contribution of entrainment and vertical plumes to the winter cascading of cold shelf waters in a deep lake observations made from vertical moorings of thermistors and current meters are used to analyze the relative contribution of the entrainment of overlying water and the cold plumes convecting from the surface to the cascade of dense shelf waters down the sides of lake geneva assuming a steady state and ignoring the effects of the earth s rotation a simple model was introduced and 25 individual slugs of gravity currents were analyzed the average drag coefficient which includes the effects of friction on the slope and the interface between the gravity current and the ambient water is found to be equal to c d 4 6 1 5 x 10 3 the volume flux in the gravity current increases with downslope distance a consequence of both the entrainment of water into the gravity current and the supply of cold water from the plumes entrainment was found to be on average 1 9 0 9 times the contribution of vertical plumes suggesting that the latter may be important in the dynamics of the cascade

oxygen isotopes in titanite and apatite and their potential for crustal evolution research abstract oxygen isotope analysis of zircon often combined with geochronology and hf isotope analysis has been pivotal in understanding the evolution of continental crust in this contribution we expand the use of underexplored accessory phases titanite and apatite by demonstrating that their oxygen isotope systems can be robust and by developing geochemical indicators involving o isotopes and trace element concentrations to better constrain magma petrogenesis these minerals have the advantage over zircon of being present in less evolved magmas and being more responsive to igneous processes and crustal metamorphism we present new data on titanite apatite and zircon from carefully selected granitoids through geological time the phanerozoic high ba sr granites caledonian province scotland archean sanukitoids karelia province finland and a neoproterozoic basalt andesite dacite rhyolite suite badr guernsey channel island we demonstrate i that 18o values of the studied accessory minerals are not affected by crystal fractionation ii a strong correlation between 18o in all three accessory minerals showing that apatite and titanite can faithfully record the magmatic 18o iii that these accessory minerals can also record metamorphic and or fluid circulation events during the syn to post magmatic history of granitoids

interannual variability of circulation under spring ice in a boreal lake a small range 1uc of under ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep oligotrophic boreal lake with the water column at 4uc melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep water temperature we attribute that to the nonlinear dependence of water density on temperature albeit further affected by stochastic weather factors in four of nine study years convection led to complete under ice overturn of the lake indicating that this may not be uncommon in similar lakes with steep topography river inflow and more intensive warming of water in the littoral zone also created a horizontal density differential convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep water layers the vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer the contribution of horizontal convection to under ice mixing was interannually variable and in one of the study years it eventually dominated under ice mixing a thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline we identified five different under ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period the dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate

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

gravity and magnetic studies in the channel islands the gravity and total magnetic field over jersey guernsey herm and jethou and the gravity field over alderney and sark have been surveyed in detail at 470 gravity stations and 847 magnetic locations as the basis of bouguer and magnetic anomaly maps which can be integrated with the marine gravity and aeromagnetic maps of the region there is a striking correlation in location and relative amplitude of gravity and magnetic anomalies on the islands the st peter port gabbro of guernsey can be modelled as a laccolith no more than 0 8 km thick and extending several km ne subsurface beyond its present outcrop limit the aeromagnetic and gravity high over se jersey can be modelled as a circular laccolithic lens c 12 km in diameter and c 2 km thick with about 1 km of cover thus the dioritic and gabbroic outcrop in that area does not reflect a great volume of such material at

magma mingling and melt modification between granitic pipes and host diorite guernsey channel islands within the northern igneous complex of guernsey cylindrical pipe intrusions 20 40 cm in diameter developed by buoyant emplacement of granite magma into overlying denser diorite magma the high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing and mesoscopically sharp well defined interfaces developed the granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re equilibration these formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and or melt interdiffusion across the pipe host interface such processes between partially crystallized magmas may in part provide an explanation for textural mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites

