surface changes in the eastern labrador sea around the onset of the little ice age despite the relative climate stability of the present interglacial it has been punctuated by several centennial time scale climatic oscillations the latest of which are often colloquially referred to as the medieval climatic anomaly mca and the little ice age lia the most favored explanation for the cause of these anomalies is that they were triggered by variability in solar irradiance and or volcanic activity and amplified by ocean atmosphere sea ice feedbacks as such changes in the strength of the atlantic meridional overturning circulation amoc are widely believed to have been involved in the amplification of such climatic oscillations the labrador sea is a key area of deep water formation the waters produced here contribute approximately one third of the volume transport of the deep limb of the amoc and drive changes in the north atlantic surface hydrography and subpolar gyre circulation in this study we present multi proxy reconstructions from a high resolution marine sediment core located south of greenland that suggest an increase in the influence of polar waters reaching the labrador sea close to mca lia transition changes in freshwater forcing may have reduced the formation of labrador sea water and contributed towards the onset of the lia cooling

subaqueous explosive eruption and welding of pyroclastic deposits silicic tuffs infilling an ancient submarine caldera at mineral king in california show microscopic fabrics indicative of welding of glass shards and pumice at temperatures 500 degrees c the occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water which would cause chilling intracaldera progressive aggradation of pumice and ash from a thick fast moving pyroclastic flow occurred during a short lived explosive eruption of approximately 26 cubic kilometers of magma in water 150 meters deep the thickness high velocity and abundant fine material of the erupted gas solids mixture prevented substantial incorporation of ambient water into the flow stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general both above the vent and along any flow path may be the main process giving rise to buoyant convective subaqueous eruption columns and attendant fallout deposits

improving the utility of ocean circulation models through adjustment of the momentum balance we propose a new method to improve the utility of three dimensional ocean circulation models the method uses climatological temperature and salinity data to adjust the momentum balance of the model while leaving the tracer equations fully prognostic and unconstrained the adjustment is accomplished by replacing density in the hydrostatic equation by a linear combination of model computed and climatological density the procedure is equivalent to adding a forcing term to the horizontal momentum equation through a modification of the model s horizontal pressure gradient term the forcing term modifies the model computed velocity field which in turn affects the model computed temperature and salinity fields through the advection term there is no adjustment of the tracer equations carried by the model assuming the linear combination coefficient to be invariant in time and space we suggest a statistical approach to estimating its optimal value we apply this semiprognostic method to the northwest atlantic a primitive equation circulation model is initialized with january climatological temperature and salinity and is forced by monthly mean comprehensive ocean atmosphere data set surface wind stress and heat flux by restoration of the surface salinity to monthly mean climatology and by flows through the open boundaries both the model computed tracer and velocity fields produced using the semiprognostic method show significant improvement over those produced by a purely prognostic calculation drift of the tracer and velocity fields away from climatology is greatly reduced further convective mixing is explicitly represented thus improving the utility of results over those obtained from pure diagnostic calculations the velocity fields obtained with the new approach are somewhat more realistic than those obtained from pure diagnostic calculations the method reproduces many well known circulation features in the region including the labrador current the gulf stream and the north atlantic current more significantly the method reproduces reasonably well the seasonal evolution of temperature and salinity in the region despite the fact that the model s tracer fields are not constrained directly by the new method this result suggests that the semiprognostic approach will be useful for examining the evolution of tracers that are not easily determined by observations

variability of sea surface height and circulation in the north atlantic forcing mechanisms and linkages abstract simulations with a coarse resolution global ocean model during 1958 2004 are analyzed to understand the inter annual and decadal variability of the north atlantic analyses of empirical orthogonal functions eofs suggest relationships among basin scale variations of sea surface height ssh and depth integrated circulation and the winter north atlantic oscillation nao or the east atlantic pattern eap indices the linkages between the atmospheric indices and ocean variables are shown to be related to the different roles played by surface momentum and heat fluxes in driving ocean variability in the subpolar region variations of the gyre strength ssh in the central labrador sea and the nao index are highly correlated surface heat flux is important in driving variations of ssh and circulation in the upper ocean and decadal variations of the atlantic meridional overturning circulation amoc surface momentum flux drives a significant barotropic component of flow and makes a noticeable contribution to the amoc in the subtropical region momentum flux plays a dominant role in driving variations of the gyre circulation and amoc there is a strong correlation between gyre strength and ssh at bermuda

multifrequency polarimetric synthetic aperture radar observations of sea ice the first known fully polarimetric airborne synthetic aperture radar sar data set of sea ice is introduced images were acquired in the beaufort bering and chukchi seas in march 1988 during a campaign for validation of defense meteorological satellite program special sensor microwave imager radiometer ice products statistics of the magnitude phase and polarization of complex backscattered signals recorded by the jet propulsion laboratory three frequency sar are examined in detail for various scenes with different ice characteristics the full stokes matrix information generated from c l and p band data characterize the scattering behavior of different ice types polarization ratios and phase differences between linear copolarized returns are used for discrimination between particular image features and mechanisms are suggested for the observed polarimetric characteristics results indicate that combinations of frequency and polarization enhance current capability to distinguish ice of different properties using single frequency fixed polarization micro wave radar a specific example is the polarimetric identification of new ice formation which may not be easily distinguished in ers 1 5 3ghz vv polarization sar data such findings are consistent with theoretical model simulations of scattering characteristics of sea ice overall these preliminary results demonstrate that radar polarimetry will likely add a new dimension to our current capability for extracting geophysically important ice variables using radar remote sensing methods

