orbits of vertical profiles for visible near ir spectra 0 4 4 0 m which are ideally suited to identifying the composition and particle sizes of mars ice and dust aerosols over 50 100 km altitudes in the mars mesosphere within the coverage limitations of the crism limb data set a distinct compositional dichotomy is found in mars mesospheric ice aerosols co2 ice clouds appear during the aphelion period of mars orbit solar longitudes ls 0 160 at low latitudes 20s 10n over specific longitude regions meridiani valles marineris and at typical altitudes of 55 75 km apart from faint water ice hazes below 55 km mesospheric h2o ice clouds are primarily restricted to the perihelion orbital range ls 160 350 at northern and southern mid to low latitudes with less apparent longitudinal dependences mars mesospheric co2 clouds are presented in crism spectra with a surprisingly large range of particle sizes cross section weighted radii reff 0 3 to 2 2 m the smaller particle sizes reff 1 m appear concentrated near the spatial latitude and altitude boundaries of their global occurrences crism spectra of mesospheric co2 clouds also show evidence of iridescence indicating very narrow particle size distributions effective variance veff 0 03 and so very abrupt co2 cloud nucleation furthermore these clouds are sometimes accompanied by altitude coincident peaks in 1 27 m o2 dayglow which indicates very dry cold regions of formation mesospheric water ice clouds generally exhibit small particle sizes reff 0 1 0 3 m although larger particle sizes reff 0 4 0 7 m appear infrequently on average water ice cloud particle sizes decrease with altitude over 50 80 km in the perihelion mesosphere water ice mass appears similar in clouds over a large range of observed cloud particle sizes with particle number densities increasing to 10 cm 3 for reff 0 2 m near coincident mars climate sounder mcs temperature and aerosol profile measurements for a subset of crism mesospheric aerosol measurements indicate near saturation h2o and co2 conditions for ice clouds and distinct mesospheric temperature increases associated with mesospheric dust loading dayside 3 pm mesospheric co2 clouds with larger particle sizes reff 0 5 m scatter surface infrared emission in mcs limb infrared radiances as well as solar irradiance in the mcs solar band channel scattering of surface infrared emission is most strikingly presented in nighttime 3 am mcs observations at 55 60 km altitudes indicating extensive mesospheric nighttime co2 clouds with considerably larger particle sizes reff 7 m mesospheric co2 ice clouds present cirrus like waveforms over extensive latitude and longitude regions 10 10 as revealed in coincident mars color imager marci nadir imaging solar tides gravity waves and the large orbital variation of the extended thermal structure of the mars atmosphere influence all of these behaviors mesospheric dust aerosols appear infrequently over the non global planet encircling dust storm era of the crism limb data set 2009 2016 and exhibit smaller particle sizes reff 0 2 0 7 m relative to dust in the lower atmosphere one isolated case of an aphelion ls 96 mesospheric dust layer with large dust particle sizes reff 2 m over syria planum may reflect high altitude non local transport of dust over elevated regions

a numerical study of stratified tidal rectification over finite amplitude banks part ii georges bank abstract tidal rectification over a two dimensional finite amplitude symmetrical bank is studied using the blumberg and mellor primitive equation coastal ocean circulation model ecom si in the homogeneous case the nonlinear interaction of tidal currents with the variable bottom topography generates an along isobath residual circulation over the bank which tends to increase as either the slope or height of the bank is increased in the stratified case internal waves at tidal and higher frequencies are generated over the sloping sides of the bank tidal mixing occurs in the bottom boundary layer leading to horizontal tidal mixing fronts the resulting stratified tidal rectification associated with the tidal mixing front the generation of internal tides and the modification of internal friction due to stratification leads to a subsurface intensification of the along isobath residual current at the front and at the top of the bottom mixed layer over the slope and a cross bank double cell circulation

occurrence and geochronology of the eoarchean 3 9 ga iqaluk gneiss in the saglek block northern labrador canada evidence for the oldest supracrustal rocks in the world abstract understanding earth s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on earth the saglek hebron block in the north atlantic craton is one of the oldest terrains in the world to better understand the eoarchean history of the saglek hebron block we undertook a comprehensive geological and geochronological investigation of this terrain the saglek hebron locality contains orthogneisses and supracrustal rocks which are classified into two groups of the eoarchean and mesoarchean suites based on field associations defined by intrusion of the mesoarchean mafic saglek dykes the eoarchean suites contain the nulliak supracrustal rocks and uivak gneisses in this paper we report cathodoluminescence cl observations and u pb ages of zircons from the eoarchean orthogneisses to constrain the evolution of the saglek hebron block we made detailed sketch maps 1 20 of critical outcrops to establish relative chronological relationships between lithologies at least seven generations of mafic and felsic units were recognized in an outcrop of the st john s harbour south area the first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses which are intruded into the supracrustal units we study cl observation geochemistry and u pb dating of zircons from the oldest and youngest tonalitic gneisses the cl observation of internal structures of the zircons showed they typically comprise three domains of core mantle and rim the cores have clear oscillatory zoning whereas the mantles lack oscillatory zoning and exhibit dark cl emission and structureless cl images the obvious correlation between the ages and chemical compositions of u contents and th u ratios indicates that the cores with low u contents and high th u ratios preserve the protolith age the cores with clear oscillatory zoning low u contents and high th u ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 ma the oldest and average ages of the six oldest spots on the concordia line are 3953 54 and 3920 49 ma respectively on the other hand the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 63 ma the geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other confirming an eoarchean age of ca 3 9 ga for the oldest suite of the uivak gneisses we name this oldest suite the iqaluk gneiss as the protolith of the iqaluk gneiss was clearly intruded into the nulliak supracrustal rocks the nulliak supracrustals must have a minimum age of ca 3 9 ga indicating that they are the oldest supracrustal rocks on earth but further studies should be necessary to obtain more precise age of the iqaluk gneiss and nulliak supracrustal rocks

ion probe u th pb zircon dating of polymetamorphic orthogneisses from northern labrador canada rounded zircons included within igneous zircons from the uivak i gneisses northern labrador have ages of up to 3863 12 ma 2 showing the former existence of rocks that predate the uivak gneisses by more than 100 ma the igneous zircons themselves have experienced varying amounts of early pb loss but age estimates based on maximum 207pb 206pb from three separate gneiss samples agree to within error at 3732 6 ma 2 which is taken as the age of emplacement of the igneous precursor to the dominant component of the uivak gneisses a new generation of zircons was formed during migmatization at ca 3620 ma the volumetrically less important lister gneiss was emplaced at 3235 8 ma 2 in the late archaean different parts of the gneiss complex were subjected to different grades of metamorphic overprinting the zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events with simultaneous recrystallization of old zircons and growth of new u poor zi