tidally induced cross frontal mean circulation analytical study an analytical model is developed to study the tidally induced mean circulation in the frontal zone four distinct forcing mechanisms are identified which result in the generation of the counterclockwise bernoulli cell the clockwise ekman cell the clockwise frontal cell and the stokes drift facing in the direction with the shallow water to the left the decomposition of the cross frontal circulation provides a dynamical framework for interpreting and understanding its complex structure to illustrate the underlying physics three model configurations are considered pertaining to a homogenous ocean and winter and summer fronts for a homogeneous ocean the circulation is dominated by three cells for the winter front the offshore bernoulli cell is strengthened and for the summer front two counterrotating cells are found in the vertical direction associated with the two branches of the front the dependence of the cell structure on the ekman burger and other dimensionless numbers is examined

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

giant ice rings on lakes baikal and hovsgol inventory associated water structure and potential formation mechanism observations of giant ice rings on lake baikal russia have recently sparked scientific and public interest however there is still no clear consensus on their origins here we provide an inventory of the ice rings based on satellite imagery and photography for 1974 2014 we have identified 45 rings on lake baikal compared with 13 previously known and also for the first time four rings for the neighbouring lake hovsgol mongolia the results of our hydrographic surveys beneath the ice rings in lake baikal in 2012 2014 and in lake hovsgol in 2015 show the presence of warm double convex lens like eddies before and during manifestation of ice rings we suggest that these eddies are the driving factor for the formation of ice rings in these lakes we reassess the existing hypotheses of ice ring formation and discuss the potential mechanisms of eddy formation

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

the front on the northern flank of georges bank in spring 2 cross frontal fluxes and mixing buoyancy fluxes are downward and most intense reaching values of 5 10 7 w kg near the bottom at the edge of the bank and decrease both on and off bank horizontal crossbank buoyancy fluxes are partitioned into mean tidal pumping and nontidal eddy components and are computed as a function of cross isobath vertical position by averaging in the along isobath direction the tidal pumping component is dominant over most of the cross bank section with a peak value of 1 10 4 w kg directed off bank near the bank edge a diagnosed tidal vertical velocity field is used with mean buoyancy gradients to compute the along isopycnal skew flux the horizontal component of this skew flux has similar spatial structure and magnitude to that of the observed tidal pumping flux the divergent component of the skew flux at depths above the bottom boundary layer appears to be convergent north of the bank edge and divergent at the bank edge suggesting that tidally driven advective processes drive buoyant bank water downward and off bank at mid depth and force the upwelling of deep dense water near the bottom at the bank edge index terms 4528 oceanography physical fronts and jets 4568 oceanography physical turbulence diffusion and mixing processes 4560 oceanography physical surface waves and tides 1255 4544 oceanography physical internal and inertial waves keywords georges bank tidal mixing front globec internal tide skew eddy flux turbulent mixing

geochemistry and u pb and 40ar 39ar geochronology of the man of war gneiss lizard complex sw england pre hercynian arc type crust with a sudeten iberian connection the man of war gneiss is a variably deformed meta igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the lizard ophiolite complex sw england u pb dating of zircon demonstrates that the parental magmas crystallized in the earliest ordovician 499t s 3 ma but 40 ar 39 ar step heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite grade metamorphic event at c 374 ma probably associated with thrust emplacement of the ophiolite the gneiss exhibits major trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase probably with minor garnet and zircon the elemental abundances and variations are characteristic of granitoids formed in supra subduction zone environments a granitoid sill within the adjacent old lizard head series schists previously inferred to be related to the man of war gneiss is trondhjemitic and exhibits higher sio 2 tio 2 and mgo but lower zr v ba and sr contents and therefore is probably not genetically related to the latter the sill may record local melting during ophiolite emplacement the man of war gneiss probably represents a structural inlier of pre hercynian basement incorporated in the hercynian nappes of sw england during nnw directed thrusting we emphasize the geological and geochronological similarities between variably deformed granitoid units of the lizard complex the sudeten massif of southern poland and the allochthonous complexes of northern portugal and nw spain these granitoid rocks may represent dispersed remnants of tremadoc arcs that formed oceanward from the northern margin of gondwana this is further convincing evidence that at this time eastern avalonia was separated from northern gondwana by an ocean basin that may have been connected with the tornquist sea