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

high energy ephemeral stream deltas an example from the upper silurian holmestrand formation of the oslo region norway abstract the late orogenic deposits of the upper silurian holmestrand formation of the oslo region norway comprises sharply alternating ephemeral stream and sandy beach face deposits the formation is divided into three units reflecting the down slope change on an extensive ephemeral stream delta system the lower unit of the formation comprises ephemeral stream sediments as stacked sheet sandstone bodies deposited on a broad coastal floodplain each sheet is bounded by a lower intraclast strewn scoured surface overlain by massive horizontally stratified low angle and trough cross bedded sandstones in the medial and upper units of the formation the ephemeral stream sediments alternate with beach face deposits forming ephemeral stream deltas the ephemeral stream deltas are marked by intraclast strewn scoured surfaces and dominated by trough cross bedded sandstones they may contain escapement burrows marine bioturbation and wave working of the delta surfaces the beach face deposits are composed of alternating irregular horizontally stratified sandstones with current and wave ripple cross lamination and tabular cross bedded sandstones deposition took place in a coastal non to micro tidal shallow water environment influenced by longshore currents and low palaeo wave energy seven ichnotaxa are recognized and support the sedimentary record of a transitional environment between a broad coastal floodplain and a shallow marine environment the presence of calcretes indicate an arid to semi arid palaeo climate

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

c band sar observations of marginal ice zone rheology in the labrador sea synthetic aperture radar sar imagery collected during the limex 87 field program in the labrador sea is used to qualitatively interpret the deformation modes in play throughout the experiment the ice cover exhibited two distinct rheologies separated by a clear line of shear a nonlinear viscous outer regime and a quasi brittle inner regime a single constitutive relation capable of effectively modeling both is unlikely within a plastic rate independent formulation rate dependent effects are discussed in relation to favoring brittle fraction in an otherwise ductile material

physical volcanology of the submarine mariana and volcano arcs narrow beam maps selected dredge samplings and surveys of the mariana and volcano arcs identify 42 submarine volcanos observed activity and sample characteristics indicate 22 of these to be active or dormant edifices in the volcano arc are larger than most of the mariana arc edifices more irregularly shaped with numerous subsidiary cones and regularly spaced at 50 70 km volcanos in the mariana arc tend to be simple cones sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 counterclockwise from that trend suggesting a strong fault control on the distribution of arc magmas volcanos in the mariana arc are generally developed west of the frontal arc ridge on rifted frontal arc crust or new back arc basin crust volcanos in the central mariana arc are usually subaerial large 500 km3 and spaced about 50 70 km apart those in the northern and southern marianas are largely submarine closer together and generally less than 500 km3 in volume there is a shoaling of the arc basement around iwo jima accompanied by the appearance of incompatible element enriched lavas with alkalic affinities the larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos if the magma supply estimated at 10 20 km3 km of arc per million years at 18 n has been relatively constant along the mariana arc we can infer a possible evolutionary sequence for arc volcanos from small irregularly spaced edifices to large over 1000 km3 edifices spaced at 50 70 km the volcano distribution and basal depths are consistent with the hypothesis of back arc propagation into the volcano arc

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

depositional facies of high energy beach to offshore sequence comparison with low energy sequence a study of the nearshore zone and continental shelf of a high energy coast was made in the ventura port hueneme area of california during the summer of 1974 to investigate depositional facies characteristics and compare them with similar facies from a low energy coastal setting analysis of box cores can cores and vibracores permits recognition of three principal zones 1 nearshore facies backshore to 9 m water depth made up primarily of parallel ripple and cross bedded clean sand bioturbation is only locally significant 2 transition facies 9 to 18 m water depth a zone of fine sand and silty sand characterized by an increase in biogenic over physical sedimentary structures that are commonly preserved as laminated to burrowed beds wave ripple bedding and parallel lamination are important structures in this facies 3 offshore facies 18 m water depth sandy silt is the primary texture and bioturbation is the dominant sedimentary structure remnant parallel lamination is the only physical sedimentary structure present comparison of the results of this study with a previous description of a low energy beach to offshore sequence at sapelo island georgia indicates that the two areas do not differ greatly in their facies or overall vertical sequence of sedimentary structures the principal difference is in the thickness of the three facies the california facies are significantly thicker than their georgia counterparts this difference is in direct response to the role of higher wave energy on the california coast