3850 ma bif and mafic inclusions in the early archaean itsaq gneiss complex around akilia southern west greenland the difficulties of precise dating of zircon free protoliths in migmatites abstract the southern part of the early archaean itsaq gneiss complex southern west greenland on akilia and adjacent islands consists of polyphase dioritic tonalitic granitic injection components with inclusions of metavolcanic amphibolites chemical sediments such as banded iron formation bif gabbros and ultramafic rocks incipient in situ partial melting and strong deformation during several archaean tectonothermal events strongly modified these injection components so that they are now mostly banded schlieric migmatites with neosome produced during several events the margins of many inclusions have been loci for either segregation of neosome and or higher strain obliterating the relationship between the inclusions and the older components of the migmatites an added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon which would provide precise direct dates instead minimum ages of inclusions are obtained by dating invasive components of the granitic sensu lato migmatites previous age determinations of the inclusions were centred on a 200 m long body of amphibolites bif and ultramafic rocks on the southwestern corner of akilia geochronology of this locality has been controversial with our proposed age of 3850 ma making the inclusion the world s oldest dated sedimentary and mafic rocks we continue the debate on the age of the inclusion on southwest of akilia and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of akilia this 1 addresses the contentious ages of these rocks 2 examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies and 3 addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths on qilanngaarsuit island a gneiss sheet cutting a metaperidotite inclusion has an age of 3850 ma on an islet 2 km west of akilia tonalite and quartz diorite which intrude and envelop two mafic amphibolite and hornblendite inclusions have dates of 3850 ma also with our proposed ages of 3850 ma these inclusions are older than similar rocks in the 3700 3800 ma isua supracrustal belt at other investigated localities 3850 ma zircon dates have not been obtained from the orthogneisses enveloping or intruding inclusions at two localities invasive leucogranite neosome and a discordant diorite sheet cutting inclusions have ages of 3600 3660 ma a period of known tectonothermal events including crustal melting and intrusion of gabbros and diorites at three localities banded schlieric orthogneisses adjacent to supracrustal gabbroic and ultramafic rocks contain 3800 3700 ma igneous components in these cases the inclusions could still be 3850 ma and be equivalent in age to similar rocks on akilia but this has been masked by the local tectonothermal history and greater amount of neosome development an alternative explanation is that some younger

relative dispersion of surface drifters in a barrier reef region 1 the spreading of two clusters of satellite tracked surface drifters was quantified by relative dispersion and relative diffusivity the time derivative of relative dispersion in two regions of the southern great barrier reef gbr a low density reef matrix of complex topography and a shelf lagoon over flat bathymetry although based on a small data set averaged values of these parameters over a 12 day period with concurrent measurements for the two regions indicated that relative dispersion and diffusivity of drifter pairs were 182 and 52 times greater respectively along the reef matrix than found in the lagoon relative diffusivities in the southern gbr were anisotropic and varied largely in space and time within the spatial scales of 1 100 km and temporal scales of hours to 16 20 days they were considerably enhanced by high frequency tidal processes submesoscale processes 20 km were important in the southern gbr particularly in areas with complex topography where secondary circulations around the reefs and regions of steep bathymetry caused abrupt increase in dispersion although the dispersion was much higher along the reef matrix the presence of ring circulations around the islands and wake eddies in the lee of islands can act as trapping mechanisms to keep particles within the reef matrix retarding the loss to open water areas this provides an efficient physical mechanism for both enhancing larval connectivity between reefs and at the same time trapping larvae when in the vicinity of reefs

reconstruction of a 3700 ma transgressive marine environment from isua greenland sedimentology stratigraphy and geochemical signatures abstract the rare preservation of a stratigraphy in the northwest of the eoarchean isua supracrustal belt greenland demonstrates the world s earliest recognised marine transgression stratigraphically lowest is the 3710 ma solvang volcanic formation of picrites basalts and basaltic andesites with arc like geochemical signatures close to its erosional top this formation s volcanic rocks show increase in k2o with depletion of na2o depletion in sr with no alteration in eu abundance and 18ovsmow increase to 16 interpreted as subaerial weathering at 3700 ma the sedimentary snowpatch formation was deposited unconformably on the solvang volcanic formation locally the unconformity is marked by the laterally discontinuous conglomerate member dominated by rounded quartz clasts but also with clasts of basalt and andesite up to 10 cm across layered chert and ultramafic rocks the conglomerate member carries sparse detrital zircons the youngest of which are 3710 ma the conglomerates are succeeded by the sandstone and dolostone member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross bedding with scoured layers tempestite breccia beds and very rare stromatolite horizons the sandstone and dolostone member is succeeded by the 3695 ma bif member the snowpatch formation quartz dolomite sandstones and interlayered stromatolites have seawater like ree y trace element patterns variably modified by a negligible to significant influx of a terrigenous component trace element signatures such as ti v ratios and normalising ree y data to a 3710 ma weathered volcanic rock rather than post archean average shale suggest this component was derived from a juvenile arc source resembling the basement solvang volcanic formation the detailed field and geochemical evidence such as positive correlation between fe mg and pr yb in the snowpatch formation dolostones and bif banded iron formation indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation during a marine transgression over a weathered 3710 ma volcanic arc basement followed by bif deposition in deeper water thus by 3700 ma shallow water exposed settings had already been established which were the site of stromatolite development