deep sea high resolution hydrography and current measurements using an autonomous underwater vehicle the overflow from the strait of sicily autosub 2 an autonomous underwater vehicle auv developed by the southampton oceanography centre was used for high resolution hydrographic surveys in the sicily strait a combination of seasoar type profiling and terrain following missions were undertaken and velocity and hydrographic measurements taken from autosub 2 were compared with concurrent shipboard hydrographic and velocity profiles even though shipboard stations were separated by just 5 to 8 km along the mission path data from the auv showed small scale variability that was missed by the shipboard sampling in this paper we present the example of an intense jet with maximum speed greater than 0 50 m s 1 less than 4 km wide

mesoscale eddies of the arctic ocean subsurface baroclinic eddies in the arctic ocean have a diameter of 10 20 km and are confined in depth between 50 and 300m during the arctic ice dynamics joint experiment aidjex in the central beaufort sea between march 1975 and may 1976 extensive information on eddy characteristics statistics and behavior was collected from four manned drifting ice camps during this 14 month period 146 eddies were encountered of which 19 were found to be repeated crossings making a total of 127 separate eddies observed during this period on the basis of the 1975 1976 aidjex data set it is concluded that eddies are 1 prevalent in the amerasia basin of the arctic ocean and in particular the beaufort sea where they may occupy up to a quarter of the available surface area 2 located in the depth range of 50 300 m although a few deeper eddies are also present 3 the second largest producers 32 of kinetic energy found within the upper 200 m of the beaufort sea and 4 predominantly anticyclonic in rotational tendency these eddies apparently originate north of point barrow alaska as a result of instability in the eastward flowing alaskan coastal current and transfer seasonally varying water properties from the chukchi sea and alaskan shelf into the arctic ocean they appear to translate in response to barotropic forcing over short time scales and over longer time periods move with the mean geostrophic field the season of formation for an eddy may be identified by the anomalous thermal properties still residing in its core

petrology of a magma chamber the plutonic complex of guernsey channel islands uk the northern plutonic complex of guernsey is a calc alkaline intrusion emplaced within a metamorphic complex composed mainly of the icart gneiss and the perelle gneiss the northern plutonic complex is made up of four distinct bodies the st peter port gabbro the bordeaux diorite group the l ancresse granodiorite and the cobo granite which are roughly contemporaneous emplaced at the end of the cadomian orogeny at around 550 560 ma the st peter port gabbro and the bordeaux diorite group are relicts of a magma chamber characterized by layered rocks along the margins and heterogeneous rock associations in the core of the chamber resulting from stirring by convection oxygen isotopic data are consistent with melting of the country rock possibly the perelle gneiss in response to the intrusion of the gabbro diorite magma chamber the resulting acidic melts mixed with crystallizing gabbro diorite magma to yield the cobo granite and the l ancresse granodiorite this mechanism of acid magma genesis can probably be extended to other late cadomian magmatic events in the northern armorican massif suggesting that basic plutonism in this area may be more important than previously considered