two examples of subaqueously welded ash flow tuffs the visean of southern vosges france and the upper cretaceous of northern anatolia turkey abstract pyroclastic deposits interpreted as subaqueous ash flow tuff have been recognized within archean to recent marine and lacustrine sequences several authors proposed a high temperature emplacement for some of these tuffs however the subaqueous welding of pyroclastic deposits remains controversial the visean marine volcaniclastic formations of southern vosges france contain several layers of rhyolitic and rhyodacitic ash flow tuff these deposits include from proximal to distal settings breccia lapilli and fine ash tuff the breccia and lapilli tuff are partly welded as indicated by the presence of fiamme fluidal and axiolitic structures the lapilli tuff form idealized sections with a lower coarse and welded unit and an upper bedded and unwelded fine ash tuff sedimentary structures suggest that the fine ash tuff units were deposited by turbidity currents welded breccias interbedded in a thick submarine volcanic complex indicate the close proximity of the volcanic source the lapilli and fine ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm wave base in northern anatolia thick massive sequences of rhyodacitic crystal tuff are interbedded with the upper cretaceous marine turbidites of the mudurnu basin some of these tuffs are welded as in southern vosges partial welding is attested by the presence of fiamme and fluidal structures the latter are frequent in the fresh vitric matrix these tuff units contain a high proportion of vitroclasis and were emplaced by ash flows welded tuff units are associated with non welded crystal tuff and contain abundant bioclasts which indicate mixing with water during flowage at the base basaltic breccia beds are associated with micritic beds containing a marine fauna the welded and non welded tuff sequences are interbedded in an alternation of limestones and marls these limestones are rich in pelagic microfossils the evidence above strongly suggest that in both examples tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment the sources of the pyroclastic material are unknown in both cases we propose that the ash flows were produced during submarine fissure eruptions such eruptions could produce non turbulent flows which were insulated by a steam carapace before deposition and welding the welded ash flow tuff deposits of southern vosges and northern anatolia give strong evidence for existence of subaqueous welding

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

beaufort chukchi ice margin data from seasat ice motion the motion of sea ice in the summer and fall marginal zone of the beaufort and chukchi seas is observed by tracking ice features in sequential seasat synthetic aperture radar images four examples of ice motion fields are shown and discussed with translation observation density finer than one observation per 5 km for the most part in this region the components of proportional relative motion are shown to commonly have values in the range 0 1 to 0 25 day throughout the marginal zone which itself has a scale of the order of 100 km

recent changes in the north atlantic it has long been recognized that the atlantic meridional overturning circulation moc is potentially sensitive to greenhouse gas and other climate forcing and that changes in the moc have the potential to cause abrupt climate change however the mechanisms remain poorly understood and our ability to detect these changes remains incomplete four main interrelated types of ocean change in particular are associated in the literature with greenhouse gas forcing these are a slowing of moc overturning rate changes in northern seas which might effect a change in atlantic overturning including changes in the freshwater flux from the arctic and changes in the transport and or hydrographic character of the northern overflows which ventilate the deep atlantic a change in the trans ocean gradients of steric height both zonal and meridional which might accompany a change in the moc and an intensification of the global water cycle though as yet we have no direct measure of the freshwater flux passing from the arctic to the atlantic either via the canadian arctic archipelago or along the east greenland shelf and no direct measure yet of the atlantic overturning rate we examine a wide range of time series from the existing hydrographic record for oceanic evidence of the other anticipated responses large amplitude and sustained changes are found or indicated by proxy over the past three to four decades in the southward transport of fresh waters along the labrador shelf and slope in the hydrography of the deep dense overflows from nordic seas in the transport of the eastern overflow through faroe bank channel and in the global hydrologic cycle though the type and scale of changes in ocean salinity are consistent with an amplification of the water cycle we find no convincing evidence of any significant concerted slowdown in the atlantic overturning circulation

sea ice motion measurements from seasat sar images satellite imaging radar is expected to become the prime instrument for mapping the distribution and motion of sea ice on a periodic basis the observational independence from weather fog and clouds and the fact that it provides its own illumination are decisive advantages that imaging radar has in the remote sensing of sea ice the seasat spacecraft launched in 1978 included such an imaging radar and approximately one fourth of all seasat sar observations are of sea ice in the beaufort sea data from seven sequential orbits of the satellite were selected to study the capabilities and limitations of spaceborne radar application to sea ice mapping the results are encouraging 1 there is no difficulty in identifying homologue ice features on sequential radar images 2 the accuracy is entirely controlled by accuracy of the orbit data and the geometric calibration of the sensor 3 conventional radargrammetric methods serve well for satellite radar ice mapping 4 ground control points can be used to calibrate the ice location and motion measurements in the cases where orbit data and sensor calibration are lacking ice motion was found to be approximately 6 4 0 5 km per day however higher accuracies are feasible with better timing calibration of the sensor events and knowledge of the orbit the accuracy of pixel location was also determined over land areas it was found that with the use of one control point in 10 000 km2 the accuracy is about 150 m with higher density of control points 7 in 1000km2 the location accuracy improves to the image resolution i e 25 m this is applicable for both optical and digital data

an ash flow tuff emplaced in shallow water early miocene koura formation southwest japan abstract the t 4 tuff in non marine shallow water deposits of the early miocene koura formation southwest japan consists mainly of poorly sorted ash pumice and lithic lapilli and is about 20 m thick the tuff grades upward into a parallel to cross stratified surge like layer and further to a fine tuff layer and locally possesses gas escape pipes extending vertically from the base to the top the basal part of the tuff includes ripped up clasts and load marks are common on the sole armored lapilli commonly occur in the upper part especially in the stratified layer the features of the t 4 tuff are mostly those of subaerial ash flow tuff but load marks on the sole and ripped up clasts in the basal part are typical of subaqueous ash flow tuff armored lapilli imply that the ash flow was wet the ash flow from which the t 4 tuff was deposited is interpreted to have advanced across a shallow water body while pushing water aside and becoming wet the t 4 tuff may be a typical facies of ash flows emplaced in quite shallow water bodies