recurrent frontal slicks of a coastal ocean upwelling shadow 1 marine ecosystems are greatly influenced by the structure and dynamics of fronts in coastal upwelling systems frontogenesis occurs frequently by upwelling and transport of cold water and warming in sheltered upwelling shadow retention sites monterey bay in the california current upwelling system hosts a dynamic upwelling shadow environment using a decade of satellite synthetic aperture radar sar observations with ancillary remote sensing and in situ data we describe recurrent surface slicks that develop along the seaward periphery of the monterey bay upwelling shadow and we examine their relationships with fronts slick median dimensions 17 5 km long and 0 8 km wide describe their elongated structure although the typical pattern is a single slick multiple slicks may concurrently develop in association with different types of fronts repeated volume surveys through a front underlying a slick revealed lateral mixing and interleaving of regional water types velocity fields from coastal hf radar show that slicks may coincide with a variety of surface circulation patterns that they may extend contiguously across regions having very different surface velocity and that they may be separated from the shear front of upwelling filaments by 5 10 km slicks occur in all seasons and may coincide with both upwelling and downwelling wind forcing surfactant accumulation in small scale convergence zones is indicated as the primary mechanism of slick formation not ocean current shear or small scale air sea coupling the results of this study emphasize the role of upwelling system fronts in creating small scale structure and dynamics that influence plankton ecology

near surface recirculation over georges bank satellite tracked drifters with drogues centered near surface 5 m and below the seasonal thermocline 50 m were launched during late winter and spring of 1988 and 1989 in the northern great south channel in the western gulf of maine to investigate the regional circulation as part of the south channel ocean productivity experiment scopex many of the near surface drifters became entrained in the clockwise gyre over georges bank and eight drifters made a total of 16 complete circuits around the bank during the stratified season the average recirculation period of these eight drifters was 48 days and the average drifter speed around the bank was 12 cm s 1 there is no clear evidence from the drifter data that the strength of the clockwise gyre over the bank increased with time during the stratified season on average these drifters i followed a relatively narrow path around the bank except over the eastern end of the bank where three preferred paths were observed ii moved fastest over the northern and southern flanks of the bank iii did not enter a core area of 3500 km2 centered at 41 17 n 68 00 w approximately 30 km southwest of the topographic center of the bank and iv stopped circling the bank by the end of november due in part to strong wind events that appeared to drive drifters off the bank curiously none of the near surface drifters moved from the southern flank of georges bank onto the new england shelf as might be expected from continuity of flow along the outer shelf instead the drifters that circled the bank tended to move off the bank along its southern flank none of the drifters with drogues centered at 50 m appeared to recirculate around georges bank

the m2 tide and its residual on the outer banks of the gulf of maine abstract tidal computations are reported for the gulf of maine with emphasis on its seaward banks georges bank browns bank nantucket shoals and the nearshore region off cape sable the model is 3d and nonlinear with quadratic vertical viscosity stratification is ignored the objective is to establish a theoretical baseline for tidal and residual currents on detailed topography against which seasonal changes may be examined two triangular meshes are used the base case corresponds to earlier studies with roughly 6 km resolution on the banks a more refined and extended mesh provides resolution at 2 3 km in critical areas and has boundaries farther removed from the banks both meshes provide very good fits with observed m2 elevations comparison of computed and observed m2 current profiles at 76 sites reveals general agreement that improves with resolution for the refined mesh and the 48 sites with records exceeding 60 days average deviations for m2 ellipse parameters are 6 1 4 7 cm s 1 for the

chaotic stirring in a tidal system an analysis of a two dimensional tidal model of the wadden sea reveals lagrangian chaos in the trajectories of water parcels the associated chaotic stirring results from the transverse intersection of the stable and unstable curves of hyperbolic fixed points in the lagrangian residual displacement field the tidal poincare map this tidal dispersion mechanism produces rapid water exchange along the channel axis and could be representative of many shallow tidal seas

petrology and geochemistry of the 2 9ga itilliarsuk banded iron formation and associated supracrustal rocks west greenland source characteristics and depositional environment abstract here we present new field petrographic and geochemical data from the 2 9 ga itilliarsuk banded iron formation bif and associated lithologies within the itilliarsuk supracrustal belt south eastern nussuuaq west greenland the supracrustals represent a volcanic sedimentary sequence which rests unconformably on a basement of tonalite trondhjemite granodiorite ttg lithologies felsic metagreywackes meta semipelites and thinly bedded ferruginous shales were identified intercalated with the itilliarsuk bif other associated rocks include metapelites acidic metavolcanics and metagabbroic sills the supracrustals have experienced amphibolite facies metamorphism which has resulted in complete resetting of the u pb system with an apparent age of 1895 48 ma this tectono metamorphic event corresponds well with the paleoproterozoic rinkian orogeny known from this region the itilliarsuk oxide facies bif has been divided into two segments on the basis of major and trace elements chemistry a shaley bif with a strong clastic component and a more chemically pure bif the shaley bif contains high terrigenous influx as reflected by elevated al 2 o 3 up to 12 wt tio 2 high field strength elements hfse and transition metals the chemically pure bif is characterised by alternating high iron 68 wt and high silica 64 wt bands with low total rare earths and yttrium rey al 2 o 3 tio 2 and hfse contents suggesting a low detrital component the least altered bands of the bif record diagnostic archaean seawater features with post archaean average shale paas normalised positive la and eu anomalies enrichment in heavy rare earth elements hree relative to light rare earth elements lree pr yb paas t dm model ages of 3 23 2 85 ga and initial nd nd i values in the range of 0 43 to 4 05 average of 1 35 in contrast the chemically pure fe rich bands yield t dm model ages of 3 61 3 22 ga and nd i values from 2 87 to 0 09 average of 1 29 the associated supracrustal rocks in the study area have significantly higher positive nd i values the 143 nd 144 nd in the itilliarsuk bif therefore contrasts world bifs by exhibiting radiogenic positive nd i values in shallow seawaters where the rey s were controlled by a local depleted continental crust whereas the negative nd i values found in the iron rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle possibly an older subcontinental lithospheric segment the felsic metagreywackes are immature first cycle sio 2 al 2 o 3 4 4 la yb 1 chon metasediments with affinities to ttg suites primarily extrusives whereas the meta semipelites and metapelites contain a larger mafic contribution with higher content of fe 2 o 3 mgo cr ni and hrees this suggests that the bif was deposited in a highly unstable basin presumably in a palaeo continental slope or outer shelf environment with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources the t dm model ages and the use of th sc zr and la th sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting

estimates of dispersion from clustered drifter deployments on the southern flank of georges bank abstract data from 16 clustered drifter deployments are used to examine horizontal dispersion on the southern flank of georges bank the spreading rates of all clusters have an average of 1 6 km d 1 with a standard deviation of 1 8 km d 1 both effective and apparent diffusivities are calculated for each cluster their ranges e g 54 to 757 m 2 s 1 for effective diffusivity are related to differences in cluster size and proximity to the tidal mixing front cross bank convergence is documented for nearly 40 of the clusters this occurs especially for clusters with centroids within 10 km of the tidal mixing front location as deduced from conductivity temperature and depth transects ctd conducted concurrently with the cluster deployments estimates of turbulent dispersion distinct from shear effects are derived by the method of a okubo and c c ebbysmeyer 1976 determination of vorticity divergence and deformation rates from analysis of drogue observations deep sea research 23 349 352 the results reveal that the effects of horizontal shear are important in spreading of larger drifter clusters often the impact of shear is evidenced by the track of a lone drifter that separates from a cluster as it is entrained into the current of the shelf edge front or the tidal mixing front cluster dispersion is time dependent as evidenced by a significant modulation of cluster size at the m 2 tidal frequency this modulation is due to the spatial variation of tidal currents over the southern flank of georges bank and is closely reproduced by immersing drifter clusters into the flow field of a georges bank tidal model

the vertical distribution of martian aerosol particle size using approximately 410 limb viewing observations from the compact reconnaissance imaging spectrometer for mars crism we retrieve the vertical distribution of martian dust and water ice aerosol particle sizes we find that dust particles have an effective radius of 1 0 m over much of the atmospheric column below 40 km throughout the martian year this includes the detached tropical dust layers detected in previous studies little to no variation with height is seen in dust particle size water ice clouds within the aphelion cloud belt exhibit a strong sorting of particle size with height however and the effective radii range from 3 m below 20 km to near 1 0 m at 40 km altitude conversely water ice clouds in the seasonal polar hoods show a near uniform particle size with an effective radius of approximately 1 5 m throughout the atmospheric column

georges bank residual circulation during weak and strong stratification periods prognostic numerical model results the seasonal extremes of the three dimensional residual circulation on georges bank are studied by numerically computing the circulation during weakly march april and strongly july august stratified periods the computations are performed using a prognostic i e including tidal time advection of the density field free surface nonlinear finite element model on realistic georges bank topography the dynamics considered result from the dominant m 2 tide baroclinic pressure gradients wind stress upstream boundary conditions and advanced turbulence closure comparison of these solutions with earlier diagnostic model results illustrates the importance of processes which could not be included in these earlier simulations such as density evolution and advanced turbulence closure in tidal time inclusion of nonlinear baroclinic effects improved treatment of other nonlinearities etc during the strongly stratified july august period the newly modeled processes intensify the residual circulation and result in recirculating transport on georges bank of 0 25 sv which is twice the diagnostic result the level of agreement between modeled and observed residual circulation improves very significantly for this period with the prognostic model predicting around bank currents as high as 46 cm s 1 in the frontal zone on the northeast peak analysis of the tidal time density and current structure in the frontal zone on the northeast peak indicates important tidal frequency dynamics and features which are supported by observational evidence during the weakly stratified march april period the dynamical impact of the newly modeled processes is small hence the prognostic and diagnostic models predict similar global structure with a recirculating transport on georges bank of 0 05 sv although there are local differences which result in the prognostic simulation agreeing more closely with the observed circulation

surf zone diffusivity on a rip channeled beach absolute and relative diffusivity are measured on a rip channeled beach using 30 position tracking drifters released in clusters 4 12 drifters deployed on 7 days with different wave forcing and tidal elevations at sand city monterey bay california diffusivity and dispersion were found to be larger on days with rip current flow patterns and larger waves rip currents cause material to diffuse quickly for t 0 9 two independent methods are used to measure the small scale turbulent diffusion contribution kxy which are found significantly correlated r2 0 95 with each other and calculated surf zone wave breaking induced turbulent eddy viscosity here kxy is small relative to the total dispersion ke kxy 3 30 indicating that the shear flow is the primary mechanism responsible for dispersion in a rip current system

possible juvenile palaeoarchaean ttg magmatism in eastern india and its constraints for the evolution of the singhbhum craton high precision shrimp u pb zircon dating yields a late palaeoarchaean age 3290 8 6 ma for a large unmetamorphosed weakly peraluminous ttg body the keonjhargarh bhaunra pluton in the singhbhum craton of eastern india one inherited subhedral zircon grain gave a concordant age of 3495 9 5 3 ma and nd isotope characteristics show a juvenile trend with end t 0 and t dm 3395 3453 ma the data support a model of typical archaean crustal evolution until late palaeoarchaean times for the singhbhum craton which is in contrast to the more southerly bastar craton where palaeoarchaean non ttg granites have been identified these data demonstrate the diachronous development of continental crustal blocks now forming the basement of the eastern and central peninsular of india

chemical sedimentary protoliths in the 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract analyses of chemical sedimentary precipitates such as banded iron formation bif provide a direct means to explore the nature and composition of the early hydrosphere the recently discovered 3750 myr old nuvvuagittuq supracrustal belt nsb in the northeast superior province quebec canada hosts a suite of iron oxide rich pyroxene and amphibole units that are interpreted to be the metamorphosed equivalents of fe oxide facies bif and a collection of bif like ca fe mg silicate rocks the nsb rocks provide a rare glimpse of trace metal availability in eoarchean ca 3800 ma seawater as they may be contemporaneous with the relatively well studied isua supracrustal belt of southern west greenland their comparison provides an opportunity to enhance our basic understanding of the eoarchean oceans at a global scale work since the initial discovery of the nsb in 2001 has established the basic lithological geochemical and petrographic characteristics of these bif and bif like rocks here we review the current state of knowledge of nsb rocks of probable chemical sedimentary origin including aspects of their geology likely origin and age we conclude by examining the implications of results thus far for our understanding of early seawater compositions and for the emergence of life in the context of early metallo enzyme evolution