model study of the cross frontal water exchange on georges bank a three dimensional lagrangian experiment 1 characteristics of the cross frontal water exchange over georges bank are investigated through a sequence of numerical model experiments featuring realistic three dimensional 3 d bathmetry bi monthly averaged climatological stratification tidal forcing and mean and observed wind forcing the model used in this study is the ecom si version of the blumberg and mellor 1987 primitive equation model with mellor yamada 1982 level 2 5 turbulence closure the model domain includes georges bank and the gulf of maine and is forced at the open ocean boundary by the semidurnal m2 tide fluid particles were tracked in the 3 d time dependent eulerian flow field to examine the lagrangian flow field and passive tracer experiments were conducted to investigate the relative roles of advection and turbulent diffusion on cross frontal exchange two distinct paths for the on bank water movement were detected on georges bank one is over the northwestern flank of the bank where the bottom topography changes sharply in both along and cross bank directions and the other is near the bottom around the bank where the tidal mixing front is located over the northern flank the cross bank component of the lagrangian residual current is generally opposite in direction to that of the eulerian residual current resulting in an on bank cross frontal water transport near the bottom over the southern flank the near bottom water tends to converge toward the tidal mixing and shelf break fronts so that the near bottom flow over the shelf between these two fronts is divergent the response to wind forcing varied with ambient stratification and water depth in winter strong winds can drive a significant off bank water transport tending to wash out the bank in summer winds are generally too weak to alter the general pattern of tidal driven particle motion within the mixed region on the crest of the bank and the surrounding tidal mixing front some wind driven off bank transport occurs near the surface in the stratified region on the outer southern flank but this has little influence on water movement near the bottom passive tracer experiments reveal that the net cross frontal water flux near the bottom is caused primarily by advection and horizontal diffusion tidal induced vertical diffusion tends to make the tracer mix rapidly upward thus reducing the percent of the cross frontal flux due to advection our new model results are consistent with previous model studies of georges bank and agree in general with drifter and dye patches observed in the u s globec nw atlantic georges bank program

radiatively driven convection in ice covered lakes observations scaling and a mixed layer model penetrative convection is discussed where the instability is driven by radiative heating of water below the temperature of maximum density convection of this type occurs in ice covered freshwater lakes in late spring when the snow cover vanishes and solar radiation is absorbed beneath the ice cover the vertical temperature structure bulk mixed layer scaling and mixed layer deepening are examined for a number of temperate and polar lakes a bulk mixed layer scaling for this type of convection is based on energy arguments underlying the classical deardorff convective scaling the depth of the convective layer serves as an appropriate length scale however a modified scale that takes account of the energetics of a distributed radiation source term replaces the surface buoyancy flux velocity scale used by deardorff the scaling compares favorably with large eddy simulations of turbulence kinetic energy tke and with both observations and large eddy simulations of the tke dissipation rate mixed layer deepening is simulated with a model of convection beneath lake ice the model describes the structure of the stably stratified layer just beneath the ice with a stationary solution to the heat transfer equation the structure of the entrainment layer is parameterized with a zero order jump approach the entrainment equation is derived from the mixed layer tke budget and bulk mixed layer scaling entrainment regimes characteristic of convection beneath ice are analyzed it is shown that if the deardorff convective velocity scale is replaced with a scale incorporating the distributed buoyancy flux the entrainment equation describing atmospheric and oceanic convective boundary layers also applies beneath the ice model predictions compare well with data from observations in a number of lakes we propose and compare with observations an extension of the mixed layer model that allows for the inclusion of salinity although the salt concentration is low in most temperate and polar lakes its dynamical effect can be significant close to the temperature of maximum density less

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 mafic silicic layered intrusions of saint jean du doigt france and north guernsey channel islands armorican massif gabbro diorite layering and mafic cumulate pegmatoid association abstract the saint jean du doigt france and north guernsey channel islands intrusive complexes hereafter referred to as sjic and ngic respectively are examples of mafic silicic layered intrusions in the armorican massif both are characterized by the occurrence of 1 a basal peripheral gabbroic unit interlayered with sheets generally dioritic in composition occasionally gabbroic and crossed by leucocratic diapirs and pipes from monzodioritic to q monzonitic in composition 2 peripheral pegmatoids associated with mafic cumulates and 3 coeval granitoids beside these main similarities some contrasted features lead us to propose two distinct models of formation the variscan sjic includes tholeiitic mafic rocks monzogabbro that locally mingle and mix with leucocratic components monzonite or q monzonite the cadomian ngic is calc alkaline the sjic sheet bearing gabbro is homogeneous from a petrologic point of view whereas the ngic exhibits gabbroic macrorhythmic sequences with mineral layering the sr nd isotopic compositions of the sjic gabbros are significantly different from those of the associated dioritic layers this is not the case in the ngic where the magmas could be cogenetic we argue that the sjic gabbro was a liquid that crystallized in situ without significant crystal settling by contrast the rhythmic sequences of the ngic are consistent with crystal accumulation subsequently both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition we interpret the sheets in the sjic as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below at levels of neutral buoyancy injections and convection in the central part of the reservoir possibly resulted in spectacular mixing mingling structures in the ngic the emplacement of the dioritic sheets was rather controlled by pre existing rhythmic cumulative structures in both intrusions late differentiated diapirs were extracted from the dioritic sheets associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase this extraction might have been enhanced by the disruption of the peripheral cumulate stack perhaps following pressure drops