wind forcing of the atlantic thermocline along 32 n at low frequencies abstract the bermuda tide gauge record extends back to the early 1930s that sea level fluctuations there are highly coherent with dynamic height from hydrographic data has two interesting implications first it should contain information about the low frequency circulation of the atlantic furthermore because dynamic height contains information on heat storage it might on the limited timescales accessible in the record also contain clues about climate a simple model of wind forcing of the atlantic from the african coast to bermuda uses the levitus mean density data to estimate the long rossby wave speed as a function of longitude sea level and thermocline variability estimated this way are in remarkably good agreement with observations at periods of more than a few years duration the peak to peak sea level signal is 18 cm which is nearly 25 of the slope across the gulf stream at this latitude the model results suggest that the variability is largest somewhat to the east of bermuda fluctuatio

fan delta sequence in the archean fig tree group barberton greenstone belt south africa abstract the fig tree group of the barberton greenstone belt in south africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism in the southern part of the greenstone belt the lower fig tree conformably overlies the onverwacht group and can be grouped into either 1 volcaniclastic deposits consisting of pyroclastic and reworked volcanic detritus or 2 non volcanogenic terrigenous deposits of sandstone and conglomerate these petrologic groups are further divided into six lithofacies the lithofacies within the lower fig tree formation can be grouped into two facies associations 1 a subaqueous delta front association made up of fine grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented land derived sandstone and conglomerate and 2 a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy ephemeral stream processes on an alluvial fan surface the stratigraphic sequence coarsens upward from predominantly siltstone and fine grained sandstone at the base to pebble and cobble conglomerate at the top this sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow quiet water composition of clasts in the conglomerates indicates that immediately underlying greenstone belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations erosion of uplifted greenstone belt terrain was apparently only to shallow levels both tectonic and volcanic activity played a role in producing several cycles of fan delta progradation

variability of the simulated meridional heat transport in the north atlantic for the period 1951 1993 a 43 year ocean model simulation for the period 1951 1993 is analyzed with a focus on the meridional heat transport mht as a proxy for the strength of meridional overturning cell moc at 25 n the surface heat flux forcing associated with the north atlantic oscillation nao pattern is related to the variability in mht both on interannual and longer timescales the manifestation of the interannual and decadal oceanic response to nao is nonlocal as evidenced by the concentrated heat content and sea level variability at the gulf stream and north atlantic current regions and shown by the comparison of the model sea level variability to the altimeter data between low mht years in mid 1980s and high mht years in early 1990s the same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level mht and time series of the leading sea level mode are highly correlated which reflects the importance of the moc in determining the variability of the heat content of the whole basin also the model suggests that the mht moc has entered a very strong phase since the mid 1980s and this trend has continued up to the end of 1993 this behavior follows the general trend in the nao index

the seasonality of convective events in the labrador sea abstractmodeling deep convection is a key challenge for climate science here two simulations of the labrador sea circulation obtained with the regional ocean modeling system roms run at a horizontal resolution of 7 5 km are used to characterize the response of convection to atmospheric forcing and its seasonal variability over the period 1980 2009 the integrations compare well with the sparse observations available the modeled convection varies in three key aspects over the 30 years considered first its magnitude changes greatly at decadal scales this aspect is supported by the in situ observations second the initiation and peak of convection i e initiation and maximum shift by 2 3 weeks between strong and weak convective years third the duration of convection varies by approximately one month between strong and weak years the last two changes are associated with the variability of the time integrated surface heat fluxes over the labrador sea during winter and spring while the first resu

decadal variabilities of the upper layers of the subtropical north atlantic an ocean model study numerical simulations of the atlantic ocean during the period 1950 to 1989 using a sigma coordinate free surface numerical model show long term variabilities in the upper ocean subtropical gyre similar to those obtained from observations the simulations show how westward propagating planetary waves originated in the eastern north atlantic affect interdecadal variabilities of ocean properties such as the bermuda sea level the gulf stream position and strength and subsurface temperature anomalies in the western north atlantic special attention is given to the dramatic sea level drop at bermuda in the early 1970s which is accompanied by cooling of subsurface layers in the western north atlantic and a northward shift and weakening of the gulf stream following these events between 1970 and 1980 the cold temperature anomalies in the upper layers of the western north atlantic slowly propagated eastward and downward the strongest propagating signal in the model is found at 200 m depth suggesting that advection of anomalies downstream by the gulf stream current and changes in winter mixing are involved significant correlations were found between the sea level anomalies at bermuda and sea level anomalies in the eastern north atlantic up to eight years earlier sensitivity experiments with different atmospheric forcing fields are used to study the ocean response to observed sea surface temperature and wind stress anomalies it is shown that on decadal timescales the ocean model responds in a linear fashion to the combined effect of sst and wind stress anomalies a fact that might be exploited in future climate prediction studies