geology of the eoarchean 3 95 ga nulliak supracrustal rocks in the saglek block northern labrador canada the oldest geological evidence for plate tectonics the earth is a unique planet which has been highly evolved diversified and complicated through geologic time and underwent many key events including giant impact magma ocean core formation large scale mantle differentiation and late heavy bombardment especially in its dawn but our knowledge of early earth is limited due to the lack of the hadean supracrustal rocks the supracrustal rocks with the eoarchean ages provide key evidence for the earth s early evolution but few supracrustal rocks have been comprehensively investigated therefore we mapped in seven areas of the saglek block northern labrador where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses early studies suggested that some of them have the mesoarchean ages because of the lack of the mesoarchean saglek dyke but we found the saglek dykes in the areas to recognize the eoarchean nulliak supracrustal rocks and uivak gneiss in all the areas recent reassessment of u pb dating and cathodoluminescence observation of zircons from the oldest suites of the uivak gneiss showed that the uivak gneiss has the eoarchean age 3 95 ga and forms the iqaluk uivak gneiss series because our geological survey clearly showed that the iqaluk uivak gneisses were intruded into the nulliak supracrustal belts the nulliak supracrustal rocks are the oldest supracrustal rock in the world the supracrustal belts consist of piles of fault bounded blocks which are composed of the ultramafic rocks mafic rocks and sedimentary rocks in ascending order similar to modern ocean plate stratigraphy ops in addition small scale duplex structures are found over the areas the presence of duplex structure and ops indicates that the 3 95 ga nulliak supracrustal belts originate from an accretionary complex the presence of the accretionary complex ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early earth

thermotectonic evolution of proterozoic and reworked archaean terranes along the nain churchill boundary in the saglek area northern labrador

on the dispersion and diffusion near estuaries and around islands abstract a series of diffusion experiments using surface drifters were performed in the coastal waters of south western taiwan near a small estuary and around an island two stages or regimes of drifter dispersion can be identified for all experiments a coherent regime in the beginning stage when the drifters remained clustered with little dispersion occurred and a rapid disperse regime in the latter stage when the separation of drifters grew to the order of a few kilometres our results show that near estuaries the extent of the dispersive influence is limited to the tidal excursion distance and the dispersion coefficient is about 12 15 m 2 s 1 on the other hand the dispersion coefficient downstream of an island in a strong flow 1 5 m s 1 can reach a much larger value 45 m 2 s 1 which can be attributed to the enhanced mixing and transport processes due to island wakes theoretical scaling parameters for coastal water flows are also presented to predict the formation of island wakes after applying three different methods to exclude the effects of shear flow the turbulent diffusivities for our experiments amount to 0 2 5 m 2 s 1 analysis of the differential kinematic properties indicates that a close relationship exists among the dispersion diffusion and the effect of shear flow

vertical distribution of dust and water ice aerosols from crism limb geometry observations 1 near infrared spectra taken in a limb viewing geometry by the compact reconnaissance imaging spectrometer for mars crism on board the mars reconnaissance orbiter provide a useful tool for probing atmospheric structure specifically the observed radiance as a function of wavelength and height above the limb enables the vertical distribution of both dust and water ice aerosols to be retrieved more than a dozen sets of crism limb observations have been taken so far providing pole to pole cross sections spanning more than a full martian year radiative transfer modeling is used to model the observations taking into account multiple scattering from aerosols and the spherical geometry of the limb observations both dust and water ice vertical profiles often show a significant vertical structure for nearly all seasons and latitudes that is not consistent with the well mixed or conrath v assumptions that have often been used in the past for describing aerosol vertical profiles for retrieval and modeling purposes significant variations are seen in the retrieved vertical profiles of dust and water ice aerosol as a function of season dust typically extends to higher altitudes 40 50 km during the perihelion season than during the aphelion season 20 km and the hellas region consistently shows more dust mixed to higher altitudes than other locations detached water ice clouds are common and water ice aerosols are observed to cap the dust layer in all seasons

moc observations of the 2001 mars planet encircling dust storm abstract from 15 september 1997 through 21 january 2006 only a single planet encircling martian dust storm was observed by mgs moc the onset of the storm occurred on 26 june 2001 l s 184 7 earliest recorded to date it was initiated in the southern mid to low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in malea and in hellas basin l s 176 2 184 4 the initial expansion of the storm though asymmetric was very rapid in all directions 3 32 m s 1 the main direction of propagation however was to the east with the storm becoming planet encircling in the southern hemisphere on l s 192 3 several distinct centers of active dust lifting were associated with the storm with the longest persisting for 86 sols syria claritas these regional storms helped generate and sustain a dust cloud haze which reached an altitude of about 60 km and a peak opacity of dust 5 0 by l s 197 0 the cloud had encircled the entire planet between 59 0 s and 60 0 n obscuring all but the largest volcanoes the decay phase began around l s 200 4 with atmospheric dust concentrations returning to nominal seasonal low levels at l s 304 0 exponential decay time constants ranged from 30 117 sols the storm caused substantial regional albedo changes darkening and brightening as a result of the redistribution removal and deposition of a thin veneer of surface dust at least 0 1 11 1 m thick it also caused changes in meteorological phenomena i e dust storms dust devils clouds recession of the polar caps and possibly surface temperatures that persisted for just a few weeks to more than a single mars year the redistribution of dust by large annual regional storms might help explain the long period 30 years between the largest planet encircling dust storms events