timing of post tectonic cadomian magmatism on guernsey channel islands evidence from 40ar 39ar mineral ages the channel island of guernsey exposes pre and post tectonic calc alkaline plutonic rocks attributed to late precambrian cadomian subduction activity 40 ar 39 ar mineral cooling ages presented here clarify the age and time span of post tectonic intrusions amphibole from the bon repos meladiorite yields an isotope correlation age of 605 3 4 6 ma indicating emplacement during late stages of the tectonothermal event recorded within its host rocks igneous hornblendes from the northern igneous complex record consistent isotope correlation ages st peter port gabbro 570 2 1 1 ma bordeaux diorite complex 569 0 1 1 and 565 3 2 2 ma and a comagmatic enclave within cobo granite 570 2 1 5 ma hornblende and biotite from contact metamorphosed host rocks adjacent to the cobo granite yield isotope correlation ages of 569 6 1 1 and 563 8 3 3 ma respectively these ages together with field data demonstrate that the various members of the northern igneous complex were emplaced contemporaneously at c 570 ma significantly earlier than a c 500 ma rb sr whole rock isochron age previously reported for the cobo granite the 40 ar 39 ar ages indicate that post tectonic magmatism on guernsey was broadly coeval with post metamorphic cooling around the baie de st brieuc on the adjacent french mainland and occurred c 30 ma prior to peak cadomian deformation and metamorphism in the st malo area

numerical study of the diapycnal flow through a tidal front with passive tracers 1 a two dimensional numerical model is used to study the diapycnal flow through a tidal front with passive tracers in a basic numerical experiment a passive tracer is released into the bottom water at the offshore edge of a tidal front and it subsequently moves on bank with a velocity that decreases with time this qualitatively agrees with a recent field experiment using a dye tracer on georges bank additional experiments are performed to investigate the sensitivity of the tracer dispersion to the tidal phase and the location of tracer release within the front as the release point is moved on bank across the front the tracer velocity decreases until it weakly reverses on the on bank edge of the front this trend can be understood by considering the structure of the lagrangian velocity field in the tidal front the degree of vertical mixing of the tracer and the concentration weighted mean patch velocity the tidal phase at the time of tracer release does not significantly affect the tracer dispersion

a fluorescein tracer release experiment in the hydrothermally active crater of vailulu u volcano samoa 1 on 3 april 2001 a 20 kg point source of fluorescein dye was released 30 m above the bottom of the active summit caldera of vailulu u submarine volcano samoa vailulu u crater is 2000 m wide and at water depths of 600 1000 m with the bottom 200 m completely enclosed it thus provides an ideal site to study the hydrodynamics of an active hydrothermal system the magmatically driven hydrothermal system in the crater is currently exporting massive amounts of particulates manganese and helium the dispersal of the dye was tracked for 4 days with a fluorimeter in tow yo mode from the u s coast guard icebreaker polar sea lateral dispersion of the dye ranged from 80 to 500 m d 1 vertical dispersion had two components a diapycnal diffusivity component averaging 21 cm2 s 1 and an advective component averaging 0 025 cm s 1 these measurements constrain the mass export of water from the crater during this period to be 8 1 3 4 6 107 m3 d 1 which leads to a turnover time for water in the crater of 3 2 days coupled with temperature data from ctd profiles and mn analyses of water samples the power output from the crater is 610 100 350 mw and the manganese export flux is 240 kg d 1 the mn heat ratio of 4 7 ng j 1 is significantly lower than ratios characteristic of hot smokers and diffuse hydrothermal flows on mid ocean ridges and points to phase separation processes in this relatively shallow hydrothermal system