a shallow marine volcaniclastic facies model an example from sedimentary rocks bounding the subaqueously welded ordovician garth tuff north wales u k abstract volcaniclastic sedimentary rocks bounding the ordovician garth tuff in north wales were deposited in a shallow marine setting adjacent to a magmatic arc the volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz angular euhedral feldspar and volcanic rock fragments these grains are of a pyroclastic origin but have been reworked to various degrees sedimentary rock fragments rounded quartz muscovite biotite iron rich chlorite and various clay minerals occur in lesser amounts the sedimentary rocks can be divided into a proximal offshore to foreshore facies association poffa and an offshore to lower shoreface facies association olsfa the sand dominated poffa exposed near capel curig is characterized by large wave ripples hummocky cross stratification co sets of trough cross beds reactivation surfaces plane beds and graded turbidite layers in suspension deposited mudstone water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross stratification and by facies association vary from a few meters to around 30 m 45 50 m theoretical maximum in a 45 m thick stratigraphic section beneath the tuff eight kilometers to the southeast the garth tuff is bounded by a thick sequence of suspension deposited laminated mudstone with thin graded beds of siltstone and fine grained sandstone this facies association olsfa represents the introduction of material by distal turbidites and by pelagic sedimentation the olsfa was deposited in water depths well below storm wave base possibly in excess of 200 m no evidence of storm waves or surface generated currents occur until over 40 m above the tuff this study documents marine sedimentary rocks deposited in water depths ranging from foreshore to offshore as the bounding facies of the garth tuff it is thus reasonable to conclude that the garth tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study

classification of the global sentinel 1 sar vignettes for ocean surface process studies abstract spaceborne synthetic aperture radar sar can provide finely resolved meters scale images of ocean surface roughness day or night in nearly all weather conditions this makes it a unique asset for many geophysical applications initially designed for the measurement of directional ocean wave spectra sentinel 1 sar wave mode wv vignettes are small 20 km scenes that have been collected globally since 2014 recent wv data exploration reveals that many important oceanic and atmospheric phenomena are also well captured but not yet employed by the scientific community however expanding applications of this whole massive dataset beyond ocean waves requires a strategy to automatically identify these geophysical phenomena in this study we propose to apply the emerging deep learning approach in ocean sar scenes classification the training is performed using a hand curated dataset that describes ten commonly occurring atmospheric or oceanic processes our model evaluation relies on an independent assessment dataset and shows satisfactory and robust classification results to further illustrate the model performance regional patterns of rain and sea ice are qualitatively analyzed and found to be very consistent with independent remote sensing datasets in addition these high resolution wv sar data can resolve fine sub km scale spatial structure of rain events and sea ice that complement other satellite measurements overall such automated sar vignettes classification may open paths for broader geophysical application of maritime sentinel 1 acquisitions

sea ice displacement from seasat synthetic aperture radar images obtained by a synthetic aperture radar on seasat have been used to measure sea ice displacements over a three day interval in october 1978 the position of a natural ice feature was measured on 5 october and again on october 8 the difference in these two positions is a displacement the displacement of many features was measured the tracked features lie roughly along a line and are quite dense about 2 km apart over a distance of 865 km the displacements are about 20 km displacement errors grow with distance from shore becoming as large as 3 km the graph of displacement versus distance has occasional discontinuities of several kilometers displacement discontinuities are accurate to 0 07 km along track and 3 of their magnitude across track

the submarine eruption and erosion of surtla surtsey iceland abstract surtla is the site of a short lived submarine vent which built basaltic elastic deposits almost to sea level in 1963 early in the eruption of surtsey since then wave and current activity have eroded the volcanic pile such that in july 1981 its top was a fairly level plateau 45 m below sea level and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules this winnowed layer was underlain by a nonreworked poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles steam explosions and quench brecciation during the eruption the explosion violence and associated comminution increased as the pile built up to shallower water depths it is argued that at times of continuous effusion a cupola of steam was situated over the vent as indicated by scoriaceous spatter which shows agglutination and bread crust features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption

modelling hydrographic changes in the labrador sea over the past five decades abstract inter annual to inter decadal changes of hydrographic structure and circulation in the subpolar north atlantic are studied using a coarse resolution ocean circulation model the study covers 1949 through 2001 inclusive a time mean state nudging method is applied to assimilate the observed hydrographic climatology into the model the method significantly reduces model biases in the long term mean distribution of temperature and salinity which commonly exist in coarse resolution ocean models by reducing the time mean biases we also significantly improve the model s representation of inter annual to inter decadal variations in the central labrador sea the model broadly reproduces the heat and salt variations of the labrador sea water lsw as revealed by hydrographic observations model sensitivity experiments confirm that the low frequency hydrographic changes in the central labrador sea are closely related to changes in the intensity and depth of deep convection changes in surface heat flux associated with the winter north atlantic oscillation nao index play a major role in driving the changes in t s and sea surface height ssh changes in wind stress play a secondary role in driving these changes but are important in driving the changes in the depth integrated circulation the total changes in both ssh and depth integrated circulation are almost a linear combination of the separate influences of variable buoyancy and momentum fluxes