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

diapycnal flow through the georges bank tidal front a dye tracer study a fluorescent dye tracer fluorescein injected into the bottom mixed layer at the seaward edge of the tidal front on georges bank has provided the first quantitative measurement of an on bank diapycnal lagrangian flow through the front from the warming of the dye patch 2 5 10 6 c s and 7 8 10 6 c s on the south flank and northeast peak respectively as it passed through the frontal temperature gradient we infer an on bank flow of 1 9 cm s on the south flank and 3 2 cm s on the northeast peak the heat flux required for this warming is predominantly due to vertical mixing within the tidal front these observations further demonstrate the utility of direct lagrangian measurements and provide quantitative estimates of the cross frontal exchange on georges bank the focus of the u s globec northwest atlantic georges bank phase iii program

the vertical structure of limb hazes in the martian atmosphere abstract profiles of scattered light above the planetary limb from over 100 viking orbiter images are used to constrain the temporal and spatial behavior of earosols suspended in the martian atmosphere the data cover a wide range of seasons locations and viewing geometry providing information about the aerosol optical properties and vertical distribution the vertical resolution of the data is about 1 5 km with an absolute elevation accuracy of 1 to 4 km viking orbiter camera colibration algorithms provided reflectance accuracies to 7 absolute allowing meaningful radiometric analysis the typical atmospheric column contains one or more discrete optically thin ice like haze layers between 30 and 90 km elevation depending on season whose composition is inferred to be water ice below the detached hazes there is a continuous haze extending to the surface the continuous haze is rarely observed above 50 km and is much redder in color than the detached hazes above implying a composition that has a strong dust component the aerosol distribution exhibits a strong seasonal control inferred to be driven by the variable solar flux around the orbit of mars

dust storm and electron density in the equatorial d region ionosphere of mars comparison with earth s ionosphere from rocket measurements in brazil we report the first model result for the dust densities and electron densities in the d region ionosphere of mars for aerosol particles of different sizes during a major dust storm that occurred in martian year my 25 at low latitude these calculations are made at latitude 10 s and solar longitudes ls 200 220 250 and 280 for high medium low and absence of dust storms respectively four corresponding dust layers were found at 50 km 50 km 38 km and 25 km during these events during high dust storm period the optical depth and dust density increased by a factor of 20 from its normal condition the electron densities estimated for the d region ionosphere of mars for submicron sized dust particles are largest as compared to that estimated for larger particles the electron density reduced by 2 orders of magnitude during high dust storm the estimated electron density in the clear atmosphere of mars is compared with measurements of earth s ionosphere at nearly the same geophysical condition

the early archaean nulliak supracrustal assemblage northern labrador the nulliak supracrustal assemblage the remains of ca 3800 ma succession of volcanic and sedimentary rocks was broken up by intrusion of the protoliths of the early archaean uivak orthogneisses and then deformed metamorphosed and variably metasomatised several times under upper amphibolite to granulite facies conditions in the archaean amphibolites of komatiitic basalt and tholeiitic chemical affinity are the most important nulliak assemblage lithologies high al2o3 metagabbroic rocks and anorthosites also occur interlayered with the amphibolites are marbles calc silicate rocks and banded iron formation interpreted as chemical sediments that were probably laid down in a shallow water environment also found are felsic rocks probably derived by reworking of penecontemporaneous felsic volcanic rocks and garnet and sillimanite bearing paragneisses derived from pelites all these lithologies are randomly interlayered on a scale down to 1 m or less the occurrence of 3850 3900 ma cores for zi

influence of diurnal heating on stratification and residual circulation of georges bank 1 the influence of the diurnal heat flux on summer stratification and residual circulation over georges bank was examined using a three dimensional primitive equation numerical circulation model for a given spatially uniform and time varying heat flux the model results show that the surface water is heated much faster on the southern flank than on the northern flank and much faster in the stratified region than in the mixed region heating significantly strengthens the tidal mixing front and intensifies the frontward convergence near the surface as seasonal stratification develops the location of the tidal mixing front gradually shifts on bank on the southern flank while remaining almost unchanged on the northern flank response of the tidal currents to the diurnal variation in the heat flux varies across georges bank it changes periodically with tidal cycles on the southern flank but is locked to the phase of the eastward tidal current on the northern flank this phase lock feature directly contributes to the intensification of the along bank residual current jet on the northern flank diagnostic analysis suggests that this intensification is mainly caused by the heat enhanced cross bank momentum flux modelcomputed variations of near surface temperature and residual currents are in good agreement with satellite derived sea surface temperature data and drifter measurements index terms 4219 oceanography general continental shelf processes 4227 oceanography general diurnal seasonal and annual cycles 4255 oceanography general numerical modeling 4279 oceanography general upwelling and convergences 4528 oceanography physical fronts and jets keywords heat flux tidal mixing front residual current stratification frontward convergence

model simulations of the bay of fundy gyre 1 climatological results 1 the characteristics of a persistent gyre in the mouth of the bay of fundy are studied using model simulations a set of climatological runs are conducted to evaluate the relative importance of the different forcing mechanisms affecting the gyre the main mechanisms are tidal rectification and density driven circulation stronger circulation of the gyre occurs during the later part of the stratified season july august and september october the density driven flow around the gyre is set up by weak tidal mixing in the deep basin in the central bay of fundy and strong tidal mixing on the shallow flanks around grand manan island and western nova scotia spring river discharge has an important influence on near surface circulation but only a small effect when averaged over the entire water column retention of particles in the gyre is controlled by the residual tidal circulation increased frontal retention during stratified periods wind stress and interactions with the adjacent circulation of the gulf of maine residence times longer than 30 days are predicted for particles released in the proximity of the gyre