the impact of scotian shelf water cross over on the plankton dynamics on georges bank a 3 d experiment for the 1999 spring bloom the 1999 march seawifs images detected an intense phytoplankton bloom on the southern flank of georges bank gb the bloom covered a large portion of the southern flank between the 60 and 200 m isobaths and later extended to and connected with an even larger patch near the northeast peak nep and browns bank a three dimensional 3 d model experiment was conducted to examine the cause of the bloom and the impact of scotian shelf water on the spring phytoplankton bloom dynamics the finite volume coastal ocean model fvcom provided the hydrodynamic fields for lagrangian particle trajectory tracer and biological model experiments process oriented modeling experiments showed that the formation and maintenance of the phytoplankton bloom on the southeastern flank of gb is related to the weak stratification caused by the transport of the colder but fresher scotian shelf water across the northeast channel nec with sufficient nutrients from the slope the bloom could result from in situ growth of phytoplankton near the slope where the stabilizing salinity front is located the model results suggest that the timing and location of the phytoplankton bloom on the southern flank of gb is sensitive to the spatial distribution of temperature and salinity on the bank the flow fields across the nec and the location of the salinity front near the shelf break r 2006 elsevier ltd all rights reserved

mixing in a coastal environment 1 a view from dye dispersion 1 dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of new england the experiments were done in 1996 and 1997 as part of the coastal mixing and optics experiment during the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 sundermeyer and ledwell 2001 diapycnal diffusivities inferred from dye dispersion range from 10 6 to 10 5 m2 s at buoyancy frequencies from 9 to 28 cycles hour diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by oakey and greenan 2004 agree closely in most cases estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by duda and rehmann 2002 and rehmann and duda 2000 are fairly consistent with the dye diffusivities the dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by mackinnon and gregg 2003 from profiling microstructure measurements obtained at the same site in august 1996 all of the measurements support the general conclusion that the diffusivity averaged over several days is seldom greater than 10 5 m2 s in the stratified waters at the site and usually not much greater than 10 6 m2 s severe storms such as a hurricane that passed over the cmo site in 1996 can dramatically increase the mixing at the site however

western arctic shelfbreak eddies formation and transport abstract the mean structure and time dependent behavior of the shelfbreak jet along the southern beaufort sea and its ability to transport properties into the basin interior via eddies are explored using high resolution mooring data and an idealized numerical model the analysis focuses on springtime when weakly stratified winter transformed pacific water is being advected out of the chukchi sea when winds are weak the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of o 25 cm s 1 and an eastward transport of 0 42 sv 1 sv 106 m3 s 1 despite the absence of winds the current is highly time dependent with relative vorticity and twisting vorticity often important components of the ertel potential vorticity an idealized primitive equation model forced by dense weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site the model boundary current is also highly v