an ice water vortex at the edge of the east greenland current during the y mer 80 cruise in august 1980 we observed an ice feature of 60 km diameter having the shape of a breaking wave it lay at the edge of the east greenland drift ice in fram strait with its center at 79 15 n 00 38 e and the wave was breaking upstream relative to the east greenland current two conductivity temperature and depth sections across the feature in n s and e w directions revealed warm up to 4 3 c water some 60 km inside the polar front in lenses centered at 40 m depth but with an effect to beyond 600 m the temperature anomalies were accompanied by salinity anomalies so that there was little net effect on the density profile the form of the sections together with the ice distribution observed by vertical photography from a helicopter and surface motion observed by tracking four radar transponders all suggest that the feature is a vortex produced by an instability in the polar front it has similar characteristics to vortices investigated experimentally and theoretically by griffiths and linden 1981a b 1982 and observationally by wadhams et al 1979

the behaviour of subaerially produced pyroclastic flows in a subaqueous environment evidence from the roseau eruption dominica west indies abstract the roseau tephra 30 000 yrs b p with its source on the island of dominica represents the largest eruption in the caribbean in the last 200 000 years pyroclastic flows generated by the eruption flowed offshore into the grenada basin giving rise to a widespread layer of sediment gravity flow deposits on land the flow deposits are valley ponded and their granulometric characteristics allow them to be classified as weakly fines depleted ignimbrites offshore the characteristics of the sediment gravity flow deposits are consistent with deposition by numerous small scale events rather than a single large scale event a comparison of the on land and offshore deposits reveals an increase in the crystal lithic ratio in the latter this change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water mixing and fragmentation occurred not at the air water interface but as flows descended the steep submarine slopes of the island of dominica

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

long term coordinated changes in the convective activity of the north atlantic the north atlantic is a peculiarly convective ocean the convective renewal of intermediate and deep waters in the labrador sea and greenland iceland sea both contribute significantly to the production and export of north atlantic deep water thus helping to drive the global thermohaline circulation while the formation and spreading of 18 degree water at shallow to intermediate depths off the us eastern seaboard is a major element in the circulation and hydrographic character of the west atlantic for as long as time series of adequate precision have been available to us it has been apparent that the intensity of convection at each of these sites and the hydrographic character of their products have been subject to major interannual change as shown by aagaard 1968 clarke et al 1990 and meincke et al 1992 for the greenland sea in the ows bravo record from the labrador sea eg lazier 1980 et seq and at the panulirus hydrostation s site in the northern sargasso off bermuda eg jenkins 1982 talley and raymer 1982 this paper reviews the recent history of these changes showing that the major convective centres of the greenland and labrador seas are currently at opposite convective extrema in our postwar record with vertical exchange at the former site limited to 1000 m or so but with labrador sea convection reaching deeper than previously observed to over 2300 m as a result greenland sea deep water has become progressively warmer and more saline since the early 70 s due to increased horizontal exchange with the arctic ocean through fram strait while the labrador sea water has become progressively colder and fresher over the same period through increased vertical exchange most recently convection has become deep enough there to reach into the more saline nadw which underlies it so that cooler but now saltier and denser lsw has resulted

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

repeat pass interferometry with airborne synthetic aperture radar it is shown that interference can be observed by coherently combining pairs of either x or c band airborne synthetic aperture radar sar images from separate passes over the same test site coherence between separate images is obtained only if the aircraft is flown and the data are processed in such a way that each resolution cell in the two images is viewed with very nearly the same geometry successful repeat pass interferometric results were obtained from those passes flown by the ccrs convair 580 aircraft with flight line offsets of less than a few tens of meters a summary of the experiment the phase correction of nonrectilinear aircraft motion and the subsequent data processing is provided

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

extracting sea ice data from satellite sar imagery with the prospect of operational satellite sar s by the end of the decade there is a clear need to develop automated algorithms for the extraction of geophysical data about sea ice from highresolution radar imagery to this end we have developed techniques for distinguishing ice from open water and for resolving the details of deformation within areas 100 km square imaged by seasat sar the classification of ice and open water is based on the creation of a second band of image data consisting of the local variance of the original brightness the first band being a local average brightness in the space of these two variables ice and open water are separated into two distinct clusters the deformation is found on a 3 4 km mesh by local cross correlations of the brightness the strategy is to find a coarse displacement field with a highly averaged image and to proceed through a hierarchy of images with increasing resolution improving the accuracy of the displacements at each step comparison with manually determined displacement shows room for improvement in regions of high deformation by using smaller areas for cross correlation the concentration and deformation data are used together to determine localized regions of the scene where open water is produced or lost