eoarchean to neoarchean evolution of the nuvvuagittuq supracrustal belt new insights from u pb zircon geochronology the nuvvuagittuq supracrustal belt in northern quebec is a rare 9 km2 fragment of the earth9s early crust the belt contains a metamorphosed volcano sedimentary sequence that is at least eoarchean in age including amphibolites of the ujaraaluk unit that may have protoliths as ancient as 4400 million years old ma o9neil and others 2012 upper amphibolite facies metamorphism and high strain deformation have obscured many primary field relationships and disturbed whole rock isotope systematics leading to debate over the interpretation of previous geochronological data we report new shrimp u pb isotopic analyses of zircons from key meta igneous and newly identified metasedimentary units of the belt the analyzed samples fall into four categories a felsic gneisses including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt b a newly identified metasedimentary unit c fuchsite bearing quartz rich layers d mafic gneisses including a garnet biotite amphibolite from the ujaraaluk unit and a meta gabbro the felsic lithologies place constraints on the minimum age of the supracrustal assemblage oscillatory zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 32 ma and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 11 ma a metasedimentary unit dominated by quartz albite and clinozoisite has been identified within a lower strain domain interfolded with amphibolites of the ujaraaluk unit a sample of this unit yielded zircons with highly variable cl structures and near concordant ages of between 3390 and 3780 ma zircons from a fuchsite bearing quartz layer have oscillatory zonation similar to the orthogneiss grains and define an upper intercept age of 3794 16 ma petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band rather than a meta sedimentary rock belonging to the supracrustal package electron microscopy of polished thin sections was used to locate zircons within 6 samples of the ujaraaluk unit amphibolites and two samples of meta gabbro large 500 m zircons separated from a garnet biotite amphibolite together with grains from a meta gabbro are sector zoned and yield ages of 2700 to 2640 ma recording the timing of pervasive high temperature metamorphism in the region the new zircon u pb data add important new detail to the geological history of the nuvvuagittuq belt the volcano sedimentary sequence and mafic ultramafic intrusive rocks were formed prior to 3800 million years ago subsequent geological evolution included extensive and protracted early archean ttg magmatism and crustal re melting paleo to mesoarchean uplift and sedimentation and neoarchean high grade metamorphism and regional magmatism during consolidation of the superior craton

near surface horizontal convergence and dispersion near the tidal mixing front on northeastern georges bank abstract horizontal dispersion and convergence are evaluated from near surface drifter data collected during the summer and autumn of 1988 and summer of 1989 in the vicinity of the tidal mixing front on northeastern georges bank the effective horizontal diffusivity coefficient k e estimated from the relative motion of the drifters over the duration of the cluster tracking increases as the cluster length scale with a range from approximately 100 to 450 m 2 s 1 for length scales of approximately 5 to 58 km at length scales k e is near zero with many negative estimates indicative of convergence at length scales 20 km k e increases in magnitude and scatter above 30 km length scales k e is typically between 200 400 m 2 s 1 the dispersion as a function of length scale is compared to published estimates using apparent diffusivities which are adjusted for the finite size of the drifter cluster at release georges bank shows lower apparent diffusivities at length scales 20 km the latter are believed to be principally due to the large horizontal gradients in the seasonal flow field although other processes perhaps associated with the large tidal currents may also contribute at the shorter length scales the low dispersion is due to convergent processes examples of convergence at different locations and under different wind and stratification conditions are presented convergence near the tidal mixing front tends to occur under light wind conditions and was observed in both seasons and in both years during two intense southwestward windstorms drifters also converged in the bank s vertically well mixed zone apparently due to a combination of spatially varying ekman depth and topographic effects

tidal front entrainment and retention of fish larvae on the southern flank of georges bank abstract from a grid of stations across the southern flank of georges bank in may 1997 larval haddock and cod were observed to be concentrated across the tidal front in a band 10 20 km wide several cross bank ctd and acoustic doppler current profiler adcp sections indicated a narrow 35 m cross isobath residual flow 0 6 1 2 cm s

the role of frontal currents in larval fish transport on georges bank abstract the hydrography and distributions of cod larvae on georges bank were surveyed during two research cruises in april and may 1993 in order to relate larval drift between cruises to the vernal intensification of the frontal component of the residual circulation we observed the transport of two patches of cod larvae one patch which had maximum larval cod densities of 45 larvae 100 m 3 in april appeared to have been advected south about 75 km between surveys while the other which had maximum larval cod densities of 20 larvae 100 m 3 in april appeared to have been advected north northeast about 25 km maximum larval densities in each patch sampled during the second cruise in may were 15 and 18 larvae 100 m 3 respectively and mean growth in total length for larvae in the two patches was approximately 5 5 mm and 4 5 mm respectively between the two cruises during the april cruise there was a large volume of anomalous cold fresh water of scotian shelf origin which occupied much of the eastern third of georges bank during may relatively cold fresh water appeared in a band from the northeast peak along the southern flank between georges bank water on the top of the bank and upper slope water offshore the distribution of cold fresh water suggests its participation in the general clockwise circulation around the bank the transport of cod larvae comprising the first patch appeared to become organized within and move along the frontal boundary established by the scotian shelf like water mass while larvae in the second patch which we assumed to have moved to the north may have been transported northward in an on bank flow of warmer and saltier upper slope water which may have originated from a gulf stream ring based upon observed transport of the first patch of larvae in relation to the frontal boundary we present a conceptual model of frontal mixing currents on georges bank where current velocities may reach 5 cm s 1 at the depth of the pycnocline we suggest that this frontal component of the residual circulation which is in addition to that resulting from tidal rectification may be important in the transport of fish larvae and that interannual variability in the degree of intrusion of extrinsic water masses may contribute to variable larval cod drift patterns to variable larval cod retention on the bank and ultimately to variable larval fish recruitment to the early juvenile stage