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

mixing between granitic and dioritic crystal mushes guernsey channel islands uk abstract the contact zone between the cobo granite and bordeaux diorite complex of guernsey channel islands uk displays numerous features which result from the interaction of these two penecontemporaneously emplaced intermediate to felsic magmas initial interaction resulted in the formation of chilled mafic enclaves in granite magma as thermal equilibrium was approached some physical mixing took place to produce a heterogeneous marginal facies continued interaction resulted in incorporation of previously mixed magma into intruding magma the early mixed material is locally preserved as enclaves but more commonly underwent disaggregation promoted by its incompletely crystallised nature the mineral components becoming distributed as xenocrysts and often as microenclaves or glomeroxenocrysts into surrounding magma further modification within the contact zone was brought about by the infiltration of melt through the interconnected pore space of the magma mushes on the scale of centimetres to hundreds of metres these processes produced geochemical profiles which do not exhibit perfect mixing trends the petrographic and geochemical features described not only demonstrate the efficacy of mixing between partially crystallised magma mushes of broadly similar composition but also provide criteria by which such interaction may be recognised elsewhere many features in particular mineral scale disequilibria and small scale modal heterogeneity bear striking similarities to those which occur widely in granite plutons where obvious evidence for magma mixing is absent as such it is possible that many granite bodies preserve a subtle record of hitherto overlooked mixing events

eddy maintenance and attrition in a vertically sheared current under arctic ice abstract steady drift of an ice cover produces a vertically sheared current in the upper ocean of the arctic under the ice cover mesoscale shallow brine and freshening sources generate submerged anticyclones and cyclones respectively a submerged eddy extending deep into the water column experiences differential advections by the vertically sheared current interaction between subsurface eddies and the sheared current is examined using a three dimensional numerical model in a coordinate system moving with the ice the initial salinity field is in geostrophic balance with the sheared current and a pulse of brine or freshening forcing produces an anticyclone or a cyclone in a coordinate system moving with the ice eddies are in a vertically sheared backward ambient current to an observer looking into the direction of the backward ambient current eddies move with the current and deflect to the right left for counterclockwise clockwise rotating eddies in both hemispheres the lower half of the eddy

internal tides in monterey submarine canyon abstract the m2 internal tide in monterey submarine canyon is simulated using a modified version of the princeton ocean model most of the internal tide energy entering the canyon is generated to the south on sur slope and at the head of carmel canyon the internal tide is topographically steered around the large canyon meanders depth integrated baroclinic energy fluxes are up canyon and largest near the canyon axis up to 1 5 kw m 1 at the mouth of the upper canyon and increasing to over 4 kw m 1 around monterey and san gregorio meanders the up canyon energy flux is bottom intensified suggesting that topographic focusing occurs net along canyon energy flux decreases almost monotonically from 9 mw at the canyon mouth to 1 mw at gooseneck meander implying that high levels of internal tide dissipation occur the depth integrated energy flux across the 200 m isobath is order 10 w m 1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head where internal ti

a cyclonic gyre in an ice covered lake observations of a cyclonic gyre in an ice covered midsize 5 km2 temperate lake are presented horizontal and vertical measurements of temperature and electrical conductivity measurements were collected using a conductivity temperature depth logger mounted on an autonomous underwater vehicle and additional instrumentation these measurements revealed a cylindrical density anomaly with a radius of 110 m extending from the surface to 14 m depth the observed radius is smaller than the internal rossby radius of deformation 200 m which suggests a cyclogeostrophic balance between centripetal coriolis and pressure forces the maximum azimuthal velocity calculated assuming this balance was 2 1 cm s21 at 6 8 m depth the rossby number associated with this velocity was 1 7 this is consistent with the cyclogeostrophic assumption i e rossby number 1 and nearly twice that of similar under ice eddies in the arctic ocean the estimated ekman spindown timescale is 1 5 15 d but despite this the gyre appeared to be relatively unchanged over 6 d of field observations this persistence implies the gyre was forced over the course of the field study however the source of the forcing is unknown horizontal temperature transects at and below the bottom of the gyre revealed coherent temperature fluctuations suggestive of vertical transport associated with the gyre

the multi year development of long lived convective chimneys in the greenland sea 1 the first convective chimney in the greenland sea to have its temperature and salinity structure fully mapped was discovered in march 2001 near 75 n 0 w wadhams et al 2002 later cruises have shown that this remarkable feature has survived for a further 26 months being remapped in summer 2001 winter 2002 summer 2002 and most recently in april may 2003 making it the longest lived chimney yet seen in the world ocean the chimney has an anticyclonically rotating core and experiences an annual cycle in which it is uniform in properties from the surface to 2500 m in winter but is capped by lower density water in summer the latest cruise also discovered a second chimney 70 km nw of the first during a thorough survey of 15 000 km2 of the gyre centre which left the existence of further chimneys unlikely we conclude that the 75 0 chimney is not unique but that greenland sea chimneys are rare and are probably rarer than in 1997 when several such features were discovered by a float survey this has implications for deep water renewal and for the role of greenland sea convection in the north atlantic circulation

spin down of baroclinic eddies under sea ice a linear model is used to examine the spin down of a baroclinic eddy under the sea ice for anti cyclonic eddies the ice stress besides directly spinning down the azimuthal flow within the mixed layer generates an ekman divergence that raises the pycnocline near the eddy axis for eddies of the size of the baroclinic radius of deformation based on the unperturbed mixed layer depth h and the density jump across the pycnocline the doming reaches a quasi stationary state on the frictional time scale t defined as h where is the resistence coefficient at the ice water interface which generally is of the order of days upward entrainment of the fluid across the pycnocline causes a flow convergence below the mixed layer that continues to spin down the deeper flow and flatten the dome the erosion of the dome however occurs over a much longer time scale of n 1t where and n are dimensionless parameters characterizing the entrainment rate and the deep stratification using realistic parameter values for the polar eddies this time scale is of the order of a year or longer the pycnocline dome observed over the antarctic warm cells is thus likely to survive into the following freezing season and provide a preconditioning for the deep convection in the weddell sea

convectively driven transport in temperate lakes penetrative convection in the surface layer of a midsize temperate lake 5 km2 was investigated in both summer and winter using a conductivity temperature depth ctd logger mounted on an autonomous underwater vehicle auv flown repeatedly along horizontal transects at selected depths in summer the epilimnion cooled differentially during a calm evening 240 and 297 w m22 on the east and west sides of the lake respectively these cooling rates agree well with the average net heat flux of 270 w m22 estimated from meteorological data density currents were driven by this differential cooling in winter ctd profiles during a sunny day showed four distinct thermal layers beneath the ice 50 cm thick consistent with radiative penetrative convection a stratified diffusive layer just beneath the ice top 1 6 m a well mixed convective layer that deepens at 1 14 m d21 and warms at 0 015u cd 21 during the observation period an entrainment layer 1 5 m thick and a weakly stratified quiescent layer to bottom auv transects flown at constant depths in each layer revealed a 150 m wide region displaying evidence of penetrative convection surrounded by regions with negligible heat changes these high resolution horizontal ctd measurements provided insight into previously unresolved physical dynamics of the well mixed layer of a temperate lake in quasi shear free conditions that would have been difficult to quantify during summer months and impossible under winter ice cover without the use of an auv platform in temperate lakes during periods of little or no wind exposure penetrative convection driven by a destabilizing buoyancy flux will be the dominant source of vertical mixing in the upper layers of the water column above the temperature of maximum density penetrative convection

axisymmetric circulation driven by marginal heating in ice covered lakes below the temperature of maximum density tmd in freshwater lakes heating at the lateral margins produces gravity currents along the bottom slope akin to katabatic winds in the atmosphere and currents on continental shelves we describe axisymmetric basin scale circulation driven by heat flux at the shorelines in polar lake kilpisjarvi a dense underflow originating near the shore converges toward the lake center where it produces warm upwelling and return flow across the bulk of lake water column the return flow being subject to coriolis force creates a lake wide anticyclonic gyre with velocities of 2 4 cm s 1 while warm underflows are common on ice covered lakes the key finding is the basin scale anticyclonic gyre with warm upwelling in the core this circulation mechanism provides a key to understanding transport processes in semi enclosed basins subject to negative buoyancy flux due to heating or cooling at temperatures above tmd at their lateral boundaries

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california