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

north atlantic oscillation induced changes of the upper layer circulation in the northern north atlantic ocean 1 hydrographic data of the 1990s along world ocean circulation experiment sections a1e greenland ireland and a2 newfoundland france indicate a redistribution of cold low saline subarctic waters and warm saline subtropical waters in the upper layer of the northern north atlantic within about 2 years after the north atlantic oscillation had turned from a period with strong westerlies until 1995 to a period with weak westerlies in 1996 and 1997 in the latter period subarctic waters spread preferably southward along the continental slope east of newfoundland rather than eastward with the north atlantic current nac into the irminger iceland and west european basins as a consequence the subarctic front and the associated nac shifted eastward in the newfoundland basin where subarctic waters accumulate and westward in the iceland basin where a large warm and saline anomaly was found in 1996 and 1997 indicating a contraction of the subpolar gyre in the irminger basin the anomaly occurred in 1998 and 1999 whereas the cyclonic circulation in the irminger and iceland basins weakened it intensified in the newfoundland basin where a strengthened labrador current retroflection joined the nac this and the increased anticyclonic recirculation of subtropical waters in the newfoundland and west european basins caused a considerable reduction of the northward heat transport across 47 n in the upper layer in 1997 the anticyclonic circulation occurring in the west european basin suggested a northward expansion of the subtropical gyre

a subaqueous welded tuff from the ordovician of county waterford ireland abstract the metal man tuff mmt from the ordovician of county waterford ireland was emplaced and welded in water depths greater than the thickness of the pyroclastic flow the mmt is the basal member of the middle tramore volcanic formation mtvf of the 5 km thick tramore group the mmt consists of a 10 m thick basal graded zone that represents a pyroclastic flow consisting of angular clasts of black mudstone pumice gray flow banded rhyolite and pink massive rhyolite set in a matrix of non deformed ash shards and pumice maximum grain size grades from large cobbles and small boulders to pebbles the basal 10 30 cm is depleted with respect to the largest boulders resulting in an inversely graded basal layer the basal graded zone passes upward into a transition zone with a strong eutaxitic foliation defined by elongated fiamme of mudstone and flattened pumice overlying this is an upper welded zone with a pronounced eutaxitic foliation columnar jointing flattened ash shards and shards deformed around phenocrysts and spheroids the presence of these features indicate that the deposit is welded was hot and was in motion as the shards deformed the mmt represents a pyroclastic flow that was a hot primary product of an eruption rather than re mobilized cold pyroclastic debris the mmt is bounded by suspension deposited fine grained tuff tuffaceous mudstone and terrigenous mudstone deposited below storm wave base many of the mudstone horizons contain brachipod faunas from shelf depth water nowhere in the 5 km thick tramore group is there terrigenous sandstone evidence of unidirectional flowing water nor any indication of shoreline alluvial environments or subaerial exposure it thus seems reasonable to conclude that the mmt was emplaced and welded subaqueously the geochemistry of the mmt is typical of other high silica 70 78 wt sio2 rhyolite from the ordovician of ireland the chemistry of the mmt is consistent from top to bottom allowing it to be distinguished from associated syndepositional intrusive rock and fine grained tuffs

hydrographic changes in the labrador sea 1960 2005 abstract the labrador sea has exhibited significant temperature and salinity variations over the past five decades the whole basin was extremely warm and salty between the mid 1960s and early 1970s and fresh and cold between the late 1980s and mid 1990s the full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s the freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the labrador sea sustained throughout the subsequent years it took only a decade for the whole water column to lose most of its excessive freshwater reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s if the recent tendencies persist the basin s storages of salt and heat will fairly soon likely by 2008 exceed their historic highs the main process responsible for the net cooling and freshening of the labrador sea between 1987 and 1994 was deep winter convection which during this period progressively developed to its record depths it was caused by the recurrence of severe winters during these years and in its turn produced the deepest densest and most voluminous labrador sea water lsw1987 1994 ever observed the estimated annual production of this water during the period of 1987 1994 is equivalent to the average volume flux of about 4 5 sv with some individual annual rates exceeding 7 0 sv once winter convection had lost its strength in the winter of 1994 1995 the deep lsw1987 1994 layer lost communication with the mixed layer above consequently losing its volume while gaining heat and salt from the intermediate waters outside the labrador sea while the 1000 2000 m layer was steadily becoming warmer and saltier between 1994 and 2005 the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air sea heat fluxes in the winter of 1999 2000 this change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent lsw class lsw2000 penetrating deeper than 1300 m this layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass implying that lsw2000 is likely to follow the fate of its deeper precursor lsw1987 1994 the increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid depth perimeter of the labrador sea the near bottom waters originating from the denmark strait overflow exhibit strong interannual variability featuring distinct short term basin scale events or pulses of anomalously cold and fresh water separated by warm and salty overflow modifications regardless of their sign these anomalies pass through the abyss of the labrador sea first appearing at the greenland side and then about a year later at the labrador side and in the central labrador basin the northeast atlantic deep water 2500 3200 m originating from the iceland scotland overflow water reached its historically freshest state in the 2000 2001 period and has been steadily becoming saltier since then it is argued that lsw1987 1994 significantly contributed to the freshening density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening lsw the hydrostatic adjustment to expanding lsw or both