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

the whole rock sm nd age for the 2825 ma ikkattoq gneisses greenland is 800 ma too young insights into archaean ttg petrogenesis abstract the ikkattoq gneisses of the archaean gneiss complex in the nuuk region southern west greenland are the orthogneiss component within the amphibolite facies tre brodre terrane they have mostly granodioritic compositions with a small amount of quartz diorite sm nd isotopic data for a quartz diorite and five granodiorite ikkattoq gneiss samples from within 5 km of the ikkattoq fjord type locality yielded a regression with a slope equivalent to 2005 52 ma mswd 0 72 regardless of the low mswd this cannot be the true age of the ikkattoq gneisses because all ikkattoq gneisses yield u pb zircon dates of c 2825 ma and they are cut by the undeformed 2560 ma qorqut granite complex this anomalously young regression age resulted instead from mixing of different nd components indicating that the ikkattoq gneisses are derived from mixed source materials taking the true age of the ikkattoq gneisses as 2825 ma from u pb zircon dating the range of initial e nd in the ikkattoq gneisses is 7 1 to 1 8 the negative initial e nd values mean that older light rare earth enriched sialic crust contributed to the igneous precursors of the ikkattoq gneisses this nd evidence for contribution of older sialic crust is supported by positive e sr values for the ikkattoq gneisses with e nd values as low as 7 1 this older crustal component has to be eoarchaean the presence of scarce quartz diorites low sio 2 high mgo suggests that ultramafic rocks upper mantle metasomatised by the passage of fluids or silicic melts were another contributing source the ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle the intercalation of tectonostratigraphic terranes during collisional orogeny at c 2720 ma destroyed the architecture of this 2825 ma magmatic system and the ikkattoq gneisses now form a slice tectonically isolated from their source region in terms of trace element parameters the ikkattoq gneisses resemble phanerozoic volcanic arc granites thus an andean style arc setting for the generation of the ikkattoq gneiss precursors is possible other archaean ttg suites of the nuuk region are generally thought to represent predominantly juvenile additions to the crust in the broadest sense they do because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust however in detail within these broadly juvenile suites a contribution from older crust can be detected thus c 3000 ma type nuk gneisses from around nuuk town show a spread in e nd values down to 1 7 in this case the likely older crustal component was 3230 ma quartz diorite that occurs as enclaves in the c 3000 ma suite thus to a lesser or greater degree some meso to neoarchaean ttg suites in the nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust

fishing and the oceanography of a stratified shelf sea fishing vessel position data from the vessel monitoring system vms were used to investigate fishing activity in the celtic sea a seasonally stratifying temperate region on the shelf of northwest europe the spatial pattern of fishing showed that three main areas are targeted 1 the celtic deep an area of deeper water with fine sediments 2 the shelf edge and 3 an area covering several large seabed banks in the central celtic sea data from each of these regions were analysed to examine the contrasting seasonality of fishing activity and to highlight where the spring neap tidal cycle appears to be important to fishing the oceanographic characteristics of the celtic sea were considered alongside the distribution and timing of fishing illustrating likely contrasts in the underlying environmental drivers of the different fished regions in the central celtic sea fishing mainly occurred during the stratified period between april and august based on evidence provided in other papers of this special issue we suggest that the fishing in this area is supported by 1 a broad increase in primary production caused by lee waves generated by seabed banks around spring tides driving large supplies of nutrients into the photic zone and 2 greater concentrations of zooplankton within the region influenced by the seabed banks and elevated primary production in contrast while the shelf edge is a site of elevated surface chlorophyll previous work has suggested that the periodic mixing generated by an internal tide at the shelf edge alters the size structure of the phytoplankton community which fish larvae from the spawning stocks along the shelf edge are able to exploit the fishery for nephrops norvegicus in the celtic deep was the only one to show a significant spring neap cycle possibly linked to nephrops foraging outside their burrows less during spring tides more tentatively the fishery for nephrops correlated most strongly with a localised shift in the tidal current polarisation suggesting that the muddy seabed required by nephrops is controlled by rotational constraints on the extent of the bottom boundary layer

recent observations of the mean circulation on georges bank abstract a clockwise circulation around georges bank was measured by means of moored current meters aircraft tracked surface drifters and satellite tracked drifters drogued at 10 m the strongest flow was in a narrow jetlike current 30 cm s 1 along the northern flank of the bank the flow of shelf water on the southern flank was westward 10 cm s 1 toward the middle atlantic bight some of this water flowed northward through the eastern side of great south channel and recirculated around georges bank the satellite tracked drifters and the moored observations indicate that the circulation around the bank was not completely closed and considerable variability occurs in the trajectory of an individual water particle

role of a thin komatiite rich oceanic crust in the archean plate tectonic process in the archean tectonic regime the time available for oceanic lithosphere to cool on its journey from ridge to convergent plate boundary was far less than at present under these conditions only an inherently dense lithosphere could have acquired the negative buoyancy necessary for it to subduct at this time plate tectonics could have operated only if 1 the oceanic crust was rich in komatiite lava and 2 the thickness of this crust was not much greater than the thickness of modern oceanic crust

circulation hydrographic structure and mixing at tidal fronts the view from georges bank and discussion the steep slope on the northern side of georges bank and its location in the fundy maine tidal system result in a persistent summertime frontal system comprising a tidal mixing front and a stratified tide topography interaction at the bank edge recent field studies have provided a high resolution description of the circulation hydrographic structure and mixing in the region frontal features include an along front residual jet a surface convergence zone regular variations in frontal structure and position over the tidal period and tidal modulation cycle large amplitude internal waves and strong spatial and temporal variations in small scale turbulence the observations suggest that the magnitude of cross front and vertical exchange in frontal regions can be site specific depending on the relative importance of the underlying physical processes

high spatial and temporal resolution sampling of martian gas abundances from crism spectra 1 the compact reconnaissance imaging spectrometer for mars on the mars reconnaissance orbiter spacecraft has been collecting spectra in the visible to near infrared wavelength range for over 5 years approximately 3 martian years observations consist of image cubes with two main selections of spectral sampling 74 and 545 spectral channels and two main selections of spatial sampling approximately 20 and 200 m pixel retrievals of gas abundances specifically co2 h2o and co are performed from spectra collected in all observation modes the retrievals are efficiently performed using a look up table where the strength of gas absorption features are precalculated for an n dimensional discrete grid of known input parameters season location environment viewing geometry etc and the one unknown parameter to be retrieved gas abundance a reverse interpolation in the look up table is used to match the observed strength of the gas absorption to the gas abundance this algorithm is extremely fast compared to traditional radiative transfer computations that seek to recursively fit calculated results to an observed spectral feature and can therefore be applied on a pixel by pixel basis to the tens of thousands of compact reconnaissance imaging spectrometer for mars images to examine cross scene structure as well as to produce climatological averages