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

the effect of possible taylor columns on the summer ice retreat in the chukchi sea the chukchi sea topography consists of a broad flat 50 m deep plain with two prominent shoals herald and hanna which have horizontal dimensions of about 100 km and rise to depths of about 30 m herald shoal in particular is a prominent isolated seamount which because of the warm water flux in summer through bering strait has a warm 0 1 m s 1 flow incident upon it examination of active and passive microwave imagery of the region for 1992 1994 shows that as the general ice cover recedes the ice remains preferentially over herald shoal for 3 4 weeks after the surrounding ice has melted a scale analysis suggests that the cause of the ice persistence is the formation of a taylor column over the shoal which traps cold water and ice above it a similar trapping of ice occurs over hanna shoal and on its eastern slope but the response is complicated by a very different topography and the northward flow down barrow canyon

north atlantic interdecadal variability oceanic response to the north atlantic oscillation 1865 1997 abstract in contrast to the atmosphere knowledge about interdecadal variability of the north atlantic circulation is relatively restricted it is the objective of this study to contribute to understanding how the north atlantic circulation responds to a forcing by the north atlantic oscillation nao on interdecadal timescales for this purpose the authors analyze observed atmospheric and sea surface temperature sst data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the nao for the period 1865 1997 in agreement with previous studies it is shown that the relationship between the local forcing by the nao and observed sst anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating sst anomalies a comparison between observed and modeled sst anomalies reveals that the model results can be used to assess interdecadal variability of the north atlantic circulation the observed

diffusion of sea ice the field of motion of sea ice is such that a lagrangian element will be progressively deformed the deformation can be resolved into a smooth part analogous to the distortion of a highly viscous fluid plus a random and discontinuous part having some similarity to the random motion of a turbulent fluid and the discontinuous motions of plate tectonics the lagrangian element spreads out at a rate determined by the mean field and by the random field but the mean field usually is more important the importance of the random field is that it gradually mixes the lagrangian element with the surrounding ice a mechanism for this mixing is discussed the simplest mode of deformation for sea ice may be a finite shear displacement along a single crack if the ice deforms in this way first along one crack then along another and eventually along a sequence of randomly oriented cracks the result will be a complete reshuffling of the ice pack nearby points disperse compact regions become fragmented this model can be tested against observations of ice motion in terms of the relative diffusion of pairs of points the model predictions have a weak resemblance to actual ice behavior

sedimentology of a lacustrine fan delta system miocene horse camp formation nevada usa a 1600 m thick succession of the miocene horse camp formation member 2 exposed in east central nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan delta environments and nearshore and offshore lacustrine environments these four depositional environments are distinguished by particular associations of individual facies 14 defined facies subaerial and subaqueous fan delta facies associations include ungraded matrix and clast supported conglomerate normally graded matrix and clast supported conglomerate ungraded and normally graded sandstone and massive to poorly laminated mudstone subaqueous fan delta deposits typically have dewatering structures distorted bedding and interbedded mudstone the subaerial fan delta environment was characterized by debris flows hyperconcentrated flows and minor sheetfloods the subaqueous fan delta environment by debris flows high and low density turbidity currents and suspension fallout the nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments high energy oscillatory wave currents probably related to a large fetch reworked grains as large as 2 cm into horizontally stratified sand and gravel offshore directed currents produced uncommonly large typically 1 2 m thick trough cross stratified sandstone in addition stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high energy conditions alternating with periods of carbonate precipitation under reduced energy conditions massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout a depositional model constructed for the horse camp formation member 2 precludes the existence of all four depositional environments at any particular time rather phases characterized by deposition in subaerial fan nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan delta subaqueous fan delta and offshore lacustrine deposition this model suggests that high energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge thus preventing development of a subaqueous fan delta environment and promoting development of a well defined nearshore lacustrine environment low energy nearshore currents induced by shallow water along the

processes and imagery of first year fast sea ice during the melt season an analysis of first year fast sea ice during the melt season has been made by using surface measurements and aircraft radar and photographic imagery obtained during a field study near prince patrick island in the canadian archipelago from june 13 to july 13 1982 and satellite imagery from landsat and seasat distinct changes observed in the properties of the snow layer and the sea ice were a temporary increase in small scale surface roughness caused by formation of nodules of ice at the snow ice interface extensive snow melt and surface flooding development of surface water drainage networks and low topography around fractures and seal breathing holes and a rapid draining of much of the surface water from the extensive salinity profiles obtained two zones of rapid desalination in the first year ice were observed one zone extending from the air ice interface downward toward the center of the ice sheet that resulted from surface warming and drainage of the surface melt water through the ice and the other zone extending from the sea ice interface upward toward the center of the ice sheet that resulted from heating and separation of seawater and ice caused by a layer of low salinity meltwater beneath the ice formed from surface meltwater runoff aircraft radar imagery detected changes in the amount of surface water and in the development of topography surrounding drainage features similar changes were detected in coincident landsat multispectral scanner mss imagery of the study area and in seasat radar imagery and landsat mss imagery of the prince of wales strait from july 1978

decadal wind forcing of the north atlantic subtropical gyre abstract in the central north atlantic ocean there are large decadal scale fluctuations of sea level and of the depth of the thermocline this variability can be explained by low frequency rossby waves forced by wind the authors have used a simple model of ocean fluctuations driven by the coads wind stress curl and find that their model results and hydrographic data to the limited extent they can be compared meaningfully agree rather well the variation in wind curl over the ocean leads to surprisingly large north south variability in the computed oceanic response over the subtropical gyre the peak to peak sea level differences are as great as 20 cm and persist for many years these large variations could induce major errors in calculations of the mean ocean flow when hydrographic sections from many years are combined it appears possible however to correct for these wind induced effects to allow the lower frequency signals to be determined the westernmost edge of our oceanic calculations is at the

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions